Elektronendichteverschiebungen und Reaktionskonstanten mit entgegengesetzten Vorzeichen bei der Analyse von Dediazonierungsreaktionen mit Hilfe dualer Substituentenparameter

Coordinate Covalent C → B Bonding in Phenylborates and Latent Formation of Phenyl Anions from Phenylboronic Acid

The results are reported of a theoretical study of the addition of small nucleophiles Nu(-) (HO(-), F(-)) to phenylboronic acid Ph-B(OH)(2) and of the stability of the resulting complexes [Ph-B(OH)(2)Nu](-) with regard to Ph-B heterolysis [Ph-B(OH)(2)Nu](-) --> Ph(-) + B(OH)(2)Nu as well as Nu(-)/Ph(-) substitution [Ph-B(OH)(2)Nu](-) + Nu(-) --> Ph(-) + [B(OH)(2)Nu(2)](-). These reactions are of fundamental importance for the Suzuki-Miyaura cross-coupling reaction and many other processes in chemistry and biology that involve phenylboronic acids. The species were characterized by potential energy surface analysis (B3LYP/6-31+G*), examined by electronic structure analysis (B3LYP/6-311++G**), and reaction energies (CCSD/6-311++G**) and solvation energies (PCM and IPCM, B3LYP/6-311++G*) were determined. It is shown that Ph-B bonding in [Ph-B(OH)(2)Nu](-) is coordinate covalent and rather weak (<50 kcal.mol(-1)). The coordinate covalent bonding is large enough to inhibit unimolecular dissociation and bimolecular nucleophile-assisted phenyl anion liberation is slowed greatly by the negative charge on the borate's periphery. The latter is the major reason for the extraordinary differences in the kinetic stabilities of diazonium ions and borates in nucleophilic substitution reactions despite their rather similar coordinate covalent bond strengths.

The Structural Influence in the Stability of Polymer-Bound Diazonium Salts

Various new diazonium ions on a polymeric support that have a variety of counterions and complexation with crown ethers have been prepared, and the thermal stability of these resins over a larger temperature interval has been investigated. Nonisothermal kinetics applied to DSC data have been used predicting the lifetime of the resins.

Ab Initio Study of the SN1Ar and SN2Ar Reactions of Benzenediazonium Ion with Water. On the Conception of “Unimolecular Dediazoniation” in Solvolysis Reactions

The nucleophilic substitution of N2 in benzenediazonium ion 1 by one H2O molecule to form protonated phenol 2 has been studied with ab initio (RHF, MP2, QCISD(T)//MP2) and hybrid density functional (B3LYP) methods. Three mechanisms were considered: (a) the unimolecular process SN1Ar with steps 1 --> Ph+ + N2 and Ph+ + H2O --> 2, (b) the bimolecular process SN2Ar with precoordination 1 + H2O --> 1 x H2O, SN reaction 1 x H2O --> [TS]++ --> 2 x N2 and dissociation of the postcoordination complex 2 x N2 --> 2 + N2, and (c) the direct bimolecular process SN2Ar that bypasses precoordination and involves just the SN reaction 1 + H2O --> [TS]++ --> 2 + N2. The SN2Ar reactions proceed by way of a Cs symmetric SN2Ar transition state structure that is rather loose, contains essentially a phenyl cation weakly bound to N2 and OH2, and is analogous to the transition state structures of front-side nucleophilic replacement at saturated centers. In solvolysis reactions, all of these processes follow first-order kinetics, and the electronic relaxation is essentially the same. It is argued that "unimolecular dediazoniations" have to proceed by way of SN2Ar transition state structures because strict SN1Ar reactions cannot be realized in solvolyses, despite the fact that the Gibbs free energy profile favors the strict SN1Ar process over the SN2Ar reaction by 6.7 kcal/mol. It is further argued that the direct SN2Ar process is the best model for the solvolysis reaction for dynamic reasons, and its Gibbs free energy of activation is 19.3 kcal/mol and remains higher than the SN1Ar value. Even though the SN1Ar and SN2Ar models provide activation enthalpies and SKIE values that closely match the experimental data, the analysis leads us to the unavoidable conclusion that this agreement is fortuitous. While the experiments do show that the solvent effect on the activation energy is about the same for all solvents, they do not show the absence of a solvent effect. The ab initio results presented here suggest that the solvent effect on the direct SN2Ar dediazoniation is approximately 12 kcal/mol, and computation of solvent effects with the isodensity polarized continuum model (IPCM) support this conclusion.