Dimerization cyclopropene

Although the reactions have been generally described in terms of a carbenium ion mechanism, this does not altogether explain the catalytic behavior of the alkali metal ion-exchanged zeolites or the selectivity behavior. An ionic mechanism of the type previously described for cyclopropene dimerization would seem to be more appropriate for the alkali metal ion-exchanged zeolites, where the activity does seem to correlate qualitatively with the electrostatic field (e/r) exerted by the cation. 

On the other hand, in the triplet state (irradiation in acetone or acetonitrile in the presence of benzophenone) the cyclopropene dimerizes into a tricyclohexane derivative. Moreover, racemization occurs about 25 times faster than conversion to the furan product, indicating the intermediacy of a vinylcarbene on the singlet surface, whereas no racemization is observed in the triplet state . 

Cyclopropene dimerization. A peculiar reaction involving alkali-zeolites (A) is the dimerization of cyclopropene... 

The monolithiated cyclopropene dimer was computed to have an unusual structure with two planar tetracoordinate carbon fragments the planar structure 6 is found to be about 11 kcal mol more stable than the perpendicular form 7 with tetrahedral tetracoordinate carbon. Calculations on model dimers indicated that the planar geometry is even stabilized further by heteroatom substituents at the vinylic Cfi, because of the formation of favorable chelate rings. The solid-state structure of a solvated cyciopropenyllithium dimer reveals the perpendicular geometry with tetrahedral tetracoordinate carbons, but lithium chelation by C/0 and C/N dianions results in dimeric cyclopropenyl structures with a high degree of planarization of the tetracoordinate carbon environments. ... 

The photochemical behavior of methylene cyclopropenes is a subject of current investigation170 Previous results with some 4,4-diacyl and 4,4-dicyano triafulvenes indicate that mainly dimerization, but sometimes additional solvent incorporation and hydrogen abstraction occurs. In the case of the photodimer of 1,2-diphenyl-4,4-diacetyl triafulvene (180) the structure 460 can be assigned from spectral evidence ... 

Reaction of the same cyclopropene with octacarbonyldicobalt resulted in a 14.6% yield of an inseparable mixture of the dimeric species (102) in which the two vinylketene moieties may adopt a cis (102.a auf Deckung ) or trans (102.b auf Liicke ) configuration. 

Dimerization is an important mode of reaction of strained alkenes, and two different modes of reaction may be found. With cyclopropene, an ene reaction occurs to give a dimer,and this in turn may further react in the same fashion to give a polymer. 

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... 

The choice of catalyst is important, for instance the use of tetrakis(triphenylphos-phane)palladium(O) complex results in the quantitative cyclotrimerization of 3,3-dimethylcy-elopropene.17 In similar fashion 3,3-dimethoxycyclopropene cyciodimerizes to 3 (R = OMe, 74%) using bis(dibenzylideneacetone)paUadium(0) [Pd(dba)2] complex.18 The trisubstituted cyclopropene 4 is transformed to the head-to-head dimer 5 in the presence of copper(I) iodide.19... 

Actually such sensitized dimerizations have been reported only for small ring, cyclic olefins, the reaction being reasonably efficient for cyclopropenes,262 cyclobutenes,263 and cyclopentenes, but rather inefficient for cyclohexene.264 The effects of ring size on triplet lifetimes... 

For a variety of methyl-substituted cyclopropene molecules, and a variety of zeolites (e.g., NaCaA, NaA, CaA, NaX, and HY), the reaction selectivity to dimer, diene, or polymer was found to be determined by at least three factors, namely ... 

Treatment of cyclopropanethiones 437 with triphenylphosphine results in dimerization to form cyclopropenes 438, which isomerize to carhenes 439. The intramolecular coupling of the carbene 439 (R = Ph) affords thieno[3,2- Ithiophene 441, whereas the carbenes 439 (R =Ph, 2-thienyl, R = SBu ) isomerize to bis(allene)s 440, whose... 

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