Chemistry and catalysis advances in organometallic chemistry and catalysis
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287 N N R' N N R ′ R ′ N N R ′ R ′ X V O R O X = C, R = R ′ = H, n = +1 (1) R'
X = C, R = SO 3 , R
′ = H, n = 0 (2) X = B, R = H, R ′ = Me, n = 0 (3)
NH N Me Me NH N Me Me NH N Me Me V O [BF 4 ] O Cl Cl N N N N N N V C HO O 5 N N N N N N B V H N N
N N B H O H
V O
O O O N NH
Cl
M C H N N N N N N n = 2, M = Fe (8) or Cu (9) n = 3, M = V (10) or Re (11) N N Re C [BF 4 ] PPh 3 Cl Cl 12 N N
N N H N N N N N N C SO 3 Cl n M M = Cu, n = 1, x = 0 (13) x M = Fe, n = 2, x = −1 (14) M = V, n = 3, x = 0 (15) N N
C N N N N O O O
O 3
N N Re C N N N N O Cl PPh
3 17 Cl O 3 S
Some scorpionate or pyrazole complexes (catalysts or catalyst precursors) mentioned in the text.
288 CARBON–SCORPIONATE COMPLEXES IN OXIDATION CATALYSIS N N
N N N C Fe HO N N N N N N C OH X X = [FeCl 4 ]Cl (18) X = Cl 2 (19) N N N N N N C Fe HO OH 2 Cl OH 2 2 (Cl) 4 N N N N N N C Fe HO N N N N N N C OH 20 21 2 [Fe 2 OCl
6 ](Cl)
2 · 4 H
2 O N N N N N N C Fe HO N N N N N N C OH N N N N N N C Cu OH Cl Cl 22 N N N N N N C Cu Cl Cl N N N N Cu C Cl Cl O S O O O S O O 23 N N Re PPh
3 N X N C Cl L O Ph H N N L = PPh 3 , X = Cl (25) L = Hpz, X = F (24) or Cl (26) Figure 22.2 (Continued) Re, V, Fe, Co or Cu catalyst aq. H
2 O 2 , MeCN, room temperature OH O
n n n n = 1,2 Scheme 22.1 Oxidation of cycloalkane to cycloalkanol and cycloalkanone catalyzed by tris(pyrazol-1-yl)methane Re, V, Fe, or Cu complexes. [CuCl
{SO 3 C(pz) 3 }] (13) (maximum TON of 43 and 32, respectively; maximum yield of circa 4%) or by the full-sandwich hydrotris(pyrazolyl)methane Cu(II) compound [Cu {HC(pz)
3 } 2 ]Cl 2 (maximum TON and yield of 18% and 2.5%, respectively), under the same experimental conditions [5c]. Moreover, 22 still exhibits quite a notorious activity in the absence of NCMe, a behavior favored by the good solubility of the catalyst in water. PEROXIDATIVE OXYGENATIONS OF ALKANES 289 TABLE 22.1 Peroxidative Oxidation of Cyclohexane to Cyclohexanol and Cyclohexanone Catalyzed by Scorpionate or Pyrazole V, Fe, or Cu Complexes a Catalyst Total b Yield, % Total b TON max References. [VO 2
3 C(pz)
3 }] (2) 19 117
5a [VO
2 {HB(3,5-Me 2 pz)
3 }] (3) 13 86
[VO 2 (3,5-Me 2 Hpz)
3 ][BF
4 ] (4) 24 113
5a [VOCl
2 {HOCH
2 C(pz)
3 }] (5) c 2
× 10 3 5b [VO {HB(pz)
3 }{H
2 B(pz)
2 }] (6) 13 75
[VCl 3 {HC(pz) 3 }] (10) 13 167
5c [VCl
3 {SO
3 C(pz)
3 }] (15) 10 121
5c [FeCl
2 {HC(pz)
3 }] (8) 13 690
5c Li[FeCl
2 {SO
3 C(pz)
3 }] (14) 25 600
5c [Fe
{HOCH 2 C(pz) 3 } 2 ]Cl 2 (19) 10 385
5d [Fe
{HOCH 2 C(pz) 3 } 2 ][FeCl {HOCH
2 C(pz)
3 }(H
2 O) 2 ] 2 (Cl) 4 (20) 14 328
5d [Fe
{HOCH 2 C(pz) 3 } 2 ] 2 [Fe 2 OCl
6 ](Cl)
2 ·4H
2 O (21) 14 317
5d [CuCl
2 {HOCH
2 C(pz)
3 }] (22) 23 81
Abbreviation: TON, turnover number. a Selected results: yields (moles of products per 100 mol of cyclohexane); TON (moles of product per moles of catalyst). Typical reaction conditions: oxidant = H
2 O 2 , n(catalyst)/n(C 6 H 12 ) = 5 ×10 −5 , 6 h, room temperature. b Moles of cyclohexanol + cyclohexanone per 100 mol of cyclohexane. c Oxidant = mCPBA. The C-scorpionate Re complexes 11 and 16, as well as the related pyrazole 24–26 complexes, are the less active of the studied compounds, with TON values up to 285 [5f]. While the tris(pyrazolyl)methanesulfonate compound [ReO
