Cyclobutane, butane from

Knowing the importance of angle and eclipsing strain in the small-ring cycloalkanes, we should expect that these strains would become still more important in going from cyclobutane to bicyclo[1.1.0]butane or from cyclooctane to pentacyclo[4.2.0.02,5.03 8.04,7]octane (cubane). This expectation is borne out by the data in Table 12-6, which gives the properties of several illustrative smallring polycyclic molecules that have been synthesized only in recent years. 

Bidentate chiral water-soluble ligands such as (S,S)-2,4-bis(diphenyl-sulfonatophosphino)butane BDPPTS (Fig. 2) or (R,R) 1,2-bis(diphenylsul-fonatophosphinomethyl)cyclobutane have been prepared [25]. Their palladium complexes catalyze the synthesis of chiral acids from various viny-larenes and an ee of 43% has been reached for p-methoxystyrene with the BDPPTS ligand. Furthermore, recycling of the aqueous phase has shown that the regio- and enantioselectivity are maintained and that no palladium leaches. 

Both cis- and frana-butene-2 are formed from each of the dimethyl-cyclobutanes. They are not however formed in equilibrium amounts. Further, more a -butene-2 than the equilibrium amoimt is formed in the decomposition of cis-l,2-dimethylcyclobutane. The fact that the cis- and cyclo-butanes, this does imply that either the lifetime of the free biradical is of the same magnitude as the time for one rotation of the groups in the biradical, or that the biradical is never strictly a free biradical . In either case the configmation of the reactant will, to some extent, determine the stereochemistry of the products. 

The formation of butanes by reduction of arylethenes may arise by radical-radical coupling of two radical-anions giving a dianion, which is then protonated. An alternative route is by nucleophilic addition onto one neutral molecule of the radical-anion, followed by further reduction and protonation. In support of this alternative, cyclobutanes have been isolated from electrochemical reduction of phenylvinylsulphones [21] and vinylpyridines [22], A mechanism for the latter process is illustrated for the case of 2-vinylpyridine 7. Nucleophilic attack of a radical-anion on the substrate gives an intermediate and this disproportionates to form the cyclobutane and a 1,4-diary Ibutane. Cyclobutanes are themselves reduced with ring opening to form the 1,4-diarylbulane. 

Likewise, a study on the bromination of these compounds also indicated that the 1,3-addition was 100% syn stercospecific.10 Interestingly, 3-phenylbicyclo[1.1.0]butane-l-carbonitrile, from which a relatively stable benzylic cation can be formed, yielded a mixture of cis- and /ram-products.10 An electron-transfer mechanism has been proposed for these reactions.10 A recent investigation on perchloric acid catalyzed methanol addition to 3-methylbicyclo[1.1.0]butane-1-carbonitrile and methyl 3-mcthylbicyclo[1.1.0]butane-l-carboxylate, however, showed that mixtures of irons- and cA-cyclobutanes were generated, with the m-isomers predominating.11... 

Cyclopropane derivatives, including spiropentanc, have proven to be virtually inert towards carbenes,1 For this reason, no literature report that describes cyclobutane synthesis from a C3 and a Cj building block by ring enlargement of cyclopropanes exists. However, due to the partial p character, as well as the increasing reactivity caused by its strain, the central bond of bicyclo[1.1.0]butane (l)2 has been found to react with carbenes.1 Photolysis of diazomethane in the presence of bicyclo[1.1.0]butane (1) at — 50 C provides a mixture of several compounds. The major fraction of the material (80%) was analyzed by means of NMR spectrometry and found to consist of penta-1,4-diene (2, 21%) and bicyclo[l.l.l]pentane (3, 1%), plus several other known compounds as well as some unidentified products.3 The mechanistic pathway for the formation of bicyclo[l.l.l]pentane (3) has not been addressed in detail, but it is believed that a diradical intermediate is involved, as shown below.3... 

Cyclobutane undergoes ring opening45 over cobalt(III) fluoride at 100CC and the products are the same as those from butane (vide supra). 

Fig. 6.3. Gas chromatogram of C,-C4 hydrocarbons on alumina-modified squalane. Peaks 1 = methane 2 = ethylene 3 = ethane 4 = acetylene 5 = propylene 6 = propane 7 = propyne 8 = propadiene 9 = cyclopropane 10 = 1-butene 11 = isobutene 12 = frans-2-butene 13 = c/s-2-butene 14 = n-butane 15 = isobutane 16 = 1,3-butadiene 17 = cyclobutane. From ref. 103.

Aldehydes show an elimination reaction (loss of carbon monoxide, CO), that is not possible with ketones. Butanal, for example, photodissociates to propane and carbon monoxide. Cyclic ketones dissociate to a diradical (41 from cyclopentanone), which then reacts in any of several ways including elimination to ethene or 42 and coupling to cyclobutane. Formation of cyclobutane and ethene is accompanied by expulsion of CO prior... 

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