Silver carbenoids, reactions

A review of silver carbenoid reactions including the Wolff rearrangement, carbene-transfer reactions, aziridination, cyclopropanation, formation and reactions of... 

A review on the Simmons-Smith cyclopropanation has appeared. The involvement of silver carbenoids in various reactions (including Wolff rearrangement, car-benoid additions, and insertions) has been discussed. Benzannulation carried out by addition of a Fischer carbene to an alkyne (Doetz reaction) has been presented in an historical perspective featuring mechanistic details and synthetic applications. ... 

So far, the reports on copper and silver scorpionate catalysis are limited to ethyl diazoacetate as the carbenoid precursor, and it is questionable whether these catalysts can be used with other classes of diazo compounds. The reaction of the more stable methyl diazomalonate resulted in the formation of a remarkable O-bound diazo complex, which was thermally stable (Equation (9)).76... 

The use of copper as a catalyst in carbenoid transfer has its roots in the Amdt-Eistert reaction, Eq. 1 (3). Although the original 1935 paper describes the Wolff rearrangement of a-diazo ketones to homologous carboxylic acids using silver, the authors mention that copper may be substituted in this reaction. In 1952, Yates (4) demonstrated that copper bronze induces insertion of diazo compounds into the X-H bond of alcohols, amines, and phenols without rearrangement, Eq. 2. Yates proposal of a distinct metal carbenoid intermediate formed the basis of the currently accepted mechanistic construct for the cyclopropanation reaction using diazo compounds. 

In addition to participating in [2 + l]-cycloaddition reactions, divalent reactive intermediates can form ylides in the presence of carbonyl or other Lewis basic functionalities.108 These ylides participate in cycloaddition or other pericyclic reactions to furnish products with dramatically increased complexity. While carbenes (or metal carbenoids) are well known to participate in these pericyclic reactions, silylenes, in contrast, were reported to react with aldehydes or ketones to form cyclic siloxanes109,110 or enoxysilanes.111,112 Reaction of silylene with an a,p-unsaturated ester was known to produce an oxasilacyclopentene.109,113,114 By forming a silver silylenoid reactive intermediate, Woerpel and coworkers enabled involvement of divalent silylenes in pericyclic reactions involving silacarbonyl ylides115 to afford synthetically useful products.82,116,117... 

The oxymercuration of 1-substituted (i,e, H, Me, and C02Me) tricyclo[4,l,0,0 ]-heptanes with mercuric acetate affords norcaranyi- and norpinyl-mercury compounds. A synthesis of 3,4-benzotricyclo[4,l,0,0 ]heptene (692) has been reported in which the usual cyclopropylcarbene C—H insertion process is employed. Isomerization of (692) with silver perchlorate gave benzocycloheptatriene which is also formed in the thermal isomerization of (692). Reaction of (692) with n-ally 1 palladium(ii) chloride dimer yielded 2-methylene-l T-naphthalene which rearranged readily to 2-methylnaphthalene at room temperature a carbenoid mechanism appears to be involved. 

There are several experimental methods for effecting the Wolff rearrangement. The classical technique for preparation of acids or esters involves thermal decomposition in the presence of silver oxide in an aqueous solution (for acids) or an alcohol (for esters). At least for unsaturated diazoketones, the silver oxide seems to direct the reaction toward the rearrangement pathway in preference to carbenoid addition.Photolysis in nonnucleophilic solvents generates the ketene, which can undergo subsequent reactions in situ or, if sufficiently stable, be isolated. 

---