Aliphatic Thiocarbonyl Ylides and Thiobenzophenone: Experimental Study of Regiochemistry and Methylene Transfer in Cycloadditions

Abstract

Abstract1,3‐Dipolar cycloadditions of aliphatic or alicyclic thiocarbonyl ylides 3A‐D – sterically hindered at least at one terminus – with thiobenzophenone produce both regioisomeric 1,3‐dithiolanes 4 and 5. According to quantum‐chemical calculations (preceding paper), a concerted cycloaddition furnishing 2,4‐substituted dithiolanes 4 competes with the formation of an intermediate C,C‐biradical 9 which cyclizes to the more crowded 4,5‐substituted dithiolanes 5. When steric hindrance of 3 increases, the cycloaddition is superseded by ‘methylene transfer’, i.e., the transfer of the less hindered terminus of 3E‐J to the S‐atom of thiobenzophenone. The thiobenzophenone S‐alkylide 11, thus formed, rapidly reacts with a second molecule ofthiobenzophenone to generate the 4,4,5,5‐tetraphenyl‐1,3‐dithiolane 12 via the highly stabilized C,C‐biradical 10. Methylene transfer occurs when the cyclization of the mixed C,C‐biradical 9 requires a higher activation barrier than its dissociation to aliphatic thioketone + 11; the threshold is surprisingly well reproduced by calculations. The structural assignment of sixteen 1,3‐dithiolanes is based on their formation from corresponding reactant pairs as well as on 1H and 13C chemical shifts. X‐ray diffraction analyses of three spiro‐1,3‐dithiolanes reveal the van der Waals strain in non‐bonded interactions, folding angles, shearing forces, and bond lengths. Comparison of the mass spectra of many 1,3‐dithiolanes allows the reconstruction of major fragmentation pathways. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)