Oxidation-resistant, sterically demanding phenanthrolines as supporting ligands for copper(I) nitrene transfer catalysts

Article abstract of DOI:10.1039/b404515g

New 1,10-phenanthroline ligands have been synthesized with C ₆F₅- or 2,4,6-(CF₃)₃C ₆H₂- groups in the 2- and 9-positions; a cationic copper(I) complex of the latter catalyses nitrene transfer to the C-H bonds of electron-rich arenes.

Full text of DOI:10.1039/b404515g

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Oxidation-resistant, sterically demanding phenanthrolines as supporting
ligands for copper( ) nitrene transfer catalysts†
I
Charles W. Hamilton, David S. Laitar and Joseph P. Sadighi*
Department of Chemistry, Massachusetts Institute of Technology, Room 2-214A, Cambridge, MA
02139-4307, USA. E-mail: jsadighi@mit.edu
Received (in West Lafayette, IN, USA) 25th March 2004, Accepted 12th May 2004
First published as an Advance Article on the web 7th June 2004
2
New 1,10-phenanthroline ligands have been synthesized with
C6F5– or 2,4,6-(CF3)3C6H2– groups in the 2- and 9-positions; a
is the solvent-free [2·Cu]+ SbF6 (4), characterised by NMR and
elemental analysis.§This complex has resisted efforts at crystallisa-
tion; however, exposure of a concentrated CH2Cl2 solution of 4 to
cationic copper( ) complex of the latter catalyses nitrene
I
transfer to the C–H bonds of electron-rich arenes.
a small concentration of benzene vapor results in crystallisation of
2
the h -benzene adduct 4A (Fig. 2).‡ Like other copper(
I
)–benzene
The efficient and selective functionalisation of carbon–hydrogen
bonds represents an ongoing challenge in inorganic and organic
chemistry.1 We are interested in low-coordinate late transition
metal complexes as catalysts for atom- and group-transfer reactions
of this type. Ligands such as 2,9-diphenyl-1,10-phenanthroline,2
and its 2,9-dimesityl analogue,3 project considerable steric bulk
about the metal, but expose C–H bonds to the metal center. We
report herein the synthesis of two new 1,10-phenanthroline ligands,
substituted in the 2- and 9-positions with heavily fluorinated aryl
adducts,13 4A shows no substantial lengthening of the coordinated
C–C bond.
Solutions of complex 4, on addition of the reagent (p-
toluenesulfonylimino)phenyliodinane (PhINTs),14 form a reactive
green intermediate, with liberation of free iodobenzene. We have as
yet been unable to preclude extensive decomposition during the
isolation of this intermediate, possibly a cationic (sulfonylimido)-
copper(III) complex. Instead, we have investigated its activity in the
transfer of tosylnitrene to arene C–H bonds.
groups4 to avoid oxidative ligand modification.5 Copper(
I) com-
The reaction of PhINTs with anisole15 (17 equiv) in PhCF3
solution, catalysed by 4 (0.5 mol%), resulted in the rapid
plexes of both phenanthrolines have been prepared and structurally
characterized; the more sterically demanding ligand gives rise to a
reactive precatalyst for the transfer of a nitrene group from
iminoiodinanes to the C–H bonds of electron-rich arenes.6
The new phenanthrolines were prepared by cross-coupling
reactions, using Pd(OAc)2 precatalyst and the 2-(dicyclohex-
ylphosphino)biphenyl ligand developed by Buchwald and cowork-
ers7 (Scheme 1). The reaction of 2,9-dichloro-1,10-phenanthroline8
with C6F5ZnBr9 affords 2,9-bis(pentafluorophenyl)-1,10-phenan-
throline (1) in good yield. The Negishi coupling has been used
previously to prepare a variety of 2,9-diaryl-1,10-phenanthro-
lines.10 The reaction of 2,9-diiodo-1,10-phenanthroline11 with
2,4,6-(CF3)3C6H2Cu,12 prepared in situ by lithiation of
2,4,6-(CF3)3C6H3 followed by addition of CuI, affords 2,9-bis-
[2A,4A,6A-tris(trifluoromethyl)phenyl]-1,10-phenanthroline (2).
Reaction of 1 with CuI in CH2Cl2 at ambient temperature
(Scheme 2) results in the formation of a homoleptic complex
Scheme 2 Synthesis of copper(I) complexes 3 and 4.
2
[(1)2Cu]+, crystallised as its SbF6 salt 3.† The X-ray crystal
structure of 3 (Fig. 1)‡shows bond lengths and bite angles similar
to those in copper( ) complexes of 2,9-diphenyl-1,10-phenanthro-
I
line;2 however, one phenanthroline is canted, permitting a p-
stacking interaction with a pentafluorophenyl group; the distance
between rings is 3.343(5) Å.
