C2-Diphosphines

Phosphine boranes 75 can be obtained pure after acidic work-up in moderate yields even when the electrophile is the very bulky chlorotriphenylsilane. Remarkably, the same reaction has been used to prepare C2 diphosphines with a central -SiR2 moiety and also the first dendrimers containing P-stereogenic phosphines, compounds discussed in the next sections. 

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. 

This reaction requires the presence of stoichiometric amounts of Cu(II) salts and is thought to proceed though a radical intermediate, implying that the rates of formation of the homochiral isomers of 77 and the meso form are very similar. This comes with a bonus even if 71 is not completely enantiopure, the optical purity of the desired / isomer of 77 increases after separation of the meso diastereomer. For example, when ( S)-71 is enriched to 89% ee the calculated product distribution for 77 is 296 34 1, meaning that after 

Reaction of 88 with. s-BuLi/CuCl2 provides 89 in good yields, but the substitution product 90 could have been formed, following the reactivity of Section 4.3.3. The balance between basicity and steric hindrance of 5-BuLi probably makes it ideal to deprotonate the methyl group but not nucleophilic enough to attack the phosphorus centre to form 90. 

This substitution reaction is low yielding (33 2%) but it occurs with inversion of configuration at the phosphorus atoms and therefore leads to the other enantiomer of 77 than the traditional oxidative coupling with Cu(II). 

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Figure 1.3 Chelating C2-diphosphines and their ee values in Rh-catalysed hydrogenation of 11.

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... 

With this method, one C2 diphosphine borane was prepared (entry 1) along with several C analogues bearing one (entries 2,4, 5 and 7-12) or two (entries 3 and 6) stereogenic phosphorus atoms. The yields were moderate to good, except when the nucleophile was sterically hindered (entries 2 and 5). 

Many chiral C2 diphosphines have been used as hgands in rhodium sterns for asymmetric hydroformylation. It can be observed that the chiral diphosphines providing the highest enantioselectivities in asymmetric hydrogenation lead to low chiral induction in hydroformylation. The C2-symmetric diphosphines, which were developed for square planar... 

Many C2 diphosphines have been used in the asynunetric hydrofonnylation reaction. The enantioselectivities, however, are always low or moderate, generally no higher than 30% [2-4]. 

Scheme 5.11 Pd- and Pt-catalysed Diels-Alder reactions with C2-symmetric biheteroaromatic diphosphines.

Cl-symmetric hgands diphosphine-thiophene ligand 264-5 monosulfoximine hgands 192-3 C2-symmetric hgands 9 biheteroaromahc diphosphines 193 bis(sulfonamide) hgands 145-6 bis(sulfoximine) hgands 42-3,190-3... 

Very recent investigations of styrene hydroboration with C2-symmetrical 1,2-diphosphines give promising regio- and stereoselectivities. As with the other ligands described above, the electronic properties of the phosphorus substituents prove paramount in determining the reactivity (Scheme 12). Hence, the combination of (li ,2i )-l,2-bis(diphenylphosphino)cyclohexane (19) with [Rh(COD)2]+... 

While Josiphos 41 also possessed an element of atom-centered chirality in the side chain, Reetz reported a new class of ferrocene-derived diphosphines which had planar chirality only ligands 42 and 43, which have C2- and C -symmetry, respectively.87 Rhodium(i)-complexes of ligands (—)-42 and (—)-43 were used in situ as catalysts (0.75 mol%) for the hydroboration of styrene with catecholborane 1 for 12 h in toluene at — 50 °C. The rhodium/ i-symmetric (—)-43 catalyst system was the more enantioselective of the two - ( -l-phenylethanol was afforded with 52% and 77% ee with diphosphines (—)-42 and (—)-43, respectively. In both cases, the regioselectivity was excellent (>99 1). With the same reaction time but using DME as solvent at lower temperature (—60 °C), the rhodium complex of 43 afforded the alcohol product with an optimum 84% ee. 

The catalyst-substrate complexes deserve some additional comments. The two possible diastereomers for C2-symmetrical diphosphines interconvert inter- and intramolecularly, the latter being the dominant mechanism [76] (Scheme 1.22). A second property - at least of some catalyst-substrate complexes - is that the reactivity of the minor diastereomer toward H2 is notably higher than that of the major diastereomer. 

A variety of C2 symmetrical diphosphine ligands with a ferrocenyl backbone (see Fig. 25.5) have recently been described and tested, with sometimes quite impressive results. Interesting examples are f-binaphane [11], ferrotane [12], L2... 

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