Novel azine reactivity: Facile N-N bond cleavage, C-H activation, and N-N coupling mediated by RhI

Article abstract of DOI:10.1002/anie.200250571

An unprecedented reaction: Dinitrogen pincer Rh complexes react with azines in a moderately catalytic fashion by nonsymmetrical N-N bond cleavage to the corresponding nitriles and imines (see scheme).

Full text of DOI:10.1002/anie.200250571

Angewandte
Chemie
Novel Azine Reactivity
ꢀ
Novel Azine Reactivity: Facile N N Bond
ꢀ
ꢀ
Cleavage, C H Activation, and N N Coupling
Mediated by RhI**
Revital Cohen, Boris Rybtchinski, Mark Gandelman,
Linda J. W. Shimon, Jan M. L. Martin,* and
David Milstein*
¼ ꢀ ¼
Azines (R2C N N CR2) are very useful compounds. For
example, they undergo unusual [1,3] dipolar cycloaddition
reactions (the crisscross addition) with dienophiles, which
provide a convenient route to five-membered rings.[1] They
also participate in [3þ2] cycloaddition reactions as an ene
fragment.[2] Applications of azines include possible nonlinear
optical materials[3] and conducting polymers (polyazines).[4]
Relevant to dinitrogen fixation, the formation of azines by
metal-mediated functionalization of N2 was demonstrated.[5]
Azine derivatives display important biological properties and
are of importance for drug development.[6] Complexes of
titanium,[7] zirconium,[7c] cobalt,[7c] uranium,[8] and iron[9] are
ꢀ
capable of cleaving the N N bond in azines. In all the
ꢀ
reported cases, the azine N N bond is cleaved symmetrically
ꢀ ¼
forming two imide units, N CR2, coordinated to a metal
center. No metal-complex-catalyzed reactions were reported.
Herein we report a novel type of azine reactivity, the
ꢀ
rhodium-promoted catalytic “nonsymmetrical” N N bond
ꢀ
cleavage accompanied by C H activation, which leads to an
imine and benzonitrile. This is the first metal-promoted
nonsymmetric cleavage of the N N bond in an azine and
the first metal-catalyzed N N activation in an azine.
[10]
ꢀ
ꢀ
When a toluene solution of the PCP-pincer rhodium
complex 1[11] was treated with one equivalent of benzalazine
at ꢀ308C, three complexes were formed (Scheme 1). One of
them, tentatively identified by NMR spectroscopy as the
[(PCP)Rh(benzalazine)] complex 2 (precedents exist for
azine complexes[7]), is an intermediate, which converts into
[*] Prof. Dr. J. M. L. Martin, Prof. Dr. D. Milstein, R. Cohen,
Dr. B. Rybtchinski, M. Gandelman
Department of Organic Chemistry
The Weizmann Institute of Science, 76100 Rehovot (Israel)
Fax: (+972)8-9344142
E-mail: comartin@wicc.weizmann.ac.il
david.milstein@weizmann.ac.il
Dr. L. J. W. Shimon
Chemical Services Unit, The Weizmann Institute of Science
76100 Rehovot (Israel)
[**] This work was supported by the Israel Science Foundation and by
the MINERVA Foundation, Munich (Germany). We thank Yehoshoa
Ben-David for technical assistance and Dr. Leonid Konstantinovsky
for helpwith NMR measurements. D.M. is the holder of the Israel
Matz Professorial Chair of Organic Chemistry.
Supporting information for this article (preparation procedures for
complexes 3, 4, and 6, and xyz coordinates of the computed
structures) is available on the WWW under http://www.angewand-
te.org or from the author.
Angew. Chem. Int. Ed. 2003, 42, 1949 – 1952
DOI: 10.1002/anie.200250571
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1949

Communications
ature, the imine complex 3 decomposed to unidentifiable
products. Complex 4 was stable at room temperature for days.
The [(PCN)Rh(N2)] complex 5 exhibits reactivity similar
to that of 1. Upon reaction with benzalazine quantitative
formation of the benzonitrile complex 6 (Table 1) was
¼
observed together with formation of free imine PhCH NH
(Scheme 2).[13] This imine is unstable and rapidly decomposes
Scheme 1. Reaction of benzalazine with 1.
Scheme 2. Reaction of benzalazine with 5.
the two new complexes 3 and 4 in a 1:1 ratio within an hour at
room temperature.