3 {SO
3 C(pz)
3 }] (16) is more active for the cyclohexane oxidation if performed in the presence of acid, the fluoro- dipyrazole complex [ReClF {N 2 C(O)Ph }(Hpz)
2 (PPh
3 )] (24) provides the best catalyst, among these rhenium compounds, for the oxidation of cyclohexane in the absence of acid. The fluoride ligand is particularly favorable as the analogous dichloro- complex [ReCl 2 {N
C(O)Ph }(Hpz)
2 (PPh
3 )] (26) exhibits circa half of the activity. This feature, which is also displayed by the supported catalysts (see Section 22.2.2), conceivably relates to the stronger electron-donor character of the F − ligand compared with Cl − [5f], thus promoting the oxidation of the complex to a higher metal oxidation state that is more favorable to the reaction. 22.2.2 In Supported Systems The oxo-vanadium(IV) complexes [VOCl 2 {HOCH
2 C(pz)
3 }] (5) and [VO(acac) 2 (Hpz)] (7), immobilized on a polydimethyl- siloxane (PDMS) membrane, act as supported catalysts for the cyclohexane oxidation (Scheme 22.2) using benzoyl peroxide (BPO), tert-butyl hydroperoxide, mCPBA, hydrogen peroxide, or the urea-hydrogen peroxide adduct as oxidants (TONs up to 620) [5b]. The best results were obtained with the less polar mCPBA or BPO on account of the hydrophobic character of the membrane that favors their sorption. Although the homogeneous reactions (Section 22.2.1) take place faster than the heterogeneous ones and a higher activity is found in the former case, the use of the PDMS-supported catalysts is promising, as it is based on cheap and easily obtained membrane and vanadium(IV) catalysts, involves a simple way to heterogenize the catalyst without its chemical modification, and allows an easy product separation [5b]. PDMS-supported VO-catalyst Room
temperature OH O + Scheme 22.2 Cyclohexane oxidation catalyzed by [VOCl 2 {HOCH
2 C(pz)
3 }] or [VO(acac) 2 (Hpz)] encapsulated in a polymeric PDMS- based membrane. 290 CARBON–SCORPIONATE COMPLEXES IN OXIDATION CATALYSIS 22.3 OXIDATION OF ALKANES BY MOLECULAR OXYGEN The use of molecular oxygen as an oxidant is particularly attractive from ecological and economic perspectives, as it is the best environmentally friendly oxidant and is cheap. It can be used with either liquid or supported systems. This section concerns the use of dioxygen as an oxidant without the assistance of any added peroxide reagent. The scorpionate vanadium complexes [VCl 3 {HC(pz) 3 }] (10) and [VCl 3 {SO
3 C(pz)
3 }] (15), which catalyze cyclohexane oxidation with H 2 O 2 (Section 22.2.1,), also operate with dioxygen under solvent-free conditions. Cyclohexane is oxidized to cyclohexanol (the main product) and cyclohexanone (13% conversion), with a high selectivity, typically at the O 2 pressure of 15 atm, at 140 ◦ C, 18 h reaction time [6]. The reaction is further promoted (to 15% conversion) by pyrazinecarboxylic acid. The reactions proceed via radical mechanisms with possible involvement of both C-centered and O-centered radicals. The pyrazole complexes [ReClF {N 2
}(Hpz) 2 (PPh 3 )] (24), [ReCl 2 {N
C(O)Ph }(Hpz)(PPh 3 )
] (25) and [ReCl 2 {N 2 C(O)Ph
}(Hpz) 2 (PPh 3 )] (26), which act as homogeneous catalysts for cyclohexane peroxidative (by H 2 O
