An RI-NMR and DFT Study of Boron Homologations: Structures of α-Halo Lithium Carbenoids and the Effect of Boronic Ester Structure on 1,2-Metalate Rearrangement Rates
- 1Department of Chemistry
- 2Texas A&M University
- 3University of Pennsylvania
Abstract
Despite the widespread utility of α-halo lithium carbenoids in boron homologations, their solution-state aggregation structures remain unresolved. In this work, a rapid-injection NMR study was undertaken to characterize unstabilized α-halo lithium carbenoids at –105 °C. 13C–6Li scalar coupling established direct carbon–lithium connectivity, and DOSY NMR revealed that the lithium carbenoids exist as monomeric species solvated by THF. Next, the 1,2-metalate rearrangement rates were measured by trapping the α-bromo lithium carbenoid with arylboronic esters and monitoring the conversion of the 8-B-4 complexes generated in situ. The kinetic studies reveal that ring size, backbone substitution, and donor identity strongly influence the rate of the 1,2-metalate rearrangement. Detailed computational studies further confirmed that the lower 1,2-metalate rearrangement barriers of the more reactive boronic esters arise from the combination of the release of steric strain (B-strain), charge redistribution, and more favorable access to the reactive antiperiplanar rotamer of the 8-B-4 complex. Finally, these structure–reactivity relationships were translated into synthetic advances where a faster-migrating 2,4dimethylpentan-2,4-diol boronic ester enabled productive homologation of carbamate-derived carbenoids under mild conditions in which the corresponding BPin boronic ester derivative afforded no detectable homologation product.
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BibTeX
@misc{kang2026rinmr,
title = {{An RI-NMR and DFT Study of Boron Homologations: Structures of α-Halo Lithium Carbenoids and the Effect of Boronic Ester Structure on 1,2-Metalate Rearrangement Rates}},
author = {Seokmin Kang and Kate V. Grimley and Jin Qian and Andrew F. Zahrt and Andy A. Thomas},
howpublished = {ChemRxiv},
year = {2026},
month = oct,
doi = {10.26434/chemrxiv.15010914/v1},
url = {https://chemrxiv.org/doi/full/10.26434/chemrxiv.15010914/v1},
note = {CC BY},
}
RIS
TY - UNPB
TI - An RI-NMR and DFT Study of Boron Homologations: Structures of α-Halo Lithium Carbenoids and the Effect of Boronic Ester Structure on 1,2-Metalate Rearrangement Rates
AU - Seokmin Kang
AU - Kate V. Grimley
AU - Jin Qian
AU - Andrew F. Zahrt
AU - Andy A. Thomas
PY - 2026
DA - 2026/10/22
PB - ChemRxiv
DO - 10.26434/chemrxiv.15010914/v1
UR - https://chemrxiv.org/doi/full/10.26434/chemrxiv.15010914/v1
AB - Despite the widespread utility of α-halo lithium carbenoids in boron homologations, their solution-state aggregation structures remain unresolved. In this work, a rapid-injection NMR study was undertaken to characterize unstabilized α-halo lithium carbenoids at –105 °C. 13C–6Li scalar coupling established direct carbon–lithium connectivity, and DOSY NMR revealed that the lithium carbenoids exist as monomeric species solvated by THF. Next, the 1,2-metalate rearrangement rates were measured by trapping the α-bromo lithium carbenoid with arylboronic esters and monitoring the conversion of the 8-B-4 complexes generated in situ. The kinetic studies reveal that ring size, backbone substitution, and donor identity strongly influence the rate of the 1,2-metalate rearrangement. Detailed computational studies further confirmed that the lower 1,2-metalate rearrangement barriers of the more reactive boronic esters arise from the combination of the release of steric strain (B-strain), charge redistribution, and more favorable access to the reactive antiperiplanar rotamer of the 8-B-4 complex. Finally, these structure–reactivity relationships were translated into synthetic advances where a faster-migrating 2,4dimethylpentan-2,4-diol boronic ester enabled productive homologation of carbamate-derived carbenoids under mild conditions in which the corresponding BPin boronic ester derivative afforded no detectable homologation product.
ER -