Gas-phase diastereoselectivity of secondary 5-substituted (X)-adamant-2-yl (X = F, Si(CH(3))(3)) cations.
Secondary 5-X-adamant-2-yl cations IX (X = F, Si(CH3)3) have been generated in the gas phase (total pressure = 760 Torr) from protonation-induced defluorination of epimeric 2-F-5-X-adamantanes 1X and their kinetic diastereoselectivity toward CH318OH investigated in the 40-160 degrees C range. The experimental results indicate that the facial selectivity of IX is insensitive to the composition of the starting 1X epimers as well as to the presence and the concentration of a powerful base (N(C2H5)3). This kinetic picture, supported by B3LYP/6-31G* calculations, is consistent with a single stable pyramidalized structure for IX, that is, (Z)-5-F-adamant-2-yl (I(Z)F) and (E)-5-Si(CH3)3-adamant-2-yl cations (I(E)Si). The temperature dependence of the IX diastereoselectivity lends support to the intermediacy of noncovalent adducts [IX*CH318OH], characterized by a specific C2-H+...O18(H)CH3 hydrogen bonding interaction. Their conversion to the covalently bonded O-methylated (Z)- (II(Z)X) and (E)-5-X-adamantan-2-ols (II(E)X; X = F, Si(CH3)3) is governed by activation parameters, whose magnitude depends on the specific IX face accommodating CH318OH. The gas-phase diastereoselectivity of IX toward CH318OH is compared to that exhibited in related gas-phase and solution processes. The emerging picture indicates that the factors determining the diastereoselectivity of IX toward simple nucleophiles in the gaseous and condensed media are completely different.
Fraschetti
Caterina
C
Dipartimento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente Attive, Sapienza Università di Roma, 00185 Roma, Italy.
Novara
Francesca R
FR
Filippi
Antonello
A
Trout
Neil A
NA
Adcock
William
W
Sorensen
Ted S
TS
Speranza
Maurizio
M
D000218:Adamantane / Q000737:chemistry*; D002412:Cations / Q000737:chemistry*; D019295:Computational Biology; D008401:Gas Chromatography-Mass Spectrometry; D005740:Gases; D007700:Kinetics; D008956:Models, Chemical; D015394:Molecular Structure; D011522:Protons; D012825:Silicon / Q000737:chemistry*; D013237:Stereoisomerism
D002412:Cations; D005740:Gases; D011522:Protons; D000218:Adamantane; D012825:Silicon