Mechanistic comparison of the nickel(0)-catalyzed homo-oligomerization and co-oligomerization of alkynes and nitriles

Article abstract of DOI:10.1002/1099-0682(20011)2001:1<77::aid-ejic77>3.0.co;2-g

A comparative mechanistic study of the nickel(0)-catalyzed homo-oligomerization and co-oligomerization of alkynes and of nitriles has been undertaken, with diphenylacetylene and benzonitrile, towards an array of nickel(0) reagents, such as finely divided nickel, (COD)₂Ni, (Bpy)(COD)Ni, (Et₃P)₄Ni, (Bpy)(PhC≡CPh)Ni and (COD)₂Ni-MeAlCl₂ combinations in donor (THF) and nondonor (PhMe or neat substrate) solvents. Special attention has been given to the detailed molecular structures of the initial 1:1 adducts, (Bpy)(PhC≡CPh)Ni, (Ph₃P)₃(PhC≡CPh)Ni and [(Ph₃P)(PhC≡N)Ni]₄ by a consideration of XRD and IR data. Data from the single crystal XRD analysis of (Bpy)(PhC≡CPh)Ni, reported here for the first time, are shown to be in excellent accord with the presence of a 2,3-diphenylnickelacyclopropene ring for the (PhC≡CPh)Ni moiety with almost coplanar chelating coordination of the bipyridyl ligand, rather than with the presence of simple "side-on" coordination of the alkyne with the metal center. A parallel analysis of XRD and IR data for the two benzonitrile-nickel(0) complexes, which were drawn from previous publications, has concluded that the nickel in (Ph₃P)₃(PhC≡N)Ni is coordinated in an "end-on" fashion and the nickel centers in [(Ph₃P)(PhC≡N)Ni]₄ are coordinated as bridges between nitrile units in both an "end-on" and "side-on" manner. The stereochemistry of the acid hydrolysis of the nickelacyclopropene complex to (E)- or (Z)-alkene was shown to depend on the structure of the cleaving acid; parallel hydrolysis of nitrile-nickel complexes has shown that "end- on" complexes regenerate the nitrile, while "side-on" complexes lead to the aldehyde. In homo-oligomerization of diphenylacetylene or other alkyne the clean cyclotrimerization to the benzene derivative was shown to proceed by way of a nickelacyclopentadiene intermediate, as was evident by chemical trapping. The homo-oligomerization of benzonitrile by nickel(0) was found not to lead to 2,4,6-triphenyl-1,3,5-triazine, as claimed in the literature, but rather solely to benzyl phenyl ketone, the dimeric hydrolysis product. The attempted co-oligomerization of diphenylacetylene and benzonitrile with ordinary nickel(0) complexes led only to the homocyclotrimer of the alkyne. Only when the alkyne was prebonded to the nickel, as in (Bpy)(PhC≡N)Ni, could significant amounts of a codimerization product with the nitrile be observed. The origin of the triazine, claimed in a previous report to form from benzonitrile and Raney nickel, has been traced to the presence of adventitious moisture and air. Other unexpected products formed from nickel(0) complexes and benzonitrile have been shown to arise from oxidative addition reactions of nickel(0) with various σ C-E bonds.

Full text of DOI:10.1002/1099-0682(20011)2001:1<77::aid-ejic77>3.0.co;2-g

FULL PAPER
Mechanistic Comparison of the Nickel(0)-Catalyzed Homo-Oligomerization
and Co-Oligomerization of Alkynes and Nitriles[‡]
John J. Eisch,*[a] Xin Ma,[a] Kyoung I. Han,[a] John N. Gitua,[a] and Carl Krüger[b]
Dedicated on the occasion of his 75th birthday to Professor Günther Wilke, an outstanding pioneer in transition
metal catalyzed organic synthesis
Keywords: Organometallics / Metallacycles / Oligomerizations / Alkynes / Nitriles / Nickel
A comparative mechanistic study of the nickel(0)-catalyzed
nickelacyclopropene complex to (E)- or (Z)-alkene was
homo-oligomerization and co-oligomerization of alkynes and shown to depend on the structure of the cleaving acid; paral-
of nitriles has been undertaken, with diphenylacetylene and
benzonitrile, towards an array of nickel(0) reagents, such as
finely divided nickel, (COD)2Ni, (Bpy)(COD)Ni, (Et3P)4Ni,
(Bpy)(PhCϵCPh)Ni and (COD)2Ni−MeAlCl2 combinations in
lel hydrolysis of nitrile-nickel complexes has shown that
‘‘end-on’’ complexes regenerate the nitrile, while ‘‘side-on’’
complexes lead to the aldehyde. In homo-oligomerization of
diphenylacetylene or other alkyne the clean cyclotrimeriz-
donor (THF) and nondonor (PhMe or neat substrate) solvents. ation to the benzene derivative was shown to proceed by way
Special attention has been given to the detailed molecular
structures of the initial 1:1 adducts, (Bpy)(PhCϵCPh)Ni,
(Ph3P)3(PhCϵCPh)Ni and [(Ph3P)(PhCϵN)Ni]4 by a consid-
of a nickelacyclopentadiene intermediate, as was evident by
chemical trapping. The homo-oligomerization of benzonitrile
by nickel(0) was found not to lead to 2,4,6-triphenyl-1,3,5-
eration of XRD and IR data. Data from the single crystal XRD triazine, as claimed in the literature, but rather solely to
analysis of (Bpy)(PhCϵCPh)Ni, reported here for the first
time, are shown to be in excellent accord with the presence
benzyl phenyl ketone, the dimeric hydrolysis product. The
attempted co-oligomerization of diphenylacetylene and
benzonitrile with ordinary nickel(0) complexes led only to the
of
a
2,3-diphenylnickelacyclopropene ring for the
(PhCϵCPh)Ni moiety with almost coplanar chelating coor- homocyclotrimer of the alkyne. Only when the alkyne was
dination of the bipyridyl ligand, rather than with the pres- prebonded to the nickel, as in (Bpy)(PhCϵN)Ni, could signi-
ence of simple ‘‘side-on’’ coordination of the alkyne with the ficant amounts of a codimerization product with the nitrile be
metal center. A parallel analysis of XRD and IR data for the
two benzonitrile-nickel(0) complexes, which were drawn
from previous publications, has concluded that the nickel in
(Ph3P)3(PhCϵN)Ni is coordinated in an ‘‘end-on’’ fashion and
the nickel centers in [(Ph3P)(PhCϵN)Ni]4 are coordinated as
bridges between nitrile units in both an ‘‘end-on’’ and ‘‘side-
on’’ manner. The stereochemistry of the acid hydrolysis of the
observed. The origin of the triazine, claimed in a previous
report to form from benzonitrile and Raney nickel, has been
traced to the presence of adventitious moisture and air. Other
unexpected products formed from nickel(0) complexes and
benzonitrile have been shown to arise from oxidative addi-
tion reactions of nickel(0) with various σ C−E bonds.
Introduction
vided researchers in this flourishing field with a stellar para-
digm for steering and optimizing such metal-catalyzed or-
The nickel-catalyzed selective cyclooligomerizations of ganic reactions.
alkynes by Walter Reppe’s group in the 1940s[1] and the
analogous cyclooligomerizations of 1,3-alkadienes by
Reppe’s empirical findings that acetylene is principally
Günther Wilke and co-workers[2] in the succeeding two dec- cyclotrimerized into benzene by nickel(0) catalysts, such as
ades were seminal discoveries in ushering in the current era (Ph3P)2(CO)2Ni, but is largely cyclotetramerized into the
of transition metal-mediated organic synthesis.[3] Wilke’s unstable cyclooctatetraene by such nickel(II) salts as
[1a,4]
studies in particular, with their focus on detecting and trap- Ni(CN)2
or by nickel(0) complexes having labile li-
ping reactive intermediates and on probing the steric and gands [(COD)2Ni][5] have long piqued the curiosity of many
electronic effects of metal ligands on reactivity, have pro- chemists. Subsequent detailed mechanistic studies of these
oligomerizations have adduced cogent and persuasive evid-
[‡]
Organic Chemistry of Subvalent Transition Metal Complexes,
20. Ϫ Part 19: J. J. Eisch, J. R. Alila, Organometallics, 2000,
19, 1211.
ence that reactive nickelacyclopropene (1) and nickelacyclo-
pentadiene (2) rings are pivotal intermediates in both cyclo-
oligomerizations and that trimerization to 3 or tetrameriz-
ation to 4 hinges on whether 2 reacts with a third alkyne
(path a) or undergoes autodimerization (path b)
(Scheme 1).[2c,6Ϫ13]
[a]
[b]
Department of Chemistry, The State University of New York
at Binghamton,
Binghamton, New York 13902Ϫ6016, U.S.A.
Max-Planck-Institut für Kohlenforschung,
45470 Mülheim (Ruhr), Germany
Eur. J. Inorg. Chem. 2001, 77Ϫ88
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
1434Ϫ1948/01/0101Ϫ0077 $ 17.50ϩ.50/0
77

