Phosphorus ylides, addition reactions

It) Addition of phosphorus ylides VVittig reaction (Section 19.11)... 

Nucleophilic Addition of Phosphorus Ylides TheWittig Reaction... 

The Julia-Lythgoc olefination operates by addition of alkyl sulfone anions to carbonyl compounds and subsequent reductive deoxysulfonation (P. Kocienski, 1985). In comparison with the Wittig reaction, it has several advantages sulfones are often more readily available than phosphorus ylides, and it was often successful when the Wittig olefination failed. The elimination step yields exclusively or predominantly the more stable trans olefin stereoisomer. 

Ditluoromeihylenaiion reactions may be effected also by the addition of chlorodifluoromethanc to a solution of nonstabilized phosphorus ylide [45, 46] (equation 44) (Table 17)... 

In addition there are certain other methods for the preparation such compounds. Upon heating of the thionocarbonate 2 with a trivalent phosphorus compound e.g. trimethyl phosphite, a -elimination reaction takes place to yield the olefin 3. A nucleophilic addition of the phosphorus to sulfur leads to the zwitterionic species 6, which is likely to react to the phosphorus ylide 7 via cyclization and subsequent desulfurization. An alternative pathway for the formation of 7 via a 2-carbena-l,3-dioxolane 8 has been formulated. From the ylide 7 the olefin 3 is formed stereospecifically by a concerted 1,3-dipolar cycloreversion (see 1,3-dipolar cycloaddition), together with the unstable phosphorus compound 9, which decomposes into carbon dioxide and R3P. The latter is finally obtained as R3PS ... 

The /J-silyl phosphorus ylides 9 undergo highly cnantioselective addition reactions with aromatic and aliphatic aldehydes51. 

Since electron-donating substituents at the phosphorus atom favor addition reactions over olefination reactions, addition of 9 to aldehydes leads to the exclusive formation of the silyl-pro-tected allylic alcohols 10. No reaction products arising from Wittig alkenylation could be detected. The ylides (R,S)-9 and (S.S)-9 and their enantiomers were prepared from the corresponding optically pure l-[2-(diphenylphosphino)ferrocenyl]-A,A -dimethylethanamine diastereomers 7 via the phosphonium salts 8. 

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. 

There are a few isolated cases of the addition of amines, thiols, carboxylic acids, and a phosphorus ylide to doubly bonded germanium compounds. Again, the reactions are regioselective, with the nucleophilic portion of the weak acid adding to the germanium and the proton adding to the heteroatom. 

The reaction of 18 and 19 with phosphorus ylides occurs as a stepwise process. Betaine (21) can be isolated when (Me2SiS)3 reacts with Ph3P=CHMe in a 3 2 ratio of the reactants (Scheme 11). This substance is quite stable in the solid state but on dissolving in pyridine it is reversibly transformed into a mixture of 20k and (Me2SiS)3. The equilibrium concentration of 21 in a solution at room temperature is at most 28% according to the NMR data, and the addition of one more equivalent of Ph3P=CHMe to the solution results in the quantitative transformation of 21 into 20k. 

As well as the Bingel reaction and its modifications some more reactions that involve the addition-elimination mechanism have been discovered. 1,2-Methano-[60]fullerenes are obtainable in good yields by reaction with phosphorus- [44] or sulfur-ylides [45,46] or by fluorine-ion-mediated reaction with silylated nucleophiles [47]. The reaction with ylides requires stabilized sulfur or phosphorus ylides (Scheme 3.9). As well as representing a new route to l,2-methano[60]fullerenes, the synthesis of methanofullerenes with a formyl group at the bridgehead-carbon is possible. This formyl-group can be easily transformed into imines with various aromatic amines. 

The second class of benzo-fused heterocycles accessible from benzofuroxans are benzimidazole oxides. In this case only one carbon from the co-reactant is incorporated in the product. With primary nitroalkanes 2-substituted l-hydroxybenzimidazole-3-oxides (46) are formed via displacement of nitrite, and / -sulfones behave similarly. The nitrile group of a-cyanoacetamides is likewise eliminated to alford 2-amide derivatives (46 R = CONRjX and the corresponding esters are formed in addition to the expected quinoxaline dioxides from acetoacetate esters. Under similar conditions secondary nitroalkyl compounds afford 2,2-disubstituted 2//-benzimidazole-1,3-dioxides (47). Benzimidazoles can also result from reaction of benzofuroxans with phosphorus ylides <86T3631>, nitrones (85H(23)1625>, and diazo compounds <75TL3577>. 

With no major research on the reaction of phosphorus ylides and aliphatic or alicyclic thiones prior to their investigation, Krapcho et where able to elaborate a useful method for the synthesis of the thietane ring system in addition to the preparation of novel types of thiocarbonyl stabilized ylides (Eq. 1). 

Heating a-hydroxy amides 135 in xylene with the cumulated phosphoms ylide 134 gives the 2(3//)-oxazolones 140. The reaction proceeds via an addition-cyclization-intermolecular-Wittig olefination sequence, which imphes three different types of phosphorus ylides, 134,136, and 137, respectively, of increasing ylide activity (Fig. 5.33 Table 5.7, Figs. 5.34, 5.35). 

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>. 

Addition of phosphorus ylide to carbonyl compounds Wittig reaction... 

Georg Wittig (1954) discovered that the addition of a phosphorus ylide (stahihzed anion) to an aldehyde or a ketone generates an alkene, not an alcohol. This reaction is known as Wittig reaction. 

CHMe, cyclopropylidene, and CMe2 to activated double bonds.1075 Similar reactions have been performed with phosphorus ylides, 076 with pyridinium ylides,1077 and with the compounds (PhS)3CLi and Me3Si(PhS)2CLi.1078 The reactions with ylides are of course nucleophilic addition. 

In the course of investigations on a-sulphanyl carbenes, a new cleavage reaction was found. After irradiation the phosphorus ylide 53 yields nearly quantitatively the thiaazaphosphetane 54, in addition to triphenylphosphine (55) (equation 20)98. An insertion of the corresponding carbene intermediate in to the methine—CH bond is plausible. The structure of 54 was confirmed by single-crystal X-ray structure analysis98. 

Reactions with sulfur ylides proceed differently. The products are oxacyclo-propanes (oxiranes) —not alkenes. The addition step proceeds as with the phosphorus ylides, but the negatively charged oxygen of the dipolar adduct then displaces the sulfonium group as a neutral sulfide. This is an intramolecular Sn2 reaction similar to the formation of oxacyclopropanes from vicinal chloroalcohols (Section 15-11C) ... 

Reaction of dialkyl arylphosphonites or dialkyl alkylphosphonites with two molar equivalents of dimethyl acetylenedicarboxylate, through an analogous pathway to the reaction of trimethyl phosphate with the acetylenedicarboxylate, proceeds smoothly for 30 min at -70 °C to give phosphorus ylide 7 initially in almost quantitative yield. The ylide (31P NMR p 69.8 ppm) converts to affords the X5-phosphole 8, and then to cyclic ylide 9. Addition reaction of hydrogen bromide to the X5 -phosphole 8 at -70 °C occurs for dealkylation of the X5-phosphole giving the oxo-phosphole 10 (8p 38.7 ppm) (Scheme 2) [3]. 

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