Cyclopropane ring, formation reductive opening

Substitution of cyclopropane rings with the alkenyl group permits unique ring transformations based on metal coordination interaction with four -electrons. The transition-metal-induced ring-opening rearrangement also results in the formation of metallacycles. Further elaboration is attained by insertion and reductive elimination. 

The Tafel rearrangement only occurs in acid medium. Simultaneous reduction of both carbonyl groups leads to interaction and formation of a cyclopropane. Acid catalysed cyclopropane ring opening follows to yield an a-diketone 28 which undergoes the electrochemical Clemmensen reduction step to the hydrocarbon. Side products include the two monoketones derived by partial deoxygenation of the a-diketone and the secondary alcohols from reduction of these raonoketones. Separate experiments show that the a-diketone 28 can be reduced to the hydrocarbon. 

The cyclopropane ring in la,9b-dihydro-l//-cyclopropa[l]phenanthrene (13) was opened in two different ways upon reduction with lithium or sodium and gave rise to the formation of dibenzocycloheptadiene (14), 9-methylphenanthrene (15), and 9-methyl-9,10-dihydrophenan-threne (16). The proportion of products formed depended on the number of equivalents of the reducing agent and the duration of the reaction. 

Lithium is the preferred metal for the reductive cleavage of cyclopropane bonds in cyclopropyl ketones. Sodium is less suited for this reaction because it tends to reduce the carbonyl group rather than open the three-membered ring. With sodium the cyclopropane ring in methyl-substituted benzoylcyclopropanes 1 was retained and the benzoyl group reduced to benzyl contrary to lithium which gave rise to the formation of cleavage products. ... 

In the Na/NH3 reduction only the ring-opened 123 was observed, while in the rather slow NaN reduction besides little cyclopropane 49 (6%) mostly the ring-opened 124 (58 %) and 123 (17 %) were formed. It has been pointed out before that the NaN reduction of 5(1-52 results only in the cyclopropane 49. As far as the formation of the cyclopropane 49 and the acid 99 in the Li reduction of 122 is concerned, a similar mechanism has been discussed as outlined in Scheme 15 for the reduction of 50-52 and 118-Hal in homogeneous solution. The marginal retentions of 49 (2.5 % o.p.) and 99 (2.6 % o.p.) are in line with the expected instability of the corresponding fluoride containing radical anion. 

The mechanism proposed involves formation of a metallacyclopentene 11.156, followed by strain-driven ring opening of the cyclopropane to form a metallacyclooctadiene 11.157 (Scheme 11.52). This ring expansion is then followed by reductive elimination. 

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