Ruthenium-Catalyzed Dimerization of 1,1-Diphenylpropargyl Alcohol to a Hydroxybenzocyclobutene and Related Reactions

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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). 89.27(11); C1–C2–C15, 133.32(14); C3– C2–C15, 137.20(15); C2–C3–C8, 93.11(12); C1–C8–C3, 93.48(12). C2–C15, 137.20(15); C2–C3–C8, 93.11(12); C1–C8–C3, 93.48(12). The catalytic formation of 2 should entail the orthometalation of one of the aryl group in 1 inin The catalytic formation of 2 should entail the orthometalation of one of the aryl group in 1 in addition to the C–C bond formation. We conﬁrmedconfirmed that a potential intermediate enyne 3 [[21],21], which addition to the C–C bond formation. We confirmed that a potential intermediate enyne 3 [21], which wouldwould be formedformed byby head-to-headhead-to-head dimerization of 1a [[22,23],22,23], doesdoes notnot undergoundergo thethe C–HC–H cleavagecleavage would be formed by head-to-head dimerization of 1a [22,23], does not undergo the C–H cleavage reaction underunder thethe presentpresent dimerizationdimerization conditionsconditions (Scheme(Scheme2 2).). TheThe resultresult maymay alsoalso excludeexclude thethe reaction under the present dimerization conditions (Scheme 2). The result may also exclude the involvementinvolvement of analogous orthometalation of the terminal monoyne 1 in thethe catalysis,catalysis, althoughalthough aa involvement of analogous orthometalation of the terminal monoyne 1 in the catalysis, although a related C–H cleavage reaction of 1a on an osmium complex was known [24]. On the other hand, a related C–H cleavage reaction of 1a on an osmium complex was known [24]. On the other hand, a

Inorganics 2017, 5, 80 3 of 8

Inorganics 2017, 5, 80 3 of 8 related C–H cleavage reaction of 1a on an osmium complex was known [24]. On the other hand, a labeling experiment using 1a-d1 with a deuterium at the acetylenic position revealed that the labeling experimentexperiment using using1a -1ad1-dwith1 with a deuterium a deuterium at the at acetylenic the acetylenic position position revealed revealed that thehydrogen that the hydrogenatom derived atom from derived the aromatic from the C–H aromatic bond cleavageC–H bond is selectivelycleavage isincorporated selectively incorporated into the vinyl into group the vinyl group adjacent to the C(OH)Ph2 moiety in 2a (Scheme 3). vinyladjacent group to the adjacent C(OH)Ph to the2 moiety C(OH)Ph in 2a2 moiety(Scheme in3 2a). (Scheme 3).

Ph HO Ph cat [Cp*RuCl(isoprene)] HO Ph cat [Cp*RuCl(isoprene)] recovery of 3 OH recovery of 3 Ph OH THF, rt Ph THF, rt 3 Ph 3 Scheme 2. Attempted reaction of an enyne with the Cp*RuCl catalyst.

Ph PhOH Ph cat [Cp*RuCl(isoprene)] OHD HO 2 cat [Cp*RuCl(isoprene)] D HO Ph 2 D OH Ph THF, rt Ph Ph THF, rt Ph 1a-d D 1a-d1 D 2a-d2 1 2a-d2 Scheme 3. Deuterium labeling experiment. Scheme 3. Deuterium labeling experiment.

