Base-promoted selective aryl carbon-bromine bond cleavage by Iridium(III) porphyrin for Iridium(III) porphyrin aryl synthesis: A metalloradical Ipso addition-elimination mechanism

Article abstract of DOI:10.1021/om200027q

K₂CO₃ was found to promote selective aryl carbon-bromine bond (Ar-Br) cleavage by a high-valent iridium(III) porphyrin carbonyl chloride (Ir^III(ttp)(CO)Cl, ttp = 5,10,15,20-tetra-p- tolylporphyrinato dianion) in benzene so

Full text of DOI:10.1021/om200027q

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pubs.acs.org/Organometallics
Base-Promoted Selective Aryl Carbon-Bromine Bond Cleavage
by Iridium(III) Porphyrin for Iridium(III) Porphyrin Aryl Synthesis:
A Metalloradical Ipso Addition-Elimination 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: K2CO3 was found to promote selective aryl carbon-bromine bond
(Ar-Br) cleavage by a high-valent iridium(III) porphyrin carbonyl chloride
(IrIII(ttp)(CO)Cl, ttp = 5,10,15,20-tetra-p-tolylporphyrinato dianion) in benzene
solvent at elevated temperature to give iridium(III) porphyrin aryls (IrIII(ttp)Ar) in
high yields. Ir(ttp)(CO)Cl is reduced in alkaline media to give an [Ir(ttp)]2 intermediate. [Ir(ttp)]2 then cleaves the Ar-Br bond
via a radical-type addition-elimination reaction (radical ipso -substitution) to yield Ir(ttp)Ar and a bromine radical.
Scheme 1. Discovery of This Work
roup 9 metal(II) complexes are metalloradicals which have
been widely studied in organometallic chemistry.1 A well-
G
known example of group 9 metal(II) complexes is the metallo-
porphyrin radical (MII(por), M = Co, Rh, Ir). MII(por) exhibits
unique chemical reactivity in the bond activation of small
molecules, such as olefins,2 carbon monoxide,2b H2,3 C(sp3)-
H,3a,4 C(sp3)-Br,5 and C(sp3)-C(sp3)6 bonds. They have also
been utilized as catalysts in various chemical transformations,
including the polymerization,7 cyclopropanation,8 and aziridina-
tion of olefins,9 as well as C-H amination.10
However, the cleavage of aryl-halogen bonds (Ar-X) by group
9 metalloporphyrin radicals is still unprecedented. To our knowl-
edge, the only reported Ar-X cleavages mediated by group 9
metalloradicals are the reaction of 2-iodopyridine with
pentacyanocobaltate(II) ([CoII(CN)5]3-) to give [CoIII(CN)5I]3-
and [CoIII(CN)5(2-pyridyl)]3- 11 and the reaction of Ar-X (X
= Cl, Br, I) with (L)Co0 (L = 2,6-bis[2,6-dimethylpheny-
liminoethyl]pyridine) to give (L)CoI-X and (L)CoI-Ar,12 via
halogen atom abstraction by Co radicals. While Ar-X cleavage
via oxidative addition of Co(I),13 Rh(I),14 and Ir(I)15 to MIII-
(Ar)(X) has been commonly reported, Ar-X cleavage with
group 9 metalloradicals remains underdeveloped.
As part of our continuing interest in bond cleavage chemistry
by high-valent iridium(III) porphyrins,4d,16 we sought to inves-
tigate the aryl-bromine bond (Ar-Br) cleavage by IrIII(ttp)-
(CO)Cl (1a; ttp = tetra-p-tolylporphyrinato dianion). We found
that ArBr reacted with Ir(ttp)(CO)Cl in the presence of base to
give ipso-substituted iridium(III) porphyrin aryls (IrIII(ttp)Ar).
The reaction is apparently intriguing, as a direct metathesis
process would give an energetically uphill BrCl coproduct.
