Phosphorus-lithium exchange

Hayashi showed that binaphthyl phosphine oxides such as 193 undergo phosphorus-lithium exchange with BuLi to generate binaphthylllithiums such as 194.162... 

The direct route involved bromine-lithium exchange in 160 followed by intramolecular attack of the carbanion at the phosphorus atom with extrusion of a phenyl group. With this procedure, 162 was obtained in 66% yield as an optically pure compound. This yield could be improved by boronation of the phosphorus atom followed by lithiation with /-BuLi, which provoked the cyclisation affording 161, which was finally deboronated with diethylamine in an overall yield of 87%. The absolute configuration of the phosphorus atom of 162 was determined by X-ray crystallography of a Pd derivative. ... 

A number of preparations of mixed halogenophosphoranes from tervalent phosphorus-fluorine compounds have been reported. For example, acyclic and cyclic fluorine compounds have been converted to phosphoranes, such as (36) and (37), by treatment with chlorine. Similar reactions leading to AA-dialkylaminodichlorodifluorophosphoranes (38) have been described and the stability of (38) to exchange processes commented upon. iVA-Dialkylaminotetraiodophosphoranes (39) have been prepared from AA-dialkylaminodichlorophosphines and lithium iodide, although no detailed physical evidence for the structure of these unusual compounds has yet been reported. The preparation of bis-(A-alkylamino)difluorophosphoranes (4) has been described above (see Section lA). 

An unusual mixed lithium phosphide/lithium alkyl aggregate has been reported as arising from an attempted synthesis of Li PH(mes ) (46). Initial treatment of phosphorus trichloride with Li(mes ) is reported to give a mixture of the desired product, (mes )PCl2, and the side product (mes )Cl (via Li/Cl exchange) in an approximately 2 1 ratio. Reduction of this mixture with LiAlH4, followed by treatment with BuLi, then leads to rapid formation of Li PH(mes ), accompa-... 

The lithiation shown in Eq. (10) takes places at 20°C in ra-pentane. Under these conditions, the reactivity of the lithimn alkyl is reduced to such an extent that only the most reactive bond in 53 (and in 59, respectively) is acted upon and therefore the selective exchange of the phosphorus hydrogen with lithium takes place. The phosphides formed can easily be separated and purified because they dissolve with difficulty in nonpolar solvents. Precipitation of these phosphides is, however, incomplete and can fail to occur if the reaction solutions contain significant concentrations of partially alkylated silylphosphanes such as (Me3C)P(SiMej)2 and MeP(SiMe3)2. 

Lithioferrocene (322) or dilithioferrocene (323) can also be prepared, and then utilized in situ, from the corresponding halo- or dihaloferrocenes by halogen-metal exchange with alkyl lithiums. Alternatively, monohthiation of l,l -dibromoferrocene gives rise to anion (351), which may be reacted with an electrophile in order to afford additional ferrocene derivatives. In this way, monophosphine oxazoline ferrocene (340) is prepared from bromo oxazoline ferrocene (352), a compound that also is used to prepare chiral-at-phosphorus ferrocenes (353). Other chiral-at-phosphoras ferrocenes (354) are made from (323) and a phosphite borane as the electrophile (equation 80). The phosphorus atoms may also be contained in a ring that possesses chirality the sequences used to prepare this family... 

Very often phosphonium ylides are generated with organolithium compounds (in particular phenyl-, methyl-, /j-butyl- and /-butyl-lithium) as bases.- However difficulties may be attached to this method in some cases. When alkyllithium compounds are used, ligand exchange at phosphorus may occur, thus giving rise to the alternative or additional formation of a second ylide. To avoid this phenomenon in the case of triphenylphosphonium salts phenyllithium has to be used as base. Ligand exchange may also be suppressed if one uses, instead of /j-butyllithium for example, the more bulky tertiary... 

Restricted Rotation and Pseudorotation. - Dynamic NMR spectroscopy of lithium bis(diphenylphosphino) amide gave an 8.1 kcal/mol rotation barrier around the PN bonds. The CP/MAS spectrum of the solvate has a single Li line, whereas the P CP/MAS spectrum reveals the chemical non-equivalence of the phosphorus sites. The appearance of two P signals indicates a minimum activation barrier for the P,P-exchange process of AG 13.4 kcal/mol. Li and P NMR spectroscopy indicated that the monomeric structure in THF solution is similar to the X-ray structure of a solid 5.3 THF solvate. 

In fact, lithium diphenylphosphide can be reacted in situ with alkyl halides to give phosphines. Obviously, this reaction could be a serious side reaction in mixtures obtained from lithium reagents and phosphorus trichloride in which residual lithium was present. It should be noted also that metalation of triphenylphosphine with butyllithium occurred at the meta position on one ring to a small extent (58). Consequently, in the condensation of phosphorus halides with lithium reagents prepared by an exchange reaction employing butyllithium, an excess of the latter could result in mixtures. 

---