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MgH2 as a reducing agent
Journal of Molecular Cafalysis, 54 (1989)
L23
L23 - L26
MgHz as a Reducing Agent Part II. Reduction of Organic Halides in Presence of Ni(0) and Pd(0) Complexes* C. CARFAGNA,
A. MUSCO, R. PONTELLINI
Istituto di Scienze Chimiche,
UniversiG degli Studi di Urbino, 61029
Urbino (Italy)
and G. TERZONI Enichem Synthesis S.p.a., 20097 S. Donato Milanese, Milan (Italy) (Received
March 20.1989;
accepted
June 16,1989)
In a previous paper, we have shown that MgH, prepared according to Bogdanovic et al. [2] reduces alkyl and aryl halides in the presence of transition metal halides [ 11. Deuterium labelling experiments have shown that the reduction of alkyl halides proceeds through a radical pathway with hydrogen abstraction from the solvent rather than from MgH,. Activation of LiAlH4 with transition metal halides produces similar results [ 31. We have now found that Ni(0) and Pd(0) tertiary phosphme complexes activate MgHz in the reduction of organic halides. We thought that the use of a soluble catalyst might have interesting implications such as suppression of the radical pathway and, consequently, hydrogen abstraction from MgH2**. Table 1 summarizes the results obtained in the hydrodehalogenation of aryl halides. From the experiments listed in Table 1, it results that reduction with MgHz followed by DzO quenching, produces mainly benzene-d, and minor quantities of benzenedll. This indicates that the reduction may be considered complete before the aqueous work-up. In contrast, reduction with MgD,+ and subsequent addition of Hz0 produces variable quantities of benzenedo and benzene-d, depending on both the aryl halide and the catalyst being used. Reduction of iodobenzene with MgHZin the presence of Pd(0) and then D20 quenching yields only benzenedo. Analogous reduction with MgDz followed by HZ0 addition yields 94% benzene-d1 tid 6% benzene-d* These results imply that the reduction of iodobenzene occurs mainly through abstracti’on of hydrogen from MgHz, probably in a step subsequent to the oxidative addition to Pd(0) (eqns. 1 - 3). *Previouswork, see [ 11. **Dehydrohalogenation of aryl halides by NaBHa actived by Ni(PPh& has been reported [4]. +MgDz was prepared according to [6] using LiAID4 (99%. C. Erba). MgH2 prepared according to this reference behaves as that prepared following [ 2 1. 0304-5102/89/$3.60
0 EIsevier Sequoia/Printed
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MgHz
Reducing agent
Pd(PPhsk Pd(PPhsk
Pd(PPh& Pd(PEtA Pd(PPhA Pd(PPhsk -
Pd(PPhsk Pd(PPhsk -
Pd(PPhsk Pd(PPhsk Ni(PPh& Ni(PPh& -
1.5 1 1
67 67 67
67
67
5 65 58
15 73 77 70 67
2 2 2 2 2
67 67 67 67 67 67
18 100 100 98 100 7 100 100
6 6 6 20 20
67 67 67 RT RT
5 80 65 100 100
Yieldsb (%)
2 2 2
6 18 20 20 20
67 RT RT RT RT
Ni( PPha)g Ni(PPh& NiClzd NiC1ze -
-
Reaction time (h)
Temp. (“C)
Catalyst
gl(do) Wdo)
1Wdo) lOWdo) lOWdo) 1Wdo)
6(do)
1Wdo)
75(do)
lOOtdo)
lOWdo)
Wdo)
lootdo) Wdo) 1Wdo) =(do)
7(dd Wdd
Wdd
‘Wdd
8(dd
Wdd
Wdd
2(dz)
2(&l
l(&)
2(h)
Deuterated products (%)
aReaction condition: organic halide/MgHz (or MgDa) 1:3 (mol); ML,/MgH2 (or MgD2) 1:50 (mol); THF, quenching with Ha0 unless otherwise stated. bThe yields are referred to the reduction product formed and are estimated by GLC with an appropriate internal standard. CQuenching with DaO. dNiCla/MgHa 1:lO (mol). eNiClz/MgDz 1:lO (mol). f Ck, tmns mixture, substrate/reducing agent 2 : 1 (mol).
