Alkyl halides preparing phosphonium salts from

Wittig reactions are versatile and useful for preparing alkenes, under mild conditions, where the position of the double bond is known unambiguously. The reaction involves the facile formation of a phosphonium salt from an alkyl halide and a phosphine. In the presence of base this loses HX to form an ylide (Scheme 1.15). This highly polar ylide reacts with a carbonyl compound to give an alkene and a stoichiometric amount of a phosphine oxide, usually triphenylphosphine oxide. 

Show a preparation of this phosphonium salt from an alkyl halide ... 

Ethylene oxide or 1,2-epoxybutane may also be used for the synthesis of ylides. The resulting ylide is in equilibrium with its conjugated salt (equation 15). The use of ethylene oxide offers some advantages over more conventional bases used in Wittig reactions. The application is simple since ylides and most often also phosphonium salts (from phosphine and alkyl halide) need not to be prepared separately. The reaction medium is neutral, so that base-induced side reactions fail to appear. The method is however less applicable to weakly acid phosphonium salts, since deprotonation requires high temperatures (150 C). 

In the Wittig reaction an aldehyde or ketone is treated with a phosphorus ylid (also called a phosphorane) to give an alkene. Phosphorus ylids are usually prepared by treatment of a phosphonium salt with a base, and phosphonium salts are usually prepared from the phosphine and an alkyl halide (10-44) ... 

The reaction of epoxides with C02 affords either CCs or polymers [119], and many reports have been made [120-125] and different active catalysts described [126-130] such as alkyl ammonium-, phosphonium-salts and alkali metal halides, in this respect. The main drawbacks here are the need for a high catalyst concentration, a high pressure (5 MPa of C02), and a temperature ranging from 370 to 400 K. The recovery of the catalysts for reuse is also a key issue, and in order to simplify the recovery process various hybrid systems have been developed, an example being that prepared by coupling 3-(triethoxysilyl)propyltriphenylphosphonium bromide with mesoporous silica [131]. In this case, the reaction was carried out in the absence of solvent, under very mild conditions (1 MPa, 263 K, 1 mol% loading of catalyst, 6h), such that the hybrid catalyst could be recovered and recycled several times. 

Preparation of alkyldiphenylphosphine oxides. General procedure from phospho-nium salts. Triphenyl phosphine is heated under reflux with an excess of alkyl halide. The precipitated phosphonium salt is filtered off, washed well with ether, and then heated with 30 per cent w/w aqueous sodium hydroxide (c. 4 ml/g) until all the benzene has distilled out. The mixture is cooled and extracted with dichloromethane, and the extracts are dried (magnesium sulphate) and evaporated to dryness. In this way ethyldiphenylphosphine oxide is obtained from triphenyl phosphine (65.6 g, 0.25 mol) and iodoethane (42.9 g, 0.275 mol) in dry toluene (250 ml) to give first the phosphonium salt (102.4 g, 97.9%) after 3.5 hours, from which the phosphine oxide is obtained as needles (53.2 g, 92.5%), m.p. 123-124 °C (from ethyl acetate) p.m.r. 5 (CDC13, TMS) 1.9-13 (m, 10H, Ph2PO), 2.3 (m, 2H, CH2) and 1.2 (dt, 3H, JHm, = 7 Hz, JMeP = 17 Hz, Me). 

The phosphorus-stabilized carbanion is an ylide (pronounced ilL-id )—a molecule that bears no overall charge but has a negatively charged carbon atom bonded to a positively charged heteroatom. Phosphorus ylides are prepared from tri-phenylphosphine and alkyl halides in a two-step process. The first step is nucleophilic attack by triphenylphosphine on an unhindered (usually primary) alkyl halide. The product is an alkyltriphenylphosphonium salt. The phosphonium salt is treated with a strong base (usually butyllithium) to abstract a proton from the carbon atom bonded to phosphorus. 

Phosphorus ylides are prepared from phosphonium salts by deprotonating them with a strong base. The method consists of the alkylation of triphenylphosphine with alkyl halide. The resulting phosphonium salt is treated with a strong base (phenyUithium or n-butyllithium) to give a phosphorus ylide. The simplest ylide is methylenetriphenylphos-phorane (3.50), which is prepared by the abstraction of a proton from methyltriphenylphos-phonium iodide. 

A publication discussing the uses of reactive arsonium ylides for the stereospecific preparation of epoxides draws attention to the fact that arsonium salts are less readily prepared than phosphonium salts because of the poorer nucleophilicity of arsenic compared to phosphorus, and suggests methods for obtaining them. Primary salts were made from alkyl triflates, while a-branched salts were prepared from alkyldiphenylarsines, obtained from iodo compounds as, for example, in equation 23. Reaction of alkyl halides with arsines to form arsonium salts is also promoted by the presence of silver tetra-fluoroborate . 

