摘要:
J. CHEM. SOC. PERKIN TRANS. JI 1982 The lsomerisationof Some ortho-Substituted Nitrobenzenes : a Structure Correlation Analysis By Michael G. Hutchings,' I.C.I. Pic., Organics Division, Research Department, Blackley, Manchester M9 3DA A search of the Cambridge Crystal Structure file has revealed a number of benzenoid molecules containing an NO2group ortho to an XYZ unit. X, Y, and Z are second row atoms, the X atom is bonded to the benzene ring, and the CXYZ unit is essentially planar. A scatter plot of the dihedral angles between the benzene ring and the NO2and XYZ groups, respectively, leads via the principle of structure correlation analysis to the conclusion that these mole- cules isomerise by disrotatory torsions of the two substituents, such that the sum of the dihedral angles is always 90".A degree of resonance stabilisation is thus retained throughout the isomerisation, while non-bonded inter- actions are minimised. Alternative mechanisms do not satisfy one or other of these conditions. A CONTINUING area of interest in organic stereochemistry concerns the study of the isomerisation pathways and energetics of molecules comprising a symmetric skeleton substituted by two or more sterically interacting aryl groups. Di-, tri-, and tetra-arylmethanes and cognates, and arylated ethanes, benzenes, cyclopentadienones, and porphyrins all fall within this overall c1assification.l Commonly, variable temperature lH and 13C n.m.r. techniques have given data which through line-shape analyses have yielded values for isomerisation barriers.Furthermore, it has frequently been possible to deduce an idealised transition state for the isomerisation in question, sometimes with the aid of a permutational analysis of all feasible isomerisations, and thus to infer a mechanism. However, the exact timing of consecutive steps in the isomerisation can never be derived from such experi- ments. As an alternative, theoretical calculations, usually of the empirical force field (EFF) type, have been applied to appropriate systems and have yielded a more detailed picture of the dynamics of an isomerising molecule. However, there exist molecules where the rotors are formally analogous to the two-fold aryl groups despite being chemically quite different,? but where both the n.m.r.and the EFF methods are difficult to apply, if not totally unsuitable. One such example is the ortho- substituted nitrobenzene system of generalised structure (1) where the two rotors are NO, and XYZ groups, pivoted about a C-N and a C-X bond, respectively. aNo2XY z (1 1 Nitroaromatic systems simpler than (1) have been studied by means of EFF calculation^,^ despite compli- cations due to differing degrees of resonance stabilisation as the substituents rotate. N.m.r. analysis of (1)would * Present address.-Institut fur Organische Chemie, Technische Universitat Miinchen, Lichtenbergstrasse 4, 8048 Garching,BRD. t Recently discussed examples are hexaethylbenzene and its M(CO), complexes, where the ethyl groups can be considered analogous to unsymmetrically substituted phenyl groups,positioned on D6h and Cs, skeletons, respectively.2 depend on the presence of suitable magnetic probes in Y and Z, and/or 170n.m.r.spectra for the NO, group. In this paper, the dynamic stereochemistry of (1) is discussed from the point of view of a third technique introduced by Burgi and Dunitz, namely, that of struc- ture correlation analy~is.~ This has been used to deduce mechanistic details of reacting systems, as for instance the nucleophilic addition to carbonyl groups, besides conformational interconventions, as in this paper? For such an approach to be applicable, it is necessary to have available solid state structural data derived by X-ray diffraction on a series of different but structurally related molecules.Trends in structural variations, if observable, can be equated to the motions which a typical molecule from the series would undergo during a reaction, based on the hypothesis that geometric de- formations in the crystal-bound molecule will correspond to the path of least resistance, i.e. just such a pathway that a free