Catalytic oxidation of benzylic amines to imines by M(TPP)CL (M = Fe, Mn) followed by reduction to secondary amines

ELSEVIER

Inorganica Chimica Acta 247 (1996) 71-74

Catalytic oxidation of benzylic amines to imines by M (TPP)CL (M = Fe, Mn) followed by reduction to secondary amines S. Tollari, A. Fumagalli, F. Porta * Dipartimento di Chimica inorganica, Metallorganica e Analitica e Centro CNR, Universitlz di Milano, Via Venezian 21, 20133 Milan, Italy Received 23 June 1995; revised 18 September 1995

Abstract

The oxidation of benzylic amines ArCH2NH2 (3--10) to the corresponding imines ArCH=NCH2Ar (3a-10a) has been observed in catalytic systems containing ButOOH and metallo(IIl) tetraphenyl porphyrins (metallo-Fe, Mn) in the presence of 1-methyl imidazole as axial ligand. Secondary amines, (ArCH2)2NH (3b--10b) have been obtained by further reduction of the reaction products with NaBH4. Keywords: Catalysis;Oxidation;Amines; Ironcomplexes;Manganese complexes;Porphyrin complexes

1. I n t r o d u c t i o n

The ability of metallo porphyrins to oxidise saturated and unsaturated hydrocarbons catalytically is well known [ l ]. We recently reported that the catalytic system 1-methyl imidazole/ButOOH/metallo-porphyfins (M = Fe, Mn) oxidises primary aromatic amines to nitro compounds with high selectivity [ 2 ]. This work deals with the oxidation of benzylic amines (3-10) to the corresponding Schiff bases (3a-10a) by the same catalytic system. The oxidation reactions were followed by subsequent reduction by NaBI-L, without isolating the imines, thus obtaining corresponding secondary amines (3b-10b). In this way a one-pot procedure to transform primary amines into secondary amines was realised. The catalytic oxidation of benzylic amines has been already reported, to give nitriles [3] or Schiff bases [4]. In this work an improvement has been achieved in the synthesis of Schiff bases; high turnovers/hour were reached (up to 500/h).

2. R e s u l t s a n d d i s c u s s i o n

To complement our studies in the field of the catalytic oxidation of amines [ 2,5 ], the oxidation of benzylic amines was studied. Iron(III) and manganes©(III) tetraaryl porphyfins, M(P)CI (1, 2) in the presence of 1-methyl imidazole, as axial ligand, catalyse the oxidation of benzyl amines, * Corresponding author. 0020-1693/96/$15.00 O 1996 Elsevier Science S.A. All fights reserved SSD10020- 1693 ( 95 )0483 6- 1

ArCH2NH2 (3-10), by ButOOH to the corresponding imines ArCH = NCH2Ar (3a-10a) (Eq. ( 1 ) ): M(P)Cl (1, 2)

2ArCH2NH2 (3- IO)

ButOOH,L

ArCH=NCH2Ar + H20 + NH3

( 1)

(3a- lOa) 1 2

M(P) = Fem(tpp); M(P) =Mnm(tpp) (H2tpp=5,10,15,20-tetraphenylporphyrin);

3 4 5 6 7 8 9 10

Ar= C6H5; Ar=4Ci~; Ar = 2CI--CeH4; Ar= (2,4)CI2--C6H3; Ar = 2(MeO)-4?~H¢; Ar= 3(MeO)--C6H4; At= (3,4) (MeO)2-C6H3; Ar = 4CH3-C6H4; L = l-methyl imidazole;Bu'OOH (3 M in isooctanesolution); solvent~ CH2CI 2

