Alkyne [2 + 2 + 2] Cyclotrimerization Catalyzed by a Low-Valent

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Alkyne [2 + 2 + 2] Cyclotrimerization Catalyzed by a Low-Valent Titanium Reagent Derived from CpTiX3 (X = Cl, O‑i‑Pr), Me3SiCl, and Mg or Zn Sentaro Okamoto,*,† Takeshi Yamada,† Yu-ki Tanabe,† and Masaki Sakai† †

Department of Materials and Life Chemistry, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan

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S Supporting Information *

ABSTRACT: Inter-, partially intra-, and intramolecular [2 + 2 + 2] cycloadditions of alkynes were catalyzed by a low-valent titanium species generated in situ from the reduction of CpTi(O-i-Pr)3, CpTiCl3, or Cp*TiCl3 with Mg or Zn powder in the presence of Me3SiCl. The role of Me3SiCl as an additive in the reaction mechanism is discussed.

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Meanwhile, alkyne cyclotrimerization, 14 which is of importance as a straightforward, atom-economic, synthetically useful means for synthesizing substituted benzenes, can be also catalyzed by titanium as well as other early transition metals.15 Thus, titanium alkoxides such as calix[4]arene-TiCl2 complexes,16 (ArO)2TiCl2,17 and a titanium bis(phenolate)pyridine complex18 have been utilized as catalysts or precursors in such reaction. Moreover, a TiCl4/i-Bu3Al reagent was reported to cyclotrimerize terminal alkynes,19 and most recently, cyclotrimerization catalyzed by Ti(II) species derived from Ti(IV) imido complexes has been developed.20 Herein, we report that a LVT species derived from Ti(O-i-Pr)4 or CpTiX3 [X = O-i-Pr, Cl] in the presence of Me3SiCl and Mg or Zn powder can catalyze the inter-, partially intra-, and intramolecular cyclotrimerizations of alkynes.

INTRODUCTION Low-valent titanium (LVT) reagents, which have been widely utilized in organic synthesis, can be generated by the reaction of TiXn (X = halogen, n = 3 or 4) with a reducing agent1 or, as more recently reported, from Cp2TiCl2 (Cp = η5-cyclopenetadienyl) or Ti(OR)4 with an appropriate reductant. Thus, Cp2Ti(III)Cl (and its dimer), Cp2TiPh, and Cp2Ti(II)(L)n (L = ligand) can be generated by reduction of Cp2TiCl2 with Zn, Na(Hg), Mg, or n-BuLi.2,3 Furthermore, the reaction of Ti(O-i-Pr)4 or ClTi(O-i-Pr)3 with an alkyl Grignard or lithium reagent can generate a divalent titanium equivalent (η2alkene)Ti(O-i-Pr)2.4 These reagents enable a variety of unique molecular transformations. In addition, we have recently reported the generation of LVTs derived from Ti(O-i-Pr)4 or titanatrane by the reaction with Me3SiCl and Mg, along with their synthetic reactions,5 such as C−O bond cleavage of allyl and propargyl ethers5a,f,g and esters,5h N−S bond cleavage of sulfonamides,5c reductive carbonyl coupling (McMurry olefination),5b imino pinacol coupling,5b and reduction of epoxides5d and oxetanes.5e On the basis of these results, we pursued further development of a new LVT and investigated an LVTgeneration method by reduction of half-titanocene trialkoxide or trichloride CpTiX3 [X = O-i-Pr, Cl]. Half-titanocenes and their derivatives having a modified Cp ligand have been mostly used as catalysts in the synthesis of polymers from alkenes and lactide.6,7 So far, the utilization of CpTiX3 in organic synthesis has been limited to the following examples: (1) LVTs generated from CpTiCl3 by reduction with Mg(Hg) or LiAlH4 were reported to promote the reductive pinacol coupling of aldehydes and ketones.8 (2) A CpTiCl3/LiAlH4 reagent deoxygenated 1,4-endoxide compounds to the corresponding benzene derivatives.9 (3) Aromatic halides were reduced by NaBH4 in the presence of a CpTiCl3 catalyst.10 (4) CpTiCl3-catalyzed cross-coupling of aryl fluorides with Grignard reagents.11 (5) Fluorination of acetoacetic acid derivatives has been reported.12 (6) Cyclopropanation of nonactivated alkenes13 have also been reported. © XXXX American Chemical Society

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RESULTS AND DISCUSSION As in our previous report,5a,i Ti(O-i-Pr)4 catalyzed cyclotrimerization of terminal alkynes such as 1-hexyne (1a) and ethynylbenzene (1b) to produce the corresponding trisubstituted benzenes in moderate to good yields (entries 1 and 18, Table 1). In these reactions, the LVT species derived from the reduction of Ti(O-i-Pr)4 after the ligand exchange from alkoxide to chloride by Me3SiCl is proposed to catalyze the trimerization of 1-alkynes (eq 1, Scheme1). On the basis of this hypothesis, we pursued our research on such reactions using CpTi(O-i-Pr)3 as a catalyst in the presence of a metal powder reductant and Me3SiCl, where a similar ligand exchange can be envisaged to promote the generation of an active low-valent complex (eq 2, Scheme 1). Thus, we found that CpTi(O-i-Pr)3 could catalyze the cycloaddition of 1-hexyne (1a) in the presence of Mg powder and Me3SiCl (entry 3). The addition Special Issue: Organometallic Complexes of Electropositive Elements for Selective Synthesis Received: September 15, 2018

