Transition metals metallocarbene formation

Transition metal-catalyzed carbenoid transfer reactions, such as alkene cyclopro-panation, C-H insertion, X-H insertion (X = heteroatom), ylide formation, and cycloaddition, are powerful methods for the construction of C-C and C-heteroatom bonds [1-6]. In contrast to a free carbene, metallocarbene-mediated reactions often proceed stereo- and regioselectively under mild conditions with tolerance to a wide range of functionalities. The reactivity and selectivity of metallocarbenes can be... 

Metallocarbenes derived from diazoacetates and the appropriate transition metal are generally thought to be electrophilic in nature that is, they are susceptible to nucleophilic addition. This reactivity manifold has been exploited with the formation of ylide species on reaction with a number of nucleophiles providing intermediates that can undergo either rearrangements (e.g., 41 > 42, Scheme 8.8)25 or cycloadditions (e.g., 43 > 44, Scheme 8.8)26 depending on the type of ylide formed... 

Initially, the most obvious mechanism for olefin metathesis was a land of molecular square dance in which two different olefin molecules join to form a cyclobutane ring and then change partners to form two new olefin molecules. While this thermal reaction is Woodward-Hoffmann forbidden, transition metals were initially perceived to allow violations of these rules. However, no cyclobutanes were detected in olefin metathesis reactions, nor did cyclobutanes produce olefins when placed into metathesis reaction mixtures. The breakthrough came in 1971 when Yves Chauvin (1930- ), at the French Petroleum Institute, made the concepmal link between the Phillips Petroleum reaction discovered in 1964 and metallocarbenes isolated in the same year by Ernest Otto Fischer (see chapter 7). Other important discoveries were made by Michael F. Lappert (1928- ) at Sussex, Charles P. Casey (1942- ) at Wisconsin, and especially Thomas J. Katz (1936- ) at Columbia. The mechanism involves formation of a metallocyclobutane (see the accompanying figure), from reaction of a metallocarbene ( M=CR 2 ) with an olefin (R2C=CR2), that splits into a new olefin (R 2C=CR2) and a new metallocarbene ( M=CR2 ). 

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