Oxazaphospholidine boranes preparation

Recently Buono has reported a new class of efficient catalyst systems which contains a chiral oxazaphospholidine-borane complex as a catalyst in the borane reduction of ketones[115, 116]. The oxazaphospholidine-borane complex 75 is easily prepared by action of 1.3 equiv. of BH3 THF on (2R, 5S)-2-phenyl-3-oxa-... 

An oxazaborolidine 65 prepared from a-pinene is a new system. More drastic ariations are the oxazaphospholidine oxides, such as 66, which have been examined for their effectiveness in catalyzing the asymmetric reduction.The corresponding oxazaphospholidine-borane complex catalyzes the reduction of imines with diminished aereocontrol. 

Juge developed a powerful method (Juge-Stephan method) [1, 51] for the preparation of / -stereogenic phosphines based on the use of ephedrine as a chiral auxiliary. The key reactants in this methodology are 1,3,2-oxazaphospholidine boranes 78, prepared by a one-pot reaction from bis(diethylamino)phenylphosphine and (—)-ephedrine, followed by protection with BH3. The cyclization of the (—)-ephedrine takes place stereoselectively, with preferential formation of the (/ p)-diastereoisomer in 90% de [52, 53]. The absolute configuration at the phosphorus atom has been determined by chemical correlations and NMR analysis, and proved by X-ray analysis [54]. Oxazaphospholidines react readily with electrophiles or nucleophiles to provide various chiral phosphorus compounds. Enantiomeric antipodes of tertiary phosphines (Sp)-79 and (Rp)-81 were obtained from (-1-)- or (-)-ephedrine, as shown in Scheme 25. The configuration at the E-atom is controlled by the configuration at the Ph-substituted Cj of (-i-)-pseudoephedrine or ( )-ephedrine, respectively. This was confirmed by X-ray crystal-structure analyses of two intermediate compounds in the synthetic route to the chiral triarylborane-phosphine adducts [54]. 

Scheme 3.17 Stepwise cyclisation of ( )-ephedrine to prepare oxazaphospholidine boranes (24).

In the last section several oxazaphospholidine oxides, obtained by oxidation of the P(III) precursor with t-BuOOH, have already been described. There is also one report by Juge and co-workers in which they prepare oxazaphospholidine oxides and sulfides by in situ deboronation/oxidation of oxazaphospholidine boranes. This section illustrates some more derivatives, prepared directly from P(V) species and ephedrine. Chronologically, these types of compounds were studied earlier than the corresponding P(III) counterparts. Nowadays oxazaphospholidine boranes, not oxides, are the most important precursors used to prepare enantiopure phosphorus ligands. However, apart from historic interest, ephedrine-derived oxazaphospholidine oxides, sulfides and selenides occupy an important place in the study of phosphorus stereochemistry and conformational analysis. Only a few examples are described here. 

Alcohol 61 is easily prepared from a chemical found in industrial waste and was used to prepare the tricyclic oxazaphospholidine borane 62 in 64% combined yield. The X-ray structure revealed that in this case the phosphorus atom had S absolute configuration. In the same paper they described the preparation of 64 in 40% yield and in optically pure form, starting from the azetidinic alcohol 63. The same authors prepared the phosphine oxide version of 62 via oxidation of the P(III) precursor and by direct reaction of 61 with PhP(0)Cl2/NEt3. 

Martens and co-workers used several oxazaphospholidine boranes to prepare methylphosphinite boranes (Scheme 4.54). 

Also interesting are the observaa ranched at both a- and a -position. S i-alcohols. An explanation based on An oxazaborolidine 65 prepared ariations are the oxazaphospholidine. heir effectiveness in catalyzing tl xazaphospholidine-borane complex c tereocontrol. 

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