8 Organometallic Chemistry By G. R. STEPHENSON School of Chemical Sciences University of East Anglia Norwich NR4 7TJ UK 1 Organometallic Complexes in New Materials In recent years there has been a boom in organic chemistry related to materials science. At the organometallic wing of the subject there has been a similar awakening to the possibilities of interdisciplinary research in which the special effects available from the use of a transition metal can offer advantages. Work in this area has been progressing over several years and looking back over 1994 at the contributions from the many research groups now active in the field the rate of conceptual advance is striking. This year's Annual Report will begin with a survey of developments in this topic. Nonlinear Optical Electrical and Magnetic Properties.-The use of transition metals to polarize extended conjugated n-systems is a typical strategy in the design of organometallic materials combining donor and acceptor groups.The dimethylamino- stilbazole complex (1) combines an organic donor (the dimethylamino group) with an beC = 61 x loao organometallic acceptor formed through coordination of the pyridine ring to tungsten. This complex was easily obtained from W(CO),.THF which is made by photolysis of tungsten hexacarbonyl.' Substantial fl values were observed for this compound. Attachment of smaller polarizable ligands around metal centres is also effective. Thiolate adducts of nickel have been used in a comparative study of the design of molecules with large nonlinearities.2 Molybdenum-based organometallics demon- strate third-order susceptibility properties3 and platinum ethynyl complexes were examined in optical limiting st~dies.~ Because of its exceptional stability ferrocene has long been popular in materials D.R. Kanis P.G. Lacroix M. A. Ratner and T.J. Marks J. Am. Chem. Soc. 1994 116 10089. J. Waite M. G. Papadopoulos S.N. Oliver and C. S. Winter Mol. Cryst. Liq. Cryst. Sci. Technol. Sect. B 1994 6 297. T. Zhai C. M. Lawson G. E. Burgess M. L. Lewis D. C. Gale and G. M. Gray Opt. Lett. 1994,19,871. J. Staromlynska P. B. Chapple J. R. Davy T.J. McKay Proc. SPIE-lnt. SOC. Opt. Eng. 1994 2229 59. 25 1 G.R. Stephenson Figure 1 applications. The bis( 1,3-dithiolylidene)dihydroanthracenecore has been functional- ized with two ferrocene units to form the structure (2) shown in Figure 1.The multi-stage redox properties of the product have been examined.' Similar building blocks are combined in the disubstituted ferrocene (4) which was easily obtained from the diformyl ferrocene (3) as shown in Scheme 1. The conductivity and magnetic properties of TCNQ complexes of (4) have been examined.6 F* BuU MF,-78" 0 Scheme 1 Organometallic Liquid Crystals and Effects Chemicals.-The ferrocene-based liquid crystal (5) has been found to form a chiral smectic C phase.7 Smectic A behaviour has been observed with rhodium adduct (6) of the salen-type ligand (shown in Figure 2)8 and its iridium counterpart. The ferrocenophane (7)also showed smectic C properties.' The phase transition for (7) (l0OOC) occurs at a rather higher temperature than observed for (6) (83"C),while the temperature of the phase transition to smectic A for (6) was higher still at 141 "C.A benzocrown ether has been transformed into a highly effective FTIR-readable molecular sensor for alkali metal ion ratioing by the attachment of a tricarbonyl- ' G. J. Marshallsay and M. R. Bryce J. Org. Chem. 1994 59 6847. A. Togni M. Hobi G. Rihs G. Rist A. Albinati P. Zanello D. Zech and H. Keller Organometallics 1994 13 1224. ' C. Imrie and C. Loubser J. Chem. SOC. Chem. Commun. 1994 2159. * P. Berdaque J. Courtieu and P. M. Maitlis J. Chem. SOC.,Chem. Commun. 1994 1313. A. Werner and W. Friedrichsen J. Chem.SOC. Chem. Commun. 1994 365. Organometallic Chemistry Figure 2 chromium group. Differential responses for different alkali metals can be distinguished by careful analysis of the metal carbonyl vibrational band envelope by principal component analysis. lo In (8) a tritiated oligodeoxyribonucleotide-basedphosphite is complexed to pentacarbonyltungsten for use in DNA-probe-based diagnostics.' FTIR-readout features in a new probe for SH groups in biomolecules which has been shown to be stable in aerated aqueous solutions despite the presence of Fe(CO),Cp moiety.'* Organoiron complexes [Fe(CO),C,H,] have also been used to tag surface NH groups on protein^.'^ Organometallic Structures.-Incorporation of c60 into an organometallic molecule is a certain way to form an eye-catching structure.A cyclic voltametric study of ferrocenyl-substituted fulleropyrrolidines shows that the reduction potentials are more negative than those measured for unsubstituted C6,.14 Organometallic salts of fullerides obtained by reaction with Mn(Cp*) have also been examined electrochemi- ~a1ly.l~ Organometallic chemistry can be used for the elaboration of the carbon lo C.E. Anson C.S. Creaser and G.R. Stephenson J. Chem. SOC.,Chem Commun. 1994 2175. l1 J. M. Dalla Riva Toma and D. E. Bergstrom J. Org. Chem. 1994 59 2418. B. Rudolf and J. Zakrzewski Tetrahedron Lett. 1994 35 9611. l3 C.E. Anson C. S. Creaser 0.Egyed M. A. Fey and G. R. Stephenson J. Chem. Soc. Chem. Commun. 1994 39. l4 M. Maggini A. Karlsson G.Scorrano G. Sandona G. Farnia and M. Prato J. Chem. SOC. Chem. Commun. 1994 589. l5 J. P. Seleque S. Dev T. F. Guarr J. W. Brill and E. Figueroa Proc. Electrochem. Soc. 1994 24 1245. G.R. Stephenson framework of fullerene structures. A palladium catalysed [3 + 21 cycloaddition adds a trimethylenemethane unit to form a fluorescent fullerene derivative.I6 Organization of organometallic complexes such as tricarbonyl(cyclopentadieny1)-manganese and ferrocene derivatives on a gold surface affords photosensitive Figure 3 self-assembled monolayers.' The reverse concept an organic framework with attached metal atoms (Figure 3) is the long term aim of work at the Max-Planck- Institut fur Polymerforschung in Mainz. Substituted tricarbonyl(cyc1obutadiene)iron complexes obtained by palladium catalysed coupling reactions with alkynes have been prepared as staging points towards these structures.'8 The supramolecular building block (9) illustrates another approach to regularly spaced metal-containing (9) structures.' Nano-sized chromium clusters have been formed by complexation of phenyltriethoxysilane with chromium hexacarbonyl to form an $-complex equipped with sol-gel functionality.Sol-gel processing combining 1,4-bis(triethoxysilyl)benzene l6 L.-L. Shiu,T.-I. Lin S.-M. Peng G.-R. Her D. D. Ju S.-K. Lin J.-H Hwang C. Y. Mou,and T.-Y. Luh,J. Chem. SOC. Chem. Commun. 1994,647. E.W. Wollman D. Kang C.D. Frisbie 1. M. Lorkovic and M. S. Wrighton J. Am. Chem. Soc. 1994,116 4395. '* J. E.C. Wiegelmann U.H. F. Bunz and P. Schiel Organometallics 1994 13 4649. l9 E. C. Constable A. J. Edwards M.d. Marcos P. R. Raithby R. Martinez-Maiiez and M. J. L. Tendero Inorg. Chim. Acta 1994 224 11. Organometallic Chemistry and the chromium complex produced extended metal-containing arrays.20 On a smaller scale four tricarbonylchromium units have been dispersed around a cyclobu- tadiene ligand in structure ( As with C,, organometallic coupling methods can be used to form metal-free products of considerable structural importance. Quinonyl- and arylporphyrins have been obtained by benzannulation of Fischer carbene complexes and palladium catalysed coupling with arylboronic acids respectively.22 Polymetallic Rods and Dipoles.-One of the beauties of this subject is that it seems possible to justify making compounds simply because they have interesting structures.Over the last year some remarkable organometallic rods have been prepared (Figure 4).The polycationic iron-based structure (11)23 and the dirhodium or diiridium bis-alkyne complex based on 4,4'-dialkyn~lbiaryls~~ provide remarkable examples. Bent rods in which metal acetylide adducts are further complexed at the alkyne links2' or in which the alkynes are alkynylferrocene moieties26 show further possibilities. + + + ke ire ke 49 49 (11) Figure 4 As with the metal-polarized n-systems prepared in search of NLO effects discussed earlier bimetallic rod-like polyene and polyyne structures offer interesting elec- trochemistry and the prospect of electrochemically generated cationic dipoles.The dimanganese complex (13) obtained from (12) (Scheme 2) contains a butadiene centre-unit linked at each end as a carbene ligand. Electrochemical reduction forms a 2o K. M. Choi and K. J. Shea J. Am. Chem. SOC. 1994 116 9052. 21 F.-E. Hong Y.-T. Chang C.-T. Chen S.-L. Wang and F.-L.Liao J. Organomet. Chem. 1994,480 75. 22 C.-S. Chan A.K.-S. Tse and K.S. Chan J. Org. Chem. 1994,59 6084. 23 A. S. Abd-El-Aziz and C. R. de Denus J. Chem. SOC.,Chem. Commun. 1994 663. 24 R. R. Tykwinski and P.J. Stang Organometallics 1994 13 3203. 25 H. Lang M. Herres and W. Imhof J. Organomet. Chem. 1994 465 283. 26 K. Onitsuka X.-Q. Tao and K. Sonogashira Bull. Chem. SOC.Jpn. 1994 67 2611. 256 G.R. Stephenson Me Me I)BuLl c 2) cut 3) 02 11) 2BuLi Me‘ - + e- MeP.(eledrochem. reduction) (13) Me’ Scheme 2 + I-.