Cobalt(II) Coordination Polymer as a Precatalyst for Selective

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Cobalt(II) Coordination Polymer as a Pre-catalyst for Selective Hydroboration of Aldehydes, Ketones and Imines Jing Wu, Haisu Zeng, Jessica Cheng, Shengping Zheng, James A. Golen, David R. Manke, and Guoqi Zhang J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b01094 • Publication Date (Web): 22 Jun 2018 Downloaded from http://pubs.acs.org on June 23, 2018

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The Journal of Organic Chemistry

Cobalt(II) Coordination Polymer as a Pre-catalyst for Selective Hydroboration of Aldehydes, Ketones and Imines Jing Wu, †,‡ Haisu Zeng,†,‡ Jessica Cheng,† Shengping Zheng,*,‡ James A. Golen,§ David R. Manke§ and Guoqi Zhang,*, † †

Department of Sciences, John Jay College and Ph.D. Program in Chemistry, The Graduate Center of the City University of New York, New York, 10019 NY, USA. ‡ Department of Chemistry, Hunter College, the City University of New York, New York, 10065 NY, USA. §

Department of Chemistry and Biochemistry, University of Massachusetts Dartmouth, North Dartmouth, MA 02747, USA.

ABSTRACT: Highly effective hydroboration precatalyst is developed based on a cobalt(II)-terpyridine coordination polymer (CP). The hydroboration of ketones, aldehydes and imines with pinacolborane (HBpin) has been achieved using the recyclable CP catalyst in the presence of an air-stable activator. A wide range of substrates containing polar C=O or C=N bonds have been hydroborated selectively in excellent yields under ambient conditions.

Catalytic hydroboration of carbonyl compounds and imines provides an important and convenient approach to functionalized alcohols and amines, respectively, comparing to the conventional stoichiometric reduction using hydride reagents (such as LiAlH4, or NaBH4), which usually suffer from poor functional group tolerance and product selectivity (aldehydes vs. ketones or over-reduction), modest reaction rates (with hindered ketones and imines) and often harsh conditions.1 Recent efforts have been made to develop efficient catalysts for the hydroboration of carbonyl compounds and imines with relatively less-active reductants, such as HBpin. In a recent review, Kinjo and coworker have documented the catalytic methods for this transformation.2 A number of discrete metal complexes have been utilized as catalysts for this reaction.3,4 Among the nonprecious metals explored, cobalt is quite attractive in catalytic hydrogenation and dehydrogenation processes.5,6 Major progress has been achieved for the observation of well-defined homogeneous cobalt hydrogenation catalysts under mild conditions, including the Co-PNP pincer complex studied by our group,7,8 and some have also found applications in catalytic hydroboration of alkenes, alkynes and nitriles. 8e,9 While homogeneous metal catalysts for hydroboration of carbonyl compounds and imines have been extensively explored, heterogeneous approaches for this conversion were little addressed. Mesoporous silica-supported amidozirconium catalysts have been previously reported for the hydroboration of carbonyl compounds.10 It was recently realized that incorporating active metal sites within a metal-organic framework (MOF) could be a viable strategy for catalytic hydroboration

of unsaturated bonds. In 2016, Lin and coworkers revealed that single-site magnesium alkyl supported on the metal nodes of TPHN-MOF [UiO-69 topology, TPHN = 4, 4bis(carboxyphenyl)-2-nitro-1,1′-biphenyl] cataylzed the hydroboration of carbonyl compounds and imines with high turnover numbers (TONs), where the catalyst was postsynthesized by reacting active Me2Mg with as-synthesized MOF materials.11 More recently, Delferro et al. reported a MOF-supported single-site Ti(IV) catalyst for the hydroboration of carbonyl compounds.12 This catalyst also utilizes the post-synthetic modification on an existing Zr-MOF with Ti(OiPr)4 to generate catalytically active Ti(IV) sites. Typically, in those limited examples the active hydroboration catalysts were generated by modifying MOF materials (constructed by complex organic linkers) with an air- and moisturesensitive metal reagent for immobilization of catalytically active sites. Herein, we report the first example of a readily available cobalt(II) coordination polymer as a precatalyst for selective hydroboration of aldehydes, ketones and imines under mild conditions with high efficiency, in the absence of air- and moisture-sensitive organometallic reagents. Previously, well-defined cobalt(II) complexes based on tridentate NNN ligands have found applications in the hydroboration of a variety of nonpolar and polar multiple bonds, in the presence of activators (NaHB(OEt)3 or LiCH2SiMe3).8e,9b,13 We also reported a manganese(II) dialkyl complex of 2,2′;6′,2′′-terpyridine (tpy) that catalyzed the Markovnikovselective hydroboration of alkenes.14 Significantly, Thomas

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and coworkers have revealed that air-stable cobalt(II) or iron(II) dihalide complexes of NNN ligands could act as effective precatalysts for alkene hydroboration and hydrosilylation while being activated by sodium tert-butoxide.15 To design extended coordination polymers as heterogeneous precatalysts, we envisioned that anchoring an additional N coordination site on the tpy backbone would form a ditopic ligand (1, Scheme 1) that is suitable for construction of extended coordination framework upon coordinating with CoII dihalide.16 Thus, the metal centers within the framework should adopt a coordination environment similar to that reported previously in discrete complexes.15 To compare, the divergent terpyridine ligand, 4′(4-dimethylaminophenyl)-4,2′;6′,4′′-tpy (2), was also employed for similar coordination framework.17 Scheme 1. Ligands 1 and 2 Derived from 2,2′;6′,2′′Terpyridine

