Mechanism allylic intermediates

Much work has been invested to reveal the mechanism by which propylene is catalytically oxidized to acrolein over the heterogeneous catalyst surface. Isotope labeling experiments by Sachtler and DeBoer revealed the presence of an allylic intermediate in the oxidation of propylene to acrolein over bismuth molybdate. In these experiments, propylene was tagged once at Ci, another time at C2 and the third time at C3. 

A proposed mechanism for the oxidation of propylene to acrolein is by a first step abstraction of an allylic hydrogen from an adsorbed propylene by an oxygen anion from the catalytic lattice to form an allylic intermediate ... 

Malacria and co-workers76 were the first to report the transition metal-catalyzed intramolecular cycloisomerization of allenynes in 1996. The cobalt-mediated process was presumed to proceed via a 7r-allyl intermediate (111, Scheme 22) following C-H activation. Alkyne insertion and reductive elimination give cross-conjugated triene 112 cobalt-catalyzed olefin isomerization of the Alder-ene product is presumed to be the mechanism by which 113 is formed. While exploring the cobalt(i)-catalyzed synthesis of steroidal skeletons, Malacria and co-workers77 observed the formation of Alder-ene product 115 from cis-114 (Equation (74)) in contrast, trans-114 underwent [2 + 2 + 2]-cyclization under identical conditions to form 116 (Equation (75)). 

Scheme 16.2 illustrates the catalytic mechanism proposed by Muetterties and coworkers [13]. Salient features of this mechanism are the coordination of benzene in the -fashion, to give a transient Col I( 4-C, iH, i)(PR3)2 complex, and the intramolecular hydride transfer to form the allylic intermediate Co(//3-Ctl l7) (PR3)2. Hydrogen addition would give an 4-1,3-cyclohexadiene complex that ultimately releases cyclohexane via H2 addition/hydride migration steps. Complete cis stereoselectivity of hydrogen addition was demonstrated by replacing H2 with D2. 

As an inversion of enantioselectivity was observed experimentally for 4-(dimethylamino)styrene, (64% R ee) as compared to styrene (64% S ee), we have recalculated the relative thermodynamic stabilities of endo and exo isomers for each step of the catalytic cycle using this second substrate. These calculations allow us to verify the quality of our findings by checking if an inversion in the relative stabilities of the endo and the exo-ri3-silyl-allyl intermediates (with the endo being more stable than the exo) is observed with 4-(dimethylamino)styrene. Using 4-(dimethylamino)styrene as the substrate, the calculated relative stabilities of the intermediates in the Chalk-Harrod mechanism are shown as parenthetic values in Figure 15. 

The mechanistic studies were carried out mainly with butadiene and two mechanisms were suggested depending first of all on the trans/cis ratio of the formed 2-butene. On Pd and sometimes on Co catalysts the trans/cis ratio is high and the mechanism is based on formating of syn- and awfi -jr-allyl intermediates which cannot interconvert on the surface. On other metals, where the trans/cis ratio is about unity, the intermediates are Tt-alkenes or cr-alkyls that may interconvert more freely36. 

In the present chapter, no explicit discussion or review of the acid-and/or base-catalyzed isomerization of olefins will be included. The discussion will be confined to isomerizations achieved with soluble transition metal complexes. However, it will be seen that addition and elimination reactions and allylic intermediates figure prominently in discussions of the mechanisms. 

The dissociative mechanism can explain both facts in that the hydrogen removed in the first step may recombine with an isomeric form of the ally lie intermediate to yield the isomeric olefin. Apparently syn and anti 7T-allylic complexes [67, 68) retain their configurations unless each may be converted into a common a-bonded complex in which the nonterminal carbon atoms of the allyl group are connected by a single bond and the isomerization of the intermediate can be represented as in Fig. 11. However, the recombination of the hydrogen atom with the allylic intermediate must be faster than the rate at which it enters the surface pool of... 

Yeast isopropylmalate isomerase of the leucine biosynthetic pathway, which catalyzes a totally analogous reaction to that of aconitase, converts 3-hydroxy-3-carboxy-4-methylpentanoate to 2-hydroxy-3-carboxy-4-methylpentanoate via an allylic intermediate. In its initial characterization by EPR spectroscopy, a high-field shift in its EPR signal from a g-average of 1.96 to 1.90 is seen upon addition of substrate (70). This result suggests that its mechanism is the same as that found for aconitase. 

