Diastereofacial Selectivity in Reactions of 5-Substituted (X) Adamant-2-yl Cations:  Manifestations of Electron Demand

Abstract

A series of 5-substituted (X) 2,2-dimethoxyadamantanes (4, X ) F, Cl, Br, COOCH 3 , OCH 3 , CH 3 , C 6 H 5 , p-NO 2 C 6 H 4 , p-BrC 6 H 4 , SiMe 3 , and SnMe 3 ) as well as 2-methylene-5-tert-butyladamantane (6, Y ) CH 2 ; X ) C(CH 3 ) 3 ) have been synthesized and characterized. Reduction of the ketals 4 under ionic conditions with triethylsilane and phenylsilane provide π-facial diastereoselectivities for hydride trapping of 5-substituted (X) 2-methoxyadamant-2-yl cations (1, R ) OCH 3 T 5). A comparison of this data with known diastereoselectivities for nucleophilic capture of tertiary and secondary 5-substituted (X) 2-adamantyl cations highlights that stereoselectivity in these systems is a function of electron demand. Diastereoselectivities for the hydrochlorination of 6 (Y ) CH 2 , X ) C(CH 3 ) 3 ) in CH 2 Cl 2 and NO 2 CH 3 have been determined and compared with the corresponding data for the silicon analogue (6, Y ) CH 2 ; X ) SiMe 3 ). Low electron demand coupled with the stereoelectronic requirement of double hyperconjugation appears the most likely explanation, rather than a long-range steric factor, for the total lack of stereoselectivity in the Clcapture of the 2-methyl-5-(trimethylsilyl)adamant-2-yl cation (1, Y ) CH 2 ; X ) SiMe 3 ) in NO 2 CH 3 as solvent.

Nature of the Electronic Factor Governing Diastereofacial Selectivity in Some Reactions of Rigid Saturated Model Substrates

“…Although these groups are similar in that they exert virtually no electrostatic field influence (σ F ≈ 0; Table ) they differ markedly with respect to their double hyperconjugative interaction mode as revealed by the 19 F SCS of 14 and 23 (Tables and , respectively). Model system reactivity studies (see later) under conditions of high electron demand have confirmed and heightened this distinction. 63i,,, Note, however, that the selectivities exerted by these groups (Tables and ) are feeble and, moreover, the observed selectivities are in accord with expectations based purely on electrostatic grounds. Thus, the overall picture portrayed by the results (Tables and ), namely, the unimportance of transition-state hyperconjugation governing diastereofacial selectivity in the nucleophilic additions of 1 (Y = O), is strongly reinforced.…”

“…The calculated diastereoselectivities for electrophilic addition (HCl in CH 2 Cl 2 ) to 1 (Y = CH 2 ) based on the ρ FS value from Figure are listed in Table together with the experimentally observed product distributions. , It can be seen that there is a quite good agreement between the observed and calculated polar-field-induced selectivities in CH 2 Cl 2 for most of the σ-electron-withdrawing substituents. Thus, the main conclusion to be drawn from these results is that the electrostatic field effect (σ F effect) of these groups plays an important role governing diastereoselectivity in the capture of 32 (R = CH 3 , X = σ-electron-withdrawing group).…”

“…There are two significant discrepancies in Table worth noting, namely, the selectivity order for the halogens (F > Cl > Br > I (observed) vs F ≈ Cl ≈ Br ≈ I (calculated)) and the unexpected lack of stereoselectivity observed for the Si(CH 3 ) 3 group in NO 2 CH 3 compared to in CH 2 Cl 2 . Particular attention has been drawn to these matters in the original reports. , One point that was emphasized in connection with the latter discrepancy is that there is a mechanistic distinction for HCl addition in CH 2 Cl 2 versus NO 2 CH 3 , whereas for the former the reaction involves the formation of tight-ion pairs, in the latter free ions are expected to mediate the reaction . On this basis long-range steric phenomena 65 as well as the demanding stereoelectronic requirement of the double hyperconjugative interaction mode 91 have been raised as possible origins of the anomaly.…”

“…On this basis long-range steric phenomena 65 as well as the demanding stereoelectronic requirement of the double hyperconjugative interaction mode 91 have been raised as possible origins of the anomaly. However, because the discussions are somewhat speculative, these will not be reiterated here and interested readers are referred to the original papers. , What can be said, however, is that more experiments are needed to resolve these and other matters.…”

Electronic Control of Facial Selection in Additions to Sterically Unbiased Ketones and Olefins

“…Subsequently, the study was extended to a variety of other substrates related to 22 . Diastereoselectivity was also examined in other types of reactions, such as epoxidation and dichlorocarbene addition, solvolysis, thermal 86 and photocycloadditions, cycloreversion, oxidation, oxy-Cope 90 and Claisen 91 rearrangements, protonation, radical capture, and iron tricarbonyl complexation . The remote substituent effects in all of these diverse reactions were qualitatively similar.…”