fl-bromostyrenef
bromocyclohexane
iodobenzene
chlorobenzene
Substrate
Reduction of organic halides with magnesium hydridea
TABLE 1
L25
C6H51 + Pd(PPh,),, C6H,Pd(PPh& &H,Pd(PPh&H
-
C,HsPd(PPh&I
+ MgHz + 2PPhs -
+ 2PPhs
C6H,Pd(PPhs)H
(1)
+ MgHI
C&H6 + Pd(PPh&
(2) (3)
Bromobenzene behaves differently. Reduction with either MgHz or MgD,, in the presence of Pd(PPh&, followed by aqueous work-up as seen above for iodobenzene, produces benzene-d,,. We conclude that an aryl radical is formed which abstracts hydrogen from the reaction medium. Partially deuterated benzene (23% d,), is obtained in the reduction of bromobenzene with MgDz in the presence of Ni(PPh&, suggesting partial activation of the substrate through oxidative addition to Ni(0). It is worth noting that aryl iodides are more reactive than a&bromides in oxidative additions to electron-rich complexes [ 6 1. Chlorobenzene is quantitatively reduced to benzene in the presence of either NiC12 or Ni(PPh&. Deuterium labelling experiments are consistent with hydrogen abstraction from both MgH, and solvent, thus oxidative addition to Ni(0) [7 3 and a radical pathway may be considered simultaneously operative. Cyclohexyl bromide is quantitatively reduced by MgHz in the presence of PdL, complexes. Minor quantities of cyclohexene and benzene are also formed. As cyclohexaned, is obtained with both MgHz and MgD,, we argue that the reduction proceeds through abstraction of hydrogen from the reaction medium. Comparable results were obtained in the reduction of cis-4-bromo-t-butylcyclohexane with MgHz in the presence of CrCls [ 11. We lastly investigated the reduction of vinyl bromides. It is known that the reduction of cis- and truns-fl-bromostyrene with LiAlH4 occurs with loss of stereochemistry [8]. Conversely, we have found that reduction with MgHz in the presence of Pd(PPh& proceeds with retention of stereochemistry. /3-bromostyrene (cis, tram mixture) is reduced to styrene in good yields by MgHz in the presence of Pd(PPh&. As styrened, is obtained in 95% yield by running the reduction with MgD2, a radical pathway may be excluded. Moreover, the reaction of cis- and truns-bromostyrene with MgD, in the presence of Pd(PPh& yields cis- and trans-fldeuterostyrene respectively. Therefore the reduction proceeds through previous oxidative addition to Pd(O), which is known to occur with retention of configuration [9]. Our work suggests that the reduction of organic halides with the system MgH2/ML, may proceed by hydrogen abstraction either from MgHz or from the reaction medium. In the former case, the activation of the organic halide proceeds through oxidative addition to the metal(O) complex. In the latter case, radical intermediates may be formed through an electron transfer process from MgH2*. The role played by ML, complexes in the electron transfer remains unclear. *Electron LiAlH4 [lo].
transfer has been proposed
in the reduction
of aryl and alkyl halides with
L26
References 1 C. Carfagna, A. Musco, R. Pontellini and G. Terzoni, j. Mol. Cat& in press. 2 B. Bogdanivic, S. Liao, M. Schwickardi, P. Sikorsky and B. Spliethoff, Angew. Chem. Int. Ed. Engl., 19 (1980) 818. 3 J. 0. Osby, S. W. Heinzman and B, Ganem, J. Am. Chem. Sot., 108 (1986) 6’7. 4 S. T. Lin and J. A. Roth, J. 0~. Chem., 44 (1979) 309. 5 E. C. Aahby and A. B. Goel, J. Org. Chem., 42 (1977) 3480. 6 P. Fitton and E. A. Rick, J. Orgunometull. Chem., 28 (1971) 287. 7 M. Foil and L. Cassar, J. Chem. Sot., Dalton Trans., (1975) 2572. 3 S. K. Chung, J. Org. Chem., 45 (1980) 3616. 9 B, E. Mann, B. L. Shaw and N. J. Tucker, J. Chem. Sot. A, (1971) 2667; L. Cassar and A. Giarrusso, Gazz. Chim. Ital., 103 (1973) 793. 10 S. Chung and F. Chung, Tetrahedron Lett., (1979) 2473; E. C. Ashby, R. N. De Priest, A. B. Gael, B. Wenderoth and T. N. Pham, J. Org. Chem., 49 (1984) 3545.
