Reduction of Pseudo-geminal Bis(ethynyl)-Substituted [2.2]Paracyclophanes

Abstract

The reduction of pseudo-geminal bis(ethynyl)-substituted [2.2]paracyclophanes provides compounds with new bridges. The type of bridging is substituent dependent. The (trimethylsilyl)ethynyl moiety induces the formation of a bridge with two semicyclic double bonds; less bulky substituents, like propynyl and phenylethynyl, lead to bridges with endo double bonds.Key words acetylenes, reduction, dienes, lithium, paracyclophanes [2.2]Paracyclophane derivatives have been the subject of particular interest since their first appearance in the literature more than six decades ago.1-3 Since then, most studies have been devoted to elucidation of the structural characteristics of [2.2]paracyclophanes, particularly their geometry and steric properties, transanular interactions, and ring strain. [4][5][6] As far as the chemical behavior is concerned the vast majority of the reported reactions has been carried out at the benzene rings. The chemistry of the molecular bridges of these molecules should also be interesting, especially when the bridges carry important functional groups. In the last several years we have developed several methods for the introduction of new bridges into [2.2]paracyclophanes. 7 Although we reported the incorporation of various functional groups into the ethano bridges of [2.2]paracyclophanes, 8 the chemistry of this type of bridge has received little attention so far.Functional groups in pseudo-geminally substituted [2.2]paracyclophanes often undergo highly specific reactions. This is due to the rigid framework and the short distance within the aromatic units. In one such application, unsaturated cyclophane bis(esters) provided the corresponding ladderanes by intramolecular photocyclization. 9-11The 4,13-bis(ethynyl)[2.2]paracyclophane and its derivatives are interesting building blocks for molecular scaffolding, 12 since the ethynyl functions can easily be connected by various coupling reactions.Pentalenes represent another class of hydrocarbons with an impressive chemical history. 13 The development of the chemistry of these antiaromatic 4n hydrocarbons was strongly correlated with the number of efficient synthetic methods for the preparation of these compounds. A general route to dibenzopentalenes is represented by the metal-induced dimerization of different types of ethynyl aromatics. 14 For example, the reduction of tris(isopropyl)silylethynylbenzene (1) with excess lithium, followed by oxidation with iodine provided the dibenzo[a,e]pentalene derivative 2 in 7% yield (Scheme 1). 15 Scheme 1Prompted by these recent developments on the chemistry of pentalenes, we decided to investigate the reactions of a series of pseudo-geminally substituted bis(ethynyl) derivatives with lithium. Thus, we are presenting here a preliminary report on the interaction of lithium with 4,13-bis(trimethylsilyl)-, 4,13-bis(propyn-1-yl)-, and 4,13-bis(phenylethynyl)[2.2]paracyclophanes.In a first experiment we have used as substrate an ethynyl aromatic compound related to 1, 4,13-bis(trimethylsilyl)[2.2]paracyclo...

Synthesis of 4-(4-Hydroxyaryl)-1,3-Dithiolium Perchlorates

“…The presence of a hydroxyl substituent in the para-position induces an extended delocalization of the negative charge up to the C(4)-C(5) bond of the dithiolium ring. In comparison with the result reported for the unsubstituted 4-hydroxyphenyl derivatives [47], the spectrum of 1,3dithiolium perchlorate 3b indicate a bathochromic shift induced by the bromine substituents on the p ® p* absorption band from 290 nm to 310 nm that overlaps the band corresponding to the n ® p* transitions. The charge transfer absorption band of mesoionic phenolates 4 (380 nm) is overlapped by the large absorption band centered at 338 nm.…”

“…Treatment of these perchlorates with a saturated NaHCO 3 solution gives the corresponding phenolates 4 as yellow crystalline products, which show mesoionic character [47,50] (scheme 2). The molecular structure of the new compounds was proved by analytical and spectral data (table 3) and by the following chemical transformation: treatment of an acetone suspension of the mesoionic compounds 4 with 70% perchloric acid regenerates the 1,3-dithiolium perchlorates 3 in quantitative yields (scheme 2).…”

“…Scheme 2. Synthesis of mesoionic phenolates 4 and their interconversion with the corresponding 1,3-dithiolium perchlorates 3 In a previous paper [47], the comparative study of UV-Vis absorption spectra of 2-, 3-, and 4-[2-(pyrrolidin-1-yl)-1,3-dithiol-2-ylium-4-yl]phenolates has shown that the yellow color of the above zwitterionic compounds is due to a charge transfer between electron-rich and electrondeficient regions of the molecules and not to the contribution of quinoid structures in the ground states. The intramolecular nature of the charge-transfer band was proved by measurements at different concentrations.…”