Ylides, reactions with carbodiimides

A modified, o-alkoxy-substituted ylide 783 has an improved solubility in nonpolar solvents, such as aromatic hydrocarbons and has a generally higher reactivity than ylide 781 [1069]. In particular, ylide 783 is a useful reagent for the preparation of oxazole derivatives 784 in the reaction with carbodiimides 782 under homogeneous conditions in the presence of Rh(II) or Cu(II) catalysts (Scheme 3.309) [1069]. 

The reaction of ylides with carbodiimides usually produces linear 1 1 adducts. The adducts derived from DMSO and carbodiimides undergo a facile reaction with primary alcohols to give an aldehyde (Moffat oxidation). With phenols and carboxyhc acids, alkylation products and esters, respectively, are formed. The oxidation proceeds under mild conditions and can be applied to sensitive compounds. Primary alcohols are oxidized solely to aldehydes without the formation of even trace amounts of carboxylic acids. The carbodiimide adducts generated from DMSO or the dimethylseleniumoxide " adducts have structure 369 (X = S, Se). 

Iminophosphoranes (96) react with primary isocyanates (97) or isothiocyanates (98) to give the betaines (99), while similar reaction with / opropyl or tertiarybutyl isocyanate gives the corresponding carbodiimides (100).52 The reactions of 1,3-ylide-anions (101) with phenylisocyanate and with dicyclohexacarbodiimide have been investigated.53 Both reactions follow a similar path to provide the adducts (102) which are themselves ylide-anions. Due to their substantial nucleophilicity, compounds (102) react with ketones as well as aldehydes to give a,p-unsaturated anilines (103) and amidines (104) with good (E)-stereoselectivity. 

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... 

In a one-pot reaction (Scheme 126) the iminophosphorane of 6-aminoura-cil (346) (92AHC129) is transformed with isocyanate in the presence of pyridine into a nonisolable carbodiimide. Spontaneous addition of pyridine follows to give a 1,6-dipolar pyridinium ylide (347), which cyclizes to the... 

Iminoalkylidenetriphenylphosphoranes result from the transylidation reaction of methylenetriphe-nylphosphorane with imidoyl chlorides (equation 76), or from Michael addition of ylides to the C—43 bond of ketenimines (formed from carbodiimide and ylides in the initial step). 

The original Pfittner-Moffatt procedure for alcohol oxidation by activated dimethyl sulfoxide utilized dicyclohexylcarbodiimide (DCC) and a source of protons such as polyphosphoric acid or pyridinium tri-fluoroacetate. The use of strong acids such as the common mineral acids must be avoided since, although acidic conditions are initially required, the reaction must readily become basic in the later stages of the process. Mechanistically it is reasonable to suggest that the activation follows the pattern whereby initial attack of the nucleophilic sulfinyl oxygen of dimethyl sulfoxide, with the protonated carbodiimide, forms a sulfonium isourea. This is followed by displacement of dicyclohexylurea by the alcohol to form an alkoxysulfonium salt. Base treatment of this salt forms an ylide, which collapses via the proven cyclic mechanism to the carbonyl compound and dimethyl sulfide (Scheme 4). 

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