Vinyl phosphonium salts

Triphenylphosphine gives Michael additions to the activated triple bond of acetylene dicarboxylic esters in presence of acidic compounds HY (Scheme 1). The reactions take place easily at room temperature, even at -10°C [1], through formation of intermediate activated vinylic phosphonium salts, which undergo a subsequent Michael addition of HY. The reactions afford various stabilized ylides which can be isolated in high yields or undergo possibly evolution, for example by intramolecular Wittig reaction [2]. 

It is interesting that the reaction of triphenylphosphine with a 1,2-allenyl ketone leads to the formation of a vinyl phosphonium salt 449, which upon protection of the carbonyl group would accept nucleophilic attack followed by elimination in the presence of Et3N to afford y-nudeophile substituted- ,/j-unsaturated enones 451 [197]. 

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]. 

In vinylic phosphonium salts, the jft-carbon is found to be substantially deshielded (A<5 = 22.4 ppm for the vinyltriphenylphosphonium bromide as compared with ethylene), although only a small inductive effect was expected. The extent of this was interpreted to be a consequence of pn-dn bonding between phosphorus and carbon39. This argument was used to identify structures when or 31PNMR spectra failed96. 

TABLE 25. Synthesis of cyclic compounds from vinylic phosphonium salts and carbonyl-containing nucleophiles... 

Yields of Products from Vinyl Phosphonium Salts... 

Reaction of vinyl phosphonium salts with sodium azide also yields triazoles via triazoline intermediates by elimination of triphenylphosphine (Scheme 130).418... 

Vinyl phosphonium salts 30 are also very efficient radical traps. As shown in Scheme 13, the corresponding adducts 31,... 

Annelation. Cleavage of l-(trimethylsilyloxy)-2-alkoxycarbonylcyclo-propanes (1) with KF-18-crown-6 generates y-oxo-a-ester enolate anions, which can be trapped by electrophiles. Trapping with a vinyl phosphonium salt results in [3 + 2] annelation to provide annelated cyclopentanones (2).1... 

Very likely the ammonium fluorides are the proton sources and therefore the reason for incomplete conversions, since potassium fluoride in acetonitrile gives high yields in a very elegant [3 + 2]-annuIation process 87). It combines a Michael addition to a vinyl phosphonium salt with an intramolecular Wittig reaction and proceeds only in the presence of 18-crown-6 with satisfying yield. This cyclopentene synthesis has been executed in a repetitive manner to prepare linear triquinanes as illustrated in Scheme 6. Unfortunately, the sequence is non-stereoselective with regard to the ethoxycarbonyl functions. 

With fluoride ion under aprotic conditions alkylating ring-opening can be performed (equation 94). Similarly, a clever [3 -I- 2] annulation combines a fluoride-induced Michael addition to vinyl phosphonium salts with a subsequent internal Wittig reaction (equation 95) . ... 

N-acylaziridines occurs on treatment with triphenylphosphine with the formation of the ylides (67), thereby providing a route to a-substituted primary allylic amines via their in situ reactions with aldehydes.The reactions of triphenylphosphine with epoxides in phenolic media provide a new access to the vinyl-phosphonium salts (68). A route to 1-hydroxyalkylphosphonium salts is provided by the reactions of trimethyl- or triethyl-phosphine with aldehydes or ketones in the presence of anhydrous acids. The related 1-trimethylsiloxyalkylphosphonium salts can be isolated in the presence of trimethylsilyl chloride.Further Wittig-products have been isolated from the reaction of 1,2,3-indanetrione with triphenylphosphine, and, in related work, it has been shown that furoin is deoxygenated on heating with triphenylphosphine to form 2-furfuryl(2-furyl)ketone and triphenylphosphine oxide.The structure of the blue product from the reaction of triphenylphosphine with 2,3,5,6-tetracyano-7-oxabicyclo(2,2,1]hepta-2,5-diene has now been shown by X-ray techniques to be the stable ylide (69). ... 

Eq. 6), allowing a regiospecific synthesis of 5-alkylimidazoles. In a rather similar reaction, acyl vinyl phosphonium salts can be condensed with amidines to give imidazolyl phosphonium salts (22), presumably by way of 21 (Eq. 7). °... 

Triphenylphosphine reacts with oxirans in the presence of phenol to form vinyl-phosphonium salts (116) in high yield. ... 

The polymer is prepared by homo- or copolymerising a vinyl phosphonium salt monomer, such as 2-methacryloxyethyltri-n-octylphosphonium chloride and has a wide antimierobial spectram and a sufficient antimicrobial effect, even with a short contact time. The lens and lens care products exhibit excellent optical properties and processabihty. 

The MIMIRC sequence with vinyl phosphonium salts... 

This is the basis of Posner s MIMIRC (Michael-Michael-Ring Closure) sequences.10 In the first example, the electrophile is a vinyl phosphonium salt 59. The first adduct 60 could cyclise, by the Wittig reaction, only to a cyclobutene 62 so it prefers to add a second molecule of 59 but the next intermediate 61 cyclises to a six-membered ring 63. 

The final product 63 retains the Ph3P+ group from the first vinyl phosphonium salt 59 and in the example below, using a boron enolate, this was hydrolysed to the stable phosphine oxide11 66. Note the creation of a quaternary carbon atom at a spiro centre. 

The interaction of (PPh3)4Pt with alkylnylvinyl triflates results in formation of butatrienyl cationic Pt(II) complexes and their reactions were also reported145,146. Vinyl triflates reacted with PPh3 in the presence of catalytic amounts of Pd(0), with formation in good yields of vinyl phosphonium salts with high stereospecificity as shown in equation 132. 

Wittig olefin syntheses in hydroxylic solvents may proceed via vinyl-phosphonium salts (14) and not via the usual oxaphosphetans when the double bond formed is stabilized by a ir-bonding substituent. In some... 

The names A -thiole and 3-thiole have previously been used for 2,5-dihydrothiophene. A general synthesis for 2,5-dihydrothiophenes with different substituents is based on the Michael addition of a-sulfanylcarbonyl compounds to vinyl phosphonium salts, followed by an intramolecular WiTTiG reaction ... 

An interesting intramolecular cyclisation of the vinyl phosphonium salt (28) affords the thiazole (29). An initial Michael addition to the vinyl phosphonium group is followed by aromatisation and loss of methylenetriphenylphosphorane <96MI153>. 

TABLE 4.14. 1,3-DIPOLAR CYCLOADDITION REACTIONS OF IN SITU-GENERATED MUNCHNONES AND VINYL PHOSPHONIUM SALTS"... 

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