On the basis basis of of these these observations, observations, we we propose propose the the mechanism mechanism for for the the catalytic catalytic dimerization dimerization of 1aof (Scheme1a (Scheme 4). 4Two). Two molecules molecules of 1a offirst1a bindﬁrst to bind the ruthenium to the ruthenium atom to form atom a toruthenacycle form a ruthenacycle A [8,9,25]. ASubsequent[8,9,25]. Subsequent σ-bond metathesisσ-bond metathesis in A would in resultA would in the result formation in theformation of the vinyl of intermediate the vinyl intermediate B, which wouldB, which then would undergo then E undergo–Z isomerization.E–Z isomerization. Reductive elimination Reductive elimination from the hydrido(vinyl) from the hydrido(vinyl) complex C affordscomplex theC affordsdimerization the dimerization product 2a product. The mechanism2a. The mechanism is consistent is consistent with the withdeuterium the deuterium labeling experimentlabeling experiment illustrated illustrated in Scheme in Scheme3. In related3. In relatedreactions reactions of propargylic of propargylic alcohols alcohols without without 1-aryl 1-arylsubstituents, substituents, Dixneuf Dixneuf and co-wor andkers co-workers obtained obtained alkylidenecyclobu alkylidenecyclobutenetene derivatives derivatives by a three- by a componentthree-component dehydrative dehydrative condensation condensation of propargylic of propargylic alcohols alcohols and carboxylic and carboxylic acid with acid a withCp*Ru a catalystCp*Ru catalyst [26]. They [26]. Theyisolated isolated a cyclobutadiene a cyclobutadiene complex, complex, which which may may be be derived derived from from reductive elimination form A, as the reaction intermediate. Trost Trost and co-workers also described a ruthenacycle similar to to AA asas a key a key intermediate intermediate in CpRu-catalyzed in CpRu-catalyzed reactions reactions of propargylic of propargylic alcohols alcohols [27,28]. [On27, 28the]. otherOn the hand, other Chan hand, and Chan co-workers and co-workers synthesized synthesized indene indene derivatives derivatives by the by iron-catalyzed the iron-catalyzed self- condensationself-condensation of 1-arylpropargyl of 1-arylpropargyl alcohols alcohols involving involving aromatic aromatic C–H C–H bond bond cleavage cleavage [29]. [ 29].

H H H H 1a H H 1a H H Ru Ph Ru Ph Ph Ph Ph Ph Ph Ph HO OH HO Ru OH HO Ph Ph OH HO Ru OH 2a Ph Ph Ph Ph 2a Ru Ph Ph Ru A A

H H H H Ph Ph Ph Ph HO Ph Ph HO OH OH OH Ru Ph OH Ru Ph H H Ph Ph Ru OH Ru OH Ph Ph H Ph Ph H Ph Ph + HO C + HO C HO Ph HO Ph Ph OH Ph Ru OH – OH Ru Ph –Ru OH Ph Ru Ph H H Ph B H H B Scheme 4. Proposed mechanism for catalytic dimerization of 1a. Ru = Cp*RuCl. Scheme 4. Proposed mechanism for catalytic dimerization ofof 1a1a.. Ru = Cp*RuCl.

2.2. Reaction of a (Hydrido)ruthenium Complex with 1,1-Diphenylpropargyl Alcohol In order to gain further insight into the mechanism of the catalytic dimerization of 1a shown in Scheme 1, we examined the reaction of 1a with a related hydrido complex. When the hydrido

Inorganics 2017, 5, 80 4 of 8

2.2. Reaction of a (Hydrido)ruthenium Complex with 1,1-Diphenylpropargyl Alcohol Inorganics 2017, 5, 80 4 of 8 InorganicsInorder 2017, 5, to 80 gain further insight into the mechanism of the catalytic dimerization of 1a shown4 of 8 incomplex Scheme [Cp*RuH(cod)]1, we examined was the treated reaction with of a 1aslightwith excess a related of 1a hydridoat 50 °C, complex.a novel ruthenocene When the complex hydrido complex [Cp*RuH(cod)] was treated with a slight excess of 1a at 50 ◦°C, a novel ruthenocene complex complex [Cp*RuH(cod)] was treated with a slight excess1 of 1a at 50 C, a novel ruthenocene complex 4 was obtained in moderate yield (Scheme 5). The 1H NMR spectrum of 4 exhibits two mutually 4 was obtained in moderate yield (Scheme5). The 1H NMR spectrum of 4 exhibits two mutually coupled doublets at δ 3.56 and 4.24 with a 1H intensity each, which are assignable to the newly coupled doublets at δ 3.56 and 4.24 with a 1H intensity each, which are assignable to the newly formed formed cyclopentadienyl ligand. Figure 2 depicts the crystal structure of 4 featuring a fused bicyclic cyclopentadienyl ligand. Figure2 depicts the crystal structure of 4 featuring a fused bicyclic hemiketal hemiketal skelton derived from dehydrative condensation of three molecules of 1a. The two skelton derived from dehydrative condensation of three molecules of 1a. The two cyclopentadinyl cyclopentadinyl rings in 4 are slightly tilted with a dihedral angle of 12.9°. The Cp-fused six- rings in 4 are slightly tilted with a dihedral angle of 12.9◦. The Cp-fused six-membered ring adopts a membered ring adopts a half-chair conformation with an axial hydroxy group, and two vicinal half-chair conformation with an axial hydroxy group, and two vicinal phenyl groups in the ring lie in phenyl groups in the ring lie in the anti configuration. the anti conﬁguration.