- Scheme 3 CP’ ,Re ON bPh3 1.5Cu(OAc) ___) Py = cP\ ‘“b Re= Ph3 NO = =Re I,PPh3 ‘CP* Scheme 4 Organometallic Chemistry 257 bimetallic radical anion.27 The diiron complex (14) provides an interesting parallel (Scheme 3) since in this case each iron atom is bound to the butadiene centre portion through a o-bond. Electrochemical oxidation leads via a radical cation (15) to the dication (16) with a biscarbene structure.28 Metal-capped polyyne rods have also been prepared. One approach to the tetrayne (18)is a copper-mediated coupling of the rhenium diyne complex (17) shown in Scheme 4.29 In a mixed rhenium/manganese analogue with a bis-alk~ne,~’ unsymmetrical charge-distribution can be anticipated in the cationic structure that is formed by removal of an OMe group from a Fischer carbene complex and uneven charge distribution would lead to a rod-like dipole structure.The product is reported to have an intense absorption at 480 nm. 2 Physical Studies of Organometallics Because of the practical difficulties involved the techniques of physical organic chemistry are relatively infrequently applied to the study of organometallic structures. When studies of this type are attempted however interesting information almost always emerges. In a stopped-flow kinetic study of reversible methoxide addition to Fischer carbene complexes equilibrium constants for the formation of the tetrahedral intermediate were found to be eight orders of magnitude higher than those for the corresponding reaction of the ester in methyl ben~oate.~’ There is also much scope for bond-energy studies.The strength of the metal-alkene bond in titanium complexes thought to be involved in Ziegler-Natta catalysts has been calculated by the MCPF method.32 X-Ray crystallography has been brought to bear on the study of molecular recognition between metal carbonyl moieties. The tetrahedra of nickel tetracarbonyl have been found to stack together neatly in a dense three-dimensional array. A no less remarkable interlocking packing pattern was observed for iron penta~arbonyl.~~ Rotational barriers have been measured in trimethylenemethane tricarbonyliron and iron carbonyl phosphine complexes.34 Voltammetric studies of metallocene~~~ and an analysis of isomerization of tricarbonylchromium phosphine complexes using cyclic voltammetry simulation36 provide further examples.3 The Role of Organometallic Complexes in Asymmetric Synthesis Where the synthesis of organic target molecules is concerned the most active frontier is the search for increasingly general and efficient enantioselective methods. Where organometallic intermediates are used in bond-forming reactions the asymmetric and enantioselective modification of these processes has been increasingly a major focus of research effort. Palladium catalysed allylic substitution and cross coupling performed 27 A. Rabier N. Lugan and R. Mathieu Organometallics 1994 13 4776.B. A. Etzenhouser Q. Chen and M. B. Sponsler Organometallics 1994 13 4176. 29 M. Brady W. Weng and J. A. Gladysz J. Chem. SOC. Chem. Commun. 1994 2655. ’O W. Weng T. Bartik and J. A. Gladysz Angew Chem. Int. Ed. Engl. 1994 33 2199. C. F. Bernasconi F. X. Flores J. R. Gandler and A. E. Leyes Organometallics 1994 13 2186. 32 V. R. Jensen M. Ystenes K. Warnmark B. Akermark M. Svensson P. E. M. Siegbahn and M. R.A. Blomberg Organometallics 1994 13 282. 33 D. Braga F. Grepioni and A.G. Orpen Organometallics 1994 13 3544. ” V. Branchadell L. Deng and T. Ziegler Organometallics 1994 13 3115. 35 D. Obendorf E. Reichart C. Rieker and H. Schottenberger Electrochim. Acta 1994 39 2367. ” A.W. Bott Curr. Sep. (Eng) 1994 13 45. G.R.Stephenson in the presence of chiral ligands and strategies for the enantioselective preparation of stoichiometric metal complexes have been particularly of note and much progress has been made during 1994. Asymmetric Allylic Substitution.-An q3-1,3-diphenylallyl electrophile has proved a popular test system for the evaluation of chiral ligands in asymmetric allylic substitution. The two ends of the allyl unit are diastereotopic while a chiral ligand is present in the palladium complex and structures of this type are easily obtained by the displacement of acetate or other leaving groups from starting materials such as (19) (Scheme 5). P AcO h d Ph [ p m h ~ p h -] Me02C CO2M62Ph \ Ph (19) (20) (21) Scheme 5 Excellent results have been obtained with ligands that contain one phosphine and one amine heterocentre.Helmchen’s oxazoline-based ligand (22) provides a typical example. Allylic displacement by the enolate formed from dimethyl malonate can often be in the high 90% range. The palladium complex of (22) has been studied in solution by NMR and in the solid state by X-ray cry~tallography.~~ Sulfur and nitrogen can also be combined and in the case of (23) the William’s group at Loughborough have obtained the product (21) in 92% yield and 96% enantiomeric excess.38 The effect of substituents on the aromatic rings flanking the allyl unit on enantioselectivity and degree of conver- sion have been e~amined.~’ In the case of the chiral ligand (24) a much closer spacing is present between the nitrogen and phosphorous atoms which are linked directly by a covalent bond.40 In contrast ligands of type (25)41 can offer much wider spacings and have the chiral element the axis of chirality in the binaphthyl unit far more remote from the ligand sites.Nonetheless enantiomeric excesses as high as 96% have been achieved though both the rate and enantioselectivity of the reaction were found to be highly dependent on spacing in the chelate chain. Diamine ligands [e.g. (26)] for asym- metric modification of this reaction have also been examined but enantiomeric excess- es are typically lower in the range 62-88%.42*43 Bisaziridines have been employed as chiral ligand~.~~ In the chiral acetal(27) sulfur and oxygen are employed as the metal- binding sites.45 Examples of chiral ligands are shown in Figure 5.For (21) (Scheme 5) the palladium-ally1 moiety is prochiral. With an unsymmetri- cally substituted ligand on the other hand planar chirality is present in the metal/q3- 37 J. Sprinz M. Kiefer G.Helmchen M. Reggelin G. Huttner 0.Walter and L. Zsolnai Tetrahedron Lett. 1994,35. 1523. ’’ J.V. Allen S. J. Coote G. J. Dawson C. G. Frost C. J. Martin and J. M. J. Williams J. Chem. Soc. Perkin Trans. I 1994 15 2065. 39 J. V. Allen J. F. Bower and J. M. J. Williams Tetrahedron Asymmetry 1994 5 1895. G. Brenchley E. Merifield M. Willis and M. Fedouloff Tetrahedron Lett. 1994 35 2791. 41 H. Kubota and K. Koga Tetrahedron Lett. 1994 35 6689. 42 P. Gamez B. Dunjic F. Fache and M. Lemaire J. Chem. Soc.Chem. Commun. 1994 1417. ” J. Kang W.O. Cho and H.G. Cho Tetrahedron Asymmetry 1994 5 1347. 44 D. Tanner P. G. Anderson A. Harden and P. Somfai Tetrahedron Lett. 1994 35 4631. 45 C.G. Frost and J. M. J. Williams Synlett 1994 7 551. Organometallic Chemistry 259 allyl moiety. The ligand MOP-phen (28) has been evaluated in the presence of formic acid to transfer a hydrogen atom to the allyl ligand.46 In the case of the vinylsilane (30) this provides an alternative asymmetric synthesis of allyl~ilanes.~’ An X-ray structure dN-SiPh213d +h (24) phh Ph phBAph 00 MeN NMe hSPh HH analysis of an intermediate palladium allyl complex has been perf~rmed.~~ In an un- usual example of asymmetric induction in the formation of axial chirality the popular chiral ligand (R)-BINAP (29) was employed to form (32) from (31) (Scheme 6).49 Pd,(dba),,-CHC13 (28) HCOY Me3Si 0 93% yield 91% 8.8.46 T. Hayashi H. Iwamura and Y. Uozumi Y. Matsumoto F. Ozawa Synthesis 1994 526. 47 T. Hayashi H. Iwamura and Y. Uozumi Tetrahedron Lett. 1994 35,4813. 48 T. Hayashi H. Iwamura M. Naito Y. Matsumoto and Y. Uozumi J. Am. Chem. SOC. 1994 116 775. 49 J. -Y. Legros and J.-C. Fiaud Tetrahedron 1994 50 465. G. R. Stephenson An alternative approach to asymmetric induction is to include a chiral centre in the nucleophile. A chiral enamine has been used in this way.” Metal-catalysed allylic acetoxylation can induce asymmetry in the preparation of starting materials for allylic substitution.Asymmetric modification of palladium copper and nickel acetoxylation catalysts has been examined but gives products with low enantiomeric excess.” Asymmetric Coupling Reactions.-Differentiation of enantiotopic ends of symmetri- cally substituted alkenes and enantiotopic alkene groups in prochiral dienes offer two distinct approaches for asymmetric Heck-coupling reactions. Coupling trifloxyben- zene with the alkene (33) under the influence of (S)-BINAP as the chiral ligand gives access to precursors for non-racemic cyclic lactones in up to 72% enantiorneric ex- c~ss.’~ With a vinyl halide as the coupling partner and norbornene as the alkene substituted chiral norbornanes such as (34) have been obtained in up to 93% enan-tiomeric excess (Scheme 7).53 In this case too BINAP was used as the chiral Ph B,/\\/Ph Pdcat.* (R)-BINAP Scheme 7 auxiliary. BINAP also figured in the enantioselective differentiation of alkenes in the diene (35) (Scheme 8). With (36) enantioface differentiation holds the key to Pdcat. & (R&,PO4 )-BINAP * H NMP 94% Scheme 8 50 K. Hiroi J. Abe K. Suya S. Sato and T. Koyama J. Org. Chem. 1994 59 203. H. Yang M.A. Khan and K.M. Nicholas J. Mol. Catal. 1994 91 319. 52 Y. Koga M. Sodeoka M. Shibasaki Tetrahedron Lett. 1994 35 1227. 53 F. Ozawa Y. Kobatake A. Kubo and T. Hayashi J. Chem. SOC.,Chem. Commun. 1994 1323 Organometallic Chemistry asymmetric induction. Full details of both these reactions have now appeared.54 Pal- ladium catalysed enyne cycloisomerizations can also be used to form chiral centres.A covalently attached chiral auxiliary in the ligand has been used in an attempt to gain diastereoselectivity in the coupling step. A 50% d.e. has now been achieved.55 Asymmetric Epoxidation.-The contemporary challenge is the asymmetric epoxida- tion of unfunctionalized alkenes. Epoxidation catalysts reported by Jacobsen were feted as reagent of the year for 1994 and throughout the year both Jacobsen and Katsuki have kept up a steady flow of papers reporting new developments in the field. Other workers (Nagata Imagawa Yamada and M~kaiyama,~~?