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Cobalt-Catalyzed Pyridine Syntheses from Alkynes and Nitriles.

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[32] F. F. Morehead, B. L. Crowder in F . H . Eisen, L. 7: Chadderton:
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Cobalt-Catalyzed Pyridine Syntheses from Alkynes and Nitriles
methods (23)
By Helmut Biinnemann[*]
Soluble organocobalt catalysts permit selective formation of substituted pyridines under
mild conditions in a single step: mono-, di-, and trisubstituted pyridines can be obtained
at will in high conversions and high yields from nitriles and alkynes. Polynuclear derivatives
such as bipyridines can also be prepared by this method.
1. Introduction
During the past 15 years, the use of homogeneous transition
metal catalysts in olefin chemistry has made numerous useful
hydrocarbons available from simple starting materials. The
relation between the structure of the catalyst and its mode
of action has, in many cases, been largely elucidated (cf. refs.
’1). In contrast, relatively few investigations have been made
on the metal-catalyzed syntheses of heterocycle^[^! This is
understandable since many metal catalysts are blocked when
hetero atoms such as nitrogen, oxygen, or sulfur are present
in the substrate.
Starting from the discovery that the perturbing influence
of polar groups on cobalt as the central atom of a homogeneous
catalyst is apparently very slightc4],heterofunctional substrates
were allowed to react in the presence of organocobalt catalysts.
Cyclotrimerization of alkynes to give benzene derivatives
[eq. (a)] was adopted as a “model reaction” for systematic
studies on the use of soluble cobalt catalysts in the synthesis
of heterocycles.
Ill
%
+
@
Transition metal
system
Many variants of this long-known reaction have been studied. It is a typical metal-catalyzed organic reaction, proceeding with a wide range of transition metal systems such as
carbonyls, metallocenes, and even simple salts (cf. refs. [5-81).
Cobalt was found to be one of the most effective catalyst
metals for the reaction shown in eq. (a).
The very first attempts to use cobalt for the synthesis of
heterocycles revealed that a C=C bond could be replaced
by a C = N bond in the standard cyclization procedure [eq.
(a)]: reaction of both alkynes and nitriles on cobalt catalysts
,R2
(b)
[*] Priv.-Doz. Dr. H. Bonnemann
Max-Planck-Institut fur Kohlenforschung
Postfach 01 13 25, D-4330 Miilheim/Ruhr (Germany)
Angew. Chem. Int. Ed. Engl. 17,505-515 (1978)
505
in homogeneous phase afforded a pyridine derivative of type
( 1 ) as the main catalytic product.
Delineation of the scope of this approach soon showed
that eq. (b) provides a new general catalytic pathway to pyridine derivatives[’, ‘I.
Before considering the preparative implications, let us first
examine the nature of the catalysts.
2. Survey of the Organocobalt Systems Employed
2.1. Organocobalt Complexes
A series of “ready-made” organocobalt complexes can be
used for the synthesis of pyridines.
Reaction usually proceeds smoothly on allylcobalt systems.
The preparation and properties of these species have been
reported elsewhere“ ‘I. A typical representative of this class
of catalysts is butadiene(5-methylheptadienyl)cobalt (2)r”I.
All the organic constituents of compound (2) are derived
from butadiene.
tion of hydrogen from (3a). (7) can also be prepared from
the n form (3b)[15].
P
O
C
O
D
C COD + O
2 H,
~
Diamagnetic cobalt([) complexes with a central unit (6)
stabilized by ene and diene systems are known in various
guises’’ 6 - 1‘’ and are readily accessible. Our previous experience has shown that all systems containing a central unit
of type (6) are effective catalysts for the pyridine synthesis,
regardless of the kind of stabilizing olefinic ligand present.
This series of “ready-made” cobalt catalysts for the pyridine
synthesis could be extended at will: the only basic requirement
is that the complex ligands should be completely or partly
displaceable from the metal by the nitrile or alkyne-the
active cobalt(]) species is liberated and catalysis occurs. Cobaltocene can also be used in this way[’’. ”I; an initial activating
step, eq. (d),gives a system containing the essential core structure (6)[”].
--.t
QCo
Co COD
-+
*H
The red crystalline complex (2) is formed on reduction
of a solution of cobalt(I1) chloride and butadiene in ethanol
at -30°C with NaBH4. Two butadiene units combine to
form a methyl-substituted C7 chain which is bonded to the
cobalt atom oia the n-allylic system and the terminal vinylic
group. Pure (2) is thermolabile and must be stored and
handled below - 30°C.