A

DOI: 10.1021/acs.organomet.8b00678 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 1. Study on the Reaction Conditions for Alkyne Cyclotrimerization

entry

1

Ti complex (mol %)

metal (equiv)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

1a 1a 1a 1a 1a 1a 1a 1a 1a 1a 1a 1a 1a 1a 1a 1a 1a 1a 1b 1b 1c 1c 1d 1d 1d 1d 1d

Ti(O-i-Pr)4 (10) Ti(O-i-Pr)4 (10) CpTi(O-i-Pr)3 (5) CpTi(O-i-Pr)3 (5) CpTi(O-i-Pr)3 (5) CpTiCl3 (5) CpTiCl3 (5) CpTiCl3 (5) CpTiCl3 (5) CpTiCl3 (5) CpTiCl3 (5) CpTiCl3 (5) CpTiCl3 (5) CpTiCl3 (5) CpTiCl3 (5) CpTiCl3 (5) Cp*TiCl3 (5) Cp*TiCl3 (5) Ti(O-i-Pr)4 (20) CpTiCl3 (5) Ti(O-i-Pr)4 (10) CpTiCl3 (5) Ti(O-i-Pr)4 (20) CpTi(O-i-Pr)3 (5) CpTi(O-i-Pr)3 (5) CpTiCl3 (5) Cp*TiCl3 (5)

Mg (0.5) Zn (0.5) Mg (0.5) Zn (0.5) Mg (0.5) Mg (0.5) Mg (0.5) Zn (0.5) Mg (0.5) Zn (0.5) Zn (0.5) Zn (0.5) Mg (0.5) Zn (0.5) Zn (0.5) Zn (0.5) Mg (0.5) Zn (0.5) Mg (1.0) Mg (0.5) Mg (0.5) Mg (0.5) Mg (1.0) Mg (0.5) Zn (0.5) Mg (0.5) Mg (1.0)

Me3SiCl (equiv)

°C, h

(0.5) (0.5) (0.5) (0.5)

40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 45, 40, 40, 45, 45, 40, 40, 40, 40, 45, 45, 45, 45, 45,

(0.5) Me3SiBr (0.5)g LiCl (0.5)g LiCl (0.5)g n-Bu4NCl (0.3)g Ph−Cl (0.5)g MgBr2 (0.5)g ZnI2 (0.3)g (1.0) (1.0) (1.0) (1.0) (0.5) (0.5) (0.5) (1.0) (1.0) (1.0) (0.5) (1.0)

12 12 12 12 24 12 12 12 24 24 24 24 24 24 12 24 12 12 12 12 12 12 24 20 24 24 20

yield (2/3)b,c 71% (41:59)d 2%d (38:62) 80% (38:62) 70% (40:60) no reaction 79% (39:61) trace−38%e,f (39:61) 60% (38:62) no reaction no reaction no reaction no reaction no reaction no reaction 80% (38:62) no reaction 68% (38:62) 62% (34:66) 65% (34:66)d 74% (37:63) 63% (30:70) ≤16%d (16:84) no reaction 82% trace 60% 35%

a

The reaction was carried out using an alkyne (1.0 mmol) in THF (2.0 mL). The reactions of entries 2, 7, 22, 26, and 27 were not completed or did not proceed, and a large or initial amount of alkynes remained unreacted. In all other cases, alkyne 1 was consumed nearly completely, where oligomeric and/or polymeric materials were coproduced. bTotal yield of 2 and 3 after column chromatography. cDetermined by 1H NMR analysis. d See,ref 5a. eA large amount of 1 remained unreacted. fNot reproducible. gAdded instead of Me3SiCl.

of Me3SiCl proved to be essential for the reaction to proceed (entry 5). Interestingly, in contrast to the Ti(O-i-Pr)4catalyzed reaction, Zn powder could be used instead of Mg (entries 2 and 4). As depicted in eq 3 of Scheme 1, it was expected that CpTiCl3 could be reduced by metal powder in the absence of Me3SiCl, since no ligand exchange would be required. Indeed, the formation of a CpTiCl2−THF complex by reduction of CpTiCl3 with Mg, Zn, or Mn in THF was reported.21 However, attempts at performing the cyclotrimerization of 1a with CpTiCl3 with Mg or Zn in the absence of Me3SiCl failed; the use of Zn powder resulted in no reaction (entry 16), whereas the reaction with Mg powder sometimes afforded the corresponding benzene derivatives, although in low yield and poor reproducibility (entry 7). These results stem most likely from the lack of catalytic activity of the in situ generated Ti(III) species, i.e., CpTiCl2−THF complex, in the cyclotrimerization of alkynes, which may not proceed through a radical pathway (vide inf ra). Nevertheless, surprisingly and delightfully, upon addition of Me3SiCl to the CpTiCl3/Mg or CpTiCl3/Zn system, catalysis

