Ruthenium-catalyzed β-alkylation of secondary alcohols with primary alcohols

Inorganica Chimica Acta 431 (2015) 234–241

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Ruthenium-catalyzed b-alkylation of secondary alcohols with primary alcohols Wei Bai, Guochen Jia ⇑ Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 13 January 2015 Received in revised form 14 March 2015 Accepted 17 March 2015 Available online 7 April 2015

The catalytic properties of a series of ruthenium complexes for b-alkylation of secondary alcohols with primary alcohols were studied. The catalytic activities of the ruthenium complexes were found to be dependent on the auxiliary ligands. The most active catalytic precursor found in this study is the ruthenium complex RuCl2(PPh3)2(2-NH2CH2Py) [2-NH2CH2Py = 2-aminomethyl pyridine], which effectively catalyzed the b-alkylation of both aryl- and alkyl-substituted secondary alcohols with benzylic and alkyl primary alcohols. Ó 2015 Elsevier B.V. All rights reserved.

Keywords: Ruthenium Alcohol Alkylation Transfer hydrogenation

1. Introduction Guerbet-type self- and cross-coupling of alcohols represents a highly attractive route to produce higher alcohols because alcohols are readily available and the reaction is highly atom efficient with water as the sole by-product [1]. The Guerbet reaction, which involves self-coupling of primary and secondary alcohols is known for more than a century [2]. The original procedure to carry out the reaction is to heat the reaction mixture in the presence of a strong base at high temperatures (>220 °C). Under the reaction condition, the desired coupled alcoholic products were formed usually in low yields along with side products. There has been much interest in developing catalysis for carrying out the reactions at lower reaction temperatures, minimizing side products and expanding the scope of the reaction [3–5]. OH

OH H

R A R'

+

R" OH

R

B

R" R'

+ H2O

(1)

C

ð1Þ This work concerns Guerbet-type cross-coupling between a secondary alcohol (A) and a primary alcohol (B) to give a coupled alcoholic product (C) resulting from b-alkylation of the secondary alcohol with the primary alcohol (Eq. (1)). The reaction can be ⇑ Corresponding author. Tel.: +852 23587361. E-mail address: [email protected] (G. Jia). http://dx.doi.org/10.1016/j.ica.2015.03.023 0020-1693/Ó 2015 Elsevier B.V. All rights reserved.

promoted by strong bases such as alkaline hydroxide at high temperatures [6]. However, the method works better for reactions between aryl-substituted secondary alcohols and primary benzylic alcohols and can give coupled ketones as side products. Base-promoted reactions involving aryl-substituted secondary alcohols and primary alkylalcohols were found to proceed slowly and no reaction was observed between alkyl-substituted secondary alcohols RCH(OH)Me (R = nPr, Et) and benzylic alcohols. To overcome the difficulty, there has been much interest in the development of catalytic version of the reaction. In the area of heterogeneous catalysis, cross-coupling reactions of secondary benzylic alcohols with primary alcohols mediated by Pd/C [7], Ag/Al2O3 [8] and AuPd (hydrotalcite supported) [9] have been investigated. However, the reactions produced coupled ketones as the major products. Homogeneous catalysis based on soluble transition metals complexes [10] has also been actively explored for the transformation, for example, with complexes of metals such as Ir [11–13], Rh [4l,14] Pd [15], Cu [16], Ni [17], and Fe [18]. Another class of complexes that have been attracted much attention as potential catalysts for the transformation are ruthenium complexes. In 2003, Cho reported that the ruthenium complex RuCl2(PPh3)3 (5 mol%, 300% KOH, 80 °C, 40 h) can catalyze the direct b-alkylation of secondary alcohols with primary alcohols in the presence of the hydrogen acceptor 1-dodecene in dioxane [19]. Since then, a number of the ruthenium complexes have been tested for the reaction, including RuCl2(@CHPh)(PCy3)2 [20], [(g6-cymene)RuCl2]2 [12,20], RuCl2(DMSO)4 [21], [(g6-C6H6)Ru(o-C6H4CH2NR2(NCMe)]PF6 [22], [(g6-cymene)RuCl(pyrimidine-NHC)]PF6 [13], RuCl2(PPh3)(2,20 :60 , 200 -terpyridine) [12], RuCl3(2,20 :60 ,200 -terpyridine) [12], Cp⁄RuCl2

