Atom transfer radical addition (ATRA) catalyzed by copper complexes with tris[2-(dimethylamino)ethyl]amine (Me6TREN) ligand in the presence of free-radical diazo initiator AIBN

Article abstract of DOI:10.1039/c1dt10189g

In this article, we focus on the evaluation of tris[2-(dimethylamino)ethyl] amine (Me₆TREN) ligand in copper catalyzed ATRA in the presence of free-radical diazo initiator AIBN (2,2′-azobis(2-methylpropionitrile)). The addition of carbon tetrac

Full text of DOI:10.1039/c1dt10189g

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PAPER
Atom transfer radical addition (ATRA) catalyzed by copper complexes with
tris[2-(dimethylamino)ethyl]amine (Me6TREN) ligand in the presence of
free-radical diazo initiator AIBN†
William T. Eckenhoff and Tomislav Pintauer*
Received 2nd February 2011, Accepted 2nd March 2011
DOI: 10.1039/c1dt10189g
In this article, we focus on the evaluation of tris[2-(dimethylamino)ethyl]amine (Me6TREN) ligand in
copper catalyzed ATRA in the presence of free-radical diazo initiator AIBN
(2,2¢-azobis(2-methylpropionitrile)). The addition of carbon tetrachloride to 1-hexene, 1-octene and
cis-cyclooctene proceeded efficiently to yield 89, 85 and 85% of monoadduct, respectively, using the
catalyst to alkene ratio of 1 : 2500. For alkenes that readily undergo free radical polymerization, such as
methyl acrylate, catalyst loadings as high as 0.4 mol-% were required. Furthermore, modest yields of
the monoadduct were obtained with less active alkyl halides (chloroform and bromoform) using 250 : 1
and 500 : 1 ratios of alkene to copper(II). Interestingly, the addition of carbon tetrachloride to
cis-cyclooctene produced only 1-chloro-4-(trichloromethyl)-cyclooctene, while carbon tetrabromide
yielded 1,2 and 1,4-regioisomers in 75 : 25 ratio. The activity of [CuII(Me6TREN)X][X] (X = Br- and
Cl-) complexes in ATRA in the presence of AIBN was additionally probed by adding excess free ligand,
source of halide anions and triphenylphosphine. The results indicated that disproportionation is a likely
cause for lower activity of Me6TREN as compared to TPMA (tris(2-pyridylmethyl)amine).
(typically between 5 and 30 mol% relative to alkene). This was
Introduction and background
mostly due to radical termination reactions, which resulted in
Discovered by Kharasch in 1945, atom transfer radical addition
accumulation of the deactivator (transition metal complex in the
(ATRA) is a fundamental reaction for the formation of carbon–
higher oxidation state). Similar problems were also encountered in
carbon bonds starting from alkyl halides and alkenes.1–6 The
a synthetically more useful intramolecular version of ATRA, also
commonly known as atom transfer radical cyclization (ATRC).28–38
The use of solid supported catalysts30,39,40 and biphasic fluorous
systems41 was examined as solutions to catalyst recycling and
recovery, but met with only limited success.
Although the catalyst in such systems could be easily separated
from the reaction mixture, the problem of recycling due to the
accumulation of the deactivator species still remained.
One of the most effective methods of diminishing catalyst
concentration in ATRA is that of in situ catalyst regeneration
in the presence of free radical diazo initiators (e.g. 2,2¢-azobis(2-
methylpropionitrile) (AIBN)) or magnesium. This method was
originally developed for mechanistically similar atom transfer
radical polymerization (ATRP),42–45 and was subsequently applied
first to Ru46–54 and then Cu55–69 based ATRA. In all of these
processes, the deactivator (transition metal complex in the higher
oxidation state) is continuously reduced to the activator (transition
metal complex in the lower oxidation state). The proposed
mechanism for copper catalyzed ATRA in the presence of free
radical diazo initiator AIBN is shown in Scheme 1. Radicals
generated from thermal decomposition of AIBN partially reduce
copper(II) to the corresponding copper(I) complex. The copper(I)
complex starts a catalytic cycle by homolytically cleaving an alkyl
reaction is typically initiated by peroxides, diazo compounds or
light. Initially, ATRA was limited to the addition of polyhalo-
genated alkanes to alkenes that do not readily undergo free
radical polymerization such as a-olefins. However, the realization
that transition metal complexes could act as better halogen
atom transfer agents than alkyl halides, and additionally catalyze
ATRA through a reversible redox process, expanded the scope of
this relatively simple organic transformation.7–11 These catalysts
provided better selectivity towards the monoadduct by reducing
the overall radical concentration, thus suppressing side reactions
such as radical termination and oligomerization/polymerization.
Complexes of Cu, Fe, Ru, and Ni were found to be particularly
active for a variety of alkyl halides and alkenes.12–27 However,
metal-mediated ATRA systems required large concentrations of
the catalyst in order to obtain high yields of the monoadduct
Department of Chemistry and Biochemistry, Duquesne University, 600
Forbes Avenue, 308 Mellon Hall, Pittsburgh, PA, 16282, USA. E-mail:
pintauert@duq.edu; Fax: +1 412 396 5683; Tel: +1 412 396 1626
† Electronic supplementary information (ESI) available: Detailed product
characterization, UV-Vis experiments and crystallographic data tables.