J. J. Eisch, X. Ma, K. I. Han, J. N. Gitua, C. Krüger
FULL PAPER
plexes but in an entirely different manner not involving the
formation of triazine 6. The structural and electronic
reasons for these divergent oligomerization pathways fol-
lowed by alkynes and by nitriles under nickel(0)-mediation
will be elucidated in light of the actual and probable or-
ganonickel intermediates.
Scheme 1
Results and Discussion
The varying prevalence of path a or path b has been sug-
gested to depend upon the ligands Ln on nickel and the
substituents R on the acetylene monomer.[12]
Structures of Initial 1:1 Complexes of Nickel(0) with
Alkynes or with Nitriles
In light of the intricate mechanistic features involved in
such oligomerizations, we were curious to learn to what ex-
tent organic nitriles or azaacetylenes, RϪCϵN, might un-
dergo similar homo-cyclooligomerization or co-cyclo-
trimerization with alkynes by nickel metal catalysis. Two
brief publications offered encouragement: benzonitrile (5)
was reported to cyclotrimerize to 2,4,6-triphenyl-1,3,5-triaz-
Such cyclotrimerizations of alkynes have as their com-
mon starting intermediate 1:1 adducts with nickel(0) that
have been isolated, where appropriate ligands are present,
and characterized by spectroscopic and single crystal X-ray
structural analyses. Here we present such characterization
of 2,2-bipyridyl(η2-diphenylacetylene)nickel (7), where an
essentially planar interaction of both the bipyridyl group
and the CϵC bond with the nickel center is evident (Fig-
ure 1). The average NiϪC (acetylenic) separation of 1.86 Ϯ
ine (6) [Equation (1)] when heated at reflux with Fe(CO)5
[14]
or Fe2(CO)9
or when allowed to reflux with Raney
nickel for many hours.[15] However, attempts by other
workers to reproduce the formation of triazine 6 over care-
fully dried and deoxygenated Raney nickel or even on pure
nickel surfaces were unsuccessful.[16] No reports of the
nickel-catalyzed co-cyclotrimerization of alkynes and ni-
triles have been found, even though such oligomerizations
by cobalt catalysis are common and have received much
attention.[17] In the present report we establish that benzon-
itrile can be co-dimerized with an alkyne but only by inter-
mediacy of an alkyne-nickel(0) complex. Furthermore,
benzonitrile can indeed be oligomerized by nickel(0) com-
˚
0.01 A is comparable with sigma NiϪC bonds involving
sp2-hybridized carbon centers, such as the NiϪC bond of
[18]
˚
1.89 Ϯ 0.01 A in structure 8.
Moreover, the supposed
˚
acetylenic CϪC separation of 1.30 Ϯ 0.005 A is more con-
gruent with the presence of a CϭC rather than a CϵC
bond in 7[19] (Table 1). Finally, complex 7 displays a sharp
absorption at 1770 cmϪ1 in its infrared spectrum, shifted
by 330 cmϪ1 to lower frequencies than the CϵC stretch of
unsymmetrically substituted acetylenes such as phenylacet-
ylene. An absorption of 1770 cmϪ1 in complex 7 is in much
closer agreement with a CϭC bond stretching vibration,
Professor Günther Wilke was born in Heidelberg, Germany, on February 23, 1925, where he was to obtain both the Abitur at the Humanistisches Gymnasium
and the doctorate degree in 1951 at the Universität Heidelberg upon completion of structural studies on lignin under the direction of Professor Karl
Freudenberg. Shortly thereafter, he joined Karl Ziegler’s research group as scientific assistant at the Max-Planck-Institut für Kohlenforschung in Mülheim
(Ruhr) and participated in bringing Ziegler’s recent discoveries in organoaluminum chemistry and low-pressure ethylene polymerization to economic fruition.
Since his curiosity was strongly aroused by the seminal discovery of the Nickel Effect by Ziegler and Holzkamp, he launched his independent research
program in 1956 with the systematic investigation of the effect of nickel and other transition metals on reactions of main-group organometallics. Over the
next 40 years he pursued the exciting and unforeseen ramifications of such research, which have led to the novel template syntheses of medium-sized
carbocycles, the regioselective oligomerizations of alkenes and alkadienes as well as the asymmetric syntheses of carbon skeletons. Interwoven with such
discoveries have been many pioneering mechanistic studies that have elucidated the governing roles of the transition metal and its ligands in these novel
carbon-carbon bond coupling processes. During the period of 1969-1993 as successor to Karl Ziegler, he directed the MPI during a flourishing period of
expansion of its research activities and facilities. In over 220 publications and numerous patents he disseminated the classic research results of his many
doctoral students and published with Peter Jolly the authoritative, two-volume treatment of “The Organic Chemistry of Nickel” (1974, Academic Press).
Among many honors for his scientific discoveries may be cited the Ruhr-Preis für Kunst und Wissenschaften and both the Emil Fischer Medaille and the
Karl-Ziegler-Preis of the Gesellschaft Deutscher Chemiker. His outstanding enthusiasm and skill in the art of laboratory experimentation finds a fitting
parallel in his keen interest in and a notable private collection of contemporary art. As a young scientist it was this senior author’s pleasure and honor to
work closely with Günther Wilke, the master experimentalist, during a yearlong postdoctoral fellowship in Professor Ziegler’s institute in 1956.
78
Eur. J. Inorg. Chem. 2001, 77Ϫ88

Nickel(0)-Catalyzed Homo-Oligomerization and Co-Oligomerization of Alkynes and Nitriles
FULL PAPER
which in unconjugated disubstituted cis-alkenes absorb Table 1. Selected bond lengths and angles of 2,2Ј-bipyridyl(η2-di-
near 1670 cmϪ1. Accordingly, a nickelacyclopropene or
phenylacetylene)nickel (7)
nickelirene structure seems to be a more important reson-
[a]
˚
Bond lengths [A]
Min. value
Max. value
Av. value
ance structure contributor (7a) with a nickel(II) oxidation
state than does resonance structure 7b where the nickel
center is viewed as in a Ni(0) oxidation state and as only
accepting electron density from its ligand donors, di-
phenylacetylene and 2,2-bipyridyl. As will be evident, the Ni1ϪC12
nickelirene structure 7a also proves to be the more useful
molecular model for understanding the courses of chemical
reactions undergone by 7 with various electrophilic or un-
saturated organic reagents.
C11ϪC12
C31ϪC32
Ni1ϪC11
Ni2ϪC31
1.296
1.289
1.854
1.843
1.866
1.856
1.445
1.458
1.446
1.450
1.306
1.299
1.860
1.849
1.872
1.862
1.455
1.468
1.456
1.460
1.301
1.294
1.857
1.846
1.869
1.859
1.450
1.463
1.451
1.455
Ni1ϪC32
C11ϪPh1
C31ϪPh21
C12ϪPh11
C32ϪPh31
Bond angles [°]
C11ϪNi1ϪC12
C31ϪNi2ϪC32
Ni1ϪC11ϪC12
Ni2ϪC31ϪC32
Ni1ϪC12ϪC11
Ni2ϪC32ϪC31
40.8
40.8
69.9
69.9
68.9
68.8
41.0
41.0
70.3
70.3
69.3
69.2
40.9
40.9
70.1
70.1
69.1
69.0
[a]
The pairwise grouping of bond lengths and bond angles in the
Table are the measured values for the two independent molecules
of 7 found in the asymmetric cell. The first value in each pair is
for the molecular and the atom numeration shown in Figure 1. The
second value in each pair is that of the second molecule not shown
in any figure. However, the atom numeration in the second molec-
ule has the following atom correspondence to molecule one/ C11 ϭ
C31; C12 ϭ C32; Ni1 ϭ Ni2; Ph1 ϭ Ph21; Ph11 ϭ Ph31.
structure of the clathrate compound, tetrakis[(benzonitrile)-
(triphenylphosphane)nickel(0)] (11),[21] incorporating vari-
ous hydrocarbon solvent molecules, actually has the Ni0
centers simultaneously coordinated to adjacent benzonitrile
units in an ‘‘end-on’’ and a ‘‘side-on’’ manner (11, schemat-
ically drawn). Since complexes 10 and 11 are in equilibrium
with each other, one can conclude that ‘‘end-on’’ coordina-
tion of the nickel center is the stronger primary binding
mode with the nitrile group and that ‘‘side-on’’ bonding is
only ancillary to it. The ‘‘end-on’’ NiϪN bond length of
Figure 1. Structure and atom numeration of one of the independent
molecules of 2,2-bipyridyl (η2-diphenylacetylene)nickel found in
the asymmetric unit cell of 7
˚
1.90 A is similar to that in 7 but the NiϪNϪC bond angle
at 167° deviates more from linearity. The ‘‘side-on’’ NiϪC
˚
bond length at 1.85 A is comparable to NiϪC bonds with
sp3-hybridized carbons (vide supra).
Infrared spectral data on complexes 10 and 11 provide a
useful insight into such ‘‘end-on’’ and ‘‘side-on’’ coordina-
tion. The nitrile stretch in uncomplexed benzonitrile at 2230
cmϪ1 is lowered to 2150 cmϪ1 by ‘‘end-on’’ coordination as
in 10.
A number of spectroscopic and single crystal X-ray struc-
tural analyses of 1:1 adducts of benzonitrile with nickel(0)
complexes have been published. The two XRD studies are
most valuable in establishing that the nickel(0) center can
coordinate with the nitrile group in both an ‘‘end-on’’ (9a)
and a ‘‘side-on’’ (9b) manner.
As was established in the structure of (benzonitrile)tris(-
triphenylphosphane)nickel(0) (10), ‘‘end-on’’ coordination
On the other hand, complex 11 displays a strong band at
of Ni° was evident (NiϪN bond length of 1.89 Ϯ 0.011 A 1850 cmϪ1. Again, such a stretching frequency lies much
and NiϪNϪC bond angle of 173 Ϯ 7°).[20] Similarly, the closer to a CϭN stretch (1470Ϫ1690 cmϪ1) than to a CϵN
˚
Eur. J. Inorg. Chem. 2001, 77Ϫ88
79

