Methylphosphine boranes

Vedejs, E. and Donde, Y., Stereogenic P-trisubstituted phosphorus by crystallization-induced asymmetric transformation A practical synthesis of phe-nyl(o-anisyl)methylphosphine borane, /. Am. Chem. Soc., 119, 9293, 1997. 

Synthesis of (R)-tert-Butyl(hydroxymethyl)methylphosphine-borane.. .. 66... 

Synthesis of (R)-Benzoyloxy(tert-butyl)methylphosphine-borane. 67... 

The combined organic extracts were washed with 2 M HCl and brine, and dried over Na2S04. The solvent was removed on a rotary evaporator, and the residue was purified by flash chromatography over silica gel (eluent hexane/ethyl acetate = 3/ 1) to give (R)-terf-butyl(hydroxymethyl)methylphosphine-borane as a colourless solid (6.48 g, 73%). -9.9° (92% ee, c = 1.0, CHCI3). 

SYNTHESIS OE (R)-BENZOYLOXY(ferf-BUTYL)METHYLPHOSPHINE-BORANE... 

Benzoyl chloride (2.61 mL, 22.5 mmol) was added to a stirred, cooled (0°C) solution of (R)-ferf-butyl(hydroxylmethyl)methylphosphine-borane (92% ee, 2.22 g, 15.0 mmol) in pyridine (10 mL) and the mixture was warmed to room temperature. After 1 h, the reaction mixture was quenched with 2 M aqueous HCl. 

To a stirred solution of benzoyloxy(tcrt-butyl)methylphosphine-borane (99% ee, 6.06 g, 24 mmol) in acetone (25 mL) was added a solution of potassium hydroxide (4.0 g, 72 mmol) in water (15 mL) dropwise. After completion of the hydrolysis (ca. 1 h), the reaction mixture was diluted with water (lOOmL) and extracted three times with diethyl ether. The combined extracts were washed with brine and dried over Na2S04. The solvent was evaporated and the resulting crude (/f)-tert-butyl(hydroxymethyl)methylphosphine-borane was dissolved in acetone (70 mL). The solution was added to a vigorously stirred, cooled (0°C)... 

In order to determine the ee of the product, it was successively reacted with n-BuLi and benzyl chloride in dry THE the product, benzyl(rert-butyl)methylphosphine-borane, was analyzed by HPLC. Chiracel OD-H, hexane 2-propanol = 9 l, flow rate 0.5mL min UV detection 254 nm, (5) ti — 14.7 min, (R) t2 16.4 min. 

Tsuruta and Imamoto disclosed a promising approach to prepare the methylphosphine borane of entry 11 in high optical purity, although it required two more steps (Scheme 2.21). 

Some of the methylphosphine boranes presented in the previous tables have been used to prepare several diphosphine boranes with an ethyl bridge, i.e. analogues of DiPAMP via Cu(II) promoted dimerisation (Scheme 2.23 and Table 2.14). 

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). 

Another set of diphosphine boranes has been prepared by double nucleophilic attack on dichlorosilanes by carbanions derived from methylphosphine boranes (Scheme 4.43 and Table 4.8). 

The yield of the boronated phosphine has not been reported, but is 56% for the free phosphine (contaminated with the methylphosphine borane precursor). 

The high reactivity of chlorosilanes towards a-carbanions derived from methylphosphine boranes and the availability of several carbosilane dendrimers possessing Si-Cl bonds made the preparation of the first dendrimers bearing P-stereogenic phosphines possible. 

The reaction of triflate 99 with a metallated methylphosphine borane followed by deboronation with morpholine yielded, after column purification, dendron 100 in 31% yield. An analogous procedure was used to prepare 101 in similar yield. Both dendrons can be considered as phosphines bearing an extremely bulky carbosilane dendrimer group. 

The deprotonation with i-BuLi of diethyl ether solutions of several aryldi-methylphosphine boranes 4 in the presence of (-)-sp generated the corresponding carbanions 5, which were quenched with benzophenone to afford derivatives 6 in good yield and optical purity (Table 5.1). 

Scheme 5.4 Enantioselective deprotonation-electrophilic quenching of aryldi-methylphosphine boranes.

The enantioselective deprotonation protocol has been expanded to alkyldi-methylphosphine boranes. Kann and co-workers subjected t-butyl- and... 

Enantioselective deprotonation followed by oxidative coupling with Cu(II) salts has been employed to prepare C2-symmetric 1,2-Z)Zj (methylphosphine borane)ethanes, described in the following paragraphs and in Section 5.2.1.3. 

Imamoto and co-workers used a few optically pure alkynyl(t-butyl)-methylphosphine boranes 47, obtained by enantioselective deprotonation, to prepare the corresponding 1,2-diphosphinoethanes 48 (Scheme 5.21). 

The practical unavailability of (+)-sparteine means that the standard enantioselective deprotonation protocol can not be used to prepare (i ,f )-BisP diphosphines (see, however. Section 5.4.1). Some work has been performed to overcome this limitation. Imamoto and co-workers were able to prepare both enantiomers of t-Bu-BisP (BH3)2 by using both enantiomers of mrt-butyl-methylphosphine borane. Unfortunately, the preparation of the latter compounds involved a preparative HPLC separation step and therefore the strategy cannot be easily adapted for large-scale synthesis. 

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