Cyclopropene carbon-alkene reactions

Two types of addition reactions involving carbenoid species have been used to synthesize cyclopropanes. The first approach, the reaction of a vinyl carbene species, obtained by a metal-catalyzed ring-opening reaction of cyclopropenes, with alkenes is discussed in Section 1.1.6.3.2., as it is, from a topological point of view, a codimerization of eyclopropene and an alkene. In this case the ring carbons of the cyclopropane are not identical to those of the eyclopropene substrate. 

The meso-ionic l,3-oxazol-5-ones show an incredible array of cycloaddition reactions. Reference has already been made to the cycloaddition reactions of the derivative 50, which are interpreted as involving cycloaddition to the valence tautomer 51.21 In addition, an extremely comprehensive study of the 1,3-dipolar cycloaddition reactions of meso-ionic l,3-oxazol-5-ones (66) has been undertaken by Huisgen and his co-workers.3-39-45 The 1,3-dipolarophiles that have been examined include alkenes, alkynes, aldehydes, a-keto esters, a-diketones, thiobenzophenone, thiono esters, carbon oxysulfide, carbon disulfide, nitriles, nitro-, nitroso-, and azo-compounds, and cyclopropene and cyclobutene derivatives.4 In these reactions the l,3-oxazol-5-ones (66)... 

Dirhodium(ll) tetrakis[methyl 2-pyrrolidone-5(R)-oarboxylate], Rh2(5R-MEPV)4, and its enantiomer, Rh2(5S-MEPY)4, which is prepared by the same procedure, are highly enantioselective catalysts for intramolecular cyclopropanation of allylic diazoacetates (65->94% ee) and homoallylic diazoacetates (71-90% ee),7 8 intermolecular carbon-hydrogen insertion reactions of 2-alkoxyethyl diazoacetates (57-91% ee)9 and N-alkyl-N-(tert-butyl)diazoacetamides (58-73% ee),10 Intermolecular cyclopropenation ot alkynes with ethyl diazoacetate (54-69% ee) or menthyl diazoacetates (77-98% diastereomeric excess, de),11 and intermolecular cyclopropanation of alkenes with menthyl diazoacetate (60-91% de for the cis isomer, 47-65% de for the trans isomer).12 Their use in <1.0 mol % in dichloromethane solvent effects complete reaction of the diazo ester and provides the carbenoid product in 43-88% yield. The same general method used for the preparation of Rh2(5R-MEPY)4 was employed for the synthesis of their isopropyl7 and neopentyl9 ester analogs. 

Isolated carbon-carbon double or triple bonds do not usually take part in inter-molecular Diels-Alder reactions, but a number of cyclic alkenes and alkynes with pronounced angular strain are reactive dienophiles. The driving force for these reactions is thought to be the reduction in angular strain associated with the transition state for the addition. Thus, cyclopropene reacts rapidly and stereoselectively with cyclopentadiene at 0°C to form the endo adduct 7 in 97% yield, and butadiene gives norcarene 8 in 37% yield (3.13). ... 

In the carbometallation reaction described previously, the regioselectivity was found to be the highest (>19 1) when aromatic substituents were present at the double bond, whereas alkyl-substituted cyclopropenes may form the opposite regioisomer. Similarly, this methodology was applied to the preparation of trisub-stituted vinylstannanes and all-carbon tetrasubstituted alkenes (Scheme 10.44). Reaction of [Fe(acac)j] with trialkylaluminum presumably leads to the generation of a low-valent iron species 135 that initiates a possible catalytic cycle for these reactions (Scheme 10.45). 

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