Alkenes allyl intermediates

Cp2Zr(H)(Cl) (8). The apparent record for catalyzed double bond movement is on 9-decene-l-ol to decanal (nine positions) using Fe3(CO)i2 (9). However, 30 mol % was required, which means that nearly a mole of metal was used per mole of alkenol. Herein we expand upon our initial report (10) of a very active catalyst (1) which has been shown to move a double bond over 30 positions. Catalyst 1 appears to have an intriguing and useful mode of action, in which the pendant base ligand performs proton transfer on coordinated alkene and Ti-allyl intermediates in a stereoselective fashion. 

The catalyst reported by Grotjahn and Lev (11-13) for alkyne hydration (2) is capable of isomerizing alkenes, but veiy slowly. Because we knew that the rate of alkyne hydration was unchanged in the presence of excess phosphine ligand, we thought that like alkyne hydration, alkene isomerization would require loss of acetonitrile ligand (14) and alkene binding. Subsequent deprotonation at an allylic position would make an q -allyl intermediate which when reprotonated at the other... 

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. 

Toyota, Ihara, and coworkers demonstrated that silyl enol ethers undergo Pd -promoted intramolecular nucleophilic attack on alkenes [18]. Allhough early examples required stoichiometric Pd [167], they have also shown that Pd(OAc)2 in DMSO is an effective catalyst in the presence of an aerobic atmosphere (Eq. 38) [168-170]. The reaction is proposed to proceed through an oxo-jt-allyl intermediate that can undergo competitive alkene insertion or P-hydride elimination (Scheme 11). The latter reaction is the basis for the synthetically useful conversion of silyl enol ethers to a,P-unsaturated carbonyl compounds (see below). Efforts to use BQ as an oxidant were not described. 

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. 

An ab initio method has been employed to study the mechanism of the thermal isomerization of buta-1,2-diene to buta-1,3-diene. The results of the study have indicated619 that the transformation proceeds in a stepwise manner via a radical intermediate. Experimental free energies of activation for the bond shift in halocyclooctatetraenes have been reported and analyzed by using ab initio MO calculations.620 The isomerization of hexene using a dihydridorhodium complex in dimethyl sulfoxide has been reported,621 and it has been suggested622 that the Pd(II)-catalysed homogeneous isomerization of hexenes proceeds by way of zr-allylic intermediates. A study has been made623 of alkene isomerization catalysed by the rhodium /-phosphine-tin dichloride dimeric complex, and the double-bond isomerization of olefinic amines over potassium amide loaded on alumina has been described.624... 

Cross-coupling of allylic compounds occurs by transmetallation between 7i-allyl intermediates and organometallic compounds of Mg, Zn, B, Al, Si and Sn, and subsequent reductive elimination. Reaction of the allylic dithioacetal 180 with MeMgBr in the presence of an Ni catalyst affords alkenes 184 bearing a tert-butyl group [90]. In this reaction, generation of the 7i-allylnickel 181 by oxidative addition and subsequent transmetallation with MeMgBr afford 182. Then the methylated product 183 is formed by reductive elimination, and finally the dimethylated product 184 is formed by the sequence of similar reactions. 

Intermolecular insertion of alkenes to a-allyl intermediates is possible with an Ru catalyst. For example, 3,5-dienecarboxamide 274 is formed in high yield by Ru(cod)(cot)-catalysed coupling of 2-butenyl methyl carbonate (273) with acrylamide in the presence of A-methylpiperidine [122], Ni-catalysed transformation of allyl 3-butenoate (275) to heptadienoic acids 276a and 276b proceeds by insertion of the double bond to 7r-allylnickel intermediate [123],... 

Although free-radical halogenation is a poor synthetic method in most cases, free-radical bromination of alkenes can be carried out in a highly selective manner. An allylic position is a carbon atom next to a carbon-carbon double bond. Allylic intermediates (cations, radicals, and anions) are stabilized by resonance with the double bond, allowing the charge or radical to be delocalized. The following bond dissociation enthalpies show that less energy is required to form a resonance-stabilized primary allylic radical than a typical secondary radical. 

Oxidative addition of HCN onto 9.54 with the elimination of COD leads to the formation of 9.56. Oxidative addition by HCN and coordination by the substrate onto 9.55 and 9.56, respectively, lead to the formation of 9.57. Insertion of the alkene functionality into the Ni-H bond leads to the formation of the p3-allyl intermediate 9.58. Substrate addition or oxidative addition of... 

The mode of COj linkage to a methylenccyclopropane involves the intermediate trimethylenemethane, which has also been Implicated in the cycloadditions of mcthylenecyclopropane to alkenes catalyzed by palladium(O). The insertion of COj into this allylic intermediate gives cyclic carboxylate species, which liberate the lactones, as shown in Scheme 11. 

Insight into the mechanism involved was obtained in two labeling studies, as shown in Eqs. 5.24 and 5.25. The former indicates that the carbon bearing the hydroxyl group preferentially forms the new C-C bond to the terminal alkyne carbon. The latter indicates that the alkene geometry is largely retained. These studies support the intervention of a jt-allyl species in which rotation around the ruthenium-allyl axis is slow relative to the rate of reductive elimination and the absence of a n-allyl intermediate. 

The dissociative mechanism of double bond migration via an allylic intermediate can occur at a site, as indicated in Scheme 2, or at a two atom site such as M—which can provide the three vacant coordination positions which are required to bind both the TT-allyl, a four-electron donor, and the hydrido group. Because the surface of a metal provides sites which differ in structure, both mechanisms may operate on a given catalyst, the relative importance being a function of the metal, the alkene and the reaction conditions. - ... 

The CT-allyl intermediate may be transformed to a ir-allyl structure with either the syn or anti configuration (relative to the methynyl hydrogen) as in equation (31), see also Scheme 2. The union with a second hydrogen leads to adsorbed alkenes 1-butene and either trans-2-buXtnt from the iyn-allyl intermediate or cw-2-butene from the anti form (Scheme 2). The desorption of the alkene competes with the further addition of hydrogen to form the alkane (equation 32). The reaction of a cr-allyl structure with hydrogen can yield the unbound alkene directly. The selectivity may depend upon the relative importance of these competitive reactions, which are likely to be a function of the metal as well as the reaction conditions. 

Fig. 15.2. Alkene hydrogenation mechanism proceeding through a n-allyl intermediate.

It has been proposed that 1,2-addition occurs through a bis Ti-adsorbed species, 60, whereas 1,4-addition occurs through an adsorbed 7t-allyl intermediate, 61. Catalysts that promote primarily 1,2-addition were referred to as Type A catalysts and those that react via a 1,4-addition mechanism as Type B catalysts. The 2-butenes formed over Type B catalysts have low cis/trans ratios with the trans 2-butene usually the predominant alkene present. Some 2-butenes are produced with Type A catalysts, but these have cis/trans ratios near unity. 

---