Synthesis of a diferrocenylvinylidene complex by migration of a ferrocenyl substituent

Article abstract of DOI:10.1039/d1cc00605c

An unusual 1,2-ferrocenyl migration has been observed following reactions of [Ru(dppe)Cp][BAr^F₄] with diferrocenylacetylene, extending the scope of group rearrangments beyond methyl (Wagner-Meerwein) and phenyl entities. Ferrocene-containing bis(alkynes) RCCArCCR (R = Fc, Ar = 1,4-phenylene; R = Ph, Ar = 1,1′-ferrocenylene) gave bimetallic bis(vinylidene) complexes following two consecutive rearrangements.

Full text of DOI:10.1039/d1cc00605c

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Synthesis of a diferrocenylvinylidene complex
by migration of a ferrocenyl substituent†
Cite this: Chem. Commun., 2021,
7, 4251
5
Marcus Korb, * Stephen A. Moggach
and Paul J. Low
Received 2nd February 2021,
Accepted 24th March 2021
DOI: 10.1039/d1cc00605c
rsc.li/chemcomm
An unusual 1,2-ferrocenyl migration has been observed following fashion, similar to those previously reported for rearrangements of
F
10
reactions of [Ru(dppe)Cp][BAr
4
]
with diferrocenylacetylene, internal alkynes.
extending the scope of group rearrangments beyond methyl
The coordinatively and electronically unsaturated complex
F
F
(
4 4
Wagner–Meerwein) and phenyl entities. Ferrocene-containing [Ru(dppe)Cp][BAr ] ([1]BAr ; dppe = 1,2-bis(diphenylphosphino)
5
F
ꢀ
ꢀ
bis(alkynes) RCRCArCRCR (R = Fc, Ar = 1,4-phenylene; R = Ph, ethane, Cp = Z -C
5 5
H , [BAr
4
]
= [B(3,5-(CF
)
–C
H
)
] ) is known
3
2
6
3
4
0
0
Ar = 1,1 -ferrocenylene) gave bimetallic bis(vinylidene) complexes to promote rearrangements of internal aromatic alkynes RCRCR
following two consecutive rearrangements.
under moderate conditions (toluene, 70 1C, 0.5–12 h) to vinylidenes:
0
CQCR(R ) via 1,2-migration reactions within the metal coordination
7–9
The construction of new molecular scaffolds by controlled sphere. Scoping reactions of ferrocenylphenylacetylene (2a) with
intramolecular migration and rearrangement processes is a [1][BAr ] (generated in situ from RuCl(dppe)Cp (1-Cl) and
powerful tool in synthetic chemistry. In the most general Na[BAr ]) gave the vinylidene complex [3a][BAr ] in quantitative
F
11
4
F
12
F
4
4
possible terms, the course of a rearrangement reaction is yield (Scheme 1).
governed by the relative migratory aptitude of the possible
Given the results of computational studies on related sys-
1
0
migrating groups, with migrations of hydrogen, alkyl fragments tems, showing migration of the less electron-rich substituent,
1
and substituted aromatic fragments as well as the skeletal it is likely that the phenyl group migrates in the formation of
2
bonds of (poly)cyclic frameworks all well known. Examples
include the Wagner–Meerwein reaction involving the 1,
2-migration of skeletal C–C bonds in carbocations and related
methyl substituent migrations observed in the Nametkin and
2
a
Pinacol rearrangements to name but a few.
Despite the prevalence of rearrangement reactions and the
close synergies in the substitution chemistry of ferrocene and
aromatic organic compounds, the migration of ferrocenyl
groups is rare and has yet only been observed within a Pinacol
3
4
rearrangement, whereas methyl migration, Wolff (FcC(O)
5
CHN to FcHCQCQO) and Curtius rearrangements (FcC(O)N
to FcNH ) around the periphery of ferrocenyl backbones are all
2
3
6
2
known. We now report the rearrangement of ferroceneyl-
substituted alkynes and diynes to ferrocenyl-substituted vinylidenes
within the coordination sphere of a half-sandwich ruthenium
7–9
fragment. The 1,2-ferrocenyl migration occurs in a nucleophilic
The University of Western Australia, School of Molecular Sciences,
3
5 Stirling Highway, Crawley, Perth, Western Australia 6009, Australia.
E-mail: marcus.korb@uwa.edu.au
Electronic supplementary information (ESI) available. CCDC 2056667–2056672.
