in diethyl ether (N2) or (Ar); addn. of Mg(C4H6)*2THF in portions to soln. of (iPr)2NBCl2 at -80°C within 1 h under stirring; stirring at -80°C for 2 h; slowly warming to 20°C; stirring overnight; filtration; washing with Et2O; evapn. of Et2O-soln. and then distn. in vac.; condensation; elem. anal.;