Alcohol Oxidations Using Reduced Polyoxovanadates

Article Type : Full Paper

Alcohol oxidations using reduced polyoxovanadates

McKenzie. L. Campbell, Dino Sulejmanovic, Jacqueline B. Schiller, Emily M. Turner, Shiou-Jyh Hwu,*

Daniel. C. Whitehead*

Department of Chemistry, Clemson University, Clemson, SC, 29634 USA

Email: SHWU@clemson.edu; dwhiteh@clemson.edu

ABSTRACT

A full account of our recently communicated room temperature alcohol oxidation using reduced

polyoxovanadates (r-POVs) is presented. Extensive optimizations revealed optimal conditions

employing 0.02 equiv of r-POV catalyst Cs₅(V₁₄As₈O₄₂Cl), 5 equiv tert-butyl hydrogen peroxide

(tBuOOH) as the terminal co-oxidant, in an acetone solvent for the quantitative oxidation of aryl-

substituted secondary alcohols to their ketone products. The substrate scope tolerates most aryl

substituted secondary alcohols in good to quantitative yields while alkyl secondary and primary

activated alcohols were sluggish in comparison under similar conditions. Catalyst recyclability was

successful on a 1.0 mmol scale of starting alcohol 1-phenylethanol. The oxidation was also

successfully promoted by the V^IV/V^Vmixed valent polyoxovanadate (POV) Cs₁₁Na₃Cl₅(V₁₅O₃₆Cl).

Finally, a third POV, Cs_2.64(V₅O₉)(AsO₄)₂, was investigated for catalytic activity using our established

reaction protocol, but proved ineffective as compared to the other two r-POV catalysts. This study

expands the field of POM-mediated alcohol oxidations to include underexplored r-POV catalysts.

While our catalysts do not supplant the best catalysts known for the transformation, their study

may inform the development of other novel oxidative transformations mediated by r-POVs.

This article has been accepted for publication and undergone full peer review but has not been

through the copyediting, typesetting, pagination and proofreading process, which may lead to

differences between this version and the Version of Record. Please cite this article as doi:

10.1002/hlca.201600338

Keywords: polyoxovanadates, oxidation, catalysis

Introduction

Molybdenum and tungsten polyoxometalates (POMs) featuring vanadium-substituted

anionic frameworks, i.e. hetero-transition-metal POMs, are important catalysts in oxidative

reactions,¹yet little focus has been given to synthesizing POMs with vanadium as the sole transition

metal cation in the polyoxo-core structure (i.e. polyoxovanadates (POVs).²There are several

examples of vanadium-substituted POM catalysts promoting highly selective, quantitative

reactions;³however, current issues associated with POM-mediated oxidation protocols include high

reaction temperatures (~ 90 – 135 ˚C),⁴high catalyst loadings (e.g. ~40 mol%),⁵and some examples

require co-catalysts for optimal performance.⁶These considerations somewhat reduce the overall

synthetic utility of the resulting methods. Conversely, we recently reported a POV-mediated

method for the oxidation of alcohols that proceeds under comparatively much milder conditions at

room temperature with only 2 mol% catalyst loading.⁷Unlike other anionic POM clusters, our

materials described herein are highly water-soluble and feature fully reduced vanadium metal

centers.

The unique incorporation of exclusively vanadium atoms as the only transition metal within

the anionic POM framework is facilitated by the use of salt-inclusion chemistry (SIC).⁸Employing

SIC, where molten halide salts can act as a high temperature solvent (i.e. > 500 ˚C), a new family of

reduced POVs (r-POVs) are accessible. Due to the presence of salt ions, two dissimilar lattices, ionic

and covalent, coexist which results in soluble r-POV-containing species. Over the years, some of us

have used salt-inclusion chemistry for the synthesis of otherwise unattainable materials featuring

novel magnetic nanostructures,⁹mesoporous materials with permanent porosity,^10–13and water-

soluble polyoxometalate-containing salt-inclusion solids.^11–13The utility of salt-inclusion chemistry

has been demonstrated in the previously documented syntheses of the polyoxovanadate systems

Cs_2.5(V₅O₉)(AsO₄)₂(1) (Figure 1A),¹¹Cs₅(V₁₄As₈O₄₂Cl) (2) (Figure 1B),¹²Cs₁₁Na₃(V₁₅O₃₆Cl)Cl₅(3)

(Figure 1C).¹³With ready access to these unprecedented POV materials, we set out to investigate

their utility as catalysts for organic transformations.