For ESI and crystallographic data in CIF or other electronic format see DOI:
0.1039/d1cc00605c
†
Scheme 1 Internal alkyne/vinylidene rearrangement reactions of alkyne
2a and bis(alkyne) 2b for the synthesis of ferrocenyl phenyl vinylidene
complexes.
1
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(typically being adjudged complete within 0.5 h at 70 1C in
dichloroethane), driven by migration of the less-electron-rich
10
substituent. Notably, symmetric alkynes, e.g. PhCRCPh, and
aliphatic, (i.e. electron rich) substrates rearrange less quickly,
8
taking 1 h and 10 d, respectively to reach completion. This
+
general pattern of reactivity is reflected in the high yield of [3a]
and its relatively rapid formation (2 h) from 2a, combining an
electron-rich ferrocenyl group and a phenyl ring. Although 1,
2-diferrocenylacetylene (5a) combines both aspects above men-
tioned that potentially lower the migration rate, it may also be that
the coordination of the alkyne substrate towards the metal centre
is less effective, due to the increased bulkiness of a ferrocenyl
compared to a phenyl substituent.
The reaction of the bis(alkyne), 1,4-bis(ferrocenylethynyl)benzene
F
F
(
3
(
5b) with [1][BAr
8%) and double ([7][BAr
Scheme 2). Although the double rearrangement product was only
isolated in trace quantities, the structure was confirmed by single
Scheme 2 Internal alkyne/vinylidene rearrangement of diferrocenyl(bis) crystal X-ray diffraction (Fig. 2). Although we cannot definitively rule
alkynes 5a,b for the synthesis of diferrocenyl vinylidene complexes.
4
] gave products arising from mono ([6b][BAr
4
],
F
4 2
]
, o3%) rearrangement reactions
F
4
out the formation of [7][BAr ] by migration of the vinylidene-
functionalised phenylene group {Ru(CQC(Fc)C H –)(dppe)Cp}
2
+
6
4
+
F
[
3a] . A similar reaction of [1][BAr ] with 2b, bearing an (which would also be unprecedented), we suggest that in this case
additional phenylethynyl substituent in the 1 -position, gave it is more likely that the smaller ferrocenyl moiety acts as the
3b][BAr ] in good (62%) yield together with a double- migrating group.
4
0
F
[
4
rearrangement product, the bimetallic bis(vinylidene) complex
The procedure outlined in Schemes 1 and 2 provides access
to ferrocenyl-substituted vinylidene complexes that are other-
F
4 2
[4][BAr ] in low yield over 18 h.
1
4
With ferrocenyl alkynes clearly demonstrated to be compa- wise only accessible by either protonation or S 2-type reac-
N
1
5
tible with the rearrangement processes within the coordination tions of ferrocenylacetylide complexes with e.g. MeI and other
+
16
sphere of [Ru(dppe)Cp] , attention was turned to the explora- alkyl halides, whereas products bearing arene entities were
+
tion of the ferrocenyl migration processes (Scheme 2). Reaction of inaccessible. Attempts to obtain [6a] by reacting the ferrocenyl
F
+
+
a mixture of 1-Cl and Na[BAr ] with 1,2-diferrocenylacetylene (5a) vinylidene [Ru{CQC(H)Fc}(dppe)Cp] ([8] ) and ferrocene (FcH)
4
F
17
gave the diferrocenylvinylidene complex [6a][BAr ], which in accordance to an intramolecular S Ar-type cyclization, were
4
E
although only isolated in moderate (37%) yield, unambiguously not successful (Scheme 3).
+
evidenced the migration of a ferrocenyl moiety (Fig. 1).
As an aside, we note that attempts to synthesise complex [8]
+
F
Mechanistically, half-sandwich species such as [Ru(dppe)Cp]
from 1, Na[BAr ] and ferrocenylacetylene FcCRCH (9) in
4
are known to stabilise the vinylidene product relative to the methanol, the common solvent for such reactions with terminal
2
18
+
Z -alkyne tautomer upon reaction with terminal and internal alkynes, resulted in formation of the carbene [10] within 2 h.
13
alkynes. Asymmetrically-substituted alkyne substrates bearing This is somewhat unusual, the formation of Fischer carbenes by
Ph and p-OMe-C H groups show the highest rates of reaction
6
4
2+
Fig. 2 ORTEP/wireframe model (30% probability level) of [7] showing the
+
F
ꢀ
Fig. 1 ORTEP/wireframe model (50% probability level) of [6a] showing
labelling of the vinylidene ligand. Hydrogen atoms, the [BAr
and two disordered molecules of CHCl are omitted for clarity. Selected bond
distances: RuQC1 1.843(3), C1QC2 1.311(4), C2–C3 1.498(5), C2–C6 1.475(5),
RuꢁꢁꢁRu 10.4738(6), C ꢁꢁꢁC 11.407(16), C3–C2–C6 118.6(3). (Symmetry
codes for generating equivalent atoms: ꢀx, ꢀy, ꢀz.)