) oxidation (Section 22.2.1), are inactive with O 2 [7]. However, when immobilized on 3-aminopropyl functionalized silica, they catalyze the cyclohexane oxidation with dioxygen to cyclohexanol and cyclohexanone (the main product) in the absence of any added solvent and additives and, under relatively mild conditions, up to 16% overall conversion toward the ketone and alcohol, with an overall selectivity of 95% at the O 2 pressure of 19 atm, at 150 ◦ C, 8 h reaction time (Scheme 22.3) [7]. The higher activity of the fluoro-complex [ReClF {N 2 C(O)Ph }(Hpz)
2 (PPh
3 )] (24), in comparison with the related chlorocomplexes, is in accord with the observed behavior for the homogeneous peroxidative oxidation of cyclohexane (Section 22.2.1), and possibly concerns the overall stronger electron-donor character of the fluoride ligand relative to chloride. Complex 24, supported on 3-aminopropyl functionalized silica, also catalyzes the oxidation of other cycloalkanes (e.g., cyclopentane, cycloheptane, and cyclooctane) to the corresponding cycloalkanols and cycloalkanones, with conversions and selectivities comparable to those for cyclohexane oxidation [7]. The use of radical traps supports the involvement of a free-radical mechanism via carbon- and oxygen-centered radicals. 22.4 CARBOXYLATION OF LIGHT ALKANES Although carboxylation reactions (which involve C–C bond formation) are not typical oxidation reactions, they are related to those we have discussed above in view of the possible use of common types of catalysts. Moreover, they are also of synthetic relevance, allowing to synthesize carboxylic acids (bearing one more carbon atom than the substrate) and their derivatives, which have several applications in our daily life [8a]. Scorpionate or pyrazole V(V) 1–4, 6, 10, and 15, and Re(III) 11, 12, 16, 17, 24–26 complexes have been used as catalysts for the carboxylation of gaseous alkanes via single-pot conversions [5a,f]. These syntheses of carboxylic acids are much simpler than those used in industry. For instance, in the case of the conversion of methane into acetic acid, the current industrial routes commonly involve three distinct stages and use more expensive catalysts and harder experimental conditions (e.g., the Mosanto and BP-Amoco Cativa processes of carbonylation of methanol, at the third stage, are based on Rh and Ir catalysts, respectively) [8b]. The alkane carboxylation reactions are typically undertaken in trifluoroacetic acid (TFA) at 80 ◦ C with a V or Re complex catalyst and peroxydisulfate as the oxidant, under a CO atmosphere, and lead to the corresponding carboxylic acids with one more carbon (Scheme 22.4 for the cases of methane and ethane). O 2
°C OH O + n n n n = 1 −4 3-aminopropyl functionalized silica Scheme 22.3 Oxidation of cycloalkane to cycloalkanol and cycloalkanone catalyzed by a Si-supported pyrazole Re complex. CH 4
CH 3 COOH Catalyst, K 2 S 2 O 8 , TFA, 80 °C C 2 H 6 C 2 H 5 COOH
CO Scheme 22.4 Carboxylation of methane and ethane catalyzed by scorpionate V or Re complexes. BAEYER–VILLIGER OXIDATION OF KETONES 291 TABLE 22.2 Carboxylation of Alkanes Catalyzed by Scorpionate or Pyrazole V or Re Complexes a Substrate CH 4 C 2 H 6 Catalyst pCO, atm MeCOOH
Yield, % EtCOOH
Yield, % MeCOOH
b Yield, %
Total Yield, %
Reference [VO
2 {SO
3 C(pz)
3 }] (2) 5 39
10 23 5a [VO 2 {HB(3,5-Me 2 pz)
3 }] (3) 5 16
11 25 5a [VO 2 {HC(pz) 3 }][BF