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[12], (g6-cymene)ruthenium complexes supported by triazolidene [23] and cymene-ruthenium NHC [24], (NHC@imidazolin-2-ylidene, imidazolin-4-ylidene, and pyrazolin-3-ylidene) ligands, [CpRu(CH3CN)(PPh3)2]BF4 [25], CpRuCl(PPh3)2 [25], CpRuCl(dppm) [25], [TpRu(CH3CN)(PPh3)2]BF4 [25], TpRuCl(PPh3)2 [25] and cis[Ru(H2O)2(6,6’Cl2bpy)2](OTf)2 [25]. These previous studies have revealed that b-alkylation of arylsubstituted secondary alcohols ArCH(OH)R with primary alcohols can be accomplished with many ruthenium complexes, although the activity varies with ligands. However, catalytic reactions involving alkyl-substituted secondary alcohols RCH(OH)R’ have been less studied and b-alkylation of alkyl-substituted secondary alcohols RCH(OH)R’ with primary alkyl alcohols RCH2OH in particular has met with limited success. In fact, there are only two reports concerning Ru-catalyzed b-alkylation of alkyl-substituted secondary alcohols RCH(OH)R’ with primary alkyl alcohols RCH2OH. Cho showed that the ruthenium complex RuCl2(PPh3)3 (5 mol%, 300% KOH, 80 °C, 40 h) catalyzed b-alkylation of RCH(OH)Me (R = CH3(CH2)4, Ph(CH2)2, Me2CH) with PhCH2CH2OH in the presence of the hydrogen acceptor 1-dodecene to give RCH(OH)CH2CH2CH2Ph in low or moderate yields (23–58%) [19]. Lau et al. showed that only trace amounts of the expected coupling products were obtained in the reactions of RCH(OH)Me (R = CH3(CH2)4, Ph(CH2)2) with PrCH2OH catalyzed by complexes such as CpRuCl(PPh3)2, CpRuCl(dppm) and cis-[Ru(H2O)2(6,60 Cl2bpy)2](OTf)2 (0.4 mol%, 120 °C, 20 h) [25]. Previous studies have also revealed that the ligand environment around ruthenium can play a significant role in determining the catalytic activity of ruthenium complexes. For example, while RuCl2(PPh3)(2,20 :60 , 200 -terpyridine) effectively catalyzed the coupling reaction between PhCH(OH)Me with PhCH2OH, the complexes [(g6-cymene)RuCl2]2 and Cp⁄RuCl2 are essentially inactive for the reaction [12]. These observations promoted us to investigate the catalytic properties of other ruthenium complexes with the intention of finding efficient catalytic systems for b-alkylation of alkyl-substituted secondary alcohols with primary alcohols. In this work, we report (i) ligand effect on the catalytic activities of ruthenium complexes for b-alkylation of secondary alcohols with primary alcohols, and (ii) new catalytic systems that work for b-alkylation of both aryl- and alkyl-substituted secondary alcohols with benzylic and alkyl primary alcohols. 2. Results and discussion 2.1. Selection of catalysts In our initial effort in searching for catalysts for b-alkylation of secondary alcohols with primary alcohols, we have studied the catalytic activities of a series of ruthenium complexes for the reaction of 1-phenylethanol (1a) with benzyl alcohol (2a) to give 1,3-diphenyl-1-propanol (3a) (Eq. (2)). OH

OH OH

+

[Ru] KOH

1a

2a

3a

ð2Þ In these screening experiments, a mixture of 1-phenyl ethanol, two equivalents of benzyl alcohol, 5 mol% of ruthenium complex and one equivalent of KOH in dioxane was heated at 80 °C for 5 h. The reaction mixture was then analyzed by 1H NMR. The results are shown in Table 1. The cyclopentadienyl complexes CpRuCl(dppe), CpRuH(dppe), Cp⁄RuCl(COD) and Cp⁄RuCl(PPh3)2 as well as the dihydride

complex RuH2(CO)(PPh3)3 are essentially inactive, giving the expected product 1,3-diphenyl-1-propanol (3a) in less than 7% yields (entries 1–5, Table 1). The complexes RuHCl(CO)(PPh3)3, {[(g6-cymene)RuCl(NH2CH2CH2O)]2H}Cl, (g6-cymene)RuCl(NH2 CH2CH2NTs)Cl and RuCl2(2,20 -bipyridine)(PPh3)2 are marginally active, giving the expected product 1,3-diphenyl-1-propanol (3a) in 12–19% yields (entries 6–8 and 11, Table 1). The complexes (g6-cymene)RuCl(PPh3) and Ru(OAc)2(PPh3)2 are moderately active, giving the expected product 1,3-diphenyl-1-propanol in 55% and 36% respective yields (entries 9 and 10, Table 1). Ruthenium complexes containing 2-aminomethylpyridine (2NH2CH2Py) such as RuCl2(dppb)(2-NH2CH2Py), RuCl2(dppf) (2-NH2CH2Py) and RuCl2(PPh3)2(2-NH2CH2Py) showed good activity for the reaction, giving the desired secondary alcohol product in 64–91% yields with RuCl2(PPh3)2(2-NH2CH2Py) being most active (entries 13–15, Table 1). Further study shows that the reaction catalyzed by RuCl2(PPh3)2(2-NH2CH2Py) also went smoothly with 1 mol% loading of RuCl2(PPh3)2(2-NH2CH2Py) in toluene at 105 °C in the presence of one equivalent of KOtBu (entry 17, Table 1). For comparison, we have also carried out catalytic reactions with complexes RuCl2(PPh3)2 [25], RuCl2(DMSO)4 [25], CpRuCl(PPh3)2 [25] and RuCl2(PPh3)(2,20 :60 ,200 -terpyridine) [12], which were previously reported to be catalytically active for the reaction. As shown in entries 18–21, Table 1, these complexes are less effective than RuCl2(PPh3)2(2-NH2CH2Py) under similar reaction condition. 2.2. Scope of substrates After finding out that the complex RuCl2(PPh3)2(2-NH2CH2Py) is the most active catalyst for b-alkylation of PhCH(OH)Me with PhCH2OH, we have explored the substrate scope of the reactions catalyzed by RuCl2(PPh3)2(2-NH2CH2Py). In these catalytic experiments, a mixture of a secondary alcohol, two equivalents of a primary alcohol, 1 mol% of the ruthenium complex and one equivalent of KOtBu in toluene was heated for a period time. The Table 1 Catalytic b-alkylation of 1-phenyl ethanol with benzyl alcohol catalyzed by selected ruthenium complexes.a Entry