Mechanistically, the formation of any Ir(V) porphyrin inter-
mediate is difficult in electronics and sterically demanding, with
three non-porphyrin ligands located in a cis position. In exploring
this bond activation, we discovered that base can reduce Ir(ttp)-
(CO)Cl to a metalloradical dimer intermediate, [IrII(ttp)]2 (step
i in Scheme 1), which then undergoes an unprecedented radical-
type ipso substitution with Ar-Br17 via an addition-elimination
reaction to give IrIII(ttp)Ar (step ii in Scheme 1). We now report
the results and mechanistic studies of the Ar-Br cleavage.
Results and Discussion. Ir(ttp)(CO)Cl (1a) initially reacted
very slowly with PhBr (1.1 equiv) in benzene solvent at 200 °C in 7
days to give Ir(ttp)Ph (2e) in only 9% yield. Upon the addition of
K2CO3 (20 equiv), the reaction was complete in 2 days to yield 2e
quantitatively (Table 1, eq 1, entry 5). 2e was formed from Ph-Br
cleavage rather than the C-H activation of benzene, since 1a reacted
with PhBr (1.1 equiv) and K2CO3 (20 equiv) in benzene-d6 at
200 °Cin10daystogive2e quantitatively without any Ir(ttp)C6D5.18
This base-promoted reaction is general for various ArBr species to
give high yields of Ir(ttp)Ar bearing different functional groups
(Table 1). 4,40-Dibromobiphenyl also reacted with 1a and K2CO3
sequentially to give Ir(ttp)(p-C6H4)2(p-Br) (2q) and thenIr(ttp)(p-
C6H4)2Ir(ttp) (3) (Scheme 2). The new convenient synthesis of
iridium(III) porphyrin aryls eliminates the use of Grignard19 or
organolithium20 reagents required in the traditional synthesis.
In order to understand the reaction mechanism, the reaction of
1a with PhBr (1.1 equiv) and the stronger base Cs2CO3 (20
equiv) in benzene-d6 at a lower temperature of 150 °C was
monitored by 1H NMR spectroscopy to observe any intermedi-
ates formed (eq 2; Table S1 in the Supporting Information).21
After 3 h, 2e in 30% yield was formed together with intermediates
IrIII(ttp)H (4) and [IrII(ttp)]2 (5) in 13% and 1% yields,
respectively (eq 2a). After 60 h, 1a, 4, and 5 were completely
consumed to yield 2e quantitatively (eq 2b). The base has
promoted the reduction of 1a to 4 and 5.
Received: January 13, 2011
Published: March 09, 2011
r
2011 American Chemical Society
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|

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Table 1. Substrate Scope of Ar-Br Cleavage
entrya
FG
time/h
product (yield/%)b
entrya
FG
p-Cl
time/h
product (yield/%)b
1
2
3
4
5
6
7
8
p-OMe
p-tBu
9
15
11
15
48
27
20
36
2a (89)
2b (86)
2c (73)
2d (80)
2e (100)
2f (68)
2g (99)
2h (76)
9
24
36
19
24
6
2i (72)
2j (84)
2k (82)
2l (72)
2m (63)
2n (83)
2o (85)
2p (84)
10
11
12
13
14
15
16
p-CO2Me
p-C(O)Me
p-CF3
p-Me
p-SiMe3
p-H
p-NO2
p-NPhthc
p-F
m-OMe
m-Me
17
14
16
p-Br
m-NO2
a The intermediate Ir(ttp)(CO)Br (1b) was observed in the course of the reaction and was completely consumed to yield Ir(ttp)Ar upon prolonged
heating. b Isolated yield. c NPhth = phthalimide.
Mechanism 1. OH-, which can be generated from the
Scheme 2. Synthesis of Ir(ttp)(p-C6H4)2Ir(ttp)
thermal hydrolysis of K2CO325 with residual H2O26 in benzene
at 200 °C (eqs ia and ib), undergoes nucleophilic attack to the CO
ligand of 1a to give Ir(ttp)CO2H and Cl- (eq ii). Ir(ttp)CO2H
then decarboxylates to give Ir(ttp)H and CO2 (eq iii),27,28 in line
with the known transition-metal-mediated water-gas shift
reaction.27-29
Ir(ttp)H further undergoes dehydrogenative dimerization via an
equilibrium to give [Ir(ttp)]2 (eq iv). [Ir(ttp)]2 finally reacts with
ArBr to give Ir(ttp)Ar and Ir(ttp)Br (eq v).
