Steric factors reaction rate, effect

The reaction of the rhenium alkylidyne complex 277 with diisopropyl-acetylene and with diethylacetylene [Eq. (196)] demonstrates the sensitivity of metathesis reactions toward steric factors (57). With diisopropylace-tylene an alkylidyne complex is obtained whereas the reaction with diethylacetylene gives a metallacyclobutadiene. In the metathesis reactions the alkyne with the bulkiest groups cleaves most easily from intermediate metallacyclobutadiene complexes. The rhenacyclobutadienes with the smallest substituents thus become sinks and slow down the effective rate of metathesis. The alkylidyne alkylidene rhenium complex 278 is an active olefin metathesis catalyst (52). Reaction with hexene transforms the neo-pentylidene group into a propylidene group as shown in Eq. (197). 

Hammett [14] has developed quantitative expressions relating the effect of substituents in either R or X on the rate of the reaction, when steric factors are negligible. He defined a reaction constant p by the equation... 

We can extend the collision theory to calculate the rate constant for bimolecular reactions of two species, A and B. Comparing observed and predicted rate constants gives the values of P shown in Table 18.1. As the colliding molecules become larger and more complex, P becomes smaller because a smaller fraction of collisions is effective in causing reaction. The steric factor is an empirical correction that has to be identified by comparing results of the simple theory with experimental data. It can be predicted in more advanced theories but only for especially simple reactions. 

Decades of work have led to a profusion of LEERs for a variety of reactions, for both equilibrium constants and reaction rates. LEERs were also established for other observations such as spectral data. Furthermore, various different scales of substituent constants have been proposed to model these different chemical systems. Attempts were then made to come up with a few fundamental substituent constants, such as those for the inductive, resonance, steric, or field effects. These fundamental constants have then to be combined linearly to different extents to model the various real-world systems. However, for each chemical system investigated, it had to be established which effects are operative and with which weighting factors the frmdamental constants would have to be combined. Much of this work has been summarized in two books and has also been outlined in a more recent review [9-11]. 

Effect on rate assuming no change in mechanism is caused steric factors upon substitution at C and rise to Cp have not been considered. The rate reductions are geared to substiment effects such as those giving rise to Hammett reaction constants on p- and a-aryl substitution. 

For removal of compounds bonded to silica or alumina, tt and ct bonds must be broken and new bonds formed. This requires energy and the correct orientation of attacking groups along with an effective attacking species. In addition, the rate of species removal will depend on the reaction path and the steric factors involved. All these put together will determine the overall rate of the reaction and the time needed for extraction. 

In heterogeneous catalytic hydrogenations suprafacial (as) addition of hydrogen would be expected, as the transfer of hydrogen atoms from the catalyst surface to the reactant is usually assumed. However, in some Pt catalyzed reactions antarafacial (trans) addition of hydrogen is also observed. The ratio of diastereomeric products formed is determined by the chemisorption equilibrium of the surface intermediates and by the relative rates of hydrogen entrance to the different unsaturated carbon sites. Both effects are influenced by steric factors. 

Rate constants and Arrhenius parameters for the reaction of Et3Si radicals with various carbonyl compounds are available. Some data are collected in Table 5.2 [49]. The ease of addition of EtsSi radicals was found to decrease in the order 1,4-benzoquinone > cyclic diaryl ketones, benzaldehyde, benzil, perfluoro propionic anhydride > benzophenone alkyl aryl ketone, alkyl aldehyde > oxalate > benzoate, trifluoroacetate, anhydride > cyclic dialkyl ketone > acyclic dialkyl ketone > formate > acetate [49,50]. This order of reactivity was rationalized in terms of bond energy differences, stabilization of the radical formed, polar effects, and steric factors. Thus, a phenyl or acyl group adjacent to the carbonyl will stabilize the radical adduct whereas a perfluoroalkyl or acyloxy group next to the carbonyl moiety will enhance the contribution given by the canonical structure with a charge separation to the transition state (Equation 5.24). 

In the intramolecular reactions studied by Bruice and Koshland and their co-workers, proximity effects (reduction in kinetic order and elimination of unfavourable ground state conformations) and orientation effects might give rate accelerations of 10 -10 . Hence, these effects can by themselves account for the enhancements seen in most intramolecular reactions. However, a factor of 10 -10 is less than the rate acceleration calculated for many enzyme reactions and certain intramolecular reactions, for example, hydrolysis of benzalde-hyde disalicyl acetal (3 X 10 ) (Anderson and Fife, 1973) and the lactonization reaction of[l] (10 ) where a trimethyl lock has been built into the system. If hydrolysis of tetramethylsuccinanilic acid (Higuchi et al., 1966) represents a steric compression effect (10 rate acceleration), then proximity, orientation, and steric compression... 

In all cases the boron goes to the side of the double bond that has more hydrogens, whether the substituents are aryl or alkyl.35B Thus the reaction of 43 with BH3 gives 98% 44 and only 2% of the other product. This actually follows Markovnikov s rule, since boron is more positive than hydrogen. However, the regioselectivity is caused mostly by steric factors, though electronic factors also play a part. Studies of the effect of ring substituents on rates and on the direction of attack in hydroboration of substituted styrenes showed that the attack by boron has electrophilic character.359 When both sides of the double bond are... 

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