摘要:

Chemical Society Reviews Vol 3 No 3 1974 Page Quantitative Drug Design 273By G. Redl, R.D. Cramer tert., andC. E. Berkoff Tunable Lasers By J. K. Burdett and M. Poliakoff 293 Formation of Hydrocarbons by Micro-organisms By C. W. Bird and J. M. Lynch 309 The Photochemistry of Olefinic Compounds ByJ. D. Coyle 329 Isomer Enumeration Methods ByD. H. Rouvray 355 NYHOLM MEMORIAL LECTURE Forward from Nyholm’s Marchon Lecture By H. Frank Halliwell 373 The Chemical Society London Chemical Society Reviews Chemical Society Reviews appears quarterly and comprises approximately 25 articles (ca. 600 pp) per annum. It is intended that each review article shall be of interest to chemists in general, and not merely to those with a specialist interest in the subject under review.The articles range over the whole of chemistry and its interfaces with other disciplines. Although the majority of articles are specially commissioned, the Society is always prepared to consider offers of articles for publication. In such cases a short synopsis, rather than the completed article, should be submitted to The Editor, Reports and Reviews Section, The Chemical Society, Burlington House, Piccadilly, London, W 1V OBN. Members of The Chemical Society may subscribe to Chemicul Society Reviews at €3.00 per mum;they should place their orders on their Annual Subscription renewal forms in the usual way. Non-members may order Chemical Society Reviews for 210-00per mum (remittance with order) from: The Publications Sales Officer, The Chemical Society, Blackhorse Road, Letchworth, Herts., SG6 lHN, England. 0Copyright reserved by The Chemical Society 1974 Published by The Chemical Society, Burlington House, London, W1V OBN Printed in England by Eyre & Spottiswoode Ltd, Thanet Press,Margate

ISSN:0306-0012    

DOI:10.1039/CS97403FP005

出版商:RSC    

年代:1974

数据来源: RSC

ISSN:0306-0012    

DOI:10.1039/CS97403FX009

出版商:RSC    

年代:1974

数据来源: RSC

ISSN:0306-0012    

DOI:10.1039/CS97403BX011

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

Quantitative Drug Design By G. Redl, R. D. Cramer tert., and C. E. Berkoff TECHNOLOGY ASSESSMENT, SMITH KLINE & FRENCH LABORATORIES, PHILADELPHIA, PENNSYLVANIA 19101, U.S.A. 1 Introduction The discovery and clinical availability of unique drug therapy are increasingly governed by a number of severe competitive and regulatory c0nstraints.l To develop a potentially useful therapeutic agent to the point of U.S. regulatory approval typically demands an investment of some $11.5 million in support of an 8-10 year programme. By 1977 the necessary investment is expected to rise to $40 million.2 With little understanding at the molecular level of how one designs a new agent in any field of clinical need, only 1 in perhaps 15000 com-pounds emerges as a commercial product.Any rational synthetic strategy for improving these odds must appeal to those concerned with the practical aspects of discovery and development of new drug therapy. Although still an emerging discipline, quantitative drug design can, in very practical terms, contribute both to the discovery of new therapeutic agents and to the progress of biomedical research in general. While, to our knowledge, there is no example of any molecule having found its way into the physician’s armamentarium via quantitative structure-activity analysis, developing tech- niques are becoming increasingly more capable of directing synthetic effort from compounds that have a low probability of success to structural variations often overlooked by the most experienced and imaginative medicinal chemist.These powerful tools become even more relevant as modern organic chemistry makes vast numbers of compounds synthetically accessible. For example, following the discovery of penicillin (1) as an effective and useful antibiotic in .CH,COHN ? I;’ H ‘C0,Na Sodium Penicillin G (1) F.A. Robinson, Chem. in Britain, 1974,10, 129. L. H. Sarett, Research Management, 1974, 18. Quantitative Drug Design the treatment of bacterial disease, many laboratories set out to modify the structure of the molecule to improve its efficacy while reducing adverse side- effects. Penicillin thus became a ‘lead’ structure -the point of departure for further synthesis and biological testing. In the typical quest for a better drug the lead structure is modified in stepwise fashion, changing both substituent groups and parent ring system.It is a sobering thought, however, that in the case of penicillin, for 20 substituents (including hydrogen) attached to 3 of the 5 available positions of the phenyl ring, the number of possible analogues is 36537. Coupling these variations with different amide side-chains, structural changes at other positions on the thia-azabicycloheptane nucleus, and variation of the heterocyclic ring system per se (including stereochemical possibilities at extant asymmetric centres), the medicinal chemist is rapidly confronted with astronomic numbers of possible structures; many of these may be arguably reasonable in terms of their biological potential, their synthetic (or semi-synthetic) accessibility, and, in very commercial terms, their patentability.Indeed, enormous effort has already been expended in the synthesis of literally thousands of penicillins and the related cephalosporins, very few of which have in fact emerged as successful therapeutic agents .The discovery of quantitative struc t ure-ac t ivi t y relationships can reduce such problems to practical dimensions and increase the chances of a synthetic programme successfully meeting its objectives. The ultimate goal of drug design is to enable the chemist to design compounds with a prescribed biological profile. The achievement of this aim still appears to be far in the future and will require breakthrough advances -especially in our understanding of biological and disease processes at the molecular level. Two approaches to the problem have emerged and both have been of significant (albeit indirect) value in the discovery and development of new drug therapy. The biochemical approach is to design new drugs on the basis of established and/or hypothesized models and mechanisms of drug action at the molecular level.The other, and the focus of this Review, is primarily a statistical approach in which a variety of computational techniques are applied to establish cor-relations between the level of a given biological activity and other measurable (or calculable) properties associated with the chemical structure of the molecule. The separation of the two approaches is, of course, artificial; they must coalesce if the ultimate goal of drug design is to be achieved. It is, in fact, an indication of the state of the art that the two approaches can be pursued with only very small degrees of overlap or cross-impact. Virtually all of the published literature in drug design has dealt with the analysis of series of chemically closely related compounds, where the primary objective is the design of the optimally active compound within a lead series, i.e.‘lead optimization’. In practice, drug-design problems cannot be restricted to a selected compound series. Although very few attempts have been described dealing with the problem of anaIysis of structurally diverse compounds, its im- portance and challenge have persuaded us to give strong emphasis in our own studies to this particular aspect of the overall problem. This we call ‘lead gen- erat ion’.Redl, Crarner and Berkof Drug design has been the subject of numerous articles and monographs.3 In the review that follows we attempt to describe the state of the art in very practical terms to a general chemical readership. In this context, it is worth noting that much of the developing methodology lends itself to the correlative analysis of chemical structure with any measurable or calculable property of a molecule, whether biological, chemical, or physical. Thus, appropriate quantitative drug- design techniques can, for example, also be applied to an understanding of chemical reactivity, interpretations of spectral behaviour, and to the design of other new chemical products, such as paints, plastics, and pesticides.2 Genera1Considerations A. Biological.-We define a drug as a chemical substance that exerts a repro-ducibly observable effect on a biological system. The effect may be called biological activity or biological response, and is usually dose-dependent. The dose is the quantity or concentration of the administered compound; the response is the observed effect, which can be as objective as the percent inhibition of an enzymatic reaction or a change in heart rate, or as subjective as a change in the mood or posture of a test animal. In view of the complexity of factors that affect the in vivo activity of a drug (absorption, transport, tissue distribution, metabolism, etc.), it is not surprising that structureactivity correlation has been most successful with data obtained from less complex biological systems such as isolated organs, cell cultures, or purified enzymes.To reduce the dimensions of dose and response to a single parameter, biolo- gical activity is usually expressed as log(l/c), where c is the minimum concen- tration required to produce a specific response. Its evaluation requires inter- polation of experimental results obtained at several concentrations; in practice this is carried out only for the relatively few compounds of special interest. In most primary test systems (screens), where compounds are initially tested at a single dose only, the activity may be expressed as a percentage (P) of some maximal response.For this type of data the logit transformation, ln[P/(100-P)], can be useful. Experimental variability of biological measurements is much larger than that of most physical or chemical measurements. For meaningful correlations, the need for reliable biological data with well-defined confidence limits is as evident as it is frequently ignored. The drug designer should satisfy himself that the experimental design of the test system, in particular the nature and repro- ducibility of the observed effects, provides data which are appropriate for analysis. 3 (a) E. J. Ariens, ‘Drug Design’, Academic Press, New York and London, 1971 ; (b) W. P. Purcell, G. E. Bass, and J.M. Clayton, ‘Strategy of Drug Design’, John Wiley & Sons, New York, 1973 ; (c) A. Burger, ‘Medicinal Chemistry’, Part I, Wiley-Interscience, New York, 1970, pp. 25-245. J. Thomas, C. E. Berkoff, W. Flagg, J. J. Gallo, R. F. Haff, C. A. Pinto, C. Pellerano, and L. Savini (manuscript submitted for publication). Quantitative Drug Design B. Computational.-In its most general sense, quantitative drug design embraces all attempts to relate biological activities mathematically to other properties of molecular structure. Inasmuch as the mathematically simplest relationships among several properties are linear equations, the majority of quantitative drug-design methods are essentially attempts to derive a linear equation of the form (l), where the xi are structural properties and the coefficients at emerge m biological activity = a0 + 2am i= 1 from the analysis.This equation allows prediction of the biological activity of any compound for which the xt are known. Procedures for obtaining the m coefficients in equation (1) require an experi- mentally determined biological activity and a group of rn structural properties for each of the n compounds in a series. Because of the relatively low precision of biological data and the uncertainty that a linear correlation model is appli- cable to any specific structure-activity relationship, good practice in drug design requires that n be considerably larger than rn, preferably by a factor of five or more. The number of compounds in excess of the minimum (m + 1) required for an analysis is called the number of degrees of freedom.The set of coefficients at that constitutes the solution for equation (1) is usually obtained from the data by the least-squares method using multiple-regression computer programs. Statisticians have developed a number of criteria5 to evaluate the appropriateness of a regression equation such as (1) for correlating the data and for extrapolating to new results. The most important of these are: the multiple correlation coefficient (R), where R2 is the proportion of variation within the observations that is explained by the equation; and the F-test, an assessment of the probability (p) that the relationship derived is actually a chance occurrence.It should be recognized that over-reliance on statistical criteria to the neglect of common sense is a dangerous and all too frequent abuse.6 Though undoubtedly an oversimplification, the assumption of a linear relation among biological activity and a few structural parameters has proved useful among series of related compounds where the biological activity can be quantified. To handle more complex drug-design problems, for example the analysis of data from the testing of structurally diverse compounds, the various methods of pattern recognition may ultimately prove useful. The recognition of patterns involving two or three variables is readily achieved by the scientist, without computer aid, using spatial representations of the data. For example, the existence of any relationship, linear or otherwise, between activity and a single function of structure is easily detected by graphical means.G. W. Snedecor and W. G. Cochran, ‘Statistical Methods’, Iowa State University Press, Ames, 1967; see also ref. 36, p. 27. S. H. Unger and C. Hansch, J. Medicin. Chem., 1973, 16, 745. Redl, Cramer and Berkof A correlation of activity with two functions of structure can still be recognized by plotting the two functions on Cartesian axes with an activity classification indicated for each compound/point. For the analysis of multivariate data exceeding three dimensions, however, human cognitive processes are poorly suited. While there are nearly as many computational methods for pattern recognition as workers in the field, those recommended for chemical problems7 and employed in drug design8-11 include various forms of cluster analysis, discriminant analysis, and linear learning machines.Cluster analysis, l2 a mathematical technique for classification, seeks similar sets of data; similarity is defined in terms of a ‘distance’ between points representing the objects (Le., compounds) in multidimensional variable space. Different clustering procedures can yield quite different classifications of the same sets of data, depending on the precise definition of similarity and the heuristic methods used to simplify the computational burdens. Discriminant analysis, a technique firmly grounded in classical statistical theory, seeks a linear equation which can be used to place an unknown object into either of two classes (Le., active or inactive). In geometric terms, it is obvious that discriminant analysis defines a hyperplane which optimally bisects a multidimensional data space.Linear learning machines,l4 heuristic methods originating in the field of artificial intelligence, are useful in deriving linear equations. The power of any of these methods is enhanced by preprocessing the data in a variety of ways which may weigh all features equally, give weight to the features expected to have particular importance, or completely remap the features. One objective of such remapping is to reduce the data to two or three dimensions while retaining as much as possible of the original information.This kind of two- or three-dimensional representation can then be displayed by the computer, allowing visual pattern recognition by the scientist.7 3 Lead-optimizing Techniques Lead optimization is the phase of the drug development process in which the principal goal is to improve the biological profile of a lead compound, typically by increasing the separation between a dose that produces desirable activity and a dose that produces undesirable side-effects. A lead-optimizing programme, which focuses on synthesizing and testing structural modifications of the lead, ‘I B. R. Kowalski and C. F. Bender, J. Amer. Chem. SOC., 1972, 94, 5632; 1973,95, 686. * Y. C. Martin, J. B. Holland, C. H. Jarboe, and N. Plotnikoff, J. Medicin.Chem., 1974, 17, 409. B. R. Kowalski and C. F. Bender, J. Amer. Chem. SOC., 1974,96,916. lo K. C. Chu, R. J. Feldmann, M. Shapiro, G. F. Hazard, jun., C. L. Chang, and R. Geran, Abstracts of 167th American Chemical Society Meeting, April 1974, CHLT 24; K. C. Chu, Analyt. Chem., 1974, in the press. l1 K. H. Ting, R. C. Lee, G. W. A. Milne, M. Shapiro, and A. M. Guarino, Science, 1973, 180, 417. l2 R. M. Cormack, J. Royal Statistical ASSOC., 1971, 134, 321. a T. W. Anderson, ‘An Introduction to Multivariate Statistical Analysis’, Wiley, New York, 1958. l4 N. J. Nilsson, ‘Learning Machines’, McGraw-Hill, New York, 1965. Quantitative Drug Design requires the identification of a particular structural moiety which is associable with the observed biological activity of a compound.This problem is by no means trivial, as is well illustrated by the belated discovery of the cephalosporin antibiotics. The principal structural difference between the penicillins and the cephalosporins is that the thiazolidine ring of the former has been expanded to the corresponding thiazine ring. Nevertheless, some fifteen years of synthetic effort failed to uncover the worth of this relatively minor structural modification; the exciting antimicrobial utility of the cephalosporins was obliged to await the testing of soil microflora samples for its discovery. Although there clearly can be no final answer to the question of which part of a structure is responsible for the observed biological effects of a molecule, in practical terms the problem can be tentatively resolved.Choice of the structural moiety defining the scope of a lead-optimizing synthetic programme usually rests on objective and very practical considerations such as synthetic accessibility and potential patent- ability; unquestionably it also embraces more subjective issues such as a scientist’s propensity for one particular type of chemistry over another. A. The Physicoche~nical Model.-Introduction of different substituents into a lead molecule alters its chemical and consequently its biological properties in ways which can often be related linearly to the physicochemical properties of the substituents themselves. If such a relation can be found, knowledge of the physicochemical properties of unexplored substituents will permit prediction of the activities of the unsynthesized members of a lead series.These considerations form the basis of the physicochemical model which underlies the development of the ‘multiple parameter’ or ‘linear free energy’ approach to drug design. The wealth of publications devoted to the physicochemical approach, generally associated with the name of Hansch, suggests it to be by far the most popular of quantitative drug-design methods. The physicochemical properties associated with a substituent may be loosely classified as electronic, steric, or solvent partitioning. However, it is not clear which laboratory measurements or calculated parameters best define a class of substituent properties.15 For example, very different steric effects can be estimated from solution kinetics, crystallography, molecular models, quantum mechanics, and polarizability data.Understandably perhaps, the number of physicochemical substituent properties that have been tried in correlation studies has now reached 32;lS many of these, however, are highly inter~orre1ated.l~ The vast majority of published studies have been based on the Hanschrr, often augmented by the Harnmett 0, and occasionally by one or more other properties. Although the effect of the oillwater distribution ratio on drug action had been recognized and even quantified in the nineteenth century,l* it was Hansch J. Shorter, Quart. Rev., 1970, 24,433. l6 Reference 36, pp. 4345. l7 A.Leo, C. Hansch, and C. Church, J. Medicin Chem., 1969, 12, 766. la E. Overton, 2. physiol. Chem., 1897, 22, 189. Red& Cramer and Berkof who constructed a theoretical rationale for the effect,l9 developed a standard reference system for its measurement,20 and demonstrated its general relevance withnumerouscorrelations.21Thenvalue of a substituent is defined as log(P/Po), where P is the partition coefficient between octanol and water for the substituted compound and POthe coefficient for the unsubstituted compound. This value not only is essentially independent of compound series, but is also well approxi- mated for an unknown substituent by summing then values of the substituent fragments.22 The additive property ofrr values is extremely useful when synthesis of a compound containing a new substituent is being considered.The classical Hammett 0, the second most widely used substituent property, is an expression of the electronic effect of a substituent. Regression equation (2), typical of correlation results involving physico- chemical substituent properties, was obtained for a series of nitroso-ureas (2) tested for their ability to delay the growth of a solid tumour, the Lewis lung carcinoma, in mice.23 IOg(l/C) = -0.08(10gP)~ + 0.14(10gP) + 1.23 (2) [Statistics: n = 13; R2 = 0.585; F2,10 = 7.1 (p <0.025)] The log P values in equation (2) are the logarithms of experimentally determined octanol-water partition coefficients for the whole molecule; log(1 /c) and the statistical indices are as explained above (cf.Section 2). While the relatively low value of R2 indicates that a substantial proportion of the variation in the observed biological data has yet to be explained, the results of the Rest and the large structural variation among the substituent groups R (from adamantyl to carboxycyclohexyl) leave little doubt that the structure-activity relationship implied by equation (2) is real. As is often the case, equation (2) contains a term in (log P)2 and thus describes a parabolic rather than a linear relationship between hydrophobicity and biological activity. The negative value of the parabolic term in (2) indicates further that there is a particular value of the partition coefficient for which lo C.Hansch, Accounts Chem. Res., 1969, 2, 232. C. Hansch and T. Fujita, J. Amer. Chern. SOC., 1964, 86, 1616. C. Hansch and W.J. Dunn, J. Pharm. Sci., 1972, 61, 1. 28 A. Leo, C. Hansch, and D. Elkins, Chern. Rev., 1971,71,525; G. G.Nys and R. F. Rekker, Chimie Therapeutique, 1973,5, 521. a3 J. A. Montgomery, J. G. Mayo, and C. Hansch, J. Medicin. Chem., 1974,17,477. Quantitative Drug Design biological activity will be maximized. Several theoretical attempts to classify series of drugs according to their optimal partition coefficient have ap- peared.19~21~ In principle, p hysicochemicall y based struc ture-ac t ivity correlation equations should be useful in lead development programmes by allowing predictions of the activity of unsynthesized compounds.Resulting data would then of course be incorporated into refined analyses. In practice, reliable equations often either fail to appear or emerge only after interest in further development of a series has waned. An important reason can be that the properties of the initial members of a series are poorly suited for analysis. Thus recent efforts to present physico- chemical data in a form that would be useful at an early stage of lead develop- ment should be welcome. For example, Craig has advocated the use of the (T versus T scatter diagram shown in Figure 1.25 Selection of representative sub- stituents from each of the four quadrants of the graph in Figure 1, in the planning CO2H -0.25 06 B.r I -2.0 -1.6 -1.2 -0.8 -0.4 0 F 0.4 0.8 1.2 1.6 I I I I I 1.1 I I 7r MeCONH 0 SMe o~ed"0.25 Me Et t-B.uty1 ---0.50 NH2 .NMeZ0 ---0.75 Figure 1 Relationship between the Hammett u and Hansch r values of some commonly used para-substituents.(Reproduced by permission from J. Medicin. Chem., 1971, 14,682.) R. Franke and W. Schmidt, Acta Biol. Med. Germ., 1973, 31, 273; T. Higuchi and S. S. Davis, J. Pharm. Sci.,1970, 59, 1376. P.N. Craig, J. Medicin. Chem., 1971,14,680. Red&Cramer and Berkof stages of a programme, increases the chances of early discovery of the o/v region of maximum activity. Generalizing this approach to five important physicochemical parameters, Hansch et aZ.26 used a hierarchical form of cluster analysis to classify substituents according to their overall similarity and dis- similarity.Most important from a practical viewpoint, Topli~s~~ has developed a set of physicochemically based decision rules whose use during a synthetic programme requires no computers or statistics. In six retrospective cases cifed,27,28 use of the Topliss decision rules to guide a lead-optimizing synthetic programme would have identified the most active compound with considerably less chemical effort than that actually expended. B. The Additive Model.-One of the simplest models that can form the basis for structure-activity correlation among series of related compounds is one that assumes that biological activity is an additive property of the substituents that vary within the series.Analyses based on this model have been able to account for a substantial part of the variation of the biological activity in numerous series of compounds. Several methods based on the additive model have been described29 but it was Free and Wilson who in 1964 developed the technique30 into an elegant and now generally accepted form. In their mathematical formulation every sub- stituent is assigned a substituent constant which represents the contribution of that substituent to the overall biological activity of the molecule in which it is present. These substituent constants are evaluated by the least-squares solution of a set of linear equations of the form (3), one for each of the molecules in the series. biological -mean biological .sum of substituent /2\\JI -activity -activity + contributions A recent analysis31 of the antimalarial activity of a series of phenanthrene-carbinols (3) illustrates both the usefulness and the problems of applying the Free-Wilson methodology. HO-CHCH RG& /R' (3) pE C. Hansch, S. H. Unger, and A. B. Forsythe, J. Medicin Chem ,1973,16, 1217. 27 J. G. Topliss, J. Medicin. Chem., 1972, 15, 1006. Y.C. Martin, J. Medicin. Chem., 1973, 16, 578. Is T. C. Bruice, N. Kharasch, and R. J. Winzler, Arch. Biochem. Biophys., 1956, 62, 305; J. Kopecky, K. Bocek, D. Vlachova, and M. Krivucova, Experientia, 1964,20,667. 30 S. M. Free, jun.,and J. W. Wilson,J. Medicin. Chem., 1964,7,395. 31 P. N. Craig and C. H. Hansch, J. Medicin. Cheltl., 1973, 16, 661.281 Quantitative Drug Design Possible combination of the substituents represented within the 43 compounds available for analysis is 3 x 3 x 3 x 6 x 6 x 3 = 2916. The minimum number required to solve the equations is 1 +(2+2+2+5 +5 +2) = 19. Thus the number of compounds available in excess of this minimum is 43 -19 = 24 (the number of degrees of freedom), which is satisfactory from a statistical point of view. However, as seen in Table 1, 6 of the 24 substituents occur only once in the series of compounds and the distribution of the other substituents is relatively uneven. This, of course, is undesirable but is not atypical of series available for retrospective analysis. Such problems could be avoided by application of the Free-Wilson methodology in the planning stages of a synthetic lead-optimization programme.The rank order of the substituent constants at a given position parallels the substituent contributions to the biological activity. To estimate the significance of the differences between values the student t test is commonly applied.5 The tempting conclusion that the optimal compound is one with substituents having the highest constant in each position would be valid only if additivity were perfect and biological variability of the test system negligibly small, conditions seldom if ever satisfied. The range of the substituent constants (Table 1) at the different positions on the molecule varies substantially (0.093-1.021); this reflects the relative sen- sitivity of the biological activity to substitution of the molecule. The larger the range the more important is that position for optimizing the biological activity.A small range suggests the position to be relatively unimportant, but it may also be that appropriate substituents have not been explored. Thus the data presented in Table 1 suggest that the substituents explored at position R6have minimal influence on the observed biological activity; in contrast, position R5appears to be most sensitive to substitution. Analyses based on regression techniques allow the comparison of calculated and experimentally determined biological activities. A useful way of examining such data is to construct a plot of calculated versus experimental values; if most of the compounds fall into a zone bisecting the axes with a zone-width comparable to a specified confidence limit (e.g.95%) of the experimental values then the additive model applied can be deemed appropriate. Compounds represented by points clearly outside this zone should be retested biologically and examined for possible error in structural assignment. If neither is responsible for the deviation, specific interaction between substituents is suggested. The identi- fication of such hidden synergisms between groups is an important point of departure for further research. Further useful information can emerge by recognizing correlations between subs t i tuen t constants and the p hy sicochemical parameters (electronic, par tit ion- ing, steric) of the substituents.The relative importance of the latter can only be established by exploring substituents covering a sufficiently wide range of para- meter values; in this regard, reference to substituent scatter diagrams of the type shown in Figure 1 can be of considerable value. If the correlation proves significant after simple or multiple regression analysis, the synthesis of com- Redl, Cramer and Berkofl Table 1 Free-Wilson analysis of antimalarial phenanthrenecarbinols (3): summary of results No. of Position Group examples R1 R1 R1 R2 R2 R2 R3 R3 R3 R3 R3 R3 R4 R4 R4 R5 R5 R5 R5 R5 R5 R6 R6 R6 c1 3 H 39 Br 1 c1 7 CF3 1 H 35 CF3 7 Br 1 c1 10 I 1 F 2 H 22 CI 1 CF3 1 H 41 CF3 18 Br 2 c1 6 F 2 H 13 OCH3 2 2-piperidyl 13 dibutylamino 13 diheptylamino 17 Substituent constant 0.130 -0.001 -0.338 0.301 0.292 -0.069 0.384 0.296 0.155 0.129 -0.193 -0.194 0.273 0.043 -0.008 0.451 0.363 -0.187 -0.196 -0.477 -0.570 0.037 0.0142 -0.056 Range 0.468 0.370 0.5781 0.2801 1.0211 0.093 Statistics: n = 43; R2 = 0.853; F = 7.82 (p<O.Ol) pounds containing previously untested substituents might be indicated.By extending the predictive potential of the analysis beyond new combinations of ‘old’ substituents, this approach constitutes a very desirable coupling of the Hansch and Free-Wilson techniques. C. Quantum Chemical Methods.-As early as 1945, the pioneering application of quantum chemical reasoning by Pullman led to recognition of the role of so-called K and L regions in the carcinogenic activity of fused aromatic hydro- Quantitative Drug Design carbons.32 The valence-bond method used originally gradually yielded to molecular orbital (MO) methods; the latter are now used almost exclusively when quantum chemistry is applied to problems of structure-activity correlation. In early studies the classical Huckel MO methods were employed; these were restricted to T electrons and thus to planar molecules or structural fragments. In the past decade, MO theory and computational techniques have advanced rapidly, andconvenient programme packages are now available for many different all-valence-electron MO methods.Particularly extensive use has been made of the iterative, semi-empirical approaches based 011 Hoffman’s Extended Huckel Theory (EHT)33and Pople’s Complete Neglect of Differential Overlap (CND0).34 Besides providing numerical values for molecular electronic parameters, the all-valence-electron MO methods allow the calculation of conformational energy profiles. Preferred (minimum energy) conformations of agonist molecules have been assumed to be those required for biological activity. For example, on the basis of comparing the minimum energy conformations of acetylcholine, muscarine, and muscarone, the muscarinic pharniacophore shown in Figure 2 was proposed.35J6 Extensive studies of this kind on nicotinic, adrenergic, hista- minic, and other agonist and antagonist molecules have been reviewed by Kier.36 Different calculations on the same molecules can yield different conformational energy profiles depending on the molecular parameters (e.g., bond lengths and bond angles) and the MO method used.There is no consensus and apparently no clear answer as to which of the semi-empirical methods is most reliable. Ab initio methods are presumably more accurate, and recent refinements based on the molecular fragment approach37 have made these methods suitable for molecules as complex as the antibiotic lincomycin (C18H34N206S).38 More important than the problem of relative accuracy of the various methods is whether the preferred conformation of an isolated molecule is likely to be involved in its interaction with a receptor.Differences between conformational energy minima are often only a few kilocalories and can be more than com- pensated by the energy of interaction between agonist and receptor. Thus delineation of the nature and magnitude of the conformational barriers might be more important than detailed knowledge of the configurations corresponding to energy minima. For example, in a recent study of conformational energy profiles of histamine and some of its methyl-substituted derivatives, Ganelli~~~~ presented good evidence to suggest that the conformation of histamine for inter- 3a A. Pullman, Compt. rend. SOC.Biol., 1945,139, 1956. 33 R. Hoffmann, J.Chem. Phys., 1963,39, 1397. 34 J. A. Pople, D. P. Santry, and G. A. Segal, J. Chem. Phys., 1965,43, S129. ss L. B. Kier, Mol. Pharmacol., 1973, 9, 820. w L. B. Kier in ‘Advances in Chemistry Series’, No. 114, American Chemical Society, Wnsh- ington, 1972, see also J. P. Green, C. L. Johnson, and S. Kang, Ann. Rev. Pharm., 1974, 14, 319. 37 R. E. Christofferson,Adv. Quantum Chem., 1972,6,333. 38 L. L. Shipman, R. E. Christofferson, and B. V. Cheney,J. Medicin. Chem., 1974,17, 583. C. R. Ganellin, J. Medicin. Chem., 1973, 16, 620. Redl, Cramer and Berkofl Figure 2 Predicted conformations of acetylcholine (a), muscarine (b), muscarone (c), and proposed muscarinic pharmacophore. (Reproduced by permission from Advances in Chemistry Series, No.114, 1972, p. 120) action with one of its two recognized receptors is in the region of a local maximum in the energy profile of the molecule. Although the computer time required for the MO calculation of a medium size (30-60 atom) drug molecule in a single conformation is moderate, a detailed Quantitative Drug Design conformational analysis involving several simultaneously variable bond para- meters can increase the demand for machine time to the limits of practicability. Increasing use of the computationally very much faster PCILO (Perturbation Configuration Interaction using Localized Orbitals) MO method40 has now greatly reduced this problem, and even the consideration of solvation effects has become p0ssible.~1 Many of the structure-activity correlations using MO calculations are based on computed indices reflecting the electronic structure of the rn0lecules.4~ Net atomic charges, frontier electron and orbital densities, superdelocalizabilities, energies of the highest occupied (HOMO) and lowest empty (LEMO) molecular orbitals are some of the most frequently used indices.A comprehensive list of MO indices used in structure-activity studies has recently been ~ompiled.~s Similar to, and sometimes in conjunction with, experimentally determined physicochemical parameters (cJ Section 3A), MO indices can be used in multiparameter regression analyses. While there is considerable question regarding the reliability of the absolute values of the calculated indices, it is generally assumed that errors due to the approximate nature of the MO methods, the lack of experimentally determined bond lengths and bond angles, the neglect of solvation effects, etc., are largely parallel within structurally closely related series.Thus, trends and differences among the indices might be meaningful even though the absolute values are not. If the molecules in a series under study are conformationally flexible, choice of the conformer for evaluation of the indices poses an additional problem. Typically, conformational energy investigation is carried out for one or two representative members, and preferred conformations are assumed to apply to the rest of the series. This is clearly an oversimplification, but separate energy minimization for every member of a series is rarely feasible.A study44 of the correlation of antihypertensive potencies with EHT MO indices in a series of benzothiadiazines clearly illustrates these problems. In this U *O S. Diner, J. P. Malrieu, F. Jordan, and M.Gilbert, Theor. Chim. Acta, 1969,15, 100. *l B. Pullman, Ph. Courriere, and H. Berthod, J. Medicin. Chem., 1974, 17, 439 48 L B. Kier, ‘Molecular Orbital Theory in Drug Research’, Academic Press, New York and London, 1971 ;A. Cammarata in ‘Molecular Orbital Studies in Chemical Pharmacology’, ed. L. B. Kier, Springer Verlag, New York, 1970. 45 Reference 36, pp. 4546. 44 A. J. Wohl, Mol. Pharmacol., 1970,6, 195. Redl, Cramer and Berkof example, the further complicating possibility of tautomerism was resolved by a separate preliminary study45 leading to the conclusion that the equilibrium strongly favours the 4H-tautomer (4). It should be borne in mind, however, that in the macrobiological milieu activity may reside in an unfavoured tautomer even if only a minute fraction of the molecules are in that form at equilibrium.Of the various regression equations examined, equation (4) was found to be most satisfactory. pA2 = 64.12 -5.16 EHOMO+ 55.09 ST + 115.78 St + 5.16q(3R) (4) [Statistics: n = 23; R2 = 0.96; F = 62.31 (p <0.0005)] [where pA2 = in vitru potency based on competitive antagonism of Ca2+; EHOMO= energy of HOMO in -eV; SF = nucleophilic superdelocalizability on atom 5; Sr = nucleophilic superdelocalizability on atom 6; and q(3R) = summed regional charge over all atoms in the 3-R group]. The statistical significance of the equation is impressive, although 5-substituted derivatives were excluded in its derivation and the predicted values for these molecules were consistently high.The activity of the 5-substituted derivatives could be satisfactorily accounted for by an alternative regression equation which, however, required 8 indices, rather too many for a series containing only 25 compounds. In general, many of the MO indices are calculated for individual atoms, and thus the parameter pool available for correlation analysis is large. Selection from this pool should ideally be based on biochemical reasoning since reliance on stepwise regression methods, which automatically select the statistically significant indices, can lead to an often overlooked pitfall.As Topliss has con- vincingly demonstrated,46 the likelihood of obtaining chance correlations in- creases considerably with the number of parameters tried. The common practice of quoting only best regression equations without mentioning the size of the parameter pool can be very misleading. Reliance on the somewhat arbitrary and artificial atom-by-atom MO indices makes less than optimal use of the informational content of the MO calculations. However, few constructive alternative approaches have yet been suggested. A recent structure-activity study4' of some anticholinergic phen- c ycl idine derivatives using CNDO-generated electrostatic pot en t ial maps appears to be a promising new departure.4 Lead-generating Techniques A research programme whose objective is the optimization of a lead represented by an existing drug product is unlikely to produce a truly novel therapeutic 45 A. J. Wohl, Mol. Pharmacol., 1970,6,189. 46 J. G. Topliss and R. J. Costello, J. Medicin. Chem., 1972,15, 1066. 47 H. Weinstein, S. Maayani, S. Srebrenik, S. Cohen, and M. Sokolovsky, Mol. Pharmacol., 1973,9,820. 287 Quantitative Drug Design agent. New therapy is more reasonably found either by utilizing established testing systems to search for new lead compounds, or by devising new test systems where a lead structure may not exist. In either of these cases, a new lead might be suggested by consideration of a biochemical model or hypothesis.For example, if the objective were to inhibit a particular enzyme, one logical strategy would be to give provisional lead status to compounds that resemble the natural enzyme substrate, either in the ground or transition state. The dis- covery of allopurinol (6), a xanthine oxidase inhibitor used in the treatment of gout, exemplifies this appr0ach.4~ OH OH allopurinol xanthine (6) (7) Unfortunately, however, available biochemical hypotheses are often inadequate to identify meaningful leads, especially in the areas of most pressing clinical need. In the absence of any hypothesis, biochemical or otherwise, lead identi- fication must depend primarily upon the screening of structurally diverse com- pounds. By virtue of the empirical and all but random character of their com- pound requirements, such screening programmes will usually have a relatively high throughput capacity.Screens capable of processing a thousand compounds per year are common, and the U.S. National Cancer Institute recently increased the target,for its primary screen to a thousand compounds per week.49 The analysis of data generated at this rate poses challenging problems -and oppor- tunities -for the drug designer: (i) Based on accumulated experience, how might priorities be established for the selection and testing of new compounds? (ii) In which of several possible competing screens should the (probably limited) supply of a novel compound be consumed? (iii) Can a large body of test data, frequently generated over long periods of time, be made to yield hitherto unidentified lead structures? While the volume of the data and the structural variations represented within high-capacity screening programmes suggest the potential for computer assis- tance in addressing these problems, useful techniques are, surprisingly, only beginning to emerge.48 G. H. Hitchings, ‘Progress in Drug Research’, P.M.A. Research Symposium, Washington, D.C., March 6, 1969. 4g T. H. Maugh, jun., Science, 1974,184,970. Redl, Cramer and Berkofl Recent independent experiments9 -ll. 50-54 aimed at developing systems capable of meeting the challenges of lead generation have been based on sub- structural features (functionality, rings, chains, hetero-atoms, and combinations thereof).55 The substructural approach has intuitive appeal.In fact, the lead concept underlying much of drug design might be paraphrased as ‘a complex substructure whose incorporation tends to confer activity on a molecule’. Although the term ‘substructural analysis’ has been used for one of the tech- niques,51*52 the phrase seems more appropriate as a general name encompassing all of these approaches. Most substructural analyses can be described formally as attempts to devise linear equations in which the likelihood of activity of a compound is related to the sum of contributions from its constituent substructures; each substructural contribution is in turn computed from past testing experience with compounds containing the substructure.Differences among the analyses result from the ways that past testing experience is allowed to influence substructural contri- butions, and in the coding of the substructures themselves. The most common procedure is to form a set of linear equations much like those used in the Free-Wilson additive model, except that, for computational tractability, the contribution of a substructure must be assumed to be indepen- dent of its molecular environment. However, inasmuch as the variety of sub- structures coded among a series of unrelated compounds of moderate complexity usually greatly exceeds the number of compounds, there initially are far too few degrees of freedom for confident regression solution of the equations.Therefore, Kowalski and Bender,g Chu et al.,lo and Hiller et aE.,50have used pattern- recognition techniques to extract only the substructures which seem most influential in determining activity. Cramer et aZ.51952 use all coded substructures, avoiding the problem of degrees of freedom by initially assigning a value to the coefficient of each substructural contribution, e.g. the proportion, of actives among tested compounds containing the substructure. Other pattern-recognition approaches have also been used in substructural analysis, in particular the k-nearest-neighbour technique and other methods of cluster analy~is.~ -11953 For reasons of expedience, the substructures employed in these analyses have generally been drawn from existing data-retrieval systems, a less than ideal situation since these systems were designed for different purposes.Nevertheless, innovative substructural descriptors have been used. For example, instead of allowing the substructural parameter simply to be the number of occurrences of 6o S. A. Hiller, V. E. Golender, A. B. Rosenblit, L. A. Rastrigin, and A. B. Glaz, Computersand Biomedical Research, 1973, 6,411. 61 R. D. Cramer tert., G. Redl, and C. E. Berkoff, Abstracts of 167th American Chemical Society Meeting, April 1974, CHLT 3. 6z R. D. Cramer tert., G. Redl, and C. E. Berkoff, J. Medicin. Chem., 1974, 17, 533; G. Redl, R. D. Cramer tert., and C. E. Berkoff, ‘Proceedings of the Conference on Chemical Struc- ture -Biological Activity Relationships’, Prague, June 1973 (in the press).63 P. J. Harrison, J. Appl. Statistics, 1968, 17, 226. 54 G. W. Adamson and J. A. Bush, Nature, 1974,248,406. 55 C. E. Granito and E. Garlield, Nuturwiss., 1973,60, 189. Quantitative Drug Design a substructure, Kowalski and Bender have defined substructural parameters having continuous properties, such as the number of sulphur atoms per carbon atom.9 Another experiment was based on the use of mass spectral fragments as substructures.11 Since general statistical criteria for analyses carried out by pattern-recognition techniques have not been developed, the validation of a substructural analysis requires empirical yardsticks. The degree of success in predicting activity within a group of compounds not used in the analysis is compared with some bench- mark success rate.Ideally this prediction set will be completely distinct from the training set used in the analysis. When the compounds are too few to permit a permanent division, ‘leave-~ne-out’~~ ‘lea~e-n-out’~~or techniques may be employed. Using these techniques, a compound or set of n compounds is excluded from the training set and the likelihood of its (their) activity is recom- puted. The leave-out procedure is repeated until all the compounds have been made members of a small prediction set, and the overall results are again compared with the benchmark rate. When making these comparisons, considera- tion should be given to existing statistical criteria for distributions, such as the x2 test.5 We note parenthetically that leave-out tests could also be used to evaluate retrospective analyses which employ regression techniques (cf.Section 3).Some very recent work51 exemplifies the broad scope and statistically signi- ficant, but limited, predictive powers of current approaches. Among 540 struc-turally diverse compounds screened for their ability to inhibit passive cutaneous anaphylaxis in the rat (a measure of their potential anti-allergic effect), 259 showed some degree of activity. For each of the 401 substructures among the tested compounds, a substructure activity score was computed as [Ar -Z(259/540)], where AI is the number of active compounds and Ti the number of tested compounds containing the ith substructure.A leave-3-out technique was employed. After exclusion of the testing results for a three-compound prediction set, the three compounds were ranked by descending values of molecular activity score (defined as the sum of the substructure activity scores). This procedure was repeated until all 540 compounds had been included in a three-compound prediction set, while a record was kept of the number of times that the first-place, second-place, and third-place compounds were actually active. As can be seen in Table 2, there is a relationship between total activity score Table 2 The distribution of activity among compounds ranked by descending values of the molecular activity score Proportion of Average molecular Rank active compounds activity score 1 100/180 = 0.56 40.6 2 95/180 = 0.53 0.5 3 64/180 = 0.36 -31.3 B.R.Kowalski and C.F.Bender, Analyt. Chem., 1972,44,1405. Red!, Cramer and Berkof and the likelihood of activity, since compounds of higher rank were active more often. The probability of obtaining a distribution this skewed by chance, were there no relationship between molecular activity score and biological activity, is less than 2%according to a x2 test. Close examination of the results from OUT substructural analysis of 771 com-pounds tested for anti-arthritic activity52 suggests that many compounds fall into a category that could be regarded as ‘anti-lead’. These compounds can be predicted to have a significantly lower than average chance of being active, since a preponderant number of their substructures have occurred mostly in inactive compounds.Exclusion of this type of compound from testing con- sideration would enhance the lead-generating efficiency of the screen. To date, two of the three published substructural analyses involving related series of compounds9J1 have been criticized for their apparent triviality; the conclusions drawn from the analyses have appeared obvious upon re-examina- ation of the data.57958 The third example, an impressive correlation obtained in a regression study of the substructures involved in penicillin binding to proteinsY5* seems less remarkable to us considering the strong dependence of drug-protein binding on hydrophobicity59 and the known additive properties of partition coefficients.22 In view of the severe approximations involved in substructural analysis, the ability to obtain any correlation is encouraging.However, the utility of the methods in terms of the problems of lead generation raised above remains to be conclusively demonstrated. The use of substructural analysis to establish screening priorities will, of course, depend upon its reliability, and on the relative costs of computer and biological testing.50 The more exciting challenge of designing new lead structures (perhaps by combination of substructural fragments into more complex moieties) is a more distant but nonetheless realistic goal as sophisticated structural representations become available.6O 5 Conclusions ‘Is quantitative drug design of any practical use?’ -the provocative question often put by the disbeliever.Despite the limited number of successful predictive analyses,sf we believe that drug design methodologies should be of great value in many of the problems faced by the medicinal chemist. At today’s unfavourable odds against any particular compound becoming a drug product, the traditional measure of ‘success’ appears to be an unrealistic challenge, rather similar to 67 S. H. Unger, Cancer Chemotherapy Reports, 1974, in the press. C. L. Perrin, Science, 1974,183, 551. 6B W. Scholtan, Arzneim.-Forsch., 1968,18, 505. Eo W. T. Wipke, S. R. Heller, R. J. Feldmann, and R. Hyde, ‘Computer Representation and Manipulation of Structural Information’, John Wiley and Sons, New York, 1974.R. W. Fuller, M. M. Marsh, and J. Mills, J. Medicin. Chem., 1968, 11, 397; J. G. Beasleyand W. P. Purcell, Biochim. Biophys. Acta, 1969, 178, 175; Y. C. Martin, T. M. Bustard, and K. R. Lynn, J. Medicin. Chem., 1973, 16, 1089; P. J. Goodford, F. E. Norrington,W. H. G. Richards, and L. P. Walls, Brit. J. PharmacoI., 1973, 48, 650; H. Cousse, G. Mouzin, and L. Dussourd #Hinterland, Chimie Therapeutique, 1973, 4,466. 291 Quantitative Drug Design expecting a professional golfer to demonstrate his superior techniques by shooting a hole-in-one. To the question put by the less disbelieving, ‘Which of the many methodologies in current use is the best?’ we respond that there is no direct answer, save that it is the wrong question to ask.Different methods require different types of data and answer different questions; all approaches must be considered when the analysis of a new problem is being planned. The application of lead-optimizing regression techniques requires series of active compounds and is restricted to relatively narrow structural classes. Identification of a lead is therefore a prerequisite. Nevertheless, by identifying the physicochemical properties that most influence biological activity in a given series, multiparameter analysis may help elucidate the biological mechanisms of action and thus contribute to the discovery of new leads as well as to the opti- mization of existing ones.This underscores the somewhat arbitrary nature of the distinction between lead-op t imizing and lead-genera t ing techniques. Until recently, quantitative drug design has not been applied to the problems of generating new structural leads. Substructural analysis now offers great promise, in particular because of its capacity to accommodate qualitative data on large numbers of diverse structures. Drug design is undeniably still in its infancy, and quanta1 improvements are needed in virtually all aspects of available methodologies. To realize its full potential, readjustment of existing attitudes towards application of the tech- niques is mandatory at critical points in a research programme. For example, its impact should be anticipated in the planning stages of any chemical prog- ramme devoted to the synthesis of an optimally active compound. Further, in the absence of rationally founded chemistry it is tempting, even wise, to be guided by the most tenuous of predictions based on structure-activity cor-relations. However, the original tenuousness of the predictions must be remem- bered, especially if negative data appear. Only with better integration into the overall research process can quantitative drug design assume its proper place and emerge as a mature technology. We thank Dr. A. D. Bender for stimulating discussion and continuing en- couragement.

