Cycloaddition of aromatic imines to a ruthenium butatrienylidene complex: Synthesis of 4-ethynylquinoline and 1-azabuta-1,3-diene complexes

Article abstract of DOI:10.1039/a700663b

Reactions of aromatic imines with the putative butatrienylidene-ruthenium cation [Ru(C=C=C=CH₂)(PPh₃)₂-(η-C ₅H₅)]⁺ gives complexes containing either 4-ethynylquinoline or 1-azabuta-1,3-diene ligands; the molecular structures of [Ru(C≡CC₉H₄MeN)(PPh₃) ₂(η-C₅H₅)] and [Ru{C≡CC(CH=CHPh)=N(C₆H₄NO₂-3)}(PPh ₃)₂(η-C₅H₅)] are reported.

Full text of DOI:10.1039/a700663b

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Cycloaddition of aromatic imines to a ruthenium butatrienylidene complex:
synthesis of 4-ethynylquinoline and 1-azabuta-1,3-diene complexes
Michael I. Bruce,*a Peter Hinterding,a Mingzhe Ke,a Paul J. Low,a Brian W. Skeltonb and Allan H. Whiteb
a Department of Chemistry, University of Adelaide, Adelaide, South Australia 5005, Australia
b Department of Chemistry, University of Western Australia, Nedlands, Western Australia 6907, Australia
Reactions of aromatic imines with the putative butatrien-
an acetylenic ligand of which the other substituent is a
4-quinolinyl group. Structural characteristics of this ligand are
not exceptional and further discussion will be deferred until the
full report. Similar tricyclic heterocycles are obtained with
naphthylimines.
A remarkable property of complexes 2 is their extreme
sensitivity towards protonation, traces of water being sufficient
ylidene–ruthenium
cation
[Ru(CNCNCNCH2)(PPh3)2-
(h-C5H5)]+ gives complexes containing either 4-ethynylqui-
noline or 1-azabuta-1,3-diene ligands; the molecular struc-
tures
of
[Ru(C·CC9H4MeN)(PPh3)2(h-C5H5)]
and
[Ru{C·CC(CHNCHPh)NN(C6H4NO2-3)}(PPh3)2(h-C5H5)]
are reported.
to
form
to
deep
claret
coloured
cations,
We have recently described the formation of an intermediate
cationic complex from the reaction between buta-1,3-diyne and
[Ru(thf)(PPh3)2(h-C5H5)]+ which, from its method of formation
and subsequent reactions, is proposed to be the butatrienylidene
cation [Ru(CNCNCNCH2)(PPh3)2(h-C5H5)]+ 1.1 We showed
that its reactivity is characterised by nucleophilic attack at Cg, as
predicted by theoretical studies.2
[Ru(C·CC9H4RNH)(PPh3)2(h-C5H5)]+ 3,† also characterised
by an X-ray study for R = H, full details of which will be
reported elsewhere.‡ N-Methylation can be achieved using
methyl triflate. Complexes 2 and 3 form a readily reversible
base–acid system, but in contrast with the well known
acetylide–vinylidene systems,3 there is no evidence for protona-
tion occurring at the acetylide b-carbon.
The availability of 1, in which the outer carbons of the four-
carbon unsaturated ligand are not sterically protected by the
bulky PPh3 ligands as are atoms Ca and Cb, encouraged us to
study its chemistry in more detail. In particular, we were
interested in the possibility of cycloaddition to the unsaturated
