Dimerization of cyclopropenes

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

A specific example is the reaction between maleic anhydride (85) and propene (86) to give 87 (equation 11.38). As in this case, the ene reaction typically involves an alkene substituted with electron-withdrawing groups. Electron-withdrawing groups are not always required, however, as shown by the dimerization of cyclopropene (88) to give 3-cyclopropylcy-clopropene (89, equation 11.39). ... 

An obvious test reaction is the dimerization of cyclopropene to tricyclo[3.1.0.0 ] -hexane (TCH) and its reversal, the fragmentation of TCH to cyclopropene. Formally, the analysis is easily seen to hold here as well, but it does not follow that the energetic advantages of this perturbation are sufficient for concerted cycloaddition to syn-TCR to overcome the normal preference for formation of a rans-biradicaloid species, illustrated profusely in the preceding chapter. 

The course of the reverse reaction, thermal dimerization of cyclopropene, was followed computationally [31]. The molecules were mutually oriented in C2v] as they were brought together the symmetry was relaxed to C, so as to bias the reaction path in favor of concerted closure of the four-membered ring to syn-TCH, in analogy with the behaviour of CBD. As illustrated in Fig. 7.13, the dimerization follows the conventional stepwise path instead, bonding across a diagonal to form a transoid biradical that is more stable thermodynamically than the reactants and can close - if at all - only to the anti isomer. 

In a very recent experimental study [32], dimerization of cyclopropene was found to yield a dienic product with two three-membered rings, that could be... 

Figure 7.13. Computed C2 reaction path for dimerization of cyclopropene.The numbering of the atoms follows that of Chemical Abstracts for tricyclo[3.1.0.0 ]hexane. The slight computational instability near r4s = 1.7A is due to a discontinuity in the limited Cl procedure used.

The reactions, in which basic sites play a principal role, indude aldol condensation of butanal and dimerization of cyclopropene and dehydrogenation of 2-propanol. ... 

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... 

Metal-catalysed rearrangements of cyclopropenes are well known and the resultant products suggest that vinylcarbenoid intermediates are involved. The Cu(I) catalysed conversion of 1,3,3-trimethylcyclopropene (271b) into triene 284 involves dimerization of complexed carbene. Related processes are promotedby Hg(II) and... 

The reaction of 3,3-disubstituted cyclopropenes with mono- and 1,2-disubstituted alkenes proceeds only with difficulty and leads to low yields of cyclopropanes. In the case of but-l-ene, an 8% yield, with hex-1-ene and hept-l-ene between 5 and 10% yield, and with cyclooctene about 10% of the cyclopropane product is formed. In these cases, the major product is the formal dimer of the intermediate ethenylcarbene complex, i.e. the corresponding (fj-hexatriene. When copper(I) chloride is used as catalyst rather than the copper halide/phosphane or phosphite system, about half the yield of the [2-f-1] cycloadduct is obtained along with an increased amount of the hexatriene. Mechanistically, these acyclic trienes could also be formed from an (alk-l-enyl)bicyclo[1.1.0]butane intermediate without any carbene being involved. Bicyclo[1.1.0]butanes are low yield (< 20%) byproducts of the thermal dimerization reaction of methyl 3,3-dimethylcyclopropenecarboxylate (1). On the other hand, bicyclo[l. 1. Ojbutanes, such as 3, are known to undergo isomerization to form 1,3-dienes. ... 

Far higher yields of the [2-fl] cycloadducts are obtained when 1,1-disubstituted alkenes or cycloalkadienes are used in the trapping reaction (Table 2). Moreover, in the presence of cyclic dienes, such as cyclopen tadiene, a high selectivity for the formation of the entfo-product is observed. Hexatrienes are obtained as a side product arising from dimerization of the ring-opened cyclopropene. The amount of [2-1-2] homo-cyclodimer of the cyclopropene is normally below 5% in the case of different substituents in the 3-position, several isomers are formed. 

With methyl 2-alkylcycloprop-2-enecarboxylates, on the other hand, only [2-1-1] cycloaddition to the tricyclic dimers of cyclopentadiene (18) occurs. Only the strained double bond in the norbornene substructure is cyclopropanated, and the exo-adducts are formed. The Z/E isomeric ratios (6 1 and 2.5 1) are comparable to the ratios obtained in the reactions with norbornadiene vide supra). A mechanism proposing ring opening of the cyclopropene and attack on the metal-coordinated alkene in a concerted way has been suggested in order to explain the stereoselectivity of the reaction. ... 

If, in an early stage of the reaction, a second cyclopropene molecule is coordinated to the nickel, homo-cyclodimerization leading to tricyclic dimers of type 28 may also occur. To prevent the formation of 28, the stationary concentration of the cyclopropene in the reaction mixture must be small, i.e. the cyclopropene must be added slowly. This is especially critical if the electron-poor alkenes are only weakly bound, as is the case with methyl acrylate and the 3-alkyl-substituted acrylates. When acrolein or acrylonitrile are employed, the cycloaddition reaction is inhibited due to the formation of stable bis(alkene)nickel complexes. 

Treatment of vicinal-substituted dibromocyclopropanes with /ert-butyllithium gives cyclopropenes, which, if strained, can react with the lithium reagent, undergo dimerization, or participate in Diels-Alder reactions if a dienophile is present. 22,737 three types of reactivity are exhibited by l,6-dibromobicyclo[4.1.0]heptane when treated with er/-butyllithium. If the reaction is carried out in the presence of 1,3-diphenylisobenzofuran, cycloaddition predominates giving 6 in 71% yield. If the same reaction is carried out in the absence of the diene, however, l-tert-butylbicyclo[4.1.0]heptane (8) and the dimer of bicyclo[4.1.0]hept-l(6)-ene 7 are the most abundant products. 

Cyclopropenes have long been known to dimerize under copper catalysis, -and trienes are often isolated as minor components of other reactions of cyclopropenes induced by copper.They can also undergo dimerization on heating or even at room temperature or below, e.g. formation of 1, 2, and... 

The experimental evidence is suggestive, but inconclusive. Although cyclopropene is a highly strained molecule, with an estimated strain energy of 26 kcal/mol,[28] it does not dimerize spontaneously. The an e-isomer of TCH, the 6z5- em-dimethyl derivative of which has been formed by catalytic dimerization of em-dimethyl cyclopropene,[29] does not revert thermally to two monomer molecules. Instead it isomerizes to vibrationally excited cyclohexadiene [30] ... 

A student wished to prepare of series of cyclobutanes that can be considered to be dimers of cycloalkenes. Sensitized photolysis of the cycloalkenes proceeded efficiently for cyclopropene, cyclobutene, cyclopentene, cyclohexene, and cyclohcptene. However, cyclooctene produced little or no dimer. Rationalize this result. 

Another possible application of cyclopropene ROMP in the investigation of conjugated polymers would be to use its dimer, bicycloprop-2-enyl (the structure on the extreme right in... 

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 ... 

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