Cyclopropene ring

Double bonds are accommodated by rings of all sizes. The smallest cycloalkene, cyclo-propene, was first synthesized in 1922. A cyclopropene ring is present in sterculic acid, a substance derived from one of the components of the oil present in the seeds of a tree (Sterculia foelida) that grows in the Philippines and Indonesia. 

The theoretically interesting phenyl hydroxy cyclopropenone (57) was prepared by Famums1, s2 according to the general principle of cyclopropene ring closure developed by Closss3) from 53 via the vinyl carbene 54 and phenyl trichloro cyclopropene (55). 

Although the original synthesis of an 774-vinylketene complex1 (see Section VI,B) had used the method of metal insertion into a cyclopropene ring, it was more than 20 years before it was applied to metals such as... 

Both sets of observations confirm that the ring-opening process is subject to both steric and electronic influences, the combination of which produces some subtle effects in product distribution. Ultimately, the regiochemistry of the product must be decided after a cyclopropene-Fe(CO)4 complex is formed, but probably during the cleavage of one of the C-C bonds in the cyclopropene ring. 

Theoretical calculations predict that the highest occupied Walsh orbital of the cyclopropene ring in 1 should stabilize a positive charge at C2, ° and this prediction was experimentally confirmed. Appearance energy measurements for the loss of Br from ionized 2-bromobenzocyclopropene (202 X = Br) suggest that the benzo-cyclopropen-2-ylium ion 291 is stabilized by more than 27.6 kcal/mol over the phenyl cation 292. Calculations (3-2IG ) give a stabilization energy of 23 and 22 kcal/mol of 291 over 292 and over the m-isomer 293, respectively. ... 

The molecular structure of the chelate complex was determined by a single-crystal X-ray diffraction study. The Mo distances to the cyclopropene ring carbon atoms, averaging 229... 

The interesting zwitterionic compound (39) with the cationic component a butadien-2-yl cation was obtained by reaction of l,4-di(t-butyl)butadiyne with 2 mol of di(r-butyl)aluminium hydride, with the structure being established by X-ray analysis.80 The reactions of the l,2-diferrocenyl-3-(methylthio)cyclopropenylium ion with carbanions derived from active methylene compounds were investigated.81 Products were derived by ring opening of the cyclopropene ring after the initial carbanion addition. The bis(ferrocenylethynyl)phenylmethylium cation (Fc-C=C-)2C+Ph (Fc = ferrocenyl) was prepared 82 This cation proved to be much less stable than its bis-ethenyl analogue (Fc-CH=CH-)2C+Ph. 

The routes available for generating a cyclopropene ring include those involving the formation of the 1,3- (or 2,3-) bond, with or without allylic rearrangement, of one or both of the 1,2-bonds, of both 1,3- and 2,3- bonds, and of both 1,3- and one of the 1,2-bonds in (1). 

The elimination is successful with a range of substituents at C-2 or C-3 of the cyclopropane, although in some cases the derived cyclopropenes ring-open even at room temprature to give vinylcarbenes, which may be trapped in inter- or intramolecular processes. It is successful when X or Y = Br even if R = H, but when X = Y = Cl and R = H, an alternative 1,2-elimination of HC1 occurs. By careful control of the quantity of reagent, it is possible to carry out the elimination in the presence of functional groups which are relatively reactive to such reagents (final two examples below)112-118,120,80. 

The alkyne is formally derived by a 1,4-dehydrochlorination with ring opening, although subsequent work has shown that the cyclopropene ring-opens even at ambient temperature to 3-chloro-3-methylbut-l-yne and so a 1,2-dehydrochlorination of this is an alternative. The carbene could be formed by several routes from the cyclopropene, and an early experiment with a 14C-labelled compound indicated that the label at C-l of the cyclopropane became C-l of the carbene ... 

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