ISSN:0306-0012    

DOI:10.1039/CS9740300273

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

Tunable Lasers By J. K. Burdett and M. Poliakoff DEPARTMENT OF INORGANIC CHEMISTRY, THE UNIVERSITY, NEWCASTLE-UPON-TYNE NE1 7RU 1 Introduction Since the review by Jones1 in 1969 there has been considerable progress in laser technology, and in particular in the field of tunable lasers. Until recently tunable laser radiation could only be obtained by changing the temperature, pressure, or magnetic field of some existing fixed-frequency laser. In every case, the tuning range was very limited. For example, the output wavelength of a ruby laser changes by -0.6 cm-1 K-1 at room temperature, and in the Zeeman tuned gas laser (ZTG) the atomic energy levels of, e.g., Ne (in an He-Ne laser) may be slightly shifted by applying an axial magnetic field. In this review we describe truly tunable lasers with much larger tuning ranges. We shall concentrate on the four main types available at the moment and will outline the physical principles of their operation, which are somewhat different from those of non-tunable lasers, familiar to most chemists.We shall describe typical tuning ranges, line- widths, and powers. Already several commercial tunable lasers are available covering parts of the ultraviolet, visible, and infrared regions. In such a brief review we can only mention a few of the potential chemical applications of these devices. We refer readers who wish to read more comprehensive accounts of tunable laser operation and applications to several reviews2-’ which contain more extensive bibliographies. 2 Dye Lasers Dye lasers4 produce tunable radiation in the visible and near4.r. regions.They exploit the very intense absorption and fluorescence of organic dyes, such as Rhodamine 6G, in solution. Figure 1 shows schematically the lowest electronic levels of such a dye. Absorption of light promotes an electron from the ground state SOto a vibrationally excited level b of the first excited state SI.There is rapid radiationless decay from b into the lowest vibrational level Byof Slyand a W. J. Jones, Quart. Rev., 1969, 23, 73. a C. K. N. Patel in ‘Coherence and Quantum Optics’, ed. L. Mandel and E. Wolf, Plenum Press, 1973. E. D. Hinkley, K. W. Nill, and F. A. Blum in ‘Laser Spectroscopy of Atoms and Molecules’, ed. H. Walther, Springer-Verlag, Heidelberg, to be published 1974.B. B. Snaveley, Proc. ZEEE, 1969, 57, 1374; ‘Topics in Applied Physics’, Vol. I, ‘DyeLasers’, ed. F. P. Schlffer, Springer-Verlag, New York, 1973. S. E. Harris, Proc. ZEEE, 1969, 57,2096. Proceedings of the Conference on Laser Spectroscopy, Vail, Colorado, 1973, ed. R. G. Brewer and A. Mooradian, Plenum Press, 1974. W. Demtroder, Phys. Reports, 1973, 7C, 225. Tunable Lasers population inversion is produced between this state and the vibrationally excited states, a, of SO,since the relaxation a4A is very fast. Thus laser action can occur on the transition B+a. Since the system involves four sorts of levels, groundandexcited states of both S1and SO,it is possible to maintain a continuous population inversion and the laser can operate in both continuous wave (Cw) and pulsed modes.The state SIcan also cross into the lowest triplet state, TI.The transition TI+SO is spin-forbidden and, upon irradiation, molecules can be 'trapped' in the relatively long-lived TIstate, preventing CW operation unless a triplet quencher, such as 02, is added to the solution.4 Experimentally the dye laser consists of a short-path-length cell, containing the dye solution, placed between two mirrors which constitute the resonant cavity. The dye is 'optically pumped' using either a flash lamp or a fixed-frequency laser (e.g. N2 or Ar ion). The dye is usually made to flow rapidly through the cell to prevent excessive heating. (Under the high illumination required for CW operation even quartz windows have been known to 'burn').The spontaneous linewidth of such a simple laser is relatively broad, 34 nm, but the linewidth can be drastically reduced by placing a Fabry-Hrot etalon inside the laser cavity. An etalon consists of a narrow air gap between two parallel transparent plates. Interference effects only allow light of a particular wavelength (dictated by the length of the air gap) to pass through the etalon undeviated. Laser action can occur only at this wavelength where the optical gain is maintained. Thus, the etalon acts not merely as a filter but cohcentrates all the latent laser power into a narrow bandwidth. Limited tuning can be achieved by rotation of the etalon relative to the axis of the laser cavity (i.e.by changing the effective length of the air gap). The dye laser is tunable because the vibrationally excited states of So (Figure 1) Lifetimes B -a T 5 x IO-~~B -5x10-8s1 T,-so -Figure 1 Schematic energy levels of an organic dye, showing transitions involved in laser action Burdett and Poliakofl are broadened by interaction with the solvent so that they virtually form a continuum and the transition from B to a large part of this continuum can be amplified. The laser threshold (the population of the excited state required for laser action) is a function of wavelength, dye concentration, and mirror reflec- tance, and, for a particular laser, light will be emitted at the wavelength cor- responding to the minimum threshold.Tuning involves changing either the dye concentration or the mirror reflectance. In practice, the latter is preferred. High wavelength dependence of reflectivity is obtained by replacing one of the cavity mirrors by a diffraction grating or mirror-prism combination, and the laser output is then tuned by rotating the grating or prism. The laser is, however, not continuously tunable. The output wavelength, A, is related to the optical length, L, of the laser cavity (physical length x refractive index) by the Fabry-P&ot relationship, mX = 2L and, since only integral values of m are allowed, mode-hopping from one value of rn to the next will be observed. Continuous tuning can only be obtained if the cavity length is also changed during tuning.This is achieved experimentally by using a piezoelectric mirror mount which can be moved through small distances by applying a voltage across it. The tuning range of a particular dye is 60-70 nm although the range can be extended by suitable solvent changes. Most dyes fluoresce at wavelengths longer than 500 nm but, by incorporating a frequency-doubling crystal (see below) into the laser cavity, near-u.v. light can also be generated. Thus a typical com- mercial pulsed system can be tuned over discrete ranges in the region 265-800 nm, and tuning ranges are constantly being extended as new dyes are discovered. Pulsed powers are high, typically as much as 25 MW. Laser-pumped dye lasers tend to have a higher peak power but lower overall pulse energies than flash- lamp-pumped systems.CW powers are usually in the range 10 mW-1 W. The linewidth of the laser output is essentially limited by mechanical vibration of the optical components and turbulence in the dye cell. Mechanical vibration is, in practice, the principal cause of frequency instability in most tunable lasers. There are CW dye lasers in regular use4 with linewidths of 1 MHz* using invar stabilization and vibration-free mountings. Pulsed lasers have somewhat larger linewidths, -10 MHz. It must be remembered, however, that linewidth and pulse length are related by the Heisenberg Uncertainty Principle. Thus a pico- second pulse (10-l2s), which can be produced by a laser operating under mode-locked conditions, is limited to a linewidth of -5 cm-I.If such a pulse is made more nearly monochromatic using an etalon it will simultaneously be stretched in time. 3 Non-linear Devices These devices305 are based on the interaction of high-intensity laser beams with * 1 cm-l = 30 GHz; 1 GHz = 3 x lo-* cm-l; 1 MHz = 3 x cm-l; 1 kHz = 3 x lo-' cm-l; 1 MHz, equivalent to a wavelength change of 10-a nm at 600 nm. Tunable Lasers non-linear materials. For such materials it is convenient to relate the dipole moment P,induced by the electric field E, and the susceptibility tensor x, by the expression P = XE = (xo + PE)E. In low fields P is linearly dependent on E, whereas in high fields, such as exist in laser beams, the total susceptibility becomes field-dependent and the second term oscillates with the frequency of the laser input.This expression for the susceptibility is reminiscent of the Raman polariza- bility tensor,* one component of which oscillates with a vibrational frequency of the molecule, giving scattered photons of frequency vinput k Vvibration. Thus, using a derivation, mathematically almost identical to the classical deriva- tion of the Raman effect, it can be shown that two input laser photons of frequency vl and v2 will give rise to v3 = v1 + v2 (the two photon mixer, TPM) and v3 = v1 -v2 (difference frequency generator, DFG). The well known effect of frequency doubling is a special case of TPM, with vl = v2. A more rigorous mathematical treatment shows that it is possible to have a process whereby a single input photon produces two photons of different frequency, v1 = v2 + v3 (by conservation of energy).This is the exact opposite of the TPM process. For a multi-photon process momentum as well as energy must be conserved. If k (k = 2nr/X, where r is a unit vector in the direction of propagation) is the wavevector of a photon, momentum conservation implies input output Momentum is normally conserved by the propagation of input and output beams in different directions. However, in an efficient non-linear device it is vital that the input and output beams should be parallel. How this is achieved is best understood by considering the frequency doubler, which, for example, converts red light into blue using a crystal of KDP (KHZPO~).~Momentum conservation dictates that kblue = 2kred and, since wavelength and frequency are related by the speed of light and the refractive index, n, of the material, nblue must equal nred.In an isotropic substance this condition cannot be fulfilled because of the dispersion of the material. In a birefringent (optically anisotropic) material, light is propagated as two rays, ordinary and extraordinary, which have different refractive indices, nard and next, and which give rise to double refraction. next varies with the orientation of the crystal whereas nard does not. In KDP, since the birefringence (nord,blue -next,blue) is greater than the dispersion (nord,blue -nord,red), it is possible to orient the crystal so that nord,red = next,blue.Figure 2 shows how rotation of the optic axis of the KDP 8 D. A. Long, Chem. in Britain, 1971, 7, 108. 9 P. D. Maker, R. W. Terhune, M. Nisenoff, and C. M. Savage, Phys. Rev. Letters, 1962, 8, 21. Burdett and Poliakof 4-504 Angle/degree Figure 2 Blue light intensity as a function of crystal orientation (angle between optic axis and red laser beam) for a KDP frequency doubler (Adapted by permission from Phys. Rev. Letters, 1962, 8, 21) crystal relative to the red laser beam affects the intensity of the blue light emitted parallel to the red. The intensity rises dramatically at the angle where the two refractive indices are equal. This process is called phase matching. TPM devices, adding together photons of different frequency, have been produced.For example, millimetre wavelength microwave radiation and the output of a CW C02 laser have been successfully mixed in a GaAs-loaded waveguide.lO The laser is tuned by changing the microwave frequency; obviously the tuning range is rather small, but eventually it should be possible to cover the entire range of the C02 laser. The power is low (-1 ,uW) and the linewidth quite narrow (-10 MHz). The advantage of this device is that the frequency of the output is very accurately known. With the majority of other tunable lasers precise frequency measurement presents a serious problem. The DFG subtracts the photons of two pump-laser beams. Tuning, in this case, is achieved by tuning one of the pump lasers, while the other remains at a fixed frequency, and rotating the crystal to preserve the phase-matching condition.Radiation above 5 prn has been generated using a dye laser and fixed-frequency loV. J. Corcoran, R. E. Cupp, J. J. Gallagher, and W. T. Smith, Appl. Phys. Lerters, 1970, 16, 316. Tunable Lasers visible laser (Ar ion or ruby). Powers have been low, 0.5 pW, with a linewidth of 50 MHz (1.6 x cm-l) CW and 1 W (linewidth 10 cm-1) pulsed. The DFG is experimentally complicated since several components have to be tuned simultaneously. A LiNbOs optical parametric oscillator (1.5-1.7 pm) (see below) and a Nd YAG laser (1.32 pm) have been successfully used with a AgGaSez crystal to produce continuously tunable radiation from 7 to 12 pm (1400-850 cm-l) with a lidewidth of 2 cm-V1 Spectra taken with this device are similar to those of a medium-resolution conventional i.r.spectrometer. The best developed tunable non-linear device is the opticalparametric oscillator, OPO, in which two i.r. photons are generated from a single input photon of visible or near4.r. radiation. The non-linear crystal is situated in a resonant cavity which amplifies the signal at either one or both of these new frequencies. The two arrangements are known as singly or doubZy resonant oscillators (SRO and DRO) respectively. The SRO is normally used as the DRO suffers from substantial mode-hopping instability ( -20 cm-I), so called clustering. The frequencies of these two beams, conventionally called signal (higher frequency) and idler (lower frequency), are dictated by phase-matching requirements in the crystal. Thus the OPO is tuned by altering the phase-matching conditions, by either rotating, heating, or compressing the crystal hydrostatically to change its refractive index.The tuning range is limited by (a) the crystal ‘running out’ of birefringence, (b) thermal decomposition, and (c) absorption of light by the crystal (e.g. LiI03 absorbs below 1750 cm-l). Figure 3 shows typical tuning characteristics of a commercially available LiNbOs OPO with temperature tuning and a Nd YAG pump laser. Etalons are required to reduce the relatively broad bandwidth of the OPO output (cf: dye laser). Linewidths better than 0.001 cm-1 (30 MHz) have been achieved at wavelengths shorter than 4 pm (2500 cm-I), with relatively low CW power (pW or mw).Piezoelectric mirror mounts (4.v.) are required to prevent mode-hopping during tuning. Pulsed lasers have higher powers, e.g. LiI03, which attains 10 kW from a 100 kW pump pulse. Tunable radiation has been generated in the entire region 1-20 pm,but OPO’s operating below 5 pm are still under development. 4 The Spin-flip Raman Laser (SFRL) This laser2s3 operates via stimulated Raman scatterings from the electronic energy levels of the conduction band of an n-type semiconductor. It has already produced tunable radiation in the i.r. around 5-6 pm, 1950-1830 cm-1 pulsed, 1950-1600 cm-I CW, and 10 pm, 995-890 cm-1 pulsed. True laser action occurs if the gain associated with the stimulated Raman process is large enough to overcome losses in the Raman scattering medium (due to absorption, reflection, etc.).In general, the output frequency of such a laser is given by the usual Raman scattering energy conservation relationship, 11 R. L. Byer, M. M. Choy, R. L. Herbst, D. S. Chemla, and R. S. Feigelson, Appl. Phys. Letters, 1974, 24,65. Burdett and Poliukofl / I /p 0.50OAO i 150 230 310 390 470 crystal temperature/OC Figure 3 Tuning curves of a commercially available parametric oscillator (Chromatix Model 1020) using a LiNbOs crystal pumped with diferent lines from a frequency-doubled Nd YAG laser (0.532, 0.562, and 0.659 pm) hvoutput = hvinput & mhvex (m= 1,2,3 ...) where vex is the frequency of the excitation involved in the scattering mechanism (for voutput c Vinput Stokes radiation is produced, for voutput > vinput anti-Stokes). Chemists will be most familiar with the situation where Vex is a molecular vibration frequency, but Raman scattering can also occur from the mobile electrons in n-type semiconductors. Stimulated ‘spin-flip’ Raman scattering from these electrons in InSb (probably the best understood of all semi-con- ductors) has been extensively developed12 since 1970. A simplified explanation of the process is as follows. In a magnetic flux density B, the conduction band of a semi-conductor is split into several well-defined states (Landau levels) with energies E = (n -k hvc vc is the cyclotron frequency and n is an integral quantum number (0, 1, 2 .. .) I* C. K. N. Pate1 and E. D. Shaw, Phys. Rev. Letters, 1970, 24,451; R.L. Allwood, S. D. Devine, R. 0. Mellish, S. D. Smith, and R. A. Wood, J. Phys. (C), 1970, 3, L186. Tunable Lasers (see Figure 4). Each Landau level is further split by electron spin into two levels (‘spin-up’ and ‘spin-down’) separated by gpBB where g is the g-factorassociated with the conduction band electrons and p~ is the Bob magneton. The spin-flip I n=2, O’I 0,t Magnetic Ef t conduction band Full valencebands Figure 4 The structure of the conduction band of an n-type semiconductor in a magnetic jield, illustrating the splitting of the Landau levels (designated by integral values n) into spin-up and spin-down levels.EFis the Fermi level transition occurs via net promotion of an electron from the ‘spin-up’ level of n = 0 to the ‘spin-down’ level of n = 0. For InSb, g is large and negative, -45 (cf. the free electron value of + 2). The Raman laser output frequency is therefore given by Youtput = Vinput & mlgpBB[ (m = 1, 2, 3 . . . .) and may be tuned by varying the applied magnetic field, with a tuning rate for InSb, since g is so large, of N 20 cm-1 T-1. The stimulated Raman effect has two features of great importance to laser physics. Firstly, there is a considerably higher ratio of Raman scattered intensity to pump (exciting) radiation than in the spontaneous (or normal) Raman effect, there being up to 50% conversion* of the incident pump radiation into Stokes Raman radiation, compared with lO-4% in the normal effect.Secondly, the process results in a sizeable narrowing of the Raman scattered linewidth Burdett and Poliakof over that associated with the spontaneous Raman effect. For CW operation the theoretical limit of the stimulated Raman scattered linewidth is as low as 1 Hz (3 x 10-11 cm-1) for an output power of 1 W. Experimentally, a rectangular single crystal of n-type InSb is located in a powerful magnetic field and is cooled to cryogenic temperatures by contact with liquid helium. Two opposite faces of the crystal are highly polished, optically flat, and almost parallel, to form the ends of the resonant cavity where the stimulated process occurs.The pump laser beam enters the crystal through one of these faces and the stimulated Raman radiation and the uncon- verted pump radiation emerge from the crystal through the opposite face. The tunable radiation has well-defined polarization and may be separated from the pump radiation by the use of filters or a monochromator. (This also serves to separate the double Stokes, Stokes, and anti-Stokes radiation which are pro- duced simultaneously.) All existing spin-flip lasers use InSb crystals. This means that any pump radiation must be lower than the InSb band gap, N 1900 cm-l,* to avoid direct absorption by the crystal. However, if the pump frequency is close to the band gap then resonant enhancement of the stimulated Raman process occurs and the conversion efficiency from pump to Raman-scattered radiation is improved enormously.This effect, equivalent to the familiar resonance Raman effect, is a vital factor in the operation of the SFRL in both the pulsed and CW modes. The pulsed SFRL is normally pumped with a pulsed or @switched COZ laser. This produces a series of lines in the region of 10.6 pm (943 cm-l), any of which can be frequency-doubled (4.v.)using a tellurium crystal to give radiation at 5.3 pm (1887 cm-1) close to the band gap of InSb. Peak SFRL output powers of 1 kW (Stokes) and 100 W (double Stokes and anti-Stokes) at frequencies of ca. 5.3 pm have been obtained for pump powers of N 1 MW. The pulse length of the SFRL output is of course related to the pulse length of the pump laser.Although the linewidth is limited by the uncertainty principle (via nanosecond ‘spikes’ in the input pulse) to 100 MHz (0.003 cm-1) the narrowest tunable line- width so far observed is -0.02 cm-1 (700 MHz). If the undoubledCO2 10.6 pm radiation is used as a pump, tunable radiation is produced in this region of the spectrum with similar power to that in the 5 r_lm region but with slightly greater linewid ths. A superconducting magnet is preferred since very high magnetic fields can be obtained. Varying the field from 0 to 10 T gives a total theoretical tuning range with a combination of anti-Stokes and double Stokes radiation of -480 cm-l for a given pump frequency. In practice the range is less than this owing to absorption of the Raman radiation across the band gap and by losses, induced by the magnetic field, which prevent the stimulated process occurring.A typical tuning curve is shown in Figure 5. * The exact position of the band gap is dependent on temperature, the applied magnetic field and carrier concentration of the semiconductor. Tunable Lasers -240 -200 -160 llIl111J 345678910 +80 +120 +160 Figure 5 Typical tuning curve of a pulsed spin-flip laser. Note that the slope of the double- Stokes curve is approximately twice that of the Stokes Since the SFRL has a finite magnetic threshold, there is an inaccessible region close to the pump frequency. This threshold decreases with decreasing carrier concentration and can be as low as -0.05 T. The frequency gaps can be covered by using adjacent lines of the pump laser. There is also a power threshold for SFRL operation.This presents no problem in pulsed operation, where the input powers are N 1 MW. Using InSb, CW operation is only possible in the 5.3 pm region, where stimulated Raman scattering is very efficient, since the threshold at 10.6 pm is so high that CW heating effects would be enormous. Normally a CW CO gas laser is used for pumping and SFRL output powers of 100 mW-1 W have been achieved. Since the CO laser has a large number of different lines, the SFRL can be tuned by using either a single CO line and a superconducting magnet or a whole series of CO lines and less powerful conventional magnets.An ingenious arrangement allows use of a permanent magnet.13 The longer-wavelength limit (-6.2 pm) is imposed by decreasing pump laser powers and increasing power thresholds for CW operation. The theoretical linewidth of this system is 1 Hz for an output power of 1 W and using the heterodyne technique a linewidth as small as 1kHz (3 x 10-8 cm-1) has been experimentally observed.2 The usable linewidth is -1-10 MHz (i.e. about 10-4 cm-1) but this may be reduced to ca. 30 kHz (-10-6 cm-l) by using 13 3. R.J. Brueck, and A. Mooradian, IEEE J. Quantum Electron., 1973, QE9, 1157. Burdett and Poliakofl a second CO laser for heterodyne ~tabilization.1~ The discrepancy between the observed linewidth and the theoretical minimum is due to pump-laser instab- ility and mechanical vibration.5 Semiconductor Diode Lasers (SDL) By pumping the band gap of a semiconductor, electrons are promoted to the conduction band leaving a hole in the valence band. Stimulated emission of radiation of frequency v = klEg may be produced by recombination of these electron-hole pairs. Population inversion can be achieved by optical pumping (at a frequency where the radiation is absorbed), by electron beam excitation, or by electron injection at a p-n junction. Tunable lasers using electron injection have been the most extensively developed and are called diode lasers.3~~Figure 6 n-p Junction P IEnergy /--n P Field on Figure 6 Schematic illustration of the method of producing a population inversion at the p-n junction of a semiconductor diode by applying an electric field (applied voltage across the junction) shows, schematically, the relative energies of the conduction and valence bands of the p and n sides of a p-n junction.When a voltage is applied across this junction, a population inversion now exists on the p side. Stimulated emission occurs simply by the ‘flow’ of electrons from the conduction band of the n side into the conduction band of the p side and across the band gap Eg. Semi-conductor materials where the band gap is in the i.r. region include such binary compounds as InAs, InSb, GaSb, PbS, and the ternary systems Pbl-zSnzTe and Pbl-zSnzSe. In the visible and near4.r. region, other materials such as GaAslz-Pz can be used.The diodes are made by cleaving single crystals of the semiconductor material such that each contains a pn junction. A typical size of such a diode is 0.1 x 0.04 x 0.02 cm. Low-resistance contacts are cold-welded to the p and n sides l4 S. R.J. Brueck, Bull. Amer. Phys. Soc., 1973, 18,400; S. R. J. Brueck and A. Mooradian, IEEE J. Quantum Electron., in the press. 303 L Tunable Lasers of the junction and the diode is attached to the cold finger of a Dewar. At present liquid helium is-used as the refrigerant for CW operation although pulsed operation of the laser is possible at 77 K. (CW operation is also possible at 10 K using a closed-cycle cooler.) A steady d.c. current is applied across the junction for CW operation or a current pulse of short duration sent through the crystal for pulsed operation3 The nominal i.r.emission frequency is set by the energy of the band gap and may be 'tailored' by altering the chemical composition of the semiconductor. Thus for Pbl-zSnzTe, emission can OCCUT anywhere between 6.5 and 32 pm (1690-312 cm-l) and crystal-growing techniques are such that a laser can be produced to emit a particular frequency with an error of less than 5 cm-l by correct choice of x. Once a diode laser has been fabricated, further adjustment of its energy gap is possible by changing the applied hydrostatic pressure, temper- ature, or magnetic field. PbSe diode lasers, for example, can be pressure-tuned from 8 to 22 pm (1250450 cm-l). The limit is set by the fact that the pressuriz- ing helium gas solidifies15 at 14 kbar at 77 K.The bandwidth associated with the spontaneous recombination radiation across the band gap may be of the order of 5-50 cm-l. However, the frequency of the stimulated (laser) radiation occurring within this bandwidth will be dictated by the optical length of the laser cavity (via the Fabry-Perot condition), formed by the cleaved ends of the semiconductor crystal. The pn junction layer runs perpendicular to these faces and the stimulated recombination occurs in this junction zone. Laser action can then only occur at wavelengths which are allowed cavity modes. Fine tuning of the laser frequency is achieved either by applying pressure to alter the physical length of the cavity or by changing the temperature or magnetic field to alter the refractive index and hence optical length of the cavity.The most frequently used method of tuning is variation of the diode current. Changes in the current alter the temperature and hence the refractive index of the diode. Because of the small thermal mass of the laser this heating is very rapid and the laser output may readily be modulated at fre- quencies3 as high as 10 kHz. A typical current tuning range for one of these crystals is ca. 40 cm-l, although only about half of this range is accessible in practice because of mode-hopping (4.v.)(Figure 7). The diode laser is a multi- mode device in that laser action can occur on several cavity modes simultaneously, although one is usually predominant in power.For a normal-size diode crystal cavity modes are separated by ca. 2 cm-1, and a simple monochromator is sufficient to separate a single frequency from the output. Gaps in the tuning range can be filled in by applying an external magnetic field or by selecting a different mode with the monochromator. The maximum power output for a diode laser to date3 has been N 1 mW in a single mode for CW operation, although typical powers are lower, N 10 pW. A peak power of 10 W has been achieved using a pulsed diode at 77 K. Higher output powers should be possible with optical pumping of the band gap, where 16 J. M. Besson, W. Paul, and A. R. Calawa, Phys. Rev., 1968, 173,699. Burdett and Poliakofl 975 970 965 200 300 A00 500 600 Diode currentlrnA Current-tuned diode laser Figure 7 Typical current-tuning curve for a diode laser indicating mode-hopping (Adapted by permission from E.D. Hinkley and A. R. Calawa, ACS Meeting Dallas,Texas, April, 1973) the output radiation is tuned by altering an applied magnetic field.16 The line- width of the CW SDL is very narrow; 54 kHz (2 x 10-6 cm-1) has been ob-tained17 as a usable bandwidth for a 0.24 mW P~O.SSS~O.IZ diode laser operating at 10.6 pm. The linewidth of the pulsed version is considerably larger, since heating of the crystal changes the temperature and the refractive index (and hence the emission wavelength) during the lifetime of the pulse. This process is called chirping and is typically 20 MHz ns-l.6 Spectroscopy using Tunable Lasers Tunable lasers are likely to revolutionize high-resolution spectroscopy, especially in the i.r. The resolution of a conventional i.r. spectrometer is usually limited R. Grisar, C. Irslinger, H. Wachering, and €I.0.Hafele. Optics Cumm., 1971,3, 415. E. D. Hinkley and C. Freed, Phys. Rev. Letters, 1969, 23, 277. Tunable Lasers by the decreasing amounts of energy reaching the detector as the slit width is reduced. The minimum practicable power at most detectors is W, and a N typical black-body source at 2000 K produces a usable power of W cm at 10 ,um (1000 cm-l), limiting the resolution to N 0.01 cm-1. On the other hand a CW diode laser can easily achieve 100 pW in abandwidth of 200kHz cm-l).Furthermore the output beam of the laser is directional and collimated, which means that its spectra2 brightness (W cm-1 sr-1) is ca. 1014 times as high as that of the black body. An i.r. spectrometer inevitably has a resolution which is much worse than the Doppler-limited width (N 100 MHz, or 0.003 cm-l) of typical gas-phase absorptions. Figure 8 gives a comparison of linewidths of the different types of tunable laser and shows that all have ‘resolution’ better than the best Wavenumberlcm-1 3~10-~ 3x10a 3x104 3 x loe2 0.3 3.0 30 I I I I 1 I 1 I 1 I Very high-r esolu tion Typical resolution of Typical range laboratory spectrometer routine spectrometers of laser line widths 1 Frequency/Hz Figure 8 Resolution requirements for spectroscopy compared with conventional and tunable laser linewidths (Courtesy E.D. Hinkley) i.r. spectrometers; most are better than the Doppler limit. This increase in resolution is amply demonstrated by Figure 9. The lower half shows the best available grating spectrum of the v3 rotation-vibration band of gas-phase SF6, while the other half shows the fine structure observed in just a small part of the spectrum using a diode laser.3~~8 (The fine structure has been partially attributed to second-order Coriolis effects.) Other impressive spectra include resolution of le E.D.Hinkley, Appl. Phys, Letters, 1970, 16, 351. 306 Burdett and Poliakof' Diode Laser Scan SF6 Pressure : 0.1 Torr Celllength : 10 cm Resohtion : 3 x lo4 an-' 1 t Grating Spectrometer Scan I I.SFg Pressure : 0.1 Torr Cell length : 25 cm 940 945 950 Wavenumber/cm-* Figure 9 A section of the i.r. vibrational spectrum of vg of SF,using a diode laser (above)compared with the best available spectrum (below), obtained using a very high-resolutionconventional spectrometer(Adapted by permission from Appl. Phys. Letters, 1970, 16, 351) nuclear hyperfine splittingslg in NO and the self-broadening20of the vibration-rotation bands of H2O. Dye lasers have already enabled saturation techniques (Lamb-dip etc.) to be used to observe fine structure normally obscured by Doppler broadening in atomic spectra.21Tunable i.r. lasers should allow these techniques to be applied to i.r., rotation-vibration spectra.Z2 l9 F.A. Blum, K. W. Nill, A. R. Calawa, and T. C. Harman, Chem. Phys. Letters, 1972, 15, 144. *O R. S. Eng, A. R. Calawa, T. C. Harman, P. L. Kelley, and A. Javan, Appl. Phys. Letters, 1972, 21, 303. p1 T. W. Hansch, I. S. Shahin, and A. L. Schawlow, Phys. Rev. Letters, 1971, 27, 707; M. S. Feld and V. S. Letokhov, Sci. Amer., 1973, 229, No. 6, p. 69. 99 A. C. Luntz and R. G. Brewer, J. Chem.Phys., 1971,54,3641; R. G. Brewer, Science, 1972, 178,247, Tunable Lasers The high powers and signal to noise ratios of tunable lasers greatly simplify the recording of spectra under adverse conditions where very weak signals are obtained. These include long path lengths (>1 km),highly absorbing solvents, very low-concentration gas mixtures (e.g.NO in city air, using the highly sensitive opto-acoustic celP) spectra of high-temperature species (in presence of strong emission from a furnace etc.), and short-lived moelcules2 (e.g. from flash photolysis where the sampling time is very short, -1 ps). The high power levels of these lasers also make possible two-step photodissociation processes,24 which may become increasingly important in the field of isotope separation.25 7 Future Developments and Conclusions It is obviously impossible to make accurate forecasts of future developments. Nevertheless it is likely that the existing tunable lasers will be further refined with increases in tuning range and power and reductions in linewidth. New dyes and non-linear materials will be exploited, and new types of tunable laser will be developed.One of the more promising of these is the high-pressure gas laser (HPG). Present-day, low-pressure gas lasers produce a series of narrow band- width lines corresponding to the rotational levels of the gas. If the pressure of the gas were to be substantially increased these rotational lines would be broadened into a quasi-continuum and continuous i.r. tuning analogous to that of the dye laser would be possible. Initial experiments have been promisingZs but much development is still needed. Nevertheless, it must be admitted that tunable lasers have already been developed to a stage where they are potentially useful to the chemist. It is to be expected that over the next few years tunable lasers will be extensively applied to chemistry. as W. R. Harshburger and M. B. Robin, Accounts Chem. Res., 1973, 6, 329; L. B. Kreuzer, J. Appl. Phys., 1971, 42, 2934. a4 R. V. Ambartzumian and V. S. Letokhov, Appl. Optics, 1972,11, 354. 96 C. B. Moore, Accounts Chem. Res., 1973, 6, 323. I6N. G. Basov, V. A. Danilychev, 0.M. Kerimov, and A. S. Podsosannyi, Zhur. exp. i teor. Fiz.Pis’ma Redukts, 1973, 17, 147. 308

