Triisopropylsilyl protected hexa-1,5-diyne-3,4-dione: A convenient precursor to 2,3-dialkynyl 1,4-diazabutadienes

Article abstract of DOI:10.1055/s-1998-1559

Diazabutadienes with a 2,3-dialkynyl substitution pattern are easily accessible by reaction of silyl-protected dialkynyl-1,2-diones with primary aromatic amines.

Full text of DOI:10.1055/s-1998-1559

64
LETTERS
SYNLETT
Triisopropylsilyl Protected Hexa-1,5-diyne-3,4-dione: A Convenient Precursor to 2,3-
Dialkynyl 1,4-Diazabutadienes
Rüdiger Faust,* Bernd Göbelt, and Christian Weber
Pharmazeutisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 364, D-69120 Heidelberg, Germany
Fax: ++49 - 6221 - 54 64 30; e mail: faust@convex.phazc.uni-heidelberg.de
Received 23 September 1997
Abstract
: Diazabutadienes with a 2,3-dialkynyl substitution pattern are
easily accessible by reaction of silyl-protected dialkynyl-1,2-diones
with primary aromatic amines.
direct the nucleophilic attack of the amine nitrogen to the ketone moiety.
4
5
Indeed, the unsymmetrically substituted diketone
,
bearing one
terminal phenyl- and one triisopropylsilyl-substituent, gave 1,4- as well
6
as 1,2 addition in xylene to furnish .
1
The development of diimine-ligated transition metal complexes as
components of efficient catalyst systems for olefin polymerisation by
1
Brookhart et al. has inspired a renewed interest in the chemistry of 1,4-
diazabuta-1,3-dienes. The steric and the electronic properties of these
2
versatile ligands can be fine-tuned by varying the substituents at the
nitrogens and/or at the internal carbon centers of the diazabutadiene
backbone.
Consequently, and most gratifyingly, shielding both alkyne units of the
5
7
diketones by triisopropylsilyl groups as in
led to the targeted
dialkynyl diazabutadiene in 70% yield after reaction with two equiv.
3,5-dimethyl aniline in refluxing xylene in the presence of catalytic
7
amounts of LiBr.
Previously, substitution at the 2,3 positions was limited to H, Me or aryl
until we recently reported a general route for the introduction of organyl
groups by a palladium-mediated cross-coupling procedure starting from
3
bis(imidoyl chlorides) and organostannanes. A particularly intriguing
result of this work was the successful preparation of the first 2,3-
2
dialkynyl 1,4-diazabutadienes as exemplified by . Our interest in small
It is evident from these findings that the preparation of acetylenic
diazabutadienes by this procedure is limited to precursors with sterically
encumbered alkynyl groups. Since terminal alkynes bearing a less bulky
substituent are readily introduced by palladium-mediated cross-
acetylenic building blocks for the assembly of alkynylated near-infrared
chromophores fueled continuous efforts to explore synthetic methods
for their generation. Reported below is an alternative route for the
preparation of dialkynyl diazabutadienes starting from
bis(triisopropysilyl)-protected dialkynyl 1,2-dione.
a
3
coupling procedures from bis(imidoyl chlorides), the above diimine
4,5
7
formation from represents a useful complementary entry to dialkynyl
7
diazabutadienes, particularly in view of the fact that can be prepared in
5
Initial attempts to generate dialkynyl diazabutadienes by simply
one step from oxalyl chloride. Furthermore, the condensation of aniline
4,5
7
3
derivatives with allows the preparation of electronically modified
condensing 1,6-diphenylhexa-1,5-diyne-3,4-dione
with primary
imine substituents for which in some cases bis(imidoyl chlorides) are
available only with difficulty (Table 1).
aromatic amines were hampered by the dominant 1,4-addition to the
8
3
reactive ynone-moiety. Hence, treatment of with 3,5-dimethylaniline
4
in toluene furnished exclusively the vinylogous bisamide , whose
double bond geometry was ascertained by NMR spectroscopy. While
this mode of addition to the dialkynyl diones points the way to a
convenient entry into bi-heterocyclic structures and β-amino acids, it
obviously obstructs the formation of diazabutadiene derivatives.
7
While in general, diimine formation from
proceeds smoothly in
refluxing xylene, the use of glacial acetic acid as a reaction medium was
found to be beneficial in terms of reaction time and yield (cf. formation
8 11 14
12
of
,
, and ). In case of the acid-sensitive methoxy derivative ,
the yield could be significantly increased by using a TiCl -mediated
4
imine-formation procedure. This Lewis acid-promoted condensation
reaction turned out to be the method of choice for generating N-(p-CF -
3
16
phenyl)-substituted dialkynyl diazabutadiene , for which all other
modes of preparation had failed. The 2,3-dialkynyl 1,4-diazabutadienes
8
12
,
are solid, air stable materials whose colors range from yellow (
-
9
14 16
15
13
,
) over orange ( ) to red ( ).
4,5
Our earlier work on dialkynyl diketones had reconfirmed the fact that
The good directing properties and the retarding influence of the
triisopropylsilyl groups in the formation of dialkynyl diazabutadienes
the bulky triisopropylsilyl group may serve as an efficient control
element by sterically shielding the alkyne unit to which it is attached.
6
7
from is also evident from the fact that the twofold imine formation can
We therefore treated triisopropysilyl protected dialkynyl-1,2-diones
with aniline derivatives, hoping that the terminal substituent would
be controlled to provide differently N-substituted push-pull
diazabutadienes.