The reaction of ylides with saturated aliphatic alkyl halides (like methyl iodide, ethyl iodide etc.) usually stops at the stage of the alkylated salt because the +/ effect of the aliphatic substituent causes the resulting salt to be a weaker acid than the conjugated salt of the original ylide (which would result in the course of a transylidation reaction). However since partial transylidation also occurs between al-kylidenephosphoranes and phosphonium salts with equal or not very different base and acid strength, mixtures may result from Ae reaction with saturated aliphatic alkyl halides. At this point it should be mentioned that the synthesis of dialkylated ylides via the salt method is also difficult since the preparation of the necessary phosphonium salt is accompanied by -elimination. The successful synthesis of dialkylated ylides may be achieved by fluoride ion induced desilylation of a-trimethylsilylphosphonium salts (see equation 18). There is no doubt about the course of ylide alkylation in cases where the inductive effect of the new substituent leads to complete transylidation (e.g. equation 54). ... 

Phosphorus ylids are prepared by treatment of phosphonium salts with bases such as phenyllithium, butyllithium. dimsyl anion, or potassium f-butoxide. They are not isolated but used directly after preparation because they are sensitive to oxygen and moisture. The requisite phosphonium salts are available in great variety from the reaction of triphenylphosphine and primary or secondary alkyl halides, usually bromides [24]. 

Phosphonium ylides (alkyhdene phosphoranes) can be prepared by a number of methods, but in practice they are usually obtained by action of a base on (alkyl)triphenylphosphonium salts, which are themselves readily available from an alkyl halide and triphenylphosphine. The phosphonium salt can usually be isolated and crystallized, but the phosphonium ylide is generally prepared in solution and used without isolation. Formation of the phosphonium ylide is reversible, and the reaction conditions and the strength of the base required depend entirely on the nature of the ylide. A common procedure is to add a stoichiometric amount of a solution of n-butyllithium to a solution or suspension of the phosphonium salt in ether or THF, followed, after an appropriate interval, by the carbonyl compound. Other bases, such as sodium hydride or sodium or potassium alkoxides, in solution in the corresponding alcohol or in dimethylformamide, are used commonly. 

Tetraphenyl phosphonium salts can be prepared from pentaphenylphosphorane by reaction with triphenyl boron, a hydrogen halide or an alkyl halide with ultraviolet radiation. 

This chapter will discuss carbanion-like reactions that utilize enolate anions. The acid-base reactions used to form enolate anions will be discussed. Formation of enolate anions from aldehyde, ketones, and esters will lead to substitution reactions, acyl addition reactions, and acyl substitution reactions. Several classical named reactions that arise from these three fundamental reactions of enolate anions are presented. In addition, phosphonium salts wiU be prepared from alkyl halides and converted to ylids, which react with aldehydes or ketones to form alkenes. These ylids are treated as phosphorus-stabilized car-banions in terms of their reactivity. 

Since the above phase transfer catalysed reactions worked particularly well with phosphonium salts prepared from benzyl halides, it was considered of interest to investigate reactions using phosphonium salts (4) prepared from chlorome thy late d polystyrenes, that is reactions where the supported species was the alkyl halide (see Scheme 2). Both linear and crosslinked chloromethylated polystyrenes reacted smoothly with triphenylphosphine to give polymers with residues (4). ° An alternative way of preparing the linear polymer is by copolymerisation of styrene and the salt (5). When the salts (4) were treated with various aldehydes in methylene chloride and... 

A very useful modification of the Wittig reaction involves the reaction of phosphonate-stabilized carbanions with aldehydes or ketones, which is known as the Homer-Wadsworth-Emmons (HWE) reaction [7, 151,152], This reaction was originally described by Homer et al. [153, 154] and further defined by Wadsworth and Emmons [155]. Phosphonate-stabilized carbanions are more nucleophilic and more basic than phosphonium ylides. They are prepared by the addition of suitable bases to the corresponding alkylphosphonates, which are readily accessible through the Michaelis-Arbuzov reaction of trialkyl phosphites with alkyl halides (usually a-halo carbonyl compounds) [143]. In contrast to the Wittig reaction, the HWE reaction yields phosphate salt byproducts that are water-soluble and hence are readily separated from the desired alkene products by simple extraction. 

F-Methylation. Phosphonium salts are prepared by the quat-ernization of phosphines with methyl iodide. The displacement reaction is usually conducted in polar solvents such as acetonitrile or DMF. Dialkyl phosphonates are prepared from the reaction of trialkyl phosphites with alkyl halides, commonly known as the Arbuzov reaction. For example, diisopropyl methylphosphonate is prepared by heating a mixture of methyl iodide and Triisopropyl Phosphite (eq 34). ... 

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