Vanadium - American Chemical Society

Chapter 2

Vanadium-Catalyzed Oxidation in Ionic Liquids

Downloaded by UCSF LIB CKM RSCS MGMT on November 19, 2014 | http://pubs.acs.org Publication Date: August 30, 2007 | doi: 10.1021/bk-2007-0974.ch002

Valeria Conte*, Barbara Floris, and Adriano Silvagni Dipartimento di Scienze e Tecnologie Chimiche, Università di Roma "Tor Vergata", via della ricerca scientifica, 00133 Roma, Italy *Corresponding author: [email protected]

Among the various interesting features of vanadium(V) there is its ability to activate hydrogen peroxide in diverse oxidation reactions. In this respect, we proposed a simple and effective procedure for oxybromination of unsaturated substrates in a biphasic system H 2 O / C H 2 C l 2 . Then, with the aim of rendering such a procedure even more interesting from the sustainability point of view, we have substituted chlorinated solvents with ionic liquids, ILs. Interestingly enough, we observed a remarkable increase of both yield and selectivity with styrene and ethynylbenzene, obtaining synthetically interesting results. Here we report results with other substrates in order to enlarge the scope of the reaction. Very preliminary data of peroxovanadium catalyzed hydroxylation of arenes in ILs are also mentioned.

28

© 2007 American Chemical Society

In Vanadium: The Versatile Metal; Kustin, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

29


st

One of the most important goals for 21 century chemistry is the achievement of sustainable synthetic procedures; this is particularly appropriate when upgrading of known processes is considered. Several reactions, in fact, have been recently tailored either by employing catalysts with better performances, in terms of both yields and selectivities, and eventually reusing them in subsequent cycles, or significantly decreasing the environment impact of the solvents. In the perspective of a more sustainable reaction medium, and considering the improvement obtained by reducing the amount of volatile organic solvents (VOCs) used in an industrial process, numerous research groups have proposed new reaction media ranging from fluorinated solvents (1,2% to supercritical C 0 (3,4,5) and ionic liquids (ILs); combination of the latter two media has also been proposed. Our attention in this field is devoted to the use of ILs which, because of their peculiar properties, have been quite recently proposed as novel and environmentally benign reaction media for several organic syntheses. Nalkylpyridium and N, W-dialkyl imidazolium cations coupled with a variety of inorganic anions have been, in fact, used as suitable solvents for several organic reactions as well as in catalytic processes: Friedel Craft reactions (6), Diels Alder cycloadditions (7), metal catalyzed hydrogenations (#), Mn-catalyzed asymmetric epoxidation (9) and bromination of double and triple bonds (10) are just a few examples in this quite new field. In the course of our studies we directed our attention toward the use of some hydrophilic and hydrophobic ILs in V(V)-catalyzed oxidations with peroxides.

cations

anions +

[bm im ]

+

[bmim ]

2

[BF -] 4

[CF3SO3]

[(CF S0 ) N-] 3

+

2

2

+

[bmim ][BF -]; [bmim ][CF S0 ] = hydrophilic 4

3

+

+

3

+

[bmim ][PF -]; [bm im ][PF -]; [bmim ][(CF S0 ) N] = hydrophobic 6

2

6

3

2

2

Figure 1. Selected hydrophilic and hydrophobic ionic liquids.