~~) have also been active in this highly competitive area. Jacobsen's research group can achieve excellent enantioselectivity (as high as 93%) for the epoxidation of 1-phenylcy~lohexene.~~ 1,3-Dienes can also be used as substrates and in the case of (37) (Scheme 9) 91% 90% 8.8.(37) Me Me 32% yield Scheme 9 enantiomeric excess was achieved for the epoxidation of the least substituted alkene linkage.59A convenient procedure for the large-scale preparation of the chiral manga- nese catalyst (38 R = 'Bu) has been reported.60 Katsuki also reports an enantiomeric 54 Y. Sato S. Nukui M. Sodeoka and M. Shibasaki Tetrahedron 1994 50 371. 55 B. M. Trost and B. A. Czeskis Tetrahedron Lett. 1994 35 211. 56 T. Nagata K. Imagawa. T. Yamada and T. Mukaiyama Inorg. Chim. Acta 1994 220 283. 5' T. Nagata K. Imagawa T. Yamada and T. Mukaiyama Chem. Lett. 1994 1259. '. '' B. D. Brandes and E.N. Jacobsen J.Org. Chem. 1994 59 4378. 59 S. Chang R. M. Heid and E.N. Jacobsen Tetrahedron Lett. 1994 35 669. 6o J. F. Larrow E.N. Jacobsen Y. Gao Y. Hong X. Nie and C. M. Zepp J. Org. Chem. 1994 59 1939. G. R. Stephenson excess in the high 90s. A series of papers in SynEett this year report the progress of this work. Enantioselective epoxidation of chromenes using hydrogen peroxide and a cata- lyst of the general type (38),6’and the more elaborate ligand (39)containing two chiral axes as well as the chiral ligand-set around manganese,62 had culminated with a dis- cu~sion~~ of the origin of chiral discrimination in enantioface recognition with simple alkenes. Two fulI papers in Tetrahedron report the details of earlier studies of epoxida- tion of substituted styrenes64 and dihydr~naphthalene~’ substrates.Use of catalysts of this type for the oxidation of aryl sulfides to sulfoxides has also been reported.66 Asymmetric Cyclopropanation Hydrogenation and Hydrosi1ation.-Compared to asymmetric epoxidation the cyclopropanation of unfunctionalized alkenes has prog- ressed more slowly. Chiral ligands based on 2,6-disubstituted pyridines have become popular. Ruthenium complexes have been employed to give an efficient carbene trans- fer from trirnethylsilyldiaz~methane.~~ Diazoesters can also be used (Scheme 10)and with 1-heptene as the substrate the product (40)was obtained as the major isomer in 99% enantiomeric excess.68 N* (40)99%e.e. Scheme 10 In asymmetric hydrogenation improved techniques and improved understanding of the hydrogenation process have been the objectives of recent work.Kinetic studies of asymmetric rhodium-based catalysts have been performed by the Brunner A water soluble version of BINAP has been developed by sulfonation of the aromatic rings. In this example four sulfonic acid groups are attached and the best results were obtained with ruthenium as the catalytically active metal.70 Attaching homogeneous catalysts to a solid support is also a commonly employed stratagem but such solid phase reagents are often less active and less enantioselective than their solution-state counterparts. Now a solvent/thin-film interface system has been developed to over- come this diffic~lty.~~ More unusual ligands and metal systems have been reported.The asymmetric hydrogenation reaction illustrated in Scheme 11 is highly 61 R. Irie N. Hosoya and T. Katsuki Synlett 1994 255. 62 H. Sasaki R. Irie and T. Katsuki Synlett 1994 356. 63 T. Hamada R. Irie and T. Katsuki Synlett 1994 479. 64 N. Hosoya A. Hatayanma R. Irie H. Sasaki and T. Katsuki Tetrahedron 1994 50 431 1. 65 H. Sasaki R. hie T. Hamada K. Suzuki and T. Katsuki Tetrahedron 50 118277. 66 K. Noda N. Hosoya R. hie Y. Yamashita and T. Katsuki Tetrahedron 1994 50 9609. 67 S.-B. Park H. Nishiyama Y. Itoh and K. Itoh J. Chem. SOC.,Chem. Commun. 1994 1315. 68 H. Nishiyama Y. Itoh H. Matsumoto S.-B. Park and K. Itoh J. Am. Chem. SOC.,1994 116 2223. 69 H. Brunner J. Furst U. Nagel and A. Fischer Z. Naturforsch. 1994,49b 1305. ’O K.Wan and M. E. Davis Tetrahedron Asymmetry 1994 4 2461. ” K.-T. Wan and M. E. Davis Nature (London) 1994 370 449. Organometallic Chemistry gpcy* re PPh2 CO Me (41) LCO2Me H2 RWnW2BF4MeOH * r:02fvle 100% yield 97% 8.8. n .H7 37% 99% e.e. + MeGPh n (42)34% 99% 8.8. Scheme 11 effective with a chiral catalyst system based on rhodium and the ligand (41).72Asym-metric hydroboration proceeds similarly. Titanium complexes are an unusual choice for hydrogenation but a system developed by Buchwald based on a C,-symmetric catalyst has proved highly effective. Hydrogenation of imine~~~ proceeds at moderate pressures (15-80 psi) to afford the chiral benzylamines with enantiomeric excesses in the range 92-99%. Kinetic resolution based on this system74 is even more effective affording (42) in 99% enantiomeric excess.BINAP is typically used in enantioselective hydr~silation.’~ In a more unusual case (Scheme 12) the chiral auxiliary (43) was constructed from two chiral ferrocene moie- ties.76 Selenium- and tellurium-bridged structures were also used. Asymmetric Induction in q6-Chromium Arene Complexes.-The use of tricarbonyl- chromium complexes of aromatic ligands in organic synthesis is the most highly developed of the stoichiometric strategies for metal-derived stereocontrol. It is thus not surprising that the induction of asymmetry in such complexes has received consider- able attention. The key theme for 1994 has been enantioselective deprotonation. 72 A. Togni C.Breutel A. Schnyder F. Spindler H. Landert and A. Tijani J. Am. Chem. SOC. 1994,116,4062. 73 C.A. Willoughby and S.L. Buchward J. Am. Chem. SOC. 1994 116 8952. 74 A. Viso N. E. Lee and S. L. Buchward J. Am. Chem. soc. 1994 116 9373. 75 X. Wang and B. Bosnich Organometallics 1994 13 4131. 76 Y. Nishibayashi J. D. Singh K. Segawa S. Fukuzawa and S. Uemura J. Chem. SOC. Chem. Commun. 1994 1375. G.R. Stephenson Me Ph,SiH,. Rh(l)cat. * ArXR 0 2) HCI MeOH HO H Scheme 12 Lithiated chromium-bound arenes are useful reagents for bond formation so the use of chiral lithium amide bases to metallate prochiral chromium arene complexes is an attractive strategy. The LDA-like chiral base (44)has been employed to good effect by the Simpkins group at Nottingham producing (45) in up to 84% enantiomeric excess by trapping the metallated aromatic ring with trimethylsilyl chloride in the presence of lithium ~hloride.~ Metallation of prochiral starting materials derived from acetals is similarly efficient though competing reaction arising through metallation at the benzylic position complicates the process.78 Kiindig has reported the use of a chiral amide base derived from borneol and l-phenylethylamine,79 and in Uemura’s group chiral diamines have been combined with butyllithium.80 Work continues on systems in which chirality present in the aromatic starting material induces asymmetry in a prochiral metal-bound aromatic ligand.In the case shown in Scheme 13 the inducing c;(CO)3 83%(45)yield 84% e.e.Scheme 13 77 D. A. Price N. S. Simpkins A.M. MacLeod and A. P. Watt Tetrahedron Lett. 1994 35 6159. ” D.A. Price N. S. Simpkins A.M. MacLeod and A. P. Watt J. Org. Chem. 1994 59 1961. 79 E. P. Kundig and A. Quattropani Tetrahedron Lett. 1994 35 3497. 8o M. Uemura Y. Hayashi and Y. Hayashi Tetrahedron Asymmetry 1994 5 1427. Organometallic Chemistry 265 chirality is removed after the asymmetric induction step to leave a 2-substituted styrene as the final product.81 Kinetic resolution8’ of the tricarbonylchromium complex of 2-methylbenzylalchol continues a line of successful investigations by a number of groups that combine biotransformations with metal-bound organic substrates. Induction of asymmetry adjacent to the chromium-bound ring has also been an important objective.The tartrate-based titanium-mediated sulfur oxidation has been used to good with a 2-substituted phenylthioether chirality at sulfur and within the q6-arene complex can be induced at the same step. Asymmetric Induction in q4-Tricarbonyliron Complexes-The lipase-mediated bio- transformation approach discussed earlier for chromium arene complexes has also been employed with the q4-diene complexes in the tricarbonyliron series.82 Induction of asymmetry by the addition of achiral nucleophiles to tricarbonyliron complexes containing chiral substituents has also yielded enantiomerically enriched diene complexes in up to 84% enantiomeric excess.84 Inclusion of the chiral auxiliary as a ligand at the metal is possibly a more subtle approach since through its presence as an auxiliary ligand the functionality on the diene complex need not be changed in order to gain access to the enantiopure series.An extensive series of chiral phos- phines has been examined.85 With electron-donor substituted complexes resolution has been performed by the chemical separation of chiral oxazolidine-substituted ligands.* Asymmetric Pauson-Khand Reaction.-The Pauson-Khand reaction which forms substituted cyclopentenones from alkynes by a cobalt-mediated cyclocarbonylation reaction has been growing in popularity and its application in organic synthesis will be discussed in detail in the next section. In 1994 considerable progress has been made with the asymmetric modification of this reaction.Currently stratagems centre on the inclusion of a chiral auxiliary in one of the organic substrates. With a non-racemic chiral alkoxyalkyne practical access to substituted cyclopentenones can be achieved. Although asymmetric induction is not complete the diastereoisomers can be separated after the cobalt-mediated cyclization step.* More elaborate chiral ligands afford organocobalt substrates of type (46) (Figure 6) which combine with norbornene to give a 92 1 ratio of diastereoisomers.88 The chiral auxiliary can also be included in the alkene ~o-reactant.