“Iron-Catalyzed C-H Borylation of Arenes” Dombray, T.; Werncke, C. G.; Jiang, S.; Grellier, M.; Vendier, L.; Bontemps, S.; Sortais, J-B.; Sabo-Etienne, S.; Darcel, C. J. Am. Chem. Soc. 2015, ASAP. DOI: 10.1021/jacs.5b00895
C-H borylation, itself a green reaction for generating useful borylated compounds, is traditionally catalyzed by Ir and Rh. Much of the work has been conducted by John Hartwig’s group at Berkeley and Mitch Smith’s group at Michigan St. French scientists have now reported an iron-catalyzed version, which complements recent reports with Co complexes and dinuclear transition metal complexes. I especially like that the reported reaction is free of H2 acceptors and utilizes light to activate the catalyst.
The authors first tested the borylation of ethylbenzene with pinacolborane. The substrate is in excess and serves as the solvent. The optimized catalyst was Fe(Me)2(dmpe)2, where dmpe is bis(dimethylphosphino)ethane, providing the borylated product in 73 % yield after 72 hours at room temperature under 350 nm light as a 68:32 mixture of the meta and para isomers. Interestingly, the dimethyl iron complex provided higher yields of the C-H borylated product than the dihydride (73 % vs 52 %, respectively).
For the scope of the reaction they report the borylation of a variety of alkylated benzenes in moderate to good yields. In addition, their reaction seems to tolerate ether and amine functionalities and they are able to borylate furans.
Mechanistically, they cashed in when they reacted the starting dimethyl complex with pinacolborane directly. The reaction proceeds to form a 1:1 mixture of the cis and trans isomers of the hydrido-(boryl)iron complex Fe(H)(Bpin)(dmpe)2. Interestingly, upon evaporating to dryness the mixture converted to solely the trans isomer, which they were able to isolate as crystals suitable for X-ray diffraction.
Lastly, the researchers performed a series of NMR experiments to elucidate some preliminary mechanistic details. Under catalytic conditions in deuterated toluene or benzene they observed evolution of HD. They also found that the hydrido(boryl) complex was catalytically competent for the borylation. Moreover, in stoichiometric reactions of the hydrido(boryl) complex (under irradiation) they observed formation of the hydrido-phenyl complex. These observations led the authors to propose the mechanism below for the borylation of arenes.
The present work is a good example of extending homogeneous catalysis to the first-row transition metals. Going forward I hope to see better regioselectivity, otherwise the gains in using a cheaper metal will be lost in the purification of the products.