Phosphonium salts deprotonation

Electro-generated bases have been employed to carry out the phosphonium salt deprotonation step of the Wittig reaction, a method that shows promise as a stereoselective electrosynthesis. 

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)... 

Silyl migrations readily occur in silylated ylides to give the ylides of optimum stability. Thus, deprotonation of the salts (21) and (23) gave the ylides (22) and (24), respectively. Intermolecular silyl transfers, from one ylide (or the corresponding phosphonium salt) to another, also lead to maximum stabilization. Silyl transfer does not occur in the product (26) from methylenetrimethylphosphorane and the chlorodisilane (25), pre-... 

Phosphonium ylides are usually prepared by deprotonation of phosphonium salts. The phosphonium salts that are used most often are alkyltriphenylphosphonium halides, which can be prepared by the reaction of triphenylphosphine and an alkyl halide. The alkyl halide must be reactive toward Sw2 displacement. 

This accounts for the considerable discrepancy between the alkene Z/E ratio found on work-up and the initial oxaphosphetan ais/trans ratio. By approaching the problem from the starting point of the diastereomeric phosphonium salts (19) and (20), deprotonation studies and crossover experiments showed that the retro-Wittig reaction was only detectable with the erythreo isomer (19) via the cis-oxaphosphetan (17). Furthermore, it was shown that under lithium-salt-free conditions, mixtures of (19) and (20) exhibited stereochemical drift because of a synergistic effect (of undefined mechanism) between the oxaphosphetans (17) and (18) during their decomposition to alkenes. 

Phosphoniosilylotion. This combination reacts with acyclic or cyclic enones to give phosphonium salts, formed by addition of P(C6H,)3 to the p-position of the enone and silylation of the carbonyl group. The products can be converted into p-substituted enones by deprotonation (BuLi), a Wittig reaction, and hydrolysis. 

Novel alkenylphosphonium salts were subjected to the Wittig reaction (Scheme 12). Allylic deprotonation took place for phosphonium salts possessing such protons, and the olefination proceeded after double bond migration. In cases where such protons were absent, allene formation was observed. 

Alternatively, phosphine imines 266 were treated with various phenylacetyl chlorides 270 (method B). Surprisingly, phosphonium salts 271 were isolated with 25 to 97% yield, which could be deprotonated by means of a base to build up the corresponding phosphoranes 272 (66-89% yield). Upon heating to... 

As shown in Scheme 1, aliphatic phosphines such as P(n-Bu)3 catalyze the addition of alcohols (2) to methyl propiolate (3) [35]. The mechanism is believed to involve an initial addition of the phosphine to the C = C moiety to give a zwitterionic allenolate (I), which then deprotonates the alcohol, yielding a vinyl phosphonium salt (II). An alkoxide addition to give an enolate (III), followed by phosphine elimination gives the product 4 and regenerates the catalyst. Several experiments suggest that when alcohols are used in excess, the catalyst rests as the original phosphine [34]. 

The formation of the heterocycle 1 from the xylylene-bis-phosphonium salt 2 and PCI3 proceeds via a detectable intermediate 3 in a cascade of condensation reactions that is terminated by spontaneous heterolysis of the last remaining P-Cl bond in a cyclic bis-ylide-substituted chlorophosphine formed (Scheme 1) [15]. The reaction scheme is applicable to an arsenic analogue of 1 [15] and to bis-phosphonio-benzophospholides with different triaryl-, aryl-alkyl- and aryl-vinyl-phosphonio groups [16, 18, 19], but failed for trialkylphosphonio-substituted cations here, insufficient acidity prohibited obviously quantitative deprotonation of the phosphonium salts, and only mixtures of products with unreacted starting materials were obtained [19]. The cations were isolated as chloride or bromide salts, but conversion of the anions by complexation with Lewis-acids or metathesis was easily feasible [16, 18, 19] and even salts with organometallic anions ([Co(CO)4] , [CpM(CO)3] (M=Mo, W) were accessible [20]. 

The bis-ylide ligands [R2P(CH2)2] can easily be prepared by treatment of the phosphonium salts [R2PMe2] (R = Me, Ph) with strong deprotonating reagents such as lithium derivatives, NaNH2, or non stabilized ylides R3P=CH2. However, Au Ag, and Cu complexes (99) were prepared by reaction of ClMPMe3 (M = Ag, Au) or CuCl with excess of R3P=CH2 [202, 203] (Scheme 30). 

As shown for the aziridines, BETMIP (68) has proved to be useful in the synthesis of azepines (Scheme 118). Treatment with methylenephosphorane leads to a phosphonium salt which in turn is deprotonated with BuLi and cyclized with benzene-l,2-dialdehyde in a Wittig and aza-Wittig step to form benzazepine 326 (93JOC1987). 

Phosphonium salts of the type Me4P+ or Ph2Me2P+, which has two carbon centers, can, after deprotonation, give the bis (ylide) species and its gold compounds... 

Good results have been achieved in phosphonio-catalysed alkylation of active methylene compounds and imides which may be steroselective873 (equation 269). Aqueous sodium hydroxide deprotonation of the phosphonium salt itself in view of a Wittig reaction is... 

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). 

Polystyrene-bound benzaldehydes can be smoothly olefinated with benzyl- or cin-namylphosphonium salts in DMF or THF using sodium methoxide as a base (Entry 1, Table 5.5 [64-67]). Alkylphosphonium salts, however, only react with resin-bound aldehydes upon deprotonation with stronger bases, such as butyllithium [30,68-70]. The more acidic acceptor-substituted phosphonium salts, on the other hand, even react with resin-bound aldehydes and ketones upon treatment with tertiary amines, DBU, sodium ethoxide, or lithium hydroxide [71-75], but stronger bases are also used occasionally [76]. 

In the first step a Wittig reaction" is used to transform the aldehyde into a terminal olefin. This requires initial preparation of a quaternary phosphonium salt. The latter is then deprotonated with sodium amide to give phosphorus ylide 46, which after nucleophilic attack on aldehyde 12 leads to the oxaphosphetane intermediate 47. This intermediate in turn decomposes into olefin 48 and triphenylphosphine oxide. 

Tetraene 12 is formed following the Willig protocol Deprotonation of the phosphonium salt 8 yields a phosphorus ylide which is subjected to condensation with aldehyde 11 (see Chapters 9 and 13). 

Convenient alternatives to direct deprotonation of ethers are tin-lithium exchange [199, 258-261], halogen-magnesium exchange [262], or reductive cleavage of 0,Se-acetals [263, 264], Another synthetic equivalent of a-metalated ethers are (alkoxymethyl)phosphonium salts [265]. 

The bulky ligands PPh3 and AsPh3 add to the unsubstituted end of 31 (180,181). The resulting phosphonium salt (32) is deprotonated by butyllith-ium at — 78°C to yield an ylid (33) which reacts with aldehydes in a Wittig reaction. Deprotonation with potassium tert-butoxide followed by addition of aldehyde 34 gives the E isomer (35) only [Eq. (20)] (182). Trimethylphosphite... 

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