Metal-diazo-compound interaction

The metal-diazo compound interaction. Several modes are conceivable for coordination of a diazoalkane molecule to a metal atom [9]. Coordination modes x]2 (N,N) X, g3 XII, and g2 (C,N) XIV demand metal complexes possessing two available adjacent coordination sites whereas for coordination modes nKN) XI and nKC) XIII one coordination site per metal atom is sufficient. 

A) interaction of the diazo compound with the catalyst leading to a metal-diazo compound complex ... 

Interaction of an electrophilic carbene or carbenoid with R—S—R compounds often results in the formation of sulfonium ylides. If the carbene substituents are suited to effectively stabilize a negative charge, these ylides are likely to be isolable otherwiese, their intermediary occurence may become evident from products of further transformation. Ando 152 b) has given an informative review on sulfonium ylide chemistry, including their formation by photochemical or copper-catalyzed decomposition of diazocarbonyl compounds. More recent examples, including the generation and reactions of ylides obtained by metal-catalyzed decomposition of diazo compounds in the presence of thiophenes (Sect. 4.2), allyl sulfides and allyl dithioketals (Sect. 2.3.4) have already been presented. 

The interaction between catalyst and diazo compound may be initialized by electrophilic attack of the catalyst metal at the diazo carbon, with simultaneous or subsequent loss of N2, whereupon a metal-carbene complex (415) or the product of carbene insertion into a metal/ligand bond (416) or its ionic equivalent (417) are formed. This is outlined in a simplified manner in Scheme 43, which does not speculate on the kinetics of such a sequence, nor on the possible interconversion of 415 and 416/417 or the primarily formed Lewis acid — Lewis base adducts. 

Tandem carbonyl ylide generation from the reaction of metallo carbenoids with carbonyl continues to be of great interest both mechanistically and synthetically. Effective carbonyl ylide formation in transition metal catalyzed reactions of diazo compounds depends on the catalyst, the diazo species, the nature of the interacting carbonyl group and competition with other processes. The many structurally diverse and highly successful examples of tetrahydrofuran formation cited in this mini-review clearly indicate that the tandem cyclization/cycloaddition cascade of metallo carbenoids has evolved as an important strategy in both carbo- and heterocyclic synthesis. 

Sulfur ylides are useful reagents in organic synthesis. The ylide is formally a zwitterion in which a carbanion is stabilized by interaction with an adjacent sulfonium centre. They are usually prepared by proton abstraction from a sulfonium salt with a suitable base or by reaction of a sulfide with an alkylating agent such as Me30+BF4 or a carbene formed, for example, by metal-catalysed or photolytic decomposition of a diazo compound (1.103). 

Considering metal earbenoid-indueed C—H bond insertions, there exists a general pattern, as shown in Seheme 1.1. The catalytic cycle is initiated by a metal complex via the deeomposition of diverse carbene precursors (such as diazo compounds) to deliver a transient metal earbenoid intermediate in situ. Subsequently, the highly reactive metal earbenoid intermediate inserts into the C—H bond to afford the corresponding product and readily regenerates the metal complex to complete the catalytie eycle. Note that the metal atom is not thought to interact with the C—H bond directly. Moreover, since the transient metal earbenoid intermediate is highly reactive, the reaction conditions are typically mild and pH neutral, which renders this method... 

It is generally accepted that a typical carbonyl yhde reaction proceeds as shown in Fig. 2. Interaction of diazo compound 1 with the metal forms diazonium complex 2, which then extrudes nitrogen forming carbenoid intermediate 3. Reaction of 3 with the carbonyl group present in the substrate forms intramolecular carbonyl yhde 4 (or an intermolecular carbonyl yhde) in which the metal catalyst may or may not remain associated with the ylide [13]. Finally, the [3+2]-cycloaddition and regeneration of the active cat-... 

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