🚀 go-pugleaf

<lucasea@sbcglobal.net> wrote in message 
news:pklCg.5341$9T3.1327@newssvr25.news.prodigy.net...
>
> "Bob" <bbx107.XYZ@excite.XYZ.com> wrote in message 
> news:1lnid2djuk34mnv43hrmq2tergs2u9tpbt@4ax.com...
>> On 8 Aug 2006 15:02:15 -0700, "Vladimirr Saigon"
>> <kantabusse@lortemail.dk> wrote:
>>
>>>Does LiAlH4 also reduce double bond carbon-carbon?
>>
>> No.  The mechanism depends on the high polarity of the C=O bond.
>
> That's generally true, but it's a bit of an oversimplification.
>
> 1)  There are several reports of LiAlH4 reducing isolated C=C with 
> transition metal all across the first row.  Reactions stoichiometric in 
> MCln (M = Ti, V, Cr, Mn, Fe, Co and Ni) give quantitative reduction to the 
> alkane.  Catalytic amounts of Ti, Ni and Co also work well.  Of course 
> it's really the transition metal hydride that's doing the reduction, but 
> the fact is that the reducting equivalents comes from LiAlH4.  It's 
> tantamount to saying that H2 can be used to reduce olefins, when in 
> reality it's the TM hydride that's doing the chemistry.
>
> 2)  There are cases of uncatalyzed reduction that depend on polarity 
> similar to what you describe.  For example, cinnamyl alcohol is reduced to 
> phenylpropanol by LiAlH4.  Here, the alcohol (alkoxide) helps deliver the 
> AlH4- to the olefin, where it can do its job.
>
> 3)  I would swear I remember reading that, under the right conditions, 
> LiAlH4 will add 1,4- to enones to give the ketone upon hydrolysis, but I 
> can't find a reference.  If this does happen, it probably involves a Cu 
> catalyst of some sort, and again, it's the Cu-H doing the addition.
>
> In all these cases, LiAlH4 is the reducing agent.  The answer to his 
> question depends on perspective.  If he's a synthetic chemist wanting to 
> use a reagent for a certain transformation, then the answer is yes, LiAlH4 
> will reduce a C=C bond under the right conditions.  If he's a mechanstic 
> chemist trying to understand how a reaction occurred, then no, the LiAlH4 
> doesn't itself reduce a C=C bond.

Oops, just found a bunch of references.  LiAlH4 (uncatalyzed) does reduce 
enones all the way to the saturated alcohol.  Similar chemistry occurs with 
conjugated unsaturated esters, nitroolefins, vinyl cyanides, etc.  They are 
all special cases, and do rely on the induced polarity of the C=C bond and 
the proximity of a group that can coordinate to Li+ and deliver the Al-H to 
the olefin.

Eric Lucas