Catalyst

NMR yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

CpRu(dppe)Cl CpRu(dppe)H Cp⁄Ru(COD)Cl Cp⁄Ru(PPh3)2Cl RuH2(CO)(PPh3)3 RuHCl(CO)(PPh3)3 {[(p-cymene)RuCl(NH2CH2CH2O)]2H}Cl (p-cymene)RuCl(NH2CH2CH2NTs)Cl (p-cymene)RuCl2(PPh3) Ru(OAc)2(PPh3)2 RuCl2(PPh3)2(2,20 -bipyridine) RuCl2(PPh3)2(NH2CH2CH2NH2) RuCl2(dppb)(2-NH2CH2Py) RuCl2(dppf) (2-NH2CH2Py) RuCl2(PPh3)2(2-NH2CH2Py) RuCl2(PPh3)2(2-NH2CH2Py) RuCl2(PPh3)2(2-NH2CH2Py) RuCl2(PPh3)3 RuCl2(DMSO)4 CpRu(PPh3)2Cl RuCl2(PPh3)(2,20 :60 ,200 -terpyridine)

<5 <5 7 <5 <5 16 19 19 55 36 12 56 64 77 91 84b >99c 13 24 19 49

a A mixture of 5 mol% of a ruthenium complex, 35 mg of KOH (0.624 mmol, 1 equiv.) 75 lL of 1-phenyl ethanol (0.620 mmol, 1 equiv.) and 130 lL of benzyl alcohol (1.256 mmol, 2 equiv.) in 1 mL of dioxane was heated at 80 °C for 5 h. b 2 mol% of the ruthenium complex was used, without change of other reagents or conditions as stated in a. c 1 mol% of the ruthenium complex and one equivalent of KOtBu in 1 mL of toluene were used. The mixture was stirred at 105 °C for 5 h. No signal of 1-phenyl ethanol was detected at the end of the reaction.

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Table 2 RuCl2(PPh3)2(2-NH2CH2Py) catalyzed b-alkylation of secondary alcohols with and primary alcohols.a Entry

Secondary alcohol

1

Primary alcohol

OH

Product (isolated yield) OH

OH

2a 1a 2

3a (82.0%) OH

OH

OH MeO

2b

OMe

1a 3

3b (84.8%) OH

OH

OH

2c 3c (68.9%)

1a 4

OH

OH OH

1a 5

2d

3d (66.0%)

OH

OH

OH 2e 1a 6

3e (66.0%) OH

OH

OH 2f 1a 7

3f (81.5%) OH

OH

OH 2g

1a 8

3g (66.0%)

OH

OH

OH

2a

Br

Br

1b 9

3h (66.2%) OH

OH

OH 2f Br

Br

1b

3i (54.6%)

a

A mixture of 1 mol% of the ruthenium complex, one equivalent of KOtBu, one equivalent of a secondary alcohol, two equivalents of a primary alcohol in 1 mL of toluene was stirred at 80–105 °C for 8–24 h (see Section 4 for details). The product was isolated by chromatography.

reaction products are purified by chromatography and analyzed by 1 H NMR. The results are shown in Table 2. As shown in Table 2, RuCl2(PPh3)2(2-NH2CH2Py) effectively catalyze the b-alkylation of PhCH(OH)Me (1a) with both benzylic and alkyl primary alcohols, giving desired products (3a-g) in 66–85% isolated yields (entries 1–6, Table 2). Our in situ 1H NMR showed that more than 99% conversions were achieved (i.e. no secondary alcohol left) and the side products ketones were not produced in all the reactions. Thus the true yields of the reactions should be higher than those listed in Table 2. 4-Bromobenzylic alcohol (1b) similar reacted with primary benzylic and alkyl alcohols, giving desired products 3h-i in 55–66% isolated yields (entries 7 and 8, Table 2). We have also carried out catalytic reactions involving secondary alkyl alcohols with primary alcohols. As shown in Table 3, in the presence of RuCl2(PPh3)2(2-NH2CH2Py), 2-pentanol (1c) reacted with two equivalents of benzyl alcohol (2a), 1-butanol (2f), and

1-octanol (2h) to give the corresponding coupling products 3j-l in moderate to excellent isolated yields (64–93%) (entries 1–3, Table 3). Under the same condition, 2-propanol (1d) reacted with two equivalents of benzyl alcohol to produce the bis-alkylated product 4a in 88.4% isolated yields (entry 4, Table 3). When 2-propanol (1d) was allowed to react with one equivalent of benzyl alcohol, the reaction produced the mono-alkylated product 3m and the bis-alkylated product 4a, which can be isolated in 30.5% and 19.7% yields, respectively (Scheme 1). 2.3. Reaction mechanism It has been proposed previously that b-alkylation of secondary alcohols with primary alcohols proceed via a ‘‘borrowing hydrogen’’ process [1]. A similar mechanism can be proposed for the present b-alkylation reactions promoted by RuCl2(PPh3)2 (2-NH2CH2Py) (5).

