Zwitterion intermediates, phosphonium

Spontaneous Altmiating Copolymerization of Cydic Phosphorus Compounds via Phosphonium Zwitterion Intermediates... 

Phosphonium ylides carrying at least one proton at the a-carbon atom react with various electrophilic reagents with formation of a-substituted phosphonium salts or zwitterionic intermediates, from which a-substituted ylides are generated by deprotonation or proton migration, the former reaction being more important (equation... 

Phosphorus ylides of a-oxotriphenylphosphoranes readily add to the azo-ene system of conjugated azoalkenes to yield the 1,6-zwitterionic intermediates. These intermediates generate the pyrrole ring by the usual intramolecular nucleophilic attack and simultaneously a phosphonium... 

The synthetically versatile anionic vinylvinylidene anion [Tp W( = C = CHCH = C5H2)(C0)2] is capable of acting as a nucleophile towards heteroatom-substituted carbynes. No reaction occurs at C5 with the electrophile Tp W( = CCl) (CO)2, however the phosphoniocarbyne [Tp W( = CPMe2Ph)(CO)2] adds to C5, forming a reactive zwitterionic phosphonium carbene intermediate that loses... 

Completing this section, it should be mentioned that nucleophilic phosphine organocatalysis generally starts via nucleophilic addition of phosphines to generate reactive zwitterionic intermediates, i.e. phosphonium salts. In rare cases such intermediate salts were either isolated or their... 

The key intermediate of the above redox copolymerization is a zwitterion 41. The propagation step involves opening of the phosphonium ring of a zwitterion by... 

Reactions of carbene complexes with phosphorous compounds are topics of current research.164-167 (l-Alkynyl)carbene complexes (CO)5M= C(OEt) —C=CR 1 (M = Cr, W R = Ph, r-Bu, SiMe3) readily add tertiary phosphanes PPhR2 (R = Ph, Me) and form stable zwitterionic phosphonium allenide complexes (CO)5M —C(OEt)=C=C(R)—P+PhR2 145, which have been characterized by X-ray diffraction (Scheme 57).55 In previous studies on Michael-type additions of nitrogen and oxygen nucleophiles to (l-alkynyl)carbene complexes 1, zwitterionic allenide complexes of type 145 have been postulated as intermediates.21,23,503 62 112,168,169 Unambiguous structural evidence of the tendency to form such intermediates is given by isolation of a stable zwitterionic adduct, such as compound 145. Zwitterionic... 

Stabilized species in the phosphorus yhde 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). 

Organozirconates. Zwitterionic zirconocene-ate see Ate Complexes) intermediates have been evoked to explain some reaction mechanisms (see Section 2.3.3). In fact, electron transfer from an organic ligand to 16-electron metal center permits the generation of a carbocationic center which can, then, undergoes isomerization and rearrangement characteristic of carbocations. It is now possible to isolate and characterize stable zwitterionic phosphonium zirconate complexes (eqnation 18). ... 

The above reaction is called a redox copolymerization reaction. The trivalent phosphorus in the monomer is oxidized to the pentavalent state in the precess of polymerization and the quinone structure is reduced to hydroquinone. The phosphonium-phenolate zwitterion is the key intermediate ... 

Subseqently, Kwon et al. described the synthesis of stable phosphonium enolate zwitterions 22, which have been proposed as intermediates in MBH... 

Most recently, Tong et al. isolated the stable phosphonium-enamine zwitterion 23, which has long been postulated as one of the key intermediates in the aza-MBH reaction, from the PPhs-catalyzed reaction between propiolate and N-tosylimine (Scheme 31.8), providing experimental evidence to support the postulated reaction mechanism of the phosphine-catalyzed MBH reaction [22]. 

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