Synthetic and theoretical studies on group-transfer imidoylation of organotellurium compounds. Remarkable reactivity of isonitriles in comparison with carbon monoxide in radical-mediated reactions

Article abstract of DOI:10.1021/ja003879r

Imidoylation of organotellurium compounds with isonitriles has been investigated in conjunction with the radical-mediated Cl homologation reaction by using CO and isonitriles. Carbon-centered radicals generated photochemically or thermally from organotellurium compounds react with isonitriles in a group-transfer manner to give the corresponding imidoylated products. Organotellurium compounds have been found to serve as effective precursors of a wide variety of stabilized radicals, namely benzyl, α-alkoxy, α-amino, and acyl radicals, which take part in the imidoylation with high efficiency. The reactions are compatible with various functional groups, and can be carried out in various solvents including environmentally benign water. The reactivity of isonitriles has been compared with that of CO through competition experiments, and the results indicate that isonitriles are superior to CO as radical acceptors in reactions with stabilized radicals. The origin of the differences has been addressed in theoretical studies with density functional theory calculations using the B3LYP hybrid functional. The calculations suggest that both carbonylation and imidoylation proceed with low activation energies, and that there are virtually no differences in the kinetic sense. Instead, it indicates that thermodynamic effects, namely differences in the stability of the acyl and the imidoyl radicals, control the overall course of the reactions.

Full text of DOI:10.1021/ja003879r

J. Am. Chem. Soc. 2001, 123, 3697-3705
3697
Synthetic and Theoretical Studies on Group-Transfer Imidoylation of
Organotellurium Compounds. Remarkable Reactivity of Isonitriles in
Comparison with Carbon Monoxide in Radical-Mediated Reactions
Shigeru Yamago,* Hiroshi Miyazoe, Ryuta Goto, Masahiro Hashidume,
Takashi Sawazaki, and Jun-ichi Yoshida*
Contribution from the Department of Synthetic Chemistry and Biological Chemistry, Graduate School of
Engineering, Kyoto UniVersity, Sakyo-ku, Kyoto 606-8501, Japan
ReceiVed NoVember 6, 2000
Abstract: Imidoylation of organotellurium compounds with isonitriles has been investigated in conjunction
with the radical-mediated C1 homologation reaction by using CO and isonitriles. Carbon-centered radicals
generated photochemically or thermally from organotellurium compounds react with isonitriles in a group-
transfer manner to give the corresponding imidoylated products. Organotellurium compounds have been found
to serve as effective precursors of a wide variety of stabilized radicals, namely benzyl, R-alkoxy, R-amino,
and acyl radicals, which take part in the imidoylation with high efficiency. The reactions are compatible with
various functional groups, and can be carried out in various solvents including environmentally benign water.
The reactivity of isonitriles has been compared with that of CO through competition experiments, and the
results indicate that isonitriles are superior to CO as radical acceptors in reactions with stabilized radicals. The
origin of the differences has been addressed in theoretical studies with density functional theory calculations
using the B3LYP hybrid functional. The calculations suggest that both carbonylation and imidoylation proceed
with low activation energies, and that there are virtually no differences in the kinetic sense. Instead, it indicates
that thermodynamic effects, namely differences in the stability of the acyl and the imidoyl radicals, control the
overall course of the reactions.
1. Introduction
1).5 This process is well-known to be reversible, and the success
Carbonylations and imidoylations have found enormous
application in organic synthesis.1,2 Many industrially important
processes have been developed as exemplified in Monsanto’s
acetic acid synthesis, and the reactions have been applied to
the synthesis of a variety of organic molecules, including
polymers and biologically active compounds. While the reaction
has been predominantly catalyzed or mediated by transition
metals, the radical-mediated carbonylation has been recognized
as an important synthetic transformation since its rediscovery
by Ryu and Sonoda in 1989.3,4
of the carbonylation relies on the increase of the equilibrium
constant to the acyl radical by applying high CO pressure. The
kinetics of the reverse reaction, namely decarbonylation, has
been extensively studied, and the rate constants depend strongly
on the nature of the substituent R.3,6 These studies revealed that
the rate of the decarbonylation becomes faster as the decarbo-
nylated radical R becomes more stable. Therefore, it is not
The success of the radical-mediated carbonylation depends
on the addition of free radicals to generate acyl radicals (eq
(4) (a) Ryu, I.; Sonoda, N. Angew. Chem., Int. Ed. Engl. 1996, 35, 1050.
(b) Ryu, I.; Kusano, K.; Ogawa, A.; Kambe, N.; Sonoda, N. J. Am. Chem.
Soc. 1990, 112, 1295. (c) Ryu, I.; Yamazaki, H.; Ogawa, A.; Kambe, N.;
Sonoda, N. J. Am. Chem. Soc. 1993, 115, 1187. (d) Ryu, I.; Muraoka, H.;
Kambe, N.; Komatsu, M.; Sonoda, N. J. Org. Chem. 1996, 61, 6369. (e)
Nagahara, K.; Ryu, I.; Komatsu, M.; Sonoda, N. J. Am. Chem. Soc. 1997,
119, 5465. (f) Ryu, I.; Okuda, T.; Nagahara, K.; Kambe, N.; Sonoda, N. J.
Org. Chem. 1997, 62, 7550. (g) Ryu, I.; Nagahara, K.; Kurihara, A.;
Komatsu, M.; Sonoda, N. J. Organomet. Chem. 1997, 548, 105. (h) Tsunoi,
S.; Ryu, I.; Yamasaki, S.; Tanaka, M.; Sonoda, N.; Komatsu, M. Chem.
