Base-promoted selective aryl C-Cl cleavage by iridium(III) porphyrins via a metalloradical ipso addition-elimination mechanism

Article abstract of DOI:10.1021/om200618m

Base-promoted aryl carbon-chlorine bond (Ar-Cl) cleavage by iridium(III) porphyrin carbonyl chloride (Ir^III(ttp)(CO)Cl; ttp = 5,10,15,20-tetrakis(p-tolyl)porphyrinato dianion) was achieved in the presence of K₂CO₃ to give iridium(III) porphyrin aryls (Ir ^III(ttp)Ar). Mechanistic studies revealed that K₂CO ₃ promotes the reduction of Ir(ttp)(CO)Cl to give the iridium(II) porphyrin dimer intermediate [Ir^II(ttp)₂. [Ir(ttp) ₂ is the source of Ir^II(ttp) metalloradical, which cleaves Ar-Cl to give Ir(ttp)Ar and a chlorine radical (Cl^?) via radical ipso substitution in an addition-elimination pathway. Cl ^? reacts with [Ir(ttp)₂ to yield Ir(ttp)Cl for subsequent base-promoted reduction and Ir(ttp) for radical chain propagation. Additionally, the base-promoted Ar-Cl cleavage of chlorobenzene (PhCl) by Ir(ttp)(CO)Cl gives both Ir(ttp)Ph and 1,4-bis-iridium(III)-porphyrin benzene, Ir^III(ttp)(p-C₆H₄)Ir^III(ttp). The reactive Cl^? can simultaneously react with PhCl via homolytic aromatic substitution to give 1,4-dichlorobenzene, which further undergoes double Ar-Cl cleavage to form Ir(ttp)(p-C₆H₄)Ir(ttp).

Full text of DOI:10.1021/om200618m

ARTICLE
pubs.acs.org/Organometallics
Base-Promoted Selective Aryl CꢀCl Cleavage by Iridium(III) Porphyrins
via a Metalloradical Ipso AdditionꢀElimination Mechanism
Chi Wai Cheung and Kin Shing Chan*
Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, People’s Republic of China
S Supporting Information
b
ABSTRACT: Base-promoted aryl carbonꢀchlorine bond (ArꢀCl)
cleavage by iridium(III) porphyrin carbonyl chloride (IrIII(ttp)-
(CO)Cl; ttp = 5,10,15,20-tetrakis(p-tolyl)porphyrinato dianion)
was achieved in the presence of K2CO3 to give iridium(III) porphyrin
aryls (IrIII(ttp)Ar). Mechanistic studies revealed that K2CO3 promotes the reduction of Ir(ttp)(CO)Cl to give the iridium(II)
porphyrin dimer intermediate [IrII(ttp)]2. [Ir(ttp)]2 is the source of IrII(ttp) metalloradical, which cleaves ArꢀCl to give Ir(ttp)Ar
and a chlorine radical (Cl•) via radical ipso substitution in an additionꢀelimination pathway. Cl• reacts with [Ir(ttp)]2 to yield
Ir(ttp)Cl for subsequent base-promoted reduction and Ir(ttp) for radical chain propagation. Additionally, the base-promoted
ArꢀCl cleavage of chlorobenzene (PhCl) by Ir(ttp)(CO)Cl gives both Ir(ttp)Ph and 1,4-bis-iridium(III)-porphyrin benzene,
IrIII(ttp)(p-C6H4)IrIII(ttp). The reactive Cl• can simultaneously react with PhCl via homolytic aromatic substitution to give
1,4-dichlorobenzene, which further undergoes double ArꢀCl cleavage to form Ir(ttp)(p-C6H4)Ir(ttp).
’ INTRODUCTION
ZrIV(Ar)(Cl).8 The selection of transition metals and reaction
conditions are thus important to achieve selective ArꢀCl cleavage.
Our group has been interested in selective bond cleavage by
high-valent rhodium(III)9 and iridium(III) porphyrin complexes.10
Recently, we have reported the base-promoted selective ArꢀBr and
ArꢀI cleavage by iridium(III) porphyrin carbonyl chloride
(IrIII(ttp)(CO)Cl) to yield iridium(III) porphyrin aryls (IrIII-
(ttp)Ar).10d,e Mechanistic studies suggested that base (OHꢀ) can
promote the reduction of IrIII(ttp)(CO)Cl to iridium(II) porphyrin
dimer ([IrII(ttp)]2) via the intermediacy of iridum(III) porphyrin
hydroxo (IrIII(ttp)OH). [Ir(ttp)]2 is the source of IrII(ttp) metal-
loradical, which cleaves the ArꢀBr and ArꢀI bonds via radical ipso
substitution in an additionꢀelimination pathway (ipso substitution,
additionꢀelimination, abbreviated as ISAE) to give Ir(ttp)Ar as a
new, alternative reactivity mode of ArꢀX cleavage.10d,e
Aryl carbonꢀchlorine bond (ArꢀCl) cleavage by transition-
metal complexes is of significant importance in organic synthesis1
and chemicalwaste treatments for environmentalprotection.2 Aryl
chlorides (ArCl) can act as a cheaper aryl source in transition-
metal-catalyzed carbonꢀcarbon1a,b and carbonꢀheteroatom1c,d
bondcoupling reactions, thoughtheArꢀClbondisstronger and is
more difficult to cleave in comparison with ArꢀBr and ArꢀI
bonds. Transition-metal-mediated hydrodechlorination of chlori-
nated dioxins and polychlorinated biphenyls also provides an
alternative treatment method to substantially lower the toxicity
of halogenated chemical wastes.2
ArꢀCl cleavage is generally achieved by using electron-rich d8
and d10 transition metals.3,4 Group 9 metal(I) complexes have
also been adopted to achieve selective ArꢀCl cleavage. For examples,
RhIPNP (PNP = bis(2-(diisopropylphosphino)-4-methylphenyl)-
amino)3c and Vaska’s3e complexes undergo oxidative addition
with ArꢀCl. Alternatively, ArꢀCl cleavage can also occur via
homolytic pathways with metalloradicals. Recently, a group 9
Co(0) metalloradical, [2,6-bis(dimethylphenyliminoethyl)-
pyridine]cobalt(0), was found to cleave ArꢀCl via chlorine atom
abstraction to yield CoIꢀAr and CoIꢀCl.5
In order to investigate the effect of ArꢀX bond strength on the
chemoselectivity of ArꢀX and ArꢀH cleavage in ArX substrates
with Ir(ttp)(CO)Cl under basic conditions, we have further
extended the substrate scope from ArBr and ArI to ArCl. We
have discovered that base-promoted, selective ArꢀCl cleavage by
Ir(ttp)(CO)Cl can also be achieved via IrII(ttp)-mediated ipso
substitution of ArꢀCl to give Ir(ttp)Ar. We now report the
reaction scope and detailed mechanistic studies of base-promoted
ArꢀCl cleavage by Ir(ttp)(CO)Cl.
One of the challenges of selective ArꢀCl cleavage is the
competitive aryl CꢀH (ArꢀH) activation of ArCl.6ꢀ8 Kinetic
CꢀH activation and thermodynamic CꢀCl cleavage of chlor-
obenzene (PhCl) by neutral (PNP)IrI,6 as well as both kinetic
and thermodynamic ortho-ArꢀH activation of PhCl by cationic
[(PNP*)IrI]+ (PNP* = 2,6-bis(di-tert-butylphosphinomethyl)-
pyridine),7 have been reported. Additionally, ortho-ArꢀH activa-
tion of PhCl by the complex [Cp*2ZrIVCH3]+ initiallyoccursto give
[Cp*2ZrIV(C6H4(o-Cl))]+, which gradually undergoes intramo-
lecular ArꢀCl cleavage via β-Cl elimination with the proposed
formation of a benzyne intermediate to give the complex
’ RESULTS AND DISCUSSIONS
Optimization of PhꢀCl Cleavage. Initially, Ir(ttp)(CO)Cl
(1a) reacted with PhCl (1.1 equiv) in benzene at 200 °C in 7 days
to give only a trace of Ir(ttp)Ph (2a; 2%) and unreacted 1a was
Received: July 10, 2011
Published: August 25, 2011
r
2011 American Chemical Society
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Table 1. Optimization of PhꢀCl Cleavage by Ir(ttp)(CO)Cl
yield, %a
entry
amt of PhCl, equiv
amt of K2CO3, equiv
time
1a (recovered)
2a
3
Ir(ttp)CH3
1
2
3
4
5
1.1
1.1
0
20
20
20
20
7 d
78
0
2b
31
57
75
74
0
0
3
5
0
0
1 d
3
3
2b,d
5b,e
50
12 h
0
200
200
7 h
22 hc
0
0
a Isolated yield. b NMR yield. c Reaction was run at 150 °C. d A trace of 1,4-Cl2C6H4 (1%) was detected by GC-MS analysis. e A trace of 1,4-Cl2C6H4
(0.4%) was detected by GC-MS analysis.
Table 2. Substrate Scope of Base-Promoted ArꢀCl Cleavage
entry
FG
time, h
Ir(ttp)Ar (yield, %)a
entry
FG
time, h
Ir(ttp)Ar (yield, %)a
1
2
3
4
5
OMe
Me
H
36
36
22
36
84
2b (76)b
2c (32)c
2a (74)d
2d (67)
2e (100)
6
7
8
9
CO2Me
CHO
CF3
46
22
2f (70)b
2g (69)e,f
2h (68)g
2i (51)h
168
24
F
CN
Cl
a Isolated yield. b Ir(ttp)CH311 in 3% yield was also isolated. c Ir(ttp)Bn(p-Cl) (2j) in 18% isolated yield and Ir(ttp)CH311 in 2% NMR yield were also
formed. d Ir(ttp)(p-C6H4)Ir(ttp) in 5% NMR yield was also formed, and a trace of 1,4-Cl2C6H4 (0.4%) was detected by GC-MS analysis.
e Ir(ttp)(PPh3)C6H4(p-CHO) was isolated by adding PPh3. f Aldehydic CꢀH activation also occurred to give Ir(ttp)(PPh3)C(O)C6H4(p-Cl) (2k)
in <1% yield. g A trace of PhCl (0.4%) was detected by GC-MS analysis. h Ir(ttp)(PPh3)C6H4(p-CN) was isolated by adding PPh3.
recovered in 78% yield (Table 1, eq 1, entry 1). When K2CO3 (20
equiv) was added, the reaction of 1a with PhCl (1.1 equiv) was
complete at 200 °C in 1 day to give 2a in 31% yield and a trace of
The chemoselectivity of ArꢀCl cleavage in ArCl becomes
lower, compared with that of the ArBr and ArI substrates.10d,e
While selective ArꢀBr cleavage of 4-bromotoluene10d and
4-bromobenzaldehyde10e with 1a and K2CO3 occurred to yield
only Ir(ttp)(p-Tol) (2c) and Ir(ttp)(PPh3)C6H4(p-CHO) (2g)
(upon addition of PPh3), competitive benzylic CꢀH bond
activation of 4-chlorotoluene occurred to give Ir(ttp)Bn(p-Cl)
(2j) in 18% yield (Table 2, entry 2),14 and competitive aldehydic
CꢀH bond activation of 4-chlorobenzaldehyde took place to
give a trace of Ir(ttp)(PPh3)C(O)C6H4(p-Cl) (2k) (Table 2,
entry 7).14
11
Ir(ttp)CH3 (3%) and the unexpected Ir(ttp)(p-C6H4)Ir(ttp)
(3; 3%) (Table 1, entry 2). In order to increase the product yields,
the PhCl loading was increased. A 50 equiv amount of PhCl
increased the yield of 2a to 57% (Table 1, entry 3). Increasing the
PhCl loading to 200 equiv further promoted the yield of 2a to 75%
after 7 h (Table 1, entry 4). The PhCl loading was thus optimized
to be 200 equiv. The reaction conditions of base-promoted PhꢀCl
cleavage by 1a were further optimized at a lower temperature of
150 °C and a longer time of 22 h (Table 1, entry 5). PhꢀCl
cleavage rather than ArꢀH activation of PhCl occurred to give 2a,
since 1a reacted with PhCl (50equiv) and K2CO3 inbenzene-d6 at
200 °C to give 2a only without any Ir(ttp)C6D5.
