Olefin radical cation cycloadditions

Abstract

Radical-cation cyclodimerizations of electron-rich cyclic 1,3dienes and radical-cation Diels-Alder reactions .of these dienes with several electron-rich olefins have been investigated. In some cases the efficiency of the electron transfer could be increased if the electron acceptors were combined with LiC104 (special salt effect). The dimerization of 1,3-cyclohexadiene (74, as well as 1 -acetoxy-and l-methoxy-1,3-cyclohexadiene (7b, 7c) with several electron acceptors yielded endo-selectively the Diels-Alder dimers. The formation of the Diels-Alder products via radical-ion intermediates could be demonstrated by quenching experiments. In addition, cyclobutane dimers were also formed, mostly through triplet-reaction channels. Only in the case of I-acetoxy-1,3-cyclohexadiene (7b) photochemically induced electron transfer is involved as well, as shown by quenching experiments. Some of these Diels-Alder dimerizations are indicated by a characteristic concentration dependence, i. e. the endo [2 + 41 dimers were preferably formed at low diene concentrations, whereas high diene concentrations favored the ex0 [2 + 4) adducts. In the reaction sensitized by 1,rl-dicyanophthalene (2) these concentration effects could be emphasized by differently strong quenching of the different products. Concentration and quenching effects indicate the involvement of different radical-ion intermediates. Crossed Diels-Alder reactions usually ran endo-selectively. Here, quenching of product formation with lJ,4-trimethoxybenzene (TMB) indicated the involvement of radical intermediates as well. D CN CN A * D tion of LiClO,.

Selectivity in Radical Cation Cycloadditions

“…Results summarized in Scheme 4.6 and Table 4.3 for the electron-rich dienophile, show an approximately 1 : 1 selectivity when using 8a. 62 Placing substituents at the 1-position as in 8b,c completely inhibits the formation of cross-adducts, while substitution at the 2-position of the dienes leads to preferential formation of the cross-adduct.…”

“…An examination of effects from substituent position is shown in Scheme 4.7. 62 The stability of the singly linked intermediate of the stepwise cycloadditions is key in understanding the selectivity. Comparing the singly linked intermediates of the crossproducts 26 þ and 27 þ , it is clear that 27 þ will be disfavored due to steric bulk at the bridgehead position and the lack of stabilization of the cation by the acetoxy group at the 2-position of the allyl system.…”

“…62 This hypothesis was the result of observing higher endo selectivity at lower concentrations, suggesting that the formation of SSIP would be favored at low substrate concentrations, producing more endo adducts. Further evidence was acquired with the addition of LiClO 4 , EtOH, and 1,2,4-trimethoxy benzene (TMB) to reacting systems.…”

“…103 A CIP formed following ET in polar solvents can either equilibrate to the SSIP or be captured by another diene forming the [2 þ 2] and exo products 10a, 11a, and 12a as described in literature. 60,62 Increasing concentration of 8a would promote capture of the CIP. The experimentally observed pressure effects can then be rationalized with the dissociation of the CIP that requires an increase in volume.…”

Oxidative Rearrangements of Tricyclic Vinylcyclobutane Derivatives

“…[4] Among those are the vinylcyclopropane [5][6][7][8] and vinylcyclobutane [9,10] rearrangements, the latter often in the context of a stepwise pathway for radical cation Diels-Alder reaction. [11][12][13] When the vinylcyclobutane moiety is part of a rigid polycyclic framework, as it is the case in a-and b-pinene, then the sigmatropic rearrangement appears to be suppressed for steric reasons (in the case of b-pinene it would lead to a bridgehead olefin). Instead one-electron oxidation leads to products derived from ring-opened distonic radical cations.…”