MeO-MOP ligands

In contrast to the formation of linear achiral allylation product on usage of the catalytic system with dppe ligand, employment of the (P)-MeO-MOP ligand gave the branched product in a high regiochemistry and high enantioselectivity. 

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-... 

Another reaction involving a carbonate is the reduction of allylic substrates in the presence of formic acid. This reaction has been described in an asymmetric manner, in the presence of aJlylpalladium chloride dimer 1 and (/J)-MeO-MOP as the chiral ligand (eq 16) The resulting ester is isolated in high yield and 87% enantiomeric excess. The mechanism was studied in the presence of an analog of (/J)-MeO-MOP ligand. ... 

Similarly, reaction of dienes with COj in the presence of Ni(COD)2 affords Jt-allylnickel complex. Upon treatment with Me2Zn, the corresponding dicer-bonxylic acid is isolated. On the other hand, when the nickel intermediate is allowed to react with Hd, only monocarboxylation product is obtained [244]. Asymmetric induction is observed when (S)-MeO-MOP ligand is used [245,246]. 

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]. 

The MOP series of ligands59 (see Section 9.5.4.2) in conjunction with standard palladium precursors has been reported to catalyze the addition of HBcat to 1,3-enynes. With 1 mol.% catalyst produced by combination of Pd2(dba)3 and the monodentate ligand (Y)-MeO-MOP (22), axially chiral allenyl-boranes are formed (Equation (3)). Subsequent oxidation affords the corresponding alcohols with moderate ee values.60... 

However, in the palladium-catalyzed addition of HSiCl3 to -substituted styrenes, the size of the substituent on the MOP ligand is crucial. While (R)-MeO-MOP/[Pd(//3-C3H5)Cl]2 yielded (R)-phenylethanol with poor enantioselectivity (14% ee),110 replacement of the methoxy group with hydrogen (H-MOP, (36d)) affords the same product with 93% ee (Equation (ll)) 111... 

The most active palladium catalyst system developed for the asymmetric hydrosilylation of cyclopentadiene (Scheme 23) involves the use of the (/ )-MOP-phen ligand (38), which shows significant enhancement of enantioselectivity compared to (R)-MeO-MOP (80% ee from (38), 39% ee from (36a)).114 Other phosphine ligands that afford active palladium catalysts for the same transformation include the /3-7V-sulfonylaminoalkylphosphine (39) and phosphetane ligand (40) (Equation (13)).115-117 A comparison of the enantioselectivities of these ligands for the palladium-catalyzed hydrosilylation of cyclopentadiene is given in Table 8. 

An asymmetric version of the Pd-catalyzed hydroboration of the enynes was reported in 1993(118]. The monodentate phosphine (S)-MeO-MOP was used as a chiral ligand for the palladium catalyst. Enantioselectivity of the asymmetric hydroboration was estimated from the enantiopurity of homopropargyl alcohols, which were obtained from the axially chiral allenylboranes and benzaldehyde via an SE pathway (Scheme 3.78). 

In 1993, Hayashi and co-workers reported a catalytic asymmetric synthesis of alle-nylboranes 256 by palladium-catalyzed hydroboration of conjugated enynes 253 (Scheme 4.66) [105]. Reaction of but-l-en-3-ynes 253 with catecholborane 254 in the presence of a catalyst, prepared from Pd2(dba)3 CHC13 (1 mol%) and a chiral mono-dentate phosphine ligand (S)-MeO-MOP 255 (1 mol%), gave an allenylborane 256. The ee of 256 was determined by the reaction with benzaldehyde affording the corresponding optically active homopropargyl alcohols 257 with up to 61% ee (syn anti= 1 1—3 1). 

The facile addition of arylboronic acids to aldehydes encouraged us to examine the asymmetric version of this protocol (Eq. 5). A monodentate ligand of (S)-MeO-MOP revealed a moderate asymmetric induction with the (/ )-(+)-1 -naphthylphenylmethanol being preferentially formed (41%ee), though the chiral bidentate ligands such as DIOP... 

Hayashi and coworkers have achieved the alkylation of 1- and 3-substituted 2-propenyl acetates with high regio- and enantioselectivities by using a palladium catalyst in the presence of a chiral ligand, (J )-2-diphenylphosphino-2 -methoxy-1,1 -binaphthyl, (.R)-MeO-MOP (Eq. 15) [32,33]. 

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]. 

Recently, a palladium complex coordinated with an axially chiral, monoden-tate phosphine ligand, MeO-MOP (7a) or its analogs [21], has been reported to be highly effective for the enantioselective hydrosilylation of alkyl-substituted terminal olefins (Scheme 4) [22,23]. Simple terminal olefins 8 were transformed efficiently into the corresponding optically active 2-alkanols 11 with enantiose-lectivities ranging between 94% and 97% ee by the catalytic hydrosilylation-ox-... 

Regioselectivity of the product can be controlled by the use of bulky mon-odentate ligand such as 2-(diphenylphosphino)-2 -methoxy-l,l -binaphtyl (MeO-MOP) as shown in Scheme 3.26 [45], The key steps in this reaction are selective anh-ehmination of the acetate, to form r -allylic complex, which is attacked by the carbanion at the site irons to the phosphine ligand from the exo-face. The observed regioselectivity may be due to the enhanced positive charge at the carbon irons to the phosphine [46]. More details on the regiochemical control of nucleophilic attack of r -allyl complexes are given in Chapter 8. 

One of the most common methods for the preparation of enantiomerically enriched organosilanes is by palladium-catalysed asymmetric hydrosilylation of alkenes in the presence of trichlorosilane. This area has been dominated by the use of monodentate phosphorus-based ligands and, in particular, Hayashi s MOP ligand/palladium catalyst combination offers a high level of enantioselectivity. The MOP ligands include MeO-MOP (2.139), H-MOP ligands such as (2.140) ... 

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