January 1998
SYNLETT
65
Acknowledgment. This work was financially supported by the Fonds
der Chemischen Industrie, Germany. We are grateful to Isabell Hamm
for skillful experimental assistance, to Dr. W. Kramer for conducting
NMR experiments, and to Prof. Dr. R. Neidlein for his continuing
interest in our work.
References and Notes
(1) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc.
1995, 117, 6414 - 6415. Johnson, L. K.; Mecking, S.; Brookhart,
M. J. Am. Chem. Soc. 1996, 118, 267 - 268. Killian, C. M.;
Tempel, D. J.; Johnson, L. K.; Brookhart, M. J. Am. Chem. Soc.
1996, 118, 11664 - 11665. Killian, C. M.; Johnson, L. K.;
Brookhart, M. Organometallics 1997, 16, 2005 - 2007. Abu-
Surrah, A. S.; Rieger, B. Angew. Chem. 1996, 108, 2627 - 2629;
Angew. Chem. Int. Ed. Engl. 1996, 35, 2475 - 2477.
(2) van Koten, G.; Vrieze, K. Adv. Organomet. Chem. 1982, 21, 151 -
239. Vrieze, K. J. Organomet. Chem. 1986, 300, 307 - 326.
(3) Faust, R.; Göbelt, B., Tetrahedron Lett., in press.
(4) Faust, R.; Weber, C. Liebigs Ann. 1996, 1235 - 1238.
(5) Faust, R.; Weber, C.; Fiandanese, V.; Marchese, G.; Punzi, A.
Tetrahedron, 1997, 53, 14655 - 14670.
(6) Rücker, C. Chem. Rev. 1995, 95, 1009 - 1064.
(7) The presence of the weak Lewis acid LiBr appears to be
mandatory for the successful conversion of 7 to 1,2-diimines by
method A.
(8) Lindauer, D.; Beckert, R.; Döring, M.; Fehling, P.; Görls, H. J.
Prakt. Chem. 1995, 337, 143 - 152.
(9) Typical procedure for method A: A mixture of dialkynyl-1,2-
5
dione 7 (1 mmol), the corresponding aniline derivative (3 mmol),
a catalytic amount of LiBr, and activated molecular sieves was
refluxed in dry xylene (25 ml) for the time specified in Table 1.
After filtration and removal of the solvent the residue was
chromatographed on SiO using hexane/ethyl acetate (20/1).
2
5
Typical procedure for method B: To a solution of 7 (1 mmol) in
glacial acetic acid (25 ml) was added the aniline derivative (3
mmol) and the solution subjected to the conditions specified in
Table 1. After completion of the reaction, the mixture was poured
into water (100 ml) and the solution extracted with diethyl ether.
The crude product remaining after drying over Na SO , filtration,
2
4
and removal of the solvent was chromatographed on SiO using
2
hexane/ethyl acetate (20/1). Typical procedure for method C: To a
5
solution of 7 (1 mmol), the aniline derivative (3 mmol), and NEt
3
(2 ml) in diethyl ether (50 ml) under an argon atmosphere was
added an etheral solution (10 ml) of TiCl (1 mmol) while cooling
4
Hence, reacting 7 with one equiv. p-dimethylamino aniline or with one
equiv. p-nitroaniline yields the iminoketones 17 and 18, respectively,
which serve as intermediates en route to unsymmetrical dialkynyl
with an ice-bath. After completion of the reaction (Table 1) the
mixture was filtered through a pad of silica gel and the solvents
were removed in vacuo. The crude product was chromatographed
diazabutadiene 19, prepared by
a subsequent TiCl -promoted
4
on SiO using hexane/ethyl acetate (20/1). All new compounds
2
condensation with one equiv. of the corresponding second aniline. It is
noteworthy that the greater nucleophilicity of p-dimethylamino aniline
combined with the tendency of 17 to decompose on silica gel renders
the preparation of 19 via 18 the synthetically more useful route.
Compound 19 is a deep red microcrystalline solid with a gold-green
metallic luster.
were fully characterized and gave correct microanalytical data.
See, for example, selected spectroscopic data for (10): yellow
-1
solid, m.p. 109 °C. – IR (KBr): ν = 2154 (w, C≡C), 1577 cm (s,
C=N). – UV (CH Cl ): λ (ε) = 252 (16400), 384 nm (9700).
max
2
2
9
1
H-NMR (250 MHz, CDCl ): δ = 7.26 (d, J = 7 Hz, 4 H), 7.15 (d,
J = 7 Hz, 4 H), 2.35 (s, 6 H, CH ), 1.03 (s, 42 H, Si(i Pr) ). – C-
3
13
3
3
NMR (90.6 MHz, CDCl ): δ = 148.7 (C), 147.5 (C), 136.0 (C),
3
129.0 (CH), 121.7 (CH), 104.0 (C≡C), 98.5 (C≡C), 21.1 (CH ,
The present work has established 1,6-bis(triisopropylsilyl)hexa-1,5-
diyne-3,4-dione 7 as a valuable precursor to various 2,3-dialkynyl 1,4-
diazabutadienes. An in-depth evaluation of their structural, chemical,
and electrochemical properties as well as an investigation of their
coordination chemistry is under way.
3
arom.), 18.5 (CH , Si(i Pr) ), 11.2 (CH, Si(i Pr) ). – MS (70 eV),
3
3
3
+
+
+
m/z (%): 596 (60) [M ], 581 (6) [M - CH ], 554 (2) [M - i Pr],
298 (100) [M /2]. – C
3
+
H N Si (596.40): calcd. C 76.46, H
38 56 2 2
9.46 N 4.70; found C 76.37, H 9.64 N 4.64. For (19): deep red

