Wittig-Horner condensation

Wittig or Wittig-Horner condensation between substituted terephthalaldehydes and p-xylene-diylphosphonium salts is also extensively used in the preparation of alternating PPV copolymers, e.g., containing different substituents in adjacent phenylene units (Scheme 2.8) [48]. 

Fermentation obviously does not afford the quantities required for clinical tests, so that a total synthesis was inevitable. Many of the methods described in earlier sections have been employed. The synthesis is convergent and combines the fragments (C-34-C-21) and (C-10-C-18) via a Wittig-Horner condensation. 

In the reaction between the (2-oxopropyl)phosphonic diesters and the aminals (167) in the presence of monochloroacetic or trifluoroacetic acid, the Wittig-Horner condensation is entirely suppressed and the (acetylstyryl)phosphonic diesters (168) can be obtained in good to excellent yields. The use of acetic acid as the catalyst led to poor results. Variations in the yields of products are to be found associated with the nature of group NO2. CN and CF3 groups favour the reaction, but reduced yields were found for 2-NO2 and 2- and 4-MeO substituents. The products (168) consist of (E) Z )mixtures of isomers, this ratio being commonly 80-90 20-10, and only rarely is the (Z ) isomer in excess. 

A second example of step-growth polycondensations with formation of the ole-finic double-bond are Wittig- and Wittig-Horner-type condensations. The Wittig-type polycondensations involve AA/BB-type reactions of aromatic bisal-dehydes with bisphosphonium ylides [99,100] with formation of PPV derivatives (75) and lead to products of only moderate molecular weight (DP 10-20). 

Recently, Meier et al. synthesized the most extended OP Vs (81) known to date [104]. Four different synthetic pathways were used for the generation of these OP Vs the final step involves the formation of one or two olefinic double bonds via a Wittig-Horner-type, a Siegrist-type or a McMurry-type condensation. 

The Orthogonal Approach (Combined Heck- and Wittig-Horner-Type Condensations) to PPM Oligomers... 

Kim et al. have introduced silicon atoms in PPV block copolymers to confine the conjugation length and achieve blue EL materials. Copolymers 188-190 [215] and 191 [216] have been synthesized by Wittig-Horner and Knoevenagel condensation, respectively. The emission band in this series can be tuned between 410 and 520 nm, and ITO/polymer/Al PLEDs with turn-on voltages 7 V have been reported (Chart 2.41). 

The condensation of aldehydes and ketones with aminoacetonitriles, although it requires more vigorous soliddiquid catalytic conditions to produce the cyano-enamines, is preferable in many respects to the traditional Wittig-Horner or Peterson procedures [45]. Hydroxyalkylphosphonates are obtained from the catalysed aldol condensation of nitromethane with acylphosphonates [46]. 

The catalyst surface contains both reducing and basic sites and experiments involving selective poisoning of the former by m-dinitrobenzene demonstrated the high probability of a SET mechanism. The same group also concluded that an SET mechanism was probably involved for Michael, Wittig-Horner, and Claisen-Schmidt condensations in the presence of the same catalyst [139,140]. 

Other nucleophilic additions conducted in aqueous media can be found in the literature (Lubineau et al., 1994 Strauss, 1999). These reactions will include the benzoin condensation, the Prins reaction, and the Wittig-Horner reaction, and the Baeyer-Villi-ger oxidation. 

Magnesium, 235 Samarium(II) iodide, 270 Titanium(IV) chloride, 304 Addition reactions to carbonyl groups—Addition of functionalized CARBON NUCLEOPHILES (see also Aldol reaction and other specific condensation reactions, Meth-ylenation, Peterson Olefination, Refor-matsky reaction, Wittig reaction, Wittig-Horner reaction)... 

Various methods have been used to lengthen a carbon side chain, in the syntheses of quinuclidyl-3-acetic acid (113) (Scheme 1). The lowest yield is in the sodiomalonic ester synthesis (13.4% ).125 Much better results are afforded by the Reformatsky reaction (40%)125 and Knoevenagel condensation (65-70%).129 The best yield (nearly quantitative) may be obtained by application of the Wittig-Horner reaction.155... 

Fig. 11.10. Wittig-Horner synthesis of alkenes with stereogenic C=C double bond via condensation of lithiated phosphine oxides B <-> B with aldehydes. In the two-step reaction the initially formed addition products syn- and anti-D are protonated and the resulting alcohols syn- and anti-C are isolated and separated the suitable diastere-omer is then stereoselectively converted into the desired alkene isomer. The one-step reaction leads to the same alkene in a cis,trans mixture.

Intramolecular Wittig-Horner reaction of the compounds 399, which are prepared by the condensation of 2-mercapto-2-arylethanols with a vinyl phosphonate followed by oxidation, gives dihydrothiophenes 400 <1996JHC687>. 

Asymmetric synthesis of a chroman. Solladie and Moine have effected an en-antiospecific synthesis of the chroman-2-carboxaldehyde 7, a key intermediate in the synthesis of a-tocopherol, from (R)-( +)-l. The phosphonate 2, derived from 1, undergoes a Wittig-Horner reaction with the dimethyl ketal of pyruvaldehyde to afford the optically active vinyl sulfoxide 3. Condensation of the aldehyde 4 with the lithio derivative of 3 affords, after silyl deprotection, the allylic alcohol 5 as the only diastereoisomer. This... 

Some further examples of the condensation of carbonyl compounds with phospho-nate carbanions (Wittig-Horner reaction) include the transformations illustrated in Scheme 6. The failure of some related reactions is not properly understood. 

The Wittig-Homer reaction between C-metallated a-phosphoryl sulfides and carbonyl compounds provides a common route to a,P-unsaturated sulfides, but one which usually results in poor yields of the desired product when enolisable ketones are used. Stephan et al. have investigated the role of boron trifluoride in promoting the Wittig-Horner reaction of enolisable ketones, which usually follows a two-step process the initial condensation of the anion followed by an elimination step. It was found that when the second step occurred at a slow rate then addition of boron triflouride was necessary for completion. ... 

The first part of this chapter deals with the effects of high pressure on cycloaddition reactions, particularly the Diels-Alder reaction, which is the most important cycloaddition reaction. The second part will illustrate applications of pressure to nucleophilic substitutions, condensations and other reactions (miscellaneous reactions), such as Mannich, Heck, ene, SeAr, Wittig, Horner-Wadsworth Emmons and multicomponent Strecker reactions. 

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