Brian F. Yates

Distonic radical cations : Guidelines for the assessment of their stability

Abstract Ab initio molecular orbital calculations on the distonic radical cations CH2(CH2)nN+H3 and their conventional isomers CH3(CH2)nNH2+ (n = 0,1, 2 and 3) indicate a preference in each case for the distonic isomer. The energy difference appears to converge with increasing n towards a limit which is close to the energy difference between the component systems CH3·H2+CH3+NH3 (representing the distonic isomer) and CH3CH3+CH3NH2+ (representing the conventional isomer). The generality of this result is assessed by using results for the component systems CH3·Y+CH3X+H and CH3YH+CH3X+. (or CH3YH+. + CH3X) to predict the relative energies of the distonic ions ·Y(CH2)nX+H and their conventional isomers HY(CH2)nX+. (X = NH2, OH, F, PH2, SH, Cl; Y = CH2, NH, O) and testing the predictions through explicit calculations for systems with n = 0,1 and 2. Although the predictions based on component systems are often close to the results of direct calculations, there are substantial discrepancies in a number of cases; the reasons for such discrepancies are discussed. Caution must be exercised in applying this and related predictive schemes. For the systems examined in the present study, the conventional radical cation is predicted in most cases to lie lower in energy than its distonic isomer. It is found that the more important factors contributing to a preference for distonic over conventional radical cations are the presence in the system of a group(X) with high proton affinity and the absence of a group (X, Y or perturbed (C—C) with low ionization energy.

Cu-Catalyzed Fluorination of Diaryliodonium Salts with KF

A mild Cu-catalyzed nucleophilic fluorination of unsymmetrical diaryliodonium salts with KF is described. This protocol preferentially fluorinates the smaller aromatic ligand on iodine(III). The reaction exhibits a broad substrate scope and proceeds with high chemoselectivity and functional group tolerance. DFT calculations implicate a Cu(I)/Cu(III) catalytic cycle.

An Assessment of Theoretical Protocols for Calculation of the pKa Values of the Prototype Imidazolium Cation

The highly accurate complete basis set method CBS-QB3 has been used in conjunction with the conductor-like polarized continuum (CPCM) method to predict the aqueous pKa values for the three different hydrogen atoms in the imidazolium cation. Excellent agreement was obtained with the available experimental values. The pKa for the deprotonation of imidazole was also calculated and found to be quite different from the experimental estimate. The protocol for the pKa calculation was carefully analyzed and some recommendations made about the choice of levels of theory.

Subtle Balance of Ligand Steric Effects in Stille Transmetalation

Experimental results have previously suggested that the transmetalation step in the Stille reaction is hindered at one extreme by very bulky ligands L on the PdL(2) catalyst, yet at the other extreme, transmetalation is also found to be slow for small ligands. Our aim in this paper is to resolve this dilemma using computational chemistry and to show which ligand is best and why. With the use of density functional theory we show that the reason why L = P(t)Bu(3) retards transmetalation is because the bulky ligand hinders the coordination of the organostannane. On the other hand a small ligand such as L = PMe(3) leads to the formation of a very stable intermediate in the catalytic cycle which then requires a large activation energy for the transmetalation to proceed. The L = PPh(3) ligand appears to provide just the right balance in that it can readily coordinate the organostannane but avoids forming the very stable intermediate, and is thus the ligand of choice. L = PPh(2)Me is predicted to be the next best option, but L = PPhMe(2) is too small and forms an intermediate whose stability prevents further reaction in the transmetalation step. Our calculations are also able to account for the accelerating role of CsF in the transmetalation step of the Stille reaction. Finally, this work demonstrates the importance of taking into account the steric properties of the full ligand in theoretical studies of such reactions, rather than using small model phosphines.

Ligand design for α1 adrenoceptor subtype selective antagonists

Abstract α1 Adrenoceptors have three subtypes and drugs interacting selectively with these subtypes could be useful in the treatment of a variety of diseases. In order to gain an insight into the structural principles governing subtype selectivity, ligand based drug design (pharmacophore development) methods have been used to design a novel 1,2,3-thiadiazole ring d analogue of the aporphine system. Synthesis and testing of this compound as a ligand on cloned and expressed human α1 adrenoceptors is described. Low binding affinity was found, possibly due to an unfavourable electrostatic potential distribution. Pharmacophore models for antagonists at the three adrenoceptor sites (α1A, α1B, α1D) were generated from a number of different training sets and their value for the design of new selective antagonists discussed. The first preliminary antagonist pharmacophore model for the α1D adrenoceptor subtype is also reported.