J. J. Eisch, X. Ma, K. I. Han, J. N. Gitua, C. Krüger
FULL PAPER
stretch (2100Ϫ2200 cmϪ1). Thus, it again appears that for
such ‘‘side-on’’ coordination, the resonance contribution of
a nickel(II) system, namely, azanickelacyclopropene (12a)
would seem to be more important than that of a nickel(0)
pi-complex (12b).
As confirmation of the importance of structure 12a is the
˚
observed CϪN bond length of 1.26 A, considerably longer
˚
than the CϵN bond (1.16 A) and very comparable with
[21]
˚
those of CϭN linkages (1.26Ϫ1.30 A).
These infrared data from nickel(0)-benzonitrile com-
plexes of known structure can now conversely be used to
evaluate the type of bonding in unknown structures. Thus
the complex, (benzonitrile) (2,2-bipyridyl)nickel(0) (13),
which has a sharp absorption at 2105 cmϪ1 must have ‘‘end-
on’’ coordination.[22] This is undoubtedly the complex
formed initially when (2,2-bipyridyl)(1,5-cyclooctadiene)
nickel(0) interacts with benzonitrile (vide infra).
Scheme 2
A congruent explanation for the retention of configura-
tion on the protonolysis of the first CϪNi bond in 7a by
acetic acid or by phosphoric acid may lie in the ability of
these acids to deliver a proton via a six-membered coordin-
ated transition state (17). By contrast, the ionic transition
state 18, formed by HCl in water, can easily isomerize by
ring-opening, to form the sterically most stable ion (19)
[Equation (2)]:
Protonolysis of the Initial 1:1 Complexes of Nickel(0) with
Alkynes or with Nitriles
The stereochemical course of the protonolysis of 2,3-
diphenylnickelirene·2,2-bipyridyl (7a) depends strikingly
upon the solvent and the specific nature of the proton
source. When 7a was treated with 85% H3PO4 in THF or
with glacial HOAc, almost quantitative yields of only cis-
stilbene (14) were obtained. On the other hand, when the
protonolysis was conducted with 6N aqueous HCl, a pale
red precipitate of PhCHϭCPh(NiCl·Bpy) (15) was formed
almost immediately in 90% yield. The remaining 10% of the
starting material had been converted into a 9:1 mixture of
cis- and trans-stilbenes. The stereochemistry of the stilbenyl
CϪNi bond in intermediate 15 was established by treating
15 first with HOAc and determining that only trans-stilbene
(16) was produced. Then in a separate experiment 15 was
Up to the present work, little has been known about the
treated with DOAc with the result that trans-stilbene-d1 course of protonolysis of ‘‘end-on’’ or ‘‘side-on’’ nickel-ni-
(16a) was formed. Since HOAc did not cause isomerization trile complexes. It has been reported that complex 13, which
of the stilbenyl group in the protonolysis of 7a, it is reason- can now be assigned an ‘‘end-on’’ structure, regenerates
able that it would not cause isomerization in the pro- benzonitrile essentially quantitatively after being treated
tonolysis of intermediate 15. Therefore, isomerization of the with a weak acid like acetylacetone. Even dilute HNO3 pro-
stilbenyl group must have occurred during the formation of duces only recovered benzonitrile.[22] Given the known res-
15 from 7a under the action of HCl. That only one proton istance of nitriles to reduction or to hydrolysis, this result
is added in the formation of 15 from 7a was verified by may not be surprising. In the present work it is therefore
treating 7a with DCl. The deuteriated 15a formed was noteworthy to report that the treatment of benzonitrile in
treated in separate experiments with either HOAc or DOAc. THF with nickel complexes, such as (COD)2Ni, (Bpy)(-
Both experiments generated only trans-stilbenes but the for- COD)Ni or (Et3P)4Ni and hydrolytic workup produces
mer experiment gave the d1-isotope (16a) while the latter benzaldehyde in yields of 5Ϫ15%. We conclude that such
only the d1,2-isotope (16b). These interrelated reactions are benzaldehyde must have arisen from the hydrolysis of a
depicted in Scheme 2.
‘‘side-on’’-coordinated nickel complex like 12a, which can
80
Eur. J. Inorg. Chem. 2001, 77Ϫ88

Nickel(0)-Catalyzed Homo-Oligomerization and Co-Oligomerization of Alkynes and Nitriles
FULL PAPER
be considered as having much azanickelacyclopropene char- hypothesis that nickelirenes, such as 7a and 28, are discrete
acter (vide supra). The intermediate benzaldehyde imine and crucial intermediates in the cyclooligomerization of al-
(20) should hydrolyze easily to benzaldehyde (21) [Equa- kynes by nickel(0) complexes.[11Ϫ13]
tion (3)].
Oligomerization Reactions of Alkynes and Nitriles by
Nickel(0) Complexes
Oligomerizations with (2,2Ј-Bipyridyl)(diphenylacetylene)-
nickel(0) (7a)
Complex 7a catalyzes the cyclotrimerization of di-
phenylacetylene (22) to hexaphenylbenzene (23) in refluxing
toluene but at a significantly slower rate than with
(COD)2Ni as catalyst, which effects comparable conver-
sions to 23 in refluxing THF in half the time (Scheme 3).
Scheme 5
Attempted Co-oligomerizations of Diphenylacetylene (22)
and Benzonitrile (5) by Nickel(0) Complexes
The interaction of a 2:1 mixture of 22 and 5 with one
equivalent of (COD)2Ni in refluxing THF led to a 65%
Scheme 3
The cyclotrimerization of dimethyl acetylenedicarboxyl-
ate (24) catalyzed by 7a to hexamethyloxycarbonylbenzene
(25) occurs in refluxing benzene faster than that of 22 in
refluxing toluene. When the cyclotrimerization of 24 with
7a was carried out at 25 °C in THF, up to 20% of the prod-
uct was the mixed cyclotrimer, 1,2,3,4-tetramethyloxycar-
bonyl-5,6-diphenylbenzene (26) (Scheme 4). This finding
supports the conclusion that at lower temperatures some of
complex 7a is stable and undergoes insertions of 24, ulti-
mately leading via 27 to 26. At higher temperatures espe-
cially, 7a decomposes with the transfer of Bpy·Ni to 24 with
the formation of 28, which leads to the formation of 25
(Scheme 5). These findings support the previously proposed
yield of hexaphenylbenzene (23) and 3% of cis-stilbene (14)
but no discernible oligomer involving incorporation of
benzonitrile, such as pentaphenylpyridine. This result im-
plies that benzonitrile cannot compete with diphenylacetyl-
ene in the crucial insertion step with the nickelirene inter-
mediate 29 (L ϭ COD) (Scheme 6 depicting the competit-
ively faster formation of 31 and hence 23 from 7a and the
much slower formation of 30, which would lead to penta-
phenylpyridine). The validity of this inference was substan-
tiated by subjecting the more stable and less reactive nickel
complex 7a to the action of benzonitrile alone in THF at
25°C, without any competing alkyne. Indeed, hydrolytic
workup gave up to 40% of the cis- and trans-isomeric 1,2,3-
triphenylpropen-1-ones (32), convincing evidence that azan-
ickelole 30 had formed from 7a by insertion of benzo-
nitrile (Scheme 7).
Attempted Homo-Oligomerizations of Benzonitrile (5) by
Nickel(0) Complexes
Many careful and systematic variations have been at-
tempted to reproduce the reported cyclotrimerization of 5
over Raney nickel in refluxing benzonitrile with the forma-
tion of 2,4,6-triphenyl-1,3,5-triazine (6) [Equation (1)].[15]
To avoid possible contaminants present in the typical
samples of Raney nickel, such as aluminum metal and
NaOH, as well as moisture and dihydrogen, the attempted
Scheme 4
Eur. J. Inorg. Chem. 2001, 77Ϫ88
81

J. J. Eisch, X. Ma, K. I. Han, J. N. Gitua, C. Krüger
FULL PAPER
plaining the origin of ketone 33, we suggest that the coup-
ling of the nitrile carbons must occur in an unusual manner,
certainly not involving a straightforward reductive dimeriz-
ation as depicted in Equation 4.[23] Were such an ordinary
reductive coupling to have occurred, an intermediate diaz-
anickelole (35) would have formed and would undoubtedly
have hydrolyzed to benzil (34), a product noteworthy by its
absence in this reaction.
Scheme 6.
To account for the formation of ketone 33, therefore, we
propose a cyclodimerization of two ‘‘end-on’’-coordinated
nickel(0) complexes of benzonitrile (36) by agency of
nickel(0) to form a (3,4,-diphenyl-1,2-diazacyclobutadi-
ene)nickel(0) complex of type 37. Protonolytic decomposi-
tion of 37 would reasonably be expected to lead via 38؊40
to ketone 33 (Scheme 8). Currently, our efforts are directed
toward isolating a suitably ligated derivative of 37 for con-
firmatory XRD and protonolysis studies. There is ample
precedent for the stable complexation of the cyclobutadiene
ring with nickel(0), typified by (η4-tetraphenylcyclobutadi-
ene)bis(triethylphosphane)nickel(0), whose structure has
been confirmed by an XRD study.[11] Also, the mode of
protonolytic cleavage of this latter structure partly re-
sembles that of 35: glacial acetic acid leads to the formation
of (E,E)-1,2,3,4-tetraphenyl-1,3-butadiene.
Scheme 7
oligomerizations were also carried out with pure nickel
metal powders, prepared under argon by the alkylative re-
duction of nickel salts with n-butyllithium in THF at
Ϫ78°C. In the complete absence of traces of oxygen and of
moisture, such nickel metal led to little or no 6 even after 2
days at 190 °C. Only when moist air was admitted during
reflux did noticeable amounts of 6 form (5Ϫ10% of 5) and
other unusual reaction products appear (vide infra). We are
forced to agree with the conclusion of Oehme and Pra-
cejus[16] that a pure nickel metal surface is incapable of cata-
lyzing the cyclotrimerization of benzonitrile under the con-
ditions reported by Heldt.[15]
Since our previous experience with the coupling and
cleavage reactions of nickel(0) complexes has shown that
such reactions are especially favored in donor solvents at or
below 25 °C, we then undertook a study of the reaction of
benzonitrile with nickel(0) complexes in THF. As reported
above, protonolysis of such reaction mixtures gave benzal-
dehyde (21), most likely stemming from the protonolysis of
a ‘‘side-on’’ complex [Equation (3), 12a]. But an additional,
most surprising hydrolysis product was also isolated,
namely benzyl phenyl ketone (33). No trace of any other
dimeric or trimeric product, such as benzil (34) or triazine 6
was detected. The yields of benzaldehyde and benzyl phenyl
ketone obtained with different ratios of (COD)2Ni and ni-
trile are noteworthy: with a 1:2 (COD)2Ni:5 ratio, the yields
of 21 and 33 were 13% and 15%, with 70% of nitrile 5 re-
maining. When a reverse ratio of 2:1 (COD)2Ni:5 was em-
ployed, all of the benzonitrile was consumed and again only
Scheme 8
Although such complexations between benzonitrile and
33 and 21 were found in the reaction products, this time in nickel(0) are relatively stable at 25°C, they are easily dis-
a ratio of 96:4. rupted at higher temperatures. For example, conducting the
These results show that the formation of ketone 33 is reaction of (Bpy)(COD)Ni with benzonitrile in THF at
fostered by the stoichiometric action of more than one 25°C gives, upon hydrolysis, 20% of ketone 33, whereas the
nickel atom per nitrile. Clearly, when a 1:2 (COD)2Ni:5 ra- same reaction conducted in refluxing THF gives no trace
tio is employed, (COD)2Ni is the limiting reagent and hence of 33. Even at room temperature, such reactions can lead
only about one-third of the benzonitrile can react. In ex- to decyanation of the nitrile, especially those nitriles with
82
Eur. J. Inorg. Chem. 2001, 77Ϫ88