2

30 The hydrophilic and hydrophobic nomenclature is normally used (//) in order to identify ILs which either form a single phase with water or dissolve very small quantity of water, thus forming two-phase systems. In particular we considered l-methyl-3-butylimidazolium [bmim ], and l,2-dimethyl-3-butylimidazolium [bm im ] cations with tetrafluoborate, [ B F ] , hexafluorophosphate, [PF -], triflate, [CF S0 "], and bistrifluoromethane-sulfonimide anions, [(CF S0 ) N"]. Accordingly, the solvents indicated in Figure 1 were tested in the title reactions, i.e. [bmim ][BF ], [bmim ][PF ], [bm im ][PF "] [bmim ][CF S0 ] and [bmim ][(CF S0 ) N]. Our interest is, since long time, mainly focused toward the oxidative functionalization of double bonds in the presence of metal catalysis (12,13). In this respect, we proposed a simple and effective procedure for oxybromination of unsaturated substrates in a biphasic system H 0 / C H C 1 where the V-catalyst, oxidant, and bromide are dissolved in the aqueous phase, while the substrate is dissolved in the organic phase. With that procedure, from one side we mimicked the activity of V-dependent haloperoxidase enzymes (13% and from another we obtained an interesting synthetic method for functionalization of organic substrates in mild conditions using a sustainable oxidant, H 0 , and a sustainable source of positive bromine such as K B r . Furthermore, by-products are bromide and water, either reusable or non-polluting. More to the point, in the course of our studies, by combining reactivity analysis with spectroscopic techniques, mainly V - N M R , we have been able to identify monoperoxo vanadium complexes as competent oxidants of Br", while diperoxo vanadium species likely act as reservoir of the active oxidant. Without entering too much in details, a good deal of mechanistic studies carried out in these last years, offered us a detailed description of our system. In particular we observed that: (14) +

+

2

6

4

3

3

2

3

2

+

+

+

4

+

3


6

2

6

5

+

3

3

2

2

2

2

2

2

2

5 1

a. b. c. d. e. f.

selectivity is strongly dependent on the rate of stirring with other brominating systems only dibromide is obtained kinetic evidence indicates substrate coordination to vanadium more nucleophilic substrates favor formation of bromohydrin Hammett correlations from competitive experiments indicate two parallel processes V-bound hypobromite intermediate can be detected via ESI-MS (15)

On this basis our mechanistic proposal involves two intermediates: the first one, i.e a V-bound hypobromite ion, is responsible for the formation of bromohydrin, the second one is simply B r , whose reaction with double bonds produces selectively dibromo derivative. More recently, a modification of the two-phase procedure for oxybromination of double and triple bonds with M / H 0 / B r " (M=V(V) or Mo(VI)) by substitution of the chlorinated solvent with hydrophobic ILs has been presented (16,17), and evidence was offered that such variation gives rise 2

2

2


31 to even more sustainable processes characterized by higher rates and better selectivities.


Results and Discussion The oxybromination reactions were carried out by using the different hydrophilic and hydrophobic ILs in order to identify, first of all, which system, homogeneous versus a two-phase one, is superior. The two-phase option is dictated, as in the case of the chlorinated solvents (18), by the use of aqueous solutions of hydrogen peroxide as primary oxidant. The ILs indicated in Figure 1 were chosen on the basis of their availability and stability in an aqueousoxidative medium. Furthermore, in order to have ILs with appropriate purity we synthesized our solvents. In fact in several instances irreproducible results were obtained by using commercial ILs (16). Reactions carried out with styrene as model substrate and hydrophilic ILs ([bmim ][BF "]; [bmim ][CF S0 "]), at 25°C, were disappointing in terms of both yield and selectivity even though a shortening of the reaction time was observed. This approach was therefore abandoned (16). Thus, we concentrated our efforts in exploring the two-phase procedure employing [bmim ][PF ], [bm im ][PF ~], and [bmim ][(CF S0 ) N"]. Figure 2 reports the schematic representation of the reaction and of the substrates used, while Table I collects the most significant results obtained. Optimization of the reaction conditions has been already reported (16) and it was made by using styrene as model substrate and [bmim ][PF "] as typical IL. +

+

4

3

3

+

+

6

+

2

6

3

2

2

+

6

vo 3

Br ; H 0 2

2

H 0 / solvent

OH

+

2

T = 25°

Br

Figure 2. Oxybromination of alkenes in a two-phase system.

