The gas-phase proton affinities (PAs) for a set of molecules with vinamidine structure are considered and their basicities in acetonitrile. It is shown that introducing double bonds to the imidazoline rings at the proton attachment site resulted in decrease in PA of the parent vinamidine. The increase in PA can be obtained by inducing modifications to the imidazoline ring at the junction of two diazepinylium rings. Placing methyl and dimethylamino substituents on the perimeter of the molecule further increased their gas-phase PAs. Studied vinamidine molecules are superbases, which possess PA values in the range between 261.0 and 284.2 kcal mol -1 in the gas phase and pK a values of 24.6-31.9 units in acetonitrile. Dismembering proton attachment site by opening the two diazepinylium and imidazoline rings resulted in the largest drop in PA values, indicating its importance in constraining the positions of imino nitrogen atoms in the neutral form of the molecule. Vinamidine molecules studied here present important pieces of the ladder of highly basic organic compounds for they possess accessible vinamidine molecular framework.
Abstract. M06-2X/6-311++G(2df,2pd)//M06-2X/6-31+G(d) computations were employed to investigate the intrinsic gas phase basicity of strained nitrogen heterocycles involving aziridine, azetidine, pyrrolidine and piperidine, together with their N-methyl and N-phenyl derivatives, NR(CH 2 ) n (n = 2-5; R = H, Me and Ph). Basicity constants were compared with the corresponding acyclic counterparts, NR(CH 3 ) 2 (R = H, Me and Ph), and were, based on triadic analysis, resolved into contributions mirroring features of both initial base and the final protonated form as well as their interplay, thus offering quantitative insight into the obtained results. In general, the N-methyl derivatives provided strongest bases investigated here, and, within each group of molecules, the basicity increased on going from three-to six-membered rings, consistent with a decrease in ring strain, with four-membered systems already surpassing or coming close to the basicity of the acyclic gauge molecule. Calculated basicities were found in a very good agreement with available experimental data, except for N-methylazetidine, where a remarkable discrepancy was revealed, suggesting that this system should be experimentally reassessed and its gas-phase basicity parameters revised. Triadic analysis showed different behaviour of individual contributions governing basicities, both among and within distinct families of molecules. It also convincingly demonstrated that, if a proper and a quantitatively accurate interpretation of observed basicity trends is desired, one should not rely only on concepts such as localized reactive hybrid orbitals (RHO) or thus derived nitrogen electron-donating ability (T. Ohwada et al., J. Org. Chem. 69 (2004) 7486), which take into account only properties of the initial base in question, but rather consider protonation reaction in its entirety. (doi: 10.5562/cca2121)
Gas-phase proton affinities (PAs) and hydride affinities (HAs) of organic bases possessing an allene moiety and substituted with methyl, dimethylamino, cyano, and vinyl substituents were examined with the B3LYP/6-311+G(2df,p)//B3LYP/6-31G(d) model. It was shown that a number of superbases and hyperbases can be obtained, as well as the potent hydride sponges. Methyl or dimethylamino substituents increased the proton affinity of the parent molecule, and the cyano substituents increased its hydride affinity. When the vinyl substituents are placed on allene, both the hydride and the proton affinities increased. A disubstituted allene with two dimethylamino groups is the smallest studied superbase, whereas the allene tetrasubstituted with four vinyl groups gives the smallest superbase possessing only alkene substituents. By introducing the vinyl group as a repeating subunit, one can obtain dendritic structures with the investigated substituents determining its properties. By changing the dimethylamino with the cyano group, a dendrimeric molecule can change from a hyperbase with a proton affinity of 324.6 kcal mol–1 to a very strong hydride ion acceptor with a hydride affinity of 205.4 kcal mol–1, while possessing the same proton or hydride ion attachment site.
Quinolino[7,quinoline is a superbasic compound, with a pK aH in acetonitrile greater than that of 1,8-bis(dimethylaminonaphthalene) (DMAN), although its synthesis and the synthesis of its derivatives can be problematic. The use of halogen derivatives 4,9-dichloroquinolino[7,8h]quinoline (16) and 4,9-dibromoquinolino[7,8-h]quinoline ( 17) as precursors has granted the formation of a range of substituted quinolinoquinolines. The basicity and other properties of quinolinoquinolines can be modified by the inclusion of suitable functionalities. The experimentally obtained pK aH values of quinolino[7,8-h]quinoline derivatives show that N 4 ,N 4 ,N 9 ,N 9 -tetraethylquinolino[7,8-h]quinoline-4,9-diamine ( 26) is more superbasic than quinolino[7,8-h]quinoline. Computationally derived pK aH values of quinolinoquinolines functionalized with dimethylamino (NMe 2 ), 1,1,3,3-tetramethylguanidino (NC(NMe 2 ) 2 ) or N,N,N′,N′,N″,N″-hexamethylphosphorimidic triamido (NP(NMe 2 ) 3 ) groups are significantly greater than those of quinolino[7,8-h]quinoline. Overall, electron-donating functionalities are observed to increase the basicity of the quinolinoquinoline moiety, while the substitution of electron-withdrawing groups lowers the basicity.
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