Carbenes, generation reaction with diazoalkanes

This problem has been overcome by incorporating 8-phenylmenthol as a chiral auxiliary into a,/3-unsaturated carbene complexes their reaction with diazoalkanes results in the formation of single pyrazoline diastereomers [37]. This concept has been successfully extended to other types of 1,3-dipoles and their reaction with chiral carbene complexes. The one-pot reaction of the chromium ( )-menthyloxy-carbene 40 with nitrilimine 41 generated in situ affords a single diastereomer of -pyrazoline ester 42 after oxidative work-up neither regioisomer 43 nor its diastereomer could be detected (Scheme 11.16) [38]. 

The reaction of diazoalkanes with acetylenes can give rise to cyclopropenes by two main routes. Some reactions involve an initial loss of nitrogen to generate a carbene which then adds to the acetylene (see Section 1.2.1.), but this section is concerned only with those reactions where the first step is a cycloaddition leading to formation of a 3//-pyrazole. Unlike the parallel series of reactions in the cyclopropane series, where the C-C double bond of the alkene requires activation by a suitable substituent or by strain. Under pressure even acetylene itself will react with diazoalkanes. For example, diphenyldiazomethane underwent addition in good yield and deazetization gave 3,3-diphenylcyclopropene (1). ... 

As with other diazoalkanes, diazomethane reacts with alkenes to form cyclopropane derivatives (sec. 13.9.C.i).272 Reaction with aromatic derivatives leads to ring expansion to cycloheptatriene derivatives.223 Both of these reactions (addition to an alkene or arene insertion) involve generation of an intermediate carbene and addition to a jt bond they will be discussed below. Many of the reactions of diazomethane tend to be ionic in nature and are, therefore, set aside from the other diazoalkane chemistry in this section. One of the commonest uses of diazomethane itself is esterification of small quantities of acids, especially acids that are precious for one reason or another. The reaction is quantitative and gives good yields of a single product, as in Tadano s conversion of 338 to the methyl ester of 339224 in a synthesis of (-)-verrucarol. 

Further data have been reported on the reactions of cobalt(m) porphyrins with diazoalkanes as a route to vinyl- or halomethyl-cobalt(m) porphyrin complexes. The reaction pathway, involving probable insertion of an initially generated carbene fragment into the Co—N bond is also discussed. ... 

The photolysis of diazoalkanes both in the gas phase and in solution is a carbenoid reaction. Moreover, the results of EPR-spectroscopic investigations (Section IIB) demonstrate that triplet carbenes can be generated by irradiation of diazoalkanes. That the reactive intermediates in carbenoid reactions are free carbenes is usually taken as self-evident. While such an assumption is probably wholly justified in most cases, it is worth remembering that both in the gas phase and in solvents such as n-hexane, the electronic absorption spectra of simple diazoalkanes show definite fine structure (Bradley etal., 1964a). This implies that the photo-excited state is bonding (Hoffmann, 1966) and consequently may have a life-time long enough to enable it to react directly with another molecule... 

Carbene itself ( CH2) is extremely reactive and gives many side reactions, especially insertion reactions (12-21), which greatly reduce yields. This competition is also true with rhodium-catalyzed diazoalkane cyclopropanations (see below). When it is desired to add CH2 for preparative purposes, free carbene is not used, but the Simmons-Smith procedure (p. 1241) or some other method that does not involve free carbenes is employed instead. Halocarbenes are less active than carbenes, and this reaction proceeds quite well, since insertion reactions do not interfereThe absolute rate constant for addition of selected alkoxychlorocar-bene to butenes has been measured to range from 330 to 1 x 10 A few of the many ways in which halocarbenes or carbenoids are generated for... 

Molecular models are available for all the reactions and intermediates invoked in these mechanisms. For example, diazoalkanes have been known to generate metal-carbene complexes, with the cyclopropanation of metal-metal double bonds under smooth conditions (compare Section 3.1.7) being of particular relevance to the chemistry of metal surfaces, cf. eq. (10) [8 a, 23],... 

Allyl sulfides and allyl amines. Rhodium-catalyzed decomposition of ethyl diazoacetate in the presence of these allyl compounds generates products 136 and 137, respectively, derived from [2,3] rearrangement of an S- or N-ylide intermediate, besides small amounts of carbene dimers No cyclopropane and no product resulting from the ylide by [1,2] rearrangement were detected. Besides RhjfOAc) and Rhg(CO)i6, the rhodium(I) catalysts [(cod)RhCl]2 and [(CO)2RhCl]2 were found to behave similarly, but yields with the only allyl amine tested, CH =CH—CH NMe, were distinctly lower with the latter two catalysts. Reaction temperatures are higher than usually needed in rhodium-promoted diazoalkane decomposition, which is certainly due to competition between the diazo compound and the allylic hetero-... 

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