Semi-ylides phosphonium

Stepwise deprotonation of methylenebis(triorgano)phosphonium salts with bases yields carbodiphosphoranes via intermediate semi-ylide salts, which may also be accessible by alkylation or phosphinol-ation of corresponding alkylidenephosphoranes (equation 118). " It depends on the starting phosphonium salt and the base whether the intermediate ylide salts can be isolated or not. Suitable bases are sodium amide, alkali metal hydrides, alkylidenetrialkylphosphoranes, potassium and lithium orga-nyls. For the synthesis of hexaphenylcarbodiphosphorane improved methods have been reported by which this compound may be generate without isolation of the ylide salt and on a large scale. 

Stepwise deprotonation of methylenebis(triorgano)phosphonium salts with bases yields caibodiphos-phoranes via intermediate semi-ylide salts, which may also be accessible by alkylation or phosphinol-ation of corresponding alkylidenephosphoranes (equation It depends on Ae starting... 

The results indicate that the kinetic acidity is markedly lower for phosphonium salts than for the corresponding nitroalkanes, but cannot be correlated with pKa values. In contrast, the measurement of kinetic acidity using double potential step chronoamperometry180 allowed the determination of pka values for a series of phosphonium salts corresponding to semi-stabilized or non-stabilized ylides ... 

The one-electron electrochemical reduction of 1,2-vinylene and buta-l,4-dienylene bisphosphonium salts at a mercury cathode produces an ylide character by the reaction pathway depicted in reactions 11—13. The mechanism is altered when OH is generated in the unbuffered aqueous-organic medium this reaction is depicted as reaction 14. The electrochemical reduction of phosphonium salt in the presence of tri-p-anisylphosphine produces a mixture of the saturated or semi-saturated bisphosphonium salts through either reaction scheme 15 or alternatively 16. 

In the first step (ij) the ylides 3 (R1 = H) can be alkylated with primary alkylhalogenide or with ethyl bromoacetate they can also be acylated with acetic anhydride in a reverse addition operation, (In spite of trans-ylidation process, the acylated phosphonium salt can be easily purified by liquid -liquid extraction with bases). Semi-stabilized ylides3(R1= 0) are not reactive enough to be alkylated or acylated in this way. 

Fig. 11.2. Working without bases in the Wittig reaction with the (semi)stabilized phosphonium ylide D in-situ formation of this reagent from the alkoxide C resulting from the SN2 ring opening of butylenoxide through the bromide ion of the phosphonium salt A.

Fig. 11.8. trans-Selective Wittig olefination of aldehydes II—Synthesis of /J-carotene from a dialdehyde. The ylide used here is already known from Figure 11.2. In a way, it is "(semi)stabilized" since it is prepared in situ like a semista-bilized phosphonium ylide, but reacts as trans-selectively as a stabilized ylide. 

All P ylides for Wittig reactions are obtained by deprotonation of phosphonium salts. Depending on whether one wants to prepare a nonstabilized, a semi-stabilized, or a stabilized ylide, certain bases are especially suitable (Table 9.1). In stereogenic Wittig reactions with aldehydes, P ylides exhibit typical stereoselectivities. These depend mainly on whether the ylide involved is nonstabilized, semi-stabilized, or stabilized. This can also be seen in Table 9.1. 

Phosphonium salts that bear substituted vinyl groups on the a-carbon atom are the usual precursors of semi-stabilized ylides in carotenoid synthesis. Alkali metal alkoxides, generally as a solution in the corresponding alcohol, are frequently the bases of choice. A two-phase system is also sometimes employed, e.g. dichloromethane/aqueous NaOH solution (see Section F). 

Stabilized and semi-stabilized ylides can also be produced under virtually neutral conditions by using oxiranes as proton acceptors [18]. This has a precondition that the anion of the phosphonium salt is a halide, since epoxide and phosphonium halide are in equilibrium with ylide and the corresponding halohydrin. The technique is particularly advantageous if base-labile functionalities are present. 

The Wittig reaction has been carried out under very mild green conditions weakly basic water, ambient temperature and overnight completion Employing silver carbonate to convert a phosphonium salt into an ylide, the reaction works for stabilized, semi-stabilized and non-stabilized ylides, using aromatic, heteroaromatic and aliphatic aldehydes (and an example of a ketone). 

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