Cooperative Friedel-Crafts catalysis in heterobimetallic regime: Alkylation of aromatics by π-activated alcohols

Article abstract of DOI:10.1021/ja0506004

The highly active Friedel-Crafts alkylation (FCA) catalyst, [Ir(COD)Cl(SnCl₃)(SnCl₄)(arene)]⁺Cl- (1-SnCl₄), is easily generated in one-pot from [Ir(COD)Cl]₂ or [Ir(COD)(μ-Cl)Cl(SnCl₃)]₂ (1) and SnCl₄. The reaction of arenes, heteroarenes with benzyl, and allyl alcohols is promoted by 1-SnCl₄ (1 mol %) with high turnover frequency. Kinetic evidence is presented to establish FCA pattern. From dual-catalyst combination studies varying the transition metal and main group metal partner, the efficiency of the present catalysts is attributed to the electrophilic "Ir^III-Sn^IV" core. Copyright

Full text of DOI:10.1021/ja0506004

Published on Web 04/06/2005
Cooperative Friedel-Crafts Catalysis in Heterobimetallic Regime: Alkylation
of Aromatics by π-Activated Alcohols
Joyanta Choudhury, Susmita Podder, and Sujit Roy*
Organometallics & Catalysis Laboratory, Chemistry Department, Indian Institute of Technology,
Kharagpur 721302, India
Received January 29, 2005; E-mail: sroy@chem.iitkgp.ernet.in
By virtue of its astonishing success with regard to versatility,
scope, and applicability, Friedel-Crafts alkylation reaction (here-
after FCA) continues to attract synthetic chemists to further enrich
a knowledge base of more than 125 years.1 The history of FCA
catalysts witnessed many landmarks in both homogeneous and
heterogeneous regimes. In the former case, transition metal halides
are long established as FCA catalysts. On the contrary, catalysts
bearing a high-valent Lewis acidic transition metal core, which are
Figure 1. ORTEP diagram of 1 with 50% probability thermal ellipsoids.
so well adapted for many organic reactions,2 remain very little
Table 1. FCA of Toluene with 4-Methylbenzyl Alcohola
explored toward FCA.3 In the course of our continuing effort to
exploit the organic reactivity of a reagent combination involving
transition metal and tin as partners,4 we became attracted to the
recent heightened interest in cooperative catalysis in heterobimetallic
regime.5 In practice, two design features exemplify such a regime.
The first type is an intramolecular version involving a single catalyst
in which two different metals are built on a single scaffold (M-
L-M′ or L-M-M′-L′). The second type is an intermolecular
version involving dual partners (M-L + M′-L′), both of which
participate in the transition state. Irrespective of their types,
heterobimetallic catalysts offer superior results in terms of efficiency
and selectivity relative to the individuals. Within an IrIII-SnIV
framework, we report here both type-I and apparent type-II catalysts,
which promote FCA of arene using π-activated 1, 2, and 3° alcohols
at very high efficiency.
The discrete heterobimetallic homogeneous type-I catalyst,
namely, [Ir2(COD)2(SnCl3)2Cl2(µ-Cl)2] 1, was easily synthesized
via the oxidative addition reaction of SnCl4 across [Ir(COD)Cl]2 2
in benzene-methylene chloride and slow crystallization. A more
efficient apparent type-II catalyst (1-SnCl4) was generated by
mixing 1 and SnCl4 in a 1:2 molar ratio at 90 °C. It can also be
generated in situ by simply mixing 2 and SnCl4 in a 1:4 molar
ratio at 90 °C. The crystal structure of 1 confirms a cis-addition of
Sn-Cl across iridium and retention of the halobridge as in the
iridium precursor (Figure 1). The presence of the discrete IrIII-
SnIV unit in 1 is noteworthy. In post-analysis, we reckon that the
high-valent “Ir-Sn” core might be responsible for the superior
electrophilicity and observed FCA reactivity in our case.