The phenylimine complex 3 and benzonitrile complex 4
were characterized by various NMR spectroscopy techniques
(10 min at room temperature), in agreement with literature
data.[14] The structure of 6 determined by crystal-structure
analysis is presented in Figure 1. Complex 6 is square planar
1
(31P, H, 13C, and 13C-1H and 15N-1H correlation experiments,
Table 1). Formation of the benzonitrile complex 4 was
verified by its independent synthesis from complex 1 and
PhCN. Complex 3 could not be isolated and was characterized
in a mixture with 4. The exact mode of imine coordination in 3
is uncertain and an h1-N/h2-C,N equilibrium of the coordi-
nated imine molecule may exist. Primary imine complexes are
known,[12] but to our knowledge, their direct synthesis from
azines has not been reported. Within a day at room temper-
Table 1: Selected spectroscopic data for compounds 3, 4, and 6. NMR
spectra in [D8]toluene (1H: 400 MHz, 13C: 100 MHz, 15N: 40 MHz, 31P:
162 MHz), d in ppm, J in Hz.
3: 31P NMR: d=61.50 (d, JRh,P =166.0); 1H NMR: d=10.25 (d,
¼
¼
J
H,H =14.7, 1H, HN CHPh), 7.96 (d, JH,H =14.7, 1H, HN CHPh);
13
C NMR: d=166.09 (brs, HN CHPh); 15N-1H correlation NMR (refer-
¼
¼
enced to liq. NH3): d=392.12 (brs, HN CHPh, JN,H =65.0); the NMR
assignment is verified by H-1H COSY, heteronuclear 13C-1H and 15N-1H
1
Figure 1. Molecular structure of 6. Selected bond lengths [Š] and bond
angles [8]: Rh2-C41=1.986(11), Rh2-N4=2.033(9), Rh2-P4=2.197(4),
Rh2-N8=2.196(11), N4-C5=1.159(15); N4-Rh2-C41=174.2(7), P4-
Rh2-N8=163.0(3). CCDC-197955 contains the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge
CB21EZ, UK; fax: (+44)1223-336-033; or deposit@ccdc.cam.ac.uk).
correlation experiments and 13C DEPT-135
4: 31P NMR: d=63.85 (d, JRh,P =162.0); 1H NMR: d=7.16–7.10 (m, 3H,
aryl), 7.02–6.92 (m, 2H, aryl), 6.77 (m, 2H, aryl), 6.68 (m, 1H, aryl), 3.10
(vt, JP,H =7.2, 4H, PCH2-aryl), 2.04 (m, 4H, (CH3)2CHP), 1.36 (dvt,
J
P,H =15.6, JH,H =7, 12H, (CH3)2CHP), 1.19 (dvt, JP,H =13.0, JH,H =7,
12H, (CH3)2CHP); 13C NMR: d=180.33 (dt, JRh,C =36.8, JP,C =12.2, Cipso),
157.17 (t, JP,C =12.8, PhCNRh), 137.40 (s, aryl), 135.83 (s, aryl), 134.52
(s, aryl), 134.35 (s, aryl), 133.93 (s, aryl), 127.01 (s, aryl), 125.78 (t,
J
J
P,C =9.6, aryl), 41.82 (dvt, JP,C =21.6, JRh,C =4.1, PCH2-aryl), 31.17 (vt,
P,C =17.2, (CH3)2CHP), 24.40 (s, (CH3)2CHP), 19.44 (s, (CH3)2CHP); IR:
with the PhCN molecule occupying the position trans to the
aryl ring. Unlike the case of the (PCP)Rh-based system, no
azine or imine complex formation was observed, probably
reflecting the more sterically congested character of the
(PCN)Rh unit in 5. Formation of the benzonitrile complex 6
was verified by independent synthesis from complex 5 and
PhCN.
