Copper-catalyzed amination of alkenes and ketones by phenylhydroxylamine

Article abstract of DOI:10.1039/b004286m

The catalytic activity of a number of copper complexes and salts toward allylic amination of alkenes using phenylhydroxylamine as the nitrogen fragment donor has been investigated. The best catalyst is CuCl₂·2H₂O, which produces moderate yields of allylamines with high regioselectivity resulting from double bond transposition. A mechanism similar to that for the molybdenum and the FePc systems is proposed. The first step in the catalytic cycle is the formation of nitrosobenzene from the oxidation of phenylhydroxylamine by Cu(II). The next step is an ene reaction of the alkene with PhNO to produce an allylhydroxylamine, which is then reduced to the allylamine product by Cu(I), thus regenerating Cu(II). The same system can also transfer the nitrogen fragment to the α-carbon of cyclic ketones; this is accompanied by dehydrogenation in some cases to produce α-aminated, α,β-unsaturated ketones.

Full text of DOI:10.1039/b004286m

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Copper-catalyzed amination of alkenes and ketones by
phenylhydroxylamine
Chi-Ming Ho and Tai-Chu Lau*
Department of Biology and Chemistry, City University of Hong Kong, T at Chee Avenue,
Kowloon T ong, Hong Kong. E-mail: bhtclau=cityu.edu.hk
Received (in Montpellier, France) 29th May 2000, Accepted 16th August 2000
First published as an Advance Article on the web 9th October 2000
The catalytic activity of a number of copper complexes and salts toward allylic amination of alkenes using
phenylhydroxylamine as the nitrogen fragment donor has been investigated. The best catalyst is CuCl É 2H O,
2
2
which produces moderate yields of allylamines with high regioselectivity resulting from double bond transposition.
A mechanism similar to that for the molybdenum and the FePc systems is proposed. The Ðrst step in the catalytic
cycle is the formation of nitrosobenzene from the oxidation of phenylhydroxylamine by Cu(II). The next step is an
ene reaction of the alkene with PhNO to produce an allylhydroxylamine, which is then reduced to the allylamine
product by Cu(I), thus regenerating Cu(II). The same system can also transfer the nitrogen fragment to the a-carbon
of cyclic ketones; this is accompanied by dehydrogenation in some cases to produce a-aminated, a,b-unsaturated
ketones.
The search for simple and selective methods for the nitrog-
enation of hydrocarbons remains a challenge to chemists. A
number of important methods have been reported, including
metal-catalyzed aziridination of alkenes with PhI2NTs,1,2
stoichiometric aziridination of alkenes with manganese nitrido
species,3 and metal-catalyzed allylic amination of alkenes with
phenylhydroxylamine.4 The latter method is particularly
interesting due to the highly regioselective migration of the
CÈC double bond. The development of this method was
stimulated by earlier reports by Sharpless et al. and by Mares
and coworkers of stoichiometric allylic amination of alkenes
by L MoO(g2-RNO)5,6 and X(2NTs) (X \ S, Se).7 Subse-
Instrumentation
1H NMR spectra were measured in CDCl on a Varian
VNMR 300 spectrometer with TMS used as an internal stan-
3
dard. Infrared spectra were recorded on a Perkin Elmer
IR-1000 FT-IR spectrometer. Gas chromatographic analyses
were performed on a HewlettÈPackard 5890 gas chromato-
graph with an HP Ultra 2 capillary column (25 m ] 0.2 mm
i.d.). GC-MS measurements were carried out on a HP 5890
gas chromatograph interfaced to an HP 5970 mass selective
detector.
Catalytic allylic amination of alkenes
n
2
quently, Nicholas et al. and JÔrgensen and colleagues have
found that dioxomolybdenum(VI) complexes,8 iron
All reactions were carried out under argon. The alkene (7.0
mmol) and CuCl É 2H O (0.1 mmol) were added to a mixture
phthalocyanine9 and Fe(II,III) salts10 catalyze the allylic amin-
ation of alkenes with phenylhydroxylamine. Recently, it was
reported that allylic amination can also be carried out with
nitrobenzene as the N-donor under high CO pressures using
ruthenium catalysts.11 We report here that simple copper salts
can also catalyze the allylic amination of alkenes with
phenylhydroxylamine. Although copper(I,II) complexes and
salts are good catalysts for the aziridination of alkenes with
PhINTs,2a,b this is the Ðrst report on their use in catalytic
allylic amination by PhNHOH. Moreover, this system is also
able to perform catalytic amination of ketones, which has not
been reported for the iron and the molybdenum systems.