Commun. 1997, 1889. (i) Nagahara, K.; Ryu, I.; Yamazaki, H.; Kambe,
N.; Komatsu, M.; Sonoda, N.; Baba, A. Tetrahedron 1997, 53, 14615. (j)
Ryu, I.; Matsu, K.; Minakata, S.; Komatsu, M. J. Am. Chem. Soc. 1998,
120, 5838. (k) Curran, D. P.; Sisko, J.; Balog, A.; Sonoda, N.; Nagahara,
K.; Ryu, I. J. Chem. Soc., Perkin Trans. 1 1998, 1591. (l) Ryu, I.; Ogura,
S.; Minakata, S.; Komatsu, M. Tetrahedron Lett. 1999, 40, 1515. (m) Ryu,
I.; Kuriyama, H.; Minakata, S.; Komatsu, M.; Yoon, J.-Y.; Kim, S. J. Am.
Chem. Soc. 1999, 121, 12910. (n) Ryu, I.; Kuriyama, H.; Minakata, S.;
Komatsu, M.; Yoon, J.-Y.; Kim, S. J. Am. Chem. Soc. 1999, 121, 12190.
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(1) (a) Colquhoun, H. M.; Thompson, D. J.; Twigg, M. V. Carbonylation;
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1991; Vol. 3, pp 1015-1043. (c) McQuillin, F. J.; Parker, D. G.; Stephenson,
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10.1021/ja003879r CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/31/2001

3698 J. Am. Chem. Soc., Vol. 123, No. 16, 2001
Yamago et al.
surprising that the reported carbonylations have been limited
to radicals which do not possess radical stabilizing groups, e.g.,
simple aryl and alkyl radicals. Thus, the difficulty in trapping
CO by stabilized radicals, e.g., R-phenyl, R-carbonyl, R-alkoxy,
and R-amino radicals, is one of the major synthetic drawbacks.
Moreover, while the addition of simple alkyl radicals to CO is
exothermic, the atom-transfer carbonylation of alkyl iodides has
been suggested to be endothermic.4f Therefore, further elucida-
tion of the mechanism of each elemental step is highly desirable.
Although a detailed reaction mechanism of the addition of
carbon-centered radicals to alkenes has been demonstrated by
using theoretical investigations,7 no similar study for carbony-
lations has been done.8
To clarify the utility of isonitriles as a C1 unit in radical-
mediated reactions, we undertook a detailed study of the group
transfer imidoylation from both synthetic and mechanistic
viewpoints. Because of their availability and the parallel
reactivity of chalcogen group transfer reactions with halogen
atom transfer reactions,13 we selected organotellurium com-
pounds as precursors for carbon-centered radicals.14,15 In this
work, we found the first example of the group-transfer imidoy-
lation of organotellurium compounds with isonitriles, as shown
in eq 3. Preliminary results have already been reported.16 We
Isonitriles are isoelectronic with CO and thus have sometimes
been used as a substitute for CO, especially in transition-metal-
catalyzed reactions. It is also known that the addition of free
radicals to isonitriles generates imidoyl radicals (eq 2), a process
now report the full details of this study, together with experi-
mental and theoretical studies, to elucidate the origin of the
reactivity difference between isonitriles and CO. The current
results clearly show that isonitriles are not simple substitutes
for CO but instead valuable supplements and complements in
the radical-mediated C1 homologation reactions.
equivalent to the acyl radical formation with CO.2a,9 The imidoyl
radicals thus formed are known empirically to be stable toward
the reverse reaction. Instead, they undergo elimination of the
R′ radical to form nitriles or addition to C-C unsaturated bonds.
These types of reactions have been applied to the synthesis of
a variety of imidoylated products and nitrogen-containing
organic compounds.10 While the features and benefits of atom
and group transfer reactions with C-C multiple bonds are well
recognized,3b only one example has been reported thus far on
the atom transfer imidoylation: the addition of a perfluoroalkyl
iodide to isonitriles.11 Therefore, the synthetic scope, as well
as the detailed mechanism of this type of reaction, remains
unknown.12
2. Results and Discussion
2-1. Imidoylation of Organotellurium Compounds. De-
tailed reaction conditions were examined by using tellurogly-
coside 1a (P ) Ac), N-phthalimide derivative 3, and acyl
telluride 5 as model compounds. We first examined the effects
of light, because irradiation is known to accelerate the generation
of radicals from organotellurium compounds.14h,g Thus, when
a solution of 1a and 2,6-xylylisonitrile (2.0 equiv) was heated
at 100 °C under UV lamp irradiation (4.5 W × 8) for 10 h, the
desired imidoylated product 2a was formed in 65% yield (Table
1, entry 1). The use of an intense light (250 W Hg lamp)
considerably shortened the reaction time, and the desired product
was obtained in 74% yield after 4 h of irradiation (Table 1,
entry 2). Although the reaction took place even in the dark, a
higher temperature (140 °C) and longer reaction time were
required. In this case, the yield of 2a was lower, probably due
to the instability of the product under the reaction conditions
(Table 1, entry 3). The same trend was also observed for 3,
which gave the imidoylated product 4 in good to excellent yield
under Hg lamp irradiation. For the imidoylation of 5, however,
the better result was obtained in the dark rather than under
irradiation. Since the consumption of 5 was accelerated by UV
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Group-Transfer Imidoylation of Organotellurium Compounds
J. Am. Chem. Soc., Vol. 123, No. 16, 2001 3699
Table 1. Imidoylation under Various Conditionsa
mechanism. It is also worth noting that the negligible solvent
effect indicates that consideration of the reaction medium is
not necessary in theoretical studies of the present reaction.
Finally, we briefly investigated the substituent effect of
isonitriles, and found that N-aryl-substituted isonitriles are more
reactive than N-alkyl-substituted ones; alkyl-substituted isoni-
triles, e.g., n-octyl- and cyclo-hexylisonitrile, gave the desired
products in only 5-10% yield under identical conditions.
Therefore, in the following sections, we routinely used 2 equiv
of an aromatic isonitrile, usually 2,6-xylylisonitrile, but the
reaction was also successful when a slight excess of the isonitrile
was used, although a longer reaction time was required (Table
1, entry 7).
