Zirconocene dichloride catalyzed hydrodefluorination of C sp 2-F bonds

Article abstract of DOI:10.1002/anie.201207036

A two-metal job: Four-coordinate aluminum dihydrides such as 1 are reported as terminal reductants for the selective title reaction. The heterobimetallic complex 2 has been isolated and shown to be catalytically competent. Copyright

Full text of DOI:10.1002/anie.201207036

Angewandte
Chemie
DOI: 10.1002/anie.201207036
Hydrides
ꢀ
Zirconocene Dichloride Catalyzed Hydrodefluorination of Csp2
bonds**
F
Shuhui Yow, Sarah J. Gates, Andrew J. P. White, and Mark R. Crimmin*
In recent years synthetic methods for the activation and
As evidenced by 19F NMR spectroscopy, the reaction of
ꢀ
functionalization of inert C F s bonds with organometallic
1a (for structure see Table 1) with C6F6 in the presence of
5 mol% [Cp2ZrCl2] in C6D6 at 808C led to the slow
production of C6F5H and p-C6F4H2. Under the same con-
ditions, a reaction with 1b appeared, qualitatively, to proceed
more rapidly, whereas 1c did not display any advantages over
1b.[13]
Control reactions conducted without [Cp2ZrCl2] or with
PhSiH3, or DIBAL-H in place of 1a–c did not lead to
significant consumption of C6F6. The complexes 1a–c were
synthesized from LiAlH4 and the pro-ligand may potentially
be recovered following the hydrodefluorination reaction.
Based upon these observations, we investigated the
complexes have attracted increased attention.[1,2] In the
presence of a reductant such as H2 or a silane, a number of
late-transition-metal and main-group catalysts are known to
[3,4]
ꢀ
ꢀ
effect the hydrodefluorination of Csp2 F and Csp3 F bonds.
While the use of early-transition-metal catalysts for this
transformation is rare,[2] in a series of seminal papers, Jones
ꢀ
and co-workers have documented stoichiometric C F bond
cleavage with group 4 metallocenes.[5–7] The related cerium
5
ꢀ
complex [{h -1,2,4-(tBu)3C5H2)}2CeH] is known to effect C F
bond activation of hydrofluorocarbons.[8] During these studies
it has often been postulated that the strength of the newly
ꢀ
formed M F bond provides not only a thermodynamic
hydrodefluorination of a series of fluoroarenes using
ꢀ
driving force for C F bond cleavage but also a barrier to
5 mol% of [Cp2ZrCl2] and 0.6–1.05 equivalents of 1b
(Table 1). The catalytic reactions were followed by a methanol
work up at 808C for 14 hours. As a result of substrate bias,
reactions were conducted at a series of temperatures [258C
(C6D6), 808C (C6D6), and 1108C ([D8]toluene)] and for each
substrate a control reaction was conducted without the
catalyst present. Data from catalyzed experiments are
included in Table 1, while full details of the control reactions
are presented in Table S1 in the Supporting Information.
Pentafluoropyridine and octafluorotoluene, known to
catalytic turnover.
Comparison of the gas-phase bond dissociation energies
ꢀ
ꢀ
ꢀ
ꢀ
for Zr F + Al H and Al F + Zr H combinations suggests
that aluminum hydrides should act as reagents for catalytic
turnover in zirconium-mediated hydrodefluorination.[9,10a]
Successful studies in this area, however, are limited to two
heteroaromatic substrates. Hence, pentafluoropyridine may
be selectively hydrodefluorinated to yield 2,3,5,6-tetrafluor-
opyridine by employing 0.5–10 mol% [Cp2ZrF2] (Cp = cyclo-
pentadienyl) or [{Cp2ZrH(m-H)}2] as a pre-catalyst and
DIBAL-H as a terminal reductant.[10a] Similarly, Red-Al has
been reported as a hydride source for the conversion of 2-
fluoropyridine into pyridine albeit with 10 mol% [Cp2MCl2]
(M = Ti, Zr) as a catalyst.[10b] In related studies, Kiplinger and
co-workers have shown that toxic M/HgCl2 (M = Mg, Al)
mixtures act as reductants for the defluorination and aroma-
tization of a handful of cyclic perfluorinated hydrocarbons
with catalytic [Cp2ZrCl2],[11] while Lentz and co-workers
reported the use of primary and secondary silanes as
reductants for the hydrodefluorination of fluoropropenes
using [Cp2TiF2] as a catalyst.[12] We now report four-coordi-
nate aluminum dihydrides as reductants for the hydrode-
fluorination of fluoroarenes catalyzed by [Cp2ZrCl2].