Coordination-driving self-assembly of known 1 and 2 with cobalt dichloride by a layering technique has afforded goodquality crystals of 3 and 4 in high yields, respectively (see SI). Both crystalline compounds were characterized by IR, elemental analysis, and X-ray crystallography. Structural analysis revealed that 3 crystallized in the orthorhombic space group Pcca. As expected, the CoII center resides within the N3 cavity of one ligand, and an extra Npyridyl atom from another ligand links to the CoII center to extend the structure. Two coordinated chloride anions are located in the axial direction of CoN4 coordination sphere, leading to a pseudo-octahedral geometry around CoII (Figure 1). The assembled structure is polymeric and propagates along the crystallographic c axis to form a onedimensional (1D) coordination polymeric chain. The chains are further packed by a strong interchain π-stacking interaction (the shortest inter-atomic C…C distance between the adjacent chains is 3.495 (5) Å), thanks to the perfect coplanarity of the aromatic region. In contrast, 4 crystallized in the orthorhombic space group Pbcn. CoII is hexacoordinate with four N atoms of four distinct ligands and two Cl atoms to form an octahedral coordination geometry. The structural expansion around CoII centers results in the formation of a 1D coordination framework composed of cage-like subunits formed by a [4 + 4] metal-ligand assembly (Figure 1). It is worth mentioning that although numerous CPs derived from related tpy ligands have been reported,18 3 and 4 are the first examples of structurally characterized CPs from the assembly of tpy with CoCl2.

3

3

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4

4

Figure 1. The ORTEP structures of 3 and 4 plotted at 50% thermal ellipsoid probability level and the ball-stick representation of the extended network in 3 and 4, respectively. H-Atoms are omitted for clarity.

To demonstrate the capability of CPs 3 and 4 in catalytic hydroboration of carbonyl compounds with HBpin, initially we employed similar reaction conditions from the known cobalt-catalyzed hydroboration involving discrete complexes.15 The results are summarized in Table 1. When only 0.1 mol% of 3 and potassium tert-butoxide (KOtBu, 2 mol %) were used, the hydroboration of acetophenone proceeded well in THF at room temperature, affording the hydroborated product 5 in >99% GC yield after 4 h. In comparison, 4 is also an active precatalyst under the same conditions, even though the yield was slightly lower. Similar results were found when using NaOtBu or KOH as an activator, while NaHB(OEt)3 acts as a poorer activator for this reaction. Control experiments show that the use of CP 3 was crucial for the excellent reactivity, whereas the combination of CoCl2 (2 mol %) and KOtBu also exhibited modest catalytic activity. Interestingly, the discrete complex, Co(tpy)Cl2, displayed inferior efficiency compared to the CP analogue. In addition, solvent effects were studied, and the results indicated that THF is more suitable than several other solvents tested for this reaction. When catecholborane was used as a boron source under the optimized conditions (as in entry 1, Table 1), the corresponding hydroborated product was found in only 25% GC yield (entry 13, Table 1). It was interesting to note that the hydroboration even proceeded smoothly in the air, giving 5a in 87% yield (entry 14, Table 1). Finally, when 0.01 mol % of 3 was used as the precatalyst, product 5 was detected in 86% yield, corresponding to a turnover number of 8,600, comparable to the MOF-supported single site catalysts reported previously.11,12 Table 1. Condition Screening for Cobalt(II) CP-Catalyzed Hydroboration of Acetophenone with Pinacolborane.a

Entry 1 2 3 4 5 6

Catalyst 3 4 3 3 3 3

Activator KOtBu KOtBu NaOtBu KOH NaHB(OEt)3 -

Solvent THF THF THF THF THF THF

Yield /%b >99 95 98 97 70 0

7 8c 9d

CoCl2 Co(tpy)Cl2

KOtBu KOtBu KOtBu

THF THF THF

8 54 71

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The Journal of Organic Chemistry 10 11 12 13e 14f 15g

3 3 3 3 3 3

KOtBu KOtBu KOtBu KOtBu KOtBu KOtBu

Et2O toluene pentane THF THF THF

88 68 65 25 87 86

a

Conditions: acetophenone (1.0 mmol), HBpin (1.1 mmol), cobalt CP (0.1 mol%), activator (2 mol%) and solvent (1 mL), 25°C, 1 h, N2. b Determined by GC analysis with hexamethylbenzene as an internal standard. c 2 mol% catalyst was used. d 1 mol% catalyst was used. e Catecholborane was used as the boron source. f Reaction run in the air. g 0.01 mol% 3 was used and reaction run for 24 h.

Scheme 2. The CP-Catalyzed Hydroboration of Ketones.a

a Conditions: ketone (1.0 mmol), HBpin (1.1 mmol), cobalt CP (0.1 mol %), KOtBu (2 mol %) and THF (1 mL), 25 °C, 4 h, N2. Yields of isolated products of alcohols (GC yields of boronate esters are shown in parenthesis). b 10.0 mmol acetophenone was used.

