Highly strained cyclopropene

The classical Hunsdiecker reaction was utilized in the laboratory of P.J. Chenier for the preparation of a highly strained cyclopropene, tricyclo[3.2.2.0 " ]non-2(4)-ene. The Diels-Alder cycloaddition was used to prepare the bicyclic 1,2-diacid, which surprisingly failed to undergo the Cristol-Firth modified Hunsdiecker reaction, most likely due to the unreactive nature of the diacid mercuric salt. However, the classical conditions proved to work better to afford the bicyclic 1,2-dibromide in modest yield. Treatment of this dibromide with f-BuLi generated the desired strained cyclopropene, which was trapped with diphenylisobenzofuran (DPIBF). 

The selectivity observed in the intramolecular aldol reaction of hexane-2,5-dione is due to the fact that all steps in the mechanism are readily reversible, so an equilibrium is reached. Thus, the relatively strain-free cyclopentenone product is considerably more stable than the highly strained cyclopropene alternative. For similar reasons, intramolecular aldol reactions of 1,5-diketones lead only to cyclohexanone products rather than to acyl cyclohutenes. 

Benzocyclopropene is an intriguing example in which the electronic structure of benzene is greatly perturbed by the fusion of the smallest alicyclic ring, cyclopropene, to the aromatic system. Benzocyclopropene thus arouses theoretical interest and the high strain energy (approximately 68 kcal./mole)3 associated with the compound suggests unusual chemical reactivity. A review article has recently appeared.4... 

Addition of distannane to alkenes has been achieved only with strained cyclopropenes (Equation (60)).158 3,3-Disubstituted cyclopropenes undergo highly face-selective distannation in the presence of the palladium-isocyanide complex to afford m-adducts. 

On the contrary, if a highly strained cyclic olefm such as the cyclopropene 16 [20] or the norbornene derivative 17 [21] is employed, the titanacycle is cleaved to form the corresponding titanocene-alkylidene 18 or 19. This reaction is clearly enhanced by the concomitant release of intrinsic strain energy (Scheme 14.10). 

To direct a solvolytic ring opening, 2-methoxycyclopropyllithium (6) was developed as a chain extension conjunctive reagent. The failure of P-elimination to occur in 6 presumably derives from the high strain of cyclopropene and poor orbital overlap for elimination. The aldehyde adducts smoothly solvolyze to give p,Y unsaturated aldehydes (Eq. 22) 23) which are best initially isolated as their hemithioacetals. 

The intermolecular reaction of alkynes with acylcarbene complexes normally yields cyclopropenes [587,1022,1060-1062]. Because of the high reactivity of cyclopropenes, however, in some of these reactions unexpected products can result. In particular intramolecular cyclopropanations of alkynes, which would lead to highly strained bicyclic cyclopropenes, often yield rearrangement products of the latter. In many instances these products result from a transient vinylcarbene complex, which can be formed by two different mechanisms (Figure 4.3). 

In contrast, isomers of 115 have so far not been isolated. An early attempt to generate cyclopropa[a,e]naphthalene (118) failed. More recently, the generation of dicyclopropa[a,c]naphthalene (119) was attempted by reaction of 120 with base. When the aromatization was carried out in the presence of DPIBF (44), stereoi-someric bis-adducts of cyclopropenes were isolated. However, the adducts provide no evidence for the formation of 119 as a reactive intermediate, since they are formed by sequential elimination-cycloaddition via 121. Cyclopropene interception of 121 is faster than further elimination to 119. The failure of the reaction to produce 119 has been attributed to the high strain energy of the product, which is estimated some 2 8 kcal/mol higher than that expected for two isolated cyclopropene units. ... 

Stabilization of the cyclopropene formed during the dehydrohalogenation may also be achieved by letting its double bond become part of an aromatic system. By this approach several highly strained hydrocarbons like cyclopropabenzene, cyclopropanaphthalene etc. have been synthesized. 

Highly strained bridged cyclopropenes have received considerable attention.52 The parent compounds undergo a rapid ene reaction leading to a dimer which reacts with itself to form a tetramer. Although bicyclo[1.1.0]but-l(3)-cne has not been observed directly,53 it has been possible to obtain information on its energy via a unique reaction of the bridgehead anion of bicyclo[1.1.0]butane with 02. 54... 

Cyclopropene cyclodimerization would give the tricyclo[3.1.0.02-4]hexane skeleton but the high temperatures required for thermal dimerizations would preclude formation of these highly strained derivatives. However, metal-catalyzed cyelodimerizations take place under milder... 

Intramolecular cycloaddition of a diazo ketone to a cyclopropene. Rhodium) II) acetate is markedly superior to copper or copper(II) sulfate as the catalyst for cyclopropanation of l,4-diacetoxy-2-butyne with /-butyl diazoacetatc. The product (1) was converted by known steps into the diazo ketone 2. In the presence of rhodium(II) acetate, 2 undergoes intramolecular cycloaddition to the cyclopropene double bond to give the highly strained tricyclic pentanone derivative 3 in 30% yield. C oppcr catalysts are less efficient for this conversion. 

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