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SYNLETT
solid, m.p. 102 °C. – IR (KBr): ν = 2145 (w, C≡C), 2136 (w,
137.5 (C), 127.4 (CH), 124.5 (CH), 120.5 (CH), 111.3 (CH),
105.9 (C≡C), 105.1 (C≡C), 100.0 (C≡C), 97.7 (C≡C), 40.3 (CH ,
3
NMe ), 18.6 (CH , Si(i Pr) ), 18.4 (CH , Si(i Pr) ), 11.3 (CH, Si(i
2 3 3 3 3
-1
C≡C), 1604 (s, C=N), 1574 cm (s, C=N). – UV (CH Cl ): λ
2
2
max
1
(ε) = 258 (17800), 302 (sh, 11500), 468 nm (18400). – H-NMR
(250 MHz, CDCl ): δ = 8.22 (d, J = 9 Hz, 2 H), 7.90 (d, J = 9 Hz,
Pr) ), 11.1 (CH, Si(i Pr) ). – MS (70 eV), m/z (%): 656 (100)
3 3
3
+
+
+
2 H), 7.10 (d, J = 9 Hz, 2 H), 6.68 (d, J = 9 Hz, 2 H), 1.14 (s, 42 H,
[M ], 613 (2) [M - i Pr], 329 (10) [M /2, NO -half], 327 (100)
2
13
+
Si(i Pr) ), 0.98 (s, 42 H, Si(i Pr) ). – C-NMR (90.6 MHz,
[M /2, NMe -half]. – C
H
N O Si (656.39): calcd. C 69.47, H
3
3
2
38 56
4
2
2
CDCl ): δ = 157.8 (C), 153.2 (C), 150.9 (C), 144.5 (C), 139.7 (C),
8.60 N 8.53; found C 69.26, H 8.54 N 8.67.
3