New efficient ligand for sub-mol % copper-catalyzed C-N cross-coupling reactions running under air

Article abstract of DOI:10.3762/bjoc.8.221

A new efficient ligand, N,N″-dimethyldiethylene triamine (DMDETA), has been synthesized and evaluated for sub-mol % coppercatalyzed C-N cross-coupling reactions. The efficiency of the ligand was determined by kinetic methods. DMDETA proved to display efficiency similar to DMEDA and, in addition, the resulting catalyst was tolerant to air.

Full text of DOI:10.3762/bjoc.8.221

New efficient ligand for sub-mol % copper-catalyzed
C–N cross-coupling reactions running under air
Per-Fredrik Larsson, Peter Astvik and Per-Ola Norrby*
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 1909–1915.
University of Gothenburg, Department of Chemistry and Molecular
Biology, Kemigården 4, SE-412 96, Gothenburg, Sweden
doi:10.3762/bjoc.8.221
Received: 24 August 2012
Accepted: 18 October 2012
Published: 09 November 2012
Email:
Per-Ola Norrby* - pon@chem.gu.se
* Corresponding author
Associate Editor: I. Marek
Keywords:
© 2012 Larsson et al; licensee Beilstein-Institut.
License and terms: see end of document.
copper; cross coupling; hyperactivity; kinetics; ligands
Abstract
A new efficient ligand, N,N’’-dimethyldiethylene triamine (DMDETA), has been synthesized and evaluated for sub-mol % copper-
catalyzed C–N cross-coupling reactions. The efficiency of the ligand was determined by kinetic methods. DMDETA proved to
display efficiency similar to DMEDA and, in addition, the resulting catalyst was tolerant to air.
Introduction
Copper-catalyzed heteroatom cross-coupling reactions (C–N, high (>20 mol %) the cross-coupling reactions can be run with
C–O and C–S) are important synthetic tools in modern organic copper concentrations down to 0.001 mol % (10 ppm). The
synthesis [1-6]. Even though copper salts have been known to temperature can be lowered to 65 °C if the reaction is run in
catalyze these transformations for a long period of time (i.e., the neat DMEDA. Using low catalytic loadings also eliminates the
Ullmann [7] and Goldberg reactions [8]) the reaction type has issue of arylation of the ligand, which is a known side-reaction
had a reputation of being inefficient, requiring high tempera- when using higher catalytic loadings with secondary amine
ture and high catalytic loading (near stoichiometric) [1]. ligands [12,13]. Several classes of ligands have proved effi-
However, the revival of copper catalysis by Buchwald and cient in copper-catalyzed cross-coupling reactions, such as
co-workers [9-24] and others [25-75] during the 1990s and phenanthrolines, alpha-amino acids, 1,3-diketones, salicyl-
2000s proved that copper is a competitive catalyst alongside the amides, and imines, but DMEDA is one of the few that works
more common choices such as palladium and nickel. With a well when sub-mol % copper is used [10,19,23,25,29,45,78-84].
few exceptions [23,45], the catalytic loading is still in the range
of 5–20 mol %. We have shown recently that in many cases Results and Discussion
sub-mol % of copper is enough to catalyze several transforma- Although DMEDA has proved to be a very efficient ligand for
tions [76,77]. By keeping the ligand (DMEDA) concentration C–N, C–O and C–S, some issues still remain, such as the sensi-
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Scheme 1: Synthetic procedure for N,N''-dimethyldiethylene triamine.
tivity to air. As the Me–NH–ethylene–NH motif seemed to be
necessary when lowering the copper concentration,
dimethyldiethylene triamine (DMDETA) was a natural choice
as ligand. The ligand was synthesized based on literature pro-
cedures starting from diethylene triamine (Scheme 1) [85,86].
Reductive methylation by using formaldehyde and NaBH4, as
well as BOC-protection followed by reduction by LiAlH4, were
also evaluated as potential synthetic routes, but both methods
proved unsuccessful. In our hands, protecting all nitrogens fol-
lowed by methylation of the two remaining free valencies on N
was the most efficient method for synthesizing DMDETA.
The removal of the tosyl groups proved to be complicated due
to issues with isolation of DMDETA during the workup proce-
dure. Steam-distillation followed by acidification by HCl (aq) to
produce the DMDETA trihydrochloride-salt 3 did solve the
Figure 1: Kinetic profile for varying [DMDETA].
problem, but at the expense of lower yield. Distillation over higher concentrations [77]. A value lower than 1 indicates that