molecule would follow during a particular interconversion in the absence of crystal packing forces. Definition of the System-Resonance stabilisation in (1)is maximal when the NO, and XYZ substituents are coplanar with the benzene ring. In this conformation, however, there is an intolerably close approach of one of the NO, oxygen atoms to either Y or 2, with the result that one or both groups must rotate out of plane to minimise the unfavourable nonbonded contact.Molecu-lar models, as well as the experimental data to be dis- cussed below, indicate that both substituents undergo torsions about the aryl-X and aryl-N bonds in the same sense, to give a ground state conformation roughly represented by (2a). For convenience, the idealised edge-on projection, (2b), will be used throughout the remainder of this paper. The 'bonds' shown between the atoms of the NO, and XYZ groups, respectively, in (2) and subsequent projections are not intended to represent bond multiplicities, but rather atom connec- tivities only. AZ1 (2a) o 1242 Projection (2b) also shows the dihedral angles, a and P, which define the degree of torsion of the two groups.These are usually measured as the angles between the planes describing the aromatic ring, and the CXYZ unit (a),and the CNO, unit (p), respectively. Other relaxation modes which are less obvious are also to be expected, including changes in the valence angles at the substituted aryl carbon atoms [0 and # in (Za)]. In fact, the two ortho-substituents would be expected to move apart such that 0 > 0' and + > #' in (2a). Thus, the quantity 6 = (0 + #) -240 can be taken as another, admittedly rough, measure of the repulsive interaction between the NO, and XYZ groups. However, even after relaxation the major nonbonded interactions continue to be between the three NO, and the three XYZ atoms, and it is these that will thus usually determine the conformation in the free state.Additional groups attached to Y and Z are assumed to play a relatively minor role in determining the conform- ation. In the solid state a molecule is exposed to ad- ditional weaker intermolecular forces which perturb the ground state geometry to a lesser or greater extent. For the purposes of the current investigation, the following restrictions have been imposed on the nature of X, Y, and 2. Each is a second row element, and thus approximately the same size throughout the series. Moreover, X and its attached atoms should be essentially J. CHEM. soc. PERKIN TRANS. 11 1982 coplanar ; most o-aminonitrobenzenes are therefore excluded.No species is included where intramolecular hydrogen-bonding is believed to determine the con-formation. By means of these restrictions, it is ensured that the XYZ group has steric requirements which are reasonably constant, at least in the all-important region where steric clashing is to be expected. Thus the mole- cules of the series are consistent with the philosophy of structure correlation analysis ; there is sufficient variation in composition to ensure different crystal packing and resultant different conformations in the crystal, but sufficient similarity in steric requirements to be regarded as equally close representatives of the idealised model molecule. Y /. /OX -N Z 0/ (3) The picture becomes more complicated if (1) is further substituted ortho to either XYZ or NO,. An edge pro- jection is shown as (3),where B is a buttressing substitu- ent and could itself be a two-fold rotor (e.g.another NO, group as in picryl derivatives). A buttressing substitu- ent of this type might be expected to play an unwanted Dihedral and valence angles of nitrobenzenes (4)-(28) Dihedral angles a Valence angles o~ b Molecule f a B \ r a+B 0 Unbuttressed species 85.6 79.6 77.4 64.7(A)68 67.1 66.2 64.0 64 62.6 46.9 41.6 40.2 39.4 24.1 22.2 21.6 7.0 4.6 9.8 8.3 26.3(B)28 34.6 36.9 47.3 26 40.7 46.6 46.6 49.1 34.7 64.3 66.6 65.3 82.5 90.1 1 24.7 (5) 89.3 118.8 86.7 126.0( 2) 91.o 122.7(6)86 124.2(4)91.7 124.6(6)92.1 123.1 (4) 101.3 125.4( 3.8) 79 123(3)93.3 123.6 (8) 92.4 126.2 (3) 88.1 121.0 d 89.3 124.2(2)74.1 123.4 78.4 124.6 (9) 87.8 122.6 (7) 86.9 122.3 (6) 