H H Ph H H Ph Ph Ph Ph Ph [Cp*RuH(cod)] + 1a Ph H O [Cp*RuH(cod)] + 1a toluene Ph H O toluene OH Ru Ph 1:3 50 °C OH Ru Ph 1:3 50 °C Ph OH Cp* Ph OH Cp* 4, 55% 4, 55% Scheme 5. Synthesis of 4. Scheme 5. Synthesis of 4.

C13 C14 C13 C14 C12 C18 C12 C18 C11 C11 C15 C43 C15 O1 C43 O1 C16 Ru1 C16 C17 Ru1 C17 O3 O3

H2 H2 H1 H1 O2 O2

Figure 2. Crystal structure of 4. Ellipsoids are drawn at the 30% probability level. Selected bond Figure 2. Crystal structure of 4. Ellipsoids are drawn at the 30% probability level. Selected bond Figuredistances 2. (Å):Crystal O1–C17, structure 1.419(2); of 4. O1–C18,Ellipsoids 1.455(2); are drawn C11–C16, at the 1.513(2);30% probability C12–C18, level. 1.515(2); Selected C16–C17, bond 1.570(2).distances (Å): O1–C17, 1.419(2); O1–C18, 1.455(2); C11–C16, 1.513(2); C12–C18, 1.515(2); C16–C17, 1.570(2).

The mechanism for the formation of 4 remainsremains openopen toto speculation;speculation; however, the reaction apparentlyThe mechanism involves 1,2-migration for the formation of the phenylof 4 remains groupin open the propragylicto speculation; alcohol. however, A plausible the reaction route is apparently involves 1,2-migration of the phenyl group in the propragylic alcohol. A plausible route suggestedis suggested in Schemein Scheme6. Formation 6. Formation of the of ruthenacycle the ruthenacycleA as A in as the in dimerization the dimerization of 1a (Scheme of 1a (Scheme4) would 4) isbe suggested followedby in protonationScheme 6. Formation of the hydrido of the ligand ruthenacycle and enyne A metathesisas in the dimerization to yield the seven-memberedof 1a (Scheme 4) would be followed by protonation of the hydrido ligand and enyne metathesis to yield the seven- ruthenacyclemembered ruthenacycleB. Subsequent B. Subsequent 1,2-migration 1,2-migration of the phenyl of the group phenyl to the group electrophilic to the electrophilic carbene atom carbene [30] wouldmembered generate ruthenacycleC. Rearrangement B. Subsequent of C1,2-migrationleads to ring of contraction the phenyl group to afford to the the electrophilic ruthenabenzene carbeneD, atom [30] would generate C. Rearrangement of C leads to ring contraction to afford the which would undergo reductive elimination. Ring closure of the resultant ruthenocene E gives rise to ruthenabenzene D, which would undergo reductive elimination. Ring closure of the resultant the formation of the hemiketal 4 as the ﬁnal product. ruthenocene E gives rise to the formation of the hemiketal 4 as the final product.

Inorganics 2017, 5, 80 5 of 8 Inorganics 2017, 5, 80 5 of 8

Ph Ph Ph Ph Ph Ph Ru 1a 1a – H Ph Ph HO Ru OH 2 HO Ru + OH H Ph Ph HO Ru OH H H H Ph Ph – H HO O Ph Ph Ph Ph A enyne metathesis Ph OH OH OH Ph Ph Ph Ph Ph Ph H Ru Ph HO 1,2-migration Ph Ph Ph Ph Ru Ph Ru Ph + O HO HO Ph Ph Ph O Ph O– C B

Ph OH H H Ph H H Ph Ph Ph Ph Ph Ph H H Ph Ph Ph OH Ph O Ru reductive OH Ru Ph OH Ru Ph HO H elimination Ph Ph Ph OH O Ph O DE4

SchemeScheme 6. 6.Proposed Proposed mechanism mechanism forfor formation of of 44.. RuRu == Cp*Ru. Cp*Ru.

