Computational studies alkylation

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... 

Di Valentin, C. Freccero, M. Zanaletti, C. Sarzi-Amade, M. o-Quinone methide as alkylating agent of nitrogen, oxygen, and sulfur nucleophiles. The role of H-bonding and solvent effects on the reactivity through a DFT computational study, j. Am. Chem. Soc. 2001, 123, 8366-8377. 

The dihalogen complexes with olefin donors were first identified spectroscopically in the mid-1960s [42-45] and extensive experimental and computational studies have been carried out by Chiappe, Lenoir and coworkers in recent years [46 - 48 ]. These systems are highly unstable, since the complexation of dihalogens with olefins is followed rapidly by the formation of ionic intermediates and further chemical transformations. Therefore, attention in the corresponding work has mostly focused on hindered olefins, although the spectral characteristics of complexes with less sterically crowded and alkyl- as well as chloro-substituted and cyclic olefins are also reported [44]. The absorption maxima for the dihalogen complexes with olefins (evaluated by the subtraction... 

Quantitative assessment of the electrophilic character of various types of phosphenium ions has been attempted using computational studies on hydride and halide exchange reactions, and the results attribute to 1,3,2-diazaphospholenium ions a lower electrophilicity (and thus higher stability) than other types of phosphenium ions [20, 66], The gain in stability due to aromatic -delocalization is predicted to be somewhat larger than inductive stabilization resulting from exhaustive A-alkylation of the parent diaminophosphenium ion, [P(NH2)2]+. 

The MacMillan laboratory has produced an interesting study on the reductive amination of a broad scope of aromatic and aliphatic methyl ketones catalyzed by ent-lk, utilizing Hantzsch ester as a hydride source (Scheme 5.26) [48]. Apphcation of corresponding ethyl ketones gave very low conversions. Computational studies indicated that while catalyst association with methyl ketones exposes the C=N Si face to hydride addition, substrates with larger alkyl groups are forced to adopt conformations where both enantiofaces of the iminium ir... 

De Andrade, J., Does, E. S., and Stassen, H., Computational study of room temperature molten salts composed by l-alkyl-3methylimidazolium cations-force field proposal and validation, /. Phys. Ghent., B, 106,13344-13351, 2002. 

A key observation was that the distribution of mono and dimagnesiated products was dependent upon the choice of magnesium alkyl, with dibutylmagnesium proving most effective for dimagnesiation. The stereoselectivity of the cyclozirconation step(s) has been rationalised with recourse to computational studies, the conclusion... 

The absorption spectra of a group of 2,6-dimethyl-4-arylpyrylium perchlorates were determined <2006PCA11338>. Changes in substituents on the aryl ring at the 4-position led to substantial changes in the absorption spectra. Computational studies provided results that were in good agreement with experiment, and for alkoxy- and alkyl-substituted systems, the spectra can be explained by Balaban s methods. 

As indicated from computational studies, the catalyst-activated iminium ion MM3-2 was expected to form with only the (E)-conformation to avoid nonbonding interactions between the substrate double bond and the gem-dimethyl substituents on the catalyst framework. In addition, the benzyl group of the imidazolidinone moiety should effectively shield the iminium-ion Si-face, leaving the Re-face exposed for enantioselective bond formation. The efficiency of chiral amine 1 in iminium catalysis was demonstrated by its successful application in several transformations such as enantioselective Diels-Alder reactions [6], nitrone additions [12], and Friedel-Crafts alkylations of pyrrole nucleophiles [13]. However, diminished reactivity was observed when indole and furan heteroaromatics where used for similar conjugate additions, causing the MacMillan group to embark upon studies to identify a more reactive and versatile amine catalyst. This led ultimately to the discovery of the second-generation imidazolidinone catalyst 3 (Fig. 3.1, bottom) [14],... 

For computational studies on the Friedel-Crafts alkylation of pyrroles and indoles catalyzed by chiral imidazolidinones, see R. Gordillo, J. Carter, K.N. Houk, Adv. Synth. Catal. 

A variety of C3 alkylated derivatives of the reversed esters of meperidine were examined (see Ref 405). Incorporation of a methyl group at C3 yields a- and jS-prodine (101 and 102, Fig. 7.23) (437). The J3 isomer (102) has five times the potency of the a isomer. X-ray crystallography and NMR indicate that the preferred conformation of the prodines is the chair form, with the phenyl ring equatorial (see Refs. 283, 438). Computational studies also indicate a preference for this conformer in the case of the reversed esters the energy differences between the equatorial and axial conformers (1.9-3.4 kcal/mol) are much greater than the differences between the two conformers of meperidine f/jand ketobemi-done(105) (410). 

One example of a bonafide bis(alkyne) complex has recently been prepared. Reaction of the in situ generated olefin complex prepared by alkylation of ( -CsHs ZrC 50 with the diaryl alkyne in Equation (7) yields 253.130 In this structure, C-C coupling has not occurred, presumably a result of the steric strain associated with the zirconacyclo-pentadienyl fragment (Equation (7)). The solid-state structure further establishes the compound as a bis(alkyne) complex. Computational studies suggest that a Zr(iv) resonance structure is the most suitable representation of the compound. However, reaction of 253 with iodine in THF yields ( -CsHs Zrle 254 and the dialkyne starting material, suggesting that the zirconium center can act as a source of Zr(n) (Equation (8)). 

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