Analysis of the reactivity of alkenylidenecyclopropanes in cycloaddition reactions

Reinterpretation of the mechanisms of concerted cycloaddition and cyclodimerization of allenes

“…Accordingly, a search was undertaken to discover other reactions in which AO's that are orthogonal in the starting state interact in the transition state leading to bond formation in the final product. Initial attention has been focused on reactions of alienes, i.e., cycloaddition and cyclodimerization reactions of alienes and hydrogen [ 1,3]-sigmatropic rearrangements. In the present paper an alternative to the ( 2, + 2 ) concerted process is described.…”

Electrophilic addition reactions of alkenylidenecyclopropanes. Formation of highly substituted, nonplanar butadienes

“…Attempted cycloaddition of 3 with maleic anhydride at 100°for 14 days resulted in no reaction. Various efforts to isomerize 3 to stereoisomer 5 for comparison of differences in chemical shifts of the isopropylidene methyls in 3 and 5 (arising from long-range shielding effects of the aromatic ring)8 have failed. Attempted iodine-catalyzed thermal isomerization at 100°failed.…”

“…Attempted iodine-catalyzed thermal isomerization at 100°failed. Heating a benzene solution of 3 in a sealed tube at 300°for 5 days in an attempt to effect a reversible [3,3] sigmatropic rearrangement equi-librium9 between 3 and 4 and hopefully 5 resulted only in partial polymerization. In the presence of strong protic acids, however, 4 cleanly rearranges to 3 (vide ante); no peaks are present in the NMR spectrum of the rearranged product which would suggest any formation of 5.…”

Felkin–Anh is not enough

“…[26] In BH 3 reductions, both BH 3 and H 2 B(OR) can act as reducing agents. [27] In many regards, LiAlH 4 reductions are mechanistically simpler than their boron containing counterparts. Under dilute conditions with excess LiAlH 4 , it is likely that only a single reducing agent is present.…”