3. Experimental3. Experimental

3.1.3.1. General General AllAll manipulations manipulations were were performed performed underunder anan atmosphereatmosphere of of argon argon using using standard standard Schlenk Schlenk techniquestechniques unless unless otherwise otherwise speciﬁed. specified. Solvents Solvents werewere dried by by refluxing reﬂuxing over over sodium sodium benzophenone benzophenone ketyl (THF, diethyl ether, diglyme, toluene, and hexane), P2O5 (dichloromethane and acetonitrile), ketyl (THF, diethyl ether, diglyme, toluene, and hexane), P2O5 (dichloromethane and acetonitrile), and Mg(OMe)2 (methanol), and distilled before use. Formic acid was dried over boric oxide and and Mg(OMe)2 (methanol), and distilled before use. Formic acid was dried over boric oxide and distilled.distilled. The The ruthenium ruthenium complexes complexes [Cp*RuCl(diene)][Cp*RuCl(diene)] [31] [31] and and [Cp*RuH(cod)] [Cp*RuH(cod)] [32] [32 were] were prepared prepared according to the literature. 1H (399.8 MHz) and 13C{1H} (100.53 MHz) NMR spectra were obtained on according to the literature. 1H (399.8 MHz) and 13C{1H} (100.53 MHz) NMR spectra were obtained a JEOL JNM-ECX-400 spectrometer (JEOL Ltd., Tokyo, Japan). 1H NMR shifts are relative to the on a JEOL JNM-ECX-400 spectrometer (JEOL Ltd., Tokyo, Japan). 1H NMR shifts are relative to residual CHCl3 (δ 7.26), while 13C shifts are referenced to CDCl3 (δ 77.0). Elemental analyses were the residual CHCl (δ 7.26), while 13C shifts are referenced to CDCl (δ 77.0). Elemental analyses performed on a 3Perkin–Elmer 2400II CHN analyzer (PerkinElmer, Waltham,3 MA, USA). ESI-MS were performed on a Perkin–Elmer 2400II CHN analyzer (PerkinElmer, Waltham, MA, USA). ESI-MS spectra were obtained on a JEOL JMS-T100LC spectrometer (JEOL Ltd., Tokyo, Japan) with a positive spectra were obtained on a JEOL JMS-T100LC spectrometer (JEOL Ltd., Tokyo, Japan) with a positive ionization mode. ionization mode. 3.2. Catalytic Dimerization of 1,1-Diarylprop-2-yn-1-ols to Give 2 3.2. Catalytic Dimerization of 1,1-Diarylprop-2-yn-1-ols to Give 2 To a solution of [Cp*RuCl(isoprene)] (10.2 mg, 0.030 mmol) in THF (6 mL) was added 1,1- diphenylprop-2-yn-1-olTo a solution of [Cp*RuCl(isoprene)] (1a, 312.0 mg, 1.5 mmol), (10.2 mg,and the 0.030 mixture mmol) was in stirred THF (6for mL) 2.5 h was at room added 1,1-diphenylprop-2-yn-1-oltemperature. After removal (1a of, the 