Alkenes Wittig

Summary Carbonyl compounds readily react with phosphonium ylides to give alkenes (Wittig reaction) ... 

Some straightforward, efficient cyclopentanellation procedures were developed recently. Addition of a malonic ester anion to a cyclopropane-1,1-dicarboxylic ester followed by a Dieckmann condensation (S. Danishefsky, 1974) or addition of iJ-ketoester anions to a (l-phenylthiocyclopropyl)phosphonium cation followed by intramolecular Wittig reaction (J.P, Marino. 1975) produced cyclopentanones. Another procedure starts with a (2 + 21-cycloaddition of dichloroketene to alkenes followed by regioselective ring expansion with diazomethane. The resulting 2,2-dichlorocyclopentanones can be converted to a large variety of cyclopentane derivatives (A.E. Greene. 1979 J.-P. Deprds, 1980). 

The Wittig reaction uses phosphorus ylides (called Wittig reagents) to convert aldehydes and ketones to alkenes... 

Identify the alkene product in each of the following Wittig ... 

Either route is feasible and indeed styrene has been prepared from both combinations of reactants Typically there will be two Wittig routes to an alkene and any choice between them is made on the basis of availability of the particular starting materials... 

The Wittig reaction (Sections 17 12-17 13) Reaction of a phosphorus ylide with aldehydes and ketones leads to the formation of an alkene A versa tile method for the regiospecific prepa ration of alkenes... 

With a regioselectivity opposite to that of the Zaitsev rule the Hofmann ehmma tion IS sometimes used in synthesis to prepare alkenes not accessible by dehydrohalo genation of alkyl halides This application decreased in importance once the Wittig reac tion (Section 17 12) became established as a synthetic method Similarly most of the analytical applications of Hofmann elimination have been replaced by spectroscopic methods... 

Wittig reaction (Section 17 12) Method for the synthesis of alkenes by the reaction of an aldehyde or a ketone with a phosphorus yhde... 

The Wittig reaction, for which George Wittig received the 1979 Nobel Prize in Chemistry, is an important synthetic procedure for converting aldehydes and ketones into alkenes. The active reagent is a phosphorous ylide which undergoes nucleophilic addition to the carbonyl carbon, e.g., for addition of triphenylphosphinemethylidene to acetone. 

The Peterson olefination can be viewed as a silicon variant of the Wittig reaction, the well-known method for the formation of carbon-carbon double bonds. A ketone or aldehyde 1 can react with an a-silyl organometallic compound 2—e.g. with M = Li or Mg—to yield an alkene 3. 

The Peterson oleflnation presents a valuable alternative to the Wittig reaction. It has the advantage to allow for a simple control of the alkene geometry. Its applicability in synthesis depends on the availability of the required silanes.2... 

The reaction of an alkylidene phosphorane 1 (i.e. a phosphorus ylide) with an aldehyde or ketone 2 to yield an alkene 3 (i.e. an olefin) and a phosphine oxide 4, is called the Wittig reaction or Wittig olefination reaction. ... 

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. 

With respect to the carbonyl substrate, a variety of additional functional groups is tolerated, e.g. ester, ether, halogen. With compounds that contain an ester as well as a keto or aldehyde function, the latter usually reacts preferentially. Due to its mild reaction conditions the Wittig reaction is an important method for the synthesis of sensitive alkenes, as for example highly unsaturated compounds like the carotinoid 17 shown above. 

Aldehydes and ketones are converted into alkenes by means of a nucleophilic addition called the Wittig reaction. The reaction has no direct biological counterpart but is important both because of its wide use in the laboratory and drug manufacture and because of its mechanistic similarity to reactions of the coenzyme thiamin diphosphate, which well see in Section 29.6. 

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