More suitable for practical applications in catalysis are
the 1,5-cyclooctadiene(cyclooctenyl)cobaltcomplexes (3 a )
and ( 3 b ) . These compounds may be stored at 20°C and
may be prepared by reduction of cobalt salts with triethylaluminum[’31,metalsr14]or electrochemically“ 51 in the presence
of 1,5-cyclooctadiene(COD).
-
11,
I
co
I
co
Similar observations have been reported for the phosphanemodified cobalt(lr1) complexes (8). Yamazaki and Wakatsuki
obtained pyridine derivatives with the aid of such systems
as early as 1973KZ3’while our work on phosphane-free systems
was in Drom-ess.
wR
R
The species (8) contains two acetylene molecules linked
together at one end and joined via a cobalt atom at the
other to form a five-membered ring. However, the Japanese
authors report the synthesis of (8) to be troublesome, and
subsequently they also adopted simple organocobalt complexes such as cobaltocene for use as catalystsi2’,23c1.
The CY form (3a) and the n isomer ( 3 b ) show the same
selectivity in pyridine synthesis.
An important role in the catalytic reactions shown in eq.
(b) is played by cyclopentadienylcobaIt(r) systems: the n-cyclopentadienyl group confers considerable thermal stability upon
the complex.
Synthesis of pyridines from alkynes and nitriles [eq. (e)]
can be carried out advantageously in a one-pot reaction;
the cobalt catalysts then being generated in situ[’~lo].
This class of compounds ( 4 ) containing a CpCo core
complex (6) is considered to include (7), which is obtained
according to eq. (c)by intramolecular ring closure and elimina-
Prior to the initiation of catalysis, a small amount of a
suitable cobalt salt is dissolved or suspended in the substrate
506
2.2. Catalysts Generated in Situ
Angew. Chem. Int. Ed. Engl. 17, 505-515 ( 1 9 7 8 )
mixture and a solvent is added if required. The next step
consists in activation with reducing agents, the anions being
removed from the cobalt and the vacant coordination sites
being occupied by substrate molecules. Metals of the 1st to
3rd Main Groups, and their hydrido or alkyl compounds,
are especially suitable as reducing agents. If complex formation
with the substrates is insufficient to stabilize the cobalt atom
in solution, additional “stabilizers” can be introduced before
the reduction step. Cyclic olefins (e.g. the chelating diolefin
1,5-cyclooctadiene) are particularly suitable as stabilizing
agents. In contrast to the situation with Ziegler catalysts, the
organometallic species serves merely as reducing agent. It
is not an integral component of the catalyst and thus of
no importance for the actual catalytic reaction. Examples
are shown in Table 1.
If catalysis [eq. (f)] is conducted in the presence of a permanent excess of nitrile and the stoichiometric amount of
alkyne is added at such a rate that the steady-state concentration of the C=C bond remains as low as possible, the heterocycles (14) can be obtained in yields exceeding 90 % without
significant formation of by-products (16). About 1 mol-%
of catalyst is first dissolved in pure nitrile to give a brown
solution without deposition of metal; these solutions are drawn
into a V4A autoclave. Acetylene is then added at 20 bar up
to a pressure of 11 bar“]. The autoclave is heated to 120130°C for 1 h, whereupon the pressure drops to ca. 9 bar.
The pressure is then raised to ca. 16 bar and the acetylene
consumed is continuously replaced. After ca. 2 h, the stoichiometric amount of acetylene has been consumed and the catalytic reaction [eq. (e)] is complete. This modified procedure
Table 1. Catalysts for pyridine synthesis generated in situ [eq. (e)]
No.
CoXJMetal
No.
CoXJMH
No.
CoXJMR
(90)
CoC12/Li [a]
CoCI2/Na [b]
CoCl21Mg [c]
Co(acac)3/Mg [c]
(lOa)
(lob)
(lor)
(IOd)
CoCI2/LiH
CoCI2/LiAIH4
CoCI2/NaBH4
CO(OAC)~/
(11 a )
(llb)
(11 c)
( I 1d)
CoC12/C4H9Li
COCI~IC~H~M~X
Co(a~ac)~/(C~H~)~Al
CO(~C~C)~/(C~H~)~AIOC~H~
(96)
19~)
(96)
[a] Reduction in the presence of the nitrile (12); the acetylene (13) is added subsequently. [b] Subsequent addition
of the nitrile ( I 2). [c] Activated with iodine.
The cobalt salts, e. g. CoC12,can be used in hydrated form.
Use of an inert atmosphere and additional stabilizing ligands
such as phosphanes, is unnecessary. The system (15) has
proved most valuable in the laboratory:
CoC1,-6 H 2 0 / 2 N a B H ,
+ Nitrile/Alkyne
readily affords 2-substituted pyridines (14) (Table 2). The
catalyst systems ( 3 ) , ( 5 ) , or ( I f ) produce 100 to 400mol
of ( 1 4 ) per mol of catalyst.
