Horner-type reagent

Tetrahydroselenopyran-4-one 100 readily reacts with Wittig-Horner-type reagents (Equation 43) <2002MCL107>. Reactions of this type provide convenient routes to precursors of selenium-containing organic conducting materials (see Section 7.11.8.2). 

Difluoroalkenes1 (cf., 11, 180). The anion (LDA) of this reagent effects difluoromethylenation of aldehydes or ketones by a Wittig-Horner type reaction. 

Simple unfunctionalized ketones such as aryl alkyl ketones 36 can be olefinated by ynolates to provide tetrasubstituted olefins 37-42 in good to excellent yield (Fig. 12) [56]. While the Wittig and the Horner-Emmons reagents are not suitable reagents for this type of olefination, especially for ferf-butyl phenyl ketone (43), the ynolate affords the corresponding olefin 41 in 74% yield (Fig. 13). Ynolates are actually much better reagents for the olefination of ketones than the conventional... 

The precursors for these Horner-Wadsworth-Emmons reagents are /f-ketophosphomc acid dialkyl esters or a-(alkoxycarbonyl)phosphonic acid dialkyl esters. The first type of compound, i.e., a /3-ketophosphonic acid dialkyl ester is available, for example, by acylation of a metalated phosphonic acid ester (Figure 6.48). The second type of compound, i.e., an a-(alkoxycarbonyl)phosphonic acid dialkyl ester, can be conveniently obtained via the Arbuzov reaction (Figure 11.12). 

Recent advances in the stereoselective olefination of phosphorus-stabilized carbon nucleophiles have been reviewed. Applications of the Horner-Wadsworth-Emmons reaction to the synthesis of natural products have been highligted. A highly Z-selective synthesis of a, -unsaturated nitriles using the Homer-Wadsworth-Emmons reaction has been reported this involves a new nitrile reagent, (o-t-BuC6H40)2P(0)CH2CN, which reacts with various types of aldehydes with 86 to >99% Z selectivity. 

Various types of phosphorus reagents have been employed for asymmetric transformations of carbonyl compounds into alkenes [2, 4, 7-11]. Depending upon the structure of these reagents, different reaction names are given [3]. Phosphonium ylides and phosphine oxide are particularly popular, and are referred to as Wittig and Horner reagents, respectively. On the other hand, phosphonates and other phosphonic acid derivatives are termed Horner-Wadsworth-Emmons (HWE) reagents. In recent years, arsonium derivatives have often been used for similar reactions, and they are also covered herein. 

By far the main interest in the reaction of halo [ C] acetates with phosphorus nucleophiles is for the preparation of phosphoryl-stabilized carbanions for use in Wittig and related reactions. The presence of the additional electron-withdrawing ester group provides additional stabilization, significantly modifying the reactivity of the ylide species and the stereochemical course of its reactions. The two phosphorus reagents discussed here include the triphenylphosphonium salt type 158. precursors of Wittig methylenetriphe-nylphosphoranes, and the trialkylphosphonoacetate type 159. applied in the Horner-Wadsworth-Emmons family of reactions . ... 

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