Exocyclic bond

Ring size Exocyclic bonds Endocyclic bonds ... 

The Ciamician-Dennstedt reaction can be thought of as the complement to the Reimer-Tiemann reaction (Scheme 8.3.2). The first step of both reactions is cyclopropanation of one of the carbon-carbon double bonds of a pyrrole with a dichlorocarbene, resulting in intermediate 3. The Ciamician-Dennstedt reaction results from cleavage of the internal C-C bond and elimination of chloride (path a), while the Reimer-Tiemann reaction results from cleavage of the exocyclic bond, and subsequent hydrolysis of the dichloromethyl moiety to furnish aldehyde 5 (path b). 

Obviously, the NMR pattern of the QM moiety depends on its coordination mode. As expected, in case of ring coordination, such as r -coordination or r -coordination,16 17 the ring signals are affected, while in the case of exocyclic bond coordination, the signals of the exocyclic bond are influenced.6 14 18 19... 

As an explanation, if was suggested that the degree of charge development in the transition state determines the preferred site of cyclopropanation A transition state with little charge development should prefer the endocyclic double bond (Pd catalysis), whereas one with much charge development should favor the exocyclic bond (Rh catalysis). 

Semi-empirical calculations at the AMI level performed on 2-amino-5-(4-pyridyl)-l,3,4-thiadiazole determined the most stable conformational arrangement between the heterocyclic unit and the central bridge as well as the rotational barrier around the C(heterocyclic)-N(exocyclic) bond <2001LC1659>. 

Nitrogen. Krygowsky et al. applied their HOMA aromaticity criterion as well as the NICS index to pyrazoles132 and found that the aromaticity of pyra-zole decreases when the double-bond character of the exocyclic bonds to substituents in the 3- and 4-positions increases. 

It is clear that the function U ( qint ) tmy be approximated by an expression of the form of eqn. (6). Whether a potential of Ais form, involving no explicit description of the solvent, is appropriate depends on the relative relaxation rates of the solvent motions and the macromolecular intramolecular coordinates. For the slow, conformationally most significant, glycosidic and exocyclic bond rotations of the carbohydrate it is apparent Aat averaging of solvent motions can occur easily on the time scale of these torsions. It is more ficult, however, to know how much important conformational detail is submerged by the averaging process. 

A pathway (Scheme I) (8.9) for the hydrolysis of oligoglycosides by lysozyme that differs from the previously accepted mechanism (Scheme II) (3,10-12) is described in this section. The alternative pathway, suggested by results of a 55-ps MD simulation of the lysozyme (GlcNAc)e complex (1), is consistent with the available experimental data and with stereoelectronic considerations. Experimental data have demonstrated that Glu 35 and Asp 52 are essential, as shown by recent site-directed mutagenesis results (13.) which corroborate chemical modification studies (3.14 and references cited therein), and that the reaction proceeds with retention of configuration at Ci Q and references cited therein). A fundamental feature of the alternative pathway is that an endocyclic bond is broken in the initial step, in contrast to the exocyclic bond cleavage in the accepted mechanism. 

The tributyltin hydride reduction of dihaloaziridines, e.g. (266), represents another example where the ring system has been maintained (79CJC1958). Especially noteworthy is the retained configuration associated with the reaction. This behavior differs from the cyclopropyl analog and was explained on the basis of increased s-character in the exocyclic bond caused by the nitrogen atom. 

The Hammett p-value for cleavage of the exocyclic bond of 2-methoxy-2-substituted-phenyl-l,3-dioxolans (—1.58 + 0.06) is a little larger than that for cleavage of the endocyclic C— bond of 2-hydroxy-2-substituted-phenyl-l,3-dioxolans (—1.24 + 0.04) (Table 9) (Chiang et al., 1983). A direct comparison between the p-values for C—OMe bond cleavage of trimethyl orthobenzoates and dimethyl hemiorthobenzoates is not possible at present since they have not been measured in the same solvent. However, that based on H+ for the breakdown of the hemiorthobenzoates (— 1.58) is less than that based on the equilibrium constants for their conversion into methyl benzoates and methanol which is —1.9 (derived from the equilibrium constants for formation of the hemiorthobenzoates, McClelland and Patel, 1981b). This implies that the development of positive charge in the transition state is less than in the final product, the ester. 

The absorption frequency of external exocyclic bonds increases with decreasing ring size. Methylene-cyclohexane absorbs at 1650 cm-1, methylenecyclopro-pane at 1781 cm-1. 

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