Radical aromatic substitutions on (η6-chloroarene)tricarbonylchromium complexes

Article abstract of DOI:10.1021/om020466l

The radical aromatic substitutions on chloroarene-Cr(CO)₃ complexes were performed by reaction with SmI₂ and ketones or aldehydes. The methyl substitution on the phenyl ring shows the meta-directing effect on the radical addition. Th

Full text of DOI:10.1021/om020466l

3848
Organometallics 2002, 21, 3848-3849
Ra d ica l Ar om a tic Su bstitu tion s on
(η6-Ch lor oa r en e)tr ica r bon ylch r om iu m Com p lexes
Hechun Lin,‡ Li Yang,‡ and Chaozhong Li*,†
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Road,
Shanghai 200032, People’s Republic of China, and State Key Laboratory of Applied Chemistry,
Lanzhou University, Gansu, Lanzhou 730000, People’s Republic of China
Received J une 12, 2002
Summary: Successful radical aromatic substitutions on
chloroarene-Cr(CO)3 complexes were carried out by
reaction with SmI2 and a carbonyl compound in THF/
HMPA at -30 to -40 °C to afford the corresponding
products in 66-89% yield. Deuterated experiments
revealed that the mechanism was preferably meta-
tele-substitution.
(CO)3 (4a ), while more than 90% of the starting material
3 was recovered. However, when we treated chloroben-
zene-Cr(CO)3 (5) with SmI2 (4 equiv) and acetone (2
equiv) in THF/HMPA with t-BuOH (2 equiv) as the
proton source at 0 °C for 4 h, 63% of the substitution
product 4a was obtained. When the reaction was carried
out at lower temperature (-30 to -40 °C) for 4 h, 4a
was achieved in 78% yield along with 14% of the
dechlorination product benzene-Cr(CO)3 and 7% of 5
was recovered (eq 1). Other ketones and even aldehydes
underwent similar reactions, and the results are listed
in Table 1.
Nucleophilic aromatic substitutions on halogenated
arene metal complexes are one of the unique charac-
teristics of organometallic chemistry and have attracted
considerable interest in organic synthesis.1 However,
successful free radical substitutions on haloarene metal
complexes have never been reported, although attempts
to carry out such transformations could be traced back
to the 1950s.2 Recently, a few examples of intramolecu-
lar radical substitutions on (η6-methoxybenzene)-Cr-
(CO)3 derivatives were reported by Schmalz et al. which
led to the demethoxylated products (e.g., 1 and 2).3
However, the general picture of the intermolecular
radical substitution chemistry remains obscure. We
report here the first examples of radical substitutions
on chloroarene-Cr(CO)3 complexes by reaction with
SmI2 and ketones or aldehydes.4
To gain more information on the radical substitution
pattern, we chose o-, m-, and p-chlorotoluene-Cr(CO)3
complexes to subject to the reaction with SmI2/acetone,
and the results are presented in Table 2. The reaction
of m-chlorotoluene-Cr(CO)3 complex 6 with Me2CO/
SmI2 afforded 7 in 74% yield as the only product, while
o-chlorotoluene-Cr(CO)3 (8) yielded two substitution
products, 7 (32%) and 9 (43%). For p-chlorotoluene-Cr-
(CO)3 (10), the reaction was slightly slower, and only
36% yield of substitution products were isolated within
4 h while 53% of the starting 10 was recovered.
However, when the reaction was carried out for a longer
period of time (12 h), the substitution products 11 (18%)
and 7 (55%) were achieved in satisfactory overall yield
(73%).
Reaction of anisole-Cr(CO)3 complex (3) with SmI2
and acetone at various reaction conditions yielded no
product or only a trace amount of PhC(OH)Me2-Cr-
As can be seen in Table 2, complexes 8 and 10 gave
both cine- and tele-substitution products, while no ipso-
substitution products could be detected. This regiose-
lectivity is different from that in ordinary homolytic
aromatic substitutions5 or that in nucleophilic aromatic
substitutions on chloroarene-Cr(CO)3 complexes,1 in
which ipso-substitutions are observed in many cases.
The results prompted us to check the reactions of 5 and
6 that appear to proceed via the ipso-substitution
process. Thus, p-D-chlorobenzene-Cr(CO)3 complex (12)6
* Corresponding author. E-mail: clig@pub.sioc.ac.cn.
† Shanghai Institute of Organic Chemistry.
‡ State Key Laboratory of Applied Chemistry.
(1) (a) Semmelhack, M. F. In Comprehensive Organic Synthesis;
Trost, B. M., Fleming, I., Eds.; Pergamon: New York, 1991; Vol. 4, p
517. (b) Semmelhack, M. F. In Comprehensive Organometallic Chem-
istry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon:
New York, 1995; Vol. 12, p 979. (c) Pape, A. R.; Kaliappan, K. P.;
Kundig, E. P. Chem. Rev. 2000, 100, 2917.
(2) Nicholls, B.; Whiting, M. C. J . Chem. Soc. 1959, 551.
(3) (a) Schmalz, H.-G.; Siegel, S.; Bats, J . W. Angew. Chem., Int.
Ed. Engl. 1995, 34, 2383. (b) Schmalz, H.-G.; Siegel, S.; Schwarz, A.
Tetrahedron Lett. 1996, 37, 2947. (c) Hoffmann, O.; Schmalz, H.-G.
Synlett 1998, 1426. (d) Schwarz, O.; Brun, R.; Bats, J . W.; Schmalz,
H.-G. Tetrahedron Lett. 2002, 43, 1009.
(5) Studer, A.; Bossart, M. In Radicals in Organic Synthesis;
Renaud, P., Sibi, M. P., Eds.; Wiley-VCH: New York, 2001; Vol. 2, p
62.
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1980, 102, 1441.
(4) For reviews on SmI2-mediated radical reactions, see: (a) Molan-
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10.1021/om020466l CCC: $22.00 © 2002 American Chemical Society
Publication on Web 08/22/2002

