Factors electronic

The concept of electronic factors in catalysis deals with the relationship between the electronic structure of solids, which depends on their physical properties, and the reactivity of adsorbed intermediates. 

The key question was how does the catal5dic activity of a solid depend on its geometrical and electronic properties  

This group of catalysts comprises solids exhibiting electrical conductivity, that is, having mobile electrons (metals and semiconductors). Many reactions proceed by the redox mechanism, for example  

These are all homolytic processes in which chemical bonds are broken with the aid of the catalyst (Eq. 5-33). This leads to formation of radicals, followed by electron transfer between the reaction partners. 

Typical redox catalysts are metals, semiconductors (e.g., metal oxides in various oxidation states), and special metal complexes. Metals that form an oxide layer on the surface under oxidizing conditions can also be regarded as semiconductors. 

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The electronic factor in the sum j(r, R) arises from the familiar BO electronic Hamiltonian defined for a fixed R. Since this Hamiltonian is independent of the nuclear set Xk(R) cany the e label. As is well known, with each k... 

If the rate constants for quaternization of 2-alkylthiazoles depended on electronic factors, they would all be greater than that of thiazole, which has the low est pK. and all of the same order. The decrease in rate constants that is observed is attributed wholly to steric effects. In Table III-50 we report the main parameters for the reaction of 2-alkylthiazoles with methyl iodide. 

Cyanohydrin formation is reversible and the position of equilibrium depends on the steric and electronic factors governing nucleophilic addition to carbonyl groups described m the preceding section Aldehydes and unhindered ketones give good yields of cyanohydrins... 

Its reactions with olefins, governed by steric rather than electronic factors, are very sluggish. Even simple 1-alkenes require 8 h at 25°C for complete reaction. In contrast, alkynes are hydroborated with great ease to alkenylboranes, high steric requirements of the reagent preventing dihydroboration (117). 

In addition to reaction sequences of type (66) -> (67), electrophilic reagents can attack at either one of the ring nitrogen atoms in the mesomeric anions formed by proton loss e.g. 70 71 or 72 see Section 4.02.1.3.6). Here we have an ambident anion, and for unsymmetrical cases the composition of the reaction product (71) + (72) is dictated by steric and electronic factors. 

Alkyl groups under nonacidic conditions sterically deflect nucleophiles from C, but under acidic conditions this steric effect is to some extent offset by an electronic one the protonated oxirane opens by transition states (Scheme 40) which are even more 5Nl-like than the borderline Sn2 one of the unprotonated oxirane. Thus electronic factors favor cleavage at the more substituted carbon, which can better support a partial positive charge the steric factor is still operative, however, and even under acidic conditions the major product usually results from Cp attack. 

TT-Conjugating groups tend to favor attack at C, but the ratio of Ca. C attack depends strongly on a balance of steric and electronic factors arising from both substituent and nucleophile (Table 4). The results can be rationalized, to a first approximation, by assuming that with good vr-donors stabilization of the incipient carbocation in (50) offsets steric hindrance. 

Benzylidene acetals have the useful property that one of the two C—O bonds can be selectively cleaved. The direction of cleavage is dependent on steric and electronic factors as well as on the nature of the cleavage reagent. 

EtSNa, DMF, reflux, 3 h, 94-98% yield.Potassium thiophenoxide has been used to cleave an aryl methyl ether without causing migration of a double bond. odium benzylselenide (PhCH2SeNa) and sodium thiocre-solate (p-CH3C6H4SNa) cleave a dimethoxyaryl compound regioselec-tively, reportedly as a result of steric factors in the former case and electronic factors in the latter case. ... 

When both the 1,3-dipoIe and the dipolarophile are unsymmetrical, there are two possible orientations for addition. Both steric and electronic factors play a role in determining the regioselectivity of the addition. The most generally satisfactory interpretation of the regiochemistry of dipolar cycloadditions is based on frontier orbital concepts. As with the Diels-Alder reaction, the most favorable orientation is that which involves complementary interaction between the frontier orbitals of the 1,3-dipole and the dipolarophile. Although most dipolar cycloadditions are of the type in which the LUMO of the dipolarophile interacts with the HOMO of the 1,3-dipole, there are a significant number of systems in which the relationship is reversed. There are also some in which the two possible HOMO-LUMO interactions are of comparable magnitude. 

Numerous observations of the effect in ionic crystals were carried out by Mineev and Ivanov in the Soviet Union [76M01]. This is a class of crystals in which a number of materials factors can be confidently varied. By choice of crystallographic orientation, various slip directions can be invoked. By choice of various crystals other physical factors such as dielectric constant, ionic radius, and an electronic factor thought to be representative of dielec-... 