2-2. Synthetic Scope of the Imidoylation. The scope of
the current imidoylation was investigated by using various
organotellurium compounds with 2,6-xylylisonitrile, and the
results are summarized in Table 2. We focused on the reaction
of organotellurium compounds that are expected to generate
stabilized radicals, because, as far as we know, there has been
no report on the carbonylation of such radicals. We were pleased
to find that a variety of stabilized carbon-centered radicals,
namely benzyl, R-amino, R-alkoxy, R-carboalkoxy, and acyl
radicals, generated from the corresponding organotellurium
compounds are effectively trapped by the isonitrile (Table 2,
entries 1-21). The successful imidoylation of R-carbomethoxy
and acyl radicals is worth mentioning, because the corresponding
carbonylation does not take place even under drastic condi-
tions.20 It is also worth noting that the reaction proceeds with
complete stereoselectivity. A single isomer, namely the syn
isomer with respect to the imidoyl telluride moiety, was formed
in all cases (see Supporting Information for the stereochemical
assignment). The observed stereoselectivity indicates that the
anti-imidoyl radical plays a significant role in the current
reaction.
temp time
yield
(%)
65 (95)c
74
43
85
91
90
91
67
85
84
86
80
73
74
41
87
entry substrate solvent
hνb (°C) (h) product
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
1a
1a
1a
3
3
3
3
3
3
3
3
3
3
5
5
5
5
5
C6D6
C6H6
C6D6
C6H6
C6H6
C6H6
C6H6
hexane
(CH2Cl)2
THF
DMF
pyridine
CH3OH
C6D6
C6D6
C6D6
hexane
CD3CN
EtOH
H2O
A
B
100
100
10
4
50
50
2
3
6
3
3
3
3
2
2
2
4
4
4
4
4
4
4
4
4
4
6
6
6
6
6
6
6
dark 140
A
B
C
C
C
C
C
C
C
C
A
B
60
80
80
80
80
80
80
80
80
3
3
80
100
100
12
12
12
12
12
12
12
dark 100
dark 100
dark 100
dark 100
dark 100
83
73
80
87
5
5
a Two equivalents of xylylisonitrile were used except in entry 7,
where 1.2 equiv of the isonitrile was used. b A: Rayonet RMR600
reactor equipped with a RMR-3500 Å lamp (4.5 W x 8) was used. B:
USHIO Optical Modulex reactor equipped with a 250 W Hg lamp was
used. C: a 100 W Hg lamp was used. c Yield based on the converted
substrate.
irradiation, the observed result was presumably due to the
instability of the product under UV irradiation (Table 1, entries
14-16).
We also investigated the imidoylation of nonstabilized radicals
and found that the progress of the reaction was slow and required
a longer reaction time. Addition of radical initiators, however,
slightly increased the efficiency of the reaction. The reaction
in the presence of 10 mol % of 1,1′-azobis(cyclohexane-1-
carbonitrile) (V-40) resulted in the clean formation of the desired
products in moderate to good yields (Table 2, entries 12-14).
The results obtained above are consistent with the well-known
radical chain mechanism of the group-transfer reaction. As
shown in Scheme 1, the radical R, which is generated by the
direct photolysis or thermolysis of organotellurium compounds,
or by the reaction with radical initiators, adds to the C-N bond
of the isonitrile. The resulting imidoyl radical undergoes a
homolytic substitution reaction with the starting organotellurium
compound to give the product, together with the regeneration
of the radical R (path a). The moderate effect of the radical
initiator indicates that the chain length of the current reaction
is short. Since a small amount of diaryl ditelluride is usually
produced as a side product, the chain may be terminated by the
reaction of the imidoyl radical with the diaryl ditelluride (path
b).
We next examined the effect of the solvent. If the transition
state or the intermediate has polar character, the efficiency of
the reaction may be affected by the polarity of the reaction
medium. However, contrary to our expectations, the solvent was
found to have only a marginal effect on the yield of the product
and the rate of the reaction. The reaction could be carried out
in various nonpolar and polar solvents including hexane,
benzene, tetrachloroethane, THF, DMF, EtCN, pyridine, alco-
hols, and water (Table 1, entries 7-20).17 Although R-addition
reactions of nucleophiles18 or electrophiles19 to isonitriles by
ionic mechanisms are well-known, the negligible solvent effects
and the insensitivity to basic and protic solvents suggest a radical
(18) (a) Niznik, G. E.; Morrison, W. H.; Walborsky, H. M. J. Org. Chem.
1974, 39, 600. (b) Murakami, M.; Ito, H.; Ito, Y. J. Org. Chem. 1988, 53,
4158. (c) Murakami, M.; Kawano, T.; Ito, H.; Ito, Y. J. Org. Chem. 1993,
58, 1458. (d) Murakami, M.; Ito, H.; Ito, Y. Bull. Chem. Soc. Jpn. 1996,
69, 25.
(19) (a) Ugi, I.; Fetzer, U. Chem. Ber. 1961, 94, 1116. (b) Capuano, L.;
Wamprecht, C.; Hell, W. Liebigs Ann. Chem. 1986, 132. (c) Westling, M.;
Smith, R.; Livinghouse, T. J. Org. Chem. 1986, 51, 1163.
(17) (a) Green Chemistry. Frontiers in Benign Chemical Syntheses and
Processes; Anastas, P. T., Williason, T. C., Eds.; Oxford University Press:
Oxford, 1998. (b) Li, C.-J.; Chan, T.-H. Organic Reactions in Aqueous
Media; John Wiley & Sons: New York, 1997.
(20) (a) Coffman, D. D.; Pinkney, P. S.; Wall, F. T.; Wood, W. H.;
Young, H. S. J. Am. Chem. Soc. 1952, 74, 3391. (b) Brubaker, M. M.;
Coffman, D. D.; Hoehn, H. H. J. Am. Chem. Soc. 1952, 74, 1509.