Structrually, these POVs feature overall 5- and 9- net anionic charges and, while the

utilization of anionic POMs as catalysts for organic oxidations is known to date back over the last

two decades,^14–15the current applications for reduced polyoxometalates (r-POM) as catalysts for

alcohol oxidations is largely unexplored in comparison to their oxidized counterparts.^2,16–17Of the

few r-POM examples found in the literature, there is a reported series of reduced catalysts that

promote the photocatalytic reductive degradation of Acid Orange 7 (AO), a common dye.¹⁸These

catalytic materials must first be activated through photo-absorption to excite the r-POM towards

oxidation of a sacrificial reducing agent, usually a small, organic alcohol. The activated r-POM, now

being available for reoxidation-recycling, reduces the azo-dye resulting in aromatic amine

derivatives.¹⁸Several other photocatalytic transformations using r-POMs require the sacrificial

reducing agent 2-propanol through similar mechanisms as discussed above.^18–19The necessity for

both light excitation (i.e. UV irradiation) and sacrificial reagents to activate these catalytic

transformations is somewhat limited by uncontrollable side reactions and therefore, reduced

A

B

C

Figure 1. Polyoxovanadate structures for catalysts A) (salt)•Cs_2.5(V₅O₉)(AsO₄)₂(1) B) Cs₅(V₁₄As₈O₄₂Cl) (2) C)

Cs₁₁Na₃(V₁₅O₃₆Cl)Cl₅(3). In 3, only [V₁₅O₃₀Cl]^9-clusters are show for clarity.

desired product yield. For example, the previously discussed photocatalytic reduction of AO suffers

from a competing process of photocatalytic oxidative degradation of the AO substrate after

continued UV exposure.¹⁸Nonetheless, using r-POMs for alcohol oxidation with non-photocatalytic

activation has not been reported to our knowledge.

In this report, we describe the details of the investigation of the catalytic aptitude of r-POV

catalysts Cs_2.5(V₅O₉)(AsO₄)₂(1),¹¹Cs₅(V₁₄As₈O₄₂Cl) (2),¹²and Cs₁₁Na₃Cl₅(V₁₅O₃₆Cl) (3)¹³for the

oxidation of alcohols. These efforts culminated in a recently communicated process that proceeds at

room temperature using only 2 mol % of catalyst 2 and tert-butyl hydrogen peroxide as the

terminal co-oxidant.⁷Herein, we present a full account of our efforts that culminated in this r-POV

promoted method for alcohol oxidation. Our efforts in this regard were motivated by several

factors: 1. as noted above, there have been relatively few explorations of the reactivity of r-POV

scaffolds as catalysts in organic oxidations, 2. the investigation of alcohol oxidations in particular

allows us to benchmark our materials in the broader context of other POM catalysts in this well-

studied transformation,¹and 3. the investigations of alcohol oxidations with r-POVs may inform

studies employing these unique catalysts for other organic oxidations of interest in the future.

Results and Discussion

Exploratory Experiments and Optimization. The synthesis and full characterization for

the POVs 1-3 discussed herein are presented in their respective manuscripts referenced below.

Briefly, molar ratios of mineralizers and inorganic salts under pressure and high temperature (SIC-

method) afford transition metal oxide frameworks with inorganic salts intercalated within the

primary structure. These salts are soluble after washing with aqueous solution leaving behind

porous metal-oxide frameworks; a property suitable for possible catalytic application. Structures of

the potential catalysts, Cs_3.5Na_1.47(V₅O₉)(AsO₄)Cl_2.33(1),¹¹Cs₅(V₁₄As₈O₄₂Cl) (2),¹²and

Cs₁₁Na₃Cl₅(V₁₅O₃₆Cl) (3),¹³were verified using powder X-ray diffraction prior to synthetic use (see

SI).

The initial focus of the present study was to analyze the efficiency of polyoxovanadates 1-3

(Table 1, entries 1-3) in the catalytic oxidation of 1-phenylethanol 4 to acetophenone 5 using the

terminal co-oxidant tBuOOH in aqueous media. Our initial investigation of the oxidation of 4 to 5

was conducted using an extraction protocol prior to GC analysis (see GC Work-Up A in

Experimental Section). In this initial evaluation, catalyst 2 proved to be the most reactive of the

three r-POV candidates, (entry 2) while r-POVs 1 and 3 (entries 1 and 3) also returned 5 in yields

that exceeded the non-catalyzed reaction promoted by tBuOOH alone (entry 4). Next, concentrating

on catalyst 2, simultaneous studies including an exploration of the influence of the nature of the

terminal co-oxidant and the solvent system were conducted. Increasing the equivalents of tBuOOH

from 1.5 to 5 equiv. increased the yield of acetophenone 5 to 64% (entry 5). Aqueous hydrogen

peroxide (H₂O₂) was investigated but returned only trace amounts of product (entries 6 and 7).

Similarly, Oxone®, returned only 15 and 13% yields of 5 at 1.5 and 5 equiv. loadings (i.e. entries 8

and 9, respectively). Having benefited from the use of urea-hydrogen peroxide complex (UHP) for

the oxidative bromolactonization of alkenoic acids promoted by V₂O₅,²⁰we investigated the use of

this reagent for under current reaction conditions. Nevertheless, when applied in the present study,

only trace amounts of product 5 were observed (entries 10 and 11). Finally, meta-

chloroperoxybenzoic acid (mCPBA) did not promote appreciable turnover to 5 as shown in entry

12.

A solvent screen revealed acetone (entry 13) as a comparable solvent to that of the initially

employed aqueous medium (entry 5). Using other solvents, such as acetonitrile (ACN), 1,4-dioxane,

and diethyl ether did not improve product yields above 64% (entries 14 – 16). Next, mixtures of

water and acetone were investigated, although, no further increase in yield of 5 was realized

(entries 17 and 18).

