Steric requirement

If acetoxylation were a conventional electrophilic substitution it is hard to understand why it is not more generally observed in nitration in acetic anhydride. The acetoxylating species is supposed to be very much more selective than the nitrating species, and therefore compared with the situation in (say) toluene in which the ratio of acetoxylation to nitration is small, the introduction of activating substituents into the aromatic nucleus should lead to an increase in the importance of acetoxylation relative to nitration. This is, in fact, observed in the limited range of the alkylbenzenes, although the apparently severe steric requirement of the acetoxylation species is a complicating feature. The failure to observe acetoxylation in the reactions of compounds more reactive than 2-xylene has been attributed to the incursion of another mechan-104... 

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). 

Dimerization is reportedly catalyzed by pyridine [110-86-1] and phosphines. Trialkylphosphines have been shown to catalyze the conversion of dimer iato trimer upon prolonged standing (2,57). Pyridines and other basic catalysts are less selective because the required iacrease ia temperature causes trimerization to compete with dimerization. The gradual conversion of dimer to trimer ia the catalyzed dimerization reaction can be explained by the assumption of equiUbria between dimer and polar catalyst—dimer iatermediates. The polar iatermediates react with excess isocyanate to yield trimer. Factors, such as charge stabilization ia the polar iatermediate and its lifetime or steric requirement, are reported to be important. For these reasons, it is not currently feasible to predict the efficiency of dimer formation given a particular catalyst. 

The spatial and steric requirements for high affinity binding to protein kinase C (PKC), a macromolecule that has not yet been crystallized, were determined. Protein kinase C plays a critical role in cellular signal transduction and is in part responsible for cell differentiation. PKC was identified as the macromolecular target for the potent tumor-promoting phorbol esters (25). The natural agonists for PKC are diacylglycerols (DAG) (26). The arrows denote possible sites of interaction. 

Steric and inductive effects determine the rate of formation of the pentacovalent siUcon reaction complex. In alkaline hydrolysis, replacement of a hydrogen by alkyl groups, which have lower electronegativity and greater steric requirements, leads to slower hydrolysis rates. Replacement of alkyl groups with bulkier alkyl substituents has the same effect. Reaction rates decrease according to ... 

Alkylation and arylation of organosilanes occur readily with alkyl and aryl alkaU metal compounds. Yields from these reactions are good but are iafluenced by steric requirements on both silane and metal compounds. There is Httie iaductive effect by the organic groups attached to siUcon, as measured by the yield of products (126,127). These reactions proceed more readily ia tetrahydrofuran and ethyl ether than ia ligroin or petroleum ether, where R and are alkyl or aryl and M is Li, Na, or K. 

Thorium compounds of anionic nitrogen-donating species such as [Th(NR2)4], where R = alkyl or sdyl, are weU-known. The nuclearity is highly dependent on the steric requirements of R. Amides are extremely reactive, readily undergoing protonation to form amines or insertion reactions with CO2, COS, CS2, and CSe2 to form carbamates. Tetravalent thorium thiocyanates have been isolated as hydrated species, eg, Th(NCS)4(H20)4 [17837-16-0] or as complex salts, eg, M4 Th(NCS)g] vvH20, where M = NH, Rb, or Cs. 

Substituents with -I,+M effects such as halogen show a similar orientation effect to that of alkyl groups. If the substituent is in the 2-position, substitution occurs at C-5 if it is in position 3, substitution occurs preferentially at C-2 or if steric requirements of the group or electrophile intervene, then substitution occurs at C-5. 

Because of the increased acidity and reduced steric requirement of the carbohydrate hydroxyl, r-BuOK can be used as a base to achieve ether for-... 

The dependence on steric bulk is attributed to the steric requirements imposed by the bulky trimefliylamine leaving group. In the transition state for anti elimination, steric repulsion is increased as R and increase in size. When the repulsion is sufficiently large, the transition state for syn elimination is preferred. 

There are many other examples of selective rearrangement/ although such strict steric requirements are not always observed/ ... 

