Understanding the competitive dehydroalkoxylation and dehydrogenation of ethers with Ti–C multiple bonds

Abstract

The divergent reactivity of a transient titanium neopentylidyne, (PNP)TiC^tBu (A) (PNP = N[2-PⁱPr₂-4-methylphenyl]₂⁻), that exhibits competing dehydrogenation and dehydroalkoxylation reaction pathways in the presence of acyclic ethers (Et₂O, ⁿPr₂O, ⁿBu₂O, ^tBuOMe, ^tBuOEt, ⁱPr₂O) is presented. Although dehydrogenation takes place also in long-chain linear ethers, dehydroalkoxylation is disfavoured and takes place preferentially or even exclusively in the case of branched ethers. In all cases, dehydrogenation occurs at the terminal position of the aliphatic chain. Kinetics analyses performed using the alkylidene-alkyl precursor, (PNP)TiCH^tBu(CH₂^tBu), show pseudo first-order decay rates on titanium (k_avg = 6.2 ± 0.3 × 10⁻⁵ s⁻¹, at 29.5 ± 0.1 °C, overall), regardless of the substrate or reaction pathway that ensues. Also, no significant kinetic isotope effect (k_H/k_D ∼ 1.1) was found between the activations of Et₂O and Et₂O-d₁₀, in accord with dehydrogenation (C–H activation and abstraction) not being the slowest steps, but also consistent with formation of the transient alkylidyne A being rate-determining. An overall decay rate of (PNP)TiCH^tBu(CH₂^tBu) with a t_1/2 = 3.2 ± 0.4 h, across all ethers, confirms formation of A being a common intermediate. Isolated alkylidene-alkoxides, (PNP)TiCH^tBu(OR) (R = Me, Et, ⁿPr, ⁿBu, ⁱPr, ^tBu) formed from dehydroalkoxylation reactions were also independently prepared by salt metatheses, and extensive NMR characterization of these products is provided. Finally, combining theory and experiment we discuss how each reaction pathway can be altered and how the binding event of ethers plays a critical role in the outcome of the reaction.