Phospholane boranes

Double deprotonation of methylphosphine borane and addition of 111 produced an equimolar mixture of phospholane boranes 112. They were separated by meticulous column chromatography, yielding 38% yield of (/ p)-112, which was oxidatively dimerised to afford 113 in 50% yield. Although (/ p,/ p)-113 can obviously be prepared from the other enantiomer of 111, it was prepared by different protocols (see Scheme 2.40 in this chapter and Chapter 5, Section 5.3.2). 

Scheme 2.40 Phospholane boranes prepared from cyclic sulfates 114.

In this case the carbanion was quenched with dry ice in order to prepare the a-carboxylated phospholane borane 123. This compound was obtained as a... 

Boranes 1 and 2 in equation 89 react with 2-fluoro-1,3-dioxa-2-phospholanes to give nng expansion products as the corresponding antimony analogues yield the products of msertion of nitrogen into the antimony-fluorme bond [775] (equation 89) (Table 31)... 

The me.vo-diphcnylphospholanomide was obtained via a cycloaddition reaction of N,N-d -methylaminophosphinodichloride and 1,4-diphenylbutadiene followed by hydrogenation. The mcw-compound was transformed into an equilibrium mixture containing predominately the rac-compound on treatment with base. After hydrolysis, the phospholanic acid was kinetically resolved using quinine. Reduction and protection of the phospholane as the borane adduct allowed the double displacement of 1,2-ethyleneglycol ditosylate. 

Vbp couplings of ca. 95 Hz have been measured for l,3-(dioxa, oxaza or diaza)-2-phospholanes and their borane adducts by Peralta-Cruz et al.p Jpse couplings of ca. 950 Hz have been found for the corresponding selenium adducts. 

Enantioselective intramolecular cyclization of secondary phosphines 216 or their boranes, catalyzed by chiral palladium(diphosphine) complexes, afforded P-stereogenic benzophospholanes 217 with moderate stereoselectivity (59-70% ee) and yields. However, the absolute configuration of compounds has not been established. This reaction allowed chiral phospholanes to be obtained, which are valuable ligands in asymmetric catalysis (Scheme 70) [115]. 

Phospholanes usually adopt a folded envelope configuration, with which there are possible alternative arrangements (6.816). Phospholane, (CH2)4PH, mp = -88°C, bp = 100-103 C, can be prepared via the dimethylamine borane adduct or the chloro derivative. 

Secondary phosphine 195 was prepared from bromide 194 (derived from (P)-camphor) as an equimolar mixture of epimers. Upon liberation of the ketone group under acidic conditions, nucleophilic addition of the secondary phosphine borane took place, giving (/ p)-196 as the sole compound. Interestingly, in non-polar solvents the phospholane ring opened affording 197 as a mixture of epimers. 

In the case of the (S,S,/ ax)-MOP phospholanes, the corresponding methoxy and phenolic compounds were also obtained, and were converted as above to their borane adducts yielding a yellow solid (39% combined mass), which was poorly soluble in most common solvents. After column chromatography, the methoxy compound (S,S,/ ax)-20b.BH3 was obtained in an impure form and in such a small quantity as to render further purification impractical. A small amount (10%) of the phenolic compound (S,S,/ ax)-21b.BH3 was isolated by column chromatography. 

Preparation of H-MOP phospholane (R,R,5ax)-22a employed the same methodology starting from (5)-6 and it was isolated as a white solid in 69% yield. For comparative purposes we also synthesized 2-naphthylphospholane (R,R)-23. For (5,5, 5 ax)-22b it was necessary to purify the compound via the borane adduct (5,S,5 ax)-22b.BH3 which again impacted on the final yield. 

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