3700 J. Am. Chem. Soc., Vol. 123, No. 16, 2001
Table 2. Imidoylation of Organotellurium Compoundsa
Yamago et al.
a The reaction was carried out with 2 equiv of 2,6-dimethyl-phenylisonitrile in C6D6 or C6H6 in a Pyrex vessel with UV irradiation. A 250 W
high-pressure Hg lamp was used except for 7 and 9, wherein a RMR-3500 Å lamp (4.5 W x 8) was used. b Isolated yield. c 1,2-Diphenylethane
formed in 30% yield. d The reaction was carried out in the dark. e Yield based on the converted substrate. f 3,4,5-Tri-O-acetylglycal formed in 13%
yield g Isopropyl tolyl telluride formed in 22% yield. h The reaction was carried out under CO pressure (50 atm). i Isopropyl tolyl telluride formed
in 4% yield. j tert-Butyl tolyl telluride formed in 78% yield. k tert-Butyl tolyl telluride formed in 7% yield. l Radical initiator (V-40, 10 mol %) was
added.
Scheme 1
class of biologically active sugar derivatives.21 The stereose-
lectivity is excellent with respect not only to the imidoyl moiety
but also to the anomeric moiety. The R-isomers were formed
selectively (>97% selectivity), except for a single case (Table
2, entry 11), where the â-isomer was formed exclusively.22
The imidoylation of acyl radicals also opens a new synthetic
route to R-acyl carbonyl compounds, which have attracted great
(21) Glycoscience; Driguez, H., Thiem, J., Eds.; Springer: Berlin, 1997.
(22) The formation of the R-isomer can be explained by considering the
anomeric effect of the glycos-1-yl radicals, wherein the reaction at the
R-radical is kinetically more reactive than that of the â-radical due to the
interaction of the SOMO and the n-orbital of the neighboring oxygen. On
the other hand, the origin of the observed â-selectivity in entry 12 is not
clear at the present time. One possible explanation may be an involvement
of an intramolecualr participation of the polar C-2 phthaloyl group with
the 1-glycosyl radical to shield the R-face, because such an acyloxy group
participation and rearrangement have been reported already. See: Giese,
B.; Dupuis, J.; Gro¨ninger, K.; Hasskerl, T.; Nix, M.; Weizel, T. In
Substituent Effects in Radical Chemistry; Viehe, H. G., Janousek, Z.,
Mere´nyi, R., Eds.; D Reidel Publishing Company: Dordecht, The Neth-
erlands, 1986; p 283. See also: Zipse, H. J. Am. Chem. Soc. 1997, 119,
1087 and references therein.
From a synthetic viewpoint, imidoylation of telluroglycosides
and acyl tellurides is particularly rewarding. The imidoylation
of the 1-glycosyl radicals provides a new synthetic route to
1-imidoyl and 1-acyl glycosides, which belong to an important

Group-Transfer Imidoylation of Organotellurium Compounds
J. Am. Chem. Soc., Vol. 123, No. 16, 2001 3701
interest as versatile building blocks in organic synthesis and as
biologically active compounds.23 In the reaction of acyl tellurides
bearing secondary and tertiary alkyl carbon residues, the
decarbonylation of CO from the acyl radical competed with the
imidoylation (Table 2, entry 15) or the decarbonylation took
place exclusively (Table 2, entry 16).24 However, the decarbo-
nylation was avoided by carrying out the reaction under CO
pressure (50 atm). The acyl telluride bearing olefin and acetylene
moieties also afforded the simple imidoylated product without
intramolecular radical cyclization of the imidoyl radical inter-
mediates (Table 2, entries 13 and 17).
Scheme 2a
2-3. Competition between Isonitriles and CO. To clarify
the difference between the reactivity of CO and that of isonitriles
in radical-mediated reactions, we next carried out competition
experiments. Prior to this, the reactions of 1 and 3 under high
CO pressure (50-100 atm) were examined, but neither gave
the desired carbonylated products, and the starting materials
were recovered quantitatively. The competition experiments with
3 in the presence of xylylisonitrile (2 equiv) under 50-100 atm
of CO gave rise to selective formation of the imidoylated product
4 (eq 4). The result clearly shows the obvious advantage of
isonitriles over CO for this type of reaction.
a Reaction conditions; (a) (TolSe)2 (2.1 equiv), C6H6, UV lamp (8
W × 6), 100 °C, 43 h, 100%; (b) Bu3SnH (1.1 equiv), AIBN (0.1
equiv), C6H6, 80 °C, 1 h, 78%; (c) -20 °C, 5 days, quant; (d) -e (2.0
F/mol), 0.2 M LiClO4/EtCN, rt, 81%; (e) 1.0 M aqueous HCl/THF
()1/10), rt - 46 °C, 45 h, 63% (f) -e (3.0 F/mol), 0.2 M LiClO4 in
H2O/EtCN ()1/9), rt, 75%.
is ascribed to a group-transfer reaction of the tert-butyl radical
with the isonitrile. The formation of pivaloyl telluride was not
detected, but it may undergo imidoylation even if it were formed
because the pivaloyl telluride is known to undergo imidoylation
under these reaction conditions (Table 2, entry 16). This result
suggests that CO and isonitriles possess competitive reactivity
toward nonstabilized radicals, e.g., tert-butyl radical.
These competition experiments clearly revealed that isonitriles
are superior C1 reagents to CO for this type of group-transfer
reaction, especially for reactions involving stabilized carbon
radicals.
