Aminophosphine boranes

The nature of donor-acceptor interactions in aminophosphine-borane adducts may be elucidated by studying the chemical behaviour of the BH3 group and H, nB, and 31P n.m.r. spectra.190 The P atom is a better acceptor than N towards B when the P and N are directly linked (as in Me2NPMe2), although the N co-ordinates preferentially to B when the P and N are separated by a methylene bridge [e.g. in (Et2NCH2)3P]. 

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... 

The diastereoselective synthesis of a 7 -chirogenic p-aminophosphine ligand 300 by carbon-carbon bond formation of the ethano bridge in a 3 1 ratio via reaction of an a-metallated f -chiral phosphine borane (5)-297 with a benzaldimine was described. The major diastereoisomeric p-aminophosphine borane (Sp)-298 was separated and decomplexed into the corresponding p-aminophosphine (5p)-300 under neutral conditions and without epimerization by heating at reflux in EtOH (Scheme 100) [198]. 

Kolodiazhnyi and co-workers reacted racemic chlorophosphines with (5)-methylbenzylamine to yield unequal mixtures of epimeric aminopho-sphines. After boronation, some aminophosphine boranes could be obtained as enantiomerically pure solids by recrystallisation as 134 and used to prepare other compounds such as 135 and 136. 

The first step is the nucleophilic ring opening of 7 with organolithiums at low temperature to produce aminophosphine boranes 8. This step occurs regiose-lectively (P-0 bond cleavage) and diastereoselectively (retention of configuration). The selective cleavage of the P-N bond is then performed in methanol with the presence of one equivalent of sulfuric acid. The products are... 

Table 4.2 shows that very often the reaetions proeeed in high yields and very high enantioselectivities. Nonetheless, the reader must be warned that the ee values usually refer to recrystallised eompounds and therefore the optieal purity of the crude products could be lower. This detail, however, is rarely discussed in the literature, although an exeeption ean be foimd in entry 33. The methanolysis of aminophosphine boranes 8 with two aromatie groups (entries 12 6) does not seem to be particularly sensitive to sterie hindering sinee it is not prevented by 0,0-disubstituted groups (entries 21 and On the other... 

The reaction is also troublesome when the precursor contains a ferrocenyl group (entries 41 and 42). Here the reaction is sluggish and low yielding, with around 35% of starting material recovered. In contrast, the methanolysis of a diastereomeric mixture of the Z)zj (aminophosphine) boranes derived from l,l -dilithioferrocene affords 20 in 90% yield but with only 20% of ee (Scheme 4.10). ... 

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. 

Table 4.2 Methylphosphinite boranes 9 from aminophosphine boranes 8.

Johansson, Kann and Andersson" transformed (5)-PAMP BH3 into several p-aminophosphine boranes via the sequence formylation-reductive amination (Scheme 4.33). 

The carbanion derived from (phosphine borane 73 in 85% yield. This compound, although rather unstable, when freshly prepared reacted smoothly with amines and subsequently sodium triacetoxyborohydride under microwave irradiation (pW) to afford the desired p-aminophosphine boranes 79 in 84-96% yields and in optically pure form. 

The carbanion derived from (S)-PAMP BH3 reacted with A-benzilidenea-niline to afford the p-aminophosphine boranes 76 as an epimeric mixture in 76% yield. The major diastereomer (shown in Scheme 4.34 after deprotection)... 

Scheme 4.33 Synthesis of P-aminophosphine boranes by formylation and subsequent reductive amination.

Ring opening of 116 with t-BuLi and subsequent acidolysis with / -toluene-sulfonic acid (PTSA) afforded the secondary phosphine oxide 117 in 40% overall yield but with an interesting 85% ee. This reaction is noteworthy because the reader will probably remember that the acidolysis (either with Me0H/H2S04 or with dry HCl) of t-Bu substituted aminophosphine boranes does not occur (see Section 4.3.2). 

Further work of the same group was focused on the obtention of P-ste-reogenic p-aminophosphine boranes. To this end, they employed a-formylated and a-carboxylated phosphine boranes (Scheme 5.18). 

Scheme 5.18 Two possible routes to prepare p-aminophosphine boranes.

Functionalised aminophosphines, e.g. (35), can be conveniently purified and stored as their air- and water-stable borane adducts (36) these with hydrogen chloride gave the chlorophosphine adducts (37) which were useful for the preparation of functionalised phosphines. An improved method to prepare N-pyrrolylphosphines (38) and (39) has appeared. The related A -indolylphos-phines (40) have been similarly prepared. Two papers describe the preparation of the tervalent phosphorus hydrazides (41) and (42) for use as chelating ligands. 

Aminophosphines 46-49 are useful starting compounds for the preparation of enantiopure compounds 4c and 50-56 (Scheme 17) [37-40]. The treatment of aminophosphines 48 with borane in THF leads to the formation of the stable... 

The opening of oxazaphospholidine rings (/ p)-122 with tert-butyllithium occurred diastereoselectively with retention of absolute configuration on the phosphorus atom, affording the borane complex of aminophosphine (Rp)-124 [67, 68]. The reaction possibly proceeds via formation of chiral o X -phosphenium cation 126, which was obtained from (Sc)-chlorophosphine 125 and then isolated as borane complex 127 (Scheme 38). 

Aminophosphines. The synthesis and use of aminophosphines as ligands have been reviewed. Raeemie ehlorophosphines of the type R R PCl have been shown to react stereoselectively with chiral amines (1-phenylethylamine or aminoacid esters) in the presence of triethylamine to give the diastereomerically enriched aminophosphines (137), which were isolated as diastereomerically pure crystalhne borane complexes. This approach has also been used in the synthesis of chiral t-butylphenylphosphine oxide, via the acid hydrolysis of an intermediate chiral aminophosphine. Among other new mon-oaminophosphines prepared by treatment of primary or secondary amines with ehlorophosphines in the presence of a base are the adenine derivatives (138), the phosphinoalkylaminophosphines (139), " the aminophosphine-phosphine... 

Reaction conditions are very straightforward, consisting of stirring the aminophosphines 8 in freshly distilled methanol in the presence of 1 equivalent of sulfuric acid for several hours at 0 °C or room temperature. Methylpho-sphinite boranes 9 are either clear liquids or crystalline solids, which can be easily purified by column chromatography or recrystallisation and, in contrast to 8, are indefinitely stable in air. Another advantage is that the reaction yields one equivalent of unchanged ephedrine, which can be recycled. The reaction is... 

After ring opening of 7 by an organolithium, the opened aminophosphine alkoxide 12 can be trapped with an equivalent of a chlorophosphine to afford 66 after boronation. Alternatively, attack of 12 on a highly enantioen-riched chlorophosphine borane 31 affords the doubly P-stereogenic compounds 67. Both 66 and 67 can be easily deprotected with DABCO in toluene, for example, with complete retention of configuration at the phosphorus atom. 

A large number of compounds in which nitrogen is (formally) singly or doubly bonded to a nonmetallic heteroelement have been studied in N or resonance. Topologic, electronic, and steric contributions to and shielding relationships in the P(III)-N bond have been deduced from aminophosphine studies, and for amine-boranes, correlations observed of and "B with C in the... 

Bar-Nir BB-A, Portnoy M (2000) Addition of borane-protected secondary phosphines to imines. A route to protected mono-Af-substituted-a-aminophosphines. Tetrahedron Lett 41 6143-6147... 

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