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

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The divergent reactivity of a transient titanium neopentylidyne, (PNP)Ti (CBu)-Bu-t (A) (PNP = N[2-P(i)Pr2-4-methylphenyl](2)), that exhibits competing dehydrogenation and dehydroalkoxylation reaction pathways in the presence of acyclic ethers (Et2O, (Pr2O)-Pr-n, (Bu2O)-Bu-n, (BuOMe)-Bu-t, (BuOEt)-Bu-t, (Pr2O)-Pr-i) 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=(CHBu)-Bu-t((CH2Bu)-Bu-t), show pseudo first-order decay rates on titanium (k(avg) = 6.2 +/- 0.3 x 10(-5) s(-1), at 29.5 +/- 0.1 degrees C, overall), regardless of the substrate or reaction pathway that ensues. Also, no significant kinetic isotope effect (k(H)/k(D) similar to 1.1) was found between the activations of Et2O and Et2O-d(10), 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=(CHBu)-Bu-t((CH2Bu)-Bu-t) 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)Ti=(CHBu)-Bu-t(OR) (R = Me, Et, Pr-n, Bu-n, Pr-i, Bu-t) 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.
Publisher
ROYAL SOC CHEMISTRY
Issue Date
2013
Language
English
Article Type
Article
Keywords

TRANSITION-METAL CATALYSIS; BETA-ALKOXIDE ELIMINATION; H ACTIVATION; ORGANOMETALLIC CHEMISTRY; GEMINAL DEHYDROGENATION; ALKANE-DEHYDROGENATION; TITANIUM ALKYLIDYNES; HYDRIDE COMPLEXES; CARBENE COMPLEXES; CROSS-METATHESIS

Citation

CHEMICAL SCIENCE, v.4, no.6, pp.2543 - 2550

ISSN
2041-6520
DOI
10.1039/c3sc22195d
URI
http://hdl.handle.net/10203/203297
Appears in Collection
CH-Journal Papers(저널논문)
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