Secondary Benzylation with Benzyl Alcohols
Catalyzed by A High-Valent Heterobimetallic
Ir-Sn Complex
SCHEME 1. Generating High-Valent Heterobimetallic
Motif
Susmita Podder, Joyanta Choudhury, and Sujit Roy*
Organometallics & Catalysis Laboratory, Department of
Chemistry, Indian Institute of Technology,
Kharagpur 721302, India
SCHEME 2. Preparation of Catalyst 1
ReceiVed December 7, 2006
features include (i) a high-valent and soft electrophilic transition
metal center (Tm) for the activation of soft nucleophiles such
as a π-system, (ii) a hard Lewis acidic main group metal center
(Mgm) for the activation of substrates having hard donor atoms,
and (iii) close proximity of Tm and Mgm centers for proximal
binding and subsequent coupling between different organic
substrates.
We present here a successful demonstration of the above
concept for the secondary benzylation of a variety of carbon
A highly efficient secondary benzylation procedure has been
demonstrated using a high-valent heterobimetallic complex
[Ir
2
(COD)
2
(SnCl
3
)
2
(Cl)
2
2
(µ-Cl) ] 1 as the catalyst in 1,2-
dichloroethane to afford the corresponding benzylated prod-
ucts in moderate to excellent yields. The reaction was
performed not only with carbon nucleophiles (arenes and
heteroarenes) but also with oxygen (alcohol), nitrogen (amide
and sulfonamide), and sulfur (thiol) nucleophiles. Mecha-
nistic investigation showed the intermediacy of the ether in
this reaction. An electrophilic mechanism is proposed from
Hammett correlation.
(
arenes and heteroarenes), oxygen (alcohol), nitrogen (amide
and sulfonamide), and sulfur (thiol) nucleophiles with secondary
benzyl alcohol derivatives employing a novel high-valent
iridium-tin heterobimetallic catalyst, namely, [Ir (COD) -
2
2
(SnCl3)2(Cl)2(µ-Cl)2] 1. In this regard, one may note that there
are only a few successful reports of secondary alkylation directly
from corresponding alcohols (having â-H atom) as the alkylating
6
agents.
Complex 1 was easily obtained by the reaction of [Ir(COD)-
(µ-Cl)]2 in dichloromethane with SnCl4 (2 equiv) in benzene at
Even after 125 years since its discovery, Friedel-Crafts
alkylation (FCA) remains a fundamental tool toward the
construction of various organic architectures of pharmaceutical
room temperature followed by slow crystallization (Scheme 2).
The structure of 1 has been unambiguously established by X-ray
crystal structure analysis (Figure 1).
Control studies on secondary benzylation were performed
using 1-phenylethanol 2 as the representative alcohol and
o-xylene as the arene in the presence of 1 mol % of catalyst 1
and in 1,2-dichloroethane as the solvent (Scheme 3). Each
1
and industrial relevance. Within the FCA domain, there has
been multi-prong development in the area of alkylation of arenes
and heteroarenes. Tuning a FCA catalyst to deliver high turnover
frequency (TOF), substrate and alkylating agent selectivity, and
environment friendliness has truly become a never-ending
(
hence, ever-young) exercise. Toward this pursuit, the resurgence/
(2) For overview, see: (a) Lewis Acids in Organic Synthesis; Yamamoto,
H., Ed.; Wiley-VCH: Weinheim, Germany, 2000; Vols. 1 and 2. (b)
Kobayashi, S. In Stimulating Concepts in Chemistry; V o¨ gtle, F., Stoddart,
J. F., Shibasaki, M., Eds.; Wiley-VCH: Weinheim, Germany, 2000; pp
evolution of d- and f-block metal catalysts (either simple salts
or designer complexes) is quite breathtaking. Our continuing
success in dual-reagent catalysis involving transition metal and
tin as the partners led us to propose a new bimetallic catalysis
2
3-12. (c) Corma, A.; Garcia, H. Chem. ReV. 2003, 103, 4307. (d) Bandini,
3
M.; Melloni, A.; Umani-Ronchi, A. Angew. Chem., Int. Ed. 2004, 43, 550.
(e) F u¨ rstner, A.; Voigtl a¨ nder, D.; Schrader, W.; Giebel, D.; Reetz, M. T.
Org. Lett. 2001, 3, 417. (f) Dyker, G.; Hildebrandt, D.; Liu, J.; Merz, K.
Angew. Chem., Int. Ed. 2003, 42, 4399. (g) Mertins, K.; Jovel, I.; Kischel,
J.; Zapf, A.; Beller, M. Angew. Chem., Int. Ed. 2005, 44, 238. (h) Iovel, I.;
Mertins, K.; Kischel, J.; Zapf, A.; Beller, M. Angew. Chem., Int. Ed. 2005,
4
concept for the alkylation of arenes. According to this proposal,
a high-valent bimetallic scaffold III could be generated by the
oxidative addition of a main group (possibly Si, Sn, In) halide
or surrogate II across a low-valent late transition metal
organometallic partner I (Scheme 1). Indeed the oxidative
addition reaction of Si-X and Sn-X across low-valent late
transition metal complexes to generate the corresponding silyl
metal or stanna metal motifs is well-known.5
4
4, 3913.
(3) (a) Sinha, P.; Roy, S. Organometallics 2004, 23, 67. (b) Banerjee,
M.; Roy, S. Org. Lett. 2004, 6, 2137. (c) Banerjee, M.; Roy, S. J. Mol.
Catal. A: Chem. 2006, 246, 231.
(4) For a preliminary report, please see: Choudhury, J.; Podder, S.; Roy,
S. J. Am. Chem. Soc. 2005, 127, 6162.
One may note that scaffold III bears interesting features for
potential application within a cooperative catalysis regime. These
(
5) (a) Corey, J. Y.; Braddock-Wilking, J. Chem. ReV. 1999, 99, 175.
b) Janzen, M. C.; Jennings, H. A.; Puddephatt, R. J. Organometallics 2001,
0, 4100. (c) Mark, S. H.; William, L. W.; John, H. N. Chem. ReV. 1989,
89, 11.
(6) (a) Yamauchi, T.; Hattori, K.; Mizutaki, S.; Tamaki, K.; Uemura, S.
(
2
(1) (a) Olah, G. A. Friedel Crafts and Related Reactions; Wiley-
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Magic Chemistry. Autobiographical Reflections of a Nobel Prize Winner;
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0.1021/jo0625094 CCC: $37.00 © 2007 American Chemical Society
Published on Web 03/17/2007
J. Org. Chem. 2007, 72, 3129-3132
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