ISSN:0306-0012    

DOI:10.1039/CS9740300293

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

Formation of Hydrocarbons by Micro-organisms By C. W. Bird DEPARTMENT OF CHEMISTRY, QUEEN ELIZABETH COLLEGE, CAMPDEN HILL, LONDON W8 7AH J. M. Lynch AGRICULTURAL RESEARCH COUNCIL, LETCOMBE LABORATORY, WANTAGE OX12 9JT 1 Introduction Alkanes and alkenes are a relatively neglected group of natural products; indeed, apart from the obligatory reference to methane in marsh gas, most standard chemistry textbooks make no reference to their biological origin. In large measure this probably reflects the difficulties in separating and identifying individual members of hydrocarbon mixtures prior to the advent of gas-liquid chromatography (g.1.c.). Studies on the biosynthesis of alkanes are also hampered by the difficulty of locating the position of isotopic labels in the carbon chain.A large portion of the work on the biological occurrence and synthesis of hydrocarbons refers to plants1 and arises partly because material on which to work is relatively easy to obtain and partly because hydrocarbons might be used in phylogenetics. However, over the past few years the production of hydro-carbons by micro-organisms has begun to receive increasing attention for reasons ranging from the recognition of ethylene as a plant hormone to the use of steranes as biological markers in ancient shales. From an experimental viewpoint the biggest continuing problem is the exclusion of adventitious hydrocarbon (gaseous, liquid, and solid) contamination. This necessitates scmpulous attention to the purity of all solvents, materials, and apparatus used in such studies.An interesting side-light on this problem is the recent suggestion,2 that the increasingly popular ‘parafilm’ can be used as an excellent reference source of alkanes for g.1.c. These problems are accentuated by the relatively low hydrocarbon content of micro-organisms, which necessitates growing them on a substantial scale. An unambiguous way of establishing the microbial origin of isolated hydrocarbons is to employ a 14C-labelled substrate and demonstrate, if possible using radio-g.l.c., that the resulting individual hydrocarbons have similar specific activities. 3 G. Eglinton and R. J. Hamilton, ‘Chemical Plant Taxonomy’, ed. T. Swain, Academic Press, 1963, p. 187; A. G. Douglas and G.Eglinton, ‘Comparative Phytochemistry’, ed. T. Swain, Academic Press, 1966, p. 57. P. Gaskin, J. MacMillan, R. D. Firn, and R. J. Pryce, Phytochemistry, 1971, 10,1155. C. W. Bird, J. M. Lynch, and S. J. Pirt, Chem.and Znd. 1974, in the press; E. Merdinger and R. H. Frye, J. Bucteriol., 1966, 91, 1831. Formationof Hydrocarbons by Micro-organisms For purposes of discussion the hydrocarbons produced by micro-organisms are conveniently sub-divided into methane, ethylene, other gaseous hydrocarbons, longer-chain hydrocarbons, and isoprenoids. 2 Methane Methane is commonly encountered in nature wherever bacterial decomposition of organic material can occur under anaerobic conditions, as in swamps or the black muds of lakes. Some idea of the potential scale of methane formation is indicated by the fact that the anaerobic fermentation of sludge in modern sewage plants provides an ample supply of methane to generate all their power requirements.It is also produced in the digestive tracts of animals, to the extent of some 200 1 per day in the case of a large cow, and a recent reviewer4 has commented: ‘if the legendary dragon ever existed it was probably a ruminant . . . however, the biochemistry of the ignition mechanism escapes modern-day biochemists’. The small group of bacteria primarily responsible for methane formation have proved difficult to obtain in pure culture. Those currently known4~5 as pure cultures are Methanobacterium rurninantium, M. formicicum, M. mobilis, Methanosarcina barkeri, Methanococcus sp., M.vannielii, and a Methanospirillum sp., which cover a variety of morphological types. The preferred substrates are generally hydrogen and carbon dioxide or formate. Exceptionally Methano-sarcina barkeri will also grow on methanol, or acetate.6 The problems which can be encountered in working in this area are illustrated by work with a culture described as Methanobacillus omelianskii which has played an important role in these studies. The culture produced methane in an ethanol-carbonate mineral medium, and the specific activity of the methane was the same as that of the added [Wlcarbon dioxide when the substrate was unlabelled ethanol.’ Thus the ethanol was simply acting as a hydrogen source. Eventually it was found8 that Methanobacillus omelianski was in fact a symbiotic association of two organisms, one of which, designated Methanobacterium M.0.H., produced methane when grown on carbon dioxide and hydrogen.The other organism, ‘S’, oxidizes ethanol to acetate and hydrogen and its growth is suppressed by accumulation of hydrogen. Little is known about the intermediate species between carbon dioxide and methane. Pyruvate, which was originally found to be active in the formation of methane by cell-free extract^,^ has subsequently been shown to function as a source of carbon dioxide resulting from decarboxylation.10 Formate functions R. S. Wolfe, Adv. Microb. Physiol., 1971,6, 107. P H. Smith, Developments Industrial Microbiol., 1966,7, 156.B. A. Blaylock and T. C. Stadtman, Arch. Biochem. Biophys., 1966,116, 138. T. C. Stadtman and H. A. Barker, Arch. Biochem., 1949,21,256. M. P. Bryant, E. A. Wolin, M. J. Wolin, and R. S. Wolfe, Archiv. Mikrobiol., 1967.59,20. E. A. Wolin, M. J. Wolin, and R. S. Wolfe, J. Biol. Chem., 1963,238,2882. loB. C. McBride, cf. ref. 4, p. 129. Bird and Lynch in a similar fashion. It has been shown11 that C-3 of serine can function as a methane precursor in Methanobacterium M.O.H. This carbon atom is trans- ferred to tetrahydrofolate, forming N-5,N-10-methylenetetrahydrofolate(l), by the enzyme serine transhydroxymethylase. The species (1) is reduced to N-5-methyltetrahydrofolate(2) by an NADH2-requiring reductase present in extracts of the micro-organism.These extracts also readily generated [14C]methane from N-5-14CH3-tetrahydrofolate.Presumably N-5-methyltetrahydrofolate provides the methyl group of methylcobalamin. Both rnethylcobalaminl2 and methyl- Co-5-hydroxybenzimidazolylcobamine(3),13 the natural cobamide of M. omelianski, are excellent sources of methyl groups for methane formation in the presence of a cell extract, ATP, and a hydrogen atmosphere. Support for the supposition that methylcobalamins are normal intermediates in methane formation is provided by the observation that dichloromethane, chloroform, and carbon tetrachloride competitively inhibit methane formation both in rumen fluids and from methylcobalamin in cell-free extracts. The inhibition results from reaction of these compounds with Bizs, forming a series of chloro- methylcobalamins.14 The 14CH3-cobaloximes (4) can also function as a source l1 J.M. Wood, A. M. Allam, W. J. Brill, and R. S. Wolfe, J. Biol. Chem., 1965, 240, 4564. laT. C Stadtman, Ann. Rev. Microbiol., 1967, 21, 121;M. J. Wolin, E. A. Wolin, and R. S. Wolfe, Biochem. Biophys. Res. Comm., 1963,12,464. l3J. M. Wood, M. J. Wolin, and R. S. Wolfe, Biochemistry, 1966, 5, 2381. l4 J. M. Wood, F. S. Kennedy, and R. S. Wolfe, Biochemistry, 1968, 7, 1707. 31 1 Formationof lYydrocarbons by Micro-organisms of methyl groups under these ~0nditions.l~ The rate of methane formation is particularly affected by the nature of the axial ligand, L, the greatest activity being observed with easily displaced ones such as water and pyridine.Some methyl-group transfer to B12 has been observed with this system in the absence of ATP, but it is not known whether this occurs under the normal conditions of methane generation. Neither ethyl- nor n-propyl-cobaloxime produce ethane or propane under comparable conditions. With Methanosarcina baukeui, for which methanol functions as the methyl donor and Bizs as the methyl acceptor, the necessary components of the methyl- l6 B. C. McBride, J. M. Wood, J. W. Sibert, and G. N. Schrauzer, J. Amer. Chem. SOC., 1968,90,5276. Bird and Lynch transfer reaction are ferredoxin, a corrinoid protein, an unidentified protein, an unknown heat-stable cofactor, ATP, magnesium ions, and a hydrogen atmos- phere.l6 So far as the subsequent formation of methane from methylcobalamin is concerned using cell-free extracts from M.omelianski, one mole of ATP is used for every mole of methane liberated.17 In addition two protein fractions were required and a reduced flavin adenine dinucleotide-generating system. Recently the synthesis of small amounts of methane by disrupted cells of Desulphovibrio desulphuricans, Desulphotomaculum ruminis, and Clostridiumpas-teurianum has been demonstrated.18 The methyl group of pyruvate is the precursor of the methane and not the carboxy-group as in Methanobacterium extract^.^ In one Desulphovibrio sp.methane formation was shown to involve B12, coenzyme A, thiamine pyrophosphate, magnesium ions, and acetyl phosphate.Replacement of pyruvate by a-ketobutyrate led to formation of ethane. 3 Ethylene The biological formation of ethylene has attracted increasing attentionlg since the accidental discovery in 1901 that it hastened the ripening of fruits.20 It is now established as a plant-growth regulator and amongst the actions attributed to the gas are the breaking of dormancy, regulation of swelling and elongation, hypertrophy, promotion of adventitious roots, modification of root growth, promotion of root hairs, epinasty, hook closure, inhibition of leaf expansion, control of flower induction, exudation, ripening, senescence and abscission, inhibition of root nodulation, and induction of soil fungistasis. In animals, very much larger concentrations of ethylene (>80 % in oxygen) are needed to produce readily observable physiological effects, primarily anaesthesia.21 Following earlier demonstrations that plants could produce ethylene, the first indications of microbial production were provided by the observation that the respiratory activity of citrus fruits was increased when they were infected by the green mould Penicillium digitatum.22 It was subsequently shown that P.digitatum formed ethylene when grown in pure culture. Ethylene has been implicated in pathogenesis by a number of other micro-organisms (see Table). In the case of Ceratocystisfimbriata infection of sweet potato roots, the ethylene produced induces the formation of isocoumarins, which provide resistance to l6 B.A. Blaylock, Arch. Biochem. Biophys., 1968, 124, 314. l7 J. M. Wood and R. S. Wolfe, J. Bacteriol., 1966, 92, 696. J. R. Postgate, J. Gen. Microbiol., 1969, 57, 293. F. B. Abeles, ‘Ethylene in Plant Biology’, Academic Press, New York and London, 1973. 2o D. Neljubov, Beih. Bot. Zentralbl., 1901, 10, 128. I1 A. B. Luckhardt and J. B. Carter, J. Amer. Med. ASSOC., 1923, 80, 1440; A. B. Luckhardt and D. Lewis, ibid., 1923, 81, 1851. ** J. B. Biale, Science, 1940,91,458; J. B. Biale and A. D. Shepherd,Amer. J. Eot., 1941,28, 263; E. V. Miller, J. R. Winston, and D. F. Fisher, J. Agric. Res., 1940, 60, 269. Formation of Hydrocarbons by Micro-organisms further fungal attack.24 Most pathogens, with the exception of P.digitaturn and P.solanacearum,32 produce little or no ethylene in pure culture so that the plant must be supplying the biosynthetically necessary substrates. The only human pathogens shown to produce ethylene are the dimorphic fungi Blastomyces dermatitidis, B. braziliensis, and Histoplasma caps~latum.~~ A recent survey found that 58 species of fungi out of 228 examined produced ethylene, Aspergilfus clavatus being by far the most prolific.34 Ethylene has been observed in anaerobic soils at concentrations sufficient, under laboratory conditions, to affect the root extension of cereals.36 The microbial origin of this ethylene was indicated by inhibition of its formation following sterilization by autoclaving or 7-irradiating the soil.36 By enriching soil with methionine and glucose, which are substrates for ethylene formation, Table Ethylene production by some plant pathogens Pathogen Host Symptom Reference Penicillium digitaturn Citrus fruits Fruit rot 22,4145, 61-64 Ceratocystis fimbriata Sweet potato Black rot 23,24,25 Erwinia carotovora Xanthomonas campestris Cauliflower Soft rot 26 Botrytis sp.Carnation Flower damage 27 Fusarium oxysporum Tomatoes Leaf wilt 28 Tulip Stunted growth, 29 blasting of flower buds Erysiphe graminis Barley Powdery mildew 30 Sclerotina fructigena Pseudomonas solanacearum AppleBanana Brown rot Early ripening 31 32 Tomato Wilt Tobacco Wilt a3 E. Chalutz and J. E. DeVay, Phytopathology, 1969, 59, 750; E. Chalutz, J.E. DeVay, and E. C. Maxie, Plant Physiol., 1969, 44,235. a4 S.Sakai, H. Imaseki, and I. Uritani, Plant and Cell Physiol., 1970, 11, 737. a6 Y.Kato and I. Uritani, Agric. and Biol. Chem. (Japan), 1972,36,2601. a6 B. M. Lund and L. W. Mapson, Biochem.J.,1970,119,251. 27 W. H. Smith, 0.F. Meigh, and J. C. Parker, Nature, 1964,204,92. as A. E. Dimond and P. E. Waggoner, Phytopathology, 1953,43,663. as W. J. de Munk and M. de Rooy, Hort. Science, 1971, 6, 40; W. J. de Munk, Netherlands J. Plant Pathol., 1973,79,41. 30 E. C. Hislop and M. A. Stahmann, Physiol. Plant Pathol., 1971,1,297. s1 E. C. Hislop, G. V. Hoad, and S. A. Archer in ‘Fungal Pathogenicity and the Plant’s Response’, ed. R. J. W. Bryde and C. V. Cutting, Academic Press, London, 1973, p.87; E. C. Hislop, S. A. Archer, and G. V. Hoad, Phytochemistry, 1973,12, 2081. 32 H. T. Freebairn and I. W. Buddenhagen, Nature, 1964,202,313. 33 W. J. Nickerson, Arch. Biochem., 1948,17,225. 34 L. Ilag and R. W. Curtis, Science, 1968,159,1357. 35 K. A. Smith and R. S. Russell, Nature, 1969,222, 769; K. A. Smith and P. D. Robertson, ibid., 1971, 234, 148. 36 K. A. Smith and S. W. F. Restall, J. Soil Sci., 1971,22,430. 314 Bird and Lynch the common soil fungus, Mucor hiemalis, and two yeasts, Trichosporon cutaneum and Candida vartiovaari, were isolated and shown to produce ethylene when grown in pure culture.37 Subsequent work by others has confirmed the substrate requirements for ethylene formation by soil micro-0rganisms.3~ Considerable variability in ethylene production was encountered during attempts to study the physiology of its formation in shaken flasks, whereas consistency was obtained with chemostat cultures.39 Higher yields of ethylene per gram of organism were obtained from the chemostat than from batch cultures and the amounts were further increased at lower growth rates.40 The latter observation provides a possible explanation for the higher ethylene yields in chemostat cultures, where the specific growth rate is usually lower.More ethylene per gram of organism is formed as the concentration of dissolved oxygen is increased, but it seems that anaerobic conditions are necessary in the soil in order to release substrates from the soil organic matter for ethylene formation.39 Undoubtedly some of the foregoing observations with M.hiemalis must also be borne in mind when considering similar physiological studies with Penicillium digitaturn. Here various worker+-43 have shown that carbon sources such as serine, sugars, malate, a-alanine, and ethanol promote ethylene formation. There is marked disagreement as to whether ethylene production is higher in stationary44 or in shaken45 cultures. An additional complication is indicated by the wide variation in ethylene production observed44 with single spore cultures. In stationary cultures at least the amount of ethylene produced is not propor- tional to growth of the organism, the maximum production occurring after completion of mycelial gro~th.~4 As in many plants,lg the precursor of ethylene formation by E.carotovora,26 P. solana~earum,~~ is methionine. In the latter case ethionine and M. hiernali~~~.~~ is equally acceptable but none of a wide range of other substrates including pyruvate and ethanol (see below). A pathway for the conversion of methionine into ethylene, established using cell-free systems extracted from cauliflower florets, is summarized in Scheme 1. The first stage is the conversion of methionine by a transaminase into 4-methylmercapto-2-oxobutyricacid (KMBA). 47 This compound is then converted into ethylene by a peroxidase enzyme in the presence of p-hydroxybenzoic acid and methanesulphinic acid.48 ~49The necessary hydrogen 37 J. M. Lynch, Nature, 1972,240,45.38 E. J. Dasilva, E. Henriksson, and L. E. Henriksson, Plant Sci.Letters, 1974,2,63. J. M. Lynch and S. H. T. Harper, J. Gen. Microbiol., 1974,80, 187. 40 J. M. Lynch and S. H. T. Harper, unpublished observations. 41 C. L. Fergus, Mycologia, 1954,46,543. 42 C. T. Phan, Rev.gdndrale Bot., 1962,69,505. 43 B. A. Sprayberry, W. C. Hall, and C. S. Miller, Nature, 1965,208, 1322. 44 D. H. Spalding and M. Lieberman, Plant Physiol., 1965,40,645. 4b M. Meheriuk and M. Spencer, Canad.J. Bot., 1964,42,337. 46 B. T. Swanson, H. F. Wilkins, and B. Kennedy, Hort. Science, 1972,7,26. 47 L. W. Mapson, J. F. March, and D. A. Wardale, Biochem. J., 1969,115, 653. L. W. Mapson and A. Mead, Biochem. J., 1968,108,875. 49 L. W. Mapson, R. Self, and D. A. Wardale, Biochem.J., 1969,111,413.Formation of Hydrocarbons by Micro-organisms 0 Transaminase II MeSCHzCHzCHCOzH + MeSCHzCHzCCOzHa-keto-acid%-==+ I +a-amino-acid NH2 Glucose oxidase .-+02+D-glucose D-gluconolactone +H202 Peroxidast H202 +MeSCH2CH2COCO2H -+CH2=CH2 +other products Scheme 1 peroxide is generated by a glucose oxidase. Other workers have questioned50951 the role of KMBA in ethylene formation, since in some instances it is a far less satisfactory precursor than methionine. The role of the soft-rot bacterium Erwinia carotovora in promoting ethylene production in cauliflower florets appears to be the generation of pectic enzymes, which release and activate a glucose oxidase enzyme from the plant cell walls, thus increasing hydrogen peroxide production.26 The bacterium does not produce ethylene in pure culture.The filtered culture medium from Mucor hiemalis grown in shaken flasks has been found to contain a species which can generate ethylene without enzymic inter~ention.~~The production of ethylene is stimulated by change of the normal pH of 6 to 1. This may be due to release of iron from the citrate chelate present in the medium since addition of ferrous iron greatly increases ethylene produc- tion. Ferric ions are rather less effective than ferrous ones, while cupric and manganous ones are without effect. Addition of ninhydrin, semicarbazide, or 2,4-dinitrophenylhydrazineto the filtered culture medium inhibits ethylene formation. So far the ethylene precursor has resisted positive identification.The foregoing observations draw attention to a basic difficulty in deciding to what extent ethylene production from methionine is enzyme-mediated. A variety of in vitro systems have been found which will effect this process including copper(11)-ascorbate,5* peroxidase-manganese(~~)-sulphite-phenol,~~~~~y-ir-radiation,55 and flavin mononucleotide53 or flavin adenine dinucle~tide~~ with light. In at least some instances it has been shown that methional or KMBA also yields ethylene. Additionally, the combination peroxidase, p-hydroxy- benzoate, indolyl-3-acetic acid, and benzenesulphinate converts KMBA into ethylene.56 It has been suggested that a free-radical mechanism53 is responsible 50 M. Lieberman and A.T. Kunishi, Plant Physiol., 1971,47, 576. 51 A. H. Baur, S. F. Young, H. K. Pratt, and J. B. Biale, Plant Physiol., 1971, 47, 696. 52 J. M. Lynch, J. Gen. Microbiol., in the press. 53 S. F. Yang, Arch. Biochem., Biophys., 1967,122,481 ;J. Biol. Chem., 1969,244,4360. 54 M. Lieberman, A. T. Kunishi, L. W. Mapson, and D. A. Wardale, Biochem. J., 1965,97, 449. 55 J. M. Lynch, unpublished observations. 56 L. W. Mapson and D. A. Wardale, Phytochemistry, 1972, 11, 1371. Bird and Lynch where a sulphinium ion is generated and breaks down to ethylene: H -I-[Mei-CH2-CH2-C-Onm] --t $(MeS)2 + C2H4 + HCOzHv I OH However, the failure to observe free radicals in the system by electron spin resonance studies appears to exclude this proposal.It seems more likely that the degradation of methionine follows the usual amino-acid oxidative route, although the mechanism of individual steps will vary according to the reagent : NH2 NH 0I 101 II H,O 11 RCHCOzH +RCCOzH +RCCOzH +RCHO The precise route from KMBA or methional to ethylene is a matter of conjecture. Experiments using the peroxidase-sulphite-manganese(@-phenol system have shown53 that whereas C-1 of methional is largely liberated as formic acid, C-2 of KMBA appears as carbon dioxide. The methylmercapto-group was detected as dimethyl disulphide, showing that prior oxidation of the sulphide group of methional or KMBA is not a prerequisite for ethylene formation. These obser- vations are in keeping with the following speculative mechanism : OH FeOOH fl I MeS-CHz-CH2CHO MeS-CH2-CH2-C-H 0-OFe13 MeS-+ C2H4 + Fe=O + HCOzH In the case of KMBA the formic acid would be replaced by oxalic acid, which would probably be oxidized further to carbon dioxide.Experiments using both crude enzyme and model systems have shown that peptides containing C-terminal methionine residues are equally acceptable substrates for ethylene production.57 Apart from its formation from methionine, there is evidence that in plants and mammalian systems ethylene can also originate from oxidative breakdown of 1inolenate.58 The extrapolation to a biological context is readily envisaged of the generation of ethylene from, inter alia, monoethyl phosphate and sulphate in the presence of ferrous sulphate and a peroxide59 and the ethylene-forming 57 H.S. Ku and A. C. Leopold, Biochem. Biophys. Res. Comm., 1970, 41, 1155; D. M. Demorest and M. A. Stahmann, Plant Physiol., 1971, 47, 450. 58 W. B. McGlasson, Biochem. Fruits and Products, 1970, 1,475. 59 J. Kumamoto, H. Dollwet, and J. H. Lyons,J. Amer. Chem. SOC.,1969,91,1207. Formationof Hydrocarbons by Micro-organisms breakdown of 2-chloroethanephosphonicacid in acid solution.60 The need to bear in mind other routes which could lead to the biogenetic formation of ethylene is indicated by reports which throw doubt on the universality of the methionine route. For example tracer studies of Ceratocystis fimbriata infection of sweet potato tissue indicate that ethylene formation occurs not only by the methionine route, but also by an acetate route and by an as yet unidentified pathway.25 There is also disagreement as to whether methionine is a precursor of ethylene in P.digitaturn.61162One study concluded that uniformly labelled methionine was not converted into labelled ethylene,61 whereas another found that it was rather poorly utilized but showed that C-3 and C-4 were specifically incorporated into ethylene.62 Several gr0ups~l-~3agree that acetate gives better incorporation and that C-2 but not C-1 is utilized. Incorporation of 14C into ethylene was also found with serine (C-3),43 succinic acid (C-2 and C-3), fumaric acid (C-2 and C-3), m-malate (C-3), p-alanipe (C-2), acrylate (C-2), propionate (C-3 % C-2),62 and pyruvate (C-3).63Presumably these intermediates are incorporated via the Krebs cycle. Suggestions that ethylene might be formed by dehydration of ethanol are apparently excluded by the observation that C-2 is incorporated more than four times as efficiently as C-1.63 The fact that all of the fore- going utilization studies use P.digitatum mycelial mats rather than cell-free systems raises the question as to whether the poor incorporation of methionine into ethylene could be due to transport problems. However, the production of ethylene was not affected64 by rhizobitoxine65 (3, which inhibits ethylene HOCH2CH(NH2)CH20CH== CHCHNH2C02H (5) biosynthesis in sorghum seedlings and in senescent apple tissues.64 Rhizo- bitoxine inhibits methionine biosynthesis by irreversibly inactivating the enzyme /3-cystathionase both in bacteria66 and plant@.Finally it must be emphasized that controls are always very important in ethylene studies as the gas can easily originate from non-biological sources such as rubber and plastic.6* 6o A. R. Cooke and D. I. Randall, Nature, 1968,218, 1974. D. L. Ketring, R. E. Young, and J. B. Biale, PZunt and CeZlPhysioZ., 1968,9, 617. 62 D. W. Jacobsen and C. H. Wang, Plant Physiol., 1968,43,1959. 63 M. S. Gibson and R. E. Young, Nature, 1966,210, 529. 64 L. D. Owens, M. Lieberman, and A. Kunishi, PZanr Physiol., 1971, 48, 1. 66 L. D. Owens, J. F. Thompson, R. G. Pitcher, and T. Williams, J.C.S. Chem. Comm., 1972, 714.L. D. Owens, S. Guggenheim, and J. Hilton, Biochim. Biophys. Acta, 1968,158,219. 67 J. Giovanelli, L. D. Owens, and S. H. Mudd, Biochim. Biophys. Acta, 1971,227,671. 68 D. F. Meigh, Nature, 1962, 196, 345; J. V. Jacobsen and W. B. McGlasson, Plant Physiol. 1970, 45, 631; E. P. Kavanagh and J. R. Postgate, Lab. Practice, 1970, 19, 159; D. W. Pritchard and A. F. ROSS,Plant Physiol., 1972, 49, 564; B. Thake and P. R. Rawle, Arch. Mikrobiol., 1972, 85, 39. Bird and Lynch 4 Other Short-chain Hydrocarbons Although gases such as ethane, propane, propylene, and higher hydrocarbons have been observed in soils35J6 and may well be of microbial origin the matter has received no attention. However, the recent identification69 of n-hexane as the gamone of the seaweed Fucus vesiculosus suggests that such unprepossessing molecules may have unsuspected physiological properties.Probably one of the most exotic microbial hydrocarbons is hexa-l,3,5-triyne, the major volatile product from the Basidiomycete Fornes annos~s.~OThis compound inhibits the growth of other f~ngi719~~ Subsequentand plant~.