CCDC reference numbers 808280–808287. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c1dt10189g
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“greener” alternative to currently available synthetic processes for
such organic transformations.
The success of ATRA in the presence of reducing agents
using copper complexes with TPMA encouraged us to seek
more active complexing ligands for this process. According to
the well established correlation between the equilibrium constant
for atom transfer (KATRA = ka/kd ) and activity in ATRP,70,71
tris[2-(dimethylamino)ethyl]amine (Me6TREN) was projected to
be an even more active ligand in copper catalyzed ATRA
(Scheme 2). Me6TREN was first introduced to copper catalyzed
ATRP of acrylates, enabling well controlled polymerization
even at ambient temperature.44,72,73 Subsequently, its use was
extended to ATRC reactions of bromoacetimides with excel-
lent results when compared to other neutral nitrogen-based
ligands.29,74
Scheme 1 Proposed mechanism for copper catalyzed ATRA in the
presence of free radical diazo initiator AIBN.
halide bond to produce an alkyl radical that adds across a carbon–
carbon double bond of an alkene. The generated secondary radical
then irreversibly abstracts halogen atom from the copper(II)
complex to form a desired monoadduct. This step regenerates the
activator or copper(I) complex, completing the catalytic cycle. As
indicated in Scheme 1, the competing side reactions in this process
include radical terminations by coupling or disproportionation, as
well as repeating radical additions to alkene to form oligomers/
polymers.
Catalyst regeneration has been shown to be highly successful
when used in copper mediated ATRA systems, producing the
highest turn-over-numbers (TON) observed for this process.59,62,64
Using [CuI(TPMA)Cl] (TPMA = tris(2-pyridylmethyl)amine) and
AIBN, TONs as high as 7200 were achieved in the addition of CCl4
to 1-hexene,58 and even more notable results were obtained with
[CuII(TPMA)Br][Br] in the addition of CBr4 to methyl acrylate
and styrene with TONs of 162 000 and 190 000 respectively.57
Controlling the single addition to highly reactive alkenes (vinyl
acetate, methyl methacrylate, methyl acrylate, and styrene), which
polymerize rapidly in the presence of free radical initiators, has
historically been a challenge for ATRA. By performing the
reactions at room temperature using 2,2¢-azobis(4-methoxy-2,4-
dimethylvaleronitrile) (V-70) and [CuII(TPMA)X][X] (X = Cl
or Br), the addition of polyhalogenated methanes proceeded
efficiently with [CuII]0 ꢀ 0.1 mol% relative to alkene.63 Copper(I)
complexes with anionic trispyrazolyl-borate (homoscorpionate)
ligands were also recently shown to be effective in ATRA systems
without the use of reducing agents. In this case, small amounts
of acetonitrile were used to coordinatively saturate copper in the
lower oxidation state, suppressing the oxidation by alkyl halide.68,69
This process reduced the overall radical concentration and thus
minimized accumulation of the deactivator (copper(II) complex).
The methodology for catalyst regeneration in the presence of
reducing agents was also extended towards more complex organic
synthesis with the addition of CCl4 to 1,6-heptadiene derivatives
followed by sequential ATRC to yield substituted cyclopentanes
in a single step with the lowest catalyst loadings reported so
far.65,67 Recently, monoadducts formed via Ru mediated ATRA
and ATRC in the presence of magnesium powder as a reducing
agent were utilized in a second reaction to synthesize cyclopropane
rings via dehalogenation.50 These examples are a visible indicator
that this methodology is on a trajectory to potentially become a
Scheme 2 ATRA equilibrium (a) and typical nitrogen based ligands
commonly used in copper catalyzed ATRA/ATRP (b). The equilibrium
constant (KATRA) was measured for ethyl 2-bromoisobutyrate in the
presence of CuIBr in CH3CN at 22 ◦C.70
In this article, we describe the use of [CuII(Me6TREN)X][X] (X =
Br- and Cl-) complexes in ATRA of polyhalogenated methanes to
various alkenes in the presence of free radical diazo initiator AIBN.
Furthermore, the activity of copper complexes in the presence of
excess free ligand, source of halide anions and triphenylphosphine
is examined.