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W. Bai, G. Jia / Inorganica Chimica Acta 431 (2015) 234–241 Table 3 RuCl2(PPh3)2(2-NH2CH2Py) catalyzed b-alkylation of secondary alkyl alcohols with primary alcohols.a Entry

Secondary alcohol

1

Primary alcohol

OH

Product (isolated yield) OH

OH

2a

1c

3j (92.6%) 2

OH

OH OH 2f

1c

3k (75.3%)

3

1-octanol 2h

OH

OH

1c 4

3l (64.0%)

OH

OH

OH

2a

1d

4a (88.4%) a

A mixture of 1 mol% of the ruthenium complex, one equivalent of KOtBu, one equivalent of secondary alcohol, two equivalents of primary alcohol in 1 mL of toluene or dioxane was stirred at 100 °C for 24 h. The product was isolated by chromatography.

OH

OH

+ 1d

OH +

RuCl2(PPh3)2(2-NH2CH2Py)

2a

OH

100 oC, 24 h

4a 19.7%

3m 30.5% Scheme 1. Alkylation of 2-propanol with benzyl alcohol.

H H2 N

PPh3

Ru

N

PPh3 Cl

H

Ph

Me

H OH

7 PPh3

PPh3 Ph

O

H Me

H N N

PPh3 10

Ru

Ph

H

Ru

N

O

H

N

- acetone

6

Me H

N

NaOCHMe2

Ph

O H

PPh3

H2 N

H

N H2

H PPh3

N

Ru

PPh3 PPh3

Ru H

PPh3 H

Me

8

OH

9 Ph

Me

Scheme 2. Reported reactions of ruthenium hydride complexes supported by 2-aminomethyl pyridine.

It is known that the monohydride complex RuHCl(PPh3)2 (2-NH2CH2Py) (6) can reacts with NaOCHMe2 to give the dihydride complex c,t-RuH2(PPh3)2(2-NH2CH2Py) (7) (Scheme 2) [26]. It has also been reported that the dichloro complex RuCl2(PPh3)2(2-NH2 CH2Py) (5) and the monohydride complex RuHCl(PPh3)2 (2-NH2CH2Py) (6) can be used as catalytic precursors for transfer hydrogenation of PhC(O)Me with isopropanol in the presence of NaOH [26], and that the monohydride complex RuHCl(PPh3)2 (2-NH2CH2Py) (6) (in the presence of KOiPr) and the dihydride complex c,t-RuH2(PPh3)2(2-NH2CH2Py) (7) are catalytically active for hydrogenation of PhC(O)Me [27]. In the catalytic hydrogenation reaction, it was proposed that the dihydride complex c,t-RuH2 (PPh3)2(2-NH2CH2Py) (7) is converted to the five coordinated amido complex 8 through its reaction with PhC(O)Me. The catalytic cycle is completed by the reaction of complex 8 with H2 to give the dihydride complex t,c-RuH2(PPh3)2(2-NH2CH2Py) (9), which react

with PhC(O)Me via transition state 10 to give PhCH(OH)Me and to re-generate the five coordinated amido complex 8. Based on these previous work, a plausible mechanism for the present b-alkylation of secondary alcohols with primary alcohols catalyzed by RuCl2(PPh3)2(2-NH2CH2Py) (5) is proposed in Scheme 3. In the presence of base, RuCl2(2-H2NCH 2Py)(PPh3)2 (5) can react with RCH(OH)Me and R’CH2OH to give the carbonyl compounds RC(O)Me and R’CH@O, and the dihydride complex c,t-RuH2(2-H2NCH2Py)(PPh3)2 (7) which may react with RC(O)Me or R’CH@O to give the five coordinated amido complex RuH (2-HNCH2Py)(PPh3)2 (8). Complex 8 may react with RCH(OH)Me and R’CH2OH via a transition state like 10 shown in Scheme 2 (an out-sphere mechanism) to give the dihydride complex t,c-RuH2(2-H2NCH2Py)(PPh3)2 (9) and the carbonyl compounds RC(O)Me and R’CH@O. The carbonyl compounds RC(O)Me and

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H2 N N

OH

Cl PPh3

R

+ Me

O

O

OH R'

+ Me R'

R

Ru

H Ru

N

PPh3 Cl

PPh3

H2 N

H

base

5 OH + Me

R

H 7 PPh3

OH

O

O

R'

or Me R'

R

H

H OH + Me

R

N

OH

H N N

R

+ Me R'

R'

H Ru

PPh3 PPh3

H 9

O

O

R

H 8 H

HO

PPh3 PPh3

Ru

N

R'

O aldol condensation H

base

R

R'

Scheme 3. A proposed mechanism for the alkylation of secondary alcohols with primary alcohols.

