Origin of Diastereofacial Selectivity in Tertiary 2-Adamantyl Cations

Abstract

The intrinsic factors governing the diastereofacial selectivity of 2-methyl-5-X-2-adamantyl cations (X = F (I(F)), Si(CH(3))(3) (I(Si))) toward a representative nucleophile, i.e., methanol, have been investigated in the gas phase at 750 Torr and in the 20-80 degrees C temperature range. The kinetic results indicate that CH(3)OH addition to I(F) proceeds through tight transition structures (TS(F)(syn) and TS(F)(anti)) characterized by advanced C-O bonding. The same interactions are much less pronounced in the comparatively loose transition structures involved in the CH(3)OH addition to I(Si) (TS(Si)(syn) and TS(Si)(anti)). The experimental evidence indicates that the activation barriers for the anti addition to I(F) and I(Si) are invariably lower than those for the syn attack. Large adverse entropic factors account for the preferred syn diastereoselectivity observed in the reaction with I(F). Entropy plays a minor role in the much looser transition structures involved in the reaction with I(Si), which instead exhibits a preferred anti diastereoselectivity. Comparison of the above gas-phase results with related theoretical and solution data suggests that the diastereofacial selectivity of I(F) and I(Si) measured in solution arises in part from the differential solvation of the two faces of the pyramidalized ions.

Facial Selectivity of the (R)‐1,3‐Dimethyl‐1‐Cyclohexyl Cation in the Gas Phase

“…In CH 4 /1/CH 3 18 OH mixtures, gradiolysis of the bulk gas CH 4 generates stationary concentrations of C n H 5 (n 1,2) ions. These strong Br˘nsted acids are able to protonate 1 [5] (path i in Scheme 1) yielding the chiral (R)-1,3-dimethyl-1-cyclohexyl cation (I) in the presence of small amounts of nucleophile (CH 3 18 OH, 18 O 94 %), an efficient radical scavenger (O 2 ), and a powerful base [(N(C 2 H 5 ) 3 )]. The base ensures that the oxonium-ion intermediates are efficiently deprotonated (k b and k' b in Scheme 1).…”

“…[6] The so formed CH 3 18 OH 2 then acts both as a Br˘nsted acid catalyst [7] (path ii in Scheme 1) and as a nucleophile (CH 3 18 OH) generator. The substantial difference between the two approaches is that, in route i (henceforth called the extracomplex route), the products (1R,3R)-1-methoxy-1,3-dimethylcyclohexane (3 E) and (1S,3R)-1-methoxy-1,3-dimethylcyclohexane (3 Z) arise when external CH 3 18 OH molecules present in the gaseous mixture react with preformed I to firstly give the encounter complexes II syn and II anti . On the other hand, in route ii (henceforth called the intracomplex route), the same products arise from the intimate II syn and II anti complexes characterized by an initial specific orientation between the incipient ion I and the incipient CH 3 18 OH molecule, which is internally generated by proton transfer from CH 3 18 OH 2 to 1.…”

“…O, O 2 , and N(C 2 H 5 ) 3 . g-Irradiation of such gaseous mixtures generates stationary concentrations of the Lewis acid (CH 3 ) 2 Cl , which barely reacts with H 2 18 O, [9] but which is strong enough to methylate the alcoholic chiral substrates to yield the corresponding oxonium intermediates III E and III Z (path iii in Scheme 1).…”

“…Radiolytic experiments: The upper part of Table 1 reports the relative distribution and the total absolute yield of the 18 Olabeled ethereal products (3 E and 3 Z) obtained from the girradiated CH 4 /1/CH 3 18 OH gaseous mixtures (the extracomplex reaction). The product yields from the CH 3 in the gaseous mixtures.…”

“…The product yields from the CH 3 in the gaseous mixtures. However, the yield of both labeled or unlabeled 3 E and 3 Z decreases substantially (by over 80 %) when the molar fraction of the strong base N(C 2 H 5 ) 3 is quintupled in the starting mixtures; this is evidence that the ethereal products in Table 1 …”