312.0 solvent mg, 1.5in vacu mmol),o, the and residue the mixture was chromatographed was stirred for on 2.5 a hcolumn at room temperature.of silica. Elution After removalwith hexane/ethyl of the solvent acetate in vacuo,(9:1 v/v the) afforded residue 2a was (302.6 chromatographed mg, 97%). Single on crystals a column of silica.suitable Elution for X-ray with analysis hexane/ethyl were obtained acetate by recrystallization (9:1 v/v) afforded from2a acetone–pentane.(302.6 mg, 97%). 1H NMR Single (CDCl crystals3) 1 suitableδ: 2.36, for 2.87 X-ray (s, 1H analysis each, OH), were 6.27 obtained (d, 3JHH by = recrystallization 15.2 Hz, 1H, CH from=C(OH)Ph acetone–pentane.2), 6.32 (d, 3JHH H= 11.3 NMR Hz, (CDCl 1H, 3) 3 3 δ: 2.36,CH=C(cyclobutene)), 2.87 (s, 1H each, 6.45 OH), (dd, 6.27 3JHH (d, = 15.2,JHH =11.3 15.2 Hz, Hz, 1H, 1H, CH–C CH=C(OH)PhH=CH), 7.20–7.352), 6.32 (m, (d, 19H,JHH aryl).= 11.3 13C{ Hz,1H} 1H, 3 13 1 CHNMR=C(cyclobutene)), (CDCl3) δ: 79.1 6.45 (C(OH)Ph (dd, JHH2), 86.2= 15.2, (C(OH)Ph), 11.3 Hz, 116.2 1H, CH–C (C=C(cyclobutene)),H=CH), 7.20–7.35 119.3, (m, 121.3, 19H, 125.4, aryl). 125.6,C{ H} NMR126.8 (CDCl (CH=CHC(OH)Ph3) δ: 79.1 (C(OH)Ph2), 126.9,2), 127.0 86.2 ((m),C(OH)Ph), 127.9, 128. 116.21, 129.5, (C=C(cyclobutene)), 130.0, 131.9, 139.3 119.3, (CHC(OH)Ph 121.3, 125.4,2), 141.9, 125.6, 126.8144.8 (C H=CHC(OH)Ph(CH=C(cyclobutene)),2), 126.9, 145.8, 127.0 145.9, (m), 149.8, 127.9, 153.4. 128.1, Anal. 129.5, Calcd. 130.0, for 131.9, C30H 139.324O2·0.5acetone: (CHC(OH)Ph C, 284.91;), 141.9, 144.8H, (CH=6.11. Found:C(cyclobutene)), C, 84.66; H, 145.8, 6.00. 145.9, 149.8, 153.4. Anal. Calcd. for C30H24O2·0.5acetone: C, 84.91; H, 6.11.Data Found: for 2b C,: 84.66;Yield 84%. H, 6.00. 1H NMR (CDCl3) δ: 2.60, 3.21 (s, 1H each, OH), 3.74 (m, 9H, OMe), 3.77 (s, 3H, OMe), 6.18 (d, 3JHH =1 15.2 Hz, 1H, vinyl), 6.20 (d, 3JHH = 11.3 Hz, 1H, vinyl), 6.42 (dd, 3JHH = 15.2, Data for 2b: Yield 84%. H NMR (CDCl3) δ: 2.60, 3.21 (s, 1H each, OH), 3.74 (m, 9H, OMe), 3.77 (s, 11.3 Hz, 1H, CH–C3 H=CH), 6.72–6.78 (m, 8H, aryl), 7.07–7.223 (m, 7H, aryl). ESI-MS (m/z): 3calcd. for 3H, OMe), 6.18 (d, JHH = 15.2 Hz, 1H, vinyl), 6.20 (d, JHH = 11.3 Hz, 1H, vinyl), 6.42 (dd, JHH = 15.2, + 11.3C34 Hz,H32 1H,O6 + CH–CNa , 559.2096;H=CH), found, 6.72–6.78 559.2091. (m, 8H, aryl), 7.07–7.22 (m, 7H, aryl). ESI-MS (m/z): calcd. for + C34 H32O6 + Na , 559.2096; found, 559.2091. Inorganics 2017, 5, 80 6 of 8