Table 2.2-Substituted pyridines ( 1 4 ) obtained according to eq. (e). [Catalyst
( I I ) or ( 5 ) : reaction temperature 120-130°C; almost complete conversion;
work-up by distillation.]
(15)
R
3. Catalytic Synthesis of &Substituted Pyridines
2-Substituted pyridines of type (14) are readily obtained
in solution from carbonitriles (12) and acetylene (13) [eq.
(e)] in the presence of “ready-made” organocobalt catalysts
(Section 2.1). Whereas the nitriles (12) undergo practically
exclusive conversion into the pyridine derivatives (14), if no
special precautions are taken“] the alkyne component always
undergoes some degree of conversion into benzene derivatives
(16). The ratio of heterocyclic to carbocyclic products (i. e.
( I 4 ) :(1 6 ) ) determines the preparative value of the synthesis
and can be directed in favor of (14) by using an excess of
the nitrile (12).
2 HGCH
-4
[*] Vollhardt’s generalization that co-cyclization of monoacetylenes with
nitriles leads to “complicated mixtures” [132] is not justified and is presumably
based on a misinterpretation of the data published by us [lo, 241 and
Wikatsuki [21, 231.
Angew. Chem. I n t . Ed. Engl. 1 7 , 505-515 (1978)
Yield
B.p.
[“Cltorr]
n6’
[%]
93
96
89
97
96
98
96
95
91
96
78
89
94
1291760
1501760
1881760
104117
111115
122113
10918
10511.5
12510.01
16617
50-60113
146116
147115
1.so05
1.4968
1.4892
1.4834
1.4837
1.4852
1.4830
1.4866
1.4810
1.4807
M’ (mle)
[a1
93
107
135
149
163
177
191
205
219
233
105
1.6182 155
1.5781 169
Product known
according to ref
[25-421
[25,31,32,36,43-461
[32, 36,47-491
[32, 501
~321
~321
151-531
[39,43,44, 54-64]
[39, 57, 65-68]
[29, 31, 69-72]
[a] Further IR and ‘H-NMR spectra are available for the major fraction.
As discussed for eq. (0, the product selectivity of the cobalt
catalyst with respect to (14) or (16) can be influenced by
a suitable choice of the nitrile/alkyne ratio. This result suggests
that the product selectivity in eq. (f) might be controlled
by further ligands complexed to the cobalt. Moreover, the
catalytic reaction could possibly show a control effect in
strongly polar solvents such as tetrahydrofuran, alcohols, or
amines which can coordinate to transition metals. Several
series of comparative studies show, however, that
~-
[*] Special safety precautions must be observed on working with acetylene
under high pressure ( > 3 bar).
501
1) The product selectivity is independent of the catalyst type;
compounds ( 2 ) to ( 4 ) and (8) to (11) were examined;
2) Phosphanes, arsanes, and Lewis acids have no effect“];
3) Strongly coordinating solvents such as cyclic ethers, chlorohydrocarbons, amines, or alcohols lower the rate of eq. (f)
but do not affect the product ratio (14) : (16).
These observations justify the conclusion that the selectivity
of the cobalt catalyst of eq. (f) is subject only to “substrate
control”.
In order to compare the turnover numbers (measured as
number of mols of pyridine per mol of cobalt and hour)
of soluble organocobalt catalysts we performed the standard
reaction of eq. (e) at 130°C and 13bar of acetylene under
the same conditions but with various catalysts. The results of
this comparative study are listed in Table 3.
Table 3. Efficacy of various catalyst types in reaction according to eq. (e)
[130°C/13 bar acetylene, R=phenyl]. As soon as a drastic drop in acetylene
uptake was observed catalysis was discontinued and the products worked
The only method so far available for selective synthesis
of 2-substituted pyridines, i. e. cobalt-catalyzed reaction of
nitriles with acetylene [eq. (e)], has also acquired industrial
importance. In this context, particular interest is attached
to 2-picoline (2-methylpyridine) that is used mainly for the
production of 2-vinylpyridine.
2-Vinylpyridine,which is also directly accessible from acetylene and acrylonitrile according to Table 2, is used as a terpolymeric grip promoter in the tire industry. Moreover, it is also
employed as comonomer in the polymerization of acrylonitrile
in order to improve the dyeing properties of acrylic fibers.
UP.