Nickel(0)-Catalyzed Homo-Oligomerization and Co-Oligomerization of Alkynes and Nitriles
FULL PAPER
electron-withdrawing substituents, as shown in Equa- amounts of benzaldehyde (5%) and benzyl phenyl ketone
tion (5). With electron-donating substituents, oxidative ad- (10%) upon hydrolysis. But, surprisingly, ethyl phenyl ke-
dition of CϪCN bonds to nickel(0) occurs more slowly tone (46) was also formed in 60% yield. Since the only dis-
[Equation (6)].
cernible source of the ethyl group is the Et3P ligand, we
are led to suggest that the Ni(0) center, in the presence of
benzonitrile (9a) undergoes an oxidative addition into the
ethyl-phosphorus bond of a Et3P unit (Scheme 10). Decom-
position of 47 in either a heterolytic or a homolytic fashion
would yield 48, whose hydrolysis would provide ketone 46.
Although an unusual reaction for phosphane complexes
themselves at 25 °C, the following CϪP bond cleavage reac-
tion has been observed in refluxing hexane [Equa-
tion (7)].[25] Clearly an oxidative addition to the Ni(0)
center must be involved. Pending further study, it remains
unclear as to whether such reactions are promoted by light
or radical sources.
The usual aryl aldehyde and substituted benzyl phenyl
ketone were also detected upon hydrolysis of the latter reac-
tion mixture.
During our attempts to reproduce the cyclotrimerization
of benzonitrile reported by Heldt[15] as occurring on nickel
metal in refluxing 5, we did observe significant decyanation,
as was evident by low recovery of 5 (Ͻ50%) and by the
detection of cyanide ion in the hydrolysis products with the
‘‘Prussian Blue’’ test.
Significant, Unexpected Reactions Occurring between
Benzonitrile and Nickel(0) Complexes
Formation of Benzamide (41) and 2,4,6-Triphenyl-1,3,5-
triazine (6)
Scheme 10
From the present study and the previous work of Oehme
and Pracejus[16] it is clear that under a completely inert at-
mosphere and anhydrous conditions neither a clean nickel
surface nor a nickel(0) complex reacts with pure benzonitr-
ile at temperatures up to 200 °C to produce any triazine
6. However, in the presence of adventitious moist air such
nickel(0) complexes convert the nitrile to benzamide (41)
even at 25 °C. Nickel(II) salts, in turn, have been shown to
convert benzamide into the triazine 6 at elevated temper-
atures.[16] The so-called catalytic hydration of nitriles to
amides, reported over 40 years ago, involves heating aque-
ous suspensions of amides with nickel metal catalysts of the
Raney or Urushibara type under an air atmosphere.[24] In
the light of our present findings, we can now propose a
rational reaction mechanism involving the formation of a
‘‘side-on’’ complex on the nickel surface (42, with 5), its air-
oxidation to 43 and the hydrolysis via enaminate 44 to the
amide 41 (Scheme 9). In keeping with the intermediacy of
42, significant reduction of the nitrile to the amine or even
reductive cleavage to ammonia has been observed.
Isobutylation of Benzonitrile (5) by Triisobutylaluminum (49)
in the Presence of Bis(1,5-cyclooctadiene)nickel(0) (50)
Nitriles react with diisobutylaluminum hydride at 20Ϫ25
°C to give aldehydes upon hydrolysis but they do not react
with triisobutylaluminum at such temperatures. Only when
nitriles are heated with Bu3Al does reaction occur but then
hydroalumination, rather than carbalumination, is the ob-
served outcome, leading again to aldehydes or primary
amines.[26] Therefore, the extraordinary reaction of benzon-
itrile with 1:1 mixture of Bu3Al and (COD)2Ni to yield iso-
butyl phenyl ketone (51) in 42% yield deserves particular
attention. Such a yield results from reactions of 1:1:1 mix-
tures of the reactants conducted in toluene either over seven
days at 25 °C or 24 hours at reflux. Traces of benzyl phenyl
ketone (33) and benzophenone (1Ϫ5%) were also detected.
To account for the formation of ketone 51, an oxidative
addition of the Ni(0) center of 9a into the isobutyl-alumi-
num bond is reasonable, similar to that depicted in
Scheme 10 to form 52 which decomposes in an analogous
fashion to 53, whose hydrolysis would yield 51 (Scheme 11).
Scheme 9
Ethylation of Benzonitrile (5) in the Presence of
Tetrakis(triethylphosphane)nickel(0) (45)
As with other nickel(0) complexes, the triethylphosphane That the yield of 51 levels off under 50% may be a con-
complex 45 reacted with benzonitrile to produce small sequence of strong Lewis base (53) complexing with half of
Eur. J. Inorg. Chem. 2001, 77Ϫ88
83

J. J. Eisch, X. Ma, K. I. Han, J. N. Gitua, C. Krüger
FULL PAPER
parently in the form of an η4Ϫ1,2-diazacyclobutadiene-
nickel(0) precursor complexed in a ‘‘corner-on’’ fashion
with two nickel(0) units.
Experimental Section
Instrumentation, Analysis and Starting Reagents: All reactions were
carried out under a positive pressure of anhydrous, oxygen-free ar-
gon. All solvents employed with organometallic compounds were
dried and distilled from a sodium metal-benzophenone ketyl mix-
ture prior to use.[28] The IR spectra were recorded with a
PerkinϪElmer instrument, model 457 and samples were measured
either as mineral oil mulls or as KBr films. The NMR spectra (1H
and 13C ) were recorded with a Bruker spectrometer, model EM-
360 and tetramethylsilane (TMS) was used as the internal standard.
The chemical shifts reported are expressed in the δ-scale and in
parts per million (ppm) from the reference TMS signal. The GC/
MS measurements and analyses were performed with a Hewlett-
Packard GC 5890/Hewlett-Packard 5970 Mass-Selective-Detector
instrument. The gas chromatographic analyses were carried out
with a Hewlett-Packard instrument, model 5880, provided with a
6 ft OV-101 packed column or with a Hewlett-Packard instrument,
model 5890, having a 30 m SE-30 capillary column, respectively.
Melting points were determined on a Thomas-Hoover Unimelt ca-
pillary melting point apparatus and are uncorrected. The
(COD)2Ni was prepared according to a procedure adapted from
those of Semmelhack[29] and of Schunn.[30] The yield of product
has been improved to 95% by purifying the starting materials with
extreme care.
Scheme 11
the available Bu3Al and preventing it from reacting with 9a.
As to the mechanism of reaction, there is evidence from
the reaction of diphenylacetylene with 1:1 combinations of
iBu3Al and (COD)2Ni that both photochemical- and rad-
ical-promoted processes may be operative.[27]
The small but significant yield of benzophenone is note-
worthy: the above-mentioned oxidative addition of Ni(0)
into aryl-cyano bonds [Equations (5) and (6)] is un-
doubtedly the cause. At 25 °C the resulting phenylnickel
cyanide (54) would be sufficiently stable to add to the nitrile
5 to form adduct 55, whose hydrolysis would lead to benzo-
phenone (Scheme 12):
The (Et3P)4Ni was prepared according to the method of Cundy[31]
and the (Bpy)(COD)Ni by the procedure of Dinjus et al.[32] The
complex, (Bpy)(PhϪCϵCϪPh)Ni (7), was prepared according to
the following modified procedure, which is based upon the method
of Eisch and Piotrowski.[33]
Scheme 12
A suspension of (COD)2Ni (2.91 g, 10.6 mmol) in 40 mL THF was
treated with 2,2Ј-bipyridyl (1.70 g, 10.7 mmol) in 10 mL THF at
25 °C and the reaction mixture stirred for 1 h. After the solvent
was removed under reduced pressure, the viscous dark purple res-
idue was dried under vacuum overnight. Then 50 mL of THF was
introduced to dissolve this black-purple solid and diphenylacetyl-
ene (1.89 g, 10.6 mmol) in 20 mL of THF was then added dropwise.
The reaction mixture was then stirred at 25 °C for 15 h, after which
time THF was removed under reduced pressure and the black vis-
cous residue dried under vacuum for 16 h in order to remove the
cyclooctadiene. Then 50 mL of THF was used to extract the residue
and the mixture filtered. The filtrate was concentrated to about 20
mL and pentane was then introduced in small portions. After the
first precipitation was observed, another 15 mL of pentane was
added. The mixture was allowed to stand at 25 °C for 2 h to com-
plete the crystallization. The black crystalline solid was then fil-
tered off and was washed with small portions of pentane three
times and then dried under vacuum for 4 h. The yield of the prod-
uct was 3.00 g (72%). Such crystals were employed for X-ray crys-
tallographic structure determination. Ϫ IR (mineral oil): ν˜ ϭ
710(s), 730(s), 755(s), 765(m), 775(m), 850(w), 1025(s), 1490(m),
1570(w), 1590(s), 1740(br, w), 1770(m).
Conclusions
These parallel studies of the homo-oligomerization and
co-oligomerization of alkynes and nitriles show how pivotal
the initial 1:1 adduct of nickel(0) and the triple-bonded sub-
strate is on the course of such oligomerizations. With al-
kynes the reactive ‘‘side-on’’ complex can best be viewed as
a nickelacyclopropene or nickelirene both in terms of its
molecular structural properties and its chemical reactions.
The 1:1 nickel(0) complex with nitriles can be ‘‘end-on’’ as
well as ‘‘side-on’’ (azanickelacyclopropene) with evidence
existing that ‘‘end-on’’ complexation is the more stable
mode and the more important precursor to the homo-oligo-
merization, nickelodecyanation and several unusual addi-
tion reactions of nitriles. In the co-oligomerization of al-
kynes and nitriles the greater stability of the ‘‘side-on’’ com-
plex of nickel(0) with the alkyne monomer over the corres-
ponding ‘‘side-on’’ complex of nickel(0) with nitriles results
in essentially exclusive homocyclotrimerization of the al-
kyne. Only in the absence of free alkyne can the nickelacy-
clopropene apparently be intercepted by the nitrile leading
to a co-dimer with the alkyne. In the homo-oligomerization
of nitriles the 1:1 ‘‘end-on’’ complexation with nickel(0) ap-
General Procedures: All steps in the preparation, transfer, and main
reactions of the metal salts and organometallic reagents studied
here were conducted under an atmosphere of anhydrous and oxy-
pears to lead to an unusual stoichiometric dimerization ap- gen-free argon. All solvents and apparatus were likewise freed of
84 Eur. J. Inorg. Chem. 2001, 77Ϫ88