R’CH@O can undergo base-catalyzed aldol condensation to form a, b-unsaturated carbonyl compounds RC(O)CH@CHR’, which then reacts with the dihydride complex t,c-RuH2(2-H2NCH2Py)(PPh3)2 (9) to give the product RCH(OH)CH2CH2R’ and regenerate the five coordinated amido complex RuH(2-HNCH2Py)(PPh3)2 (8) to start another catalytic cycle. We have tried to detect the active species involved in the reaction of PhCH(OH)Me with PhCH2OH catalyzed by RuCl2(PPh3)2 (2-NH2CH2Py) (5) using in situ NMR spectroscopy. In agreement with our proposed mechanism, the dihydride complex c,t-RuH2 (PPh3)2(2-NH2CH2Py) (7) was detected in the early stage of the reaction. After a 1:1 mixture of PhCH(OH)Me and PhCH2OH in C6D6 in the presence of 10 mol% of RuCl2(PPh3)2(2-NH2CH2Py) and one equivalent of KOtBu was heated at 60 °C for 10 min., the 1 H NMR spectrum of the mixture showed the 1H NMR signals of the dihydride complex c,t-RuH2(PPh3)2(2-NH2CH2Py) (7) at 18.2 and 16.3 ppm, and the 31P NMR spectrum showed a singlet 31 1 P{ H} signal of the dihydride complex c,t-RuH2(PPh3)2 (2-NH2CH2Py) (7) at 66.4 ppm. It should be noted that the in situ 1 H NMR spectrum also showed minor hydride signals of RuH2(CO)(PPh3)3 at 8.3 and 6.5 ppm and unidentified hydride signals at 7.0 (br) ppm, 12.7 ppm. 3. Summary We have investigated the catalytic properties of a series of ruthenium complexes for b-alkylation of secondary alcohols with primary alcohols. The catalytic activities of the ruthenium complexes were found to be dependent on the auxiliary ligands. The ruthenium complexes RuCl2(PPh3)2(2-NH2CH2Py) was found to be the most active catalytic precursor, which effectively catalyzed the b-alkylation of both aryl- and alkyl-substituted secondary alcohols with benzylic and alkyl primary alcohols.

Solvents were distilled under nitrogen from sodium benzophenone (hexane, ether, THF), or calcium hydride (CH2Cl2). Other solvents were purged with nitrogen for 10 min before use. CpRuCl(dppe) [28], CpRuH(dppe) [29], Cp⁄RuCl(COD) [30], Cp⁄RuCl(PPh3)2 [31], RuHCl(CO)(PPh3)3 [32], RuH2(CO)(PPh3)3 [32], {[(p-cymene) RuCl(NH2CH2CH2O)]2H}Cl [33], (p-cymene)RuCl(NH2CH2CH2NTs) Cl [34], (p-cymene)RuCl2(PPh3) [35], Ru(OAc)2(PPh3)2 [36], RuCl2 (PPh3)2(2,20 -bipyridine) [37], RuCl2(NH2CH2CH2NH2)(PPh3)2 [38], RuCl2(dppf)(2-PyCH2NH2) [39], RuCl2(dppb)(2-NH2CH2Py) [26], RuCl2(2-H2NCH2Py)(PPh3)2 [26], RuCl2(PPh3)3 [40], RuCl2(DMSO)4 [41], CpRuCl(PPh3)2 [42], and RuCl2(PPh3)(2,20 :60 ,200 -terpyridine) [43] were prepared following the procedures described in the literature. All other reagents were used as purchased from Aldrich Chemical Co., Acros Organics, International Laboratory (USA), or Kodak. Microanalyses were performed by M-H-W Laboratories (Phoenix, AZ). 1H, 13C{1H} and 31P{1H} NMR spectra were collected on a Bruker-400 spectrometer (400.1 MHz) or a Bruker ARX-300 spectrometer (300.1 MHz). 1H and 13C NMR shifts are relative to TMS, and 31P chemical shifts are relative to 85% H3PO4. 4.1. General procedure for ruthenium catalyzed alkylation reactions between 1-phenyl ethanol and benzyl alcohol in the screening experiments (Table 1) A mixture of 5 mol% of a ruthenium complex, 35 mg of KOH (0.624 mmol, 1 equiv.), 75 lL of 1-phenyl ethanol (0.620 mmol, 1 equiv.) and 130 lL of benzyl alcohol (1.256 mmol, 2 equiv.) in 1 mL of dioxane was stirred at 80 °C for 5 h. The reaction mixture was then analyzed by NMR. 4.2. General procedure for RuCl2(PPh3)2(2-NH2CH2Py) catalyzed b-alkylation reactions of secondary alcohols with primary alcohols (Tables 2 and 3)

4. Experimental section All manipulations were carried out under a nitrogen atmosphere using standard Schlenk techniques unless otherwise stated.

A mixture of RuCl2(PPh3)2(2-NH2CH2Py) (1 mol%), KOtBu (1 equiv.), a secondary alcohol (1 equiv) and a primary alcohol (2 equiv.) in 1 mL of toluene or dioxane was heated for a period

W. Bai, G. Jia / Inorganica Chimica Acta 431 (2015) 234–241

of time. The product was extracted with diethyl ether, purified by column chromatography on silica gel using petroleum ether/ diethyl ether 6:1 and 4:1 as the eluents, and identified by NMR.