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Table 1 ATRA of polyhalogenated methanes to various alkenes cat-
alyzed by [CuII(Me6TREN)X][X] (X = Cl- or Br-) in the presence of free-
radical diazo initiator AIBN
The addition of CCl4, CHCl3, and CHBr3 to cis-cyclooctene
produced predominantly 1,4-regioisomers, resulting from a 1,5-
hydrogen shift occurring before the radical trapping by copper(II)
complex (Scheme 3). For the molecular structure of cis-1-
chloro-4-(trichloromethyl)-cyclooctane, see supporting informa-
tion. Clearly, with these alkyl halides, the rate of 1,5-hydrogen
shift (kH-shift[R∑]) exceeded the rate of radical trapping (kd [CuII][R∑]),
consistent with previous studies in which initiation was achieved
by either thermal or photoinitiated means.76,77 Taking into account
the ratio of 1,4- to 1,2-regioisomers (~98 : 2), copper(II) concentra-
tion (~5.0 ¥10-3 M), and approximate deactivation rate constant
for copper(II) complex (kd ª 5.0 ¥107 M-1s-1),55,56 the rate constant
Entrya Alkene
[Alkene]0 : [Cu]0 R-X
Conv. (%)/Yield (%)b
1
2
3
4
5
6
7
8
1-Hexene
1000 : 1
2500 : 1
1000 : 1
2500 : 1
CCl4 100/100
CCl4 89/89
1-Hexene
1-Octene
1-Octene
cis-Cyclooctene 1000 : 1
cis-Cyclooctene 2500 : 1
Methyl Acrylate 250 : 1
1-Hexene
1-Octene
cis-Cyclooctene 250 : 1
Styrene 250 : 1
Methyl Acrylate 250 : 1
1-Hexene
1-Octene
cis-Cyclooctene 500 : 1
Styrene
Styrene
Methyl Acrylate 250 : 1
Methyl Acrylate 1000 : 1
CCl4 99/99
CCl4 85/85
CCl4 95/95c
CCl4 85/85c
CCl4 100/67
CHCl3 51/51
CHCl3 46/46
CHCl3 26/26c
CHCl3 77/40
CHCl3 100/32
CHBr3 46/46
CHBr3 39/39
CHBr3 37/37c
CHBr3 98/80
CHBr3 93/69
CHBr3 100/51
CBr4 100/87
250 : 1
250 : 1
9
10
11
12
13
14
15
16
17
18
19
for 1,5-hydrogen shift can be estimated to be on the order of kH-shift
ª
1.2 ¥107 s-1. This value is in good agreement with literature values
for similar substrates involving 1,2- and 1,5-hydrogen shifts.78,79
Certainly, the control over product distribution could be achieved
by increasing the copper concentration in the system. For example,
at copper(II) loadings as high as 10 mol-% (relative to alkene),
the ratio of 1,4- to 1,2-regioisomers is expected to decrease to
approximately 65 : 35. Similar effects were previously observed in
ruthenium catalyzed ATRA reactions.80
500 : 1
500 : 1
250 : 1
500 : 1
a T = 60 ◦C, time = 24 h, solvent = CH3CN, [Alkene]0 : [R-X]0 : [AIBN]0 =
1 : 1.1 : 0.05, [Alkene]0 = 1.34 M (CCl4, CHCl3 and CHBr3) or 1.13
M (CBr4). b Yield is based on the formation of monoadduct and was
determined using 1H NMR spectroscopy. c 1,4-regioisomer resulting from
a 1,5-hydrogen shift.
Results and discussion
ATRA catalyzed by copper complexes with Me6TREN ligand
The activity of copper complexes with Me6TREN ligand was
first evaluated in ATRA of polychlorinated and polybromi-
nated methanes to a variety of alkenes (Table 1). Overall,
[CuII(Me6TREN)X][X] (X = Br- or Cl-) complex in the presence
of AIBN was found to be less active than its pyridyl counterpart,
tris(2-pyridylmethyl)amine (TPMA). At alkene to catalyst ratios
of 2500 : 1, ATRA of CCl4 to 1-hexene, 1-octene, and cis-
cyclooctene proceeded efficiently to yield between 85 and 89%
of monoadduct (entries 2, 4, and 6). Much higher yields of
monoadduct were obtained at higher catalyst loadings (entries
1, 3, and 5). Poor control was attained in the case of the addition
of CCl4 to methyl acrylate, which required a catalyst loading as
high as 0.4 mol-% in order to suppress competing free radical
polymerization initiated by AIBN (entry 7). As expected, the
additions of chloroform to all alkenes produced significantly lower
yields of the monoadduct (entries 8–12). This was presumably due
to a slower activation rate constant, as a result of stronger C–Cl
bonds.75 The discrepancy between conversion and percent yield
of the monoadduct in the case of styrene (entry 11) and methyl
acrylate (entry 12) can be attributed to AIBN-initiated free radical
polymerization. Similar results were also obtained in the case of
bromoform (entries 13–18), with the exception of styrene which
yielded 80% of the monoadduct (entry 16).
Scheme 3 Formation of 1,2- and 1,4-regioisomers during copper cat-
alyzed ATRA of CX3Y (X = Cl or Br, Y = Cl, Br or H) to cis-cyclooctene.
The situation with carbon tetrabromide is expected to be quite
different because of the well known fact that it is an excellent
halogen atom transfer agent.81,82 Indeed, ATRA reactions pro-
ceeded efficiently in the presence of AIBN only, affording nearly
quantitative yields of the monoadduct. However, the addition
of CBr4 favored the formation of 1,2-regioisomers (75%), with
the remaining 25% being attributed to products resulting from
the 1,5-hydrogen shift. Following the analogy discussed above
and assuming that kH-shift ª 1.2 ¥ 107 s-1, the rate constant for
halogen atom transfer from CBr4 to cyclooctene radical can be
approximated to be on the order of 3.2 ¥ 107 M-1s-1.
Due to the large chain transfer constant of carbon tetrabromide,
ATRA reactions with most alkenes proceeded efficiently in the
absence of copper catalyst, with the exception of methyl acrylate
which polymerized rapidly, affording only 12% yield of the
monoadduct. However, in the presence of [CuII(Me6TREN)Br][Br]
(0.10 mol-%), the yield increased to 87% (entry 19).
