Ligand monophosphines

The search for even more active and recyclable ruthenium-based metathesis catalysts has recently led to the development of phosphine-free complexes by combining the concept of ligation with N-heterocyclic carbenes and benzyli-denes bearing a coordinating isopropoxy ligand. The latter was exemplified for Hoveyda s monophosphine complex 13 in Scheme 5 [12]. Pioneering studies in this field have been conducted by the groups of Hoveyda [49a] and Blechert [49b], who described the phosphine-free precatalyst 71a. Compound 71a is prepared either from 56d [49a] or from 13 [49b], as illustrated in Scheme 16. 

The hydroboration of enynes yields either of 1,4-addition and 1,2-addition products, the ratio of which dramatically changes with the phosphine ligand as well as the molar ratio of the ligand to the palladium (Scheme 1-8) [46-51]. ( )-l,3-Dienyl-boronate (24) is selectively obtained in the presence of a chelating bisphosphine such as dppf and dppe. On the other hand, a combination of Pdjldba), with Ph2PC6p5 (1-2 equiv. per palladium) yields allenylboronate (23) as the major product. Thus, a double coordination of two C-C unsaturated bonds of enyne to a coordinate unsaturated catalyst affords 1,4-addition product On the other hand, a monocoordination of an acetylenic triple bond to a rhodium(I)/bisphosphine complex leads to 24. Thus, asymmetric hydroboration of l-buten-3-yne giving (R)-allenyl-boronate with 61% ee is carried out by using a chiral monophosphine (S)-(-)-MeO-MOP (MeO-MOP=2-diphenylphosphino-2 -methoxy-l,l -binaphthyl) [52]. 

Phosphine sulfides or selenides are well-known ligands in gold(I) chemistry. With monophosphine complexes of the type [AuX(SPR3)] (X = C1, Br, CN PR3 = PPh3, PCy3, PPh2Py,... 

The MOP range of ligands designed by Hayashi has proved remarkably useful for asymmetric hydrosilylation reactions.59 MOP ligands are a series of enantiomerically pure monophosphine ligands whose chirality is due to 1,1 -binaphthyl axial chirality. 

Generally, monophosphine complexes can be generated by decomposition of suitable precursors, among which the most notable are palladacycles (Section 9.6.3.4.7). A spectacular example makes use of spontaneous disproportionation of a dimeric complex of Pd1 with very bulky ligands to give one of the most reactive catalytic systems known so far, which catalyzes the fast crosscoupling of arylboronic acids with aryl chlorides and hindered aryl bromides at room temperature (Equation (28)) 389... 

Bidentate ligands with very wide bite angles (like dpephos or xantphos, Scheme 4) are likely to form unstable chelates prone to dechelation at elevated temperatures, thus giving another route to monophosphine species. The application of such ligands to the crosscoupling of sterically hindered bromides and arylboronic acids under strictly anhydrous conditions enforced by the addition of molecular sieves has been shown to be advantageous (118).398... 

Figure 3 Monophosphine ligands for the etherification of aryl halides.

A variety of triazole-based monophosphines (ClickPhos) 141 have been prepared via efficient 1,3-dipolar cycloaddition of readily available azides and acetylenes and their palladium complexes provided excellent yields in the amination reactions and Suzuki-Miyaura coupling reactions of unactivated aryl chlorides <06JOC3928>. A novel P,N-type ligand family (ClickPhine) is easily accessible using the Cu(I)-catalyzed azide-alkyne cycloaddition reaction and was tested in palladium-catalyzed allylic alkylation reactions <06OL3227>. Novel chiral ligands, (S)-(+)-l-substituted aryl-4-(l-phenyl) ethylformamido-5-amino-1,2,3-triazoles 142,... 

This reaction has lent itself to the development of its asymmetric version (Scheme 88). The trick here is to remove the choride ligands from the coordination sphere of the platinum-chiral ligand complex. This makes the metal center more electrophilic, thus reactive reactions can be run at lower temperature. Interestingly, the best ligand was found to be the atropisomeric monophosphine (fJ)-Ph-BINEPINE.312 Enantiomeric excess up to 85% was observed. Very recently, enantioselectivity up to 94% ee has been achieved using [(AuCl)2(Tol-BINAP)] as pre-catalyst for the reaction of another enyne.313... 

The asymmetric hydrosilylation that has been most extensively studied so far is the palladium-catalyzed hydrosilylation of styrene derivatives with trichlorosilane. This is mainly due to the easy manipulation of this reaction, which usually proceeds with perfect regioselectivity in giving benzylic silanes, 1-aryl-1-silylethanes. This regioselectivity is ascribed to the formation of stable 7t-benzylpalladium intermediates (Scheme 3).1,S Sa It is known that bisphosphine-palladium complexes are catalytically much less active than monophosphine-palladium complexes, and, hence, asymmetric synthesis has been attempted by use of chiral monodentate phosphine ligands. In the first report published in 1972, menthyldiphenylphosphine 4a and neomenthyldiphenylphosphine 4b have been used for the palladium-catalyzed reaction of styrene 1 with trichlorosilane. The reactions gave l-(trichlorosilyl)-l-phenylethane 2 with 34% and 22% ee, respectively (entries 1 and 2 in Table l).22 23... 

This achievement was unique in two respects 1) it was the first example of industrial application of a homogeneous enantioselective catalysis methodology and 2) it represented a rare example of very quick convergence of basic knowledge into commercial application. The monophosphine ligand CAMP was shortly replaced by the related diphosphine ligand DIPAMP which improved the selectivity for the I-DOPA system up to 95% ee [45]. 

Recently, the first report was made on the ruthenium-catalyzed enantioselective hydrogenation of aryl-methyl ketones using monodentate phosphonites (Scheme 28.18). In particular, ligand 15f induced excellent ee-values. One very early report on rhodium-catalyzed hydrogenation of ketones using the monophosphine bmpp 1 f met with a low e.e. [95]. 

The enhanced synthetic potential of rhodium-complex-catalyzed enantioselective hydrogenation provided by these advances in ligand design has led to renewed interest in the reaction mechanism, and here we highlight four recent topics (i) the extended base of reactive intermediates (ii) an improved quadrant model for ligand-substrate interactions (iii) computational approaches to mechanism and (iv) (bis)-monophosphine rhodium complexes in enantioselective hydrogenation. These are discussed in turn. 

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