Molybdenum-Catalyzed Dehydrogenative Synthesis of Imines from Alcohols and Amines

Article abstract of DOI:10.1002/cctc.201800677

A molybdenum N-heterocyclic carbene catalyst has been developed for the synthesis of imines from primary alcohols and amines with the liberation of dihydrogen. The catalyst is generated in situ from molybdenum hexacarbonyl, 1,3-dicyclohexylimidazolium chloride and potassium tert-butoxide and is further stabilized by the phosphine ligand dppe. Imines are formed in moderate to good isolated yields and a variety of alcohols and amines can be employed in the reaction including anilines. The transformation constitutes the first example of a homogeneous molybdenum-catalyzed acceptorless dehydrogenative coupling with alcohols and is believed to proceed by formation of a cis-coordinated molybdenum bis-N-heterocyclic carbene complex, which performs an oxidative addition to the alcohol, β-hydride elimination and reductive elimination of dihydrogen.

Full text of DOI:10.1002/cctc.201800677

Full Papers
DOI: 10.1002/cctc.201800677
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Molybdenum-Catalyzed Dehydrogenative Synthesis of
Imines from Alcohols and Amines
Kobra Azizi[a] and Robert Madsen*[a]
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A
molybdenum N-heterocyclic carbene catalyst has been
anilines. The transformation constitutes the first example of a
homogeneous molybdenum-catalyzed acceptorless dehydro-
genative coupling with alcohols and is believed to proceed by
formation of a cis-coordinated molybdenum bis-N-heterocyclic
carbene complex, which performs an oxidative addition to the
alcohol, b-hydride elimination and reductive elimination of
dihydrogen.
developed for the synthesis of imines from primary alcohols and
amines with the liberation of dihydrogen. The catalyst is
generated in situ from molybdenum hexacarbonyl, 1,3-dicyclo-
hexylimidazolium chloride and potassium tert-butoxide and is
further stabilized by the phosphine ligand dppe. Imines are
formed in moderate to good isolated yields and a variety of
alcohols and amines can be employed in the reaction including
20 Introduction
variety of reactions ranging from olefin metathesis and allylic
alkylations to oxidation of different functional groups with
hydroperoxides.[9] Several low-valent molybdenum complexes
have been developed as catalysts for the hydrogenation of
alkenes, ketones, imines and nitriles with dihydrogen.[10]
Furthermore, molybdocene hydrides have been shown to
catalyze the transfer hydrogenation of ketones with alcohols[11]
while molybdenum sulfide clusters have been used for the
reductive amination between aldehydes and nitrobenzenes
under a dihydrogen atmosphere.[12] Based on these results, we
speculated that homogeneous molybdenum complexes would
be able to catalyze the acceptorless dehydrogenation of
alcohols.
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22 Since the turn of the century, acceptorless dehydrogenation of
23 alcohols has been developed as an atom-economical method
24 for the synthesis of imines, amides, esters, heterocycles and
25 various aldol condensation products.[1] In these reactions, the
26 alcohol is dehydrogenated to the corresponding carbonyl
27 compound, which then undergoes further transformations in
28 the same pot. The reactions do not require any stoichiometric
29 additives or oxidants and only releases dihydrogen (and water)
30 as the byproduct(s). The most commonly used catalysts have
31 been complexes based on the platinum-group metals ruthe-
32 nium and iridium.[1] Our group has developed the ruthenium N-
33 heterocyclic carbene complex [RuCl2IiPr(p-cymene)] for the
34 dehydrogenations and investigated the reaction mechanisms.[2]
35 We have also shown that the iridium complex IrCl(CO)(BINAP) is
36 able to perform a consecutive dehydrogenation and decarbon-
37 ylation reaction of primary alcohols.[3] However, recently
38 attention has shifted towards earth-abundant metals where a
39 number of complexes based on iron, cobalt and manganese
40 have been developed as catalysts for alcohol dehydrogen-
41 ations.[4] Furthermore, these transformations have also been
42 achieved with nickel and copper catalysts in several cases.[5]
43 Notably, homogeneous catalysts based on molybdenum have
44 to the best of our knowledge not been employed for the
45 acceptorless dehydrogenation of alcohols.[6]
Herein, we describe the development of a molybdenum-
catalyzed protocol for the dehydrogenative synthesis of imines
from alcohols and amines. Imines are key intermediates in
organic synthesis and the direct preparation from alcohols
constitutes an environmentally benign route from inexpensive
starting materials.[13]
Results and Discussion
The initial experiments were performed with benzyl alcohol
and cyclohexylamine as the substrates and focused on the low-
valent molybdenum complex Mo(CO)6 as the catalyst. Interest-
ingly, refluxing this mixture in mesitylene produced the
corresponding imine in 50% yield (Table 1, entry 1). Traces of
the secondary amine N-benzylcyclohexylamine was also ob-
served, but not further quantified. Unfortunately, no improve-
ments were observed by adding a phosphine (PCy3, PPh3,
dppe) or an amine (pyridine, DABCO, phenanthroline, 2,2’-
bipyridine) as a ligand for the reaction (results not shown).