Data collected in Table I clearly indicate that substitution of the chlorinated solvent with hydrophobic ILs, regardless to the nature of the substituent present on the double bond, results in faster reactions and higher selectivity toward the


32 formation of bromohydrins. Very impressive is the case of the oxybromination of 1-octene, the less nucleophilic substrate used. In this case the selectivity A : B = 9:91 obtained in CH C1 is completely reversed both in [bmim ][PF ], and [bmim ][(CF S0 ) N], reaching a value of A : B = 87:13 rarely achievable with other reagents and in such mild conditions. +

2

2

6

+

3

2

2

Table I. V(V)-catalyzed Oxybromination of alkenes with Br7H 0 in a two-phase System H 0/solvent 2

2

a


2

Substrate

Solvent

A:B

V(V)

Time

Yield

moir'

hrs

%

0.02

2

81

43:57

styrene

CH C1

it

[bmim ][PF ]

0.02

2

>99

98: 2

99

94: 6

[bmim ][(CF S0 ) N]

0.01

6

92

97: 3

CH C1

0.01

6

76

9:91

0.02

4

74

8 7 : 13

[bmim ][(CF S0 ) N]

0.02

4

70

87: 13

CH CI

0.02

4

64

0.02

1

>99

42:58

0.01

1

81

61 :39

2

2

+

6

u

u 14

+

[bm im ][PF ] 2

u

6

t

3

1-octene

2

u

2

2

2

+

[bmim ][PF ] 6

u

+

3

/-stilbene

2

2

2

2

+

[bmim ][PF ] 6

+

[bmim ][(CF S0 ) N] 3

a

2

2

k

1

b

1

H 0/Solvent 1: 1 mL, T = 25°C, rpm 1000, Substrate 0.02 moll/ ; KBr 0.1 molL , H 0 0.02 molL . 2

1

2

2

b

Quantitave HPLC analysis was possible only for dibromide, likely both stereoisomers of bromohydrin were formed and overlapping of the peaks did not allow a precise determination of them, estimated amount of both bromohydrins is ca. 10%.

Explanation to the results here presented, provided that our reaction mechanism still holds in the ionic environment, can be found taking into consideration that transfer of molecular bromine in the IL phase, where the oxidation takes place, is disfavoured by the medium. In addition, on the basis of the results shown above for the oxybromination of double bonds, we have now further support to the proposal that the ionic environment produces both a higher concentration of the active species in the IL


33


phase (favouring the first equilibrium indicated in Figure 3) (16), and facilitates, because of a slower escape from the organized solvent cage, the reaction between the bromiranium species (formed upon reaction of the vanadium-bound hypobromite intermediate and the substrate (19)), and the vanadium coordinated water molecule, to form the bromohydrin. The net result is thus a more selective and faster formation of bromohydrins.

O 11/OH

O 11/OH (H,OV'

H

( 2°)

n

O—Br

R'

OBr w

IL

products

Figure 3. Essential steps for the V-catalysed oxybromination of double bonds in a two-phase system water/IL

In this respect the data obtained in the reaction with /raws-stilbene in molecular solvent, where both stereoisomers of the halohydrin were likely formed, suggest that in chlorinated solvent the more stable carbocation is formed, while, the formation of bromiranium is more favored in ionic liquids, even though the selectivity achieved (A:B) is lower than that observed with the other substrates. In view of these results, investigation of a similar reaction with phenylethyne, as model alkyne, appeared very interesting. However, since bromination of a triple bond is expected to be more difficult than that of a similarly substituted double bond, the more reactive Mo(VI) species was used as catalyst (17) even though also V(V) was tested. Here we report only the data obtained with vanadium. Being the process not previously reported, oxybromination reaction of phenylethyne was first carried out at room temperature in a two-phase system H 0 / C H C 1 . Figure 4 sketches the outcome of the reaction, while Table II collects the experimental results. A number of interesting aspects emerge. First, the reaction performed in water/[bmim ][PF "] 2

2

2

+

6


34 is faster than that in the halogenated solvent. Second, the selectivity is definitely shifted toward the dibromoketone F. This is a significant result, because ot,a-dibromoacetophenone is a key molecule, with antibacterial, fungicidal and algicidal properties. Moreover, it is a valuable intermediate for further transformations, for example, to ct-haloenolates or biologically active heterocyclic compounds. Br


