Azo Bridges from Azines, XXIV. Tuning the Addition of Thiols to Parallel CC/NN Bonds from Transannular to Normal Addition

Abstract

As an example of system A, compound 5 was reacted with several thiols. A radical route procedures transannular 1,4-addition products (6a/b). In the presence of sulfur, the addition reaction is strongly enhanced, and a polar route yields 1,2-adducts (?a/b). Structures 6a/b and ?a/b are supported by X-ray analysis. The different reaction modes are discussed.Recently, systems of type A were shown to yield half-cage products of type D on reaction with dialkylborane~[~1. These transannular reactions start with an electrophilic attack of borane on the azo group. We now report on different transannular reactions of the same system A with thiols, which start at the C = C bridge and result in half-cages of type B. Unexpectedly, however, by the addition of elemental sulfur, the reaction of systems A with thiols can be tuned to result in normal addition at the olefinic bridge, thereby yielding systems C. Such a modifying effect by submolar amounts of sulfur is unprecedented. D Aof thiophenol occurs at the norbornene double bond in both cases. From 1 the expected exo adduct 2 is isolated. The addition of the thiophenol radical to 3 obviously starts in the same way. However, instead of a normal addition to the C = C bond, a transannular reaction is observed, which even involves the much less reactive [2.2.2]bicyclooctene moiety, thereby yielding the tandernL9] addition product 4. Both reactions were run under nonoptimized conditions and the pure products 2 and 4 resulted from one recrystallization of the crude material without chromatographic purification. Therefore, isomers of 2 and 4 are hence absent. Results & -PhSH . " S & 10aIt is well-known that under radical initiation thiols react highly strained norbornene adds thiophenol smoothly to For a more detailed study we concentrated our work on 5[*1, which contains the olefinic bridge in a norbornene moiety. Both thiophenol and phenylmethanethiol were tested, together with ethanethiol as a representative of aliphatic thiols. The rather puzzling results are collected in Table 1.

A Homage to Siegfried Hünig and His Research

“…(4)], [30] and the transannular addition of thiophenol to give an octacyclic product [Eq. (5)] [31] are presented here. The preparation and the (3 + 2) cycloaddition of the azoxy compound furnished two regioisomeric products that differ in the location of the double bond in the anellated cyclopentene ring.…”

Eine Hommage an Siegfried Hünig und seine Forschung

“…(4)] [30] und die transannulare Addition von Thiophenol zu einem octacyclischen Produkt [Gl. (5)] [31] . Bei der Herstellung und der (3+2)‐Cycloaddition der Azooxyverbindung [Gl.…”

“…(5)]. [31] Bei der Herstellung und der (3 + 2)-Cycloaddition der Azooxyverbindung [Gl. (4)] entstehen zwei Regioisomere, die sich in der Lage der Doppelbindung im anellierten Cyclopentenring unterscheiden.…”