Cyanide abstractions from benzyl isocyanides by phenyl and tri-n-butyltin radicals: New examples of SH2 reactions

Full text of DOI:10.1021/jo940292k

J . Org. Chem. 1996, 61, 4827-4829
4827
Ta ble 1. Rela tive Ra tes, Ha m m ett Cor r ela tion s, a n d
Cya n id e Abstr a ction s fr om Ben zyl
Isocya n id es by P h en yl a n d Tr i-n -Bu tyltin
Ra d ica ls: New Exa m p les of SH2 Rea ction s†
Secon d a r y r-Deu ter iu m Kin etic Isotop e Effect for
Cya n id e Abstr a ction s fr om Ben zyl Isocya n id es by P h en yl
Ra d ica l a t 100 °C
Y
Sung Soo Kim,* Ki Woong Yang, and Chang Soo Lee1
relative
rates
p-CH3
H
m-OCH3
p-Cl
m-CN
p-CN
Department of Chemistry, Inha University,
Incheon 402-751, South Korea
a
a
kYH/kCl
4.02
4.76
4.24
5.16
5.24
6.11
6.64
k
YD/kCl
F ) 0.24b (r ) 0.982);c kYH/kYD ) 1.075
d
Received February 24, 1994
a
Error limits are less than 2% with average deviations of more
An isonitrile2 can be considered as a hybrid of reso-
nance structures of 1 and 2 with the former being
responsible for the radical reactions. Various radicals3
add to the terminal carbon atom to produce imidoyl
radicals.4 The presence of imidoyl radicals was, however,
detected only at quite low temperatures i.e. -60 °C. The
b
than four runs. The values of σ were taken from Ritchie, C. D.;
Sager, W. F. Prog. Phys. Org. Chem. 1964, 2, 334. c Correlation
d
coefficient. kYH/kYD ) kYH/kCl × kCl/kYD
.
Sch em e 1
‚ ‚
+
-
RsNdC: T RsNtC:
1
2
reactions of benzyl isocyanides5 with phenyl radical at
100 °C take place in a concerted manner.6 Tri-n-butyltin
(at 120-130 °C)3b and phenyl (at 100 °C)5 radicals attack
benzyl isocyanides to give corresponding cyanides and
benzyl radicals via eq 1. The benzyl radicals were then
p
m
p
p
m
nides. The reactivities of phenyl and tri-n-butyltin
radicals toward benzyl isocyanides are now to be com-
paratively discussed.
YC6H4CH2NC + R‚ f YC6H4CH2‚ + RCN (1)
R‚: n-Bu3Sn‚ and C6H5‚
Resu lts
Reactions at 100 °C of the ampoules containing YC6H4-
CH2NC (0.5-1.0 M), CCl4 (1.5-2.0 M), C6H5Br (3mM,
internal standard), benzoyl peroxide (BP, 5 mM), and
C6H6 (sovlent) gave C6H5CN and C6H5Cl as the principal
products according to eqs 2-4 in Scheme 1. The molar
ratios of [YC6H4CH2NC + CCl4]/[BP] g 400 could realize
pseudo-first-order kinetics, whereby eq 5 has been de-
rived from reactions 2-4. The relative rates (kYH/kCl and
trapped by tri-n-butyltin hydride and carbon tetrachlo-
ride to yield toluenes3b and benzyl chlorides,5 respectively.
In our own hand,7 the material balance was excellent
(over 98%) between consumption of benzyl isocyanides
and formation of toluenes. These outcomes6,7 could imply
that the homolytic R-additions via imidoyl radicals are
unimportant at the temperatures. Reaction 1 is therefore
the major sink for the radical reactions of benzyl isocya-
k
YD/kCl) were then determined using eq 5. A Hammett
correlation was obtained
† Dedicated to Professor Lawrence A. Singer on the occasion of his
60th birthday.
(1) A postdoctoral fellow supported by a grant from Korea Science
and Engineering Foundation from 1995 to 1996.
(2) Ugi, I. Isonitrile Chemistry; Academic Press: New York and
London, 1971.
(3) (a) Shaw, D. D.; Pritchard, H. O. Can. J . Chem. 1967, 45, 2749.
(b) Saegusa, T.; Kobayashi, S.; Ito, Y.; Yasuda, N. J . Am. Chem. Soc.
1968, 90, 4182. (c) Saegusa, T.; Ito, Y.; Yasuda, N. Polymer J . 1970,
1, 591. (d) Banks, R. E.; Haszeldine, R. N.; Stephens, C. W. Tetrahe-
dron Lett. 1972, 13, 3699. (e) Singer, L. A.; Kim, S. S. Tetrahedron
Lett. 1974, 15, 861. (f) Kim, S. S. Tetrahedron Lett. 1977, 18, 2741.
(g) Meier, M.; Ruchardt, C. Tetrahedron Lett. 1983, 24, 4671.
(4) (a) Danen, W. C.; West, C. T. J . Am. Chem. Soc. 1973, 95, 6872.
