Sterically inhibited isomerization reactions

The correlation between selectivity and intracrystalline free space can be readily accounted for in terms of the mechanisms of the reactions involved. The acid-catalyzed xylene isomerization occurs via 1,2-methyl shifts in protonated xylenes (Figure 3). A mechanism via two transalkylation steps as proposed for synthetic faujasite (8) can be ruled out in view of the strictly consecutive nature of the isomerization sequence o m p and the low activity for disproportionation. Disproportionation involves a large diphenylmethane-type intermediate (Figure 4). It is suggested that this intermediate can form readily in the large intracrystalline cavity (diameter. 1.3 nm) of faujasite, but is sterically inhibited in the smaller pores of mordenite and ZSM-4 (d -0.8 nm) and especially of ZSM-5 (d -0.6 nm). Thus, transition state selectivity rather than shape selective diffusion are responsible for the high xylene isomerization selectivity of ZSM-5. 

Grigg and co-workers (310) recently examined the 1,3-APT reaction of various aldoximes (270) (R or R = H) with divinyl ketone (Scheme 1.56). While ketoximes 270 (R = R) form a mixture of adducts, 271 and 272 via nitrone 273, the aldoximes selectively afford 272 (as a mixture of endo and exo diastereoisomers). Under the thermal reaction conditions, the oxime starting materials can undergo ( /Z) isomerization, while the nitrone intermediate was expected to be unaffected and the isolated cycloadducts showed no interconversion via cycloreversion. Thus, the increasing selectivity for endo-212 [via ( )-273, R = H] over exo-212 [via (Z)-273, R = H] with the increasing size of the aldoxime substituent was attributed primarily to the inhibition of oxime isomerization by steric clash between R or R and the oxime OH. In contrast, Lewis acid catalysis, in particular by hafnium (iv) chloride, of the cycloaddition of various aldoximes with this dipolarophile gave exo-271 exclusively (216). 

The primary isomer distribution, which is the result of the disproportionation reaction, may deviate significantly from the thermodynamic equilibrium composition, for two reasons. First, the reaction may be controlled by the kinetics rather than the thermodynamics, i.e. mechanistic reasons may exist which cause the reaction to proceed along a certain path. Second, in the case that the reaction obeys a bimolecular mechanism, it may pass through a transition state which would presumably favor the (taller) para isomer. Hence, it is possible that the primary product contains an enhanced fraction of the para isomer. The departure from the equilibrium composition then gives the driving force for the subsequent (monomolecular) isomerization reaction. This will reduce the fraction of the para isomer, provided the formation of the bulky ortho and meta isomers are not inhibited by sterical effects, i.e. when the micropore diameter is sufficiently large or there is a chance for the isomerization reaction to take place at the outer surface of the crystallites. Thus, the secondary isomer distribution may approach the thermodynamic equilibrium composition, as a limiting case. 

A further improvement of the approach of Wei [107] was reported in 1989 by Hashimoto et al. [42], which considered not only adsorption effects, but also the nonselective reactions occurring at the outer surface of the crystallites. The nonselective influence of these reactions has also been recognized by Fraenkel [35] in 1990, who studied the formation of xylene from toluene on a HZSM-5 catalyst. Fraenkel assumed that inside the crystallite only />-xylene is formed, whereas the ortho and meta isomers are sterically inhibited there. Hence, he concluded that the amount of o- and m-xylene observed during his experiments must be due to the isomerization of p-xylene at the outer surface of the crystallites. This two-step mechanism was first suggested in 1987 by Paparetto et al. [82] for the ethylation of toluene. It may also be worth noting that Fraenkel s model took into account not only the isomerization but also the nonselective alkylation at the outer crystallite surface. 

Ag+-catalysed isomerization of 4-substituted homocubanes (5) to norsnoutanes (6) also proceeds via pre-equilibrium complex formation and follows second-order kinetics. In the case R= Me adherence to Michaelis-Menten kinetics could also be demonstrated. C-4 Substituents capable of resonance interaction in the cationic transition state promote deviations in the rate of reaction relative to substituents which exhibit inductive effects only. With R=Bu bond-switching is reduced in rate, presumably because of steric inhibition of Ag+ attack on the homocubane to give an intermediate analogous to (4). Placement of deuterium or CDg at C-4 produces only a minor inverse kinetic deuterium isotope effect (kH/kD=0.97) which implies that a completely free carbonium ion intermediate is not involved and so argues in favour of a delocalized species analogous to (4). 

A very different type of catalyst was developed by Buchwald et al. [6] the chiral Ti complex with Brintzinger s ansa-metallocene ligand (ebthi) is extraordinarily effective for the enantioselective hydrogenation of cychc imines with high optical yields (>97% ee). Unfortunately, the activity and productivity of this Ti catalyst are relatively low compared to Rh- and Ir-diphosphine catalysts. The stereochemical outcome of the reaction can be predicted by straightforward steric arguments. Acyclic imines are reduced with lower enantioselectivity, probably due to isomerization problems and the presence of syn/anti isomers. Studies with multifunctional imines or in presence of other substrates revealed that the scope of the Ti-ebthi catalyst is rather Hmited. Partial or total catalyst inhibition is observed in presence of most functional groups, expect amines, alcohols, acetals, and halides [39]. 

The substitution of the cyclobutanone plays an important role in the outcome of the reaction as shown in a later report by Murakami [34]. 2-Substituted cyclobutanone (31, Scheme 6b) afforded benzocyclooctenones 34 under the reaction conditions. It was proposed that Rh is directed by the terminal olefin to insert into the more hindered a C-C braid, followed by migratory insertion to form intermediate 33. Non-selective p-H eUmination of either Ha or Hb afforded the isomeric mixture of olefins 34a and 34b. Further exploration of the substrate scope found that additional steric bulk inhibits the reactitai as neither substrate 35 nor 36 reacted. 

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