Alkenes, isomerization

The blue solutions have been found to catalyse alkene isomerization and hydrogenation and have very considerable synthetic utility (Figure 1.8). 

L. Ploense, M. Salazar, B. Gurau, and E. Smotkin, Spectroscopic study ofNEMCA promoted alkene isomerizations at PEM fuel cell Pd-Nafion cathodes, Solid State Ionics 136-137, 713-720(2000). 

Cobalt catalysts such as HCo(CO)4 are widely used for hydroformyla-tion of higher alkenes, despite the higher temperatures and pressures required. The main reason for this is that these catalysts are also efficient alkene isomerization catalysts, allowing a mix of internal and terminal alkenes to be used in the process. Catalyst recovery is more of a problem here, involving production of some waste and adding significantly to the complexity of the process. A common recovery method involves treating the catalyst with aqueous base to make it water soluble, followed by separation and subsequent treatment with acid to recover active catalyst (4.3). 

With the renaissance in alkene chemistry engendered by the rising versatility of olefin metathesis in both fine chemical and commodity production, new methods for alkene isomerization are of increasing interest and importance. Alkene isomerization can be performed using Bronsted-Lowry acid or base catalysis (1). However, these reactions are limited to substrates which tolerate carbanionic or carbocation intermediates, and are susceptible to undesired side reactions. 

Transition metal catalysts offer a way to isomerize alkenes under neutral conditions. There are numerous catalysts capable of moving a double bond over a few positions (2-7). When it comes to more extensive movement, however, the apparent record for alkene isomerization is 20 positions on the hydrocarbon CH3(CH2)i9CH=CH(CH2)i9CH3 by stoichiometric amounts of the reagent... 

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... 

OH must help subsequent isomerization, because ( )-30 is stable in the presence of 1, even after 2 d at 70°C. To test whether the bulk of the silyl ether protecting group was responsible for this reactivity difference, compound 32 was allowed to isomerize, forming a high yield of (ii)-33 at room temperature. In this case, heating at 70°C for 15 h does seem to lead to traces of what is tentatively identified as 34, but the second isomerization is clearly much slower than the first. Further studies are planned to elucidate the role of a hydroxyl substituent in alkene isomerization... 

In Section 3.5 on alkene isomerization, it was mentioned that Li and co-workers reported a RuCl2(PPh3)3-catalyzed shuffling of functional groups of allylic alcohols in water (Eq. 3.35).140 Since the reaction proceeds through an enol intermediate, allyl alcohols can thus be considered as enol equivalents.203 This has been developed into an aldol-type reaction by reacting allyl alcohols with aldehyde (Scheme 3.11).204 The presence of In(OAc)3 promoted the aldol reaction with a-vinylbenzyl alcohol and aldehyde.205... 

Recently, a new class of phosphabarrelene/rhodium catalysts has been developed, which for the first time allows for hydroformylation of internal alkenes with very high activity and which proceeds essentially free of alkene isomerization [36-38]. Two examples, results of hydroformylation of an acyclic and a cyclic internal alkene substrate, are depicted in Scheme 2. 

The reaction can be combined with an alkene isomerization, which requires the use of the more electron-withdrawing XANTPHENOXAPHOS ligand. Thus, starting from internal alkenes, linear amines can be obtained in quite reasonable yields and high n/iso selectivity (Scheme 15) [60]. 

Double bonds tend to migrate to more highly substituted positions within a substrate that is, terminal alkenes isomerize to disubstituted or trisubstituted alkenes, disubstituted alkenes tend to migrate to trisubstituted, and trisubstituted to tetrasubstituted alkenes. Of course, migration can go both ways, and adsorbed surface species may not exhibit the same thermodynamic stability as their desorbed relatives. (The rate of migration is strongly catalyst dependent for example, it frequently occurs rapidly on Pd and slowly on Pt.)... 

On the basis of these new results, and that fact that no alkene isomerization was observed under these reaction conditions, it was concluded that this reaction proceeds in a nonstereoselective manner, via a stepwise mechanism. This conclusion was further supported by an ab initio study conducted on bicyclic S-S dications as outlined in Section 12.11.5.1.1(1). 

However, the pathways for these reactions, particularly in the gas phase, have been only -.rtially characterized. In a wide variety of these reactions, coordinatively unsaturated, highly reactive metal carbonyls are produced [1-18]. The products of many of these photochemical reactions act as efficient catalysts. For example, Fe(C0)5 can be used to generate an efficient photocatalyst for alkene isomerization, hydrogenation, and hydrosilation reactions [19-23]. Turnover numbers as high as 3000 have been observed for Fe(C0)5 induced photocatalysis [22]. However, in many catalytically active systems, the active intermediate has not been definitively determined. Indeed, it is only recently that significant progress has been made in this area [20-23]. 

Recently, we have demonstrated another sort of homogeneous sonocatalysis in the sonochemical oxidation of alkenes by O2. Upon sonication of alkenes under O2 in the presence of Mo(C0) , 1-enols and epoxides are formed in one to one ratios. Radical trapping and kinetic studies suggest a mechanism involving initial allylic C-H bond cleavage (caused by the cavitational collapse), and subsequent well-known autoxidation and epoxidation steps. The following scheme is consistent with our observations. In the case of alkene isomerization, it is the catalyst which is being sonochemical activated. In the case of alkene oxidation, however, it is the substrate which is activated. 

Alkene isomerization has both positive and negative aspects. The positive aspect is where isomerization is needed prior to, for example, hydro formylation to give the desired product. The negative aspect of alkene isomerization is similar to that described in Section 2.6.2.1 on hydrogenation. The byproduct must be separated from both catalyst and product, and recycle opportunities may be limited. Not only is isomerization a direct efficiency loss, but when the isomerised alkene is purged, desired reactants will likely also be lost. 

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