Enol ethers (15) and mixed acetals (16) are readily obtained from secondary but not from tertiary alcohols, whereas tetrahydropyranyl ethers can be formed even from tertiary alcohols. This is a result of the greater steric requirements of the reagents (17) and (18) as compared to (19). 

While the reductive procedure appears simpler, it also provides a higher proportion of the A -double bond isomer on dehydration than is obtained from the tertiary carbinol. This may be a consequence of the lower steric requirements of a A -olefin having only one alkyl group attached to C-20. In either case, all other double bonds must be protected before the ozonolysis and other free hydroxyls should be esterified prior to dehydration at C-20. 

These Br nsted-type plots often seem to be scatter diagrams until the points are collated into groups related by specific structural features. Thus, p-nitrophenyl acetate gives four separate, but parallel, lines for reactions with pyridines, anilines, imidazoles, and oxygen nucleophiles.Figure 7-4 shows such a plot for the reaction of trans-cmmm c anhydride with primary and secondary aliphatic amines to give substituted cinnamamides.All of the primary amines without substituents on the a carbon (R-CHi-NHi) fall on a line of slope 0.62 cyclopentylamine also lies on this line. If this line is characteristic of normal behavior, most of the deviations become qualitatively explicable. The line drawn through the secondary amines (slope 1.98) connects amines with the structure R-CHi-NH-CHi-R. The different steric requirements in the acylation reaction and in the model process... 

Many complexes have more than one coordination mode of BH4 featured in their structure, e.g. [U ()9 -BH4)()9 -BH4)2(dmpe)2]. Likewise, whereas [M(BH4)4] are monomeric 12-coordinate complexes for M = Zr, Hf, Np, Pu, they are polymeric for M = Th, Pa, U the coordination number rises to 14 and each metal centre is coordinated by two r) -BH4 and four bridging r) -BH4 groups. It is clear that among the factors which determine the mode adopted are the size of the metal atom and the steric requirements of the co-ligands. Many of the complexes... 

CH3I should approach the enolate from the direction that simultaneously allows its optimum overlap with the electron-donor orbital on the enolate (this is the highest-occupied molecular orbital or HOMO), and minimizes its steric repulsion with the enolate. Examine the HOMO of enolate A. Is it more heavily concentrated on the same side of the six-membered ring as the bridgehead methyl group, on the opposite side, or is it equally concentrated on the two sides A map of the HOMO on the electron density surface (a HOMO map ) provides a clearer indication, as this also provides a measure of steric requirements. Identify the direction of attack that maximizes orbital overlap and minimizes steric repulsion, and predict the major product of each reaction. Do your predictions agree with the thermodynamic preferences Repeat your analysis for enolate B, leading to product B1 nd product B2. 

If we are comparing reactions which have approximatively the same steric requirements, the first term is roughly constant. If the species are very polar the second term will dominate, and the reaction is charge controlled. This means for example that an electrophihc attack is likely to occur at the most negative atom, or in a more general sense, along a path where the electrostatic potential is most negative. If the molecules are non-polar, the third term in (15.1) will dominate, and the reaction is orbital controlled. 

The position of aniline in the above reactivity order deserves special comment. Aniline is less basic than pyridine by a relatively small factor, 0.65 pA units, but is appreciably more polarizable it then seems likely that the inverted order of reactivity is caused by the polarizability term in accordance with Edwards equation. If this is correct, in the reactivity order piperidine > aniline > pyridine, inversion with respect to basicity appears to result from an abnormally high reactivity of aniline rather than from a particularly low reactivity of pyridine. This view differs from that based on relative steric requirements of the reagents, but other factors besides basicity and polarizability may well contribute to the quantitative experimental picture. 

As pointed out by Chapman et the steric requirements of the reagents and the degree of solvation of the substrate at the reacting center should also be considered when comparing the nucleophilicities of different amines toward different substrates. The large number of factors which may be involved clearly call for much more work in this area. 

The good correlation found between the reactivities of 2-chloro-3-cyano-5-nitropyridine and the polar effects of the substituents on the aniline reagent has enabled Chapman and his co-workers to illustrate the steric requirements of the reagent by including in their studies... 

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