2
2
of 1,4-dioxane (9 ml) and dichloromethane (1 ml) and heated
to reÑux (ca. 100 ¡C). PhNHOH (1.0 mmol) dissolved in 1,4-
dioxane (9 ml) and dichloromethane (1 ml) was then slowly
added over a period of 30 min and the resulting mixture was
further reÑuxed for 2 h. The reaction mixture was analyzed by
GC and GC-MS using naphthalene as the internal standard.
The allylamine products were identiÐed by comparing with
authentic samples prepared and characterized according to
published methods.8h10
For competitive experiments a mixture of a-methylstyrene
(1.0 mmol) and para-substituted a-methylstyrene (1.0 mmol)
were used as substrates. The relative reactivities were deter-
mined using the equation: k /k \ [log(Y /Y )]/[log(X /X )],
Experimental
Materials
X
Y
f
i
f
i
where Y and Y are the initial and Ðnal concentrations of sub-
i
f
stituted a-methylstyrene, and X and X are the initial and
i
f
Ðnal concentrations of a-methylstyrene.
Solvents and alkenes were of reagent grade and were puriÐed
by standard methods.12 1,4-Dioxane was of HPLC grade
and was used as received. para-Substituted a-methylstyrenes
were prepared from the corresponding para-substituted
Catalytic a-amination of ketones
The procedure is similar to that for the allylic amination of
alkenes. The cyclic a-ketoenamine products were also isolated
by precipitating the copper complex using hexane, followed by
silica column chromatography using hexaneÈethyl acetate as
eluent. The products were characterized by 1H NMR, IR and
GC-MS. The spectral data of some of the cyclic a-
ketoenamines are given below.
acetophenone.13
Cu(salen),14
[Cu(MeCN) ](ClO ) ,15
4
4 2
[Cu(tren)(H O)](ClO ) ,16
[Cu(bpy) ](ClO ) ,17
o-
2
4 2
2
4 2
(hydroxyamino)biphenyl,18 PhNHOH19 and N-phenyl-N-
(1,1,2-trimethylprop-2-enyl)hydroxyamino9b were prepared
according to literature methods. All other chemicals were of
reagent grade and were used as received.
DOI: 10.1039/b004286m
New J. Chem., 2000, 24, 859È863
859
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2-(Phenylamino)cyclopent-2-enone. TLC: R 0.27 (hexaneÈ
nitrogen fragment donor, and a higher yield of 45% was
obtained. Apart from molybdenum and iron, this is the third
metal system that catalyzes this amination reaction with rea-
sonable yields. The yields for the copper system are compara-
ble to the molybdenum (42%)8 and the FeCl ] FeCl
f
EtOAc 10 : 1). IR (neat): 3327, 3056, 2922, 1693, 1631, 1598,
1537, 1502 cm~1. 1H NMR (CDCl , 300 MHz): d 7.30È6.95
3
(m, 5H), 6.75 (t, 1H, J \ 3.2 Hz), 6.24 (br s, 1H), 2.68È2.63 (m,
2H), 2.49È2.46 (m, 2H). EI-MS (m/z): 173 (100), 144 (95), 130
2
3
(20), 117 (20), 90 (21), 77 (27%).
systems (41%),10 but are lower than the FePc system (76%).9
Experiments were also done with other alkenes using
2-(Phenylamino)cyclohex-2-enone. TLC: R 0.50 (hexaneÈ
CuCl É 2H O as the catalyst and the results are shown in
f
2
2
EtOAc 8 : 1). IR (neat): 3364, 3053, 2926, 1674, 1630, 1597,
Table 2. Similar to the iron and the molybdenum systems,
higher yields were obtained with 1,1-disubstituted or trisubsti-
tuted alkenes; a single regioisomer was produced from double
bond transposition and small or zero yields were obtained if
product formation resulted in the lost of conjugation (e.g. no
allylamine product could be detected with b-methylstyrene).
The highest yield obtained was 72% (Table 2, entry 5).
