Kinetic Control of Surface Reactions:  Regioselectivity in the Reaction of 2-Methylcyclopropylmethanol on Mo(110)−(6×1)−O

Abstract

The reactions of a diastereomeric mixture of 2-methylcyclopropylmethanol are investigated on oxygen-modified Mo(110) as a means of probing the influence of lifetime and stability of hydrocarbon radicals formed in a surface reaction on selectivity. 2-Methylcyclopropylmethanol forms 2-methylcyclopropylmethoxide at ∼250 K. The C−O bond of 2-methylcyclopropylmethoxide subsequently breaks near 350 K, yielding the (2-methylcyclopropyl)methyl radical. The (2-methylcyclopropyl)methyl radical can undergo fast rearrangement to two different linear C5 homoallyl radical species. The two different ring-opening pathways are due to the asymmetric methyl substitution of the cyclopropyl unit. Only one pathway results in a surface-bound product: adsorbed 4-penten-2-oxide is formed via rearrangement to 4-penten-2-yl radical and subsequent trapping on surface oxygen. The 4-penten-2-oxide is identified by comparison of infrared and temperature-programmed reaction data for the reaction product to analogous data for the reference compound. Evolution of C5 hydrocarbons competes with trapping at 350 K. Careful analysis of temperature-programmed reaction data indicates that 3-methyl-1-butene, 2-methyl-1,3-butadiene, and 1,2-dimethylcyclopropane contribute to the 410 K product. However, the close resemblance of the fragmentation patterns for all C5 hydrocarbons renders quantitative analysis impossible. At elevated temperatures the C−O bond of 4-penten-2-oxide breaks and C5 hydrocarbon evolution sets in. 1-Pentene and 1,3-pentadiene, as well as propene, are formed in a ratio of 20:70:10 at ∼520 K. Above 900 K CO is formed from recombination of adsorbed carbon and oxygen. Our results are discussed in the context of relative reaction rates for different pathways available to hydrocarbon radicals and their relation to product selectivity.

Cyclopropylmethyl/Cyclobutyl Rearrangements on Surfaces: Evidence for Transient Cation Formation near O-Covered Mo(110)

“…Organic radicals and cations are often proposed as intermediates in important heterogeneous reactions, especially in catalysis, but direct evidence for these short-lived intermediates is seldom available . Recently, the rearrangement reactions of cyclopropylmethoxide on Mo(110) (Mo−O−CH 2 −c-C 3 H 7 ) were investigated . Alkoxides on Mo(110) are of special interest because they undergo heteroatom removalthe reverse of hydrocarbon oxidationto provide free hydrocarbons upon strong heating …”

The Cyclopropylmethyl−3-Butenyl Rearrangement on Mo(110): A Radical Clock on a Surface?

“…It seems then possible to use radical clocks as mechanistic and kinetic probes for surface reactions. Related radical clock systems with slightly different lifetimes in the gas phase have been studied on this surface as well and show the analogous rearrangement . This development is particularly interesting, as radicals are invoked as key intermediates for a number of catalytically relevant surface reactions, while information on the mechanisms of these reactions is still rare.…”

“…The formation of the cyclic alkoxide on the surface was observed for different precursor molecules, cyclopropylmethylbromide 9 and cyclopropylmethanol, 5 which were dosed in small concentrations onto the surface. In the first case, the alkyl moiety is transferred, obviously without rearrangement, onto a surface oxygen.…”

“…Related radical clock systems with slightly different lifetimes in the gas phase have been studied on this surface as well and show the analogous rearrangement. 5 This development is particularly interesting, as radicals are invoked as key intermediates for a number of catalytically relevant surface reactions, 6 while information on the mechanisms of these reactions is still rare. It has been shown recently that the lifetime of the radical clock systems is affected by the influence of metal ions.…”

“…Thus, it is crucial to learn more about the reaction of the cyclic alkoxide 3 to the ring-opened alkoxide 4, both proven spectroscopically to exist on the surface. The formation of the cyclic alkoxide on the surface was observed for different precursor molecules, cyclopropylmethylbromide 9 and cyclopropylmethanol, 5 which were dosed in small concentrations onto the surface. In the first case, the alkyl moiety is transferred, obviously without rearrangement, onto a surface oxygen.…”