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functionalized, and because the sulfonyl group can be easily cleaved without causing any racemization of the chiral center. Very recently, we prepared optically active β-hydroxy sulfones in 97−99% ee by the catalytic reduction of the corresponding β-keto sulfones using 11b as the catalyst and BACH-EI TM as the borane source (eq 23). 62 The reduction is also very effective for hindered aliphatic analogs. 62 4. Asymmetric Reduction of Prochiral Ketimines In contrast to the large number of 12 VOL. 35, NO. 1 • 2002 α-Hydroxy Esters n Reduction Conditions Yield (%) % ee 0 4 (1.1 equiv), -20 °C 58 60, S 0 11c (0.12 equiv), CB (1.1 equiv), -20 °C 92 65, S 1 11c (0.12 equiv), CB (1.1 equiv), -20 °C 66 91, R 2 11c (0.12 equiv), CB (1.1 equiv), -20 °C 58 68, R Reagents % ee of the Amine Products References 34-BH3, 30 °C 99 (52) a 62 73 14a, 64a-c 34-NaBH4-ZrCl4, b rt (90) a 14c 11a-BH3 86 (58) a 40 78 (37) a 64b,c 14-BH3 58 (36) a 90 79 (38) a 64c,d 16 (0.1 equiv), BMS (1.2 equiv) (33) a (51) a 22a 19b (2.2 equiv), BH3 (3.0 equiv), 50 °C 96 25a 6 (5.0 equiv), MgBr2 (1.2 equiv) NR 56 10 35 (0.1 equiv)-BMS, 110 °C (63) a 65 a The figures in parentheses indicate % ee obtained from the use of 0.1 equiv of catalyst or ligand. b [34]:[NaBH4]:[ZrCl4]:[imine] = 15:40:40:12. asymmetric ketone reductions that are known, only limited success has been achieved in the reduction of ketimines. This is due to the low electrophilicity of the imine carbon and the rapid equilibration between the E and Z isomers. 63 In addition, most chiral Lewis acids including oxazaborolidines are trapped by the basic nitrogen atoms of imines and/or product amines leading to less effective catalytic reactions. 4.1. Acyclic Prochiral Ketimines Itsuno and coworkers reported the successful asymmetric reduction of aromatic eq 22 eq 23 eq 24 ketone oxime O-alkyl ethers with excess borane-THF 14a,b or NaBH4-ZrCl4, 14c in the presence of 1.2 equiv of 34, to produce the corresponding amines in good-to-high ee’s. They also obtained high ee’s for the same reduction using a polymer-supported sulfonamido alcohol, 19b, as a chiral inducer. 25b We found that the stoichiometric borane reduction of aromatic N-phenyl imines using 10a or 11a proceeded with good enantioselectivity. 64a When 0.1 equiv of 10a or 11a was used, lower enantioselectivities were obtained in cases of both the oxime ethers and N-phenyl imines. 14b,64b,c
Other reductions using 14, 64c,d 16, 22 or 35 65 as chiral catalysts afforded moderate enantioselectivities. Reagent 6 reduced N-phenylimines to N-phenylamines with moderate ee’s, but did not reduce oxime ethers. 10 Reagents 1, 4, and 5 did not reduce oxime ethers or N-phenylimines. 42,64a,c All of these results are summarized in eq 24. In the borane reduction of oxime ethers using 14a or 16 as chiral inducers, Sakito 66 and Bolm 22 independently found that the absolute configurations of the amines formed are dependent on the geometry of the oxime ethers: the E oxime led to the S amine, while the Z oxime gave rise to the R amine (Scheme 1). The asymmetric reduction of activated imines such as N-silyl-, N-sulfenyl-, N-sulfonyl-, or N-diphenylphosphinylimines under stoichiometric or catalytic