ꢀ
contain activated C F bonds, reacted readily within 1 hour
at 258C in the presence of 1.05 equivalents of 1b and 5 mol%
of [Cp2ZrCl2] (Table 1, entries 1 and 6), while more challeng-
ing substrates required elevated temperatures before hydro-
defluorination was observed. Under the catalytic conditions
C6F6 was completely consumed after 24 hours at 808C and
both C6F5H and p-C6F4H2 were observed after protic work up
ꢀ
(Table 1, entry 10). Although the high C F bond dissociation
energy of hexafluorobenzene has meant that it is often
employed as a substrate in hydrodefluorination catalysis,[1–2]
the stabilization of the {C6F5H2} anion may result in a rela-
ꢀ
tively low activation energy for C F bond cleavage by
nucleophilic aromatic substitution.
A similar argument
could not be made for 1-fluoronaphthalene, which underwent
hydrodefluorination albeit at a higher temperature of 1108C
and a longer reaction time of 4 days (Table 1, entry 13). An
attempt to improve the efficiency of the latter reaction using
[(h5-C5Me4H)2ZrCl2], a pre-catalyst with a more electron-rich
cyclopentadienyl ligand,[7] failed, yielding naphthalene in only
25% conversion after 5.5 days at 1108C.
[*] Dr. S. Yow, S. J. Gates, Dr. A. J. P. White, Dr. M. R. Crimmin
Department of Chemistry, Imperial College
South Kensington, London, SW7 2AZ (UK)
E-mail: m.crimmin@imperial.ac.uk
[**] We are grateful to the Royal Society for provision of a URF (M.R.C.),
Imperial College for support of an undergraduate project student-
ship (S.J.G.), and the EPSRC (S.Y.) for funding. Peter Haycock is
thanked for invaluable assistance with NMR spectroscopy. We are
grateful to the referees for insightful comments and suggestions.
Ozerov and co-workers have documented silylium and
alumenium cations associated with noncoordinating carbor-
ane anions for the mild and selective hydrodefluorination of
[4a–d]
ꢀ
ꢀ
Csp3 F bonds in the presence of Csp2 F bonds.
In the
ꢀ
current case, a selective hydrodefluorination of Csp2 F bonds
occurs and the data are reminiscent of those reported for the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207036.
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Table 1: Zirconocene-catalyzed hydrodefluorination.
copy. For example, the hydrode-
fluorination of C5F5N using
1.05 equivalents of 1b (Table 1,
entries 1 and 3) proceeded with
formation of 3b, whereas use of
0.6 equivalents of 1b produced 2b
as the major reaction by-product.
The by-products have been assigned
by the independent synthesis of
each. Although reaction of 1b with
BF3·OEt2 in Et2O at ꢀ788C gave 2b
in 86% yield upon isolation
Entry
Substrate
Product(s)
Cat. (mol%)
Equiv.