Substrate Scope of ArCl. Under the optimal reaction condi-
tions, 1a generally reacted with a variety of ArCl species to give
Ir(ttp)Ar in moderate to high yields via selective ArꢀCl cleavage
with high functional compatibility (Table 2, eq 2). Notably, in the
reactions with 4-chlorobenzaldehyde and 4-chlorobenzonitrile,
brown precipitates were initially formed without the successful
isolation of Ir(ttp)Ar. Presumably, coordination oligomers of
Ir(ttp)C6H4(p-CN)10e,12 and Ir(ttp)C6H4(p-CHO)10e,13 at the
trans vacant site of the neighboring Ir(ttp)Ar form. Addition of
PPh3 resulted in the isolation of monomeric Ir(ttp)(PPh3)Ar
(Table 2, entries 7 and 9).10e
IrII for PhꢀCl Cleavage. To investigate the reaction mechan-
ism of base-promoted ArꢀCl cleavage by 1a, the reaction of 1a
with Cs2CO3 (20 equiv)15 and PhCl (50 equiv)16 in benzene-
d6 at 150 °C was monitored in a sealed NMR tube. After 3.5 h,
Ir(ttp)H (4a) and [Ir(ttp)]2 (5) in 26% and 7% yields,
respectively, were observed (eq 3a). After 18 days, 1a, 4a,
and 5 were consumed to give Ir(ttp)Ph (2a), Ir(ttp)(p-
C6H4)Ir(ttp) (3), and Ir(ttp)CH311 in 56%, 6%, and 6% yields,
respectively (eq 3b). Ir(ttp)H and [Ir(ttp)]2 were the observed
intermediates.
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ARTICLE
Table 3. Relative Reactivities of Possible Ir(ttp) Species in
PhꢀCl Cleavage
Scheme 3. Conversion of (H2O)(ttp)IrOIr(ttp)(OH2) to
Ir(ttp)H via Ir(ttp)OH
yield, %a
entry
Ir(ttp)X0
time
Ir(ttp)Ph
Ir(ttp)Cl
1
2
3
Ir(ttp)ꢀK+ (6)b
Ir(ttp)H (4a)
4 d
4 d
5 h
50
67
57d
4c
4c
Scheme 4. Base-Promoted Reduction of Ir(ttp)(PPh3)Cl
1/2[Ir(ttp)]2 (5)
a Isolated yield. b In the form of Ir(ttp)ꢀ[K(18-crown-6)]+. c Ir(ttp)-
(CO)Cl (1a) was isolated. d Ir(ttp)(p-C6H4)Ir(ttp) (3) was also formed
in 3% NMR yield.
Scheme 1. General Mechanism of Base-Promoted ArꢀCl
Scheme 2 depicts the mechanism of base-promoted reduction
of 1a to [Ir(ttp)]2 for ArꢀCl cleavage.
Cleavage
OHꢀ, which is generated from the thermal hydrolysis of K2CO318
with the residual water19 in benzene solvent (eq i in Scheme 2), reacts
with 1a via ligand substitution to give Ir(ttp)OH (7a) and gaseous
CO (eq ii).10d,e Ir(ttp)OH then converts to [Ir(ttp)]2 and H2O2
via the reduction of the Ir(III) center by the hydroxo ligand
(eq iii).10d,e In the absence of or at a low concentration of ArCl,
[Ir(ttp)]2 simultaneously undergoes rapid disproportionation
with OHꢀ to give Ir(ttp)OH and Ir(ttp)ꢀ (eq iv).20 Ir(ttp)OH is
recycled for subsequent redox reaction, whereas Ir(ttp)ꢀ is
protonated by residual water to give Ir(ttp)H (eq v).21 Ir(ttp)H
can undergo thermal and base-promoted dehydrogenative di-
merization to regenerate [Ir(ttp)]2 via an equilibrium (eq vi).10d
To support the above proposal (Scheme 2), a sample of Ir-
(ttp)OH (7a) is needed to demonstrate the conversion of Ir-
(ttp)OH intermediate to Ir(ttp)H. However, Ir(ttp)OH could not
be prepared by reacting the electrophilic “Ir(ttp)SbF6” (1b) (as a
inseparable mixture of Ir(ttp)SbF6 and Ir(ttp)(CO)SbF6 prepared
in irreproducible ratios)10e with CsOH in benzene-d6 at room
temperature, due to the instantaneous formation of Ir(ttp)H in 95%
yield.10e Previously, we have used (H2O)(ttp)IrOIr(ttp)(OH2) (8)
as a precursor of Ir(ttp)OH via KOH-promoted hydrolysis
(Scheme 3, path ii).10e 8 reacted with KOH (20 equiv) and residual
water in benzene-d6 at 200 °C in 10 days to give Ir(ttp)H in 58%
yield (Scheme 3, path i), supporting the intermediacy of Ir(ttp)OH
for Ir(ttp)H formation (Scheme 3, paths ii and iii).10e
To independently validate the proposed reaction steps in
Scheme 2, we further adopted the coordinatively saturated and
less reactive Ir(ttp)(PPh3)Cl (1c) to study the base-promoted
reduction processes. 1c was prepared in 65% yield by reacting 1a
with excess PPh3 (5 equiv) in benzene at 200 °C for 1 day.
(a). Evidence for Scheme 2, eq ii. 1c reacted with KOH in
benzene-d6 at 200 °C in 4 h to give the more stable, coordina-
tively saturated Ir(ttp)(PPh3)OH (7b) in 96% yield accompa-
nied by a trace amount of Ir(ttp)(PPh3)H (4b) (2%) (Scheme 4,
path i). Ligand substitution of 1a with OHꢀ to form Ir(ttp)OH is
thus supported.
Scheme 2. Reduction of Ir(ttp)(CO)Cl to [Ir(ttp)]2 and
Ir(ttp)H
In principle, Ir(ttp)ꢀ (6) could also be generated via the
deprotonation of Ir(ttp)H with K2CO3.10d To identify the iridium
porphyrin intermediate for ArꢀCl cleavage, the relative reactivities
of Ir(ttp)ꢀ, Ir(ttp)H, and [Ir(ttp)]2 were compared by reacting
with PhCl (50 equiv) in benzene-d6 at 200 °C (Table 3, eq 4).
Ir(ttp)ꢀK+ and Ir(ttp)H reacted slowly in 4 days to give Ir(ttp)Ph
(Table 3, entries 1 and 2). [Ir(ttp)]2 reacted the fastest (5 h) to give
Ir(ttp)Ph in 57% yield with a trace of Ir(ttp)(CO)Cl (4%)17 and
Ir(ttp)(p-C6H4)Ir(ttp) (3%) (Table 3, entry 3). [Ir(ttp)]2 is thus
the most probable intermediate for PhꢀCl cleavage.
Mechanism (Base-Promoted). The mechanism of base-
promoted ArꢀCl cleavage by 1a (Scheme 1) is generally
similar to the previously reported mechanism of ArꢀBr and
ArꢀI cleavage,10d,e involving (i) the base-promoted reduction
of 1a to [Ir(ttp)]2, (ii) ArꢀCl cleavage by [Ir(ttp)]2 to give
Ir(ttp)Ar and Ir(ttp)Cl, and (iii) recycling of Ir(ttp)Cl
via base-promoted reduction. The detailed mechanisms are
described below.
(b). Evidence for Scheme 2, eq iii. In the reaction of 1c with
KOH, 7b, once formed, was further converted to 4b in 33%
yield22 in 13 h (Scheme 4, path ii). The hydroxo ligand acts as
a reducing agent to reduce the Ir(III) center of 7b to give
IrII(ttp)(PPh3) or [IrII(ttp)PPh3]2 for subsequent rapid reaction
to yield 4b (Scheme 2, eqs iiiꢀv). The conversion of 7b to
Ir(ttp)(PPh3)/[Ir(ttp)(PPh3)]2 and H2O2 can be supported by
Base-Promoted Reduction of Ir(ttp)(CO)Cl to [Ir(ttp)]2 and
Ir(ttp)H. Previously, we ruled out the possibility of reduction of
1a to Ir(ttp)H and then [Ir(ttp)]2, via the nucelophilic attack of
OHꢀ to the CO ligand in 1a, followed by the decarboxylation of
Ir(ttp)CO2H.10d,e Instead, base most likely acts as a reducing
agent to reduce the Ir(III) center of 1a to [IrII(ttp)]2.10d,e
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Scheme 5. Reaction of ArCl with [Ir(ttp)]2 in Benzene
(Ar = Ph), Cl• also reacts with excess PhCl to yield dichlor-
obenzene via HAS (Scheme 5, eq v).28,30 1,4-Dichlorobenzene
can further react with [Ir(ttp)]2 to give Ir(ttp)(p-C6H4)Ir(ttp)
(3) via double ArꢀCl cleavage (Scheme 5, eq vi).31
The following two lines of evidence10d,32 can support the ISAE
mechanism of ArCl by [Ir(ttp)]2:
(a). Detection of Organic Coproducts Originating from
Cl•.32 In the base-promoted (p-CF3)C6H4ꢀCl cleavage by 1a, a
trace of PhCl was detected by GC-MS analysis (Table 2, entry 8).
This supports the HAS of (p-CF3)C6H4ꢀCl with [Ir(ttp)]2
intermediate to give Cl•, which further reacts with benzene to
give PhCl (Scheme 5, eqs ii and iv).29,33
Additionally, in the base-promoted PhꢀCl cleavage by 1a, a
trace of 1,4-dichlorobenzene (1,4-Cl2C6H4) was detected by
GC-MS analysis (Table 1, entries 4 and 5). Most likely, the HAS
of PhCl with [Ir(ttp)]2 gives Cl•, which further reacts with excess
PhCl to give 1,4-Cl2C6H4 (Scheme 5, eqs ii and v).30,34 Indeed,
[Ir(ttp)]2 reacted with neat PhCl at 200 °C to give Ir(ttp)Ph,
Ir(ttp)(CO)Cl,17 Ir(ttp)(p-C6H4)Ir(ttp), and 1,4-Cl2C6H4 in
39%, 1%, 4%, and 2% yields, respectively (eq 8). Most likely, the
coproduct 1,4-Cl2C6H4 further reacts with [Ir(ttp)]2 to give a
trace of 3 (Scheme 5, eq vi). This can be supported by the
reaction of 1a with 1,4-Cl2C6H4 (1 equiv) and K2CO3 in
benzene at 200 °C in 7 h to give a trace of 3 (eq 9).35 The
formation of 3 via HAS of Ir(ttp)-Ph with [Ir(ttp)]2 is excluded,
as 5 did not react with 2a to give 3 (eq 10).
the detection of H2O2 (Scheme 2, eq iii). The direct detection of
H2O2, however, is difficult, as it can rapidly disproportionate into
H2O and O2 in the presence of a base catalyst.23 Instead, H2O2
can be indirectly detected by reacting with excess PPh3 as a H2O2
trap to produce P(O)Ph3.24,25 Indeed, 1a reacted with KOH and
residual water in the presence of excess PPh3 (5 equiv) in
benzene-d6 at 200 °C in 3 h to give Ir(ttp)(PPh3)H (4b; 42%)
and a moderate yield of P(O)Ph3 (21%) (eq 5). Without KOH,
1a only reacted with excess PPh3 under identical conditions in 12
h to yield 1c only without any 4b (eq 6). The results reveal that
OHꢀ is oxidized to produce H2O2, whereas Ir(III) is reduced to
Ir(II) which finally gives Ir(ttp)H.