Ligand 2 presents considerably greater steric demand than 1, and
reacts with CuI in CH2Cl2 to afford trigonal 2·CuI. The iodide
ligand is readily abstracted by AgSbF6, and the product after drying
2
Fig. 1 Representation of 3, shown as 50% ellipsoids. The SbF6 ion, one
molecule of CH2Cl2 and hydrogen atoms have been omitted for clarity.
Selected interatomic distances (Å) and bond angles (°): Cu–N(1) 2.109(3),
Cu–N(2) 2.025(3), Cu–N(3) 2.079(3), Cu–N(4) 2.048(3), C(12)–C(42)
3.343(5); N(1)–Cu–N(2) 83.39(11), N(3)–Cu–N(4) 82.77(11), N(1)–Cu–
N(3) 102.6(11), N(2)–Cu–N(3) 119.89(11).
Scheme 1 Synthesis of fluorinated 2,9-diaryl-1,10-phenanthrolines.
† Electronic Supplementary Information (ESI) available: Synthesis of 1–4A,
with spectroscopic and analytical details; crystallographic data plus
additional structures. See http://www.rsc.org/suppdata/cc/b4/b404515g/
1628
C h e m . C o m m u n . , 2 0 0 4 , 1 6 2 8 – 1 6 2 9
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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dissolution of the iminoiodinane, with the formation of o- and p-
sulfonamidation products, TsNH2, and poorly soluble oligomers
that were removed during NMR and GC-MS sample preparation.
The substrate 1,3-dimethoxybenzene was chosen next, to sidestep
temporarily the issue of o/p selectivity, and to examine whether the
desired reaction would occur more readily with this doubly
activated nucleus, minimizing the subsequent side-reactions
(Scheme 3).¶Indeed, the reaction of PhINTs with this substrate (20
equiv in PhCF3 at ambient temperature), catalysed by 4 (1.8 mol%)
led to the formation of p-tosyl-1,3-dimethoxyaniline in an isolated
yield of 63% based on iminoiodinane.
A change in stoichiometry resulted in a notably different
outcome. With arene as the limiting reagent, the major product was
the N,N-diarylsulfonamide. Only a trace of monoarylsulfonamide
was observed; the other products appear to be sulfonated oligoar-
ylamines. This seemingly paradoxical result suggests that the
initially formed N-arylsulfonamide can be activated under the
reaction conditions to generate a nitrogen-based electrophile.
In conclusion, we have prepared new 1,10-phenanthroline
ligands substituted with heavily fluorinated aryl rings to confer
steric bulk while resisting oxidation. With the C6F5-substituent, the
Notes and references
¯
‡
Crystal data. 3: C49H14N4F26Cl2CuSb, M = 1408.83, triclinic, P1, a =
12.5042(13) Å, b = 14.2178(15) Å, c = 14.2393(15) Å, a = 74.479(2)°,
b = 85.194(2)°, g = 74.395(2)°, V = 2349.1(4) Å3, Z = 2, Dcalc = 1.992
Mg m23, MoKa l = 0.71073 Å, T = 193 K, m = 1.289 mm21. A total of
9642 reflections were collected in the q range 1.97–23.30° of which 6649
were unique (Rint = 0.0338). No absorption correction was applied. The
least squares refinement converged normally with residuals of R (based on
F) = 0.0355, wR2 (based on F2) = 0.0895, and GOF = 1.041 based on I
> 2s(I).
¯
4A: C37H18N2F24Cl2CuSb, M = 1202.72, triclinic, P1, a = 12.8498(11)
Å, b = 17.2274(14) Å, c = 19.3942(16) Å, a = 88.865(2)°, b =
85.991(2)°, g = 72.624(2)°, V = 4087.3(6) Å3, Z = 4, Dcalc = 1.955 Mg
m23, MoKa l = 0.71073 Å, T = 194 K, m = 1.455 mm21. A total of 25968
reflections were collected in the q range 1.62–28.32° of which 18329 were
unique (Rint = 0.0186). No absorption correction was applied. The least
squares refinement converged normally with residuals of R (based on F) =
0.0687, wR2 (based on F2) = 0.1748, and GOF = 1.124 based on I >
2s(I).
CCDC 234760 and 234762. See http://www.rsc.org/suppdata/cc/b4/
b404515g/ for crystallographic data in .cif or other electronic format.
§
Synthesis and characterization data for 4: Reactions were carried out at
ambient temperature, in dried solvents under inert atmosphere. CuI (1.05
mmol) and 2 (0.696 mmol) were stirred overnight in CH2Cl2 (7 mL).
Filtration of the mixture and concentration in vacuo afforded 2·CuI as a red
powder in 80% yield. The reaction of 2·CuI (0.210 mmol) with AgSbF6
(0.216 mmol) in CH2Cl2 (5 mL) for 5 min, followed by filtration and
ligand is small enough to form a homoleptic copper(
whereas the larger 2,4,6-(CF3)3C6H2-substituted ligand supports
the formation of a reactive copper( ) cation. This complex catalyses
I) complex,
I
nitrene transfer to the C–H bonds of an electron-rich arene, leading
to either of two major products depending on the reaction
stoichiometry. Near-future goals include the elucidation of relevant
reaction mechanisms, and the development of more versatile arene
functionalization reactions.