Instead, several N-heterocyclic carbene ligands were investi-
gated based on the previous experience in the group.[2] The
carbenes were generated in situ from the corresponding
imidazol(in)ium salts by deprotonation with potassium tert-
butoxide. A total of 14 different salts were investigated
46
Molybdenum is an essential trace element for all living
relatively non-toxic metal.[7]
47 organisms and constitutes
a
48 Molybdenum is a rather inexpensive transition metal where the
49 major application is found in the production of steel.[8] In
50 homogeneous catalysis, molybdenum has been used for a
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[a] Dr. K. Azizi, Prof. Dr. R. Madsen
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Department of Chemistry
Technical University of Denmark
2800 Kgs. Lyngby, Denmark
E-mail: rm@kemi.dtu.dk
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.201800677
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H containing different substituents or counter ions (entries 4–
9). With methyl-substituted salts I and J traces of the secondary
amine was also detected (entries 10 and 11). Imidazolinium salts
K–N were not good carbene precursors since they all afforded
the secondary amine as a byproduct in addition to moderate
yields of the imine (entries 12–15). Therefore, imidazolium salt
A was selected for general use with potassium tert-butoxide as
the base (entry 16) which gave higher yields than by deproto-
nating with sodium hydride or sodium tert-butoxide (entries 17
and 18). The role of the base is only to generate the carbene
in situ and not to promote the imination. When the corre-
sponding carbenes of salts D and G were prepared separately
and included in the reaction in the absence of a base, the imine
was formed in 54 and 48% yield, respectively (entries 19 and
20).
Table 1. Optimizing molybdenum-catalyzed alcohol dehydrogenation.[a]
Entry
X
Ligand
Base
t [h]
Yield [%][b]
1
2
3
4
5
6
7
8
10
5
5
5
5
5
5
5
5
5
5
5
5
5
5
10
10
10
5
–
–
60
45
45
45
45
45
45
45
45
45
45
45
45
45
45
60
60
60
45
45
60
60
60
60
60
60
60
60
60
50[c]
62
57
48
43
44
47
46
10% A
10% B
10% C
10% D
10% E
10% F
10% G
10% H
10% I
10 % J
10% K
10% L
10% M
10% N
20% A
20% A
10% KOtBu
10% KOtBu
10% KOtBu
10% KOtBu
10% KOtBu
10% KOtBu
10% KOtBu
10% KOtBu
10% KOtBu
10% KOtBu
10% KOtBu
10% KOtBu
10% KOtBu
10% KOtBu
20% KOtBu
20% NaH
20% NaOtBu
-
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
9
55
10
11
12
13
14
15
16
17
18
19
20
21
22[e]
23
24
25
26
27
28
29
54[c]
37[c]
41[c]
38[c]
40[c]
57[c]
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Further optimization of the reaction was performed by
reinvestigating the influence of different phosphine and amine
ligands. No improvement was observed by adding 20% of
pyridine, DABCO, P(Cyp)3, PPh3, P(o-tolyl)3, P(tBu)3 or 10% of
BINAP or dppp (results not shown). However, with 20% of PCy3
the yield improved to 91% (entry 21) indicating that additional
stabilization by a phosphine ligand can be beneficial. Changing
the solvent to toluene or decreasing the amount of A gave
significantly less imine formation (entries 22 and 23). Further-
more, two equivalents of phosphine per equivalent of
molybdenum seems necessary since lower or higher amounts
of PCy3 also decreased the yield (entries 24 and 25). Therefore,
bidentate phosphine ligands were considered again and with
10% of dppe the yield increased to 96% (entry 26). The yield
was reduced when lower amounts of dppe, the carbene or
Mo(CO)6 were employed (entries 27–29). As a result, it was