n˜ =2183.8 cmꢀ1 (m, nCꢁ
)
N
6: 31P NMR: d=7.51 (d, JRh,P =231.3); 1H NMR: d=7.19–7.16 (m, 2H,
aryl), 6.98–6.93 (m, 3H, aryl), 6.52 (brs, 1H, aryl), 3.90 (s, 2H, aryl-
CH2N), 3.07 (d, JP,H =8.2, 2H, PCH2-aryl), 2.86 (m, 2H, CH3CH2N), 2.76
(m, 2H, CH3CH2N), 2.36 (s, 3H, aryl-CH3), 2.24 (s, 3H, aryl-CH3), 1.63
(t, JH,H =7.2, 6H, CH3CH2N), 1.48 (d, JP,H =11.7, 18H, (CH3)2CHP);
13C NMR: d=178.82 (dd, JRh,C =37.7, JP,C =10.0, Cipso), 146.55 (dd,
When benzalazine was used in excess (10 equiv) with
either complex 1 or 5, an unprecedented catalytic reaction
producing 7 equivalents of benzonitrile took place
J
Rh,C =19.1, JP,C =6.7, PhCNRh), 145.86 (s, aryl), 130.81 (s, aryl), 130.55
(s, aryl), 129.57 (s, aryl), 127.95 (s, aryl), 123.81 (s, aryl), 118.99 (s, aryl),
115.12 (s, aryl), 65.12 (s, aryl-CH2N), 54.48 (s, NCH2CH3), 35.76 (dd,
JP,C =26.1, JRh,C =5.6, aryl-CH2P), 35.19 (dd, JP,C =11.7, JRh,C =1.9,
¼
(Scheme 3). The imine PhCH NH could not be detected,
(CH3)2CP), 30.42 (d, JP,C =5.8, (CH3)2CP), 22.18 (s, aryl-CH3), 20.96 (s,
aryl-CH3), 13.46 (s, NCH2CH3)
probably because of its fast decomposition.[14] The same
turnover number (TON) was observed upon addition of
1950
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Angewandte
Chemie
Scheme 3. Catalytic reaction of 1 or 5 with benzalazine.
100 equivalents of the azine, which indicates that the catalysis
is hampered by the benzonitrile product in both the PCP and
PCN cases; PhCN coordinates to the rhodium centers to form
the complexes 3 and 6, and interferes with the azine activation
process, which causes low TONs. Optimization of the catalysis
ꢀ
Scheme 5. Computational analysis of N N cleavage in azine in a
model system.
found to have a kinetic barrier of 63 kcalmolꢀ1 (and to be
endergonic by 11.6 kcalmolꢀ1), grossly inconsistent with the
experimental observation that the reaction takes place even
at ꢀ308C. The optimized geometry of the bisimide complex
exhibits a large deviation from planarity in the chelate core,
the two phosphanyl groups acquiring a close to cis config-
uration (P-Rh-P angle is 1088). This is likely to result from the
strong trans effect of the imide ligands, which destabilizes the
complex and causes a high activation barrier as a consequence
of the large change in geometry compared to the azine
is currently being sought in our group. Stoichiometric (not
[7]
ꢀ
ꢀ
catalytic) C H and N N activation in azines are reported,
but not the combination of both in one system as in the
(PCP)Rh-based one.[15]
¼ ꢀ
Treating complexes 1 or 5 with the ketazine Me(Ph)C N
N C(Ph)Me revealed no reactivity even at elevated temper-
¼
atures. Interestingly, when the nonsymmetrical azine
¼ ꢀ ¼
Me(Ph) N N C(Ph)H was reacted with complex 1 in
toluene, the same organometallic products as in the benza-
lazine case, complexes 3 and 4, were formed in a 1:1 ratio
within one hour at room temperature together with an
ꢀ
complex. Thus, most probably, direct N N activation is not
operative in our systems—in contrast to the reported
examples.[7–9] Computational investigation of the mechanism
is currently under way.
¼ ꢀ ¼
equivalent amount of the ketazine Me(Ph)C N N
C(Ph)Me, as observed by NMR spectroscopy and confirmed
by GC-MS (Scheme 4).[16]
In conclusion, we have reported a novel type of azine
ꢀ
reactivity, the nonsymmetrical rhodium-mediated N N bond
ꢀ
cleavage, coupled with C H activation to form a nitrile and an
imine. The reaction is catalytic, providing the first example of
ꢀ
metal-complex-catalyzed N N bond cleavage in an azine,
although deactivation by the nitrile product takes place. It
ꢀ
most probably does not proceed through direct N N bond
ꢀ
cleavage, but through a mechanism involving a C H activa-
ꢀ
ꢀ
tion step. A metal-promoted N N bond cleavage/N N
coupling sequence was also demonstrated that led to a
symmetric ketazine from a mixed aldazine/ketazine molecule.
These reactions provide new insight into metal-promoted
transformations of azines. The synthetic implications of these
findings are being explored.
Scheme 4. Reactivity of 1 towards the nonsymmetrical azine
¼ ꢀ ¼
Me(Ph) N N C(Ph)H.
Received: November 18, 2002 [Z50571]
ꢀ
Keywords: azines · C H activation · density functional
.
ꢀ
calculations · N N activation · pincer ligands
Thus, a symmetric ketazine molecule can be formed from
two nonsymmetric aldazine/ketazine molecules. Such a metal-
ꢀ
promoted apparent N N bond cleavage/coupling sequence is
[1] a) “Azomethine Imines”: R. Grashey, A. Padwa in 1,3-Dipolar
Cycloaddition Chemistry, Vol. 1 (Eds.: D. C. Taylort, A. Weiss-
berger), Wiley, New York, 1984, p. 733; b) S. Radl, Aldrichimica
Acta 1997, 30, 97.
