Horner-Wittig modification

Horner-Wittig modification Alternatively, phosphine oxide reacts with aldehydes in the presence of a base (sodium amide, sodium hydride or potassium t-butoxide) to give an alkene. The phosphine oxide can be prepared by the thermal decomposition of alkyl-triphenylphosphonium hydroxide. Deprotonation of phosphine oxide with a base followed by addition to aldehyde yields salt of (3-hydroxy phosphineoxide, which undergoes further syn-elimination of the anion Ph2P02. The lithium salt of (3-hydroxy phosphineoxide can be isolated, but Na and K salt of (3-hydroxy phosphine oxide undergoes in situ elimination to give alkene (Scheme 4.26). 

When the halogen compound employed in the first step has an activated halogen atom (RCH CHCHjX, QHjCHjX, XCH2CO2H) a simpler procedure known as the Horner phosphonate modification of the Wittig reaction is applicable. When benzyl chloride is heated with triethyl phosphite, ethyl chloride is eliminated from the initially formed phosphonium chloride with the production of diethyl benzylphosphonate. This phospho-... 

Poly(p-arylene vinylene)s were synthesized with low molecular weight via the Horner-Emmons modification of the Wittig reaction [12] and via the precursor route [13]. Efforts to synthesize poly(p-anthrylene vinylene) via a Heck reaction failed [15]. We chose ROMP followed by a dehydrogenation step to produce poly(2-alkyl-9,10-anthrylene vinylene) as has been shown before for poly(l,4-naphthylene vinylene) by Pu et al. [16]. The initiator used was the Schrock molybdenum carbene shown in Figure 1 and our goal was to produce a soluble poly(anthrylenevinylene). 

A one-carbon homologation of aldehydes and ketones to carboxylic acids has been reported to involve the Horner-Emmons modification of the Wittig reaction using diethyl-/er/-butoxy(cyano)-methylphosphonate (EtO)2 P0CH(CN)0/-Bu to produce ethyl-/pr/-butoxyacrylonitriles. The /er/-butyl ether group is cleaved by zinc chloride in refluxing acetic anhydride, and the a-acetoxyacrylonitrile is converted to the acid by solvolysis. 

Wittig-Horner reaction (see p. 241). Photolysis of the nitrite of the 21-chloro-20-hydroxy-compound (194) gave the 18-oximino-compound (195) which on Jones oxidation was converted into the chlorolactone (196). Hypoiodite reaction of the 21-chloro-20-hydroxy-compound (194) also gives the chloro-lactone (196) which with modification gives 18-hydroxy-11-deoxycorticosterone (see also ref. 198). 

The preparation of a,/ -unsaturated aldehydes and ketones is exemplified by the following four important methods (a) the oxidation of the corresponding unsaturated primary or secondary alcohol (b) the Horner-Emmons or Wadsworth-Emmons modification of the Wittig reaction248 (c) the aldol con-... 

THE HORNER-EMMONS OR WADSWORTH-EMMONS MODIFICATION OF THE WITTIG REACTION... 

Step 1 (3-keto phosphonates often are used for olefmation of ketones (Horner-Wadsworth-Emmons modification of the Wittig reaction). Steps 2 + 3 The cuprate-mediated 1,4-addition and subsequent Lemieux-Johnson oxidation of a vinyl group are excellent procedures for the introduction of the p-formyl group. 

Horner-Wadsworth-Emmons (HWE) reaction There is a close relationship between the original Wittig alkenation and its phosphonate ester modification which is known as... 

In conclusion, we would thank our team of contributors for their efforts in writing for these volumes in recent years. As noted above, this volume may represent the end of an era which has extended over more than 35 years, the first volume having appeared in 1970 under the editorship of Professor Stuart Trippett. It would be a serious omission in writing this preface if we failed to note the passing of Professor Leopold Horner (1911-2005) who contributed so much to the development of organophosphorus chemistry, particularly to the development of routes to chiral phosphines and, of course, to the Horner modification of the Wittig reaction. 

Horner-Wadsworth-Emmons Reactions of Phosphonate Anions. - As with the Horner modification of the Wittig reaction, the principal focus of papers that mention the Horner-Wadsworth-Emmons reaction relate to synthetic applications. The use of pressure to induce the synthesis of P-amino esters, p-thioesters and P-thionitriles via tandem Horner-Wadsworth-Emmons and Michael reactions has been reported. The reagent (l-tritylimidazol-4-yl)methylphosphonate (99) has been prepared and, when treated with aldehydes and ketones, affords (E)-vinylimidazoles in high yields. ... 

Related reactions Horner-Wadsworth-Emmons olefination, Homer-Wadsworth-Emmons olefination - Still-Gennari modification, Julia-Lithgoe olefination, Takai-Utimoto olefination, Tebbe oiefination, Wittig reaction, Wittig reaction - Schiosser modification ... 

The past year has seen continued interest in the structural nature of phosphorus ylides with the aim of gaining greater insight into their stabilities, electronic distributions and conformations, which ultimately affect the reactivities of these species. However, there have been fewer mechanistic studies of the Wittig reaction itself although several studies into the closely related aza-Wittig and Horner-Wadsworth-Emmons modifications have appeared. 

The Horner-Wadsworth-Emmons modification of the Wittig reaction continues to find wide application in contemporary organic synthesis. Two reviews, looking at different aspects of the stereoselectivity of this procedure, have been published. One considers approaches to improve the Z-selectivity of the Horner-Wadsworth-Emmons reaction, and the other the use of heteroatoms to improve the stereoselectivity of the Horner-Wadsworth-Emmons reaction. A combined experimental and computational study of the selectivity of the Horner-Wadsworth-Emmons reaction of a series of phosphonoacetates (172) has... 

The Horner modification " of the Wittig reaction uses substituted lithio-(dialkylaminomethyl)diphenylphosphine oxide(49, = H, alkyl, aryl = Me, Et, Ph ... 

The Wittig reaction along with its modification (Horner-Emmons-Wadsworth) has been employed to generate a variety of interesting structures. For example, conjugated polyenals have been synthesized as shown in nation (46) <88SC51>. 

Some of the disadvantages of the Wittig reaction can be overcome by use of the Horner-Wadsworth-Emmons reaction or, in brief, the Horner-Emmons reaction [6,14-16]. The modification of the Wittig process makes use of a phosphonate-stabilized carbanion instead of a phosphorus ylide it is discussed in Section E. 

Purpose To demonstrate the synthesis of a stilbene by the Horner-Wadsworth-Emmons modification of the Wittig reaction. 

---