Mechanism 2. 1a undergoes CO dissociation to give Ir(ttp)Cl
(eq vi), which further reacts with OH- to give the proposed
Ir(ttp)OH via ligand substitution (eq vii).30 Ir(ttp)OH then gives
[Ir(ttp)]2 and H2O2 via the reduction of the IrIII center by the
IrII for Ar-Br Cleavage. In the base-promoted Ar-Br cleavage,
Ir(ttp)H and [Ir(ttp)]2 were the observed intermediates (eq 2a).
Ir(ttp)H rapidly underwent dehydrogenative dimerization in benzene
to give [Ir(ttp)]2 both on heating and as promoted by K2CO3, as
shown by independent experiments (eqs 3a and 3b).4d,22 Ir(ttp)-
(6), although it is not observed, could also be produced in a low
concentration in benzene via the deprotonation of Ir(ttp)H with base,
as Ir(ttp)H is acidic enough.16b Thus, Ir(ttp)H, [Ir(ttp)]2, and
Ir(ttp)- can coexist in equilibria in benzene.16b Their relative
reactivities with ArBr (1.1 equiv) were thus compared independently
at 200 °C in benzene-d6 to establish the intermediate for Ar-Br
cleavage (Table S4 in the Supporting Information). In Ph-Br
cleavage, the relative reaction rates of [Ir(ttp)]2, Ir(ttp)H, and
Ir(ttp)- were estimated to be about 10 000:20:1, by comparing the
time required for complete consumption of iridium porphyrin species.
Such a reactivity trend was also observed in the reactions with both
electron-rich (p-MeO)C6H4Br and electron-poor (p-NO2)C6H4Br.
Additionally, Ir(ttp)-Kþ was nonproductive in the Ar-Br cleavage
of (p-MeO)C6H4Br but rather reacted to give Ir(ttp)CH3, probably
via nucleophilic substitution of Me-OC6H4(p-Br).23 Thus, [Ir(ttp)]2
is the most probable intermediate for Ar-Br cleavage to give
Ir(ttp)Ar and Ir(ttp)Br24 (Table 2, entries 3, 6, and 9).
hydroxo ligand (eq viii),16c,31 since the one-electron reduction of
first-row transition-metal porphyrins by OH- in aprotic solvents
has been reported.32 The H2O2 formed rapidly disproportionates
into H2O and O2, catalyzed by OH- (eq ix).33 In a low
concentration or the absence of ArBr, [Ir(ttp)]2 can react compe-
titively with residual water in benzene to give Ir(ttp)OH and the
observed Ir(ttp)H (eq x).16c,34 In a productive process, [Ir(ttp)]2
reacts with ArBr to give Ir(ttp)Ar and Ir(ttp)Br (eq xi). Ir(ttp)Br,
once formed, can further react with CO dissociated from 1a to form
Ir(ttp)(CO)Br (1b) (eq xii). Both Ir(ttp)Br and Ir(ttp)(CO)Br
continue to undergo base-promoted Ar-Br cleavage to give
Ir(ttp)Ar (eqs vi-viii and xi).
Mechanism 1 is unlikely to operate. Since OH- reduces 1a to
Ir(ttp)H via the consumption of CO to give CO2 (eqs ii and iii in
Scheme 3), subsequent reaction of [Ir(ttp)]2 with ArBr would
only give both Ir(ttp)Ar and Ir(ttp)Br with each of them in a
maximum yield of 50% (eq v in Scheme 3). However, no
Ir(ttp)Br (δ(pyrrole H) ∼8.9 ppm)24 was observed in the crude
reaction mixtures of the base-promoted Ar-Br cleavage by 1a by
1H NMR spectroscopy, and the isolated yields of Ir(ttp)Ar are
consistently much higher than 50% (Table 1).
