Stereo-electronic influences

Isocyanates and amines react together to form ureas. Primary aliphatic amines react very quickly at temperatures down to ambient, whereas secondary aliphatic and primary aromatic amines react less quickly. The reaction rate of secondary aromatic amines is the slowest. The speed of the reaction can further be modified by the addition of substitutes near the amine group. The control of the speed can either be electronic, as illustrated by the effect of the chlorine in the MOCA ring, or by stereo chemical influences where the groups next to the amine group have a very strong hindrance to the curing. This is... 

The first DFT studies performed on the PK reaction pathway were published in 2001 by Nakamura and co-workers.81 They used ethyne and ethene as substrates and confirmed that the alkene insertion step is the critical stereo- and regiochemical determining step in the mechanism and that it is reversible. They also showed that although the coordination and insertion steps take place on one cobalt atom, the other cobalt atom exerts an electronic influence through the Co Co bond by donating and excepting electron density. 

Special steric or electronic influences may thwart the expected reaction pathway, as in the regio- and stereo-selective conversion of (86) to the pyrrolizidine (87 Scheme 42). This surprising formation of a five- instead of a six-membered ring is interpreted in terms of an aza-Cope equilibrium between the expected iminium salt (88) and its isomer (89 The latter finally cyclizes regioselectively in an exo-type mode to give (87). Further support for this assumption was obtained from quenching experiments with related compounds. ... 

To successfully correlate thermodynamic or kinetic parameters with stereo-electronic effects, there must be a clean separation of the electronic from the steric effects (Eq. 1). Since there is no necessary reason for an experimentally based steric parameter to be free of electronic effects, workers have turned to applications of computational chemistry to achieve a quantitative measure of pure steric influence of a ligand. 

Model compound studies reveal factors which influence free—radical additions of ethers, polyethers, and crown—polyethers, to fluorinated alkenes. Stereo-electronic effects significantly affect relative reactivities of cyclic amines. Novel dienes are obtained by reactions of some oligomers of fluorinated alkenes with sodium amalgam and some reactions of these systems with nucleophiles are described. 

Since the importance of entropic factors in the kinetics of reactions controlled by stereo-electronic interactions discussed in this chapter is well recognized now (6h, 10,11,315a), the influence of these interactions on the rates should preferably be discussed in terms of enthalpies of activation (cf. Section II.F). This seems to be especially important when stereoelectronic effects are deduced from rates of reactions of nonisomeric compounds. 

Reaction between fullerene and carbanion intermediates of PS [22], poly(bromostyrene) [23] or poly(vinylbenzyl chloride) [24] has afforded the corresponding polymers fullerenated directly on the main chain (26a-c). In these cases, polymer stereo-electronic effects as well as the steric hindrance of C > units greatly influence the structure and physical properties of the polymers. 

A related situation is found in the case of P-substituted cycloketones here, the electronic difference between the two a-carbons is almost insignificant, resulting in unselective migration upon chemical oxidation. BVMOs have a particularly different behavior, as they can influence the stereo- and/or regioselectivity of the biooxidation. In the latter case, the distribution of proximal and distal lactones is affected by directing the oxygen insertion process either into the bond close or remote to the position of the P-substituent. Consequently, a regioisomeric excess (re) can be defined for this biotransformation, similar to enantiomeric excess or diastereomeric excess values [143]. 

The 1,3-dipolar cycloaddition of 7V-benzyl-C-ethoxycarbonylnitrone with (5)-5-hydroxymethyl-(577)-fiiran-2-one is regio- and stereo-selective. The intramolecular 1,3-dipolar cycloaddition of sugar ketonitrones (50) provides a convenient method for the stereoselective formation of carbohydrate derivatives (51) possessing nitrogenated quaternary centres. This methodology has been successfully used to prepare synthetic precursors of (—)-tetrodotoxin (52) (Scheme 18). The hydrophobic effect has been shown to influence the rate and selectivity of 1,3-dipolar cycloaddition reactions of C,iV-diphenylnitrone with electron-deficient dipolarophiles. ... 

It has so far only been possible to solve the complex problem associated with chemical sialylation in a few examples in a convincing way [44]. Due to the lack of a neighboring group at C-3 there is little possibility to influence the stereochemistry at the anomeric center except by solvents. Further, the marked reverse anomeric effect of the carboxyl group leads to the electronically preferred -configuration. Thus, this area would be ideally suited for the apphcation of stereo- and regioselective biocatalysts. 

Molecular Weight and Molecular Weight Distribution Effects of Electron Donors. The production of broad molecular weight distributed polypropylene is a fimction of the multiplicity of active centers, which differ in stereo- and regiospecificity, and the propagation rate. Lewis bases in general, and external alkoxy silane donors in particular can influence the molecular weight... 

Interpretation of the stereo- and chemoselectivity of oxidation reactions using thianthrene oxide must consider its low barrier to ring-inversion, steric effects, and the role that solvents may play in the product distribution. All of these possibilities could influence the Xso value of oxidants. Specifically, mechanistic consideration of electrophilic oxidation of thianthrene oxide in the presence of protic solvents or acid leads to diminished yields of SSO2. Despite aU of these concerns, benzothianthrene oxide is still considered a reliable probe for the electronic character of oxidants. 

---