decided to use 10% of Mo(CO)6, 20% of the carbene from
imidazolium salt A and 10% of dppe in refluxing mesitylene as
the optimum protocol for the molybdenum-catalyzed dehydro-
genative synthesis of imines. All the reactions after 60 h in
Table 1 showed full conversion of benzyl alcohol. For compar-
ison, imination reactions catalyzed by ruthenium, osmium,
cobalt and manganese catalysts have typically used a 0.2–5%
loading at 110–1408C for 18–60 h.[2d,14]
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68
54
48
91
40
64
78
67
96
77
84
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20% A
10% D[d]
5
10% G[d]
-
10
10
10
10
10
10
10
10
5
20% A+20% PCy3
20% A+20% PCy3
10% A+20% PCy3
20% A+10% PCy3
20% A+40% PCy3
20% A+10% dppe
20% A+5% dppe
10% A+10% dppe
10% A+5% dppe
20% KOtBu
20% KOtBu
10% KOtBu
20% KOtBu
20% KOtBu
20% KOtBu
20% KOtBu
10% KOtBu
10% KOtBu
[a] Reaction conditions: Benzyl alcohol (1 mmol), cyclohexylamine
˚
(1 mmol), Mo(CO)6, ligand, base, mesitylene (4 mL), 4 A MS (150 mg),
reflux. [b] GC yield. [c] Traces of N-benzylcyclohexylamine was also formed.
[d] As the free carbene. [e] Toluene was used instead of mesitylene.
35 containing different wingtip groups and counter ions (Figure 1).
36 Notably, with 1,3-dicyclohexylimidazolium chloride (A) the yield
37 increased to 62% with 5% of Mo(CO)6 and the reaction was
38 now completely selective for the imine with no concomitant
39 formation of the secondary amine (entry 2). A slightly lower
40 yield was obtained with the analogous benzimidazolium salt B
41 (entry 3) and the same was observed with imidazolium salts C–
The procedure was then subjected to a variety of primary
alcohols and amines to explore the substrate scope and
limitations of the transformation. Cyclohexylamine was first
reacted with different alcohols and the corresponding imines
were purified by flash chromatography (Table 2). This produced
the product from benzyl alcohol in 89% isolated yield (entry 1).
The influence of various groups in the para position of benzyl
alcohol were subsequently investigated where the p-methyl-
and p-methoxy-substituted substrates afforded 80% and 87%
yield, respectively (entries 2 and 3). p-Methylthiobenzyl alcohol
gave 70% yield of the imine while p-phenylbenzyl alcohol
furnished 62% yield (entries 4 and 5). In the latter case, small
amounts of the starting alcohol could still be observed after the
reaction. p-Nitrobenzyl alcohol afforded the product in 58%
yield after an extended reaction time (entry 6) and traces of
several byproducts could be observed due to competing
reduction of the nitro group. The four p-halobenzyl alcohols
gave 37–79% yield (entries 7–10) where no dehalogenation
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Figure 1. Imidazol(in)ium salts as ligand precursors.
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Table 2. Imine formation from different alcohols and cyclohexylamine.[a]
Table 2. continued
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Entry
Alcohol
Product
Yield
[%][b]
Entry
13
Alcohol
Product
Yield
[%][b]
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
75
80
1
89
80
14
2
[a] Reaction conditions: Alcohol (1 mmol), cyclohexylamine (1 mmol),
Mo(CO)6 (0.1 mmol), A (0.2 mmol), dppe (0.1 mmol), KOtBu (0.2 mmol),
mesitylene (4 mL), 4 A MS (150 mg), reflux, 60 h. [b] Isolated yield. [c]
Reaction time 72 h.
3
87
70
62
58
79
65
68
37
70
32
˚
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could be detected with the chloro and the bromo substrate.