1514, 1495 cm~1. 1H NMR (CDCl , 300 MHz): d 7.30È6.88
3
(m, 5H), 6.43 (t, 1H, J \ 4.8 Hz), 6.36 (br s, 1H), 2.58È2.54 (m,
2H), 2.48È2.42 (m, 2H), 2.06È1.98 (m, 2H). EI-MS (m/z): 187
(100), 158 (48), 130 (70), 104 (22), 93 (21), 77 (42%).
2-Methyl-2-(phenylamino)cyclohexanone. TLC:
R
0.5
f
(hexaneÈEtOAc 1 : 8). IR (neat): 3384, 3050, 2928, 2858, 1711,
Mechanistic considerations
1601, 1503, 1288 cm~1. 1H NMR (CDCl , 300 MHz): d 7.16È
3
6.52 (m, 5H), 4.25 (br s, 1H), 2.85È2.75 (m, 1H), 2.37È2.21 (m,
A proposed mechanism similar to that for the molybdenum
and the FePc systems is shown in Scheme 1. The Ðrst step in
the catalytic cycle is the formation of PhNO from the oxida-
tion of PhNHOH by Cu(II). The next step is an ene reaction of
the alkene with PhNO to produce an allylhydroxylamine,
which is then reduced to the allylamine product by Cu(I), thus
regenerating Cu(II).
1H), 2.08È1.75 (m, 6H), 1.41 (s, 3H). EI-MS (m/z): 203 (37), 175
(30), 160 (23), 146 (100), 134 (76), 118 (42), 93 (70), 77 (52%).
Results and discussion
Allylic amination of alkenes
Investigations of the catalytic activity of a number of copper(I)
and copper(II) complexes and salts were carried out by slow
addition (approx. 30 min) of a solution of PhNHOH (1.0
mmol) in dioxane (10 ml) to a reÑuxing solution of the copper
catalyst (0.1 mmol) and a-methylstyrene (7 mmol) in dioxane
(10 ml). After 2 h, analysis by GC and GC-MS reviewed the
formation of products shown by the following equation:
Formation of PhNO. When PhNHOH (1 mmol) was
allowed to react with 2 mol equiv. of CuCl É 2H O at room
2
2
temperature in dioxane for 3 h, PhNO (70%), PhNH (10%)
2
and azoxybenzene (10%) were detected by HPLC. PhNO is
most likely produced from the oxidation of PhNHOH by
Cu(II):
PhNHOH ] 2Cu2` ] PhNO ] 2Cu` ] 2H`
(2)
On the other hand, PhNH is probably formed from the
2
deoxygenation of PhNHOH by Cu(I) according to eqn. (3):
(1)
PhNHOH ] 2Cu` ] 2H` ] PhNH ] 2Cu2` ] H O (3)
2
2
As observed for the molybdenum and iron-catalyzed systems,
the formation of the allylamine product 1 was accompanied
by the side products 2 and 3. The allylamine product 1 was
also isolated and characterized by NMR. From the results
shown in Table 1, it can be seen that copper complexes con-
taining unidentate ligands are in general better catalysts than
those containing multidentate ligands. The best catalyst is
CuCl É 2H O, which gives a 32% yield of the desired product
Combining eqn. (2) and (3) gives eqn. (4), which represents
copper-catalyzed disproportionation of PhNHOH:
2PhNHOH ] PhNO ] PhNH ] H O
(4)
2
2
Although the disproportionation of PhNHOH can also
occur thermally,20 we found that it is much slower than the
copper-catalyzed reaction. The third product, azoxybenzene,
is believed to arise from the reaction of PhNH and PhNO.21
2
2
1. The yield is increased to 40% by using a dioxaneÈCH Cl
2
From the distribution of these three products, it can be esti-
2
2
(9 : 1) mixed solvent system. The use of other solvents such as
CH CN, THF or CH Cl , however, resulted in yields below
mated that 60% of PhNO arises from the oxidation pathway
[eqn. (2)], while the remaining 40% comes from dispro-
portionation.