(c). Evidence for Scheme 2, eqs iv and v. In the presence of
PPh3 as a stabilizing ligand, [Ir(ttp)]2 reacted with KOH and
residual water in benzene-d6 at 200 °C in 4 h to yield 7b (16%)
and 4b (15%) in around a 1:1 ratio.20 Without KOH, [Ir(ttp)]2/
PPh3 only reacted with residual water in 4 h to give only a trace of
7b and 4b (eq 7b), supporting the promoting effect of OHꢀ in
the disproportionation of [Ir(ttp)]2. We are not very clear about
the reason of low yields of 7b and 4b. Probably, the coordina-
tively saturated [IrII(ttp)(PPh3)]2 decomposes much faster
either with heat at 200 °C or in the presence of H2O2.26
(b). Rate Enhancement by Para Substituents (p-FGs) in
ArCl.32. In the competition reaction of an equimolar ratio of para-
substituted ArCl (p-FG-C6H4Cl) and PhCl with 1a and K2CO3
(eq 11), both the electron-donating and -withdrawing p-FGs
promoted the rate of ArꢀCl cleavage, as shown in the V-shaped
Hammett plot (Figure 1).36 The results suggest that the Ir(ttp)ꢀ
cyclohexadienyl radical intermediate is formed and stabilized by
resonance with the p-FG group (Figure 2).37
ArꢀCl Cleavage by [IrII(ttp)]2 in Benzene. The mechanism of
[Ir(ttp)]2-mediated ArꢀCl cleavage is similar to that of ArꢀBr and
ArꢀI cleavage.10d,e We propose that [Ir(ttp)]2 initially dissociates
into IrII(ttp) (Scheme 5, eq i).27 Ir(ttp) then reacts with ArCl via
radical ipso substitution in an additionꢀelimination pathway
(ISAE)10d,e to give Ir(ttp)Ar and a very reactive chlorine radical
(Cl•) via the intermediacy of an Ir(ttp)ꢀcyclohexadienyl radical
(I) (Scheme 5, eq ii). Cl• reacts with [Ir(ttp)]2 to give Ir(ttp)Cl
and another Ir(ttp) (Scheme 5, eq iii). Ir(ttp)Cl is recycled via a
base-promoted reduction (Scheme 2), whereas Ir(ttp) further
reacts via radical chain propagation (Scheme 5, eq ii).
Other Mechanistic Possibilities of ArꢀCl Cleavage. Two
other possible mechanisms of ArꢀCl cleavage to give Ir(ttp)Ar
are (A) the eliminationꢀaddition reaction of ArCl with Ir(ttp)ꢀ
(Scheme 6, mechanism A) and (B) chlorine atom abstraction by
[Ir(ttp)]2 (Scheme 6, mechanism B).
Cl• can leak out from the chain reaction and reacts with
benzene solvent to give PhCl via homolytic aromatic substitution
(HAS) (Scheme 5, eq iv).28,29 When PhCl is used as a substrate
Mechanism A. The intermediate Ir(ttp)ꢀ can in principle
abstract a proton from the para-substituted ArCl, and a chloride
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Figure 1. Hammett plot of ArꢀCl cleavage using the Hammett constants σop.
the residual water19 in benzene hydrolyzes 1a slowly to give
Ir(ttp)OH, HCl, and CO (Scheme 7, eq i). Ir(ttp)OH then
rapidly converts to [Ir(ttp)]2 and H2O2 (Scheme 7, eq ii).
[Ir(ttp)]2 further cleaves the PhꢀCl bond via an ISAE mechan-
ism to give 2a and Ir(ttp)Cl (Scheme 7, eq iii). Ir(ttp)Cl finally
reacts with CO to give back 1a (Scheme 7, eq iv).
Figure 2. Stabilization of the Ir(ttp)ꢀcyclohexadienyl intermediate by
para substituents.
’ CONCLUSION
Scheme 6. Other Possible Mechanisms of ArꢀCl Cleavage
In summary, we have discovered base-promoted selective
ArꢀCl cleavage by high-valent Ir(ttp)(CO)Cl in benzene sol-
vent. Mechanistic studies reveal that OHꢀ promotes the reduc-
tion of Ir(ttp)(CO)Cl to [Ir(ttp)]2, which then cleaves the
ArꢀCl bond via a radical ipso additionꢀelimination mechanism
to give Ir(ttp)Ar and a chlorine radical. The chlorine radical
further reacts with [Ir(ttp)]2 to give Ir(ttp)Cl and simultaneously
reacts with excess benzene and ArCl via homolytic aromatic
substitution to give the chlorinated aromatic compounds.
Scheme 7. Mechanism of PhꢀCl Cleavage without Base
’ EXPERIMENTAL SECTION
General Procedures. Unless otherwise noted, all reagents were
purchased from commercial suppliers and directly used without further
purification. Hexane was distilled from anhydrous calcium chloride.
Benzene and benzene-d6 were distilled from sodium and were stored in a
Teflon screw capped tube under nitrogen prior to use. All reactions were
carried out without light irradiation by wrapping with aluminum foil.
The reactions in Teflon screw capped Schlenk tubes were heated in heat
blocks on heaters, whereas the reactions in sealed NMR tubes were
heated in GC ovens. The reaction mixtures should be handled with care
and with safety precautions (e.g., protective shield), as explosion may
occur at high temperatures (150ꢀ200 °C). Thin-layer chromatography
was performed on precoated silica gel 60 F254 plates. Ir(ttp)(CO)Cl
(1a),17 “Ir(ttp)SbF6” (1b),10e Ir(ttp)(p-TolCN)Cl,10e Ir(ttp)CH3,17
Ir(ttp)H (4a),10a Ir(ttp)(PPh3)H (4b),10c [Ir(ttp)]2 (5),10a Ir(ttp)ꢀ-
[K(18-crown-6)]+ (6),10c Ir(ttp)OH (7a),10e and (H2O)(ttp)IrOIr(ttp)-
(H2O) (8)10e had been characterized and were prepared according to the
literature procedures. Ir(ttp)Ar (2aꢀi) and Ir(ttp)(p-C6H4)Ir(ttp) (3)
have been characterized.10d,e Silica gel (Merck, 70ꢀ230 mesh), and neutral
alumina (Merck, activity I, 70ꢀ230 mesh) added with H2O (∼10/1 v/v),
were used for column chromatography.
ion is then eliminated to give an aryne (Scheme 6, path i).
Subsequent reaction of the aryne with Ir(ttp)ꢀ and residual water
yields both iridium(III) porphyrin para- and meta-substituted aryls
(Scheme 6, path ii). Mechanism A is ruled out due to the absence
of Ir(ttp)C6H4(m-FG) in the base-promoted ArꢀCl cleavage of
para-substituted ArCl by 1a (Table 2).
Mechanism B. Ir(ttp), dissociated from the intermediate
[Ir(ttp)]2, can directly abstract a chlorine atom (Cl•) from ArCl
to give Ir(ttp)Cl and Ar• (Scheme 6, path iii). Ar• further reacts
with [Ir(ttp)]2 to give Ir(ttp)Ar (Scheme 6, path iv) or leaks from
the chain reactions and dimerizes to yield biaryls (Scheme 6, path v).
Mechanism B is also ruled out, since the ratio of Ir(ttp)Cl to
Ir(ttp)Ar is much less than 1 (Table 3, entry 3)10d and no biphenyl
was detected in the reaction of [Ir(ttp)]2 with PhCl (eq 8).
Mechanism (without Base). Without base, 1a reacted slowly
and incompletely with PhCl (1.1 equiv) in benzene at 200 °C in
7 days to give a trace of 2a (2%) (Table 1, entry 1). It is likely that
Unless otherwise noted, all the reaction mixtures were degassed for
three freezeꢀthawꢀpump cycles and were then back-filled with N2 in
the Teflon screw capped Schlenk tubes, or flame-sealed under vacuum in
the Teflon screw capped NMR tubes. In sealed NMR tube experiments, the
residual benzene proton signal was used as an internal standard to
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1
estimate the NMR yields of iridium porphyrin species by H NMR
spectroscopy.
50 °C under N2 for 2 h to give a deep brown solution. The mixture was
then cooled to room temperature, and p-chlorobenzyl bromide (92 mg,
0.45 mmol) was added under N2. The reaction mixture was further
heated at 50 °C for 2 h. The reaction mixture was dried and purified by
column chromatography with alumina using CH2Cl2/hexane (1/2) as
eluent. The major deep brown fraction was collected and dried. The
product was recrystallized in CH2Cl2/hexane to obtain a deep brown
solid of Ir(ttp)Bn(p-Cl) (2j; 61 mg, 0.062 mmol, 68%). Rf = 0.67
(CH2Cl2/hexane 1/1). 1H NMR (CDCl3, 300 MHz): δ ꢀ4.02 (s, 2 H),
2.69 (s, 12 H), 3.04 (d, 2 H, J = 8.4 Hz), 5.84 (d, 2 H, J = 8.4 Hz), 7.53 (d,
4 H, J = 7.8 Hz), 7.55 (d, 4 H, J = 8.1 Hz), 7.94 (d, 4 H, J = 7.5 Hz), 8.02
(d, 4 H, J = 7.8 Hz), 8.49 (d, 8 H). 13C NMR (CDCl3, 75 MHz): δ
ꢀ15.7, 21.7, 124.1, 125.3, 126.2, 127.7, 131.4, 133.7, 133.9, 137.3, 138.8,
140.3, 143.2. HRMS (FABMS): calcd for [C55H42N4ClIr]+ ([M]+) m/z
986.2722, found m/z 986.2730.
A trace of residual water was always present in benzene-d6 (δ(H2O)
∼0.4 ppm).19 The amount of residual water (∼0.2ꢀ2 equiv with
reference to the iridium porphyrin species) in benzene-d6 in sealed
1
NMR tube experiments was estimated by H NMR spectroscopy by
taking the ratio between the proton signal of residual water and that of
porphyrin’s pyrrole of iridium porphyrin species. The yields of Ir(ttp)-
(p-C6H4)Ir(ttp) (3) and [Ir(ttp)]2 (5) were based on the number of
moles of iridium porphyrin ring (Ir(ttp)) incorporated into the complexes.
Experimental Instrumentation. 1H, 13C, and 31P NMR spectra
were recorded on a Br€uker DPX-300 instrument at 300, 75, and 122 MHz,
respectively, or a Br€uker AV-400 instrument at 400, 100, and 162 MHz,
respectively. Chemical shifts were referenced with the residual solvent
protons in C6D6 (δ 7.15 ppm), CDCl3 (δ 7.26 ppm), or tetramethylsi-
lane (TMS) (δ 0.00 ppm) in 1H NMR spectra as the internal standards
and in CDCl3 (δ77.16ppm), C6D6 (δ128.06 ppm), or THF-d8 (δ(β-CH2)
25.62 ppm) in 13C NMR spectra as the internal standards. H3PO4 (85%
solution) (δ 0.00 ppm) in C6D6 was referenced as the external standard
in 31P NMR spectra. Chemical shifts (δ) are reported as parts per million
(ppm) in the δ scale downfield from TMS. Coupling constants (J) are
reported in hertz (Hz).
Preparation of (p-Chlorobenzoyl)(triphenylphosphine)(5,10,15,20-
tetrakis(p-tolyl)porphyrinato)iridium(III), Ir(ttp)(PPh3)C(O)C6H4(p-Cl)
(2k). Ir(ttp)CH3 (62 mg, 0.072 mmol), 4-chlorobenzaldehyde (506 mg,
3.6 mmol, 50 equiv), and benzene (2 mL) were heated in a Teflon screw
capped tube at 200 °C under N2 for 14 days. The reaction mixture was
then dried and further purified by column chromatography over silica gel
using CH2Cl2/hexane (2/1) as eluent to obtain an orange solid of
Ir(ttp)C(O)C6H4(p-Cl)10f (42 mg, 0.042 mmol, 58%). Ir(ttp)C(O)-
C6H4(p-Cl) was then reacted with PPh3 (110 mg, 0.42 mmol, 10 equiv)
in CHCl3 (4 mL) in a Teflon screw capped tube in air at 50 °C for 30
min. The deep reddish brown reaction mixture was dried and purified by
column chromatography over alumina using CH2Cl2/hexane (1/3) as
eluent to obtain a reddish brown solid of Ir(ttp)(PPh3)C(O)C6H4(p-Cl)
(2k; 45 mg, 0.036 mmol, 50% with reference to Ir(ttp)CH3). Rf = 0.67
(CH2Cl2/hexane 1/1). 1H NMR (CDCl3, 300 MHz): δ 2.22 (d, 2 H, J =
8.4 Hz), 2.23 (s, 12 H), 4.05 (br, 6 H), 5.85 (d, 2 H, J = 8.0 Hz), 6.54 (t, 6
H, J = 6.8 Hz), 6.85 (t, 3 H, J = 7.2 Hz), 7.48 (d, 8 H, J = 8.0 Hz), 7.69 (d,
4 H, J = 8.0 Hz), 7.80 (d, 4 H, J = 7.6 Hz), 8.48 (s, 8 H). 13C NMR
(CDCl3, 75 MHz): δ 21.6, 118.5, 118.6, 122.8, 125.3, 127.0 (d, J = 7.6
Hz), 127.1, 127.6, 127.9, 128.1, 128.3, 130.0 (d, 1JPC = 30.5 Hz), 130.8
High-resolution mass spectra (HRMS) were performed on a Ther-
moFinnigan MAT 95 XL mass spectrometer in fast atom bombardment
(FAB) mode using 3-nitrobenzyl alcohol (NBA) matrix and CH2Cl2 as
solvent or electrospray ionization (ESI) mode using MeOH/CH2Cl2
(1/1) as solvent.