(b) Blum, P. M.; Roberts, B. P. J . Chem. Soc., Chem. Commun. 1976,
535. (c) Blum, P. M.; Roberts, B. P. J . Chem. Soc. Perkin Trans. 2
1978, 1313. (d) Davies, A. G.; Nedelec, J . Y.; Sutcliffe, R. J . Chem.
Soc. Perkin Trans. 2 1983, 209.
kYH(YD)/kCl ) [C6H5CN]/[C6H5Cl] ×
[CCl4]o/[YC6H4CH2NC]o (5)
from the plot of kYH/kCl against σ. A secondary R-deute-
rium kinetic isotope effect has been also obtained as
follows, kYH/kYD ) kYH/kCl × kCl/kYD. These are shown in
Table 1.
Reactions at 80 °C of the ampoules containing YC6H4-
CH2NC (1 M), C6H5CH2NC (1 M), n-Bu3SnH (2.5 M),
naphthalene (3 mM, internal standard), azoisobutyroni-
trile (AIBN, 3 mM), and C6H6 (solvent) gave rise to
formations of YC6H4CH3 and C6H5CH3 through reactions
6-11 (Scheme 2). The disappearance of benzyl isocya-
(5) Kim, S. S.; Lee, K. S.; Hwang, S. B.; Kim, H. J . Tetrahedron Lett.
1990, 31, 3575.
(6) A control experiment strongly excludes the occurrence of the
R-addition. tert-Butyl isocyanide (25 mmol), tert-butyl chloride (50
mmol), and AIBN (2.5 mmol) were dissolved in 5 mL of benzene. The
mixture was flushed with nitrogen and heated at 75 °C for 22 h. The
NMR spectrum of the resulting mixture showed only the formation of
pivalonitrile and absence of the R-addition product, (CH3)3CNdC(Cl)C-
(CH3)3. The present concerted mechanism could be compared to the
two-bond homolysis of the peresters (Bartlett, P. D.; Simons, D. M. J .
Am. Chem. Soc. 1960, 82, 1753 and Bartlett, P. D.; Ru¨chart, C. J . Am.
Chem. Soc. 1960, 82, 1756). The benzyl radical possesses sufficient
stability to cause the thermolysis of tert-butyl phenylperacetates to
take place with synchronous cleavage of an O-O and a C-C bond
producing carbon dioxide and the benzyl and tert-butoxy radicals:
C6H5CH2C(O)O2C(CH3)3 f C6H5CH2• + CO2 + •OC(CH3)3.
(7) Benzyl isocyanides were reacted with tri-n-butyltin hydride at
80 °C using AIBN as the initiator (refer to the Experimental Section).
The excellent material balance indicates that reaction 1 must be the
sole pathway for the reactions of benzyl isocyanides.
Sch em e 2
S0022-3263(94)00292-6 CCC: $12.00 © 1996 American Chemical Society

4828 J . Org. Chem., Vol. 61, No. 14, 1996
Notes
Ta ble 2. Rela tive Ra tes, Ha m m ett Cor r ela tion s, a n d
Secon d a r y r-Deu ter iu m Kin etic Isotop e Effect for th e
Cya n id e Abstr a ction s fr om Ben zyl Isocya n id es by
Tr i-n -bu tyltin Ra d ica l a t 80 °C
ing carbanions which experience dispersion of the nega-
tive charge into nitro group. However, such delocalization
could not occur and the charge localizes instead on the
carbon atom for the corresponding TS. The fractional
benzylic anion moiety of TS 3 could behave similarly so
that the negative charge may not conjugate with the
phenyl ring. The validity of F-σ correlation in Table 1
could be thus soley derived from the inductive effects
caused by the substituents (Y). On the other hand, the
structure of cationic portion of 3 could be equivalent to a
resonance hybrid of A, B, and C. In particular, C could
significantly reduce the free energy of activation for 3.
Y
relative
rates
p-OCH3
p-CH3
H
1
p-Cl
p-CN
a
kY/kH
0.63
0.80
2.04
3.12
F ) 0.77b (r ) 0.979);c kH/kD ) 1.112
d
a
Error limits are less than 3% with average deviations of more
b
than three runs. The values of σ were taken from Ritchie, C. D.;
Sager, W. F. Prog. Phys. Org. Chem. 1964, 2, 334. c Correlation
coefficient. kH/kD ) kH/kp-CH × kp-CH /kD ) 0.89/0.80 ) 1.112.
