Chiral monophosphine

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]. 

A catalytic asymmetric [4+2]-cydoaddition of a vinylallene with butadiene has been achieved successfully, in which a palladium complex modified by a ferrocene-derived chiral monophosphine ligand proved to be a superior catalyst transferring chirality to the product (Scheme 16.80) [90],... 

The Ci bidentate ligand system may be as simple as a chiral monophosphine and a chloride ion, as shown in Figure 13.12. Suppose that in a P-X ligand system the nucleophilic attack takes place at the position trans to the phosphine, then we expect on the basis of micro reversibility that also the... 

Asymmetric hydrosilylation of 1,3-dienes provides convenient access to optically active a-chiral allylsilanes.107 1073 1071 The combination of 7r-allylpalladium chloride dimer with axially chiral monophosphine ligand 17 realizes high catalytic activity and enantioselectivity in the reaction of cyclic 1,3-dienes with HSiCl3.108,108a The allyltrichlorosilanes obtained react with aldehydes in a syn-Se mode to give homoallyl alcohols with high diastereo-and enantioselectivity (Scheme 9). 

Thus, chiral monophosphine auxiliaries have a real potential for various catalytic processes and certainly deserve further attention. In addition, their synthesis allows a modular approach to a variety of structures. Due to the importance of the binaphthol framework for efficient enantioselection, the latter has been the most frequently used for the construction of monophosphine monodentate auxiliaries. 

Optically active alcohols, amines, and alkanes can be prepared by the metal catalyzed asymmetric hydrosilylation of ketones, imines, and olefins [77,94,95]. Several catalytic systems have been successfully demonstrated, such as the asymmetric silylation of aryl ketones with rhodium and Pybox ligands however, there are no industrial processes that use asymmetric hydrosilylation. The asymmetric hydrosilyation of olefins to alkylsilanes (and the corresponding alcohol) can be accomplished with palladium catalysts that contain chiral monophosphines with high enantioselectivities (up to 96% ee) and reasonably good turnovers (S/C = 1000) [96]. Unfortunately, high enantioselectivities are only limited to the asymmetric hydrosilylation of styrene derivatives [97]. Hydrosilylation of simple terminal olefins with palladium catalysts that contain the monophosphine, MeO-MOP (67), can be obtained with enantioselectivities in the range of 94-97% ee and regioselectivities of the branched to normal of the products of 66/43 to 94/ 6 (Scheme 26) [98.99]. 

Figure 4.39 The peripherally chiral monophosphine-functionalized carbosilane dendrimers.

The application of a mixture of a chiral and an achiral monophosphorus ligand for the rhodium catalyzed asymmetric hydrogenation of enamides was tested by Beller and coworkers [45]. By using a mixture of a chiral monophosphine 36a and an achiral ligand tris(4 methoxyphenyl)phosphine [P(4 MeOCf,H4)3] (1 1), the N (1 phenylvi nyl)acetamide (7a) was hydrogenated to amine 8a with 88% ee, but this enantios electivity is inferior to that obtained with single monophosphine 36a (93% ee). 

Some chiral monophosphines were also prepared during the period 1971-1973. Camp (cyclohexyl o-anisyl methyl phosphine) led to a quite high ee (up to 90%) in asymmetric hydrogenation of W-acetyl dehydrophenylalanine [29] and neomenthyl diphenyl phosphine gave modest ee s in the reduction of various conjugated acids [30]. 

The axially chiral, monophosphine ligand, MeO-MOP (7a), was not as effective for styrene derivatives as for simple terminal olefins [31]. The palladium-catalyzed hydrosilylation of styrene (13) with trichlorosilane in the presence of the (H)-MeO-MOP ligand (7a) under standard conditions (without solvent) followed by oxidation gave (H)-l-phenylethanol (16) with only 14% ee (Scheme 8). Use of benzene as solvent for the hydrosilylation reaction improved the enanti-... 

HAYASHI-UOZUMI Asymmetric Functionalization Catalytic asymmetric synthesis of optically active alcohols via hydrosilylation of aikenes catalyzed by chiral monophosphine-palladium. 

Metalation of sulfinylferrocene (338) to give the lithiated species, followed by transmetalation to give either the isolable boron analog (348) or the nonisolable zinc analog (349), is an approach used to access yet another class of ferrocene derivatives (equation 79)Palladium-catalyzed coupling of (348) or (349) with aryl iodides affords planar chiral aryl ferrocenes that are converted by standard transformations into planar chiral monophosphine aryl ferrocenes (350) that are also useful ligands in asymmetric catalysis. With appropriate ort/ o-substitution on the aryl ring, ferrocenes that possess axial chirality as well as planar chirality are prepared if the orfAo-substitutent is a second phosphine unit, access to a bisphosphine aryl ferrocene is achieved. ... 

The formation of branched esters suggests the potential for asymmetric carbonylation. Only a few successful asymmetric carbonylations are known. Zhou and co-workers reported the highly successful asymmetric carbonylation of styrene. They obtained the branched ester 6 with 99.3 % ee and high regioselectivity of the branched ester using the Alper catalyst, PdCl2-CuCl2 in the presence of the chiral monophosphine DDPPI 9 in methyl ethyl ketone [6]. 

As noted earlier in this chapter, the enantioselective hydrosilylation of olefins could be a useful method to prepare chiral, non-racemic alcohols. A.lthough the scope of highly enantioselective hydrosilylations is limited, high enantioselectivities have been obtained for the asymmetric hydrosilylation of alkenes and vinylarenes. A majority of the most selective chemistry has been conducted using a palladium catalyst containing an axially chiral monophosphine ligand. 

Their preliminary studies discovered that the two phosphines of (i )-131 operate independently. When the intermediate (i )-133 was isolated and reused in the resolution of racemic 134, the corresponding starting hydroperoxide was obtained in 84% ee with good selectivity. These results suggest that the chiral monophosphines may also be useful for such type of KRs. 

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