Letter pubs.acs.org/acscatalysis
High-Selectivity Bis(imino)pyridine Iron Catalysts for the Hydrosilylation of 1,2,4-Trivinylcyclohexane Crisita Carmen Hojilla Atienza,† Aaron M. Tondreau,† Keith J. Weller,‡ Kenrick M. Lewis,§ Richard W. Cruse,§ Susan A. Nye,‡ Julie L. Boyer,‡ Johannes G. P. Delis,⊥ and Paul J. Chirik†,* †
Department of Chemistry, Princeton University, Princeton, New Jersey, United States 08544 Momentive Performance Materials, 260 Hudson River Road, Waterford, New York, United States 12188 § Momentive Performance Materials, 769 Old Saw Mill River Road, Tarrytown, New York, United States 10591 ⊥ Momentive Performance Materials bv., Plasticslaan 1, Bergen op Zoom, The Netherlands 4612PX ‡
S Supporting Information *
ABSTRACT: Aryl-substituted bis(imino)pyridine iron dinitrogen complexes are active for the hydrosilylation of 1,2,4trivinylcyclohexane with tertiary alkoxy silanes, a process used in the manufacture of low rolling resistance tires. The iron compounds exhibit unprecedented selectivity for the monohydrosilylation of the desired 4-alkene that far exceeds results obtained with commercially used platinum compounds. KEYWORDS: iron, catalysis, hydrosilylation, silane, trivinylcyclohexene
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tire chemistry requires the addition of a single [Si−H] from an alkoxysilane to a single alkene. Two vinyl groups must be left intact, ideally those that are adjacent, for subsequent functionalization with sulfur and cross-linking into the rubber matrix. Thus, it is advantageous to have the monohydrosilylation occur exclusively at the 4-position (Chart 1). 9 Alkoxysilanes are required for bonding to a silica surface. Performing the hydrosilylation of the isomers of TVCH with commercially employed platinum catalysts such as Karstedt’s and Speier’s catalysts, Pt2 {[CH2 CH)SiMe2 ]2 O} 3 and H2PtCl6·6H2O/iPrOH,10,11 yields a mixture of products arising from multiple silylations with poor regioselectivity among the various alkene positions.8 Even when a slow addition of a substoichiometric amount of trialkoxysilane ((RO)3Si−H/ TVCH ratio of 0.8:1) is used, a 50:30 mixture (by weight) of mono to disilylated products is obtained, with the balance being unreacted TVCH isomers. Within the monoaddition products, the hydrosilylation occurs statistically at the three different alkene positions. This lack of selectivity requires purification by
electivity and specificity are often enabling features of homogeneous catalysis.1,2 Structure−activity relationships and rational ligand design are well-demonstrated approaches to modulate and improve reactivity to achieve a desired outcome. A particularly intriguing challenge for selective catalysis is the anti-Markovnikov hydrosilylation of the mixture of the four stereoisomers (A−D) present in commercial samples of 1,2,4trivinylcyclohexane (TVCH, Chart 1). These hydrosilylation products have a wide range of applications, including silicone coatings,3−5 aerospace lubricants,6 cross-linkers for contact lens materials,7 and coupling agents in silica-filled automotive tires with low rolling resistance and improved wear.8 While the other applications necessitate hydrosilylation of all three alkenes, the Chart 1. Four Stereoisomers (A−D) of 1,2,4Trivinylcyclohexane
Received: September 5, 2012 Published: September 7, 2012 © 2012 American Chemical Society
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dx.doi.org/10.1021/cs300584b | ACS Catal. 2012, 2, 2169−2172
ACS Catalysis
Letter
Table 1. Evaluation of Bis(imino)pyridine Iron Dinitrogen and Traditional Pt Complexes for the Hydrosilylation of the Isomers of TVCH with MD′M or TES.a compound
TVCH
silane
time (min)
remaining silane (mol %)
remaining TVCH (mol %)
mono-silylation (mol %)
disilylation (mol %)
(iPrPDI)Fe(N2)2 (4-NMe2-iPrPDI)Fe(N2)2 [(iPr,EtPDI)FeN2]2(μ2-N2) [(EtPDI)FeN2]2(μ2-N2) [(MePDI)FeN2]2(μ2-N2) (iPrPDI)Fe(N2)2 (4-NMe2-iPrPDI)Fe(N2)2 [(iPr,EtPDI)FeN2]2(μ2-N2) [(EtPDI)FeN2]2(μ2-N2) [(MePDI)FeN2]2(μ2-N2) [(EtPDI)FeN2]2(μ2-N2) [(EtPDI)FeN2]2(μ2-N2) [(EtPDI)FeN2]2(μ2-N2) [(EtPDI)FeN2]2(μ2-N2) Speier’s catalystb Speier’s catalystb Speier’s catalyst (optimized)d
A A A A A A A A A A B B C C A A A
MD′M MD′M MD′M MD′M MD′M TES TES TES TES TES MD′M TES MD′M TES MD′M TES TES
180 180 60
