Cyclooctatrienes Bicyclo octa-1,3,6-triene

Selective thermal isomerization of 5,8-di(l-cyano-l-methylethyl)-l,3,6-cyclooctatriene (10), obtained from 1,4-addition ofa-cyanoisopropyl radicals to 1,3,5,7-cyclooctatetraene, to homogeneous 3,8-di(l-cyano-l-methylelhyl)bicyclo[4.2.0]octa-2,4-diene (12) has been observed in refluxing benzene (75 h) or xylene (20 h)87. The 1,3,5-triene 11 formed by an initial [1,5] hydrogen shift is regarded as an intermediate in this conversion87. 

A mixture of cycloocta-l,3,5-triene and cycloocta-l,3,6-triene formed by partial reduction of cyclooctatetraene is the usual starting point for the study of cyclooctatriene complexes, and obviously complexes may be derived from both isomers. Also, the 1,3,5-isomer is in thermal equilibrium with its valence tautomer, bicyclo[4.2.0]octa-2,4-diene (XXVII), at 100° C (30), from which complexes may also be formed. Although silver nitrate... 

The behavior of the cyclooctatrienes as four-ir-electron donors is complicated by the existence and ready interconversion of the various isomers. Early work on the cyclooctatrieneiron carbonyls has been summarized 118). The yellow oil produced in the Fe(CO)s-isomeric cyclooctatriene reaction, and initially formulated as 7r-l,3,6-CgHioFe(CO)3 61), is now known to be (bicyclo[4.2.0]octa-2,4-diene)Fe(CO)3 (XXVIII) on the basis of chemical and NMR studies 104, 105, 108, 116). A less stable isomer formed from Fe(CO)s or Fe3(CO)i2 and the isomeric trienes is formulated as 7t-1,3,5-CgH]oFe(CO)3, with coordination from a pair of conjugated double bonds (XXIX) 105,108, 116). 

The cyclooctatriene mixture also reacts with 4r-C5HsC CO)2 to give two isomers of formula n CgHsCo-ir-CgHio (97,108,117). In this case, the less stable isomer, a red solid m.p. 46° C, is derived from bicyclo[4.2.0]octa-2,4-diene, while the more stable isomer, a yellow solid m.p. 98° C, is derived from cycloocta-l,3,5-triene. NMR studies show that, in the latter complex, coordination takes place via a pair of unconjugated, chelate double bonds, the olefin being in a tub conformation (XXXI). In agreement with this, the protonated species is very unstable. This work, and the related studies on... 

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