~~ A further variant of this process employs unsaturated sugar in the cycloreaction with a cobalt-bound alkyne attached via an acetal at the aldehyde carbon P. W. N. Christian R. Gil J.Muiiiz-Fernandez S. E. Thomas A. T. Wierzchleyski J. Chem. Soc. Chem. Commun. 1994 1569. ‘2 M. Uernura H. Nishirnura S. Yarnada Y. Hayashi K. Nakamura K. Ishihara and A. Ohno Tetrahedron Asymmetry 1994 5 1673. 83 S. L. Griffiths S. Perrio and S. E. Thomas Tetrahedron Asymmetry 5 1847. 84 C. W. Ong and R. H. Hsu Organometallics 1994 13 3952. J. A. S. Howell A. D. Squibb A. G. Bell P. McArdle D. Cunningham Z. Goldschmidt H. E. Gottlieb and D. Hezroni-Langerman Organometallics 1994 13 4336. 86 J. A. S. Howell A. G. Bell P. J. O’Leary P McArdle D. Cunningham G. R. Stephenson and M. Hastings Organometallics 1994 13 1806. ” V. Bernardes X. Verdaguer N. Kardos A. Riera A. Moyano M. A. Pericas and A. E. Greene Tetrahedron Lett. 1994 35 575. ” X.Verdaguer A.Moyano M. A. Pericas A. Riera V. Bernardes A. E. Greene A. Alvarez-Larena and J. F. Piniella J. Am. Chem. Soc. 1994 116 2153. 89 J. Castro A. Moyano M. A. Pericas A. Riera and A. E. Greene Tetrahedron Asymmetry 1994 5 307. G.R. Stephenson Figure 6 of the sugar chain." The alkyne can also be attached by an ether link to an alcohol adjacent to the unsaturated portion of the sugar unit." In a total synthesis of (-)-a-kainic acid (47)' Pauson-Khand cyclization of an enyne complex with a chiral spacer between the two unsaturated ligands affords a key intermediate which was elaborated as shown in Scheme 14.92 1) PdC HS Me3N0 2) H + ___t 3) TsCl 4) MOMCl 5) Fe(0). TMSCI Me& P OMOM Ts + isomer (47) Scheme 14 4 Stoichiometric Organometallic Complexes in Organic Synthesis Pauson-Khand Cyc1izations.-The mechanism of the Pauson-Khand reaction sug- gests that the process should be catalytic in the cobalt carbonyl but normally a stoichiometric hexacarbonyl dicobalt alkyne complex is employed.A catalytic version of an intramolecular Pauson-Khand reaction has now been described.93 Another variant is to form the cobalt carbonyl complex in situ from cobalt dibromide and zinc Alternatives to the use of cobalt are also under investigation. Pearson's group W. E. Lindsell P. N. Preston and A. B. Rettie Carbohydr. Res. 1994 254 311. 91 J. Marco-Contelles Tetrahedron Lett. 1994 35 5059. 92 S. Yo0 and S.H. Lee J. Org. Chem. 1994 59 6968. 93 N. Jeong S.H. Hwang Y. Lee and Y.K. Chung J.Am. Chem. SOC. 1994 116 3159. 94 M. Periasamy H. R. Reddy and A Devasagayaraj Tetrahedron 1994 50 6955. Organometallic Chemistry at Case Western Reserve have reported the use of iron pentacarbonyl in the cyclization of an alkyne and alkene to form a cy~lopentenone.’~ More normally it is dicobalt octacarbonyl that is used to form cyclopentenones in this fashion. This reaction is highly versatile and even the presence of a cyclopropane ring on the alkene does not interfere with the efficiency of the cobalt-mediated rea~tion.’~ The Pauson-Khand reaction has reached the stage of development where it can be employed in target molecule synthesis. A 6,5,5-ring system terminating in a trisubstituted cyclopen- tenone has been formed in this way in a model study for the CDE rings of xest~bergsterol.~~ Diastereoselectivity greater than 14 1 was notable in this case.The Nicholas Reaction.-The binding of two cobalt atoms to an alkyne also holds the key to the Nicholas reaction in which a metal-stabilized propargyl cation is combined with a nucleophile. Two examples of intramolecular electrophilic aromatic substitu- tion using this approach have been described. Reactions of this type can occur at temperatures as low as -78 OC9*and have been shown to be enantiospecific.” The use of sugar-derived starting materials such as (48) features in studies leading towards enantioselective synthesis of oxepane sub-units of marine polyether toxins. loo Considerable manipulation via (49) however is needed to achieve the required building block which is liberated from the complex (50)by oxidation of Co,(CO) with I (Scheme 15).A more straight-forward application is the use of a propargyl aldehyde complex with a silyl enol ether nucleophile in a stereoselective total synthesis of bengamide E. Scheme 15 Because complexation to cobalt bends the alkyne away from a linear structure the Nicholas reaction has found considerable application in the synthesis of cyclic 95 A. J. Pearson and R. A. Dubbert Organometallics 1994 13 1656. 96 A. Stolle H. Becker J. Salaiin and A. de Meijere Tetrahedron Lett. 1994 35 3517. 97 M.E. Krafft and X. Chirico Tetrahedron Lett. 1994,35 4511. 98 D. D. Grove J. R. Corte R. P. Spencer M. E. Pauly and N. P. Rath J. Chem. SOC. Chem. Commun.1994 49. 99 A. V. Mueheldorf A. Guzman-Perez and A. F. Kluge Tetrahedron Lett. 1994 35 8755. loo S. Tanaka N. Tatsuta 0.Yamashita and M. Isobe Tetrahedron 1994 50 12883. lo’ C. Mukai 0.Kataoka and M. Hanaoka Tetrahedron Lett. 1994 35 6899. G. R. Stephenson enediynes. A typical strategy is to employ a propargyl leaving group in an intramolecular reaction with an enol ether-based nucleophile. In the case of (51) however cyclization ultimately gives rise to the rearranged allenic structure (52) (Scheme 16).'02 Since both the Pauson-Khand and the Nicholas reactions employ stoichiometric transition metal complexes for activation and control these approaches to carbon- carbon bond formation are particularly attractive when combined in a single synthesis.In this way the cobalt complex can play a double role and so best reward the effort of its attachment to synthetic intermediates. Indeed when viewed as a decomplexation method the Pauson-Khand reaction is extremely attractive since key skeletal bonds are formed during the decomplexation event. The total synthesis of (+ )-epoxydic-TBSOvfl;3a C OR 1 n 0 (co)~co-co(co)~ 0 TBSO (51) (52) Scheme 16 Me Steps -OMe Scheme 17 tymene (Scheme 17) from Schreiber's group provides an excellent illustration of this approach. The synthetic route begins by the combination of two chiral building blocks (53)and (54) by a conventional acetylide displacement of a triflate. The triflate (53)is in enantiopure form and (54) was racemic hence the product (55) was formed as a 1 :1 M.E. Maier and D. Langenbacher Synlett 1994 713-716. Organometallic Chemistry mixture of diastereoisomers. The undefined chiral centre however is corrected in the Nicholas step. After complexation with dicobalt octacarbonyl the Nicholas reaction is promoted with trimethylsilyl triflate. There are two propargyl leaving groups leading to an issue of chemoselectivity. Under the conditions used the major product is the ether (56) containing the terminal alkene needed for the intramolecular Pauson- Khand step. This compound like the ethyl ether by-product was formed as a single diastereoisomer illustrating the point that the cobalt-mediated step was stereoconver- gent. Thus chirality originating in (53) which itself was prepared in five steps from the terpene natural product @)-pulegone has been successfully relayed to this point in the target structure.Removal of the metal by the Pauson-Khand cyclization completes the final two rings of the target molecule again in a stereocontrolled fashion.'03 Scheme 18 illustrates an unusual variant of hexacarbonyl dicobalt alkyne chemistry in which radical-mediated steps are performed adjacent to the cobalt-bound portion of the ligand.lo4 52% Scheme 18 The Dotz-Wulff Cyc1ization.-It was Dotz working as a Ph.D. student in the laboratory of E.O. Fisher who first discovered that vinyl and aryl-substituted Fischer carbenes can be induced to cyclize with alkynes to give 1,4-dioxygenated aromatic products.Since then a good many groups have employed this reaction though it is work in the two laboratories of Dotz and Wulff which has had the greatest impact defining the scope and selectivity of processes of this type. In contrast to the Pauson-Khand reaction in which carbon-carbon bonds are formed during metal removal the Dotz-Wulff cyclization forms carbonsarbon bonds during the attach-ment of the metal. The strategic importance of this point is discussed later in the Report. Typically at the present time the cyclization reaction is combined with an oxidative work-up to produce a metal-free product. With a relatively vigorous oxidative procedure quinone products can be obtained. lo' Progress has been made in the search to define intermediates in the mechanism of this multistep cyclization.In Oviedo the Barluenga group has characterized tetracarbonylvinylcarbene(57) and metallohexa- triene (58) intermediates as shown in Scheme 19.'06 Low temperature infrared studieslo7 have also been used to probe the reaction sequence that leads to the cyclization products. Intramolecular reactions are often used to ensure regiocontrol. Intramolecular benzannulation has been achieved by the use of a dialkoxysilane link in the tether. T. F. Jamison S. Shambayati W. E. Crowe and S.L. Schreiber J. Am. Chem. SOC. 1994 116 5505. Io4 G. G. Melikyan 0.Vostrowsky W. Bauer H. J. Bestmann M. Khan and K. M. Nicholas J. Org. Chem. 1994 59 222. S. Chamberlin and W. D. Wulff J. Org. Chem. 1994 59 3047. J. Barluenga,F. Azner A.Martin S. Garcia-Granda and E. Perez-Carreiio,J. Am. Chem. SOC.,1994,116 11 191. lo' J. R. Knorr and T. L. Brown Organometallics 1994 13 2178. 270 G.R. Stephenson r 1 I C02Me \ Scheme 19 These reactions proceed most efficiently when a further alkene is added to the reaction mixture.'" The use of stannyl substituents on the alkyne has been found to reverse the normal regioselectivity in intermolecular benzann~lation.''