Using 5.0 equiv of aq. tBuOOH as a co-oxidant (entry 5), two final preliminary investigations

were run. The necessity of any additional solvent was probed by simply conducting the

transformation with aq. tBuOOH as the only solvent (entry 19). Once again, comparable yields were

realized to that of entry 5. Finally, acetone was employed as a co-solvent in conjunction with 5.0

equiv of aq. tBuOOH. Under these conditions, an approximate 20% increase in product yield was

observed (83%, entry 20).

Table 1. Exploratory experiments and optimization of 1-phenylethanol oxidation using r-POV

Cs₅(V₁₄As₈O₄₂Cl) (2).

OH

O

POV cat. (equiv.)

co-oxidant (equiv.)

solvent, rt, time

4, 0.1 mmol

5

Solvent^a[M]

Entry

Catalyst (equiv.)

Co-oxidant (equiv.)

Yield (%)

1

TBHP (aq.)^b(1.5)

TBHP (aq.) (1.5)

TBHP (aq.) (5.0)

H₂O₂(aq.)^c(1.5)

H₂O₂(aq.) (5.0)

Oxone® (1.5)

Oxone® (5.0)

UHP (1.5)

28% +/- 4

56% +/- 10

22% +/- 2

15% +/- 3

64% +/- 5

2% +/- 0

Cs_3.5Na_1.47(V₅O₉)(AsO₄)₂Cl_2.33(0.05) (1)

Cs₅(V₁₄As₈O₄₂Cl) (0.05) (2)

Cs₁₁Na₃Cl₅(V₁₅O₃₆Cl) (0.05) (3)

none

H₂O [0.3]

2

3

4

5

Cs₅(V₁₄As₈O₄₂Cl) (0.05)

6

7

7% +/- 3

8

15% +/- 3

13% +/- 0

5% +/- 0

9

10

11

12

13

14

15

16

17

18

19

20

8% +/- 0

UHP (5.0)

29% +/- 2

64% +/- 11

57% +/- 8

39% +/- 7

15% +/- 3

53% +/- 5

60% +/- 3

60% +/- 8

83% +/- 2

mCPBA (1.5)

H₂O [0.3]

TBHP (aq.) (1.5)

TBHP (aq.) (5.0)

acetone [0.3]

ACN [0.3]

1,4-dioxane [0.3]

Et₂O [0.3]

acetone:H₂O (5:1) [0.3]

acetone: H₂O (1:5) [0.3]

none

acetone [0.3]

^a[M]=(0.1 mmol starting material / x mL solvent). ^bTBHP (aq.) denotes a 70% aq solution of TBHP empolyed as a co-oxidant. ^cH₂O₂(aq.) denotes a

30% aq solution of H₂O₂empolyed as a co-oxidant.

Further Optimization, Additive Investigation and Control Reactions. Next, we studied the

effects of solvent concentration and catalyst loading in order to further optimize the transformation

of alcohol 4 to its corresponding ketone 5. First, the optimal solvent concentration was determined

by screening the reaction at six concentrations ranging from 1 M to 0.1 M (Table 2; entries 1-6). The

yield of 5 was reduced at higher concentrations (entries 1 and 2). These experiments revealed a

concentration of 0.25 M to be optimal.

We then turned to an optimization of catalyst loading. Recall that the highest observed

yield of 5 from the previous round of optimization was 83% while employing a 5 mol% loading of

Table 2. Further optimization of 1-phenylethanol oxidation using r-POV Cs₅(V₁₄As₈O₄₂Cl) (2).

OH

O

POV cat. (equiv.)

co-oxidant (equiv.)

solvent, rt, time

4, 0.1 mmol

5

Entry

Catalyst (equiv.)

Acetone [M]^a

[1.00]

Co-oxidant (equiv.)

TBHP (aq.)^c(5.0)

TBHP (aq.) (5.0)

Yield (%)^b

49% +/- 5

51% +/- 1

74% +/- 8

80% +/- 2

52% +/- 4

77% +/- 4

85% +/- 4

60% +/- 1

1

2

3

4

5

6

7

8

9

10

0.05

0.04

0.03

0.02

0.01

[0.50]

[0.33]

[0.25]

[0.20]

[0.10]

[0.25]

^a[M]=(0.1 mmol starting material / x mL solvent). ^bYields isolated via acid/base extraction.

^cTBHP (aq.) denotes a 70% aq solution of TBHP empolyed as a co-oxidant.

Cs₅(V₁₄As₈O₄₂Cl) (2) (Table 1, entry 20). Reducing the catalyst loading to 4 and 3 mol% of the

catalyst did not drastically affect the product yield (Table, 2, entries 7 and 8). The highest product

yield was observed at a 2 mol % catalyst loading (entry 9). Finally, further adjusting the catalyst

loading to 1 mol % resulted in a reduced 62% yield of product 5.

