Redox isomerizations are examples of atom-economical processes 1 in which one site in an organic substrate is oxidized with concomitant reduction of another site. One especially well-studied example is the conversion of allylic alcohols to aldehydes or ketones, 2,3 which involves movement of the alkene double bond 4-7 over two positions (eq 1, n) 2). Far fewer catalysts exist for the movement of a more remote double bond (n > 2). For example, Kirchner et al. reported 8 that, although [CpRu(PR 3)(CH 3 CN) 2 ] + (1a-1c) were improved catalysts for allylic alcohol isomerization relative to CpRu(PR 3) 2 Cl, it failed in the case of 3-buten-1-ol (n) 3) or alkenes devoid of alcohol functionality. The apparent record for alkene isomerization of any kind is over 20 positions on the hydrocarbon CH 3 (CH 2) 19 CHdCH(CH 2) 19 CH 3 by stoichiometric amounts of the reagent Cp 2 Zr(H)(Cl). 9 The apparent record for catalyzed double bond movement is on 9-decene-1-ol (nine positions, n) 9) using Fe 3 (CO) 12. 10,11 However, 30 mol % was required, which means that nearly a mole of metal was used per mole of alkenol.

Although controlling both the position of the double bond and E:Z selectivity in alkene isomerization is difficult, 1 is a very efficient catalyst for selective mono-isomerization of a variety of multifunctional alkenes to afford >99.5% E-products. Many reactions are complete within 10 min at room temperature. Even sensitive enols and enamides susceptible to further reaction can be generated. Catalyst loadings in the 0.01-0.1 mol% range can be employed. E-to-Z isomerization of the product from diallyl ether was only <10(-6) times as fast as its formation, showing the extremely high kinetic selectivity of 1.

After searching for the proper catalyst, the dual challenges of controlling the position of the double bond, and cis/trans-selectivity in isomerization of terminal alkenes to their 2-isomers are finally met in a general sense by mixtures of (C5Me5)Ru complexes 1 and 3 featuring a bifunctional phosphine. Typically, catalyst loadings of 1 mol % of 1 and 3 can be employed for the production of (E)-2-alkenes at 40-70 °C. Catalyst comprising 1 and 3 avoids more than any other known example the thermodynamic equilibration of alkene isomers, as the trans-2-alkenes of both nonfunctionalized and functionalized alkenes are generated.

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A tandem catalytic reaction has been developed as part of a process to discover tungsten-based olefin metathesis catalysts that have a strong preference for terminal olefins over cis or trans internal isomers in olefin metathesis. This tandem isomerization/terminal olefin metathesis reaction (ISOMET) converts C n trans internal olefins into C2n–2 cis olefins and ethylene. This reaction is made possible with Ru-based “alkene zipper” catalysts, which selectively isomerize trans olefins to an equilibrium mixture of trans and terminal olefins, plus tungsten-based metathesis catalysts that react relatively selectively with terminal olefins to give Z homocoupled products. The most effective catalysts are W­(NAr)­(C3H6)­(pyr)­(OHIPT) (Ar = 2,6-diisopropylphenyl; pyr = pyrrolide; OHIPT = O-2,6-(2,4,6-i-Pr3C6H2)2C6H3) and various [CpRu­(P–N)­(MeCN)]­X (X– = [B­(3,5-(CF3)2C6H3)4]–, PF6 –, B­(C6F5)4 –) isomerization catalysts.