Next, we sought to investigate the application of 3catalyzed hydroboration under optimized conditions. A variety of ketones were first examined, and the results are summarized in Scheme 2. For all reactions indicated here, the hydroborated products were confirmed by GC-MS analysis. Purification of the products through silica gel column chromatography afforded hydrolyzed products, the alcohols (6) in high yields, consistent with the results reported previously. 14 The hydroboration of acetophenone was readily approached in a gram scale, yielding 1-phenylethanol (6a) in 94% isolated yield. Functionalized aryl ketones were smoothly hydroborated under the standard conditions. Halogenated acetophenones were completely converted to the corresponding boronate esters and isolated as alcohols in excellent yields (6b-d). Methyl phenyl ketones containing electron-donating groups and a bulkier isopropyl phenyl ketone also proceeded well (6e-g). Cyclopropyl phenyl ketone was used to produce 6h in a reasonable yield without ring-opening, indicating that no radical mecha-

nism was involved. In addition, relatively lower yields were obtained for 2-acetylnaphthalene (6i) and diaryl ketones (6j and 6k). However, the cyclic ketone, tetralone, was hydroborated completely (6l) to afford the corresponding cyclic alcohol in 97% yield. In addition, the steric hindered ketone, 2′methylacetophenone also proceeded well with 6m being isolated in 90% yield. Unfortunately, ketones containing a reducible functional group were not suitable substrates for effective ketone hydroboration under standard conditions. For example, when 4′-nitroacetophenone was used, a complicated mixture of hydroborated products was detected (~ 50% conversion of ketone) and no desired alcohol product could be isolated. Finally, more challenging alkyl ketones proved to be suitable substrates (6n-q). The heterogeneous hydroboration of ketones catalyzed by 3 allows us to study the recycling and reuse of the catalyst. In a recycling reaction using acetophenone as a model substrate (Figure S3, see SI), catalyst 3 (2 mol %) was filtered out of the reaction mixture and reused for 5 more reaction cycles (the activator was reloaded in each reuse cycle). The results showed no obvious drop in the yields determined by GC, indicating excellent recyclability of the CP pre-catalyst. Under the same conditions, we further investigated the hydroboration of aldehydes to prepare primary alcohols. Generally, aldehyde hydroboration proceeded smoothly in 16 h, with the alcohol products being isolated in good to excellent yields after column chromatography (Scheme 3). Benzaldehydes bearing halogen, electron-withdrawing or electrondonating groups were hydroborated to the corresponding boronate esters in almost quantitative conversions, and alcohols were isolated in high yields (7a-i), except that in the case of p-cyanobenzaldehyde the reaction was a little sluggish and the desired product 7e was obtained in a modest yield. In addition, benzaldehyde containing an ester functional group was selectively hydroborated on the aldehyde, affording 7j in a reasonable yield. Furthermore, trans-cinnamaldehyde containing conjugated C=C and C=O bonds was selectively hydroborated on the C=O part (7k), even though 2.0 equiv. of HBpin was used. While the heterocyclic aldehyde, 2thiophenecarboxaldehyde is a suitable substrate for the hydroboration (7l), 2-pyridinecarboxaldehyde did not react at all (7m), probably due to the strong binding ability of the substrate to metal centers, which deactivates the catalyst. Finally, aliphatic aldehydes were found to be readily hydroborated in high efficiencies (7n and 7o). Scheme 3. The CP-Catalyzed Hydroboration of Aldehydes.a



a Conditions: aldehydes (1.0 mmol), HBpin (1.1 mmol), cobalt CP (0.1 mol %), KOtBu (2 mol %) and THF (1 mL), 25°C, 16 h, N2. Yields of isolated products of alcohols (GC yields of boronate intermediates are shown in parenthesis). b 2.0 eq. of HBpin was used and ~34% starting material was detected by GC. c ~20% starting material was detected by GC. d 2.0 eq. of HBpin was used. e n. r.: no reaction.

Encouraged by these results, we studied the hydroboration of imines with the cobalt CP catalyst. While there was no reaction observed when N-benzylideneaniline was mixed with HBpin under the standard condition at room temperature, at elevated temperature (70 oC) complete hydroboration of the imine occurred, with 8a being isolated in 96% yield (Scheme 4). Accordingly, several substituted aromatic imines containing fluoro, nitro or ester groups have been converted to the corresponding amines (8b-d) in good yields by the same protocol, demonstrating better applicability of the CP precatalyst. Scheme 4. The CP-Catalyzed Hydroboration of Imines.a

a

Conditions: imines (1.0 mmol), HBpin (1.1 mmol), cobalt CP (0.1 mol %), KOtBu (2 mol %) and THF (1 mL), 70°C, 16 h, N2. Yields of isolated products of amines (GC conversion of imines are shown in parenthesis).

The chemoselectivity of CP-catalyzed hydroboration of ketones and aldehydes was further examined. First, the hydroboration of equimolar acetophenone and benzaldehyde with HBpin (1.0 equiv.) was conducted in a competition study (Scheme S1), and the results indicated that the hydroboration of aldehyde is much preferred, providing only hydroborated product of benzaldehyde. In contrast, the discrete catalyst, Co(tpy)Cl2/KOtBu is much less active, whereas the trend of chemoselectivity remains the same (Scheme S1A). This trend was further confirmed in a similar experiment using substituted ketone and aldehyde as substrates when 3 was the precatalyst. It was also observed that hydroboration of acetophenone was not affected by the presence of equimolar N-

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benzylideneaniline at room temperature, showing good chemoselectivity for ketones over imines. In addition, 4acetylbenzaldehyde was used to study the intramolecular selectivity. It was found that the hydroboration reaction mainly occurred at the aldehyde, affording 9 in 81% yield after column chromatography. The chemoselectivity on aldehydes over ketones observed here is similar to those found in ruthenium-, lanthanide-, or aluminum-catalyzed hydroboration of carbonyl compounds.4,19 In summary, we report the first cobalt-based coordination polymer-catalyzed selective hydroboration of ketones, aldehydes and imines under mild, heterogeneous conditions. In comparison with other nonprecious metal catalysts, the CoII CP catalysis system is easy-to-make, air-stable, highly efficient, reusable, and tolerant of a wide range of substrates. This work implies new catalytic applications of simple coordination polymers based on Earth-abundant metals, paving the way for development of practical and sustainable catalysts for key organic transformations.

