Iron-Catalyzed Dehydrocoupling/Dehydrogenation of Amine鈥揃oranes

详细信息    查看全文

• 作者：James R. Vance ; Andr茅 Sch盲fer ; Alasdair P. M. Robertson ; Kajin Lee ; Joshua Turner ; George R. Whittell ; Ian Manners
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：February 26, 2014
• 年：2014
• 卷：136
• 期：8
• 页码：3048-3064
• 全文大小：759K
• 年卷期：v.136,no.8(February 26, 2014)
• ISSN：1520-5126

文摘

The readily available iron carbonyl complexes, [CpFe(CO)₂]₂ (1) and CpFe(CO)₂I (2) (Cp = 畏-C₅H₅), were found to be efficient precatalysts for the dehydrocoupling/dehydrogenation of the amine鈥揵orane Me₂NH路BH₃ (3) to afford the cyclodiborazane [Me₂N鈥揃H₂]₂ (4), upon UV photoirradiation at ambient temperature. In situ analysis of the reaction mixtures by ¹¹B NMR spectroscopy indicated that different two-step mechanisms operate in each case. Thus, precatalyst 1 dehydrocoupled 3 via the aminoborane Me₂N鈺怋H₂ (5) which then cyclodimerized to give 4 via an off-metal process. In contrast, the reaction with precatalyst 2 proceeded via Me₂NH鈥揃H₂鈥揘Me₂鈥揃H₃ (6) as the key intermediate, affording 4 as the final product after a second metal-mediated step. The related complex Cp₂Fe₂(CO)₃(MeCN) (7), formed by photoirradiation of 1 in MeCN, was found to be a substantially more active dehydrocoupling catalyst and not to require photoactivation, but otherwise operated via a two-step mechanism analogous to that for 1. Significantly, detailed mechanistic studies indicated that the active catalyst generated from precatalyst 7 was heterogeneous in nature and consisted of small iron nanoparticles (鈮?0 nm). Although more difficult to study, a similar process is highly likely to operate for precatalyst 1 under photoirradiation conditions. In contrast to the cases of 7 and 1, analogous experimental studies for the case of photoactivated Fe precatalyst 2 suggested that the active catalyst formed in this case was homogeneous. Experimental and computational DFT studies were used to explore the catalytic cycle which appears to involve amine鈥揵orane ligated [CpFe(CO)]⁺ as a key intermediate.