Dimethylamino-group, stabilizing effect

Full -polarization in diradicals can give rise to zwitterionic products. First examples were studied in detail by Carpenter and coworker who investigated solvent effects on rates and product distribution in Myers-Saito cyclizations.64 Polar solvents and substitution patterns that stabilize either positive or negative charges (or both) favor the zwitterionic products. For example, the presence of a dimethylamino group leads to stabilization of cations and isolation of pyrrolo-quinolines, rather than pyrido-indoles from eneyne-carbodiimides, as reported by Wang and coworkers (Scheme 14).65... 

Only one other substituent is capable of stabilizing trityl cations more effectively than amino or dimethylamino groups namely, the oxyanion substituent O . The stabilization of the trityl cation form of the dianion of the indicator dye phenolphthalein depends on the presence of two such substituents ... 

Finally, two or even three amino or dimethylamino groups in the para position stabilize trityl cations so efficiently because of their pronounced electron-donating resonance effect that the associated non-ionized neutral compounds are no longer able to exist at all but heterolyze quantitatively to salts. These salts are the well-known tri-phenylmethane dyes. 

It is tempting to assume, that the facile formation of silylene 2 from cyclotrisilane 1 is due to the effective stabilization of 2 by intramolecular coordination of the dimethylamino group to the silicon centre [10], which should lower the activation energy of a dissociation process from 1 to 2. Reaction of 1 with benzylvinylether resulted in a complex reaction mixture, from which 12 % of vinylsilane 4 was isolated 4 is presumably formed by rearrangement of the unstable oxy-substituted silacyclopropane 3c. 

The inhibitions described above occurred only when the analog and polynucleotide contained complementary bases. These combinations are not the only ones in which the interaction can occur, e.g., affinity methods detect some interaction between the non-complementary poly-9-vinyladenine and polyadenylate Apparently, such complexes are too unstable to affect the enzymatic reactions nevertheless, extensive modification of the analog can increase the stability of the polymer-polynucleotide complex to the point where such a polymer can effectively inhibit the reaction. Thus, omisssion of the amino group from poly-9-vinyladenine leads to poly-9-vinylpurine and the latter polymer inhibits the reverse transcription of polyadenylate and polyuridylate The introduction of a dimethylamino group in place of the amino group of poly-9-vinyladenine abolMies all of its inhibitory effects All these effects can be correlated with the ability of polymers to form complexes with templates. 

The stabilizing effect of the 7t-donor dimethylamino group is clearly reflected in the preparation of the heptahexaenylidene complexes M C C C C C C C (NMe2)2 (CO)5] (M = Cr, W) (75), which represent the longest metallacumulenylidenes isolated to date (Scheme 23) [69,70]. They were synthesized from triyne Me SiiCsCj SiMe by a sequential five step reaction (overall yield 3%), analogous to that discussed previously in the preparation of their pentatetraenylidene counterparts 72. 

The stabilizing role of other functional groups can also be described in resonance terms. Both electron-attracting groups such as carbonyl and cyano and electron-donating groups such as methoxy and dimethylamino have a stabilizing effect on a radical intermediate at an adjacent carbon. The resonance structures that depict these interactions indicate delocalization of the unpaired electron onto the adjacent substituents ... 

Although steric effects and strain always work against basicity in simple molecules and monocyclic compounds, there are a few compounds in which the steric and strain effects stabilize the H adduct, increasing the basicity. An example is the 1.8-bis(dimethylamino)naphthalene molecule in which steric hindrance of the methyl groups and repulsions of lone pairs on the nitrogen destabilize the free base ... 

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