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叔丁醇铝:一种可以高效地提高MPV还原的催化剂

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OPRD-2012-1301-Al(OtBu)3 as an effective catalyst for the enhancement of Meerwei.pdf

Al(OtBu)3as an effective catalyst for the enhancement of Meerwein-Ponndorf-

Verley(MPV) reductions. Org. Process Res.Dev. 2012, 16, 13011306.

 

本文献通过比较叔丁醇铝与异丙醇铝对MPV还原的影响,得出前者比后者的催化效果更好:需要的量更少(低至20mol%);时间短。此外,还根据两者的聚集状态,分析了没有α-H的叔丁醇铝有更好的催化效果的原因:由于叔丁醇铝是以二聚体的形式存在,处于非桥接位置、可发生交换的叔丁氧配体较多(2/3)。而异丙醇铝是以四聚体的形式存在,使得处于非桥接位置、可发生交换的异丙氧配体较少(1/2);另外六价配位的Al中心由于位阻的作用使得周围的异丙氧难与其它异丙氧发生交换。所以,叔丁醇铝有更好的催化效果。

 

Herein we report the use of Al(OtBu)3 instead of the commonlyused Al(OiPr)3 which results in a dramatic rate increase and significantly lower catalyst loading for the reduction of (1)model compounds benzaldehyde and acetophenone, and (2)

(S)-CMK, a key intermediate inHIV protease inhibitor synthesis.

One of the most chemoselective and mild reactions for these types ofreductions is the MeerweinPonndorfVerley(MPV) reduction, which involves the reaction of aldehydes or ketones withan aluminum alkoxide in an alcohol solvent.

It should bepointed out that, in principle, catalytic quantitiesof aluminum reagent can be employedsince ligand exchange can occur after hydride transfer such that another carbonylgroup can coordinate with the aluminum.

It is often thecase that Al(OiPr)3-catalyzed MPV reductions can take long periods of time to reachcompletion.

Because the concentration of isopropyl alcohol isdecreased in the mixed-solvent systems, the reductions require longer periods of time to reach completion with bothcatalysts compared to the reductions in neat isopropyl alcohol.

It is conjecturedthat the trifluoroacetic acidcatalyzes the exchange of ligands attached to the aluminum centers.

At first glance the use ofAl(OtBu)3 may be surprising since the tert-butoxide ligand has no α-hydrogens. Oncloser inspection, however, it isclear from the proposed mechanism that ligand exchange takes place suchthat a tertbutoxide ligand can be replaced by an isopropoxide group.

In all cases, the rates of reduction employing Al(OtBu)3 were significantly greater than the rates using Al(OiPr)3.

It is interestingto speculate why the tert-butoxy derivative of aluminumis a much more reactive reagent than its iso-propoxy counterpart.

It is believed that the difference inaggregation state results in the superior activity of aluminum tert-butoxide.

Thus, if it isassumed that the same states of aggregation hold for these aluminumalkoxide catalysts in isopropyl alcohol, then there would be more exchangeableligands with the former catalyst compared to the latter.

It should be notedthat on a small scale with 5 mol % loading, reduction failedto reach completion.

This was attributedto the aluminum catalyst being quenched by inherent moisture.

It has beendemonstrated that Al(OtBu)3in isopropyl alcohol dramatically enhances the rate of reduction compared tothe classical aluminum reagent Al(OiPr)3.

Workhorse. adj. 吃苦耐劳的

Uncontested. adj. 无竞争的

Conjecture. vivt. 推测;猜测

Batch mode. n. 批处理模式;成批处理方式

Judicious. adj. 审慎的;合理的;考虑周到的

Drop in. n.

Carousel. n. 传送带?

 



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