At this point, we settled on the optimized reaction conditions for the POV-catalyzed

oxidation of 4 to 5 using 2 mol % of catalyst Cs₅(V₁₄As₈O₄₂Cl) (2) with 5.0 equiv of aqueous tBuOOH

as a co-oxidant in acetone (0.25 M) for 24 hours at room temperature. These conditions reliably

returned approximately 85% yields of product 5 (Table 2, entry 9). Next, the influence of both

acidic and basic additives was investigated (Table 3). In this study, 3.0 equiv of para-

toluenesulfonic acid (p-TSA) returned the highest yields of 5 (80%) (Table 3, entry 1). Other

organic acid additives, i.e.

Table 3. Additive investigation for the oxidation of 1-phenylethanol

oxidation using r-POV Cs₅(V₁₄As₈O₄₂Cl) (2).

citric acid, acetic acid, and

benzoic acid did not effect an

OH

O

appreciable

increase

of

Cs₅(V₁₄As₈O₄₂Cl) (0.02)

acetophenone

production

TBHP (70% aq.) (5.0 eq.)

acetone, rt, 24h

(entries 2 through 4). Base

additives including

4

5

potassium carbonate, sodium

bicarbonate, and sodium

carbonate, gave reduced

yields of 5 ranging from 33 to

48% (entries 5-7).

% Yield^a

Entry

Additive

p-TSA (3.0)

TBHP equiv.

1

5.0

80% +/- 5

2

citric acid (3.0)

acetic acid (3.0)

5.0

67% +/- 13

3

4

5.0

68% +/- 16

53% +/- 12

benzoic acid (3.0)

K₂CO₃(3.0)

At this point in our

optimization efforts, our GC

work-up protocol involving

an acid-base extraction prior

to GC analysis (i.e. GC Work-

Up A) was abandoned in

favor of a simplified reaction

5

6

7

5.0

48% +/- 0

38% +/- 5

33% +/- 3

NaHCO₃(3.0)

Na₂CO₃(3.0)

^aIsolated via acid/base extraction.

work-up (GC Work-Up B). When the reaction medium was sampled directly without an intervening

extraction, our optimized conditions returned a quantitative yield of 5 after just 12 h (Table 4, entry

1). Evidently, the quantitative conversion of 4 to 5 under our previously optimized conditions was

obscured by product loss due to the extraction and concentration steps in our initial work-up.

Employing the new work-up (i.e. GC Work-Up B), we next conducted a brief evaluation of the

parameters of our optimal reaction conditions. Specifically, lowering the equivalents of co-oxidant

resulted in reduced yields of 87% when 1.5 equiv. were used (entry 2); yet the use of 3.0 equiv. did

not result in significant loss in yield (i.e. 95% yield of 5, entry 3). Similar to our results with GC

Work-Up A (cf. Table 2, entry 10), further reduction of the catalyst loading below 2 mol % resulted

in an unacceptable reduction in product yield (entries 4 and 5).

Next, several control reactions were conducted in order to rule out other oxidation

pathways. First, to negate the possibility of the POV serving as a Lewis acid to promote an

Oppenauer oxidation of 4 by acetone,²¹the transformation was investigated in the absence of the

tBuOOH co-oxidant. Under these conditions, acetophenone 5 was isolated in an insignificant 7%

yield in both acetone and acetone/water solvent systems (entries 6 and 7) indicating that the

oxidation of 1-phenylethanol is not promoted by acetone in the presence of the POV 2 catalyst.

Table 4. Final optimization and control reactions for the oxidation of 1-

phenylethanol using r-POV Cs₅(V₁₄As₈O₄₂Cl) (2).

OH

O

POV cat. (equiv.)

co-oxidant (equiv.)

solvent, rt,12 h

4, 0.1 mmol

5

Entry Catalyst (equiv.)

Acetone [M]^a

Co-oxidant (equiv.)

Yield (%)^b

TBHP^c(aq.) (5.0)

TBHP (aq.) (1.5)

TBHP (aq.) (3.0)

TBHP (aq.) (5.0)

0.02

0.01

0.005

0.02

100% +/- 3

87% +/- 5

95% +/- 2

62% +/- 4

47% +/- 2

7% +/- 0

1

2

3

4

5

6

[0.25]

H₂O^d

7% +/- 0

none

7

8^e

9^f

100% +/- 1

6% +/- 2

TBHP (aq.) (5.0)

none

^a[M]=(0.1 mmol starting material / x mL solvent). ^bYields isolated via GC. ^cTBHP (aq.)

denotes a 70% aq solution of TBHP empolyed as a co-oxidant. ^dEqual volume to TBHP. ^eRan

f

under N₂. Ran under O₂balloon.

Conducting the reaction under a dry N₂atmosphere (anoxic conditions, entry 8) did not reduce the

yield of acetophenone 5, thus negating the possibility of atmospheric O₂acting as a competing co-

oxidant in the POV-promoted oxidation of 4. Further, conducting the reaction under an O₂

atmosphere (entry 9) without the presence of aq. tBuOOH returned only trace amounts of

acetophenone.

Substrate Scope with Catalyst 2. Next, an in-depth substrate scope was investigated using

the optimal reaction conditions (and GC Work-Up B) described above. In brief, the transformation

tolerates a variety of activated aryl alcohols in quantitative yields; their products are shown in

Figure 2, compounds 5 – 7. Substituted benzylic alcohols returned their ketones in 90% to

quantitative yields regardless of the electronic nature of the para substituents (compounds 8 - 13).