Gas chromatographyꢀmass spectrometric (GC-MS) analysis was
conducted on a Shimadzu GCMS-2010Plus instrument using a Rtx-5MS
column (30 m ꢁ 0.25 mm). The column oven temperature and injection
temperature were 50.0 and 250 °C, repsectively. Helium was used as a
carrier gas. Flow control mode was chosen as linear velocity (36.3 cm
s
ꢀ1) with a pressure of 53.5 kPa. The total flow, column flow, and purge
flow were 24.0, 1.0, and 3.0 mL minꢀ1, respectively. Split mode inject
with split ratio 20.0 was applied. After injection, the column oven
temperature was kept at 50 °C for 5 min and the temperature was then
elevated at a rate of 20 °C minꢀ1 for 10 min up to 250 °C. The temperature
of 250 °C was kept for a further 5 min.
2
(d, JPC = 11.5 Hz), 131.7, 133.8, 134.5, 137.0, 139.1, 142.4. HRMS
(FABMS): calcd for [C73H56N4ClOPIr]+ ([M + H]+): m/z 1263.3503,
found m/z 1263.3500.
Experimental Procedures. Preparation of Chloro(triphenylpho-
sphine)(5,10,15,20-tetrakis(p-tolyl)porphyrinato)iridium(III), [Ir(ttp)-
(PPh3)Cl] (1c). Ir(ttp)(CO)Cl (1a; 100 mg, 0.11 mmol), PPh3 (142 mg,
0.54 mmol, 5 equiv), and benzene (4 mL) were heated in a Teflon screw
capped tube at 200 °C under N2 for 1 day. The solvent was dried, and the
reaction mixture was purified by column chromatography with alumina
using CHCl3 as the eluent. The major red solution was collected and then
dried under vacuum. The product was further recrystallized using CH2Cl2/
hexane to obtain a red solid of Ir(ttp)(PPh3)Cl (1c; 81.5 mg, 0.070 mmol,
65%). Rf = 0.15 (CHCl3). 1H NMR (C6D6, 300 MHz): δ 2.39 (s, 12 H),
4.27 (dd, 6 H, 3JPH = 9.0 Hz, 3JHH = 8.6 Hz), 6.33 (t, 6 H, 3JHH = 7.4 Hz),
6.55 (t, 3 H, 3JHH = 7.2 Hz), 7.14 (d, 4 H, 3JHH = 7.9 Hz), 7.35 (d, 4 H,
3JHH = 7.5 Hz), 7.78 (d, 4 H, 3JHH = 7.4 Hz), 7.93 (d, 4 H, 3JHH = 7.3 Hz),
8.84 (s, 8 H). 13C NMR (THF-d8, 300 MHz): δ 21.9, 123.5, 126.0 (d,
1JPC = 53.2 Hz), 127.9 (d, 2JPC = 15.7 Hz), 128.1, 128.6, 130.2, 132.1 (d,
3JPC = 9.0 Hz), 132.4, 134.8, 135.6, 138.1, 140.5, 143.4. 31P NMR (C6D6,
162 MHz): δ ꢀ37.5 (br) (Figure S2d in the Supporting Information).
HRMS (FABMS): calcd for [C66H51N4PClIr]+ ([M]+) m/z 1158.3164,
found m/z 1158.3199.
Preparation of Phenyl(triphenylphosphine)(5,10,15,20-tetrakis-
(p-tolyl)porphyrinato)iridium(III), Ir(ttp)(PPh3)Ph (2l). Ir(ttp)Ph (21.6
mg, 0.023 mmol), triphenylphosphine (60 mg, 0.23 mmol, 10 equiv), and
benzene (2 mL) were heated in a Teflon screw capped tube at 120 °Cunder
N2 for 30 min. The reaction mixture was then dried, purified by column
chromatography over alumina using CH2Cl2/hexane (1/1) as an eluent,
and further recrystallized with CH2Cl2/CH3OH to obtain a deep reddish
brown solid of Ir(ttp)(PPh3)Ph (2l; 21.9 mg, 0.018 mmol, 79%). Rf = 0.51
1
(CH2Cl2/hexane 1/1). H NMR (CDCl3, 400 MHz): δ 0.37 (dd, 2 H,
3JHH = 7.0 Hz, 4JPH = 6.4 Hz), 2.65 (s, 12 H), 4.12 (dd, 6 H, 3JPH = 8.1 Hz,
3JHH = 8.1 Hz), 4.71 (t, 2 H, 3JHH = 7.1 Hz), 5.13 (t, 1 H, 3JHH = 7.0
Hz), 6.54 (t, 6 H, 3JHH = 7.0 Hz), 6.85 (t, 3 H, 3JHH = 7.3 Hz), 7.41
(d, 4 H, 3JHH = 7.7 Hz), 7.45 (d, 4 H, 3JHH = 7.7 Hz), 7.63 (d, 4 H, 3JHH
=
7.4 Hz), 7.79 (d, 4 H, 3JHH = 7.5 Hz), 8.42 (s, 8 H). 13C NMR (CDCl3, 100
MHz): δ 21.6, 119.2, 122.8, 122.9, 126.9 (d, 3JPC = 8 Hz), 127.0, 127.4,
128.0, 128.9 (d, 1JPC = 25 Hz), 130.9 (d, 2JPC = 11 Hz), 131.7, 133.8, 134.6,
136.9, 139.2, 142.4. HRMS (FABMS): calcd for [C72H56N4PIr]+ m/z
1200.3866, found m/z 1200.3861.
Preparation of Triphenylphosphine Oxide, P(O)Ph3 (9). The typical
synthetic method for the preparation of P(O)Ph3 is the oxidation of
PPh3 by H2O2.25a PPh3 (64 mg, 0.24 mmol), H2O2 (30% solution, 2.5 mL,
24.4 mmol of H2O2 (100 equiv)), and benzene (2 mL) were heated in a
Teflon screw capped tube under N2 at 200 °C for 12 h. The reaction mixture
was then purifiedbycolumnchromatographywithsilica gel using CHCl3 as
eluent. The product was crystallized by CH2Cl2/hexane to yield P-
Preparation of p-Chlorobenzyl(5,10,15,20-Tetrakis(p-tolyl)porphyrinato)-
iridium(III), Ir(ttp)Bn(p-Cl) (2j). A suspension of Ir(ttp)(CO)Cl (84 mg,
0.091 mmol) in THF (20 mL) in a Teflon screw capped 250 mL round-
bottomed flask and a solution of NaBH4 (68.7 mg, 1.82 mmol) in
aqueous NaOH (1.0 M, 1.5 mL) were purged with N2 for 15 min
separately. The solution of NaBH4 was added slowly into the suspension
of Ir(ttp)(CO)Cl via a cannula under N2. The mixture was heated at
1
(O)Ph3 (9; 50.7 mg, 0.18 mmol, 76%). H NMR (C6D6, 400 MHz):
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ARTICLE
δ 6.96ꢀ7.03 (m, 9 H), 7.74 (ddd, 6 H, 3JPH = 11.5 Hz, 3JHH = 8.1 Hz,
in a single fraction. The ratio 2b:Ir(ttp)CH3 was estimated by using the
porphyrin’s pyrrole proton ratio 2b:Ir(ttp)CH3 in the isolated product
mixture by 1H NMR spectroscopy.
4
4JHH = 1.5 Hz). The observation of JHH is dependent on the con-
centration of P(O)Ph3. 31P NMR (C6D6, 162 MHz): δ 25.2 (hept,
+
3JPH = 11.3 Hz). HRMS (ESIMS): calcd for [C18H15OPNa] ([M +
(b). Reaction with 4-Chlorotoluene. Ir(ttp)(CO)Cl (9.5 mg, 0.010
mmol), K2CO3 (28 mg, 0.21 mmol, 20 equiv), 4-chlorotoluene (260 mg,
243 μL, 2.1 mmol, 200 equiv), and benzene (1 mL) were heated at 150 °C
under N2 for 36 h to give Ir(ttp)(p-Tol) (2c;10d 3.1 mg, 0.003 mmol,
32%), Ir(ttp)Bn(p-Cl) (2j; 1.8 mg, 0.002 mmol, 18%), and Ir(ttp)CH3
(0.3 mg, 0.0003 mmol, 3%) in a single fraction by column chromatography.
(c). Reaction with 1-Chloro-4-fluorobenzene. Ir(ttp)(CO)Cl (10.4
mg, 0.011 mmol), K2CO3 (31 mg, 0.23 mmol, 20 equiv), 1-chloro-4-
fluorobenzene (294 mg, 240 μL, 2.3 mmol), and benzene (1 mL) were
heated at 150 °C under N2 for 36 h to give Ir(ttp)C6H4(p-F) (2d;10d
7.2 mg, 0.008 mmol, 67%).
(d). Reaction with 1,4-Dichlorobenzene. Ir(ttp)(CO)Cl (10.7 mg,
0.012 mmol), K2CO3 (32 mg, 0.23 mmol, 20 equiv), 1,4-dichloroben-
zene (340 mg, 2.3 mmol), and benzene (1 mL) were heated at 150 °C
under N2 for 3.5 days to give Ir(ttp)C6H4(p-Cl) (2e;10d 11.2 mg, 0.012
mmol, 100%).
(e). Reaction with Methyl 4-Chlorobenzoate. Ir(ttp)(CO)Cl (10.2
mg, 0.011 mmol), K2CO3 (30.5 mg, 0.22 mmol, 20 equiv), methyl
4-chlorobenzoate (376 mg, 2.2 mmol), and benzene (1 mL) were heated
at 150 °C under N2 for 46 h to give Ir(ttp)C6H4(p-C(O)Me) (2f;10d 7.7
mg, 0.008 mmol, 70%) and Ir(ttp)CH3 (0.3 mg, 0.0003 mmol, 3%).
(f). Reaction with 4-Chlorobenzaldehyde. Ir(ttp)(CO)Cl (10.8 mg,
0.012 mmol), K2CO3 (32 mg, 0.023 mmol, 20 equiv), 4-chlorobenzal-
dehyde (328 mg, 2.3 mmol), and benzene (1 mL) were heated at 150 °C
under N2 for 22 h. PPh3 (30.6 mg, 0.12 mmol, 10 equiv) was then added
under N2, and the reaction mixture was further heated at 120 °C for 1 h
to give Ir(ttp)(PPh3)C6H4(p-CHO) (2g;10e 9.9 mg, 0.008 mmol, 69%)
and a trace of Ir(ttp)(PPh3)C(O)C6H4(p-Cl) (2k; <1%).
Na]+) m/z 301.0753, found m/z 301.0748.
Optimization of Base-Promoted PhꢀCl Cleavage by Ir(ttp)(CO)Cl.
The experimental procedures of the reaction of Ir(ttp)(CO)Cl with
PhCl (1.1 equiv) in benzene at 200 °C without and with K2CO3 are
described as typical examples.
(a). Reaction of Ir(ttp)(CO)Cl with PhCl (1.1 equiv) at 200 °C.
Ir(ttp)(CO)Cl (20 mg, 0.022 mmol), PhCl (2.7 mg, 2.4 μL, 0.024
mmol), and benzene (2 mL) were heated in a Teflon screw capped tube
at 200 °C under N2 for 7 days. The solvent was then removed under vacuum,
and the crude products were purified by column chromatography over
silica gel with CH2Cl2/hexane (2/1) as eluent. The major red fraction
was collected to give unreacted Ir(ttp)(CO)Cl (15.7 mg, 0.017 mmol,
78%). Ir(ttp)Ph was also formed in a trace amount, but it could not be
isolated by column chromatography, due to its decomposition in silica
gel. The yield of Ir(ttp)Ph (2a; 2%, NMR yield) was estimated by using
the isolated yield of unreacted Ir(ttp)(CO)Cl and the porphyrin’s
pyrrole proton ratio Ir(ttp)(CO)Cl:Ir(ttp)Ph (∼39:1) in the crude
products estimated by 1H NMR spectroscopy.
