Phosphonium ylides bonding

Phosphonium hexafluorophosphate, benzotriazolyl-N-hydroxytris(dimethylamino)-in peptide synthesis, 5, 728 Phosphonium salts chromene synthesis from, 3, 753 reactions, 1, 531 Phosphonium salts, vinyl-in pyrrole synthesis, 4, 343 Phosphonium ylides in heterocyclic synthesis, 5, 165 Phosphoramide, triethylene-as pharmaceutical, 1, 157 Phosphoramide, triethylenethio-as pharmaceutical, 1, 157 Phosphorane, pentaphenyl-synthesis, 1, 532 Phosphoranes, 1, 527-537 Berry pseudorotation, 1, 529 bonding, 1, 528... 

Arylmethylene-2-thioxo-4-thiazolidinones (34) react with phosphonium ylides to give dihydrofuro[2,3-rl]thiazol-2(3 -ones (35) in refluxing ethyl acetate, while performing the reaction in refluxing toluene led to the pyrone derivative (36) both of these products result from an initial 1,4-addition to the exocyclic double bond <95T11411>. 

Particularly interesting are the results obtained with the phosphonium ylides including an acyl rest derived from aminoacid if the N-H bond reactivity is blocked by an amide protection, the alkyne formation takes place [25,27], but if the N-H bond is not deactivated, an intramolecular cyclization occurs to give a new stabilized ylide [27,28]. 

Reaction of Stabilized Phosphonium Ylides with Activated C=C Double Bonds... 

However an unexpected new cyclic ruthenium phosphorus ylide half-sandwich complex 42 has been obtained by reaction of 41 with dichloromethane as solvent [79]. The cyclisation involves a C-Cl activation and corresponds to the incorporation of the methylene moiety in the P-C bond and to the ortho-metal-lation of one phenyl of the phosphine. An other novel unusual phosphonium ylide ruthenium complex 43 has also recently been described [80]. 

Olefination Reactions Involving Phosphonium Ylides. The synthetic potential of phosphonium ylides was developed initially by G. Wittig and his associates at the University of Heidelberg. The reaction of a phosphonium ylide with an aldehyde or ketone introduces a carbon-carbon double bond in place of the carbonyl bond. The mechanism originally proposed involves an addition of the nucleophilic ylide carbon to the carbonyl group to form a dipolar intermediate (a betaine), followed by elimination of a phosphine oxide. The elimination is presumed to occur after formation of a four-membered oxaphosphetane intermediate. An alternative mechanism proposes direct formation of the oxaphosphetane by a cycloaddition reaction.236 There have been several computational studies that find the oxaphosphetane structure to be an intermediate.237 Oxaphosphetane intermediates have been observed by NMR studies at low temperature.238 Betaine intermediates have been observed only under special conditions that retard the cyclization and elimination steps.239... 

Among the olefination reactions, those of phosphonium ylides, phosphonate anions, silylmethyl anions, and sulfone anions are discussed. This chapter also includes a section on conjugate addition of carbon nucleophiles to a, (J-unsaturated carbonyl compounds. The reactions in this chapter are among the most important and general of the carbon-carbon bond-forming reactions. 

There have been ab initio studies of the bonding for a series of imino and amino phosphorus molecules,15 and for the phosphonium ylides (8).14 The anion character of the ylidic carbon is intermediate between that of olefinic and substituted carbanions. Polarisation of the H3P group has a stabilising effect. The influence of the substituent X on the inversion of the carbanion and also the tendency of the molecule to dissociate to carbene and phosphine is also discussed.14 CNDO/S MO calculations on the... 

Phosphonium ylides, which can be written in the two familiar canonical forms, are available with a wide variety of substituents both at the phosphorus and at the carbon atoms (Scheme 30). In gold complexes, without any exceptions, they function as two-electron donors, as proposed by the dipolar form to give discrete Au-C cr-bonds (771, monohapto). No side-on, 7r-coordination (t 2), as might be expected out of the ylene form, has been observed to date. 

Polyenes are most often synthesized by cross-coupling reactions between unsaturated systems. Typically these reactions require an activated carbon, often in the form of an organometallic reagent. Enolates and phosphonium ylides, Wittig-type reagents, are also commonly employed in carbon-carbon bond formation. Pericyclic rearrangements also result in the generation of new carbon-carbon bonds and will be treated separately. 

The Wittig reaction is one of the most important reactions in organic chemistry for synthesizing alkenes with unambiguous positioning of the double bond. The process involves a reaction between a phosphonium ylide and an aldehyde or ketone 150). The reacting ylide is formed from a phosphonium salt in a solution of a base such as NaH, t-BuOK, or NaOH 151) (Scheme 19). 

Structure and bonding in phosphonium ylides, salts and phosphoranes... 

In phosphonium ylides, as we shall see in Section II. A, the phosphorus atom is tetracoordi-nate and the unique (anionic) carbon is tricoordinate. For exactly the same reasons as in the oxides2, all previous discussions3-14 of the P—C bond have been in terms of a resonance hybrid between a dipolar form 1A and a double bond form IB ... 

TABLE 2. Bond lengths0 in some conjugatively stabilized phosphonium ylides R3P=CX—C-(=Y)Z... 

For reasons similar to those for the oxides2, studies of the bonding in phosphonium ylides have concentrated on the nature of the P=C bond regarding the following issues (i) the strength of the bond, although in reality this has meant its reactivity (see below) (ii) the exact distribution of the electron density with discussions very similar to those for the oxides and (ii) the difference between phosphorus, nitrogen and (to a lesser extent) arsenic ylides. 

Structure, bonding and spectroscopic properties of phosphonium ylides... 

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