catalysts Article Synthesis and Application of Novel Ruthenium Catalysts for High Temperature Alkene Metathesis Tegene T. Tole, Jean I. du Toit, Cornelia G. C. E. van Sittert, Johan H. L. Jordaan and Hermanus C. M. Vosloo * Research Focus Area for Chemical Resource Beneficiation, Catalysis and Synthesis Research Group, North-West University, Hoffmann Street, 2531 Potchefstroom, South Africa; [email protected] (T.T.T.); [email protected] (J.I.d.T.); [email protected] (C.G.C.E.v.S.); [email protected] (J.H.L.J.) * Correspondence: [email protected]; Tel.: +27-18-299-1669 Academic Editors: Albert Demonceau, Ileana Dragutan and Valerian Dragutan Received: 24 November 2016; Accepted: 3 January 2017; Published: 10 January 2017 Abstract: Four pyridinyl alcohols and the corresponding hemilabile pyridinyl alcoholato ruthenium carbene complexes of the Grubbs second generation-type RuCl(H 2 IMes)(OˆN)(=CHPh), where OˆN = 1-(2 ′ -pyridinyl)-1,1-diphenyl methanolato, 1-(2 ′ -pyridinyl)-1-(2 ′ -chlorophenyl),1-phenyl methanolato, 1-(2 ′ -pyridinyl)-1-(4 ′ -chlorophenyl),1-phenyl methanolato and 1-(2 ′ -pyridinyl)- 1-(2 ′ -methoxyphenyl),1-phenyl methanolato, are synthesized in very good yields. At high temperatures, the precatalysts showed high stability, selectivity and activity in 1-octene metathesis compared to the Grubbs first and second generation precatalysts. The 2-/4-chloro- and 4-methoxy-substituted pyridinyl alcoholato ligand-containing ruthenium precatalysts showed high performance in the 1-octene metathesis reaction in the range 80–110 ◦ C. The hemilabile 4-methoxy-substituted pyridinyl alcoholato ligand improved the catalyst stability, activity and selectivity for 1-octene metathesis significantly at 110 ◦ C. Keywords: alkene metathesis; hemilabile ligand; pyridinyl alcohol; ruthenium carbene; Grubbs-type precatalyst; 1-octene 1. Introduction The well-defined Grubbs first ( 1 and 2 ) [ 1 – 4 ] and second ( 3 ) [ 5 – 7 ] generation precatalysts are found to be very active and robust toward a large number of substrates. These precatalysts however showed activity at room temperature in alkene metathesis reactions [ 2 , 8 , 9 ]. Although they perform excellent at room temperature, they have a relatively short catalytic lifetime.   Catalysts   2017 ,   7 ,   22;   doi:10.3390/catal7010022   www.mdpi.com/jou rnal/catalysts   Article   Synthesis   and   Application   of Catalysts 2017 , 7 , 22 2 of 21 also reported [ 12 ] that the electronic and steric effects, together with the ring size and rigidity of the hemilabile bidentate ligands, can possibly influence the stability of the complex to which the bidentate ligand is attached and consequently the catalytic performance. Different groups followed different design concepts of initiators in the various precatalysts to obtain thermally-switchable initiators (Scheme 1) [13]. Catalysts   2017 ,   7 ,   22   2   of   21   Raubenheimer   et   al.   [10].   Grubbs ‐ type   precatalysts   with   hemilabile   ligands   showed   greater   lifetimes   and   stability   [10,11].   It   was   also   reported   [12]   that   the   electronic   and   steric   effects,   together   with   the   ring   size   and   rigidity   of   Catalysts 2017 , 7 , 22 3 of 21 1-(2 ′ -pyridinyl)-1,1-diphenyl-methanols having chlorine and/or methoxy substituents on one of the phenyl rings. Catalysts   2017 ,   7 ,   22   3   of   21     Scheme   2.   Synthesis   of   pyridinyl   alcohols   5 – 8 .   The   synthesis   of   the   alcohols   was   performed   in   a   dry   and   inert   three ‐ neck   round ‐ bottomed   flask   by   stirring   the   mixture   of   nBuLi   solution   with   2 ‐ bromopyridine,   in Catalysts 2017 , 7 , 22 4 of 21 Catalysts   2017 ,   7 ,   22   4   of   21   density   around   the   atom   [30].   Therefore,   the   chelation   of   the   pyridinyl ‐ alcoholato   ligands   to   the   ruthenium   metal   is   evident.   The   precatalysts   13 – 15   were   all   synthesized   for   the   first   time,   as   we   did   not   find   any   reported   in   the Catalysts 2017 , 7 , 22 5 of 21 Table 2. Summary of products of 1-octene metathesis in the presence of ruthenium alkylidene precatalysts. PMPs (primary metathesis products); IPs (isomerization products); SMPs (secondary metathesis products). Reaction Substrate 1 Products 1 Abbreviation Primary metathesis - - - Self-metathesis C=C 7 C=C + C 7 =C 7 PMPs Isomerization C=C 7 C 2 =C 6 + C 3 =C 5 + C 4 =C 4 IPs Secondary metathesis 2 - - - Self-metathesis C 2 =C 6 C 2 =C 2 + C 6 =C 6 SMPs Cross-metathesis C=C 7 + C 2 =C 6 C 2 =C 7 + C=C 6 + C=C 2 + C 6 =C 7 1 Hydrogens are omitted for simplicity; 2 only representative examples of SMPs are shown. 2.3.1. Effect of Temperature on Catalyst Activity and Selectivity in 1-Octene Metathesis The effect of temperature on the catalytic activities of precatalysts 13 – 15 was investigated for temperatures of 70, 80, 90, 100 and 110 ◦ C. The results of the metathesis reaction of 1-octene with 13 and 14 at 70 ◦