Scheme 1. Possible Paths for Generation of LVTs

B


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Organometallics proceeded smoothly (entries 6 and 15). Similarly, Cp*TiCl3 (Cp*: pentamethylcyclopentadienyl) with Mg or Zn could catalyze the cyclotrimerization, albeit in somewhat lower yields (entries 17 and 18). Meanwhile, ethynylbenzene (1b) was cyclotrimerized to triphenylbenzenes by the present system, with CpTiCl3 affording better yield than Ti(O-i-Pr)4 (entries 19 and 20). In contrast, the CpTiCl3-catalyzed reaction of ethynyltrimethylsilane (1c) resulted in the inefficient production of the corresponding benzene derivative, albeit with better regioselectivity (entry 22), and Ti(O-i-Pr)4 was found to be a much better catalyst for the cyclotrimerization of 1c (entry 21). These results suggest that the CpTiCl3 catalytic system might be more sensitive to sterically demanding group(s) than the reaction with Ti(O-i-Pr)4, most likely due to steric repulsion between the Cp ligand and the alkyne substituent. Besides 1-alkynes, internal alkynes such as 4-octyne (1d) could be cyclotrimerized by CpTiX3/Mg/Me3SiCl catalyst (entries 24 and 26), whereas Ti(O-i-Pr)4 did not catalyze the reaction at all (entry 23). The reaction with Zn instead of Mg also failed (entry 25). The Cp*TiCl3-catalyzed reaction proceeded but resulted in a low yield, due presumably to steric factors (entry 27). In summary, 1-alkynes can be cyclotrimerized by Ti(O-iPr)4/Mg, CpTi(O-i-Pr)3/Mg, CpTiCl3/Mg, CpTiCl3/Zn, or Cp*TiCl3/Mg in the presence of Me3SiCl. However, for the reaction of 1-alkyne having a sterically demanding substituent such as trimethylsilyl acetylene, a Ti(O-i-Pr)4/Mg catalyst gives better results. The cycloaddition of internal alkynes to hexa-substituted benzenes requires the use of CpTi(O-i-Pr)3/ Mg or CpTiCl3/Mg catalyst. With the CpTiX3/Mg or Zn/Me3SiCl (X = Cl or O-i-Pr) catalytic systems in hand, the substrate scope in the alkyne [2 + 2 + 2] cycloaddition reactions was investigated, and the results are summarized in Scheme 2. In addition to 1-alkynes 1a, 1b, and 1c, siloxy, benzyloxy, and phenoxy derivatives 1e, 1f, and 1g were effectively cyclotrimerized and gave the corresponding trisubstituted benzenes as a mixture of 1,3,5and 1,2,4-isomers in moderate to good yields. In a 20 mmol scale reaction of 1-hexyne (1a), the reaction with the reduced amounts of catalyst reagents (2.5 mol %, 45 °C, 36 h) proceeded smoothly to give a mixture of 2a and 3a in 77% yield (Scheme 2, 1a with reagents E). Internal alkynes 1d and 1h could also be converted to the corresponding hexasubstituted benzene derivatives by a CpTiCl3-catalyzed reaction. Besides intermolecular reactions, a CpTiCl3/Mg/ Me3SiCl reagent catalyzed partially and fully intramolecular reactions. Thus, the cross-addition of diyene 1i and 1d (4 equiv) in the presence of the catalyst proceeded to give 4 in 85% isolated yield. Meanwhile, triyne 1j was effectively cyclotrimerized by the catalyst to provide bis-annulated benzene 5 in 84% isolated yield. These results indicate that silyl and benzyl ethers, and the ketal functionality were tolerated under the reaction conditions. To gain more insight into the role of the silyl chloride additive in the CpTiCl3-catalyzed reaction, a gas chromatography analysis was performed on the reaction mixture (diluted with hexane) of the CpTiCl3/Mg/Me3SiCl- or CpTiCl3/Zn/ Me3SiCl-catalyzed 1-hexyne cyclotrimerization. Comparing with authentic samples, it was found that Me3Si-SiMe3 was not produced at all. After aqueous workup, the analysis of the organic layer showed formation of silanol (Me3SiOH) and siloxane (Me3SiOSiMe3), which were generated by the reaction of Me3SiCl with water. These results suggest that

Scheme 2. LVT-Catalyzed Inter-, Partially Intra- and Intramolecular Alkyne Cyclotrimerization Reactions