Instead, mechanism 2 operates. The Ir(ttp)(CO)Br (1b)35
intermediate was formed in the base-promoted Ar-Br cleavage
by 1a (Table 1), strongly supporting the CO dissociation path-
way (eq vi in Scheme 3). 1b was completely consumed to yield
Ir(ttp)Ar (Table 1), most likely because 1b is further reduced by
OH- to yield [Ir(ttp)]2 for further Ar-Br cleavage (eqs vi-viii
and xi in Scheme 3). Further studies are ongoing to gain the
understanding of the reduction of 1a and 1b by OH-.
Mechanism of Ir(ttp)(CO)Cl Reduction. We propose two
possible mechanisms for base-promoted Ar-Br cleavage by
Ir(ttp)(CO)Cl (1a) to give Ir(ttp)Ar (Scheme 3).
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Scheme 3. Possible Mechanisms of Base-Promoted Ar-Br
Cleavage
Scheme 4. Radical Ipso Substitution of ArBr by [Ir(ttp)]2
Figure 1. Stabilization of the Ir(ttp)-cyclohexadienyl intermediate by
various substituents.
and -withdrawing p-FGs only, but not with the m-FGs
(Figure 1; Table S6 and Schemes S3-S7 in the Support-
ing Information).39 Indeed, such a reactivity pattern has
also been observed in radical ipso substitution of Ar-Br
with sulfur-centered radicals.17e
Conclusion. In summary, we have discovered base-promoted
selective Ar-Br cleavage by high-valent Ir(ttp)(CO)Cl. Me-
chanistic studies suggest that OH- likely promotes the reduction
of Ir(ttp)(CO)Cl to [Ir(ttp)]2, which then cleaves the Ar-Br
bond via radical ipso substitution to give Ir(ttp)Ar. Further
mechanistic studies are ongoing.
Ar-Br Cleavage Mechanism. Ar-Br cleavage by [Ir(ttp)]2
likely goes through radical ipso substitution via the addition-
elimination reaction (ISAE) (Scheme 4) rather than a direct
bromine atom abstraction.36 We propose that [Ir(ttp)]2 initially
dissociates into an Ir(ttp) metalloradical (eq i).2b,3a Ir(ttp) then
attacks the ipso carbon of ArBr to give an Ir(ttp)-cyclohex-
adienyl radical intermediate (I), which subsequently dissociates
into Ir(ttp)Ar and a bromine atom (Br•) via radical ipso
substitution (eq ii).17 Br• further reacts with [Ir(ttp)]2 to form
Ir(ttp)Br and another Ir(ttp) for chain propagation (eq iii).
Thus, the Ar-Br cleavage operates in a radical-chain mechanism.
Consequently, some Br• can leak out from the chain reaction and
react with benzene solvent to give PhBr (eq iv).37
’ ASSOCIATED CONTENT
S
Supporting Information. Sequence of reactions moni-
b
1
tored by H NMR spectroscopy, supplementary experimental
results, table and figure of crystallographic data for complex 3
(CIF and PDF), GC-MS analysis, and 1H and 13C NMR spectra.
This material is available free of charge via the Internet at http://
pubs.acs.org.
The ISAE of ArBr with [Ir(ttp)]2 (Scheme 4) can be sup-
ported by three lines of evidence.
’ AUTHOR INFORMATION
(1) Higher ratio of Ir(ttp)Ar to Ir(ttp)Br. In the reactions of
[Ir(ttp)]2 with various (p-FG)C6H4Br (FG = H, OMe,
NO2), the ratios of Ir(ttp)Ar to Ir(ttp)Br are consistently
greater than 1 (Table S4 in the Supporting Information)
due to the Br• leakage (eq ii in Scheme 4).
Corresponding Author
*E-mail: ksc@cuhk.edu.hk.
’ ACKNOWLEDGMENT
(2) Detection of PhBr. [Ir(ttp)]2 reacted with (p-tBu)C6H5Br
(1.1 equiv) in C6H6 at 200 °C to give Ir(ttp)C6H4(p-tBu)
(47%), Ir(ttp)Br (∼39%), and PhBr (1%),38 as detected
by GC-MS analysis (eq 4). The detection of PhBr firmly
supports the ISAE of Ar-Br by Ir(ttp) to give Br• for
PhBr formation (eqs ii and iv in Scheme 4).
