Assymetric Alkylation Reactions

Assymetric Alkylation Reactions - Myers Brown Allylation...

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CH 3 BH CH 3 BOCH 3 CH 3 B 2 2 2 R OH (–)-Ipc 2 B CH 3 n -C 3 H 7 n -C 4 H 9 t -C 4 H 9 C 6 H 5 R RH O MgBr O B H 3 C H 3 C CH 3 H H CH 3 CH 3 H H R H 3 C H H H H CH 3 CH 3 CH 3 H 3 C Chem 2 15 Asymmetric Allylation Reactions Myers Synthesis of B -Allyldiisopinocampheylborane Brown, H. C.; Desai, M. C.; Jadhav, P. K. J. Org. Chem. 1982 , 47 , 5065-5069. Brown, H. C.; Singaram, B. J. Org. Chem. 1984 , 49 , 945-947. Jadhav, P. K.; Bhat, K. S.; Perumal, P. T.; Brown, H. C. J. Org. Chem. 1986 , 51 , 432-439. + Et 2 O –78 A 23 °C; NaOH, H 2 O 2 yield (%) ee (%) a 74 71 72 88 81 93 86 87 83 96 ee (%) b a Allylboration carried out without filtration of Mg salts. b Allylboration carried out at –100 °C under Mg-salt free conditions. ¡ 99 - 96 ¡ 99 96 Enantioselective Allylboration Brown, H. C.; Jadhav, P. K. J. Am. Chem. Soc. 1983 , 105 , 2092-2093. Brown, H. C.; Bhat, K. S. J. Am. Chem. Soc. 1986 , 108 , 5919-5923. Racherla, U. S.; Brown, H. C. J. Org. Chem. 1991 , 56 , 401-404. The reaction is quite general; the stereochemistry of the addition is the same in all cases examined. Lower reaction temperatures (0 A –78 A –100 °C) lead to increased enantioselectivity. Only Mg-salt free reagent can be used at –100 °C because the reactive borane is sequestered by ate complex formation with CH 3 OMgBr at this temperature. Allylboration of aldehydes is essentially instantaneous at –78 or –100 °C in the absence of Mg salts. Reviews: Srebnik, M.; Ramachandran, P. V. Aldrichimica Acta 1987 , 20 , 9. Roush, W. R. In Comprehensive Organic Synthesis , Trost, B. M.; Fleming, I., Eds., Pergamon Press: New York, 1991 , Vol. 2 , pp. 1-53. Brown Allylation and Crotylation Reactions (1 R )-(+)- _ -Pinene = 91.3% ee H 3 B•S(CH 3 ) 2 THF, 0 °C 72 h,72% 98.9% ee CH 3 OH, 1 h 0 °C, 100% 98.9% ee –78 A 25 °C 25 °C, 1 h 98.9% ee Allylation of aldehydes proceeds through a chair-like TS where R occupies an equatorial position and the aldehyde facial selectivity derives from minimization of steric interactions between the axial Ipc ligand and the allyl group. Prolonged incubation at 0 °C affords enantiomerically enriched Ipc 2 BH. This is due to equilibration of tetraisopinocampheyldiborane with _ -pinene and triisopinocampheyl- diborane; the symmetrical dimer crystallizes preferentially. Both enantiomers of _ -pinene are commercially available and inexpensive (Aldrich: (1 R )-(+)- _ -pinene, 91% ee, $100/500mL; (1 S )-(–)- _ -pinene, 87% ee, $42/100mL). B -Allyldiisopinocampheylborane can be prepared and used in situ after filtration of the magnesium salts produced during its formation. M. Movassaghi (–)-Ipc 2 BH
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(+)-Ipc 2 BC H 3 CH 3 (+)-Ipc 2 BOCH 3 (+)-Ipc 2 B CH 3 CH 3 Li (+)-Ipc 2 H 3 CH 3 CH 3 CH 3 R CH 3 n -C 4 H 9 CH 2 =CH (CH 3 ) 2 C=CH R CH 3 n -C 3 H 7 n -C 4 H 9 t -C 4 H 9 CH 2 =CH R OH CH 3 H 3 C R OH CH 3 H H 3 C O OBz H H 3 C O H 3 C H H 3 C O Ph H 3 C OH OBz H 3 C OH H 3 C H 3 C OH Ph H 3 C OH H 3 C H 3 C OH OBz H 3 C OH Ph RCHO, Et 2 O –78 °C, 12 h; NaOH, H 2 O 2 yield (%) 73 79 70 85 ee (%) 91 92 95 96 Brown, H. C.; Jadhav, P. K. Tetrahedron Lett. 1984 , 25 , 1215-1218.
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Assymetric Alkylation Reactions - Myers Brown Allylation...

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