4
]
counter ions
F
ꢀ
the labelling of the vinylidene ligand. Hydrogen atoms and the [BAr
4
]
3
counter ion are omitted for clarity. Selected bond distances (Å):
RuQC1 1.840(3), C1QC2 1.337(11), C2–C3 1.482(11), C2–C13 1.468(11),
C3–C2–C13 121.6(7).
5 5
H
5 5
H
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Scheme 4 Schematic representation of the mechanism for the synthesis
+
F
ꢀ
of diferrocenylvinylidene complex [6a] . ([Ru] = [Ru(dppe)Cp]; the [BAr
4
]
counter ions are omitted.)
Scheme 3 Reaction of 1 with ethynylferrocene (9) and attempted synth-
+
+
esis of diferrocenyl vinylidene [6a] by S
E
Ar from vinylidene [8] . (i)
F
Na[BAr
4
], reflux, 2 h, solvent; (ii) CH
2
Cl
2
, 25 1C, 7 d.
Analysis of the electrochemical properties showed (electro)-
+
chemically reversible ferrocenyl-related redox events for [3a,b] ,
2
+
+
+
+
[
4] , [6a,b] and [10] , whereas [8] slowly decomposed (Table 1
addition of MeOH to sterically encumbered vinylidene complexes
19
and Fig. S1; for more information see the ESI†). The gem-
diferrocenyl vinylidene [6a] reveals the lowest (most thermo-
dynamically favourable) redox potentials. Replacement of one
usually requiring reaction periods of 2 days. To avoid this issue,
+
+
the synthesis of [8] was carried out in toluene.
+
2+
+
2+
+
The vinylidene complexes [3a,b] , [4] , [6a,b] , [7] , [8] ,
ꢂ 0
+
+
and carbene [10] were characterised by various NMR techni- Fc by a phenyl group increases E1 from 31 ([6a] ) to 55 mV
+
ques, in addition to mass spectrometry and IR spectroscopy. ([3a] ). Additional FcCRC groups in para position of the
3
1
1
+
The P{ H} NMR resonances of the dppe ligands appear at phenyl moiety ([6b] ) resulted in essentially no change in the
B77 ppm for vinylidene complexes and at 89 ppm for carbene first oxidation potential.
+
[
10] . Resonances of the a carbon of the vinylidene ligand were
In contrast, functionalization of the ferrocene moiety with
1
3
1
0
observed between 352 and 364 ppm in the C{ H} NMR spectra an electron-withdrawing PhCRC fragment in the 1 -position
see ESI†). The carbene carbon in [10] is significantly more (Fc , [3b] ) shifts the redox potential to 121 mV, whereas a
shielded and resonates at 302 ppm.
+
0
+
(
0
0
2+
cationic vinylidene fragment (Fc , [4] ) is less electron with-
F
4
F
4
F
ꢂ 0
Vinylidene complexes [3a][BAr
], [6a][BAr
], [6b][BAr
4
], drawing (E1 ¼ 93 mV). The relative oxidation potentials of
F
F
+
2+
[
4 2 4
7][BAr ] and carbene [10][BAr ] could further be analysed [3a] (55 mV) and [4] (93 mV) are readily attributed to the
by single crystal X-ray diffraction (Fig. 1, 2 and Fig. S2, S3, ESI†). differences in charge on the complex, but surprisingly small. All
The vinylidene complexes display RuQC (1.840(3)–1.857(5) Å) told, inspection of the trends in the first oxidation potential
and C QC (1.31 Å) bond lengths entirely consistent with the indicate that the ferrocene moiety is the redox site, that the
a
a
b
2
0
description as vinylidene structures (Table S1, ESI†). The formal complex charge is predominantly located at ruthenium,
RuQC bond length in carbene [10] is elongated to 1.948(2) Å. and that conjugation between the vinylidene entity and the
+
+
The C –R1/R2 distances in vinylidene complexes are consistent ferrocenyl group is of minor importance. Carbene [10] shows
b
with the C–C single bond nature of these connections.