We thank the Research Grants Councils of Hong Kong SAR,
People’s Republic of China (No. 400308), for financial support.
’ REFERENCES
(1) The chemistry of group 9 metalloradicals other than metallo-
porphyrin radicals has been reported. For examples, see: de Bruin, B.;
Hetterscheid, D. G. H. Eur. J. Inorg. Chem. 2007, 211–130.
(2) For examples, see: (a) Zhai, H.; Bunn, A.; Wayland, B. B.
Chem. Commun. 2001, 1294–1295. (b) Cui, W.; Li, S.; Wayland, B. B.
J. Organomet. Chem. 2007, 629, 3198–3206.
(3) For examples, see: (a) Del Rossi, K. J.; Wayland, B. B. J. Chem.
Soc., Chem. Commun. 1986, 1963–1965. (b) Wayland, B. B.; Ba, S.;
Sherry, A. E. Inorg. Chem. 1992, 31, 148–150.
(4) For examples, see: (a) Sherry, A. E.; Wayland, B. B. J. Am. Chem.
Soc. 1990, 112, 1259–1261. (b) Wayland, B. B.; Ba, S.; Sherry, A. E.
J. Am. Chem. Soc. 1991, 113, 5305–5311. (c) Chan, Y. W.; Chan, K. S.
Organometallics 2008, 27, 4625–4635. (d) Cheung, C. W.; Chan, K. S.
Organometallics 2008, 27, 3043–3055.
(3) Rate enhancement by para substituents (p-FGs) in ArBr.
Both the electron-rich and -poor para-substituted ArBr
promoted the rate of base-promoted Ar-Br cleavage by
1a, as shown by the V-shaped Hammett plots (Table S5
and Figure S5 in the Supporting Information). Such a
reactivity trend is attributed to the stabilization of the
Ir(ttp)-cyclohexadienyl radical intermediate via reso-
nance interactions with both the electron-donating
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(5) Paonessa, R. S.; Thomas, N. C.; Halpern, J. J. Am. Chem. Soc.
1985, 107, 4333–4335.
presumably due to the proton exchange of the sixth coordinated
water in Ir(ttp)(H2O)Br.
(6) For examples, see: Chan, Y. W.; Chan, K. S. J. Am. Chem. Soc.
2010, 132, 6920–6922.
(25) Thomas, A. M., Jr. J. Chem. Eng. Data 1963, 8, 51–54.
(26) A trace of residual water (∼0.2-2.0 equiv with regard 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-capped Schlenk tube under N2.
(27) The decarboxylation of Ir-CO2H to Ir-H and CO2 has been
proposed. See: Ziessel, R. J. Am. Chem. Soc. 1993, 115, 118–127.
(28) For the reported decarboxylation of the analogous Rh(por)-
CO2H to Rh(por)H and CO2, see: Wayland, B. B.; Woods, B. A.; Pierce,
R. J. Am. Chem. Soc. 1982, 104, 302–303.
(7) For examples, see: Li, S.; de Bruin, B.; Peng, C.-H.; Fryd, M.;
Wayland, B. B. J. Am. Chem. Soc. 2008, 130, 13373–13381.
(8) For examples, see: Fields, K. B.; Engle, J. T.; Sripothongnak, S.;
Kim, C.; Zhang, X. P.; Ziegler, C. J. Chem. Commun. 2011, 47, 749–751.
(9) For examples, see: Ruppel, J. V.; Jones, J. E.; Huff, C. A.; Kamble,
R. M.; Chen, Y.; Zhang, X. P. Org. Lett. 2008, 10, 1995–1998.
(10) For examples, see: Lu, H.; Tao, J.; Jones, J. E.; Wojtas, L.;
Zhang, X. P. Org. Lett. 2010, 12, 1248–1251.
(11) Halpern, J.; Maher, J. P. J. Am. Chem. Soc. 1965, 87, 5361–5366.
(12) Zhu, D.; Budzelaar, P. H. M. Organometallics 2010, 29,
5759–5761.
