Chromium-vinyl ketene

Intramolecular insertion of carbon monoxide into the metal-carbene bond of the (Ej-isomer of D leads to the t/4-vinyl ketene complex intermediate E. Experimental support for this type of intermediate has been provided by the isolation of Cr( CO) 3-coordinated dienyl ketenes related to 5 (Scheme 4) [15a], and by trapping the vinyl ketene intermediates as vinyl lactone derivatives in the course of the reaction of chromium carbene complexes with 1-alkynols [15b]. 

C-3 of the naphthalene nucleus that controls the electrophilic ring-closure of the vinyl ketene intermediate moreover, the degree of aromaticity of the angular rings in the phenanthrene skeleton exceeds that in the anthracene analogues. This regioselectivity observed in the benzannulation of chromium carbenes is paralleled by results observed for 2-naphthyl cyclo-butenones [77] and for the palladium-catalyzed cyclocarbonylation of 2-naphthyl allyl acetates [78]. 

To date, reports of linear benzannulation are very limited [55b, 79]. One example referred to the annulation of a 2-naphthohydroquinoid chromium carbene, which was employed in the synthesis of anthracycline (see Section 8.6.2) [79]. Later, a complementary approach in this area started from 2-naphthylcyclopropenes which, in the presence of hexacarbonyl-molybdenum, served as precursors for the vinyl ketenes required for linear annulation [55b],... 

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