Stereoselective Bromoboration of Acetylene with Boron Tribromide: Preparation and Cross-Coupling Reactions of (Z)-Bromovinylboronates
| Authors | |
|---|---|
| Year of publication | 2020 |
| Type | Article in Periodical |
| Magazine / Source | The Journal of Organic Chemistry |
| MU Faculty or unit | |
| Citation | |
| Web | https://pubs.acs.org/doi/10.1021/acs.joc.0c00341 |
| Doi | http://dx.doi.org/10.1021/acs.joc.0c00341 |
| Keywords | HALOBORATION REACTIONS; UNSATURATED-COMPOUNDS; BUILDING-BLOCKS; HYDROBORATION; ISOMERIZATION; CYCLIZATION; CATALYST; ALKYNES; ROUTE |
| Description | The mechanism of acetylene bromoboration in neat boron tribromide was studied carefully by means of experiment and theory. Besides the syn-addition mechanism through a four-center transition state, radical and polar anti-addition mechanisms are postulated, both triggered by HBr, which is evidenced also to take part in the Z/E isomerization of the product. The proposed mechanism is well supported by ab initio calculations at the MP2/6-31+G* level with Ahlrichs’ SVP all-electron basis for Br. Implicit solvation in CH2Cl2 has been included using the PCM and/or SMD continuum solvent models. Comparative case studies have been performed involving the B3LYP/6-31+G* with Ahlrichs’ SVP for Br and MP2/Def2TZVPP levels. The mechanistic studies resulted in development of a procedure for stereoselective bromoboration of acetylene yielding E/Z mixtures of dibromo(bromovinyl)borane with the Z-isomer as a major product (up to 85%). Transformation to the corresponding pinacol and neopentyl glycol boronates and stereoselective decomposition of their E-isomer provided pure (Z)-(2-bromovinyl)boronates in 57–60% overall yield. Their reactivity in a Negishi cross-coupling reaction was tested. An example of the one-pot reaction sequence of Negishi and Suzuki–Miyaura cross-couplings for synthesis of combretastatin A4 is also presented. |
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