3
2 2
20%. 4-Fluorophenylhydroxylamine was also used as the
To further test for the presence of PhNO in the catalytic
system, a catalytic amount of CuCl É 2H O (0.01 mmol) was
mixed with PhNHOH (0.1 mmol) and 2,3-dimethylbutadiene,
DMB (0.5 mmol), in dioxane at 75 ¡C for 3 h. DMB is known
to react rapidly with aromatic nitroso compounds such as
PhNO.22 Analysis of the reaction mixture by GC indicated
the formation of 21% of the hetero-DielsÈAlder adduct, 7% of
Table 1 Allylic amination of a-methylstyrene by phenylhydroxyla-
mine catalyzed by various copper complexesa
2
2
Yieldb(%)
Catalystc
1
2
3
CuCl
[Cu(MeCN) ](ClO )
6
14
9
46
42
35
45
40
38
41
50
42
50
11
2
9
4
2
the allylamine of DMB and 33% of PhNH [eqn. (5)]. Only a
4
4
2
Cu(ClO ) É 6H O
4 2
Cu(CH CO ) É H O
2 2
trace amount of the adduct was detected in the absence of the
2
2
3
2
Cu(OTf)
14
32
3
7
6
2
CuCl É 2H O
Cu(salen)
[Cu(tren)(H O)](ClO )
[Cu(bpy) ](ClO )
[Cu(phen)Cl ]
1
2
2
12
18
13
6
2
4 2
2
4 2
7
2
a See experimental section for conditions. b Yields are based on
phenylhydroxylamine and were determined by GC with naphthalene
as the internal standard. Products were also characterized by IR,
1H-NMR and GC-MS. c Abbreviations: salen \ N,N@-bis(salicyli-
dene)ethylenediamine
bpy \ 2,2@-bipyridine, phen \ 1,10-phenanthroline.
dianion,
tren \ tris(2-aminoethyl)amine,
Scheme 1 Proposed catalytic cycle for copper-catalyzed allylic amin-
ation of alkenes.
860
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Table 2 CuCl É 2H O catalyzed allylic amination of alkenes by PhNHOHa
2
2
Entry
Substrate
Product
Yieldb(%)
N-selectivityc(%)
1
40
52
36
31
15
70
12
2
3
4
5
6
22
22
9
72
6
7
8
9
2
9
5
13
21
39
a See
experimental
section
for
conditions.
b Yields
are
based
on
phenylhydroxylamine.
c N-selectivity \ allylamine/
(allylamine ] aniline ] azoxybenzene).
copper catalyst. This suggests that free PhNO is produced
substituted a-methylstyrenes were carried out. A plot of
from Cu(II) and PhNHOH.
log(k/k ) vs. the Hammett constant p gives a fairly linear
0
relationship with o \ [1.1 (Fig. 1). Such a small negative
value indicates that a small positive charge was developed at
the a-carbon in the transition state. This o value is compara-
ble to that of the thermal ene reaction of para-substituted 1-
arylcyclopentenes with diethyl oxomalonate (o \ [1.2),24 but
is very di†erent from that of iron-salt-catalyzed allylic amina-
tions (q \ [3.9).10b These results support free PhNO as the
active aminating agent in the copper-catalyzed reaction.
(5)
Ene reaction between PhNO and alkene. Alkenes are known
to react with PhNO to produce allylhydroxylamines.8b,9,23
The observed regioselectivity and relative alkene reactivity in
the copper-catalyzed reactions are consistent with this ene-
type reaction. The higher yield obtained when 4-
Ñuorophenylhydroxylamine replaces phenylhydroxylamine is
due to the formation of the stronger enophile F-PhNO. Since
this step takes place in the presence of the copper catalyst,
there is the possibility that the PhNO is coordinated to the
metal during the ene reaction. Strong evidence for “on-metalÏ
transfer is found for the iron-salt-catalyzed reactions, where
the active intermediate, an iron complex having a C-nitroso
dimer ligand, has been isolated and structurally character-
ized.10b This compound can aminate alkenes to produce ally-
lamine exclusively, even in the presence of DMB. In the case
of the copper-catalyzed system, a hetero-DielsÈAlder adduct is
produced when DMB is added, suggesting an “o†-metalÏ
mechanism involving free PhNO as the aminating agent.
In order to obtain more information on the nature of the
transition state, competitive kinetic studies with a set of para-
Fig. 1 Hammett plot for copper-catalyzed allylic amination of para-
substituted a-methylstyrenes.