89.6 1 24.9( 7) Buttressed species (22)(23)(24) (26) (26) (27)(28) 61 61 66 66 42.6 42.6 61 63 64 36 44 26 49 63 41 32 19(482 83 121 131 124 116 121* O(2)97 123.6( 2) 109 119.0(3)90 124.4(3)91.6 124.9 96.6 122.6 102 116(2)96 120 See text for definitions.8 E.s.d.s,where known, are recorded in parentheses. and adjacent CC bonds, and NO bonds and adjacent CC bonds, respectively. published values lead to 6 -3.9*.16 L \4 6 Ref. 119.8(6) 4.5 5 124.8 3.6 6 1 19.9( 2) 6.9 7 120.6(6) 3.3 8 120.7 (4) 4.9 9 120.1(6) 4.7 10 1 20.4( 4) 3.6 11 120.4( 3.5) 6.9 12 121(2) 4 6 120.3 (8) 3.8 13 119.8( 3) 6.0 14 120.3 0.7 16 1 19.4( 2) 3.6 16 123.2 6.6 17 119.2 (9) 3.8 18 12 1.1(7) 3.7 19 122.7 (6) 6.0 20 124.0(6) 8.9 21 122 3 22 118 2 23 118.7(2) -0.3 24 121.O( 2) 4.6 120.2(3) -0.8 26 12 1.6(3) 6.0 120.1 6.0 26 120.0 2.6 114(2) -10 27 126 6 28 Average of dihedral angles between XY-XZ Taken directly from the CSSR print-out.The J. CHEM. SOC. PERKIN TRANS.n 1982 role in determining the conformation of the molecule. However, we have felt justified in including such species in the current discussion for the following reason. Ir. (3),B will perturb the conformation of XYZ, but NO, is still free to adopt an orientation in sympathy with that adopted by XYZ. Thus, B can be viewed as a special case of an external perturbing force on the XYZ-NO, system in just the same way that intermolecular forces due to adjacent molecules are assumed to perturb the conformation in the crystal.Clearly those molecules where neither NO, nor XYZ is free to relax (doubly buttressed) cannot justifiably be included in the series. X b""' (9) X=NH2 (14) X =MeCO (16) (17) F$H Cl NO2 RESULTS A search of the Cambridge Crystal Data Centre data base via the Crystal Structure Search Retrieval (CSSR)system revealed 18 unbuttressed ortho-substituted nitrobenzenes (4)-(21) (Scheme) which fall within the classification described above and a further seven buttressed species (22)- (28).K-as Relevant geometric details abstracted from the X-ray structure analyses are ;&corded in the Table.The 1,2-disubstituted species are considered first, and since any isomerisation must take place through group torsions, we concentrate on the relationship between the dihedral angles a and p. The over-riding conclusion of this (13) (19) X = Br (21) Cl (20) x = Cl cL*NPrn2 (22) (23) (24)X=N(Me)COMe (25) X = N (Me) NO2 (27) X 0 C(N02)z'Rb' (Ci 1 (28) X =: C (NO,),' K ' (C,) SCHEMEortho-Substituted nitroaromatic compounds of type (1) for which structural data are available study immediately emerges from the Table, namely, that the sum of u and p is approximately constant at 90". To emphasise this relationship, a scatter plot of a against a is shown in Figure 1. The straight line derived by least squares correlation of the 18 pairs of values obeys equation (1) a + 1.024p = 89.1 (n 18; Y 0.96; s 5.9) (1) From the Table, the valence angle distortions, as reflected by 6, are reasonably constant between 3.3 and 8.9", with the exception of the value for (15) which is anomalously low at 0.7'.J. CHEM. SOC. PERKIN TRANS. I1 1982 Inspection of the Table reveals that three NO,-XYZ pairs have 6 < 0. That is, the NO, and XYZ groups are forced by the third, buttressing, substituent toward one another so that they would therefore be expected to have to undergo relatively greater overall. torsion to maintain non- bonded contacts within the same acceptable limits. Grati-fyingly, the a + p data for these three molecules are in fact 72-0084*00h160.00 n\ 048.00 m Q 36.00 24.00 12,oo1 \ 0.00' I 12.00 1 I 36.00 1 60.00 I \ 84.00 O(') FIGURE1 Plot of dihedral angle of XYZ (a) versus dihedral angle of NO, (p).0, Unbuttressed species; 0,buttressed species. The A/B pair o-dinitro-system of (22) is represented by two points In contrast to molecules (4)-(21), the sums of the di- hedral angles, a + p, of (29) and (30) are only ca. 60 and 30", respecti~ely.