Br Ar-^i V

f~\

_

°3Br;H Q 2

Br 2

D

c nt H 0 / solvent 2

T = 25°

•A-

Ar Br

Br F Figure 4. V-catalysed oxybromination of phenylacetylene in a two-phase system: Molecular solvent vs ILs

Table II. V(V)-catalyzed Oxybromination of phenylacetylene with B r 7 H 0 in a two-phase System H 0/solvent 2

2

a

2

Solvent

Sub. moll"

CH CI 2

2

+

[bmim ][PF -] 6

it

a

V(V) 1

moir

1

H 0

KBr moll"

2

1

moll"

0.01

0.01

0.05

0.01

0.02

0.02

0.1

0.02

0.02

0.02

0.1

Time Yield

2

0.02

1

b

C:D:E:F

hrs

%

22

30

14:43: 4:39

4

22

16:24:16:43

24

70

13:15:9:63

1

H 0/Solvent 1: 1 tnL, T = 25°C, rpm 1000; KBr 0.05 molL , H 0 0.01 m o l L added in two portions 2

2

2

l b

H 0 2

2

It is however important to underline that, in order to reach an interesting total yield of products of 70%, hydrogen peroxide needs to be added portionwise, so that its vanadium catalyzed decomposition can be kept under control. The pathways of phenylethyne oxybromination have been proposed (17) in analogy to that found in alkene oxybromination (15,16). That mechanistic


35 scheme well explains the experimental results. In fact, the disfavoured formation of 1,2-dibromostyrene on going from dichloromethane to [bmim ][PF '] can be ascribed to a slowed down - or even inhibited - formation of molecular bromine in the latter medium, where the functionalization of the substrate is likely to occur. Moreover, the internal structure of IL may help in keeping the reactive species within the organized solvent cage, thus rendering the reaction faster. Accordingly, also with phenylethyne the oxybromination reaction by H 0 / K B r was successful the main points being: catalysis by Mo(VI) is more effective than that by V ( V ) ; conversion of phenylethyne can be made synthetically interesting by adding H 0 in portions; and selectivity can be diverted from 1,2-dibromoalkene to the synthetically useful dibromoketone, changing the solvent from dichloromethane to ionic liquids. To finish this short survey regarding the vanadium catalyzed oxidation with hydrogen peroxide in ionic liquids is worthy of mention here some very preliminary, yet disappointing up to now, data obtained in the attempt to extend this approach also to the hydroxylation of benzene. Such a reaction has been very well studied (20) since the initial work of Mimoun (21) who published the synthesis and the reactivity of the peroxo vanadium picolinato complex VO(0 )pic, which reacts with benzene, in acetonitrile, producing good yields of phenol (up to 70% in stoichiometric conditions). At the same time, by using an appropriate phase transfer agent, namely 4-(3heptyl)-pyridine-2-carboxylic acid, we settled a two-phase procedure (22) where benzene and substituted benzenes are hydroxylated to monophenols with fair yields. Considering that inorganic salts as well as organic compounds are generally much more soluble in ionic liquids than in molecular solvents, we hoped that hydroxylation of aromatics could take place in the biphasic system H 0 / I L s where the aqueous phase contains V-catalyst and picolinic acid and benzene is dissolved in the IL phase, while slow addition of hydrogen peroxide would assure a better performance in terms of products vs peroxide decomposition. Unfortunately, very small amounts of phenol were detected both in [bmim ][PF "], and [bmim ][(CF S0 ) N]. Also attempts to use hydrophilic ILs, i.e. [bmim ][BF "] [bmim ][CF S0 "], thus obtaining homogeneous reaction mixtures, failed. These results may be due to the fact that hydroxylation reaction proceedes through a radical mechanism, at variance with the polar mechanism of the oxybromination of double and triple bonds. However, making the effort to understand why VO(0 )pic was unreactive in the presence of IL we have observed that vanadium peroxocomplexes in ILs change dramatically their spectral (UV-VIS and V - N M R ) behavior. Therefore we are currently carrying out speciation studies in order to understand which kind of interactions are taking place between vanadium species, both in the presence and in the absence of hydrogen peroxide, and the ionic components of +

6


2

2

2

2

2

2

+

+

6

3

+

2

2

+

4

3

3

2

51


36 ILs, with the ultimate aim to synthesize an ionic liquid with appropriate characteristics for the oxidative functionalization of benzene.