~~1~3 screening of some 37 further species of the genus Fornes indicated that formation of hexatriyne was restricted to some strains of F. annosus.73 Substantial amounts of methyl chloride were produced by some of the other Fornes species examined. Acetylene is not produced in appreciable concentrations by micro-organisms or soils but it is used as a substrate to form ethylene in the now well-established test74 for the nitrogen-fixing enzyme, nitrogenase.5 Longer-chain Hydrocarbons Longer-chain hydrocarbons have a role as potential microbial f0ssils7~ and the study of the microbial origin of these, together with the isoprenoid ones consider- ed in the following section, has greatly accelerated since this was recognised. The rationale behind this proposition is that hydrocarbons are the most stable group of naturally occurring compounds and are likely to retain much of their original architecture over very long periods of time. As the oldest fossils are of bacteria and algae,76 some more than three billion years old, there has been particular incentive to compare their chemical composition with those of modern micro- organisms. Plant hydrocarbon fractions generally exhibit a marked predominance of alkanes possessing odd numbers of carbon at0ms.l With certain exceptions, micro-organisms do not show this marked alternation in relative abundances between odd and even carbon number hydrocarbons.As far as alkanes are OD J. R. Hlubucek, J. Hora, T. P. Toube, and B. C. L. Weedon, Tetrahedron Letters, 1970, 5163. 70 A. T. Glen, S. A. Hutchinson, and N. J. McCorkindale, Tetrahedron Letters, 1966,4223. ‘l C. M. Dick and S. A. Hutchinson, Nature, 1966,211,868. 72 A.T.Glen and S. A. Hutchinson, Trans. Brit. Mycol. Sac., 1973, 61, 583. 73 M. I. Cowan, A. T. Glen, S. A. Hutchinson, M. E. MacCartney, J. M. Mackintosh, and A. M. Moss, Trans. Brit. Mycol. SOC.,1973, 60,347. 74 J. R.Postgate, in ‘The Chemistry and Biochemistry of Nitrogen Fixation’, ed. J. R. Post-gate, Plenum Press, London, 1971, p. 311. 75 M. Calvin, ‘Chemical Evolution’, Oxford University Press, 1969. 76 E. S. Barghoorn, ScientiJic American, 1971, 224, 30. Formationof Hydrocarbons by Micro-organisms concerned algae"-92 generally have a range of ca. Cl6-c33 with n-heptadecane as the major component. FungiSs* 93-98 have a similar range of alkanes with nonacosane as the most abundant member, but the range can depend on the carbon source used for growth.99 The yeastss8JOOJ01 and bacteria~2~83~8~~89~102-105 with similar ranges of alkanes often have ca. c18and ca. c27 as the most abundant members. The qualitative and quantitative distribution of alkanes and alkenes in micro-organisms has been tabulated.106 There are few reports of hydrocarbons below c14; however, this may partly reflect on evaporation losses during extraction. In the wheat fungal pathogens, TiZZetia sp., the spores have a similar alkane distribution to that of the uninfected wheat kernel, suggesting that the TiZZetia alkanes are derived from the host,94 whereas that of the spores of the smut UstiZago maydis is quite different from those observed for healthy or infected corn tissues.95 The sclerotia of the fungus Sclerotinia sclerotiorum, as obtained from pea- and bean-cleaning operations, were found to contain a hydrocarbon in the c25-c29 range in addition to its normal C13-cl9 range of n-alkanes.107 77 R.C. Clark and M.Blumer, Limnol. Oceanog., 1967,12,79. 78 S. W. G. Fehler and R. J. Light, Biochemistry, 1970,9,418. 79 E. Gelpi, J. Orb, H. J. Schneider, and E. 0. Bennett, Science, 1968,161,700. E. Gelpi, H. Schneider, J. Mann, and J. Orb, Phytochemistry, 1970,9,603. J. Han, E. D. McCarthy, M. Calvin, and M. H. BeM, J. Chem. SOC.(0,1968,2785.J. Han, E. D. McCarthy, W. Van Hoeven, M. Calvin, and W. H. Bradley, Proc. Nat. Acad. Sci. U.S.A., 1968, 59, 29. 83 J. Han and M. Calvin, Proc. Nat. Acad. Sci. U.S.A., 1969,64,436. 84 J. Han and M.Calvin, Nature, 1969,224,576. 85 J. Han and M. Calvin, Chem. Comm., 1970, 1490. 86 I. Iwata, H. Nakata, M. Mazushima, and Y. Sakurai, Agric. and Biol. Chem. (Japan), 1961, 25,319. 87 I. Iwata and Y. Sakurai, Agric. and Biol.Chem. (Japan), 1963,27, 253. 88 J. G. Jones, J. Gen. Microbiol., 1969, 59, 145. J. Orb, T. G. Tornabene, D. W. Nooner, and E. Gelpi, J. Bacteriol., 1967,93, 1811. G. W. Patterson, J. Phycol., 1967,3,22. O1 K. Stransky, M. Streibl, and F. Sorm,Coll. Czech. Chem. Comm., 1968,33,416. K. Winters, P. L. Parker, and C. V. Baalen, Science, 1969,163,467. 93 C. W. Bird, J. M. Lynch, and S. J. Pirt, Chem. and Znd., 1974, in the press. O4 J. L. Laseter, W. M. Hess, J. D.Weete, D. L. Stocks, and D. J. Weber, Canad. J. Microbiol., 1968,14,1149. 96 J. L. Laseter, J. Weete, and D. J. Weber, Phytochemistry, 1968, 7, 1177; J. L. Laseter, J. Orb, and D. J. Weber, Phytopathology, 1966,56,886. 96 J. Orb, J. L. Laseter, and D. Weber, Science, 1966,154,399.g7 J. D. Weete, J. L. Laseter, D. J. Weber, W. M. Hess, and D. L.Stocks, Phytopathology,1969,59,545. so D. J. Fisher, P. J. Holloway, and D. V. Richmond, J. Gen. Microbiol., 1972,72,71. J. D. Walker and J. J. Cooney, Appl. Microbiol., 1973,26,705. looJ. Baraud, C. Cassagne, L. Genevois, and M. Joneau, Compt. rend., 1967, 265, D, 83; M. Fabre-Joneau, J. Baraud, and C. Cassagne, Compt. rend., 1969,268, D, 2282. lol E. Merdinger and E. M. Devine, J. Bacteriol., 1965, 89, 1488. lo*C. W. Bird, J. M. Lynch, S.J. Pirt, W. W. Reid, C. J. W. Brooks, and B. S. Middleditch, Nature, 1971, 230, 473. lo3J. B. Davis, Chem. Geol., 1968,3, 155. lo4 J. G. Jones and B. V. Young, Arch. Mikrobiol., 1970,70,82. lo6G. Rebel, J. Barth, J. Viret, and P. Mandel, BUN.SOC. Chim. biol., 1969,51, 1001. lo6J. M. Lynch, Ph.D. Thesis, University of London, 1971. lo' J. D. Weete, D. J. Weber, and D. LeTourneau, Arch. Mikrobiol., 1970,75, 59. Bird and Lynch As this long-chain hydrocarbon was not formed during growth of the fungus on normal laboratory media the investigators suggested that it is specifically produced by a cell-fungal association; they do not seem to have considered the possibility that it is squalene derived from the plants. Apart from Tilletia foetida, T. caries, and T. controversa, which contain small amounts of i-heptacosane, i-nonacosane, and i-hentriacontaneY94 i-alkanes have also been noted in Sphacelothica reiliana96,97 (c25, C27, c29, and c31), Ustilago mydis (c27, c29, and c31), and Urocystis agropyiis7 (c25).By far the most prominent producers of branched-chain alkanes are the blue-green algae. In Nostoc mscorum,81-85 Anacystis cyaneay80 Chroococcus turgidusY8O Lyngbya aestuarii,80 and Phormidium luridUm81~83 the branched-chain alkanes are 7- and 8-methylheptadecanes, while in addition to a small amount of these Chlorogloea fritschii81--83 contains a major amount of 4-methylheptadecane. Additionally Chroococcus turgidus80 produces a small quantity of 6-and 7-methylhexa- decanes. Simple alkenes are also frequently encountered in yeast and algal hydro- carbons. In particular n-heptadec-1 -ene is the principal hydrocarbon of Chlorella pyrenoidosasOJ08 and occurs in lesser amounts in Anacystis nidulans and Scenedesms quadricauda.80 The principal hydrocarbon of the latter organism is a heptacosene, and several alkenes (c23, c25, c26, and c27) are found in Anacystis m0ntana.7~18~ The formation of pentacos-l-ene and heptacos-l-ene by Chlorella vulgaris under heterotrophic conditions in addition to the c17-c36 alkanes formed autotrophicallyg0 emphasizes the potential role of growth conditions in determining the composition of hydrocarbon fractions. A more complex situation is presented by the widely distributed fresh-water alga Botryo- coccus braunii, of which three distinct physiological states are known.lOg Very little hydrocarbon synthesis occurs in large green cells, but in green active-state colonies the hydrocarbon fraction is ca.20% of the dry weight, the principal components being cis-heptacosa-1 ,18-diene, cis-nonacosa-lY20-diene,and cis- hentriaconta-l,22-diene.109J10In the brown resting state, where hydrocarbons account for some 70 % of dry weight, the predominant species are botryococcene and isobotryococcene in 9: 1 ratio.logJll The structure deduced112 for botryo- coccene (6) indicates that it is best regarded as a tetramethylated acyclic triterpene.lo8C. W. Bird and R. A. Khan, unpublished observations. loSA. C. Brown, B. A. Knights, and E. Conway, Phytochemistry, 1969, 8, 543. 110 B. A. Knights, A. C. Brown, E. Conway, and B. S. Middleditch, Phytochemistry, 1970, 9, 1317. ll1 J. R. Maxwell, A. G. Douglas, G. Eglinton, and A. McCormick, Phytochemistry, 1968, 7, 2157. 11* R.E. Cox, A. L. .Burlingame, D. M. Wilson, G. Eglinton, and J. R. Maxwell, J.C.S. Chem. Comm., 1973,284. Formation of Hydrocarbons by Micro-organisms Considerable attention has been paid to the olefinic hydrocarbons of Sarciria lutea and other members of the family Microco~cacae.ll3-~~~Apart from the variations between different investigators who have used the same strain, considerable confusion arose because the micro-organism now provided by the American Type Culture Collection as ATCC 533 is not S. Zutea and not the same as that used by the earlier workers.l13 The original strain is now referred to as FD 533.117 In the case of this bacterium the alkenes account for over 90% of the hydrocarbon fraction with the c27, c28, and c29 families predominating.The C2 9 alkene family is comprised of cis-dimethylheptacos-13-eneshaving di-iso, iso-anteiso and di-anteiso terminations. 124 Similar is0 and anteiso arrangements are present in the c27 and CZS alkenes. The position of the double bond in the CZS hydrocarbons is either dl1, d12, or A13, while for the c27 ones it is ,411 or A12. In ATCC 533 grown on the same medium the predominant families are c25, c26, and c27, while for S. Zutea ATCC 382, S. jlava, and S. subflava the major alkenes are c27 and c29.122The c29 alkenes are predom- inant in other S. lutea and Micrococcus Zysodeikticus strains. It has been noted that the relative proportions of the members of each alkene family vary depen- ding on whether a chemically defined medium, trypticase soy broth or nutrient broth, is employed.115 Presumably this reflects the relative availabilities of isoleucine and valine in the growth media (see below).It has also been noted that, whereas in the early stationary phase only ca. 10% of the hydrocarbons were saturated, this proportion rises to 89 % in the late stationary phase.ll5Jl7 Despite the aforementioned isolated observations that growth conditions can have marked effects on microbial hydrocarbon production, no systematic study has been reported. The authors’ attempts to study such aspects of alkane production by AspergiZlus nidulans were thwarted by the discovery that alkane synthesis and dissimilation occurred concurrently.93 It seems likely that this could be a general phenomenon. An overall picture is beginning to emerge of the biosynthetic pathways leading to hydrocarbon formation.Although much of the initial work was carried out on plants such as Brassica oleracea and Nicotiana tabacum,f25 some of the most informative results have been obtained with micro-organisms. Two pathways have been widely discussed. One of these, termed the elongation-decarboxylation 113 P.W.Albro and C. K. Huston, J. Bacteriol., 1964, 88, 981. 114 T. G. Tornabene, E. Gelpi, and J. Or6, J. Bacteriol., 1967, 94, 333. 115 T. G. Tornabene, E. 0.Bennett, and J. Or6, J. Bacteriol., 1967, 94, 344. 116 T. G. Tornabene and J. Orb, J. Bacteriol., 1967, 94, 349. 11’ P. W. Albro and J. C. Dittmer, Biochemistry, 1969, 8, 394. 11* P.W. Albro and J. C. Dittmer, Biochemistry, 1969, 8, 953. llDP. W. Albro and J. C. Dittmer, Biochemistry, 1969, 8, 1913. laoP. W. Albro and J. C. Dittmer, Biochemistry, 1969, 8, 3317. lS1P. W. Albro, T. D. Meehan, and J. C. Dittmer, Biochemistry, 1970, 9, 1893. laaT. G. Tornabene, S. J.Morrison, and W. E. Kloos, Lipids, 1970, 5,929; T. G. Torna- bene and S. P. Markey, ibid., 1971, 6, 190. la3P.W.Albro, J. Bacteriol., 1971, 108, 213. la4 P. W. Albro and J. C. Dittmer, Lipids, 1970, 5, 320. la6P. E.Kolattukudy, Lipids, 1970, 5, 259. Bird and Lynch pathway, envisages the biosynthesis of long-chain fatty acids (ca. ClS) which are elongated in CZ units and subsequently decarboxylated. Until recently this pathway appeared to be strongly supported by labelling experiments but there now appears to be an alternative explanation.A principal objection to the pathway was the general failure to observe the long-chain (> CZO) fatty acids which were postulated intermediates. However, it does seem to be responsible for the formation of the shorter-chain hydrocarbons, as isotopic labelling studies indicate126 that palmitic acid and stearic acid are the respective precursors of the n-pentadecane and n-heptadecane of Nostoc muscorum. The accompanying 1:1 mixture of 7-and 8-methylheptadecanes is derived from cis-vaccenic acid (octadec-1 l-enoic acid) by addition of a methionine methyl group to the double bond followed by reduction and decarboxylation (Scheme 2).126J27 In the case Me(CH2) sCH-CH(CH2) KOZH MeI 2;""Me(CH2)5C=CH(CH2) 9C02H M~(CH&CH=C(CHZ)~COZH Me Me I I Me(CH2) ~CH(CH~)IOCO~H Me(CH2) sCH(CH2) gCO2H .1 J.Me Me I I Me(CH2) 5CH(CH2)9Me Me(CH3) sCH(CH2)sMe Scheme 2 of Anabaena variabilis it has been established that the methyl group of [Me--2H3]methionine is incorporated intact, thereby excluding the intermediary formation of 11,12-rnethylenestearic a~id.1~7 Nothing is known about the de- carboxylation step. In view of the availability of a double-bond-reducing system in these algae and the co-occurrence of heptadec-l-ene with this system, it is tempting to suggest that the decarboxylation step involves the concerted fragmentation of a p-hydroxy-acid, q*nj0RCH-CHPC +RCH=CH2 + H2O + C02 \?0-H J.Han, H. W.4. Chan, and M. Calvin,J. Amer. Chem. SOC.,1969, 91, 5156. la' S. W. G. Fehler and R. J. Light, Biochemistry, 1970, 9, 418. Formation of Hydrocarbonsby Micro-organisms or a suitable derivative such as a phosphate ester. However, it should be noted that several micro-organisms are able to introduce a terminal128 or internal129 double-bond into an alkane substrate. The other widely considered pathway for alkane biosynthesis commences with a Claisen-like condensation of two fatty acids, or suitable derivatives, followed by decarboxylation to the ketone and subsequent reduction of the carbonyl group : CO2H I RlCHzCO2H + R2CH2COzH+R1CHCOCH2R2 J. -cos R1CHzCHzCHzR2+R1CH2COCH2R2 It will be noted that alkanes with an even number of carbon atoms in their chains can only result from coupling of fatty acids containing an even and an odd number of carbon atoms.While the relative rarity of odd carbon number fatty acids explains the predominance of odd carbon number alkanes in plants, no satisfactory explanation has been offered as to why the more equal distri- bution of odd and even carbon number alkanes occurs in micro-organisms, whose fatty-acid distributions are very similar to those of plants. The same strictures of course apply equally to the elongation-decarboxylation pathway. Convincing evidence demonstrates that the long-branched-chain alkenes of S. Zutea are formed by a type of head-to-head condensation. Initial labelling studies showed that the anteiso groupings were derived from isoleucine and the is0 groups from valine.ll* It is noteworthy that neither the resulting anteiso nor the is0 fatty acids were incorporated into the alkenes to a degree propor- tional to their concentration in the total lipids.This has been interpreted as indicating that either a specific pool of fatty acids, possibly determined by the fatty-acid composition of a particular class of lipids, participates in the bio- synthesis, or less likely that the enzymes involved have specificity for certain fatty acids. Experiments in which palmitate was added to S. Zutea in the presence of substantial amounts of acetate resulted in its incorporation into hydrocarbons without any loss of its carboxy carbon.l19 The double bond in the resulting hydrocarbon is between C-1 and C-2 of the incorporated palmitic acid moiety.The decarboxylated fatty acid is derived from the lipid: Ci4H2gCH2COzH + RCH2COa __+ C14H29CH=CHCHzR + C02 Work with cell-free preparations showed that palmitoyl coenzyme A was incorporated faster into the hydrocarbons than was the free acid in the absence lZ8 F. Wagner, W. Frahn, and U. Buhring, Angew. Chem. Znternat. Edn., 1967, 6, 359; H. Iizuka, M. Iida, and S. Fujita, Z. allg. Mikrobiol., 1969,9,223. lZ9 B. J. Abbott and L. E. Casida, J. Bacteriol., 1968, 96, 925. Bird and Lynch of coenzyme A, but little incorporation of the carboxyl carbon occurred.120 Further studies showed that the acyl moiety of triglycerides, fatty-acid methyl esters, and wax esters is incorporated only with decarboxylation.121 Additionally it was found that palmitaldehyde was incorporated into hydrocarbons in preference to palmitic acid.Thus the long-chain ketones and alcohols which would result from head-to-head condensation of two molecules of fatty acid, and are present in S. Zutea, would not be expected to be intermediates in hydro- carbon biosynthesis.121 This expectation is confirmed by failure to observe their incorporation into the corresponding olefins. A future paper is due to appear121J24 showing that the alk-l-enyl aliphatic group of a neutral plasma- logen is normally incorporated into the hydrocarbon rather than the aldehyde. The process now envisaged can be summarized in Scheme 3.As mentioned earlier this work has necessitated a reappraisal125 of the evidence cited in support of the elongation-decarboxylation pathway for the formation of nonacosane in young pea and spinach leaves where the incorporation of [l-14C]palmitic acid without loss of 14C appeared to exclude the head-to-head condensation pathway, which in its original form required the loss of half the activity. Scheme 3 The roles, if any, of these hydrocarbons in micro-organisms is uncertain. Fungi which are airborne have spore wall surfaces which are difficult to wet, and hydrocarbons probably contribute to this property.98 This is important in plant disease because it means that the spores are more resistant to desiccation and to fungicidal sprays. It may be conjectured that hydrocarbons also play a part in the microbial cell wall. In this respect it has been shownlOo that in Candida utilis the alkanes and alkenes are located largely in the cell wall and cell contents.6 Isoprenoid Hydrocarbons Apart from the tetraterpenoid carotenoidsl30 and botryococcene (see above) several triterpene hydrocarbons have been found in micro-organisms. The detection in yeasts and other fungi of squalene where it is clearly a sterol precursor 130 B. C. L. Weedon in ‘Carotenoids’, ed. 0.Isler, Birkhauser Verlag, Basel, 1971, p. 29. Formationof Hydrocarbonsby Micro-organisms is not particularly surprising except that its reported occu~encesQ~~~~~~~~~-~~~ are far fewer than might have been expected. This may be due in part to analy- tical problems since confusion with n-alkanes can easily occur during g.1.c.analysis unless the hydrocarbon fraction is separated into straight-chain and branched-chain fractions by molecular sieving. Another factor may be the choice of growth conditions. It has been shown for example that Saccharomyces cerevisiae accumulates substantially more squalene when grown anaerobically than aerobically.100J33 Another interesting observation is that in Candida utilis the squalene is located solely in the cytoplasmic membrane whereas the accom- panying alkanes and alkenes are mainly in the cell walls and contents.100 Until very recently it was generally believed that prokaryotic organisms, the bacteria and blue-green algae, were incapable of synthesizing sterols and by implication squalene, in contradistinction to eukaryotic organisms.This is compatible with the important role of steroids in membrane formation and the apparent absence of internal membranes in prokaryotes. The recent observations of sterols in various blue-green algae134~l~~ and bacferia102,132,136 most of which are known to have extensive internal membrane systems, has been accompanied by the detection of squalene per se in these pr0kary0te~.~3J0~J32J3~J~~J~~ Particularly noteworthy is the methane-utilizing bacterium, Methylococcus capsu2atas,lO2 in which squalene accounts for 0.55 % of the microbial dry weight! In Staphylococcus aureus squalene is accompanied by cis- 12,13-dehydrosqualene but no sterol formation has been dete~ted.13~ The extremely halophilic bacterium Halobacterium cutirubrum produces not only dehydrosqualene and squalene but also 2,3-dihydro- and 2,3,22,23-tetrahydro-squalene.l38The proportion of reduced squalenes increases with age of the culture, suggesting that they result from stepwise terminal reduction.Even more unexpected was the disco~ery13~ of the pentacyclic triterpene hop-22(29)-ene (7) in M. capsulatus and its recognition as the previously un- identified80 hydrocarbon in several blue-green algae. Concurrently140 it was found in the thermophilic bacterium Bacillus acidocaldurius, accompanied by minor amounts of hop-1 7(21)-ene (8),hopane, and possibly c31 homologues. 131 K. J. Stone and F. W. Hemming, Biochem.J., 1965, 96, 14C; W. W. Epstein and G. V. Lear, J. Org. Chem., 1966,31,3434; H. P. Kaufmann, A. K. S. Ahmad, and S. S. Radwan, Fette, Seifen, Anstrichm., 1966, 68, 1010. 139 K. Schubert, G. Rose, H. Wachtel, C. Horhold, and N. Ikekawa, European J. Biochem., 1968,5,246. Is*D. Jollow, G. M. Kellerman, and A. W. Linnane, J. Cell. Biol., 1968,37, 221. ls4 N. J. de Soma and W. R. Nes, Science, 1968,162,363. ls5 R. C. Reitz and J. G. Hamilton, Comp. Biochem., Physiol., 1968,25,401. 136 K. Schubert, G. Rose, and C. HorhoId, Biochim. Biophys. Actu, 1967, 137, 168. lJ7 G. Sue, K. Tsukada, C. Nakai, and S. Tanaka, Arch. Biochem. Biophys., 1968,123,644; G. Suzue, K. Tsukada, and S. Tanaka, Biochim. Biophys. Acta, 1968,164,88. 13* T. G.Tornabene, M. Kates, E. Gelpi, and J. Orb, J. Lipid Res., 1969, 10, 294; S. C. Kushwaha, E. L. Pugh, J. K. G. Kramer, and M. Kates, Biochim. Biophys. Actu, 1972, 260,492; J. K. G. Kramer, S. C. Kushwaha, and M. Kates, ibid., 1972,270, 103. lsD C. W. Bird, J. M. Lynch, S. J. Pirt, and W. W. Reid, Tetrahedon Letrers, 1971, 3189. lP0M. de Rosa, A. Gambarcorta, L. Minale, and J. D. Bu'Lock, Chem. Comm., 1971, 619; Phytochemistry, 1973,12,1117. Bird and Lynch From a biosynthetic viewpoint these hopenes are almost certainly derived by acid-catalysed cyclization of squalene141 and suggest the operation of a simpler process, as might have been anticipated on evolutionary considerations, than the squalene oxide cyclization which is generally responsible for steroid and triterpene synthesis. It should be noted in this context that M.capsulatus is the only one of these hopene-forming bacteria known to form sterols. The first indication that the hopenes may fulfil important physiological roles was provided by the isolationlg2 of the compound (9) and a monounsaturated analogue, as well as 22-hydroxyhopane, from Acetobacter xylinum. These compounds were found specifically to promote the alignment of the extracellular cellulose micro- fibrils produced by this bacterium.143 7 Geochemical Aspects Until fairly recently it was widely assumed that the organic compounds of sediments and oils were derived from plants and other higher organisms es-pecially because prokaryotic organisms were believed incapable of producing pre- cursors of the petroleum steranes and triterpanes.As more detailed know- ledge has accrued of the chemical composition of the deposits on the one hand and of micro-organisms on the other, it has become apparent that there ldl D. H. R. Barton, A. F. Gosden, G. Mellows, and D. A. Widdowson, Chem. Comm., 1969, 184. ldaH. J. Forster, K. Biemann, W. G. Haigh, N. H. Tattrie, and J. R. Colvin, Biochem. J., 1973,135,133. 14a W. G. Haigh, H. J. Forster, K. Biemann, N. H. Tattrie, and J. R. Colvin, Biochem. J., 1973,135,145. Formationof Hydrocarbons by Micro-organisms has been a substantial microbial contribution in many instances. Two recent reviews144 on organic geochemistry render a detailed review of the subject in this article unnecessary and only particularly apposite examples will be men-tioned.Brief reference145 to the role of Botryococcus braunii in the formation of bog- head coals has already been made. It has also been concluded146 that this alga is responsible for the oil content inter alia of certain Palaeozoic oil-bearing rocks. An illustration of the potential value of hydrocarbons as chemical fossils is provided by the obser~ation~~J~~ that the Pre-Cambrian (>2700 x 106 years) Soudan Shale appears to contain the mixture of 7-and 8-methylheptadecanes that is a unique feature of the hydrocarbons of several blue-green algae. As the bluegreen alga Nostoc is one of the types of microfossil believed to be present in some other ancient rocks it seems likely that an organism of this type con- tributed to the organic contents of the Soudan Shale.Recently the Eocene Messel oil shale (50 x 106 years), which formed in shallow lakes, has been found to contain both hopane and homohopane.14* Hopane is also one of the main triterpanes of the Green River Shale (60 x lo6 years)l49 and triterpanes of the hopane type are widely present in and a wide range of other sediments.lS1 Until recently the hopane triterpenes had only been found in primitive plants, i.e. ferns, mosses, and lichens,152 but their recent detection in bluegreen algae and other bacteria (see above) suggests139 that an appreciable proportion of the triterpanes and other com- ponents found in sediments and oil could be of bacterial origin.14* J. R. Maxwell, C. T. Pillinger, and G. Eglinton, Quart. Rev., 1971,25,571; P. Albrecht and G. Ourisson, Angew. Chem. Internat. Edn., 1971, 10, 209. l4ti K. B. Blackburn and B. N. Temperley, Trans. Roy. SOC.Edinburgh, 1936, 58, 841. 146 A. Traverse, Micropaleontology, 1944, 1, 343. u7M. Calvin, ‘Chemical Evolution’, Oxford University Press, 1969, p. 82. 14* A. Ensminger, P. Albrecht, G. Ourisson, B. J. Kimble, J. R. Maxwell, and G. Eglinton,Tetrahedron Letters, 1972, 3861. 149 W. Henderson, V. Wollrab, and G. Eglinton, ‘Advances in Organic Geochemistry’, ed. P. A. Schenk and I. Havenaar, Pergamon Press, Oxford, 1968, p. 181. 160 E. V. Whitehead, Chem. and Ind., 1971, 11 16. A. van Dorsselaer, A. Ensminger, C. Spyckerelle, M. Dastillung, 0. Sieskind, P. Arpino, P. Albrecht, G. Ourisson, P. W. Brooks, S. J. Gaskell, B. J. Kimble, R. P. Philp, J. R. Maxwell, and G. Eglinton, Tetrahedron Letters, 1974, 1349. 168 G. Berti and F. Bottari, ‘Progress in Phytochemistry’, ed. L. Reinhold and Y. Liwschitz, Wiley, London, 1968, p. 589.

