Failure and Redemption of Statistical and Nonstatistical Rate Theories in the Hydroboration of Alkenes

Abstract

Our previous work found that canonical forms of transition state theory incorrectly predict the regioselectivity of the hydroboration of propene with BH3 in solution. In response, it has been suggested that alternative statistical and nonstatistical rate theories can adequately account for the selectivity. This paper uses a combination of experimental and theoretical studies to critically evaluate the ability of these rate theories, as well as dynamic trajectories and newly developed localized statistical models, to predict quantitative selectivities and qualitative trends in hydroborations on a broader scale. The hydroboration of a series of terminally substituted alkenes with BH3 was examined experimentally, and a classically unexpected trend is that the selectivity increases as the alkyl chain is lengthened far from the reactive centers. Conventional and variational transition state theories can neither predict the selectivities nor the trends. The canonical competitive nonstatistical model makes somewhat better predictions for some alkenes but fails to predict trends, and it performs poorly with an alkene chosen to test a specific prediction of the model. Added nonstatistical corrections to this model make the predictions worse. Parameterized RRKM-master equation calculations correctly predict the direction of the trend in the selectivity versus alkene size but overpredict its magnitude, and the selectivity with large alkenes remains unpredictable with any parameterization. Trajectory studies in explicit solvent can predict selectivities without parameterization but are impractical for predicting small changes in selectivity. From a lifetime and energy analysis of the trajectories, “localized RRKM-ME” and “competitive localized noncanonical” rate models are suggested as steps toward a general model. These provide the best predictions of the experimental observations and insight into the selectivities.

VRAI-selectivity: calculation of selectivity beyond transition state theory

“…The proportion of the reaction trajectory passing over TS1 and TS1d is estimated using the transition state theory. The increase in free energy going from the starting material to TS1 and TS1d is 84.0 and 84.6 kJ mol −1 respectively for reaction (10). Therefore, the use of TST is justifiable.…”

trans-Hydroboration–oxidation products in Δ5-steroids via a hydroboration-retro-hydroboration mechanism

“…25)/def2-TZVP level at 273.15 K. Hydroboration has been theoretically studied by several groups. [26][27][28] Validation of this methodology and further details are provided in the ESI. † All these computations were done in Gaussian 16 (ref.…”

Selective Hydrodefluorination of Hexafluoropropene to Industrially Relevant Hydrofluoroolefins

“…Due to the extremely low energy local barrier for the hydroboration step, Singleton and others have commented that the approximations of transition state theory may not hold. [25][26][27] Precise determination of the regioselectivity of the addition of boranes to even simple alkenes requires the consideration of reaction dynamics and modelling of reaction trajectories in place of more standard computational approaches. [28][29][30] As such, we are reluctant to use DFT calculations to model the complete reaction mechanism of the HDF of HFP with BH 3 .…”