Catalyst type
Conversion of
RCN [ %]
mol ( 1 4 )
mol C 0 . h
Co(OAc)z/Co(acac)2/
CO(HCO~)~/N~BH~
Co(aca&/Et3Al
Co(acac)2/Et2A10Et
CoC12- 6 H 2 0 / 2NaBH4
CoCI,/Li
CpCo(C0D)
C8H13Co(COD)
CaHi sCOC4H6
Cp(R4C4Co)PPh3,R = phenyl
CPZCO
CPCO(CO)Z[731
93
595
96
94
94
95
97
94
92
90
92
40
621
640
590
595
618
4. Catalytic Synthesis of Lutidines
Reaction according to eq. (e) can also be conducted as
a co-cyclotrimerization by using two different alkynes [eq.
(h)]. For instance, simultaneous reaction of acetylene, propyne,
and acetonitrile in presence of catalyst ( 5 ) affords a mixture
of dimethylpyridines (lutidines) (18) together with 2-methylpyridine and isomeric trimethylpyridines.
580
400
120
CH3CGN
12
+
CH3CSCH
+
HCSCH
2
Table 3 also clearly reveals the inhibitory action of carbonyl
and phosphane groups on the catalysis [eq. (e)]. The best
catalysts are systems reduced in situ as well as compounds
containing the cyclopentadienylcobalt group.
Table 4. Lutidines (18) and ethylmethylpyridines obtained according to eq. (h) [catalyst (5)].
Products
Isomeric
distribution
Yield
[%] [b]
B. p.
[.C/torr]
nD
M + (m/4
Products known
according to ref.
(180)
(186)
(18~)
65
28
7
33
1431770
159/760
1601760
1.4953”
1.4984’’
1.505720
107
[31-33, 76781
[31-33,45,46, 76, 79-82]
[32, 33, 45, 46, 80-831
2-Ethyl-6-methylpyridine
2-Ethyl-4-methylpyridine
65
35
31
1601760
1731748
1.4941l6
121
[45, 46, 80, 82, 841
[45, 82, 85, 861
r %I [a1
~~
[a] According to gas chromatography. [b] Identified by ‘H-NMR spectroscopy, separated by gas chromatography.
Prompted by a preliminary report[741of our new catalytic
synthesis of 2-substituted pyridines, Botteghi applied the
technique to the catalytic synthesis of analogous optically
active derivatives[75! Thus optically pure sec-butyl cyanide,
which is readily accessible by classical methods, on cyclization
with acetylene above 7 bar at 140°C with catalyst ( 5 ) affords
the corresponding optically active 2-sec-butylpyridine (1 7 )
in over 90 % yield. For example, (S)-(+)-(17) was obtained
in 96 % enantiomeric excess.
[‘I Addition of more than two molecules of phosphane, arsane, and CO
or isocyanide per cobalt atom merely blocks the catalyst.
508
However, the catalytic reaction does not yield lutidines
selectively: Table 4 shows that the yields of lutidines formed
catalytically according to eq. (h)correspond to the statistically
expected distribution.
5. Catalytic Synthesis of 2,3,6- and 2,4,6-Trisubstituted
Pyridines
A wide field of application of the one-step catalytic synthesis
is found in the field of trisubstituted pyridines of types (19)
and f2’). They can be prepared, with hi@
numbers, according to eq. (i) at ca. 130°C using cobalt comAngew. Chem. I n t . Ed. Engl. 17, 505-515 (1978)
6. Catalytic Synthesis of Bipyridine
plexes of type ( 4 ) or catalysts generated in situ, such as
( 9 ) , ( l o ) , (II), or ( 1 5 ) .
RI-C-N
+
2 R~-C=CH
- A RaRI
Polynuclear pyridine derivatives can also be synthesized
in high yields using cobalt catalysts[24! Starting from the
readily available pyridinecarbonitriles (21 ), reaction with terminal alkynes leads to the bipyridines (22) and (23) [eq.
[COI
(i)
+
R2
(12)
N
R'
R
(ill.
(20)
(19)
Table 5. Trisubstituted pyridines ( 1 9 ) and (20) obtained according to eq. (i) [catalyst ( 4 ) , ( 9 ) - ( 1 1 ) ,
by distillation].
R'
R2
M.p.
["CI
Cdl
Isomeric ratio
[a1
(19)
(20)
Separation
or identification
Yield
61
39
GC
71
174-1 79/760
69
80
75
31
20
25
58
62
84
8496110170-1 90/10-4
69-7311 3
65
60
71
68
77
35
40
29
32
23
GC
' H-NMR
GC
IH-NMR
'H-NMR
GC
'H-NMR
GC
GC
55
180-2 lOj10 4
4741/10-4
81/12
58-95110-4
[ %] [b]
B.p.