Me3SiCl is not involved in any redox processes of the CpTiCl3−metal catalytic system22 and exists in situ in its initial form. Addition of a halogen ion source such as LiCl, n-Bu4NCl, PhCl, MgBr2, and ZnI2 instead of silyl chloride was found to be ineffective (entries 9−14, Table 1). In contrast, Me3SiBr was effective as well as Me3SiCl (entry 8). As shown in the Supporting Information (S-13), CpTiCl3 was reduced by the reaction with Zn powder in THF with a concomitant change in the reaction mixture color to blue, which could be due to the formation of a CpTiCl2−THF complex, as previously reported in the literatures.21 However, the Ti(III) species could not cycloadd alkynes. We previously reported that titanatrane complex 6 was reduced by Mg powder in the presence of silyl chloride, and the resulting LVT species could reduce epoxides and oxetanes through a single electron transfer process, but the titanium could not effectively catalyze the cyclotrimerization of triyne 1i (Scheme 3).5e,i From these results, it can be concluded that 6/Me3SiCl/Mg might generate the corresponding Ti(III) species selectively, which was not active for alkyne cyclotrimerization. Addition of Me3SiCl or 1-hexyne to the CpTiCl3/Zn system showed a similar blue color, albeit somewhat darker, and no product was obtained from 1-hexyne. To test the activation effect of Me3SiCl on Zn or Mg powder,23 Zn or Mg powder C


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Organometallics

effect of these activations, further reduction of II by metal powder could take place to generate a lower-valent titanium species such as complex III. This cooperative activation may be important because as shown in the above experimental results adding one of trimethylsilyl chloride and alkyne alone could not promote the reduction of complexes II′ and II′′. Subsequently, reduced complex III would undergo oxidative addition of alkynes to afford titanacylopentadiene V through IV. Further insertion of alkyne and reductive elimination would provide the corresponding substituted benzenes. Furthermore, it can be also proposed that silyl chloride would have a stabilizing effect for a metastable Ti(II) complex such as III, which generated in situ by the reductive elimination of VI, by the coordination of the Cl atom to the titanium center. The stabilization of the transition-metal complexes with coordinating chlorosilanes has been reported. Instead of silyl chloride, addition of trimethysilyl triflate (Me3SiOTf, 0.5 equiv), which is a strong Lewis acid, to 1-hexyne/CpTiCl3/Zn (1.0/0.05/0.5 equiv) did not afford the corresponding trisubstituted benzenes, where the reaction mixture turned brown but the color disappeared immediately.26 Use of Mg powder which is stronger reductant than Zn could reduce CpTiCl2−(THF) to a Ti(II) species in the absence of Me3SiCl, but the reaction gave the corresponding cyclotrimerization products only in trace to low yields (Table 1, entry 7), presumably due to lack of the stabilization by silyl chloride coordination such as complex III. As shown in entry 14 in Table 1, we employed ZnI2 instead of silyl chloride as an additive, which may be expected as an agent to exchange halogen between CpTiCl 3 and ZnI 2. It can generate CpTiInCl(3‑n) that may be reduced easier than CpTiCl3. Indeed, adding Zn powder to premixed solution of CpTiCl3 and ZnI2 promoted the reduction and the color turned dark blue quickly within a few minutes, which may be a Ti(III) species. However, the reaction of this mixture with 1-hexyne did not proceed at all.

Scheme 3. Results of the Reaction with a Titanatrane Catalyst

was treated with Me3SiCl in THF and then silyl chloride and the solvent evacuated. In the presence of the pretreated metal powder, a mixture of CpTiCl3 and 1-hexyne did not afford the corresponding benzenes. Meanwhile, the mixture of other coordinative halogen sources, such as LiCl, n-Bu4NCl, or PhCl, with CpTiCl3/Zn/1-hexyne turned light blue, purple, or bluegray, respectively, and these resulted in no cycloaddition reaction (entries 10−12 in Table 1). In contrast, CpTiCl3/Zn/ 1-hexyne was mixed with Me3SiCl as well as Me3SiBr to turn brown and promote the cycloaddition reaction giving tributylbenzenes (entries 4 and 8 in Table 1). As shown in the Supporting Information (S12), visible-light absorption spectra for the reacting mixture of CpTiCl3/Zn with or without an additive(s) indicate that only Me3SiCl exhibited a different behavior from other additives. Thus, an addition of Me3SiCl to CpTiCl3/Zn bathochromically shifted the absorption peak compared to that of CpTiCl3/Zn, while the hypsochromic shift was observed by other additives such as LiCl and n-Bu4NCl. These results suggest that, in the presence of both Me3SiX (X = Cl or Br) and alkyne, CpTiCl3 could be reduced by Zn in THF to a lower-valent species that would be active for the cyclotrimerization. On the basis of these results, we propose the reaction mechanism illustrated in Scheme 4.

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CONCLUSION We have reported that CpTiX3 (X = Cl or O-i-Pr) could catalyze alkyne [2 + 2 + 2] cycloaddition reactions in the presence of Mg or Zn powder and Me3SiCl to afford the corresponding substituted benzenes as a mixture of regioisomers in moderate to good yields. The presence of Me3SiCl proved to be essential for the success of the reaction. Further optimization to avoid the formation of oligomeric byproducts and control the regioselectivity is underway in our laboratories.