~ In many reactions of this type indene formation occurs in competition with benzannulation. The effect of the metal in directing the reaction towards phenolic products has been assessed. The rank order for efficiency of access to phenolic products is chromium > tungsten > molyb-denum."' With alkyne-substituted Fischer carbene complexes quite a variety of product types can be obtained.With chromium and tungsten reaction with alkynes can afford tricyclic structures,' ' while enaminones' l2 can give rise to (59). Scheme 20 also shows an ingenious enyne metathesis in which the substituents on a catalytic quantity of the Fischer carbene complex are duplicated in an enyne co-reactant (60).' ' Metal-mediated [2 + 2 + 2) Cycloadditions.-Dicarbonyl(cyclopentadienyl)cobalt has typically been used to promote the [2 + 2 + 21 co-cyclization of a,o-diynes and alkynes. Nickel'14 catalysts offer an alternative procedure. A nitrile in the place of one of the alkynes affords substituted pyridines. This has been applied in Vollhardt's group in routes to ergot alkaloids." The use of the bistrimethylsilylethyne as a co-reactant is a typical stratagem.Ethyne itself under pressure has also been used and in the reaction leading to the formation of (61) offered a product with high optical purity.'16 A procedure for the general application of these methods to form lo' M. F. Gross and M. G. Finn J. Am. Chem. SOC.,1994 116 10 921. lo9 S. Chamberlin M.L. Waters and W.D. Wulff J. Am. Chem. Soc. 1994 116 3113. 'lo W. D. Wulff B. M. Bax T. A. Brandvold K. S. Chan A. M. Gilbert and R. P. Hsung Organornetallics 1994 13 102. '11 A. Segundo J.M. Moreto J.M. Vifias and S. Ricart Organometallics 1994 13 2467. '12 R. Aurnann M. Koprneier K. Roths and R. Frohlich Synlett 1994 1041. 11' S. Watanuki N. Ochifuji and M. Mori Organometallics 1994 13 4129.'14 Y.Sato T. Nishirnata and M. Mori J. Org. Chem. 1994 59 6133. '15 C. Saa D. D. Crotts G. Hsu and K. P.C. Vollhardt SynIett 1994 487. '16 G. Chelucci M. A. Cabras and A. Saba Tetrahedron Asymmetry 1994 5 1973. Organometallic Chemistry 271 EtO W(CO)5 ? p yo-E'oso + Ph Ph (59) Scheme 20 alkyl-substituted 2-cyano-6-(2-pyridyl)pyridineshas been described.' It is interesting to compare these reactions with alternative cyclization strategies to produce aromatic rings. For example Fischer carbenes of the type discussed in the preceding section can be combined with two alkynes by heating in acetonitrile to produce an aromatic ring. An intramolecular synthesis of the steroid ring system has been achieved in this way (Scheme 21)."* Palladium catalysed coupling can also give rise to aromatics.The combination of tributylvinylstannane with the bistriflate (62) affords a chiral arene."' OTBDMS TBSO 63% eSnBu Pd(PPh& OTf TfO NMP Me Scheme 21 11' G. Chelucci M.A. Cabras and A. Saba J. Heterocyclic Chem. 1994 31 1289 J. Bao W.D. Wulff V. Dragisich S. Wenglowsky and R.G. Ball J. Am. Chem. SOC. 1994 116 7616. '19 J. Barry and T. Kodadek Tetrahedron Lett. 1994 35 2465. G.R. Stephenson Photochemistry of Carbene Complexes-Access to amino acids and 8-lactams by photolysis of carbene complexes with suitable unsaturated co-reactants has continued to develop particularly through the efforts in the Hegedus group at Colorado State. Cyclization (Scheme 22) with a substituted imidazole gives access to the 8-lactam (63).120 Thio-substituted 8-lactams can also be obtained.I2' Using an aldehyde as a co-reactant p-lactones are formed.' 22 Scheme 22 Iron Acyl Enolates.-The highly successful chiral enolate equivalents developed some years ago in the groups of Davies and Leibeskind continue to find favour in natural product synthesis.Davies has published two alternative resolution procedures to form the enantiopure reagent (Figure 7). Menthol'23 and provide the source of the chiral auxiliaries in these processes. Enolates developed from the metal acyl species have been used to open epoxides derived from protected sugars.'25 Figure 7 1,CDifunctionalization of Dienes.-Palladium catalysed oxidation of 1,4-dienes in the presence of nucleophiles typically gives rise to disubstituted alkenes.Intramolecular delivery of one of the nucleophiles has been used to form oxaspirocyclic products. 26 New developments allow carbonsarbon bond formation based on these principles. In the synthesis of (64) (Scheme 23) intramolecular nucleophile attack is followed by interception of the organopalladium intermediate by carbon monoxide to form the amide substituent. Since nucleophile addition occurs trans and carbonyl insertion proceeds on the face carrying the metal this reaction is highly stereocontrolled but regioselectivity was incomplete. The best results were obtained in dichloromethane with one atmosphere pressure of carbon monoxide. At high pressure in THF the L.S. Hegedus and W.H. Moser J. Org. Chem. 1994 59 7779. B. Alcaide L. Casarrubios G. Dominguez and M. A. Sierra J. Org. Chem. 1994 59 7934. P.-J. Colson and L. S. Hegedus J. Org. Chem. 1994 59 4972. S. J. Cook J. F. Costello S. G. Davies and H.T. Kruk J. Chem. Soc. Perkin Trans. I 1994 2369. S. C. Case-Green J. F. Costello S.G. Davies N. Heaton C.J. R. Hedgecock V. M. Humphreys M. R. Metzler and J.C. Prime J. Chem. Soc. Perkin Trans. I 1994 933. S.G. Davies H. M. Kellie and R. Polywka Tetrahedron Asymmetry 1994 5 2563. P.G. Anderson Y.I.M. Nilsson and J.-E. Backvall Tetrahedron 1994 50 559. Organometallic Chemistry 0 (64)+ regiobomer Scheme 23 alternative regioisomer was referr red.'^^ In a variant of this procedure again using a substituted cyclohexadiene starting material external nucleophile attack (by chloride) is combined with intramolecular capture of the palladium by a pendent alkyne.Finally halogen addition to the palladium centre and reductive elimination complete a vinyl halide. Stereochemistry around the six-membered ring is fully controlled in this reaction but a mixture of E and 2 exocyclic alkenes was obtained.'28 Deprotection using Transition Metal Chemistry.-Palladium catalysed allylic dis- placement can be used as a method to detach allyl protecting groups from synthetic intermediates. Typical examples appearing this year employ allyl ester' *' and carbon- ate~'~' in the presence of an appropriate nucleophile (morpholine and diethylamine respectively). Phenol leaving groups allow the development of a palladium catalysed reductive deprotection of aryl allyl ethers.In the case of 4-t-butylphenol exceptionally efficient deprotection was possible.' 31 The allyloxycarbonate version of the reaction has been used in the synthesis of phosphonopeptides from Fmoc phosphonodipept- ides.' 32 Allylic anchoring groups can also be combined with palladium catalysed de- tachment in work related to solid-phase synthesis of pep tide^.'^^ In these cases tributyltin hydride was used in conjunction with the palladium catalyst. In a further example trimethylsilylazide was chosen as the nucleophile.' 34 The application of water-soluble catalysts in palladium catalysed detachment of allyl protecting groups illustrates a further important step forward.' 35 Homogeneous or biphasic media can be used and diethylamine is employed as the allyl scavenger.Transition Metals in Amino Acid Chemistry.-The examples of peptide deprotection and detachment from solid phases presented in the preceding section draw attention to the compatibility of transition metal based methods with amino acid chemistry. In Scheme 24 through the use of LDA to generate an anionic intermediate the allyl group detached from the ester becomes incorporated stereoselectively as a substituent on the glycine component of the dipeptide.' 36 Elaboration of side-chain functionality has also 12' P.G. Anderson and A. Aranyos Tetrahedron Lett. 1994 35 4441. 12' J.-E. Backvall Y. I. M. Nilsson P. G. Anderson R. G. P. Gatti and J.Wu Tetrahedron Lett. 1994,35 5713. 129 S. Okamoto N. Ono K. Tani Y. Yoshida and F. Sato J. Chem. SOC.,Chem. Commun. 1994 279. lJo J.P. GenCt E. Blart M. Savignac S. Lemeune S. Lemaire-Audoire J.-M. Paris and J.M. Bernard Tetrahedron 1994 50 497. 13' R. Beugelmans S. Bourdet A. Bigot and J. Zhu Tetrahedron Lett. 1994 35 4349. lJ2 D.. Maffre-Lafon R. Escale P. Dumy J.-P. Vidal and J.-P. Girard Tetrahedron Lett. 1994 35 4097. lJ3 P. Loyd-Williams A. Merzouk F. Guibe F. Albericio and E. Giralt Tetrahedron Lett. 1994 35,4437. lJ4 G. Shapiro and D. Buechler Tetrahedron Lett. 1994 35 5421. 13' S. Lemaire-Audoire M. Savignac E. Blart G. Pourcelot and J. P. Genet Tetrahedron Lett. 1994 35 8783. 13' U. Kazmaier J. Org. Chem. 1994 59 6667. G.R.Stephenson Scheme 24 been examined. Phosphorus-substituted arylamino acids are popular targets. Pallad- ium catalysed coupling between an iodoarene and H,PO,Me produces an arylphos- phinate side-chain.' 37 Alternatively the use of a 4-iodoarylphosphinate ester in pallad- ium catalysed coupling with the organozinc reagent derived from iodoserine can also produce novel tyrosine-like structures with an acidic phosphonomethyl centre in place of the phenolic OH group.' 38 Carbonylation of a trifluoromethylsulfonate derivative of tyrosine produces the corresponding benzoic acid derivative.' 39 Palladium coupling with ~inylhalide'~' and ~inyltin'~' side-chain groups produces elaborated diene enyne styryl and alkyl side-chains. A synthesis of (+)-bulgecenine in Hegedus's laboratory provides a fine illustration of the advantages of making use of anions stabilized by stoichiometric metal com- plexes.Deprotonation of an aminocarbene complex and reaction of the resulting anion with an aldehyde provide a key intermediate for the synthetic route.'42 Electrophilic metal complexes can also be employed with amino acids. Nucleophile addition by the amino substituent in amino acid esters in reactions with electrophilic tricarbonyliron cyclohexadienyl complexes have been examined. 