["C/torr]
[CI
Products known according to ref.
M C (m/e)
121
56
233
245
135
(19)
(20)
[45, 46, 80-82,
88-93]
[45,46, 80-82, 841
[94, 951
1961
259
~
62
85
54
51
or ( 1 5 ) : reaction temperature ca. 130°C; work-up
138
133
183
307
[97-991
[68,87,100, 1011
[a] On 'H-NMR spectroscopic analysis, the isomeric ratio was determined from the intensity ratios of the signals characteristic for each of the structures.
[b] Based on reacted alkyne, not optimized. [c] B.p. of mixture of isomers. [d] M.p. of mixture of isomers; purification by acid-base separation.
The catalytic reaction of terminal alkynes always leads to
two isomeric pyridine derivatives: the symmetrically substituted type (19) is formed as the major product, while the
asymmetric type (20) is formed in about 30 % relative yield.
The substituents R' and R2 can be varied within wide limits
and combined at will. The nitrile component may be an alkyl
derivative of any length or an aryl compound. In this last
case phenylpyridines are formed in a single step. This appears
particularly remarkable because no single, general method
was previously available for the production of phenylpyridines
(cf., e.g., ["I).
The results obtained so far are summarized
in Table 5.
QCSN
-
+ 2 R-C-CH
[COI
(j)
(21)
&bR&
+
N
N,
(22)
R
(23)
Use of acetylene as the alkyne component gives the respective parent compound, e. g . 2,2'-bipyridine. Up to 350 catalytic
cycles [eq. (i)] can be performed on a cobalt compound such
Table 6. Bipyridine derivatives (22) and (23) obtained according to eq.
120-130°C; almost complete conversion: work-up by distillation].
6 ) [catalyst,
e.9.. ( 5 ) ; reaction temperature
R
X-CN
+ 2 R-C-CH
+
(21)
X
2-Pyridyl
3-Pyridyl
4-Pyridyl
X
R
Isomeric
ratio
[a1
(22) (23)
N
R
Yield
R
Major fraction
M i (m/e)
[XI
Products kiiown
a c c i ~ l c i l l l g111 I.cI
B. p.
[Tjtorr]
H
CH3
C6Hs
72 : 28
60 : 31
95
91
79
273/760
122/10-3
120110-4
H
CH3
62 : 38
92
94
286/760
123/10-3
H
CH3
54 : 46
93
90
281/760
125/10-3
niO
156
184
308
[102-1 231
156
184
[105-1091
1.589
156
184
[l09, 124, 1251
1.589
1.589
[a] Determined by 'H-NMR spectroscopy
Angew. Chem. I n f . E d . Engl. 17,505-515 ( 1 9 7 8 )
509
as (5). Substituted alkynes give two positional isomers, with
type ( 2 2 ) usually predominating. The bipyridines ( 2 2 ) and
( 2 3 ) bearing different substituents on the two rings, which
were inaccessible by classical methods, could be of interest
for the synthesis of transition metal bipyridine complexes.
Some typical examples are shown in Table 6.
Particular industrial interest attaches to the new synthesis
of 2,T-bipyridines. The quaternization product of bipyridine
and 1,2-dibromoethane is a commonly used herbicide.
7. Catalytic Reaction of cl,o-Dinitriles with Alkynes
The procedure corresponding to eq. 6 ) is also applicable
to m,o-dinitriles (24). The bifunctional starting materials ( 2 4 )
(25)
(26)
(27)
Table 7.2,2’-Oligometbylenedipyridines and 2,2’-(1,4-phenylene)dipyridine (25)-(27) [catalyst, e.g., ( 5 ) ; reaction temperature 140-150°C; almost complete conversion; work-up by distillation].
Z
R
Yield
[%I
Major fraction
B. p.
nL0
[.C/torr]
CH3
C6H5
H
CHI
C6H5
H
CH3
H
CH3
C6H5
H
CsH5
H
CH3
H
CH3
C6H5
H
CH3
H
72 [a]
90/10-3
160110-4
70CaI
97
94
83
92
96
98
90
86
92
81
97
92
92
94
79
90
96
94
M f (m/e)
Products known
according to ref.
170
105-1 10J0.5
414
184
240
488
198
254
212
268
516
226
530
240
296
254
310
558
268
324
232
1.578
143-14610.1
170110-4
135125
155JO. 1
12510.2
160J0.1
178/W4
126/10-3
189/10-4
128/10-’
137/10-4
138/10-3
160110
230/t0-4
1.536
~
1oojto - 4
122110 230/2.10-3
[126,1271
[1261
r1271
~271
~1271
~271
~271
~271
[1281
~~
[a] 4-Amino-2,6-bis(cyanomethyl)-5-pyrimidinecarbonitrile
is formed as a by-product in a yield of preparative interest.