ISSN:0306-0012    

DOI:10.1039/CS9740300309

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

The Photochemistry of Olefinic Compounds By J. D. Coyle DEPARTMENT OF PHYSICAL SCIENCES, THE POLYTECHNIC, WOLVEKHAMPTON, WV1 1LY 1 Introduction The extensive studies of photochemical reactions in recent years enable a systematic description of the photoreactions of organic compounds to be made on the same basis as for their thermal reactions. A compound with a particular functional group undergoes characteristic ground-state (thermal) processes and characteristic, often different, excited-state (photochemical) processes. The differences between thermal chemistry and photochemistry arise largely because the excited state has a higher internal energy than the ground state (and therefore a wider range of reactions is thermodynamically feasible), and because the electron distribution is different in the various electronic states. An attempt is made in this review1 to give a brief survey of the major classes of photochemical reaction of compounds containing the C=C functional group.Examples are taken mainly from the chemistry of alkenes and dienes. The photoaddition of such compounds as polyhalogenated alkanes and thiols to alkenes2 is omitted, in which excitation of the non-olefin component causes homolytic cleavage of a bond in the excited state to give radicals which add to the alkene. Certain reactions of unsaturated carbonyl compounds are not dealt with, which involve mainly the C=O group. 2 Excited States For compounds which contain only C=C chromophores the lowest energy electronic transition involves promotion of an electron from the highest occupied w molecular orbital to the lowest unoccupied w* orbital.The intense absorption in the spectra of simple alkenes which corresponds to this allowed transition has its maximum around 170-180 nm in the vacuum u.v., and the singlet (~,w*) excited state is of fairly high energy (-700 kJ mol-l for the planar singlet of ethylene). The corresponding triplet (w, w*) excited state is considerably lower in energy (-350 kJ mol-1 for the planar triplet of ethylene, -250 kJ mol-1 for the non-planar triplet), and the large singlet-triplet energy difference is the result of extensive spatial overlap of thew andw* orbitals. This large difference in energy causes intersystem crossing from S1(w, w*) to TI(r,v*)to be very slow, so that direct irradiation of alkenes and dienes normally gives rise only to chemical reactions of the singlet excited state. Reactions of the triplet excited For a similar review for carbonyl compounds, see J.D. Coyle and H. A. J. Carless, Chem. SOC. Rev., 1972, 1, 465.’C. J. M. Stirling, ‘Radicals in Organic Chemistry’, Oldbourne, London,1965, p. 75. The Photochemistry of Olefinic Compounds state must be brought about by the use of sensitizers. The reactions of the two (T,n*) states (singlet and triplet) often differ considerably, partly because the states differ widely in energy, and partly because the triplet-state behaviour bears some resemblance to that of a carbon biradical whereas the singlet state is capable of undergoing concerted reaction.Conjugated dienes and polyenes exhibit U.V. absorption at longer wavelength than simple alkenes, and the excited-state energy becomes progressively lower as the extent of conjugation increase^.^ Of particular interest in the photo- chemistry of conjugated dienes is that the central bond of the diene, which is of largely single-bond character in the ground electronic state, has much more double-bond character in the first excited state, so that there are two geo- metrically isomeric excited states corresponding to the s-cis- and s-trans-con- formations of the ground state. The singlet states are often represented in dipolar form (1) and the triplet states in biradical form (2), although undue significance should not be attached to such representations.L 1 s-trans singlet s-cis singlet (la) (1 b> .A. s-cis triplet8-trans triplet (2b)(2a) In the absorption spectra of ethylene and other alkenes there is a series of sharp bands superimposed on the broad r* +T absorption envelope. The bands are the first members of a Rydberg series which corresponds to promotion of an electron from theworbital of the alkene to cr-type orbitals associated with the whole C2H4 unit (for ethylene itself). The series converges to the ionization limit. Rydberg excited states seem to play a part in the photochemistry of certain types of alkene, but their role is not yet fully estabIished. 3 cis-trans Isomerization In both the singlet and triplet excited states of an alkene, rotation about the central C=C bond occurs freely, since there is effectively no n-bond in the (T, T*) state.The lowest-energy conformation of the excited states is normally A. Streitwieser, ‘Molecular Orbital Theory’, Wiley, New York, 1967, pp. 208-214. Coyle that in which rotation has occurred by 90" from the planar (ground-state) geometry (3). The existence of these 'non-vertical' ('phantom') states accounts preferred conformation of (Pn*)state Newman projection (34 (3.b) for the long, weak absorption tail in the U.V. spectrum of an alkene, the very short triplet lifetime (intersystem crossing from TI to ground state is rapid because the energy of the phantom triplet is very close to the ground-state energy in this particular geometry), and the lack of phosphorescence.On direct irradiation the cis or the trans ground state of an alkene is excited to the first singlet excited state with the same geometry, and this relaxes to a common, lower-energy state of different geometry. For stilbene this is probably the non-vertical singlet state, though for 1 -phenylprop-l-ene direct irradiation seems to lead to isomerization through a triplet state.* The common inter- mediate decays non-radiatively to either cis or trans ground state. When a photostationary state is achieved, the composition of the cis-trans mixture depends on the extinction coefficients of cis- and trans-alkene at the wavelength employed and on the partitioning of the common excited state to cis or trans ground state.In the near-u.v. region trans-stilbene has a higher extinction coefficient than cis, and since the common excited state decays equally to cis or trans ground state the photostationary state contains a high proportion of cis-stilbene (4) as a result of selective isomerization of the trans-isomer.5 +ph A& wI Ph PhPh Ph N 10% -90% (313nm) (4) The triplet-sensitized cis-trans isomerization of alkenes follows a similar pattern, and if the sensitizer has a sufficiently high triplet energy (ET)the photo- & C. s. Nakagawa and P. Sigal, J. Chem. Phys., 1973,58, 3529. J. Saltiel and E. D. Megarity, J, Amer, Chem, Soc.. 1972. 94,2742. 33 1 The Photochemistry of Olefnic Compounds stationary-state composition depends only on the partitioning of the phantom triplet state.For lower energy sensitizers the photostationary-state composition depends on the actual value of ET (Figure 1for stilbene). A high-energy sensitizer 10-cisltrans 5-I I I 200 250 300 €*(sensitizer) / kJ mol-' Figure 1 Photosensitized cis-trans isomerizationof stilbene populates both cis and trans triplet excited states efficiently, but as the sensitizer energy is reduced the efficiency of sensitization of the cis-isomer falls before that of the trans. As a result there is a region where the trans-isomer is selectively excited and the proportion of cis-isomer at photoequilibrium is greater. At even lower ET values the efficiency of sensitizing the trans-isomer is also low, and the isomer ratio at equilibrium approaches the value obtained with high-energy sensitizers, probably because the energy transfer now produces a non-vertical excited state directly.6 cis-trans Isomerization of conjugated dienes and polyenes occurs on direct irradiation or in a sensitized reaction.Direct irradiation leads to isomerization about one of the double bonds, whereas triplet sensitization can lead directly to isomerization about two double bonds in a primary process, as shown in Scheme 1.7 S. Yamauchi and T. Azumi, J. Amer. Chem. SOC.,1973,95,2709.'R. S. H. Liu and Y. Butt, J. Amer. Chern. Suc., 1971, 93, 1532. 332 Coyle sensitizer‘“i ‘b-Scheme 1 The effect of triplet sensitizer energy on the photostationary-state composition for the penta-1,3-dienes (Figure 2) is the reverse of that for stilbene.8 As ET is 1.0 cis /trans 0.5 0.0 I I r I 200 250 300 E, (sensitizer) / kJ mol-’ Figure 2 Photosensitized cis-trans isomerization of penta-l,3-diene (a) G.S. Hammond, J. Saltiel, A. A. Lamola, N. J. Turro,J. S. Bradshaw, D. 0. Cowan, R. C. Counsell, V. Vogt, and C. Dalton, J. Amer. Chern. Soc., 1964, 86, 3197; (b)J. Saltiel, L. Metts, A. Sykes, and M. Wrighton, ibid., 1971, 93, 5302. The Photochemistry of Olefinic Compounds reduced, energy transfer first becomes less efficient to the trans-diene, and only at lower ET values is the efficiency of transfer to the cis-isomer similarly affected. The processes for conjugated dienes are complicated because different configura- tions (s-cis and s-trans) of each excited state can exist, and there are different possibilities for the non-vertical states because of the presence of two double bonds about which rotation can occur.Simple acyclic alkenes also undergo cis-trans isomerization, through the (n, n*) singlet state on direct irradiati~n,~ or through a chemical intermediate with organic triplet sensitizers (rather than by energy transfer to give the alkene triplet state). Cycloalkenes behave in a similar manner, though the highly strained trans-isomers of cyclohexenes (5) and cycloheptenes cannot be isolated. These isomers can be trapped with added diene to give (@,lo by forming a dimer with a ground-state molecule of cis-cycloalkene, or by protonation with an added reagent such as methanol to give (7).11The formation of a carbenium ion in the Q Kc=0, a,,+ reaction with methanol is supported by results with O-deuteriomethanol which gives (8).P. Borrell and F.C. James, Trans. Faraday SOL, 1966, 62, 2452. loP.E. Eaton, Accounts Chem. Res., 1968, 1, 50. l1 P.J. Kropp, Pure Appl. Chem., 1970, 24, 585. Coyle Cyclopentenes behave differently and lead to products (9)-(11) from radical reactions, presumably because the trans-isomer is not formed at all. Tetra-alkyl-ethylenes are also different and react by a radical pathway." In this case the lowest excited state is not (7r,T*)but Rydberg (w, 34, and the considerable electron deficiency near the carbon atoms invites nucleophilic attack by methanol as shown in Scheme 2.4 Cyclization Reactions Concerted Cyc1ization.-The excited states of conjugated dienes and polyenes can cyclize to a monocyclic or bicyclic product. A major group of these cycliza- tions are electrocyclic reactions, in which a a-bond is formed in a concerted process between the two ends ofa conjugated mystem. The archetypal electro- cyclic reaction is the cyclization of buta-1,3-diene to cyclobutene (12), and the The Photochemistry of Olefnic Compounds reverse ring-opening process. This reaction can occur in the ground state (i.e. thermally) or in the first singlet (T,n*)excited state (i.e. on direct irradiation). The position of thermal equilibrium is determined by the difference in free energy between the open-chain and cyclic compounds, and in the butadiene reaction the equilibrium strongly favours the acyclic diene.The position of photochemical equilibrium is determined by the absorption characteristics of the two compounds at the particular wavelength of radiation employed, and since butadiene absorbs much more strongly than cyclobutene in the normal U.V. region, the photochemical reaction favours production of cyclobutene. The thermal and photochemical equilibria for conjugated trienes and higher polyenes are not always so favourable to one side of the equilibrium. With substituted compounds, different geometrical relationships can exist between open-chain and cyclic compound. These are defined as conrotatory if a two-fold axis of symmetry is preserved in the transformation of one to the other, or as disrotatory if a mirror plane is preserved (Scheme 3).For a given /-7 nAn=p c2 RK R m 3 disrotatory conrotatory Scheme 3 reaction one particular stereochemical course of reaction is very strongly favoured over the other, and this is rationalized, as for all concerted reactions, on the basis of some concept of orbital interaction and conversion such as the concept of the conservation of orbital symmetry.12 The ‘Woodward-Hoffmann’ rules for an electrocyclic reaction in which k electrons undergo major reorganiza- tion are set out in Table 1. Table 1 Woodward-Hoflmann rules for electrocyclic reactions k thermal (So) reaction photochemical (SI)reaction 4q conrotatory disrotatory 4q + 2 disrotatory conro tat ory The stereoselectivity of the reactions is seen in the photochemical ring-closure R.B. Woodward, and R. Hoffmann, ‘The Conservation of Orbital Symmetry’, VerlagChemie, Weinheim, 1970. CoyZe of trans,trans-hexa-2,4-diene(13)13 and of cis-stilbene (14).1* The latter reaction accompanies cis-trans isomerization on direct irradiation of cis-stilbene, and similar reactions provide a convenient route to the helicenes (15).15 The sequence of thermal and photochemical reactions associated with ergosterol provides good examples of electrocyclic reactions.16 When a particular geometrical mode of ring-opening can lead to two different geometrical isomers of product, the major pathway is governed by steric effects in the product, or by conformational preferences in the reactant, as shown in Scheme 4.173 la J.Saltiel, L. Metts, and M. Wrighton, J. Amer. Chem. SOC., 1970, 92, 3227. lo K. A. Muszkat and E. Fischer, J. Chem. Sac. (B), 1967, 662. l6 R. H. Martin and J. J. Schurter, Tetrahedron, 1972, 28, 1749. If) G. M. Sanders and E. Havinga, Rec. Trav. chim., 1964, 83, 665. l7 P. Courtot and R. Rumin, Buff.SOC. chim. France, 1972, 4238. la W. G. Dauben, M. S. Kellogg, J. I. Seeman, N. D. Vietmeyer, and P. H. Wendschuh, Pure Appf. Chem., 1973,33, 197. 3* The Photochemistry of Olefinic Compounds Ph Ph Php+ ($Q butno Ph Ph Ph Scheme 4 Often there is a choice of different possible electrocyclic reactions.For instance, cyclohexa-l,3-dienes can in principle undergo six-electron ring-opening or four-electron ring-closure. Ring-opening is illustrated in the photochemical preparation of all-cis-[lolannulene (16).19 If the ring-opened product is highly strained and absorbs strongly, ring-closure can predominate, and this is employed in the preparation of 'Dewar benzene' (17).20 Non-concerted Cyc1ization.-Direc t irradiation of conjugated dienes can give rise to bicyclo[l,l,0]butanes as well as to cyclobutenes.21 The former seem to be formed in a two-step process from the s-trans excited state of the diene (18), whereas cyclobutenes are formed from the s-cis excited state (19).The bicyclo- butanes can be isolated or converted in situ in to a mixture of cyclobutyl, cyclo- propylmethyl, and homoallyl ethers by reaction with added alcohol. E. E. van Tamelen, T. L. Burkoth, and R. H. Greeley, J. Amer. Chem. SOC.,1971,93,6120. E. E. van Tamelen, S. P. Pappas, and K. Kirk, J. Amer. Chem. SOC.,1971,93, 6092. *I J. A. Barltxop and H. E. Browning, Chem. Comm., 1968, 1481. Coyle The same type of biradical intermediate is postulated22 to account for three- membered-ring products which are among those formed in Hg(Y~)-sensitized reactions of dienes (20). 5 Cycloaddition Reactions Concerted Cyc1oaddition.-Direct irradiation of a non-conjugated alkene can bring about a concerted cycloaddition between the (n,n*) singlet excited state of one molecule and the ground state of another.The stereospecificity can be rationalized on the basis of the conservation of orbital symmetry or some equivalent concept. The geometrical relationship between reagents and product is defined by the mode of addition to each component, whether suprafacial (in which both new bonds are formed to the same face of thensystem) or antara-facial (in which the new bonds are formed to opposite faces of then-system). The rules12 for cycloadditions involving components with rn and n n-electrons, respectively, are shown in Table 2. Most examples of concerted cycloaddition occur in a suprafacial-suprafacial manner, with an approach of reagents which ea S.Boue, and R. Srinivasan, Mol. Photochem., 1972, 4, 93.The Photochemistry of Olefnic Compounds normally allows maximum interaction of the r-systems. Hence many concerted thermal cycloadditions involve six r-electrons (e.g. the Diels-Alder reaction), and many concerted photochemical cycloadditions involve four r-electrons. Table 2 Woodward-Hoflmann rules for cycloaddition reactions m + n thermal (SO)reaction photochemical (SI)reaction 4q suprafacial-an t ara facial suprafacial -supra facial antarafacial-suprafacial ant arafacial-antarafacial 4q + 2 suprafacial-suprafacial suprafacial-antarafacial ant ar afacial-an t ara facial antarafacial-suprafacial Cyclodimerization of cis- and trans-but-2-ene (21) gives 1,2,3,4-tetramethyI- cycl~butanes,~~and the stereochemical course of the reaction strongly supports a concerted mechanism involving distinct cis-and trans-excited states.The formation of quadricyclane from norbornadiene (22) on direct irradiation is probably a concerted reaction, although it also occurs on triplet sensitization.24 The reverse of a cycloaddition reaction follows the same steroechemid pattern, and an example of the stereochemical exclusiveness of these concerted a3 H. Yamazaki and R.J. CvetanoviC,J. Amer. Chem. SOC.,1969,91, 520. a4 W. G. Dauben and R.L.Cargill, Tetrahedron, 1961,15, 197. Coyle reactions is seen in the cycloregression of the fused cyclobutene (23), where each isomer of the tricyclic compound forms only one isomer of the monocyclic enyne on irradiation.25 Cycloaddition with Aromatic Compounds.-The irradiation of alkenes with benzenoid compounds gives rise to products of cycloaddition across the 1,2-, 1,3-,or 1,4-positions of the aromatic ring.Simple alkenes with benzene give all three types of product, the 1,3-adduct (24) normally predominating, and the reactions are stereospecific.26 1,2-Cycloaddition is thought to occur in these simple systems through an exciplex (excited complex) formed from singlet-state excited benzene (lBzu state) and ground-state alkene. It is an important reaction route when the alkene or the aromatic compound is electron-deficient, e.g. acrylonitrile (25),27 but the mechanism of reaction is not always straightforward. 0+ JL WCNfN J. Saltiel and L.S. Ng Lim, J. Amer. Chem. SOC.,1969, 91, 5404. (a) K. E. Wilzbach and L. Kaplan, J. Amer. Chem. Soc., 1971,93,2073;(6) A. Morikawa, S. Browmstein, and R. J. CvetanoviC, J. Amer. Chem. SOC.,1970,92, 1471. 27 B. E. Job and J. D. Littlehailes,J. Chem. SOC.(0,1968, 886. The Photochemistry of Olefinic Compounds In the photocycloaddition of maleimides to benzene2* a 2:l adduct (26) is produced as a result of a thermal Diels-Alder reaction of the 1:1 photoadduct. The photochemical reaction in this system begins with excitation of the imide. The reaction of maleic anhydride with benzene seems similar, affording (27), but there is no evidence for the formation of an intermediate 1:1 cycloadduct. The initial excitation is of a complex formed between ground-state reagents to give a charge-transfer excited state of the complex. This gives a dipolar inter- mediate (28), which can go on to react with maleic anhydride to form the observed product.29 The additions of both maleimides and maleic anhydrides occur on direct or sensitized irradiation.D. Bryce-Smith and M. A. Hems, Tetraheciron Letters, 1966, 1985. as D. Bryce-Smith, Pure Appl. Chem., 1968,16,47. Coyfe The reported examples of 1,3-photocycloaddition involve mainly alkyl- substituted ethylenes26 or cycloalkenes (29).30It is the benzene which is initially excited, but the subsequent steps may involve direct concerted addition to the alkene (this is the only cycloaddition of 1BzUbenzene and ground-state alkene which is allowed on orbital symmetry grounds), or addition through an exciplex, or addition through an intermediate biradical (30).8' 1,4Photocycloaddition is a major process for allenes (31),31 and for conjugated dienes. With dienes (47~ + 47~) addition products (32) may be formed instead of (2n + 4~)products.32 Non-concerted Cyc1oaddition.-There are many cycloaddition reactions of alkenes which are not concerted but which follow a two-step pathway through a R. Srinivasan, I.B.M. J. Res. Develop, 1971, 15, 34; J. Amer. Chem. SOC.,1971, 93, 3555. 31 D. Bryce-Smith, B. E. Foulger, and A. Gilbert, Chem. Comm., 1972, 664. 31 K. Kraft and G. Koltzenburg, Tetrahedron Letters, 1967, 4357, 4723. The Photochemistry of Olefinic Compounds biradical intermediate. These very often involve the triplet (T,n*) state of one alkene molecule, and of the simple alkenes only three-, four-, and five-membered cycloalkenes give dimers such as (33) through their triplet excited state obtained by energy transfer from organic triplet sensitizers.33 Higher yields and greater selectivity result if a copper(1) catalyst is employed.34 The triplet states of more flexible alkenes relax very rapidly to a non-vertical state and from this to cis or trans ground state.When one alkene group is part of a conjugated system, cycloaddition through the triplet state is much more readily achieved. If the conjugated system is an unsaturated carbonyl compound, direct or sensitized irradiation leads to the triplet state, and dimerization to give (34) or cross-addition with an alkene to give (35) can be efficient .lo935 Intramolecular cycloadditions of this type provide C02CH3 + C2H4 hV W CO,CH, routes to bicyclic systems36 such as bicyclo[2,1, llhexanes (36). 33 D.R. Arnold, D. J. Trecker, and E. B. Whipple, J. Amer. Chem. SOC.,1965, 87, 2596. 34 R. G. Salomon and J. K. Kochi, Tetrahedron Letters, 1973, 2529. 35 P. de Mayo, Accounts Chem. Res., 1971, 4, 41. 36 J. R. Scheffer and R. A. Wostradowski, J. Org. Chem., 1972, 37,4317. 344 Coyle 6c0zcHsCOPCHS hP __)I &co'cHs acetone If the conjugated system is a diene, triplet sensitization gives cycloaddition products together with products of cis-trans isomerization where appropriate.Diene dimers are produced by addition of s-cis or s-trans triplet-state diene to s-trans ground-state diene. This leads to di-allylic radicals (37), and ring-closure H-&+@ gives either 1,2-divinylcyclobutanes or 4-vinylcyclohexene.37 The participation of non-equilibrating configurations of the biradical is required to account for the variation of product ratio with the energy of the triplet sensitizer, since the s-cis-and s-trans-diene triplets are of different energy. Those dienes which have a rigidly held configuration, such as cyclohexa-1,3-diene, give dimers in a ratio which is independent of the sensitizer energy. Heterocyclic Cyc1oaddition.-The formation of heterocyclic products can be a major reaction pathway when alkenes are irradiated with compounds containing groups such as C=O, C=S, C=N-, or NO2.The reactions are usually initiated by excitation of the compound containing the heteroatom, and in many cases the formation of an exciplex involving both reagents is the next step in the process. The irradiation of alkenes with carbonyl compounds gives oxetans as cyclo- addition products.38 Efficient reaction is associated with those carbonyl com- pounds which have a lowest (n,n*) excited state. Simple alkenes (alkyl-substituted ethylenes) react with the triplet (n, v*)excited state of aliphatic or aromatic 37 G. S. Hammond, N. J. Turro, and R. S. H. Liu, J .Org. Chem., 1963,28, 3297. 38 D. R. Arnold, Adv. Photochem., 1968, 6, 301. 345 The Photochemistry of Olefinic Compounds ketones to give products (38) in a non-stereospecific process.The intermediacy of a relatively long-lived 1,6biradical accounts for both the loss of stereo-chemistry and for the preferred orientation of addition to give the isomers (39).39 TA -(39 4 4.5-I .o t-/-4 The ketone excited state and the alkene first form an excited charge-transfer complex. This exciplex in its triplet state gives the biradical, whereas the singlet state reacts stereospecifically to give oxetan either by a concerted process or through a short-lived biradical. Such singlet-state mechanisms predominate when the alkene, e.g. (40), is substituted with electron-withdrawing groups.40 39 H. A. J. Carless, Tetrahedron Letters, 1973, 3173.40 J. C.Dalton, P. A. Wriede, and N. J. Turro, J. Amer. Chem. SOC.,1970,92, 1318. Coyle The reaction of conjugated dienes (41) with aliphatic ketones41 seems similar. In this system energy transfer from the triplet state of the ketone to the ground- state diene is another major reaction pathway, though it is not in direct compe-tition with the cycloaddition reaction of diene and singlet-state ketone. Alkenes also undergo photocycloaddition with thiocarbonyl compounds,42: and two types of product are formed, thietan and 1,6dithian. With styrene the products seem to be formed through a common intermediate biradical (42),42,43 w, and the ratio of products varies with the concentration of ground state thio- ketone. The situation is more complicated with the electron-deficient alkene acrylo- nitrile.44 The thietan is produced when radiation of shorter wavelength is employed, and the dithian (43) (together with another product) when longer CcN+Ph2Cs=S. (366nm) PhxITcN + &lSPh Ph Ph Ph (43) 41 R.R. Hautala, K. Dawes, and N. J. Turro, Tetrahedron Letters, 1972, 1229, 4a A. Ohno, Internat. J. Surf;r Chem., B, 1971, 6, 183. 43 P. de Mayo and A. A. Nicholson, Israel J. Chem., 1972,10,341. 44 P. de Mayo and H. Shizuka J. Amer. Chem. Soc., 1973,95, 3942. 347 The Photochemistry of Olefinic Compounds wavelength radiation is used. A possible explanation is that the lowest (n,m*) excited state is responsible for dithian production, whilst a different excited state gives thietan.Photocycloaddition between alkenes and C-N compounds has not been widely reported. One example of azetidine formation is shown for the product (44).45 A different type of product results from irradiation of an alkene and a nitro- benzene. The excited state of the nitrobenzene reacts with alkene to give a dioxazolidine (45) at low temperat~e.~~ 0 6 Sigmatropic Reactions Sigmatropic Shifts.-A rearrangement reaction in which, formally, a a-bond migrates across one or twov-systems is called a sigmatropic shift. If the termini of the a-bond migrate over rn and n atoms, respectively (inclusive of the initially and finally bonded atoms), the shift is said to be of order (m, n). Many of these rearrangements are concerted and are subject to stereochemical restrictions as a result of orbital interaction.The geometrical relationship between reactant and product is defined as suprafacial if the migrating group is attached to the same face of thewsystem after migration as before, or antarafacial if it is attached to the opposite face after migration. The rules covering the ‘allowed’ modes of sigmatropic shift of order (myn) are the same12 as those for concerted cyclo- addition (see Table 2). As with cycloaddition reactions, many (though by no means all) of the examples reported involve the most straightforward geometrical relationship and are suprafacial(-suprafacial) shifts. Thus there are many photochemical (1,3) 45 T.H. Koch and R.M. Rodehorst, Tetrahedron Letters, 1972, 4039.45 J. L.Charlton, C. C. Liao, and P. de Mayo, J. Amer. Chem. Soc., 1971,93,2463. CuyZe sigmatropic shifts such as47 those of 1,5-dienes (46), and many photochemical (46) (1,7) shifts such as48 those of cycloheptatrienes (47). Photochemical (1,5) hydrogen H Ph Ph Ph Ph (474 (47 b> (474 (474 shifts are observed with some conjugated dienes (48),49 when the geometry of (48) the system is favourable for an antarafacial migration. Di-7r-Methane Rearrangement.-Many 1,4-dienes and 3-phenylalkenes undergo a rearrangement in the excited state which involves a 1,Zshift accompanied by ring-closure to form a vinyl- or phenyl-cyclopropane.50 With acyclic compounds (49) and most monocyclic compounds (50) the reactive excited state is the singlet, 47 R.C. Cookson, Quart. Rev., 1968, 22, 423. 48 A. P. Ter Borg and H. Kloosterziel, Rec. Trav. chim., 1963, 82, 741. 40 W. G. Dauben, C. D. Poulter, and C. Suter, J. Amer. Chem. SOC.,1970, 92, 7408. r,O H. E. Zimmermann and J. A. Pincock, J. Amer. Chem. SOC., 1973, 95,2957. 349 The Photochemistry of Olejinic Compounds and the reaction is stereospecific with retention of configuration at C(l) and C(5)and with inversion at C(3) of the 1,Cdiene unit. The triplet excited state of these compounds generally leads to cis-trans isomerization where it is possible. The singlet excited state of more rigid (especially bicyclic) systems often under- goes some other reaction. However, a reaction is possible for the triplet state analogous to that observed with the singlet state of more flexible compounds.The triplet state of the rigid compound is unable to relax to a non-vertical geometry, and a non-concerted di-r-methane rearrangement occurs to give (51).51 (51) The reaction pathway has been established by labelling studies, as shown for barrelene (52) (Scheme 5).52 A =C-H ;all others are C-D Scheme 5 7 Fragmentation Reactions Monoalkenes are essentially transparent to near-u.v. radiation, and shorter wavelengths are required to effect direct photochemical reaction. The excited states formed on direct irradiation undergo fragmentation or rearrangement, s1 H. E. Zimmermann and G. L. Grunewald, J. Amer. Chem. SOC.,1966, 88, 183. 5a H. E. Zimmermann, R.W Binkley, R. S. Givens, and M. A. Sherwin, J. Amer. Chem. SOC. 1967, 89, 3932. Coyle and these processes are usually very efficient in the gas phase. In solution the efficiency is much lower, because energy dissipation is very rapid and reaction from vibrationally excited levels of the electronically excited state is suppressed. Solvent cage effects may also play a part in reducing the efficiency of reaction in solution. Direct irradiation of ethylene leads mainly to intramolecular cleavage53 to give acetylene and molecular hydrogen, possibly by way of a carbene (53). (53) Smaller amounts of products from hydrogen atoms and vinyl radicals are also formed. With alkenes other than ethylene (e.g. 54), allylic C--H or C--C cleavage is the major process, apart from cis-trans isomerization, on direct ?& G& * Cafb + (54) IL-) irradiation.54 This is because these particular bonds are relatively weak. The above processes derive from a (T,T*)state, probably a singlet.For tetra-alkyl-substituted ethylenes, e.g. (59, different reactions occur which involve an intramolecular (1,2)-hydrogen, (1,2)-alkyl, or (1,3)-hydrogen shift. These take place readily in solution as well as in the gas phase, and they may involve a Rydberg excited state.11 The triplet states of monoalkenes can be obtained only by energy transfer since intersystem crossing is slow from the singlet states obtained on direct b3 P. Borrell, A. Cervenka, and J. W. Turner, J. Chem. Soc. (B), 1971, 2293.J. P. Chesick, J. Chem. Phys., 1966, 45, 3934. 35 1 The Photochemistry of Olefinic Compounds irradiation. In solution, acetone sensitizes the dimerization of rigid cycloalkenes (see Section 5). In the gas phase, sensitization of any alkene can be effected using Hg(3P1) or another triplet excited atomic species such as cadmium or zinc. The major quenching process of Hg(3P1) by an alkene goes through an inter- mediate complex (exciplex) and gives ground-state Hg(lS0) and triplet-state alkene. The reactions of the triplet state resemble those of the excited singlet state except that a much higher degree of selectivity is found. Triplet-state ethylene gives acetylene and hydrogen by an intramolecular process (cf.ref. 53), and there is very little radical formation.Higher alkenes undergo allylic cleavage with considerable selectivity in their triplet excited states, though many minor rearrangement products are also formed, particularly at higher gas pressures.55 This suggests that rearrangement is more important than cleavage from the lower vibrational levels of the excited state. 8 Photo-oxidation Ultraviolet or visible irradiation of alkenes with molecular oxygen in the presence of a low-energy triplet sensitizer such as methylene blue leads to oxidation products. The role of the sensitizer is to generate singlet (Ids)oxygen. Alkyl-substituted ethylenes with (ld,) oxygen give an allylic hydroperoxide as shown for (+)-limonene (56).56 The reaction is concerted and stereospecific, unlike thermal autoxidation, which gives racemic products by way of a sym-metrical allylic radical.0-0 55 J. R. Majer, J. F. T. Pinkard, and J. C. Robb, Trans. Farahy SOC.,1964, 60, 1247. 56 (a) C. S. Foote, T. T. Fujimoto, and Y. C. Chang, Tetrahedron Letters, 1972, 45; (b)N. Hasty, P. B. Merkel, P. Radlick, and D. R. Kearns, ibid., p. 49. Cuyie Alkenes such as dialkoxyethylenes give a 1,Zdioxetan (57) in a stereospecific cycloaddition reaction,57 and conjugated dienes undergo a (47r + 27~)cycloaddition which leads to a 1,2-dioxene (58).58 67 P. D. Bartlett and A. P. Schaap, J. Amer. Chem. SOC.,1970, 92, 3223. 68 K. Gollnick and G. 0.Schenck in '1,4-Cycloaddition Reactions', ed. J. Hamer, Academic Press, New York, 1967, p.255. 353

