1,4-addition transition states

Scheme 54 Difference of conformational change of the thiophene 1-oxide at the anti and syn addition transition states...

The potential energy surface for the reaction between ethylene and ClCH2ZnCl has been investigated, by a DFT (B3LYP) approach, as a model for the Simmons-Smith cyclopropanation reaction " the addition transition state corresponds to a three-centered structure and is 11 kcalmol" more favourable than for competing insertion. 

Interaction 7a features net electron donation from the alkene n orbital (HOMO) to the vacant carbene p orbital (LUMO), and tracks the electrophilic character of the carbene. Interaction 7b represents electron donation from the filled carbene a orbital (HOMO) to the vacant alkene ti orbital (LUMO) and reflects the carbene s nucleophilic character. Both interactions operate simultaneously in the addition transition state, but which one is dominant ... 

The energetically preferred direction of the asynchronous CCI2 addition transition state with propene (20) places the carbene s a orbital closer to Cl, and the substituents on the carbene directed toward The asymmetry of bonding to... 

Extensive mechanistic studies on the effect of substituents on hydride reductions of 2,4,6-triarylpyryliums with NaBELj and NaCNBH3 have been described. These studies determined first- and second-wave reduction potentials and also rates for each reaction. Comparisons of the data sets leads to the conclusion that charge neutralization in the hydride addition transition state precedes B-H bonding changes <2002JP0689>. 

The predicted isotope effects for the addition transition states are of about 1.030 at C3 and 1.008 at C2 and small KIEs for remaining carbons, as can be expected for a major bonding change at C3 and minor at C2. Predicted isotope effects for the ring-closure step are 1.012 at C2 near unity at C3. The value of 1.012 is near the low end of primary 13C KIEs, but consistent with relatively low isotope effects observed in other epoxidation reactions (vide infra). Overall, KIEs support the addition of a peroxy anion to the enone as the rate-limiting step. 

Ti-Beta was also applied in the selective transformation of a-pinene oxide to camphenolenic aldehyde.78 A selectivity of over 98% was observed in the gas-phase reaction that was explained as a combination of a Lewis catalysed reaction in the absence of Brpnsted acid sites. Furthermore the concentration of a-pinene oxide in the zeolite pores was found to be an important factor not ruling out additional transition state selectivity as well. 

SCHEME 107. Relative energies and geometries for the O- and C-addition transition state calculated for the addition of formaldehyde to the lithium enolate of acetaldehyde and the preferred conformations of aldol535... 

The Next Important Elements to Life Occur in Period 3 P and S are the smallest elements capable of multiple covalent bonds to C, O and N, and which also have available d-shells. The d-shells allow additional transition states and reaction mechanisms. P and S are particularly important in the capture, storage, and distribution of chemical energy. 

The oxidative addition transition state has a three-centered M H C geometry with an Ir-H-C angle of 93°. The activation barrier to C-H oxidative addition... 

A singlet carbene is inherently both an electrophile and a nucleophile, what is behaviorally decisive is whether, in the carbene/alkene addition transition state, it is the LUMO(carbene)/HOMO(alkene) or HOMO(carbene)/LUMO(alkene) interaction (cf., Fig. 5) which dominates and determines the electronic distribution. If the former interaction dominates, the carbene will exhibit electrophilic selectivity if the latter interaction is more important, nucleophilic selectivity will be observed. If both interactions are comparable, the carbene will display an ambiphilic selectivity pattern, in which it acts as an electrophile toward electron-rich alkenes, but as a nucleophile toward electron-poor alkenes. [8,69]... 

As a final reaction mechanism catalyzed by water, we consider the hydration/hydrolysis reaction of an imine, CH2NH. The reaction energetics are shown in Fig. 8. As a simple bimolecular reaction, the four-centered addition transition state structures leading to hydration and hydrolysis have high activation barriers. The presence of an additional water molecule, providing six-centered transition state structures, lowers the barrier heights significantly. It should be noted that the hydrated form of the imine is the more stable form of the imine in aqueous solutions, as indicated in Fig. 8. This is also true of formaldehyde, for which the hydrated form, CH2(0H)2, is the more stable. 

The role of steric interactions on anomalous stereoselectivity in the Wittig reaction has been examined. " DFT calculations combined with a distortion/interaction energy analysis show that the anomalous Z selectivity observed in Wittig reactions of o-substituted benzaldehydes is not caused by phosphorus-heteroatom interactions in the addition transition state but is predominantly steric in nature. The calculations reproduced correctly the stereoselectivity preferences for a wide range of reactant pairs as well as relative reactivities for different substituent types. An efficient synthesis of olefins by the coupling of stabilized, semi-stabilized, and non-stabilized phosphorus ylides with various carbonyl compounds in the presence of silver carbonate has been reported. Wittig olefination of aromatic, heteroaromatic, and aliphatic aldehydes (yields up to 97%) and a ketone (yield 42%) was demonstrated. 

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