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Each regioisomer in the addition of CBr4 to cis-cyclooctene
consisted of approximately 50 : 50 ratio of diastereomers, which is
typical for radical addition reactions. The molecular structures of
cis and trans 1,4-regioisomers obtained by slow evaporation from
chloroform at -10 ◦C are shown in Fig. 1.
up to 20 equivalents of excess Me6TREN were added to the
reaction mixture (Fig. 2a and 2b). Reactions of styrene and
CCl4 showed a very slight decrease in yield with increasing
free ligand concentration. Interestingly, in the case of methyl
acrylate, a significant decrease in both the alkene conversion and
monoadduct yield was observed on addition of excess Me6TREN.
For example, with 20 eq. of ligand, the conversion and product
yield after 24 h decreased from 100 to 76 and 65 to 32%,
respectively. Most likely, Me6TREN acts as a radical inhibitor
when present in large excess. Furthermore, precipitation of amino
halide salts was also observed, likely arising from a reaction with
alkyl halide (see supporting information).
Fig. 1 Molecular structures of cis-1-bromo-4-(tribromomethyl)-cyclooc-
tane (a) and trans-1-bromo-4-(tribromomethyl)cyclooctane (b) collected
at 150 K, shown with 50% probability displacement ellipsoids. Selected
˚
bond distances [A] for (a) Br1–C1 1.951(7), Br2–C1 1.942(7), Br3–C1
1.954(7), Br4–C5 1.992(7). For (b) Br1–C1 1.958(2), Br2–C1 1.960(2),
Br3–C1 1.944(2), Br4–C5 1.988(2).
The drastic differences in product distribution between CCl4,
CHBr3 and CBr4 can clearly be attributed to different halogen
atom transfer rate constants. To test this observation further,
CBrCl3 was used in ATRA to cis-cyclooctene in the absence of
copper catalyst. This substrate should have a deactivating ability
(i.e. halogen atom transfer rate constant) between that of CCl4 and
CBr4. Indeed, the product distribution in this case consisted of a
52 : 48 ratio of 1,2- to 1,4-regioisomers. For the molecular structure
of cis-1-bromo-4-(trichloromethyl)-cyclooctane see supporting
information.
Fig. 2 Effect of added Me6TREN ligand, tetrabutylammonium chlo-
Optimization of [Cu(Me6TREN)Cl][Cl] mediated ATRA reactions
ride (TBACl), and triphenylphosphine (PPh3) on [CuII(TPMA)Cl][Cl]
catalyzed ATRA reactions.
T = = 24 h, solvent =
60 ◦C, time
Available electrochemical and kinetic data on copper complexes
with Me6TREN indicate that they should be considerably more
active than corresponding complexes with TPMA ligand.70,71,83,84
Possible reasons for this lack of activity could include: (a) ligand
dissociation, (b) halide anion dissociation, (c) disproportionation
of copper(I) to copper(II and 0), and/or (d) alkene coordination.85
Aliphatic amine ligands are well known to have weaker binding
constants to copper when compared to pyridine based ligands,86,87
so ligand dissociation was investigated first. By adding excess
of Me6TREN, the equilibrium of dissociation could be shifted
towards the complexed and more active copper(I) complex. No
significant effects on the alkene conversion and the overall product
yield were observed in the case of CCl4 and 1-octene when
CH3CN, [Alkene]0 : [R-X]0 : [AIBN]0 = 1 : 1.1 : 0.05, [Alkene]0 = 1.34 M,
[Alkene]0 : [CuII]0 = 5000 : 1 (1-octene), 250 : 1 (styrene and methyl acrylate).
In order to additionally examine the effect of free Me6TREN
ligand, the conversion of 1-octene (which does not undergo
free radical polymerization in the presence of AIBN at 60 ◦C)
was monitored as a function of time. The results for 0, 5, and
20 equivalents of free ligand relative to [CuII(Me6TREN)Cl][Cl]
are shown in Fig. 3. As expected, in the presence of 0 equiv.
of Me6TREN, the first order kinetic plot was linear confirm-
ing constant radical concentration in the system (d[1-octene]/
dt = -kadd [R∑][1-octene]).55,56,62 However, significant deviations were
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Fig. 4 First order kinetic plots for ATRA of 1-octene to CCl4 cat-
