Cycloadditions of two thiocarbonyl ylides with α,β-unsaturated esters and nitriles: steric course and mechanism

New Thiocarbonyl Ylides Derived from 3,3‐Dichloro‐2,2,4,4‐tetramethylcyclobutanethione; Generation and Reactions.

“…The formation of the same mixture of two stereoisomeric products trans-and cis-23 from (E)-22 as well as from (Z)-22 is a clear evidence for the two-step mechanism analogous to that reported previously [13]. The initially formed zwitterionic intermediate 24a exists in an equilibrium with 24b, and the 1:1 ratio of trans-and cis-23 indicates that 24a and 24b equilibrate completely before the ring closure takes place, i.e., the ring closure is slower than the rotation about the C,C bond in the zwitterion.…”

“…As the only product, the crystalline 2,5-dihydro-1,3,4-thiadiazole 3a was obtained (Scheme 1). The 1 H-NMR spectrum showed the characteristic signal of the CH 2 group at 5.67 ppm; in the 13 C-NMR spectrum, this group resonates at 82.8 ppm. Analogous reactions with diazoethane (2b) and 2-diazopropane (2c) afforded the corresponding 2-methyl and 2,2-dimethyl derivatives (3b and 3c, respectively), which could be stored in the refrigerator for several days without decomposition.…”

“…Fractional crystallization gave one of the cycloadducts in pure form. The comparison of its 1 H-NMR data with those reported for analogous products [13] allowed to identify this cycloadduct as trans-23 (CH 2 appears as AB system with d = 3.51 and 3.59, J AB = 11.4 Hz). The cis-isomer was isolated from the mother liquor in ca.…”

“…In general, the sterically more hindered '2-CH2 1,3-dithiolane', formed via a 1,5-diradical intermediate, is the major product. The reactions of electron-rich and sterically hindered thiocarbonyl S-methanides with electron-deficient C,C-dipolarophiles showed that formal [2+3] cycloadducts can be formed by a stepwise reaction mechanism via zwitterionic intermediates [12][13][14]. One crucial criterion is the formation of stereoisomeric cycloadducts in the reaction with stereochemically pure dipolarophiles.…”

“…One crucial criterion is the formation of stereoisomeric cycloadducts in the reaction with stereochemically pure dipolarophiles. The relatively easily available dicyanofumarates and dicyanomaleates have been shown to act as very reactive dipolarophiles, which allow to examine the stereochemical course of the cycloaddition [13]. The reaction of 4a with diethyl dicyanofumarate (E-(22)) occurred without formation of thiirane 5a, and the presence of two AB-systems between 4.0 and 3.2 ppm indicated that two tetrahydrothiophenes cis-23 and trans-23 (Scheme 6) have been formed in a ratio of ca.…”