C4 chain through reactions which might involve successive
electro- and nucleo-philic attack on a suitable substrate. We
chose to examine first N-benzylideneaniline and were pleased to
discover that the sought-after reaction occurred readily and in
high yield.
Thus, addition of N-benzylideneaniline (2 equiv.) to a
solution of 1 in thf at 250 °C resulted in an immediate colour
change to orange–red. Work up by preparative thin layer
chromatography (TLC) afforded the yellow complex 2a
(Scheme 1), which was initially characterised as a 1:1 adduct
by mass spectrometry.† The reaction can be extended to other
imines containing substituted C-bonded aryl groups and/or
electron-donating substituents on the N-bonded aryl group. The
X-ray determined structure of 2b, obtained from 1 and
PhCHNNC6H4Me-4, is shown in Fig. 1 and important bond
parameters are collected in the caption. A conventional
Ru(PPh3)2(h-C5H5) fragment is attached by an Ru–C s bond to
A likely pathway for this reaction is (i) attack of Cd of cation
1 on the electron-deficient carbon of the imine, (ii) electrophilic
attack of Cg on the N-bonded aryl group (Scheme 1). The
resulting dihydroquinoline is dehydrogenated to form 2 by a
second molecule of imine which is thereby reduced to the
corresponding benzylamine, which has been isolated and
characterised. Consequently, optimum yields are obtained from
reactions between 1 and 2 equiv. of imine.
The presence of electron-withdrawing substituents on the
N-bonded aryl group results a different reaction course being
followed.
Thus,
the
reaction
between
1
and
PhCHNN(C6H4NO2-3) gives 4 (Scheme 2)† which was also
fully characterised by a single-crystal X-ray study.‡ A molecule
of 4 is shown in Fig. 2 and significant bond parameters are
included in the caption. Again an Ru(PPh3)2(h-C5H5)-substi-
tuted alkynyl fragment is present, now attached to a 1-azabuta-
1,3-diene ligand through C(3). No cyclisation involving either
aryl group is found in this case, nor does any dehydrogenation
take place. Similar complexes have been obtained from
03
02
04
01
05
H
C6H4R′
H
[Ru]+
C
Cβ
1
C
CH2
112
Ru
Cδ
H
α
γ
111
δ
[Ru]+ Cα Cβ Cγ
H
231
232
1
131
221
P(1)
121
HC C6H4R′
2
P(2)
31
N
39
122
32
332
222
132
212
211
33
R
C
R
381
38
–2H
331
37
N(34)
H
C6H4R′
36
C6H4R′
H
C
[Ru]
C
N
[Ru]+
C
C
C
N
– H+
Fig. 1 Plot of a molecule of [Ru(C·CC9H4MeN)(PPh3)2(h-C5H5)] 2b
showing the atom numbering scheme; carbon atoms are designated by
number only. In both Figures, non-hydrogen atoms are shown as 20%
thermal ellipsoids; hydrogen atoms have arbitrary radii of 0.1 Å. Significant
bond parameters: Ru–P(1) 2.308(2), Ru–P(2) 2.300(2), Ru–C(1) 1.997(7),
C(1)–C(2) 1.19(1), C(2)–C(31) 1.42(1) Å; Ru–C(1)–C(2) 175.6(7),
C(1)–C(2)–C(31) 170.2(8)°.
R
H, 3- or 4-NO2
R
2
Scheme
1
R
=
H, 4-Me, 4-OMe; RA
=
[Ru] = Ru(PPh3)2(h-C5H5)
Chem. Commun., 1997
715