NC-Z-CN
containing any number of bridging members Z afford the
bis(bpyridy1) derivatives (25), (26), and (27). The reaction
proceeds stepwise: in the first step the monopyridyl derivatives
( 2 8 ) and ( 2 9 ) are formed; this is followed by reaction of
the cyano group with further alkyne [eq. (k)].
As shown by Table 7, the otherwise rather inaccessible
or unknown oligomethylenepyridines (25)-(27) can be prepared in good yield, without about 450 cyclization steps being
achieved per cobalt atom of the catalysts (5) or (14).
+ 2 HCZCH
(24)
Q 2-CN
23 %
conversion
QZ-COOH
(32)
QZ-NH,
Table 8. o-(2-Pyridyl)alkanonitriles (28) and (29) obtained according to eq. (k) at 25 % conversion of dinitrile
(24) [catalyst, e.g. ( 5 ) ; reaction temperature 80-90°C; work-up by distillation].
Z
R
Isomeric ratio [a]
Major fraction
Yield
r%i
B. p.
1110
[“Citorr] [b]
72
70
71
72
77
69
72
74
62
28
30
29
28
23
31
28
26
38
19
81
89
83
84
59
68
78
85
969812
95-9711
135-1 3810.4
142-14510.5
15&152/0.2
17&1 85jl0 - 4
177-196110-4
9&99/10 - 4
i1~120/10-4
1.518
160
1.519
1.489
1.484
1.463
174
188
216
244
270
312
186
208
[a] Determined by ‘H-NMR spectroscopy. [b] B. p. of mixture of isomers.
510
Angew. Chem. Int. Ed. Engl. i 7, 505-51 5 ( 1978)
If the catalytic reaction of eq. (k) is interrupted after ca.
25% of the dinitrile has been consumed then monopyridyl
derivatives of type (30) may be obtained without difficulty
on a preparative scale [eq. (I); cf. Table 81.
As suggested in eq. (I), the cyano compound (30) is readily
transformed into the acid (31 ) or reduced to the amine (32).
8. Catalytic Synthesis of Pentasubstituted Pyridines;
Synthesis of Derivatives with Functional Groups
Pyridine derivatives having a higher degree of substitution
can also be prepared catalytically [eq. (m)]. 2-Butyne or
diphenylacetylene, for example, reacts with nitriles to give
pentasubstituted pyridines (33) (Table 9).
tution of ready-made pyridine rings (cf. ref.
steps.
['301),
in several
A catalytic cyclotrimerization reaction, using a modification
of eq. (m), leads to pyridine rings having ether functions (36).
These are useful for pharmaceutical applications (Table 9).
Alkyl thiocyanates can also be used as the cyano component;
one-step reactions according to eq. (0)give alkylthiopyridines
(35) which are otherwise accessible only with diffi~ulty['~~][*1.
Although most of the Co' systems are blocked within a few
steps by deposition of free sulfur so that true catalysis according
to eq. (0)cannot occur, use of catalyst ( 5 ) in toluene permits
synthesis of ca. 40 to 50 mol(35)/mol(5). Some of these results
are compiled in Table 9.
(33)
R2
R' = CH,, C2HSetc.; R2 = CH,, C6H5 etc.
It has become apparent in the course of our studies that
cobalt-catalyzed pyridine synthesis is by no means limited
to carbonitriles. Even extensive modifications of the cyano
components in the pyridine synthesis are tolerated by cobalt
catalysts of types ( 5 ) , (15), and (4). Monomeric cyanamide[*]
yields 2-aminopyridines (34) according to eq. (n) with the
catalyst systems mentioned.
Aminopyridines of type ( 3 4 ) (Table 9) possess considerable
preparative interest but can usually only be prepared by substi-
Hong and Yarnazaki[' recently reported the catalytic reaction of acetylene derivatives with isocyanates and carbodiimides. Phosphane-substituted systems of type (8) and
cobaltocene served as catalysts.
The reaction of isocyanates and alkynes according to eq.
(p) can also be conducted with a similar degree of success
Table 9. Pyridine derivatives of type (33) [eq. (m)], (34) [eq. (n)], (35) [eq. (o)], and (36) [modification of eq. (m)] [work-up by distillation].
Yield
["/.I 13
.