The reaction, carried out in CH2C12at low temperature (ice bath), needs an appropriate amount of l-methyl imidazole as axial ligand (see later). Previous experience in the oxidation of aromatic amines [2] had already shown 1-methyl imidazole to be the best nitrogen base to compete with amines for the coordination to the metal atom in complexes I and 2. To ascertain the optimum amount of axial ligand to use, experiments were carded out with PhCH2NH2 (3) as substrate, and Fe(tpp)Cl (1) as catalyst. To obtain 3a, the best results were achieved with a l-methyl imidazole/metalioporphyrin molar ratio of 10:1 (see Fig. 1), which corresponds to the molar ratio of l-methyl imidazole:amine 3 of nearly

72

S. Tollari et al. / lnorganica Chimica Acta 247 (1996) 71-74

Blank experiments carried out on the reaction of 3 with 1 showed that, in the absence of Bu~)OH, no oxidation to 3a occurs. In the absence of the catalyst, the conversion of 3 is total after 60 min, but the yield of 3a is low (22.1%), indicating a poor selectivity in the absence of metallo-porphyrins or a different mechanism. In this case a black oily residue was found mixed with the reaction solution and no further attempts were carried out to characterise the unknown oxidation products. In the absence of the axial ligand 3 gave the expected product, but in lower yields (58%), suggesting that benzylamine can behave as an axial ligand, but does so less efficiently than imidazole. We have previously observed [2], that a strong electrondonating axial substituent ( 1-methyl imidazole) coordinated to the metal is essential to force the reaction pathway toward the desired products. In addition, it is known [7] that, small amounts of axial ligand are generally required to avoid the formation of poorly reactive bis-ligated M (P)L2 species (Eq.

100-

80

60.

40.

20-

0

=

i 0

0.03

0.05

0.10

= 0.15

=

""~

0.4

0.8

(4)):

U g a n d / Amine

Fig. 1. Yields of 3a from the catalytic oxidation of 3 by Fe(tpp)Cl and Mn(tpp)Cl (dotted line) in the presence of varying amounts of l-methyl imidazole. I and 2 ( 10 -2 mmol);3 ( 1 retool); ButOOH ( 1 mmol); solvent: CH2Cl2 ( l0 nil).

We used this imidazole/amine molar ratio of 0.1:1 for all further oxidations of the amines (4-10). The imine derivatives (3a-10a) were detected and quantified by GC-MS chromatography and their identities were established by comparison with authentic samples, prepared as described in the literature [5c,6]. The disappearance of 3-10 in the reaction medium was detected by GC-MS spectrometry and all the reactions were stopped immediately after total consumption of the benzyl amines in order to obtain the best secondary amine (3b-10b) yields (second step of the reaction). Further oxidation products (nitriles, amides) were detected only as traces even when conducting the reaction at - 80 °C or using lower amine/oxidant molar ratios (up to 0.2). The presence of NH3 in the catalytic oxidation reaction of 3 was detected by gas chromatography. These results suggest that the oxidation path may follow the steps (Eqs. (2) and (3)): 0.1:1.

H20 ArCH2NH2 --*ArCH=NH ~ ArCHO + NH3

(2)

ArCHO + ArCH2NH2 ~ ArCH--NCH2Ar + H20

(3)

In this hypothesis the role of the system ButOOH/metalloporphyrin is to catalyse the dehydrogenation reaction yielding 1 equiv, of imine and 1 equiv, of H20 for each reacted amine; thus water can hydrolyse the intermediate imine to aldehyde and ammonia as depicted in Eq. (2). The further condensation reaction between aldehyde and benzylamine produces the Sehiff base and another equivalent of H20 (water was detected by gas chromatography).

