Simultaneous and stereoselective construction of planar and axial centers of chirality.

Atropisomerism resulting from hindered rotation about single bonds can lead to molecules with centers of axial chirality and are widely used in asymmetric catalytic synthesis.1 Molecules containing planar centers of chirality have also been used as chiral ligands, particularly those containing η6-benzene and η5-cyclopentadienyl complexes.2 To our knowledge, there are no known synthetic methods that produce both types of chiral centers in a single step. The reaction of Fischer carbene complexes with aryl alkynes has the potential for the synthesis of molecules that contained both planar and axial centers of chirality (Scheme 1). We report here the first examples of the simultaneous synthesis of centers of planar and axial chirality and furthermore that this can be accomplished with high levels of relative diastereoselection. Aside from the issue of the stereoselectivity of the benzannulation of carbene complexes with aryl alkynes of the type 2, we were concerned about the facility of the reaction in general since the benzannulation of hindered alkynes is known to lead to the formation of several different types of side products including indenes and cyclobutanones.3 We were thus delighted to see that the reaction of complex 1a with alkyne 2a gives good yields of the arene chromium tricarbonyl complexed product after trapping of the phenol function with tert-butyldimethylsilyl chloride (Table 1, entries 1 and 2).4 It was quite remarkable to find that either diastereomer 3a or 4a could be obtained selectively, depending on the reaction conditions. If tert-butyldimethylsilyl chloride and Hunig’s base are added at the beginning of the reaction (Method A, one pot), a 89:11 mixture of isomers is obtained. On the other hand, if the silylation is performed in a separate step after the benzannulation reaction is complete (Method B, seq), then a 97:3 selectivity is obtained for the other diastereomer. Similar results are seen for the reaction of complex 1a with the alkynes 2b, 2c, and 2d as indicated by the data in Table 1. For the alkynes 2c and 2d bearing larger groups in the ortho position, the temperature needs to be increased 120 °C to effect complete conversion to the thermodynamic products 4c and 4d (entries 6 and 8). To explain the different stereochemical outcomes of this reaction, it was reasoned that isomerization about the axial center may be occurring prior to protection of the phenol with the tertbutyldimethylsilyl group. Clearly the hydroxyl group would present less of a steric encumbrance to rotation about the bond than a tert-butyldimethylsilyloxy group. Thus, under the conditions of Method B, the phenol function remains unprotected until the completion of the reaction. This provides time for isomerization to the more stable thermodynamic product which is expected to be the phenol chromium tricarbonyl complex 6 with the ortho substituent of the alkyne anti to the chromium tricarbonyl group to minimize steric interactions. If this were true, the kinetic product must be the phenol complex 5 with the ortho substituent of the alkyne syn to the metal center. Only the reaction of the trans-tert-butylvinyl complex 1d gives a stable phenol chromium tricarbonyl complex. These reactions were performed at 50 °C in the absence of a trapping agent to give a 65:35 mixture in favor of 5n, but at 120 °C exclusive formation of 6n is observed. That the thermodynamic product is the anti product 4 was confirmed in the reaction of the cyclohexenyl carbene complex 1c with alkyne 2a. In this case, the same isomer predominated under both the one-pot and the sequential conditions. This stereoisomer was formed in a 21:79 ratio under the one-pot conditions at 50 °C (entry 11). Although this mixture of compounds was slow to isomerize at 50 °C under the conditions of Method B, it did completely undergo conversion to a single atropisomer () 1:99) under Method B when the reaction time was extended to 48 h for the first step (entry 13 vs 12). Alternatively, complete conversion to 4f could be effected by raising the temperature to 80 °C (entry 15). The stereochemistry of the thermodynamic product from this reaction was determined to be the anti isomer 4f by X-ray diffraction analysis on a single crystal. The assignment of the thermodynamic isomers obtained from the reaction of the cyclohexenyl complex 1c with alkynes 2c, 2e, 2d, and 2k were assigned as the anti isomers 4 in analogy with 4f and by 1H NMR correlation with 4f. For all of these alkynes, the reaction products could be completely isomerized to the anti isomer by performing the reaction at 120 °C except for the smaller ortho methoxyl complex which only required 50 °C for complete conversion. This includes the very hindered tertbutyl complex 3i which was formed as a slightly preferred kinetic product having the syn stereochemistry which was determined by X-ray diffraction. The reactions of the alkynes 2g and 2h with complex 1c gave a single diastereomer with the one-pot conditions of Method A at 50 °C, and thus it was not possible to determine if the observed isomer is also the thermodynamic isomer. Therefore, the assignment of the stereochemistry of 4l and 4m was determined by X-ray diffraction analysis of a single crystal. While high selectivities could be obtained for the thermodynamic anti product from the reactions of all of the carbene (1) Stanforth, S. P. Tetrahedron, 1998, 54, 263. (b) Bringmann, G.; Walter, R.; Weirich, R. Angew. Chem., Int. Ed. Engl., 1990, 29, 977. (c) Bringmann, G.; Breuning, M.; Tasler, S. Synthesis, 1999, 4, 525. (d) Durairaj, K. Curr. Sci. 1994, 66, 833. (e) Bringmann, G.; Walter, R.; Weirich, R. In Methods of Organic Chemistry (Houben Weyl), 4th ed.; Helmchen, G., Hoffmann, R. W., Mulzer, J., Schaumann, E., Eds.; Thieme, Stuttgart, 1995; Vol. E21a, p 567. (2) Hegedus, L. S. Transition Metals in the Synthesis of Complex Organic Molecules; University Science Books: Mill Valley, CA, 1994; Chapter 10 and references therein. (b) Uemura, M. I. In Stereochemistry of Organometallic and Inorganic Compounds; Zanello, P.; Ed., Elsevier: Amsterdam, 1994; Vol. 5, p 507. (c) Togni, A., Hayashi, T., Eds. Ferrocenes, Homogeneous Catalysis, Organic Synthesis; Material Science, VCH: Weinheim, 1995. (d) Fu, G. C. Acc. Chem. Res. 2000, 33, 412. (3) For recent reviews on the chemistry of carbene complexes, see: (a) Wulff, W. D. In ComprehensiVe Organometallic Chemistry II, Abel, E. W., Stone, R. G. A., Wilkinson, G., Eds.; Pergamon Press: Elmsford, NY, 1995; Vol. 12, p 469. (b) Bernasconi, C. F. Chem. Soc. ReV. 1997, 26, 299. (c) Hegedus, L. S. Tetrahedron 1997, 53, 4105. (d) Wulff, W. D. Organometallics 1998, 17, 3116. (e) Dötz, K. H.; Tomuschatt, P., Chem. Soc. ReV. 1999, 28 187. (f) Herndon, J. W. Coord. Chem. ReV. 1999, 181, 177. (g) Dörwald, F. Z. Metal Carbenes in Organic Synthesis; Wiley-VCH: Weinheim, New York, 1999. (4) Chamberlin, S.; Wulff, W. D.; Bax, B. Tetrahedron 1993, 49, 5531. Scheme 1 5580 J. Am. Chem. Soc. 2001, 123, 5580-5581