Kanbur Uddhav, Sadow Aaron D
Department of Chemistry, Iowa State University, 1605 Gilman Hall, 2415 Osborn Dr, Ames, IA, 50011, USA.
Chemistry. 2020 Apr 24;26(24):5479-5493. doi: 10.1002/chem.202000325. Epub 2020 Apr 7.
Organoaluminum reagents' application in catalytic C-H bond functionalization is limited by competitive side reactions, such as carboalumination and hydroalumination. Herein, rare-earth tetramethylaluminate complexes are shown to catalyze the exclusive C-H bond metalation of terminal alkynes with the commodity reagents trimethyl-, triethyl-, and triisobutylaluminum. Kinetic experiments probing alkyl-group exchange between rare-earth aluminates and trialkylaluminum, C-H bond metalation of alkynes, and catalytic conversions reveal distinct pathways of catalytic aluminations with triethylaluminum versus trimethylaluminum. Most significantly, kinetic data point to reversible formation of a unique Ln ⋅AlR adduct, followed by turnover-limiting alkyne metalation. That is, C-H bond activation occurs from a more associated organometallic species, rather than the expected coordinatively unsaturated species. These mechanistic conclusions allude to a new general strategy for catalytic C-H bond alumination that make use of highly electrophilic metal catalysts.
有机铝试剂在催化C-H键官能团化反应中的应用受到竞争性副反应的限制，如碳铝化反应和氢化铝化反应。在此，稀土四甲基铝酸盐配合物被证明可以催化端炔与市售试剂三甲基铝、三乙基铝和三异丁基铝发生专一的C-H键金属化反应。动力学实验探究了稀土铝酸盐与三烷基铝之间的烷基交换、炔烃的C-H键金属化反应以及催化转化反应，揭示了三乙基铝与三甲基铝催化铝化反应的不同途径。最为重要的是，动力学数据表明会可逆形成一种独特的Ln·AlR₃加合物，随后是限制反应速率的炔烃金属化反应。也就是说，C-H键活化发生在一个关联性更强的有机金属物种上，而非预期的配位不饱和物种。这些机理结论暗示了一种利用高亲电性金属催化剂进行催化C-H键铝化反应的新通用策略。