M(P) + L ~ M ( P ) L + L ~ M ( P ) L 2

(4)

(L = 1-methyl imidazole) Because the amine coordinates to the M(P) species, 1methyl imidazole will compete for the coordination with the amine (Eq. (5)): M(P) (amine) + L ~ M ( P ) L + a m i n e

(5)

Thus the equilibrium (F,q. (5)) is shifted far to the right only in the presence of a large excess of L. In our studies on the catalytic oxidation of primary aromatic amines to the corresponding nitro derivatives [2] the optimum l-methyl imidazole/metallo-porphyrinmolar ratio was found to be 30 (compared with 10 found in this case). If electronic effects only are considered, it might be expected that the more basic benzylic amines need a greater amount of axial ligand than the aromatic ones. Thus steric influences should also be considered in order to explain the shift of equilibrium (5) to the right. The relationships between the yields of 3a-10a and the reaction times was studied. The reaction time for the best yields was found to depend on the nature of the amines (310) (Table 1). We observed that the reaction time increases with substitution of the amine, however, it does not appear to be a straightforward relationship between the type and position of substituent with the observed reaction time. The catalytic system with Fe(tpp)Cl still proved to be efficient when l 0 3 tool equiv, of amine with respect to catalyst were used (Table 2). High turnovers/hour (up to 1969) were observed, but the best conditions (total conversion, 100% selectivity in Schiff base and 500 turnovers/hour) are obtained when the catalyst:amine:oxidant:axial iigand molar ratios are 1:500:500:10. The reaction of manganese (III) tetra-phenylporphyrin (2) with 3 was carried out under the same conditions as for the

S. Tollari et al. / Inorganica Chiraica Acta 247 (1996) 71-74 Table 1 Yields of ArCH=,NCHzAr (3a-10a) and ( ARCH, ) 2NH (3b-10b) Schiff bases

Yields (%)

t (min)

Amine

Yield (%)

3a 4a $a 6a 7a 8a 9a lOa

I00 I00 50 60 I00 83 72 54

60 120 120 150 150 120 120 120

3b 4b [7] b'b[7] 6"o[8] 7b[9a] 8b [9b] 9b [10] 10b [II]

95 90 91 89 92 96 86 96

Yields of Schiff bases (3a--10a) from the catalytic oxidation Of (3-10) by Fe (tpp) CI ( 1 ); ( 3-10 ( 1 retool); 1 ( I 0 - 2 mmoi); ! methyl imidazole ( 0.1 mrnol); ButOOH ( I nunol); solvent: CH2CI2 ( 10 ml), t= 1 h), and yields of the corresponding secondary amines (3b--10b) obtained by reduction. Table 2

Yields of C~IsCH=NCH2CsHs (3a), from the catalytic oxidation of 3 in the presence of varying amounts of Fe(tpp)Cl Cat./3/ButOOH/L

1:50:50:10 1:100:I00:10 1:200:200:10 1:500:500:10 1:1000:1000:10 1: 10000:10000:10

Conversion

Yield of 3a

Turnover

(%)

(%)

(h -~)

100 100 100 100 100 100

40.4 1130 99.2 ! 00 77.4 19.7

20 100 198 500 774 1969

3 (1 mmoi); ButOOH (1 mmoi); l-methyl imidazole (L) (0.2, 0.1, 5 × 10 -2, 2 × 1 0 -2 , 10 -2 and 10 -3 retool); Fe(tpp)Ci ( 2 × 1 0 -2, 10 -2 , 5 × 10- 3, 2 x 10- 3, I 0 - 3 and I 0 - 4 mmol ); solvent: CH2C12 ( i 0 ml), t = I h. Yields are for isolated 3a. Turnover= tool of 3 converted into 3a/mol of catalyst. Table 3 Yields of C~'IsCH-NCH2CeHs (3a), from the catalytic oxidation of 3 in the presence of varying amounts of Mn(tpp) CI Cat./3/ButOOH/L

1:50:50:10 1:100:!00:10 1:200:200:10 1:500:500:10 1:1000:1000:10

Conversion

Yield of 3a

Turnover

(%)

(%)

(h -~)