Nickel(0)-Catalyzed Homo-Oligomerization and Co-Oligomerization of Alkynes and Nitriles
FULL PAPER
traces of dissolved or adsorbed moisture and oxygen by published
procedures and then maintained under argon. Methods and tech-
niques for working under anhydrous and anaerobic conditions have
been described previously.[28] In the hydrolysis procedure employed
for the workup of all nickel(0) reaction mixtures, it proved essential
that the aqueous HCl be thoroughly free of oxygen and that both
freeze-thaw cycles in vacuum and readmission of argon be con-
ducted to that end. Because of the small-scale runs the presence of
oxygen during hydrolysis greatly reduced the amounts of benzal-
dehyde and benzyl phenyl ketone isolated.
benzene (65%) (25); 1,2,3,4-tetramethyloxycarbonyl-5,6-di-
phenylbenzene (26) (20%), m.p. 221Ϫ222°C. Ϫ 1H NMR (CDCl3):
δ ϭ 3.42 (s, 6 H), 7.03 (s, 5 H). Ϫ IR (mineral oil): ν˜ ϭ 710(s),
1000(m), 1100(s), 1210(m), 1460(s). Ϫ dimethyl fumarate (10%). Ϫ
m.p. 102Ϫ103°C. Ϫ H NMR (CDCl3): δ ϭ 3.78 (s, 6 H), 6.80 (s,
2 H); and trans-stilbene (5%).
1
Attempted Co-oligomerization of Diphenylacetylene (22) and
Benzonitrile (5) in the Presence of (COD)2Ni: To a solution of
(COD)2Ni (62.0 mg, 0.226 mmol) in 5 mL THF was added a mix-
ture of benzonitrile (232 mg, 2.26 mmol) and diphenylacetylene
(805 mg, 4.52 mmol) in 10 mL of THF. The reaction mixture was
then stirred at 25 °C for 24 h. A 2-mL sample of the reaction
mixture was hydrolyzed with 4 mL of 3N HCl. The organic layer
was then neutralized with sodium bicarbonate and dried over mag-
nesium sulfate. The solvent was removed under reduced pressure
and the product mixture was analyzed by GC. Diphenylacetylene
(conversion of 5%), benzonitrile (conversion of less than 1%) and
cis-stilbene (yield of 3%) were identified by the co-injection of au-
thentic samples.
Protonolysis of (Bpy)(Ph؊CϵC؊Ph)Ni (7): A solution of 7 (1.00
g, 2.55 mmol) in 20 mL of THF under argon was treated with 10
mL of degassed 85% H3PO4 at 25 °C for 4 h to form an off-white
precipitate and a blue solution. Hydrolytic workup gave an organic
extract containing only cis-stilbene (14) (0.73g, 95%), as determined
1
by IR, H and 13C NMR spectroscopy.
A similar solution of 7 was treated with degassed 6  aqueous
HCl with prompt formation of a pale red-colored precipitate, which
corresponded to a 90% yield of PhCHϭCPh(NiCl·Bpy) (15) based
upon a Ni analysis of 14.0% (calcd. 13.63%), as well as a positive
test for Cl anion upon sodium fusion, and IR spectroscopy: ν˜ ϭ
700(s), 740(s), 790(s), 910(m), 1040(m), 1080(m), 1500 (m, occur-
ring in CϭCϪM groupings), 1540 (vw), 1580(w), 1600(m). The fil-
trate was shown by GC/MS analysis to contain cis- and trans-stil-
benes (14 and 16) in a 9:1 ratio.
The rest of the reaction mixture was heated at reflux for 24 h. After
one hour of reflux a white solid separated and the color of the
solution turned brown. When the reaction mixture was cooled to
25 °C, the crystalline solid was filtered off from the solution and
washed twice with small amounts of THF. The organic layer was
hydrolyzed with 20 mL of 3  aqueous HCl and neutralized with
aqueous NaHCO3. The organic layer was dried over MgSO4 and
the solvent was removed under reduced pressure. Analysis by GC
showed the presence of diphenylacetylene (22) (33%), benzonitrile
(5) (85%) and cis-stilbene (3%) (14). The white solid, 512 mg (65%),
was confirmed to be hexaphenylbenzene (23) by the following spec-
tral characterization: 1H NMR (CDCl3, TMS): δ ϭ 6.84 (m). Ϫ
13C NMR (CDCl3): δ ϭ 125.13, 126.54, 131.36, 140.28 and 140.60
(m). Ϫ IR (KBr): ν˜ ϭ 559(vs), 690(vs), 728(vs), 780(vs), 1028(s),
1072(s), 1406(vs), 1448(vs), 1500(vs), 1607(vs), 3040(vs), 3070(s),
3090 (m) cmϪ1. Ϫ MS (70eV): m/z ϭ 534 [Mϩ]. A mass spectral
search for the parent ion of pentaphenylpyridine, 548, was negative.
Treatment of the pale red solid 15 with glacial acetic acid in THF
gave, upon hydrolytic workup, a quantitative yield of only trans-
stilbene.
Finally, a 1.00-g sample of 7 in THF was treated with degassed
6  DCl in D2O to give the deuteriated pale red solid, PhCDϭ
CPh(NiCl·Bpy) (15a). In separate reactions then, 15a was treated
either with HOAc in THF or with DOAc in THF. In the former
reaction, only trans-stilbene-d1, C6H5CDϭCHC6H5 (16a), was
formed. In the latter reaction, only trans-stilbene-d1,2, C6H5CDϭ
CDC6H5 (16b), was found.
Reaction of (Bpy)(PhCϵCPh)Ni (7) with Benzonitrile (5): A solu-
tion of (Bpy)(PhCϵCPh)Ni (228 mg, 0.580 mmol) in 10 mL THF
was cooled to Ϫ78 °C, after which pure benzonitrile (107 mg,
0.598 mmol) was added dropwise. The reaction mixture was
warmed to 25 °C and stirred at this temperature for 2 days. The
reaction mixture was then heated at reflux for 6 h and then
quenched with 20 mL of 2 aqueous HCl. The organic layer was
neutralized with aqueous NaHCO3 and dried over anhydrous
MgSO4. According to GC/MS analysis and co-injection with au-
thentic samples (provided by American Peptid Corp., Sunnyvale,
CA), the cis- and trans-1,2,3-triphenylpropen-1-ones (32) were
Cyclotrimerization
of
Diphenylacetylene
(22)
by
(Bpy)(Ph؊CϵC؊Ph)Ni (7): A sample of 7 (395 mg, 1.0 mmol) and
diphenylacetylene (1.00 g, 5.6 mmol) was heated in refluxing tolu-
ene for 2 days. During this time a white precipitate formed in the
dark red solution. After hydrolytic workup the solid was filtered
off and shown to be hexaphenylbenzene (23) (0.75g, 75%). Ϫ IR
(mineral oil): ν˜ ϭ 700(m), 735(w), 790(w), 1380(m), 1465(s). Ϫ m.p.
Ͼ400°C. The organic layer contained mainly unchanged 22 and
traces of stilbenes 14 and 16.
Cyclotrimerization of Dimethyl Acetylenedicarboxylate (24) by
(Bpy)(Ph؊CϵC؊Ph)Ni(0): A sample of 7 (5.5 mmol, 2.17 g) and found to be the main products. Total yield of the cis- and trans-
24 (15 mmol, 2.13 g) was heated in refluxing benzene for 36 h.
isomers was 40%. cis-Stilbene (14) (53%) was also one of the main
Usual hydrolytic workup provided an 80% yield (1.70 g) of hexa-
products. Minor products (ഠ2%) were 1,2,3,4-tetraphenyl-1,3-buta-
methyloxycarbonylbenzene (25) , m.p. 186Ϫ188°C (from THF/ diene isomers and trans-stilbene. Spectral characterization of the
H2O), identified by MS and 1H NMR spectroscopic data. Traces
of stilbenes, but no hexamethylbenzene, were observed.
products by GC-MS analysis follows: cis- and trans-1,2,3-triphenyl-
propen-1-ones: C21H16O: m/z ϭ 284 [M]·ϩ, 179 [C14H11]·ϩ, 105
[C7H5O]·ϩ, 77 [C6H5]·ϩ, isomeric butadiene derivatives of MF,
C28H22: m/z ϭ 358 [M]·ϩ, 267 [C21H15]·ϩ, 179 [C14H11]·ϩ, 178
[C14H10]ϩ. Ϫ IR spectrum (thin film): CϭO absorption of 1,2,3-
Co-cyclotrimerization of Dimethyl Acetylenedicarboxylate (24) and
Diphenylacetylene Derived from (Bpy)(Ph؊CϵC؊Ph)Ni (7): A
sample of 7 (5.9 mmol, 2.33g) and 24 (13 mmol 1.85g) dissolved in
30 mL of THF was stirred for 18 h at 25 °C. After hydrolytic
workup and column chromatographic separation of the organic
products on silica gel (40Ϫ60 mesh) with ethyl ether/hexane as the
eluting solvent, the following products were obtained and identified
triphenylpropen-1-one at 1652 cmϪ1
.
Similar reactions were carried out in toluene both at 25 °C and
under reflux. The workup procedures were identical with that of
the reaction carried out in THF. For the reaction in toluene at 25
°C for 8 days, the cis- and trans-stilbenes 14 and 16 were the main
1
by MS and H NMR spectroscopic data: hexamethyloxycarbonyl-
Eur. J. Inorg. Chem. 2001, 77Ϫ88
85