1b
t
T
[8C]
Yield
[%][a]
(19F NMR
d = ꢀ174.5 ppm,
1[b]
2[b]
3[b]
4[b]
5[b]
[Cp2ZrCl2] (5)
[Cp2ZrCl2] (1)
[Cp2ZrCl2] (5)
[{Cp2ZrH2}2] (5)
[Cp2ZrF2] (5)
1.00
1.05
0.6
1.05
1.05
10 min
24 h
1 h
24 h
30 min
25
25
80
25
25
92
64
83
93
77
Figure 1),[14] the synthesis of 3b
proved more complicated. As evi-
denced by NMR spectroscopy,
a metathesis reaction of a 1:1 mix-
ture of 1b with 2b in a C6D6 solu-
tion at 258C resulted in incomplete
consumption of the starting materi-
als along with the formation of 3b
(19F NMR d = ꢀ157.4 ppm). The
6[b]
7[b]
[Cp2ZrCl2] (5)
4b (5)
1.05
1.05
10 min
1.5 h
25
25
79
81
equilibrium,
represented
in
Scheme 1, suggested by these data
could be shifted toward the product
3b by using a 2:1 stoichiometry of
1b to 2b. Because of the propensity
of 3b to co-crystallize with the
starting materials, attempts to iso-
late pure samples of this compound
have, so far, failed.[15]
8
9
[Cp2ZrCl2] (5)
1.05
1.05
3 h
80
80
82
82
[Cp2ZrCl2] (5)
1.5 h
In many instances, in addition to
the fluoroarenes 2b and 3b, exami-
nation of reaction mixtures using 1H
and 19F NMR spectroscopy prior to
the methanol work-up revealed the
presence of nonvolatile aluminum
fluoroaryl complexes.[16] Specifi-
cally, the reaction of 1.05 equivalent
of 1b with C6F6 and 5 mol% of
[Cp2ZrCl2] gave the fluoroarene-
containing products C6F5H, p-
C6F5H (49%)
p-C6F4H2 (41%)
10
11
[Cp2ZrCl2] (5)
[Cp2ZrCl2] (5)
1.05
1.05
24 h
18 h
80
80
90
77
C6F5H (40%)
p-C6F4H2 (37%)
12
[Cp2ZrCl2] (5)
[Cp2ZrCl2] (5)
[Cp2ZrCl2] (5)
1.05
1.05
1.05
24 h
4 d
80
81
76
18
C6F4H2,
[{(MesNCMe)2CH}Al-
(C6F5)F], and [{(MesNCMe)2CH}-
Al(p-C6F4H)F] in 2, 23, 47, and 18
yield, respectively (Scheme 2). The
latter complexes were characterized
by peaks for m/z 398 and 546 in the
EI mass spectrum. Upon treatment
with excess methanol at 808C, the
aluminum complexes undergo pro-
tonolysis to yield the corresponding
13[b,c]
110
110
14[b,c]
4 d
[a] For 0.2m of substrate, the yield was determined by NMR analysis of the reaction mixture relative to
a capillary containing a 1.0m solution of C6H5F in [D6]benzene. Yields of the individual products are
given with in the parentheses. [b] Yields recorded prior to (or without) MeOH work-up. [c] Reaction
conducted in [D8]toluene.
fluoroarenes
Table 1).
(Scheme 2
and
stoichiometric reactions of [Cp*2ZrH2] and [{Cp2ZrH(m-H)}2]
Facile transfer of not only fluoride but also fluoroaryl
ligands from zirconium to aluminum explains the observed
reaction intermediates, the aluminum fluoride by-products,
and the ease of catalyst turnover. This hypothesis was probed
by an NMR-scale reaction between a mixture of [Cp2ZrF-
with hydrofluorocarbons.[2,7]
During
catalytic
preparations,
both
[(MesNC-
Me)2CH}AlF2] (2b) and [{(MesNCMe)2CH}AlHF] (3b)
were observed as reaction by-products by 19F NMR spectros-
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Scheme 3. Reaction of [Cp2ZrCl2] with 10 equiv 1b.
Figure 1. a) Crystal structure of 2b. Selected bond angles [8] and bond
lengths [ꢀ]: Al–N1 1.8594(9), Al–N3 1.8594(9), Al–F1 1.6637(8), Al–F2
1.6647(8); N1-Al-N3 99.37(4), F1-Al-F2 107.91(5).[21]
Scheme 4. Reaction of [{Cp2ZrH(m-H)}2] with 1 equiv 1b.
Scheme 1. Reaction of 1b with 2b.