However, with p-iodobenzyl alcohol the transformation was
accompanied by both dehalogenation, reduction of the imine
to the secondary amine and deoxygenation to p-iodotoluene
leading to a low yield of 37% (entry 10). The ortho-substituted
substrates o-methyl- and o-hydroxybenzyl alcohol produced
the corresponding imines in 70% and 32% yield, respectively
(entries 11 and 12). The phenolic moiety clearly impaired the
reaction since several byproducts were observed in addition to
unreacted starting alcohol. The two naphthalenemethanols
gave the products in 75% and 80% yield (entries 13 and 14)
and in both cases a slight amount of methylnaphthalene was
observed indicating that some deoxygenation of the alcohol is
also occurring in these cases. Aliphatic primary alcohols such as
hexan-1-ol and 2-phenylethanol were also subjected to the
reaction conditions, but in these cases no conversion of the
alcohol was observed.
5
6[c]
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Next, different amines were submitted to the reaction with
benzyl alcohol to study the influence of this substrate in the
transformation (Table 3). Benzylamine afforded the imine in
88% yield (entry 1) while the corresponding p-chloro com-
pound only gave 15% yield due to competing dehalogenation,
secondary amine formation and self-condensation into N-(p-
chlorobenzylidene) p-chlorbenzylamine (entry 2). Optically ac-
tive (R)-1-phenylethylamine and (R)-1-(1-naphthyl)ethylamine
furnished the products without any sign of racemization
although a longer reaction time was necessary in the latter case
(entries 3 and 4). The sterically hindered amines 1-adamantyl-
amine and benzhydrylamine afforded the imines in moderate
yields (entries 5 and 6) and traces of benzyl benzoate could be
observed as a byproduct in both cases. The reaction was also
applicable to aromatic amines where aniline and p-anisidine
gave 73% and 70% yield, respectively (entries 7 and 8). Full
conversion of benzyl alcohol was observed in all the reactions
in entries 1–8. With o-phenylenediamine and o-aminothiophe-
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1-phenylethanol gave 73% conversion into acetophenone after
60 h with no sign of any byproducts. When PhCD2OH was
reacted with BnNH2 (as in Table 3, entry 1), the product was a
mixture of PhCD=NBn and PhCH=NBn in a 1:4 ratio according
to 1H NMR. The result is not due to an exchange with the
solvent since the ratio was the same in both mesitylene and
mesitylene-d12. The scrambling in the benzylic position implies
that the dehydrogenation to the aldehyde is a reversible
reaction and that it involves a molybdenum dihydride species.
We have previously observed the same scrambling and
dihydride pathway in our mechanistic analysis of the ruthe-
nium-catalyzed alcohol dehydrogenation.[2c,d]
Table 3. Imine formation from benzyl alcohol and different amines.[a]
Entry
1
Amine
Product
Yield [%][b]
88
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
2
3
15
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The gas evolution was measured by connecting the Schlenk
tube from the reaction in Table 1, entry 26 to a burette filled
with water.[3a] Under these conditions, the transformation with
1 mmol of benzyl alcohol produced the imine in 67% GC yield
and 0.57 mmol (14 mL) of gas was collected indicating the
release of one equivalent of dihydrogen. Essentially the same
lower yield of the imine was obtained when the reaction was
performed in a closed vial, which illustrates the importance of
removing the gaseous co-product from the reaction with a
stream of nitrogen in order to obtain the yield in Table 1,
entry 26. The identity of the liberated gas was confirmed by
trapping the released substance in a sealed two-chamber
system.[3a,15] A Schlenk tube with the reaction mixture was
connected to another tube with diphenylacetylene and Pd/C in
methanol. After running the dehydrogenation for 36 h, a GC
sample from the second tube showed the formation of stilbene
as a 16:1 mixture of the Z and the E isomers. In addition, a
NMR sample was taken from the reaction that was run in a
closed vial. The sample was transferred to benzene-d6 in a
closed NMR tube which gave rise to a broad singlet a 4.47 ppm
in the NMR spectrum corresponding to H2.[16] This signal could
not be detected when NMR samples were taken from reactions
that were run under a flow of nitrogen.
