Competing intermolecular and intramolecular hydride transfers in the ionic hydrogenation of (2-alkoxyphenyl)di(1-adamantyl)methanols †

Abstract

ortho-Lithiation of anisole followed by reaction with di(1-adamantyl) ketone gives syn-(2-anisyl)di(1-adamantyl)methanol with the C-OH proton intramolecularly hydrogen-bonded to the methoxy group. Reaction of the alcohol with trifluoroacetic acid (TFA) in dichloromethane leads to a trifluoroacetate and a substituted phenol. These are formed via a carboxonium ion, resulting from an intramolecular 1,5-hydride shift of the initially formed carbocation. Ionic hydrogenation of the alcohol with TFA and a hydrosilane in dichloromethane results in the expected anti and syn deoxygenation products as well as the trifluoroacetate and the phenol. anti-(2-Anisyl)diadamantylmethane (benzylic hydrogen remote from methoxy, confirmed by a single crystal X-ray diffraction study) is formed directly from the carbocation while the syn isomer results from reduction of the carboxonium ion. Reduction of the carbonium ion is more hydrosilane-selective than that of the carboxonium ion. Kinetic isotope effects (k H /k D ) on the reaction of the carbocation at room temperature average 1.50 for triethylsilane and dimethylphenylsilane. Analogous reaction of the (2-ethoxyphenyl) derivative gives only syn-(2-ethoxyphenyl)diadamantylmethane and the substituted phenol. Kinetic isotope effects on the reduction of the corresponding carboxonium ion average 1.34 for the same hydrosilanes.

Competing inter- and intramolecular hydrogen bonding: solvent-driven rotamerization in 3,4-(ethylenedioxy)-2-thienyldi(tert-alkyl)methanols

“…1- 3 To date, however, when there is an alkoxy group close to the OH group, as in 2-anisyl-and 3-alkoxy-2-thienyldi(1-adamantyl)methanols, only the intramolecularly hydrogen-bonded rotamer has been found. 4, 5 The existence of the hydrogen bond and favourable steric effects would appear to be an adequate explanation for this phenomenon. Nevertheless, 2-anisyl(alkyl)methanols and 2-anisyldi(alkyl)methanols, with alkyl groups as large as tert-butyl, are reported to occur as hydrogen-bonded and "free" forms in equilibrium at room temperature, the latter form being favoured by hydrogen-bonding solvents.…”

Trifluoroacetylation and ionic hydrogenation of [2-(3-alkoxythienyl)]di(1-adamantyl)methanols †

“…Hydride transfers involving 1,2 and 1,3 shifts are well documented, occurring mainly in norbornane derivatives, 1 but reports of higher-order transfers are comparatively rare. [2][3][4][5] If, however, one generates a carbocation in which all other options are closed, it is possible to observe 1,4-and 1,5-hydride transfers provided that there is a suitable substituent located at an appropriate distance 6 from the electron-deficient centre. ortho-Substituted aryldi(1-adamantyl)methyl cations generated in non-nucleophilic solvents meet these criteria, insofar as the adamantyl groups hinder nucleophilic attack, have no proton accessible for elimination and are highly reluctant to rearrange or to cleave.…”

“…For direct electronic access see http://www.rsc.org/suppdata/p2/b0/b002186p/ faster for ethoxy than for methoxy, to give a carboxonium ion. 5 This latter reacts with nucleophiles to give a trifluoroacetate and a phenoxymethanol, which is isolated as the corresponding phenol. In the presence of an organosilane, intermolecular reduction of the carbocation competes with the intramolecular reaction.…”

Competing β-scission and hydrogen transfer to the pinanyl radical in the addition of methyl thioglycolate to β-pinene derivatives

“…The product ratios for runs with only a small excess of thiol were converted to rate constant ratios by simulation of the reaction using the KINAL programme associated with a Simplex routine. 7 Initial concentrations were calculated on the assumption that all substrates have the same density : molecular weight ratio as β-pinene. This is clearly an approximation, but tests over a range of plausible values showed that there is no effect upon the enthalpy term and not more than 0.1 cal mol Ϫ1 K Ϫ1 on the entropy term.…”