~~~so In these molecules, the substantially smaller hydrogen atom permits the NO, group to remain more nearly coplanar with the benzene ring. Clearly, here, the different overall steric picture precludes the inclusion of (29) and (30)in the series (4)-(21). For a different reason the sum of a and p in (31) is only 64°.31 In a system super- ficially representative of ( 1), the larger external valence angles of the five-membered ring cause 6 to be 20.4".The non-bonded interactions are correspondingly less because of the greater inherent separation of the groups, and thus the latter need to undergo less torsion despite the NO, group being buttressed by an oxygen atom. Turning attention now to the buttressed series of mole-cules (22)-(28), the Table reveals that the sum of the di- hedral angles, a + p, is on average higher than in the un- buttressed series. In an attempt to bring the conform- ational analysis of the buttressed molecules on to the same basis as that of the unbuttressed, a criterion for accepta- bility of comparison was sought.The quantity 6 has been selected for this purpose with the condition that it should be near the same range as for the unbuttressed series. This ensures that whatever additional distorting effects are present due to the third substituent, they leave the steric environment of the two groups of interest, NO, and XYZ, relatively constant and comparable with the unbuttressed series. If '0-higher than the expected value of 90". A further anomaly is (23) where the sum of a and p is 131". The conformation of the NO, group is determined by the buttressing o-chloro- group (p 82'). A torsional angle of ca. 8" would therefore be anticipated for u, in contrast to the experimentally observed value of 49". However, in a near-planar conformation, the hydrogen atom in the pyrrole 1-position would be particu- larly unfavourably placed with respect to the 0-NO, group.Here, then, is the first and only breakdown of the assump- tion that ligands attached to Y and 2 are of little conse- quence in determining the stereochemistry in these systems. The other seven data are considered acceptable within the limitations laid down, and are included in the correlation shown graphically in Figure 1. DISCUSSION A relationship has been derived between the angles of torsion of a nitro-group and an ortho-substituent in the benzenoid molecules of type (1). How does this relate to the main question of this paper, viz., the isomerisation pathway of such species? According to the principle of structure correlation analysis, the structures of mole-cules (4)-(28) can be taken as still-frame pictures of a molecule (1) undergoing isomerisation in the free state (i.e.not constrained in a crystal lattice). The correlation line of Figure 1 is thus the trajectory of an isomerising molecule in the two-dimensional space defined by the two dihedral angles a and p. Therefore, it can be con- cluded that (1) isomerises by disrotation of the two rotors, in such a manner that the sum of their dihedral angles is 90". The overall isomerisation pathway is depicted in Figure 2. Torsion of one of the two groups is not inde- pendent of the other; their rotations are said to be correlated.32 To develop the discussion further we begin from the top central conformation in Figure 2, on which one possible direction of group torsions has been arbitrarily J.CHEM. SOC. PERKIN TRANS. II 1982 indicated. All the conformations above the line, m, which represents a mirror plane, are chiral. The tran- sition point between one conformation and its enantiomer is idealistically represented by the extreme right con- formation straddling m (achiral; C, symmetry). Fur-ther torsion of the two groups leads into the enantiomeric manifold of conformations below m, and thence into the diastereoisomeric manif old, which again eventually enantiomerises via an idealised C, conformation with the nitro-group perpendicular to the aryl plane. The com- plete isomerisation sequence could occur, of course, with equal likelihood by the reverse pathway.Y P / 0 2'0 0 Y \ ,XLN i'X-N 'z \ I00 00 I IY-X-Z-N y-J Z-X-Y-N 00 II 00/ \ y-x--' N '\X-N I 'Y \ FIGURE Pathway of isomerisation of (1). The arrows on the 2 top central conformation indicate one of the two possible disrotatory motions of the rotors; the clockwise arrow at the centre of the Figure