Catalyst versus Substrate Control of Forming (E)-2-Alkenes from 1-Alkenes Using Bifunctional Ruthenium Catalysts

Abstract

Here we examine in detail two catalysts for their ability to selectively convert 1-alkenes to (E)-2-alkenes while limiting overisomerization to 3-or 4-alkenes. Catalysts 1 and 3 are composed of the cations CpRu(κ 2 -PN)(CH 3 CN) + and Cp*Ru(κ 2 -PN) + , respectively (where PN is a bifunctional phosphine ligand), and the anion PF 6 − . Kinetic modeling of the reactions of six substrates with 1 and 3 generated firstand second-order rate constants k 1 and k 2 (and k 3 when applicable) that represent the rates of reaction for conversion of 1-alkene to (E)-2-alkene (k 1 ), (E)-2-alkene to (E)-3-alkene (k 2 ), and so on. The k 1 :k 2 ratios were calculated to produce a measure of selectivity for each catalyst toward monoisomerization with each substrate. The k 1 :k 2 values for 1 with the six substrates range from 32 to 132. The k 1 :k 2 values for 3 are significantly more substratedependent, ranging from 192 to 62 000 for all of the substrates except 5-hexen-2-one, for which the k 1 :k 2 value was only 4.7. Comparison of the ratios for 1 and 3 for each substrate shows a 6−12-fold greater selectivity using 3 on the three linear substrates as well as a >230-fold increase for 5-methylhex-1-ene and a 44-fold increase for a silyl-protected 4-penten-1-ol substrate, which are branched three and five atoms away from the alkene, respectively. The substrate 5-hexen-2-one is unique in that 1 was more selective than 3; NMR analysis suggested that chelation of the carbonyl oxygen can facilitate overisomerization. This work highlights the need for catalyst developers to report results for catalyzed reactions at different time points and shows that one needs to consider not only the catalyst rate but also the duration over which a desired product (here the (E)-2-alkene) remains intact, where 3 is generally superior to 1 for the title reaction.

Dynamic π-Bonding of Imidazolyl Substituent in a Formally 16-Electron Cp*Ru(κ2-P,N)+ Catalyst Allows Dramatic Rate Increases in (E)-Selective Monoisomerization of Alkenes

“…Selective monoisomerization was also seen with 1,2-epoxy-5-hexene (Table , entry 20) with very little enal formation due to the deactivation of catalyst by decarbonylation of the aldehyde . In entry 18, 5-hexen-2-one suffers from more rapid overisomerization with a maximum yield of monoisomerized product at 66% at 4 h, but monoisomerization is still favored over conjugation by around 3:1 at this point. The conversion of 1,7-octadiene to (2 E ,6 E )-2,6-octadiene requires the migration of two separate double bonds; nevertheless, with higher 2 mol % loading, the doubly monoisomerized product yield reaches >94% within 10 min, but the conjugated diene does not appreciably increase within 24 h.…”

Selecting Double Bond Positions with a Single Cation-Responsive Iridium Olefin Isomerization Catalyst

“…The butene-containing substrates, in contrast, have only a moderate thermodynamic preference for the E isomer (ca. E : Z 4:1). , …”

“…E:Z 4:1). 54,56 The high stereoselectivity of 2-18c6b in the absence of salts is proposed to originate from steric congestion of the crown ether (Scheme 1). Computational studies have shown that the Ir−O bond trans to phenyl is easiest to dissociate, which can open up an alkene binding site.…”

Selecting Double Bond Positions with a Single Cation- Responsive Iridium Olefin Isomerization Catalyst

“…E:Z 4:1). 5,58 The high stereoselectivity of 2-18c6b in the absence of salts is proposed to originate from steric congestion of the crown ether (Scheme 1). Computational studies have shown that the Ir-O bond trans to phenyl are easiest to dissociate, which can open up a binding site for alkene to bind.…”