239

b-CH2), 1.26–1.47 (m, 2H, c-CH2), 0.93 (t, J = 7.0 Hz, 3H, Me). The NMR data are consistent with the reported ones [44]. 4.8. Reaction of 1-phenyl ethanol with butanol (Table 2, entry 6)

4.3. Reaction of 1-phenyl ethanol with benzyl alcohol (Table 2, entry 1) t

RuCl2(PPh3)2(2-NH2CH2Py), 7 mg (0.0087 mmol); KO Bu, 93 mg (0.829 mmol); 1-phenyl ethanol, 100 lL (0.827 mmol); benzyl alcohol, 171 lL (1.652 mmol); toluene, 1 mL; temperature, 105 °C; reaction time, 22 h. Yield of 3a (1,3-diphenyl-propanol): 144 mg, 82.0%. 1H NMR (400 MHz, CDCl3): d 7.15–7.36 (m, 10H, Ph), 4.66 (t, J = 8.0 Hz, 1H, a-CH), 2.61–2.77 (m, 2H, c-CH2), 1.96–2.16 (m, 3H, b-CH2 and OH). The NMR data are consistent with the reported ones [44]. 4.4. Reaction of 1-phenyl ethanol with 4-methoxyphenyl methanol (Table 2, entry 2) RuCl2(PPh3)2(2-NH2CH2Py), 7 mg (0.0087 mmol); KOtBu, 93 mg (0.829 mmol); 1-phenyl ethanol, 100 lL (0.827 mmol); 4-methoxyphenyl methanol, 228 mg (1.650 mmol); toluene, 1 mL; temperature, 105 °C; reaction time, 22 h. Yield of 3b (3-(4-methoxyphenyl)-1-phenyl-1-propanol): 170 mg, 84.8%. 1H NMR (400 MHz, CDCl3): d 6.80–7.35 (m, 9H, Ph), 4.67 (dd, J1 = 8 Hz, J2 = 5.6 Hz, 1H, a-CH), 3.80 (s, 3H, OMe), 2.55–2.75 (m, 2H, c-CH2), 2.00–2.15 (m, 2H, b-CH2), 1.95 (br s, 1H, OH). The NMR data are consistent with the reported ones [21]. 4.5. Reaction of 1-phenyl ethanol with 2-naphthyl methanol (Table 2, entry 3) RuCl2(PPh3)2(2-NH2CH2Py), 5 mg (0.0062 mmol); KOtBu, 70 mg (0.624 mmol); 1-phenyl ethanol, 75 lL (0.620 mmol); 2-naphthyl methanol, 98 mg (0.619 mmol); toluene, 1 mL; temperature, 100 °C; reaction time, 22 h. Yield of 3c (3-(2-naphthyl)-1-phenyl1-propanol): 112 mg, 68.9%. 1H NMR (400 MHz, CDCl3): d 7.25– 8.00 (m, 12H, Ph and naphthyl), 4.74 (dd, J1 = 8 Hz, J2 = 5.6 Hz, 1H, a-CH), 3.05–3.30 (m, 2H, c-CH2), 2.10–2.30 (m, 2H, b-CH2), 1.95 (br s, 1H, OH). The NMR data are consistent with the reported ones [21]. 4.6. Reaction of 1-phenyl ethanol with 1-pyrenyl methanol (Table 2, entry 4) RuCl2(PPh3)2(2-NH2CH2Py), 5 mg (0.0062 mmol); KOtBu, 70 mg (0.624 mmol); 1-phenyl ethanol, 75 lL (0.620 mmol); 1-pyrenyl methanol, 173 mg (0.745 mmol); toluene, 1 mL; temperature, 80 °C; reaction time, 21 h. Yield of 3d (3-(1-pyrenyl)-1-phenyl-1propanol): 138 mg, 66.0%. 1H NMR (400.1 MHz, CDCl3): 8.23–7.88 (m, 9H, pyrenyl ring), 7.42–7.30 (m, 5H, Ph), 4.82 (dd, J1 = 7.6 Hz, J2 = 4.8 Hz, 1H, a-CH), 3.52–3.39 (m, 2H, c-CH2), 2.38–2.25 (m, 2H, b-CH2), 1.86 (br s, 1H, OH). 13C{1H} NMR (100.62 MHz, CDCl3): 143.91, 135.55, 130.79, 130.28, 129.24, 128.05, 127.95, 127.10, 126.87, 126.66, 126.60, 126.00, 125.33, 125.18, 124.48, 124.37, 124.22, 124.09, 122.73 (pyrenyl and phenyl rings), 73.43 (a-C), 40.19 (b-C), 28.97 (c-C). Anal. Calc. for C25H20O: C, 89.25; H, 5.99. Found: C, 89.17; H, 6.16%. 4.7. Reaction of 1-phenyl ethanol with ethanol (Table 2, entry 5) RuCl2(PPh3)2(2-NH2CH2Py), 7 mg (0.0087 mmol); KOtBu, 93 mg (0.829 mmol); 1-phenyl ethanol, 100 lL (0.827 mmol); ethanol, 96 lL (1.646 mmol); toluene, 1 mL; temperature, 105 °C; reaction time, 22 h. Yield of 3e (1-phenyl-1-butanol): 82 mg, 66.0%. 1H NMR (400 MHz, CDCl3): d 7.27–7.36 (m, 5H, Ph), 4.63 (t, J = 6.4 Hz, 1H, a-CH), 2.45 (br s, 1H, OH), 1.64–1.84 (m, 2H,