Acknowledgment Financial support from MIUR, Prin 2003 Project "Development of new recyclable catalysts for oxidation processes with hydrogen peroxide" is gratefully acknowledged. Work carried out in the frame of ESF COST D29 action: Sustainable/Green Chemistry and Chemical Technology; W G 0016-04 "Novel Sustainable Metal Catalyzed Oxidations with Hydrogen Peroxide and Molecular Oxygen". Experimental contribution by the undergraduate students A . Coletti and M . L . Naitana is acknowledged.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

14. 15. 16.

Horvath, I.T., Pure Appl. Chem., 1998, 31, 641. de Wolf, E.; Van Koten, G.; Deelman, B.-J. Chem. Soc. Rev., 1999, 28, 37. Jessop, P.G.; Ikariya, T.; Noyori, R. Chem. Rev., 1999, 99, 475. Leitner, W. Acc. Chem. Res., 2002, 35, 746. Oakes, R.S.; Clifford, A . A . ; Rayner, C . M . J. Chem. Soc. Perkin Trans. I, 2001, 917. Boon, J. A . ; Levisky, J. A . ; Pflug, J. L . ; Wilkes J. S. J. Org. Chem. 1986, 51, 480. Howarth, J.; Hanlon, K . ; Fayne, D.; M c Cormac P. Tetrahedron Lett. 1997, 38, 3097. Chauvin, Y . ; Mussmann, L . ; Oliver H . Angew. Chem. Int. Ed. 1995, 34, 2698. Song, C.E.; Roh E.J. Chem. Commun. 2000, 837. Chiappe, C.; Conte, V . ; Pieraccini D. Eur. J. Org. Chem. 2002, 2831. Ionic Liquids in Synthesis, Wasserscheid, P.; Welton, T. Wiley-VCH, Weinheim, 2003. Conte, V . ; D i Furia, F.; Moro, S. J. Phys. Org. Chem. 1996, 9, 329. Conte, V . ; D i Furia, F.; Moro, S. in Vanadium Compounds: Chemistry Biochemistry and Therapeuthic Applications, (Eds. D.C. Crans, A . Tracey), A C S Symposium Series 711, 1998, chap. 10 and refs. cited therein Bortolini, O.; Conte, V . J. Inorg. Biochem. 2005, 99, 1549, and refs. cited therein. Bortolini, O.; Conte, V . ; Carraro, M.; Moro, S. Eur. J. Inorg. Chem. 2003, 42. Conte, V . ; Floris, B.; Galloni, P.; Silvagni, A . Pure & Appl. Chem. 2005, 77, 1575.



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17. Conte, V.; Floris, B.; Galloni, P.; Silvagni, A . Adv. Syn. & Catal. 2005, 347, 1341. 18. Conte, V.; Di Furia, F.; Moro, S. Tet. Lett. 1994, 35, 7429. 19. O. Bortolini, C. Chiappe, V . Conte, M . Carraro. Eur. J. Org. Chem. 1999, 3237. 20. Bonchio, M.; Conte, V . ; Di Furia, F.; Modena, G.; Moro, S. J. Org. Chem. 1994, 59, 6262. 21. Mimoun, H.; Saussine, L.; Daire, E.; Postel, M.; Fisher, J.; Weiss, R. J. Am. Chem. Soc. 1983, 105, 3101. 22. Bianchi, M . ; Bonchio, M.; Conte, V . ; Coppa, F.; D i Furia, Modena, G.; Moro, S.; Standen, S. J. Molec. Catal. 1993, 83, 107.


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