Study Of Several Metal Complexes Catalyzed Carbon Dioxide Activation By Theoretical Calculation

Carbon dioxide(CO2) is considered to be an important cause of climate change, because of its accumulation in the atmosphere and greenhouse properties. Chemical use of CO2not only can slow the rate of accumulation of carbon dioxide in the atmosphere but also can deal with the depletion of raw materials in coal chemical and petrochemical industry as a new sustainable chemical raw material, having special significance for the development of C1chemistry. The organic reactions utilizing carbon dioxide include the reactions with unsaturated compounds, epoxides, etc. Various catalysts have been developed, however, the mechanism of these catalyst systems does not understand deeply. Nonetheless, the comprehensive understanding of the reaction mechanisms by experimental methods presents several challenges. Computational chemistry is considered as a new tool to complement the drawbacks of experimental measurement. More importantly, we can provide new insight into reaction mechanisms by use of computational chemistry.In this article, we seek to study the reaction mechanism on three important reaction in carbon dioxide fixation by employing density functional theory calculations and interpret that the origin of selectivity. The main contents and results are as follows.1. Domino reaction of asymmetrical alkynes with carbondioxide catalyzed by (NHC)Cu(I) complexThe calculations show that coupling of alkynes with (NHC)Cu(I)complex ([LCuH]) was the selectivity-determining step, though notthe rate-determining step. The results of calculations indicate thatthe coupling reactions of LCuH with the four model alkynes(PhC≡CMe (1A), PhC≡CtBu (1B), AnC≡CtBu (1C), and MbC≡CtBu (1D))give the same regioselectivity, although the four alkynes are verydifferent in terms of their electronic properties and steric properties.Regarding the regioselectivity observed in coupling reactions ofLCuH with a given alkyne substrate, the electronic factors are notthe sole reason. We found that the observed regioselectivity isdominated by both electronic factors and steric factors.Furthermore, the computational results are in good agreement withthe experimental results.2. Insertion of CO2into (PSiP)Palladium Allyl σ-bondWe have considered three possible modes of CO2insertion into(PSiP)Pd-allyl bond when phosphine substituent PPh2, that is, direct1,2-insertion mode, metallo-ene mode, and SE2mode. Themetallo-ene mode is the most favorable via the six-member ringtransition state, and the corresponding activation barrier iscomputed to be21.1kcal/mol at the level of M06/6-311+G(2d,p)(def2-QZVP for Pd). Our computation results are consistent with theexperimental product of β,γ-unsaturated carboxylic acids.Calculations by the ONIOM and EDA methods evaluate the stericand electronic effects induced by different phosphine substituents in (PSiP)Pd allyl complexes. As for the CO2insertion into(PSiP)Pd-allyl bond, the contribution of electronic effect is greaterthan that of steric effect when the phosphine substituents areP(i-Pr)2and PPh2, while with the phosphine substituent PMe2, thecontribution of steric effect is slightly greater than that ofelectronic effect.3. The cycloaddition of CO2to epoxides catalyzed byMagnesium(II) porphyrin complexThe reaction mechanism of CO2to epoxides catalyzed byMg(TPP)/TBAI has been studied using the DFT calculations at theM06level. Two possible mechanisms have been considered andcompared. The calculation results show that the cycloadditionproceeds through an easier way synergistically catalyzed byMg(TPP) and TBAI. The favorable reaction mechanism includes fourelementary steps: i) the coordination of epoxide to Mg(TPP), ii) thenucleophilic ring-opening of epoxide by I-, iii) the CO2insertion into ametal-alkoxy complex, and iv) the SN2-type ring-closing step. Inaddition, the other possible mechanism involving iodoformateattack is not feasible.For all interest reactions, the ring-closing from metalcarbonate species3is the rate-determining step. Noticely, thepreferred reaction pathway (Iαor Iβ) is substrate-dependent. Thesteric factors are predicted to be dominant for alkyl-substitutedepoxides (A-B). For substrates A-B, the ring-opening andring-closing favor at the unsubstituted carbon atom (β pathway),and this preference becomes more obvious upon increasing thesteric bulk at substituted carbon. This can be supported by a clear increase in the energy difference for ring-closing TS(3A-4A)α/βâ†'TS(3B-4AB) α/β(1.6â†'4.2kcal/mol). The epoxide with strongelectron-withdrawing group (CF3) also prefers β pathway. Forepoxide D (with strong electron-donating group) and styrene oxide E,the reaction is mainly controlled by electronic factors and prefers αpathway. It is explained that the resonance effect may beresponsible for stabilization of the charge resulting of iodide. Thereactivity of different epoxides with CO2catalyzed by Mg(TPP)/TBAIbased on our calculations is in good agreement with theexperimentally observed trend.