RuCl2(PPh3)2(2-NH2CH2Py), 7 mg (0.0087 mmol); KOtBu, 93 mg (0.829 mmol); 1-phenyl ethanol, 100 lL (0.827 mmol); butanol, 151 lL (1.650 mmol); toluene, 1 mL; temperature, 105 °C; reaction time, 22 h. Yield of 3f (1-phenyl-1-hexanol): 120 mg, 81.5%. 1H NMR (400 MHz, CDCl3): d 7.30–7.38 (m, 5H, Ph), 4.63 (t, J = 8.00 Hz, 1H, a-CH), 2.59 (br s, 1H, OH), 1.69–1.84 (m, 2H, b-CH2), 1.29–1.44 (m, 6H, other CH2), 0.94 (t, J = 8.00 Hz, 3H, Me). The NMR data are consistent with the reported ones [45]. 4.9. Reaction of 1-phenyl ethanol with 3-methylbutanol (Table 2, entry 7) RuCl2(PPh3)2(2-NH2CH2Py), 7 mg (0.0087 mmol); KOtBu, 93 mg (0.829 mmol); 1-phenyl ethanol, 100 lL (0.827 mmol); 3-methylbutanol, 180 lL (1.652 mmol); toluene, 1 mL; temperature, 105 °C; reaction time, 22 h. Yield of 3g (5-methyl-1-phenylhexanol): 105 mg, 66.0%. 1H NMR (400 MHz, CDCl3): d 7.24–7.36 (m, 5H, Ph), 4.64 (t, J = 7.5 Hz, 1H, a-CH), 1.18–1.79(m, 8H), 0.850 (d, J = 6.5 Hz, 3H, Me), 0.846 (d, J = 6.5 Hz, 3H, Me). The NMR data are consistent with the reported ones [44]. 4.10. Reaction of 4-bromo-1-phenyl ethanol with benzyl alcohol (Table 2, entry 8) RuCl2(PPh3)2(2-NH2CH2Py), 6 mg (0.0075 mmol); KOtBu, 81 mg (0.722 mmol); 4-bromo-1-phenyl ethanol, 100 lL (0.726 mmol); benzyl alcohol, 150 lL (1.450 mmol); toluene, 1 mL; temperature, 90 °C; reaction time, 8 h. Yield of 3h (1-(4-bromophenyl)-3phenylpropan-1-ol): 140 mg, 66.2%. 1H NMR (300 MHz, CDCl3): d 7.15–7.46 (m, 9H, Ph), 4.64 (dd, J1 = 7.2 Hz, J2 = 6 Hz, 1H, a-CH), 2.63–2.71 (m, 2H, c-CH2), 1.95–2.09 (m, 3H, b-CH2 and OH). The NMR data are consistent with the reported ones [11c]. 4.11. Reaction of 4-bromo-1-phenyl ethanol with butanol (Table 2, entry 9) RuCl2(PPh3)2(2-NH2CH2Py), 6 mg (0.0075 mmol); KOtBu, 81 mg (0.722 mmol); 4-bromo-1-phenyl ethanol, 100 lL (0.726 mmol); benzyl alcohol, 140 lL (1.530 mmol); toluene, 1 mL; temperature, 100 °C; reaction time, 23 h. Yield of 3i (1-(4-bromophenyl)-hexanol): 102 mg, 54.6%. 1H NMR (400 MHz, CDCl3): d 7.17–7.43 (m, 4H, Ph), 4.56 (t, J = 6.8 Hz, 1H, a-CH), 2.38 (br s, 1H, OH), 1.58–1.75 (m, 2H), 1.20–1.38 (m, 6H), 0.86 (t, J = 7.0 Hz, 3H, Me). The NMR data are consistent with the reported ones [46]. 4.12. Reaction of pentan-2-ol with benzyl alcohol (Table 3, entry 1) RuCl2(PPh3)2(2-NH2CH2Py), 7 mg (0.0087 mmol); KOtBu, 103 mg (0.918 mmol); pentan-2-ol, 100 lL (0.921 mmol); benzyl alcohol, 95 lL (0.918 mmol); toluene, 1 mL; temperature, 100 °C; reaction time, 24 h. Yield of 3j (1-phenyl-3-hexanol): 101 mg, 61.5%. 1H NMR (300 MHz, CDCl3): d 7.33–7.18 (m, 5H, Ph), 3.66–3.63 (m, 1H, a-CH), 2.82–2.67 (m, 2H,), 1.83–1.72 (m, 2H), 1.68 (br s, 1H), 1.49–1.35 (m, 4H), 0.99 (t, J = 6.4 Hz, 3H, Me). The NMR data are consistent with the reported ones [11c]. 4.13. Reaction of pentan-2-ol with 1-butanol (Table 3, entry 2) RuCl2(PPh3)2(2-NH2CH2Py), 7 mg (0.0075 mmol); KOtBu, 103 mg (0.918 mmol); pentan-2-ol, 100 lL (0.921 mmol); 1-butanol, 170 lL (1.858 mmol); dioxane, 1 mL; temperature, 100 °C;

