Elimination Transition State Optimization

A transition state for the direct methane elimination from the Pt(IV) complex having two PH3 ligands was not observed. Phosphine loss occurred concomitantly with the reductive elimination. However, the authors were able to estimate an activation barrier of ca. 16 kcal/mol for direct elimination from this Pt(IV) complex (PH3)2Cl2PtCH3(H) using artificial restraints for the geometry optimization. This value is very close to the 16.5 kcal barrier obtained for reductive elimination... 

One of the steric requirements of E2 elimination is the need for periplanar geometry, which optimizes orbital overlap in the transition state leading to alkene product. Two types of periplanar arrangements of substituents are possible — syn and anti. 

The explanation is the same a solvent of low polarity favours ion pairing. Thus, in solvents of high polarity, the base forms a cyclic transition state with the substrate directly, without the involvement of its corresponding cation, i.e. the transition state is now a five- and not a six-membered ring, and so the base is now in an optimal position to remove the syn-hydrogen. Conversely, in solvents of low polarity with large amounts of ion pairing, the presence of the cation interferes with the abstraction of the proton and so anti-elimination once more predominates. 

The optimized transition states for the elimination of H2 from XH5 (D31,) possessed Cs symmetry. In the case of X = P, As and Sb, elimination occurs by an equatorial-equatorial process. In the case of X = Bi, elimination occurs by a zwitterionic apical-equatorial process, this being related to the high exothermicity (- 73 kcal/mol) of the reaction BiHs (Dsh) —BiHs (Csv) + H2, instead of - 45 to - 50 kcal/mol for PH5, AsHs and SbH5.49... 

An E2 reaction involves the removal of two groups from adjacent carbons. It is a concerted reaction because the two groups are eliminated in the same step. The bonds to the groups to be eliminated (H and X) must be in the same plane because the sp orbital of the carbon bonded to H and the sp orbital of the carbon bonded to X become overlapping p orbitals in the alkene product. Therefore, the orbitals must overlap in the transition state. This overlap is optimal if the orbitals are parallel (i.e., in the same plane). 

Another catalytic cycle studied by Matsubara, Morokuma, and coworkers [77] is the hydroformylation of olefin by an Rh(I) complex. Hydroformylation of olefin by the rhodium complex [78-80] is one of the most well known homogeneous catalytic reactions. Despite extensive studies made for this industrially worthwhile reaction [81, 82], the mechanism is still a point of issue. The active catalyst is considered to be RhH(CO)(PPh3)2, 47, as presented in Fig. 25. The most probable reaction cycle undergoes CO addition and phosphine dissociation to generate an active intermediate 41. The intramolecular ethylene insertion, CO insertion, H2 oxidative addition, and aldehyde reductive elimination are followed as shown with the surrounding dashed line. Authors have optimized the structures of nearly all the relevant transition states as well as the intermediates to determine the full potential-... 

Figure 11 shows the fully optimized geometry of the transition state of reaction (17) as well as the calculated energies. The results indicate that the transition state is quite similar to that of reaction (16), with a long transitional C-C bond (2.256 A) and productlike structure. The calculated Ea is 34.96 kcal/mol at the PMP2/6-31G level, and 33.24 kcal/mol at the B3LYP/6-31G level. Both of them are very close to that of reaction (16). It can be concluded that without other reactions (H-transfer reaction, termination reaction, and addition reaction), the radical decomposition will proceed in a chain reaction fashion until the P position is eliminated. 

Under the optimized conditions, allyl 2-bromophenyl ethers were treated with 2.0 equiv. of tert-butyllithium in diethyl ether at —78 °C to afford allyl 2-lithiophenyl ethers 166, which were stable at low temperature. Addition of TMEDA to the reaction mixture, followed by warming to 0°C, and subsequent treatment with various electrophiles led to the functionalized dihydrobenzofuran derivatives 165 in moderate to good yields (Scheme 10.53, path A). It is interesting to note that the 1,3-elimination pathway (Scheme 10.53, path B) could be avoided. Furthermore, this intramolecular carbolithiation reaction was completely diastereoselective, and only the trans diastereomers were obtained. This stereochemistry was explained considering the chairlike transition state 166 in which the a-substituent predominantly occupies a pseudoequatorial position resulting in high levels of... 

A theoretical investigation has been performed by Bickelhaupt et al. [88] on the gas phase reaction of F + Ca/fsF. For this reaction the geometries of the reactants, reactant complexes, transition states, product complexes and the products for anti-E2 and syn-E2 as well as for Sn2 pathways were optimized with the Xa potential [23]. The energetically favoured products FHF + C2-H4 are formed in an anti as well as in a syn E2 mechanism. In their study Bickelhaupt et al. presented a qualitative MO theoretical analysis enabling to understand and to predict which reaction - the elimination E2 or the substitution 5W2 - dominates for a given substrate C2HSL. From their results they concluded that the base plays a key role as a catalyst which strongly influences the competition between syn and anti elimination. Its catalytic effect consists in a considerable stabilization of the transition state. 

Fig. 6.11 Optimized transition state structure for the reductive elimination reaction through the route A-syn. Only the Me group of 2 is displayed using a baU-and-stick representation for clarity. Relevant distances ate shown in A...

Fig. 6.13 Optimized structures for the intermediates and transition states involved in the reductive elimination in B-syn route. Only the Me group of 2 is represented in ball-tind-stick for clarity. The stereochemistry of the product that results from each transition is shown in parenthesis...

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