Scheme 4. Proposed Mechanism

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EXPERIMENTAL SECTION

General. NMR spectra were recorded in CDCl3 at 600 and 500 MHz for 1H and 150 and 125 MHz for 13C, respectively, on JEOL JNM-ECA600 and 500 spectrometers. Chemical shifts are reported in parts per million (ppm, δ) relative to Me4Si (δ 0.00), residual CHCl3 (δ 7.26 for 1H NMR), or CDCl3 (δ 77.0 for 13C NMR). IR spectra were recorded on an FT-IR spectrometer (JASCO, FT/IR 4100). High-resolution mass spectra (HR-MS) were measured on a JEOL Accu TOF T-100 equipped with an ESI ionization unit. All reactions sensitive to oxygen and/or moisture were performed under an argon atmosphere. Dry solvents [tetrahydrofuran (THF) and toluene] were purchased from Kanto Chemicals. Mg powder (FUJIFILM Wako Pure Chem. Co., min. 98.0%) and Zn powder (Kanto Chem. Co. Inc., min. 90%) were used as purchased. Cyclopentadienyltitanium(IV) trichloride [CpTiCl3] and pentamethylcyclopentadienyltitanium(IV) trichloride [Cp*TiCl3] were purchased from Strem Chemicals and

Accordingly, we assume that the alkyne would coordinate to CpTiCl2 to form complex II, in which silyl chloride would act as a Lewis acid to coordinate to the Cl atom of CpTiCl2. It has been reported that the reduction of Ti(III) to Ti(II) could be promoted by the coordination of π-accepting molecule.24 It can be expected that the withdrawal of electron by the silyl Lewis acid would make the titanium more electron-deficient and favored to its reduction.25 Then, owing to a synergistic D


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Organometallics

tris(trimethylsilyl)benzenes30,32 (30:70, total 124 mg) in 63% isolated yield. 1,3,5- and 1,2,4-Tris(2-(benxyloxy)ethyl)benzenes (2f and 3f). 1H NMR (500 MHz, CDCl3) δ 7.23−7.33 (m, 15H, Ar), 7.10 (d, J = 6.5 Hz, 1H for Ar of 1,2,4-isomer), 7.00−7.04 (m, 2H for Ar of 1,2,4isomer), 6.93 (s, 3H for Ar of 1,3,5-isomer), 4.48, 4.49, and 4.50 (3s, 6H, benzylic CH2O), 3.60−3.68 (m, 6H, CH2CH2O), 2.866, 2.874, and 2.94 (3t, J = 6.0 Hz, J = 6.0 Hz, J = 6.3 Hz, respectively, total 6H, CH2CH2O). 13C NMR (125 MHz, CDCl3) δ 138.88, 138.39, 138.25, 136.92, 136.83, 134.75, 130.43, 129.82, 128.30, 127.57, 127.48, 127.46, 126.97, 72.93, 72.89, 71.23, 70.94, 36.18, 35.85, 33.07, 32.70. IR (neat) 3029, 2854, 1602, 1495, 1454, 1361, 1205, 1099, 1027, 734, 697 cm−1. HRMS calcd for C33H36O3Na [N + Na+] 503.2557, found 503.2566. (((2,4,6-Triethylbenzene-1,3,5-triyl)tris(ethane-2,1-diyl))tris(oxy))tris(tert-butyldimethylsilane) (2h) and (((3,5,6-Triethylbenzene1,2,4-triyl)tris(ethane-2,1-diyl))tris(oxy))tris(tert-butyldimethylsilane) (3h). 