inorganics Article Ruthenium-Catalyzed Dimerization of 1,1-Diphenylpropargyl Alcohol to a Hydroxybenzocyclobutene and Related Reactions Hoang Ngan Nguyen 1, Naoto Tashima 1, Takao Ikariya 1 and Shigeki Kuwata 1,2,* ID 1 Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 2-12-1 E4-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan; [email protected] (H.N.N.); [email protected] (N.T.); [email protected] (T.I.) 2 Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan * Correspondence: [email protected]; Tel.: +81-3-5734-2150 Received: 28 October 2017; Accepted: 14 November 2017; Published: 16 November 2017 Abstract: Propargyl alcohol is a useful synthon in synthetic organic chemistry. We found that the 5 ruthenium(II) complex [Cp*RuCl(diene)] (Cp* = η -C5Me5; diene = isoprene or 1,5-cyclooctadiene (cod)) catalyzes dimerization of 1,1-diphenylprop-2-yn-1-ol (1,1-diphenylpropargyl alcohol, 1a) at room temperature to afford an alkylidenebenzocyclobutenyl alcohol 2a quantitatively. Meanwhile, a stoichiometric reaction of the related hydrido complex [Cp*RuH(cod)] with 1a at 50 ◦C led to isolation of a ruthenocene derivative 4 bearing a cyclopentadienyl ring generated by dehydrogenative trimerization of 1a. Detailed structures of 2a and 4 were determined by X-ray crystallography. The reaction mechanisms for the formation of 2a and 4 were proposed. Keywords: ruthenium; alkyne; propargyl alcohol; C–H cleavage; benzocyclobutene 1. Introduction Propargyl alcohol and their derivatives have been attractive starting materials in synthetic organic chemistry [1–7]. As one of the various transition-metal catalysts, ruthenium complexes with cyclopentadienyl co-ligands have been used effectively for their catalytic transformations involving carbon–carbon and carbon–heteroatom bond formations [8,9]. For example, the Hidai’s 5 thiolato-bridged dinuclear Cp*Ru (Cp* = η -C5Me5) complexes catalyze nucleophilic propargylic substitution of terminal propargylic alcohols owing to facile and reversible formation of an allenylidene intermediate [10–12]. Recently, Fürstner and co-workers described that regioselective hydrometalation [13,14] and ene–yne coupling [15] of propargylic alcohols are catalyzed by Cp*RuCl complexes. The latter reactions are guided by the intramolecular hydrogen bonds between the hydroxy group in the alcohol and the chlorido ligand. As an extension of our study on catalytic transformation of propargylic and structurally related allylic compounds [16–19], we report here the reactions of the Cp*Ru complexes [Cp*RuX(diene)] (X = Cl, H) with a 1,1-diphenylprop-2-yn-1-ol (1,1-diphenylpropargyl alcohol) HC≡CC(OH)Ph2 (1a). The phenyl substituent in 1a proved to undergo unexpectedly facile C–H bond cleavage or to migrate in the coordination sphere of the Cp*Ru complexes, leading to catalytic formation of a benzocyclobutene as a dimerization product and a novel ruthenocene complex bearing a highly functionalized cyclopentadienyl ring derived from dehydrogenative trimerization of 1a. Inorganics 2017, 5, 80; doi:10.3390/inorganics5040080 www.mdpi.com/journal/inorganics InorganicsInorganics 20172017,, 55,, 8080 2 of 8 Inorganics 2017, 5, 80 2 of 8 2. Results and Discussion 2. Results and Discussion 2.1. Ruthenium-Catalytic Dimerization of 1,1-Arylpropargyl Alcohol 2.1. Ruthenium-Catalytic Dimerization of 1,1-Arylpropargyl Alcohol The addition of the ruthenium(II) complex [Cp*RuCl(diene)] (diene = isoprene or 1,5- The addition addition of of the the ruthenium(II) ruthenium(II) complex complex [Cp*RuCl(diene)] [Cp*RuCl(diene)] (diene (diene = isoprene = isoprene or 1,5- or cyclooctadiene (cod)) to 50 equiv. of 1,1-diphenylpropargyl alcohol 1a in THF at room temperature 1,5-cyclooctadienecyclooctadiene (cod)) (cod)) to 50 to 50equiv. equiv. of of1,1-diphenylpropargyl 1,1-diphenylpropargyl alcohol alcohol 1a1a inin THF THF at at room room temperature temperature resulted in full conversion of the alcohol. A subsequent chromatographic workup afforded a novel resulted in full conversion of the alcohol. A subsequentsubsequent chromatographic workup afforded a novel alkylidenebenzocyclobutenyl alcohol 2a in 97% yield (Scheme 1). The p-methoxyphenyl analogue 1b alkylidenebenzocyclobutenyl alcohol alcohol 2a2a inin 97% 97% yield yield (Scheme (Scheme 1).1). The The p-methoxyphenylp-methoxyphenyl analogue analogue 1b was also converted to 2b. The products 2 were characterized by X-ray analysis of 2a as well as 1H and 1bwaswas also also