Phosphinite boranes

Treatment of Cjq with lithiated secondary phosphine boranes or phosphinite boranes followed by removal of the BHj group afforded 1,2-hydrophosphorylated 27 in good yields (Scheme 3.14) [114, 117]. The phosphorus nucleophiles were generated by deprotonation of the corresponding borane complexes with Bull in THF-HMPA, and added to toluene solutions of Cjq at -78 °C. Complexes 26 are stable in air at room temperature for months. In the NMR spectrum, the proton of 26b appears... 

Figure 1.52. Racemic fullerenyl-substituted phosphane ( )-244, and stereoisomerically pure P-chiral fullerenyl-substituted phosphinites 245 and 246, obtained via addition to Cgo of diastereoisomerically pure, (+)-menthyl phosphinite-borane complexes 247 and 248, respectively.

Polymer-bound piperazine can also be used for the deprotection of phosphane-, phosphate- and phosphinite-borane complexes to give the corresponding free... 

Chiral phosphines are widely used as auxiliaries for various metal-catalyzed asymmetric reactions and can be prepared from stable phosphine-borane complexes. Secondary P-chiral phos-phine-boranes can be prepared by reductive lithiation of the corresponding tertiary phosphine-borane using LN (eq Likewise, P-chiral tertiary phosphine ligands can be produced by the reductive lithiation of phosphinite-boranes followed by alkylation, both proceeding with retention of configuration (eq 18). ... 

The transformation of readily available enantiopure //-menthylphosphinates 2 into chiral phosphinous acid boranes 5 permits the elaboration of bulky P-stereogenic secondary phosphine boranes. Taking advantage of the synthetic potential of these compounds, abroad range of hindered P-chiral tertiary phosphine boranes 6 were prepared with excellent enantiomeric excesses [12,13]. Phosphinous acid 5 can easily be converted into one or the other enantiomer of the secondary phosphines boranes (Sp)- or (/ p)-6 by stereoselective reduction or substitution of the phosphinite borane derivatives, respectively (Scheme 4 and Table 1). 

Scheme 5 Preparation of enantiopuie (l/ ,25,5/f)-menthyl phosphinite boranes 6-8...

The menthyl phosphonites and menthyl phosphinites were used for the preparation of F-chiral monodentate ligands. For example, Imamoto used menthyl phosphinite borane complexes for the synthesis of DIPAMP analogues, as shown in Scheme 6 [23]. The treatment of phosphinite borane (/ p)-8 with methyllithium. 

Oxazaphospholidine boranes 82 react regio- and stereoselectively with alkyl lithiums or aryl lithiums in THE at —78°C, with formation of acyclic phosphinite boranes 83. Various substituents R =n-alkyl, c-alkyl, aryl, or ferrocenyl were introduced into aminophosphine boranes 82 in high yield (93-97%) and with high diastereoselectivity dr >98 2). The reaction proceeded with retention of configuration at phosphorus. RecrystaUization of aminophosphine boranes 83 in propanol gave the diastereoisomerically pure products [53]. Acid methanolysis of aminophosphine boranes 83 led to the formation of phosphinite boranes 84 with inversion of configuration on the E-center to yield the compotmds 84 in high... 

Scheme 4.9 Preparation of phosphinite boranes 9 by acidic methanolysis of 8.

The reaction of aminophosphine borane 21 with o-iodophenethyl alcohol yields the phosphinite borane 22, with the expected inversion of configuration, although only in 24% yield. 

Methylphosphinite boranes react smoothly with organolithium reagents to afford the corresponding tertiary phosphine boranes, as will be discussed in the next section. However, phosphinite boranes are not electrophilic enough to react with other weaker nucleophiles such as alcohols, amines or thiols. More reactive precursors, capable of producing a wide variety of phosphorus compounds, were needed. In phosphorus chemistry halophosphines, and chlorophosphines 27 in particular, are essential synthons (Scheme 4.12) as nucleophilic (after transformation into metal phosphides 28) and electrophilic building blocks. 

As suggested by Brown and Laing, the reaction of phosphinite boranes with l,l -dilithioferrocene produces Ca-symmetric diphosphine boranes 41 (Scheme 4.21). 

Reaction of phosphinite borane 42 (Table 4.2, entry 43) with 1-bromo-l -lithioferrocene afforded the monophosphine borane 43 (Table 4.4, entry 50), which still possesses a bromine atom susceptible to metallation on the non-phosphinated Cp ring. After lithiation, the ferrocenyl anion reacted with the second phosphinite borane 44 (Table 4.2, entry 44), affording the desired optically pure dissymmetric diphosphine borane 45 in 29% yield from 42. During the synthesis impurities such as undesired monosubstituted ferrocene derivatives could be separated by column chromatography. 

Chlorophosphine boranes are more reactive than phosphinite boranes and therefore are easily attacked by carbanionic reagents such as organolithiums, organosodiums or Grignard reagents. " However, they can lead to... 

In contrast to phosphinite boranes, the high electrophilicity of chloropho-sphine boranes make them reactive towards a variety of non-carbanionic nucleophiles such alcohols, amines and thiols (Scheme 4.27). ... 

Ring opening of 7 with vinyllithium, followed by hydrophosphination of the vinyl group and borane protection afforded 102 in 89% yield. Acidic metha-nolysis and reaction of the obtained phosphinite borane with n-anisyllithium produced the desired compound 103 in 35% yield over the last two steps. Compound 103 can be easily deboronated with diethylamine. ... 

Gouverneur and co-workers used several phosphine boranes and phosphinite boranes bearing alkenyl groups prepared by the Juge-Stephan method in metathesis reactions (see Chapter 6, Section 6.2.6), but they were prepared as racemates intentionally. 

Scheme 4.54 Use of several oxazazaphospholidine boranes to prepare phosphinite boranes.

With phosphine boranes, a more complicated scenario was discovered by Imamoto and co-workers.Stereochemistry of the Pd-catalysed P-C bond formation ranges from almost complete retention to inversion, depending on the solvent and base used. A dramatic example of the effect of the solvent in the arylation of phosphinite borane 46 is shown in Scheme 6.21. 

Busacca and co-workers have prepared menthol-derived phosphinite boranes 101 and 102, by direct addition of anionic secondary phosphine borane 100 to carbodiimides, yielding the chiral phosphinite boranes under ambient temperature conditions (Scheme 25). These and other derivatives, prepared by hydrophosphination of carbodiimides, were used in the synthesis of enantiomerically enriched phosphaguanidines with potential use as ligands in asymmetric catalysis. 

Scheme 1-42. Optically active phosphines from menthyl phosphinates (54) or menthyl phosphinite-borane complexes (55) by stereocontrolled R O/R displacement.

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