Group 10 computational study

A recent paper by Singh et al. summarized the mechanism of the pyrazole formation via the Knorr reaction between diketones and monosubstituted hydrazines. The diketone is in equilibrium with its enolate forms 28a and 28b and NMR studies have shown the carbonyl group to react faster than its enolate forms.Computational studies were done to show that the product distribution ratio depended on the rates of dehydration of the 3,5-dihydroxy pyrazolidine intermediates of the two isomeric pathways for an unsymmetrical diketone 28. The affect of the hydrazine substituent R on the dehydration of the dihydroxy intermediates 19 and 22 was studied using semi-empirical calculations. ... 

The C2 symmetry of sulfide 13 means that a single diastereomer is formed upon alkylation (Scheme 1.10). Attack from the Si face of the ylide is preferred as the Re face is shielded by the methyl group cis to the benzylidene group (28). Metzner postulates that this methyl group also controls the conformation of the ylide, as a steric clash in 27b renders 27a more favorable [16]. However, computational studies by Goodman revealed that 27a was not particularly favored over 27b, but it was substantially more reactive, thus providing the high enantioselectivity observed... 

The detailed mechanism of this enantioselective transformation remains under investigation.178 It is known that the acidic carboxylic group is crucial, and the cyclization is believed to occur via the enamine derived from the catalyst and the exocyclic ketone. A computational study suggested that the proton transfer occurs through a TS very similar to that described for the proline-catalyzed aldol reaction (see page 132).179... 

Olefination Reactions Involving Phosphonium Ylides. The synthetic potential of phosphonium ylides was developed initially by G. Wittig and his associates at the University of Heidelberg. The reaction of a phosphonium ylide with an aldehyde or ketone introduces a carbon-carbon double bond in place of the carbonyl bond. The mechanism originally proposed involves an addition of the nucleophilic ylide carbon to the carbonyl group to form a dipolar intermediate (a betaine), followed by elimination of a phosphine oxide. The elimination is presumed to occur after formation of a four-membered oxaphosphetane intermediate. An alternative mechanism proposes direct formation of the oxaphosphetane by a cycloaddition reaction.236 There have been several computational studies that find the oxaphosphetane structure to be an intermediate.237 Oxaphosphetane intermediates have been observed by NMR studies at low temperature.238 Betaine intermediates have been observed only under special conditions that retard the cyclization and elimination steps.239... 

The reaction shows a dependence on the E- or Z-stereochemistry of the enolate. Z-enolates favor anti adducts and E-enolates favor syn adducts. These tendencies can be understood in terms of an eight-membered chelated TS.299 The enone in this TS is in an s-cis conformation. The stereochemistry is influenced by the s-cis/s-trans equilibria. Bulky R4 groups favor the s-cis con former and enhance the stereoselectivity of the reaction. A computational study on the reaction also suggested an eight-membered TS.300... 

The reactions occur preferentially though an endo TS in which the sulfur substituent is oriented toward the allylic group.273 Computational studies (MP2/6-31G ) found the endo TS to be favored over the exo by 1.5-2.2. kcal/mol.274... 

Perhaps the most extensive computational study of the kinetics of NO reactions on Rh and Pd surfaces has been provided by the group of Zgrablich. Their initial simulations of the NO + CO reaction on Rh(lll) corroborated the fact that the formation of N-NO intermediate is necessary for molecular nitrogen production [83], They also concluded that an Eley-Rideal mechanism is necessary to sustain a steady-state catalytic regime. Further simulations based on a lattice-gas model tested the role of the formation of... 

Chorismate mutase catalyzes the Claisen rearrangement of chorismate to prephenate at a rate 106 times greater than that in solution (Fig. 5.5). This enzyme reaction has attracted the attention of computational (bio)chemists, because it is a rare example of an enzyme-catalyzed pericyclic reaction. Several research groups have studied the mechanism of this enzyme by use of QM/MM methods [76-78], It has also been studied with the effective fragment potential (EFP) method [79, 80]. In this method the chemically active part of an enzyme is treated by use of the ab initio QM method and the rest of the system (protein environment) by effective fragment potentials. These potentials account... 

Substituents at the allene in general and methyl groups in particular favor /3-addition of radicals, which leads to the formation of allylic and therefore stabilized intermediates. Results from computational studies predict that CH3 addition to Cp of... 

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