Chalcogenate Esters

Tris(trimethylsilyl)silane is found to be an efficient reducing agent for a variety of functional groups. In particular, the reduction of halides, chalcogen groups, thiono esters and isocyanides are the most common ones. The efficiency of these reactions is also supported by available kinetic data. The rate constants for the reaction of (TMS)3Si radicals with a variety of organic substrates are collected in Table 2. 

The same photolytic treatment of O-acyl esters (2) in the presence of disulfides, diselenides, and ditellurides effectively produces the corresponding alkyl sulfides, alkyl selenides, and alkyl tellurides respectively, through SHi reaction on the chalcogen atoms by alkyl radicals, as shown in eq. 8.9. The reactivities somewhat depend on the kind of chalcogenides. Thus, the effective formation of alkyl sulfides requires 30 eq. of disulfides, that of alkyl selenides requires 10 eq. of diselenides, and that of alkyl tellurides requires 2 eq. of ditellurides [27, 28]. 

ArsBi bearing ortho methoxy groups mediates dehydrative condensation of a-monosubstituted carboxylic acids with alcohols and amines (Scheme 14.139) [288]. Macrocyclic esters can be synthesized by the ArsBi-templated reaction of diols with dicarboxylic acid derivatives [289]. The Bi-C bonds of Ar Bi are cleaved by diphenyl diselenide and ditelluride to give aryl phenyl selenides and tellurides, respectively (Scheme 14.140) [290]. The reaction of ArsBi with elemental chalcogen (E Se, Te) affords a mixture of the respective dichalcogenides (ArEEAr) and monochalcogenides (ArEAr). 

Despite statements made in Chapter 2 about the unpopularity of this procedure for the preparation of phosphonic and phosphinic esters from those of phosphonous and phos-phinous acids by the addition of oxygen, partly because of the lack of availability of the phosphorus(III) esters, but also because of the high reactivity exhibited by those esters towards oxidizing agents, the reduced reactivity shown towards the higher chalcogens by phosphonous and phosphinous halides (dihalo- and monohalo-phosphines), and even by some of the more reactive phosphorus(III) species, e.g. amides, makes such addition reactions feasible propositions. 

Historically, the first chalcogenocarboxylic acid discovered was thiocarboxylic acid - thioacetic acid - reported by Kekule in 1854 [1]. Since then, chalcogenocarboxylic acids, and particularly numerous thio- and dithiocarboxylic acid esters, have been synthesized and summarized in several review articles [2-9]. In contrast, until very recently, little has been known about the chemistry of the congeners containing heavier chalcogen atoms, such as selenium and tellurium, probably due to their instability and the handling difficulties associated with them. In this chapter, the chemistry of chalcogenocarboxylic acids, their syntheses, structures, spectral features and reactions are reviewed. 

There are a few reports of syntheses of thioacylsulfenyl chalcogen compounds of oxygen, selenium and tellurium, such as the thioacylsulfenate ester 107 [169] and its heavier chalcogen derivatives 108 [169], the haloselenium-and halotellurium dithiocarboxylates 109 [170, 171], and the selenium- and tellurium bis(dithiocarboxylates) 110 (Fig. 22) [171,172]. The chemistries of oxygen, selenium, and tellurium compounds bearing dithiocarboxylate ligands have not been studied as closely as those of the sulfur compounds. 

Y. Takashima, Y. Nakayama, K. Watanabe, T. Itono, N. Ueyama, A. Nakamura, H. Yasuda, A. Harada, J. Okuda, Polymerizations of cyclic esters catalyzed by titanium complexes having chalcogen-bridged chelating diaryloxo ligands. Macromolecules 35 (2002) 7538-7544. 

---