(13) Chen, Y.; Sun, H.; Fl€orke, U.; Li, X. Organometallics 2008,
27, 270–275.
(14) For examples, see: Willems, S. T. H.; Budzelaar, P. H. M.;
Moonen, N. N. P.; de Gelder, R.; Smits, J. M. M.; Gal, A. W. Chem. Eur. J.
2002, 8, 1310–1320.
(15) For examples, see: Blum, J.; Weitzberg, M. J. Organomet. Chem.
1976, 122, 261–264.
(29) Esswein, A. J.; Nocera, D. G. Chem. Rev. 2007, 107,
4022–4047.
(30) 1a reacts with NaOPh to yield Ir(ttp)OPh.4d It is proposed that
1a can also react with OH- to yield Ir(ttp)OH.
(31) OH- is reported as an efficient one-electron-reducing agent in
aprotic solvents. See: Sawyer, D. T.; Roberts, J. L., Jr. Acc. Chem. Res.
1988, 21, 469–476.
(32) First-row metalloporphyrins have been reported to be reduced
by OH-. as follows. (i) Fe(III) to Fe(II): (a) Shin, K.; Kramer, S. K.;
Goff, H. M. Inorg. Chem. 1987, 26, 4103–4106. (ii) Mn(III) to Mn(II):
(b) Jeon, S.; Lee, H. K.; Choi., Y. K. Bull. Kor. Chem. Soc. 1996,
17, 929–934.
(16) For examples, see: (a) Song, X.; Chan, K. S. Organometallics
2007, 26, 965–970. (b) Cheung, C. W.; Fung, H. S.; Lee, S. Y.; Qian,
Y. Y.; Chan, Y. W.; Chan, K. S. Organometallics 2010, 29, 1343–1354. (c)
Li, B. Z.; Song, X.; Fung, H. S.; Chan, K. S. Organometallics 2010,
29, 2001–2003.
(17) For reviews of the radical ipso substitution of aryl halides, see:
(a) Traynham, J. G. Chem. Rev. 1979, 79, 323–330. (b) Tiecco, M. Acc.
Chem. Res. 1980, 13, 51–57. (c) Tiecco, M. Pure Appl. Chem. 1981,
53, 239–258. For ipso substitution of ArX by carbon-centered
radicals, see: (d) Henriquez, R.; Nonhebel, D. C. Tetrahedron 1993,
49, 6497–6500. For ipso substitution of ArX by sulfur-centered
radicals, see: (e) Benati, L.; Camaggi, C. M.; Zanardi, G. J. Chem. Soc.,
Perkin I 1972, 2817–2819.
(18) No electrophilic aromatic substitution (SEAr) of benzene
solvent by 1a occurs in basic media to yield 2e. We reasoned that 1a
rapidly reacts with base to yield Ir(ttp)OH, which contains a poor
leaving hydroxo ligand and thus is not Lewis acidic enough to undergo
SEAr of benzene. Furthermore, Ir(ttp)OH undergoes rapid reduction to
[IrII(ttp)]2, which is not reactive toward benzene.
(19) Rhodium porphyrin aryls (Rh(por)Ar) can be prepared by the
reaction of Rh(por)Cl with a Grignard reagent (ArMgBr). See: Chan,
K. S.; Chiu, P. F.; Choi, K. S. Organometallics 2007, 26, 1117–1119.
(20) Metalloporphyrin aryls (M(por)Ar, M = Co, Rh) can be
prepared by the reaction of M(por)Cl with an aryllithium. See: (a)
Ogoshi, H.; Setsune, J.; Omura, T.; Yoshida, Z. J. Am. Chem. Soc. 1975,
97, 6461–6466. (b) Callot, H. J.; Cromer, R.; Louati, A.; Metz, B.;
Chevrier, B. J. Am. Chem. Soc. 1987, 109, 2946–2955.
