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Chiral Synthesis via Organoboranes 23. Enantioselective Ring Opening of meso ‐Epoxides with B ‐Halodiisopinocampheylboranes. The First General Synthesis of Optically Active 1,2‐Halohydrins
Author(s) -
Srebnik M.,
Joshi N.N.,
Brown Herbert C.
Publication year - 1989
Publication title -
israel journal of chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.908
H-Index - 54
eISSN - 1869-5868
pISSN - 0021-2148
DOI - 10.1002/ijch.198900031
Subject(s) - chemistry , epoxide , cyclohexene oxide , cleave , cyclohexene , cyclopentene , enantiomer , enantioselective synthesis , stereochemistry , optically active , enantiomeric excess , cleavage (geology) , medicinal chemistry , ring (chemistry) , organic chemistry , catalysis , enzyme , geotechnical engineering , fracture (geology) , engineering
B ‐Halodiisopinocampheylboranes, Ipc 2 BX, where X is Cl, Br, or I, cleave cyclohexene oxide at low temperatures to give the 1,2‐halohydrins, in 22, 84, and 91% enantiomeric excess (ee), respectively. 1,2‐Epoxycyclohex‐4‐ene is converted to the bromohydrin and iodohydrin in 84 and 91% ee. cis ‐2,3‐Epoxybutane, cis ‐3,4‐epoxyhexane, and cyclopentene oxide were also studied with all three halides. In general, optical induction increases in the order I > Br > Cl for any given epoxide. Thus cis ‐2,3‐epoxybutane furnishes the corresponding chlorohydrin in 35% ee, the bromohydrin in 69% ee, and the iodohydrin in 78% ee. In certain cases recrystallization provides essentially optically pure material, e.g., (1 R , 2 R )‐2‐bromocyclohexanol, (1 R , 2 R )‐2‐iodocyclohexanol and (1 R , 2 R )‐2‐iodocyclohex‐4‐en‐1‐ol. In all cases examined d Ipc 2 BX (derived from (+)‐α‐pinene) provide (1 R , 2R) halohydrins, in which the enantiotopic S C—O bond is cleaved. Ring cleavage occurs in an anti‐periplanar manner, consistent with an S N 2 type reaction pathway. Modified B ‐chloromonoisopinocampheylboranes, IpcBCl(OR), cleave cyclohexene oxide at a slower rate but furnish the chlorohydrin in up to 35% ee (R = Bnz). This study is not only a novel application of chiral haloboranes, but constitutes the first general synthesis of symmetrical optically active 1,2‐halohydrins.