Many reactions of Fischer carbene complexes occur with the retention of the chromium-carbon double-bond in the product.  Most reactions in this category involve a reaction of the carbon substituent of the carbene carbon.  One such process is the aldol reaction of alkyl carbene complexes as illustrated in Scheme VIII.  Most of the reactions in this category have counterparts in the

chemistry of esters. The aldol reaction of esters is made possible by the acidity of protons alpha to the ester carbonyl which in the case of methyl acetate is approximately pKa = 25. The acidity of the methyl carbene complex 76 is actually many orders of magnitude greater than an ester. The acidity of 76 was found by Bernasconi to be pKa = 12 and is similar to a methylene hydrogen stabilized by two carbonyl groups, i.e., a malonate [1].[One of the reasons for the high acidity of this complex is undoubtedly due to the resonance delocalization of the anion over many resonance structures including localization on the chromium 76b and over the five CO ligands (only one shown, 76c). The aldol reaction of Fischer carbene complexes was first examined by Casey and he found that, as expected for such a stable anion, the addition of 76a to a carbonyl compound was unfavorable (76a to 79) [2].iThe only examples where addition products could be observed were when the elimination occurred to give unsaturated complexes of the type 80. This could drive the unfavorable equilibrium between 76a and 79 such that reasonable yields of 80 could be realized. The formation of unsaturated complexes by this method was limited to aryl aldehydes and non-enolizable aldehydes. We were able  to isolate aldol addition products from these reactions for the first time if 

Lewis acids were used to activate the carbonyl compound towards addition of the enolate of the carbene complex. The best Lewis acid for the addition to ketones is boron trifluoride – etherate [3][iand the best Lewis acid for the addition to aldehydes is tin tetrachloride [4].iOne of the important applications of this aldol reaction is in the preparation of unsaturated carbene complexes which are starting materials for the benzannulation and cyclohexadienone annulation reactions (vide supra) and for the Diels-Alder reaction of carbene complexes (vide infra).  As an example, the reaction of the complex 82, derived from cyclopentenone, with trimethylsilylacetylene gives the spirocyclohexadienone 83 in 71 % yield [3]. Asymmetric aldol reactions are also possible with chiral imidazolidinone complexes of the type 86 [5].[The anion derived from 86 is much more reactive and its addition to carbonyls occurs without the need for Lewis acid activation. The aldol addition of 86 occurs to a variety of aldehydes with greater than 95 : 5 facial selectivity on the aldehyde. Workup of the reaction includes an oxidative removal of the metal to give the aldol adduct 87 from which can be liberated optically pure b-hydroxy acids of the type 88. It should be pointed out that the Evans oxazolidinone 89 does not give high selectivity in aldol reactions. The Evans aldol reaction is limited alpha-substituted enolate equivalents.