Adamantanes analysis

The unexpectedly complex product was isolated as an almost colourless air-stable powder, and a single-crystal X-ray analysis showed that it had the molecular adamantane-like structure (5). This is very similar to the structure of the iso-electronic compound P4O10 (p. 504). 

Calixarenes, which are macrocyclic compounds, are one of the best building blocks to design molecular hosts in supramolecular chemistry [158]. Synthesis of calix[4]arenes, which have been adamantylated, has been reported [105, 109]. In calix[4]arenes, adamantane or its ester/carboxylic acid derivatives were introduced as substituents (Fig. 29). The purpose of this synthesis was to learn how to employ the flexible chemistry of adamantane in order to construct different kinds of molecular hosts. The X-ray structure analysis of p-(l-adamantyl)thiacalix[4]arene [109] demonstrated that it contained four CHCI3 molecules, one of which was located inside the host molecule cavity, and the host molecule assumed the cone-like conformational shape (Fig. 30). 

Rhodopsin is a seven ot-helix trans-membrane protein and visual pigment of the vertebrate rod photoreceptor cells that mediate dim light vision. In this photoreceptor, retinal is the chromophore bound by opsin protein, covalently linked to Lys296 by a Schiff base linkage. Kpega et al.64 have studied NMR spectra of Schiff bases being derivatives of all-frans retinal and amino-p-cyclodextrins as a model of rhodopsin, where p-cyclodextrin plays a role of a binding pocket. On the basis of analysis of the chemical shift differences for the model compound in the presence and in the absence of adamantane carboxylate, it has been shown that the derivative of 3-amino-p-cyclodextrin forms dimer in water and retinoid is inserted into p-cyclodextrin cavity [31]. 

Dimerization and condensation of pentane-2,4-dione by molybdenum(VI) oxide tetrachloride (MoOC14) affords l,3,5,7-tetramethyl-2,4,6,8-tetraoxa-adamantane, the structure of which is determined by elemental analysis, IR, mass, and PMR spectroscopy.188... 

Analysis of the reaction KIE using monodeuterated sulfamate 106 reinforces the proposed rhodium-nitrene model (Eq. 9). The ratio of H/D products 107/108 is a direct measure of the KIE and has been found to equal 1.5 0.2 by integration of the C NMR spectrum. An equivalent value is obtained for nitrene C-H insertion upon photolysis of a carbamoyl azide, and is taken as evidence for a nonlinear transition state. As noted previously, Muller s use of NsN=lPh with partially deuterated adamantane gives... 

Gas-chromatographic and infrared analysis indicate no detectable amount of starting alcohol. Proton magnetic resonance of adamantane 2 (chloroform-d) yields a series of multiplets centered at 8 1.62, 1.86, 2.18. 

Early structural information about the simple alkyl radicals was obtained by Herzberg, who observed the ultraviolet spectra of CH3 and CD3 in the gas phase by flash photolysis.9 Analysis of the fine structure led to the conclusion that the methyl radical is planar, a deviation from planarity of not more than 10° being consistent with the data.10 The infrared spectrum of GH3 has also been recorded it does not yield conclusive structural information.11 Radicals trapped in a matrix of solid adamantane can be formed by X-irradiation of suitable precursors. In this way, spectra can be obtained at room temperature.12... 

Molecular mechanics calculations14 ) of the strain in adamantane support this interpretation. Since such calculations are based on empirically derived parameters, however, a unique set of parameters for the representation of physical reality cannot be derived. Nevertheless, all available conformational analysis calculations demonstrate that the strain in adamantane may be quantitatively accounted for in terms of angle strain, which implicitly includes strain due to... 

Oxygenation of 2-substituted adamantanes with methyl(trifluoromethyl)dioxirane showed a reaction constant, p = -2.31, consistent with a strongly electron-demanding transition state. Analysis of the effect of solvents on the rate yielded a positive regression coefficient with Dimroth-Reichardt Ej solvent polarity parameter. A mechanism involving an electrophilic O atom insertion has been postulated for the formation of alcohols and carbonyl compounds.201... 

The e.s.r. spectrum of a deposit formed by the deposition of hydrogen atoms on propylene in a matrix of adamantane is shown in Fig. 18. Comparison with the spectra of the two possible product radicals, n-propyl and isoprop3d, shows clearly that addition has occurred almost exclusively to the terminal CH2-group to form the isopropyl radical. The possibility that a small amount of n-propyl radical is present cannot be ruled out, but from an analysis of the e.s.r. spectrum it is conservatively 3... 

While cation 55 was discovered almost by chance, the dicationic species 57 was prepared by deliberate efforts. Theoretical analysis of the specific configuration of the orbitals at the bridgehead carbons of the adamantane framework encouraged these studies. The analysis predicted a very substantial degree of stabilization for 57, owing to the possibility of efficient overlap of all four orbitals of the bridgehead carbons inside the adamantane cavity. After some searching for an appropriate precursor, it was established that l,3-dehydro-5,7-... 

X-Ray structure analysis of 14a, 15a, and 15b has established the central adamantane cage which is characteristic of the unsolvated triamides [6, 8], Whereas there is little appreciable interaction between the periphery of the molecules in the structures of the t-butyl substituted species 15a and 15b, the two-coordinate Li-atoms are "internally solvated" by the phenyl groups in the chiral triamide 14a (Fig. 3). The orientation of the aryl rings towards the puckered six-membered (LiN)s-ring and the short contact between Li and C7 indicates a direct interaction between the metal atoms and the periphery. 

This theory is simple to apply and accounts for many experimental observations, with the proviso that there are likely to be situations where steric effects predominate. The Cieplak hypothesis is also supported by le Noble s work on 5-substituted adamantan-2-one attack of nucleophiles occurs at the syn face if the 5-substituent is an electron-withdrawing group [33] and at the anti face if it is a donor (see later section) [34]. A serious problem with the Cieplak analysis, which is relevant to all systems where this hypothesis has been applied, concerns the large energy difference between the adjacent CT-orbital(s) and the a orbital of the transition state, which will result in a minimum of orbital mixing and stabilisa-tion/destabilisation, and this is discussed in more detail later [35-38]. 

Another well-known rearrangement, that of hetisine (43) by acid catalysis, has been re-investigated in a similar fashion. In fact two products are formed, both adamantane-1ike in structure, in ratio 19 1. Their structures, settled by nmr-spectral and X-ray analysis, are (44) and (45) respectively. The former was isolated as an alkaloid of Aaonitum hetevophyl-Iwn earlier. A pathway for the rearrangement has been sugqested (Pelletier et al., Chem. Comm., 1981, 327 Hetero-cycles, 1983, 20, 413). 

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