Trans-l,2-divinylcyclobutane

SCHEME 25. Activation and reaction volumes of the ring enlargment of trans-l,2-divinylcyclobutane... 

Von besonderer praparativer und theoretischer Bedeutung sind die Er-gebnisse der photosensibilisierten Dimerisation von 1,3-Dienen. Hier konkurrieren C4-Cyclodimerisation und C6-Cyclodimerisation (Diels-Alder-Typ). So bildet sich aus Butadien bei der photosensibilisierten Dimerisation neben cis- und trans-l,2-Divinylcyclobutan (Formeln 16 tmd 17) auch 4-Vinylcyclohexen (Formel 18) (134, 139). Isopren liefert... 

In the absence of added phosphorus ligands, the principal product (80-90%) is trans, trans, rran5-l,5,9-cyclododecatriene (ttt-CDT 49), with smaller amounts of trans, trans, cw-l,5,9-cyclododecatriene ttc-CDT 50), trans, cis, c/5-l,5,9-cyclododecatriene (tcc-CDT 51), 1,5-cyclooctadiene (COD 52), 4-vinylcyclohexene (VCH 53), and c/5-l,2-divinylcyclobutane (DVCB 54). With the addition of phosphorus ligands, especially bulky phosphites, COD can become 96% of the product, and the rate of butadiene consumption can be increased by a factor of 10 (at 80°). ... 

Sodium-ammonia reductions of the cis-and trons-divinylcyclopropanes (2S1) and (252) do not produce identical product mixtures, as is observed for cis- and trans-divinylcyclobutanes. The stereospecificity exhibited by (251) in producing only trans-propenylcyclopentenes (Scheme 37) could be due to charge-transfer stabilization in the anion radical (253). The reduction of exo-7-phenylbenzo[2,3]bicyclo[4,l,0]-heptene (Na-NH ) has now been shown to give 2-benzyltetralin, and not 4-phenyl-benzocycloheptene as previously reported a series of 2-substituted tetralins and their 4-oxa-analogues have been obtained by this procedure. Reduction of (93 X = Cl)... 

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