L23
L23 - L26
MgHz as a Reducing Agent Part II. Reduction of Organic Halides in Presence of Ni(0) and Pd(0) Complexes* C. CARFAGNA,
A. MUSCO, R. PONTELLINI
Istituto di Scienze Chimiche,
UniversiG degli Studi di Urbino, 61029
Urbino (Italy)
and G. TERZONI Enichem Synthesis S.p.a., 20097 S. Donato Milanese, Milan (Italy) (Received
March 20.1989;
accepted
June 16,1989)
In a previous paper, we have shown that MgH, prepared according to Bogdanovic et al. [2] reduces alkyl and aryl halides in the presence of transition metal halides [ 11. Deuterium labelling experiments have shown that the reduction of alkyl halides proceeds through a radical pathway with hydrogen abstraction from the solvent rather than from MgH,. Activation of LiAlH4 with transition metal halides produces similar results [ 31. We have now found that Ni(0) and Pd(0) tertiary phosphme complexes activate MgHz in the reduction of organic halides. We thought that the use of a soluble catalyst might have interesting implications such as suppression of the radical pathway and, consequently, hydrogen abstraction from MgH2**. Table 1 summarizes the results obtained in the hydrodehalogenation of aryl halides. From the experiments listed in Table 1, it results that reduction with MgHz followed by DzO quenching, produces mainly benzene-d, and minor quantities of benzenedll. This indicates that the reduction may be considered complete before the aqueous work-up. In contrast, reduction with MgD,+ and subsequent addition of Hz0 produces variable quantities of benzenedo and benzene-d, depending on both the aryl halide and the catalyst being used. Reduction of iodobenzene with MgHZin the presence of Pd(0) and then D20 quenching yields only benzenedo. Analogous reduction with MgDz followed by HZ0 addition yields 94% benzene-d1 tid 6% benzene-d* These results imply that the reduction of iodobenzene occurs mainly through abstracti’on of hydrogen from MgHz, probably in a step subsequent to the oxidative addition to Pd(0) (eqns. 1 - 3). *Previouswork, see [ 11. **Dehydrohalogenation of aryl halides by NaBHa actived by Ni(PPh& has been reported [4]. +MgDz was prepared according to [6] using LiAID4 (99%. C. Erba). MgH2 prepared according to this reference behaves as that prepared following [ 2 1. 0304-5102/89/$3.60
0 EIsevier Sequoia/Printed
in The Netherlands
MgHz
Reducing agent
Pd(PPhsk Pd(PPhsk
Pd(PPh& Pd(PEtA Pd(PPhA Pd(PPhsk -
Pd(PPhsk Pd(PPhsk -
Pd(PPhsk Pd(PPhsk Ni(PPh& Ni(PPh& -
1.5 1 1
67 67 67
67
67
5 65 58
15 73 77 70 67
2 2 2 2 2
67 67 67 67 67 67
18 100 100 98 100 7 100 100
6 6 6 20 20
67 67 67 RT RT
5 80 65 100 100
Yieldsb (%)
2 2 2
6 18 20 20 20
67 RT RT RT RT
Ni( PPha)g Ni(PPh& NiClzd NiC1ze -
-
Reaction time (h)
Temp. (“C)
Catalyst
gl(do) Wdo)
1Wdo) lOWdo) lOWdo) 1Wdo)
6(do)
1Wdo)
75(do)
lOOtdo)
lOWdo)
Wdo)
lootdo) Wdo) 1Wdo) =(do)
7(dd Wdd
Wdd
‘Wdd
8(dd
Wdd
Wdd
2(dz)
2(&l
l(&)
2(h)
Deuterated products (%)
aReaction condition: organic halide/MgHz (or MgDa) 1:3 (mol); ML,/MgH2 (or MgD2) 1:50 (mol); THF, quenching with Ha0 unless otherwise stated. bThe yields are referred to the reduction product formed and are estimated by GLC with an appropriate internal standard. CQuenching with DaO. dNiCla/MgHa 1:lO (mol). eNiClz/MgDz 1:lO (mol). f Ck, tmns mixture, substrate/reducing agent 2 : 1 (mol).
