SYN-[2.2]METACYCLOPHANE - ISOLATION, NMR PROPERTIES, AND FACILE ISOMERIZATION TO ANTI-[2.2]METACYCLOPHANE - A SYNTHESIS INVOLVING BRIDGE REACTIONS OF CHROMIUM TRICARBONYL COMPLEXED DITHIAMETACYCLOPHANES

Chromium tricarbonyl complexation of 2,11-dithia[3.3]metacyclophane (11) holds it in the syn form. Ring contraction using a Stevens rearrangement of the derived mono- or bis(tricarbonylchromium)-complexed sulfonium salts 21 or 39 yields tricarbonylchromium complexed syn-[2.2]metacyclophanes with methylthio bridge substituents. Removal of the chromium with Ce(IV) gives free substituted syn-[2.2]metacyclophanes, which isomerize to the analogous anti derivatives below 0 °C. The complexed jyn-phane (η6-syn-2(a)-10(e)-bis(methylthio)[2.2]metacyclophane)tricarbonylchromium(0) (22) does not isomerize until about 80 °C, making handling of the complexed phanes easier. Removal of the substituents from 22 to give 35 could not be achieved directly; however, biscomplexation did allow successful isolation of the parent syn-[2.2]metacyclophane (1) and its mono- and bischromium complexes (35 and 48, respectively). Preliminary thermodynamic studies indicate that ΔH* and ΔG*298 for the isomerization of a syn- to an anti-[2.2] metacyclophane are about 17-18 and 20-21 kcal/mol, respectively, and that chromium tricarbonyl complexation of one of the rings raises these values by about 4 kcal/mol. The 1H and 13C NMR data of the syn and anti metacyclophanes are compared and discussed relative to the X-ray structures, which are presented for 22 and 28. The first example of a complexed metacyclophanediene 34, (η6-am/-[2.2]metacyclophane-1,9-diene)tricarbonylchromium(O), is reported, and interestingly it does not reduce with H2/catalysts. © 1990, American Chemical Society. All rights reserved.