Olah, in a seminal paper, classified 127 FCA catalysts based on
their activity in benzylation reactions; SnCl4 is placed in a weak
group (very low TOF), while IrCl3 is placed in Very weak group
(<1% yield).6,7 We reasoned that a high-valent “Tm-Sn” core (Tm
is Ir here) is well poised to test the principle of cooperativity
originating from the interplay of π-acidity/weak π-basicity at the
Tm center, electrophilicity at the Sn center, and a metal-metal
bond. Would cooperativity impart high Lewis acidity in such a
coresan essential criterion for a successful FCA-catalyst? To test
this hypothesis, we selected the FCA of aromatics with benzyl
alcohol as the alkylating agent. This classical and industrially useful
FCA process has been studied with traditional FC catalysts,
time
TOF
yield
(%)b
1
entry
catalyst
(min)
(h-
)
1
2
3
4
5
6
7
8
9
SnCl4
IrCl3
IrCl3 + 4SnCl4
[Ir(COD)Cl]2 2
360
360
360
360
15
360
15
30
15
30
15
360
<0.2
<0.2
0.3
0
<1
<1
2
0
10
[Ir2(COD)2(SnCl3)2Cl2(µ-Cl)2] 1
40
7.5
324
190
380
192
8
45 (36)c
81 (75)
95 (84)
95 (87)
96 (89)
2
d
1-SnCl4
e
1-SnCl4
10
11
12f
Sc(OTf)3
5
30 (29)
a Conditions: toluene (18.8 mmol), alcohol (1 mmol), catalyst (0.01
mmol), T ) 90 °C. b Determined by GC, isolated yield in parentheses.
c Along with 26% p-tolualdehyde. d As benzene adduct. e Generated in situ
f
from 2. With 0.05 mmol Sc(OTf)3 as catalyst.
lanthanide/actinide triflates, H2PtCl6‚6H2O, modified clays, and
Nafion-H, at 10-120% loading and a temperature of 80-120 °C.7,8
Model study on the benzylation of toluene with 4-methylbenzyl
alcohol at 90 °C shows that as little as 1 mol % of catalyst 1 can
promote FCA, affording 45% of ditolylmethane (o/p 19/81) after
6 h (Table 1, entry 6). Under similar conditions, even 5 mol %
scandium triflate afforded only 30% of the alkylated product (entry
12). It is noteworthy that a mere combination of IrCl3 and SnCl4 is
ineffective (entry 3), suggesting the importance of the high-valent
“Ir-Sn” core. The apparent type-2 catalyst, 1-SnCl4, shows
markedly superior catalytic activity affording 81-95% of product
(o/p 17/83) just after 15 min without any byproduct (entries 7 and
9). The catalytic activity was undeterred even after six repetitive
cycles without any loss in TOF.
The above results prompted us further to test whether cooperative
activation of iridium can be done by other FCA catalysts.
Accordingly, benzylation of toluene was carried out with a dual-
catalyst combination of [Ir(COD)Cl]2 2 (1 mol %) and a FCA
catalyst (4 mol %), representing each of the four groups in Olah’s
seminal paper. Surprisingly, all of the combinations tested with
AlCl3, Sc(OTf)3, InCl3, TiCl4, and ZnCl2 failed highlighting the
importance of the “Ir-Sn” core (Supporting Information, Table S1).
9
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J. AM. CHEM. SOC. 2005, 127, 6162-6163
10.1021/ja0506004 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Competitive and Noncompetitive Benzylation of Toluene
and Benzene with Substituted Benzyl Alcohol 4-RC6H4-CH2OH
The FCA reactivity of 1 and 1-SnCl4 has been extended to
various arenes, heteroarenes, and π-activated 1°/2°/3° alcohols
(Supporting Information, Table S5). The distinct superiority of
“IrIII-SnIV” catalysts over homogeneous FCA catalysts is ascer-
tained by lower catalyst loading, reduced reaction time, low alcohol:
arene ratio, high conversion, and absence of byproduct(s). Spec-
troscopic and kinetic studies in line with the works of Olah,
Shibasaki, and others are in progress to understand the role of the
two cooperative metals.13
Competitive
Noncompetitive
kT
×
(s-
104
kB
×
(s-
104
1
1
entry
R
kT/kB
o:m:p
)
)
kT/kB
kR/kH
σp+
1
2
3
H
6.0 37:5:58
Me 16.8 33:0:67
Cl 5.6 51:0:49
21.1
86.1
7.4
3.2
4.2
1.6
6.6
1
0
20.5 1.3 -0.17
4.6 0.5 0.23
To our knowledge, this represents the first example of a
cooperative FCA catalysis concept in heterobimetallic regime.