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861

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Table 3 CuCl É 2H O catalyzed allylic amination of ketones by
Deoxygenation of hydroxylamines by Cu(I). When N-
2
2
PhNHOHa
phenyl-N-(1,1,2-trimethylprop-2-enyl)hydroxylamine (4a,
1
mmol) was treated with [Cu(MeCN) ]` (0.1 mmol) under
Substrates
Products
Yieldb(%)
N-selectivityc(%)
4
reÑuxing conditions in dioxane (10 ml), 2,3-dimethyl-3-
(phenylamino)but-1-pene (4b) was formed in 10% yield:
25
34
(6)
7
2
14
This shows that Cu(I) is capable of deoxygenating an allylhy-
droxyamine to give an allylamine. Although attempts to
isolate the allylhydroxylamine formed from nitrosobenzene
and a-methylstyrene were not successful, the reaction of a-
methylstyrene (1 mmol) and nitrosobenzene (5 mmol) in the
Ph NH
38
80
2
presence of [Cu(MeCN) ]` (0.1 mmol) produced the deoxy-
4
genated product 2-phenyl-3-(phenylamino)propene in 11%
yield. No allylamine product was observed in the absence of
Cu(I).
44
6
78
13
(7)
a See experimental section for conditions. b Yields are based on
phenylhydroxylamine.
aniline ] azoxybenzene).
c N-selectivity \ allylamine/(allylamine ]
Relative reactivities of the copper catalysts. Since this
mechanism requires the copper catalyst to shuttle between
Cu(II) and Cu(I), the rates and the yields are expected to be
very sensitive to the redox potential of the copper catalyst.
Copper catalysts with relatively high Cu(II)/Cu(I) redox poten-
tials are expected to be efficient in the oxidation of PhNHOH
to PhNO, the Ðrst step of the catalytic process. However, the
last step of the process, the deoxygenation of allylhydroxyl-
amine, should be rather sluggish with these catalysts. The
reverse situation should occur with catalysts of relatively low
Cu(II)/Cu(I) redox potentials. Presumably, among the various
copper catalysts, CuCl É 2H O has the most appropriate
as substrates. Acetophenone, which gives a higher equilibrium
concentration of the enol form than cyclohexanone but lacks
an allylic CÈH bond, also gives no product. Mares et al. have
communicated that the stoichiometric reaction of
Mo(O)L@L(g2-PhNO)
(L@ \ pyridine-2,6-dicarboxylato;
L \ hexamethylphosphoric triamide) with cyclohexanone pro-
duced a mixture of 2-(N-phenyl)iminocyclohexanone and azo-
benzene;6 however, the yield was not reported and apparently
no dehydrogenation occurred. Although the yields of the
copper-catalyzed reactions are not yet satisfactory, this is the
Ðrst report of a catalytic system for the amination of ketones
using phenylhydroxylamine.
2
2
redox potential in dioxane so that the Ðrst and the last steps
occur with reasonable efficiencies, thus giving the highest
yield.
Detection of aryl nitrene intermediate. In the Mo-catalyzed
reaction, aryl nitrene was found to be one of the intermediates
through the detection of carbazole when o-(hydroxyamino)
biphenyl was used as the aminating agent. The carbazole
A mechanism similar to that for the amination of alkenes is
proposed. The Ðrst step is the formation of PhNO from the
oxidation of PhNHOH by Cu(II) and the copper-catalyzed
disproportionation of PhNHOH [eqns. (2) and (4)]. The
PhNO then undergoes an ene reaction with the enol form of
the ketone:
product arises from intramolecular CÈH insertion of
a
biphenylnitrene.25 We have performed similar experiments; o-
(hydroxyamino)biphenyl (1mmol) in dioxane (10 ml) was
slowly added to a reÑuxing solution of CuCl É 2H O (0.1
2
2
mmol) in dioxane (10 ml) and the mixture was further reÑuxed
for 20 h. No carbazole was detected by GC, suggesting that a
free arylnitrene intermediate is rather unlikely in the copper-
catalyzed reaction.
(8)
a-Amination of ketones
5a is then converted to the ketone product by Cu(I):
The same catalytic system can also transfer the nitrogen frag-
ment to the a-carbon of cyclic ketones, accompanied by dehy-
drogenation in some cases to produce a-aminated, a,b-
unsaturated ketones. Thus, reaction of cyclohexanone with
PhNHOH in reÑuxing dioxane in the presence of a catalytic
amount of CuCl É 2H O produced 2-(phenylamino)-2-cyclo-
hexenone in 25% yield (Table 3). In the case of 2-
methylcyclohexanone, amination occurs at the tertiary CÈH
bond without dehydrogenation. However, cyclohex-2-enone
gave diphenylamine as the product. No products were
detected when linear ketones such as pentan-2-one were used
(9)
2
2
The involvement of free PhNO as the aminating agent in
the reaction was demonstrated by the addition of DMB to the
catalytic amination of cyclohexanone. It was found that the
yield of a-ketoenamine was decreased to 5% but 8% of
862
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3
(a) J. DuBois, J. Hong, E. M. Carreira and M. W. Day, J. Am.