J. J. Eisch, X. Ma, K. I. Han, J. N. Gitua, C. Krüger
FULL PAPER
products with a cis- to trans ratio of 1.3:1.0 and the total yield in
stilbenes of 68%. Also 31% of diphenylacetylene was recovered
from the nickel complex. The reaction carried out in toluene under
reflux for 8 h gave only 10% of cis-stilbene with 85% of the di-
phenylacetylene recovered. Traces (Ͻ2%) of hexaphenylbenzene
were found but neither the 1,2,3,-triphenylpropen-1-ones nor the
1,2,3,4-tetraphenyl-1,3-butadienes were detected.
By (Bpy)(COD)Ni: To the freshly made complex 7 (0.82 mmol)
were added 15 mL of THF and benzonitrile (0.166 mL, 168 mg,
1.63 mmol). The reaction mixture was stirred at 25 °C for 7 days
with no apparent color change of the dark solution. The reaction
was quenched with 20 mL of 2  aqueous HCl and then neutralized
with aqueous NaHCO3. The GC analysis of the dried organic layer
showed that benzyl phenyl ketone 33 had formed in 20% yield,
but benzaldehyde was found in only 5% yield. The conversion of
benzonitrile was 30%.
Oligomerization of Benzonitrile (5) Mediated by Various Nickel(0)
Complexes
The same reaction carried out at reflux for 24 h, but only benzal-
dehyde was obtained in a yield less than 5%; no deoxybenzoin
was detected.
By (COD)2Ni: To a solution of (COD)2Ni (289 mg, 1.05 mmol) in
10 mL THF was added benzonitrile (0.215 mL, 216 mg,
2.10 mmol) dropwise at 25 °C and the reaction mixture then stirred
for 7 days. The color of the solution changed immediately from
yellow to red and then slowly to dark-brown. Samples withdrawn
at reaction times of 24 h, 48 h and 7 days were hydrolyzed with 3
 aqueous HCl, neutralized with aqueous NaHCO3 and finally
dried over anhydrous MgSO4. The resulting product mixtures were
analyzed by GC and GC-MS. After 7 days at 25 °C the main prod-
ucts were identified as benzaldehyde (21) (13%) and benzyl phenyl
ketone (33) (15%) with the conversion of benzonitrile at about 30%.
When small amounts of dry air were slowly admitted to the reac-
tion mixture, variable amounts of benzamide were detected in the
hydrolysis products. Characterization of benzyl phenyl ketone by
By Ni(acac)2/Et3Al: To
a mixture of Ni(acac)2 (203 mg,
0.791 mmol) and benzonitrile (0.161 mL, 163 mg, 1.58 mmol) in 15
mL of toluene was added slowly triethylaluminum (0.216 mL, 180
mg, 1.58 mmol). The color of the solution turned immediately from
green to dark brown. After the reaction mixture had been stirred
at 25 °C for 24 h, hydrolytic workup and analysis showed that 33
was formed in 15% yield.
By (COD)2Ni/MeAlCl2: a) Adding Benzonitrile to the Mixture of
(COD)2Ni-MeAlCl2 in Toluene: One milliliter of a 1  hexane solu-
tion of MeAlCl2 was added dropwise to a solution of (COD)2Ni
(276 mg, 1.00 mmol) in 10 mL of toluene. The formation of a nickel
mirror and a black precipitate were immediately formed. To this
mixture was added benzonitrile (0.10 mL, 1 mmol) and the reaction
mixture stirred at 25 °C for 24 h. The usual workup procedure and
subsequent GC analysis showed that no reaction between benzon-
itrile and the nickel complex had occurred. Of interest is that about
90% of the 1,5-cyclooctadiene had been isomerized to its 1,3-iso-
mer.
GC:MS: C14H12O: m/z ϭ 196 [M]·ϩ, 105 [C7H5]·ϩ, 91 [C7C7]·ϩ
77 [C6H5]·ϩ
,
.
A similar reaction run to the foregoing was conducted, except that
a 2:1 ratio of (COD)2Ni:5 was allowed to react in THF for 24
h. Hydrolytic workup showed that an 80% material recovery was
obtained with the complete consumption of benzonitrile and the
formation of a 96:4 molar ratio of 33:21.
The reaction of benzonitrile with (COD)2Ni was also conducted in
neat benzonitrile. Thus, freshly distilled and degassed benzonitrile
(3 mL, 29.4 mmol) was added to (COD)2Ni (245 mg, 0.892 mmol)
at 0 °C whereupon the reaction mixture changed immediately from
yellow to bright red. After 10 min at 0 °C 15 mL of 3 hydrochloric
acid was added. The usual workup procedure showed that 10% of
benzaldehyde but less than 5% of benzyl phenyl ketone was formed.
The same reaction carried out at 25 °C over 24 h yielded no 33
and only 10% of benzaldehyde.
b) Adding (COD)2Ni to the Mixture of Benzonitrile and MeAlCl2 in
Toluene: A solution of (COD)2Ni (291 mg, 1.06 mmol) in 8 mL of
toluene was added to
a mixture of benzonitrile (0.11 mL,
1.06 mmol) and 1.06 mL of a 1  hexane solution of MeAlCl2 in
7 mL toluene and the reaction mixture then stirred for 24 h at 25
°C. After hydrolysis and GC analysis benzyl phenyl ketone (33) was
found in 20% yield but no trace of benzaldehyde was detected.
By comparison, a similar reaction run in which the MeAlCl2 was
added to a mixture of (COD)2Ni and benzonitrile in toluene led to
the formation of no organic products.
By (COD)2Ni and iBu3Al: The dropwise addition of iBu3Al (0.25
mL, 1.00 mmol) to a solution of (COD)2Ni (275 mg, 1.00 mmol)
in 10 mL of toluene turned the initially yellow solution brown.
Subsequent introduction of benzonitrile (0.10 mL, 1.00 mmol)
caused the gradual development of a red color. The resulting mix-
ture was stirred for 7 days, hydrolyzed and after the usual workup
analyzed by GC-MS procedures. A 42% yield (68 mg) of isobutyl
Oligomerization of Substituted Benzonitriles by (COD)2Ni
Pentafluorobenzonitrile:
A
reddish solution of (COD)2Ni
(1.27 mmol) and 2.54 mmol of the nitrile in 10 mL of THF was
stirred at 25 °C for 24 h, during which time it turned pale yellow.
Usual hydrolytic workup permitted the recovery of only 20% of the
nitrile and yielded only pentafluorobenzene and no other organic
products. It is assumed that any nickel(0) complex with the nitrile
underwent decyanation with the loss of the C6F5 group (vide infra).
phenyl ketone (49) was obtained: MS (70 eV): m/z ϭ 162 [M]·ϩ
,
120 [C8H8O]·ϩ, 105 [C6H5CO]·ϩ, 77 [C6H5]·ϩ. Ϫ IR (Film): ν˜ ϭ
1680 cmϪ1. Benzyl phenyl ketone (5%) and benzophenone (1%)
were also identified.
4-Methoxybenzonitrile: A red solution of (COD)2Ni (1.00 mmol)
and 2.0 mmol of the nitrile in 10 mL of THF was stirred at 25 °C
for 24 h, during which time it turned dark brown. Usual hydrolytic
workup and GC-MS and NMR analysis showed the presence of
75% of the nitrile, 15% of 4-methoxybenzyl 4-methoxyphenyl ke-
tone and 10% of 4-methoxybenzaldehyde with only a trace of ani-
sole (material balance of 90%).
By (Et3P)4Ni: To a solution of the phosphane complex (458 mg,
0.914 mmol) in 15 mL of THF was added benzonitrile (0.186 mL,
188 mg, 1.83 mmol) of. The color of the solution changed from
red-purple to green immediately. The reaction mixture was stirred
at 25 °C for 7 days. Usual hydrolytic workup and a GC/MS analysis
of the organic products showed that ethyl phenyl ketone (44) was
the main product (60%, 130 mg) and 21 (5%) and 33 (10%) were
minor products.
In an identical reaction extended over 6 days, the material balance
fell to 60% and anisole was now a significant product (25%).
44: MS: m/z ϭ 134 [M]·ϩ, 105 [PhCO]·ϩ, 77 [C6H5]ϩ. Ϫ IR (film): Reaction of Nickel Metal with Benzonitrile at Reflux: To a solution
ν˜ ϭ 1684 cmϪ1
.
of Ni(acac)2 (2.56 g 10.0 mmol) in 30 mL of THF was added n-
86
Eur. J. Inorg. Chem. 2001, 77Ϫ88

