I know Borohydride forms a complex of sorts with alcohols that is more reactive than the borohydride itself, but is there any reason why borohydride
cannot be used in a polar aprotic like DMSO of THF, by itself? THF alone is used for LAH, while borohydride is weaker, there are a number of compounds
it should have no problem reducing. Why when I search can I only find BH4 + MeOH in THF solutions? I wouldn't assume that the borohydride is so weak
that it can't reduce without methanol being present.AvBaeyer - 3-9-2014 at 18:32
The reason THF (or sometimes t-butanol) is used with sodium borohydride - methanol is a practical one. NaBH4 reacts very rapidly with methanol and
thus using methanol as a single solvent often requires large excesses of borohydride because of the rapid decomposition of the borohydride-methanol
complexes in the presence of excess methanol. If the borohydride is first suspended in THF (or t-butanol) in which the compound to be reduced is
disolved, the methanol is then slowly added over a lengthy time period to effect the desired reduction. In this manner, large excesses of borohydride
can be avoided and advantage taken of the more powerful reducing properties of the borohydride-methanol adducts which are more stable with limited
amounts of methanol.
There is much information about the reducing properties of NaBH4 in various solvents and solvent combinations on the web.
AvB
AvBLeben - 3-9-2014 at 18:44
Thank you for the reply,
What I was basically wondering is: Is there any reason why I cannot just use borohydride dissolve in THF or a similar solvent? Borohydride is fairly
reactive, depending on the substrate being reduced, I am just curious as to why I need to dirty my reaction with methanol (yes I know it can easily be
removed) if the borohydride would react with the compound regardless. THF or DMSO dissolve the borohydride and the organics, so I am not really
understanding the purpose of methanol - is its requirement purely situational? DJF90 - 3-9-2014 at 22:15
THF does not dissolve sodium borohydride, but you can use it as a suspension.forgottenpassword - 4-9-2014 at 01:50
THF does not dissolve sodium borohydride, but you can use it as a suspension.
Surely it must dissolve it ever so slightly? If not, and if it is active in its undissolved form, then couldn't I just create a slurry of it in
toluene and use that?
I've never worked with THF but I was thought it was used to dissolve ionic compounds being polar apriotic? forgottenpassword - 4-9-2014 at 23:57
If you'd have clicked the link you would know that it is soluble to the extent of 0.1 g in 100g of THF at 20 C.FireLion3 - 5-9-2014 at 13:26
I bet using even a smidgen of phase transfer catalyst with a suspension of borohydride would drastically enhance the reactivity.Nicodem - 5-9-2014 at 22:39
Surely it must dissolve it ever so slightly? If not, and if it is active in its undissolved form, then couldn't I just create a slurry of it in
toluene and use that?
THF and toluene are not protic so they cannot participate in the reduction of the carbonyl group. See the reaction mechanism for the reduction of
carbonyl groups with NaBH4 to understand why methanol is most commonly used. Off course, if you would have read the review that
forgottenpassword provided you, you would have known that already. So, in order for the reduction of properly reactive carbonyl groups to occur at
room temperature at an useful rate, you need the presence of water, methanol or ethanol.
Nevertheless, NaBH4 in diglyme, DMSO, pyridine and some other aprotic solvents is actually able to reduce even poorly electrophilic
carbonyl groups, it can even reduce some amides. But this is something quite different, as it takes temperatures of above 100 °C and the mechanism is
different. NaBH4 in hot DMSO is even able to participate as a nucleophile in some SN2 substitutions.
Quote:
I've never worked with THF but I was thought it was used to dissolve ionic compounds being polar apriotic?
THF is relatively poorly polar, but it is a base and it is therefore only able to dissolve acidic salts or salts with highly polarizable ions. For
example, it dissolves LiBH4 quite well (having a strongly acidic cation), but not even nearly as well NaBH4. It does not
dissolve neutral and nopolarizable salts (like NaCl and similar).
