Highly regio-, stereo-, and chemoselective Diels–Alder reaction of monothiomaleimide, an ambident CS and CC dienophile

Abstract

The thiocarbonyl group of monothiomaleimide 1 serves as a more reactive dienophile than the electron-deficient C=C double bond in the same molecule for the Diels-Alder reaction with dienes 2c-g and provides ortho-endo products 3 exclusively or predominantly over the other possible adducts 4-10.

Competition between Diradical Stepwise and Concerted Mechanisms in Chalcogeno-Diels−Alder Reactions: A Density Functional Study

“…The Diels−Alder (DA) reaction (Scheme ) is symmetry allowed according to the Woodward−Hoffmann rules and, thus, is presumed to proceed synchronously in a single kinetic step via a concerted transition state. A single-step mechanism for the DA reaction has been verified in many theoretical studies. − A few exceptions from the “rule of synchronicity” have been predicted theoretically by Houk and co-workers 5 for hetero-DA reactions. According to a valence bond (VB) investigation of pericyclic reactions by Bernardi, Olivucci, and Robb,2c the synchronicity in the formation of cyclic transition structures may be destroyed easily.…”

“…A single-step mechanism for the DA reaction has been verified in many theoretical studies. − A few exceptions from the “rule of synchronicity” have been predicted theoretically by Houk and co-workers 5 for hetero-DA reactions. According to a valence bond (VB) investigation of pericyclic reactions by Bernardi, Olivucci, and Robb,2c the synchronicity in the formation of cyclic transition structures may be destroyed easily. There is a very delicate balance between the valence bond coulomb terms (which slightly prefer synchronicity) and the valence bond exchange terms (which slightly prefer asynchronicity).…”

“…An ionic stepwise pathway via a dipolar intermediate is also possible. Such a mechanism operates, for example, in the addition of singlet oxygen to cis,trans- and trans, trans- 2,4-hexadiene 2i or in the triazolinedione DA reactions with sterically hindered dienes 2f 1 …”

“…For the parent DA reaction, a stepwise mechanism was predicted theoretically by Houk and co-workers 2e,f and by Sakai 2h to be energetically slightly less favorable than the concerted mechanisms. One might anticipate that a stepwise mechanism would compete favorably with the concerted reaction if the diene or dienophile is so sterically hindered as to effectively block the formation of the concerted transition structure. 2f, The diradical stepwise mechanism is often invoked whenever there is evidence of a nonconcerted reaction i.e., when the DA 1,4-adduct formed does not follow from the anticipated concerted transition state. There is probably only one experimentally known example of a DA reaction where a diradical mechanism clearly operates and yields 1,4-adducts, namely the reactions of furanones with dienes.…”

Synthesis of N-substituted-4-methylene-oxazolidinones via base-catalyzed cyclization of propargylic alcohols with p-toluenesulfonyl isocyanate

“…Consequently, there is increasing interest being paid toward the synthesis of N -substituted-4-methylene-oxazolidinones. 5–11 Most of these approaches are focused on nucleophilic cyclization via transition-metal-catalyzed strategies. However, the high catalyst loading in some of these processes results in a high economic cost.…”