Quaternary exocyclic carbon

In the case of r)2-coordination of the exocyclic C=C bond, it becomes substantially elongated compared with the double bond of free alkenes, as a result of back donation from the metal to the 7t orbitals of the double bond. For instance, in complex 17b the coordinated bond length is 1.437 A (see Fig. 3.2).18 This is also reflected in the loss of planarity around the quaternary exocyclic carbon, the methylenic carbon being bent out of the ring plane by 10.78°.18 Similar structural features were also observed with other P2Pd conjugated olefin complexes.39... 

The p-carbon signals in heterocyclohexanes 52 (143) respond to the electronegativity of endocyclic substituents similarly to those of carbon atoms in p positions with respect to exocyclic substituents. The slopes of chemical shift vs. electronegativity plots for the p-methylene groups in 52 and 53 are negative (Table 10). For the quaternary P-carbon atom in 53, however, a slight positive slope is observed (164). 

The intermediacy of an anhydro base (57) was referred to in Scheme 46. Analogous anhydro bases (pyridone methides) can be formed by deprotonation of quaternary salts of 2- and 4-benzylpyridines and the like. The pyridone methides are usually highly reactive and not readily isolable some stable examples are shown in Scheme 49. Pyridine methides are intermediates in the base-catalyzed alkylation and acylation reactions of pyridinium salts at the exocyclic carbon. Compounds of type (60) have been estimated to have 25-30% dipolar character. Protonation of (60) occurs at the 2 - and 3 -positions in the ratio 4 1 respectively (70JCS(C)800). 

In cases of complexes bearing an exocyclic double bond directly coordinated to the metal center, the carbons of the double bond usually exhibit coupling with NMR-active metal centers and/or auxiliary ligands.6 14 18 19 The chemical shifts of the quaternary carbon atom vary from 66.976 to 82.2818 ppm, while the methylene group gives rise to signals at 29.16,14 41.91,6 or 51.3418 ppm in the 13C 1H NMR spectra. As one can see, the chemical shift variation is relatively broad and significantly affected by the nature of the metal center. 

Quaternary salts (90) from azalenes obtained from heterocyclic bases and phenacyl halides or derivatives of a-halo carbon acids also contain an acidic exocyclic CH2 group. Therefore, during the deprotonation, mixtures of pseudoazulenes 91 and ylides 98 can be formed, especially if Y = CH. °5 Quaternary salts (90) containing one pyrrole-type nitrogen (Y = N) de-protonate exclusively to pseudoazulenes166 (Scheme 6). Analogous behavior... 

Hetisine Reaction. In an attempt to convert hetisine to kobusine the former was transformed into the mesylate (40). When this was treated with lithium aluminium hydride the mesylate was removed as expected but the exocyclic methylene group was replaced by a deshielded methyl group on quaternary carbon (5 1.18). The product appeared to be the cyclopropane derivative (41), since oxidation of the hydroxy-group gave a ketone with v, ax 1680 cm and Amax 204 nm ( 5400). This was confirmed by an X-ray analysis of the methiodide of (41). The authors suggest that co-ordination of the mesylate groups with some aluminium species assists the process illustrated in (40). 

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