boron trifluoride etherate boron trifluoride tetrahydrofuran complex boron trifluoride tert butyl methyl ether complex boron trifluoride dibutyl ether complex boron trifluoride dihydrate boron trifluoride di acetic acid complex boron trifluoride dimethyl sulfide complex boron trichloride boron trichloride dimethyl sulfide complex boron tribromide boron tribromide dimethyl sulfide complex boron triiodide triimethoxyborane triethoxyborane trimethylaluminum triethylaluminum aluminum trichloride aluminum trichloride tetrahydrofuran complex aluminum tribromide titanium tetrachloride titanium tetrabromide titanium iodide titanium tetraethoxide titanium tetraisopropoxide scandium iii trifluoromethanesulfonate yttrium iii trifluoromethanesulfonate ytterbium iii trifluoromethanesulfonate lanthanum iii trifluoromethanesulfonate zinc ii chloride zinc ii bromide zinc ii iodide zinc ii trifluoromethanesulfonate zinc ii sulfate magnesium sulfate lithium perchlorate copper ii trifluoromethanesulfonate copper ii tetrafluoroborate and the like certain lewis acids may have optically pure ligands attached to the electron acceptor atom as set forth in corey e j angewandte chemie international edition 2002 41 10 1650 1667 aspinall h c chemical reviews washington d c united states 2002 102 6 1807 1850 groger h chemistry u2014 a european journal 2001 7 24 5246 5251 davies h m l chemtracts 2001 14 11 642 645 wan y chemtracts 2001 14 11 610 615 kim y h accounts of chemical research 2001 34 12 955 962 seebach d angewandte chemie international edition 2001 40 1 92 138 blaser h u.