CaH2 gave the pure DMDETA (4) in moderate yield.
an active monomeric catalyst is in equilibrium with inactive
dimers or higher oligomers. The reaction order never drops to
To test the efficiency of DMDETA a kinetic study was zero, as it did with DMEDA, showing that the formation of
performed by varying the concentration of the ligand and insoluble aggregates at high concentration [87] is avoided.
copper. As a standard reaction the copper-catalyzed cross
coupling of pyrazole with iodobenzene was chosen (Scheme 2).
Scheme 2: Standard reaction for the kinetic study.
The kinetic profile when varying the concentration of
DMDETA proved to be more or less the same as for our previ-
ously reported results concerning DMEDA (Figure 1) [77]
The reaction order in copper was determined to be 0.7
(Figure 2). This is different from DMEDA, where the order was
Figure 2: Reaction order for copper from 0.025–0.64 mol %.
found to be 1 at Cu concentrations below ca. 1 mM, and 0 at
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Beilstein J. Org. Chem. 2012, 8, 1909–1915.
Since DMDETA seems superior to DMEDA in solubilizing the None of the ligands in Scheme 3 showed satisfactory perfor-
catalyst and hence generating a more stable active catalyst, we mance except neat polyethyleneimine (70% yield). A few
decided to challenge the reaction with conditions where selected solid-supported aliphatic amines (StratoSphere™
DMEDA fails, by testing the reaction without an inert atmos- PL-EDA resin, StratoSphere™ PL-DETA, tris(2-
phere. Somewhat to our surprise, we found that the DMDETA- aminoethyl)amine polymer-bound) were also tested as ligands
based catalyst was efficient even under air (Figure 3).
but gave zero yield under standard reaction conditions. The fact
that StratoSphere™ PL-DETA did not give any yield compared
to the free amine (DETA, 41% yield after 24 h) [78] indicates
that the amine needs to be in its free form and could be involved
in the mass transfer of the heterogeneous base.
Conclusion
In summary, a new and efficient ligand (DMDETA) for sub-
mol % copper-catalyzed C–N cross coupling has been synthe-
sized and evaluated in kinetic studies. DMDETA, together with
DMEDA, is one of the few reported ligands that work at sub-
mol % levels of copper catalyst [23,45,77,78,88]. DMDETA
proved to be at least as efficient as DMEDA and in addition has
the ability to render the catalyst tolerant to air. The reaction
order of copper (0.7) lacked the nonlinear behavior previously
reported for DMEDA [78], indicating that the ligand is more
effective at solvating the active catalyst. The fact that
DMDETA proved to be an effective ligand in sub-mol %
Figure 3: DMEDA versus DMDETA under air and nitrogen atmos-
phere.
As can be seen from Figure 3, the standard reaction run with copper catalysis further indicates the necessity for the
DMEDA is greatly hampered by the presence of air and stops Me–N–ethylene–N–Me motif in the ligand design of these
after 8 h compared to that with DMDETA, which gives 90% systems. However, in our hands the synthetic procedures for
yield after 24 h under the same conditions. To further investi- mono-methylating ethylene amines were ineffective for the
gate the ligand scope a range of long-chained aliphatic amines ethylenediamine motif; hence, there is an opportunity for
were evaluated in the standard reaction (Scheme 3).
further ligand synthesis development.
Scheme 3: Long-chained aliphatic amines as ligands.
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Beilstein J. Org. Chem. 2012, 8, 1909–1915.
Experimental
48.22 (2C), 49.74 (2C), 127.40–127.51 (6C), 129.82–129.91
General: All the experiments were carried out under inert (6C), 133.94 (2C), 135.30 (3C), 143.61–143.87 (1C) ppm.
atmosphere (nitrogen). DMEDA, iodobenzene and dodecane
were distilled over calcium hydride. Toluene was dried through N,N’’-dimethyldiethylene triamine trihydrochloride (3):
distillation and stored under nitrogen. DMF was distilled over 2 (43.48 g, 73 mmol) was dissolved in conc. H2SO4 (90 mL)
calcium hydride and stored under nitrogen. CuCl2 (Aldrich, under heating, with continued stirring at 120 °C for 18 h. A
purity of 99.999% metal basis), K3PO4 (98%), and pyrazole saturated KOH/H2O solution was added slowly to the reaction
(98%) were stored under air- and moisture-free conditions. All mixture (cooled on ice) until it was strongly alkaline. The mix-
other chemicals were used as received. A gas chromatograph ture was then steam-distilled, and the distillate was acidified