Nickel(0)-Catalyzed Homo-Oligomerization and Co-Oligomerization of Alkynes and Nitriles
FULL PAPER
butyllithium (12.5 mL, 20.0 mmol, 1.6  in hexane) at Ϫ78 °C, and 130.76, 137.63, 164.82. Ϫ DEPT (CH only): δ ϭ 110.30, 127.29,
the resulting mixture was allowed to come to 25 °C over 2 hours.
The black suspension was then evaporated to dryness under re-
duced pressure. Under argon dry benzonitrile (degassed under Ar
and freshly distilled from P2O5, 13 mL, 0.13 mol) was added and
the reaction mixture was heated at reflux (190Ϫ195 °C) for 40
hours. The resulting dark gray suspension was evaporated to dry-
ness under vacuum and the solid residue was then slurried with
toluene and hydrolyzed with 3  aqueous HCl. Concentration of
the dried toluene extract led to the deposition of 2,4,6-triphenyltri-
128.44, 128.49, 128.92, 130.63, 130.76. Ϫ CIMS: 309 [MH]ϩ, 205,
102).
The product mixture from the last chromatographic fraction was
concentrated and the residue was analyzed by 1H and 13C NMR
spectroscopy. Small amounts of hexaphenylbenzene and benzamide
(Ͻ1%) were observed. The origin of these products can be reason-
ably ascribed to the benzamide formed from benzonitrile by adven-
titious moist air[35] and detailed consideration of their generation
will be the subject of future publications.
1
azine (6) (50 mg, 0.5%): H NMR (CDCl3): δ ϭ 7.61Ϫ7.58 (m, 9
H), 8.80Ϫ8.77 (d of d, 6 H). Ϫ 13C NMR (CDCl3): δ ϭ 128.64,
128.99, 132.49, 136.31, 171.71). Evaporation of the toluene filtrate
left a residue of 45% of recovered benzonitrile.
X-ray Crystallographic Study of 2,2Ј-Bipyridyl(η2-diphenylacetyl-
ene)nickel (7): Crystal Mounting and Data Collection: A black
crystal of 7, 0.58 ϫ 0.58 ϫ 0.14 mm in size, was mounted with a
perfluorinated polyether oil on the tip of a glass fiber on the goni-
ometer head and cooled immediately by an argon current to Ϫ173
°C. Intensity data were collected with an EnrafϪNonius CAD4
diffractometer by a coupled ω-2θ scan technique with speeds vary-
ing from 1.0 to 10.0°/min depending on the standard deviation to
intensity ratio of a preliminary 10°/min scan. The radiation em-
ployed was Mo-Kα with a graphite monochromator. The crystals
of molecular weight 393.1 g molϪ1 have two independent molecules
per unit cell and have a calculated density of 1.35 Mg mϪ3; they
belong to the monoclinic crystal system and the space group of
P21/c, No. 14. The two independent molecules in the asymmetric
unit cell of 7 have the same atom separations within the margin of
experimental error. They differ only with respect to their torsion
angles. Both Ni centers are absolutely planar (sum of angles equals
360°. All other geometrical data correspond to expected values.
The reaction of nickel metal with refluxing benzonitrile was re-
peated six times, with the nickel metal being generated initially
from either NiBr2 or Ni(acac)2 and two equivalents of n-butylli-
thium in THF, as described above. In each case, only traces of 6,
or none at all, were detected in the benzonitrile residue.
The low recovery of benzonitrile in certain runs (Ͻ50%) was an
indication of decomposition of 5 by catalytic decyanation.[34] Veri-
fication of this side reaction was obtained by cooling the unhy-
drolyzed reaction mixture and filtering off the yellow brown precip-
itate. This solid was freed of organic matter by extraction with tolu-
ene to leave a residue of nickel metal and Ni(CN)2. The presence
of Ni(CN)2 was established by a ‘‘Prussian Blue’’ test. The addition
of aqueous ammonium hydroxide dissolved the Ni(CN)2, leaving
the Ni metal residue. To this ammoniacal extract were added, suc-
cessively, aqueous FeSO4 and then aqueous FeCl3, whereupon a
deep blue precipitate formed.
The absorption corrections were made by the ψ-scan method. Ob-
served reflections were defined as those reflections with I Ն 2σ(I).
Only these were used in the solution and refinement of the structure
(F refinement). For refinement based on F2 all reflections were
used. Refinement was done by least-squares; the quantity minim-
ized was Σw(|Yo| Ϫ |Yc|)2, with Y ϭ F or F2. Computations were
done using VAX, Sun4, and Silicon Graphics computers. In addi-
tion to several locally written programs, the following software was
used: S. L. Lawton, R. A. Jacobson, TRACER (cell reduction),
United States Energy Commission, Report IS-1141, Iowa State
University, USA, 1965. P. Coppens, L. Leiserowitz, D. Rabinovich,
DATAP (data reduction), Acta Crystalogr. 1965, 18, 1035. G. M.
Sheldrick, SHELXS-86 (crystal structure determination), Crystal-
lographic Computing 3 (Eds: G. M. Sheldrick, C. Krüger, R. God-
dard), Clarendon Press, Oxford, 1985, dp. 185. G. M. Sheldrick,
SHELXL-93 (least-squares refinement), University of Göttingen,
1993. E. Egert, G. M. Sheldrick, PATSEE (structure solution),
Acta Crystallogr. 1985, A41, 262. P. Main, S. J. Fiske, S. E. Hull,
L. Lessinger, G. Germain, J.-P. Declercq, M. M. Woolfson, MUL-
TAN80 (structure solution), University of York, England and Lou-
vain, Belgium, 1980. W. R. Busing, K. O. Martin, H. A. Levy,
GFMLX a highly modified version of ORFLS (full-matrix least-
squares refinement), Report ORNL-TM-305, Oak Ridge National
Laboratory, Tennessee, USA, 1962; H. D. Flack, (enantiomorph-
polarity estimation), Acta Crystallogr. 1983, A39, 876. P. Roberts,
G. M. Sheldrick, XANADU (calculation of best planes, torsion
angles and idealized hydrogen atom positions), University of Cam-
Reaction of Nickel Metal with Benzonitrile at Reflux (Presence of
Oxygen and Moisture): In conducting the foregoing reaction it was
observed in certain runs that the initially dark gray nickel suspen-
sions turned to a yellow-green color. This color change was shown
to be caused by adventitious entry of moist air into the reaction
vessel. In such runs, 2,4,6-triphenyl-1,3,5-triazine and other reac-
tion products were found upon hydrolysis. Thus a reaction run as
the foregoing with Ni(acac)2 as the source of Ni metal gave a yel-
low-brown suspension after 50 h at 195 °C under an argon atmo-
sphere. The toluene extract was concentrated to 25 mL and then
was allowed to stand at Ϫ10 °C for several days to allow crystalliza-
tion. After the supernatant liquid was removed with a cannula the
cluster of deposited red crystals, which proved unsuitable for XRD
analysis, was quenched with degassed 6  HCl. After Et2O extrac-
tion the organic layer was neutralized with sodium bicarbonate,
dried over anhydrous magnesium sulfate and evaporated under re-
duced pressure. The major organic product proved to be 1,3-di-
phenyl-1,3-propanedione (30 mg, 5%). Ϫ 1H NMR (CDCl3) δ ϭ
4.63 (s, 2 H, diketone tautomer), 6.86 (s, 1 H, enolate vinyl hydro-
gen), 7.57Ϫ7.47 (m, 6 H), 8.11 (d, 4 H), 16.85 (s, 1 H, enolate OH).
Ϫ
13C NMR (CDCl3): δ ϭ 93.17, 127.17, 128.68, 132.47, 135.61,
185.78. Ϫ MS (Positive Ion Electrospray): m/z ϭ 225 [MH]ϩ, 105.
After the usual hydrolytic workup of the supernatant liquid the
organic residue obtained was purified by flash chromatography [sil-
ica gel, gradient elution with hexanes, hexanes/dichloromethane bridge, England, 1976. R. E. Davis, D. R. Harris, DAESD (calcula-
(4:1), and hexanes/ethyl acetate (3:1)] to give 2,4,6-triphenyl,1,3,5- tion of distances and angles), Roswell Park Mem. Inst., USA, 1970.
triazine (6, 2%, based on starting 5) and 4,5,6-triphenylpyrimidine V. Schomaker, K. N. Trueblood, RIGID (rigid-body analysis) Acta
(0.3%), for which latter product the following spectral data were
Crystallogr. 1968, B24, 63. C. K. Johnson, ORTEP (thermal ellips-
obtained: 1H NMR (CDCl3): δ ϭ 7.60Ϫ7.52 (m, 9 H), 8.03 (s, 1 oid plot program), Report ORNL-5138, Oak Ridge National Lab.,
H), 8.31Ϫ8.29 (d of d, 4 H), 8.74Ϫ8.72 (d of d, 2 H). Ϫ 13C NMR Tennessee, USA, 1976. E. Keller, SCHAKAL (molecular drawing),
(CDCl3): δ ϭ 110.31, 127.31, 128.45, 128.50, 128.92, 130.62,
Eur. J. Inorg. Chem. 2001, 77Ϫ88
Chem. Unserer Zeit 1986, 20, 178. B. W. van der Waal, FSYN (slant
87

