Esters asymmetric hydrosilylation

Cyano compounds liquid crystals, 12, 278 in silver(III) complexes, 2, 241 Cyanocuprates, with copper, 2, 186 Cyano derivatives, a-arylation, 1, 361 Cyanosilanes, applications, 9, 322 Cyclic acetals, and Grignard reagent reactivity, 9, 53 Cyclic alkenes, asymmetric hydrosilylation, 10, 830 Cyclic alkynes, strained, with platinum, 8, 644 Cyclic allyl boronates, preparation, 9, 196 Cyclic allylic esters, alkylation, 11, 91 Cyclic amides, ring-opening polymerization, via lanthanide catalysis, 4, 145... 

The marked increase in optical yield in the reaction of pyruvates compared with simple prochiral ketones can probably be ascribed to a ligand effect of the ester moiety in the key intermediate or transition state. Further support of this hypothesis comes from the results for asymmetric hydrosilylation of levulinates153 which, followed by acid solvolysis, affords 4-methyl-y-butyrolactone with more than 80% e.e. through the silyl ether of 4-hydroxybutyrates (e.g. equation 83). 

Alkyl/alkyl ketones are challenging substrates for rhodium-catalysed asymmetric hydrosilylation and are generally reduced with low enantioselectivities using oxazoline-based hgands. However a number of alternative ligand systems have been used successfidly in the rhodium-catalysed hydrosilylation of alkyl/alkyl ketones. For example, the 8-keto ester (3.158) undergoes enantioselective... 

Full accounts have appeared concerning the asymmetric reduction of ketones " and keto-esters " through hydrosilylation employing a rhodium(l) catalyst possessing chiral phosphine ligands. 

The asymmetric reduction of keto-esters via hydrosilylation has also been achieved in the presence of chiral rhodium catalysts. a-Keto-esters give the corresponding lactates after hydrolysis, and by varying the hydrosilane the optical yield can be increased to 85%. Acetoacetates give the corresponding 3-hydroxy-butyrate, but in much lower optical yield (ca. 20%), whereby levulinates give chiral 4-methyl-y-butyrolactones with optical yields of up to 84% [equation (4)]. 

The asymmetric hydrosilylation of synthesized a-acetoxy-y3-enamino esters proceeded smoothly in the presence of a chiral Lewis base catalyst, (282), to provide a wide range of chiral a-acetoxy jS-amino acid derivatives in high yields with good diastereoselectivities and enantioselectivities." ° ... 

A stable, ready-to-use, asymmetric hydrosilylation reagent, referred to as Cu—H in a bottle, which is copper hydride complexed with (i )-DTBM-SEGPHOS, has been prepared and proved highly active and enantioselective in 1,4-hydrosilylation of cyclic a,/3-unsaturated ketones and o ,jS-unsaturated esters and for hydrosilylation of simple aryl alkyl ketones (261). 

ASYMMETRIC REDUCTION OF KETO ESTERS VIA HYDROSILYLATION CATALYZED BY CHIRAL RHODIUM(I) COMPLEXES. 211... 

In Section 4, it is described that chlorotris(triphenylphosphine)rhodium(I) (7) is quite an effective catalyst for the hydrosilylation of carbonyl compounds. For this reason, extensive studies on asymmetric hydrosilylation of prochiral ketones to date have been based on employing rhodium(I) complexes with chiral phosphine ligands. The catalysts all prepared in situ are rhodium(I) complexes of the type, (BMPP>2Rh(S)a (8) [40] and (DIOP)Rh(S)Cl (6) [41], and a cationic rhodium(III) complex, [(BMPP)2lUiH2(S)2] Q04 (5) [42], where S represents a solvent molecule. An interesting polymer-supported rhodium complex (V) [41], and several chiral ferrocenylphosphines [43], recently developed as chiral ligands, have also been employed for asymmetric hydrosilylation of ketones. Included in this section also are selective asymmetric hydrosilylation of a,0-unsaturated carbonyl compounds and of certain keto esters. 

Asymmetric reduction of keto esters via hydrosilylation catalyzed by chiral rhodium(I) complexes... 

