Radical carboxylation: Ester synthesis from alkyl iodides, carbon monoxide, and alcohols under irradiation conditions [11]

Full text of DOI:10.1021/ja964124g

J. Am. Chem. Soc. 1997, 119, 5465-5466
5465
Table 1. Ester Synthesis via Catalyst-Free Radical Carboxylation
Radical Carboxylation: Ester Synthesis from Alkyl
Iodides, Carbon Monoxide, and Alcohols under
Irradiation Conditions
of Alkyl Iodides under Irradiation Conditionsa
Kiyoto Nagahara, Ilhyong Ryu,* Mitsuo Komatsu, and
Noboru Sonoda*
Department of Applied Chemistry
Faculty of Engineering, Osaka UniVersity
Suita, Osaka 565, Japan
ReceiVed December 2, 1996
There is little doubt that transition metal-catalyzed carbony-
lation of organic halides with CO to give carboxylic acids and
their derivatives is a basic and important synthetic process (eq
1).1,2 It is well-known that the Monsanto Chemical Company
produces acetic acid from methanol and CO and that a key step
in this process involves the Rh-catalyzed carbonylation of
methyl iodide to give acetyl iodide.3 In this paper, however,
we report a conceptually new method for the synthesis of
carboxylic acid esters from the combination of alkyl iodides,
alcohols, and CO that can be carried out with photoirradiation
in the absence of a metal catalyst (eq 2).4 In our previous paper,
we reported the synthesis of acyl selenides by the photolysis of
R-(phenylseleno)carbonyl compounds in the presence of an
alkene and CO.5 Direct substitution of acyl radical on selenium
atom to liberate R-acyl alkyl radical is the key step in the
process. Unlike this case, transfer of iodine atom from alkyl
iodide to acyl radical is regarded as inefficient, since the
transformation is obviously endothermic. However, we thought
that the iodine transfer reaction could be driven by a following
ionic reaction of the acyl iodide with alcohol to furnish the ester.6
We applied successfully this key concept for the present new
carbonylation system, which we refer to as catalyst-free radical
carboxylation.
a Unless otherwise noted, the following reaction conditions were
used: alkyl iodide (1 mmol), hexane (0.5 mL), K2CO3 (2 mmol), alcohol
(1.4-3 equiv), CO (20-55 atm), under Xe irradiation (Pyrex) 12-50
h. b Isolated yield after flash chromatography on silica gel. Values in
parentheses represent the NMR yield. c EtOH was used as a solvent.
d KOH (2 mmol) was used in place of K2CO3. e 91% conversion.
f Carried out on a 2 mmol scale. g 74% conversion. h The alcoholysis
product was also obtained in 14% yield.
When a hexane solution of 2-iodooctane (1a) and ethanol (3
equiv) was irradiated with a xenon lamp through a pyrex glass
tube under CO pressure (20 atm, 15 h), 1a was largely recovered
unchanged (89%) and no carbonylated product was obtained.
However, the simple addition of a base to this reaction system
resulted in a dramatic change. Thus, when the same reaction
was carried out in the presence of anhydrous K2CO3 (2 equiv),
the desired ethyl ester 3a was obtained in 72% yield, after
isolation by flash chromatography on silica gel (eq 3). Octenes
were formed as byproducts in this reaction (1-octene, 10%;
2-octene, 6% (trans/cis ) 73:27)).7 Lower CO pressures (<10
atm) and lower substrate concentrations ([RI] < 1 M) increased
these side reaction products.8 The use of potassium hydroxide
and sodium hydroxide was equally effective.
(1) Carbonylation: Direct Synthesis of Carbonyl Compounds; Colqu-
houn, H. M., Thompson, D. J., Twigg, M. V., Eds.; Plenum Press: New
York, 1991.
(2) For recent examples of ester synthesis by transition metal-catalyzed
carbonylation of alkyl iodides, see: (Pd) (a) Urata, H.; Maekawa, H.;
Takahashi, S.; Fuchikami, T. J. Org. Chem. 1991, 56, 4320 and references
cited therein. (Pt) (b) Takeuchi, R.; Tsuji, Y.; Fujita, M.; Kondo, T.;
Watanabe, Y. J. Org. Chem. 1989, 54, 1831. (Pt and hν) (c) Kondo, T.;
Sone, Y.; Tsuji, Y.; Watanabe, Y. J. Organomet. Chem. 1994, 473, 163.
(Pd-Rh) (d) Buchan, C.; Hamel, N.; Woell, J. B.; Alper, H. J. Chem. Soc.,
Chem. Commun. 1986, 167. (e) Woell, J. B.; Fergusson, S. B.; Alper, H. J.
Org. Chem. 1985, 50, 2134. Also see a pertinent review covering earlier
work: (f) Cassar, L.; Chiusoli, G. P.; Guerrieri, F. Synthesis 1973, 509.
(3) It should be noted that for the slow oxidative addition of alkyl iodides
and isomerization problem, an extension of Monsanto’s acetic acid synthesis
to higher alcohols appears less feasible, see: (a) Dekleva, T. W.; Forster,
D. J. Am. Chem. Soc. 1985, 107, 3565. (b) Dekleva, T. W.; Forster, D. J.
Am. Chem. Soc. 1985, 107, 3568.
(4) For cationic carbonylation, see: Bagno, A.; Bukala, J.; Olah, G. A.
J. Org. Chem. 1990, 55, 4284.
(5) Ryu, I.; Muraoka, H.; Kambe, N.; Komatsu, M.; Sonoda, N. J. Org.
Chem. 1996, 61, 6396.
(6) For an example of ionic trap by an internal carbonyl group, see:
Tsunoi, S.; Ryu, I.; Yamasaki, S.; Tanaka, M.; Komatsu, M.; Sonoda, N.
J. Am. Chem. Soc. 1996, 118, 10670.
Various alkyl iodides and alcohols were tested and the
reaction was found to be general (Table 1). Thus, alkyl iodides
S0002-7863(96)04124-8 CCC: $14.00 © 1997 American Chemical Society