conditions has also been studied (Table 1). The borane reduction of acetophenone N-tert-butyl- Scheme 1. Effect of Oxime Ether Geometry on Enantioselection. Table 1. The Asymmetric Reduction of Activated Ketimines Amine Y Reduction Conditions Yield (%) % ee SPh 16a (0.1 equiv), BMS (1.2 equiv), rt 64 70, S SO2Tolyl-p 16a (0.1 equiv), BMS (1.2 equiv), rt 77 22, S TBDMS 12a (2.0 equiv), BH3:THF, -10°C 33 79, R TBDMS 14a (2.0 equiv), BH3:THF, -10 °C 71 50, S POPh2 16a (0.1 equiv), BMS (1.2 equiv), rt 70 16, S POPh2 1 (1.2 equiv), -78 °C 54 15, R POPh2 20a (0.01 equiv), NaBH2(OR)2, 0 °C 90 97, S TBDMS = t-BuMe2Si. 20a: Ar = 2,4,6-Me3C6H2 dimethylsilylimine using 14a or 12a under stoichiometric conditions provided the desired amine in moderate yields and ee’s. 67 Reduction of acetophenone N-phenylsulfenylimine using 16 as catalyst gave 70% ee, while the N-tosyl- and N-diphenylphosphinylimine analogs provided much lower ee’s under the same conditions. 22 The reduction of N-diphenylphosphinyl derivatives of alkyl methyl ketimines to the corresponding aliphatic amines has been successfully achieved in moderate ee’s by using 1. 68 Recently, Mukaiyama reported the very efficient reduction of N-diphenylphosphinylimines using the chiral cobalt(II) complex 20a and sodium alkoxyborohydride as catalyst and hydride source, respectively. The reduction provided the desired amines with very high enantioselectivity even in the presence of 1 mol % of 20a. 26 Such reductions have been effectively applied to the asymmetric synthesis of the herbicide metolachlor (36), 69 3,3,3-trifluoroalanine (37), 70 γ-amino alcohol 38, 71 and C2symmetric 1,2-diamine 40 72 (Scheme 2). 4.2. Endocyclic Imines The asymmetric reduction of 1substituted 3,4-dihydroisoquinolines, 41, is the most straightforward method for preparing nonracemic 1-substituted tetrahydroisoquinoline alkaloids such as salsolidine (42a), norlaudanosine (42b), and norcryptostylines (42c,d) (Scheme 3). Recently, a very effective and convenient dihydroisoquinoline reduction, using chiral sodium acyloxyborohydride (“Mix C”, 9b) modified with (S)-N,N-phthaloylisoleucine, gave the desired alkaloids in moderate yields and 94−99% ee. 13a This reducing system has been successfully applied to the synthesis of VOL. 35, NO. 1 • 2002 13
Page 1 and 2: DEDICATED TO PROFESSOR H. C. BROWN
Page 3 and 4: Aldrich Chemical Co., Inc. 1001 Wes
Page 5 and 6: Boron-Based Reducing Agents for the
Page 7 and 8: Figure 3. Stoichiometric Boron-Base
Page 9 and 10: orane reductions 29,30 of 1,4-diphe
Page 11 and 12: α-Hydroxy Acetals R Reduction Cond
Page 13: and 5 provided moderate ee’s in t
Page 17 and 18: Masuda, H.; Ito, K.; Hirao, A.; Nak
Page 19 and 20: The synthesis of biaryl compounds v
Page 21 and 22: 52,396-8 [87199-18-6] 3-Hydroxyphen
Page 23 and 24: Josiphos is a family of chiral phos
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Page 35 and 36: B-O bond. 15 The mechanism involves
Page 37 and 38: Scheme 4. Tartrate Ester Auxiliary
Page 39 and 40: eagent with aldehydes at low temper
Page 41 and 42: from 2- and 3-carene turned out to
Page 43 and 44: Scheme 22. Application of Crotylbor
Page 45: Abstr. 1968, 68, 29743j. (11) Refer
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