A DFT Study on the Regioselectivity of the Reaction of Dichloropropynylborane with Isoprene

Abstract

This theoretical study deals with the reaction of isoprene and dichloropropynylborane. We report the results of the DFT calculations applied to the two processes involved, Diels-Alder cycloaddition and 1,4-alkynylboration. The boron influences both the chemoselectivity and the regioselectivity of this reaction through secondary orbital interactions (SOI hereafter) that give rise to transition structures with strong [4 atom + 3 atom] character. The "meta" regioselectivity observed experimentally for the reaction between 2-substituted 1,3-dienes and alkynyldihaloboranes has been explained as a result of the higher stabilization of these transition structures with "meta" orientation. Intrinsic reaction coordinate calculations were performed to determine connectivities and established the remarkable result that the geometrically very similar transition structures leading to both regioisomeric 1,4-alkynylboration products correspond to different pathways. For the "meta" orientation a direct alkynylboration of the diene through a concerted transition structure was found.

Competing mechanisms for the reaction of dichloropropynylborane with 2-tert-butylbutadiene. Diels–Alder reaction versus alkynylboration

“…However, TSAs are classical [4 + 2] structures. 20 However, in our case IRC studies for TSC-m and TSC-p showed similar pathways, connecting the intermediates with the corresponding AB products 5 and 6, respectively. Both mechanisms share the first step of the reaction with a TS in which the boron atom of the dienophile portion approaches the C 6 which lead to the [4 + 2] cycloadducts, have asynchronous structures being the C 1 -C 6 distances 2.60 Å, and the C 2 -C 3 distances 2.02 and 2.14 Å for TSB-m and TSB-p, respectively.…”

“…The more energetically favored structure had classical [4 + 2] character with a C-B secondary orbital interaction while the other one showed strong [4 + 3] character. 20 Again, two TSs were found for the cycloaddition, one of them showed a classical [4 + 2] structure with a significant C-B secondary orbital interaction, and the other TS had high [4 + 3] character and it was energetically favored for the meta mode of addition, which explained the experimental regioselectivity for similar systems. Instead, a TS that connects the enyne product of the AB process with the DA product via a zwitterionic structure was located.…”

“…17,20 We analyzed the reaction coordinates associated with TSB-m and TSC-m which are depicted in Fig. 17,20 We analyzed the reaction coordinates associated with TSB-m and TSC-m which are depicted in Fig.…”

“…3. 19,20 The IRC that connects TSC-m with enyne product 5 exhibits a rather flat area that corresponds to the six-membered ring structure Cm-p-1. In early studies, a zwitterionic sevenmembered ring which was associated with both the [4 + 3] cycloaddition and 1,4-alkynylboration pathways was also located and characterized.…”

Alkylhalovinylboranes: a new class of Diels–Alder dienophiles

“…The selection of this level of theory was based on existing records in the literature for the modelling of Diels-Alder reactions. 19,[26][27][28][29][30][31][32] Additionally, solvent effects in dichloromethane (3 ¼ 8.93) and heptane (3 ¼ 1.91) were calculated for the most stable geometries of reagents, transition structures (TSs), and products using the Polarized Continuum Model of Self-Consistent Reaction Field method (PCM method). 33,34 Furthermore, the molecular orbitals (MOs) of reagents were calculated to analyse the Frontier molecular orbital interactions and Natural Bond Orbitals (NBOs) analysis of TSs were performed using Wiberg bond indices for interpreting most relevant electronic interactions.…”