Comprehensive Organometallic Chemistry 3ed Vol 07 Grp 9 by D M P Mingos; Robert H Crabtree

By D M P Mingos; Robert H Crabtree

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This reaction is particularly facile for multichlorinated vinylcobalamins, but not for non-chlorinated vinylcobalamins. 284–288 In order to obtain information on the mechanism involved in these processes, the synthesis of chloroethenylcobaloxime has been performed. The products have been made via a reduction by NaBH4 or by H2 of in situ-generated CoII(dmgH)2py in the presence of TCE or PCE. The isolation of chlorinated vinylcobaloximes has been achieved in low yields as a mixture of products. These complexes were stable in the presence of ethanolic NaBH4 medium unless external cobaloximes were added.

Rubin, Y. J. Am. Chem. Soc. 1996, 118, 3775. ª 1996 American Chemical Society. R OC SO2 Co SO2 R = H, Me, SiMe3, CO2CH3, Me4Et, (n-allyl)5 220 Figure 50 53 54 Cobalt Organometallics Another important intermediate in the trimerization of alkynes that was never isolated is the cobaltacyclopentadiene unit to which an alkyne is 2- bound to Co. 336 succeeded in obtaining a series of such compounds 221 by treating CpCo(C2H4)2 with a triyne as depicted in Equation (37). Cp CpCo(C2H4)2 Co R R R R ð37Þ R = H, i -Pr Cp = C5H5, C5Me5, C5H4CH3, indenyl 221 Rather good yields (72–91%) of 221 were obtained, and their relevance to the mechanism of the CpCo-catalyzed cyclization of the triyne to the angular [3]-phenylene was established by allowing 221 to rearrange thermally to 222 (see Scheme 36).

High regio- and stereoselectivity has been observed to produce air stable cobalt-substituted cyclohexenes in good yields. It has been rationalized that the exo-diastereoselectivity observed was based on steric interactions between the dienophile and the cobaloxime ligand set. The demetallation reaction has led to an organic product as well as to a cobalt complex, which could be recycled into the synthesis of the starting dienyl complexes. 267 The ease of synthesis of diene–cobalt complexes depends on the position of the cobalt on the dienyl fragment (position 1 or 2) and it also depends on the substituent on the carbon atoms (1–3).

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