Isolation of compound 14 in an 87% was of particular interest due to its retention of the α-

cyclopropyl moiety, suggesting that the transformation does not involve radical intermediates.

Heterocyclic products such as 15 and 16 and α,β-unsaturated ketone 18 were recovered in

serviceable yields, while products 17 and 19 were returned in disappointing 20% and 30% yields,

respectively. A non-activated secondary alcohol (cyclohexanol) was successfully oxidized in an 89%

yield of 20. An aliphatic secondary and symmetrical alcohol were investigated and while good to

excellent yields were observed (compounds 21 and 22) an increased catalyst loading of 0.1 equiv.

was required for successful conversion. A series of primary alcohols were also investigated;

however, their compatibility with the reaction conditions was limited to benzylic alcohols and the

transformation of those substrates were inferior to that of the secondary alcohols, leading to the

isolation of multiple products (see SI for details). Also, the potential for C-H activation of

cyclohexane was investigated under the optimal conditions, however, no oxidation product was

observed via GC. Therefore, our method is selective for secondary alcohol oxidation and is not

OH

O

Cs₅(V₁₄As₈O₄₂Cl) (0.02 equiv)

TBHP (5.0 equiv)

acetone, rt, 48 h

4, 0.1 mmol

5

O

Me

7^a, 93% +/- 3

8^a, 90% +/- 4

5, 100% +/- 3

6, 100%+/- 5

O

MeO

Cl

Br

F

9^a, 98% +/- 2

10^a, 94% +/- 3

11^a, 100% +/- 2

12^a, 100% +/- 1

O

F₃C

N

13^a, 100% +/- 1

14^a, 87% +/- 3

15^a, 62% +/- 1

16^a, 68% +/- 2

O

18^a, 66% +/- 1

20^a, 89% +/- 4

17^a, 20% +/- 0

17^b, 12% +/- 2

19^a, 31% +/- 1

19^b, 30% +/- 1

O

21^a,b, 97% +/- 2

21^c, 70% +/- 5

22^a,b, 67% +/- 0

22^c, 79% +/- 9

^a48h reaction time. ^b10 mol % catalyst loading. ^c72 h stir.

Figure 2. Substrate scope for Cs₅(V₁₄As₈O₄₂Cl) (2) oxidation of secondary

alcohols.

readily amenable for primary alcohols or C-H oxidation. Having successfully uncovered an

oxidation protocol using catalyst Cs₅(V₁₄As₈O₄₂Cl) (2) for a variety of substituted alcohols, our

efforts shifted to a more thorough investigation of the other two r-POVs (i.e. 1 and 3) as catalysts

using our optimized conditions.

Further Investigation of r-POV Catalyst 3. With catalyst Cs₁₁Na₃Cl₅(V₁₅O₃₆Cl) (3), alcohol

oxidation proceeded in quantitative yields after a prolonged reaction time of 48 h at room

temperature (Table 5, entry 1). Removal of the catalyst resulted in insignificant recovery of product

5 (entry 2). Similar to the reaction catalyzed by POV 2 (vide supra), the presence of acetone as a co-

solvent is critical for the promotion of high yields of acetophenone as shown by comparing entries 3

and 4. Reducing the equivalence of the co-oxidant (entry 4) as compared to the established 5.0

equiv (entry 1) resulted in moderate product yields. Lowering the catalyst loading to 1 mol% of 3

also resulted in lower yields of acetophenone (entry 5).

Conducting control experiments with r-POV catalyst 3 yielded interesting results. In sharp

contrast to the Oppenauer-like conditions investigated for catalyst 2 (cf. Table 2, entries 17 and 18),

conversion of 1-phenylethanol to acetophenone was indeed observed in the presence of r-POV 3 in

an acetone/water solvent system without the inclusion of the aq. tBuOOH co-oxidant, albeit in a

moderate 45% yield (Table 5, entry 6). This surprising result indicates that unlike r-POV 2, catalyst

Table 5. Further investigation of Cs₁₁Na₃Cl₅(V₁₅O₃₆Cl) (3) as a catalyst in the oxidation of 1-

phenylethanol.

OH

O

POV cat. (equiv.)

co-oxidant (equiv.)

solvent, rt, time

4, 0.1 mmol

5

Solvent [M]^a

Entry Catalyst (equiv)

Co-oxidant (equiv)

Time Yield (%)

100% +/- 3

TBHP (aq.)^b(5.0)

1

2

3

Cs₁₁Na₃Cl₅(V₁₅O₃₆Cl) (0.02) (3)

none

48 h

acetone [0.25]

H₂O [0.3]

48 h 15% +/- 1

48 h 22% +/- 2

TBHP (aq.) (5.0)

TBHP (aq.) (1.5)

TBHP (aq.) (5.0)

Cs₁₁Na₃Cl₅(V₁₅O₃₆Cl) (0.05)

4

5

Cs₁₁Na₃Cl₅(V₁₅O₃₆Cl) (0.02)

Cs₁₁Na₃Cl₅(V₁₅O₃₆Cl) (0.01)

48 h 61% +/- 3

48 h 62% +/- 1

acetone [0.25]

H₂O^c

6

Cs₁₁Na₃Cl₅(V₁₅O₃₆Cl) (0.02)

48 h

48h

45% +/- 3

0%

7

none

8^d

9^e

48 h 72% +/- 4

48 h 0%

TBHP (aq.) (5.0)

none

^a[M]=(0.1 mmol starting material / x mL solvent). ^bTBHP (aq.) denotes a 70% aq solution of TBHP empolyed as a co-oxidant. ^c

Equal volume to TBHP. ^dRan under N₂. ^eRan under O₂balloon.

