Substrate, Catalyst, and Solvent: The Triune Nature of Multitasking Reagents in Hydroboration and Cyanosilylation

Abstract

A truly green chemical process would avoid the use of an external catalyst, while still achieving high efficiency. This has been realized in the very recent past for hydroboration, cyanosilylation, acetalization, and the aza-Michael addition, among other reactions. The current combined computational and experimental study unlocks the secret to how this highly desirable outcome is accomplished: one of the reactants in the process also acts as the catalyst. Specifically, this is shown (i) for the important hydroboration reaction, with pinacolborane (HBpin) as the hydroborating reagent and benzaldehyde, acetophenone, benzoic acid and p-methoxyphenylacetylene as the hydroborated substrates, and (ii) for cyanosilylation, with trimethylcyanosilane (TMSCN) as the cyanosilylating agent and benzaldehyde as the substrate. The mechanistic understanding thus gained has then been further exploited experimentally to bring hydroboration and cyanosilylation closer to experimental conditions in catalysis. These insights can potentially be expanded to the rapidly growing area of solvent-free and internal catalyst chemistry.

Hydroboration of isocyanates: cobalt-catalyzedvs.catalyst-free approaches

“…Notably, considering conversion of 3a to 5a , similar mechanism and free energy barriers have been recently reported by Vanka and Sen et al for the catalyst-free hydroboration of benzamide with HBPin to give N,N -bis(pinacolboryl)benzylamine. 27 g It is noteworthy that, for the reaction of phenyl isocyanate with HBPin, higher barriers were found for alternative pathways that either start with the addition of HBPin to the CO bond of PhNCO (42.7 kcal mol −1 vs. 33.0 kcal mol −1 for addition to NC) or involve the reaction of PhNCO with two molecules of HBPin, affording the six-membered transition state (54.5 kcal mol −1 ), similar to those proposed previously by Sen and Vanka et al for the catalyst-free and solvent-free hydroboration of aldehydes and terminal alkynes 41 (for DFT calculations of these alternative pathways, see Scheme S1 in the ESI†).…”

“…Additional single point calculations were performed via optimized geometries with the aid of two functionals, B3LYP-D3 39 and ωB97M-V, 40 and the def2-TZVPP 38 basis set, which showed very similar results. for the catalyst-free and solvent-free hydroboration of aldehydes and terminal alkynes 41 (for DFT calculations of these alternative pathways, see Scheme S1 in the ESI †).…”

Catalytic Hydroboration of Esters by Versatile Thorium and Uranium Amide Complexes

“…The analysis of TS3B geometry (actual interatomic distances and planarity around the C3 atom, Scheme ) allows us to interpret TS3B as a complex of [L 3 Th] + with [metoxyborohydride] − and aldehyde. Anionic alkoxy-substituted pinacol–borohydrides are widely known as active species in hydroboration reactions, indicating the ability of these species to stabilize the negative charge developed on them, which explains the stabilization of the transition state TS3B relative to TS3A . Furthermore, the Int4B complex liberates MeOBpin, producing the thorium ethoxide, Int5B , which is a global minimum with an energy of −72.7 kcal/mol relative to Int1 .…”

“…Anionic alkoxy-substituted pinacol−borohydrides are widely known as active species in hydroboration reactions, indicating the ability of these species to stabilize the negative charge developed on them, which explains the stabilization of the transition state TS3B relative to TS3A. 70 Furthermore, the Int4B complex liberates MeOBpin, producing the thorium ethoxide, Int5B, which is a global minimum with an energy of −72.7 kcal/mol relative to Int1. To evaluate the rate-controlling energy, we applied the energetic span model, 71 according to which "there are no ratedetermining steps, but rather rate-determining states": TOFdetermining transition state (TDTS) and TOF-determining intermediate (TDI) form the highest energy gap between them within the studied catalytic cycle.…”

Theoretical Insights into Aluminum-Catalyzed Cyanosilylation of Aldehydes and Ketones

“…15 Later on, Sen, Vanka, and co-workers proposed a reaction mechanism of this reaction, considering additional TMSCN as the catalyst. 117 We have revised this mechanistic route in Pathway-3 UC (Figure 1, dashed line). Our computations suggest the activation barriers of 33.6 and 34.5 kcal/mol for R1 and R2, respectively.…”