Sharmistha Karmakar

Understanding the Mechanisms of Unusually Fast HH, CH, and CC Bond Reductive Eliminations from Gold(III) Complexes

Carbon-carbon bond reductive elimination from gold(III) complexes are known to be very slow and require high temperatures. Recently, Toste and co-workers have demonstrated extremely rapid CC reductive elimination from cis-[AuPPh3 (4-F-C6 H4 )2 Cl] even at low temperatures. We have performed DFT calculations to understand the mechanistic pathway for these novel reductive elimination reactions. Direct dynamics calculations inclusive of quantum mechanical tunneling showed significant contribution of heavy-atom tunneling (>25 %) at the experimental reaction temperatures. In the absence of any competing side reactions, such as phosphine exchange/dissociation, the complex cis-[Au(PPh3 )2 (4-F-C6 H4 )2 ](+) was shown to undergo ultrafast reductive elimination. Calculations also revealed very facile, concerted mechanisms for HH, CH, and CC bond reductive elimination from a range of neutral and cationic gold(III) centers, except for the coupling of sp(3) carbon atoms. Metal-carbon bond strengths in the transition states that originate from attractive orbital interactions control the feasibility of a concerted reductive elimination mechanism. Calculations for the formation of methane from complex cis-[AuPPh3 (H)CH3 ](+) predict that at -52 °C, about 82 % of the reaction occurs by hydrogen-atom tunneling. Tunneling leads to subtle effects on the reaction rates, such as large primary kinetic isotope effects (KIE) and a strong violation of the rule of the geometric mean of the primary and secondary KIEs.

Tunneling Assists the 1,2‐Hydrogen Shift in N‐Heterocyclic Carbenes

At room temperature, 1,2-hydrogen-transfer reactions of N-heterocyclic carbenes, like the imidazol-2-ylidene to give imidazole is shown to occurr almost entirely (>90 %) by quantum mechanical tunneling (QMT). At 60 K in an Ar matrix, for the 2, 3-dihydrothiazol-2-ylidene→thiazole transformation, QMT is shown to increase the rate about 10(5)  times. Calculations including small-curvature tunneling show that the barrier for intermolecular 1,2-hydrogen-transfer reaction is small, and QMT leads to a reduced rate of the forward reaction because of nonclassical reflections even at room temperature. A small barrier also leads to smaller kinetic isotope effects because of efficient QMT by both H and D. QMT does not always lead to faster reactions or larger KIE values, particularly when the barrier is small.

Role of quantum mechanical tunneling on the γ-effect of silicon on carbenes in 3-trimethylsilylcyclobutylidene.

Quantum mechanical tunneling (QMT) is increasingly being realized as an important phenomenon that can enhance the rate of reactions even at room temperature. Recently, the ability of a trimethylsilane (TMS) group to activate 1,3-H shift to a carbene from a γ-position has been demonstrated. Direct dynamical calculations (using canonical varitational transition state theory) inclusive of small curvature tunneling (CVT-SCT) show that QMT plays a decisive role in such 1,3-hydrogen migration in both the presence and absence of TMS. The presence of a TMS group reduces the activation energy of 1,3-H shift reaction via 1,3-equatorial interaction of the TMS group with the carbene. Tunneling across the smaller barrier enhances the overall forward rate of the reaction. The Arrhenius plot for the reaction shows substantial curvature in comparison to the CVT mechanism at room temperature. Arrhenius plots for the kinetic isotope effects (KIEs) for the γ-deuterated and per deuterated 3-trimethylsilylcyclobutylidene also show strong deviations from the classical over the barrier mechanism. The magnitude of the KIE is suggestive of QMT from the vibrational excited states of the carbenes.

Strain Control: Reversible H2 Activation and H2/D2 Exchange in Pt Complexes

Experiments have indicated that bulky ligands are required for efficient H2 activation by Pt-Sn complexes. Herein, we unravel the mechanisms for a Pt-Sn complex, Pt(Sn(t)Bu3)2(CN(t)Bu)2 (1a), catalyzed reversible H2 activation. Among a number of Pt-Sn catalysts used to model H2 activation and H2/D2 exchange reactions, only 1a with large strain was found to be suitable because the addition of H2 to 1a requires lowest distortion energy, minimal structural changes, and smallest entropy of activation. The activity of this Pt-Sn complex was compared vis-à-vis its Pt-Ge and Pt-Si analogues, and we predicted that strained Pt-Ge complex can efficiently activate H2 reversibly. Direct dynamics calculations for the rate of reductive elimination of H2, HD, and D2 from Pt(Sn(t)Bu3)(CN(t)Bu)2H3 (4a) and Pt(Sn(t)Bu3)(CN(t)Bu)2HD2 (4a([2D])) shows that H/D atom tunneling contributes significantly, which leads to an enhanced kinetic isotope effect. Strain control is suggested as a design concept in H2 activation.