J. J. Eisch, X. Ma, K. I. Han, J. N. Gitua, C. Krüger
FULL PAPER
[13]
J. J. Eisch, A. A. Aradi, M. A. Lucarelli, Y. Qian, Tetrahedron
Symposium-in-Print 1998, 54, 1169.
plane Fourier syntheses), Technical University of Enschede,
Netherlands, 1975. SYBYL (molecular calculations), Tripos Asso-
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[14]
[15]
[16]
[17]
S. F. A. Kettle, L. E. Orgel, Proc. Chem. Soc. 1959, 307.
W. Z. Heldt, J. Organomet. Chem. 1966, 6, 292.
G. Oehme, H. Pracejus, Z. Chem. 1969, 9, 140.
Leading references: 17aH. Yamazaki, Y. Wakatsuki, Tetrahed-
Crystallographic data (excluding structure factors) for the structure
reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication
no. CCDC-144795. Copies of the data can be obtained free of
charge on application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK (Fax: (internat.) ϩ 44-1223/336-033; E-mail:
deposit@ccdc.cam.ac.uk].
17b
ron Lett. 1973, 3383. Ϫ
Y. Wakatsuki, H. Yamazaki, Syn-
[17c]
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[18]
[19]
W. Keim, F. H. Kowaldt, R. Goddard, C. Krüger, Angew.
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The reported CϭC separation in cyclopropene is 1.304 Ϯ
˚
0.003 A: L. V. Vilkov, V. S. Mastryukov, N. I. Sadova, Deter-
mination of the Geometrical Structure of Free Molecules, Mir
Publishers, Moscow, 1983, p. 122.
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I. W. Bassi, C. Benedicenti, M. Calcaterra, G. Rucci, J. Or-
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Acknowledgments
I. W. Bassi, C. Benedicenti, M. Calcaterra, R. Intrito, G. Rucci,
C. Santini, J. Organomet. Chem. 1978, 144, 225.
M. Uchino and S. Ikeda, J. Organomet.Chem.1971, 33, C41.
J. J. Eisch, Y. Qian, M. Singh, J. Organomet. Chem. 1996, 512,
207: 9-Fluorenone reacts with Ni(0) complexes to yield, upon
hydrolysis, a 25% yield of the dimeric pinacol, 9,9-dihydroxy-
9,9Ј-bifluorenol.
The foregoing research was initiated under a grant from the U. S.
National Science Foundation, fostered with support of the U. S.
Department of Energy, and brought to a successful conclusion with
financial aid to the principal investigator through an Alexander
von Humboldt Senior Scientist Award. Valuable orienting experi-
ments and technical assistance have been provided by Drs. Mona
Singh and Yun Qian.
[22]
[23]
[24] [24a]
[24b]
K. I. Watanabe, Bull. Chem. Soc. Japan 1959, 32, 1290. Ϫ
K. I. Watanabe, Bull. Chem. Soc. Japan 1964, 37, 1325. Ϫ
K. I. Watanabe, K. Saki, Bull. Chem. Soc. Japan 1966,
[24c]
39, 8.
[25]
[26]
D. R. Fahey, J. E. Mahan, J. Am. Chem. Soc. 1976, 98, 4499.
H. Lehmkuhl, K. Ziegler, H. G. Gellert in Methoden der Or-
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XIII/4, 1970, pp. 239Ϫ241.
[1]
[1a]
Leading literature citations:
W. Reppe, O. Schlichting, K.
Klager, T. Toepel, Justus Liebigs Ann. Chem. 1948, 560, 1. Ϫ
[1b]
W. Reppe, O. Schlichting, H. Meister, Justus Liebigs Ann.
Chem. 1948, 560, 93. Ϫ [1c] W. Reppe, W. Schweckendiek, Justus
Liebigs Ann. Chem. 1948, 560, 104.
[27]
J. J. Eisch, X. Ma, M. Singh, G. Wilke, J. Organomet. Chem.
1997, 527, 301. This communication was put into print without
the Elsevier Publishers sending the galley proofs to the senior
author for his approval as they had promised. As a result, the
published communication contains many errors in typography
and layout. Since Elsevier has declined to publish this version
as a corrigendum, the senior author (JJE) has been forced to
prepare a completely corrected and emended version of this
communication. Interested researchers can receive a copy of
this corrected version by writing to the senior author.
J. J. Eisch, Organometallic Syntheses, Vol. 2, Academic Press,
New York, 1981.
M. F. Semmelhack, Organic Reactions, Vol. 19, Wiley, New
York 1972, p. 178.
R. A. Schunn, Inorganic Syntheses, Vol. 15, Wiley, New York,
1974, p. 5.
C. S. Cundy, J. Organomet. Chem. 1974, 69, 305.
E. Dinjus, I. Gorski, E. Uhlig, H. Walther, Z. Anorg. Allg.
Chem. 1976, 75, 422.
[2]
[2a]
Comprehensive reviews of such oligomerizations:
B. Bog-
danovic, B. Henc, A. Lösler, B. Meister, H. Pauling, G. Wilke,
[2b]
Angew. Chem. 1974, 85, 39. Ϫ
G. Wilke, J. Organomet.
[2c]
Chem. 1980, 200, 349. Ϫ
J. J. Eisch, S. R. Sexsmith, Res.
Chem. Intermed. 1990, 13, 149.
[3]
[4]
[5]
Comprehensive Organometallic Chemistry II, Vol. 12 (Eds.: E.
W. Abel, F. G. A. Stone, G. Wilkinson), Pergamon Press, Ox-
ford, 1995.
[28]
[29]
[30]
Overviews: [4a] J. W. Copenhaver, M. H. Bigelow, Acetylene and
Carbon Monoxide Chemistry, Reinhold, New York, 1960. Ϫ [4b]
G. N. Schrauzer, Adv. Organomet. Chem.1964, 2, 1.
The actual catalyst for the cyclotetramerization of acetylene is
thought to be ‘‘naked’’ nickel(0) in all cases (‘‘naked’’ meaning,
in G. Wilke’s view, that the ligands available to nickel, such as
COD, are easily displaced by the alkyne.) The induction period
observed for the oligomerization with nickel(II) salts is thought
to entail the necessary reduction of nickel(II) to nickel(0), the
actual catalyst. If nickel(0) is added at the start, there is no
induction period and the oligomerization starts promptly (Pro-
fessor Günther Wilke, observations and views communicated
personally to J. J. E., 1985). In addition, cf. G. Wilke, Pure
Appl. Chem. 1978, 50, 677.
J. J. Eisch, J. E. Galle, J. Organomet. Chem. 1975, 96, C23.
J. J. Eisch, G. A. Damesevitz, J. Organomet. Chem. 1975, 96,
C19.
J. J. Eisch, A. A. Aradi, K. I. Han, Tetrahedron Lett. 1983,
24, 2073.
J. J. Eisch, A. M. Piotrowski, K. I. Han, C. Krüger, Y. H. Tsay,
Organometallics 1985, 4, 225.
[31]
[32]
[33]
[34]
J. J. Eisch, A. M. Piotrowski, unpublished studies.
For leading references to reductive cyanation by main-group
and transition metal reagents, cf. J. March, Advanced Organic
Chemistry, 4th ed., Wiley, New York, 1992, p. 634.
Precedents for the transformations of benzamide into the prod-
ucts observed in this reaction have been observed: 1) into triaz-
ine 6: ref. [16]; 2) into diphenylacetylene (22) by subvalent tita-
nium reagents: R. Dams, M. Malinowski, H. J. Geise, Recl.
Trav. Chim. Pays-Bas 1982, 101, 112; 3) the co-oligomerization
of benzonitrile, diphenylacetylene and possibly Ni(CN)2 into
4,5,6-triphenylpyrimidine: Scheme 7, the present work; 4) the
cyclotrimerization of 22 into hexaphenylbenzene (23) by
nickel(0): ref.[11] and the present work; and 5) the possible
coupling of two molecules of benzonitrile and Ni(CN)2 by
nickel(0) to form precursors to 1,3-diphenyl-1,3-propanedione
coupling related to that in Scheme 8, the present work.
Received March 29, 2000
[35]
[6]
[7]
[8]
[9]
[10]
[11]
J. J. Eisch, Pure & Applied Chem. 1984, 56, 35.
J. J. Eisch, A. M. Piotrowski, A. A. Aradi, C. Krüger, M. J.
Romano, Z. Naturforsch. 1985, 40b, 624.
J. J. Eisch, J. E. Galle, A. A. Aradi, M. P. Boleslawski, J. Or-
ganomet. Chem. 1987, 312, 399.
[12]
[I00121]
88
Eur. J. Inorg. Chem. 2001, 77Ϫ88