EXPERIMETAL SECTION General Information. The catalytic reactions were carried out under a nitrogen atmosphere using standard glove-box technique. Anhydrous grade solvents and liquid reagents used were obtained from Aldrich or Fisher Scientific and stored over 4 Å molecular sieves. FT-IR spectra were recorded on a Shimadzu 8400S instrument with solid samples under N2 using a Golden Gate ATR accessory. Elemental analyses were performed by Midwest Microlab LLC in Indianapolis. 1H NMR and 13C NMR spectra were obtained at room temperature on a Bruker AV 500 or 600 MHz NMR spectrometer, with chemical shifts (δ) referenced to the residual solvent signal. GC-MS analysis was obtained using a Shimadzu GCMS-QP2010S gas chromatograph mass spectrometer. 2,2′;6′,2′′-Terpyridine was purchased from Sigma-Aldrich. 4′(4-Pyridyl)-2,2′;6′,2′′-tpy (1), 4′-(4-dimethylaminophenyl)4,2′;6′,4′′-tpy (2) and Co(tpy)Cl2 were prepared according to the literature procedure.8d, 16, 17 Experimental Procedure. Synthesis of 3. A solution of 1 (62.0 mg, 0.200 mmol) in MeOH/CH2Cl2 (10 mL, 1 : 3, v/v) was placed in a test tube. A blank solution of MeOH/CH2Cl2 (6 mL, 1: 1, v/v) was layered on the top of the ligand solution, followed by a solution of CoCl2·6H2O (47.4 mg, 0.200 mmol) in MeOH (10 mL). The tube was sealed and allowed to stand at room temperature for a week, during which time X-ray quality light brown crystals grew on the wall and bottom of the tube. The crystals were collected by decanting the solvent and washed with MeOH and then dried in vacuo. Yield: 55.0 mg (89%). FT-IR (solid, cm-1) 3481br, 3057m, 1599s, 1569w, 1539s, 1472m, 1408s, 1346w, 1247s, 1169m, 1079m, 1012s, 897w, 842s, 792s, 748m, 730m. Anal. Calcd. for C20H14Cl2CoN4·2H2O: C 50.44, H 3.81, N 11.77%. Found C 50.31, H 3.79, N 11.52. Synthesis of 4. A solution of 2 (70.4 mg, 0.200 mmol) in MeOH/CH2Cl2 (10 mL, 1 : 3, v/v) was placed in a test tube. A blank solution of MeOH/CH2Cl2 (6 mL, 1: 1, v/v) was layered on the top of the ligand solution, followed by a solution of CoCl2·6H2O (47.4 mg, 0.200 mmol) in MeOH (10 mL). The tube was sealed and allowed to stand at room temperature for a week,


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The Journal of Organic Chemistry during which time X-ray quality light brown crystals grew on the bottom of the tube. The crystals were collected by decanting the solvent and washed with MeOH and then dried in vacuo. Yield: 76.6 mg (92% based on 2). FT-IR (solid, cm-1) 3050m, 2900m, 1608s, 1589s, 1557w, 1525s, 1497m, 1434w, 1399s, 1363s, 1216m, 1168m, 1063s, 1012s, 946m, 844s, 817s, 727s, 674s. Anal. Calcd. for C46H40Cl2CoN8: C 66.19, H 4.83, N 13.42%. Found C 66.05, H 4.90, N 13.30. General Procedure for CP-Catalyzed Hydroboration of Ketones. In a glovebox under N2 atmosphere, catalyst 3 (0.44 mg, 1.0 μmol) and KOtBu (2.2 mg, 20 μmol) was dissolved in THF (1.0 mL) in a 3.8 mL glass vial equipped with a stir bar. Ketone (1.0 mmol) and pinacolborane (140.8 mg, 1.1 mmol) were then added. The reaction mixture was allowed to stir at room temperature for 4 h. After completion of the reaction, the solvent was evaporated and the crude reaction mixture was first analyzed by GC-MS using a dilute CH2Cl2 solution, and then the product was isolated by flash column chromatography with SiO2 using ethyl acetate/hexane as eluent. The pure products were characterized by 1H and 13C NMR spectroscopies. General Procedure for CP-Catalyzed Hydroboration of Aldehydes. In a glovebox under N2 atmosphere, catalyst 3 (0.44 mg, 1.0 μmol) and KOtBu (2.2 mg, 20 μmol) was dissolved in THF (1.0 mL) in a 3.8 mL glass vial equipped with a stir bar. Aldehyde (1.0 mmol) and pinacolborane (140.8 mg, 1.1 mmol) were then added. The reaction mixture was allowed to stir at room temperature for 16 h. After completion of the reaction, the solvent was evaporated and the crude reaction mixture was first analyzed by GC-MS using a dilute CH2Cl2 solution, and then the product was isolated by flash column chromatography with SiO2 using ethyl acetate/hexane as eluent. The pure products were characterized by 1 H and 13C NMR spectroscopies. General Procedure for CP-Catalyzed Hydroboration of Imines. In a glovebox under N2 atmosphere, catalyst 3 (0.44 mg, 1.0 μmol) and KOtBu (2.2 mg, 20 μmol) was dissolved in THF (1.0 mL) in a 20 mL Schlenk tube equipped with a stir bar. Imine (1.0 mmol) and pinacolborane (140.8 mg, 1.1 mmol) were then added. The reaction mixture was sealed and allowed to heat to 70 ºC for 16 h. After completion of the reaction, the solvent was evaporated and the crude reaction mixture was first analyzed by GC-MS using a dilute CH2Cl2 solution, and then the product was isolated by flash column chromatography with SiO2 using ethyl acetate/hexane as eluent. The pure products were characterized by 1 H and 13C NMR spectroscopies. Competing Experiment for Selective Hydroboration of Ketones vs. Aldehydes. In a glove box under N2 atmosphere, catalyst 3 (0.44 mg, 1.0 μmol) and KOtBu (2.2 mg, 20 μmol) was dissolved in THF (1.0 mL) in a 3.8 mL glass vial equipped with a stir bar. The Ketone (1.0 mmol), aldehyde (1.0 mmol) and pinacolborane (128.0 mg, 1.0 mmol) were then added sequentially. The reaction mixture was allowed to stir at room temperature for 16 h. The reaction was exposed to the air and the solvent was evaporated. The products were analyzed by GC-MS using hexamethylbenzene as an internal reference. Recycling and Reusing Experiment for Hydroboration of Acetophenone. In a glovebox under N2 atmosphere, catalyst 3 (4.4 mg, 10 μmol) and KOtBu (2.2 mg, 20 μmol) was dissolved in THF (1.0 mL) in a small vial equipped with a stir bar. Acetophe-

