Phosphorus ylides generation

In 2004, Whittlesey and Williams demonstrated that the reversible C-H activation of Ru-NHC complexes (e.g. 32a, Scheme 13.14) provides an effective manifold for tandem dehydrogenation/Wittig reaction/hydrogenation of alcohols, thus generating alkanes from alcohols and phosphorus ylides [56]. 

The reactions of dichlorocarbene with phosphorus ylides result in the corresponding olefins and phosphines.66-68 In the reaction of dichlorocarbene generated in situ with tributyl- and triphenylmethylenephosphoranes or triphenylethylidenephosphorane, the olefin yield increases as the nucleo-philicity of phosphorus ylide increases. According to,67 the reaction starts from the electrophilic attack of carbene at the a-C atom of phosphorus ylide. Then the intermediately formed betaine (28) (Scheme 14) decomposes to eliminate the phosphine molecule and form dichloroolefin (29). 

Non-heteroatom-substituted carbene complexes can also be generated by treatment of electrophilic transition metal complexes with ylides (e.g. diazoalkanes, phosphorus ylides, nucleophilic carbene complexes, etc. Section 3.1.3). Alkyl complexes with a leaving group in the a-position are formed as intermediates. These alkyl complexes can undergo spontaneous release of the leaving group to yield a carbene complex (Figure 3.2). 

The most recent development concerns the heterocyclic (amino)(ylide)carbenes AYC. Such compounds have been known for some years [203] but so far had little impact compared to their diamino stabilized relatives. Both phosphorus ylide (86) and sulfur ylide (87) stabilized AYC ligands have been generated in situ and were stabilized at suitable metal centers (Fig. 27) [204, 205]. The palladium complex 88 with an anionic (amino) [bis(ylide)]carbene is also known [206]. 

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>. 

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. 

Table 3.42. Generation of alkenes from support-bound phosphorus ylides.

The ability of carbanions to react with elemental selenium can be advantageously used for the synthesis of selenocarbonyl compounds. For example, sulfur ylides 201 (E = +SMe2) have been reacted with elemental selenium to generate the corresponding selenocarbonyl compounds 202 (Scheme 59).374,375 But Staudinger selenylation also has been applied to the synthesis of selenoketones 202 from phosphorus ylides 201 (E = +PPh3), which have been trapped by dienes in hetero-Diels-Alder reactions.376-383... 

In 1979 Vedejs and Martinez reported a terse communication on a new concept for the generation of nitrogen, sulfur, and phosphorus ylides. Their method consists of initial alkylation of the heteroatom of amines, imines, sulfides, or phosphines with trimethylsilylmethyl triflate, and subsequent desilylation of the resulting salts with fluoride ion (79JA6452). For the... 

Another arsonium ylide reaction involves a notable transylidation reaction between a phosphorus and an arsenic ylide (Scheme 6). A useful arsenic ylide which provides a hydroxymethyl epoxide has been reported (equation IS) note the use of biphasic reaction conditions for ylide generation. 

Most alkylidenecyclopropanes have been prepared by reacting cyclopropylidenetriphenyl-.i -phosphane with aldehydes and ketones. The phosphorus ylide is either prepared by treating cyclopropyltriphenylphosphonium bromide, a stable compound, with base, e.g. phenyl-lithium, potassium er/-butoxide, sodium hydride, or by generating both the phos-phonium salt and the ylide in situ from (3-bromopropyl)triphenylphosphonium bromide employing two equivalents of base. ° The latter method seems to give somewhat better yields, as indicated by the synthesis of l-diphenylmethylene-2-methylcyclopropane (1) from ben-zophenone. ° The yield has also been increased by adding a catalyst. Considerable improvements have in particular been observed by using tris[2-(2-methoxyethoxy)ethyl]amine, a phase-transfer catalyst, e.g. formation of The use of several phosphorus ylides and bases is summarized in Table 25. 

Carbonyl-stabilized phosphorus ylides are less nucleophilic and hence do not react with (phosphine)gold(I) halides, but their gold(I) complexes can be generated from precursors such as [Au(acac)(PPh3)] or [AuCl(SC4Hg)] by reaction with phosphonium salts and ylides, respectively, and again both mono- and bis(yhde) complexes have been obtained (equation 48 and Scheme 8)206 207. The ylide carbon atoms in these complexes are centres of chirality, but no stereospecificity was observed in the coordination process and racemic mixtures are formed throughout. 

However, evidence opposing the intermediacy of a carbene species in the Corey-Winter olefin synthesis also exists.3 Thermal decomposition of hydrazone salt 14, which is proposed to proceed via carbene 12, leads to a mixture of products 13, 15, and 16.7 Thus, an alternative mechanism has been proposed for the Corey-Winter reaction that invokes a phosphorus ylide. In this mechanistic scenario, initial reaction of thionocarbonate 10 with trimethylphosphite affords zwitterion 11. Cyclization generates... 

Stabilized species in the phosphorus ylide category are normally generated in a two-step sequence beginning with the formation of the quaternary phosphonium salt. This is usually accomplished with ease by the reaction of triphenylphosphine with the appropriate haloalkane. Salt formation is followed by deprotonation at the carbon adjacent to phosphorus using an appropriate base to generate a zwitterionic species stabilized by the adjacent functionality (illustrated in equation 20). The resultant phosphorus species reacts with an introduced carbonyl compound to generate an intermediate oxaphosphatane that undergoes decomposition to produce alkene and phosphine oxide at relatively low temperatures (equation 21). 

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