Carbenoids addition-fragmentations

Two approaches have been used in the synthesis of these types of compounds. Small boron-phosphorus ring compounds can serve as building blocks, and addition and elimination reactions with other main group elements can then extend the cage structure (see Schemes 23 and 24, Section 12.12.6.4.5). Alternatively, an unsaturated carbenoid fragment can be added to the bicyclic fragment as illustrated in Scheme 31 <1998IC490>. 

The other important type of reaction leading to the formation of two bonds between a metal and four-atom fragment is addition of carbenoids, i.e. germylenes and stannylenes, to a-dicarbonyl and related compounds. The process can be classified as redox (4 + 1) cycloaddition since the dicarbonyl fragment is reduced while the carbenoid is oxidized. Formally the simplest example of such reactions is the interaction of methylglyoxal with activated tin metal to give a cyclic stannylene, Sn(II) enediolate (see Scheme 34) <83CL1825>. 

In view of the smooth addition of carbanions to the electrophilic P atom of phosphorins (see p. 78 and 66), Markl and Merz attempted the addition of carbenoids (carbenes ) by reacting dichloromethane, trichloromethane or dichloro-phenylmethane with potassium-tert-butoxide in the presence of 2.4.6-triphenyl-X -phosphorin 22 or 2.4.6-tri-tert-butyl-X -phosphorin 24. The desired bicyclocompound 7/5 or the X -phosphepin 116 were not obtained. Instead, benzene derivatives were formed by loss of a PCI fragment, the fate of which was not determined. Markl has proposed a mechanism in which intermediates 114,115 and 116 ZK invoked ... 

Carbenic fragmentation is formally the reverse of addition of carbenes to the sulfur atom of the thiophene ring, and has been observed only with 2,5-dichlorothiophenium bis(alkoxycarbonyl)methylides (30). When 30 (R = CHj) is heated at 110°C in refluxing toluene in the presence of rhodium(II) acetate or copper(II) acetylacetonate, fragmentation of the ylid to 2,5-dichlorothiophene and the carbenoid occurs. The bis(methoxycarbonyl)-carbene has been trapped with alkenes to produce high yields of the cyclopropanated products (78CC641). Since the ylid is a stable crystalline solid with a long shelf life, it represents a convenient source of bis-(methoxycarbonyl)carbene. 

A number of 1,1-diheteroatom-substituted cyclopropanes have been prepared by the addition of one-carbon fragments (carbenes or carbenoids) to olefins. Since the geminally... 

Volume 9 deals with the majority of addition and elimination reactions involving aliphatic compounds. Chapter 1 covers electrophilic addition processes, mainly of water, acids and halogens to olefins and acetylenes, and Chapter 2 the addition of unsaturated compounds to each other (the Diels-Alder reaction and other cycloadditions). This is followed by a full discussion of a-, y- and S-eliminations (mainly olefin and alkyne forming) and fragmentation reactions. In Chapter 4 carbene and carbenoid reactions, and in Chapter 5 alkene isomerisation (including prototropic and anionotropic, and Cope and Claisen rearrangements), are discussed. 

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