none (120.0 mg, 1.0 mmol) and pinacolborane (140.8 mg, 1.1 mmol) were then added. The reaction mixture was allowed to stir at room temperature for 4 h. Then, the solid was centrifuged out of suspension and extracted with hexane three times. The combined organic extracts were analyzed by GC-MS using hexamethylbenzene as an internal reference. The recovered solid catalyst in 1.0 mL THF was charged into a small vial, to which KOtBu (2.2 mg, 20 μmol) was added. Then, acetophenone (120.0 mg, 1.0 mmol) and pinacolborane (140.8 mg, 1.1 mmol) were added. The resultant mixture was stirred at room temperature in the glovebox for 4 h. The solid was centrifuged out of suspension and extracted with hexane three times. The combined organic extracts were analyzed by GC-MS. The results of five recycling experiments are summarized in Figure S1. Spectroscopic Data for Isolated Products. 1-Phenylethanol (6a). 14 Colorless oil. Yield: 117.0 mg (96%). 1H NMR (600 MHz, CDCl3) δ 7.47 – 7.31 (m, 4H), 7.29 (ddd, J = 8.8, 4.8, 3.4 Hz, 1H), 4.85 (q, J = 6.5 Hz, 1H), 2.63 (s, 1H), 1.47 (d, J = 6.4 Hz, 3H) ppm; 13C NMR (151 MHz, CDCl3) δ 145.9, 128.5, 127.4, 125.5, 70.3, 25.2 ppm. 1-(4-Fluorophenyl)ethanol (6b).14 Colorless oil. Yield: 133 mg (95%). 1H NMR (600 MHz, CDCl3) δ 7.35 – 7.33 (m, 2H), 7.05 – 7.01 (m, 2H), 4.89 (q, J = 6.5 Hz, 1H), 1.82 (br, 1 H), 1.48 (d, J = 6.5 Hz, 3H) ppm; 13C NMR (151 MHz, CDCl3) δ 162.4 (d, J = 244.9 Hz), 141.9 (d, J = 2.4 Hz), 127.4(d, J = 8.1 Hz), 115.6 (d, J = 21.2 Hz), 70.1 (d, J = 2.3 Hz), 25.6 ppm. 1-(4-Chlorophenyl)ethanol (6c). 14 Colorless oil. Yield: 150.3 mg (96%). 1H NMR (600 MHz, CDCl3) δ 7.33 – 7.28 (m, 4H), 4.86 (q, J = 6.5 Hz, 1H), 2.23 (br, 1H), 1.47 (d, J = 6.5 Hz, 3H) ppm; 13C NMR (151 MHz, CDCl3) δ 144.3, 133.0, 128.6, 126.8, 69.7, 25.2 ppm. 1-(4-Bromophenyl)ethanol (6d). 14 Colorless oil. Yield: 191.0 mg (95%). 1H NMR (600 MHz, CDCl3) δ 7.46 – 7.43 (m, 2H), 7.22 – 7.20 (m, 2H), 4.81 (q, J = 6.6 Hz, 1 H), 2.34 (br, 1H), 1.43 (d, J = 6.6 Hz, 3 H) ppm; 13C NMR (151 MHz, CDCl3) δ 145.1, 131.8, 127.4, 121.4, 70.0, 25.5 ppm. 1-(4-Methoxyphenyl)ethanol (6e). 14 Yellowish oil. Yield: 149 mg (98%). 1H NMR (600 MHz, CDCl3) δ 7.30 (d, J = 8.7 Hz, 2H), 6.89 (d, J = 8.7 Hz, 2H), 4.84 (q, J = 6.5 Hz, 1 H), 3.81 (s, 1H), 2.27 (br, 1H), 1.45 (d, J = 6.5 Hz, 3 H) ppm. 13C NMR (126 MHz, CDCl3) δ 158.9, 138.1, 126.7, 113.8, 69.9, 55.3, 25.1 ppm. 1-(3-Methoxyphenyl)ethanol (6f).7a Yellowish oil. Yield: 144.6 mg (95%). 1H NMR (600 MHz, CDCl3) δ 7.29 – 7.27 (m, 1H), 6.97 – 6.96 (m, 2H), 6.84 – 6.82 (m, 1H), 4.88 (q, J = 6.7 Hz, 1H), 3.83 (s, 3H), 2.07 (br, 1H), 1.50 (d, J = 6.7 Hz, 3 H) ppm; 13C NMR (151 MHz, CDCl3) δ 160.1, 147.9, 129.8, 118.0, 113.2, 111.2, 70.6, 55.5, 25.4 ppm. 2-Methyl-1-phenylethanol (6g).19a Colorless oil. Yield: 120 mg (80%). 1H NMR (500 MHz, CDCl3) δ 7.42 – 7.32 (m, 4H), 7.32 – 7.28 (m, 1H), 4.39 (d, J = 6.7 Hz, 1H), 1.99 (qd, J = 6.8, 1.5 Hz, 1H), 1.89 (s, 1H), 1.03 (d, J = 6.7 Hz, 3H), 0.83 (d, J = 6.9 Hz, 3H) ppm; 13C NMR (126 MHz, CDCl3) δ 143.7, 128.3, 127.5, 126.7, 80.2, 35.4, 19.1, 18.4 ppm. 1-Cyclopropyl phenylmethanol (6h).14 Yellowish oil. Yield: 106.7 mg (72%). 1H NMR (600 MHz, CDCl3) δ 7.43 – 7.42 (m, 2H), 7.37 – 7.35 (m, 2H), 7.31 – 7.28 (m, 1H), 4.00 (d, J = 8.3 Hz, 1H), 2.21 (br, 1H), 1.25 – 1.19 (m, 1H), 0.66 – 0.61 (m, 1 H), 0.58 – 0.53 (m, 1 H), 0.49 - 0.45 (m, 1 H), 0.40 – 0.35 (m, 1 H) ppm; 13C NMR (151 MHz, CDCl3) δ 144.2, 128.6, 127.8, 126.3, 78.8, 19.4, 3.9, 3.1 ppm. 1-(2-naphthalenyl)ethanol (6i).20 White solid. M.p.: 69 ˗ 70 °C. Yield: 114 mg (66%). 1H NMR (600 MHz, CDCl3) δ 7.85 – 7.83 (m, 3H), 7.80 (s, 1H), 7.51 – 7.47 (m, 3H), 5.04 (q, J = 6.5



