Spiro-nonadienes

Diisobutylaluminium hydride catalyses the ring-closure of various dienes. It is proposed that the process involves addition of the aluminium hydride to a terminal double bond, followed by ring-closure and, finally, elimination of the catalyst (equation 106). Thus 1,5-hexadiene gives methylenecyclopentane (213) (equation 107), 1,6-heptadiene gives methylenecyclohexane (214) (equation 108), 4-vinylcyclohexene gives bicyclo[3.2.1]oct-2-ene (215) (equation 109) and the spiro compound 217 is obtained from 5-methylene-l,8-nonadiene (216) (equation 110)112. 

Figure 8. (+)-(55)-l,6-spiro[4,4]nonadiene configuration (A) and schematic representation of the puckering of the various conformations (B). (Reproduced with permission from tef. 78. Copyright 1979 American Chemical Society.)... 

Synthesis af okfins from stericaUy hindered akohols, Gerlach el al. have reported a successful variation of theChugaev reaction applicable to the preparation oftemperature-sensilive olefins from sterically hindered alcohols. Thus /ron.t,/ran.s-spiro[4.4jnonane-1,6-diol (2) is converted into the thiocarbonate 0-ester (3) by reaction with (1) in pyridine for 4 hr. at room temperature (80% yield). This derivative decomposes smoothly into the diene, 1,6-spiro[4.4]nonadiene (4), at a temperature of > 135 . In this case pyrolysis of the diacetate or of the di-S-methyIxanthate failed to give the diene. 

However, Willcott found evidence for a radical chain process in these reactions. Thus,l,5-dimethyl-5-deuteriomethyl-CPD gives 23% do and 15% d2 product in addition to the expected di material. Nonetheless, spiro[4.4]nona-1,3-diene undergoes ring expansion to bicyclo[4.3.0]nonadienes with allowed retention of stereochemistry at the migrating carbon, so the reaction appears to be concerted in these cases (see Chapter 10, Section 3). 

Upon heating, spiro[4.4]nona-l,3-diene gives a number of bicyclo[4.3.0]nonadiene isomers which interconvert by 1,5-hydrogen shifts, but at higher temperatures, a 1,3-shift occurred to give bicydo[4.3.0]nona-1,6-diene which at still higher temperatures ultimately gave indane (Scheme 10.59). ... 

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