Intramolecular vs. intermolecular alkyl carbon-hydrogen bond activation. Complete thermodynamic and kinetic parameters for a reversible cyclometalation

Abstract

6805in methanol to afford 1-phenyl-1-methoxycyclohexane (2), via the 1 -phenylcyclohexyl carbocation.lOJ1 Extrapolation of the double-reciprocal plot of the quantum yield of formation of 2 vs.[H2S04], after correction for 3Ph2C0 quenching by H2SO4 and for the small (2.2%) inefficiency of conversion of the carbocation to ether,19 affords = 0.36 f 0.01 ( 2 0 )~~ and thus a value of 44.7 f 5 kcal/mol for E,/. The sum of the amplitude factors, & + 4, + q53, equals unity, indicating all the photon energy is recovered as heat within experimental error. Results in cyclohexane are almost identical. However, the inability to measure in aprotic solvent prevents a computation of Et-, in cyclohexane.The values of E T and E, , are the first experimental measurements of the relaxed energy of a short-lived twisted alkene triplet21 and of the heat of geometric isomerization to the most strained cycloalkene known, respectively. The E T value of 56 f 3.4 kcal/mol is but slightly lower than the expected22 spectroscopic triplet energy of 60 kcal/mol. We believe that this corresponds to partial relaxation (Le., <90° rotation) of the tri~1et.I~ Therefore, the triplet surface is nearly flat, similar to the trans to perpendicular region for stilbene triplet.23The energy E,, of 44.7 kcal/mol is remarkably close to the prediction of molecular mechanics (MM) for trans-cyclohexene (E,/ = 42.4 k~a l / m o l ) .~~ Given the AH,-(c-1) = -4.0 f 1.6 kcal/m01~~ and the strain energy E,(c-1) = 1.2 kcal/mo1,26 then AHdt-1) = 41 f 5 kcal/mol and E,(t-1) = 46 f 5 kcal/mol. We note the structural prediction by M M of substantially pyramidalized vinyl carbons in these and related compound^.^,^^ The implications of this for the chemistry of trans-cyclohexenes and for dynamics on the ground-state surface will be discussed in a future report.In conclusion, we have demonstrated the applicability of time-resolved photoacoustic calorimetry to the study of alkene ground and triplet surfaces. Further studies should allow a wealth of thermochemical information on relaxed olefin triplets and strained ground-state isomers. (19) Had we chosen neat alkyl silane as the standard state, PS would have In (Cnat/Cl~) = AS,M -4.0 eu, where (20) (a) Meyers, R. T. Inorg. Chem. 1978, 17,952-958. (b) Martell, A. been even smaller (ASnmt = AS,M C,,, and CIM are concentrations). E.There is considerable uncertainty about the rate constant for a "diffusion-controlled" unimolecular reaction. For a bimolecular

Mechanistic Studies of the Thermolysis of Tetraneopentyltitanium(IV). 1. Solution Evidence That Titanium Alkylidenes Activate Saturated Hydrocarbons

“…The resulting increase in steric crowding may reduce the rotational degrees of freedom of the neopentyl groups in the transition state. Similarly large entropies of activation are seen for some cyclometalation processes at d 0 metal centers. − …”

“…Similarly large entropies of activation are seen for some cyclometalation processes at d 0 metal centers. [56][57][58][59][60] Titanium Alkylidene Trapping Experiments. The conclusion that TiNp 4 decomposes via an intramolecular R-hydrogen abstraction process implies that the first step in the thermolysis of TiNp 4 results in the liberation of 1 equiv of neopentane and the formation of a titanium alkylidene intermediate "Ti(dCHCMe 3 )Np 2 ":…”

Molecular Routes to Metal−Carbides, −Nitrides, and −Oxides. 3.1 Chemical Vapor Deposition Employing X3W⋮*CCMe3, Where X = CH2CMe3, OtBu, and OtBu-d9 and *C = 12C or 13C, and a Comparison with the Chemistry of (tBuO)3W⋮N

“…On the basis of the established organometallic chemistry of metal−neopentyl ligands, one can speculate that α-CH activation 16,19-21 occurs on the surface and that this leads to the loss of the original meaning of α and γ, as both become equivalent. As seen in the chemistry of Th(IV) and Pt(II), for example, dineopentyl complexes liberate neopentane (sometimes reversibly) with the formation of a metallacycle − (eq 4).

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Mechanisms of intramolecular activation of carbon-hydrogen bonds in transition-metal complexes

“…In order to obtain the effective molarity accepting this approach, the direct and reverse rate constants of reaction 30 should be compared. 126 In this case the effective molarity is equal to the ratio &i/fe-i = K multiplied by the statistical correction (4/3). The equilibrium constants K increase with the temperature, but even at 100 °C, the K is only 8.4 M, the effective molarity being 11.2 M!…”