2
+
Especially in [7b] , the similarity of the C2–C6 (1.475(5) Å)
and C2–C3 (1.498(5) Å) distance confirms the absence of a
cumulene character of the bridging phenylene. Together with a Table 1 Ferrocenyl-related redox events (E1 ) in vinylidene complexes [3a,b] ,
0
+
2
+
+
+
1 2
[
4]
, [6a,b] , and [8] (general substitution pattern [Ru {Q CQCR R }
rather perpendicular vinylidene/phenylene plane intersection
68.3(3)1) delocalisation over the entire p-system, and hence
communication between both metal centres, can be excluded.
+
+ a
(
dppe)Cp] ), and carbene [10]
(
1
2
ꢂ 0
ꢂ 0
ꢂ 0
1;2
Compd.
R
R
E
E
DE
1
2
1
0
Based on the observations described here and elsewhere
Vinylidene
the rearrangement of 1,2-diferrocenylacetylene (5a) within the
+
[
[
3a]
3b]
Ph
Ph
Ph
Fc
Fc
Fc
Fc
Fc
Fc (Ph)
Fc (Ph)
Fc
Fc
Ph (Fc)
H
55
121
93
—
—
—
476
110
174
—
+
coordination sphere of [1] can be envisioned in a series of
+
0 0 0
2
+
00
simple steps (Scheme 4). Coordination of the alkyne results in a [4]
+
+
2
[6a]
6a]
31
445
150
114
—
Z -complex from which a subsequent 1,2-sigmatropic shift of a
b
[
[
ꢀ40
ferrcenyl group takes place. Migration of one of the ferrocenyl
groups from the alpha towards the beta carbon is the first of [8]
+ c
0 0 0
6b]
60
+
14
its kind and results in the diferrocenylvinylidene complex
Carbene
[10]
F
[
6a][BAr ]. The formation of the bimetallic, dicationic
+
4
CH Fc
OMe
172
—
—
2
2
+
2+
bis(vinylidene) complexes [4] and [7] requires coordination
of [Ru(dppe)Cp] ([1] ) to the positively charged alkynes [3b]
and [6b] prior to the second rearrangement. The low yields of solutions containing 0.1 mol L of [NBu
+
ꢀ1
Potentials vs. FcH/FcH except otherwise noted, scan rate of 100 mV s
glassy carbon (GC) working electrode; anhydrous dichloromethane
;
+
+
+
+
ꢀ1
F
4
][BAr
4
] as supporting elec-
0
0 0
00
2
+
2+
trolyte at 25 1C;
vinylidene substituent. All potentials are given in mV. In 0.1 mol L
– featuring an alkynyl substituent; – featuring a
[4] and [7] might therefore be reasonably explained by the
a
b
ꢀ1
electrostatic repulsion that must be overcome to form the
c
[
4 6
NBu ][PF ]. Obtained via square wave voltammetry (fitted
initial p-complex intermediates.
voltammogram).
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separated from the cationic carbene entity.
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3
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+
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1
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ꢂ 0
F
separated by DE1 ¼ 445 mV in the [NBu
featuring a bulky and weakly-coordinating anion. Measurement
4 4
][BAr ] electrolyte
;2
5
4
5
6
of the voltammetry in a 0.1 M [NBu
4
][PF
6
] electrolyte solution
9
ꢂ 0
reduces the redox separation to DE1 ¼ 150 mV, due to a better
;2
4
ion-pairing in solution and consequent shielding of the developing
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ꢀ
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]
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ꢂ 0
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8
Y. Mutoh, Y. Ikeda, Y. Kimura and Y. Ishii, Chem. Lett., 2009, 38,
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redox separation of 150 mV is similar to gem-diferrocenyl alkenes
5
2
1
22
23
bearing H and alkyl groups, as well as tetraferrocenyl allene.
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9
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24
+
10 (a) M. Otsuka, N. Tsuchida, Y. Ikeda, N. Lambert, R. Nakamura,
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[
4
6
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25
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1
ꢂ 0
+
Insertion of a phenylethynyl spacer ([6b] ) further reduced DE
Austr. J. Chem., 2017, 70, 113.
1;2
F
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4
4
+
+
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(
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+
+
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2
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3
1
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separation of up to 445 mV for a QCQCFc substitution
2
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authors gratefully acknowledge the facilities, and the scientific
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University of Western Australia, a facility funded by the Uni-
versity, State and Commonwealth Governments. S. A. M. thanks
the Australian Research Council (ARC) for a Future Fellowship
1
1
2
(FT200100243).
(
2
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There are no conflicts to declare.
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