3- Vinylcyclopropenes

The vinylcyclopropenes that bear an alkyl group cis to the double bond relieve the strain of the three membered via prototropic shift. Because of the geometric constraints, olefin geometry is controlled (Eq. 99). Olefination of the bifunctional... 

The structure of 3-vinylcyclopropene (157) has been determined at 103 K by XD5. The vinyl group is in ap position favorable for conjugation with the ring orbitals. The distal... 

Subsequent experimental work benefited from new synthetic approaches for the C5-chain, Hughes and coworkers used vinylcyclopropenes [277, 278] Bleeke et al. used potassium 2,4-dimethylpentadienides [279-281] and Jia with coworkers used... 

A transition metal mediates rearrangement of a substituted (noncyclo) C5 species. This includes ring expansion reactions of, for example, substituted 3-vinyl- 1-cyclopropene (cf. Scheme 22) or cyclobutene, both bonded to a metal to give supracyclopentadiene Rh and Ru complexes (92- 94). The rearrangement of vinylcyclopropenes to cyclopentadienes can also occur photolytically (66). 

A number of photochemical 1,3-shifts are known that cannot occur by a concerted (sigmatropic) mechanism, such as the sensitized rearrangement a 3-vinylcyclopropene to a cydopentadiene (2.42). There are also a group of reactions for which the term bicycle ... 

II-6) (Table 6, entries 1 and 3). This indicates an enhanced stability, and consequently a higher state of order for the liquid crystalline phase. This is confirmed by the fact that polymers with a higher number of Z-double bonds (Table 6, entries 4 and 5) show a smectic mesophase. Smectic mesophases were also found in polymers with poly(acrylate) A, poly(methacrylate) B, poly(silox-ane) C, and poly(vinylcyclopropene) D backbones (Table 6 entries 13,15,17, 19). The isotropization temperatures of these polymers were approximately the same. 

Another type of carbenes, the vinylcyclopropenes play an important role in the transformations of aUcynes coordinated to the ReCp (CO)2 fragment as mentioned in Section 5.3. 

Vinylcyclopropenes have been found to rearrange photochemically to cyclopen-tadienes. Thus, irradiation of l,2,3-triphenyl-3-vinylcyclopropene (233) in t-BuOH using a 450 W medium pressure lamp and a Pyrex filter for 3.5 h afforded 1,2,3-triphenylcyclopen-tadiene (234) in 87% yield (equation 153) . 

The photochemical rearrangement of 3-vinylcyclopropenes exhibits some regioselec-tivity. Thus, contrary to the selectivity observed with the thermal rearrangement of cyclopropenecarboxylates (see equation 149), the major product obtained from the irradiation of unsymmetrical substituted cyclopropenes was derived from the preferential cleavage of the cyclopropene single C-C bond which is methyl rather than phenyl substituted (equation 154). ... 

Interestingly, the carbene precursors to 42a-c undergo competitive intermolecular addition to alkynes to provide vinylcyclopropenes (e.g. equation 17). The sulphur-... 

Vinylcyclopropenes, e.g. 245a-c, undergo rearrangements analogous to those of their 3-aryl counterparts Direct photolyses of the cyclopropenes 127 afford preparatively... 

Examples of the vinylcyclopropene photorearrangement are known where the chromophore is external to the three-membered ring. The results suggest that the carbene mechanism operates and that it is not contingent upon the chromophore being a part of the carbocycle. For example, direct photolysis of 245d leads to three photoproducts (equation 79) and the presence of an allene requires a [1,2]H shift in the carbene intermediate. 

Methylcyclopropene is reported to react with activated alkynes at -30 °C via an ene reaction to give vinylcyclopropene derivatives 30). In the presence of silver perchlorate, bicyclopropenyl derivatives such as 15 isomerize to bicyclo[2.2.0]hexa-2,5-diene derivatives (e.g. 16), which can be isolated 78-79)... 

Another interesting class of catalysts for this rearrangement are palladium(0) complexes, e.g. Pd(dba)2 . These complexes also catalyze the isomerization of vinylcyclopropenes to cyclopentadienes72) (Eq. 33). 

A mixture of 3-cyano-3-methylcyclopropene (0.04 mol) and the amine (0.10 mol) was kept for 7 d at rt in a sealed ampule. For 3-methyl-3-phenyl- and 3-methyl-3-vinylcyclopropenes the reactions were carried out at 60 C and 80 C, respectively, in the presence of NaOMe (0.004 mol). The excess of amine was removed under vacuum and the products were isolated by column chromatography (silica gel). 

The stereoselectivity of the cycloaddition of arylcarbonitrile (V-oxides to 3,3-dimethyl-, 3-aryl-3-methyl-, 3-methyl-3-vinyl- and 3-cyano-3-methylcyclopropenes has been studied. In the case of the 3-vinylcyclopropene, addition to the external double bond can occur in competition with addition to the cyclopropene. 

In enynes, both the double and the triple bond can compete for the carbenoid that is generated from alkyl diazoacetate under copper or rhodium catalysis. The chemoselectivity is sometimes not very pronounced, but but-l-en-3-yne is selectively attacked at the triple bond [methyl diazoacetate, RhjCOAc), 70% yield], whereas in 2-methylhexa-l,5-dien-3-yne only the double bonds accept the carbenoid (Table 12, entry 2). The vinylcyclopropene obtained by addition to the triple bond may dimerize to form a 3,6-dialkoxycarbonyltricyclo-[3.1.0.0 ]hexane.2 "... 

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