fl-bromostyrenef
bromocyclohexane
iodobenzene
chlorobenzene
Substrate
Reduction of organic halides with magnesium hydridea
TABLE 1
L25
C6H51 + Pd(PPh,),, C6H,Pd(PPh& &H,Pd(PPh&H
-
C,HsPd(PPh&I
+ MgHz + 2PPhs -
+ 2PPhs
C6H,Pd(PPhs)H
(1)
+ MgHI
C&H6 + Pd(PPh&
(2) (3)
Bromobenzene behaves differently. Reduction with either MgHz or MgD,, in the presence of Pd(PPh&, followed by aqueous work-up as seen above for iodobenzene, produces benzene-d,,. We conclude that an aryl radical is formed which abstracts hydrogen from the reaction medium. Partially deuterated benzene (23% d,), is obtained in the reduction of bromobenzene with MgDz in the presence of Ni(PPh&, suggesting partial activation of the substrate through oxidative addition to Ni(0). It is worth noting that aryl iodides are more reactive than a&bromides in oxidative additions to electron-rich complexes [ 6 1. Chlorobenzene is quantitatively reduced to benzene in the presence of either NiC12 or Ni(PPh&. Deuterium labelling experiments are consistent with hydrogen abstraction from both MgH, and solvent, thus oxidative addition to Ni(0) [7 3 and a radical pathway may be considered simultaneously operative. Cyclohexyl bromide is quantitatively reduced by MgHz in the presence of PdL, complexes. Minor quantities of cyclohexene and benzene are also formed. As cyclohexaned, is obtained with both MgHz and MgD,, we argue that the reduction proceeds through abstraction of hydrogen from the reaction medium. Comparable results were obtained in the reduction of cis-4-bromo-t-butylcyclohexane with MgHz in the presence of CrCls [ 11. We lastly investigated the reduction of vinyl bromides. It is known that the reduction of cis- and truns-fl-bromostyrene with LiAlH4 occurs with loss of stereochemistry [8]. Conversely, we have found that reduction with MgHz in the presence of Pd(PPh& proceeds with retention of stereochemistry. /3-bromostyrene (cis, tram mixture) is reduced to styrene in good yields by MgHz in the presence of Pd(PPh&. As styrened, is obtained in 95% yield by running the reduction with MgD2, a radical pathway may be excluded. Moreover, the reaction of cis- and truns-bromostyrene with MgD, in the presence of Pd(PPh& yields cis- and trans-fldeuterostyrene respectively. Therefore the reduction proceeds through previous oxidative addition to Pd(O), which is known to occur with retention of configuration [9]. Our work suggests that the reduction of organic halides with the system MgH2/ML, may proceed by hydrogen abstraction either from MgHz or from the reaction medium. In the former case, the activation of the organic halide proceeds through oxidative addition to the metal(O) complex. In the latter case, radical intermediates may be formed through an electron transfer process from MgH2*. The role played by ML, complexes in the electron transfer remains unclear. *Electron LiAlH4 [lo].
transfer has been proposed
in the reduction
of aryl and alkyl halides with
L26
References 1 C. Carfagna, A. Musco, R. Pontellini and G. Terzoni, j. Mol. Cat& in press. 2 B. Bogdanivic, S. Liao, M. Schwickardi, P. Sikorsky and B. Spliethoff, Angew. Chem. Int. Ed. Engl., 19 (1980) 818. 3 J. 0. Osby, S. W. Heinzman and B, Ganem, J. Am. Chem. Sot., 108 (1986) 6’7. 4 S. T. Lin and J. A. Roth, J. 0~. Chem., 44 (1979) 309. 5 E. C. Aahby and A. B. Goel, J. Org. Chem., 42 (1977) 3480. 6 P. Fitton and E. A. Rick, J. Orgunometull. Chem., 28 (1971) 287. 7 M. Foil and L. Cassar, J. Chem. Sot., Dalton Trans., (1975) 2572. 3 S. K. Chung, J. Org. Chem., 45 (1980) 3616. 9 B, E. Mann, B. L. Shaw and N. J. Tucker, J. Chem. Sot. A, (1971) 2667; L. Cassar and A. Giarrusso, Gazz. Chim. Ital., 103 (1973) 793. 10 S. Chung and F. Chung, Tetrahedron Lett., (1979) 2473; E. C. Ashby, R. N. De Priest, A. B. Gael, B. Wenderoth and T. N. Pham, J. Org. Chem., 49 (1984) 3545.