Stability Wittig reaction

For a summary of current work on the mechanism of phosphoryl-stabilized Wittig reactions as well as the preparation of these reagents, see ref. 114. 

Use ofNon-stabiUzed YUdes. TOP sequences using non-stabilized Wittig reactions have also been carried out to great effect, generating the ylide in situ from the corresponding phosphonium salt and the bicyclic-guanidine base, MTBD (1-methyl-l,5,7-triazabicyclo[4.4.0]dec-5-ene) (5) (eqs 82 and 83).3 ... 

A nice substituted carbazole synthesis begins with installation of propargyl alcohol via its Sonogashira coupling with aryl iodide 69. Subsequent oxidation of propargylic alcohol 70 to the aldehyde, followed by stabilized Wittig reaction, affords a conjugated intermediate ene-yne 71,... 

With "non-stabilized" ylides the Wittig Reaction gives predominantly Z-olefins. Seebach et al... 

Aryl and alkyl trimethylsilyl ethers can often be cleaved by refluxing in aqueous methanol, an advantage for acid- or base-sensitive substrates. The ethers are stable to Grignard and Wittig reactions and to reduction with lithium aluminum hydride at —15°. Aryl -butyldimethylsilyl ethers and other sterically more demanding silyl ethers require acid- or fluoride ion-catalyzed hydrolysis for removal. Increased steric bulk also improves their stability to a much harsher set of conditions. An excellent review of the selective deprotection of alkyl silyl ethers and aryl silyl ethers has been published. ... 

The (Horner-)Wadsworth-Emmons reaction generally is superior to the Wittig reaction, and has found application in many cases for the synthesis of a ,/3-unsaturated esters, a ,/3-unsaturated ketones and other conjugated systems. Yields are often better then with the original Wittig procedure. However the Wadsworth-Emmons method is not suitable for the preparation of alkenes with simple, non-stabilizing alkyl substituents. 

Maleic anhydride is a convenient dienophile because of its rapid reaction with most dienes as well as its stability and ease in handling (although it is poisonous). The diene for this reaction, 1,4-diphenyl-1,3-butadiene, is readily prepared by the Wittig reaction with benzyltriphenylphosphonium chloride and cinnamaldehyde (Chapter 11, Section I). 

Because of the strongly basic conditions needed for Wittig reactions with now-stabilized ylides, one-pot reactions have not been carried out [57]. Ley s recent report of a TPAP-Wittig oxidation performed in a sequential one-pot manner is thus a promising alternative [58]. The reaction is fast and straightforward the... 

Triphenylphosphine gives Michael additions to the activated triple bond of acetylene dicarboxylic esters in presence of acidic compounds HY (Scheme 1). The reactions take place easily at room temperature, even at -10°C [1], through formation of intermediate activated vinylic phosphonium salts, which undergo a subsequent Michael addition of HY. The reactions afford various stabilized ylides which can be isolated in high yields or undergo possibly evolution, for example by intramolecular Wittig reaction [2]. 

There have been many applications of the Wittig reaction in multistep syntheses. The reaction can be used to prepare extended conjugated systems, such as crocetin dimethyl ester, which has seven conjugated double bonds. In this case, two cycles of Wittig reactions using stabilized ylides provided the seven double bonds. Note the use of a conjugated stabilized ylide in the second step.250... 

Several steps are involved in these reactions. First, the enolate of the (1-kelocstcr opens the cyclopropane ring. The polarity of this process corresponds to that in the formal synthon B because the cyclopropyl carbons are electrophilic. The product of the ringopening step is a stabilized Wittig ylide, which can react with the ketone carbonyl to form the carbocyclic ring. 

As mentioned in Sections 1 and 3, one of the most important problems in the study of the Wittig reaction mechanism is the determination of the relative stability of betaines with the open structure and cyclic oxapho-sphetanes as intermediates. The solution to a similar problem in chemistry... 

Tellurium sources, 22-24 Thermodynamics in cyclo-oligomerization, 185-186 butadiene insertion, 187-188 reductive elimination, 193, 194 selectivity control, 212 polysilane isomerisation, 158-160 see also Stability Thermolysis, 135, 136, 158 THF (tetrahydrofuran), 97, 150, 153 Thio-Wittig reaction, 37 Tin, 121... 

An important modification to the Wittig reaction is the use of stabilized phosphonate carbanions in olefin synthesis. This reaction, originally discovered by Homer but developed by Wadsworth and Emmons, is used extensively for transformation of a carbonyl... 

Intramolecular Wittig reaction of keto-stabilized ylide 28 took place in refluxing toluene leading to the 1H-dihydropyrrolizines 31 in the case of a-ketocarboxylic derivatives <2001T5873>, while trifluoroacetyl ylide 28c afforded exclusively the 3//-dihydropyrrolizine 30c (Scheme 26) <2006ARK55>. 

Since the Wittig reaction involves ionic intermediates it is ideally suited to the ionic liquid environment. Wittig reactions using stabilized ylides have been... 

Formate esters behave as typical carbonyl compounds in reactions with a number of ylides, eliminating phosphine oxide and forming vinyl ethers, e.g. (33).35 Stabilized phosphoranes are able to condense with the carbonyl group of cyclic thioanhydrides (34).38 Quinoline derivatives, e.g. (35), are obtained from the condensation of dicar-boalkoxy-ylides with isocyanates.37 Benzoyl isothiocyanates and keto-phosphoranes give quantitative yields of (36), which are unreactive in Wittig reactions but can be readily oxidized by selenous acid.38 The products obtained from reactions (Scheme 9) with the triazolinedione (37) depend upon the stability of the ylide used.39... 

The stereochemical outcome of the Wittig reaction can depend on the presence or absence of lithium salts. This may be due to a betaine intermediate stabilized by lithium cation. A stable adduct of this type has now been observed during a Wittig reaction. When Ph3P=CH2 is treated with 2,2 -dipyridyl ketone, P NMR shows the formation of an oxaphosphetane (72) and addition of lithium bromide gives the chelation-stabilized betaine lithium adduct (73). 

Phosphorus ylides have been reviewed and an intermediate betaine lithium salt adduct (stabilized by complexation with lithium ions and by the chelating effect of pyridyl ligands) has been observed spectroscopically for the first time during the course of a Wittig reaction. ... 

The results of ab initio (HF and MP2) and MNDO-PM3 molecular modelling of the Wittig reaction of unstabilized (Me3P=CHMe), semistabilized (Me3P=CHC=CH) and stabilized (Me3P=CH-CN) ylides with ethanal have been compared with experimental findings. ... 

It has been suggested that the preferential formation of ( )-alkene on Wittig reaction of amide-substituted phenyl 3-pyridyl ketones with non-stabilized phosphorus ylides which contain a carboxyl terminus is a consequence of either hydrogen bonding or salt... 

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