(b). Reaction of Ir(ttp)(CO)Cl with PhCl (1.1 equiv) and K2CO3
at 200 °C. Ir(ttp)(CO)Cl (19.7 mg, 0.021 mmol), K2CO3 (59.0 mg,
0.43 mmol), PhCl (2.6 mg, 2.4 μL, 0.023 mmol), and benzene (2 mL)
were heated at 200 °C under N2 for 1 day to yield Ir(ttp)Ph (2a; 6.2 mg,
0.007 mmol, 31%), Ir(ttp)(p-C6H4)Ir(ttp) (3; 0.5 mg, 0.0006 mmol,
3%), and Ir(ttp)CH3 (1.1 mg, 0.0006 mmol, 3%) isolated in a single
fraction (alumina, CH2Cl2/hexane (1/2)). The yields of the three
products were estimated by using the total isolated yield and the pyrrole
1
proton ratio 3:2a:Ir(ttp)CH3 in the isolated products by H NMR
(g). Reaction with 4-Chlorobenzotrifluoride. Ir(ttp)(CO)Cl (10.6
mg, 0.011 mmol), K2CO3 (33 mg, 0.23 mmol, 20 equiv), 4-chloroben-
zotrifluoride (414 mg, 2.3 mmol), and benzene (1 mL) were heated at
150 °C under N2 for 7 days to give Ir(ttp)C6H4(p-CF3) (2h;10d 7.9 mg,
0.008 mmol, 68%). PhCl (0.4%) was also detected by GC-MS analysis
using 4-chlorotoluene as an internal standard (GC5 in the Supporting
Information).
spectroscopy.
(c). Reaction of Ir(ttp)(CO)Cl with PhCl (50 equiv) and K2CO3
at 200 °C. Ir(ttp)(CO)Cl (9.7 mg, 0.010 mmol), K2CO3 (29 mg,
0.21 mmol), PhCl (59 mg, 53 μL, 0.53 mmol), and benzene (1 mL) were
heated at 200 °C under N2 for 12 h to give Ir(ttp)Ph (5.6 mg, 0.006
mmol, 57%), Ir(ttp)(p-C6H4)Ir(ttp) (0.6 mg, 0.0003 mmol, 3%), and
Ir(ttp)CH3 (0.5 mg, 0.0005 mmol, 5%).
(h). Reaction with 4-Chlorobenzonitrile. Ir(ttp)(CO)Cl (10.6 mg,
0.011 mmol), K2CO3 (32 mg, 0.23 mmol, 20 equiv), 4-chlorobenzoni-
trile (315.6 mg, 2.3 mmol, 200 equiv), and benzene (1 mL) were heated
at 150 °C under N2 for 1 day. PPh3 (30.1 mg, 0.11, 10 equiv) was then
added under N2, and the reaction mixture was further heated at 120 °C
for 1 h to give Ir(ttp)(PPh3)C6H4(p-CN) (2i; 7.3 mg, 0.006 mmol, 51%).
Monitoring the Intermediates in Base-Promoted PhꢀCl Cleavage
by Ir(ttp)(CO)Cl in Benzene-d6. (a). With K2CO3. Ir(ttp)(CO)Cl (4.6
mg, 0.005 mmol), K2CO3 (13.7 mg, 0.10 mmol, 20 equiv), PhCl (28 mg,
25 μL, 0.25 mmol, 50 equiv), and benzene-d6 (0.5 mL) were heated in a
sealed NMR tube under vacuum at 200 °C. The course of the reaction
was monitored by 1H NMR spectroscopy. Only Ir(ttp)H was observed
as an intermediate in the course of the reaction. After 4 days, all Ir(ttp)-
(CO)Cl and Ir(ttp)H were consumed to give Ir(ttp)Ph, Ir(ttp)(p-C6H4)-
Ir(ttp), and Ir(ttp)CH3 in 91%, 1%, and 1% NMR yields, respectively.
No Ir(ttp)C6D5 was formed, as the proton signal ratio pyrrole H:meta
phenyl H of IrꢀPh is 8:2 in the isolated product of Ir(ttp)Ph by column
chromatography.
(b). With Cs2CO3. Ir(ttp)(CO)Cl (4.3 mg, 0.005 mmol), Cs2CO3
(30 mg, 0.09 mmol, 20 equiv), PhCl (26 mg, 24 μL, 0.23 mmol, 50
equiv), and benzene-d6 (0.5 mL) were heated in a Teflon screw capped
NMR tube at 150 °C. The course of the reaction was monitored by 1H
NMR spectroscopy. Ir(ttp)H and [Ir(ttp)]2 were observed as the
intermediates. After 18 days, the reaction was incomplete, as unreacted
Ir(ttp)H was still observed. The reaction mixture was further heated
at 200 °C for 60 h to form Ir(ttp)Ph, Ir(ttp)(p-C6H4)Ir(ttp), and
(d). Reaction of Ir(ttp)(CO)Cl with PhCl (200 equiv) and K2CO3
at 200 °C. Ir(ttp)(CO)Cl (10.9 mg, 0.012 mmol), K2CO3 (33 mg, 0.24
mmol), PhCl (265 mg, 240 μL, 2.4 mmol), and benzene (1 mL) were
heated at 200 °C under N2 for 7 h to give Ir(ttp)Ph (8.3 mg, 0.009 mmol,
75%) and Ir(ttp)(p-C6H4)Ir(ttp) (2%, NMR yield). 1,4-Dichloroben-
zene in 1% yield was also detected by GC-MS analysis using 4-chlor-
otoluene as an internal standard (GC6 in the Supporting Information).
(e). Reaction of Ir(ttp)(CO)Cl with PhCl (200 equiv) and K2CO3 at
150 °C. Ir(ttp)(CO)Cl (9.8 mg, 0.011 mmol), K2CO3 (29 mg, 0.21 mmol),
PhCl (239 mg, 216 μL, 2.1 mmol), and benzene (1 mL) were heated at
200 °C under N2 for 22 h to give Ir(ttp)Ph (7.4 mg, 0.008 mmol, 74%) and
Ir(ttp)(p-C6H4)Ir(ttp) (5%, NMRyield). 1,4-Dichlorobenzenein0.4%yield
was also detected by GC-MS analysis using 4-chlorotoluene as an internal
standard (GC7 in the Supporting Information).
Scope of Base-Promoted Aryl CꢀCl Cleavage: Reactions of 4-Sub-
stituted Aryl Chlorides (200 equiv) with Ir(ttp)(CO)Cl and K2CO3 (20
equiv). The experimental procedure of the reaction of Ir(ttp)(CO)Cl
with 4-chloroanisole (200 equiv) and K2CO3 (20 equiv) is described as a
typical example.
(a). Reaction with 4-Chloroanisole. Ir(ttp)(CO)Cl (9.6 mg, 0.010
mmol), K2CO3 (29 mg, 0.21 mmol, 20 equiv), 4-chloroanisole (296 mg,
254 μL, 2.1 mmol), and benzene (1 mL) were heated in a Teflon screw
capped tube at 150 °C under N2 for 36 h. The crude product was dried
under vacuum and then purified by column chromatography over alumina
with CH2Cl2/hexane (1/2) as eluent to give Ir(ttp)C6H4(p-OMe) (2b;10d
7.6 mg, 0.008 mmol, 76%) and Ir(ttp)CH3 (0.3 mg, 0.0003 mmol, 3%)
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Ir(ttp)CH3 in 71%, 6%, and 4% NMR yields, respectively (Table S1 and
Figure S1 in the Supporting Information).
to HRMS analysis, and no (Ph3P)(ttp)IrOIr(ttp)(PPh3), (Ph3P)-
(ttp)IrOIr(ttp), or (ttp)IrOIr(ttp) was detected.
Relative Reactivities of Possible Iridium Porphyrin Intermediates in
PhꢀCl Cleavage. (a). Reaction with Ir(ttp)ꢀ. Ir(ttp)ꢀ[K(18-crown-
6)]+ was prepared quantitatively from the reaction of Ir(ttp)H (4.6 mg,
0.005 mmol) with KOH (3.0 mg, 0.05 mmol, 10 equiv) and 18-crown-6
ether (4.3 mg, 0.016 mmol, 3 equiv) in THF (0.5 mL) at 150 °C under
N2 for 45 min in a Teflon screw capped NMR tube.10c THF was then
dried under vacuum, and PhCl (30.0 mg, 27 μL, 0.27 mmol, 50 equiv)
and benzene-d6 (0.5 mL) were added under N2. The reaction mixture
was then heated in a sealed NMR tube at 200 °C, and the reaction was
monitored by 1H NMR spectroscopy. After 4 days, all Ir(ttp)ꢀ[K(18-
crown-6)]+ was consumed to give Ir(ttp)Ph (2.5 mg, 0.003 mmol, 50%),
which was isolated by column chromatography.
Reaction of Ir(ttp)(PPh3)H with KOH (20 equiv). Ir(ttp)H (4a; 4.7 mg,
0.005 mmol), PPh3 (1.4 mg, 0.005 mmol), KOH (6.1 mg, 0.11 mmol), and
benzene-d6 (0.5 mL) were added to a Teflon screw capped NMR tube
under N2. The reaction mixture changed to reddish brown instantaneously
at room temperature to generate in situ Ir(ttp)(PPh3)H (4b) quantitatively.10c
The reaction mixture containing Ir(ttp)(PPh3)H and KOH was then
heated in a sealed NMR tube at 200 °C for 3 days. Ir(ttp)(PPh3)H was
stable on heating and in basic media, as it was recovered in 98% NMR yield.
Reaction of PPh3 with Possible Oxygen Sources (Table S2 in the
Supporting Information). The experimental procedure of the reaction
of PPh3 with oxygen in air is described as a typical example.
(a). With Oxygen in Air. PPh3 (8.6 mg, 0.033 mmol) and benzene
(1 mL) were heated in air (∼3 equiv of O2) at 200 °C in a Teflon screw
capped tube in 12 h. The reaction mixture was dried under vacuum. A
quantified amount of TMS4Si was added as an internal standard, and
benzene-d6 was further added for 1H NMR spectroscopy. No P(O)Ph3
was formed (<1% NMR yield), and unreacted PPh3 was recovered in
89% NMR yield. The number of moles of O2 in air (n) was calculated by
using the ideal gas equation: n = (PV)/(RT) ꢁ 21%, where P = 101325
N m2ꢀ, V ≈ 13 ꢁ 10ꢀ6 m3, R = 8.314 J Kꢀ1 molꢀ1, T = 298 K, and 21%
is the percentage of O2 in air. Thus, n is found to be 0.11 mmol (∼3
equiv with reference to PPh3).
(b). With H2O. PPh3 (8.6 mg, 0.033 mmol), water (3.0 μL, 0.16 mmol,
5 equiv), and benzene (1 mL) were heated in a Teflon screw capped tube
under N2 at 200 °C in 12 h. No P(O)Ph3 was formed (<1% NMR yield),
and unreacted PPh3 was recovered in 94% NMR yield.
(c). With KOH. PPh3 (8.7 mg, 0.033 mmol), KOH (37.2 mg, 0.66
mmol, 20 equiv), and benzene (1 mL) were heated in a Teflon screw
capped tube under N2 at 200 °C in 12 h. No P(O)Ph3 was formed (<1%
NMR yield), and unreacted PPh3 was recovered in 75% NMR yield.
(d). With H2O2. PPh3 (17.5 mg, 0.067 mmol), H2O2 (30% solution,
15.1 mg, 13.6 μL, 0.1334 mmol of H2O2 (2 equiv)), and benzene (2 mL)
were heated in a Teflon screw capped tube under N2 at 200 °C. The
reaction was complete in 1 h, as determined by thin-layer chromatog-
raphy. P(O)Ph3 (9) was formed in 86% NMR yield.
(b). Reaction with Ir(ttp)H.10b Ir(ttp)H (4.8 mg, 0.006 mmol), PhCl
(31.3 mg, 28 μL, 0.28 mmol, 50 equiv), and benzene-d6 (0.5 mL) were
heated in a sealed NMR tube at 200 °C, and the reaction was monitored
by 1H NMR spectroscopy. After 4 days, all Ir(ttp)H was consumed to
form Ir(ttp)Ph (3.5 mg, 0.004 mmol, 67%) and Ir(ttp)(CO)Cl (0.2 mg,
0.0002 mmol, 4%), which were isolated by column chromatography.