ISSN:0306-0012    

DOI:10.1039/CS9740300329

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

Isomer Enumeration Methods By D. H. Rouvray CHEMISTRY DEPT., UNIVERSITY OF THE WITWATERSRAND, JOHANNESBURG, SOUTH AFRICA 1 Prologue As the year 1974 marks the centenary of the first application of a mathematical technique for the enumeration of chemical isomers, it was felt to be an appropri- ate time to review the development of isomer enumeration methods. The field has never been adequately reviewed, the only sources presently available being some cursory accounts of aspects of the field in elementary texts, an incomplete survey of methods used up to 1931,l two unpublished surveys, by Cam in 1942,2 and Wiswesser in 1958,3and two introductory accounts4 of the principal methods now in use. We remedy this situation here by presenting a review of all the significant work carried out in this field up to the end of 1973.Because much effort and ingenuity has been expended on the enumeration of the structural isomers of organic species, a considerable portion of the review relates to this area. The enumeration of stereoisomers in both organic and inorganic species is also covered, as are the techniques employed for enumerat- ing the boron hydrides, cluster compounds, and topological and valence isomers. One problem which must be settled before any isomer enumeration can be carried out is that of deciding exactly what is to be enumerated. This involves us in the formal definition of the term ‘isomer’, and the assignment of isomers to ‘classes’ or ‘categories’. These deceptively simple questions have been investigated by several workers.5 For our purposes isomers will be regarded as individual chemical species of identical molecular formulae which display at least some differing physico-chemical properties, and which are stable for periods of time long in comparison with those during which measurements of their properties are made.We thereby exclude relatively unstable species such as mesomers, and polymeric species of identical empirical formulae. The assignment of isomers to classes is also often fraught with difficulties. If, for instance, an isomer is assigned to one of the many homologous series, we are then confronted with the problem of defining precisely what is meant by such H. R. Heme and C. M. Blair, J. Amer. Chem. SOC.,1931,53, 3077. * R.E. Cam, ‘The Number of Isomeric Alkenes’, Thesis, Iowa State College, Ames, Iowa, 1942. a W. J. Wiswesser, unpublished paper, 1958. (Q) S. Rios,GQZ.Mat., 1945, 6, 1; (b) D. H. Rouvray, Chemistry, 1972, 45, 6. (a)P. Gordan and W. Alexejeff, 2. Phys. Chem., 1900,35, 610; (b) A. C. Lunn and J. K. Senior, J. Phys. Chem., 1929,33,1027; (c) S. W. Golomb, in ‘Information Theory, Fourth London Symposium’, ed. C. Cherry, Butterworths, London, 1961, p. 404; (d) G. Ege,Naturwiss., 1971,58,247; (e) I. Ugi, P. Gillespie, and C. Gillespie, Trans.New York Acad. Sci., 1972, 34, 416. 355 4 Isomer Enumeration Methods a series.6 We shall avoid such problems here by assuming that classification is both a feasible operation and one that will be intuitively understood on the part of the reader.2 The Mathematical Backdrop We consider first the basic mathematical tools necessary for all isomer enumera- tion studies. These tools are the closely inter-related disciplines of graph theory and combinatorial theory and, to a lesser extent, group theory. Graph theory has been adapted mainly for the concise representation of chemical species, whereas combinatorial theory has been applied in the actual enumeration process. Group theory has been used wherever the symmetry of species plays an important role. Graph theory’ is the study of the nature and properties of topological graphs. A topological graph, which should not be confused with the more common A Cartesian variety, consists of a pair (X, r),where Xrepresents a set of points, and rA is an operator which maps points in X into other points of the set.This 0 In=l n=2 +Yn=3 n=& Figure 1 Graph-theoretical trees representing the structural isomers of the first six mem- bers of the alkane series (a)J. K. Senior,J. Org.Chem., 1938,3, 1 ;(b) F.L. Breusch, Fortschr Chem. Forsch., 1966, 12,119; (c) I. Motoc, Rev. Roumaine Chim., 1973, 18, 1419.’(a) C. Berge, ‘The Theory of Graphs and its Applications’, Methuen, London, 1963; (b) F. Harary, ‘Graph Theory’, Addison-Wesley, Reading, Massachusetts, 1969; (c) R. J. Wilson, ‘Introduction to Graph Theory’, Oliver and Boyd, Edinburgh, 1972; (d)F. Hararyand D. Palmer, ‘Graphical Enumeration’, Academic, New York, 1973.Rouvray mapping process generates unordered pairs of points or lines in the set X. The points are usually referred to as vertices, whilst the lines are known as the edges of the graph. Graphs may be used for the representation of molecular species: the vertices depict the time-averaged positions of the atomic nuclei, the edges the valence bonds existing between the nuclei. In Figure 1 are depicted examples of tree graphs, representing the structural isomers for the first six members of the alkane series. Combinatorial theory8 is the study of the possible configurations of a finite set of objects, and normally involves an investigation of the number ofpartitions or dasses into which the set may be partitioned by some well-defined mathematical operation.It has proved an indispensable aid in determining the numbers of molecules belonging to specified classes of isomer. In the history of combinatorial theory are to be found many interesting interactions with chemistry, this resulting in a fair amount of cross-fertilization of both disciplines. As our narrative reveals, mathematicians have produced novel results whilst working under the challenge of chemical problems, and chemists working on isomer enumeration have developed several new combin- atorial techniques. The alliance of group theory with chemistry has proved even more rewarding than that of combinatorial theory. Since the main outlines of group theory are widely known among chemists today,s we shall not discuss them here.3 The Curtain Raiser The concept of isomerism is very old, being first adumbrated in the writings of Democritus around 420 B.C.10 From the sixteenth century onwards numerous early pioneers, including Jungius,ll von Humboldt,l2 Thomson,l3 Dalton,14 and Gerhardt’ls brought increasing clarification of the concept. The existence of chemical isomers was first demonstrated experimentally in 1811by Gay-Lussac.16 (a) J. Riordan, ‘An Introduction to Combinatorial Analysis’, Wiley, New York 1958; (6) E. F. Beckenbach (ed.), ‘Applied Combinatorial Mathematics’, Wiley, New York, 1964; (c) C. L. Liu, ‘Introduction to Combinatorial Mathematics’, McGraw-Hill, New York, 1968; (d) M. Eisen, ‘Elementary Combinatorial Analysis’, Gordon and Breach, New York, 1969.(a) F. A. Cotton, ‘Chemical Applications of Group Theory’, Interscience, New York, 1963; (6) D. S. Schonland, ‘Molecular Symmetry’, Van Nostrand, London, 1965; (c) L. H. Hall, ‘Group Theory and Symmetry in Chemistry’, McGraw-Hill, New York, 1969;(d)G. Davison, ‘Introductory Group Theory for Chemists’, Elsevier, London, 1971. lo H. A. M. Snelders, Chem. Weekblad, 1964, 60,217. l1 R. Hooykaas, in ‘Die Entfaltung der Wissenschaft’, Jungius Memorial Volume, Augustin, Hamburg, 1957, p. 47. l2 E. 0. von Lippmann, Chem.-Ztg., 1909, 33, 1. l3 T. Thomson, ‘The History of Chemistry’, Vol. 2, Colburn and Bentley, London, 1831, p. 304. l4 W. V. Farrar, in ‘John Dalton and the progress of science’, ed. D. S. L. Cardwell, Manchester University Press, Manchester, 1968, p.291. lS C. F. Gerhardt, ‘Prkis de Chimie Organique’, Fortin, Masson, Paris 1844, Vol. 1, p. 22, l6 (a) J. L. Gay-Lussac and L. J. Thenard, ‘Recherches Physico-Chimiques’, Deterville. Paris, 1811, Vol, 11, p. 340; (6) J. L. Gay-Lussac, Ann. Chim., 1814, 91, 149. Isomer Enumeration Methods Shortly thereafter Faraday made the prophetic observation17 on isomers that ‘now we are taught to look for them, they may probably multiply upon US’. In 1830 the term isomerismwas coined by Berzelius from the Greek roots ‘~os, equal and ‘pep&’, part; he also gave the first modern definition18 of isomers as compounds ‘possessing the same chemical constitution and molecular weight but differing properties’.Structural isomers were first fully recognized ca. 1862 by Butlerov.19 He is usually also credited with the founding of modem structural theory in organic chemistry.20 His investigations embraced several isomeric systems,21 and he succeeded in obtaining the number of isomers for the chloro- substituted methanes.lg The earliest notion of stereoisomerism is probably due to Wollaston,22 who in 1808 wrote of atoms that ‘the arithmetical relation alone will not be sufficient to explain their mutual interaction, and that we shall be obliged to acquire a geometrical conception of their relative arrangement in all the three dimensions’. The concept of the asymmetric carbon atom stems from the work of Paste~r,~3 though its importance was first appreciated in the influential works of Le Be124 and van’t H0ff.~5It was shown by van’t H~ff~~that the number of stereoisomers which can be formed from a molecule containing n asymmetric carbon atoms is 2n, though he indicated that in highly symmetric species this total may not be achieved.The first attempts at isomer classification were made by Keku16,26 B~tlerov,~’ Crum Brown,28 and Berthel~t.~~ Following Werner’s classic studies on co- ordination cornpounds,30 schemes including inorganic isomers were introduced.31 In more recent years, when over thirty varieties of isomerism have been recog- 17 M. Faraday, Phil. Trans. Roy. SOC.,1825, 115,460. 18 J. J. Berzelius, Pogg. Ann., 1830, 19, 326. 19 A. M.Butlerov, 2. Chem., 1862,5, 298.20 A. M.Butlerov, 2. Chem., 1861,4,549. a1 (a) A. M. Butlerov, Z. Chem., 1865,8, 614;(6) A. M.Butlerov, Bull. Sac. Chim. Paris, 1866,5, 17;(c) A. M.Butlerov, Ann. Chem., 1867, 144, 1. Ba W. H. Wollaston, Phil. Trans. Roy. SOC.,1808,98, 101. Z3 L.Pasteur, Compt. Rend. Paris, 1850, 31, 480. 24 J. A. Le Bel, Bull. SOC. chim. France: 1874,22, 337. 2s (a) J. H. van’t Hoff,Arch. Nker. Sci. Exact. Nut., 1874,9,445;(6)J. H. van’t Hoff,‘Voorstel tot Uitbreiding der . . . . Structuur Formules in de Ruimte’, Greven, Utrecht, 1874; (c) J. H.van’t Hoff, (Translated by J. E. Marsh), ‘Chemistry in Space’, Clarendon, Oxford, 1891. 2s F. A. Kekul6, Ann. Chem., 1858,106, 129. 27 A. M.Butlerov, 2. Chem., 1863,6,500. 28 A. Crum Brown, Trans. Roy. SOC.Edin., 1864, 23, 707.M. Berthelot, ‘LeCons de Chimie’, SOC. Chim. Paris, Hachette, Paris, 1866,p. 1. 3O (a) A. Werner, 2.anorg. Chem., 1893, 3, 267; (6) A. Werner and A. Vilmos, ibid., 1899, 21, 145;(c) G. B. Kauffman, ‘Classics in Coordination Chemistry, Part I, The Selected Papers of Alfred Werner’, Dover, New York, 1968. 31 (a) A. W. Stewart, ‘Stereochemistry’, Longmans Green, London, 1907;(6) M. Delepine, Bull. Sci. Pharmacol., 1907,14, 75;(c) C. Laar, Ion, 1909,78, 195;(d) C. Laar, J.prakt.Chem., 1909,78, 165;(e) 0.De Vries, Chem. Weekblad, 1909,6, 387;(f)H. R.Kruyt,Chem. Ber., 1910,43,540;(f)J. R.Mourelo, Rev. Sci., 1919,57,65. Rouvray nized, more complex classification schemes have had to be devised,32 and resort is now being made to set-theoretical procedures.33 4 The Opening Scene The first applications of combinatorial techniques to isomer enumeration were made in 1874.In that year investigations were carried out by the noted mathemati- cian Cayley on alkyl radicals,34 and by the chemist Korner on the substitutional isomers of benzene.35 Both workers thereby initiated studies which were to remain of importance for at least the next seventy years. Cayley’s limited success was later to inspire the valuable iterative procedures, whilst Korner’s was a forerunner of the very powerful methods of enumeration which use group theory. Cayley made the innovation of representing the carbon frameworks of alkane molecules by trees, as shown in Figure 1, and thus first introduced graph theory into the field.He attempted to find some general formula which would yield the number of isomeric alkanes of given carbon content n, but concluded that no such formula could be found.36 Cayley then directed his efforts to determining the number of rooted treeson n vertices. A rooted tree is one in which a particular vertex is singled out and called the root. He enumerated rooted trees37 with a function of the type (1 -x)-l(l -$)-A1 (1 -$)--%...(1 -~n)-A,-l = = 1 -AIX + A~x’ + .. . (1) where x is a variable, n the number of vertices, and An the number of rooted trees on n vertices. By considering the number of different centric and bicentric rooted treesCayley obtained isomer counts for all the alkanes up to the thirteenth member.Examples of centric and bicentric trees are shown in Figure 2. Cayley further investigated trees in which all vertices were of valence three or one (the so-called ‘boron trees’), and trees having vertices of valence two and one (the so-called ‘oxygen trees’).36b In both cases he determined isomer counts for small values of n. He also gave isomer counts for the alkyl radicals CnHzn+l (a) I. W. D. Hackh, Chem. News (London), 1920,121, 85; (6) E. Hertel and J. Mischnat, Ann. Chem., 1926,451, 179; (c) K. Weissenberg. Chem. Ber., 1926,59,1526; (d)P. Pfeiffer in ‘Stereochemie :Eine Zusammenfassung der Ergebnisse, Grundlagen und Probleme’, Deuticke, Leipzig, 1933; (e) F. P. A. Tellegen, Chem. Weekblad, 1935,32,3; (f) P. Niggli, ‘Grundlagen der Stereochemie’, Birkhiiuser, Basel, 1945;(g)M. Bargallo, Ciencia (Mexico), 1950, 10, 257; (h) R.L. Bent, J. Chem. Educ., 1953, 30, 220, 284, 328; (i) J. J. Jennen, Ind. Chim. Belge, 1954,19, 1051; (j) J. J. Jennen, ibid., 1955,20, 1067; (k) R. T. M. Fraser, Adv. Chem. Ser. No. 62, 1967, 295. 33 (a) J. J. Mulckhuyse, ‘Molecules and Models: Investigations on the Axiomatization of Structure Theory in Chemistry’, Thesis, University of Amsterdam, Amsterdam, 1960; (b) E. Ruch, Theor. Chim. Acta, 1968, 11, 183; (c) E. Ruch, et al., ibid., 1970, 19, 288; (d) I. Ugi, et al., Angew. Chem. Internat. Edn., 1970, 9, 703; (e) Ref. 5d; cf) Ref. 5e; (g)W. Htisselbarth and E. Ruch, Theor. Chim. Acta, 1973, 29, 259. 34 A. Cayley, Phil. Mag., 1874, 47, 444.36 W. Korner, Gazzetta, 1874, 4, 305. as (a)A. Cayley, Chem. Ber., 1875; 8, 1056, (b) A. Cayley, Rept. Brit. Assoc. Adv. Sci., 1875, 257. s7 (a)A. Cayley, Phil. Mag., 1857, 13, 172; (6)A. Cayley, ibid., 1859, 18, 374. Isomer Enumeration Methods up to the thirteenth member,38 though in this, as for the alkanes, his results were correct only as far as the eleventh member. Centric Bicenttic h 0110 Figure 2 Examples of centric and bicentric trees. A centric tree has one vertex for its centre whereas a bicentric tree has two 5 The Early Developments The limited success achieved by Cayley stimulated several other workers to try to improve upon his methods. In general, however, little progress was made as the procedures introduced were cumbersome and frequently led to erroneous results. None of these early methods succeeded in enumerating the alkanes beyond the fourteenth member.In 1875, Schiff developed a method39 in which trees were drawn in the form of symmetric main chains with shorter side chains attached, but got no further than the twelfth member. The errors in both Cayley’s and Schiff’s work were corrected five years later by Hermann,40 who enumerated the alkanes by classify- ing the trees into types according to the number of branches attached to the main chain. At a congress held in 1893 Tiemann described an abbreviated numerical system for enumerating the alkanes.41 However, in spite of earlier work, he gave incorrect answers for the eleventh and twelfth members.Several more sophisticated approaches fared no better. These were made by Delannoy,42 who developed a formula to which additional terms were attached 3a (a) Ref. 34; (b) A. Cayley, Phil. Mag., 1877, 3, 34. 39H. Schiff, Chem. Ber., 1875, 8, 1542. 40 (a) F. Hermann, Chem. Ber., 1880, 13. 792: Ih) F. Hermann, ibid., 1897, 30, 2423; (c) F. Hermann, ibid., 1898, 31, 91. 41 F. Tiemann, Chem. Ber., 1893,26, 1595. 4a M. Delannoy, Bull. SOC.chim. France,1894, 11, 239. 360 Rouvruy for succeeding members, Losanitsch,43 and G01dberg~~ both of whom used a combinatorial approach. In the present century Trautz published a combinatorial analysis of the~holeproblem,4~ and David attempted to improveuponDelannoy’s f0rrnula.4~ But the problem remained essentially unresolved until the develop- men t of it er at ive techniques.6 Iteration’s DCbut The first significant advance in enumeration techniques came in 1931 when the chemists Hem and Blair started to develop recursion f~rmulae.~‘ This approach proved to be very successful and results were obtained for all the common homologous series. Several accounts of their work have been given in the chemical literature.48 In essence their method rested upon three postulates : (i) Cayley’s conclusion that no general formula could be found giving isomer counts for members of homologous series; (ii) free rotation about single C-C bonds did not yield new isomers; and (iii) if the isomer count for a given member of an homologous series having n carbon atoms could be ascertained, an appropri- ate recursion formula would yield the isomer count for the succeeding member containing n + 1 carbon atoms.We examine the application of this method to enumeration of the alcohols, CnH2n+10H.47a All alcohol molecules were represented by the general formula R1 I R2-C-OH IR3 where the R groups represented either hydrogen atoms or alkyl radicals. The symbolspn, sn, and tn were used to represent, respectively, the number of primary, secondary, and tertiary alcohols containing n carbon atoms. The total number of alcohols of all types Tn was thus given by pn f sn + tn = Tn (2) Since the replacement of an -OH group in any alcohol molecule by a 43 S. M. Losanitsch, Chem. Ber., 1897, 30, 1917, 3059.44 A. Goldberg, Chem.-Ztg., 1898,22, 395. 45 M.Trautz, ‘Lehrbuch der Chimie’, De Gruyter, Berlin, 1924,Vol. 111, p. 23. 46 L. David, Rev. Gdn. Sci.,1928, 39, 142. 47 (a) H.R. Henze and C. M. Blair, J. Amer. Chem. SOC.,1931,53, 3042, 3077;(b) H.R. Henze and C. M. Blair, ibid., 1932,54, 1098, 1538; (c) C. M.Blair, ‘The hydrolysis of hydantoinoxindoles and the number of structurally isomeric aliphatic compounds’, Thesis University of Texas, Austin, Texas, 1933;(d) D. D. Coffman, et al., J. Amer. Chem. SOC., 1933,55, 252;(e) H.R.Henze and C. M. Blair, ibid., p. 680. 48 (a)E. J. Leeming, Sch. Sci. Rev., 1935,16,412;(b)J. Zuidweg, Faruday, 1943, 13,94, 132; (c) J. Zuidweg, Chem. Weekblad, 1951,47, 686;(d) J. Zuidweg, Faraday, 1951, 6, 81; (e) T.Benfey, ‘The Names and Structures of Organic Compounds’, Wiley, New York, 1966;cf) R.W. Payne, Sch. Sci.Rev., 1969,51, 374. Isomer Enumeration Methods -CH20H group must always result in the formation of a primary alcohol, Pn will be given by the simple expression pn = Tn-1 (3) The evaluation of Sn is made by imagining the joining of two alkyl radicals R1 and \ R2 to the free bonds in the CHOH group, keeping the total carbon content in /R1 and R2 fixed at n -1. When the resulting alcohol molecule has an even number of carbon atoms sn is given by the equation sn = T1.Tn-2 + T2.Tn-3 + . . . +T(n-2)/2.Tn/2 (4) whereas, when n is odd, Sn = T1.Tn-2 + T2.Tn-3 + .. . . + T(n-sp.T(n+1)/2+ 9T(n-1)/2(1+ T(n-1)/2) (5) In the case of tertiary alcohols formation of the molecule is regarded as arising from the attachment of the radicals R1, R2, and RSto the three bonds in the \ -COH group; again the carbon content of the radicals is fixed at n -1.The /equations for tn are (i) when R1 # R2 # R3 tn(') = Tz.TU.Tz (6) where x, y, and z are positive integers totalling n -1;(ii) when R1 = R2 # R3 tn(ii)= iZTz(1 + Tz) Tg (7) where 2x + y now equals n -1; and (iii) when R1 = R2 = R3 tn(iii)= &Tz(l + Tz) (2 + Tz) (8) where 3x equals n -1. The advantage of the iterative approach is that there is no need to consider centric and bicentric symmetry centres in molecules, or the addition of new terms to generating functions for succeeding members of series.A possible drawback that the method involves much tedious calculation has been overcome by the use of modern computers (see Section 11). It has been shown that the results for the hydrocarbon series may be used for enumerating many other series, including the organic acids, aldehydes, alkyl halides, amines, ethers, and nitriles.49 Extensions of this approach have been made by Perry on the alkanes and alcohols,50 Coffman on the alkynesY5l Allen and Diehl on the stereoisomeric 49 H. R. Henze and C. M. Blair, J. Amer. Chem. SOC.,1934, 56, 157. 50 D. Perry, J. Amer. Chem. SOC.,1932, 54, 2918. 61 D. D. Coffman, J. Amer. Chem. SOC.,1933,55,695. Rouvray alcohols,52 Rancke-Masden on the alkanes, alkenes, and alkynes,53 and Kornilov on the tertiary alc0hols.5~ 7 The Role of Symmetry After the development of the iterative approach, increasing attention was focused on the role that symmetry might play in enumeration. The mathematical discipline involved was group theory and this now occupied a position of some prominence.Utilization of the symmetry of a species proved particularly effective in the enumeration of substitutional isomers. The earliest investigations usually included an actual preparation of the predicted isomers in an attempt to confirm their number. Thus, many stereoisomers were prepared by Auwers and Me~er~~ to confirm the predictions of van’t H~ff.~~ After Kekul6 had postulated the existence of only one isomer for mono-, penta-,and hexa-substituted benzene and three isomers for the cases of di-, tri-, and tetra-substitution,56 the predictions were confirmed by Korner.57 Although this approach initially encountered some oppositi~n,~~ it was not long before similar studies were undertaken on the naphthalene59 and other substitutional isomers.60 However, the lack of any well-defined mathemati- cal technique for approaching this type of problem resulted in very little activity for the next few decades.In an extensive treatise in 1929 the reasons for this limited progress were examined by Lunn and Senior.5b They pointed out that a prime reason was the considerable confusion existing in the definition of the basic concepts such as that of the term ‘isomer’. After providing many new definitions, they went on to stress the importance of using permutation groups for the characterization of molecular configuration.A general mathematical formula for all classes of molecules was devised for the enumeration of substitutional isomers. This was of the form where N&) is the isomer count for molecules of permutation group G within the classp, g is the order of the group, nt is the number of symmetry operations of type t which may be performed on the molecule, and K(p, t) is the number of invariant configurations left in p when operation t is carried out. 5a E. S. Allen and H. Diehl, Iowa State Coll. J. Sci., 1942, 16, 161. 63 E. Rancke-Madsen, Acta Chem. Scand., 1950,4, 1450. 64 M. Y. Kornilov, Zhur. strukt. Khim., 1967, 8, 373. b6 K. Auwers and V.Meyer, Chem. Ber., 1888, 21, 784. 66 F. A. Kekulk, Ann. Chem., 1866, 137, 129. 67 (a) Ref. 35; (6) W. Korner and V. Wender, Gazetta, 1887, 17, 5486. 68 A. Ladenburg, Chem. Ber., 1874,7, 1684. 6s (a) F. Reverdin and H. Fulda, Tabellarische ubersicht der Naphtalin-derivate, Georg, Basel, 1880; (b) M. E. Noelting, Mon. Sci., 1894, 8, 178; (c) H. Kaufmann, Z. angew.Chem., 1900,209; (d)H. Kauffmann, Chem. Ber., 1900,33,2131; (e)H. Rey, ibid., p. 1910. (a)Ref.59c; (b)Ref.59d;(c) H. Kauffmann, ‘DieValenzlehre’, Enke, Stuttgart, 191 1, p. 127. 