Dialkyl pyrocarbonates

Cobalt(II) alkoxides are known and monomeric forms are part of a wider review.413 The interest in these compounds pertains to a potential role in catalysis. For example, a discrete cobalt(II) alkoxide is believed to form in situ from a chloro precursor during reaction and performs the catalytic role in the decomposition of dialkyl pyrocarbonates to dialkyl carbonates and carbon dioxide.414 A number of mononuclear alkoxide complexes of cobalt(II) have been characterized by crystal structures, as exemplified by [CoCl(OC(t-Bu)3)2 Li(THF)].415 The Co ion in this structure and close relatives has a rare distorted trigonal-planar coordination geometry due to the extreme steric crowding around the metal. 

An additional minor source of urethane can be the reaction of unconsumed reagent with V-nucleophile (path C). Aminolysis of chloroformate occurs if there is an excess of reagent or if the anhydride-forming reaction is incomplete. The latter is more likely when the residues activated are hindered. This side reaction can be avoided by limiting the amount of reagent and extending the time of activation. A third side reaction that is of little consequence is disproportionation of the mixed anhydride to the symmetrical anhydride and dialkyl pyrocarbonate (see Section 7.5). 

The protocol described also can be used for the acylation of ketone enolates with carbonic acid derivatives (Figure 13.62). Especially good acylating agents are cyanocarbonic acid methyl ester (Mander s reagent, Figure 13.62, top) and dialkyl pyrocarbonates (bottom). Usually it is not possible to use dimethyl carbonate for the acylation of ketone enolates because dimethyl carbonate is a weaker electrophile than cyanocarbonic acid methyl ester or diethyl pyrocarbonates. 

Fig. 13.62. Acylation of ketone enolates with carbonic acid derivatives. Especially good acylation reagents are cyanocarbonic acid methyl ester (top) and dialkyl pyrocarbonates (bottom).

The action of sodium alkyl carbonates on 3-chloro-l,2-benzisothiazole 1,1-dioxide (64) produces mixtures of AT-alkoxycarbonylsaccharins (65) and pseudosaccharin anhydride (66), together with dialkyl pyrocarbonates. The product ratio of (65) to (66) decreases in the order (R = ) Me > Et > Pr > Bu. The initial step in the reaction is considered to be a nucleophilic displacement of chlorine by ROCOO ion. ... 

FMF Chen, M Slebioda, NL Benoiton. Mixed carboxylic-carbonic acid anhydrides of acylamino acids and peptides as a convenient source of 2,4-dialkyl-5(4//)-oxazo-lones. (pyrocarbonates for mixed anhydrides) Int J Pept Prot Res 31, 339, 1988. 

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