4[c]
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5
6[c]
45
7
8
73
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A small trace of tert-butyl formate was detected in the
imination which is due to a reaction between coordinated
carbon monoxide and tert-butoxide.[17] No other byproducts
from the release of the CO ligands could be detected. A 13C
NMR spectrum of Mo(CO)6 in toluene-d8 showed a signal at
201 ppm. A small singlet from the CO ligands could also be
observed at 203 ppm when Mo(CO)6, A, dppe and KOtBu were
stirred in mesitylene for 2 h at 508C. However, no CO signal
could be detected upon further stirring this mixture at 1008C
for an additional 2 h. The same was observed when benzyl
alcohol and cyclohexylamine were also added to the mixture
indicating that a majority of the CO ligands are readily released
at the beginning of the transformation.
9
65
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[a] Reaction conditions: Benzyl alcohol (1 mmol), amine (1 mmol), Mo(CO)6
(0.1 mmol), A (0.2 mmol), dppe (0.1 mmol), KOtBu (0.2 mmol), mesitylene
˚
(4 mL), 4 A MS (150 mg), reflux, 60 h. [b] Isolated yield. [c] Reaction time
72 h.
47 nol additional dehydrogenation took place to furnish the
48 corresponding benzimidazole and benzthiazole in 65% and
49 54% yield, respectively (entries 9 and 10). o-Aminophenol, on
50 the other hand, was completely unreactive which again shows
51 the impeding effect of the phenol moiety. Even with one equiv.
52 of potassium tert-butoxide o-aminophenol only afforded a trace
53 amount of 2-phenyl benzoxazole.
No imine was formed in the absence of Mo(CO)6 under the
conditions in Table 1, entry 26. The presence of trace metal
impurities have been a serious concern in several metal-
catalyzed reactions or transformations presumed to be metal-
free.[18] Especially, some reactions believed to be catalyzed by
iron or manganese have turned out to be mediated by trace
amounts of another metal most likely copper or palladium.[19]
Therefore, it was decided to analyze Mo(CO)6 by inductively
coupled plasma mass spectrometry (ICP-MS) for traces of
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No benzaldehyde was detected by GCMS in any of the
55 experiments in Table 1 and 3. In the absence of an amine,
56 benzyl alcohol was converted into benzyl benzoate in 45% GC
57 yield with the optimized protocol. Under the same conditions,
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elements known to perform alcohol dehydrogenations. How-
ever, none of these metals could be detected beyond their
detection limit of typically 1 ppm. Consequently, we do not
believe that the findings in Table 1 – 3 are due to traces of
another element since alcohol dehydrogenations are not
known to occur with extremely low loadings of catalyst.
alcohols and liberate both hydrogen gas and form hydrido
molybdenum species.
Conclusions
The transformation is presumed to proceed through the
formation of a molybdenum bis-N-heterocyclic carbene inter-
mediate. Mo(CO)6 is known to react with two equivalents of N-
In summary, we have described the first homogeneous
molybdenum catalyst system for acceptorless dehydrogenation
of alcohols and applied the reaction to the synthesis of imines.
The catalyst is generated in situ from Mo(CO)6 with two
equivalents of 1,3-dicyclohexylimidazol-2-ylidene and one
equivalent of dppe. The catalytically active species is believed
to be a molybdenum(0) complex with cis coordination of the
two carbenes.
10 heterocyclic carbenes (or precursors) at about room temper-
11 ature to afford cis-Mo(CO)4(NHC)2 species as the thermodynami-
12 cally most stable products.[20] In fact, trans-Mo(CO)4(NHC)2
13 complexes will isomerize thermally to the corresponding cis
14 compounds[21] and Mo(CO)5(NHC) complexes will split upon
15 heating to cis-Mo(CO)4(NHC)2 and Mo(CO)6.[22] Likewise, further
16 displacement of CO ligands in these complexes with phos-
17 phines can occur at elevated temperature[20] making Mo(CO)2
18 (ICy)2dppe the most likely compound formed initially in the
Experimental Section
19 reaction. The coordination of water and alcohols to the related
General Information
20 molybdenum(0) complex Mo(CO)3(PCy3)2 has been studied
21 previously, and with water it has been debated whether an
22 oxidative addition occurs to form a hydrido-hydroxo com-
23 plex.[23] The oxidative addition is known to occur with thiols
24 where Mo(N2)2(dppe)2 was shown to react with RSH to form
25 MoH(SR)(dppe)2 complexes.[24] If this oxidative addition also
26 takes place with an alcohol (e.g. at elevated temperature) a
27 hydrido-alkoxo species 2 will be formed (Scheme 1). b-Hydride
28
29
30
31
32
33
34
35
36
All commercially available reagents were purchased from Sigma-
Aldrich or Stem Chemicals and were not further purified.
