Reduction of Adamantanone with

The equatorial transition state shows a Cf-C/j bond extension to 1.560 A (0.84%) compared with the calculated bond length of cyclohexanone (1.547 A), with no extension of the C -Hax bond. Similarly, the C -Cc=o bond length shortened in both axial (1.509 A, 0.4%) and equatorial structures (1.499 A, 1.1%) compared to the corresponding calculated bond length in cyclohexanone (1.515 A). 

It is of considerable interest to establish the magnitude of electronic effects on facial selectivity. The symmetry of 2-adamantanone (1 X=H) makes this structure ideal [61] to investigate electronic effects on transition state energy, since the faces of the carbonyl are little affected by steric and torsional effects with substitution at C5. The donor and acceptor ability of the four adjacent carbon-carbon bonds to the carbonyl can be varied without significantly altering the molecular structure. 

The results of experiments on the reduction on 5-substituted adamantanones 1 and 5-azaadamantan-2-one N-oxide (2) with NaBHq show that electron-withdrawing substituents favour attack by the complex hydride syn to the substituent or nitrogen [30, 34, 51] (Fig. 6-17). In the case of 2, the effect is striking, with a synlanti attack ratio of 96/4 for the formation of the anti syn alcohols respectively (Table 6-1). Electron-donating substituents show a marginal preference for anti attack. Similarly, syn facial selectivity is found in free-radical reactions [63], ther- 

This principle is applied in Fig. 6-20 to the reduction of substituted adamanta-nones, which shows that electron-donating groups decrease the HOMO/LUMO gap and favour reaction anti to the more donating adjacent C-C bonds. The reverse applies for electron-withdrawing substituents. 

There are problems with the Cieplak hypothesis. A major problem is that donation by the donor cr-bonds to the cr orbitals of the C-Nu forming bond would be expected to be small as these orbitals are markedly different in energy (Figs. 6-18, 6-19 and 6-20). Furthermore, while a donor substituent attached to the carbon a to the carbonyl will interact with the (J nu orbital to give stabilization 

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Figure 6-22. Transitions structure geometry for reduction of adamantanone (1 X=H) with AlHj.

Dehydroadamantanes are most readily obtained from either carbene insertion reactions or from 1,3-reductive eliminations. Pyrolysis of the dry sodium salt of the tosylhydrazone of adamantanone gives good yields of 2,4-dehydroada-mantane 133>. The unstable 1,3-dehydroadamantane is obtained from the treatment of 1,3-dibromoadamantane with sodium (Eq. (43)) 134>. 

Figure 6-17. Facial selection observed in the reduction of 2-adamantanones with NaBH4.

Studies of adamantanone reduction have been interpreted by le Noble [8 a, 8 b, 9] as consistent with the Cieplak hypothesis, since the reaction occurs preferentially from the face opposite the more electron-rich cr-bond. The Cieplak hypothesis considers interactions of adjacent c-bonds adjacent to the transition state with the cr orbital of the forming C-Nu bond (see Fig. 6-18). Figure 6-19 shows that the mixing of the adjacent antiperiplanar C-D (D=donor) with the (7 c-nu ot the transition state lowers the energy of the transition state. 

Calculations on the Reduction of 5-Substituted Adamantanones with AlHj 177... 

Dodecamethylcyclohexasilane can now be prepared in very good yield by the one step reduction of MeaSiCla using lithium in excess. With a deficiency of lithium, however, larger cyclopolysilanes resulted (MeaSi) (x = 7, 8, or 9). A series of silyl and phenyl substituted cyclopolysilanes have been synthesized, and while ethylene oxide inserts into Li(SiPha)5Li, dimethylsilylene, generated from (MeaSi)6, will additively cyclodimerize adamantanone and norbornone through a mechanism thought to involve the formation of an oxasilacyclopropane intermediate. ... 

Typical preparation of naphthopyran 32 involves Fries rearrangement of 1-acetoxynaphthalene 30. Condensation of 2-acetyl- 1-naphthol 31 with adamantanone, followed by usual reduction and dehydration gives 32 (Scheme 16).70... 

On p. 1023, it was mentioned that electronic effects can play a part in determining which face of a carbon-carbon double bond is attacked. The same applies to additions to carbonyl groups. For example, in 5-substituted adamantanones (2) electron-withdrawing (-/) groups W cause the attack to come from the syn face, while electron-donating groups cause it to come from the anti face. In 5,6-disubstituted norborn-2-en-7-one systems, the carbonyl appears to tilt away from the 7i-bond, with reduction occurring from the more hindered face. An ab initio study of nucleophilic addition to 4-ferf-butylcyclohexanones attempted to predict 7i-facial selectivity in that system. ... 

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. 

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