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reactions between 1 and PhCHNN(C6H4R-4) (R = NO2,
CO2Et) and PhCHNN(1-C10H7).
Preliminary studies show that other 1,3-dynyl complexes
undergo similar reactions. Further discussion of these interest-
ing transformations will be given in the full report.
We thank the Australian Research Council for support of this
work and the Deutsche Forschunsgemeinschaft for a Fellowship
(P. H); P. J. L. held an Australian Postgraduate Award.
In this variant, we propose that a formal [2 + 2] cycloaddition
of the CgNCd double bond in 1 to the NNCH of the imine occurs,
possibly by attack of the imine N atom on Cg rather than the aryl
carbon, after initial Cd–C(imine) bond formation as in (i) above.
There is little precedent for this reaction, although it resembles
the [2 + 2] cycloadditions of tetracyanoethene (tcne) to
transition-metal s acetylide complexes.4 In the present case, we
do not observe any deep coloured intermediates, but never-
theless, it is unlikely that a concerted addition occurs (which
would contravene the Woodward–Hoffmann rules). The initial
four-membered C3N ring formed in such a reaction could
undergo ring cleavage (also found with the alkyne–tcne
cycloadducts) to give the observed products (Scheme 2).
Footnotes
* E-mail: mbruce@chemistry.adelaide.edu.au
† Selected spectroscopic data: for 2b. IR: n(C·C) 2051 cm21. 1H NMR: d
2.36 (Me), 4.43 (C5H5), 7.05–8.24 (aromatics). FABMS: m/z 933, M+; 671,
[M 2 PPh3]+; 429, [Ru(PPh3)(C5H5)]+. For 4. IR: 2048s (C·C), 1530,
1351s (NO), 831 (CN). 1H NMR: d 4.56 (s, 5 H, C5H5), 7.0–8.3 (m, 41 H,
atomatic). FABMS: m/z 966, M+; 702, [M 2 2 H 2 PPh3]+; 429,
[Ru(PPh3)(C5H5)]+.
‡ Crystal data and refinement details: 2b; [Ru(C·CC9H4MeN)(PPh3)2(h-
–
[Ru]+
C
C
C
CH2
H
H
C5H5)]·C59H47NP2Ru, M = 933.1; triclinic, space group P1 (Ci1, no. 2),
γ
α
β
δ
C
N
a
= 18.067(9), b = 11.472(3), c = 11.279(8) Å, a = 86.08(4),
H
1
[Ru]+
C
C
C
C
b = 82.47(5), l = 87.06(3)°, U = 2310 Å3, Z = 2. Dc = 1.34 g cm23
;
Ph
Ph
F(000) = 964. m(Mo-Ka) 4.5 cm21; specimen: 0.18 3 0.65 3 0.04 mm;
A*(min., max.) = 1.02, 1.08.
N
C
H
RC6H4
– H+
RC6H4
4: [Ru{C·CC(CHNCHPh)NN(C6H4NO2-3)}(PPh3)2(h-C5H5)]·C58H46
-
–
N2O2P2Ru, M
b
g
=
966.0; triclinic, space group P1, a
10.688(3) Å, a 98.26(2), b
2333 Å3, Z
2. Dc
=
=
19.787(8),
101.18(2),
=
=
11.684(3), c
101.39(3)°, U
=
=
=
Ph
=
=
1.375 g cm23
;
[Ru]
F(000) = 966. m(Mo-Ka) 4.5 cm21; specimen: 0.40 3 0.12 3 0.24 mm;
A*(min., max.) = 1.06, 1.15.
HC
C
C
H
C
H
[Ru]
C C
C
N
C Ph
Unique data sets were measured at ca. 295 K within the limit 2qmax = 50°
using an Enraf-Nonius CAD4 diffractometer (2q–q scan mode; mono-
chromatic Mo-Ka radiation, l = 0.71073 Å); for 2b, 7404 independent
reflections were obtained [8225 for 4], 4645 [4544] with I > 3s(I) being
considered ‘observed’ and used in the full-matrix least-squares refinement
after gaussian absorption correction. Anisotropic thermal parameters were
N
C
C
H
RC6H4
RC6H4
4
Scheme 2 R = NO2-3 or -4, CO2Et-4; [Ru] = Ru(PPh3)2(h-C5H5)
refined for the non-hydrogen atoms; (x, y, z, Uiso H were included
)
constrained at estimated values. Conventional residuals R, RA on ıFı are
03
02
04
0.058, 0.059 (for 2b) and 0.059, 0.058 (for 4), statistical weights derivative
2
2
4
of s (I) = s (Idiff) + 0.0004s (Idiff) being used. Computation used the
XTAL 3.0 program system5 implemented by S. R. Hall; neutral atom
complex scattering factors were employed.
01
05
O(3131)
Ru
226
221
112
Atomic coordinates, bond lengths and angles, and thermal parameters
have been deposited at the Cambridge Crystallographic Data Centre
(CCDC). See Information for Authors, Issue No. 1. Any request to the
CCDC for this material should quote the full literature citation and the
referenece number 182/387.
131
111
1
311
P(1)
P(2)
132
312
2
N(313)
232
3
121
N(31)
O(3132)
122
211
32
212
References
31
1 M. I. Bruce, P. Hinterding, P. J. Low, B. W. Skelton and A. H. White,
Chem. Commun., 1996, 1009.
2 B. E. R. Schilling, R. Hoffmann and D. L. Lichtenberger, J. Am. Chem.
Soc., 1979, 101, 585.
321
322
3 A. Davison and J. P. Selegue, J. Am. Chem. Soc., 1978, 100, 7763;
M. I. Bruce and R. C. Wallis, J. Organomet. Chem., 1978, 161, C1.
4 M. I. Bruce, T. W. Hambley, M. R. Snow and A. G. Swincer,
Organometallics, 1985, 4, 501.
5 XTAL Users’ Manual, Version 3.0, ed. S. R. Hall and J. M. Stewart,
Universities of Western Australia and Maryland, 1990.
Fig. 2 Plot of a molecule of [Ru{C·CC(CHNCHPh)NN(C6H4NO2-3)}
(PPh3)2(h-C5H5)] 4 showing the atom numbering scheme. Significant bond
parameters: Ru–P(1) 2.319(2), Ru–P(2) 2.293(2), Ru–C(101) 1.978(8),
C(1)–C(2) 1.22(1), C(2)–C(3) 1.43(1), C(3)–C(32) 1.48(1), C(3)–N(31)
1.280(9), C(31)–C(32) 1.32(1) Å; Ru–C(1)–C(2) 171.2(6), C(1)–C(2)–C(3)
176.9(6)°.
Received in Cambridge, UK on 29th January 1997; Com.
7/00663B
716
Chem. Commun., 1997