10b
(c). Reaction with [Ir(ttp)]2. [Ir(ttp)]2 was prepared quantita-
tively from the reaction of Ir(ttp)H (4.5 mg, 0.005 mmol) with (2,2,6,6-
tetramethylpiperidin-1-yl)oxyl (TEMPO; 0.9 mg, 0.006 mmol) in
benzene (0.5 mL) in a Teflon screw capped NMR tube under N2.
The solvent was removed under vacuum, and PhCl (29.4 mg, 26.5 μL,
0.26 mmol, 50 equiv) and benzene-d6 (0.5 mL) were added into the tube
under N2. The reaction mixture was then heated in a sealed NMR tube
under vacuum at 200 °C, and the reaction was monitored by 1H NMR
spectroscopy. After 5 h, all [Ir(ttp)]2 was consumed to form Ir(ttp)Ph
(2.8 mg, 0.003 mmol, 57%) and Ir(ttp)(CO)Cl (0.2 mg, 0.0002 mmol,
4%), which were isolated by column chromatography. Ir(ttp)-
(p-C6H4)Ir(ttp) (3%, NMR yield) was also formed, and its yield was
estimated by using the yield of Ir(ttp)Ph and the pyrrole proton ratio
1
Ir(ttp)Ph:Ir(ttp)(p-C6H4)Ir(ttp) in the crude reaction mixture by H
NMR spectroscopy.
Reaction of Ir(ttp)(PPh3)Cl with KOH. (a). In Sealed NMR Tube.
Ir(ttp)(PPh3)Cl (1c; 4.5 mg, 0.004 mmol), KOH (4.4 mg, 0.08 mmol),
and benzene-d6 (0.5 mL) were heated in a sealed NMR tube at 200 °C
for 4 h to form Ir(ttp)(PPh3)OH (7b; δ(pyrrole) 8.80) and Ir(ttp)-
(PPh3)H (4b; δ(pyrrole) 8.74 ppm) in 96% and 2% NMR yields,
respectively, using residual benzene as an internal standard. The reaction
mixture was further heated for 13 h to form Ir(ttp)(PPh3)H (7b) in
(e). Without Oxygen Sources. PPh3 (8.6 mg, 0.033 mmol) and
benzene (1 mL) were heated in a Teflon screw capped tube under N2 at
200 °C in 12 h. No P(O)Ph3 was formed (<1% NMR yield), and
unreacted PPh3 was recovered in 75% NMR yield.
Reaction of Ir(ttp)(CO)Cl (1a) with KOH and Excess PPh3 (5 equiv).
Ir(ttp)(CO)Cl (10 mg, 0.011 mmol), KOH (12.1 mg, 0.22 mmol),
PPh3 (14.2 mg, 0.054 mmol), and benzene-d6 (1 mL) were heated in a
Teflon screw capped tube under N2 for 3 h, and the reaction mix-
ture changed from deep red to deep brown. Tetrakis(trimethylsilyl)-
methane (TMS4Si) in a quantitative amount was added into the reaction
mixture as an internal standard. Ir(ttp)(PPh3)H (4b) and P(O)Ph3 (9)
were formed in 42% and 21% NMR yields, respectively, with reference to
Ir(ttp)(CO)Cl added. Unreacted PPh3 was recovered in 70% yield with
reference to PPh3 added (Figure S3a in the Supporting Information).
The formation of P(O)Ph3 in the reaction was confirmed by 1H NMR
spectroscopy using the authentic sample of P(O)Ph3. Characterization
data for P(O)Ph3 formed in the reaction are as follows. 1H NMR (C6D6,
400 MHz): δ(o-phenyl H) 7.75 (dd, 3 JPH = 12.0 Hz, 3J HH = 7.6 Hz).
The P(O)Ph3’s phenyl meta and para protons are obscured by the excess
PPh3’s phenyl meta and para protons. 31P NMR (C6D6, 162 MHz): 24.9
(br) (Figure S3b in the Supporting Information). HRMS (ESIMS):
1
∼33% NMR yield. Residual water (∼1.7 equiv) was observed by H
NMR spectroscopy. Characterization data for Ir(ttp)(PPh3)OH (7b)
are as follows. 1H NMR (C6D6, 300 MHz): δ ꢀ9.35 (s, 1 H, 3JPH = 7.5
Hz), 2.40 (s, 12 H), 4.29 (dd, 6 H, 3J PH = 8.9 Hz, 3JHH = 8.7 Hz), 6.36 (t,
6 H, 3JHH = 7.2 Hz), 6.56 (t, 3 H, 3JHH = 7.1 Hz), 7.17 (d, 4 H, 3JHH = 7.2
Hz), 7.35 (d, 4 H, 3JHH = 7.5 Hz), 7.82 (d, 4 H, 3JHH = 7.5 Hz), 7.92 (d, 4
H, 3JHH = 7.5 Hz), 8.81 (s, 8 H). 13C NMR (C6D63, 100 MHz): δ 21.2,
1
122.8, 126.7, 126.9 (d, JPC = 17 Hz), 131.0, (d, JPC = 9 Hz), 131.7,
133.8, 134.9, 136.7, 139.5, 142.9. One set of porphyrin’s tolyl meta
carbon signals and PPh3’s ipso and para phenyl carbon signals were
obscured by residual benzene carbon signals. 31P NMR (C6D6, 122
3
MHz): δ ꢀ34.9 (d, JPC = 7.2 Hz) (Figure S2e in the Supporting
Information). HRMS (FABMS): calcd for [C66H52N4OPIr] + ([M]+)
m/z 1140.3502, found m/z 1140.3545.
(b). In Teflon Screw Capped Tube. Ir(ttp)(PPh3)Cl (1c; 5.2 mg,
0.004 mmol), KOH (5.0 mg, 0.10 mmol), and benzene-d6 (0.5 mL)
were heated in a Teflon screw capped tube at 200 °C for 30 min to form
Ir(ttp)(PPh3)H (4b; δ(pyrrole H) 8.75 ppm), Ir(ttp)(PPh3)OH (7b;
δ(pyrrole H) 8.81 ppm), and unreacted Ir(ttp)(PPh3)Cl (1c; δ(pyrrole
H) 8.84 ppm) in 31%, 17%, and 29% NMR yields, respectively, by 1H
NMR spectroscopy using a quantified amount of tetrakis(trimethylsilyl)-
silane (TMS4Si) as an internal standard. The reaction mixture was subjected
+
calcd for [C18H15OPNa] ([M + Na]+): m/z 301.0753, found m/z
301.0750.
Reaction of Ir(ttp)(CO)Cl (1a) with Excess PPh3 (5 equiv) without
Base. Ir(ttp)(CO)Cl (1a; 10.3 mg, 0.011 mmol), PPh3 (14.6 mg, 0.056
mmol), and benzene-d6 (1 mL) were heated in a Teflon screw capped
tube under N2 for 12 h. TMS4Si in a quantitative amount was added to
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Organometallics
ARTICLE
the reaction mixture as an internal standard. Ir(ttp)(PPh3)Cl was formed
quantitatively, and P(O)Ph3 was observed in ∼5% NMR yield, with
reference to Ir(ttp)(CO)Cl added. Unreacted PPh3 and P(O)Ph3 were
observed in 69% and 1% NMR yields, respectively, with reference to
PPh3 added. The formation of P(O)Ph3 was presumably due to the
oxidation of PPh3 with residual O2 catalyzed by iridium porphyrin species.
Reaction of [Ir(ttp)]2/PPh3 in Benzene-d6. (a). Without Addition of
KOH. Ir(ttp)H (4a; 3.8 mg, 0.004 mmol), (2,2,6,6-tetramethylpiperidin-
1-yl)oxyl (TEMPO; 1.0 mg, 0.005 mmol), and benzene (0.5 mL) were
mixed in a Teflon screw capped NMR tube under N2 to form [Ir(ttp)]2
quantitatively.10a The reaction mixture was dried under vacuum for a few
hours to remove the solvent, unreacted TEMPO, and TEMPOH
coproduct. PPh3 (1.2 mg, 0.004 mmol) and benzene-d6 (0.5 mL) were
added into the tube under N2. The reaction mixture changed to red
instantaneously. Reddish brown precipitate was formed, probably due to
the formation of insoluble [Ir(ttp)(PPh3)]2. The reaction mixture was
TEMPO, and TEMPOH coproduct were removed under vacuum. PhCl
(1 mL) was then added under N2. The reaction mixture was heated in a
Teflon screw capped tube at 200 °C for 30 min. The organic fraction was
collected under vacuum by a cold trap with liquid N2 and was then
subjected to GC-MS analysis. 1,4-Cl2C6H4 (2% yield, with reference
to Ir(ttp) monomer used) was detected using 4-chlorotoluene as an
internal standard (GC8 in the Supporting Information). The crude
products were purified by column chromatography in a single fraction to
yield Ir(ttp)Ph (2.4 mg, 0.003 mmol, 39%) and Ir(ttp)(CO)Cl (<0.1
mg, 0.000 01 mmol, ∼1%). Ir(ttp)(p-C6H4)Ir(ttp) (4%, NMR yield)
was also formed, and its yield was estimated by using the isolated yield of
Ir(ttp)Ph and the pyrrole proton ratio Ir(ttp)Ph:Ir(ttp)(p-C6H4)Ir(ttp)
in the reaction mixture by 1H NMR spectroscopy.
Reaction of Ir(ttp)(CO)Cl with 1,4-Dichlorobenzene (1 equiv) and
K2CO3. Ir(ttp)(CO)Cl (11.0 mg, 0.012 mmol), K2CO3 (33 mg, 0.24
mmol, 20 equiv), 1,4-dichlorobenzene (1.8 mg, 0.012 mmol, 1 equiv),
and benzene (1 mL) were heated in a Teflon screw capped tube at
200 °C in 7 h under N2 to give Ir(ttp)C6H4(p-Cl) (1.1 mg, 0.001 mmol,
9%), Ir(ttp)CH3 (0.1 mg, 0.0002 mmol, 1%), and Ir(ttp)(p-C6H4)Ir-
(ttp) (3%, NMR yield) by column chromatography. Unreacted Ir(ttp)H
(7%, NMR yield) was also recovered. The NMR yields of products were
estimated by using the isolated yield of Ir(ttp)C6H4(p-Cl) and the
pyrrole proton ratio Ir(ttp)H:Ir(ttp)(p-C6H4)Ir(ttp):Ir(ttp)C6H4-
(p-Cl) in the crude reaction mixture by 1H NMR spectroscopy.
Reaction of Ir(ttp)Ph with [Ir(ttp)]2. Ir(ttp)H (4a; 8.4 mg, 0.010
mmol), (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO; 1.5 mg, 0.010
mmol), and benzene (1 mL) were mixed in a Teflon screw capped tube
under N2 to form [Ir(ttp)]2 quantitatively.10a The reaction mixture was
dried under vacuum for a few hours to remove the solvent, unreacted
TEMPO, and TEMPOH coproduct. Ir(ttp)Ph (10.6 mg, 0.011 mmol,
1.2 equiv) was added into the tube under N2, and the reaction mixture was
further heated at 200 °C under N2 for 30 min. No Ir(ttp)(p-C6H4)Ir(ttp)
1
then sealed in an NMR tube under vacuum. As shown by H NMR
spectroscopy, all unreacted [Ir(ttp)]2 was consumed, and scarce Ir(ttp)-
(PPh3)OH (7b) was observed. The reaction mixture was then heated at
200 °C for 4 h to yield Ir(ttp)(PPh3)H (4b) and Ir(ttp)(PPh3)OH (7b)
in trace amounts (<5% NMR yields) estimated by adding TMS4Si
(0.000 29 mmol, from standard TMS4Si/benzene solution) as the internal
standard (Table S3 and Figure S4a in the Supporting Information).
Residual water (∼1 equiv) was estimated by 1H NMR spectroscopy.