363 Isomer Enumeration Methods Starting from equation (9) several other formulae were derived relating to specific classes of molecule. In one instance a formula was derived which was virtually identical with one originally used by Kaufmann in his enumeration of the substitutional isomers of naphthalene.59 Lunn and Senior also gave extensive tables of K(p, t) values for different molecular configurations. In fact, it was the considerable success achieved by these workers which was to inspire the next major step forward.8 The Turning Point The most notable advance in the history of isomer enumeration came in 1936, when the mathematician P6lya developed his Enumeration Theorem.61 So important has this become in recent years that it is now referred to as the funda- mental theorem in all enumeration work. Although the theorem had been foreshadowed in certain aspects, notably in the work of the mathematician RedfieldG2 and the chemists Lunn and Senior,5b it exploited for the first time the full power of group theory by making an integrated use of symmetry classes of molecules, generating functions, and weighting factors.Several elementary accounts of the theorem are now available.63 As development of the underlying mathematics would be inappropriate here, we illustrate the use of the theorem by applying it to the enumeration of the substitutional isomers of benzene. Our exposition is based on the results con- tained in Table 1. There are listed all the symmetry operations which bring the benzene ring into coincidence with itself. For each of these twelve operations a listing of the vertex positions both before and after the operation is given. This listing is then rewritten to reveal the permutation groupings existing among the vertices, and from this the important cycle index is deduced for each operation.The cycle index is formally defined by the equation where Z(G)represents the cycle index for a group G of order g, p is the number of vertices permuted, the& are variables, and hjl...jpis the number of permutations of G consisting ofjl cycles of order one, j2 cycles of order two, etc. A cycle of order q is one involving the interchange of q vertices after performance of the symmetry operation. The prime over the summation signifies that the condition P Chji = P c= 1 "((a) G. Pblya, Compt. rend., 1935, 201, 1167; (b) G. Pblya, ibid., 1936, 202, 1554; (c) G. Pblya, Helv. Chim. Acta, 1936, 19, 22; (d) G.Pblya, Vierteljahrsschr. Naturforsch. Ges. Ziirich, 1936, 81, 243; (e) G. Pblya, 2. Krist., 1936, 93, 415; cf) G. Pblya, Acta Math., 1937, 68, 145. 62 J. H. Redfield, Amer. J. Math., 1927, 49, 433. 83 (a) N. G. De Bruijn, ref. 8b, p. 144; (b) N. G. De Bruijn, Nieuw. Arch. Wisk., 1963, 11, 142; (c) F. Harary, in 'Graph Theory and Theoretical Physics', ed. F. Harary, Academic, London, 1967, p. 5; (d) Ref. 8c, p. 126; (e) Ref. 7b, p. 180. Rouvray Table 1 The derivation of the cycle index for each symmetry operation which may be performed on the benzene ring Permutation groupings Cycle index flS f6' f51 f12 f32 f13 fl2f22 fl2f22 f12f22 f13 fa3 f13 always holds, where P is the total number of permutations.The cycle indices for all the symmetry operations are added and the total divided by the number of operations for a given species. Isomer Enumeration Methods For the benzene molecule the index has the form: The number of isomers formed by successive substitution of a univalent radical X into benzene is obtained by making substitutions of the type fnm = (1 + xn)m (13) in equation (12). This in turn yields the polynomial Z(C6H6) = 1 x -k 3x2 + 3x3 + 3x4 + x5 -k x6 (14) The coefficients of x in equation (14) give the isomer counts directly. Thus, because the coefficient of x4is three, tetrasubstitution of benzene by the radical X will produce three isomers. The isomers for all the possible cases are illustrated in Figure 3.I> ox0;oxOxxOx xx XX X X X X (j*OXxOXX X Figure 3 A representationof all the positional isomers formed when benzene is substituted by a univalent radical X To date there have been comparatively few direct applications of the theorem. It was employed by HW4 and Taylore5 in a general discussion on isomerism, by Huebner66 and Balaban67 for enumerating isotopic isomers, by Riem-schneider68 and Balaban and Harary69 on cyclic molecules, by Kornilov54 on tertiary alcohols, and by Rouvray70 on the arenes.Applications to inorganic systems are given in Section 10. 84 T. L. Hill, J. Chem. Phys., 1943, 11, 294. 65 W. J. Taylor, J. Chem. Phys., 1943, 11, 532. H. Huebner, A6h. Deutsch. Akad. Wiss. Berlin, KI. Chem., Geol., Biol., 1964, 7, 701. 67 A.T. Balaban, J. Labelled Compounds, 1970, 6, 211. 6a R.Riemschneider, tfsterr. Chem.-Ztg., 1956, 57, 38. 69 A. T. Balaban and F. Harary, Rev. Roumaine Chim., 1967,12, 1511. 'O (a) D. H. Rouvray,J. S. African Chem. Inst., 1973, 26, 141;(6) D. H. Rouvray, ibid., 1974, 27, 20. Rouvray 9 The Unfolding Panorama Significant generalizations and extensions have been made in recent years to all of the methods discussed thus far. The work of Cayley37 and Henze and Blair47 on the enumeration of trees has been extended by the mathematicians Otter,71 Clarke,72 and De Bruijn.73 An improved generating function for the alkyl halides has been devised by Fisher.74 The assumptions that no general formula could be found for enumerating the alkanes was shown to be invalid by Wiswesser,75 who devised a new method of enumeration based on a partition- \ ing of the CH2 groups around and between branched carbon atoms.The method /was successful because each branched atom arrangement established a partition- counting series. The well-known 2n formula of van’t Hoff, giving the number of stereoisomers in species containing n asymmetric carbon atoms, has been modified by several workers to allow for cases when this total is not attained for any reason. These include Fi~cher,~~ Branch and Hill,79 and Weissenberg,32c Senior,77 Hahr~,~~ Feldman,80 who studied systems ranging from the sugars to chain compounds. New formulae for determining the stereoisomers in a variety of systems have been proposed by Snell,81 Dienske,82 Carr,2 Mai,83 Elie1,84 and Breusch.85 After Lunn and Senior had stressed the importance of symmetry in the enumeration of species, many formulae were developed which used this sym- metry.These included the work of Ramirez,86 Hill,87 Riemschneider,88 and De Loach et aLs9on cyclic organic species, Evans and Le Quesne,So yo shin^,^^ R. Otter, Ann. Math., 1948, 49, 583. 72 L. E. Clarke, Quart. J. Math., 1959, 10, 43. 73 N. G. De Bruijn, Proc. Ned. Akad. Wetensch., 1959, 21, 59. 74 R. A. Fisher, Ann. Eugenics, 1942, 11, 395. 75 W. J. Wiswesser, ‘Structure Counting Functions: A New Synthesis of Chemistry and Arithmetic’, Wilson Research Center, Reading, Pennsylvania, 1958. 7eE.Fischer, Chem. Ber., 1891, 24, 1836.77 J. K. Senior, Chem. Ber., 1927, 60B, 73. 78G.Hahn, Chem. Ber., 1927,60B, 1362. 79 G. E. K. Branch and T. L. Hill, J. Org. Chem., 1940,5, 86. A. Feldman, J. Org. Chem., 1959,24, 1556. 81 J. F. Snell, Chem. News (London), 1932,144,321. 8a J. W. Dienske, Chem. Weekblad, 1938, 35, 243. 83 L. A. Mai, Zhur. obshchei Khim., 1958, 28, 2860. 84 E. L. Elie1,‘Stereochemistry of Carbon Compounds’, McGraw-Hill, New York, 1962, p.180. 85 F. L. Breusch, Fette, Seijh, Anstrichm., 1970, 72, 1. M. A. M. Ramirez, Anal. Fis. Quim., 1941, 37, 594. 87 T. L. Hill, J. Phys. Chem., 1943, 47, 253, 413. R. Riemschneider, Z. Naturforsch., 1956, 11B, 228, 291, 675. 89 (a) M. L. Shivar and W. S. De Loach, J. Elisha MitchellSci. SOC., 1968,84,367;(6)E. F. Wells and W.S. De Loach, ibid., 1969,85,45; (c) W. S. De Loach and J. B. Levy, ibid., 1970, 86,38. R. F. Evans and W. J. Le Quesne, J. Org. Chem., 1950,15, 19. alT.Yoshino, J. Chem. SOC.Japan, 1951,72, 501. Isomer Enumeration Methods and Papulov and Lesnyak92 on aromatic molecules, Pishnamazzade93 on a variety of systems including the substituted alcohols, the ethers, and the esters, Vladimirskayag4 on the alkenes, and Breuschs5 on the fatty acids. Since its inception P6lya’s Enumeration Theorem has been generalized by several mathematiciansg5 They have succeeded in extending the power and range of applicability of the theorem. In the chemical context the theorem has been elaborated by Kennedy et aLg6 and Leonardg7 who used point groups rather than permutational groups for species, by Balabang8 who studied mono- cyclic aromatic species formed from three different types of atom, by Sala-Pala and Guerchajsg9 who devised rules for enumerating geometric and optical isomers, by McDaniellOO who restated the theorem to be applicable to the enumeration of inorganic species, by Leonard97 and Mason101 who studied many different systems including stereoisomers, and by Lloyd102 who investi- gated cyclic molecules.Furthermore, in addition to the original work of P61ya61f on asymptotic values of isomer counts approached in various series, Francislo3 has studied the alkylbenzenes and Goncharovl04 the alkanes. The latter worker concluded that for the C400 alkane there would be ca. 1040isomers.10 The Inorganic Scene Following on from Werner’s work,30 co-ordination chemistry soon developed into an important branch of chemistry. Many investigations of the isomers involved have been made. Isomers for the five-co-ordinate complexes have been enumerated by Gielen et aZ.1°5 and Muetterties,lo6 those for six-co-ordinate ga Y.G. Papulov and G. N. Lesnyak, Uch. Zap. Kalinin, Gos. Pedagog. Inst., 1970, 76, 86. O3 (a) S. Mamedov and B. F. Pishnamazzade, Izvest. Akad. Nauk Azerb. S.S.R., 1950, 27; (b) B. F. Pishnamazzade, ibid., 1952,43; (c) B. F. Pishnamazzade, ibid., 1957, 35; (d)B. F. Pishnamazzade, Azerb. Khim. Zhur., 1962, 27, 41, 77; (e) B. F. Pishamazzade, ibid., 1963, 61,69; (f)B. F. Pishnamazzade, ibid., 1964, 49, 63. g4 G. N. Vladimirskaya, Nauch.Doklady Vyss. Shkoly, Khim. i Khim. Tekhnol., 1958, 1, 86. (a) J. Riordan, J. SOC. Zndus. Appl. Math., 1957, 5, 225; (b) Ref. 73; (c) F. Harary and E. Palmer, J. Comb. Theory, 1966,1, 157; (d)J. Sheehan, Canad. J. Math., 1967,19, 792; (e) S.G. Williamson, J. Comb. Theory, 1970, 8, 162. g6 B. A. Kennedy, C. H. McQuarrie, and C. H. Brubaker, Inorg. Chem., 1964, 3, 265. O7 J. E. Leonard, ‘Studies in Isomerism: Permutations, Point Group Symmetries, and Isomer Counting’, Thesis, California Inst. Technol., California, 1971. g8 A. T. Balaban, Studii Cercetari Chim., 1959, 7, 257. gg G. Sala-Pala and J. E. Guerchais, Compt. rend., 1969, 268C,2192. loo D. H. McDaniel, Znorg. Chem., 1972, 11,2678. lol K. H. Mason, ‘Theoretical Aspects of Natural Optical Activity’, Thesis, University of Kent, Canterbury, 1973, p.159. lo2 E. K.Lloyd, 1974, in press. lo3A. W. Francis, J. Amer. Chem. SOC., 1947, 69, 1536. lo4 M. A. Goncharov, Vestnik Moskov Univ., Geol., 1973, 28, 117. 105(a)M. Gielen and J. Nasielski, Bull. SOC. chim. belges, 1969, 78, 339; (b) M. Gielen, C. Depasse-Delit, and J. Nasielski, ibid., p. 357. lo8 E.L. Muetterties, Znorg. Chem., 1967, 6,635. Rouvray complexes by Trimble,107 Bailar,los Mayper,log Muetterties,llo Gielen et al.,lll Menez et aZ.,112 and Musher,113 and those for eight co-ordinate complexes by Marchi et aZ.,114 and Porai-Koshits and Aslanov.ll5 Other types of complex have been enumerated by Fernelius and Bryant,1l6 Gaz0,117 Freeman and Liu,l18 Youinou et aZ.,119 and King.120 Co-ordination polyhedra and polynuclear structures have been studied by Niggli,l21 Krivoshei and Vvendenski,l22 Block and Maguire,l23 Muetterties and Wright,l24 Baker and Figgi~,l~~ King,126 and Ha~th0rne.l~~ Most of this enumeration work was based upon a group-theoretical analysis of the various structures investigated.Certain workers merely adapted the approach of Polya to their particular studies.121J22 Reformulations of the Enumeration Theorem in an inorganic context were made by Kennedy et aLg6 and McDaniel.loO Use of the symmetry properties of species has also been evi- denced in the enumeration of the ferrocenes by Rinehart and Motz,128 Rosen- blum and Wo~dward,l~~ Schlogl et aZ.l30 and Leonard;97 and in enumeration of the clathratesl31 and cluster compounds132 by King.lo7R. F. Trimble, J. Chem. Educ., 1954, 31, 176. loSJ. C. Bailar, J. Chem. Educ., 1957, 34, 334. logs.A. Mayper, J. Chem. Educ., 1957, 34, 623. IloE. L. Muetterties, J. Amer. Chem. SOC.,1968, 90, 5097. ll1 (a) M. Gielen, Bull. SOC.chim. belges, 1969, 78, 351; (b) M. Gielen, G. Mayence, and J. Topart, J. Organometallic Chem., 1969, 18, 1;(c) M. Gielen and J. Topart, ibid., p. 7; (d)M. Gielen, Med. Vlaam. Chem. Ver., 1969,31,201; (e) M. Gielen and C. Depasse-Delit, Theor. Chim. Acta, 1969, 14, 212. 11* (a) A. J. Menez, J. Sala-Pala, and J. Guerchais, Bull. SOL chim. France, 1969, 1115; (6)A. J. Menez, J. Sala-Pala, and J. Guerchais, ibid., 1970, 46. 113 (a) J.I. Musher, U.S. Nat. Tech. Inform. Sew. AD Rept. No. 733689, 1971 ; (b) J. I. Musher, Znorg. Chem., 1972, 11, 2335. 114 (a) L. E. Marchi, W. C. Fernelius, and J, P. McReynolds, J. Amer. Chem. SOL,1943, 65, 329; (b) L. E. Marchi and J. P. McReynolds, ibid., 1943, 65, 333; (L) L. E. Marchi, ibid., 1943, 65, 2257; (d) L. E. Marchi, ibid., 1944, 66, 1984. M. A. Porai-Koshits and L. A. Aslanov, Zhur. strukt. Khim., 1972, 13, 266. W. C. Fernelius and B. E. Bryant, J. Amer. Chem. SOC.,1953,75, 1735. 117 J. Gazo, Chem. Zvesti, 1966, 20, 212. W. A. Freeman and C. F. Liu, Znorg. Chem., 1968, 7, 764. M. Youinou, F. Petillon and J. E. Guerchais, Bull. Soc. chim. France, 1968, 503. lZ0(a)R. B. King, J. Amer. Chem. SOC.,1969, 91, 7217; (b) R. B.King, U.S. Nat. Tech. Inform. Serv. A.D. Rept. No. 746829, 1972. lZ1P. Niggli, Helv. Chim. Acta, 1946, 29, 991. lZ2 (a) I. V. Krivoshei, Zhur. strukt. Khim., 1963, 4, 757; (6) I. V. Krivoshei, ibid., 1965, 6, 322; (L) I. V. Krivoshei, ibid., 1966, 7, 430, 638; (d) I. V. Krivoshei, ibid., 1967, 8, 321; (e) I. V. Krivoshei and V. Y. Vvendenski, Theor. i eksp. Khim., 1967, 3, 508. (a) B. P. Block and K. D. Maguire, Inorg. Chem., 1967,6,2107; (6) B. P. Block and K. D. Maguire, U.S.Govf.Res. Dev. Reports, 1967, 67, 64. lZ4E. L. Muetterties and C. M. Wright, Quart. Rev., 1967, 21, 109. lZ5L. C. W. Baker and J. S. Figgis, J. Amer. Chem. SOC.,1970, 92, 3794. (a) R. B. King, J. Amer. Chem. SOL.,1969, 91, 7211; (b) R. B. King, ibid., 1970, 92, 6455, 6490.Iz7 M. F. Hawthorne, Pure Appl. Chem., 1972, 29, 547. lZ8K. L. Rinehart and K. L. Motz, Chem. and Znd., 1957, 1150. lZ9 M. Rosenblum and R. B. Woodward, J. Amer. Chem. SOC.,1958, 80, 5443. 130 (a) K. Schlogl, M. Peterlik, and H. Seiler, Monatsh., 1962,93, 1309; (b) K. Schlogl, TopicsStereochem., 1967, 1, 39. 131 R. B. King, Theor. Chim. Acta, 1972, 25, 309. 132 R. B. King, J. Amer. Chem. SOC., 1972, 94, 95. Isomer Enumeration Methods Enumeration of the boranes and carbaboranes has proved dficult mainly because of the three-centre bonds found in these species.133 Simple enumeration of the graphs representing these molecules will not suffice. A decision must be taken for each species whether its bonding scheme is feasible or not by considering the conservation of electrons and orbitals, and the requirement that all pairs of neighbours have at least one common bond.At present results are available for boranes and carbaboranes containing up to about 12 boron atoms.lS4 Because the sorting of feasible structures becomes exceedingly tedious for the higher boranes, resort is now being made to computer programmes.135 11 The Advent of the Computer The appearance of high-speed computers in recent years has considerably facilitated several types of isomer enumeration. In addition to the work on the boranes mentioned above, the iterative procedures have proved readily adaptable to the computer. Enumeration programmes for the alkanes have been written by Davis et aZ.136and Lederberg,13' and a general programme for enumerating co-ordination compounds has been devised by Bennett.138 Programmes are now also available for the representation and storage of structural and other isomers,l39 and for the searching of given structures within a storage file.140 Much progress has also been made in writing programmes which list all of the possible isomers for a given chemical formula.141 133 (a) W.N. Lipscomb, Pure Appl. Chem., 1972, 29, 493; (6) R. E. Williams, ibid., p. 569; (c) W. N. Lipscomb, Accounts Chem. Res., 1973, 6, 257. n4(a) B. V.Nekrasov, Zhur. obshchei Khim., 1940,10,1021; (b) R.P. Bell and H. J. Emeleus, Quart. Rev., 1948,2,132; (c) J. R. Platt, J. Chem. Phys., 1954,22,1033; (d)R. E Dickerson and W.N. Lipscomb, ibid., 1957, 27, 212; (e) T. E. Haas, Inorg. Chem., 1964, 3, 1053;cf) W. N. Lipscomb, ibid., 1964, 3, 1683; (g) W. N. Lipscomb, Science, 1966, 153, 373; (h) W. N. Lipscomb, U.S. Clearinghouse Fed. Sci. Tech. Inform. A.D. Rept. No. 662761, 1967; (i) H. D. Kaesz, R. Bau, H. A. Beall, and W. N. Lipscomb, J. Amer. Chem. Soc., 1967,89,4218; (j) N. V. Emelyanova and I. V. Krivoshei,Zhur. strukt. Khim., 1968,9,881; (k)S. F. A. Kettle and V.Tomlinson,J. Chem.Soc. (A),1969,2002, 2007, (I) A. T. Balaban, Cull. Int. Cent. Nut. Rech. Sci., 1970, 191, 233; (m) R. E. Williams, Inorg. Chem., 1971, 10, 210; (n) Ref. 132. 135 (a) I. R.Epstein and W. N. Lipscomb, Inorg. Chem., 1971, 10, 1921; (b) I. R. Epstein,D. S. Marynick, and W. N. Lipscomb, J.Amer. Chem. Soc., 1973, 95, 1760. 13*C. C. Davis, K. Cross, and M. Ebel, J. Chem. Educ., 1971, 48, 675. 137 (a)J. Lederberg, N.A.S.A. Report CR-57029,1964; (b)J. Lederberg, ibid., CR-68899,1966. 138 W. E. Bennett, Inorg. Chem ,1969, 8, 1325 138 (a) Ref. 137; (b)J. Lederberg, N.A.S.A. Report CR-68898, 1965; (c) D. J. Gluck, J. Chem. Docum., 1965,5,43, (d)H. L. Morgan, ibid., 1965,5,107; (e)A. E. Petrarca, M. F. Lynch,and J. E. Rush, ibid., 1967, 7, 154; tf) A. E. Petrarca, M. F. Lynch, and M. F. Brown, Amer. Chem. SOC. 156th Meeting, INOR 147, Atlantic City, 1968; (g)A. E. Petrarca and J. E. Rush, J. Chem. Docum., 1969, 9, 32; (h) W. S. Hoffman, ibid., 1968, 8, 3. lro(a) W.E.Cossum, M. L. Krakiwsky, and M. F. Lynch, J. Chem. Docum., 1965, 5, 33; (6) Ref.139d; (c) D. Gould, E. B. Gasser, and J. F.Nan, ibid., 1965,5, 24; (d) Ref. 139h; (e) E. H. Sussenguth, ibid., 1965, 5, 36. 141(a) G. L. Sutherland, U.S.Govr. Res. Dev. Reports, 1968,68,82; (b)Ref. 137a; (c) Ref. 139b; (d)J. Lederberg et al., J. Amer. Chem. Soc., 1969, 91,2973; (e) Y. M. Sheikh et al., Org. Mass Spectrometry, 1970, 4, 493; cf) V. V. Raznikov and V. L. Talroze, Zhur. strukt. Khim., 1970, 11,357; (g) D. Mitchie, Nature, 1973, 241, 507. Rouvray 12 The Last Word Over the past two decades several challenging new problems have been tackled in the field of isomer enumeration, and some new methods have also been devel-oped. For instance, a number of the less frequently studied isomers have been investigated. For hydrocarbon species this includes the work of Balaban142 and others143 on the enumeration of valence isomers, of Pinkus and M~Lachlan1~~ and others145 on isotopic isomers, of R0uvrayl4~ and others147 on topological isomers, and of Andreenkol48 and others149 on rotational isomers.All of these workers employed an essentially combinatorial approach to the enumeration. Other species which have been enumerated include the isomers in polymeric materials and those arising in biochemical systems. Polymers have been classified by Conix,150 Elias,l51 Huggins et aZ.,152 and Lewis.lS3 Enumeration in chain polymers has been carried out by Schultz,154 in vinylic polymers by Schildknecht et aZ., l55 and in phenolic resins by Hollingdale and Megson.156 Stereopolymers were studied by Costescu,l57 Hat~,l~~ Isomerism in its biochemical and Bre~sch.~~ manifestations has been investigated by Bla~kmanl~~ on the porphyrins, Hirsch- mann and HansonlG0 on stereoisomerism, PlouvierlS1 on enantiomorphism, and Urrnantsev1G2 on living systems.The arenes have proved surprisingly difficuIt to enumerate. Graph-theoretical 142 (a) A. T. Balaban, Rev. Roumaine Chim., 1966, 11, 1097; (b) A. T.Balaban, ibid., 1970, 15,463;(c)A. T.Balaban, ibid., 1972, 17, 865,883; (d)A.T.Balaban, ibid., 1973,18, 635. 143 (a) H. P. Schultz, J. Org. Chem., 1965, 30, 1361; (b) E. E.Van Tamelen, Angew. Chern., 1965,77,759;(c) H. G.Viehe, ibid., 1965,77,768;(d)G. Maier, Chem. Unser. Zeit., 1968, 1968,2,35; (e) J. W. F. K. Barnick, Chem. Techn., 1969,24,96,129;(f)L.I. Scott and M. Jones, Chem. Rev., 1972, 72, 181. 144 (a) A. G. Pinkus, J. Chem. Educ., 1957, 34,299; (b) A. G.Pinkus and E. K. McLachlan, Chem. and Ind., 1960, 1186. 145 (a) Ref. 66;(b) Ref. 67;(c) R. C. Read in 'Graph Theory and Applications', ed. Y.Alavi, D. R. Lick, and A. T. White, Springer, Berlin, 1972,p. 243. 146 (a)D. H. Rouvray, Compt. rend., 1973,277,C, 239;(6)D.H. Rouvray, ibid., 1974,278,C, 331. 14' (a) H. L. Frisch and E. Wasserman, Amer. Chem. SOC., Div. Polym. Chem., Preprints, 1960,93;(b) H. L. Frisch and E. Wasserman,J.Amer. Chem. SOC.,1961,83,3789;(c) S.J. Tauber, J. Res. Nat. Bur. Stand., 1963, 67A 591; (d) H.L. Frisch and D. Klempner,Adv. Macromol. Chem., 1970, 2, 149; (e) R.Wolovsky, G.P. 2108965, C07C, 1971; (f)G. Schill, 'Catenanes, Rotaxanes, and Knots', Academic, New York, 1971. 14* (a) V. A. Andreenko, Zhur. $2. Khim., 1971, 45, 1813, 1818; 1819, (b) V. A. Andreenko, ibid., 1972, 46, 1862. 149 (a) E.Funck, 2.Electrochem., 1958, 62,901 ;(b) Ref. 97. lsoA.Conix, Med. Vlaam. Chem. Ver., 1958, 20, 102. 151 H. G.Elias, Chimia, 1968, 22, 101. 152 (a) M. L. Huggins et al., J.Polymer Sci., 1952,8,257;(6) M. L.Huggins et al., Chim. Id., 1964, 46, 536. 153 R. N. Lewis, Amer. Chem. SOC., Div. Polym.Chem., Preprints, 1970, 852. lB4 G.V. Schultz,J. Makromol. Chem., 1943, 1, 35. 165 C. E. Schildknecht, S.T. Gross, and A. 0.Zoss, Ind. Eng. Chem., 1949,41, 1998. 156 S. H.Hollingdale and N. J. L. Megson, J. Appl.Chem., 1955,5,616. 15' D. C. Costescu, Rev. Chim., 1958,4,193. 158 R. Hatz, Gem.-Ztg., 1964,88, 855, 887. '"(a) D. Blackman, J. Chem. Educ., 1973,50,258 ;(b) W.H.'Eberhardt, ibid., p. 728. 180 H. Hirschmann and K. R. Hanson, EuropeanJ. Biochem., 1971, 22, 301. 161 V. Plouvier, Phyrochemisrry, 1966,5, 955. 162 (a)Y.A. Urmantsev, Uspekhi Sovrem Biol., 1966,61,374;(b)Y.A.Urmantsev, Bot. Zhur., 1970, 55, 153. 371 Isomer Enumeration Methods listings of all arenes containing up to eight benzene rings have been given by the chemists Wiswesserls3 and Balaban and Hara~y.l~~ An attempt has been made by the mathematicians Harary and Readl65 to find general formulae for this purpose, but to date success has been achieved only with cata-condensed species which contain no rings of hexagons.The isomers formed in intramolecular reaction processes have been studied in detail over the last decade. In this category mention may be made of the work of Dunitz and PreloglSs and Lauterbur and Ramirezls7 on trigonal-bipyramidal systems, of Gielen et aZ.16* on five- and six-co-ordinate complexes, of Balaban et aZ.169on 1,Zshifts in carbonium ions, of Muetterties170 on polytopal rearrange- ments, of Gillespie et aZ.171 on Berry-type pseudo-rotation reactions, of Klem- ~erer17~on permutational isomerization reactions, and of Mu~her"~ on the phosphoranes. As the literature in this area is too extensive to review fully here, the reader is referred to review articles on the subject.174 The author is very grateful to Professor W.J. Wiswesser for provision of un-published work on isomer enumeration, and to Mrs. E. Fernandes for assistance in the literature search that this work entailed. 163 W.J. Wiswesser, unpublished paper, 1964. (a) A. T. Balaban and F. Harary, Tetrahedron, 1968,24, 2505; (v) A. T. Balaban, ibid., 1969,25,2949;(c)A. T.Balaban, Rev. Roumaine Chim., 1970,15,1251;(d)A. T.Balaban, Tetrahedron, 1971, 27, 6115; (e) Ref. 142d. lo6F. Harary and R. C. Read, Proc. Edin. Math. SOL, 1970, 17, 1. lB6 J. D.Dunitz and V. Prelog, 4ngew. Chem. Znternut. Edn., 1963,7, 725. la' P. C.Lauterbur and F. Ramirez, J. Amer. Chem. SOC.,1968,90, 6722. lB8 (a)Ref. 105;(b) Ref. 11 1 ;(c) M. Gielen, M. De Clercq, and J. Nasielski, J. Orgunometuffic Chem., 1969,18,217;(d)M.Gielen, Med.Vfaum. Chem. Ver., 1969,31, 185; (e) M. Gielen et al., Proc Symp Coord. Chem., 1970,1,495;cf) M.Gielen and N. Vanlautem, Bull. SOC. chim. belges, 1970,79,678. lBDA.T.Balaban, D.Farcasiu, and R. Biiriic8, Rev. Roumanie Chim., 1966, 11, 1205. 170E.L.Muetterties, J. Amer. Chem. SOC.,1969, 91, 1636, 3098, 4115. 171 (a)Ref. 5e; (6)Ref. 33d; (c) P. Gillespie et al., Angew. Chem. Znternut. Edn., 1971, 10,687. 172 (a) W.G. Klemperer, J. Amer. Chem. SOC., 1972,94,6940; 8360, (6) W.G.Klemperer, J. Chem. Phys., 1972, 56, 5478; (c) W.G.Klemperer, Znorg. Chem., 1972, 11, 2668. 173 (a) J. I. Musher, U.S. Nat. Tech. Inform. Serv. AD Rept. No. 757416, 1973; (6) J. I. Musher, J. Amer. Chem. SOC., 1972, 94, 5662. 174 (a) Ref. 32k;(b) S. J. Lippard, Progr. Inurg. Chem., 1967,8, 109;(c) Ref. 124;(d) E. L. Muetterties and R. A. Schunn, Quart. Rev., 1966,20, 245; (e) E. L. Muetterties, Accounts Chem. Res., 1970,3,266;cf) E.L.Muetterties, Rec. Chem. Progr., 1971, 31, 51; (g) J. Brocas, Fortschr. Chem. Forsch., 1972,32,43;(h) H.Beall and C. H. Bushweiler, Chem. Rev., 1973, 73, 465. 372

