Michael reactions triphenylphosphine

Barbas developed this procedure further by introducing an asymmetric three-component Michael reaction that should be applicable to many other conjugate addition reactions. He used a Wittig olefmation to prepare, in situ, an a,P-unsatu-rated ketone that subsequently underwent a conjugate addition with malonates (Scheme 21) [94]. The rate of the conjugate addition process was observed to be considerably faster than the analogous reaction reported by Jprgensen which was attributed to the presence of triphenylphosphine oxide within the reaction mixture. 

When 5,5-dimethyl-1,3-hexadiynyl(phenyl)iodonium triflate is mixed with two equivalents of triphenylphosphine in toluene, a mixture of bisphosphonium triflates is produced (equation 76)34. This transformation apparently involves the initial formation of (5,5-dimethyl-l,3-hexadiynyl)triphenylphosphonium triflate followed by competing Michael and conjugate Michael reactions of this compound with a second mole of triphenylphosphine. The ( -geometry about the carbon-carbon double bonds in both products has been established by NMR analysis34. 

An enantioselective Michael reaction of aldehydes and maleimides gives up to 99% yield and ee, using a cooperative catalysis by a chiral primary amine and triphenylphosphine. Evidence for a supramolecular assembly of the catalysts is presented, including UV/visible, fluorescence, CD and NMR spectroscopy, and ESI-MS. 

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]. 

Highly stabilized phosphorus ylides are prepared from acetylenic esters, a carbon-based nucleophile, and triphenylphosphine in aqueous media.40 In acetone-water (2 1) solvent, the reaction proceeds via the conjugate addition of triphenylphosphine to dialkyl acetylenedicarboxy-lates the resulting vinyl triphenylphosphonium salts undergo Michael addition reaction with a carbon-nucleophile to give the corresponding highly stabilized phosphorus ylides. 

A fundamentally different approach to the synthesis of 3-pyrrolines is evidenced in the annulation in Eq. 13.50 [58]. Ethyl 2,3-butadienoate 150 reacts with N-sulfony-limine 151 in the presence of triphenylphosphine under very mild conditions to give JV-protected 3-pyrroline 152 in 90% yield. The mechanism that has been postulated is related to that of the Baylis-Hillman reaction. Michael addition of triphenylphosphine to the allenyl ester generates a zwitterion that combines with the imine to give 153 in a non-concerted process. This is followed by ring closure, proton exchange and expulsion of triphenylphosphine to give 152. This annulation is successful only for aromatic or cinnamyl imines [59]. 

Phosphorus ylides can be generated from triphenylphosphine, 3-chloro-(3//, 5//)-furan-2,4-dione, and alkynyl esters. Additional alkynyl ester, acting as a Michael acceptor, reacts with the ylides in a [4+2] cycloaddition reaction that results in the formation of furo[2,3-3]pyran derivatives (Equation 30) <2000T5221>. 

The reaction of triphenylphosphine and dimethyl acetylenedicarboxylate (DMAD) in the presence of either 2-, 3-, or 4-aminophenol and 2-amino-3-hydroxypyridine gives a vinylphosphonium salt which undergoes Michael addition to give a range of highly functionalized heterocyclic systems <2002T6895>. 

Enones with a pendant aldehyde, RC(=0)-CH=CH-(CH2)2-CH0, have been cyclized via an intramolecular MBH reaction in a study of the influence of Michael acceptor stereochemistry on yield.164 Using triphenylphosphine as catalyst, the Z-isomer consistently gave 2.5-8.5 times higher yield of the product (55), using reaction times of 1-3 days. It is unclear whether this is due to the relative accessibility of the /3-positions of the isomers to the nucleophilic catalyst, or differential stability in the enolate intermediates. 

Treatment of the ot/ o-quinones 687 with 2equiv of carbethoxymethylenetriphenylphosphorane 688 affords coumarins in good yield. The reaction proceeds via Wittig olefination of the C-6 carbonyl to form the intermediate ort o-quinone methanides 689. Michael addition of a second equivalent of the phosphorane 688 followed by a Hoffmann type elimination of triphenylphosphine affords the intermediate 690. Intramolecular lactonization then forms the desired coumarins (Scheme 167) <2002J(P1)1455>. 

As shown in Equation (17), 2-trimethylsilyloxyfuran also participated in a triphenylphosphine-catalyzed substitution reaction with Morita-Baylis-Hillman acetates to provide interesting 7-butenolides regio- and diastereoselec-tively <2004AGE6689>. However, the reaction mechanism (vinylogous Michael vs. Diels-Alder) has not been distinguished. 

The reaction of triphenylphosphine and an alkyl halides is facilitated by the use of microwave irradiation. Indeed, the Wittig reaction itself is assisted by micro-wave irradiation. Phosphonium salts are also prepared by addition of phosphines to Michael alkenes (like 15-8) and in other ways. The phosphonium salts are most often converted to the ylids by treatment with a strong base such as... 

Stable phosphorus ylides (50) and (51) have been prepared from the reaction of electron-deficient acetylenic esters, such as dialkyl acetylenedicarboxylates or alkyl propiolates and triphenylphosphine in the presence of 3-chlorotetrahyd-rofuran-2,4-dione (Scheme 10). These reactions are thought to proceed via vinylphosphonium salt intermediates which undergo Michael addition with the conjugate base of the CH-acid. Similar methodology has been used to prepare phosphonium ylide (52) from triphenylphosphine, isatin (indoline-2,3-dione) and dimethyl acetylenedicarboxylate. " ... 

JCS(P1)415>. The simple double Wittig Reaction was observed only in one instance. In most cases, an initial mono Wittig was followed by Michael addition of a second ylide molecule, with subsequent elimination of triphenylphosphine. 

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