100 100 100 100 100

46.6 91.3 82.1 18.7 10.8

23 91 164 94 108

3 (1 mmol); ButOOH (1 mmoi); l-methyl imidazole (L) (0.2, 0.1, 5 × 10 -2. 2 × 10 -2, 10 -2 and 10 -3 mmel); Mn(tpp)CI ( 2 × 10 -2, 10 -2, 5 × 10- 3, 2 × 10 -3, 10- 3 and 10 -4 mmoi); solvent: CH2Ci2 ( 10 ml ). Turnover = tool of 3 converted into 3a/mol of catalyst.

iron (IlI) complex. A better reaction time ( 30 min ) and good yields (100%) were observed, but the efficiency of the catalyst is clearly lower (turnover/h= 164), than that of the ferric porphyrin complex 1 (Table 3). The reaction times listed in Table 1, represent the best conditions to obtain the maximum yield of the imine derivatives (3~-10a). After that, the imines (3a-10a) were converted quantitatively into the secondary amines

73

(ArCH2)2NH (3b--10b) without further work up and isolation of the rather unstable Schiff bases. Thus these reactions represent a fair method for the onepot transformation of benzylic amines to the corresponding secondary symmetric amines.

3. E x p e r i m e n t a l

Unless otherwise noted all the reactions were carried out under nitrogen. Solvents were dry and purified by standard methods. Fe(tpp)Cl (1) and Mn(tpp)Cl (2) were commercial products (97%(wt./wt.)), purchased by Aldrich and were used as received. Benzylamines (3-10) were distilled before use and stored under nitrogen. The solution of Bu~DOH (3 M in isooctane) was purchased by Fluka and was used as received. Naphthalene, used as internal standard, was high purity grade reagent (99%, Aldrich). The GC-MS chromatographic analyses were carried out on a Hewlett Packard 5890 gas chromatograph with an HP 5971 mass selective detector coupled with a HP Vectra QS/20 PC. The chromatographic analyses were carried out on a Perkin Elmer 8420 capillary gas chromatograph. Elemental analyses were carried out in the Analytical Laboratories of Milan University. IH and 13C NMR spectra were recorded on a Brucker VP 80.

3.1. General procedure for the oxidation of amines (3-10) to Schiff bases (3a-lOa) A 4 × 10 -3 mol 1-1 stock solution of 1 or 2 was prepared and stored under nitrogen, in the dark at 5 °C in a refrigerator. 2.5 ml (10 -2 mmol) of the stock solution of 1 or 2 were dissolved in 10 ml of CH2C12. To the obtained solution (brown for iron, green for manganese), 1-methyl imidazole (0.1 mmol) was added and the mixture then cooled to 0 °C (ice bath). After 5 min, amines (3-10) (1 mmol) and ButOOH (0.33 ml of a 3 mol I - l isooctane solution) were added. After 10 min of reaction, naphthalene (internal standard) was added. The reaction times range from 30-120 min depending on the amine (3-10) and on catalyst (1-2) (see Table 1). The products were quantified by GC-MS chromatography. Amine conversions: 100% by weight. The Schiff bases yields were calculated as mmol of 3a-10a/mmol of converted amines 3-10 (see Table 1 ).

3.2. General procedure for the reduction of Schiff bases (3a-lOa) to the corresponding secondary amines (3b-lOb) To the reaction solutions of the imine derivatives ( 3 a I0a), previously described, tetrabutylammonium hydrogen sulfate ( l % ( w t . / w t . ) ) and NaBH4 (2 equiv.) were added. The reactions were stirred at room temperature for 3 h. The separated organic solutions were washed with water (20 ml), dried (Na2SO4) and evaporated under reduced pressure. The residues were chromatographed using CH2CI2 on a silica gel

74

S. Tollari et el./lnorganica Chimica Acta 247 (1996) 71-74

column giving 3b--10b that have been quantified by weighing (see Table 1). Their characterisation was done by comparison of their physical, chemical and spectroscopic properties with those of the authentic compounds reported in the literature [7-11].

Acknowledgements We thank Progetto Strategico del CNR 'Tecnologie Chimiche Avanzate', for financial support.

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