1H NMR (500 MHz, CDCl3) δ 3.68−3.80 (m, 6H, CH2O), 2.88−2.95 (m, 6H, CH2CH2O), 2.63−2.73 (m, 6H, CH2CH3), 1.20−30 (m, 9H, CH2CH3), 0.93, 0.92, and 0.916 (3s, total 27H, t-Bu), 0.08, 0.07, 0.068, and 0.065 (4s, total 18H, SiCH3). 13 C NMR (125 MHz, CDCl3) δ 140.58, 139.91, 139.54, 138.95, 133.49, 132.77, 132.34, 131.72, 63.93, 63.81, 33.13, 33.09, 32.98, 32.92, 26.03, 22.73, 22.69, 22.44, 22.40, 18.42, 18.38, 15.58, −5.26, −5.28. IR (neat) 2930, 1471, 1386, 1361, 1254, 1075, 1005, 922, 835, 775, 663 cm−1. HRMS calcd for C36H72O3Si3Na [M + Na+] 659.4682, found 659.4706. 4′,7′-Dibutyl-2,2-dimethyl-5′,6′-dipropyl-1′,3′dihydrospiro[[1,3]dioxane-5,2′-indene] (4). Under an argon atmosphere, to a mixture of diyene 1i (91.3 mg, 0.30 mmol), alkyne 1d (176 μL, 1.2 mmol), and Mg powder (12.1 mg, 0.50 mmol) in THF (2.0 mL) was added Me3SiCl (126 μL, 1.0 mmol). After being stirred 15 min, a THF solution of CpTiCl3 (1.0 mL, 0.1 M, 0.10 mmol) was added. After being stirred for 20 h at 45 °C, the mixture was poured into a mixture of aqueous 0.5 M NaHCO3 (3 mL) and Et2O (6 mL). The organic layer was separated and aqueous layer extracted with Et2O (2 × 5 mL). The combined organic layers were dried over anhydrous Na2SO4, filtered through a pad of Celite, and concentrated under reduced pressure. The residue was chromatographed on silica gel (hexane/Et2O) to isolate 4 (105.5 mg, 85% yield) and hexapropylbenzene 2d (66 mg). Compound 4. 1H NMR (500 MHz, CDCl3) δ 3.77 (s, 4H, CH2O), 2.85 (s, 4H, cyclic benzyl CH2), 2.48−2.55 (m, 8H, acyclic benzyl CH2), 1.51 (s, 6H, ketal CH3), 1.40−1.55 (m, 12H, CH2), 1.06 (t, J = 6.0 Hz, 6H, CH2CH3), 0.98 (t, J = 5.5 Hz, 6H, CH2CH3). 13 C NMR (125 MHz, CDCl3) δ 137.49, 137.34, 134.96, 97.81, 69.44, 40.55, 39.38, 32.61, 31.66, 30.36, 25.17, 23.84, 23.42, 15.04, 13.87. IR (neat) 2956, 2870, 1646, 1456, 1381, 1248, 1198, 1156, 1059, 932, 832, 732, 670 cm−1. The structure of 4 was confirmed by converting the corresponding diol, (4,7-dibutyl-5,6-dipropyl-2,3-dihydro-1Hindene-2,2-diyl)dimethanol, by removal of a ketal moiety (aqueous HCl−THF, room temperature, 95% yield). (4,7-Dibutyl-5,6-dipropyl2,3-dihydro-1H-indene-2,2-diyl)dimethanol: 1H NMR (500 MHz, CDCl3) δ 3.77 (s, 4H, OCH2), 2.77 (s, 4H, cyclic CH2), 2.45−2.53 (m, 8H, benzylic CH2), 1.37−1.54 (m, 14H, OH and CH2), 1.04 (t, J = 7.5 Hz, 6H, CH3), 0.95 (t, J = 7.0 Hz, 6H, CH3). 13C NMR (125 MHz, CDCl3) δ 137.66, 137.31, 134.95, 70.04, 47.54, 37.69, 32.65, 31.67, 30.40, 25.19, 23.42, 15.06, 13.92. IR (film) 3316, 2954, 2928, 2869, 1464, 1028, 697 cm−1. HRMS calcd for C25H42O2Na [M + Na+] 397.3077, found 397.3078.

