Diradical reaction mechanisms in [3 + 2]-cycloadditions of hetaryl thioketones with alkyl- or trimethylsilyl-substituted diazomethanes.

Grzegorz Mlostoń,Paulina Pipiak,Heinz Heimgartner

doi:10.3762/bjoc.12.71

Grzegorz Mlostoń, Paulina Pipiak + Show 1 more

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https://doi.org/10.3762/bjoc.12.71

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Abstract

Reactions of dihetaryl and aryl/hetaryl thioketones with 2-diazopropane, diazoethane, and (trimethylsilyl)diazomethane were studied at variable temperature. The experiments showed that reactions with 2-diazopropane carried out at –75 °C occur mainly via the initially formed, relatively stable 1,3,4-thiadiazolines as products of the [3 + 2]-cycloaddition of the diazo dipole onto the C=S bond. The latter decompose only at higher temperature (ca. −40 °C) to generate thiocarbonyl S-isopropanide. In the absence of the starting thioketone, the corresponding thiiranes and/or ethene derivatives, formed from them via spontaneous desulfurization, are the main products. In contrast, reactions with diazoethane occurred predominantly via initially formed diradicals, which in cascade processes gave sterically crowded 4,4,5,5-tetrahetaryl-1,3-dithiolanes as major products. Finally, the reaction of dihetaryl thioketones with (trimethylsilyl)diazomethane occur smoothly at −75 °C leading to the corresponding 4,4,5,5-tetrahetaryl-1,3-dithiolanes as the exclusive [3 + 2]-cycloadducts formed via a cascade of postulated diradicals. The presence of S or Se atoms in the hetaryl rings is of importance for stabilizing diradical intermediates. Remarkably, in no single case, the ‘head-to-head dimerization’ of aryl/hetaryl and dihetaryl substituted thiocarbonyl ylides was observed.

Highlights

Cycloaddition reactions belong to the most important classes of organic reactions, and [3 + 2]- cycloadditions, known as 1,3-dipolar cycloadditions or Huisgen reactions, offer a universal tool for the preparation of five-membered heterocycles with a variable number of heteroatoms in the ring [1,2].In addition to their practical importance, discussions on the mechanism contribute significantly to the development of fundamental concepts in organic chemistry [3,4,5,6,7]
In our ongoing studies on thioketones and their applications in the cycloaddition chemistry, we described in a recent publication the unexpected course of the reaction of diazomethane with aryl/selenophen-2-yl thioketones of type 1c, leading to unusual dimers 5 of intermediate thiocarbonyl ylides of type 3c [22] (Scheme 2)
A similar tendency was observed in other studied cases with hetaryl thioketones and 7a. The replacement of the latter by diazoethane (7b), leading to the less stable 1,3,4thiadiazolines 2, resulted in a general increase of the corresponding 1,3-dithiolanes 10, which were formed in substantial amounts, even in experiments performed at −75 °C

Summary

Introduction

Cycloaddition reactions belong to the most important classes of organic reactions, and [3 + 2]- cycloadditions, known as 1,3-dipolar cycloadditions or Huisgen reactions, offer a universal tool for the preparation of five-membered heterocycles with a variable number of heteroatoms in the ring [1,2].In addition to their practical importance, discussions on the mechanism contribute significantly to the development of fundamental concepts in organic chemistry [3,4,5,6,7]. Scheme 3: Formation of thiiranes 8 and/or 1,3-dithiolanes 10 in the reaction of aryl/aryl, aryl/hetaryl and dihetaryl thioketones 1 with 2-diazopropane (7a), diazoethane (7b), and (trimethylsilyl)diazomethane (7c) (Table 1).

Results

Conclusion