Fig. 3 First order kinetic plots for ATRA of 1-octene to CCl4 catalyzed
alyzed by [CuII(Me TREN)Cl][Cl] in the presence of 0 ( ), 5 (ꢀ),
᭹
6
by [CuII(Me TREN)Cl][Cl] in the presence of 0 ( ), 5 (ꢀ), and 20
᭹
and 20 (᭡) equivalents of TBACl. T = 60 ◦C, solvent = CH3CN,
6
(᭡) equivalents of free Me6TREN. T = 60 ◦C, solvent = CH3CN,
[1-octene]0 : [CCl4]0 : [AIBN]0 : [CuII]0 = 5000 : 5500 : 250 : 1, [1-octene]0
=
[1-octene]0 : [CCl4]0 : [AIBN]0 : [CuII]0 = 5000 : 5500 : 250 : 1, [1-octene]0
=
1.34 M.
1.34 M.
observed in the case of 5 and 20 equiv., which also showed faster
reaction rates. The acceleration in the latter two cases can be
attributed to increased copper(I) concentrations in the system,
indicating that free Me6TREN might act as an additional reducing
agent (apart from radicals generated by thermal decomposition
of AIBN).88 To test this hypothesis, UV-Vis spectroscopy was
employed to monitor the ratio of equilibrium concentrations
of copper(II) and copper(I) complexes in the presence of AIBN
at 60 ◦C. Without externally added free ligand, only 10% of
[CuII(Me6TREN)Cl][Cl] was reduced to CuI(Me6TREN)Cl, even
after six days. However, upon the addition of 20 equivalents
of Me6TREN, most of the copper(II) complex was found to be
reduced to copper(I) (see supporting information). These results
further indicate that free Me6TREN can indeed act as reducing
agent in these systems.
The possibility of chloride anion dissociation from
[CuII(Me6TREN)Cl][Cl] was also examined by the addition of
tetrabutylammonium chloride (TBACl, Fig. 2c and 2d). Overall,
the addition of TBACl was found to have a slightly negative effect
on the conversion of alkene and the yield of monoadduct. In the
ATRA of 1-octene to CCl4, the yield decreased from 69% to 52%
in the presence of 20 equivalents of TBACl. Similarly, decrease
in the conversion of styrene from 66% to 52% was accompanied
by a decrease in the monoadduct yield from 22% to 12%. Lastly,
in the addition of CCl4 to methyl acrylate in the presence of 20
equiv. of TBACl, nearly quantitative conversions were observed as
a result of free radical polymerization initated by AIBN. However,
the yield of the desired monoadduct decreased from 66% to 55%.
Generally, the addition of TBACl to ATRA reactions catalyzed
by [CuII(Me6TREN)Cl][Cl] in the presence of AIBN resulted in
slower reaction rates. This is demonstrated in Fig. 4 which shows
the first order kinetic plots for the ATRA of CCl4 to 1-octene in
the presence of 1,5 and 20 equivalents of TBACl. The decreased
activity is most likely due to the formation of complexes that are
not active in ATRA systems. Such complexes can be formed by
multiple halide substitutions resulting in the formation of higher
order halocuprates, which are well known and documented in the
literature.85
Having probed the ligand and/or halide anion dissociation
from [CuII(Me6TREN)Cl][Cl], our studies have focused on dis-
proportionation, which is known to occur readily with copper(I)
complexes with Me6TREN ligand. Neutral phosphines, such as
PPh3 and P(OBu)3, have been shown to stabilize copper(I) com-
plexes and suppress this side reaction. However, their coordination
to copper complexes might result in significantly lower ATRA
activity. Both phosphines were found to produce very similar
results when added to ATRA reactions involving 1-octene, styrene,
and methyl acrylate. Therefore, only data with PPh3 will be
discussed. The yield of monoadduct in the case of 1-octene was
found to drastically decrease in the presence of more than 10
equivalents of PPh3 (Fig. 2e and 2f). Under such conditions,
the results mimicked non-metal catalyzed Kharasch addition,
indicating the complete inactivity of the copper complexes to
catalyze ATRA. Product yields for both styrene and methyl
acrylate were also reduced to negligible amounts by the addition
of more than 10 equivalents of PPh3 relative to copper(II). The
reason for this decreased yield is clearly the formation of ATRA
inactive copper complexes. Therefore, although auxiliary ligands
such as PPh3 or P(OBu)3 might suppress disproportionation of
copper(I) to copper(II) and copper(0), their use in ATRA catalysis
should be avoided.
Aside from CH3CN, ATRA reactions were also performed in
DMF, MeOH, and MeOH–H2O mixtures. Disproportionation is
known to occur more rapidly in polar solvents,87 and would thus
have a more noticeable effect on ATRA. This was indeed observed
experimentally. Using 0.2 mol-% of [CuII(Me6TREN)Cl][Cl] (rel-
ative to alkene), 1-octene was previously shown to produce
quantitative yields of monoadduct in CH3CN. However, the yield
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decreased to 50% in both MeOH and DMF. A further decrease
was also observed on the addition of small amounts of H2O.
A similar trend was noted for methyl acrylate, which showed
complete conversion in each solvent, but the yields of monoadduct
decreased from 67% in CH3CN, to 52% and 37% in MeOH
and DMF respectively. Clearly, the increased polarity of the
reaction medium decreases the yield of the desired monoadduct
as a result of disproportionation. Disproportionation decreases
the overall copper(I) concentration in the system, resulting in
slower activation rates (CuI + RX → CuII-X + R∑). Additionally,
polar protic media might favor halide anion dissociation from
[CuII(Me6TREN)Cl][Cl], which would result in even further
decrease in product selectivity.87
The results presented thus far indicate that copper complexes
with Me6TREN ligand can be used in ATRA reactions in the
presence of free-radical diazo initiator AIBN. However, despite
higher equilibrium constant for atom transfer (Fig. 1), their
activity appears to be approximately 10 times lower than with
TPMA analogue.57,58 This decrease can be attributed to concurrent
side reactions associated with the catalyst such as ligand and/or
halide anion dissociation and disproportionation. Based on the
above studies, the latter one was found to have the most significant
effect in lowering the product selectivity.