Product
77
B. p. [T/torr]
(M. P. WI)
57/tO-'
Product known
according to ref
163
246
87
78
M t( 4 4
153-1 5 9 / W 4 [c]
277
R = CAI
(35c)
R = 1-C3H1
83
t60-167/10-4 [c]
29 1
(3Sb)
R = C2H5
85
171&178/10-4 [c]
305
(360)
85
67112
123
72
181
68 [h, c]
231
11291
[a] Based on reacted nitrile component, not optimized. [b] Purified by acid-base separation. [c] Isolation of mixture of isomers.
[*] Monomeric cyanamide is available in various grades from Siiddeutsche
Kalkstickstoffwerke AG, D-8223 Trostherg.
Angew. Chem. I n t . E d . Engl. 1 7 , 505-515 ( 1 9 7 8 )
['I This variant was developed by Dr. G. S. Natarajan, Nagpur University,
India, 1974-1976 DAAD Guest Scientist at the Max-Planck-Institut f i r
Kohlenforschung.
511
(P)
R
R
using our catalysts ( 5 ) and (11) to give 2-pyridones. About
200 catalytic steps take place per cobalt atom.
In the presence of complex (8) or of cobaltocene, the carbodiimides (37) react catalytically to form the imines (38) [eq.
(q)][ 3 1'.
2
RI-C~C-R~
+
PhN=C=NPh
ICOI
(37)
(4)
A highly interesting variation becomes possible by reaction
of acetylenes on CpCo'(diene) systems ( 4 ) [eq. (r)]. 1,7-Octadiyne initially gives the non-isolable intermediate ( 3 9 ) containing a cyclohexane ring in an intramolecular process. In
double reaction cycle (Fig. 1).The left-hand cycle (bold arrows)
represents the principal reaction leading to pyridine as the
catalytic reaction product; the right-hand cycle explains the
formation of carbocyclic by-products.
Both cycles start with a CpCo' complex (6) which initially
coordinates an acetylene molecule to form ( 4 2 ) . This coordinatively unsaturated species can accept a further molecule of
acetylene, under formation of a cobaltacyclopentadiene. This
cobalt-containing intermediate, shown enclosed in a box, may
be regarded as a junction between the pyridine and benzene
cycles.
The product-forming step of the catalysis consists in insertion of a third triple bond. In the case of a nitrile molecule
(left-handcycle), the pyridine derivative ( 1 4 ) is formed catalytically via ( 4 3 ) while coordination of a third acetylene molecule
(right-hand cycle) leads catalytically to benzene.
The phosphane-stabilized complexes (44), reported by
Yarnazaki et a/. in another context in 1970['331, can serve
as model compounds for the intermediates shown in Figure 1.
( M a ) can be compared with species ( 4 2 ) , while ( 4 4 b )
resembles the postulated intermediate ( 4 3 ) [cf. also catalyst
(811.
cR'
\i'J
(41)
the presence of an excess of nitrile, a second ring closure
takes place at the cobalt and leads to derivatives of tetrahydroisoquinoline (40) in ca. 60% yield. Compound ( 4 1 ) is obtained analogously from 1,6-heptadiyne.
The annelated pyridines (40) and ( 4 1 ) are also obtained
with C ~ C O ( C Oas) ~catalyst['32!
9. Mechanism of the Cobalt-Catalyzed Pyridine Synthesis
The product formation using catalysts containing the CpCo'
group as central moiety, can be rationalized in terms of a
An intermediate of type ( 4 5 ) might be considered as an
alternative to the central intermediate shown in Figure 1.
(45)
c p - c p
R
Results of kinetic
1
ofcatalytic pyridine formation is in
tion of nitrile in the reaction mixti
formation were to proceed via .
of type (45), then high nitrile conc
the catalytic reaction. The avail;
Iwever, show that the rate
ependent of the concentra-e. If the catalytic pyridine
-containing intermediates
ntrations would accelerate
Ae experimental evidence
CpCo' ( 6 )
Rc"'
(43)
Fig. 1. Cobalt(1)-catalyzed re:iction of alkyne and nitrile [cf. eq.
512
(01.
Angew. Chem. Int. Ed. Engl. 17.505-515 (1978)
shows that this is not the case and intermediates of type
( 4 5 ) are thus highly unlikely.
Kinetic measurements also revealed that the reaction rates
of both catalytic cycles show a quadratic dependence upon
the acetylene concentration.
If a large steady-state concentration of acetylene is made
available to the central intermediate shown in Figure 1, which
is evidently formed in the rate-determining step of the catalysis,
then the reaction is increasingly diverted into the right-hand
cycle and benzene formation predominates.
Two characteristics of the catalytic reaction course appear
to be important:
1) Cyclic alternation of the formal valency between cobalt(1)
and cobalt(rI1);
2) cyclic alternation of the coordination numbers and coordination geometries of the central atom.