(33) Roberts, J. L., Jr.; Sugimoto, H.; Barrette, W. C., Jr.; Sawyer,
D. T. J. Am. Chem. Soc. 1985, 107, 4556–4557.
(34) It is proposed that [IrII(ttp)]2 undergoes disproportionation with
water to give Ir(ttp)OH and Ir(ttp)H. The analogous disproportionation of
monomeric rhodium(II) porphyrins (RhII(por)) with D2O to give
Rh(por)D and Rh(por)OD and with MeOH to give Rh(por)OMe and
Rh(por)H have been reported; see: (a) Fu, X.; Li, S.; Wayland, B. B.
Inorg. Chem. 2006, 45, 9884–9889. (b) Sarkar, S.; Li, S.; Wayland, B. B.
J. Am. Chem. Soc. 2010, 132, 13569–13571.
(35) 1b and 1a were well-separated by TLC for identification.
(36) When the halogen atom abstraction of Ar-X occurs, the ratio
of M-X to M-Ar would be larger than 1, and Ar-Ar would be formed
due to the Ar• radical leakage.12 However, it does not occur in the Ar-Br
cleavage with [Ir(ttp)]2, since the ratio of Ir(ttp)Br to Ir(ttp)Ar is
consistently less than 1 (Table S4 in the Supporting Information), and
no Ar-Ar was detected via the dimerization of Ar• by GC-MS analysis in
the reaction of [Ir(ttp)]2 with ArBr (Ar = p-tBuC6H4) (eq 4).
(37) Benzene was shown to react with Br2 (as a source of Br•) at
200 °C in 90 min to form PhBr in 83% yield, likely via homolytic
aromatic substitution. For the mechanism of homolytic aromatic
substitution, see: Smith, M. B.; March, J. March’s Advanced Organic
Chemistry: Reactions, Mechanisms and Structure, 6th ed.; Wiley: New York,
2007; Chapter 14.
(38) The experiment has been done twice to ensure the formation of
a trace of PhBr (1%).
(21) The original reaction conditions (K2CO3, 200 °C) have been
adopted initially in sealed NMR tube experiments, but no intermediates
were observed. Thus, the stronger base Cs2CO3 at 150 °C was used to
achieve the successful observation of reaction intermediates.
(22) Within 2 h, [Ir(ttp)]2 was formed and gradually converted back
to Ir(ttp)H, presumably via the reaction of [Ir(ttp)]2 with H2 formed
(see the Supporting Information for details of the proposed intercon-
version between Ir(ttp)H and [Ir(ttp)]2/H2). The reaction of [Ir(oep)]2
(oep = octaethylporphyrinato dianion) with H2 to give Ir(oep)H has also
been reported.3a
(39) In the thermal rearrangements of arylmethylenecyclopropanes,
all the electron-donating and -withdrawing para substituents (p-FGs) on
the aryl rings promote the rearrangement rates by stabilizing the benzyl
radical intermediates via resonance, but the meta substituents (m-FGs)
generally do not promote the rates. See: (a) Creary, X. J. Org. Chem.
1980, 45, 280–284. (b) Creary, X.; Mehrsheikh-Mohammadi, M. E.;
McDonald, S. J. Org. Chem. 1987, 52, 3254–3263. The reactivity trends
brought by the p-FGs and m-FGs in the thermal rearrangements of
arylmethylenecyclopropanes are similar to those observed in base-
promoted Ar-Br cleavage by 1a (Table S5 and Figure S5 in the
Supporting Information), further suggesting the ISAE of Ar-Br by
[Ir(ttp)]2 to give Ir(ttp)Ar.
(23) Nucleophilic substitution (SN2) of CH3OH by Ir(ttp)- to give
Ir(ttp)CH3 and OH- has been reported.16b It is likely that SN2 of
CH3OC6H4(p-Br) by Ir(ttp)- to form Ir(ttp)CH3 and the better arylate
ion leaving group, ArO-, is even more favorable.
(24) Ir(ttp)Br (δ(pyrrole) (C6D6) ∼8.9 ppm) was observed by
1H NMR spectroscopy (Figure S4 in the Supporting Information)
but was too unstable to be purified by column chromatography.
Its formation could only be confirmed by HRMS analysis. The
variable chemical shifts of the broad pyrrole signals of Ir(ttp)Br are
1771
dx.doi.org/10.1021/om200027q |Organometallics 2011, 30, 1768–1771