We also examined the competition by using tert-butyl tolyl
telluride (7). The reaction afforded the R-acyl imidoyltelluride
8 and the simple imidoylation product 9 in 19% and 36% yield,
respectively (26% and 50% yield, respectively, based on the
converted 7, eq 5). The formation of 8 can be ascribed to the
2-4. Transformations of the Products. The synthetic
potential of the imidoylated product is noteworthy, because the
carbon-tellurium bond is utilized for several synthetic trans-
formations.25 We first examined the possibility of an imidoyl
radical generation from imidoyl telluride 2a, which contains a
sugar moiety.26 Thus, the photothermal reaction of 2a and ditolyl
diselenide at 100 °C under Hg lamp irradiation was carried out
to obtain the corresponding imidoyl selenide 11 in quantitative
yield (Scheme 2). The tin-hydride mediated radical reduction
of 2a took place smoothly in the presence of AIBN to give the
glycosyl aldehyde imine 12, which was further converted to
glucal 13 under mild conditions.27 In both radical-mediated
reactions, products derived from the reverse imidoylation were
not observed, and the result is consistent with the theoretical
prediction (vide infra). The imines 12 and 13 would be useful
for further synthetic transformations for the synthesis of new
types of C-glycosides.
carbonylation of the tert-butyl radical generated from 7. The
imidoylation of the resulting acyl radical and subsequent group-
transfer reaction led to the formation of 8. The formation of 9
(23) Review: Kova´cs, L. Recl. TraV. Chim. Pays-Bas 1993, 112, 471.
Recent examples: (a) Wasserman, H. H.; Chen, J.-H.; Xia, M. J. Am. Chem.
Soc. 1999, 121, 1401. (b) Wasserman, H. H.; Xia, M.; Petersen, A. K.;
Jorgensen, M. R.; Curtis, E. A. Tetrahedron Lett. 1999, 40, 6163. (c) Singh,
R. P.; Kirchmeier, R. L.; Shreeve, J. M. J. Org. Chem. 1999, 64, 2579. (d)
Katritzky, A. R.; Oniciu, D. C.; Ghiviriga, I.; Soti, F. J. Org. Chem. 1998,
63, 2110. (e) Wasserman, H. H.; Petersen, A. Tetrahedron Lett. 1997, 38,
953. (f) Baldino, C. M.; Casebier, D. S.; Caserta, J.; Slobodkin, G.; Tu, C.;
Coffen, D. L. Synlett 1997, 488.
(24) It is noteworthy that decarbonylation did not take place in the
cyclopropyl-substituted acyl telluride (entry 14). This is probably because
cyclopropyl radicals, which possess sp3 radical character, are less stable
than the conformationally unrestrained secondary alkyl radicals, since the
rate of the decarbonylation becomes faster when the decarbonylated radical
becomes more stable. For the stability of cyclopropyl radicals, see;
Walborsky, H. M. Tetrahedron 1981, 37, 1625.
The oxidative transformation of 2a also provides an important
class of C-glycosides. Thus, the imidic ester 14 was obtained
in good yield by electrochemical oxidation of 2a in the presence
(25) Petragnani, N. Tellurium in Organic Synthesis; Academic Press:
London, 1994.
(26) Leardini, R.; Nanni, D.; Pareschi, P.; Tundo, A.; Zanardi, G. J. Org.
Chem. 1997, 62, 8394 and references therein. Bachi, M. D.; Denenmark,
D. J. Am. Chem. Soc. 1989, 111, 1886. Bachi, M. D.; Denenmark, D. J.
Org. Chem. 1990, 55, 3442. Bachi, M. D.; Melman, A. J. Org. Chem.,
1995, 60, 6242. Hofmann, J.; Schulz, K.; Zimmermann, G. Tetrahedron
Lett. 1996, 37, 2399.
(27) Clive, D. L. J.; Chittattu, G. J.; Farina, V.; Kiel, W. A.; Menchen,
S. M.; Russell, C. G.; Singh, A.; Wong, C. K.; Curtis, N. J. J. Am. Chem.
Soc. 1980, 102, 4438.

3702 J. Am. Chem. Soc., Vol. 123, No. 16, 2001
Yamago et al.
Scheme 3
H, Y ) O) were calculated, and the results are summarized in
Figure 1. TS 16A is calculated to be very early as judged by
the small structural changes from the reactants, the long newly
forming C-C bond length, and the lack of atomic spin density
at the CO moiety. TS 16A leads to the formation of the σ-type
acyl radical 17A, whose O-C-C bent angle (127.9°) agrees
with the proposed structure from ESR studies33 and previous
computational studies.8,34 The calculated activation energies of
the forward reaction (carbonylation) are 10.2 and 18.4 kJ/mol
by B3LYP/631HW and B3LYP/6311HW//B3LYP/631HW,
respectively, values which are in good agreement with the
experimental ones (13-18 kJ/mol).35 The calculated activation
energies of the reverse reaction (decarbonylation) are 73.1 and
70.2 kJ/mol by B3LYP/631HW and B3LYP/6311HW//B3LYP/
631HW, respectively, values which are also in good agreement
with the experimental value (72 kJ/mol).35b The Gibbs free
energies are also calculated at 298.15 K under 1 atm of pressure
by B3LYP/631HW. The results suggest that the entropic term
considerably destabilizes both the TS and the product by ca.
26.8 and 33.6 kJ/mol, respectively. The Mulliken charge does
not change significantly during the course of the reaction, but
the C-O bond becomes slightly polarized as the reaction
progresses. This change may account for the observed small
solvent effects in the decarbonylation.36
The geometry and energies of the TSs and products for the
imidoylation were also obtained by calculations and the results
are summarized in Figure 2. TSs 16B and 16C are also very
early, and the C-N-C bent angles remain almost 180°. TS
16B leads to syn-17B, which further isomerizes to the energeti-
cally more stable anti-17B, although there is no direct pathway
to the anti-isomer from the reactants.37 In sharp contrast to the
reaction with methyl isonitrile, the reaction of phenyl isonitrile
directly gives the anti-imidoyl radical 17C via TS 16C. These
results are consistent with the experimental results showing that
the syn-imidoyl tellurides were formed exclusively from the anti-
imidoyl radicals. The overall energetics from the reactants to
anti-17B and 17C is very similar regardless of the nature of
the substituent on the nitrogen. It should be emphasized that
the imidoylation is far more exothermic than the carbonylation,
whereas the activation energies of the imidoylation are slightly
of MeOH (LiClO4 solution of MeOH/EtCN).28 The imidic ester
14 was further hydrolyzed to 1-carbamoylglycoside 15 by acidic
treatment. Alternatively, the same transformation from 2a to
15 was achieved in a single step by the electrolysis of 2a in
aqueous EtCN.