Hz, 1H), 2.22 (br, 1H), 1.58 (d, J = 6.5 Hz, 3H) ppm; 13C NMR (151 MHz, CDCl3) δ 143.5, 133.6, 133.2, 128.6, 128.2, 128.0, 126.4, 126.1, 124.1, 124.1, 70.8, 25.4 ppm. Diphenylmethanol (6j).4f White solid. M.p.: 64 ˗ 65 °C. Yield: 147.4 mg (80%). 1H NMR (600 MHz, CDCl3) δ 7.43 – 7.41 (m, 4H), 7.40 – 7.37 (m, 4H), 7.33 – 7.30 (m, 2H), 5.85 (s, 1H), 2.38 (br, 1H) ppm; 13C NMR (151 MHz, CDCl3) δ 143.9, 128.5, 127.6, 126.6, 76.3 ppm. Di-p-tolylmethanol (6k).20 White solid. M.p.: 66 ˗ 68 °C. Yield: 157 mg (74%). 1H NMR (600 MHz, CDCl3) δ 7.28 (d, J = 8.4 Hz, 4H), 7.17 (q, J = 8.4 Hz, 4H), 5.78 (br, 1H), 2.36 (s, 7H) ppm; 13C NMR (151 MHz, CDCl3) δ 141.4, 137.4, 129.4, 126.8, 76.2, 21.4 ppm. 1,2,3,4-Tetrahydronaphthalen-1-ol (6l)20 Colorless oil. Yield: 143.7 mg (97%). 1H NMR (600 MHz, CDCl3): δ 7.44 – 7.42 (m, 1H), 7.22 – 7.20 (m, 2H), 7.12 – 7.11 (m, 1H), 4.77 – 4.75 (m, 1H), 2.86 – 2.81 (m, 1H), 2.76 – 2.71 (m, 1H), 2.11 (br, 1H), 2.02 – 1.95 (m, 1H), 1.92 – 1.87 (m, 1H), 1.82 – 1.76 (m, 1H). 13C NMR (151 MHz, CDCl3): δ 139.1, 127.4, 129.2, 128.9, 127.8, 126.4, 68.4, 32.5, 29.5, 19.0 ppm. 1-(2-Methylphenyl)ethanol (6m).20 Colorless oil. Yield: 122.0 mg (90%). 1H NMR (600 MHz, CDCl3) δ 7.54 (dd, J = 7.8, 1.7 Hz, 1H), 7.27 (td, J = 7.5, 1.7 Hz, 1H), 7.21 (td, J = 7.4, 1.6 Hz, 1H), 7.17 (dd, J = 7.7, 1.8 Hz, 1H), 5.12 (q, J = 6.5 Hz, 1H), 2.37 (s, 3H), 2.26 (s, 1H), 1.49 (d, J = 6.5 Hz, 3H) ppm; 13C NMR (151 MHz, CDCl3) δ 143.9, 134.2, 130.4, 127.2, 126.4, 124.6, 66.8, 24.0, 19.0 ppm. 1,3-diphenylpropan-2-ol (6n).14 Colorless oil. Yield: 201.7 mg (95%). 1H NMR (600 MHz, CDCl3) δ 7.39 – 7.36 (m, 4H), 7.31 – 7.28 (m, 6H), 4.13 – 4.09 (m, 1H), 2.93 – 2.90 (m, 2H), 2.84 – 2.80 (m, 2H), 1.80 (br, 1H) ppm; 13C NMR (151 MHz, CDCl3) δ 138.8, 129.7, 128.8, 126.8, 73.8, 43.7 ppm. 2-Heptanol (6o).20 Colorless oil. Yield: 72 mg (62%). 1H NMR (500 MHz, CDCl3) δ 3.82 (m, 1 H), 1.47 (m, 4 H), 1.33 (m, 5 H), 1.21 (d, J = 6.0 Hz, 3 H), 0.91 (t, J = 7.0 Hz, 3 H) ppm; 13C NMR (126 MHz, CDCl3) δ 68.2, 39.3, 31.9, 25.5, 23.5, 22.7, 14.1 ppm. Benzyl alcohol (7a).14 Colorless oil. Yield: 102 mg (94%). 1H NMR (600 MHz, CDCl3) δ 7.46 – 7.38 (m, 2H), 7.38 – 7.30 (m, 2H), 4.63 (s, 2H), 3.35 (s, 1H); 13C NMR (151 MHz, CDCl3) δ 140.9, 128.5, 127.5, 127.0, 64.9 ppm. 4-Chlorobenzyl alcohol (7b).14 Colorless oil. Yield: 135.4 mg (95%). 1H NMR (600 MHz, CDCl3) δ 7.35 – 7.33 (m, 2H), 7.30 – 7.28 (m, 2H), 4.65 (s, 1H), 2.45 (s, 1 H), 2.12 (br, 1H) ppm; 13C NMR (151 MHz, CDCl3) δ 139.6, 133.6, 129.0, 128.6, 64.7 ppm. 2-Chlorobenzyl alcohol (7c).21 Colorless oil. Yield: 136.9 mg (96%). 1H NMR (600 MHz, CDCl3) δ 7.49 (d, J = 7.7 Hz, 1H), 7.38 (d, J = 7.7 Hz, 2H), 7.30 (t, J = 7.4 Hz, 1H), 7.25 (t, J = 7.4 Hz, 1H), 4.79 (s, 2H), 2.29 (s, 1H) ppm; 13C NMR (151 MHz, CDCl3) δ 138.5, 133.0, 129.6, 129.1, 129.0, 127.3, 63.1 ppm. 4-Nitrobenzyl alcohol (7d).14 Yellowish oil. Yield: 140.9 mg (92%). 1H NMR (600 MHz, CDCl3) δ 8.21 (d, J = 8.4 Hz, 2H), 7.53 (d, J = 8.4 Hz, 2H), 4.83 (s, 2H), 1.97 (br, 1H) ppm; 13C NMR (151 MHz, CDCl3) δ 148.5, 147.6, 127.3, 124.1, 64.3 ppm. 4-(Hydroxymethyl)benzonitrile (7e).21 Yellowish oil. Yield: 79.8 mg (60%). 1H NMR (600 MHz, CDCl3) δ 7.64 (d, J = 8.0 Hz, 2H), 7.47 (d, J = 8.0 Hz, 2H), 4.77 (s, 2H), 1.96 (br, 1H) ppm; 13 C NMR (151 MHz, CDCl3) δ 146.6, 132.6, 127.3, 119.2, 111.5, 64.5 ppm. 4-Methoxybenzyl alcohol (7f).4f Colorless oil. Yield: 131.2 mg (95%). 1H NMR (600 MHz, CDCl3) δ 7.28 (d, J = 8.6 Hz, 2H), 6.89 (d, J = 8.6 Hz, 2H), 4.60 (s, 2H), 3.80 (s, 3H), 1.81 (br, 1H) ppm; 13C NMR (151 MHz, CDCl3) δ 159.5, 133.5, 129.0, 114.3, 65.3, 55.6 ppm.

