Carbenoids, vinyl cycloaddition reactions

Padwa A, Price AT (1995) Tandem cyclization-cycloaddition reaction of rhodium carbenoids as an approach to the aspidosperma alkaloids. J Org Chem 60 6258-6259 59. Padwa A, Price AT (1998) Synthesis of the pentacyclic skeleton of the aspidosperma alkaloids using rhodium carbenoids as reactive intermediates. J Org Chem 63 556-565 Mejia-Oneto JM, Padwa A (2004) Intramolecular [3+2]-cycloaddition reaction of push-pull dipoles across heteroaromatic p-systems. Org Lett 6 3241-3244 61. Padwa A, Lynch SM et al (2005) Cycloaddition chemistry of 2-vinyl-substituted indoles and related heteroaromatic systems. J Org Chem 70 2206—2218... 

A third mechanistically distinct [3 -1- 2] cycloaddition between vinyl ethers and vinyl-carbenoids was discovered and reported in 2001 [26]. This reaction is remarkable because when Rh2(S-DOSP)4 is used as the catalyst, the cis-cyclopentenes 142 are formed in up to 99% enantiomeric excess. The reaction occurs between vinylcarbenoids unsubstituted or alkyl-substituted at the vinyl terminus and vinyl ethers substituted with an aryl or vinyl group. Some illustrative examples are shown in Tab. 14.12. The reaction is considered to be a concerted process, which would be consistent with the highly stereoselective nature of the reaction [26]. Contrary to the [3-1-2] cycloaddition derived by means of vinylogous carbenoid reactivity, this latest [3 -1- 2] cycloaddition is not influenced by solvent effects. Due to steric demands on the carbenoid, the [3-1-2] cycloaddi-tion only occurs with cis-vinyl ethers. 

The structure of the carbenoid has considerable effect on the outcome of the reaction with vinyl ethers. Unlike the case with diazoacetate, reaction with diazopyruvate resulted in the formation of a dihydrofu-ran (17) rather than a cyclopropane (equation 8).26d The reaction is a formal [2 + 3] cycloaddition but it... 

Bis(phenylsulfanyl)](trimethylsilyl)methyllithium and trimethylsilyloxirane do not afford a homo-Peterson reaction product, but a cyclopropane 4a with a shifted phenylsulfanyl group. [Bis(phenylsulfanyl)](trimethylsilyl)methyllithium may be looked on as a carbenoid species which is in equilibrium with carbene and phenylsulfonate. This equilibrium may lie towards the carbanion. On addition of trimethylsilyloxirane, phenylsulfonate is trapped with formation of an alkoxide, which corresponds to the intermediate of a Peterson olefination of formaldehyde, and leads to phenyl vinyl sulfide. This provides a reaction partner for the liberated carbene giving cA-l,2-bis(phenylsulfanyl)-l-trimethylsilylcyclopropane (4a) in a stereospecific [2 + 1] cycloaddition.16 17... 

Pirrung MC, Zhang J et al (1995) Reactions of a cyclic rhodium carbenoid with aromatic compounds and vinyl ethers. J Org Chem 60 2112-2124 55. Pirrung MC, Zhang J et al (1991) Dipolar cycloaddition of cyclic rhodium carbenoids to aromatic heterocycles. J Org Chem 56 6269-6271... 

Recently, Chen and coworkers reported an efficient new method to construct a series of densely functionalized 4,5-dihydro-IH-azepine products 41 from the intermolecular reaction of alkyl azides with propargylic esters. In this approach, sequential reaction of vinyl-gold carbenoids 42 with allqrl azides and formation of vinyl imine intermediates 43 may be involved fScheme 4.14). Then a subsequent formal [4+3] cycloaddition with another molecule of vinyl-gold carbenoid 42 afford the desired azepine 41. 

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