Steric effects combination

For most vinyl polymers, head-to-tail addition is the dominant mode of addition. Variations from this generalization become more common for polymerizations which are carried out at higher temperatures. Head-to-head addition is also somewhat more abundant in the case of halogenated monomers such as vinyl chloride. The preponderance of head-to-tail additions is understood to arise from a combination of resonance and steric effects. In many cases the ionic or free-radical reaction center occurs at the substituted carbon due to the possibility of resonance stabilization or electron delocalization through the substituent group. Head-to-tail attachment is also sterically favored, since the substituent groups on successive repeat units are separated by a methylene... 

The more stable the LUMO, the stronger is the interaction with the HOMO of the approaching nucleophile. The observed (Cram s rule) stereoselectivity is then a combination of stereoelectronic effects ftiat establish a preference for a perpendicular substituent and a steric effect that establishes a preference for the nucleophile to approach from the direction occupied by the smallest substituent. 

When this stereoelectronic requirement is combined with a calculation of the steric and angle strain imposed on the transition state, as determined by MM-type calculations, preferences for the exo versus endo modes of cyclization are predicted to be as summarized in Table 12.3. The observed results show the expected qualitative trend. The observed preferences for ring formation are 5 > 6, 6 > 7, and 8 > 7, in agreement with the calculated preferences. The relationship only holds for terminal double bonds. An additional alkyl substituent at either end of the double bond reduces the relative reactivity as a result of a steric effect. 

Note that hydrozirconation of 2-vinylfuran gives only the internal product [86] (Scheme 8-16) which probably is the result of the combination of the effects described in this section (i) O-coordination, (ii) aromatic stabihzation, (iii) reduced steric effect of the flat furan ring, which favors the reverse expected regiochemistry in the hydrozirconation reaction of alkenes with [Cp2Zr(H)Cl] (1). 

If the substituents are nonpolar, such as an alkyl or aryl group, the control is exerted mainly by steric effects. In particular, for a-substituted aldehydes, the Felkin TS model can be taken as the starting point for analysis, in combination with the cyclic TS. (See Section 2.4.1.3, Part A to review the Felkin model.) The analysis and prediction of the direction of the preferred reaction depends on the same principles as for simple diastereoselectivity and are done by consideration of the attractive and repulsive interactions in the presumed TS. In the Felkin model for nucleophilic addition to carbonyl centers the larger a-substituent is aligned anti to the approaching enolate and yields the 3,4-syn product. If reaction occurs by an alternative approach, the stereochemistry is reversed, and this is called an anti-Felkin approach. 

The combined influences of polar and steric effects and of the strength of the newly formed bond93 was also recognized in the reaction of a,0-unsaturated carbonyl compounds and similar electron deficient alkenes95 with organomercurials and NaBH4. For the addition of alkyl radicals to substituted styrenes, p assumed a... 

Another hypothesis was provided by Mikio Shimitso (1982) on the basis of studies of steric effects in molecular models. It had been noted years previously that the fourth nucleotide at the 3 end of the tRNA molecules (referred to as the discrimination base) might have a recognition function. In the case of certain amino acids (i.e., their tRNA-amino acid complexes) this base pair, in combination with the anticodon of the tRNA molecule, can select the amino acid corresponding to the tRNA species in question this is done on the basis of the stereochemical properties of the molecule. Since the anticodon of a tRNA molecule and the fourth nucleotide of the acceptor stem are far apart in space, two tRNA molecules must complex in a head-to-tail manner. The pocket thus formed can then fit specifically to the corresponding amino acid. 

The selectivity in the Heck reaction of allylic alcohol 111 is interesting, and the factors that lead to the observed preference for (3-hydride elimination toward nitrogen in this system are unclear, although a combination of steric effects and stereoelectronic factors (i.e., alignment of C-H and C-Pd bonds, nN a c H interactions) is likely involved. Examination of related examples from the literature (Scheme 4.20) reveals no clear trend. Rawal and Michoud examined substrate 115, which lacks the influence of both the amine and hydroxyl substituents and also seems to favor (3-hydride elimination within the six-membered ring over formation of the exocyclic olefin under standard Heck conditions [18a]. However, under... 

Steric effects and FMO control have been combined in an elegant way to achieve regiospecific synthesis of pyrazole inhibitors of dihydroorotate dehydrogenase <2006SL901>. When the size of the propargylic acid ester 86 is increased from ethyl to diphenylmethyl, pyrazole 87 is formed from compound 85 regiospecifically (Scheme 3 Table 4) <2006H(68)1007>. 

A combination of steric and electrostatic factors is presumably decisive with regard to the form of the acid most stable in sulfuric acid solution. The simple protonated form XX of benzoic acid is stabilized by resonance structures sterically prohibited in mesitoic acids. The ortho methyl groups of mesitoic acid would interfere with a coplanar dihydroxymethylene group. On the other hand, the inductive and resonance effects of the methyl groups help stabilize the acylium ion form of mesitoic acid as in the formulae XXI. In the case of 2,4,6-tribromobenzoic acid the steric effect and its abetting electronic effects are not sufficient, and this acid behaves like benzoic acid.17 >177... 

The regioselectivity in radical addition reactions to alkenes in general has successfully been interpreted by a combination of steric and electronic effects1815,47. In the absence of steric effects, regiochemical preferences can readily be explained with FMO theory. The most relevant polyene orbital for the addition of nucleophilic radicals to polyenes will be the LUMO for the addition of electrophilic orbitals it will be the HOMO. Table 10 lists the HOMO and LUMO coefficients (without the phase sign) for the first three members of the polyene family together with those for ethylene as calculated from Hiickel theory and with the AMI semiempirical method48. 

The electrostatic and steric effects can be combined to stabilize nanoparticles in solution. This type of stabilization is generally provided by means of ionic surfactants such as alkylammonium cations (Scheme 9.3). These compounds bear both a polar head group which is able to generate an electrical double layer, and a lipophilic side chain which is able to provide steric repulsion [14, 15]. 

Two separate computational investigations of the Rh/dppms catalyst and related systems have appeared in the literature. One study [40] concluded that steric effects were important in promoting migratory CO insertion in [Rh(CO)(dppms)l2Me], while the other [41] proposed that an electronic effect, arising from the sulfur donor atom of dppms, was responsible. It is likely that a combination of steric and electronic effects result in the observed reactivity. 

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