F and B NMR spectroscopy. The rate of propene polymerisation with this system was only three times faster than that of 1-hexene. This slow rate contributes to the high regioselectivity of the polymerisation no 2,1-propene misinsertions were detected. H and NMR spectroscopy also provided information about the chain termination mechanism here this occurred by p-H elimination in a first-order process. Polymer chain-end epimerisation, i.e. chirality inversion at the P-carbon of the polymer chain (Scheme 8.31), proceeded via a zirconium tert-alkyl (rather than tt-allyl) intermediate [96c]. 

The termination step for 1-alkene formation is now the reaction of the surface alkenyl with surface H instead of the p-elimination step. Chain branching can proceed by the involvement of allylic intermediates. Since this new mechanism involves different types of reactions to form C2 and C2< hydrocarbons, it is not expected that the amounts of C2 products will lie on the normal curve of the Ander-son-Schulz-Flory distribution. 

The relative contribution of the two mechanisms to the actual isomerization process depends on the metals and the experimental conditions. Comprehensive studies of the isomerization of n-butenes on Group VIII metals demonstrated179-181 that the Horiuti-Polanyi mechanism, the dissociative mechanism with the involvement of Jt-allyl intermediates, and direct intramolecular hydrogen shift may all contribute to double-bond migration. The Horiuti-Polanyi mechanism and a direct 1,3 sigma-tropic shift without deuterium incorporation may be operative in cis-trans isomerization. 

Fig. 10. Mechanism for the isomerisation of the n-butenes involving 7T-allylic intermediates.

Conclusive evidence for the participation of 7r-allylic intermediates in double bond migration has been obtained from a study of the nickel-catalysed hydrogenation of the isomeric olefinic esters methyl oleate and methyl elaidate using tritium as a tracer [147]. It was also concluded that in this system cis—trans isomerisation occurred by an addition—abstraction mechanism. 

Mechanism. The mechanism outlined for the propene oxidation over metal oxides is, in general, fully applicable to bismuth molybdate. The occurrence of a symmetrical allyl intermediate and the participation of lattice oxygen is well established (Hucknall [160], Voge and Adams [343]). 

The conversion of isobutene to methacrolein is closely related to the selective oxidation of propene to acrolein and demands similar catalysts. It has been verified that the same mechanism applies, involving a symmetrical allylic intermediate, viz. 

Kinetics of reaction must be considered when attempting to postulate mechanisms, but kinetic equations alone are unreliable in fixing mechanism. For example, in the oxidation of propylene to acrolein, cuprous oxide and bismuth molybdate have very different kinetics, yet the studies of Voge, Wagner, and Stevenson (18), and especially of Adams and Jennings (1, 2) show that in both cases the mechanism is removal of an H atom from the CH3 group to form an allylic intermediate, from which a second H atom is removed before the O atom is added. The orders of the reactions and the apparent optimum catalysts (16) are as follows ... 

For specific cases such as olefin oxidation over Bi-Mo oxide combinations some information concerning the oxidation mechanism is available. The work of Adams and Jennings (2), of Sachtler (16), and of Adams (1) has led to the general acceptance of an allylic intermediate. The discoverers of the Bi-Mo catalyst system (21) showed that propene is converted to acrolein, while Hearne and Furman (9) proved that butene forms butadiene. The allylic intermediate therefore can in principle react in two different ways (1) formation of a conjugated diene... 

The catalytic isomerization reation of olefins is caused by either an associative or a dissociative mechanism. The associative mechanism involves either dissociative mechanism involves allylic intermediates, as described in Scheme 2, where M—H and M represent active sites. 

Recently it has been reported that the catalytic isomerization of allylic alcohols is promoted by [Rh(diphosphine)(solvent)2]+ at 25°C yields synthetically useful quantities of the corresponding simple enols and that the transformation of allylic alcohols to enols and thereby to ketonic products proceeds catalytically via hydrido-7t-allylic and hydrido-7t-oxy-allylic intermediates, respectively [20]. Consistently observed, enantioselection has been in the process of conversion of a prochiral enol to a chiral aldehyde. Thus, the prochiral substrate 32 is transformed to the optically active aldehyde 34 with 18% ee by using [Rh(BINAP)]+ catalyst (eq 3.13). Accordingly, this isomerization proceeds via a different mechanism from that of the isomerization of allylamine. For the reaction mechanism of the... 

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