Chem. Soc., 1996, 118, 915; (b) J. DuBois, C. S. Tomooka, J.
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T. Ando, M. Nishimura, I. Ryu and M. Komatsu, Angew. Chem.,
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Am. Chem. Soc., 1978, 100, 7061.
the hetero-DielsÈAlder adduct of DMB was produced. The
role of Cu(I) in the conversion of 5a to 5b is not clear, since
this process involves only an overall loss of an H O molecule.
2
However, no product was detected when PhNO was mixed
with cyclohexanone in the absence of a copper catalyst. On
the other hand, a-ketoenamine was readily detected in the
presence of a Cu(I) species, such as Cu(MeCN) `:
4
4
5
6
7
D. A. Muccigrosso, S. E. Jacobson, P. A. Apgar and F. Mares, J.
Am. Chem. Soc., 1978, 100, 7063.
(a) K. B. Sharpless and T. J. Hori, J. Org. Chem., 1976, 41, 176; (b)
K. B. Sharpless, T. Hori, K. Truesdale and C. O. Dietrich, J. Am.
Chem. Soc., 1976, 98, 269.
(a) R. S. Srivastava, Y. Ma, R. Pankayatselvan, W. Dinges and
K. M. Nicholas, J. Chem. Soc., Chem. Commun., 1992, 853; (b)
R. S. Srivastava and K. M. Nicholas, J. Org. Chem., 1994, 59,
5365.
(a) M. Johannsen and K. A. JÔrgensen, J. Org. Chem., 1994, 59,
214; (b) M. Johannsen and K. A. JÔrgensen, J. Org. Chem., 1995,
60, 5979.
(10)
This process, however, can also be catalyzed by Cu(II):
8
9
10 (a) R. S. Srivastava, M. A. Khan and K. M. Nicholas, J. Am.
Chem. Soc., 1996, 118, 3311; (b) R. S. Srivastava and K. M.
Nicholas, J. Am. Chem. Soc., 1997, 119, 3302.
(11)
Summary
11 S. Cenini, F. Ragaini, S. Tollari and D. Paone, J. Am. Chem. Soc.,
1996, 118, 11964.
In conclusion, CuCl É 2H O catalyzes the amination of
12 D. D. Perrin and W. L. F. Armarego, PuriÐcation of L aboratory
Chemicals, 3rd edn., Pergamon Press, Oxford, 1988.
13 A. I. Vogel, T extbook of Practical Organic Chemistry, 5th edn.,
Longman, New York, 1989, p. 495.
2
2
alkenes and ketones by phenylhydroxylamine. The catalyst
appears to act as a redox reagent, oxidizing the starting
hydroxylamine to the reactive nitrosobenzene and then
reducing the resulting allylhydroxylamine. The major factor
limiting the yields seems to be copper-catalyzed dispro-
portionation of the starting phenylhydroxylamine. The a-
amination of ketones, which is also accompanied by
dehydrogenation, appears to have a more complicated mecha-
nism, and this reaction will be further studied.
14 C. Root, Inorg. Chem., 1972, 11, 1489.
15 G. J. Kubas, Inorg. Synth., 1979, 19, 90.
16 P. C. Jain and E. C. Lingafelter, J. Am. Chem. Soc., 1967, 89,
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17 J. Foley, D. KenneÐck, D. Phelan, S. Tyagi and B. Hathaway, J.
Chem. Soc., Dalton T rans., 1983, 2333.
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19 A. I. Vogel, T extbook of Practical Organic Chemistry, 5th edn.,
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20 W. A. Waters, J. Chem. Soc., Perkin T rans. 2, 1976, 732.
21 J. W. Timberland and J. C. Stowell, in T he Chemistry of Hydrazo,
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1975, ch. 4.
Acknowledgements
Financial support from the Hong Kong Research Grants
Council (CityU 964/96P) and the City University of Hong
Kong (DAG 7100042) are gratefully acknowledged.
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