Studies are underway to exploit the general applicability of the said
concept and extend it further to enantioselective FCA.
Scheme 1 a
Acknowledgment. S.R. dedicates this work to Prof. R. J.
Puddephatt, FRS. We thank CSIR, DST, and UGC for financial
support, Professors S. Chandrasekaran, S. Sengupta for stimulating
discussion, and Professors A. Basak, M. Bhattacharjee for help.
a (a) 2 equiv of SnCl4, CH2Cl2-C6H6, RT; (b) 2 equiv of SnCl4, ArH,
90 °C; (c) 4 equiv of SnCl4, ArH, 90 °C.
Supporting Information Available: Experimental procedures,
spectral and analytical data, tables, and figures. This material is available
free of charge via the Internet at http://pubs.acs.org.
An obvious question at this stage is which other “Tm-Sn” motif
can show promising FCA activity. Catalyst screening with a dual-
catalyst combination of SnCl4 (4 mol %) and a Tm partner (1 mol
%) shows that CoCl(PPh3)3, RuCl2(PPh3)3, and NiCl2(PPh3)2 are
inactive, while PdCl2(PPh3)2 and PtCl2(PPh3)2 are very weakly
active (up to 5% FCA product). Promising activity is shown by
RhCl(PPh3)3 and RhCl(CO)(PPh3)2, but TOFs are much lower
compared to those in “Ir-Sn” catalysts discussed earlier (Supporting
Information, Table S2).
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In the context of the present investigation, it must be emphasized
that although the organic reactivity of catalytic Ir(I) complexes is
so well established, that of catalytic Ir(III) is only beginning to
emerge.9,10 The latter includes LA-type catalysis, catalytic C-H
activation, and cycles involving Ir(III)/Ir(I) or Ir(III)/Ir(IV). Interest-
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pathway.10 In view of the above, we felt it appropriate to carry out
preliminary kinetic studies to evaluate whether the present reaction
follows FCA pattern.6a The data for competitive and noncompetitive
benzylation reactions of toluene and benzene with para-substituted
benzyl alcohols using in situ generated 1-SnCl4 as catalyst at 90
°C indeed show familiar FCA behavior (Table 2, and Supporting
Information, Tables S3 and S4). The noteworthy points are (1) that
the reactions give predominantly ortho-para-substitution, (2) that
kT/kB ratios are similar in competitive and noncompetitive runs,
(3) that competitive kT/kB ratio and isomer distribution are
independent of time within a span of 1-20 min, (4) that electron
donating Me substituent in the benzyl alcohol increases the kT/kB
ratio but lowers the ortho/para ratio,6a and (5) that the log(kR/kH)
+
values linearly correlate with Hammett σp constants.
The apparent type-II catalyst was isolated as a benzene adduct
(1-SnCl4‚PhH). The isolated adduct is highly moisture sensitive,
soluble in benzyl alcohol, behaves as a 1:1 electrolyte in solution,
and shows excellent FCA reactivity in the present benzylation
reaction. From elemental analyses, conductivity measurement, FT-
IR, Raman, TGA, NMR studies of the adduct, we conclude that
the type-II catalyst (1-SnCl4) is [Ir(COD)(SnCl3)Cl(SnCl4)-
(arene)]+Cl- (Scheme 1 and Supporting Information).11 This
suggestion is further supported by an early work of Meyer on the
existence of an “Ir-SnCl4” donor-adduct isomer in [Ir(SnCl4)Cl-
(CO)(PPh3)2].12 Attempts to grow crystals of 1-SnCl4‚ArH for
structure determination are underway.
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(13) See Supporting Information for preliminary results.
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