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4-(Dimethylamino)benzyl alcohol (7g). 21 Colorless oil. Yield: 137.6 mg (91%). 1H NMR (500 MHz, CDCl3) δ 7.27 (d, J = 8.5 Hz, 2 H), 6.77 (d, J = 9.0 Hz, 2 H), 4.58 (s, 2 H), 2.98 (s, 6 H), 1.97 (br., 1 H) ppm; 13C NMR (126 MHz, CDCl3) δ 150.3, 129.0, 128.7, 112.7, 65.3, 40.8 ppm. Piperonol (7h).14 Colorless oil. Yield: 147.6 mg (97%). 1H NMR (600 MHz, CDCl3) δ 6.85 (s, 1H), 6.79 – 6.76 (m, 2H), 5.94 (s, 2H), 4.55 (s, 2H), 1.95 (br, 1H) ppm; 13C NMR (151 MHz, CDCl3) δ 148.1, 147.4, 135.2, 120.8, 108.5, 108.2, 101.3, 65.5 ppm. 2-(Methylthio)benzyl alcohol (7i).22 Colorless oil. Yield: 137.2 mg (89%). 1H NMR (500 MHz, CDCl3) δ 7.15 (d, J = 7.5 Hz, 1 H), 7.05 (m, 2 H), 6.96 (m, 1 H), 4.52 (d, J = 2 Hz, 2 H), 2.26 (d, J = 1.5 Hz, 3 H), 2.18 (br., 1 H) ppm; 13C NMR (125 MHz, CDCl3) δ 138.8, 136.6, 128.4, 127.9, 126.4, 125.5, 63.4, 16.0 ppm. Methyl 4-(hydroxymethyl)benzoate (7j). 21 Colorless oil. Yield: 118 mg (71%). 1H NMR (500 MHz, CDCl3) δ 8.00 (d, J = 8.0 Hz, 2 H), 7.41 (d, J = 8.0 Hz, 2 H), 4.74 (s, 2 H), 3.91 (s, 3 H), 2.52 (br., 1 H) ppm; 13C NMR (126 MHz, CDCl3) δ 167.1, 146.2, 129.8, 129.1, 126.5, 64.5, 52.2 ppm. Cinnamyl alcohol (7k).14 Yellowish oil. Yield: 104.6 mg (78%). 1H NMR (500 MHz, CDCl3) δ 7.41 – 7.40 (m, 2H), 7.35 – 7.33 (m, 2H), 7.28 – 7.26 (m, 1H), 6.63 (d, J = 16 Hz, 1H), 6.97 (dt, J = 16, 5.8 Hz, 1H), 4.33 (dd, J = 5.8, 1.7 Hz, 2H), 2.49 (br, 1H) ppm; 13C NMR (126 MHz, CDCl3) δ 137.0, 131.2, 128.8, 127.9, 126.7, 63.8 ppm. 2-Thiophenylmethanol (7l).14 Yellowish oil. Yield: 107.3 mg (94%). 1H NMR (600 MHz, CDCl3) δ 7.28 – 7.27 (m, 1H), 7.01 – 6.97 (m, 2H), 4.81 (s, 2H), 2.07 (br, 1H) ppm; 13C NMR (150 MHz, CDCl3) δ 144.3, 127.2, 125.9, 125.8, 60.2 ppm. N-benzylaniline (8a). 7a Yellowish oil. Yield: 176 mg (96%). 1 H NMR (600 MHz, CDCl3) δ 7.55 – 7.43 (m, 4H), 7.42 – 7.36 (m, 1H), 7.29 (dd, J = 9.0, 7.2 Hz, 2H), 6.84 (t, J = 7.4 Hz, 1H), 6.78 – 6.70 (m, 2H), 4.42 (s, 2H) ppm; 13C NMR (151 MHz, CDCl3) δ 148.2, 139.5, 129.4, 128.7, 127.6, 127.3, 117.7, 113.0, 48.4 ppm. N-(4-fluorobenzyl)aniline (8b).7f Yellow oil. Yield: 145 mg (72%). 