[2] See, for example: E. E. Schweizer, Z. Cao, A. L. Rheingold, M.
Bruch, J. Org. Chem. 1993, 58, 4339.
[3] H. S. Nalwa, A. Kakatu, A. Mukoh, J. Appl. Phys. 1993, 73, 4743.
[4] E. C. Kesslen, W. B. Euler, Chem. Mater. 1999, 11, 336.
[5] Y. Nishibayashi, S. Iwai, M. Hidai, Science 1998, 279, 540; S. M.
Rocklage, R. R. Schrock, J. Am. Chem. Soc. 1982, 104, 3077.
[6] a) A. I. Kholdair, P. Bertrand, Tetrahedron 1998, 54, 4859; b) K.
Yasumura, K. Miyajima, T. Nagahama, S. Ishikawa, Y. Tohyama,
K. Sugiyama. PCT, WO 94/19335.
[7] a) M. Rep, J.-W. F. Kaagman, C. J. Elsevier, P. Sedmera, J. Hiller,
U. Thewalt, M. Horacek, K. Mach, J. Organomet. Chem. 2000,
597, 146; b) A. Ohff, T. Zippel, P. Arndt, A. Spannenberg, R.
unprecedented to our knoweledge. Importantly, only the
aldazine “half” gives rise to the imine (and nitrile) complexes.
ꢀ
These observations imply involvement of a C H activation
and suggest that a complex reaction mechanism, probably
bimolecular, is operative. We believe that the azine splitting
observed in the (PCP)Rh- and (PCN)Rh-based systems does
ꢀ
not proceed through direct metal insertion into the N N
bond. Further confirmation was obtained from a computa-
[17]
ꢀ
tional DFT study of the N N cleavage step (at the mPW1k
/
LANL2DZ + P//mPW1k/LANL2DZ[18] level of theory[19]).
Our model system included a PCP ligand with H-substituted
phosphanyl groups and a formaldazine molecule. (No spin-
ꢀ
state crossing was considered.) The direct N N bond cleavage
producing the bisimide rhodium complex (Scheme 5) was
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ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1951

Communications
Kempe, U. Rosenthal, Organometallics 1998, 17, 1649; c) T.
Zippel, P. Arndt, A. Ohff, A. Spannenberg, R. Kempe, U.
Rosenthal, Organometallics 1998, 17, 4429.
[8] J. L. Kiplinger, D. E. Morris, B. L. Scott, C. J. Burns, Organo-
metallics 2002, 21, 3073.
[9] A. Zimniak, G. Bakalarski, J. Mol. Struct. 2001, 597, 211.
ꢀ
[10] Photolytic N N bond cleavage in benzalazine to form benzoni-
trile and benzalimine is known: R. W. Binkley, J. Org. Chem.
1969, 34, 2073.
[11] M. Gandelman, B. Rybtchinski, N. Ashkenazi, R. Gauvin, D.
Milstein, J. Am. Chem. Soc. 2001, 123, 5372.
[12] See, for example: a) W.-Y. Yeh, C.-S. Ting, S.-M. Peng, G.-H.
Lee, Organometallics 1995, 14, 1417; b) D. A. Knight, M. A.
Dewey, G. A. Stark, B. K. Bennet, A. M. Arif, J. A. Gladysz,
Organometallics 1993, 12, 4523, and references therein.
1
¼
[13] PhCH NH was identified by comparison of its H NMR data to
the reported ones: D. R. Boyd, P. B. Coulter, R. Hamilton, N. T.
Thompson, N. D. Sharma, M. E. Stubbs, J. Chem. Soc. Perkin
Trans. 1 1985, 2123.
¼
[14] PhCH NH was reported to be unstable at room temperature; it
decomposes forming oligomers (see ref. [13]).
[15] We detected only traces of benzonitrile upon reaction of
benzalazine with [(iPr3P)2RhCl] or [(iPr3P)2Rh(N2)Cl].
[16] In the absence of 1, no reaction was observed after several hours
at room temperature. Acid- and water-catalyzed disproportion-
ation of the nonsymmetrical azine to the aldazine and ketazine
was reported: D. H. Kenny, J. Chem. Educ. 1980, 57, 462.
[17] B. J. Lynch, P. L. Fast, M. Harris, D. G. Truhlar, J. Phys. Chem. A
2000, 104, 4811.
[18] P. J. Hay, W. R. Wadt, J. Chem. Phys. 1985, 82, 299.
[19] See Supporting Information for full details.
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