Tunneling Control: Competition between 6π-Electrocyclization and [1,5]H-Sigmatropic Shift Reactions in Tetrahydro-1H-cyclobuta[e]indene Derivatives

Direct dynamics calculation using canonical variational transtition state theory (CVT) inclusive of small curvature tunneling (SCT) reveals the influential role of quantum mechanical tunneling (QMT) for 2,2a,5,7b-tetrahydro-1H-cyclobuta[e]indene derivatives (2a-2j) in governing their product selectivity. 2a-2j follow two distinct reaction channels, namely, 6π-electrocyclization (2 → 3) and [1,5]H-sigmatropic shift (2 → 4), among which the activation barrier is higher for [1,5]H-shift (2 → 4), thereby favoring the kinetically controlled product (3a-3j) as anticipated. However, SCT calculations show that a narrower barrier and smaller mass of participating atoms make QMT more pronounced for [1,5]H-shift reaction despite its higher activation energy, which results in a competition between kinetic controlled (2 → 3) and tunneling controlled (2 → 4) products. At low temperature (T ≤ 170 K), when QMT is the dominant pathway, the tunneling controlled product (4a-4j) is formed exclusively. As the reaction temperature increases, the role of QMT becomes less prominent and eventually gets kinetically controlled at room temperature. Nevertheless, QMT strongly tunes the product ratio at ambient temperatures by favoring the [1,5]H-shift reaction over 6π-electrocyclization. For 2a, k[1,5]H-shift:k6π-electrocyclization increases from 1:13 at CVT level to 1:2 at CVT+SCT level for room temperature.

Role of Heavy Atom Tunneling in Myers–Saito Cyclization of Cyclic Enyne-Cumulene Systems

Direct dynamics calculation using canonical variational transtition state theory (CVT) inclusive of small curvature tunneling (SCT) reveals heavy atom tunneling in Myers-Saito cyclization of 10- and 9-membered cyclic enyne-cumulene systems like 1,6-didehydro[10]annulene and derivative of neocarzinostatin, respectively. The pure density functional theory functional, BLYP at a 6-31+G (d,p) basis set reproduce the observed reaction energies and barriers within 1.0 kcal/mol. The calculated rate constants of cyclization inclusive of heavy atom tunneling (k(CVT+SCT) = 3.26 × 10(-4) s(-1) at 222 K; t1/2 = 35 min) are in excellent agreement with experiments (t1/2 ∼ 21-31 min). Both primary and secondary kinetic isotope effect (KIE) become enhanced significantly upon inclusion of quantum mechanical tunneling. An Arrhenius plot of KIE shows measurable curvature at the experimental temperature of 222 K. The translation vector for the cyclization reactions in the transition-states (TS) show significant motion of primary and secondary carbon atoms explaining the origin of large KIE.

Understanding the Reactivity of CO3·– and NO2· Radicals toward S-Containing and Aromatic Amino Acids

The reactivity of CO3·- and NO2· radicals toward six amino acid side chains namely, cysteine (Cys), methionine (Met), phenylalanine (Phe), tyrosine (Tyr), histidine (His), and tryptophan (Trp), has been explored using state-of-art density functional theory (DFT) and transition state theory (TST). Three reaction mechanisms, namely hydrogen atom abstraction (HAT), radical adduct formation (RAF), and single electron transfer (SET), have been considered for detailed study. While CO3·- radical is highly reactive toward majority of amino acids, the reactivity of NO2· radical is limited. The CO3·- radical creates oxidative damage to amino acid residues predominantly via HAT mechanism with moderate to high rate constant. Kinetic data suggest that tryptophan and tyrosine moiety possess the highest reactivity while the phenylalanine furnishes slow reaction. On the other hand, NO2· radical cannot produce direct damage toward most of the amino acids except tryptophan and histidine. The NO2· radical reacts exclusively by SET mechanism with 6.01 × 106 M-1 s-1 and 4.69 × 102 M-1 s-1 rate constant for Trp and His, respectively. Therefore, the CO3·- radical may cause severe damage to amino acid side chains during oxidative stress conditions, whereas the NO2· radical is mostly inert. Moreover, the reaction of CO3·- and NO2· radicals with amino acid radical intermediates generate variety of oxidation and nitro products which explain the formation of different experimentally characterized biomarkers during oxidative stress.