3 must be sufficiently Lewis acidic to allow for the acetone-promoted Oppenauer oxidation of 1-

phenyl ethanol. This result serves to highlight that structural perturbations of the r-POV scaffold,

achieved by the salt-inclusion synthesis method, may allow for significant changes in organic

reactivity. Interestingly, this novel Oppenauer oxidation was not observed in the absence of the

water co-solvent (entry 7). In further contrast to catalyst 2, a significant, ~28% reduction in the

yield of acetophenone 4 was observed when 3 was employed as the catalyst under an N₂

atmosphere (entry 8, cf. entry 1). The reason for this marked reduction in yield is unclear at this

point, and warrants further investigation. Finally, no product formation was observed via GC

analysis when using O₂as the only oxidant source (entry 9).

Next, we conducted an abbreviated evaluation of the substrate scope with the

Cs₁₁Na₃Cl₅(V₁₅O₃₆Cl) (3) catalyst. In the event, 0.02 equiv of 3 promoted quantitative oxidations of

1-phenyl ethanol, diphenyl methanol, and 1-phenyl propanol returning acetophenone 5,

OH

O

Cs₁₁Na₃Cl₅(V₁₅O₃₆Cl) (0.02 equiv)

TBHP (5.0 equiv)

acetone, rt, 48 h

4, 0.1 mmol

5

O

F₃C

Me

MeO

5^a, 100% +/- 3

6, 100% +/- 3

7^a, 100% +/- 3

8^a, 67% +/- 5

9^a, 54% +/- 6

13^a, 54% +/- 1

O

23^a, SM

cyclohexane- 20^a, SM

cyclohexanol- 20^a, 48% +/- 8

21^a, 30% +/- 2

22^a, 25% +/- 2

^a48h reaction time.

Figure 3. Substrate scope for secondary alcohol oxidation using r-POV Cs₁₁Na₃Cl₅(V₁₅O₃₆Cl) (3).

benzophenone 6, and 1-phenylpropanone 7 respectively (Figure 3). The para-substituted

analogues of compound 7 returned only moderate yields of approximately 60% yield the desired

ketone products 8, 9, and 13 regardless of electron donating or electron withdrawing character.

Secondary aliphatic alcohols reacted sluggishly, returning moderate-to-low yields of compounds

20, 21, and 22. Further, only starting material was recovered from the attempted C-H oxidation of

cyclohexane as well as the attempted oxidation of the primary alcohol, 1-octanol.

Further Investigation of r-POV catalyst 1. We next turned to a more thorough evaluation of the

catalytic efficiency of r-POV Cs_2.5(V₅O₉)(AsO₄)₂(1) under our optimized conditions. Employing r-

POV 1, 1-phenylethanol was

OH

O

converted

to

acetophenone

Cs_2.64(V₅O₉)(AsO₄)₂(0.02 equiv)

(Figure 4, 5a) in an 80% yield

under our optimized conditions.

Using higher catalyst loading did

not afford an increase in product

formation (5b). Screening other

TBHP (5.0 equiv)

acetone, rt, 48 h

4, 0.1 mmol

5

O

7, 10% +/- 2

5a, 80% +/- 3

5b, 63% +/- 5^b

substituents with catalyst

1

O

returned puzzling results. Upon

evaluating

similar

the

structurally

F₃C

13, 11% +/- 4

Me

8, 15% +/- 1

MeO

1-phenylpropanol

9, 8% +/- 1

20a, SM

20b, 20% +/- 1

substrate, a dramatic decrease in

the yield of the corresponding

ketone was reliably revealed (i.e.

10% yield of 7 versus 80% yield

of 5). Similarly, other para-

O

21, 5% +/- 0

^b10 mol% catalyst loading.

22, SM

23, SM

Figure 4. Further investigation of Cs_2.5(V₅O₉)(AsO₄)₂(1) as a catalyst

in the oxidation of 1-phenylethanol.

1-phenylpropanol

substituted

derivatives also returned poor

yields (see products 8-13). Similarly, substrates lacking an arene were also poorly reactive (20-23).

Currently, it is unclear why there is such a dramatic decrease in yield upon extending the carbon

chain of the substrate alcohol by one unit. It is noteworthy, however, that unlike catalyst 2 and 3, r-

POV 1 bears 2 nm pores (see Fig. 1C). If the catalytic site on 1 resides within the pore cavity, more

sterically demanding substrates may be precluded.

Recyclability Study for r-POV Catalysts 2 & 3. To demonstrate the potential for recycling

our catalysts under our optimized conditions, both Cs₅(V₁₄As₈O₄₂Cl) (2) and Cs₁₁Na₃Cl₅(V₁₅O₃₆Cl)

(3) were impregnated on celite (see SI for details) to aid in filtration during the recovery process.

