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Issue 6, 2013
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Understanding the competitive dehydroalkoxylation and dehydrogenation of ethers with Ti–C multiple bonds

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Abstract

The divergent reactivity of a transient titanium neopentylidyne, (PNP)Ti[triple bond, length as m-dash]CtBu (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)Ti[double bond, length as m-dash]CHtBu(CH2tBu), show pseudo first-order decay rates on titanium (kavg = 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 (kH/kD ∼ 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)Ti[double bond, length as m-dash]CHtBu(CH2tBu) with a t1/2 = 3.2 ± 0.4 h, across all ethers, confirms formation of A being a common intermediate. Isolated alkylidene-alkoxides, (PNP)Ti[double bond, length as m-dash]CHtBu(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.

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

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Publication details

The article was received on 10 Dec 2012, accepted on 26 Mar 2013 and first published on 26 Mar 2013


Article type: Edge Article
DOI: 10.1039/C3SC22195D
Chem. Sci., 2013,4, 2543-2550

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

    M. G. Crestani, A. Olasz, B. Pinter, B. C. Bailey, S. Fortier, X. Gao, C. Chen, M. Baik and D. J. Mindiola, Chem. Sci., 2013, 4, 2543
    DOI: 10.1039/C3SC22195D

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