2-5. Theoretical Model. Theoretical calculations were carried
out to understand carbonylations and imidoylations, especially
in terms of the differences in the reactivity of CO and isonitriles
toward radicals. Addition of substituted-methyl radical (XCH2
radical) to CO and methylisonitrile (CNMe) was used as a model
reaction (Scheme 3). The addition of the XCH2 radical to CO
or isonitriles generates the acyl or imidoyl radicals 17 by way
of transition states 16, and they undergo homolytic substitution
with (XCH2)TeMe via transition states 18 to produce the acyl
or imidoyl telluride 19 with generation of the XCH2 radical.
We also investigated the reaction pathway of methyl radical
addition to phenylisonitrile (Y ) NPh) to understand the effect
of the substituent on the nitrogen atom.
All theoretical calculations were carried out with Gaussian
94 and Gaussian 98 programs.29 Geometry optimizations were
performed with the hybrid B3LYP density functional30 with a
Hey-Wadt effective core potential (ECP) and the outermost
valence electron basis set31 for tellurium and the 6-31G(d)32
basis set for the rest (denoted as B3LYP/631HW). Single-point
energy calculations were performed with the hybrid B3LYP
density functional with a Hey-Wadt ECP and the outermost
valence electron basis set for tellurium and the 6-311+G(2df,2p)
basis set for the rest (denoted as B3LYP/6311HW). Details are
described in the Supporting Information.
(33) Bennett, J. E.; Mile, B. Trans. Faraday Soc. 1971, 67, 1587. Paul,
H.; Fischer, H. HelV. Chim. Acta 1973, 56, 1575.
(34) Guerra, M. J. Chem. Soc., Perkin Trans. 2 1996, 779.
(35) It is worth noting that the experimental values are obtained under
the assumption of the normal preexponential factors. (a) Watkins, K. W.;
Thompson, W. W. Int. J. Chem. Kinet. 1973, 5, 791. (b) Watkins, K. W.;
Word, W. W. Int. J. Chem. Kinet. 1974, 6, 855 (c) Bakac, A.; Espenson, J.
H. J. Chem. Soc., Chem. Commun. 1991, 1497. (d) Bakac, A.; Espenson,
J. H.; Young, V. G., Jr. Inorg. Chem. 1992, 31, 4959. (e) Nagahara, K.;
Ryu, I.; Kambe, N.; Komatsu, M.; Sonoda, N. J. Org. Chem. 1995, 60,
7384.
2-6. Addition of methyl radical to CO and isonitriles. The
geometry and energies of the transition state (TS) 16A for the
methyl radical addition to CO and the acyl radical 17A (X )
(36) Tsentalovich, Y. P.; Fischer, H. J. Chem. Soc., Perkin Trans. 2 1994,
729.
(37) Two transition structures for the isomerization were obtained in close
activation energies. The TS i, which is corresponding to the isomerization
of the nitrogen substituent, is found to be slightly lower in energy than the
TS ii corresponding to the isomerization of the carbon imidoyl carbon
substituents.
(28) Yamago, S.; Kokubo, K.; Yoshida, J. Chem. Lett. 1997, 111.
(29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; V. G. Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-
Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe,
M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.;
Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian
98, Revision A.7; Gaussian, Inc.: Pittsburgh, PA, 1998.
(30) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785.
(31) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270, 284.
(32) Hehre, W. J.; Radom, L.; von Rague´ Schleyer, P.; Pople, J. A. Ab
Initio Molecular Orbital Theory; John Wiley: New York, 1986; references
therein.

Group-Transfer Imidoylation of Organotellurium Compounds
J. Am. Chem. Soc., Vol. 123, No. 16, 2001 3703
Figure 1. Geometry (A, deg) and relative energies (total energy + ZPE, kJ/mol) of the reactants, TS, and product for the addition of methyl radical
to CO obtained by B3LYP/631HW calculations. Mulliken spin densities and charges of the heavy atoms are in boxes. Relative Gibbs energies at
298.15 K under 1 atm of pressure are shown in square brackets. The energies obtained by B3LYP/6311HW//631GHW + ZPE are shown in
parentheses.
Figure 2. Geometry (A, deg) and relative energies (kJ/mol) of the reactants, TSs, and products for the addition of methyl radical to CNMe and
CNPh obtained by B3LYP/631HW calculations. Mulliken spin densities and charges of the selected heavy atoms are in boxes. Relative Gibbs
energies at 298.15 K under 1 atm of pressure are shown in square brackets. The energies obtained by B3LYP/6311HW//631GHW + ZPE were
shown in parentheses.
higher than those of the carbonylation. Therefore, the net
activation energies of the reverse imidoylation from the anti-
radicals are 121-123 kJ/mol by B3LYP/6311HW//B3LYP/
631HW, which are significantly higher than those of the
decarbonylation (70 kJ/mol). The trend of the relative Gibbs
free energies of the TSs and the products is similar to that of
the carbonylation, and the entropic term considerably destabi-
lizes both the TSs and the products.
