Mechanisms for both the (i) formal Rh- catalyzed [4+1] cycloaddition of vinyl isocyanates and vinyl ketenes to afford spirooxindole cyclopentenones and (ii) Ni- catalyzed 1,4-addition of trialkylboranes to α, β-unsaturated esters have been investigated computationally. A collaborative effort between the Wiest and Ashfeld groups studies the formal Rh- [4+1] cycloaddition to rationalize the retention of stereochemistry through a diradicaloid concerted mechanism. Hammett studies have been conducted for the ketene and isocyanate at the para position of the phenyl group and 6- position of the oxindole moiety. The ketene Hammett plots suggest that the effects of para-substitution are due to electron stabilization rather than radical stabilization. The isocyanate Hammett plots have the same general trend however there is a V-shape in the σ* plots; this non-linearity suggests a change in mechanism. Initial insights into the previously proposed mechanism for the Ni- conjugate addition yielded a calculated transmetallation barrier of 52.3 kcal/mol. The 7- membered transition state for this pathway features an unfavorable geometry the where the B – C – Ni angle is 95.3° and the C – B distance is 2.6 Å that suggests that this pathway is not feasible. New investigations studies a range of alternative pathways including base-mediated transmetallation pathways and open-shell derivatives. A base-mediated pathway is envisioned where carbonate coordinates to both the trialkylborane and the Ni-π complex that allows a 6- membered transition state that facilitates the transmetallation of the alkyl group. This pathway allows for a more favorable geometry than the previously calculated 7- membered transition state. Open-shell pathways can, at this point, not be ruled out and will be studied further when a favorable transmetallation geometry is discovered. Experimental mechanistic studies are also being devised that will be used to guide further computational elucidation efforts.

Utilizing DFT to gain mechanistic insight into nickel sp<sup>2</sup>-sp<sup>3</sup> catalysis and [4+1] cycloadditions

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ACS Fall 2022

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Utilizing DFT to gain mechanistic insight into nickel sp2-sp3 catalysis and [4+1] cycloadditions

Utilizing DFT to gain mechanistic insight into nickel sp²-sp³ catalysis and [4+1] cycloadditions