Fig.
5
Molecular structures of [CuII(Me6TREN)Cl][Cl] (a) and
[CuII(Me6TREN)Br][Br] (b) collected at 150 K, shown with 50% prob-
ability displacement ellipsoids. H-atoms have been omitted for clarity,
[symmetry codes: (i) -z + 3/2,-x + 1,y + 1/2, (ii) -y + 1,z - 1/2,-x + 3/2
for (a) and (i) y,z,x, (ii) z,x,y for (b)].
previously reported structures for [CuII(Me6TREN)Br][Br]85,89 and
[CuII(Me6TREN)Cl0.63/Br0.37][Br]90 at room temperature. Addi-
tionally, the structural features of 1 and 2 were also similar to other
copper(II) complexes with tetradentate neutral nitrogen based
ligands.57,58,60,85
Structural characterization of copper complexes with Me6TREN
ligand
Only a few examples of copper(I) complexes with Me6TREN
ligand have known molecular structures.91,92 This is mostly due
to their oxidative instability and propensity to disproportionate
in a variety of solvents. To prevent this, a single equivalent of
PPh3 was added to a solution of [CuI(CH3CN)4][ClO4] in CH3CN
before the addition of Me6TREN. Salt metathesis with NaBPh4
resulted in the formation of [CuI(Me6TREN)PPh3][BPh4] (3). No
disproportionation was observed during the reaction and the
resulting complex was relatively air stable. The copper(I) center in 3
was distorted tetrahedral in geometry as a result of displacement of
one arm of Me6TREN ligand by PPh3 (Fig. 6). The remaining three
nitrogen atoms from Me6TREN were found to be coordinated
The copper(II) complexes, [CuII(Me6TREN)Cl][Cl] (1) and
[CuII(Me6TREN)Br][Br] (2), were synthesized by reacting CuIICl2
or CuIIBr2 with stoichiometric amounts of Me6TREN. The
same complexes can alternatively be prepared by oxidizing
CuI(Me6TREN)Cl or CuI(Me6TREN)Br with excess CCl4 or
CBr4, respectively. 1 and 2 were found to be distorted trigonal
bipyramidal in geometry with crystallographically imposed C3
symmetry (Fig. 5). Each complex was coordinated by four nitrogen
atoms from Me6TREN ligand and either a chloride (1) or bromide
(2) anion. The axial CuII–N bond lengths in both complexes were
˚
approximately 2.05 A, and the equatorial bonds were slightly
II
II
˚
longer (~2.15 A, Table 2). The Cu –Cl and Cu –Br bond lengths
were found to be 2.2589(5) and 2.4016(4) A, respectively. All of
˚
the angles in the coordination sphere of both complexes were close
to a trigonal bipyramidal geometry. This was also confirmed by
a t factor which was calculated to be 1.00 for both complexes
(trigonal bipyramidal (t = 1) and square pyramidal (t = 0)).85
The structures reported herein were in close agreement with
Table 2 Selected bond distances (A) and angles (◦) for
˚
[CuII(Me6TREN)Cl][Cl] (1) and [CuII(Me6TREN)Br][Br] (2)a
1
2
Bond distances
Cu–N1
Cu–N2
2.0545(15)
2.1489(9)
2.2589(5)
2.046(2)
2.1527(13)
2.4016(4)
Cu–X
Fig. 6 Molecular structure of [CuI(Me6TREN)PPh3][BPh4] (3) col-
Bond angles
N1–Cu–N2
N2–Cu–X
N2–Cu–N2i
N1–Cu–X
lected at 150 K, shown with 50% probability displacement ellipsoids.
84.62(3)
95.38(3)
119.132(8)
180.00(2)
84.81(4)
95.19(4)
119.191(11)
180.00(4)
-
H-atoms and counterion BPh4 have been omitted for clarity. Se-
◦
˚
lected bond distances [A] and angles [ ]: Cu1–N1 2.1450(14), Cu1–N2
2.1753(17), Cu1–N3 2.1865(18), Cu1–P1 2.1910(5), N1–Cu1–N2 85.79(6),
N1–Cu1–N3 83.87(6), N2–Cu1–N3 113.40(8), N1–Cu1–P1 136.92(4),
N2–Cu1–P1 111.80(6), N3–Cu1–P1 119.80(4).
a X = Cl for 1 and Br for 2.
4914 | Dalton Trans., 2011, 40, 4909–4917
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˚
at distances of 2.1450(14), 2.1753(17) and 2.1865(18) A. Finally,
Conclusions
the tetrahedral geometry was completed by the coordination of a
Copper(II) complexes with the Me6TREN ligand were thoroughly
investigated as catalysts for ATRA in the presence of free-
radical diazo initiator AIBN. These complexes were found to
catalyze the addition of CCl4, CHCl3, CBr4, and CHBr3 to a
series of alkenes with good efficiency, albeit lower than expected
due to their large ATRA equilibrium constants. Attempts were
made to increase their activity towards ATRA by addressing lig-
and/halide anion dissociation and disproportionation equilibria.
Catalytic activity could not be increased, and disproportionation
of CuI(Me6TREN)X to Cu0 and [CuII(TPMA)X][X] (X = Br- or
Cl-) was found to be the most likely cause.