Obviously the structures and yields of Birch reduction products are determined at the two protonation stages. The ring positions at which both protonations occur are determined kinetically the first protonation or 7t-complex collapse is rate determining and irreversible, and the second protonation normally is irreversible under the reaction conditions. In theory, the radical-anion could protonate at any one of the six carbon atoms of the ring and each of the possible cyclohexadienyl carbanions formed subsequently could protonate at any one of three positions. Undoubtedly the steric and electronic factors discussed above determine the kinetically favored positions of protonation, but at present it is difficult to evaluate the importance of each factor in specific cases. A brief summary of some empirical and theoretical data regarding the favored positions of protonation follows. 

Protecting groups are generally formed by nucleophilic attack on the carbonyl group and the rate of this process is determined by steric and electronic factors associated with the ketone. In steroid ketones steric effects are usually more important due to the rigid tetracychc skeleton. 

The stereochemistry of the 1,4-addition to A -octal-l-one and 1,1-di-methyl-A -octal-2-one has been investigated by House and Marshall, respectively. In summary, steric and stereoelectronic factors play a part in the mechanism of conjugate addition of Grignard compounds. With methylmagnesium iodide, the introduction of an axial methyl group into steroidal 5a-A -3-ketones (3) and 5 -A -3-ketones (6) is favored by stereo-electronic factors in the transition state. 

The reaction rates and product yields of [2+2] cycloadditions are expectedly enhanced by electronic factors that favor radical formation. Olefins with geminal capto-dative substituents are especially efficient partners (equations 33 and 34) because of the synergistic effect of the electron acceptor (capto) with the electron donor (dative) substituents on radical stability [95]... 

The increase in the proportion of the tetrasubstituted isomer in the cases of the morpholine and piperidine enamines of 2-methylcyelohexanone has been ascribed to both steric and electronic factors. The authors propose that the overlap of the electron pair on the nitrogen atom and the v electrons of the double bond is much more important in the case of the pyrrolidine enamines and much less with the others. Support for this postulate was provided by the NMR spectra of these enamines, wherein the chemical shifts of the vinylic protons of the pyrrolidine enamines were at a higher field than those of the corresponding morpholine and piperidine enamines by 20-27 Hz. The greater amount of overlap or electron delocalization, in the case of pyrrolidine enamine, is in accord with the postulate of Brown et al. (7- ) that the double bond exo to the five-membered ring is more favored than the double bond exo to the six-membered ring. 

Phosphinamides are stable to catalytic hydrogenation, used to cleave benzyl-derived protective groups, and to hydrazine. The rate of hydrolysis of phosphinamides is a function of the steric and electronic factors around the phosphorus. This derivative has largely been used for the protection of amino acids and occurs few, if any, times in the general synthetic literature. 

This reaction, now termed hydroboration, has opened up the quantitative preparation of organoboranes and these, in turn, have proved to be of outstanding synthetic utility. It was for his development of this field that H. C. Brown (Purdue) was awarded the 1979 Nobel Prize in Chemistry . Hydroboration is regiospecific, the boron showing preferential attachment to the least substituted C atom (anti-Markovnikov). This finds ready interpretation in terms of electronic factors and relative bond polarities (p. 144) steric factors also work in the same direction. The addition is stereospecific cis (syn). Recent extensions of the methodology have encompassed the significant development of generalized chiral syntheses. 

The conformation of alkylcyclohexanes is determined largely by steric repulsion (see Chapter 5, Problems 6 and 7). More polar substituents may show different conformational preferences due to a combination of steric and electronic factors. 

In contrast to the facile condensation of o-nitrotoluene with diethyl oxalate, other a-alky] nitrobenzenes are sluggish to react with diethyl oxalate or fail to react at all. It has been suggested that this is due both to steric and electronic factors effected by the alky] group, which destabilizes the methylene group in regard to formation of the carbanion. ... 

Both steric and electronic factors have been claimed to control the selectivity in the cyclization step. Not only the control of the selectivity on the ring closure but also the lack of activity toward cyclization was observed. In one example of this, methyl substituted aminoindole 96 provided cyclization product 99 while attempted cyclization of methyl ether 98 led to decomposition. ... 

However, the 0-alkyl derivatives are potentially unstable with respect to thermal elimination of a carbonyl compound and consequent reduction to the corresponding lactam. A combination of steric and electronic factors may permit this decomposition, i.e., 133 -a- 134, to occur at quite moderate temperatures. The 0-methyl derivative of the benzalphthalimidine (132) undergoes slow loss of formaldehyde at 177° (Ti/a in dimethyl sulfoxide 40 minutes), but this elimination is much faster in certain thiohydroxamic acid derivatives, e.g., 135, which lose benzaldehyde readily at 139° in dimethyl sulfoxide (T1/2 6 minutes). The outstanding example of this decomposition, however,... 

It is known that the ability of nitrotolane to cyclize depends on electronic factors (69MI2) hence l,3-dimethyl-4-nitro-5-phenylethynylpyrazole, whose acetylene group is in the most electron-accepting position of the pyrazole ring, i.e., favorable for nucleophilic addition, was introduced into the reaction of cyclization. Thus,... 

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