Activation phosphine-borane

P-Chirogenic diphosphine 19, which rhodium-chelate complex forms a seven-membered ring (rare case for P-stereogenic ligand), was also prepared in reasonable yield (68%) using the wide chemistry of secondary phosphine borane [37]. Deprotonation of the enantiomerically enriched ferf-butylmethylphos-phine-borane 88 (Scheme 15) followed by quenching with a,a -dichloro-o-xylene and recrystallization afforded optically active diphosphine-borane 89 (precursor of free phosphine 19). 

Scheme 17. Improved synthesis of optically active secondary phosphine-boranes...

The bridging coordination of M—X bonds represents a preliminary stage in their intramolecular activation and may ultimately lead to zwitterionic complexes upon heterolytic cleavage. Such a process has been observed and unambiguously authenticated with simple phosphine-boranes of type 4 as well as di- and tri-phosphine derivatives of the heavier group 13 elements 7 and 8. 

The air-stable phosphine-borane la was evaluated in the Pd-catalyzed Suzuki-Miyaura reaction. Typically, 4-bromoanisole and phenyl boronic acid were efficiently coupled using 1 mol% of PdCOAc) and 2 mol% of ligand la (Scheme 6). The BMes moiety is compatible with the cross-coupling and it actually improves catalytic activity (under the same conditions, PPh gives a notably lower yield). Interestingly, a catalytically competent Pd(0) complex 39 was isolated and fully characterized. The phosphine-borane adopts a new coordination mode besides phosphorus, the Pd center is coordinated by one of the Mes groups at B interaction). 

Stereochemistry — The conformational analysis of various deuteriated ethyl-phosphine-borane adducts and dimethyl methylphosphonates were based on vibrational spectral data. The stereochemistry of diethylphosphonyl acetamide,the unusual vinyl compounds (60 X = L.E.P., O, and a number of l,3,2-dioxaphosphorinanes have been studied. It was found that the Raman-active ring vibration is related to the orientation of the phosphoryl group. Conformational data for dioxaphosphepane was compared with calculated parameters. A low-temperature study of several cyclopropyl-phosphonates (61) revealed rotational isomerization about the P-O bonds but not about the P-C bond. The variable temperature i.r. and Raman spectra of the silyl phosphates (62) also revealed rotational isomerism. ... 

Pd-catalyzed cross-coupling of secondary phosphine-boranes and aryl iodides has been carried out in ionic liquids using a ligand immobilized with a pyridinium substituent (Fig. 3). Catalyst recycling at least six times without significant loss of activity was possible [93]. 

The unique reactivity of the above system with H2 appears to arise from the unquenched Lewis basicity and acidity of the respective donor P and the acceptor B centers. This inference prompted questions about the nature and reactivity of other phosphine-borane systems and, more broadly, of Lewis acid/base combinations. Is it necessary to have a link between the donor and acceptor sites Could similar H2 activation arise from combinations of donors and acceptors in which steric encumbrance frustrates Lewis acid-base adduct formation If indeed such frustrated Lewis pairs could be uncovered, could one exploit them for the activation of small molecules and applications in catalysis ... 

There are no reports on the enantioselective reaction of the carbanion a to non-activated phosphines due to the low acidity of their a-protons. However, formation of phosphine-borane complexes enables deprotonation at the a position [83]. Evans and coworkers have foimd that enantiotopic methyl groups of phos-phine-boranes can be efficiently discriminated by s-BuLi-(-)-sparteine [Eq. (30)] [84]. This result is in accord with many other reports that the chiral complex composed of s-BuLi and (-)-sparteine is generally the most efficient enantioselective deprotonating combination [1,2,3,85]. However, in the reaction of phosphine sulfides this is not the case. The n-BuLi-(-)-sparteine combi-... 

Kobayashi and coworkers have demonstrated that optically active phos-pholane-2-carboxylic acid [n=l,Eq. (33)] and phosphorinane-2-carboxylic acid (n=2) can be prepared by asymmetric carboxylation of the phosphine-borane complexes. High trans selectivity and excellent enantiomeric excesses of the trans isomers were achieved on treatment of 1-phenylphospholane-borane and 1-phenylphosphorinane-borane complexes with s-BuLi-(-)-sparteine and subsequently with CO2. Notably, enantioselectivity was improved up to 92% ee when 1-phenylphospholane-borane was treated with s-BuLi-(-)-sparteine at 25°C, compared with 83% ee at -78°C [87]. 

Phosphorylation of At2PH-BH3 with aryl halides proceeds under mild conditions using Pd-Cu catalyst and P-chiral phosphine-boranes of high enantiopurity are prepared by this method. As an effective ligand, MePPh2 is used with Pd(OAc)2. The phosphine-borane 50 was prepared in 68 % yield by phosphorylation of optically active (Sp)-methylphenylphosphine-borane (47) via 48 with the iodide 49 in the presence of the Pd(OAc)2-MePPh2 catalyst and Cul as a cocatalyst at 0°C for 3 days. The reaction proceeded with retention of stereochemistry and the asymmetrically substituted phosphine 51 with 99% ee was obtained [15]. 

Scheme 1 Reversible H2 activation by the phosphine-borane species...

Pd/L ratio and the source of the metal, but with little success. It was found that increasing the temperature to 50 °C tripled the conversion from 12 to 35% and, unexpectedly, slightly improved the enantioselectivity (from 30 to 42%). Further increase of the temperature is detrimental for both activity and enantioselectivity. This is probably due to deprotection of the phosphine borane 41a to the free phosphine, which can compete with L in palladium complexation. 

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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