143 The same elec- trophiles can be used in the synthesis of uncommon amino acids by combination with a Schiff-base derived n~cleophile.'~~ A total synthesis of K-13145 has been based in Pearson's laboratory on an electrophilic arylruthenium complex.A related q6-elec- trophile in the tricarbonylmanganese series forms the basis for model studies for the construction of the CFG rings of ristocetin Heterocyclic Synthesis.-Organometallic chemistry is employed surprisingly frequent- ly in the synthesis of heterocycles. Typical of such procedures are the cyclization routes to substituted indoles starting from aniline derivatives. A 2-alkynyl aniline is a com- mon starting material.'47 This procedure can be modified to include carbonyl insertion to form in a methanol solvent 2-substituted 3-carbomethoxyindoles.'48 With alkyl halides present 3-ketoindoles are formed.'49 The formation of (65) in 13' H. Lei M.S. Stoakes K. P. B. Herath J. Lee and A. W. Schwabacher J. Org. Chem. 1994 59 4206. 13* R.L. Dow and B. M. Bechle Synlett 1994 293. 139 R. G. Franz J. Weinstock R. R. Calvo J. Samanen and N. Aiyar Org. Prep. Proced. Int. 1994,26 533. 140 G. T. Crisp and P. T. Glink Tetrahedron 1994 50 2623. 14' G. T. Crisp and P. T. Glink Tetrahedron 1994 50 3213. 142 G. Schmeck and L.S. Hegedus J. Am. Chem. SOC.,1994 116 9927. 143 L.A. P. Kane-Maguire R. Kanitz P. Jones and P. A. Williams J. Organomet. Chem. 1994 464,203. 144 J. P. Genet R. D. A. Hudson W.-D. Meng E. Roberts G. R. Stephenson and S. Thorimbert Synlett 1994 631. 14' A.J. Pearson and K. Lee J. Org. Chem. 1994,59 2304. 146 A.J. Pearson and H. Shin J. Org. Chem. 1994 59 2314. 14' S. Cacchi V. Carnicelli and F. Marinelli J. Organomet. Chem 475 289. 14* S. Kondo F. Shiga N. Murata T. Sakamoto and H.Yamanoka Tetrahedron 1994 50 11 803. A. Arcadi S. Cacchi V. Carnicelli and F. Marinelli Tetrahedron 1994 50 437. Organometallic Chemistry Scheme 25 Scheme 25 is a typical example. A 2-iodoaniline can combine intermolecularly with an alkyne again affording a 2,3-disubstituted product. A 5-HT receptor agonist has been obtained in this way.’5o With an enamine derived from a 2-iodoaniline an intra- molecular coupling exploiting the carbon-iodine bond provides an alternative.’” Less common routes to indoles start from 2-nitro~tyrenes”~ with a palladium cata- lyst or aryl isonitriles with a ruthenium cataly~t.’’~ These routes to indoles involve a disconnection that forms bonds in the heterocyclic ring. Grigg has used palladium catalysed cyclization/anion capture to build bonds in both carbocyclic and aromatic rings of tetrahydroindoles.The example shown in Scheme 26 combines the Pd(0)cat NaBPh4 PhOMe AC AC 69% Scheme 26 process with a final carbon-carbon bond forming step during removal of the palladium from the reaction intermediate.’ 54 Seven-membered partially saturated heterocyclic rings have also been formed using palladium catalysis. Nucleophilic addition to an q3-7c-allyl intermediate figured in the formation of (66),’” and in the absence of the acetate leaving group stoichiometric palladium chloride ally1 complexes have been used instead (Scheme 27).ls6 Alkenes bound to transition metals are also electrophilic. Palladium catalysed intra- molecular nucleophile addition combined with interception of the a-bound or-ganometallic intermediate with a vinylhalide affords a pyrrolidine.’57 For pyrrole ‘’O C. Chen D. R. Lieberman R. D. Larsen R.A. Reamer T. R. Verhoeven P. J. Reider I. F. Cottrell and P. G. Houghton Tetrahedron Lett. 1994 35 6981. ‘’I K. Koerber-Ple and G. Massiot Synlett 1994 759. 152 M. Akazome T. Kondo and Y. Watanabe J. Org. Chem. 1994 59 3375. lS3 G.C. Hsu W. P. Koser and W. D. Jones Organornetallics 1994 13 385. R. Grigg J. Heterocyclic Chem. 1994 31. 631. M. Grellier M. Pfeffer and G. van Koten Tetrahedron Lett. 1994 35, 2877. 156 P. A. van der Schaaf J.-P. Sutter M. Grellier G. P. M. van Mier A. L. Spek G. van Koten and M. Pfeffer J. Am. Chem. SOC. 1994 116 5134. ’’’ R.C. Larock H. Yang S.M. Weinreb and R.J. Herr J. Org. Chem. 1994 59 4127. G. R. Stephenson OAc I Scheme 27 -Ph Ph Ph Scheme 28 ring formation carbenes are popular starting materials. Reaction with an a,b-un- saturated imine affords (67) under thermal conditions (Scheme 28).' 58 Aminoalkenyl substituted carbenes and acid chlorides also react to form substituted pyrroles.' 59 Alkynyl carbenes and imines form five-membered heterocycles.' 6o With diazo- methane the carbene unit remains in place and cycloaddition occurs at the alkyne again forming a five-membered heterocycle.'61 Aminocarbenes react intermolecularly with alkynes to afford furans with the extra CO unit originating from a carbon monox- ide molecule.'62 Other cyclizations are possible. Allenyl imines can be cyclized by reactions via pentacarbonyliron to form highly unsaturated lactams,' 63 and palladium catalysed carbonylation of 2-bromoacetophenones in the presence of an isocyanide affords alkylidene-substituted lactams.' 64 Palladium coupling also has a role in het- erocyclic synthesis.The iodopyridine (68) combines with a 2-substituted phenyl- boronic acid to afford (69) (Scheme 29).'65 Five-membered rings are formed from 2-aminothiophenol and aryl halides by carbonyl insertion.'66 Oxygen heterocycles are accessible in a similar way. 3,4-Disubstituted coumarins are formed from unsaturated esters of 2-iodophenol phenylacetylene and carbon monoxide.'67 Coupling reactions can be used to functionalize existing heterocycles. For example vinyltin coupling to a chloro-substituted heterocycle allows efficient carbonsarbon T.N. Danks and D. Velo-Rego Tetrahedron Lett. 1994 35,9443. R. Aumann B. Jasper R. Goddard and C. Kriiger Chem. Ber. 1994 127 717. 160 F. Funke M. Duetsch F. Stein M. Noltemeyer A. de Meijere Chem. Ber. 1994 127 911. 16' C. Baldoli P. Del Buttero E. Licandro S. Majorana A. Papagni and A. Zanotti-Gerosa J.Organomet. Chem. 1994,476 C27. 162 C. Bouancheau A. Parlier M. Rudler H. Rudler J. Vaissermann and J.-C Daran Organometallics 1994 13 4108. M.S. Sigman and B.E. Eaton J. Org. Chem. 1994 59 7488. 164 S. Hamaoka M. Kawaguchi and M. Mori Heterocycles 1994 37 167. 16' F. Guillier F. Nivoliers A. Godard F. Marsais and G. Queguiner Tetrahedron Lett. 1994 35 6489. R. J. Perry and B.D. Wilson Organometallics 1994 13 3346. 16' M. Catellani G. P. Chiusoli M. C. Fagnola and G. Solari Tetrahedron Lett. 1994 35 5923. 277 Organometallic Chemistry Scheme 29 bond formation.'68 A similar reaction with an iodoheterocycle in this case protected as its N-benzyl derivative and vinylzinc as the co-reactant has been re~0rted.l~~ 3-Stannylindoles can be elaborated by coupling with vinyl triflates.' 70 Pyrimidinyl trif- lates behave similarly. ' N-Protected iodoimidazoles are dimerized by palladium catalysis to form bisimidazoles. 72 Thienothiophenes can be metallated and coupled with aryl halides173 and bromoindoles can be combined with arylboronic acids.'74 In this regard the cross-coupling procedures that attach aromatic rings are similar to those discussed for vinyl groups earlier in this section.Heck coupling is also possible as shown by the formation of (71) from (70). (Scheme 30).'75 bCN M e \ N b N Pd(OAC)~ HOAc OAN IMe A IMe (70) (711 Scheme 30 Mention of two heterocyclic n-complexes concludes this section. The tricarbonyl- chromium complex of thiophene has been metallated and combined with benzal- dehyde' 76 and the problem of chromium complexation of pyridines originally over- come by silylation on the aromatic ring has now been extended albeit in low yield to 2,6-dimethylpyridine.' 77 L.-L. Gundersen Tetrahedron Lett. 1994 35 3155. 169 L.-L. Gundersen A. K. Bakkestuen A. J. Aasen H. Bverls and F. Rise Tetrahedron 1994 50 9743. 170 P. G. Ciattini E.Morera and G. Ortar Tetrahedron Lett. 1994 15 2405. 17' J. Sandosham and K. Undheim Heterocycles 1994 37 501. 17' M. D. Cliff and S.G. Pyne Synthesis 1994 681. 173 D. Prim and G. Kirsch J. Chem. Soc. Perkin Trans. 1 1994 2603. 174 G. M. Carrera Jr and G. S. Sheppard Synlett 1994 93-94. 175 K. Hirota H. Kuki and Y. Maki Heterocycles 1994 37 563. '16 M. Struharik and S. Toma J. Organomet. Chem. 1994 464,59. 177 A. Goti and M. E. Semmelhack J. Organomet. Chem. 1994 470 C4. G. R. Stephenson Carbon-Carbon Bond Formation during the Formation of q" n-Complexes.-In principle when stoichiometric transition metal complexes are used in organic synthesis multiple use of the transition metal should be the goal. Otherwise a catalytic procedure would be more appropriate.During the preceding sections examples have been seen where carbon<arbon bond formation has occurred during the formation of a n-complex and during its decomplexation reaction. If these steps can be combined with metal-mediated reactions exploiting properties of the intermediate metal complexes then the decision to include a transition metal will no longer involve unavoidable redundant steps which fail to advance the skeletal bond formations needed to complete the target molecule. From this stand-point it can be seen that complexation steps which combine the two objectives of attachment of the metal and skeletal bond formation are of particular strategic importance. Once rare these types of bond forming reactions are now becoming more common.Examples arising in chromium carbene benzannulation reactions have already been mentioned. A stereoselective example has now been deve10ped.I~~ In this case (Scheme 31) an optically pure protected propargyl alcohol (72) serves as the co-reactant and the chiral q6-chromium complex is formed with high diastereoselectivity. The chiral centre in the propargyl unit controls the induction of planar chirality in the n-complex. r OMe (-) 68% Scheme 31 Cobalt-mediated benzannulation reactions can be modified to give access to metal n-complexes. For example the use of an allene as a reaction partner with dicarbonyl- (cyc1opentadieny)cobalt and an x,w-alkyne affords q4-CoCp polycyclic products. In the case of (73) (74) and (75) were formed as a 7 3 mixture of diastereoisomers.A variety of complexed ring systems are accessible (Scheme 32).'79 Vollhardt has also reported' 8o new cyclization reactions of furan- and thiophene-containing alkynes which allow the isolation of q4-cobalt complexes. Two alkynes can be combined by reaction with iron pentacarbonyl to form q4-complexes of cyclobutadienone.' 