used as they stand. CpTi(O-i-Pr)3 was prepared by addition of a THF-solution of CpTiCl3 to a THF/hexane-solution of i-PrOLi, generated in situ from i-PrOH and n-BuLi (in hexane) in THF, according to the modified method in the literature,27 and the concentration of the resulting THF(and hexane) solution was adjusted to 0.1 M by adding an additional THF. The CpTi(O-iPr)3 solution (dark red), in which a part of LiCl was precipitated, could be stored for more than a month under inert atmosphere at ambient temperature and used as the solution. Other chemicals which are commercially available, unless otherwise indicated, were used as received. Diyne 1i28 and triyne 1j29 were prepared according to the reported procedures. Structures of the known compounds, i.e., tributylbenzenes (2a and 3a), 5a,29,30,32 triphenylbenzenes (2b and 3b), 5a,30−32 tri(trimethylsilyl)benzenes (2c and 3c),30,32 tri(4-phenoxyphenyl)benzenes (2g and 3g),31 hexa(n-propyl)benzene (2d),32 and 1,3,6,8-tetrahydrobenzo[1,2-c:3,4-c′]difuran (5),5a,29,31 were confirmed by comparison of their spectral with those in the literatures and/or our authentic samples. Typical Procedure for CpTiCl3-Catalyzed Cyclotrimerization of Alkyne. Under an argon atmosphere, to a mixture of Mg powder (24.3 mg, 1.0 mmol) or Zn powder (65.4 mg, 1.0 mmol) in THF (3.0 mL) was added Me3SiCl (126 μL, 1.0 mmol). After being stirred 15 min, 1-hexyne (228 μL, 2.0 mmol) and a solution of CpTiCl3 (1.0 mL, 0.1 M in THF, 0.10 mmol, 5 mol %) were sequentially added. After being stirred for 12 h at 40 °C, the mixture was poured into a mixture of aqueous 0.5 M HCl (3 mL) and hexane (6 mL). The organic layer was separated and the aqueous layer extracted with hexane (2 × 5 mL). The combined organic layers were dried over anhydrous Na2SO4, filtered through a pad of silica gel, and concentrated under reduced pressure. The residue was chromatographed on silica gel (hexane) to give a mixture of 1,3,5 and 1,2,4tributylbenzenes5a,29,30,32 (39:61, total 130 mg) in 79% isolated yield. Typical Procedure for CpTi(O-i-Pr)3-Catalyzed Cyclotrimerization of Alkyne. Under an argon atmosphere, to a mixture of Mg powder (12.1 mg, 0.5 mmol) and (but-3-yn-1-yloxy)(tert-butyl)dimethylsilane (184. mg, 1.0 mmol) in THF (1.5 mL) was added Me3SiCl (63 μL, 0.5 mmol). After being stirred 15 min, a solution of CpTi(O-i-Pr)3 (0.5 mL, 0.1 M in THF, 0.05 mmol, 5 mol %) was added. After being stirred for 12 h at 40 °C, the mixture was poured into a mixture of H2O (2 mL) and hexane (3 mL). The organic layer was separated and aqueous layer extracted with hexane (2 × 3 mL). The combined organic layers were dried over anhydrous Na2SO4, filtered through a pad of silica gel, and concentrated under reduced pressure. The residue was chromatographed on silica gel (hexane/ ether) to give a mixture of 1,3,5- and 1,2,4-tris(2-((tertbutyldimethylsilyl)oxy)ethyl)benzenes (2e and 3e) (39:61, total 146 mg) in 79% isolated yield. Compounds 2e and 3e. 1H NMR (500 MHz, CDCl3) δ 7.07 (d, J = 6.5 Hz, 1H for Ar of 1,2,4-isomer), 6.97−7.00 (m, 2H for Ar of 1,2,4-isomer), 6.88 (s, 3H for Ar of 1,3,5-isomer), 3.73−3.80 (m, 6H, CH2O), 2.86−2.90 and 2.75−2.79 (2m, 6H, benzylic), 0.86 and 0.89 (2s, 27H, t-Bu), 0.002, 0.006, 0.010, and 0.014 (4s, 18H, CH3). 13C NMR (125 MHz, CDCl3) δ 138.85, 136.90, 134.80, 130.90, 129.99, 127.81, 127.03, 64.61, 64.33, 64.26, 61.82, 39.53, 39.22, 36.36, 36.03, 25.94, 25.89, 18.31, −5.27, −5.38. IR (neat) 2928, 2857, 1472, 1386, 1255, 1096, 1005, 937, 837, 775, 662 cm−1. HRMS calcd for C30H60O3Si3Na [M + Na+] 575.3743, found 575.3746. Typical Procedure for Ti(O-i-Pr)4-Catalyzed Cyclotrimerization of Alkyne. Under an argon atmosphere, to a mixture of Mg powder (24.3 mg, 1.0 mmol) in THF (4.0 mL) was added Me3SiCl (126 μL, 1.0 mmol). After being stirred 15 min, ethynyltrimethylsilane (277 μL, 2.0 mmol) and Ti(O-i-Pr)4 (59 μL, 0.20 mmol, 10 mol %) were sequentially added. After being stirred for 12 h at 40 °C, the mixture was poured into a mixture of H2O (3 mL) and hexane (3 mL). The organic layer was separated and aqueous layer extracted with hexane (2 × 3 mL). The combined organic layers were dried over anhydrous Na2SO4, filtered through a pad of silica gel, and concentrated under reduced pressure. The residue was chromatographed on silica gel (hexane) to give a mixture of 1,3,5- and 1,2,4-