(b). With Addition of KOH (20 equiv). [Ir(ttp)]2 was prepared in a
Teflon screw capped NMR tube similar to the procedure as shown in
(a). PPh3 (1.2 mg, 0.004 mmol), KOH (5.0 mg, 0.088 mmol), and
benzene-d6 (0.5 mL) were added into the NMR tube under N2. The
reaction mixture was then sealed in the NMR tube under vacuum. The
reaction mixture changed to red instantaneously. Reddish brown pre-
cipitate was formed, probably due to the formation of insoluble [Ir-
(ttp)(PPh3)]2. As shown by 1H NMR spectroscopy, all unreacted
[Ir(ttp)]2 was consumed, and scarce Ir(ttp)(PPh3)OH (7b) was
observed. The reaction mixture was heated at 200 °C for 4 h to yield
Ir(ttp)(PPh3)H (4b) and Ir(ttp)(PPh3)OH (7b) in 15% and 16% NMR
yields, respectively. The yield of Ir(ttp)(PPh3)OH (7b) was estimated
by adding TMS4Si (0.000 29 mmol, from standard TMS4Si/benzene
solution) as the internal standard. The yield of Ir(ttp)(PPh3)H (4b) was
estimated by using the yield of Ir(ttp)(PPh3)OH (7b) and the ratio of
porphyrin’s pyrrole proton signals for Ir(ttp)(PPh3)HandIr(ttp)(PPh3)OH
(1.0:1.1) (Table S3 and Figure S4b in the Supporting Information). Residual
water (∼1 equiv) was estimated by 1H NMR spectroscopy.
1
was observed in the crude reaction mixture by H NMR spectroscopy.
Unreacted Ir(ttp)Ph (8.2 mg, 0.009 mmol, 79%) was isolated by column
chromatography.
Competition Reactions with Para-Substituted ArCl and PhCl. The
experimental procedure of the competition reaction between 4-chlor-
oanisole and PhCl with Ir(ttp)(CO)Cl and K2CO3 is described as an
example.
(a). 4-Chloroanisole. Ir(ttp)(CO)Cl (8.5 mg, 0.009 mmol), K2CO3
(25 mg, 0.18 mmol, 20 equiv), 4-chloroanisole (131.1 mg, 113 μL, 0.92
mmol, 100 equiv), PhCl (103.5 mg, 93.5 μL, 0.92 mmol, 100 equiv), and
benzene (1 mL) were heated in a Teflon scew capped tube at 150 °C
under N2 for 35 h. The kinetic ratio Ir(ttp)C6H4(p-OMe):Ir(ttp)Ph was
estimated to be 4.39:1.00 by 1H NMR spectroscopy by taking the ratio of
the meta aryl proton signal ratio of (ttp)IrꢀAr. The crude product was
then purified by column chromatography over alumina with CH2Cl2/
hexane (1/1) as eluent to yield Ir(ttp)C6H4(p-OMe) (5.9 mg, 0.006
mmol, 66%), Ir(ttp)Ph (1.7 mg, 0.002 mmol, 19%) (ratio 3.5:1), and
Ir(ttp)CH3 (0.2 mg, 0.0003 mmol, 3%).
(b). 1-Fluoro-4-chlorobenzene. Ir(ttp)(CO)Cl (8.6 mg, 0.009
mmol), K2CO3 (25.7 mg, 0.19 mmol, 20 equiv), 1-fluoro-4-chloroben-
zene (121.4 mg, 99 μL, 0.93 mmol, 100 equiv), PhCl (104.7 mg, 95 μL,
0.93 mmol, 100 equiv), and benzene (1 mL) were heated at 150 °C
under N2 for 38 h. The kinetic ratio Ir(ttp)C6H4(p-F):Ir(ttp)Ph was
found to be 3.29:1.00. Ir(ttp)C6H4(p-F) (4.4 mg, 0.005 mmol, 50%),
Ir(ttp)Ph (1.4 mg, 0.001 mmol, 16%) (ratio 3.1:1), and Ir(ttp)CH3 (0.3
mg, 0.0003 mmol, 4%) were isolated.
Reaction of Sulfuryl Chloride with Benzene. SO2Cl2 (25.0 mg, 15 μL,
0.19 mmol) and benzene (1 mL) were heated in a Teflon screw capped
tube under N2 at 200 °C in 90 min. The PhCl formed (20%) was
quantified by GC-MS analysis using 4-chlorotoluene as an internal
standard (GC3 in the Supporting Information). The product yield was
calculated with reference to the number of Cl atoms in SO2Cl2. The
formation of PhCl was confirmed by the commercial sample of PhCl.
Reaction of Sulfuryl Chloride with PhCl. SO2Cl2 (25.0 mg, 15 μL,
0.19 mmol) and excess PhCl (1 mL) were heated in a Teflon screw
capped tube under N2 at 200 °C for 90 min. 1,4-Cl2C6H4 (6%), 1,2-
Cl2C6H4 (4%), 1,3-Cl2C6H4 (<1%), and chloro-substituted biphenyls
(1%) were detected by GC-MS analysis using 4-chlorotoluene as an
internal standard, in which the product yields were calculated with
reference to the number of Cl atoms in SO2Cl2 (GC4 in the Supporting
Information). The formation of 1,4-Cl2C6H4 and 1,2-Cl2C6H4 in the
reaction was confirmed by the commercial samples, whereas the
formation of 1,3-Cl2C6H4 and chloro-substituted biphenyls was con-
firmed by the GC-MS library.
(c). 1,4-Dichlorobenzene. Ir(ttp)(CO)Cl (10.8 mg, 0.012 mmol),
K2CO3 (32 mg, 0.23 mmol, 20 equiv), 1,4-dichlorobenzene (171.7 mg,
1.17 mmol, 100 equiv), PhCl (131.5 mg, 119 μL, 1.17 mmol, 100 equiv),
and benzene (1 mL) were heated at 200 °C under N2 for 60 h. The
kinetic ratio Ir(ttp)C6H4(p-Cl):Ir(ttp)Ph was found to be 5.74:1.00 by
averaging the original kinetic ratio (11.48:1) due to the presence of two
Reaction of [Ir(ttp)]2 with PhCl. [Ir(ttp)]2 was prepared quantita-
tively from the reaction of Ir(ttp)H (5.6 mg, 0.006 mmol) with (2,2,6,6-
tetramethylpiperidin-1-yl)oxyl (TEMPO; 1.1 mg, 0.007 mmol) in benzene
(0.5 mL) in a Teflon screw capped tube.10a The solvent, unreacted
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ARTICLE
CꢀCl bonds in 1,4-dichlorobenzene for reaction. Ir(ttp)C6H4(p-Cl)
(8.0 mg, 0.008 mmol, 70%), Ir(ttp)Ph (0.9 mg, 0.001 mmol, 8%) (ratio
8.8:1), Ir(ttp)(p-C6H4)Ir(ttp) (3%, NMR yield), and Ir(ttp)CH3 (3%,
NMR yield) were isolated. The NMR yields of Ir(ttp)(p-C6H4)Ir(ttp)
and Ir(ttp)CH3 were estimated by using the isolated yield of Ir-
(ttp)C6H4(p-Cl) and the ratio of pyrrole proton signals Ir(ttp)C6H4-
(p-Cl):Ir(ttp)(p-C6H4)Ir(ttp):Ir(ttp)CH3 in the crude products.
(d). 4-Chlorobenzonitrile. Ir(ttp)(CO)Cl (10.3 mg, 0.011 mmol),
K2CO3 (31 mg, 0.22 mmol, 20 equiv), 4-chlorobenzonitrile (153.3 mg,
1.11 mmol, 100 equiv), PhCl (125.4 mg, 113 μL, 1.11 mmol, 100 equiv),
and benzene (1 mL) were heated at 200 °C under N2 for 3.5 days. PPh3
(29.2 mg, 0.11 mmol, 10 equiv) was then added under N2, and the
reaction mixture was further heated at 120 °C for 30 min. The kinetic
ratio Ir(ttp)(PPh3)C6H4(p-CN):Ir(ttp)(PPh3)Ph (2l) was found to be
29.8:1.00. Ir(ttp)(PPh3)C6H4(p-CN) (4.9 mg, 0.004 mmol, 40%) and a
trace of Ir(ttp)(PPh3)Ph were isolated.
(iii) Cu(I): (d) Surry, D. S.; Buchwald, S. L. Chem. Sci. 2010, 1, 13–31.
(e) Casitas, A.; King, A. E.; Parella, T.; Costas, M.; Stahl, S. S.; Ribas, X.
Chem. Sci. 2010, 1, 326–330.
(5) Zhu, D.; Budzelaar, P. H. M. Organometallics 2010, 29, 5759–
5761.
(6) (a) Fan, L.; Parkin, S.; Ozerov, O. V. J. Am. Chem. Soc. 2005,
127, 16772–16773. (b) Wu, H.; Hall, M. B. Dalton Trans. 2009,
5933–5942.
(7) (a) Ben-Ari, E.; Gandelman, M.; Rozenberg, H.; Shimon, L. J. W.;
Milstein, D. J. Am. Chem. Soc. 2003, 125, 4714–4715. (b) Ben-Ari, E.;
Cohen, R.; Gandelman, M.; Shimon, L. J. W.; Martin, J. M. L.; Milstein, D.
Organometallics 2006, 25, 3190–3210.
(8) Wu, F.; Dash, A. K.; Jordan, R. F. J. Am. Chem. Soc. 2004, 126,
15360–15361.
(9) For recent bond-cleavage reactions by high-valent rhodium(III)
porphyrins, see the following. (a) CꢀH activation of alkanes: Chan,
Y. W.; Chan, K. S. Organometallics 2008, 27, 4625–4635. (b) Benzylic
CꢀH activation of toluenes: Choi, K. S.; Chiu, P. F.; Chan, K. S.
Organometallics 2010, 29, 624–629. (c) CꢀC activation of cyclooctane:
Chan, Y. W.; Chan, K. S. J. Am. Chem. Soc. 2010, 132, 6920–6922.
(d) C(O)ꢀC(R) activation of acetophenones: Fung, H. S.; Li, B. Z.;
Chan, K. S. Organometallics 2010, 29, 4421–4423. (e) CꢀO cleavage of
methanol: Fung, H. S.; Chan, Y. W.; Cheung, C. W.; Choi, K. S.; Lee,
S. Y.; Qing, Y. Y.; Chan, K. S. Organometallics 2009, 28, 3981–3989.
(10) For recent bond-cleavage reactions by high-valent iridium(III)
porphyrins, see the following.(a) Benzylic CꢀH activation of toluenes:
Cheung, C. W.; Chan, K. S. Organometallics 2008, 27, 3043–3055.
(b) C(O)ꢀC(R) activation of acetophenones: Li, B. Z.; Song, X.; Fung,
H. S.; Chan, K. S. Organometallics 2010, 29, 2001–2003. (c) CꢀO
cleavage of methanol: Cheung, C. W.; Fung, H. S.; Lee, S. Y.; Qian, Y. Y.;
Chan, Y. W.; Chan, K. S. Organometallics 2010, 29, 1343–1354.
(d) ArꢀBr cleavage: Cheung, C. W.; Chan, K. S. Organometallics
2011, 30, 1768–1717.(e) ArꢀI cleavage: Cheung, C. W.; Chan, K. S.
Organometallics 2011, 30, 4269ꢀ4283. (f) Aldehydic CꢀH activation
of benzaldehydes: Song, X.; Chan, K. S. Organometallics 2007, 26,
965–970.
’ ASSOCIATED CONTENT
S
Supporting Information. Text, tables, and figures giving
b
the sequence of reactions monitored by 1H NMR spectroscopy,
1
supplementary experimental results, GC-MS analysis, and H
and 13C NMR spectra. This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: ksc@cuhk.edu.hk.
’ ACKNOWLEDGMENT
We thank the Research Grants Councils of Hong Kong SAR,
the People’s Republic of China (No. 400308), and a Special
Equipment Grant (Project No. SEG/CUHK09) for financial
support.
(11) Ir(ttp)CH3 is likely formed via the base-promoted reduction of
CO ligand of Ir(ttp)(CO)Cl by the Ir(ttp)H intermediate. See ref 10a
for the proposed mechanism of Ir(ttp)CH3 formation.
