Selective Aerobic Oxidation of Benzylic Alcohols Catalyzed by a

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Selective Aerobic Oxidation of Benzylic Alcohols Catalyzed by a Dicyclopropenylidene-Ag(I) Complex Roya Mir, Rozhin Rowshanpour, Katie Dempsey, Melanie Pilkington, and Travis Dudding J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b00624 • Publication Date (Web): 21 Mar 2019 Downloaded from http://pubs.acs.org on March 24, 2019

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The Journal of Organic Chemistry

Selective Aerobic Oxidation of Benzylic Alcohols Catalyzed by a Dicyclopropenylidene-Ag(I) Complex Roya Mir, Rozhin Rowshanpour, Katie Dempsey, Melanie Pilkington and Travis Dudding* Department of Chemistry, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, Ontario, Canada, L2S 3A1

(iPr)2N

N(iPr)2 Ag

(iPr)2N OH

(1 mol%)

CF3CO2

o

2.65 A o

N(iPr)2

via O

toluene, tert-BuOK, rt, 4 - 8 h dry air

R

o

1.46 A 1.19 A

o

2.99 A

R

ABSTRACT: The unprecedented synthesis, single-crystal X-ray structure and first catalytic application of a dicarbene-Ag(I) complex [Ag(BAC)2][CO2CF3] (BAC = bis(diisopropyl)aminocyclopropenylidene) is reported. This novel complex provides a versatile catalytic platform for selective aerobic oxidation of benzylic alcohols to aldehyde or ketone products in high yields. Ease of experimental execution coupled with the use of abundant atmospheric molecular oxygen as an oxidant and low catalyst loading are inherit strengths of these oxidations. Introduction Silver catalyzed processes currently find widespread use by a diverse range of scientists, although not so long ago this group 11 metal was thought to have low catalytic activity.1 Past notions aside, the advancement of homo- and heterogeneous silver catalyzed transformations continue to gain traction,2 e.g., in Ag(I)-catalyzed aldol and allylation reactions of carbonyls and imines, C‒ H activation enabling carbon-carbon (C‒ C)/carbon-heteroatom (C‒ X) bond formation, intramolecular (hetero)cyclizations, and cycloadditions.3 By the same token, Ag(I)-catalyzed alcohol oxidations furnishing carbonyls are paramount for upgrading raw commodity chemicals to versatile synthetic building blocks and value-add ACS Paragon Plus Environment

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materials.4 Often associated with these oxidations, however, are limitations including poor substrate scope, low catalyst efficiencies, as well as challenges related to experimental execution and product isolation (Scheme 1a).5 An additional obstacle to carbonyl synthesis from alcohols is selectivity. For instance, the selective oxidation of benzylic alcohols can produce aldehyde, ketone and/or carboxylic acid products. To overcome these limitations several Ag(I)-catalyzed protocols have emerged over the years, including the use of SiO2 or CaO supported silver catalysts as oxidizing agents for the gas-phase oxidation of benzyl alcohols.6 Further the homogenous use of unsupported silver nitrate (AgNO3) at low catalyst loadings (2 mol%) under reflux conditions was reported for the liquid-phase oxidation of benzylic alcohols.5a Moreover, as a recent entry the catalytic use of N-heterocyclic carbene (NHC) Ag(I) [Ag(NHC)2][AgBr2] complexes and stoichiometric amounts of base (BnMe3NOH) were employed for the selective oxidation of alcohols to aldehydes.4d

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Results and Discussion In reflecting upon these previous works and with the intent of expanding the scope of Ag(I) catalyzed methodologies targeting selective alcohol oxidations we envisioned merging the redox activity of Ag(I) with the strong donor ability of carbene bis(diisopropyl)aminocyclopropenylidene (BAC) as a stabilizing ligand to provide dicarbeneAg(I) complex [Ag(BAC)2]+ as a catalytic platform for selective alcohol oxidation (Scheme 1b). In acting upon this concept and furthering our program interest in cyclopropenium and cyclopropenylidene chemistries,7 herein we report the synthesis and first catalytic application of homoleptic dicarbene-Ag(I) complex [Ag(BAC)2][CO2CF3] (1). As reported vide infra dicarbene-Ag(I) complex 1 catalyzes the selective oxidation of benzylic alcohols to aldehydes or ketones in the presence of abundant atmospheric molecular oxygen as an oxidant and inexpensive base (Scheme 1c). Scheme 1. Silver Catalyzed Oxidation of Alcohols (a) Limitations Poor catalyst efficiency Limited substrate scope Lack of product selectivity (aldehydes vs. carboxylic acids) (b) Concept

+ N(iPr)2

(iPr)2N

(iPr)2N + Ag(I)

Ag(I) N(iPr)2

(iPr)2N

(iPr)2N Stabilizing Carbene Ligand

Redox Active Ag(I)