The formation of positional isomers in reactions involving
terminal alkynes is readily explained by assuming the intermediacy of the cobalt-containing five-membered ring. Interme-
-0,
R2
-+
R2
R
bco
H'
R2
N
R2
1
(471
noR2
R2
1
.2i0
Received: June 6, 1977 [A 219 IE]
German version: Angew. Chem. 90, 517 (1978)
diate (46) results from head-to-tail linkage at cobalt and,
after insertion of the third triple bond, affords compounds
( 4 8 ) and ( 4 9 ) with symmetrical substitution. Intermediate
( 4 7 ) results from tail-to-tail linkage of the alkynes to cobalt,
and leads to the asymmetrically substituted products (50)
and ( 5 1 ) .
The trisubstituted pyridines obtained by cobalt catalysis
provide further information about the course of reaction at
the cobalt. This is shown schematically in equations (s) to
(v).
Insertion of the e N triple bond of the nitrile into the
CoC bond evidently occurs in such a manner that the nitrogen
atom initially interacts with the cobalt. This is supported
in particular by the findings corresponding to eq. (s).
Products which would arise from intermediates with a
vicinal substitution pattern (head-to-head linkage of the acetylenes [eq. (v)]) are not formed.
Angew. Chem. Inr. Ed. Engl. 17. 505-515
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C 0 M M U N I CAT10N S
recognized['], the o type undergoes this cleavage reaction
while the K type does not.
More about n- and o-Succinimidyl Radicals: Ring
Opening R e a c t i o n s [ * * ]
By Philip S. Skell, James C . Day, and Joseph P . Slangs"]
Until recentlyf1- 31 the reactions of the succinimidyl radical
were virtually unknown, or unrecognized, with the exception
of its intermediacy in the rearrangement of N-bromosuccinimide (NBS) to P-bromopropionyl isocyanate (I ), a reaction
which had defied ~nderstanding'~].
O
n
0 -.+
0
II
Br<H2CH2C-N=C=0
I)r
6,
(1)
NBS
The various parts of the puzzle are now accommodated
by the hypothesis that succinimidyl radical undergoes rapid
and reversible ring cleavage, and that trapping of the openchain form leads to the P-halopropionyl isocyanate. Of the
two types of succinimidyl radicals which have been
-~
[*I Prof. Dr. P. S . Skell, Dr. J. C. Day, J. P. Slanga
Department of Chemistry, The Pennsylvania State University
University Park, Pennsylvania 16802 (USA)
[**I This work was supported by the Air Force Office of Scientific Research
(2748C).
Angew. Chem. Int. Ed. Engl. 17 (1978) No. 7
The assemblage of experimental results which are to be
accommodated by this hypothesis are summarized below.
1) Irradiation (or heating, with benzoyl peroxide initiation)
in the presence of alkenes results in 25-90% yields of acyl
isocyanate (I ) for N-bromo- (NBS) and N-iodosuccinimide
(NIS); no isocyanate is formed if N-chlorosuccinimide (NCS)
is used, or any of the N-halophthalimides or N-haloglutarimides, despite the fact that the same H-abstractions, additions
to alkenes, and additions to arenes are observed as for NBS
and NIS.
2) Within large limits the above results are insensitive to
changes in olefin concentrations (see also [4b]).
3) In the presence of large concentrations of alkenes (or
benzene) no isocyanates are obtained; the succinimidyl moieties are found as succinimide (H-abstraction) and in 1,2adducts to the olefin (or benzene)[3!
4) In the presence of Br2 or Iz no isocyanates are formed;
only H-abstractions occur, among them abstractions which
cannot be attributed to Br' or 1'.
5) One or more alkyl substituents on NBS have only a
small effect on its reactivity in the presence of alkenes, as
determined in competition reactions in which the rate of disappearance of the NBS derivatives is monitored[5'. However,
the product is 100 % isocyanate when the alkyl-substituted
NBS is used, whereas under the same circumstances NBS
itself gives 25-60 % yields of isocyanate (and succinimide
arising by H-abstraction). In the presence of alkenes the succinimidyl radicals are exclusively 0 type.
6) On the other hand, in the reaction of N-bromo-3,3dimethylsuccinimide with neopentane in CHZCl2in the presence of Brz, no isocyanate formation is observed (absence
of 2230cm- band). The reaction products are neopentyl
bromide, BrCHCl', and 3,3-dimethylsuccinimide. In the presence of Br2 the succinimidyl intermediate is exclusively of
7c type. There seems to be no significant interconversion of
o and 7c types under these reaction conditions.
515
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