4 ] (1) 5 16
11 23 5a [VO 2 (3,5-Me 2 Hpz)
3 ][BF
4 ] (4) 5 25
11 17 5a [VO {HB(pz)
3 }{H
2 B(pz)
2 }] (6) 5 31
3 7 5a [VCl 3 {SO 3 C(pz)
3 }] (10) 5 37
2 8 5a [VCl 3 {HC(pz) 3 }] (15) 5 20
3 5 5a [ReO 3 {SO 3 C(pz)
3 }] c (16) 0 1 28 29 5f [ReOCl {SO
3 C(pz)
3 }(PPh
3 )]Cl (17) 0 0
18 5f [ReCl 2 {HC(pz)
3 }(PPh
3 )][BF
4 ] (12) 0 0
16 5f [ReCl 3 {HC(pz)
3 }] c (11) 0 0 5 5 5f [ReClF {N 2 C(O)Ph }(Hpz)
2 (PPh
3 )] (24) 0 5
46 5f [ReCl 2 {N 2 C(O)Ph }(Hpz)
2 (PPh
3 ) c (26) 0 1 24 25 5f [ReCl 2 {N 2 C(O)Ph
}(Hpz)(PPh 3 ) 2 ] c (25) 0 0 4 4 5f Abbreviation: TON, turnover number. a Selected results: product yields (moles of product per 100 mol of alkane). Typical reaction conditions for carboxylation: p(alkane) = 5 atm (1.02 and 1.53 mmol for CH 4 and C
2 H 6 , respectively), n(catalyst) = 0.020 mmol, 80 ◦ C, 20 h, K 2 S 2 O 8 (4.00 mmol), CF 3 COOH (7.5 ml for CH 4 and 5.5 mL for C 2 H 6 ). b Formed by oxidation of ethane, rather than carboxylation. c
2 H
) = 3 atm.
The dioxo-vanadium(V) [VO 2 {HC(pz) 3 }][BF
4 ] (1), [VO 2 {SO
3 C(pz)
3 }] (2), [VO 2 {HB(3,5-Me 2 pz)
3 }] (3), and [VO 2 (3,5-
Me 2 Hpz) 3 ][BF
4 ] (4); the oxo-vanadium(IV) [VO {HB(pz) 3
2 B(pz)
2 }] (6); and the non-oxo-vanadium(III or IV) [VCl 3 {HC(pz)
3 }] (10) and [VCl 3 {SO
3 C(pz)
3 }] (15) compounds, bearing scorpionate or pyrazole ligands, exhibit catalytic activity in the single-pot carboxylation of methane and ethane to the corresponding carboxylic acids (yields up to 40%, TONs up to 157, Table 22.2) under mild conditions [5a]. For the alkane carboxylation reaction, the dioxo-scorpionate-V(V) complexes are usually more active than the monooxo-discorpionate-V(IV) and the non-oxo catalysts [VCl 3 {HC(pz) 3 }] (10) and [VCl 3 {SO 3 C(pz)
3 }] (15) [5a]. In addition, the sulfonate derivative is water soluble, which is favorable toward application in a green system. Radical trap experiments suggest the involvement of radical mechanisms for the carboxylations [5a]. The rhenium tris(pyrazolyl)methane compounds [ReCl 3 {HC(pz)
3 }] (11), [ReCl 2 {HC(pz)
3 }(PPh
3 )][BF
4 ] (12), [ReO 3 {SO
3 C(pz)
3 }] (16), and [ReOCl{SO 3 C(pz)
3 }(PPh
3 )]Cl (17), and the Re pyrazole complexes [ReClF {N 2
}(Hpz) 2 (PPh 3 )] (24), [ReCl 2 {N 2 C(O)Ph }(Hpz)(PPh 3 )
] (25), and [ReCl 2 {N 2 C(O)Ph
}(Hpz) 2 (PPh 3 )] (26) also act as selective catalysts (or catalyst precursors) in the K 2 S 2 O 8 /TFA system at 80 ◦ C in the absence of CO, for the single-pot oxidation of ethane to give acetic acid (Scheme 22.4), in a yield up to circa 40% (Table 22.2). Propionic acid can also be formed but normally in a much lower yield [5f]. The formation of propionic acid is, however, promoted by using CO gas which, in addition, hampers the production of acetic acid, thus the selectivity being controlled by adjusting the pressure of this gas. The fluoro-dipyrazole complex [ReClF {N 2
}(Hpz) 2 (PPh 3 )] (24) provides the best catalyst for the oxidation of ethane to acetic acid. The presence of the fluoride ligand, as indicated above for the peroxidative oxidation reactions, is particularly favorable as the analogous dichloro-complex [ReCl 2 {N
C(O)Ph }(Hpz)
2 (PPh
3 )] (26) exhibits circa half of the activity [5f]. Download 11.05 Mb. Do'stlaringiz bilan baham: 
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