3.3. Synthesis of 4 A mixture of [Cp*RuH(cod)] (52.0 mg, 0.151 mmol) and 1,1-diphenylprop-2-yn-1-ol (1a, 94.0 mg, 0.451 mmol) in toluene (5 mL) was heated at 50 ◦C for 17 h. After removal of the solvent in vacuo, the resultant solid was recrystallized from THF–diethyl ether to give 4 as yellow crystals (70.9 mg, 0.0824 1 mmol, 55%). H NMR (CDCl3, δ): 1.70 (s, 15H, C5Me5), 2.01, 2.11 (s, 1H each, OH), 3.31 (s, 1H, CH), 3 3.56, 4.24 (d, 1H each, JHH = 2.5 Hz, C5H), 4.91 (d, 1H, JHH = 7.6 Hz, aryl), 6.35 (m, 1H, aryl), 6.91–7.04 (m, 11H, aryl), 7.12–7.43 (m, 13H, aryl), 7.57 (m, 2H, aryl), 8.21 (d, 1H, JHH = 7.9 Hz, aryl). Anal. Calcd. for C55H50O3Ru: C, 76.81; H, 5.86. Found: C, 76.53; H, 5.84.

3.4. Crystallography Single crystals suitable for X-ray analyses were mounted on a fiber loop. Diffraction experiments were performed on a Rigaku Saturn CCD area detector with graphite monochromated Mo Kα radiation (λ = 0.710 73Å). Intensity data were corrected for Lorentz–polarization effects and for absorption. Structure solution and refinements were carried out by using the Crystal Structure program package [33]. The heavy-atom positions were determined by a direct methods program (SIR92 [34]) and the remaining non-hydrogen atoms were found by subsequent Fourier syntheses and refined by full-matrix least-squares techniques against F2 using the SHELXL-2014/7 program [35]. The hydrogen atoms were included in the refinements with a riding model. The hydroxy hydrogen atoms in 2a·0.5acetone were placed at two disordered positions linked to hydrogen acceptors (carbonyl and hydroxy groups) with 50% occupancies. Details of crystallographic data are summarized in Supplementary Materials. Crystal Data for 2a·0.5acetone (Supplementary Materials) : C31.5H27O2.5 (M = 445.56 g/mol), triclinic, space group P1 (no. 2), a = 10.337(6) Å, b = 10.816(6) Å, c = 12.119(7) Å, α = 72.216(16)◦, β = 84.13(2)◦, γ = 69.554(17)◦, V = 1209.0(12) Å3, Z = 2, T = 93 K, µ(Mo Kα) = 0.076 mm−1, 3 ◦ ◦ Dcalc. = 1.224 g/cm , 9706 reflections measured (6.2 ≤ 2Θ ≤ 55.0 ), 5322 unique (Rint = 0.0336) which were used in all calculations. The final R1 was 0.0510 (I > 2σ(I)) and wR2 was 0.1477 (all data). Crystal Data for 4:C55H50O3Ru (M = 860.07 g/mol), monoclinic, space group P21/c (no. 14), a = 17.089(4) Å, b = 12.512(3) Å, c = 20.001(4) Å, β = 107.378(2)◦, V = 4081.5(15) Å3, Z = 4, T = 123 K, −1 3 ◦ ◦ µ(Mo Kα) = 0.431 mm , Dcalc. = 1.400 g/cm , 31910 reflections measured (6.4 ≤ 2Θ ≤ 55.0 ), 9326 unique (Rint = 0.0300) which were used in all calculations. The final R1 was 0.0332 (I > 2σ(I)) and wR2 was 0.0795 (all data).

4. Conclusions We found that the ruthenium(II) complex [Cp*RuCl(diene)] catalyzed a novel mode of dimerization of 1,1-diarylpropargyl alcohols 1 to afford the highly functionalized cyclobutenes 2, which has been unambiguously characterized by NMR spectroscopy and X-ray crystallography. The proposed mechanism for the catalytic formation of 2 involves the ordinary five-membered ruthenacycle as a primary intermediate. However, the presence of the aryl substituents in 1 led to unexpected aromatic C–H bond cleavage to afford the benzocyclobutenes 2 with high efficiency and selectivity. Formation of the five-membered ruthenacycle intermediate before the C–H bond cleavage was supported by the reaction of the related hydrido complex [Cp*RuH(cod)]. The dehydrogenative cyclization of three molecules of 1a therein took place without the C–H bond cleavage to generate the fused cyclopentadienyl ring in the product 4. These results added new aspects to the coordination chemistry and catalysis of the Cp*Ru complexes [8,9,12,36,37].

Supplementary Materials: The following are available online at www.mdpi.com/2304-6740/5/4/80/s1. Cif and cif-checked ﬁles. CCDC-1582525 (2a·0.5acetone) and CCDC-1582526 (4) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam. ac.uk/conts/retrieving.html. Acknowledgments: This work was supported by a Grant-in-Aid for Scientiﬁc Research (S) (no. 22225004) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. Inorganics 2017, 5, 80 7 of 8

Author Contributions: Hoang Ngan Nguyen and Naoto Tashima performed the experiments; Takao Ikariya supervised the research; Shigeki Kuwata initiated and coordinated the research and wrote the manuscript. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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