The Cycloaddition-Cycloelimination Pathway to Homotropilidenes − Synthesis and Properties of Homotropilidenes

Abstract

The cycloaddition‐cycloelimination reaction sequence between 3,6‐disubstituted 1,2,4,5‐tetrazines 9 and various cyclopropenes 10 provides 3,4‐diazanorcaradienes 11, which undergo [4+2] cycloadditions with additional cyclopropenes 10 to form tetracyclic azo compounds 12. Photolysis of these compounds, with accompanying loss of nitrogen, affords homotropilidenes (bicyclo[5.1.0]octa‐2,5‐dienes) 13−26. Substituents at various positions of the homotropilidene skeleton have been observed to exert significant influences on the Cope rearrangement rate and, in cases of unsymmetrically substituted homotropilidenes, on the equilibrium positions.

An One-Pot Synthesis of Semibullvalenes and Its Mechanism

“…[5] Of special interest are the socalled degenerate Cope rearrangements, [6] as observed in homotropilidenes, [7,8] bullvalenes, [6,9] barbaralanes, [10,11] and semibullvalenes. [12Ϫ14] In this contribution, we give a full account of our investigations involving the synthesis of symmetrically substituted semibullvalenes by means of a simple cycloadditionϪ cycloelimination sequence utilizing 1,2,4,5-tetrazines and bifunctional cyclopropene derivatives.…”

“…After photochemically induced denitrogenation, this afforded homotropilidenes in great variety. [8] Consequently, use of the bifunctional 3,3Ј-bicyclopropenyls as dienophiles would be expected to give semibullvalenes, which in principle represent bridged homotropilidenes.…”

A Simple Route tocis- andtrans-Bis-σ-homobenzenes

“…[37] In general, a wide variety of azo compounds provided homotropilidenes smoothly on photolysis. [38] When 3,3-dimethylcyclopropene was used as a dienophile under forcing conditions (high pressure, for example), azo compounds of structure 15 were isolated. Quite unexpectedly, direct photolysis of 15 produced isomers of homotropilidenes.…”

“…Only a signal for one olefinic proton (δ ϭ 6.62Ϫ7.96) and three signals for cyclopropane protons (2-H, 4-H, 7-H) are found, with the expected coupling constants, thus ruling out the expected homotropilidene photoproducts. [38] NOEs were of special significance for evidence of the proximity of the corresponding functional groups, with the strong NOE between 10-CH 3 and 11-CH 3 for the bis-σ-homobenzene 16d, as listed in Figure 2, convincingly attesting to the cis configuration.…”