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reaction time, 24 h. Yield of 3k (nonan-4-ol): 100 mg, 75.3%. 1H NMR (400 MHz, CDCl3): d 3.66–3.56 (m, 1H, a-CH), 1.57–1.23 (m, 13H, CH2 and OH), 0.93 (t, J = 7.1 Hz, 3H, Me), 0.89 (t, J = 7.0 Hz, 3H, Me). The NMR data are consistent with the reported ones [47]. 4.14. Reaction of pentan-2-ol with 1-octanol (Table 3, entry 3) RuCl2(PPh3)2(2-NH2CH2Py), 7 mg (0.0087 mmol); KOtBu, 103 mg (0.918 mmol); pentan-2-ol, 100 lL (0.921 mmol); 1-octanol, 300 lL (1.905 mmol); dioxane, 1 mL; temperature, 100 °C; reaction time, 24 h. Yield of 3l (tridencan-4-ol): 60 mg, 118 mg, 64.0%. 1H NMR (300 MHz, CDCl3): d 3.60 (m, 1H, a-CH), 1.10– 1.60 (m, 21 H, CH2 and OH), 0.93 (t, J = 3.6 Hz, 3H, Me), 0.87 (t, J = 6.8 Hz, 3H, Me) The NMR data are consistent with the reported ones [48]. 4.15. Reaction of isopropanol with two equivalents of benzyl alcohol (Table 2, entry 4) RuCl2(PPh3)2(2-NH2CH2Py), 10 mg (0.0124 mmol); KOtBu, 147 mg (1.310 mmol); isopropanol, 100 lL (1.308 mmol); benzyl alcohol, 270 lL (2.610 mmol); toluene, 1 mL; temperature, 100 °C; reaction time, 24 h. Yield of 4a (1,5-diphenyl-3-pentanol): 278 mg, 88.4%. 1H NMR data of 4a (400 MHz, CDCl3): d 7.11–7.32 (m, 10H, Ph), 3.68–3.70 (m, 1H, a-CH), 2.78–2.85 (m, 2H, c-CH2), 2.66–2.73 (m, 2H, c-CH2), 1.79–1.86 (m, 5H, b-CH2 and OH). The NMR data are consistent with the reported ones [49]. 4.16. Reaction of isopropanol with one equivalent of benzyl alcohol (Scheme 2) RuCl2(PPh3)2(2-NH2CH2Py), 10 mg (0.0124 mmol); KOtBu, 147 mg (1.310 mmol); isopropanol, 100 lL (1.308 mmol); benzyl alcohol, 135 lL (1.305 mmol); toluene, 1 mL; temperature, 100 °C; reaction time, 24 h. Yield of 3m, 62 mg 19.7%; yield of 4a, 60 mg, 30.5%. 1H NMR data of 4a (400 MHz, CDCl3): d 7.15–7.32 (m, 5H, Ph), 3.83 (m, 1H, a-CH), 2.62–2.82 (m, 2H, c-CH2), 1.70– 1.85 (m, 2H, b-CH2), 1.58 (br s, 1H, OH), 1.23 (d, J = 6.0 Hz, 3H, Me). The NMR data are consistent with the reported ones [50]. Acknowledgements This work was supported by the Hong Kong Research Grant Council (Project No.: 602611, 601812, 602113, CUHK7/CRF/12G-2). References [1] For reviews, see: (a) G. Guillena, D.J. Ramón, M. Yus, Angew. Chem., Int. Ed. 46 (2007) 2358; (b) M.H.S.A. Hamid, P.A. Slatford, J.M.J. Williams, Adv. Synth. Catal. 349 (2007) 1555; (c) T.D. Nixon, M.K. Whittlesey, J.M.J. Williams, Dalton Trans. (2009) 753; (d) G.E. Dobereiner, R.H. Crabtree, Chem. Rev. 110 (2010) 681. [2] (a) M.C.R. Guerbet, Acad. Sci. 149 (1909) 129; (b) E. Reid, H. Worthington, A.W. Larchar, J. Am. Chem. Soc. 61 (1939) 99; (c) H. Machemer, Angew. Chem. 64 (1952) 213; (d) S. Veibel, J.I. Nielsen, Tetrahedron 23 (1967) 1723; (e) T. Koto, K. Yoshinaga, N. Hatanaka, J. Emoto, M. Yamaye, Macromolecules 18 (1985) 846. [3] Examples of work on self-coupling of alcohols with homogeneous catalysts (a) G. Xu, T. Lammens, Q. Liu, X. Wang, L. Dong, A. Caiazzo, N. Ashraf, J. Guana, X. Mu, Green Chem. 16 (2014) 3971; (b) Y. Obora, Y. Anno, R. Okamoto, T. Matsu-ura, Y. Ishii, Angew. Chem., Int. Ed. 50 (2011) 8618; (c) K. Koda, T. Matsu-ura, Y. Obora, Y. Ishii, Chem. Lett. (2009) 838; (d) T. Matsu-ura, S. Sakaguchi, Y. Obora, Y. Ishii, J. Org. Chem. 71 (2006) 8306; . Rh based catalysts(e) P.L. Burk, R.L. Pruett, K.S. Campo, J. Mol. Catal. 33 (1985) 1; . Ru based catalysts(f) I.S. Makarow, R. Madsen, J. Org. Chem. 78 (2013) 6593; . Rh, Ru, Pt, Pd and Au based catalysts(g) G. Gregorio, G.F. Pregaglia, R. Ugo, J. Organomet. Chem. 37 (1972) 385.

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