(c) Solution ArCH2-OH

Dicarbene-Ag(I) Complex

[Ag(BAC)2][CO2CF3] 1 (1 mol%) toluene, tert-BuOK, rt, 4 - 8 h dry air

O Ar

H



At the outset of our study, with ease of experiment and practicality in mind, the synthesis of homoleptic dicarbene-Ag(I) complex [(BAC)2Ag]CO2CF3 (1) was realized by an inventive approach involving stirring known carbene precursor bis(diisopropylamino)cyclopropenium (BAC)− H•Cl− (2)8 with silver trifluoroacetate and K2CO3 in acetone for 4 hours at reflux (Figure 1a). Subsequent X-ray diffraction analysis of a single crystal of [Ag(BAC)2][CO2CF3] obtained by vapor diffusion of ethyl acetate into an acetonitrile solution of 1 revealed half of the molecule was crystallographic unique with the Ag(I) cation sitting on an inversion centre, coordinated to cyclopropenylidene ligands in a linear fashion (Figure 1b). Meanwhile, the triflate counterion was highly disordered and was modeled over four positions. A summary of the experimental crystal data for 1 together with an ORTEP plot of the cation can be found in the Supporting Information (see Figure S1 and Table S1).

Figure 1. (a) Synthesis of [Ag(BAC)2][CO2CF3] (1) and (b) ORTEP plot of the molecular structure of 1 with appropriate labelling scheme. Thermal ellipsoids are plotted at 50%. (Hydrogen atoms and disordered counterion are omitted for clarity) Next, to better understand the underlying structural and electronic facets of [Ag(BAC)2][CO2CF3] single point density functional theory (DFT) calculations at the uB3LYP-


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D3/LACV3P**++ level of theory were performed using the X-ray coordinates of the cation [Ag(BAC)2]+ of 1 to provide 1+DFT. Clear from the HOMO – 12 of this cationic complex was a stabilized in-phase bonding interaction between a dz2 orbital of the two-coordinate silver metal ion and the carbenic carbon lone pairs of the cyclopropenylidene (BAC) ligands (Figure 2, top). On the other hand, the plotted average local ionization energy and computed NBO charge of 0.415 e at the silver atom of 1+DFT offered clues to applying 1 in catalysis (Figure 2, bottom), namely, as a catalyst for oxidation-reduction (redox) reactions. DFT Insight to Applications - oxidation-reduction (redox) reactivity

Stability Ag-Carbene Stabilization

HOMO-12

Reactivity local Ionization potential NBO charge Ag = 0.415 e

Figure 2. Single point uB3LYP-D3/LACV3P**++ computed HOMO-12 (isovalue = 0.04) of cation [Ag(BAC)2]+ (1+DFT) and depicted electrostatic potential plotted onto the local ionization (isovalue minimum = -0.09 and maximum = 0.1). From this basis and in building on our interest in coinage metal catalysis we explored the use of homoleptic dicarbene-Ag(I) complex 1 as a catalyst for aerobic alcohol oxidation.9 To this end, initial optimization studies employing benzyl alcohol (3a) in dichloromethane (DCM) with dry atmospheric air as an oxidant and a low loading of 1 (1 mol%) in the absence of light resulted in the recovery of unreacted starting alcohol (Table 1, entry 1). The addition of base tert-



BuOK (2 equiv.) to assist in alcohol activation was then examined, which after 20 hours generated oxidation product benzaldehyde (4a) in 10% yield along with unreacted alcohol 3a (Table 1, entry 2). Motivated by the selective formation of aldehyde 4a and absence of overoxidation product benzoic acid the use of tetrahydrofuran (THF) as a solvent was examined resulting in a two-fold increase in the yield of 4a (Table 1, entry 3). Next, increasing the equivalents of base tert-BuOK (4 equiv.) led to the formation of aldehyde 4a in 67% yield (Table 1, entry 4). Meanwhile, the use of bases KOH or Na2CO3 resulted in a dramatic decrease in reaction yield (Table 1, entries 5 and 6). Finally, exchanging the solvent for toluene and using base tert-BuOK (2 equiv.) to our delight afforded aldehyde 4a in 95% yield in 4 hours, while the use of KOH as a base provided 4a in low 14% yield (Table 1, entries 7 and 8). Table 1. Screening of Aerobic Oxidation Conditionsa,b [Ag(BAC)2][CO2CF3] 1 (1 mol%) OH

O

solvent, base, time, rt dry air

3a

entry

solvent

base (equiv.)

1

DCM

—

2

DCM

3

4a

time (h)

yield (%)b

20

0

tert-BuOK (2 equiv.)

20

10

THF


20

20

4

THF


20

67

5

THF

KOH (5 equiv.)

20

22


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6

THF

Na2CO3 (5 equiv.)

20

Selective Aerobic Oxidation of Benzylic Alcohols Catalyzed by a

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