The reactions of CCl4 to cis-cyclooctene were found to produce
solely the 1-chloro-4-(trichloromethyl)cyclooctane, as a result of
intramolecular 1,5-hydrogen transfer. Interestingly, CCl3Br and
CBr4 produced both the 1,2- and 1,4-regioisomers in 48 : 52 and
75 : 25 ratios, respectively. These results were attributed to different
halogen atom transfer constants of alkyl halides.
The molecular structures of [CuII(Me6TREN)Cl][Cl] and
[CuII(Me6TREN)Br][Br] were determined and found to be nearly
isostructural with distorted trigonal bipyramidal geometries and
perfect C3 symmetry. A rare example of a copper(I) com-
plex with Me6TREN was prepared using PPh3 to stabilize the
complex against disproportionation. The molecular structure of
[CuI(Me6TREN)PPh3][BPh4] was found to be distorted tetrahe-
dral as a result of the dissociation of a single arm of Me6TREN
ligand. Variable temperature 1H NMR studies confirmed that the
structure in solution was consistent with the solid state.
˚
phosphorus atom from PPh3 at the distance of 2.1910(5) A. The
arm displacement from Me6TREN, induced by the steric bulk of
PPh3, was previously observed in structurally related copper(I)
complexes with tetradentate tris(2-pyridylmethyl)amine (TPMA)
ligand.60,93
From the solid-state structure of 3, Me6TREN is clearly
a tridentate ligand, similar to N,N,N¢,N¢,N¢-
pentamethyldiethylenetriamine (PMDETA). Therefore, the so-
acting as
1
lution structure was probed by variable temperature H NMR
spectroscopy. At room temperature, only a single resonance was
observed for the methyl protons of the ligand at 2.20 ppm and two
signals for the methylene ones at 3.04 and 2.68 ppm (Fig. 7). All
three proton resonances showed a downfield shift relative to free
Me6TREN (2.56 and 2.31 ppm for methylene and 2.16 ppm for
methyl protons), consistent with coordination to copper(I). Upon
cooling, the resonances for Me6TREN significantly broadened
until finally at 170 K the signals for the methylene protons
coalesced and the singlet for the methyl protons at 2.09 ppm
became very broad. Resonances corresponding to PPh3 (7.57–
-
7.55 ppm and 7.49–7.44 ppm) and BPh4 anion (7.33, 6.92, and
6.77 ppm) were observed to show very little change as a result
of cooling. The observed broadening and shifting of Me6TREN
resonances in 3 towards free ligand can be best explained as
the result of rapid dissociation/association of at least one ligand
arm. We ruled out the possibility for complete ligand dissociation
because experiments in the presence of 3 and one equivalent
of free Me6TREN clearly indicated resonances for both free
and complexed ligand in the temperature range 298–180 K.
Furthermore, the ligand exchange occurred only at temperatures
above 360 K (see supporting information). Therefore, the solution
structure of 3 is consistent with the solid state (Fig. 6).
Experimental section
Materials
All chemicals were purchased from commercial sources and used
as received. Tetrakis(acetonitrile)copper(I) perchlorate94 was syn-
thesized according to literature procedures. Warning: Although we
have experienced no problems, perchlorate metal salts are potentially
explosive and should be handled with care. All manipulations
involving copper(I) complexes were performed under argon in the
dry box (<1.0 ppm O2 and <0.5 ppm H2O) or using standard
Schlenk line techniques. Solvents (pentane, acetonitrile, acetone,
and diethyl ether) were degassed and deoxygenated using an
Innovative Technology solvent purifier. Methanol was vacuum
distilled and deoxygenated by bubbling argon for 30 min prior
to use. The corresponding copper(II) complexes were synthesized
under ambient conditions.
Instrumentation
1H NMR spectra were obtained using Bruker Avance 400 and
500 MHz spectrometers and chemical shifts are given in ppm
relative to residual solvent peaks [CDCl3 d 7.26 ppm; (CD3)2CO
d 2.05 ppm]. iNMR software was used to generate the images of
NMR spectra. Temperature calibrations were performed using a
pure methanol sample.
The X-ray intensity data were collected at 150 K using graphite-
˚
monochromated Mo-K radiation (0.71073 A) with a Bruker Smart
Apex II CCD diffractometer. Data reduction included absorption
corrections by the multi-scan method using SADABS.95 Structures
were solved by direct methods and refined by full matrix least
squares using the SHELXTL 6.1 bundled software package.96
Fig. 7 Variable temperature 1H NMR (400 MHz, acetone-d6) of
[CuI(Me6TREN)PPh3][BPh4] (3).
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The H-atoms were positioned geometrically (aromatic C–H 0.93,
methylene C–H 0.97, and methyl C–H 0.96) and treated as riding
atoms during subsequent refinement, with Uiso(H) = 1.2Ueq(C)
or 1.5Ueq(methyl C). The methyl groups were allowed to rotate
about their local threefold axes. ORTEP-3 for Windows97 and
Crystal Maker 7.2 were used to generate molecular graphics.