We thank Mr Gergely Sirokman for improvements to the initial
preparation of 1, and Prof. Daniel G. Nocera for helpful
discussions. CWH gratefully acknowledges a Lester Wolfe pre-
doctoral fellowship. We thank the MIT Department of Chemistry
for startup funding, and the NSF (Awards CHE-9808061 and DBI-
9729592) for support of our NMR facilities.
1
concentration in vacuo, afforded 4 as a yellow powder in 87% yield. H
NMR (300 MHz, acetone-d6): d 9.20 (d, J = 8.4 Hz, 2H), 8.63 (s, 4H), 8.57
(s, 2H), 8.50 (d, J = 8.4 Hz, 2H). 19F NMR (282 MHz, acetone-d6): d
257.14 (s, 12 F), 262.90 (s, 6F). Anal. calcd for C30H10N2F24CuSb: C,
34.66; H, 0.97; N, 2.69. Found: C, 34.46; H, 1.00; N, 2.65%.
¶
General procedure for sulfonamidations: Reactions were carried out at
ambient temperature, in dried solvent under inert atmosphere. PhINTs and
activated 3 Å molecular sieves were suspended, and catalytic amounts of 4
dissolved, in PhCF3. 1,3-Dimethoxybenzene was added via syringe. Upon
completion of the reactions, the mixtures were concentrated in vacuo and
the organic products isolated by column chromatography on silica gel.
1 Selected reviews: (a) F. Kakiuchi and N. Chatani, Adv. Synth. Catal.,
2003, 345, 1077–1101; (b) J. A. Labinger and J. E. Bercaw, Nature,
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2 M. T. Miller, P. K. Gantzel and T. B. Karpishin, Inorg. Chem., 1998, 37,
2285–2290.
3 M. Schmittel and A. Ganz, Chem. Commun., 1997, 999–1000.
4 An analogous ligand, bearing 3,5-(CF3)2C6H3 groups in the flanking
positions, has been prepared: D. D. Lecloux and Y. Wang, E. I. Du Pont
de Nemours and Company, USA, PCT Int. Appl., 2004.
5 Selected examples of this strategy: (a) A. Mahammed, H. B. Gray, A. E.
Meier-Callahan and Z. Gross, J. Am. Chem. Soc., 2003, 125,
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8961–8962. See also ref. 6(a).
6 (a) The analogous reaction with benzene, using a copper(I) catalyst with
a brominated scorpionate ligand, was reported recently: M. M. Díaz-
Requejo, T. R. Belderraín, M. C. Nicasio, S. Trofimenko and P. J. Pérez,
J. Am. Chem. Soc., 2003, 125, 12078–12079; (b) Copper-catalyzed
alkane dehydrogenation and olefin aziridination by PhINTs: A. N.
Vedernikov and K. G. Caulton, Chem. Commun., 2004, 162–163.
7 J. P. Wolfe, R. A. Singer, B. H. Yang and S. L. Buchwald, J. Am. Chem.
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2
Fig. 2 Representation of 4A, shown as 50% ellipsoids. For clarity, the SbF6
ion, solvent, and hydrogen atoms have been omitted, and only one molecule
in the asymmetric unit is shown. A more extensive structure, showing p-
stacking interactions among the cations, is available in the ESI.† Selected
interatomic distances (Å) and bond angles (°): Cu(1)–N(1) 2.108(4), Cu(1)–
N(2) 2.026(4), Cu(1)–C(31) 2.232(5), Cu(1)–C(32) 2.113(5), C(31)–C(32)
1.397(9); N(1)–Cu(1)–N(2) 82.71(15), N(1)–Cu(1)–C(31) 127.1(2), N(1)–
Cu(1)–C(32) 117.2(2).
10 J. C. Loren and J. S. Siegel, Angew. Chem., Int. Ed., 2001, 40,
754–757.
11 S. Toyota, C. R. Woods, M. Benaglia and J. S. Siegel, Tetrahedron Lett.,
1998, 39, 2697–2700.
12 G. E. Carr, R. D. Chambers, T. F. Holmes and D. G. Parker, J.
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13 See: D. S. Laitar, C. J. N. Mathison, W. M. Davis and J. P. Sadighi,
Inorg. Chem., 2003, 42, 7354–7355and references cited therein.
14 Y. Yamada, T. Yamamoto and M. Okawara, Chem. Lett., 1975,
361–362.
Scheme 3 Divergent outcomes for nitrene transfer to C–H bonds.
15 Under these conditions, benzene was not observed to react.
C h e m . C o m m u n . , 2 0 0 4 , 1 6 2 8 – 1 6 2 9
1629