with flame ionization detector and a 30 m × 0.25 mm EQUITY- with HCl and evaporated to dryness to yield the HCl salt of
5 fused silica capillary column was used, with hydrogen as dimethyldiethylene triamine (12 g, crude). The salt was further
carrier gas. General temperature program: 100 °C for 14 min, purified by heating under reflux in EtOH (99.8%), and, upon
then up to 300 °C at 50 °C min−1 for 2 min. Dodecane was used cooling on ice, was filtrated and washed with ice cold EtOH to
as an internal standard. The NMR spectroscopic analysis was give the pure HCl-salt of N,N’’-dimethyldiethylene triamine (3)
carried out on a Varian 400 MHz instrument by using CDCl3 or (5.28 g, 31%). 1H NMR (400 MHz, D2O) δ 2.69 (s, 6H), 3.38
D2O as solvent, with shifts measured against TMS.
(m, 8H) ppm; 13C NMR (400 MHz, D2O) δ 33.23 (2C), 43.32
(2C), 44.06 (2C) ppm.
Synthesis of N,N’’-dimethyldiethylene
triamine (DMDETA)
N,N’’-dimethyldiethylene triamine (4): The HCl-salt 3 was
4-Methyl-N,N-bis(2-(4-methylphenylsulfonamido)- distilled over CaH2 by Kugelrohr distillation, at 160–200 °C
ethyl)benzenesulfonamide (1): Diethylene triamine (10 g, 97 under vacuum, to yield a clear oil of 4 (40–60%). 1H NMR
mmol, 1 equiv) and NaOH (11.65 g, 291 mmol, 3 equiv) were (400 MHz, CDCl3) δ 1.22 (s, 3H), 2.39 (s, 6H), 2.68 (m, 8H)
added to a round-bottom flask, and H2O was added under stir- ppm; 13C NMR (400 MHz, CDCl3) δ 36.45 (2C), 49.27 (2C),
ring until the hydroxide was dissolved. Tosyl chloride (57.2 g, 51.71 (2C) ppm.
300 mmol, 3.1 equiv) was dissolved in DEE (300 mL) and
added over 1 h to the reaction mixture, followed by stirring at General procedure for the kinetic experiments: The initial
ambient temperature for 4 h. The resulting white solid was rate log plots were constructed from data points ranging from 0
collected by filtration, washed repeatedly with hot water, and to a maximum of 25% yield. The kinetic investigation was
then with EtOH (3 × 50 mL), and dried in vacuo to yield 1 carried out by varying the concentration of one component and
(44 g, 80%). 1H NMR (400 MHz, CDCl3) δ 2.43 (s, 9H), 3.16 keeping the rest of the components constant under standard
(m, 8H), 7.31–7.77 (m, 12H) ppm; 13C NMR (400 MHz, reaction conditions; pyrazole (136 mg, 2 mmol, 1 equiv),
CDCl3) δ 21.55 (3C), 42.65 (2C), 50.56 (2C), 127.16–127.31 iodobenzene (334 μL, 1.5 equiv), CuCl2 (40 μL, 0.01 mol %)
(6C), 129.83–130.00 (6C), 134.67 (2C), 136.63 (3C), 143.65 K3PO4 (849 mg, 2 equiv), DMDETA (26 mg, 0.2 mmol,
(1C) ppm.
10 mol %) and dodecane (50 μL, 0.22 mmol). Into a microwave
vial was added pyrazole (A mmol) and K3PO4 (B mmol). The
N,N-bis(2-(N,4-dimethylphenylsulfonamido)ethyl)-4-methyl- vial was sealed and a CuCl2 solution (C μL, 5 mM in dry THF)
benzenesulfonamide (2): 1 (44 g, 78 mmol, 1 equiv) was was added. The THF was removed by three cycles of vacuum
dissolved in dry DMF (500 mL) whereupon NaH (9.3 g, 234 followed by nitrogen, whereupon toluene (2 mL), DMEDA
mmol, 60% susp., 3 equiv) was added in portions under stirring, (D mmol), iodobenzene (E mmol), and dodecane (50 mL,
and stirring was then continued for 30 min at ambient tempera- 0.22 mmol) were added. The closed vial was placed in a
ture. Excess NaH was separated by allowing the solids to settle, preheated aluminum block at 135 °C. Samples (50 μL) were
whereupon the solution was removed with a syringe. The solu- collected at intervals, filtered through a small silica plug, and
tion was heated to 120 °C. Iodomethane (15 mL, 234 mmol, analyzed by GC. The yield from the GC was determined by
3 equiv) was added slowly under stirring at 120 °C and then using dodecane as the internal standard.
heated under reflux for 24 h. Half of the solvent was evapo-
rated, whereupon water was added (400 mL), resulting in a Procedure for running under air or nitrogen: The same
white/yellow precipitation. The solids were collected by filtra- procedure as the kinetic experiment was used; however, the
tion, washed repeatedly with hot water, and dried in vacuo to reactions were not run in closed vials but under reflux in air or
yield 2 (43.48 g, 94%). 1H NMR (400 MHz, CDCl3) δ 2.43 (s, nitrogen. In this case 20 mol % of the ligands were used,
9H), 2.78 (s, 6H), 3.29 (m, 8H), 7.33 (m, 6H), 7.69 (m, 6H) DMEDA (43 μL, 0.4 mmol, 20 mol %) and DMDETA (52 mg,
ppm; 13C NMR (400 MHz, CDCl3) δ 21.53 (3C), 36.28 (2C), 0.4 mmol, 20 mol %). Samples were taken (50 μL) at certain
1912

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