Mesitylene was stored over activated 4 A molecular sieves and
˚
degassed with N2 before being used. Gas chromatography was
performed on a Shimadzu GCMS-QP2010S instrument fitted with
an Equity 5, 30 mꢁ0.25 mmꢁ0.25 mm column. Flash column
chromatography separations were performed on silica gel 60 (40–
63 mm). NMR spectra were recorded on a Bruker Ascend 400
spectrometer. Chemical shifts were measured relative to the signals
of residual CHCl3 (dH =7.26 ppm) and CDCl3 (dC =77.16 ppm).
Analysis for trace metals by ICP-MS was performed by ALS
Scandinavia AB and had a detection limit of 0.5 ppm for Co, 1 ppm
for Pd, Pt, Ir, Rh, Ru, and Os, 4 ppm for Ni, 5 ppm for Mn, 10 ppm
for Cu and 20 ppm for Fe.
General Procedure
Mo(CO)6 (26.4 mg, 0.10 mmol), 1,3-dicyclohexylimidazolium
chloride (53.6 mg, 0.20 mmol), 1,2-bis(diphenylphosphino)ethane
(39.8 mg, 0.10 mmol), KOtBu (22.4 mg, 0.20 mmol) and pre-acti-
Scheme 1. Proposed mechanism.
37
˚
vated 4 A molecular sieves (150 mg) were placed in an oven-dried
38
tube (20 mL). The tube was placed in a Radley carousel on a
hotplate, subjected to vacuum and then filled with N2 (repeated 3
times). Freshly degassed mesitylene (4 mL) was injected into the
mixture, which was then heated to 1648C under a N2 atmosphere.
After 10 min at this temperature, the alcohol (1 mmol), the amine
(1 mmol) and tetradecane (0.5 mmol, internal standard) were added
and the reaction was refluxed for 60 h. The mixture was cooled
down and the solvent was removed under vacuum at 608C. The
residue was purified by silica gel column chromatography (98/2
hexane/Et3N) to obtain the desired product.
39
40 elimination from this intermediate will afford the carbonyl
41 compound and dihydrido complex 3. Reductive elimination of
42 dihydrogen from the latter will then regenerate the starting
43 complex 1 with an available coordination site. Dihydrogen is
44 known to be a weak ligand for molybdenum(0) complexes such
45 as Mo(CO)(dppe)2 and the coordination to molybdenum has
46 been studied in detail showing a very small difference between
47 h2-dihydrogen and dihydride bonding.[25]
48
The mechanistic proposal in Scheme 1 receives further
49 support from a reported stoichiometric reaction at room
50 temperature between benzyl alcohol and the complex Mo(N2)2
51 (dppe)2.[26] This transformation produced one equivalent of
52 benzene (due to aldehyde decarbonylation), one equivalent of
53 dihydrogen gas, and a total of one equivalent of molybdenum
54 complexes Mo(CO)(N2)(dppe)2, Mo(CO)2(dppe)2 and MoH4
55 (dppe)2 in a 5:4:2 ratio.[26] This experiment shows that
56 molybdenum(0) compounds are capable of dehydrogenating
57
Acknowledgements
We thank the Torkil Holm Foundation for financial support.
Conflict of Interest
The authors declare no conflict of interest.
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Keywords: alcohols
catalysis · imines · molybdenum
·
dehydrogenation
·
homogeneous
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Manuscript received: May 15, 2018
Version of record online: &&&, &&&&
ChemCatChem 2018, -53, 1–7
www.chemcatchem.org
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These are not the final page numbers!

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FULL PAPERS
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Dr. K. Azizi, Prof. Dr. R. Madsen*
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Molybdenum-Catalyzed Dehydro-
genative Synthesis of Imines from
Alcohols and Amines
Now with molybdenum: A homoge-
neous molybdenum catalyst has been
developed for the acceptorless dehy-
drogenative synthesis of imines from
alcohols and amines. The catalyst is
generated in situ from molybdenum
hexacarbonyl, 1,3-dicyclohexylimida-
zol-2-ylidene and 1,2-bis(diphenyl-
phosphino)ethane.
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