Diphosphine boranes

We further synthesized unsymmetrical MiniPHOS derivatives 13b (Scheme 13) [30]. Thus, enantioselective deprotonation of l-adamantyl(dimethyl)phos-phine-borane (74, R = 1 -Ad), followed by treatment with ferf-butyldichlorophos-phine or 1-adamantyldichlorophosphine, methylmagnesium bromide and bo-rane-THF complex afforded the optically active diphosphine-boranes 82b as a mixture with the corresponding raeso-diastereomer. Enantiomerically pure unsymmetrical MiniPHOS-boranes 82b were obtained by column chromatography on silica gel or separation by recycling preparative HPLC. 

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). 

In sharp contrast to the intensely studied reactions of dipenteles with transition metal compounds, reactions with group 13 metal compounds are almost unknown. Only two diphosphine-borane bisadducts of Type C ([H3B]2[Me4P2], [H2(Br)B]2[Me4P2]) have been synthesized and structurally characterized65 but no diarsine, distibine or dibismuthine adducts. We, therefore, became interested in the synthesis of such compounds, focusing... 

Figure 13 Molecular structures of the diphosphine-borane 8a (open form, left) and triphosphine-borane 9 (closed form, right) with selected geometric and spectroscopic data.

Diphosphaferrocene, with chromium carbonyls, 5, 220 l,l -Diphospha[2]ferrocenophane, synthesis, 6, 210 Diphosphazanes, in dinuclear Ru complexes, 6, 674 l,l -Diphosphetanylferrocenes, preparation, 6, 200-201 Diphosphine borane complex, polypyrrole support for,... 

Polypropylenes, and metallocene catalysts amorphous PP, 4, 1052 flexible and elastomeric isotactic PP, 4, 1064 isotactic PP, 4, 1056 syndiotactic PP, 4, 1070 Polypyrroles, with ferrocene groups, 12, 305 Polypyrrole supports, for diphosphine borane complex,... 

A polypyrrole-supported diphosphine borane complex has been prepared by electrochemical polymerization of a diphosphine-substituted monomer and used as a supported ligand for Rh and Pd catalysis. 

Table 2.6 Mono- and diphosphine boranes prepared by the dynamic resolution of rac-21 (Scheme 2.6).

With this strategy, tert-butylmethylphenylphosphine borane and PAMP-BH3 were obtained, although the latter presented low optical purity (41% ee) due to racemisation in the decarboxylation step, which had been performed in xylene at 130 °C for 1 h. The menthyl ester intermediate 60, for R = o-An, was also reduced to the alcohol, which was mesylated and iodinated affording the (I-iodophosphine borane 62 in 62% combined yield. This compound was reductively dimerised by activated copper to diphosphine borane 63 (64% yield), which is similar to DiPAMP but with a butyl bridge. 

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). 

All the phosphine boranes in Table 2.14 were obtained as optically pure compounds after removal of the small quantity of meso compound, if any. Compounds of entries 1-4 bear an ort/zo-substituted phenyl group and were prepared in order to compare their performance in Rh-catalysed hydrogenation with DiPAMP and to shed light into the role of the methoxy group in the catalysis. The menthyldiphosphinite borane of entry 7 is interesting, because it has been used to prepare other diphosphine boranes (Table 2.11, entry 11, and Table 2.12, entry 19). 

Scheme 2.22 Preparation of optically pure methylphenylphosphine boranes and a diphosphine borane.

Treatment of 75 (with a 3 1 epimeric ratio) with vinylmagnesium chloride allowed the isolation of crude 76 as a 2 1 mixture of epimers, which could be separated by column chromatography and recrystallisation (22% yield for the major epimer, which corresponded to the i p). Each diastereomer was reacted with lithium diphenylphosphide and borane to afford 77 in good yield. The diphosphine borane was deprotected and complexed to a chiral palladium complex to determine the absolute configuration at the P atom by careful NMR studies. 

Table 2.14 Diphosphine boranes 64 with an ethylene bridge by the menthol methodology.

Scheme 2.27 Preparation of a diphosphine borane containing a P-stereogenic atom with a menthyl substituent. Only the Rp diastereomer is represented.

Monoferrocenyllithium was prepared by direct metallation of ferrocene with t-BuLi, a procedure that resulted in the formation of traces of l,l -dilithio-ferrocene, explaining the obtention of the interesting diphosphine borane 40 in 3% yield. This has been exploited to prepare bidentate ligands, as detailed below. Other groups prepared other ferrocenyl monophosphines as can be... 

Scheme 4.21 Preparation of C2-symmetric diphosphine boranes based on the ferro-cenyl backbone.

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

Mezzetti s" and van Leeuwen s " groups followed this method to prepare a small family of diphosphine boranes. For = Ph, diphosphines with R = o-An and R = 1-naphthyl have been named BPAF and BPNF respectively." Table 4.5 lists the diphosphine boranes of this type reported to date. 

Table 4.5 Ferrocenyl-based diphosphine boranes 41 prepared from 9.

The preparation of a very unusual diphosphine borane, reported by van Leeuwen and co-workers, is depicted in Scheme 4.22. 

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. 

Mezzetti and co-workers, in their exploration of phosphines bearing highly symmetric and bulky substituents, used (5)-chloromethylphenylphosphine borane (Scheme 4.26) to prepare a rare example of non-C2 symmetric methylene-bridged P-stereogenic diphosphine borane. 

From a-lithiated carbanions 72, formylated (73), carboxylated (74), silylated (75) and aminated (76) P-stereogenic phosphine boranes have been prepared without loss of optical purity. Some of them can be further functionalised to obtain other interesting compounds. Furthermore, oxidative coupling with Cu(II) salts affords C2 diphosphine boranes of the DiPAMP family (77) and... 

The Juge-Stephan methodology has been used to prepare many Cj diphosphine boranes bearing at least one aryl group at each phosphorus atom. In particular, it is probably the best method to prepare not only DiPAMP itself, but also a large family of analogues with dilferent substituents at the phosphorus atoms and/or different bridges, which have found application in many catalytic processes. 

In all the entries but the first two, the phosphorus atoms contain two aryl groups. Some crowded diphosphine boranes have also been reported (entries 2, 6, 10, 14 and 16-21). In general, good yields for the coupling reaction are observed, even in the latter cases. The diphosphine boranes are obtained as enantiomerically pure white solids or oils, which can be easily deboronated by amines. The diphosphinite borane of entry 1 also deserves some comment because another product could have been formed (Scheme 4.40). 

Table 4.7 Diphosphine boranes with an ethylene bridge prepared by the Juge-Stephan method.

Nucleophilic displacement of both methoxy groups of compound 89 has been used by Zupancic, Mohar and Stephan " as an alternative route to diphosphine boranes (Scheme 4.41). 

Scheme 4.42 Preparation a diphosphine borane by double nucleophilic substitution.

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