Figure 2. Crystal structure of 4c. Selected bond angles [8] and bond
lengths [ꢀ]: Al–N3 1.9464(10), Al–N1 1.9493(10), Zr–C 2.4964(14)–
2.5219(13); N1-Al-N3 93.14(4).[21]
Scheme 2. Catalytic hydrodefluorination of C6F6 with 1b.[17]
(C6F5)]/[Cp2ZrF2], formed from the reaction of [{Cp2ZrH(m-
H)}2] with C6F6,[7c] and excess 1b. In a C6D6 solution after
30 minutes at 258C, the aforementioned reaction yielded
a mixture of [{(MesNCMe)2CH}Al(C6F5)X] (X = H or F) and
3b as the major fluorine-containing products as evidenced by
19F NMR spectroscopy.[18] An alternative explanation for the
formation of aluminum fluoroaryl complexes under the
Upon dissolving crystalline samples of 4b in C6D6 an
equilibrium between 4b and 1b/[{Cp2ZrH(m-H)}2] was
observed; [{Cp2ZrH(m-H)}2] was characterized by diagnostic
1H NMR resonances at d = ꢀ3.62 (t, J = 7.2 Hz) and
+ 3.68 ppm (t, J = 7.2 Hz) for the bridging and terminal
hydrides, respectively.[20] The reversibility of alane binding to
zirconocene dihydride was confirmed by a crossover experi-
ment in which 1 equivalent of 4b was reacted with 1 equiv-
alent of 1c and shown to form 4c. Variable-temperature
1H NMR data recorded across the ꢀ80 to + 808C range in
[D8]toluene revealed a series of fluxional processes consistent
with fast exchange between all possible hydride positions of
4b. In the high temperature regime (>+ 208C) a single broad
hydride resonance is observed for 1b at d = ꢀ0.71 ppm
corresponding to a time-averaged combination of bridging
(Hb) and terminal (Ht) hydride resonances. Upon cooling, two
decoalescence events are observed. The first (Tc =+ 108C)
resolves Ht and Hb, and the second (Tc = ꢀ108C) resolves Hb
into two independent resonances. In the lowest temperature
regime, the magnetically non-equivalent bridging hydrides
are observed at d = ꢀ2.42 and ꢀ2.64 ppm. Although the
terminal hydrides of 1b could not be observed directly in the
1H NMR data, ROESY and NOESY experiments conducted
at ꢀ408C revealed chemical exchange of the bridging
hydrides with two resonances at d =+ 1.10 and + 4.75 ppm,
ꢀ
catalytic conditions is C H bond activation of the organic
reaction products. Heating p-C6F4H2 in the presence of 1b
and 5 mol% of [Cp2ZrCl2] in C6D6 for 24 hours at 808C,
however, did not result in the formation of
ꢀ
[{(MesNCMe)2CH}Al(p-C6F4H)H] by C H activation of the
fluoroarene.
The catalyst resting state at the early stages of the reaction
was investigated by an additional series of experiments.
Reaction of [Cp2ZrCl2] with 1b in a 1:10 ratio resulted in
formation of the heterobimetallic complex 4b and [{(MesNC-
Me)2CH}AlHCl] (Scheme 3).[19] The assignment of
[{(MesNCMe)2CH}AlHCl] was confirmed by spiking reac-
tions with authentic samples, while 4b was synthesized on
a preparative scale by reaction of 1b with [{Cp2ZrH(m-H)}2]
(Scheme 4). Although attempts to obtain single crystals of 4b
was
unsuccessful,
the
analogue
[{(2,6-Me2C6H3-
NCMe)2CH}Al(H)(m-H)2Zr(H)Cp2] (4c) was the subject of
an X-ray diffraction study. The results of this experiment are
presented in Figure 2.
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[2] For review articles specific to group 2–5 metals, see: a) W. D.
Jones, Dalton Trans. 2003, 3991; b) M. Klahn, U. Rosenthal,
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[3] For selected examples of late-transition-metal-catalyzed hydro-
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which were assigned as the zirconium and aluminum terminal
hydrides, respectively. These data are consistent with a “freez-
ing-out” of the solid-state structure of 4b, presumed by
analogy to 4c, in which the asymmetric environment at
zirconium results in the magnetic non-equivalence of the
bridging hydrides.
In the presence of 1.0 equivalent of 1b, 5 mol% of 4b
proved kinetically competent for the hydrodefluorination of
octafluorotoluene to p-CF3C6F4H in 81% yield within
90 minutes at 258C (Table 1, entry 7).
In summary, we have discovered that four-coordinate
diketiminate stabilized aluminum dihydrides are efficient
terminal reductants for the zirconocene-catalyzed hydrode-
fluorination of fluoroarenes and drastically increased the
scope of zirconocene hydrodefluorination catalysis. Initial
data are consistent with catalyst turnover by fast ligand
transfer between the catalyst and terminal reductant. While
a heterobimetallic complex has been observed as a catalyst
resting state it remains unclear whether such intermediates
ꢀ
are involved in C F bond cleavage.
