Electronic effects in iridium C–H borylations: insights from unencumbered substrates and variation of boryl ligand substituents

Abstract

Experiment and theory favour a model of C–H borylation where significant proton transfer character exists in the transition state.

Distortion/Interaction Analysis Reveals the Origins of Selectivities in Iridium-Catalyzed C–H Borylation of Substituted Arenes and 5-Membered Heterocycles

“…19,50,51 Indeed, Smith et al have shown that more negatively charged aryl groups stabilize the aryl iridium intermediates. 17 In addition, we also observed a moderate correlation between the Ir−C bond strength and the energy needed to homolytically cleave the C−H bond (R 2 = 0.75; Figure S5), similar to the case for palladium. 18 At first glance, this correlation suggests that activation of the strongest C−H bond will occur since it will lead to the most stable Ir−C bond.…”

“…Our computed activation energies for 16 (23.0 and 25.5 kcal/mol for 3-and 4-borylation, respectively; predicted selectivity 98.5%:1.5%) also agree well with Smith's experimental observations for the borylation of benzodioxole. 17 Experimentally, 91% and 3% 3-and 4-borylation products are observed with 16, along with 6% diborylated product. 17 Computed activation energies for the C−H activation of 17 at 2-and 3-positions are 18.4 and 16.1 kcal/mol, respectively (predicted selectivity 2%:98%).…”

“…17 Experimentally, 91% and 3% 3-and 4-borylation products are observed with 16, along with 6% diborylated product. 17 Computed activation energies for the C−H activation of 17 at 2-and 3-positions are 18.4 and 16.1 kcal/mol, respectively (predicted selectivity 2%:98%). Experimentally, Smith et al showed that borylation of 17 leads to high selectivity for the 3-position, i.e., ortho to the fluorine group (2-:3-borylation 8%:92%, isolated yield).…”

“…40 Compositions of molecular orbitals were calculated using the AOMix program. 41,42 ■ RESULTS AND DISCUSSION The complex lacking the four methyl groups on each Bpin, ( b p y ) I r ( B e g ) 3 ( b p y = 2 , 2 ′ -b i p y r i d i n e ; B e g = (ethyleneglycolato)boron) 9,17,43 was used as a model for (dtbpy)Ir(Bpin) 3 . Although Beg is a poorer electron donor 44 and thus expected to be less reactive in C−H oxidative addition than the Bpin-ligated catalyst, 45 the effects on regioselectivity employing Beg in place of Bpin are expected to be small (see page S5 in the SI).…”

Design principles in frustrated Lewis pair catalysis for the functionalization of carbon dioxide and heterocycles

“…However, a key difference is the carbophilic nature of the borane compared to the palladium center, which makes the B-C interaction the key component of the activation process. By contrast, the deprotonation of the reactive site is equally or more important in transition metal chemistry, with iridiumboryl catalysts that favor activation at the most acidic site [101][102] and palladiumcarboxylate complexes for which the selectivity is determined by both the nucleophilicity and the acidity of the position [89]. One of the limitations in the FLP mediated process is the dimeric nature of the catalyst that prevents the reaction to occur at ambient temperature.…”