converted converted to 2b to. The2b. products The products 2 were 2characterizedwere characterized by X-ray by analysis X-ray analysisof 2a as well of 2a as as1H welland 13C{1H} NMR spectroscopy. Figure 1 clearly shows the benzocyclobutene framework of 2a. The C–C 13asC{11HH} and NMR13 C{spectroscopy.1H} NMR spectroscopy. Figure 1 clearly Figure shows1 clearly the benzocyclobutene shows the benzocyclobutene framework of 2a framework. The C–C bond distances in the six-membered ring of the benzocyclobutene core fall in the range of 1.383(2)– bondof 2a. distances The C–C in bond the six-membered distances in the ring six-membered of the benzocyclobutene ring of the benzocyclobutenecore fall in the range core of fall 1.383(2)– in the 1.398(2) Å, indicating the delocalization of the double bonds. Meanwhile, the short C2–C15 and C16– 1.398(2)range of Å, 1.383(2)–1.398(2) indicating the delocalization Å, indicating theof the delocalization double bonds. of theMeanwhile, double bonds. the short Meanwhile, C2–C15 and the shortC16– C17 distances (1.337(2) Å) as well as the long C15–C16 distance (1.456(2) Å) in 2 are in agreement C17C2–C15 distances and C16–C17 (1.337(2) distances Å) as well (1.337(2) as the long Å) as C15–C16 well as thedistance long C15–C16(1.456(2) distanceÅ) in 2 are (1.456(2) in agreement Å) in 2 with the butadiene skeleton derived from dimerization of 1. The hydroxy groups is preserved withare in the agreement butadiene with skeleton the butadiene derived skeleton from dimerization derived from of dimerization 1. The hydroxy of 1. The groups hydroxy is preserved groups is throughout the reaction despite of their ease of dehydration in the coordination sphere [20]. The 1H throughoutpreserved throughout the reaction the despite reaction of despitetheir ease of theirof dehydration ease of dehydration in the coordination in the coordination sphere [20]. sphere The [ 201H]. NMR spectrum of 2a displays three mutually coupled vinyl resonances at δ 6.27, 6.32, and 6.45; these NMRThe 1H spectrum NMR spectrum of 2a displays of 2a displays three mutually three mutually coupled coupled vinyl resonances vinyl resonances at δ 6.27, at δ6.32,6.27, and 6.32, 6.45; and these 6.45; signals are assigned to the H14, H12, and H13 atoms shown in Figure 1, respectively, by HMQC and signalsthese signals are assigned are assigned to the toH14, the H12, H14, and H12, H13 and atoms H13 atoms shown shown in Figure in Figure 1, respectively,1, respectively, by HMQC by HMQC and HMBC experiments. HMBCand HMBC experiments. experiments. Ar Ar OH Ar cat [Cp*RuCl(diene)] OH HO 2 H ArOH cat [Cp*RuCl(diene)] HO Ar 2 Ar H ArOH THF, rt Ar Ar THF, rt Ar 1a: Ar = Ph R 1a: Ar = Ph R 1b: Ar = C6H4OMe-p 2a: Ar = Ph, R = H; 97% 1b: Ar = C6H4OMe-p 2a: Ar = Ph, R = H; 97% 2b: Ar = C6H4OMe-p, R = OMe S/C = 50 2b: Ar = C H OMe-p, R = OMe S/C = 50 84% 6 4 84% Scheme 1. Catalytic dimerization of 1,1-diarylpropargyl alcohols 1 with the Cp*RuCl complex. SchemeScheme 1. Catalytic 1. Catalytic dimerization dimerization of 1,1-diarylpropargyl of 1,1-diarylpropargyl alcohols alcohols 1 with1 with thethe Cp*RuCl Cp*RuCl complex. complex. H2 H1 H2 H1 O2 O2 O1 H13 O1 H13 C8 C1 C7 C8 C1 C18 C7 C18 C17 C15 C16 C17 C2 C15 C16 C6 C2 C6 C3 C3 H14 C4 H12 H14 C5 C4 C5 H12 Figure 1. Crystal structure of 2a·0.5acetone. The solvated molecule is omitted for clarity. Ellipsoids Figure 1. Crystal structure of 2a·0.5acetone. The solvated molecule is omitted for clarity. Ellipsoids Figureare drawn 1. Crystal at the30% structure probability of 2a·0.5acetone. level. Selected The bond solvated distances molecule (Å) and is omitted angles (deg.): for clarity. C1–C2, Ellipsoids 1.565(2); are drawn at the 30% probability level. Selected bond distances (Å) and angles (deg.): C1–C2, 1.565(2); areC1–C8, drawn 1.539(2); at the 30% C2–C3, probability 1.473(2); level. C2–C15, Selected 1.337(2); bond distances C3–C8, 1.390(2);(Å) and angles C15–C16, (deg.): 1.456(2); C1–C2, C16–C17, 1.565(2); C1–C8, 1.539(2); C2–C3, 1.473(2); C2–C15, 1.337(2); C3–C8, 1.390(2); C15–C16, 1.456(2); C16–C17, 1.337(2);C1–C8, 1.539(2); C17–C18, C2–C3, 1.516(2); 1.473(2); C2–C1–C8, C2–C15, 84.14(11); 1.337(2); C1–C2–C3,C3–C8, 1.390(2); 89.27(11); C15–C16, C1–C2–C15, 1.456(2); 133.32(14);C16–C17, 1.337(2); C17–C18, 1.516(2); C2–C1–C8, 84.14(11); C1–C2–C3, 89.27(11); C1–C2–C15, 133.32(14); C3– 1.337(2);C3–C2–C15, C17–C18, 137.20(15); 1.516(2); C2–C3–C8, C2–C1–C8, 93.11(12); 84.14(11) C1–C8–C3,; C1–C2–C3, 93.48(12).