Phosphonium ions, deprotonation

These compounds, which are often called phosphine ylides, are prepared by deprotonation of a quaternary phosphonium ion, namely,... 

EGB) is seen to be the carbanion derived from the probase (PB), which, in this case, is also the substrate to be deprotonated (the phosphonium ion). 

Thephosphoms ylide starting material is synthesized from the corresponding phosphonium ion, which is deprotonated by a strong base such as n-butyllithium ... 

Fluonnated ylides have also been prepared in such a way that fluonne is incorporated at the carhon P to the carbamonic carbon Vanous fluoroalkyl iodides were heated with tnphenylphosphine in the absence of solvent to form the necessary phosphonium salts Direct deprotonation with butyUithium or hthium dusopropy-lamide did not lead to yhde formation, rather, deprotonation was accomparued by loss of fluonde ion Flowever deprotonation with hydrated potassium carbonate in thoxane was successful and resulted in fluoroolefin yields of45-S0% [59] (equation 54) P-Fluorinated ylides may also be prepared by the reaction of an isopropyli-denetnphenylphosphine yhde with a perfluoroalkanoyl anhydnde The intermediate acyl phosphonium salt can undergo further reaction with methylene tnphenylphosphorane and phenyUithium to form a new yhde, which can then be used in a Wittig olefination procedure [60] (equation 55) or can react with a nucleophile [6/j such as an acetyhde to form a fluonnated enyne [62] (equation 56)... 

Trialkyl- and triaryl-phosphines react with 1,3-benzodithiolylium ions to give a phosphonium salt which is deprotonated by n-butyllithium to give (282) (76TL3695). 

The triol 16 can be deprotonated according to Scheme 6. The initial step involves met-allation and this is followed by a coproportionation on addition of two further equivalents of 16. Finally, addition of an ammonium or phosphonium salt gives 36. The stability of the ion has implications for the surface chemistry of silica where similar multiply hydrogen bonded anion sites are likely to be more acidic than isolated silanol groups144. The related cubic polyanion [SigOigtOHji]6- can be prepared directly from aqueous solutions of silica and a quaternary ammonium hydroxide derivative such as [NPhMe3]OH145 146. 

In addition to the generation of phosphonium ylides from phosphonium salts by deprotonation with bases in some instances ylides may result from pyrolysis of phosphonium salts, especially silylated salts (equation 17). Similar fluoride ion induced desilylation (equation 18) of phosphonium salts proved to be a very useful alternative for the synthesis of ylides which are difficult to synthesize by the conventional salt method (as in the case of R, R = alkyl). - The most effective fluoride source is cesium fluoride and the reaction proceeds at room temperature. 

Because of the strong basicity of the fluoride ion, in nucleophilic Sn2 reactions elimination usually occurs as a dominant side-reaction. On the other hand, the basicity of the naked fluoride can be used for synthetic purposes, e. g. for deprotonation of phosphonium salts to the corresponding ylides in a system based on potassium fluoride with catalytic amounts of 18-crown-6 in acetonitrile. 

There is no general solvent that is useful for all reactions, and BTF naturally has its limitations. In addition to the limitations posed by the freezing point, boiling point and chemical stability mentioned before, BTF is not very Lewis-basic and therefore is not a good substitute for reactions that require solvents like ethers, DMF, DMSO, etc. Not surprisingly, ions are not readily dissolved in BTF and many types of anionic reactions do not work well in BTF. For example, attempted deprotonations of esters and ketones with LDA in BTF were not successful. Reaction of diethyl malonate with NaH (5 equiv) and reaction with Mel[72] (6 equiv) in BTF was very heterogeneous and yielded 60% of the di-methylated product, compared to 89% in THF. No reaction was observed if the same malonate anion was used as a nucleophile in a Pd-catalyzed allylic substitution reaction in BTF (see 3.7). Wittig reactions also did not work very well in BTF. The ylid of ethyl triphenyl phosphonium bromide [73] was formed only slowly in BTF, and the characteristic deep red color was never obtained. 

The diphosphinomethanes (70 R = Me, Ph) react with 1,2-xylylene dibromide to give the bis(phosphonium) salts (71 R = Me, Ph) <820M1266>. By stepwise deprotonation of the salts, the mono-, bis- <81CB1428>, and tris-ylides <820M1266> (72)-(74) have been prepared (74) has been used as a ligand for transition metal ions <820M1266,83AG(E)907>. 

The positively charged P atom stabilizes the negative charge on carbon, making phosphonium salts another class of carbon acids (to add to those you met In Chapter 8) that can be deprotonated by strong base. The hydride ion is the conjugate base of H2 which has a p/Tg of about 35. 

Newly designed chiral tetraaminophosphonium salt 184 possessing an organic anion cooperatively catalyzes asymmetric Mannich-type reaction of azlactones (182) with N-sulfonyl imines (183) (Scheme 28.20). The basic carboxylate anion deprotonates the active methine proton of the azlactone (182) to form the corresponding chiral phosphonium enolate with a defined hydrogen bonding network (ion pair), and then highly stereoselective bond formation proceeds [92]. 

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