8' Allenes react with Fe,(CO) to produce bimetallic metal-substituted allyl structures and traces of back-to-back n-allyls bound to Fe2(C0)6.'82 Yields are very low but in the Fe,(CO) case the central carbon-carbon bond between the two allyl portions has been formed in the process providing an unusual route to unusual structures and an interesting comparison with the alkyne series (Scheme 33).Reactions which rearrange one type of n-system to another are also relevant to this 178 R. P. Hsung and W. D. Wulff .I.Am. Chem. Soc. 1994 116 6449. 17' C. Aubert D. Llerena and M. Malacria Tetrahedron Lett. 1994 35 2341. 180 R. Boese D. F. Harvey M. J. Malaska and K. P.C. Vollhardt J. Am. Chem. Soc. 1994 116 11 153. A. J. Pearson and R. J. Shively Jr Organometallics 1994 13 578. lE2 A.M. Kuonen J. Raemy and T. A. Jenny Chimia 1994 48 362. Organometallic Chemistry (73) (74) (75) 7:3 Scheme 32 R RR 1439% 1-9% Scheme 33 issue. For example the carbene complex (76),accessible via iron-mediated opening of vinyl epoxides can be combined with a trimethylsilylenol ether to form an q4-diene complex (Scheme 34).' 83 Another curious example affording an q4-diene complex is provided by the photoloysis of (77) with cycl~hexene.'~~ Molybdenum carbyne complexes carrying cyclopropyl substituents rearrange to form formyl-substituted q4-diene complexes,' 85 and allenyl tungsten complexes undergo carbonyl insertion upon reaction with amines to form l-amide-substituted q3-allyl complexes.' 86 An Mn(CO) acyl complex rearranges to form the Mn(CO) complex of an q3-allyl with lB3 W.Fortsch F. Hampel and R. Schobert Chem. Ber. 1994 127 711. 184 C.-H. Sun N.-C. Shang L.S. Liou and J.-C. Wang J. Organomet. Chem. 1994 481 179. 18' M. D. Mortirner J.D. Carter K. B. Kingsbury K.A. Abboud and L. McElwee-White J. Am. Chem. SOC. 1994 116 8629. 186 T.-W. Tseng I.-Y. Wu J.-H. Tsai Y.-C. Lin D.-J.Chen G.-H. Lee M.-C. Cheng and Y. Wang Organometallics 1994 13 3963. G.R. Stephenson OSiMe3 Med 91% Ph. Ph. \ hv F~(co) Scheme 34 intramolecular coordination of an adjacent amide carbonyl The asymmetric induction methods that open up new routes to optically pure complexes with planar chirality (discussed in section 3) and complex-forming reactions that also form skeletal bonds (discussed above) constitute crucial strategic objectives that will facilitate more efficient applications of stoichiometric metal complexes in organic synthesis. Widely applicable ‘set-piece’ reactions such as the Pauson-Khand and Nicholas reactions the Dotz-Wulff cyclization metal-mediated [2 + 2 + 21 steps and metal-stabilized anions such as the Davies chiral enolate system will have a major role to play in the development of these opportunities in future years.Also of importance are varied and versatile bond-forming methodologies based on particular metal n-complex classes. Indeed the ‘virtuoso capabilities’ of these metal/ligand assemblies may ultimately hold the key to the truly general application of these methods in organic synthesis. The 1994 Report ends with a selection of some of the best examples to appear during the year of the use of metal complexes in organic synthesis and a selection of reactions that will enhance versatility. Tricarbonyliron Chemistry.-The 24-membered polyyne macrolide macrolactin A (78) (Scheme 35) offers a suitable challenge to put the control effects of q4-diene tricarbonyliron complexes to good use.Results from the laboratories of Donaldson at Marquette University and Grte at Rennes concentrate on approaches in which two of the three diene linkages originate as tricarbonyliron complexes. In both sections control of functionality adjacent to the tricarbonyliron group is an issue and in the lower portion a remote chiral centre must also be controlled. Addressing the top (Cl-Cll) segment Donaldson had made progress with a route that commenced with a heterocycloaddition. After ring opening this affords an aldehyde intermediate which is extended by addition of carbons 1-3 by a Wittig-Horner step producing predomi- nantly the required Z isomer (Scheme 36). The lower (ClGC24) portion of the target has also been addressed in this work.Nucleophile addition to aldehyde groups adjacent to tricarbonyliron diene complexes A. AbuBaker C. D. Bryan A. W. Cordes and N. T. Allison Organometallics 1994 13 3375. Organometa11ic Chemistry 28 1 OH I 11 Q 16 Scheme 35 52% Scheme 36 typically lack stereocontrol. In the case shown in Scheme 37 product (80)was formed as a 1 :1 mixture of stereoisomers but hydrolysis of the acetal and formation of the cyclic hemiacetal in (81) proceeded with epimerization at the carbon atom a to the metal complex providing a 3 :1mixture which can be separated by chromatography. Introduction of the methyl group (C24 in the target) completed the lower carbon chain and provided a means to relay chirality to C23.Cyclization to form the ether (82) and ionic reduction next to the tricarbonyliron group complete a sequence of reactions in which the chirality of attachment of the tricarbonyliron group in (79) has been used to control asymmetry at a carbon atom four centres away from the q4 portion of the ligand.' ** In Grke's laboratory work with optically pure metal complexes has also addressed the lower portion of the macrolactin target structure culminating in the preparation of the intermediate (84) with the aldehyde present that would be needed for an aldol approach to the C13-Cl5 sequence of two stereocentres. In this convergent approach an optically pure building block introduces the C23 stereocentre in pre-built form and W.A. Donaldson P.T.Bell Z. Wang and D.W. Bennett Tetrahedron Lett. 1994 35 5829. G.R. Stephenson (79) Scheme 37 ,P h (83) 1) Swap protecting groups 12) Et. SiH. TFA 13) Fthctionalgroup interconversion Scheme 38 Organometallic Chemistry issues of remote asymmetric induction do not arise. Conversion of (83) into (84)again employs an ionic reduction. To complete the reaction sequence shown in Scheme 38 it is necessary to swop protecting groups and effect a two-step functional group interconversion to achieve the required structure.' 89 In a synthesis of an optically pure sample of Streptomyces by-product SS20846A (which proved the absolute configuration of this metabolite) the sorbaldehyde complex (79) (X = Me) (which is also available in optically pure form) was again employed.After formation of the imine hetero-Diels-Alder functionalization was optimized to afford diastereoselective access to (85),for which the relative stereochem- istry was proved by X-ray crystallography. A sequence of two reduction steps afforded the alcohol (86) as a 7 :3 mixture of diastereoisomers. The target (87) was completed by simultaneous deprotection of the nitrogen and detachment of the metal by oxidation with ceric ammonium nitrate at low temperature. The specific rotation of the (-) isomer (87) obtained in this way matched the -15" value for the [aIDvalue for the natural product (Scheme 39).I9O Scheme 39 Because of the predictability of diastereoselective reactions using tricarbonyliron complexes and the fact that optically pure examples of these compounds are now widely available these methods have become attractive for enantioselective synthesis and for the proof of absolute configurations.Recently a detailed circular dichroism study of the metal complexes has been described."' An empirical rule relates the cyclic q4-diene complexes to the CD curves (analysed earlier by other researchers) in the acyclic series. T. Benvegnu L. Schio Y. Le Floc'h and R. Gree Synlett 1994 505. 190 Y. Takernoto S. Ueda J. Takeuchi T. Nakarnoto and C. Iwata Tetrahedron Lett. 1994 35 8821. 19' G.R. Stephenson and P.W. Howard J. Chem. Soc. Perkin Trans I 1994 2873. G.R. Stephenson The lability of alcohol leaving groups adjacent to the metal complexes exploited to gain diastereoselectivity in Scheme 37 can also be employed in carbon-carbon bond formation.Examples which make use of intermediates available through highly effective functionalization of aldehydes using chiral organoboron reagents have been reported from the Roush group.'92 This approach is now being applied the asymmetric synthesis of ikar~gamycin.'~~ In another case a diene complex obtained from a ligand constructed from ( +)-L-arabinose has been elaborated diastereoselectively to form (88).' 94 Opening of ally1 epoxides with iron carbonyl reagents affords ferralactone Scheme 40 intermediates. Diastereoselective functionalization adjacent to the q3-allyl unit in these structures has been examined in the Ley group in Cambridge. The products can be converted by simple treatment with barium hydroxide into the corresponding q4-diene complexes with the asymmetry of the carbon bearing the alcohol group still intact.19' Functionalization of substituents at C2 of a tricarbonyliron-bound diene has also been examined and intermediates obtained in this way can be further cyclized by intramolecular anion addition reactions that proceed via intermediate Fe(C0); c~rnplexes.'