2-7. Homolytic Substitution Reaction. Homolytic substitu-
tion reactions of the acyl radical 17A and the anti-imidoyl
radical 17B with Me2Te (X ) H in Scheme 3) were also
analyzed by calculations, and the results are summarized in
Figure 3. The reactions were calculated to take place via the
T-shaped TSs 18A and 18B as shown in Figure 3. While two
isomeric TSs with respect to the relative orientation of the acyl
and the imidoyl radicals to Me2Te were obtained for each
reaction, only the lower energy TSs are shown here (geometries
and energies of all calculated TSs and products are shown in
the Supporting Information). Although formation of the hyper-
valent chalcogen-centered radical intermediate has been pro-
posed for the simple model reaction, no such intermediate could
be located by using the present calculations.38,39
The overall energetics of the two processes is very similar,
and late TSs are obtained in both cases. Thus, the forming C-Te
bond is shorter than the cleaving C-Te bond, and the spin
density is mainly located on the leaving methyl group. Both
processes are endothermic, because the starting acyl and imidoyl
radicals are more stable than the forming methyl radical.
Analysis of the Gibbs free energies indicates that the entropic
(38) Schiesser, C. H.; Smart, B. A. Tetrahedoron 1995, 51, 6051. Smart,
B. A.; Schiesser, C. H. J. Comput. Chem. 1995, 16, 1055. Schiesser, C. H.;
Wild, L. M. J. Org. Chem. 1999, 64, 1131.
(39) Homolytic substitution reaction of 17A and Me2Te at the MP2-
(FU)/631HW level of theory also gave the similar T-shaped transition state.
Although the activation energy (57 kJ/mol) was slightly higher than that
obtained by B3LYP/631HW, we could not locate the hypervalent intermedi-
ate as discussed in ref 38.

3704 J. Am. Chem. Soc., Vol. 123, No. 16, 2001
Yamago et al.
Figure 3. Geometry (A, deg) and relative energies (total energy + ZPE, kJ/mol) of Me2Te, TSs, and products of the addition of acyl radical 17A
and anti-imidoyl radical 17B to Me2Te obtained by B3LYP/631HW calculation. While two isomeric TSs were obtained, only the TSs of lower
energy are shown. Mulliken spin densities and charges of the heavy atoms are in boxes. Relative Gibbs energies at 298.15 K under 1 atm of
pressure are shown in square brackets. The energies obtained by B3LYP/6311HW//631GHW + ZPE were shown in parentheses.
differences between the two reactions derive from the first
elemental step, and that the imidoylation is more exothermic
than the carbonylation due to the formation of the stable imidoyl
radicals.
The overall chain cycle is exothermic (41.4 and 69.8 kJ/mol
for the carbonylation and the imidoylation, respectively). In
addition, since the activation energies of both TSs 16 and 18
are very low, the kinetics, as well as the thermodynamics,
suggests that both reactions must take place spontaneously. The
competitive carbonylation and imidoylation of the tert-butyl
radical, as in eq 5, are consistent with the theoretical results.
Conversely, the imidoylation took place exclusively instead
of the carbonylation for stabilized radicals as shown in Table 2
and eq 4, and the results cannot be explained by the energy
profile shown in Figure 4. Therefore, we next examined the
effects of the substituent X, because the energy profiles of the
radical addition to CO are well-known to be related to the
stability of the starting radicals.
Figure 4. Energy profile of the group-transfer carbonylation and
imidoylation reactions of methyl radical (X ) H) and CO and methyl
isonitrile with Me2Te (X ) H) obtained by B3LYP/6311HW//B3LYP/
631HW + ZPE. The energy corresponding to the anti-imidoyl radical
(anti-17B) is shown for Y ) NMe.
2-9. Carbonylation and Imidoylation of Substituted Meth-
yl Radicals. The reactions of the substituted methyl radical
XH2C• (X ) H, Me, OH, CN), CO or methyl isonitrile, and
(XH2C)TeMe were calculated, and the relative energies of the
TS 16, radical intermediate 17, and final product 19 are
summarized in Table 3.41 For the imidoylation, only the anti-
imidoyl radicals and the syn-imidoyl tellurides are shown here,
and the energies corresponding to the syn-imidoyl radicals are
shown in the Supporting Information. We did not calculate TS
18 primarily because of limited computational expenses. The
activation energies of the reaction of the substituted derivatives
of 17D-I (X ) Me, OH, and CN) with (XH2C)TeMe are
expected to be very similar to or lower than those of 17A and
17B with Me2Te, because elimination of the stabilized radicals
XH2C• (X ) Me, OH, and CN) occurs as the reaction progresses.
term also significantly destabilizes both the TSs and the
products. Mulliken charge analysis indicates a small amount of
electron flow from the leaving methyl group to the acyl and
the imidoyl carbon. This may be the origin of the reported minor
solvent effect in the group transfer reaction.40
2-8. Energetics of the Group Transfer Carbonylation and
Imidoylation. The energetics of the radical-chain reaction of
the methyl radical, Me2Te, and CO and that of the methyl
radical, Me2Te, and methyl isonitrile (Scheme 3, X ) H)
obtained by B3LYP/6311HW//B3LYP/631HW is summarized
in Figure 4. The initial addition reaction of the methyl radical
to CO and methyl isonitrile is highly exothermic to form the
acyl and imidoyl radicals 17 via TS 16, and the subsequent
slightly endothermic homolytic substitution reaction affords the
products 19 via TS 18. Figure 4 clearly indicates that the striking
(41) For the ethyl radical addition to CO, the activation energies for the
forward reaction and the reverse reaction are in good agreement with the
experimental data. See ref 35a.
(40) Curran, D. P.; Martin-Esker, A. A.; Ko, S.-B.; Newcomb, M. J.
Org. Chem. 1993, 58, 4961.

