DMSO activators carbodiimide

Note Pfitzner-Moffatt reaction represents oxidation of primaiy and secondary alcohols by dimethyl sulfoxide (DMSO) activated with carbodiimides, usually dicyclohexylcarbodiimide (DCC). Intermediary alkoxysulfonium ylides rearrange to aldehydes or ketones. This reaction yields urea as a by-product that is difficult to... 

The activation of DMSO toward the addition step can be accomplished by other electrophiles. All of these reagents are believed to form a sulfoxonium species by electrophilic attack at the sulfoxide oxygen. The addition of the alcohol and the departure of the sulfoxide oxygen as part of a leaving group generates an intermediate comparable to C in the carbodiimide mechanism. 

The Pfitzner Moffatt oxidation is performed in the presence of a carbo-diimide that is transformed into a form of activated DMSO . As both the carbodiimide and the activated DMSO are strong electrophiles, it would seem reasonable to expect that nucleophilic sites in a molecule would interfere with the oxidation. Nevertheless, Pfitzner-Moffatt oxidations very often can be carried out in the presence of thiols,14b amines40 and amides.23c,d... 

Some amines react under Pfitzner-Moffatt conditions, yielding an adduct with the carbodiimide or a S, S -dimethy lsulfilim i ne, resulting from attack of the amine on activated DMSO. The reactivity of different amines is very diverse, and observed in amines, which are not substantially protonated under the reaction conditions, while they still posses enough nucleophilicity. Thus, tertiary amines do not interfere, while hindered secondary ones seldom do it. 

A variety of methods have been described to solve the task in solution.16 Common oxidative agents for this transformation include various heavy-metal reagents such as chromium-or ruthenium-based oxides, pyri-dine-S03, and dimethylsulfoxide (DMSO) in combination with acetic anhydride, carbodiimide, or oxalyl chloride for activation. One of the most prominent methods for the reliable conversion of sensitive compounds is the Dess-Martin reagent or its nonacetylated equivalent, 1-hydroxy-(17/)-benzo-l,2-iodoxol-3-one-l-oxide (2-iodoxybenzoic acid, IBX). 

The mechanism of the P-M oxidation consists of three distinct steps 1) activation of the DMSO by a protonated dialkyl carbodiimide 2) activation of the alcohol substrate and the formation of the key alkoxysulfonium ylide intermediate and 3) the intramolecular decomposition of the alkoxysulfonium ylide to afford the product ketone or aldehyde and the dialkyl urea by-product (established by isotopic labeling studies). The alkoxysulfonium ylide is a common intermediate in all other oxidations using activated DMSO. 

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