For detailed crystallographic data tables refer to supporting
information. Crystallographic data (excluding structure factors)
for the structure reported in this article have been deposited with
Cambridge Crystallographic Data Centre (CCDC) as supplemen-
tary publications nos. CCDC 808280–808287. Copies of the data
can be obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB21EZ, UK (fax(+44)1223-336-033; e-mail:
deposit@ccdc.cam.ac.uk).
IR spectra were recorded in the solid state using a Nicolet Smart
Orbit 380 FT-IR spectrometer (Thermo Electron Corporation).
Elemental analyses for C, H, and N were obtained from Midwest
Microlabs, LLC. UV-Vis spectra were recorded using a Beckman
DU-530 spectrometer in 1.0 cm path-length airtight quartz
cuvettes.
solution was stirred at ambient temperature for 15 min. The
complex was precipitated by the addition of 50 mL of petroleum
ether. The blue powder was collected by filtration and dried under
vacuum (yield = 1.33 g, 98%). UV-Vis (CH3CN): lmax = 938 nm,
emax = 451 Lmol-1cm-1. Anal. Calcd. for C12H30N4CuCl2 (364.85):
C, 39.50; H, 8.29; N, 15.36. Found: C, 38.81; H, 7.97; N, 14.98.
Synthesis of [CuII(Me6TREN)Br][Br] (2)
CuIIBr2 (0.83 g, 3.71 mmol) was dissolved in methylene chloride
and Me6TREN (0.860 g, 1 mL, 3.71 mmol) added. The green
solution was stirred at ambient temperature for 15 min. The
complex was precipitated by the addition of 50 mL of petroleum
ether. The green powder was collected by filtration and dried under
vacuum (yield = 1.63 g, 96%). UV-Vis (CH3CN): lmax = 973 nm,
emax = 426 Lmol-1cm-1. Anal. Calcd. for C12H30N4CuBr2 (453.75):
C, 31.76; H, 6.66; N, 12.35. Found: C, 31.40; H, 6.46; N, 12.20.
Synthesis of [CuI(Me6TREN)PPh3][BPh4] (3)
[CuI(CH3CN)4][ClO4] (100 mg, 0.306 mmol) and PPh3 (80 mg,
0.306 mmol) were dissolved in 3.0 mL of MeOH. Me6TREN (70
mg, 82 mL, 0.306 mmol) was then added, followed by NaBPh4
(105 mg, 0.306 mmol). A colorless powder began to precipitate
immediately. The solution was left at -35 ◦C overnight to complete
precipitation. The methanol was removed and the powder was re-
dissolved in 3.0 mL acetone. The complex was crystallized by slow
General procedures for ATRA reactions
0.033 g AIBN (0.20 mmol), p-dimethoxybenzene (internal stan-
dard), 4.03 mmol alkene (630 mL 1-octene, 525 mL cis-cyclooctene,
500 mL 1-hexene, 462 mL styrene, 362 mL methyl acrylate), and
4.43 mmol alkyl halide (430 mL CCl4, 355 mL CHCl3, 387 mL
CHBr3, 1.470 g CBr4) were combined in a vial. Additives, such
as tetrabutylammonium chloride (TBACl), PPh3, or Me6TREN,
were added as solutions (32.2 mM for 1-octene and 645.0 mM for
styrene and methyl acrylate). Solvent was then added (acetonitrile,
methanol, or dimethylformamide) in order to maintain a constant
volume of 2.20 mL for 1-hexene, 1-octene, and cis-cyclooctene or
1.39 mL for styrene and methyl acrylate. The reaction mixture
was then divided equally into 5 NMR tubes (439 mL for 1-hexene,
1-octene and cis-cyclooctene and 278 mL for styrene and methyl
acrylate). A 0.01 M solution of [CuII(Me6TREN)X][X] (X = Cl-
or Br-) in acetonitrile, methanol, or dimethylformamide was then
added to accommodate various catalyst loadings (250 : 1–322 mL,
500 : 1–161 mL, 1000 : 1–81 mL, 2500 : 1–32 mL, 5000 : 1–16 mL,
and 10 000 : 1–8 mL). The total volume in the NMR tube was then
adjusted by the addition of solvent so that the total concentration
of alkene was maintained at 1.34 M. Reaction tubes were flushed
with argon for 30 s, sealed with a plastic NMR tube cap, wrapped
with Teflon tape, and placed in an oil bath thermostated at 60 ◦C
for 24 h.
1
diffusion of diethyl ether (yield = 0.213 g, 80%). H NMR (400
MHz, acetone-d6, 298 K): d7.57–7.55 (m, 9H), d7.49–7.44 (m,
6H), d7.36–7.32 (m, 8H), d6.92 (t, J = 7.4 Hz, 8H), d6.79–6.76 (m,
4H), d3.04 (t, J = 5.4 Hz, 6H), d2.68 (t, J = 5.6 Hz, 6H), d2.20 (s,
18H). Anal. Calcd. for C54H65BCuN4P (875.45): C, 74.08; H, 7.48;
N, 6.40. Found: C, 74.09; H, 7.65; N, 6.87.
Acknowledgements
Financial support from a National Science Foundation Career
Award (CHE-0844131) is greatly acknowledged.
Notes and references
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to be 0.860 g mL-1. H NMR (400 MHz, CDCl3, 298 K): d2.52
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The Royal Society of Chemistry 2011
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