Theoretical Investigation into the Mechanism of Reductive Elimination from Bimetallic Palladium Complexes

Reductive elimination of C-Cl and C-C bonds from binuclear organopalladium complexes containing Pd-Pd bonds with overall formal oxidation state +III are explored by density functional theory for dichloromethane and acetonitrile solvent environments. An X-ray crystallographically authenticated neutral complex, [(L-C,N)ClPd(μ-O(2)CMe)](2) (L = benzo[h]quinolinyl) (I), is examined for C-Cl coupling, and the proposed cation, [(L-C,N)PhPd(1)(μ-O(2)CMe)(2)Pd(2)(L-C,N)](+) (II), examined for C-C coupling together with (L-C,N)PhPd(1)(μ-O(2)CMe)(2)Pd(2)Cl(L-C,N) (III) as a neutral analogue of II. In both polar and nonpolar solvents, reaction from III via chloride dissociation from Pd(2) to form II is predicted to be favored. Cation II undergoes Ph-C coupling at Pd(1) with concomitant Pd(1)-Pd(2) lengthening and shortening of the Pd(1)-O bond trans to the carbon atom of L; natural bond orbital analysis indicates that reductive coupling from II involves depopulation of the d(x(2)-y(2)) orbital of Pd(1) and population of the d(z(2)) orbitals of Pd(1) and Pd(2) as the Pd-Pd bond lengthens. Calculations for the symmetrical dichloro complex I indicate that a similar dissociative pathway for C-Cl coupling is competitive with a direct (nondissociative) pathway in acetonitrile, but the direct pathway is favored in dichloromethane. In contrast to the dissociative mechanism, direct coupling for I involves population of the d(x(2)-y(2)) orbital of Pd(1) with Pd(1)-O(1) lengthening, significantly less population occurs for the d(z(2)) orbital of Pd(1) than for the dissociative pathway, and d(z(2)) at Pd(2) is only marginally populated resulting in an intermediate that is formally a Pd(1)(I)-Pd(2)(III) species, (L-Cl-N,Cl)Pd(1)(μ-O(2)CMe)Pd(2)Cl(O(2)CMe)(L-C,N) that releases chloride from Pd(2) with loss of Pd(I)-Pd(III) bonding to form a Pd(II) species. A similar process is formulated for the less competitive direct pathway for C-C coupling from III, in this case involving decreased population of the d(z(2)) orbital of Pd(2) and strengthening of the Pd(I)-Pd(III) interaction in the analogous intermediate with η(2)-coordination at Pd(1) by L-Ph-N, C(1)-C(2).

Experimental and computational study of a reductive elimination mechanism in a methyl–Pd(II)–CNC carbene complex

Experimental and density functional studies on the decomposition of a novel palladium-methyl complex of the rigid CNC ligand 2,6-bis(1-alkylimidazolin-2-yliden-3-yl)pyridine show that reductive elimination to give 2-methylimidazolium species is a facile reaction.

Cleavage of Carbon Dioxide by an Iridium-Supported Fischer Carbene. A DFT Investigation

The reaction of CO(2), OCS, and PhNCO with an iridium-supported Fischer alkoxycarbene has been investigated with density functional theory. We have confirmed the mechanism for the important CO(2) reaction and successfully rationalized the selective cleavage of the CS and CN bonds in OCS and PhNCO. Armed with this information we have used our model to predict that the same iridium system will preferentially cleave the CS bond in methyl thiocyanate (MeNCS) rather than the CN bond. The formation of the iridium-supported carbene itself has also been investigated and a fascinating autocatalytic mechanism has been discovered which nicely fits the observed experimental behavior.

Theoretical investigation into the mechanism of Au(I)-catalyzed reaction of alcohols with 1,5 enynes.

Density functional theory has been used to investigate the reactions of 1,5 enynes with alcohols in the presence of a gold catalyst. We have compared the mechanism of the alcohol addition reaction for the enyne with that of the enyne where the carbon at position 3 is replaced with silicon. We find that different intermediates are present in both cases, and in the case of the silicon analogue, the intermediate that we find from the calculations is different from any that have previously been proposed in the literature. For the silicon analogue we have been able to rationalize the observed effects of alcohol concentration and nucleophilicity on the product distribution. For the carbon-based enyne we have shown why different products are observed depending on the substitution at position 3 of the enyne. Overall, we have provided for the first time a consistent explanation and rationalization of several different experiments that have been previously published in the literature. Our mechanism will assist in predicting the outcome of experimental reactions involving different alcohols, reagent concentrations, and substitution patterns of the 1,5 enynes.

Ligand effects in bimetallic high oxidation state palladium systems

Ligand effects in bimetallic high oxidation state systems containing a X-Pd-Pd-Y framework have been explored with density functional theory (DFT). The ligand X has a strong effect on the dissociation reaction of Y to form [X-Pd-Pd](+) + Y(-). In the model system examined where Y is a weak σ-donor ligand and a good leaving group, we find that dissociation of Y is facilitated by greater σ-donor character of X relative to Y. We find that there is a linear correlation of the Pd-Y and Pd-Pd bond lengths with Pd-Y bond dissociation energy, and with the σ-donating ability of X. These results can be explained by the observation that the Pd d(z(2)) population in the PdY fragment increases as the donor ability of X increases. In these systems, the Pd(III)-Pd(III) arrangement is favored when X is a weak σ-donor ligand, while the Pd(IV)-Pd(II) arrangement is favored when X is a strong σ-donor ligand. Finally, we demonstrate that ligand exchange to form a bimetallic cationic species in which each Pd is six-coordinate should be feasible in a high polarity solvent.