[Dan]

I'm trying Reformatsky reactions at the moment but I'm having alot of problems seemingly with the generation of the Reformatsky reagent.

The literature procedure I'm following is as follows:

Zinc powder (1 eq) and 0.1 eq CuCl is suspended in dry THF and heated at reflux for 30 min under Ar in dried apparatus.
The reaction mixture is then taken off the heat, and 1 eq tert-butylbromoacetate is added at a rate that maintains a gentle reflux.
Once things calm down, heating is resumed for a further hour.

At this point, the literature describes a green solution. I obtained a green solution the first time I attempted this reaction, and the reagent worked well on my substrate.

However, on repeating this, I always end up with a colourless solution with a purple-ish sort of burgandy precipitate. Adding my substrate to this does result in the desired reaction to some extent, but side products are more prominent and yields are extremely low.

Could this be a moisture problem? The anhydrous THF might not be so anhydrous any more, for the first attempt it was from a new bottle, but it's now been open for a few days - I always remove the THF under Ar, but I have caught the masters students withdrawing solvent without doing this. The air humidity has also rocketed over the last couple of days.

I can't think of what this purple precipitate is. Anyone got any thoughts?

[Shaun]

You always have to use dry THF.  Do you have stills set up?  You should always use THF drawn under argon from a well maintained still using sodium potassium amalgam with a benzophenone indicator.  The indicator will impart a blue color on the THF if it is dry/oxygen free.  If the still is yellow or colorless, the THF isn't dry.

You must predry your glassware and syringes in a drying oven and cool them under argon in a desiccator before use.

If you don't have a still, you can distill enough THF for the reaction by refluxing THF over NaH for a while, distilling the THF, and using immediately.  Everything must be kept under argon at all times.  If you have some THF left over, don't be tempted to try to use it the next day (or even several hours later).  It probably will have gone bad already and you will waste your time and reagents trying to use it.

THF is notorious for soaking up moisture from the air.  It is significantly more hygroscopic than diethyl ether.

[kiwi]

dry solvent is a reasonable idea, although bear in mind allyl/propargyl halides will apparently react with zinc powder/aldehydes in THF:aq. ammonium chloride (eg. http://www.mdpi.org/molecules/papers/60800655.pdf). 
one trick i have seen (and have used) is to spike your nice clean zinc with some catalytic impurities. your method calls for copper but i have used 0.05 mol% Pb(OAc)2-3H2O with good success for diiodomethane -> bis(iodozinc)methane. some people like zinc-silver couple (Zn + AgNO3) for various purposes too.
the other thing you want to be doing is burning the crap off your zincs surface before use - typically i will swirl zinc in a conical with 2M HCl for about 2 min; then vacuum filter, wash with a 2 portions each of water then ethanol then ether, the nice bright zinc is then dried on the frit then under vacuum on your manifold (with gentle heat if need be). the resulting zinc powder will keep in a parafilmed vial for a fair while.
one thing you might like to consider if your substrates allow it is to do the reaction barbier-style ie: add bromide to a suspension of RCHO/Zn powder/THF. that way your zinc reagent won't build to the concentration where it can get up to mischief.
just a few ideas for you; good luck (there are many other ways to do a reformatsky other than Zn too)

[DaveD]

In my experience Reformatsky reactions are capricious due to the quality of the zinc used (and also as the previous post mentioned, due to the presence of trace amounts of metal impurities).
Usually the zinc metal needs to be activated somehow, and variabilities in the activation protocal can result in capricious results.
One thing to try is Rieke zinc - a highly active form of zinc metal which could give you more uniform results in your Reformatsky reaction.  You should be able to either purchase some of this or find an article on how to prepare some yourself.  The author's name is Reuben Rieke and he had some papers from the late 1980's/early 1990's (JACS and JOC I believe).