Group-Transfer Imidoylation of Organotellurium Compounds
J. Am. Chem. Soc., Vol. 123, No. 16, 2001 3705
Table 3. B3LYP-Calculated Relative Energies (kJ/mol) and Gibbs
Energies (kJ/mol) for TSs and Products Related to H2XC• Addition
Reactions to CO, CNMe, and CNPh
and CN, although the relative total energies were calculated to
be exothermic in all cases. Interestingly, formation of the
imidoyl tellurides is more exothermic (61-73 kJ/mol) than that
of the acyl tellurides (24-45 kJ/mol). The relative Gibbs free
energies reveal that carbonylations of nonstabilized radicals (X
) H and Me) are slightly exothermic while those of stabilized
radicals (X ) OH and CN) are endothermic. The calculated
results are consistent with the current experimental results in
that carbonylation takes place only with nonstabilized radicals
but not with stabilized radicals. The relative Gibbs free energies
also reveal that the formation of the imidoyl tellurides is
exothermic in all cases, as much as 17-28 kJ/mol, suggesting
that imidoylation proceeds with both nonstabilized and stabilized
radicals.
total energya
Gibbs energyb
X
Y
compd
631HW 6311HW
631HW
transition state 16
10.2
H
O
16A
18.4
16.8
14.1
42.9
29.6
31.5
29.9
28.3
21.8
37.0
42.9
39.0
68.3
48.9
58.3
54.7
59.1
44.8
c
CH3
O
O
O
16D
16E
16F
8.7
3.9
3.9
21.1
22.2
17.1
18.7
14.3
OHc
CNc
H
NMe 16B
NMe 16G
NMe 16H
NMe 16I
c
CH3
OHc
CNc
H
NPh
16C
The substituent effects on the TSs are small for both
carbonylation and imidoylation, but more definitive electronic
effects are observed in carbonylation. Thus, as the radical
becomes electron rich, the activation energy decreases due to
the electron-deficient character of CO. For imidoylation,
however, no electronic effects are observed, while polar
substituent effects have been proposed in the reaction of
electrophilic radicals.12
product 17
-62.9
-52.8
-39.0
2.0
-107.5
-95.5
-87.7
-45.0
-114.5
H
O
O
O
O
17A
17D
17E
17F
-51.8
-40.4
-26.4
12.5
-93.4
-80.5
-72.3
-38.3
-98.9
-29.3
-9.4
1.9
c
CH3
OHc
CNc
H
CH3
OHc
CNc
H
41.3
NMe anti-17B
NMe anti-17G
NMe anti-17H
NMe anti-17I
-65.3
-46.4
-41.8
-4.8
-74.1
c
NPh
anti-17C
product 19
-42.2
-49.2
-32.9
-28.2
-74.5
-79.6
-68.6
3. Conclusion
H
O
O
O
O
19A
19D
19E
19F
-41.4
-45.1
-29.5
-23.6
-69.8
-72.6
-62.1
-6.1
-5.2
We have demonstrated that a variety of organotellurium
compounds undergo radical-mediated group-transfer reactions
with isonitriles. Imidoylation takes place under mild photother-
mal conditions with complete stereoselectivity to give a single
stereoisomer in all cases. The reaction is especially useful when
stabilized radicals are formed, because the synthetically equiva-
lent reaction with CO, namely carbonylation, does not take place
in such cases. The synthetic utility of the present group-transfer
imidoylation is obvious. Theoretical calculations of the carbo-
nylation and the imidoylation processes revealed that both
reactions proceed with low activation energies, and that kinetic
factors cannot explain the differences in the reactivity between
CO and isonitriles. Instead of kinetic factors, the calculations
suggest that thermodynamic factors play a crucial role, namely
imidoylation is highly exothermic because stable imidoyl
radicals are formed. Therefore, isonitriles are not simple
substitutes for CO but are useful complements in radical-
mediated C1 homologation reactions.
CH3
OH
CN
H
CH3
OH
11.6
14.8
NMe syn-19B
NMe syn-19G
NMe syn-19H
-24.8
-28.3
-17.2
a δEa ) δE + δZPE. Single-point calculation for 6311HW. b Energy
obtained at 298.15 K under 1 atm of pressure. c Two isomeric stationary
points were obtained within close energy differences, and the value is
taken from the lowest energy TS. Optimized structures and energies
are shown in the Supporting Information.
The nature of the substituent X is found to strongly influence
the stability of the products 17 and 19, while the effect on the
TS 16 is small in all cases. The relative total energies of the
acyl and imidoyl radicals 17 decreased dramatically as the
substituent X changed from H to Me, OH, and CN. Indeed, the
carbonylation of CN-substituted methyl radical to form 17F is
calculated to be an endothermic process. Despite the same
substitution effects, formation of the imidoyl radicals anti-
17G-I is still highly exothermic. The entropic terms consider-
ably destabilize 17, and the relative Gibbs free energies of the
products 17 indicate that the formation of the acyl radicals 17E
and 17F is an endothermic process, although the formation of
the imidoyl radicals anti-17G-I is still exothermic. The
substituent effects on the stability of the acyl radicals 17A-F
toward decarbonylation are consistent with the previous kinetic
experiments in that decarbonylation proceeds more rapidly when
the decarbonylated radical becomes more stable.6 Conversely,
the activation energies of deimidoylation are, in all cases, very
high (67-123 kJ/mol). These results are also consistent with
the fact that imidoyl radicals are stable toward reverse reactions.
The effects of the substituent X on the stability of the product
19 are also noteworthy. The relative total energies of 19 also
decreased as the substituent X changed from H to Me, OH,
Acknowledgment. This work is partly supported from a
Grant-in-Aid for Scientific Research from the Ministry of
Education, Culture and Sports (Monbusho). Generous allowance
of computational time from the UJI Super computer laboratory
in the Chemical Institute of Kyoto University is greatly
appreciated. We also thank Dr. K. Itami in our group for the
X-ray analysis.
Supporting Information Available: Crystal date for the
imidoylated product (Table 2, entry 2), experimental procedures
and characterization of all new compounds, and optimized
geometries and energies for the theoretical calculation (PDF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
JA003879R