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The stability vs. reactivity of electrophilic 3-(triphenylphosphonio)-cyclopropenium salts towards a neutral nucleophile, such as triphenylphosphane, is reported. Depending on the nature of cyclopropenyl substituents (R), the three-membered cyclic structure is preserved (R = Ph) or evolves by ring opening to the isomeric linear allene (R = Mes). The respective formation of 1,3-bis(triphenylphosphonio)-2,3-diphenylcyclopropene and 3,3-bis(triphenylphosphonio)-1,1-dimesitylallene products is rationalized on the basis of steric and electrostatic constraints.

The first example of aromatic cation-activated nucleophilic acyl substitution has been achieved. The conversion of carboxylic acids to their corresponding acid chlorides occurs rapidly in the presence of 3,3-dichlorocyclopropenes via the intermediacy of cyclopropenium carboxylate complexes. The effect of cyclopropene substituents on the rate of conversion is examined. The addition of tertiary amine base is found to dramatically accelerate reaction, and conditions were developed for the preparation of acid sensitive acid chlorides. Preparative scale peptide couplings of two N-Boc amino acids were achieved with this method.

UV irradiation of alkyl-, aryl-, and heteroatom-substituted cyclopropenones results in the loss of carbon monoxide and the formation of quantitative yields of corresponding alkynes. The quantum yield of the photochemical decarbonylation reaction ranges from 20% to 30% for alkyl-substituted cyclopropenones to above 70% for the diphenyl- and dinaphthylcyclorpopenones. Rapid formation (5 ns) and then a somewhat slower decay (ca. 40 ns) of an intermediate in this reaction was observed by using laser flash photolysis. The DFT calculations allowed us to identify this intermediate as a zwitterionic species formed by a cleavage of one of the carbon-carbon bonds of the cyclopropenone ring. The latter then rapidly loses carbon monoxide to produce the ultimate acetylenic product. Despite their high photoreactivity, cyclopropenones were found to be thermally stable compounds with the exception of hydroxy- and methoxy-substituted cyclopropenones. The latter undergo rapid solvolysis in hydroxylic solvents even at room temperature. The application of this reaction to the in situ generation of the enediyne strucutre was illustrated by the photochemical preparation of benzannulated enediyne 12.

Phase transfer catalytically generated chlorofluorocarbene adds to alkynes much more readily than dichlorocarbene.Some sterically shielded or critically substituted compounds that do not give addition with CCl2 can be reacted with CClF.Chlorofluorocyclopr

If a diarylalkyne 4-XC6H4CCR (R=Ph, 4-MeC6H4, or 4-ButC6H4) or 1-phenylpropyne in trifluoroacatic acid containing mercury(II) trifluoroacetate is irradiated with u.v. light filtered through Pyrex glass, the e.s.r. spectrum of the corresponding azulene can be obsreved.The azulenes have been isolated and converted back into their radical cations by irradiation in trifluoroacetic acid, or in dichloromethane containing (4-BrC6H4)3N+. or (2,4-Br2C6H3)3N+..Possible mechanisms by which the azulenes are formed the alkynes are discussed.

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