Cation Coordination Model for Diastereoselectivity

In 12a, the empty orbital is antiperiplanar to crC2 ri and rC6-Hax bonds. In 12b, the empty orbital is antiperiplanar to crC2 ca and crC5 ce bonds. Since the CTc h bond is a better electron-donor than the crc c bond, the principle of stereoelectronic effect dictates that 12a is of lower energy than 12b. The electron-rich nucleophile (a Lewis base) is expected to be electrostatistically drawn toward the empty orbital (a Lewis acid) on the axial face in 12a, leading to the formation of the equatorial 

D1 = 011-C2-C3-C4, D2 = 011-C2-C3-C10 The ax- and eq-attacks are in respect of the heteroatom-containing cyclohexanone unit. 

Like the N- m e t h y I - 2 - ad a m a n ta n o ne species above, 5-aza-2-adamantanone N-oxide exhibits axial selectivity by a margin of 96 4 on reduction with NaBH4 in isopropanol. Cieplak model predicts axial selectivity for the overall electron-attracting character of the N+-0 bond. The Anh-Felkin model fails as it is opposite of the Cieplak model in concept and allows attack of a nucleophile anti to the more electron-deficient bond on the a carbon. The computed 3D structures of 5-aza-2-adamantanone N-oxide and its protonated derivative are shown in Fig. 12. 

This molecule is truly appealing because a nucleophile is expected to add on to the boron atom before any addition to the carbonyl carbon to give 22 (Fig. 9) [16]. This is so because the boron atom is 1.5 times more electron-deficient than the carbonyl carbon. This will have consequences on the observed selectivity. On using hydride ion as the nucleophile to add to boron and H+ for carbonyl group protonation, D2 is discovered to be 40° larger than D1 and, hence, an eminent equatorial attack [17]. This analysis is in agreement with the result observed for 

5-pyridyl-5-bora-2-adamantyl radical in its capture of deuterium from n-Bu3SnD, the ax eq selectivity is 35 65. An orbital with one electron is very similar to the empty orbital on a carbonyl carbon because both are electron-poor and, therefore, similar stereoelectronic arguments apply. 

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In the conformer 24a, crcx C9 and ctc1() ci i bonds are antiperiplanar to crC2 c3 bond. In the conformer 24b, the <5c -C9 bond is almost antiperiplanar to crcl C2 bond, and ctc10 cii bond is almost synperiplanar to the exo ctC3-h bond. The conformer 24a is more stable than 24b by almost 3.40 kcal mol-1. In application of the cation coordination model, the torsion angles D1-D4, both before and after carbonyl protonation, are found to support anti pyramidalization [42]. These torsion angles are collected in Table 10. The calculated 3D structures of 23 and 24, and their H+- and Li+-coordinated species are shown in Fig. 15 for a ready visualization of the expected diastereoselectivity. 

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