Cyclopropene, cleavage reactions

The singlet excited state photochemistry of cyclopropenes is dominated by ring cleavage reactions (Section IV.B.2), but in the triplet manifold [2 -I- 2] cycloaddition is the major pathway. Because of inefficient intersystem crossing the first excited triplet state of a cyclopropene is best populated by sensitization. Despite the added energy involved with these species there is no effective pathway available for ring cleavage in the absence of added olefin stereospecific dimerization to a cis-transoid-cis-tricyclohexane, e.g. 145, occurs. The smallest of the C(3) substituents is endo in the product ". A stepwise... 

Because of the high strain energy 164,285 associated with the cyclopropenes (228 kJ mol" thermal ring cleavage reactions are easily brought about as discussed in the following pages. 

As noted above (Section IV.B.5) various metal complexes including Ni(0)(cyclo-octadiene)2 catalyse cyclopropene ring cleavage reactions and deliver products by way of... 

In light of the above results it is interesting to note that the reaction of diphenylcyclo-propenone dimer spirolactone with ironenneacarbonyl yields a mixture of ring-opened vinyl carbene and -vinylketene complexes, and these interconvert under addition (or removal) of CO (equation 225) . A possible pathwav to vinylketene Fe-complexes, prepared earlier from cyclopropenes and ironcarbonyls " , may thus involve initial f -coordination, followed by ring cleavage to vinyl carbene and finally carbonylation to the ketene iron // -complexes. An analogous // -manganese complex is prepared similarly by the reaction of CpMn(CO),THF with 3,3-dimethylcyclopropene complex (equation 226) . ... 

Cyclopropenes undergo an oxidative cleavage to yidd substituted enones, as shown in equation (27). The reaction is believed to proceed through the unstable qwxide. The regioselectivity is genoaOy low if R 9 R, but if one of the substituents is trimethylsilyl a hi y selective conversion to the a-silyl enone takes place. There is one report of a similar oxidative cleavage that takes place with thallium(ni). ... 

Singlet excited state reactions of cyclopropenes proceed with low quantum efficiencies due to thermal return of carbene 193 to starting material as elegantly illustrated with optically active esters 234. On photolysis racemization of 234 occurs four times faster than ring cleavage. The photorearranged furan product results from six-electron cyclization of the intermediate carbene and such species have been invoked to account for the interconversion of variously substituted furans By comparison... 

The reactions of cyclopropenes with metal complexes proceed either with ring retention or with cleavage of the C(1 C(3) cr bond. 

Nucleophiles react with cyclopropenium 2C and diphosphirenylium salts 2Pa affording the corresponding cyclopropenes 1C and lH-diphosphirenes l Px (Scheme 14), respectively. Nucleophiles such as lithium salts also react at the dicoordinate phosphorus atom of the diphosphirenium salt 2 Pa [22b, 45], but in this case ring opening reactions occur affording the phosphaalkenes 15 rather than the thermodynamically unfavored cyclic 47r-electron ylides 19. Cleavage of... 

The reaction apparently proceeds by an initial monodehydrochlorination followed by cleavage of the derived cyclopropene to a diradical which is converted into the product by loss of Cl and addition of a hydrogen atom. The vinylcyclopropene may also be obtained from 2-chloro-l-vinylcyclopropane by dehydrochlorination under the same conditions. The NMR signals for the product persisted at — 100 C in tetrahydrofuran, but diminished at temperatures above — 70 C, and at room temperature a [2 - - 2] dimer was observed. 

Similar to the addition of halogens, cycloproparenes react under hydrogenation conditions either by initial attack at the cyclopropene bond or via cleavage of one of the lateral cyclopropane bonds. In the former case, the intermediate norcaradiene opens to a cycloheptatriene which is further reduced to cycloheptane. In the latter, the reduction leads to toluene or methyl-cyclohexane. For example, benzocyclopropene, on reaction with platinum oxide in acetic acid, was hydrogenated to cycloheptane (49%) and methylcyclohexane (51%). 1,1-Difluoroben-zocyclopropene, under the same conditions, gave cycloheptane exclusively (85%). Hydrogenation of benzocyclopropene with Raney nickel gave toluene in 90% yield. 

A variety of diverse synthetic methods have been empioyed for the preparation of cyclopropane (1 j. Schlatter and Demjanov and Dojarenko pyrolyzed cyclopropyltrimethylammonium hydroxide at 320°C using platinized asbestos as the catalyst. About equal amounts of cyclopropene (1) and cyclopropyidimethylamine are formed, contaminated with some dimethyl ether and ethylene. Treatment with dilute hydrochloric acid removed the amine from the gas stream and 1 was separated from the other products by gas chromatography. Alder-Rickert cleavage of the Diels-Alder adduct formed from cycloheptatriene and dimethyl acetylenedicarboxylate resulted only in the formation of a polymer and trace amounts of 1. A simple approach by Closs and Krantz based on the synthesis of 1-methylcyclopropene involved the addition of allyl chioride to a suspension of sodium amide in mineral oil at 80°C. Under the conditions employed, 1 could readily escape from the reaction mixture. Though a number of variations were tried, the yield of 1 never exceeded 10%. 

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