Cyclohexadienones of the type 42 are produced from the reactions of unsaturated carbene complexes and alkynes when the beta-carbon of the carbene complex is disubstituted (Scheme V) [1].  This is presumed to be the result of the fact that if substituents R¹ and R² in intermediate 16 (see also Scheme II) are both non-hydrogen, then tautomerization to the aromatized phenol is not possible and as a result the cyclohexadienone 42 is isolated after the metal is lost.  This reaction is quite general for a variety of alkenyl complexes as is illustrated by the reaction of complex 43 with alkyne 44 [2]. This process can also be extended to indole carbene complexes [3]. The reaction of complex 47 with 3-hexyne gives the cyclohexadienone annulated product 48 in 95 % yield.  In this reaction, part of the aromaticity of the indole ring is lost.  The aromaticity can be restored by a 1,5-sigmatropic shift in the cyclohexadienone.  This is illustrated in the reaction of complex 49 with a protected 5-amino-1-pentyne to give the cyclohexadienone 50.  This compound can be isolated, or if heated to 140 ^oC it will undergo a rearrangement to the isomeric cyclohexadienone 51 in 61 % overall yield.  This combination of cyclohexadienone annulation and 1,5-sigmatropic shift allows for an efficient entry to the aspidospermadine alkaloids [4].

Asymmetric induction in the cyclohexadienone annulation can occur in several different modes (Scheme VI). The cyclohexadienone annulation produces a new chiral center at carbon 6 in the cyclohexadienone. Therefore, there can be a central-to-central chirality transfer from a center of chirality that is present in either the alkyne, the carbene carbon substituent or in the heteroatom stabilizing substituent on the carbene complex. We have reported a number of examples of chirality transfer from a chiral center in the carbene ligand [5].  As illustrated by the reaction of complex 55, there can be good asymmetric induction from a chiral center present on the carbene carbon substituent.  In this case, there is a 92 : 8 selectivity for the trans-decalindieone 56 with trimethylsilyl acetylene and similar selectivities were noted with other alkynes. A reduced selectivity is observed if the methyl group is in the 3-position of the cyclohexene ring. 

The only examples of asymmetric induction from chiral heteroatom substituted carbene complexes involve chiral imidazolidinone complexes [6]. Complex 58 will react with 1-pentyne to give the cyclohexadienone 59 with greater than 98 : 2 selectivity.  Imidazolidinone complexes of the type 58 exist as isolable atropisomers.   The atropisomer of 58 reacts with 1-pentyne to give the epimer of 59 also with a 98 : 2 selectivity.   As in the benzannulation reaction, the asymmetric cyclohexadienone annulation occurs with high inductions with chiral propargylic ethers [7]. The reaction of the E-carbene complex 60 with alkyne 40 gives a 92 : 8 selectivity for the cyclohexadienone 61 in 73 % yield. This reaction is also stereospecific since reaction of alkyne 40 with the Z-isomer of carbene complex 60 gives the diastereomer 62 as the major product with similar stereoselectivity ( 91 : 9).  This reaction provides for a unique method for 1,4-asymmetric induction in the construction of highly functionalized cyclohexadienone and its utilization in total synthesis is currently ongoing in our research group.

[1] Tang, P. C.; Wulff, W. D., J. Am. Chem. Soc., 1984, 106, 1132.

[2] Gilbertson, S. R.; Wulff, W. D., Synlett, 1989, 47.

[3] Bauta, W. E.; Wulff, W. D.; Pavkovic, S. F.; Zaluzec, E. J., J. Org. Chem., 1989, 54, 3249.

[4] Quinn, J. F.; Bos, M. E.; Wulff, W. D., Org. Lett., 1999, 1, 161.

[5] Hsung, R. P.; Wulff, W. D.; Challener, C. A., Synthesis, 1996, 773.

[6] Quinn, J. F.; Powers, T. S.; Wulff, W. D.; Yap, G. P. A.; Rheingold, A. L., Organometallics, 1997, 16, 4945.

[7] Hsung, R. P.; Quinn, J. F.; Weisenberg, B. A.; Wulff, W. D.; Yap, G. P. A.; Rheingold, A. L., J. Chem. Soc., Chem. Commun., 1997, 615.