5466 J. Am. Chem. Soc., Vol. 119, No. 23, 1997
Communications to the Editor
Scheme 1
Judging from the fact that the irradiation of 5-iodononane (1b)
in the presence of CO (78 atm) did not give appreciable amounts
of acyl iodide, an esterification process by an alcohol and a
base to shift the equilibrium may be critical for this reaction to
occur. A relatively slow reaction for the case of a primary alkyl
iodide (entry 8) may be the result of its slower atom transfer
ability, compared with secondary and tertiary iodides.11
Ester synthesis from secondary alkyl iodides by transition
metal-catalyzed carbonylation often results in the formation of
positional isomers associated with â-elimination of the key metal
alkyl species,2a and for similar reasons, the carbonylation of
tertiary iodides is thought to be attainable only with great
difficulty. The present ester synthesis, however, takes place
selectively at the carbon to which the iodine had been attached,
and thus, a wide range of aliphatic iodides including tertiary
iodide can be used. The catalyst-free process is clean, simple,
and environmentally friendly, since the recovery of neither
precious transition metals nor toxic ligands is necessary.
Thus, we have shown that an efficient conversion of aliphatic
iodides to the corresponding esters is possible by simply using
irradiation in the visible region without the use of a metal
catalyst. A free radical mechanism for this reaction is highly
likely, but gratifyingly, the process requires neither radical
mediator nor photosensitizer. Importantly, the basic principle
of photoinduced catalyst-free carboxylation demonstrated here
may have promise in terms for a wide range of other carbonyl
compounds.
1 were converted into the corresponding esters 3 in good yields.
Nearly stoichiometric amounts of alcohols suffice for this ester
synthesis. An ω-chloro substituent on an alcohol and a
γ-phenylthio group on an alkyl iodide can also tolerate this
carbonylation (entries 6 and 7). 1-Iodoadamantane (1g) also
works well for this reaction. In this case to avoid direct SN1
type alcoholysis of the starting iodide, a nearly equimolar
amount of alcohol should be used.
The mechanism of this catalyst-free radical carboxylation is
outlined in Scheme 1.9 In the initiation step, the C-I bond of
an alkyl iodide would be cleaved homolytically by irradiation.10
(7) For photoirradiation-promoted elimination of HI from alkyl iodides,
see: (a) Kropp, P. J. Acc. Chem. Res. 1984, 17, 131. (b) Kropp, P. J.;
Poindexter, G. S.; Pienta, N. J.; Hamilton, D. C. J. Am. Chem. Soc. 1976,
98, 8135.
Supporting Information Available: Typical experimental proce-
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(8) Dilution may disfavor the intermolecular atom transfer step.
(9) For reviews of radical carbonylations, see: (a) Ryu, I.; Sonoda, N.
Angew. Chem., Int. Ed. Engl. 1996, 35, 1050. (b) Ryu, I.; Sonoda, N.;
Curran, D. P. Chem. ReV. 1996, 96, 177.
JA964124G
(11) Newcomb, M.; Sanchez, R. M.; Kaplan, J. J. Am. Chem. Soc. 1987,
109, 1195.
(10) UV irradiation gave inferior results.