Olefins migration isomerization

In the presence of transition-metal complexes, organic compounds that are unsaturated or strained often rearrange themselves. One synthetically useful transition-metal catalyzed isomerization is the olefin migration reaction. Two general mechanisms have been proposed for olefin migrations, depending on the type of catalyst employed (A and B) (Scheme 3.8).137... 

Recently, Grubbs138 demonstrated that olefin isomerization of allyl-lic ethers and alcohols is catalyzed by Ru(II)(H20)6(tos)2 (tos = p-toluenesulfonate) in aqueous medium. The olefin migration products, enols, and enol ethers thus generated are unstable and are hydrolyzed instantly to yield the corresponding carbonyl compounds (Eq. 3.34). 

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. 

To distinguish between 4PN or 3PN precursors to MGN, further experiments were done using DCN, followed by analysis of dinitrile products and recovered 3PN by GC/MS and H NMR. The simplest results were obtained with ZnCl2, where little or no deuterium incorporation in the recovered 3PN was observed. The ability of this NiL /DCN/ZnC catalyst system to isomerize double bond migration without deuterium incorporation in the isomerized olefin is reminiscent of the isomerization of 1-butene with Ni[P(0Et)3]4/D2S04, where the ratio of isomerization to deuteration rates was 170 (at 0°C). Figure 12 shows the possible pathways to DN-d, products from PN-d0 precursors. Essentially all of the MGN comes from wrong-way addition of DCN to 4PN, rather than from 3PN. This is consistent with rate measurements which show that addition of HCN to 4PN is 500-600 times faster than addition to 3PN (47). 

Indeed, a ruthenium-catalyzed tandem olefin-migration/aldol-type reaction has been realized when an aldehyde is present in aqueous media [18,19]. For 3-butene-2-ol (9), the tandem isomerization/aldol-type reaction was examined. The mixture of 9, aldehyde (10), and a catalytic amount of RuCl2(PPh3)3 in H20/toluene(4/l) (Eq. 6) or H20 alone (Eq. 7) was stirred for 5 h at 110 °C (oil bath temperature) and afforded the aldol adduct 11. 

Example 11.2. Streamlined network for hydroformylation of n-heptene catalyzed by phosphine-substituted cobalt hydrocarbonyl. In hydroformylation of straight-chain olefins with a phosphine-substituted cobalt hydrocarbonyl catalyst, the model must account for three complications that are absent with cyclohexene isomerization by migration of the double bond along the hydrocarbon chain, formation of isomeric aldehydes and alcohols, and condensation of the straight-chain aldehyde to "heavy ends (chiefly an alcohol of twice the carbon number, such as 2-ethylhexanol from propene via n-butanal and a C8 aldol). A streamlined network for n-heptene is ... 

The immediate product from migration of the angular methyl group to Cp ) is a Cps)-carbonium ion (25), the fate of which varies according to the compound. Solvolysis of a secondary i7j -tosylate, or acid-catalysed dehydration of the i7j3 alcohol, leads to a mixture of olefins containing the isomer 26) as the major product, with minor amounts of j i3(i4) 27) and other olefins. An elaborate analysis by Johns [126] of the olefins obtained by the boric add-catalysed dehydration of oestradiol 3-methyl ether at high temperatures led to the characterisation of at least six isomeric olefins in the mixture. The precise mechanism of the action of boric acid has not been defined. It may involve thermal decomposition of a borate ester or merely the expulsion of the protonated... 

Depending on the metal and the specific nature of the ligand sphere (especially its steric bulk), the hydride migration ( insertion , step A) in the product-determining step can follow either the anti-Markovnikov or the Markovnikov path (Scheme 2). Only in the latter case and if subsequent y-H elimination takes place (step B), does an isomerized olefin result. 

On heating, alkylborons may undergo isomerization by migration of the boron to a less sterically encumbered position by a series of reversible j8-hydride elimina-tion-hydroboration steps. Displacement of the isomerized olefin may be effected by addition of a second olefin, which forms a more stable alkylboron than the one displaced. This reaction has been used to effect the contrathermodynamic isomerization of olefins. 

In the study of Ru-BINAP catalyzed asymmetric hydrogenation of geraniol 10, a striking isomerization to y-geraniol 11 was observed, Eq. (5) [24]. The reaction is noteworthy in that it promotes the olefin migration from an inner to an exo position. Originally the presence of 11 was supposed to be an intermediate... 

This possibility is not realized because the double bond rearranges or migrates after ionization but prior to fragmentation. As a result, isomeric olefins tend to give nearly identical spectra. Examples of this kind constitute a serious drawback of electron-impact ionization if complete sample identification is required. 

The addition of 1,4-benzoquinone was also found to prevent olefin isomerization in a number of ruthenium-catalyzed olefin metathesis reactions of allylic ethers (Scheme 12.32) [57]. When the siloxy ether 107, which bears a ds-olefin, was treated with Ru catalyst 3 (5 mol%) in CD2CI2 at 40 C for 24 h, a mixture of 107 and the corresponding tram isomer of 107 was observed in 19% yield, while 81% of the reaction mixture was the isomerized silyl enol ether product 108. The addition of 1,4-benzoquinone or acetic acid completely suppressed olefin migration, and mixtures of cis- and tram-105 were the major products (> 95%). Phenol, another common additive in olefin metathesis reactions, failed to inhibit olefin migration, and enol ether 108 was formed as the major product. 

Vilar and coworkers [59] have investigated the use of additives to suppress olefin migration in the self-metathesis of urea derivatives (Scheme 12.33). The Af-allyl urea 109 was found to undergo self-metathesis in the presence of Ru catalyst 3 in 33% yield. The isomerized species 110 was isolated as the major product (55% yield, Z= 1 1). Vilar and coworkers identified phenylphosphoric acid (112) as an effective inhibitor of the undesired alkene isomerization. When the metathesis of Af-allyl urea 109 was performed with phenylphosphoric acid (50mol%), the desired dimer 111 was isolated in 56% yield, and 30% of the starting material 109 could be recovered. The authors also investigated the use of benzoquinone derivatives, and found that 2,6-dichloro-l,4-benzoquinone (10 mol%) was equally... 

In 2007, Cossy and coworkers [61] demonstrated that small-molecule additives could be used to help inhibit olefin migration in challenging CM reactions (Scheme 12.35). Investigating the CM of lactone 117 with olefin 118 by H-NMR, the desired product 119 was formed in 50% yield, while a 13% yield of the isomerized lactone (120) was formed. A variety of small-molecule additives (5mol%) were then studied to suppress the undesired olefin isomerization. While a number of phosphorous-derived additives were surveyed, two additives clearly stood out as being the most efficient 2,6-dichloro-l,4-benzoquinone and -chlorocatechol. In particular, the 2,6-dichloro-l,4-benzoquinone was able to... 

Palladium complexes are able to assist in the Z/ -isomerization and migration of a variety of olefins. Heteroatoms in the olefin and trialkyl- or dialkylphosphines as ligands for palladium support isomerization [134]. 

The double bond migration in steroid hydrocarbons catalyzed by acids or noble metals (see, for example, ref. 185) will not be discussed here. A general review of nonsteroid olefin isomerization has recently been published. Iron carbonyl has been used to isomerize steroidal dienes. 

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