(12) The nitrile group in 4-bromobenzonitrile can coordinate
strongly to iridium(III) porphyrin species. Indeed, 1a has been reported
to react with p-tolunitrile (p-TolCN) to yield stable, coordinatively
saturated Ir(ttp)(p-TolCN)Cl.10e
’ REFERENCES
(1) For examples, see: (a) Martin, R.; Buchwald, S. L. Acc. Chem. Res.
2008, 41, 1461–1473. (b) Hartwig, J. F. Acc. Chem. Res. 2008, 41, 1534–
1544. (c) Surry, D. S.; Buchwald, S. L. Chem. Sci. 2011, 2, 27–50. (d)
Maiti, D.; Fors, B. P.; Henderson, J. L.; Nakamura, Y.; Buchwald, S. L.
Chem. Sci. 2011, 2, 57–68.
(2) For examples of hydrodechlorination of aryl chlorides, see: (a)
Hara, R.; Sun, W.-H.; Nishihara, Y.; Takahashi, T. Chem. Lett. 1997,
26, 1251–1252. (b) Alonso, F.; Beletskaya, I. P.; Yus, M. Chem. Rev.
2002, 102, 4009–4091. (c) Chen, J.; Zhou, S.; Ci, D.; Zhang, J.; Wang,
R.; Zhang, J. Ind. Eng. Chem. Res. 2009, 48, 3812–3819. (d) Akzinnay, S.;
Bisaro, F.; Cazin, C. S. J. Chem. Commun. 2009, 5752–5753. (e) Cannon,
K. A.; Geuther, M. E.; Kelly, C. K.; Lin, S.; MacArthur, A. H. R.
Organometallics 2011, 30, 4067–4073.
(13) Water-soluble iridium(III) porphyrin, [Ir(tspp)(H2O)2]3ꢀ
(tspp = tetrakis(p-sulfonatophenyl)porphyrinato dianion) has been
reported to undergo thermodynamically favorable ligand substitution
with CH3OH to form [Ir(tspp)(CH3OH)2]3ꢀ. See: Bhagan, S.; Sarkar,
S.; Wayland, W. W. Inorg. Chem. 2010, 49, 6734–6739. We proposed
that the nonbulky aldehydic carbonyl group (ꢀCHO) can also bind
strongly to the iridium center of Ir(ttp)Ar.
(14) The benzylic CꢀH bond of toluene (88.5 kcal molꢀ1) and the
aldehydic CꢀH bond of benzaldehyde (88.7 kcal molꢀ1) are weaker
than the ArꢀCl bond of chlorobenzene (95.5 kcal molꢀ1). See: Luo,
Y.-R. Comprehensive Handbook of Chemical Bond Energies; CRC Press:
Boca Raton, FL, 2007. Thus, competitive cleavage between benzylic/
aldehydic CꢀH bonds and ArꢀCl bonds can occur.
(3) For examples of ArꢀCl cleavage by d8 transition metals, see the
following. (i) Fe(0): (a) Shi, Y.; Li, M.; Hu, Q.; Li, X.; Sun, H.
Organometallics 2009, 28, 2206–2210. (ii) Co(I): (b) Chen, Y.; Sun,
H.; Fl€orke, U.; Li, X. Organometallics 2008, 27, 270–275. (iii) Rh(I): (c)
Gatard, S.; C-elenligil-C-etin, R.; Guo, C.; Foxman, B. M.; Ozerov, O. V.
J. Am. Chem. Soc. 2006, 128, 2808–2809. (d) Ito, J.; Miyakawa, T.;
Nishiyama, H. Organometallics 2008, 27, 3312–3315. (iv) Ir(I): (e)
Blum, J.; Weitzberg, M. J. Organomet. Chem. 1976, 122, 261–264. (f)
Douvris, C.; Reed, C. A. Organometallics 2008, 27, 807–810.
(15) Cs2CO3 has been adopted for the successful observations of
iridium porphyrin intermediates in base-promoted ArꢀBr cleavage by
1a.10d Thus, it is also used in base-promoted PhꢀCl cleavage to observe
the intermediates formed.
(16) A lower loading of PhCl (50 equiv) was adopted to
(1) minimize the obscuration of proton signals of the iridium porphyrin
intermediates by the intense and broad proton signals of a large excess of
PhCl (when 200 equiv of PhCl was added) and (2) reduce the rate of
consumption of iridium porphyrin intermediates for successful observation.
(17) Ir(ttp)(CO)Cl (1a) rather than Ir(ttp)Cl was isolated after
column chromatography. The source of CO is unclear, similar to the
situation of 1a being isolated from the reaction of [IrCOD)Cl]2 (COD =
(4) For examples of ArꢀCl cleavage by d10 transition metals, see the
following. (i) Ni(0): (a) Tsou, T. T.; Kochi, J. K. J. Am. Chem. Soc. 1979,
101, 6319–6332. (b) Cariou, R.; Graham, T. W.; Dahcheh, F.; Stephan,
D. W. Dalton Trans. 2011, 5419–5422. (ii) Pd(0): (c) Barrios-Landeros,
F.; Carrow, B. P.; Hartwig, J. F. J. Am. Chem. Soc. 2009, 131, 8141–8154.
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Organometallics
ARTICLE
1,5-cyclooctadiene) with 5,10,15,20-tetrakis(p-tolyl)porphyrin in reflux-
ing p-xylene, even though no CO was introduced into the reaction
mixture. See: Yeung, S. K.; Chan, K. S. Organometallics 2005,
24, 6426–6430.
(32) Three lines of evidence support the ISAE mechanism of ArꢀX
cleavage by [Ir(ttp)]2: (1) higher Ir(ttp)Ar:Ir(ttp)X ratio due to the X•
leakage, (2) the detection of PhX coproducts formed by the reaction of
X• with benzene solvent, and (3) the rate enhancement in ArꢀX
cleavage by all the electron-donating and -withdrawing para substituents
in ArX, as shown by the V-shaped Hammett plots in the competition
experiments. See ref 10d for further details.
(18) (a) Frary, F. C.; Nietz, A. H. J. Am. Chem. Soc. 1915, 37, 2268–
2273. (b) Thomas, A. M., Jr. J. Chem. Eng. Data 1963, 8, 51–54.
(19) A trace of residual water (∼0.2ꢀ2 equiv with reference to
iridium porphyrin, δ(H2O) ∼0.4 ppm) remained in benzene-d6 even
though benzene-d6 had been distilled over sodium and stored in a Teflon
screw capped Schlenk tube under N2. Adventitious water was also
inevitable. For the reported chemical shift of H2O in benzene-d6, see:
Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.;
Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I. Organome-
tallics 2010, 29, 2176–2179.
(20) Thermodynamically favorable σ-donor ligand induced dispro-
portionation has been reported for rhodium(II) porphyrins as follows.
(a) Pyridine as ligand: Wayland, B. B.; Sherry, A. E.; Bunn, A. G. J. Am.
Chem. Soc. 1993, 115, 7675–7684. (b) D2O as ligand: Fu, X.; Li, S.;
Wayland, B. B. Inorg. Chem. 2006, 45, 9884–9889. (c) CH3OH as ligand:
Sarkar, S.; Li., S.; Wayland, B. B. J. Am. Chem. Soc. 2010, 132, 13569–
13571.
(21) The pKa value of Ir(por)ꢀH is generally higher than that of
H2O, as Ir(oep)H (oep = octaethylporphyrin dianion) remains un-
reacted in NaOH/ethanol solution to give the soluble Ir(oep)ꢀ. Thus,
the protonation of Ir(por)ꢀ with water would be favorable to give
Ir(por)H. See: Ogoshi, H.; Setsune, J.-I.; Yoshida, Z.-I. J. Organomet.
Chem. 1978, 159, 317–328.
(22) The low yield of 4b formed was due to neither the formation of
(Ph3P)(ttp)IrOIr(ttp)(PPh3) (which was not detected by HRMS) nor
the decomposition of Ir(ttp)(PPh3)H (which remained unreacted
under basic conditions at 200 °C). Probably, the coordinatively satu-
rated 7b, or the [IrII(ttp)(PPh3)]2 intermediate formed, is less reactive
and decomposes considerably at 200 °C prior to the subsequent reaction
to give 4b.
(33) The reaction of Cl• with excess (p-CF3)C6H4Cl to yield
dichloro-substituted benzotrifluorides (p-CF3-C6H3Cl2) could not be
excluded. However, detection of (p-CF3)C6H3Cl2 in the reaction
mixture by GC-MS analysis was not carried out.
(34) Only 1,4-Cl2C6H4 was detected from the reaction mixture
without 1,2-Cl2C6H4. It is proposed that the low concentration of Cl•
generated favors the regioselective attack of Cl• at the less sterically
hindered para ArꢀH of PhCl to yield only 1,4-Cl2C6H4 due to the steric
strain.
(35) No Ir(ttp)C6H4(p-Cl)10d was observed in the reaction of 1a
with PhCl (200 equiv) and K2CO3 in benzene (Table 1, entries 4 and 5)
1
or in the reaction of [Ir(ttp)]2 with neat PhCl (eq 8) by H NMR
spectroscopy; only 3 was isolated. The reason is unclear. We proposed
that the electrophilic, σ-withdrawing IrIII(ttp) group promotes the
radical ipso substitution of IrIII(ttp)C6H4(p-Cl) by the more nucleo-
philic IrII(ttp) metalloradical to rapidly yield 3, whereas the π-donating
p-Cl in 1,4-Cl2C6H4 leads to a slower reaction of ipso attack by IrII(ttp)
to give Ir(ttp)C6H4(p-Cl).
(36) A V-shaped Hammett plot was also obtained when using the
Hammett constants σ+p (Figure S5b in the Supporting Information).
(37) For the details of the stabilization of Ir(ttp)ꢀcyclohexadienyl
radical by para-FG, see the Supporting Information in ref 10d.
(23) Roberts, J. L., Jr.; Sugimoto, H.; Barrette, W. C., Jr.; Sawyer,
D. T. J. Am. Chem. Soc. 1985, 107, 4556–4557.
(24) KOH, residual water, H2O2, and O2 are possible oxygen
sources for the oxidation of PPh3 to form P(O)Ph3. As control
experiments, only H2O2 was shown to efficiently promote the oxidation
of PPh3 to P(O)Ph3 (Table S2 in the Supporting Information). Hence,
any P(O)Ph3 detected can be attributed to the reaction of H2O2 with
PPh3.
(25) (a) H2O2 has been reported to react with PPh3 to give
P(O)Ph3. See: Abu-Omar, M. M.; Espenson, J. H. J. Am. Chem. Soc. 1995,
117, 272–280. (b) In the quantitative reduction of Mo(VI) and W(VI)
complexes by OHꢀ to give Mo(V) and W(V) complexes, the coproduct
H2O2 is trapped by PPh3 to give P(O)Ph3 in 50% yield. See: Cervilla, A. C.;
Pꢀerez-Plꢀa, F.; Llopis, E.; Piles, M. Dalton Trans. 2004, 1461–1465.
(26) No precipitate was observed after the reaction. The side
products from the decomposition of the proposed [Ir(ttp)(PPh3)]2
remain unclear.
(27) Del Rossi, K. J.; Wayland, B. B. J. Chem. Soc., Chem. Commun.
1986, 1963–1965.
(28) For the mechanism of HAS, see:Smith, M. B.; March, J. March’s
Advanced Organic Chemistry: Reactions, Mechanisms. And Structure, 6th
ed.; Wiley,: New York, 2007; Chapter 14.
(29) As a control experiment, SO2Cl2 (as a source of Cl•) reacted
with excess C6H6 at 200 °C in 90 min to give PhCl in 20% yield.
(30) As a control experiment, SO2Cl2 (as a source of Cl•) reacted
with excess PhCl at 200 °C in 90 min to give 1,4-Cl2C6H4 (6%), 1,2-
Cl2C6H4 (4%), 1,3-Cl2C6H4 (<1%), and various dichlorinated biaryls
(2%). 1,4-Cl2C6H4 and 1,2-Cl2C6H4 are generated more than 1,3-
Cl2C6H4, since all the substituents in monosubstituted arenes are ortho
and para directing.28
(31) With K2CO3, 1a reacted with excess 1,4-dibromobenzene and
1,4-diiodobenzene to yield Ir(ttp)C6H4(p-X) (X = Br, I), which further
reacted with 1a to yield 3.10e The K2CO3-promoted conversion of 1,4-
dichlorobenzene to 3 most likely follows analogous reaction pathways.
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