Catalyst (2) was successfully

O

OH

r-POV-celite (0.02 equiv)

used for three consecutive

reactions in the oxidation of 1-

TBHP (70% aq.) (5.0 equiv)

acetone, rt, time

5

phenylethanol

to

4, 1.0 mmol

100% 100%

100%

acetophenone at 1 mmol scale

(Figure 5). Catalyst 3 was

recyclable over four oxidative

100%

80%

60%

40%

20%

0%

59%

cycles before showing

a

54%

dramatic decrease in activity

with

only

17%

(5)

of

17%

acetophenone

being

isolated after a 48 h reaction

time in the fifth run.

RUN 1

RUN 2

RUN 3

RUN 4

RUN 5

Catalyst (3)

Cs₁₁Na₃Cl₅(V₁₅O₃₆Cl) (3)

Cs₅(V₁₄As₈O₄₂Cl) (2)

Catalyst (2)

Qualitative

Light

Figure 5. Recyclability of r-POV catalysts at a 1.0 mmol reaction scale.

Scattering Experiments. We

suspected that r-POVs 1-3

might aggregate in solution, a phenomenon that is easily illustrated on a qualitative level with light

scattering (Figure 6A-C). In contrast to the cuvettes containing just acetone (left most cuvette in

each image), a clear indication of light scattering of the green laser upon irradiation is evident

during when the r-POVs are dissolved in solution (second cuvette from the left). This phenomenon

is still apparent in the third and fourth cuvettes during the course of the oxidative transformation,

suggesting that the POV scaffolds maintain their structural integrity and the likely micron-scale

aggregates throughout the catalytic cycle.

Conclusions

A

B

In

conclusion,

our materials are the first POVs

of their kind (i.e. fully reduced

vanadium clusters) used for

organic oxidations. A detailed

investigation of the catalytic

aptitude of reduced POV

C

catalysts

Cs_2.5(V₅O₉)(AsO₄)₂

Light scattering experiment

(1),¹⁰Cs₅(V₁₄As₈O₄₂Cl) (2),¹¹and

Cs₁₁Na₃Cl₅(V₁₅O₃₆Cl) (3)¹²for

the oxidation of alcohols was

conducted. Catalysts 2 and 3

showed the greatest efficiency

for product formation under the

optimized conditions. Unlike

other previously reported POM-

showing the aggregation of the

(A) Cs₅V₁₄As₈O₄₂Cl (2), (B)

Cs₁₁Na₃Cl₅(V₁₅O₃₆Cl) (3), and

(C) Cs_2.5(V₅O₉)(AsO₄)₂(1),

clusters in solution from left to

right; 1. acetone 2. acetone +

POV 3. acetone + POV +

substrate + TBHP 4. acetone

+POV +substrate + TBHP after

15 minute stir.

Figure 6. Qualitative light-scattering by r-POV catalysts.

mediated oxidation protocols, our method proceeds at room temperature using only 2 mol% of the

catalyst to facilitate the oxidation of a range of secondary alcohols. The recyclability of these

materials under optimized reaction conditions was successful (1.0 mmol starting alcohol) using

both catalyst (2) and (3). Catalyst 2 does act as a more efficient catalyst by promoting quantitative

conversion for a larger variety of secondary alcohols and in shorter reaction times as compared to

catalyst 3, which only allows for quantitative conversion of certain aryl activated alcohols. Catalyst

3 is limited in oxidation of alkyl secondary alcohols and as with catalyst 2, no activation for C-H or

primary alcohol oxidation. Conversely, catalyst 1 proved to be virtually inactive as a catalyst for the

oxidation of alcohols. The investigation of these novel r-POV materials adds to the impressive suite

of POM catalysts¹that are capable of promoting the oxidation of alcohols. While our materials do

not supplant the best POM catalysts known for this transformations in terms of catalyst turnover

and most benign co-oxidant, their study may inform the development of new, more reactive r-POV

catalysts using the unique SIC synthetic technique. Additionally, the knowledge gained from the

studies described herein will undoubtedly inform future investigations of r-POV catalysts like 1-3

for other important organic oxidations of general interest. Current efforts are focused on probing

the mechanism of catalysis by r-POVs 1-3, as well as investigating other organic transformations of

interest. The results of the efforts will be reported in due course.

Experimental

General Material and Methods. All reagents were purchased from commercial suppliers

and used without further purification. The synthetic protocol and requisite reagents used in the

preparation of Cs_2.5(V₅O₉)(AsO₄)₂(1),¹¹Cs₅(V₁₄As₈O₄₂Cl) (2),¹²and Cs₁₁Na₃Cl₅(V₁₅O₃₆Cl) (3)¹³were

reported previously. The purity of catalysts 1-3 was assessed by X-ray powder diffraction (Figures

S1, S2, and S3).

