Rapid Entry to Functionalized Boratabenzene Complexes through Metal-Induced Hydroboration at the Anionic 1-H-Boratabenzene Ligand

The hydroboration of alkenes, alkynes, imines, and carbodiimides using the anionic 1-H-boratabenzene ligand bound to rare-earth (RE = Y, Lu), transition (Zr and Rh), and main-group (Li) metals is reported. This hydroboration is metal ion dependent; in the case of 1-H-boratabenzene transition metal complexes, the reactivity follows the trend RE > Zr > Rh. Hydroboration with 1-H-boratabenzene rare-earth metal complexes works well for a range of unsaturated substrates, including 1-hexene, allyl propyl ether, allyl ether, 3-hexyne, benzylidene-n-propylamine, and N,N′-diisopropylcarbodiimide, thus generating a series of new alkyl-, alkenyl-, amino-, or amidino-functionalized boratabenzene rare-earth metal complexes in high yields. The reactions are highly anti-Markovnikov selective, and the mechanism has been investigated by deuterium-labeling experiments. In comparison, a 1-H-boratabenzene Zr complex reacts with benzylidene-n-propylamine and N,N′-diisopropylcarbodiimide, and a 1-H-boratabenzene Rh complex reacts with N,N′-diisopropylcarbodiimide. In contrast, the 1-H-boratabenzene lithium salt reacts only with the activated substrate benzylidene-n-propylamine at elevated temperature to give the corresponding hydroboration product. Boratabenzene Y complexes undergo ligand redistribution with Rh chlorides to give boratabenzene Rh complexes. Studies of the novel monoanionic amidino-boratabenzene ligand by X-ray diffraction and DFT calculations have revealed interesting structural features.