M. Bilodeau. D.A. Evans A Survey of Soft Enolization Techniques Chem 115 Dialkylboron Triflates Dialkylboron Triflates Evans, Nelson, Vogel, Taber J. Am. Chem. Soc. 1981, 103, 3099-3111 ■ Di-n-butylboron triflate 76.559562. Bull. Chem. Soc. Jpn Lut: ZE= 69: 31 Ether,-78°c Enolizes ketones with 2.6-lutidine or DIPEa in ethereal solvents Diastereoselective Aldol reactions of boron enolates DIPEA R=cC5H11ZE=82:18 J. Am. chem. Soc. 1979. 101 3099-3111 n-Bu2BOTf Bartroli. Shih J. Am. Chem. Soc. 1981. 103 2127. 人 Ether,-78°c Masamune. S. et al. J. Am Chem. Soc. 1981. 103 1566- Borane and lutidine or DIPEA form 1: 1 complex. Complexation reversible as enolization will occur upon addition of ketone Less hindered nitrogen bases- pyridine, Dabco, DBU, and tetramethylguanidine innefective Chiral dialkylboron triflate The more hindered triflate- enolization must be carried out at oc Masamune. Sato. Kim Wollmann J. Am. chem. Soc. 1986. 108. 8279-8281 LaBOTf 、BL2OTf Paterson, l. et. al. Tetrahedron 1990, 46, 4663-4684 lining step rather than complexati (C) All factors being equal anti deprotonation preferred over syn deprotonation. Polniazek, R. Research Report 94:6ZE 22A-09 Ketone Acidity 11/1/01 3: 43 PMM. Bilodeau, D.A. Evans A Survey of 'Soft' Enolization Techniques Chem 115 (-)-(Ipc)2BOTf ■ Chiral dialkylboron triflates Masamune, S. et. al. Tetrahedron Lett. 1979, 2225, 2229, 3937. Masamune, S. et. al. J. Am Chem. Soc. 1981, 103, 1566-1568. ■ Diastereoselective Aldol Reactions of Boron Enolates. ■ Di-n-butylboron triflate Evans, Vogel, Nelson J. Am. Chem. Soc. 1979, 101, 6120. Evans, Nelson, Vogel, Taber J. Am. Chem. Soc. 1981, 103, 3099-3111. Evans, Bartroli, Shih J. Am. Chem. Soc. 1981, 103, 2127. Dialkylboron Triflates Enolizes ketones with 2,6-lutidine or DIPEA in ethereal solvents. Mukaiyama, Inoue Chem. Lett. 1976, 559-562. Bull. Chem. Soc. Jpn. 1980, 53, 174-178. Masamune, Sato, Kim, Wollmann J. Am. Chem. Soc. 1986, 108, 8279-8281. Paterson, I. et. al. Tetrahedron 1990, 46, 4663-4684. Tetrahedron Lett. 1989, 30, 997-1000. Tetrahedron Lett. 1986, 27, 4787-4790. ■ Borane and lutidine or DIPEA form 1:1 complex. Complexation reversible as enolization will occur upon addition of ketone. ■ Less hindered nitrogen bases - pyridine, Dabco, DBU, and tetramethylguanidine innefective in the enolization. Attributed to irreversible amine-borane complexation. ■ The more hindered triflate - enolization must be carried out at 0° C. R=Et: Z:E = >97:3 R= i-Pr: Z:E = 45:55 R= t-Bu: Z:E = 25:75 n-Bu2BOTf DIPEA Ether, -78° C R2BOTf DIPEA Ether, -78° C R= n-Bu: Z:E = >97:3 R= c-C5H11: Z:E = 82:18 DIPEA: Z:E = >97:3 Lut: Z:E = 69:31 n-Bu2BOTf Base Ether, -78° C Evans, Nelson, Vogel, Taber J. Am. Chem. Soc. 1981, 103, 3099-3111. Dialkylboron Triflates + L2BOTf + - + - NR'3 NR'3 (A) E and Z enolates derived from deprotonation of syn and anti complexes respectively. (B) Deprotonation is rate determining step rather than complexation. (C) All factors being equal anti deprotonation preferred over syn deprotonation. 1. n-Bu2BOTf Et3N 2. n-BuLi, TMSCl Polniazek, R. Research Report 94:6 Z:E TfOB Me Me BOTf O Me Me Me Me O Me R O O R Me H Me H R O BL2OTf O BL2OTf R H Me H Me R Me OBL2 R OBL2 O Me Et O H H O Et Me TMSO 22A-09 Ketone Acidity 11/1/01 3:43 PM