ASYMMETRIC REDUCTION OF AND 7 KETO ESTERS via HYDROSILYLATION USING... 

Asymmetric hydrosilylation of acetophenone, methylbenzyl ketone, and isobutyro-phenone can be achieved using soluble chiral rhodium(i)-diop complexes (49) and polymer-supported catalyst. The optical yield is dependent on the silane used, dihydrosilanes giving higher yields than monohydrosilanes. Similar results are obtained with a-keto-esters. ... 

In a different approach to the problem an asymmetric hydrogenation of the pyruvamides (214) was achieved using Pd-C as catalyst the best diastereoisomer ratio was 98 2. Asymmetric hydrosilylation and asymmetric hydrogenation have been applied to the N-(a-ketoacyl)-a-amino-esters (215) the former produced good to high diastereoselectivity whereas the latter method was less selective. ... 

Palladium-catalyzed asymmetric cyclization/hydrosilylation tolerated a number of functional groups including benzyl and pivaloyl ethers as well as benzyl and methyl esters (Table 8, entries 1-4). Furthermore, the protocol tolerated substitution at one of the two /ra/zi -terminal alkenyl positions and at one of the two allylic positions of the 1,6-diene (Table 8). As was the case with diene cyclization/hydrosilylation catalyzed by achiral palladium... 

Stereoselective reduction of a-alkyl-3-keto acid derivatives represents an attractive alternative to stereoselective aldol condensation. Complementary methods for pr uction of either diastereoisomer of a-alkyl-3-hydroxy amides from the corresponding a-alkyl-3-keto amides (53) have been developed. Zinc borohydride in ether at -78 C gave the syn isomer (54) with excellent selectivity ( 7 3) in high yield via a chelated transition state. A Felkin transition state with the amide in the perpendicular position accounted for reduction with potassium triethylborohydride in ether at 0 C to give the stereochemi-cally pure anti diastereoisomer (55). The combination of these methods with asymmetric acylation provided an effective solution to the asymmetric aldol problem (Scheme 6). In contrast, the reduction of a-methyl-3-keto esters with zinc borohydride was highly syn selective when the ketone was aromatic or a,3-unsaturated, but less reliable in aliphatic cases. Hydrosilylation also provided complete dia-stereocontrol (Scheme 7). The fluoride-mediated reaction was anti selective ( 8 2) while reduction in trifluoroacetic acid favored production of the syn isomer (>98 2). No loss of optical purity was observed under these mild conditions. 

This volume begins with two procedures in the area of catalytic asymmetric synthesis. The first procedure describes the synthesis of (R)-2-Dl PH ENYLPHOSPHI NO-2 -METHOXY-1,1 -BINAPHTHYL (MOP), a chiral ligand that has proven very useful in palladium-catalyzed hydrosilylation of olefins and palladium-catalyzed reduction of allylic esters by formic acid. The next procedure describes the catalytic asymmetric synthesis of nitroaldols using a chiral LANTHANUM-LITHIUM-BINOL COMPLEX, illustrated by the synthesis of (2S,3S)-2-NITRO-5-PHENYL-1,3-PENTANEDIOL. 

Since CIgSiH is known to be activated by DMF and other Lewis bases to effect hydrosilylation of imines (Scheme 4.2) [8], it is hardly surprising that chiral formamides, derived from natural amino adds, emerged as prime candidates for the development of an asymmetric variant of this reaction [8]. It was assumed that, if successful, this approach could become an attractive altemative to the existing enzymatic methods for amine production [9] and to complement another organo catalytic protocol, based on the biomimetic reduction with Hantzsch ester, which is being developed in parallel [5]. 

Asymmetric reduction of a-keto esters, typically pyruvates and phenylglyoxylates, is effected by chiral rhodium complex-catalyzed hydrosilylation . Optical yields of lactates are higher than those obtained for simple prochiral ketones. The ester group as well as the hydrosilane used effects the extent of asymmetric induction. A high optical yield is attained for M-propyl pyruvate using a-naphthylphenylsilane (85.4% e.e.p ... 

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