Steric crowding reactions

Clearly the steric crowding that influences reaction rates in 8 2 processes plays no role in Stvfl reactions The order of alkyl halide reactivity in 8 1 reactions is the same as the order of carbocation stability the more stable the carbocation the more reactive the alkyl halide... 

FIGURE 16.15 Orientation effects in intramolecular reactions can be dramatic. Steric crowding by methyl groups provides a rate acceleration of 2.5 X 10 for the lower reaction compared to the upper reaction. (Adaptedfrom Milstien,. S., and Cohen, L. A., 1972. Stereopopnlation control I. Rate enhancements in the laetonization of o-hyelroxyhyeJroeinnamie acid. Journal of the American Chemical Society 94 9158-9165.)... 

The Corey-Winter reaction provides a useful method for the preparation of olefins that are not accessible by other routes. For instance it may be used for the synthesis of sterically crowded targets, since the initial attack of phosphorus at the sulfur takes place quite distantly from sterically demanding groups that might be present in the substrate molecule. Moreover the required vicinal diols are easily accessible, e.g. by the carbon-carbon bond forming acyloin ester condensation followed by a reductive step. By such a route the twistene 10 has been synthesized ... 

A most attractive feature of radical reactions that recommends their use in the synthesis of complex molecules is that steric crowding, particularly on the radical center, is tolerated in many instances. Indeed, radical reactions are ideally suited for the con-... 

Rates of addition to transfer agents 11,12 are determined by the same factors that determine rates of addition to monomers Section 23). Substituents on the remote terminus of a double bond typically have only a minor influence. Thus, in most cases, the double bonds of the transfer agents have a reactivity towards propagating radicals that is comparable with that of the common monomers they resemble. With efficient fragmentation, transfer constants can be close to unity. The radicals formed by addition typically have low reactivity towards further propagation and other intermolecular reactions because of steric crowding about the radical center. 

A mechanistic study6 5 of the reaction of sulphoxides with the sodium salt of 1 -methyl-1 -phenylethyl hydroperoxide in benzene-alcohol solutions at room temperature showed that the reaction is first order with respect to sulphoxide and the hydroperoxide salt. The rate was also shown to be inversely proportional to the alcohol concentration in the solvent and this dependence was shown to increase with increasing pH. Also, more sterically crowded alcohols caused a decreased rate of reaction. 

E) alkenes. One explanation for this is that the reaction of the ylid with the carbonyl compound is a 2-1-2 cycloaddition, which in order to be concerted must adopt the [rt2s+n2al pathway. As we have seen earlier (p. 1079), this pathway leads to the formation of the more sterically crowded product, in this case the (Z) alkene. If this explanation is correct, it is not easy to explain the predominant formation of ( ) products from stable ylids, but (E) compounds are of course generally thermodynamically more stable than the (Z) isomers, and the stereochemistry seems to depend on many factors. 

First, the order of ky values in different solvents is quite reasonably interpreted as a nucleophilicity order - e.g. ( 113)280 > HjO CH3NO2 > C2H5OH) and second, the k rate is (as is the k2) greatly reduced by steric crowding . Of course, a nucleophilic attack by solvent is a very likely process, a priori. In any solvent, the solvent itself will be the poorest nucleophile that can be studied since poorer ones will not effectively compete. Thus the k term of equation (21) corresponds to the ki value. The aquo intermediate of scheme (22) has been trapped by using reactions in the presence of OH , a poor nucleophile but good base . ... 

Diphosphenes stabilised by bulky groups, e.g. (157, R = 2,4,6-tri-t-butylphenyl), can be isolated in high yield from the reactions of alkyl- or aryl-(trichlorogermyl)phosphines with an excess of the base DBU. The formation of less sterically crowded systems, e.g. (157, R = t-butyl), is also possible by this route,... 

Unexpectedly, tho hydrolysis of jV-(2,4-dinitrobenzoyl)imidazole at 25 °C was found to be slower by a factor of 25 in comparison to iV-(4-nitrobenzoyl)imidazole. This lower reactivity of iV-(2,4-dinitrobenzoyl)imidazole was explained by a combination of steric crowding at the reaction center and intramolecular stabilization of the reactant state.[19]... 

For the preparation of sterically crowded amides amino magnesium salts have been recommended for the reaction with imidazolides in order to increase the nucleophilicity of the amine moiety. Amino magnesium salts are prepared from the appropriate amines and ethyl magnesium bromide in tetrahydrofuran [90]... 

Because in the case of a sterically crowded imidazolide the formation of a carbinol is more difficult, reaction with the titanium reagent or the corresponding Grignard compound produces the allyl ketone in about the same yield [96]... 

Cobalt(II) alkoxides are known and monomeric forms are part of a wider review.413 The interest in these compounds pertains to a potential role in catalysis. For example, a discrete cobalt(II) alkoxide is believed to form in situ from a chloro precursor during reaction and performs the catalytic role in the decomposition of dialkyl pyrocarbonates to dialkyl carbonates and carbon dioxide.414 A number of mononuclear alkoxide complexes of cobalt(II) have been characterized by crystal structures, as exemplified by [CoCl(OC(t-Bu)3)2 Li(THF)].415 The Co ion in this structure and close relatives has a rare distorted trigonal-planar coordination geometry due to the extreme steric crowding around the metal. 

The effect of steric crowding on the rates of reaction of the Ni1 tetraaza macrocycle complex [Ni(dmc)]+ with organic halides and hydroperoxides has also been examined. Reaction with this complex was found to be about 104 times slower than with the corresponding [Ni(tmc)]+ complex.2329... 

Sn2 attack on the CH2 in (10) is found to proceed at very much the same rate as on that in MeCH2Cl, suggesting that any adverse steric crowding in the T.S. by the bulky C6H5 group is compensated by a small electronic (inductive ) effect promoting reaction. 

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