Phosphorus-carbon ylides

Griitzmacher, H. and Pritzkow, H., 3 Independent molecules in the unit-cell of a phosphorus ylide - changes in the molecular geometry on rotation about the phosphorus carbon ylide bond, Angew. Chem. Int. Ed. Engl. 31 (1), 99-101 (1992). 

The bisylide 31 has been prepared by the salt method (equation 36) it is unstable as a solid and especially in solution Its central carbon atom could not be detected in its C-NMR spectrum. The mixed phosphorus-arsenic ylide (32) was also prepared it formed yellow crystals which decomposed easily ". ... 

Nonstabilized Ylides. These ylides are devoid of electron-v ithdrawing substituents at the anionic center they carry hydrogen or electron-donating alkyl groups on the ylide carbon and phenyl groups on the phosphorus. Nonstabilized ylides are very nucleophilic and react with CO2, O2, and H2O hence they must be handled in an inert atmosphere. Primary as well as secondary alkyl halides may be used for the preparation of phosphonium salts and subsequently ylides. 

Although those Michaelis-Arbuzov reactions which involve acyl halides and phospho-rus(III) esters are yet a further route to phosphorus-carbon bond formation and will be discussed later in Section VI, the use of halogenated acyl halides has led to some unusual results which, conveniently, can be summarized here. The products obtained from reactions between trialkyl phosphites and perfluoroacyl chlorides contain both phosphonate and phosphate moieties and are structurally dependent on reaction temperature. The initial product (Scheme 4) is thought to be the ylide 66. In an ethereal solvent at low temperature, decomposition of the ylide yields [l-(dialkoxyphosphinoyl)oxy-l/f-perfluo-roalkyl]phosphonates (67) exclusively, but at -20 °C and above, and in the absence of a solvent, the products consist of (Z)-[l-(dialkoxyphosphinoyl)oxyperfluoroalkene]phos-phonates (68) . The treatment of the compounds 67 with Ida yields 68, and the action of BuLi-CuI on 68 results in loss of the phosphate moiety to give the esters 69 The structural isomers 70 of the compounds 68 have been obtained as illustrated in equation... 

Probably the best known of all the many reactions which result in the cleavage of a phosphorus-carbon bond is one which is based on the Wittig reaction. The Wittig reaction consists in the interaction of a triphenylphosphonium ylide and an aldehyde or ketone, the outcome of the reaction being the formation of a tertiary phosphine oxide together with an... 

More recent developments are based on the finding, that the d-orbitals of silicon, sulfur, phosphorus and certain transition metals may also stabilize a negative charge on a carbon atom. This is probably caused by a partial transfer of electron density from the carbanion into empty low-energy d-orbitals of the hetero atom ( backbonding ) or by the formation of ylides , in which a positively charged onium centre is adjacent to the carbanion and stabilization occurs by ylene formation. 

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 initial step of olefin formation is a nucleophilic addition of the negatively polarized ylide carbon center (see the resonance structure 1 above) to the carbonyl carbon center of an aldehyde or ketone. A betain 8 is thus formed, which can cyclize to give the oxaphosphetane 9 as an intermediate. The latter decomposes to yield a trisubstituted phosphine oxide 4—e.g. triphenylphosphine oxide (with R = Ph) and an alkene 3. The driving force for that reaction is the formation of the strong double bond between phosphorus and oxygen ... 

The reactivity of the phosphorus ylide 1 strongly depends on substituents R R. For preparative use R often is a phenyl group. When R or R is an electron-withdrawing group, the negative charge can be delocalized over several centers, and the reactivity at the ylide carbon is reduced. The reactivity of the carbonyl compound towards addition of the ylide increases with the electrophilic character of the C=0 group. R R are often both alkyl, or alkyl and aryl. 

A comparative study on ylide stability as a function of the heteroatom type was carried out by Doering et al. [3,4]. They concluded that the phosphorus and sulfur ylides are the most stable ones. The participation of three-dimensional orbitals in the covalency determines the resonance stabilization of the phosphorus and sulfur ylides [5-8]. The nitrogen ylides are less stable from this point of view. The only stabilization factor involves electrostatic interactions between the two charges localized on adjacent nitrogen and carbon atoms [9]. 

O The nucleophilic carbon atom of the phosphorus ylide adds to the carbonyl group of a ketone or aldehyde to give an alkoxide ion intermediate. 

An aldehyde or ketone reacts with a phosphorus ylide to yield an alkene in which the oxygen atom of the carbonyl reactant is replaced by the =0 2 of the ylide. Preparation of the phosphorus ylide itself usually involves reaction of a primary alkyl halide with triphenylphosphine, so the ylide is typically primary, RCH = P Ph)3-This means that the disubstituted alkene carbon in the product comes from the carbonyl reactant, while the monosubstituted alkene carbon comes from the ylicle. 

Previous syntheses of terminal alkynes from aldehydes employed Wittig methodology with phosphonium ylides and phosphonates. 6 7 The DuPont procedure circumvents the use of phosphorus compounds by using lithiated dichloromethane as the source of the terminal carbon. The intermediate lithioalkyne 4 can be quenched with water to provide the terminal alkyne or with various electrophiles, as in the present case, to yield propargylic alcohols, alkynylsilanes, or internal alkynes. Enantioenriched terminal alkynylcarbinols can also be prepared from allylic alcohols by Sharpless epoxidation and subsequent basic elimination of the derived chloro- or bromomethyl epoxide (eq 5). A related method entails Sharpless asymmetric dihydroxylation of an allylic chloride and base treatment of the acetonide derivative.8 In these approaches the product and starting material contain the same number of carbons. 

Usually, C-mercury substituted phosphorus ylides are monomers and in order to stabilize these complexes the presence of a second substituent on the carbon is necessary to balance the electron-donating effect of the metal. However a dimeric complex 85 has been obtained by the reaction of mercuric halides HgX2... 

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... 

A number of A5 n3 compounds (18) have chemical shifts in the range 127 to 178.37 A theoretical study supports the suggestion made in the last volume that these compounds are represented as phosphonium ylides with the negative charge distributed between the two sp carbon atoms bound to the phosphorus atom.3 ... 

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. 

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. 

This is an important parallel to the analogous process first discovered for phosphorus ylides in the late 1980s,166 and extended to simple hydrocarbons in the mid-1990s.15 Note that carbon is again found to become hypercoordinate in a square-pyramidal geometry (Scheme 37). 

In contrast, the corresponding phosphorus ylides show insertion of the ylidic carbon fragment into the C /C3 bond of 4,4-diacyi triafulvenes giving rise to a-acyl diacyl-methylene cyclobutenes 561269 which are isomerized thermally to the 2-methylene pyranes 560, probably via the allene 562. 

Organometallic betaines of type I can be considered as the closest structural analogs of carbon betaines of the (+ )P-C-C-X( ) type (IV), which were regarded for a long time as possible intermediates in classical reactions of carbonyl and thiocarbonyl compounds with phosphorus ylides (Wittig and Corey-Chaykovsky reactions and related processes,5,6 Scheme 1). Vedejs and coworkers7,8 proved unambiguously that oxapho-sphetanes (III) are true intermediates in the reactions of nonstabilized phosphorus ylides with carbonyl compounds. The formation of oxabetaines (+)p-c-c-o(-> was detected only in the form of their adducts with lithium salts.9,10... 

Reactions of Phosphorus Ylides with Carbon Disulfide, Stable Thiocarbonyl Compounds, and Thiosilanones... 

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