Natural Science Forum / Chemistry / Organic Synthesis / March 2006

Hi, I'm new to this, so I hope I'm not addressing a question that's
posted on weekly...

I am following H.C. Brown's procedure for the preparation of
2-bromo-4,4-dimethyl-2-pentene from the analogous catecholborane alkene
(JACS, 1973, 95, 6456-7).  2 equiv of bromine are added to a 1 M
solution of the substrate at -20 C.  After one hour, sodium methoxide
in MeOH (I'm using a ~25% solution) is added, maintaining cold temp for
an additional hour.  The literature then reports an aqueous workup,
separation and extraction of the aqueous phase, followed by drying and
distillation of the product.  Most of Brown's results are actually only
GC yields, but the one compound that he isolates, 1-bromo-1-octene, is
reported obtained in 82% yield.  When I follow this procedure, a golden
brown organic phase is obtained upon extraction and "drying", but this
quickly turns to dark purple as the CH2Cl2 is stripped off on a
rotovap.  When I distill the residue (~50 C at 50 Torr) I only get
about 30% contaminated with a significant amount of a second phase
(which I believe is an aqueous azeotrope - obviously not very dry).  A
viscous purple mass is left in the pot.

So my main problem seems to be unreacted bromine, possibly further
oxidizing the product after separation from the aqueous phase, with a
secondary problem of an unexpectedly wet crude product.  I would
appreciate any suggestions from anyone with experience with this
reaction or in general with removing excess bromine.

Thanks in advance!

dan