~~ Ally1 anion complexes of Fe(CO) have been examined directly in reactions with carbon-based electrophiles.a$-Unsaturated ketones can be obtained in this way by carbonyl insertion. Replacing a metal-bound carbon monoxide by triphenylphosphine gives rise to heterodiene complexes containing a dicarbonylphos- phine-iron group.' 97 Electrophilic $-cyclohexadienyl complexes are also important intermediates in organic synthesis.Precursors for a route to aranorosin have been obtained by intramolecular nucleophile addition affording spirocyclic products. 19' An alternative cyclization procedure extends the oxidative methods for heteroatom addition adjacent to q4-cyclohexadiene tricarbonyliron complexes. The addition of benzyl amines mediated by oxidation using either very active manganese dioxide or the ferrocinium cation has been re~0rted.l~~ New types of nucleophiles are also under examination for the functionalization of q'-dienyl complexes. This is important when weak directing groups are present on the metal-bound portion of the ligand.'" W. R. Roush and C. K. Wada Tetrahedron Lett.1994,35 7347. W. R. Roush and C.K. Wada J. Am. Chem. SOC.,1994 116 2151. 194 E. Hessler H.-G. Schmalz and G. Durner Tetrahedron Lett. 1994 35,4547. 19s S. V. Ley G. Meek K.-H. Metten and C. Pique J. Chem. SOC. Chem. Commun. 1994 1931. 196 J.-L. Wang C.-H. Ueng S.-J. Cheng and M.-C. P. Yeh Organometallics 1994 13 4453. S. Chang J. Yoon and M. Brookhart J. Am. Chem. SOC. 1994 116 1869. 19* H.-J. Knolker G. Baum and M. Kosub Synlett 1994 1012. H.-J. Knolker A.-A. El-Ahl and G. Weingartner Synlett 1994 194. G.R. Stephenson and K. Milne Aust. J. Chem. 1994 47 1605. Organometallic Chemistry Tricarbonylchromium Chemistry .-Synthetic applications of arene complexes bearing the tricarbonylchromium group have been advanced by the preparation of analogues of dihydroheliporin E and dihydropseudopterosin G.201Several aspects of tricar- bonylchromium chemistry have been discussed earlier in this report where methods of asymmetric induction were described.Another important development which has gained momentum this year has been the use of metal-catalysed coupling reactions in the presence of the tricarbonylchromium group particularly in reactions that elaborate functionality attached to the metal-bound arene itself. Typical examples are shown in Scheme 41. Palladium catalysed cross-coupling between complexed aryl 91% triflates and vinyl tin reagents,202 and the use of Suzuki-type coupling combining complexes of aryl bromides with arylboronic a~ids~~~*~~~ and carbonylation of the chromium-bound aryl chloride205 have all been reported.Work on stereocontrolled elaboration of functionality adjacent to tricarbonyl- chromium complexes has also advanced and parallel developments in the tricar- bonyliron series discussed in the preceding section. An asymmetric Baylis-Hillman coupling reaction promoted with DABCO gives diastereoselective access to (89)206 (Scheme 42). Nucleophile addition to an q'-tropylium complex using copper/zinc- Scheme 42 201 H.-G. Schmalz A. Schwarz and G. Diirner Tetrahedron Lett. 1994. 35,6861. 202 A.M. Gilbert and W. D. Wulff J. Am. Chem. Soc. 1994 116 7449. 203 M. Uemura and K. Kamikawa J. Chem. Soc. Chem. Commun. 1994 2697. 204 M. Uemura H. Nishimura K. Kamikawa K. Nakayama and Y. Hayashi Tetrahedron Lett. 1994 35 1909.205 J.-F. Carpentier E. Finet Y. Castanet J. Brocard and A Mortreux Tetrahedron Lett. 1994 35,4995. 206 E. P.Kiindig L. Hii Xu,and B. Schnell Synlett 1994 413. 286 G.R. Stephenson based nucleophiles to introduce a functionalized side-chain prepares the way for a further example of intramolecular nucleophile addition this time to an q6 cyclohepta-triene ligand.207 Nucleophile addition to neutral q6-arene complexes affords q5-anions that can be trapped with electrophiles. Carbonyl insertion can also be promoted in this way. Other Metal n-Complexes as Synthetic Intermediates..-Although tricarbonyliron and tricarbonylchromium complexes are now the most widely used organometallocar- bony1 n-complexes quite a wide variety of other metal/ligand systems are popular.Electrophilic tricarbonylmanganese arene complexes have been combined with nuc- leophiles stabilized by metal ~arbenes.~'~ Phosphonate reagents have been obtained by reaction with trimethylphosphite,2 lo shadowing access to phosphonium salts in the tricarbonylchromium series through reactions of the metal-bound tropylium ca-tion.21 A more unusual study of cycloaddition reactions to neutral q5-tricarbonyl- manganese complexes has been reported.212 Also popular for the stabilization of q6-cation complexes is the FeCp unit. This has been employed to promote the displacement of aryl chlorides by hydroquinone in a route to polyaromatic ethers.21 Selective arylation of diols can be performed using FeCp-bound arylpropyl ethers to transfer the aryl group,214 and 1,6dichlorobenzene can be selectively elaborated by displacement of one halide with a phenoxide n~cleophile.~'' Substituent effects in q6-arene complexes of FeCp have been examined.2 l6 Ruthenium analogues employing the pentamethylcyclopentadienyl ligand (Cp*) have received attention.21 Access to cationic complexes of this type by direct complexation of enones combined with deoxygenation has been reported.218 The chemistry of cyclopentadienone ligands bound to RuBrCp* has been examined,219 and unusual cationic alkyl nitrosyl ruthenium complexes of Cp* can be elaborated by direct reaction with a$-unsaturated esters.220 An unusual ring-opening reaction in cationic (Cp*)Co complexes gives rise to ring-opened products of type (90)via an intermediate with an agostic hydrogen.221 The Fe(CO),Cp metal/ligand system retains popularity with work examining [3 + 21 ~y~loadditions~~'~~'~ and electrophilic ring-closure reactions224 being reported this year.The molybdenum counterparts have been utilizedz2' in enolates that are '07 M.-C. P. Yeh and C.-N. Chuang J. Chem. SOC. Chem. Commun. 1994 703. '08 E. P. Kiindig A. Ripa R. Liu and G. Bernardinelli J. Org. Chem. 1994 59,4773. '09 F. Rose-Munch C. Susanne F. Balssa and E. Rose J. Organomet. Chem. 1994 476 C25. 'lo T. Lee H.B. Yu Y.K. Chung W.A. Hallows and D.A. Sweigart Inorg. Chim. Acta 1994 224 147. 'I1 D.A. Brown J. Burns W.K. Glass D. Cunningham T. Higgins P. McArdle and M.M. Salama Organometallics 1994 13 2662. 212 C. Wang J.B. Sheridan H.-J. Chung M. C. Cote R. A. Lalancette and A. L. Rheingold 1. Am. Chem. SOC. 1994 116 8966. 'I3 A. S. Abd-El-Aziz D. C. Schriemer and C. R. de Denus Organometallics 1994 13 374. 214 A. J. Pearson and A. M. Gelormini J. Org. Chem. 1994 59 281. 215 A. J. Pearson and A.M. Gelormini J. Org. Chem. 1994 59 4561. '16 P. G. Gassman and P. A. Deck Organometallics 1994 13 2890. '17 D.T. Glatzhofer Y. Liang G.P. Funkhouser and M.A. Khan Organometallics 1994 13 315. '18 R Carreno F. Urbanos F. Dahan and B. Chaudret New J. Chem. 1994 18 449. '19 K. Kirchner K. Mereiter K. Mauthner and R. Schmid Organometallics 1994 13 3405. 220 E. Hauptman M. Brookhart P. J. Fagan and J. C. Calabrese Organometallics 1994 13 774. 221 J.C. Nicholls and J.L. Spencer Organometallics 1994 35 7889."'S. Jiang and E. Turos Tetrahedron Lett. 1994 35 7889. 223 T. Chen S. Jiang and E. Turos Tetrahedron Lett. 1994 35 8325. 224 D. P. Dawson W. Yongskulrote J. M. Bramlett J. B. Wright B. Durham and N. T. Allison Organometal-lics 1994 13 3873. 225 M.-F Liao G.-H. Lee S.-M. Peng and R.-S. Liu Organometallics 1994 13 4973. Organometallic Chemistry 287 reminiscent of the Davies enolate system in stereoselective reactions that form 4-pentene-1,3-diols. Cyclopentadienone chemistry exploits a cationic q4-complex,226 and (in an acyclic series) cationic q4-complexes of isoprene have been selectively elaborated with alkyllithium and Grignard reagents.227Stereocontrolled functionaliz-ation of enolates in q2-complexes of cyclopentenone has been examined.228 Or-ganocopper reagents have been added to enones adjacent to neutral ally1 CpMo(CO) complexes and related conjugate addition reactions to extended n-systems have been shown to give moderate diastereosele~tivity.~~~ In the tungsten carbonyl series nucleophiles have been reacted with cationic q4-2-methoxycarbonyl-1,3-pentadiene complexes.230 Me Me ($+= d (90) Me S02Ph BnO HPF e I ie+(co) 1 Nu-MeY+@So2Ph Nu Scheme 43 Fe(CO) complexes are more typical for stoichiometric attachment to alkenes.A bis Fe(CO) complex of a divinylketone has been described.231 Optically pure q2-complexes carrying an allylic leaving group can be converted into cationic q3 226 R. H. Yu J. S. McCallum and L.S. Liebeskind Organornetallics 1994 13 1476. 227 A. J. Pearson and M. K. Babu Organornetallics 1994 13 2539. 228 D. Schinzer T. Blume and R.U. R. Wahl Synlett 1994 297. 229 S.-H. Lin S.-F. Lush W.-J. Cheng G.-H. Lee S.-M. Peng Y.-L. Liao S.-L. Wang and R.-S. Liu Organornetallics 1994 13 1711. 230 M.-H. Cheng Y.-H. Ho Chen,G.-H. Lee S.-M.Peng S.-Y Chu,and R.-S. Liu Organornetallics 1994,13 4082. L31 A. C. C. Cano N. Zufiiga-Villarreal,C. T. Alvarez R. A. Toscano M. Cervantes A. Daz and H. Rudler JZ3’ A. J. Pearson and R.J. Shively Jr Organornetallics 1994 13 578. G.R. Stephenson intermediates and further elaborated by regio- and stereoselective nucleophile addi- The product (91) was obtained in >96% enantiomeric excess in this way (Scheme43).Rhenium nitrosyl complexes have been extensively studied in the Gladysz group at Utah.233*234 232 A.M. Kuonen J. Raemy and T.A. Jenny Chimia 1994,48 362. 233 W. Fortsch F. Hampel and R. Schobert Chern. Ber. 1994 127 711. 234 C.-H. Sun N.-C. Shang L.S. Liou and J.-C. Wang J. Organomet. Chem. 1994 481 179.