Allylic anions boron-substituted

Anions formed from group 6 and manganese Fischer carbene complexes undergo aldol condensations with aldehydes and ketones. Allylic carbenes exclusively react in the y position with aldehydes affording dienyl-substituted carbenes. For alkoxy-substituted carbenes, the presence of an excess Lewis acid see Lewis Acids Bases), such as boron trifluoride etherate, titanium tetrachloride, or tin tetrachloride is required for the reaction to proceed in reasonable yield. The initial aldol product can be isolated without elimination (Scheme 12). ... 

Addition of triethylaluminum or triethylborane to (61f) produces the ate complexes, which react with aldehydes at the a-position. In contrast to the ate complex of the oxygen-substituted anion, the ate complex of (61f) produces low diastereoselectivity. Reaction of the boron ate complex with y,y-dimethylal-lyl chloride and bromide occurs at the a-position with inversion of the allyl unit. 

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