Steric effects in the ionic hydrogenation of aryldi(1-adamantyl)methanols to the corresponding methanes by trifluoroacetic acid and hydrosilanes or sodium borohydride

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

“…The isomeric (2-anisyl)diadamantylmethanes, 7A-Me and 7S-Me, are associated with benzylic carbon 13 C NMR shifts of 72.3 and 55.3 ppm, respectively. This finding confirms that the differences observed for other aryldiadamantylmethanes 12,15 apply also in the present series.…”

“…† Values of k 2 /k 1 calculated from the single-concentration experiments for the other hydrosilanes range from zero (TPS and TIPS) to 0.36 M Ϫ1 (TTMSS) and follow no obvious trend in steric or polar effects. As has been observed in other work on the ionic hydrogenation of aryldi(1-adamantyl)methanols, 15 there is no correlation of the relative rates of reduction of the carbocation with the rates of hydride transfer from these hydrosilanes to diarylcarbenium ions, which are a reflection of almost pure polar effects upon the stability of the incipient silylium ion. 28 The two bulky hydrosilanes, TPS and TIPS, of moderate reactivity do not react at all with the carbocation, whereas the bulky but highly reactive TTMSS does.…”

“…It should be remarked that in the analogous reaction of (2-alkylphenyl)diadamantylmethanols intermolecular hydride transfer does not occur, 12 except when sodium borohydride is used. 15 The fact that 7A-Me is formed at all indicates that the intermolecular/ intramolecular rate ratio is higher in the present system. Heating the isolated mixture of anisyldiadamantylmethanes for 8 h at 240 ЊC results in complete conversion to the syn isomer, 7S-Me, the half-life being about 1.5 h at this temperature; this corresponds to an activation energy of around 40 kcal mol Ϫ1 .…”

“…14 This has proved to be an interesting means of investigating the stereochemistry of intermolecular hydride transfer in conformationally locked systems. 15…”

Rotation barriers in aryl- and heteroaryldi(1-adamantyl)methyl systems; the ionic hydrogenation of heteroaryldi(1-adamantyl)methanols

“…Carbocations derived from congested aryl-substituted tertiary alcohols in dichloromethane-TFA mixtures can be reduced stereoselectively by hydrosilanes or sodium borohydride, the steric requirements of the reactants determining how the hydride donor approaches. 25 Though the intermediate carbocation cannot be observed directly, certain heteroaryl derivatives undergo the same reaction. 15 The 2-pyridyl and thiazol-2yl derivatives, 2S-a and 6, could not be reduced in this way, the electron-attracting nitrogen atom presumably preventing carbocation formation.…”