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00678. NMR spectra for compounds 2−5 (PDF). Visible-light absorption spectra for the reaction mixture (PDF) E


Article

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Reagents in Carbohydrate Chemistry: Glycal and C-Glycoside Synthesis. Tetrahedron 2000, 56, 2103−2112. (h) Yamamoto, Y.; Hattori, R.; Itoh, K. Highly trans-selective intramolecular pinacol coupling of dials catalyzed by bulky Cp2TiPh. Chem. Commun. 1999, 825−826. (i) Yamamoto, Y.; Hattori, R.; Miwa, T.; Nakagai, Y.; Kubota, T.; Yamamoto, C.; Okamoto, Y.; Itoh, K. Diastereoselective Inter- and Intramolecular Pinacol Coupling of Aldehydes Promoted by Monomeric Titanocene(III) Complex Cp2TiPh. J. Org. Chem. 2001, 66, 3865−3870. (j) Handa, S.; Kachala, M. S.; Lowe, S. R. Synthesis of N-heterocyclic diols by diastereoselective pinacol coupling reactions. Tetrahedron Lett. 2004, 45, 253−256. (g) Okamoto, S.; Sato, F. Biscyclopentadienyltitanium (III) chloride dimer. eEROS Encycl. Reagents Org. Synth. 2002, rn00196. (3) (a) Takeda, T.; Miura, I.; Horikawa, Y.; Fujiwara, T. Desulfurizative titanation of allyl sulfides. Regio and diastereoselective preparation of homoallyl alcohols. Tetrahedron Lett. 1995, 36, 1495− 1498. (b) Horikawa, Y.; Watanabe, M.; Fujiwara, T.; Takeda, T. New Carbonyl Olefination Using Thioacetals. J. Am. Chem. Soc. 1997, 119, 1127−1128. (c) Takeda, T.; Yamamoto, M.; Yoshida, S.; Tsubouchi, A. Highly Diastereoselective Construction of Acyclic Systems with Two Adjacent Quaternary Stereocenters by Allylation of Ketones. Angew. Chem., Int. Ed. 2012, 51, 7263−7266. (d) Urabe, H.; Sato, F. Titanocene (Cp2Ti(II)). e-EROS Encycl. Reagents Org. Synth. 2003, rn00195. (4) (a) Sato, F.; Urabe, H.; Okamoto, S. Synthetic reactions with divalent titanium complex. Yuki Gosei Kagaku Kyokaishi 1998, 56, 424−432. (b) Sato, F.; Urabe, H.; Okamoto, S. Bicyclization of dienes, enynes, and diynes with Ti(ii) reagent. New developments towards asymmetric synthesis. Pure Appl. Chem. 1999, 71, 1511− 1519. (c) Sato, F.; Urabe, H.; Okamoto, S. Synthesis of Organotitanium Complexes from Alkenes and Alkynes and Their Synthetic Applications,. Chem. Rev. 2000, 100, 2835−2886. (d) Sato, F.; Urabe, H.; Okamoto, S. Synthetic Reactions Mediated by a Ti(O-i-Pr)4/2 iPrMgX Reagent. Synlett 2000, 2000, 753−775. (e) Sato, F.; Okamoto, S. The Divalent Titanium Complex Ti(O-i-Pr)4/2 i-PrMgX as an Efficient and Practical Reagent for Fine Chemical Synthesis. Adv. Synth. Catal. 2001, 343, 759−784. (f) Sato, F.; Urabe, H. In Titanium and Zirconium in Organic Synthesis; Marek, I., Ed.; Wiley: Weinheim, 2002; pp 319−354. (g) Okamoto, S.; Livinghouse, T. Titanium(IV) Aryloxide Catalyzed Cyclization Reactions of 1,6- and 1,7-Dienes. J. Am. Chem. Soc. 2000, 122, 1223−1224. (h) For Kulinkovich reaction: Kulinkovich, O. G.; de Meijere, A. 1,n-Dicarbanionic Titanium Intermediates from Monocarbanionic Organometallics and Their Application in Organic Synthesis. Chem. Rev. 2000, 100, 2789− 2834. (i) Eisch, J. J. Early transition metal carbenoid reagents in epimetallation and metallative dimerization of unsaturated organic substrates. J. Organomet. Chem. 2001, 617−618, 148−157. (j) Wolan, A.; Six, Y. Synthetic transformations mediated by the combination of titanium (IV) alkoxides and Grignard reagents: selectivity issues and recent applications,. Tetrahedron 2010, 66, 15−61 and 2010, 66, 3097−3183. . (5) (a) Ohkubo, M.; Mochizuki, S.; Sano, T.; Kawaguchi, Y.; Okamoto, S. Selective Cleavage of Allyl and Propargyl Ethers to Alcohols Catalyzed by Ti(O-i-Pr)4/MXn/Mg. Org. Lett. 2007, 9, 773− 776. (b) Okamoto, S.; He, J.-Q.; Ohno, C.; Oh-iwa, Y.; Kawaguchi, Y. McMurry coupling of aryl aldehydes and imino pinacol coupling mediated by Ti(O-i-Pr)4/Me3SiCl/Mg reagent. Tetrahedron Lett. 2010, 51, 387−390. (c) Shohji, N.; Kawaji, T.; Okamoto, S. Ti(O-iPr)4/Me3SiCl/Mg-Mediated Reductive Cleavage of Sulfonamides and Sulfonates to Amines and Alcohols. Org. Lett. 2011, 13, 2626−2629. (d) Kawaji, T.; Shohji, N.; Miyashita, K.; Okamoto, S. Non-Cp titanium alkoxide-based homolytic ring-opening of epoxides by an intramolecular hydrogen abstraction in β-titanoxy radical intermediates. Chem. Commun. 2011, 47, 7857−7859. (e) Takekoshi, N.; Miyashita, K.; Shoji, N.; Okamoto, S. Generation of a Low-Valent Titanium Species from Titanatrane and its Catalytic Reactions: Radical Ring Opening of Oxetanes. Adv. Synth. Catal. 2013, 355, 2151−2157. (f) Moriai, R.; Naito, Y.; Nomura, R.; Funyu, S.; Ishitsuka, K.; Asano, N.; Okamoto, S. Design and synthesis of 2-(1,3-

AUTHOR INFORMATION

Corresponding Author

*E-mail for S.O.: [email protected]. ORCID

Sentaro Okamoto: 0000-0002-3187-1496 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) [no. 17K05869], Japan, for financial support.

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Alkyne [2 + 2 + 2] Cyclotrimerization Catalyzed by a Low-Valent

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