Analytical gas chromatography (GC) was performed on a SHIMADZU GC-2014

chromatograph equipped with a SHIMADZU AOC-20i autosampler, a split mode capillary injection

system, a flame ionization detector and a GS-Tek stationary phase GsPB-5 GC column. GC analyses

were carried out within the following parameters: inlet temperature: 200.0 ˚C; split injection with a

20:1 split ratio at 60 mL/min; injector sampling depth: 5 mm; column flow: 2.68 mL/min, constant

pressure; carrier gas: helium; FID temperature: 220 ˚C; oven temperature ramp: 100 ˚C for 1 min,

20 ˚C/min ramp to 220 ˚C, hold for 2 min. GC yields were determined using tetraglyme as the

1

internal standard. H and ¹³C NMR spectra were collected on a 300 MHz Bruker NMR spectrometer

using CDCl₃solvent. Chemical shifts are reported in parts per million (ppm) with spectra referenced

1

to the residual solvent peak. An isolated sample of acetophenone (5) was characterized by H and

¹³C NMR and was in complete agreement with samples reported in literature.

GC Work-Up A: Representative procedure for the catalytic oxidation of alcohols using acid-

base work-up for isolation of product in triplicate. A 3 mL screw-capped vial was charged with

5.4 mg of Cs₅(V₁₄As₈O₄₂Cl) catalyst (0.002 mmol; 0.02 equiv.), 12.7 μL of 1-phenylethanol (0.1

mmol; 1.0 equiv.), and 400 μL of acetone. To the stirring solution, 69 μL of 70% aqueous tert-butyl

hydrogen peroxide (tBuOOH) (0.5 mmol; 5.0 equiv.) was added and allowed to stir for 12 hours at

room temperature. After the 12h reaction time, the solution was first diluted with 5 mL of distilled

water and 2 mL of saturated sodium meta-bisulfite solution was added to quench any remaining

tBuOOH. The liquid was transferred to a 60 mL separatory funnel and the aqueous layer was

extracted with 10 mL of DCM (X3). The combined organics were washed with 10 mL of saturated

brine. The resulting organic layer was dried over sodium sulfate for twenty minutes, filtered, and

concentrated in vacuo before being subjected to GC analysis.

Into a 1 mL volumetric flask, product residue and the internal standard tetraglyme (11 μL,

0.05 mmol) was added. The solution was diluted to 1 mL using acetone then the full volume of

liquid was transferred to a screw-cap GC vial and analyzed by GC to determine the yield. Yields

were calculated by means of product standard curves equating GC peak area to product

concentration. Reported yields are triplicate averages with standard deviations.

GC Work-Up B: Representative procedure for the catalytic oxidation of alcohols ran in

triplicate. A 3 mL screw-capped vial was charged with 5.4 mg of catalyst (0.002 mmol; 0.02 equiv.),

12.7 μL of 1-phenylethanol (0.1 mmol; 1.0 equiv.), and 400 μL of acetone as the solvent. To the

stirring solution, 69 μL of 70% aqueous tert-butyl hydrogen peroxide (tBuOOH) (0.5 mmol; 5.0

equiv.) was added and allowed to stir for a set time at room temperature. After the allotted reaction

time, the solution was transferred to a 1 mL volumetric flask and the internal standard tetraglyme

(11 μL, 0.05 mmol) was added to the vial. The solution was diluted to 1 mL then the full volume of

liquid was transferred to a screw-cap GC vial and analyzed by GC to determine the yield. Yields

were calculated by means of product standard curves equating GC peak area to product

concentration. Reported yields are triplicate averages with standard deviations.

Procedure for catalyst recyclability study. A 54 mg sample of Cs₅(V₁₄As₈O₄₂Cl) catalyst was first

impregnated on 100 mg of celite by mixing of the solids in 4.0 mL of acetone before the addition of

1.0 mmol of 1-phenylethanol (121 μL; 1.0 equiv.) The co-oxidant tBuOOH (0.7 mL; 5.0 mmol; 5.0

equiv.) was introduced and the mixture was allowed to stir for 24 hours at room temperature. The

magnetic stir bar was then removed and the heterogeneous solution was filtered through a fritted

glass funnel and allowed to dry overnight. The remaining, clear liquid was concentrated in vacuo to

approximately 5 mL before being diluted with 5 mL of water and quenched with saturated sodium

meta-bisulfite. The aqueous mixture was extracted with ethyl acetate (3 x 15 mL), washed with

saturated sodium bicarbonate (15 mL), and dried over anhydrous sodium sulfate. The collected

aqueous layers were then back extracted with another 15 mL of ethyl acetate that was added to the

drying organic layers. The organics were then filtered and concentrated in vacuo. Silica gel column

chromatography

(20%

ethyl

acetate/hexanes)

returned

the

desired

product.

Acknowledgments

We would like to thank the Clemson University Chemistry Department for their support.

Author Contributions

DS prepared and characterized the polyoxovanadates. MLC, JBS, and EMT conducted the oxidation

experiments and characterization. MLC, DCW, and SJH conceptualized the study, analyzed the data,

and prepared the manuscript.

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Article Doi

Alcohol Oxidations Using Reduced Polyoxovanadates

DOI: 10.1002/hlca.201600338

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Article abstract of DOI:10.1002/hlca.201600338

Full text of DOI:10.1002/hlca.201600338

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