Communications
Organometallics, Vol. 21, No. 19, 2002 3849
Ta ble 1. Rea ction s of Ch lor oben zen e-Cr (CO)3
Sch em e 1. P r op osed Mech a n ism for
Com p lex 5 w ith Sm I2/R1R2CO
m eta -tele-Su bstitu tion
all the chlorotoluene cases also implies the strong
directing effect of the methyl group on radical addition
at the meta-position.
a
Reaction conditions: SmI2 (4 equiv), a carbonyl compound (2
On the basis of the above results, a plausible mech-
anism could be drawn for the aromatic substitution
reactions as outlined in Scheme 1. The probably revers-
ible radical addition to the arene ring generates the 17-
VE intermediate 14, which is then reduced by SmI2 to
give the 18-VE intermediate 15.3,7,8 Acidification of 15
to 16 followed by rearrangement to 17 and subsequent
elimination of HCl affords the product 4a , which
resembles the mechanism of nucleophilic aromatic
substitutions.9 The cine-substitution might follow a
similar mechanism.
equiv), t-BuOH (2 equiv) in THF/HMPA, -30 to -40 °C, 4 h.
b
Isolated yield based on 5. Dechlorination products were also
formed in 5-15% yield along with the starting complexes recovered
(6-15%).
Ta ble 2. Rea ction s of Ch lor oa r en e-Cr (CO)3
Com p lexes w ith Sm I2/Me2CO
In conclusion, we demonstrate that chloroarenetri-
carbonylchromium complexes readily undergo radical
aromatic substitution reactions with SmI2 and an al-
dehyde or ketone. The reactions exhibit a unique regi-
oselectivity in that meta-tele-substitution is strongly
preferred, while no ipso-substitution can be observed.
The methyl substitution on the phenyl ring shows the
meta-directing effect on the radical addition. Detailed
investigations of the above radical substitution mech-
anism and its implications in organic synthesis are
currently in progress.
Ack n ow led gm en t. This project is supported by the
Hundreds of Talent program of the Chinese Academy
of Sciences, by the National Natural Science Foundation
of China, and by the QiMingXing program of the
Shanghai Municipal Scientific Committee.
Su p p or tin g In for m a tion Ava ila ble: Typical experimen-
tal procedure for the synthesis of 4, 7, 9, and 11, characteriza-
tion of 4, 7, 9, 11, and 13, and X-ray crystal structures of 4a ,
7, 9, and 11. This material is available free of charge via the
Internet at http://pubs.acs.org.
a,b
See Table 1. c Reaction time: 12 h.
was subjected to the reaction with SmI2/Me2CO under
the same experimental conditions above and o-D-PhC-
(OH)Me2-Cr(CO)3 (13) was achieved in 77% yield as
the sole product, demonstrating that the substitution
on 5 occurred at the meta-position exclusively. This
result, along with the formations of 7 (from 6), 9, and
11, indicates that meta-tele-substitution is the preferred
mode in the radical aromatic substitution reactions of
chloroarene-Cr(CO)3 complexes. The formation of 7 in
OM020466L
(7) Merlic, C. A.; Miller, M. M.; Hietbrink, B. N.; Houk, K. N. J .
Am. Chem. Soc. 2001, 123, 4904.
(8) Lin, H.; Zhang, H.; Yang, L.; Li, C. Org. Lett. 2002, 4, 823.
(9) For examples, see: (a) Rose-Munch, F.; Rose, E.; Semra, A. J .
Chem. Soc., Chem. Commun. 1987, 942. (b) Rose-Munch, F.; Rose, E.;
Semra, A.; J eannin, Y.; Robert, F. J . Organomet. Chem. 1988, 353,
53. (c) Rose-Munch, F.; Rose, E.; Semra, A.; Mignon, L.; Garcia-Oricain,
J .; Knobler, C. J . Organomet. Chem. 1989, 363, 297.