ISSN:0306-0012    

DOI:10.1039/CS9740300355

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

NYHOLM MEMORIAL LECTURE* Forward from Nyholm’s Marchon Lecture7 By H. Frank Halliwell EMERITUS PROFESSOR, UNIVERSITY OF EAST ANGLIA; In years to come it is certain that, whether they are concerned with chemical education or inorganic chemistry, these Nyholm Memorial lectures will become increasingly focused on what, at that particular time, is a current problem. I thought it appropriate therefore that in this first lecture, which it was decided should be a tribute from chemical education, there should be a greater concern with Ronald Nyholm’s own impact on that activity and his place in it, particularly with the impact he made on it during the decade before his death. Yet at the same time I knew it would be no tribute to him to look back merely nostalgically or reminiscently.That would not be in keeping. I am sure he would have said that any looking back must be a deliberate part of a programme for moving on, and that is what I had in mind in deciding on the content of my lecture and is my excuse for the title’s apparent flippancy. There is no doubt that Ronald Nyholm was at the centre of great change and development both here and abroad, and in trying to get his influence into some kind of perspective I was helped by a comment recently made on the Nuffield Chemistry Project with which he was so intimately linked. The comment was that the project was not the beginning of a new era; it was the end of an old one. This I believe to be true -I also think it false. This paradox (which if sub- stantiated would hardly endear itself to the setters of some of our modern examination questions) arises for me because I seemany of the developments and chemistry teaching projects in those years (including the Nuffield Science Pro- jects) as marking a watershed in science education.These various projects arose out of, and were predominantly based on, a past that had been increasingly active, but then, I think, they themselves, together with powerful, contemporary but extraneous social forces, resulted in the development of such new perspectives and contexts, and opened up such new possiblities that the period following them can be seen as qualitatively different. In this sense these science Teaching Projects, often cast in a mould appropriate to the earlier period, helped to catalyse the transition to a new period: they were both an end and a beginning.That the decade from 1960 to 1970 marked a watershed for us in science * Delivered at a Meeting of the Education Division of The Chemical Society on 10th April 1974 at University College, London. t The 1964 Marchon Lecture was delivered by Professor R. S. Nyholm, D.Sc., F.R.I.C., F.R.S., at the University of Newcastle-upon-Tyne, 9th March 1964. $ Present address: 53 Newmarket Road, Norwich NOR 22~. Nyholm Memorial Lecture education I see as a key perspective in future development :and I also see Ronald Nyholm as one of the outstanding men who by their efforts created the entry into a new era, a new era of which I know he was in fact beginning to get a glimpse.I therefore propose to develop this theme, and in doing so to put some of the new perspectives and contexts with which those going on will have to cope, into juxtaposition with the changes and developments of the past. In this Janus- like exercise let me start with a brief survey of that side of the watershed which was explored and developed by earlier generations and from which Ronald Nyholm and the rest of us started. As a preliminary to this survey I would like to draw your attention to a remark of a social historian about a comparable situation. In his book, George Ewart Evans is commenting on changes in village life during the last hundred years. What I shall have to say is so much in keeping with his comments that I will quote them.He says this period is ‘. . . . a century to which science has given those innumerable skills and techniques that make the control of large sectors of our physical environment a reality; and -perhaps most important of all -has given us a confidence that falls short only of the awareness that now for the first time we are called not merely to suffer our own history but to make it. But make it on what? This question immediately points to a sense in which the study of the old, traditional culture is not simply a praiseworthy academic exercise but an essential preliminary to the building of a new order. For-------the old frame- work-- - ----did house something permanent -------and without an apprecia- tion of these (permanent characteristics) no attempt to make a new community here in Britain or elsewhere is likely to survive the present century’.l So in the development of science curricula should we beware of the danger of regarding as outmoded the values and characteristics and the changes and developments of the past.Indeed we should carefully judge afresh the extent to which they are appropriate in a context that may be radically changed. When we do look back on past changes one pattern at least stands out: the recurrent upsurging of progressive-type exhortations. It is indeed salutary to realize that some of the innovations which give us so much pleasure and which even provoke pride of achievement equally gave pleasure and were equally a source of pride to someone in our grandfather’s and even in our great-grand- father’s day.I strongly recommend to you the review of a century of science teaching which appeared in 1960 as the first chapter of ‘Science in Secondary Schools’,2 issued by the Ministry of Education. It is brief and it is full of most enjoyable and encouraging detail, e.g. the work in science in the village school of King’s Somborne in 1847 under the inspiration of the Rector, the Rev. Richard Dawess but for my purpose here, I am more concerned with the pattern, § An interesting view of the influence of Richard Dawes and other initiators in science education is to be found in D. Layton, ‘Science for the People’, George Allen and Unwin, London, 1973. G. E. Evans, ‘The Pattern under the Plough’, Faber and Faber, London, 1966, Introduction.‘Science in Secondary Schools’, Ministry of Education Pamphlet No. 38, London, H.M.S.O., 1960, Chapter I. Halliwell which I see in its account, of a recurrent upsurging of new intentions over a period of a century and a quarter. Before the middle of the past century science was a part of the educational experience of very few people: the few well-educated and interested adults influenced by the activities of the Royal Institution in London and by those of a few similar centres in educated circIes elsewhere; a few pupils in schools such as Stonyhurst, Mill Hill, University College School, Harrow, Rugby, and others where the physical sciences were being taught before 1850; and a very few quite young school children such as those who, by chance, were influenced by such men as the Rev.Dawes in the village school at King’s Somborne. Now when I say that I see this time as a period of the socially minded scholar, I am in no way pretending to an historical analysis but I am merely, in line with my theme, giving a cry of recognition of a phenomenon which, in a somewhat disguised form perhaps, we have seen held up for our approval in our own time. In a very general way we can see the major theme here as ‘Know and Understand and Enjoy’. Indeed, in order to underline the structure of my theme, I hope I may be allowed to use mild slogans of this sort to pin-point the characteristics of these exhortations. Then in the middle of the century a new social pattern burst into blossom, and the needs of a technological society and the importance, for the good economy of the state, of a scientifically literate work force, became the emphasis.Mechanics Institutes, established a decade or so earlier but which until then had shown rather weak growth on the science side, suddenly had support and encouragement. It was realized that in science education the French and the Germans were ahead of us, and this and the focal point offered by the Exhib- ition of 1851 seemed to have had the effect in the mid-nineteenth century that the Sputnik had in the mid-twentieth. In the same period and for related reasons two new factors appeared: the written examination, and the Revised Code of 1861 with its subsequent payment by results.The written examination did away with patronage and gave opportunity for selection by merit. One phrase used by Rev. J. Booth, one of its early protagonists, I particularly like. He referred to ‘the reservoir of unbefriended talent’. But cries that we ourselves have heard in the last decade or so were soon heard then -nearly a century ago. Booth himself in 1861 was speaking of ‘a general mania for examining everybody by means of written answers to printed questions’, and Todhunter by 1873 was bothered that written examinations were becoming instruments of specialization. Kelvin told the Royal Commission on Scientific Instruction and the Advancement of Science (1872) that examinations exerted a ‘fatally injurious tendency’ on the higher parts of science,and Huxley in 1877 described competitive examinations as ‘the educational abomination of desolation of the present day’.The second factor appearing at the same time as the largescale emergence of written examinations was the Revised Code of 1861 which, by its formula of payment by results, led to primary schools (and hence training establishments) being, from then on for quite a while, concerned almost entirely with proficiency in the 3 Rs. As the Ministry’s own Pamphlet2 comments a century later, ‘a promising beginning Nyholm Memorial Lecture in the elementary schools (in science) was cut short’ (by the Code). The second stage, the aftermath of the Great Exhibition, cannot be looked back upon with any great affection.Its main theme, however, which can be character- ized perhaps as ‘Know and profitably Use’, while so different from the ‘Know and Understand and Enjoy’ of the previous stage, is not fundamentally unaccept- able: it was its implementation at that time that turned out to be so disastrously unimaginative. Not surprisingly there was a revolt against it, a slow-moving revolt which was now fostered in the public and grammar schools. This revolt, by no means wide- spread but certainly influential, is associated with the work of Sanderson and particularly with that of Armstrong in the two decades bridging the turn of the century. In this, the principal drive was directed towards learning through experi- mental exploration followed by argument.The details of this heuristic method of science education are well documented, and an edited edition of some of Armstrong’s essays has appeared recently.3 I do not propose on this occasion to go further into details of this phase. Its later influence, after a period of eclipse, was considerable. What interests me here is that I can see in its characteristics a recurrence of the ideals of science teaching advocated by the Rev. Richard Dawes -now modified and strengthened by the tradition of mental training and character formation of the established schools for an adolescent group. I will use the phrase ‘Know and be Educated’ to indicate this movement. Then came the First World War and a resurgence of the previous technological purpose and intention.The Ministry, in its Pamphlet,2 speaks of the change as ‘a swinging away of the emphasis again from method to the matter of the studies’, (my italics) and I have spoken of the recurrent upsurgence of exhortation. Yet the resurgence of ideals was never just a repeat. Each stage had progressively left a mark on its successors. We see this particularly during the first thirty years of the present century. The work of Armstrong, of Sanderson, of numerous experienced teachers of physics and chemistry in the public schools was beginning to spread into the wider educational field of State Grammar Schools. The Association of Public Schools Science Masters expanded and became The Science Masters Association.Again, the first number of the School Science Review (June 1919) had an article on ‘Research Work in Schools’ and the sixth issue (December 1920) had an article on ‘Science for All: a plea for General Science’ -yet we have seen that this period following the First World War was not the first time that either the educational value of investigation or a doubt about the educational value of specialization had been the centre of discussion. But now there was a difference. Not only were many more pupils than in the previous century now involved, but in addition, education in the last decades of the nineteenth and the first two or three decades of the twentieth century was an instrument of social mobility -two facts that are not unrelated.The theme therefore for many, although not for all, in those years was ‘Know and so get on -and out’. ‘H. E. Armstrong and Science Education’, ed. G. van Praagh, Murray, London, 1973. Halliwell I must repeat that what I am saying is in no way an attempt at historical analysis. It is just a statement of recognition that many of the slogans which in the last decade we have heard, or have even voiced, had during the last hundred and thirty years their earlier counterpart. Nor, I would hasten to stress, is what I have so far said a prologue to a jeremaid. The recurrent upsurging of progressive-type exhortations (there is an almost ‘Old Faithful’ regularity of thirty years interval) is not a repetition of despair, is not a fruitless, sisyphean repetition.Rather has each generation found that it in turn must cope with the needs and problems thrown up by its own idiosyncrasies and historical development. When we look at the last three decades -so coming to the present time -we realize that the great expansion of the group of pupils and students concerned has resulted in such an inhomogeneity of ability and ambition that all the various, and even contradictory, slogans of the past may well now be appropriate to some sections of the group. Further it is now clear that the problems are not unique to us -they are international and the inhomogeneity of educational needs thus becomes even more marked. The development and elaboration of this inter- national and multifaceted aspect of educational needs is indeed one of the characteristics of science education after the Second World War -the period in which Ronald Nyholm played so stimulating and so important a part both at home and abroad -and it is a characteristic of very considerable volume and complexity and one which is still awaiting a critical survey.I certainly have no intention here of offering even a sketch map of its ramifications for it is my pur- pose only to indicate generally the way affairs were leading up to, and were helping to bring about, a change which in retrospect I see as a watershed, and a change in which Nyholm played a vigorous and constructive part. At home and abroad this time has been a period of greatly increased activity and voluminous proposals.Not only was there, at home, a spontaneous upsurging of demands for a new look at science education, natural after another thirty years quiet, but North America and parts of Europe had felt the impact of what in educational terms can be regarded as the mid-twentieth century counterpart of that stimulus which the Great Exhibitions and outpourings of the first flowerings of the Industrial Revolution had given in the mid-nineteenth : countries were now ‘sputniked’ into support of science education. The first and important step was that administrations were sputniked into support but soon the educational activities, now with many more resources available to them, demonstrated their intrinsic worth, and, receiving calmer and more considered support, entered a vigorous phase of production and propagation. From the ideas of earlier years there developed and blossomed the American, the Scottish, the Nuffield, the Australian Schemes, first in chemistry, physics, and biology and then later in science in some integrated form.So too, but later, came schemes for South America, South East Asia, East and West Africa, schemes fed from the initiating schemes but sponsored and supported by such international organizations as U.N.E.S.C.O. and The British Council, with field support from the young people of the American Peace Corps and the British Voluntary Service Overseas. In Nyholm Memorial Lecture fact we witnessed what can be described as the twentieth century secular counterpart of the ninteenth century Christian missionary drive -there was even a faint suggestion of ill-defined salvation about it.Science education was an International Good Thing. But in our home area we saw what at that time many thought to be an in- surmountable obstacle to change. It arose from the establishment a century earlier of the written examination. This obstacle Nyholm showed was removable. Elsewhere4I have indicated how in the Nuffield Foundation’s Chemistry Project Nyholm and the rest of us stressed very early that examination demands should be designed to assess and also encourage those intentions and aims which a particular innovation was striving to achieve. But by the middle of the twentieth century the pattern of examination demands (Imean what a candidate has to do in order to receive approval and achieve success) had, in the U.K., been fairly set for a long time.The fulminations of Huxley, of Todhunter, and of Rolleston (‘men get demoralized by the process’), the opinions of Sir William Ramsay in the nineties that an ability to pass examinations might be a good qualification for a barrister or a government official, but not for a scientist (he needed developed inventive powers) -none of these fulminations in their own days had much effect. Nyholm’s quiet but formidable administrative persistence was successful in his. As Moderator for the G.C.E. ‘0’and ‘A’ level examinations in Chemistry of the University of London Examination Board he and his colleagues in the Nuffield Foundation were able to persuade the various Examination Boards to experiment with new types of papers without handicap to the candidates involved. I personally consider this achievement as one of Nyholm’s most important in the area of educational reform.He reminded the rest of us that examinations were the servants of education and that a contrary approach had been allowed to grow up over many years and to have become established. He also showed in a practical way that examination systems were not unalterable modes of procedure. I also know he was very concerned that we should see we had carefully considered criteria thought out before we exer- cised the power to change. In this area of chemical education Ronald Nyholm was more of an outstanding and isolated figure than in others.Not of course that he ever was a lone figure. One’s memory of him is that of a centre of activity in a group, fanning this section into flame or cooling that section into calmer thinking. But in his effort through administrative means to get those responsible for examination questions to think critically about the purpose and efficacy of their demands, I do see him as the outstanding contributor. In other areas of chemical education Nyholm seemed to find it more effective if there were a number working at a problem, sometimes as a team, sometimes independently. The result is that he achieved a profitable atmosphere of pro- gressive thought which had the strength of individual contribution and also the valuable bulk property of concerted action.The steps he and others took H. F. Halliwell, ‘Chemical Education, Problems of Innovation’, R.I.C. Reviews, 1968, 1, 205 378 Halliwell to achieve some of the goals stated in the Marchon Lecture5 illustrate this. He was very committed to seeing that the science education which a pupil experienced really did help him to cope with the problems of modern life. So of course were many others similarly committed, but through his enthusiasm and vigour he had the knack of getting groups to be vocal and eventually to accept as normal, a discussion and a concern that previously would have been rejected as out-of- place: he was a major factor in getting the academic community interested in chemical education.Details of the ways in which he helped the development of secondary and tertiary education are to be given and discussed by subsequent speakers. My point here is that Nyholm was one of the most vigorous influences in this last period of recurrent upsurgence to which I have been drawing your attention, and it is my belief that at this stage we reached the watershed in science education. Certainly two factors were aiding and abetting this recurrence to break through into a new era: the one was the increased volume of the clamour -and this came from those involved in teaching at secondary and tertiary level; the other was the change in the socio-educational framework -a factor from outside the teaching community. Nyholm had contributed in positive, encouraging but disciplining ways to the first factor: he was also aware of the onset of the second and was beginning to think of the quite new types of problems which would arise.What justification do I see for thinking of developments in science education as entering at this stage into a new era? The line of demarcation, supposing my proposition to be justified, is unlikely to be sharp, but, accepting a gradual change, often with different areas of thought out of step, what differences should be detectable which would be qualitatively sufficiently marked to justify the statement? I have spent some time viewing very briefly some of the changes which science education has been through in the past century and a half. The point I now want to make is a two-fold one: partly it is that I see those earlier changes as ones which, over a century or more, took place within a well-established framework of educational beliefs that was widely accepted; andpartly it is that in the future the acceptance will be much more parochial, and the older framework, already being antagonistically questioned in some quarters, may well be replaced for some by a contrary one.We are close to the occasion, and only a provisional delineation is possible. It seems to me that the widely accepted framework to which I have just referred was one of transmission and communication from a knowledgeable and wise older group to an innocent and needy younger group who were clamorous for what the older had.The beliefs that were associated with that framework include, among others, belief in salvation through enlight- enment by reason, belief in what is a Good Thing for the Chosen will be a good thing for the rest, belief that it is shameful for the older to acknowledge ignorance and shameful for the younger not to (in spite of ignorance and fallability being a common characteristic of the human race), and a belief that the acquisition of R. S. Nyholm, Marchon Lecture, ‘Education in Science -for whom and for what purpose?’, delivered at the University of Newcastle-upon-Tyne on 9th March 1964. Nyholm Memorial Lecture past knowledge automatically develops both judgement and informed adapta- bility. I think it true to say that all the changes we have looked at so far have been urged by people who in spite of their mutual opposition would have subscribed to these values.Some changes in the future are likely to be made outside them. I am not saying that the older framework and beliefs will be utterly rejected. I am saying that they are being questioned. The impact of this questioning is difficult to foretell, but where the old values and framework continue, their continuation, because they are no longer the only possibility, will be based on a different criterion of acceptability, and the parochial nature of their acceptance (and it may be within a big parish) will mean that new techniques and intentions will develop as well as perspectives not previously imagined. Nyholm was aware of the onset of the change of framework of beliefs and values, for he discussed with me the significance of the findings of a carefully and professionally conducted survey of opinions of undergraduates and staff of the purpose and worthwhileness of the undergraduate work in the chemistry department of one of the U.K.universities. There was evidence that the group was a capable one and, judging by details of its admittance, above average and able to stand comparison with many decades of undergraduates which the department had had. Nevertheless the academic staff felt that there were too many undergraduates who did not understand the value and importance of commitment to a problem of scholarship in a chosen area or of intellectual persistence and other traits and behaviours held dearly by the academic world.What was a shock, and I believe a valuable shock, was the evidence, not that these undergraduates did not understand, but that they had weighed these ideals in their contemporary balance and found them wanting: some of them, in fact, seemed to regard such attitudes as evidence of uneducated narrow-minded- ness. Disturbing, if not painful, as we knew this must have been to those con- cerned, and limited in its coverage as it was, it fitted into what we knew of the contemporary pattern of student dissatisfaction and unease which was beginning to be worldwide. But the dissatisfaction of university students with academic fare has been common since this type of education arose many centuries ago.What was now appearing seemed somehow to be different. The impact of the variety of needs which arose from the explosive increase in the numbers involved in education all over the world -this impact, aided by the technological ease with which ideas spread round the world, brought about for many of the teaching world the need for reappraisal and a change of their own assumptions. It is unlikely, because of choice and selection, that this need for reappraisal will be as strongly felt in university science circles as in other university circles, or as it already is felt, and will be felt, in pre-university circles. Nyholm was certainly aware of the onset of this rejection of old values by some, and was aware that it would make new demands in his own university area -and also that it would make new demands in general on those involved in educational alteration and (hopefully) reform.It is these types of changes that make me speak of the water- Halliwell shed: the questioning of basic assumptions has, I believe, finally left an indelible mark. It is the exploring, mapping, and inhabiting of what I see is for us older folk the other side of the watershed that is the major task for the oncoming generation. Trying to distinguish between profitable reconnaissance on the one hand, and foolish crystal-gazing and Polonius-like admonition on the other, I propose to look briefly at two of the broad areas of growth and change in science education which I have seen emerging since we were involved with Nyholm in the production of the Nuffield Project. I must, however, recall a comment which I made with the help of a quotation at the beginning of my lecture, and that is to stress that although new and unforeseen ideas will undoubtedly arise, there is still much of value in some of the older ones-providing, as we now see,that they are no longer regarded by their advocates as of universal application.For example, the separate sciences as we have known them will undoubtedly continue in some form to be of interest and value to a minority. However, their value for the majority will surely need to be re-assessed. Science education for the latter group may well take on a quite unpredicted form, and in exploring this un- mapped area it would be well not to ignore a guide line offered by Huxley: ‘What men need is as much knowledge as they can assimilate and organize into a train for action; give them more and it may become injurious’.Let me then turn finally to these two areas where I think further exploration and development will be needed. If for this purpose I may refer to the four areas of decision making (Aims, Action, Assessment, and Adjustment) which I think must be the basis of any eff~rt,~ then the two on which I want to comment are in the areas, not of action or assessment to which our attentions rather naturally first turn, but in those of intentions and of reappraisal. Let me con- sider the latter first, because I think it raises problems which are fundamentally less complex.The Machinery and Techniques for Reappraisal and Reform.-That some form of machinery which will enable adjustment to be made should be set up, we have long recommended. The form it will take I do not know, but the form it should not take I would have thought obvious: it must not be an imposition from out- side. I shall want to draw your attention on two later occasions to the work of Edward de Bono on Lateral Thinking,G but at the moment I want to disagree with his statement (unless I have quite misunderstood his use of the word ‘conflict’) that ‘we have never developed any tool for changing ideas except conflict’. I think that the serious offering of options and the setting up of ad- visory working parties are two such tools.I would have thought the Asso- ciation for Science Education (A.S.E.) itself was a prime example. Indeed, in that particular body (in which Nyholm was so interested and involved, and of which he was President for two years) we surely have the foundation on which to set up this machinery for adjustment. If the Association were to develop, to working- party and advisory level, that side of science education concerned with further (I E. de Bono, ‘The use of Lateral Thinking’, Jonathan Cape, London, 1967. Nyholm Memorial Lecture education, technical college education, and university education (and that side is already represented in the Association), then the internal machinery for re- appraisal of the whole spectrum of science education could easily be available in an experienced and acceptable form.When it comes to techniques of reappraisal, then I think that one of de Bono’s comments6 is of the greatest importance. He says ‘being right at each stage is not enough in a sequential change’, and I think this important because curriculum development is a sequential change. The importance that Nyholm and the Nuffield Chemistry team placed on the fourth area of decision making (Adjust- ment) meant that they were aware, though perhaps not so sharply as is de Bono, of the need to be ready to rethink completezy anew,not just because feed-back shows one to be wrong in techniques, but because there may be evidence that from the beginning one had intentions which later (and only later) were seen to be inappropriate.As an example of the need for reappraisal let me offer two items requiring re-exploration and re-development. The first is the question whether a system of assessment which has the community’s confidence must be based on the import- ance of failure or whether it should or could be replaced by something based on achievement. At present, at each step of the ladder, 40% or so of the entry must be rejected: it is not a ladder -it is a sieve. Could it be that here is an undesirable hang-over from more than a century ago? Could it be that it is not the business of an Examination Body to ‘pass’ or ‘fail’ but to report on achievement and (hopefully) potential? We all know that a ‘pass’ in subject X at ‘A’-level is a ‘pass’ but that a ‘pass’ in subject Y is often a ‘fail’ when it comes to University Entrance, In suggesting this problem for immediate attention I must point out that the production of a differently orientated examination system will not be difficult-it is the making it saleable and selling it that will cause bother.A second problem needing attention is that caused by the great inhomo- geneity of the groups now regarded normal as teaching units. Our university colleagues may think they have met this problem, but I assure them that they don’t realize how easy life is for them, in this respect, compared with that in some school classes. The sample scheme of class-room action in the Nuffield proposals was based on the assumption that one would be teaching fairly homo- geneous groups of fairly willing pupils: this was the normal pattern of classes till then.It will no doubt continue to be the pattern in some schools, but the pattern in many has drastically changed. Not only is there often a wide range of ability in many classes, there is at the same time a wide range of willingness-and the two ranges may show little correlation. The techniques of teaching and learning under these conditions are naturally only just beginning to be tackled : teachers -whatever their programme -need immediate help here, I turn now to the second of these two areas where I think further exploration and development will be needed. Resolving the Uncertainty of Purpose.-The natural questioning of direction which must accompany any new exploration was an initiating factor for this Halliwell uncertainty.So were the cries for greater pertinence from pupils and students (although pertinence for what was often not clear), but the uncertainty was undoubtedly enhanced by the neo-missionary activity to which I have referred. It is amazing how helpful to the diagnosing of one’s own troubles can be the diagnosing of other peoples’ -if undertaken with a proper sense of humility and awareness of ignorance. Those who have had experience of the growth of science education in the so-called developing countries have been lucky in having the opportunity to see the problem of purpose in their own country because they have seen educational activity in a sociological setting that is foreign to them.I believe that the NufEeld Chemistry Project’s reformulation of the A.S.E. Policy Statement, namely that our purpose was to help children to know when and how to be scientific about a problem, I believe that intention still to be a helpful and worthy one. The teaching of how to be scientific presents less of a problem than does the teaching of when to be scientific, which needs close co-operation between the teaching of science and the teaching of non-science. I can also see in our emphasis of the importance of a critical imagination and of the disciplined hunch, a near cry to Ramsay’s for ‘inventive powers’. Neverthe- less we still need to evolve techniques for helping this imaginative thinking to develop in the laboratory and classroom -although I think that de Bono with his ‘Lateral Thinking’ is on the way to breaking through. But if (and neither Nyholm nor we would agree) it is taken that being scientific is the same thing as always questioning, then such an approach could be regarded as pernicious, and the cup of hemlock appropriately produced. But such a misinterpretation does occur; I would draw your attention to a report7 of a comment by Douglas Whiting, Director of the V.S.O.,speaking of products of our educational system that he has met in Asia and Africa.He says that many young people who come to him query everything -institutions, faiths, laws, and social structure. He goes on: “For this the schools must take credit because you have inculcated the critical approach, the ability to discuss and to express.Dr. Banda is constantly on edge about the tendency of volunteers to suggest to their pupils that they, too, should think for themselves, and he is not alone in this. Neither a one-party state nor Presidential rule thrives on ‘NufEeld-type’ education -which is a strong recommendation for it. But one sometimes feels that the question is eroding the powers of decision making”.fl Although the above is based on what I see as a misinterpretation of what Nyholm and the rest of us were trying to do, yet Mr. Whiting raises what I think is a very important point. What, in each subsection of man’s communal living, is a wise balance between enquiry and decision-making (and how these opposing reactions are to be achieved) I believe to be a question which the next generation must answer.It is obviously linked with the question whether different 7 The inverted commas round ‘Nuffield-type’ are my addition for reasons given above. H.F.H. D. Whiting, reported in Review, December 1971, Headmasters’ Association, p, 168, 1969. Nyholm Memorial Lecture children should have different kinds of science education or whether all must have the same, regardless of its pertinence to their more balanced living. If we go from decision-making on to exercising social responsibility we are entering an even newer field. There is a movement, and the Schools Council is concerned, which has devised a course of science designed to that end, And much opposition it is creating, for it makes quite new demands on teacher and pupil alike.The extent to which such a course will have to be redesigned, I cannot tell, but I am sure that for many if not for most, a course along some such lines will be the pattern in time to come. It is obvious that in the education of the non-specialist, i.e. in the education of the great majority of young people, there is a movement to expand the confines of science learning so as to join up with, and fuse with, the areas of non-science learning, with the areas of subjective and irrational value judgement, with the areas of ‘ought’ and ‘ought not’, and of purpose in life. We hear much of integrated science (for the third or fourth time?) but this is a development in but part of the area.If, however, we begin to develop science integrated with non-science then I believe we shall be walking into an area of light. When I was at a conference on science education in Canberra, I heard a comment which impressed me considerably. The professor of physics at Brisbane, speaking of our pride of achievement in science, remarked that ‘man prides himself on his logicality -in that [area], he is being surpassed by the computer. We should focus more on that which makes man unique -his illogicality’. Dr. Ronayne of Manchester in a recent article has gone further :he says,8 ‘While there is neglect of the social aspects of science in the education of the scientist the idea of responsibility will remain an unattainable goal’.In a way the problem of variability of opinions of worthwhileness is but the other side of the problem we have just discussed. The former was regarded from the point of view of the seller, this from that of the buyer. Obviously these two points of view should match, but the latter group varies enormously in maturity, in ambition, in social and cultural background. Whether a young person con- siders his or her educational experience as worthwhile or not arises, I suspect, from sharply different sources according to whether the young person is one of the group going on, more or less voluntarily, to further education, or whether he or she is one of a still younger group trying to escape from a compulsory education system.Any problems in the former area are likely to be the more easily solved through a variety of optional courses: the problems in the latter area are going to be more difficult to overcome. I can point to them: I do not know their solution. So for those going on from here there are new problems and new perspectives. I have indicated my reasons for thinking that the terrain on their side of the watershed will often be fundamentally different from what it was on our side. Should this turn out to be so, then many of the patterns of subjects, of examina-8 J. Ronayne, Times Higher Education Supplement, No.125, p. 12, Mc.rch 8 1974. Halliwell tions, and of administration on which we were brought up, will have to go. They will have served their day and those going on will have to devise and probe, to meet success and failure in areas we never thought of. A remark attributed to John F. Kennedy sums up the dual needs: Some men see things as they are and say why? I dream things that never were and say why not? Ron Nyholm had both these capacities.

ISSN:0306-0012    

DOI:10.1039/CS9740300373

出版商:RSC    

年代:1974

数据来源: RSC