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[6] X. Yang, C. L. Stern, C. J. Marks, J. Am. Chem. Soc. 1994, 116,
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Experimental Section
Preparative scale synthesis of 4c: In a glovebox, 1c (450 mg,
1.34 mmol, 1 equiv) and [{Cp2ZrH2}2] (300 mg, 0.67 mmol,
0.5 equiv) were weighed and transferred to a Schlenk tube. The
Schlenk was sealed and removed from the glovebox and attached to
a vacuum line. Toluene (15 mL) was added by cannula and the
mixture stirred for 24 h at 258C. The reaction mixture settled out and
unreacted zirconocene dihydride removed by filtration. The solvent
was removed from the filtrate under reduced pressure and n-hexane
(20 mL) added to the crude mixture upon which point 4c crystallized
from solution. The product was isolated by filtration, dried under
vacuum, and collected as colorless crystals (308 mg, 0.55 mmol, 41%).
X-ray quality crystals were obtained from storage of a concentrated n-
hexane solution at 58C. 1H NMR (C6D6, 400 MHz, 298 K): d = ꢀ0.71
(broad s, 1H, MH bridging), 1.48 (s, 6H, CMe), 2.30–2.32 (broad s,
12H, o-ArMe), 4.99 (s, 1H, CH), 5.19 (s, 10H, CpH), 7.01–7.03 ppm
(m, 4H, ArH); 13C NMR (C6D6, 298 K, 125.7 MHz): d = 19.5, 23.5,
97.8, 98.6, 125.9, 128.8, 134.4, 146.1, 168.7 ppm; Infrared (solid):
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n ¼2839, 1773 (wsh), 1527, 1376 cmꢀ1. Elemental analysis calc. for
~
C31H39AlN2Zr: C, 66.74; H, 7.05; N, 5.02 found C, 66.72; H, 7.18; N,
5.12.
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(Ed.: D. R. Lide), CRC, Boca Raton, FL, USA, 2000.
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Received: August 30, 2012
Revised: September 24, 2012
Published online: November 6, 2012
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ꢀ
Keywords: aluminum · C F activation · hydrides ·
synthetic methods · zirconocene
.
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[1] For review articles on C F activation and functionalization see,
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Chemie
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[18] In this experiment we cannot discriminate between X = H or F.
Furthermore based on the reaction between 1b and 2b it
remains probable that hydride and fluoride ligands are exchang-
ing under the reaction conditions.
[15] Crystallization of the product from a 1:1 reaction of 1b and 2b
from a toluene/n-hexane mixture allowed isolation of colorless
crystals which upon dissolving in C6D6 showed 19F NMR
resonances at both d = ꢀ157.4 and ꢀ174.2 ppm. Although we
cannot unambiguously rule out the presence of 1b from these
single crystals, the data are most simply modeled as a mixture of
2b and 3b. Hence, the thermal parameters of the fluorine atoms
in the crystal structure of 2b/3b indicate a mixture of species
present in a ratio consistent with that observed by 19F NMR
spectroscopy (see the Supporting Information for more details).
[19] For related zirconocene/aluminum hydride complexes, see:
a) S. M. Baldwin, J. E. Bercaw, H. H. Brintzinger, J. Am.
Chem. Soc. 2008, 130, 17423; b) A. I. Sizov, T. M. Zvukova,
V. K. Belsky, B. M. Bulychev, J. Organomet. Chem. 2001, 619, 36;
c) R. Charles, R. Gonzꢂlez-Hernꢂndez, E. Morales, J. Revilla,
L. E. Elizalde, G. Cadenas, O. Pꢃrez-Camacho, S. Collins, J. Mol.
Catal. A 2009, 307, 98; d) L. I. Shoer, K. I. Gell, J. Schwartz, J.
Organomet. Chem. 1977, 136, c19.
[20] D. G. Bickley, N. Hao, P. Bougeard, B. G. Sayer, R. C. Burns,
M. J. McGlinchey, J. Organomet. Chem. 1983, 246, 257.
[21] CCDC 896065 (2b), 896067 (3b), and 896068 (4c) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
ꢀ
[16] For C F borylation, see: a) M. Teltewskoi, J. A. Panetier, S. A.
Macgregor, T. Braun, Angew. Chem. 2010, 122, 4039; Angew.
Chem. Int. Ed. 2010, 49, 3947; b) T. Braun, M. Ahijado Saolo-
mon, K. Altenhçner, M. Teltewsjkoi, S. Hinze, Angew. Chem.
2009, 121, 1850; Angew. Chem. Int. Ed. 2009, 48, 1818.
[17] Reaction scheme not balanced for dihydrogen, 2b/3b, and the
aluminum products following methanol work-up.
Angew. Chem. Int. Ed. 2012, 51, 12559 –12563
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 12563