Publication details
Orbital interactions between C2H2, BBr3, and HBr influencing stereospecificity of acetylene bromoboration
| Authors | |
|---|---|
| Year of publication | 2017 |
| Type | Article in Proceedings |
| Conference | ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY |
| MU Faculty or unit | |
| Citation | |
| Keywords | acetylene; alkyne; anti; addition; BBr3; bromoboration; BX3; C2H2; DFT; HBr; izomerisation; mechanism; MP2; MO analysis; PES; propyne; solvent; syn; vinylbromide |
| Description | Abstract: Haloboration of C-C triple bonds, developed by many groups over several decades, has become an effective protocol for constructing multisubstituted alkenes with high selectivity and wide substrate range. In spite of this, MP2 study of Wang and Uchyiama from 2012 is the only available theoretical work related to its mechanism. In the vast majority of cases, alkyne bromoboration runs as a syn-addition of BBr3 on the alkyne triple bond. However, anomalous reactions are known as well whose products are those of anti-addition, including the reaction of C2H2 with BBr3. This observation was explained via a subsequent isomerization of the syn-adduct promoted by another BBr3 molecule. However, as early as in 1973, a closely related experimental study of phenylacetylene bromoboration by Blackborrow has shown that a mixture of syn and anti products can be obtained, with relative population of the anti-adduct increasing with the dilution, temperature, and solvent polarity. Therefore, it can be asumed that also acetylene bromoboration can be influenced by the presence of other substances potentially present in the reaction mixture like HBr or Br - anions. In our contribution, we present a comparative MP2, DFT and qualitative-MO analysis of the addition and izomerisation steps of acetylene and propyne bromoboration in the presence of HBr and/or Br - ions. We suggest a low-barrier creation (and back-addition) of vinylbromide from syn-dibrom(2-bromvinyl)borane and HBr, which is mechanistically close to the reaction route from propyne and BBr3 towards alkenyl halides presented by Wang, Tobrman, Xu, and Negishi in 2009. We also analyse and interpret PES scans related to the influence of X- ions on the mechanism of BX3 (X=Cl, Br, and I) addition to acetylene and propyne. We suggest a step-vise mechanism for X=Br, I while a concerted mechanism for X=Cl. |