indicates the corresponding sequence of isomerisation steps It is important to note that although some 1,2,3-trisubstituted systems have been included in the struc- ture correlation, the derived conclusions regarding the dynamic stereochemistry of systems of type (1) in the free state are not applicable to these buttressed species. In fact, derivatives (24)-(28) can be considered repre- sentatives of C% skeletons substituted with three interacting rotors.An independent structure correlation for such systems is unfortunately inhibited by a lack of structural data. The molecules under discussion are representative of an idealised system which has the same point group symmetry (C,) and dynamic symmetry group (S2[S2]+ S,) as the broad class of two-bladed propellers, typified by such chemically disparate systems as diarylmethanes and spirocyclic phosphoranes. The permutational analyses of their dynamic stereochemistry are identical,= and they are thus said to be stereochemically s or respondent.^^ Besides the lowest energy pathway deduced from the 1245 solid state data, other feasible isomerisation pathways are intuitively possible.These comprise conrotation via an idealised planar structure (an enantiomerisation), and conrotation via an idealised conformation with both substituents perpendicular to the benzene plane (a diastereoisomerisation) (Figure 3). The former is dis-favoured for obvious steric reasons, while the latter presumably entails unnecessary loss of resonance stabil- isation at the transition state. In fact, the experimen- tally deduced mechanism appears to minimise non-bon- ded interactions while attempting to maximise resonance stabilisation. Y 0I I 2-X yo N-0 I I z 01 1 FIGURE Isomerisation of (1) by mechanisms involving con- 3 rotation. The Figure shows idealised transition states as well as the overall consequences of the isomerisation While useful for indicating geometric trends, the whole principle of structure correlation says less about the energetics associated with the process under consider- ation. Thus it is still not known which of the structures drawn in Figure 2 most accurately represents the tran- sition state of the isomerisation.Judging from the wide spread of a/p combinations observed in the various molecules studied, the barrier to isomerisation is fairly low in (1). Furthermore, from the relative paucity of data corresponding to the conformations with a 0, p 90" and to a lesser extent a90, p 0",it might be surmised that these are energetically disfavoured and therefore corres- pond most closely to the transition state structures.Consistent with this hypothesis are results of extended Hiickel calculations 35 on an analogue of (19) and (20) which can be interpreted in terms of the disrotatory mechanism deduced in this work. The potential barrier to enantiomerisation via correlated rotation was calcu- lated to be 530 kJ mol-l, with transition state near the conformation a 0, p QO",while that for diastereoisomer- isation is 55 kJ mol-l, near 01 90, p 0". Additional reported barriers to rotation of a nitro-group in nitroaryl systems range from 13 to as high as 59 kJ mo1-1.36-43 Steric inhibition to torsion would result in yet higher barriers for (l),whereas lower barriers would be expec- ted if the ground state conformation were sterically destabilised. On the basis of the highest value so far reported,43 dynamic n.m.r. delineation of stereochemical exchange processes in (1) is feasible.1246 Comments on the manuscript by Dr. J. Gasteiger and Professors K. Mislow and J. D. Dunitz, and assistance with the correlation by Dr. D. B. Baird, are gratefully acknow- ledged. [2/045 Received, 1lth January, 19821 REFERENCES 1 R. Willem, H.Pepermans, C. Hoogzend, K. Hallenga, and M. Gielen, J. Am. Chem. Soc., 1981, 108, 2297 and references cited therein. D. J. Iverson, G. Hunter, J. F. Blount, J. R. Damewood, jun., and K. Mislow, J. Am. Chem. Sac., 1981, 103, 6073. 3 V. G. Dashevskii, Y.T. Struchkov, and Z. A. Akopyan, Zh. Strukt. 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ISSN:1472-779X
DOI:10.1039/P29820001241
出版商:RSC
年代:1982
数据来源: RSC