After heating to complete the [borane] reduction, the reaction mixture was quenched into methanol and distilled to remove some of the boron as trimethylborate.Trimethylborate has a boiling point of 68°C-69°C, according to Wikipedia. A perusal of the literature provides a nice explanation of how this works :
In this salt-free process, all traces of boron are conveniently removed from the reaction mixture as trimethylborate by full displacement in methanol. The trimethylborate forms an azeotrope with methanol (70% B(OMe)3 at 59 °C), and under these conditions, even boric acid can be removed from the reaction mixture. (Schlesinger, H. I.; Brown, H. C.; Mayfield, D. L.; Gilbreath, J. R. J. Am. Chem. Soc. 1953, 75, 213.)Maybe everyone else knew about this, but I didn't.
1. Sieser, J.E. et al. Org. Process. Res. Dev. 10.1021/op2001854
2. Couturier, M. et al. Org. Process. Res. Dev. 2002, 6, 42-48.