1H NMR (600 MHz, CDCl3) δ 7.34 (ddd, J = 8.6, 5.4, 2.9 Hz, 2H), 7.27 (d, J = 5.3 Hz, 1H), 7.19 (td, J = 7.5, 2.7 Hz, 2H), 7.02 (td, J = 8.6, 3.1 Hz, 2H), 6.77 (t, J = 7.3 Hz, 1H), 6.68 (dd, J = 8.3, 5.1 Hz, 2H), 4.31 (s, 2H) ppm; 13C NMR (151 MHz, CDCl3) δ163.1, 161.5, 129.5, 129.4, 129.4, 118.7, 115.7, 115.5, 113.7, 48.3 ppm. N-(4-nitrobenzyl)aniline (8c).23 Yellow oil. Yield: 180 mg (79%). 1H NMR (600 MHz, CDCl3) δ 8.23 – 8.17 (m, 2H), 7.54 (d, J = 8.6 Hz, 2H), 7.24 – 7.15 (m, 2H), 6.77 (s, 1H), 6.61 (d, J = 8.0 Hz, 2H), 4.48 (s, 2H) ppm; 13C NMR (151 MHz, CDCl3) δ 147.3, 129.5, 128.0, 124.0, 118.8, 113.5, 100.1, 48.0 ppm. Methyl 4-((phenylamino)methyl)benzoate (8d).24 Yellowish oil. Yield: 169 mg (70%). 1H NMR (600 MHz, CDCl3) δ 8.08 – 7.99 (m, 2H), 7.45 (d, J = 8.4 Hz, 2H), 7.18 (dd, J = 8.7, 7.2 Hz, 2H), 6.79 – 6.70 (m, 1H), 6.66 – 6.57 (m, 2H), 4.41 (s, 2H), 3.92 (s, 3H) ppm; 13C NMR (151 MHz, CDCl3) δ167.1, 147.8, 145.0, 130.0, 129.4, 129.1, 127.2, 117.9, 113.0, 52.2, 48.0 ppm. 4-Acetylbenzyl alcohol (9).25 Colorless oil. Yield: 121.5 mg (81%). 1H NMR (600 MHz, CDCl3) δ 7.90 (d, J = 8.2, 2H), 7.42 (d, J = 8.2, 2H), 4.73 (s, 2H), 2.56 (s, 3H), 2.50 (br, 1H) ppm; 13C NMR (151 MHz, CDCl3) δ 198.5, 146.8, 136.5, 128.9, 126.9, 64.7, 26.9 ppm.

AUTHOR INFORMATION Corresponding Author * Email address: [email protected];


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The Journal of Organic Chemistry [email protected] ORCID Guoqi Zhang: 0000-0001-6071-8469 Notes

The authors declare no competing financial interest.

ASSOCIATED CONTENT Supporting Information. The Supporting Information for this article is available free of charge via the Internet at http://pubs.acs.org. CIF files of crystallographic data, additional experimental details and copies of NMR spectra (PDF).

ACKNOWLEDGMENT We are grateful to donors of the American Chemical Society Petroleum Research Fund for partial support of this work (54247UNI3). We also acknowledge the support from the CUNY Collaborative Research Incentive Program, the PSC-CUNY awards (69069-0047, 60328-0048) and the Seed grant from the Office for Advancement of Research at John Jay College. The National Science Foundation (CHE-1429086) is acknowledged for the X-ray diffractometer.

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