d
3
3
nides quantitatively (over 98%) accounts for the produc-
tion of corresponding toluenes. Application of steady
state approximation to the radical concentrations for
YC6H4CH2• and C6H5CH2• could lead to eq 12. The
The cyanide abstractions by tri-n-butyltin radical show
a relatively large Hammett constant (F ) 0.77) which
indicate also the similar polarization to that of 3, that is
[YC6H4Cδ-H2‚‚‚NdC‚‚‚Snδ+(n-Bu)3], 4. TS 4 could draw
more charge separations than 3. Here again the negative
charge stays on the benzylic carbon atom. The cationic
part of 4 resembles tri-n-butyltin cation. The bond
dissociation energy of n-Bu3Sn-H is 74 kcal.13 The
ionization potential of n-Bu3Sn• could be roughly 143 kcal
which is actually the oxidation potential of Me3Sn•.14 The
bond strength of C6H5-H is 112 kcal15 and ionization
potential of C6H5• is 212 kcal.16,17 Tri-n-butyltin cation,
n-Bu3Sn+, could be thereby 107 kcal below the phenyl
cation, C6H5+, in terms of the heat content. The vigorous
stabilization of tri-n-butyltin cation could provoke decent
NdC‚‚‚Snδ+(n-Bu)3 bond formation for 4, which may be
then translated into the equivalent YC6H4Cδ-H2‚‚‚NdC
bond breaking. Accordingly, 4 could involve more bond
cleavage than 3. This is eminently consistent with the
larger figures of Hammett constant and secondary R-deu-
terium kinetic isotope effect for the tin radical (F ) 0.77;
kY/kH ) [YC6H4CH3]/[C6H5CH3] ×
[C6H5CH2NC]o/[YC6H4CH2NC]o (12)
relative rates (kY/kH) and Hammett correlations have
been accordingly calculated (refer to Table 2). Tri-n-
butyltin radical could react with C6H5CD2NC and
p-CH3C6H4CH2NC with rate constants kD and kCH
respectively. The relative rates (kH/kCH and kD/kCH ) were
,
3
3
3
similarly obtained by the competition method using
p-CH3C6H4CH2NC as the standard. The secondary R-deu-
terium kinetic isotope effect has been thus available from
kH/kD ) kH/kCH × kCH /kD and is shown in Table 2.
3
3
Discu ssion
kH/kD ) 1.112) than for the phenyl radical (F ) 0.24; kYH
kYD ) 1.075).
/
When phenyl radical was derived from thermolysis of
benzoyl peroxide, weakly nucleophilic character had been
disclosed for hydrogen atom abstractions from toluenes
(F ) 0.18)8 and for bromine atom abstractions from benzyl
bromides (F ) 0.13).9 Our F ) 0.24 of Table 1 is thus
consistent with those observations8,9 and suggests the
The structure of polar TS i.e. 3 and 4 must not be a
resonance hybrid of the reactants and the products of eq
1. The polar TS is thereby displaced from the “interme-
diate configuration”. The phenomenon of such displace-
ment has been rationalized as the Perpendicular Effect.18
Due to the Perpendicular Effect,18 the polarity of 3 and
4 may not be adequately correlated with the entities
inherent in the corresponding reactants and products.
The Marcus theory19 also maintains that the intrinsic
barrier is independent of the free energy of reaction.
When a σ bond is located at position â to a phenyl ring
and involved with the atom transfer reactions, the TS
could take a polar structure. The direction of the
polarization could be critically influenced by the character
polar transition state (TS) structure: [YC6H4Cδ-
-
H2‚‚‚NdC‚‚‚C6δ+H5], 3. The small F value could be
associated with modest degree of the polarization. In
case of electrophilic radicals,10,11 the hydrogen abstraction
reactions usually exhibit F+-σ+ Hammett correlations
with F+ < 0. The negative sign of F+ tells that the polarity
of the TS is opposite to that of 3. F+-σ+ relations also
testify to the conjugation of the positive charge on the
benzylic carbon atom with the adjacent phenyl ring.
The deprotonations of nitroalkanes (CH3NO2, MeCH2-
NO2, Me2CHNO2) by the bases12 could yield correspond-
(12) (a) Bordwell, F. G.; Boyle, W. J ., J r.; Yee, K. C. J . Am. Chem.
Soc. 1970, 92, 5926. (b) Bordwell, F. G.; Boyle, W. J ., J r. J . Am. Chem.
Soc. 1975, 97, 3447.
(13) CRC Handbook of Chemistry and Physics, 1st Student ed.; CRC
Press: Boca Raton, FL, 1988.
(14) Chatgilialoglu, C.; Guerra, M. J . Am. Chem. Soc. 1990, 112,
2854.
(8) Suehiro, T.; Suzuki, A.; Tsuchida, Y.; Yamazaki, J . Bull. Chem.
Soc. J pn. 1977, 50, 3324.
(9) Migita, T.; Nagai, T.; Abe, Y. Chem. Lett. 1975, 543.
(10) Russell, G. A. Free Radicals; Kochi, J . K., Ed.; Wiley: New York,
1973; Vol. 1, Chap. 7.
(11) (a) Kim, S. S.; Sohn, S. C. Tetrahedron Lett. 1982, 23, 3703.
(b) Kim, S. S.; Koo, H. M.; Choi, S. Y. Tetrahedron Lett. 1985, 26, 891.
(c) Kim, S. S.; Choi, S. Y.; Kang, C. H. J . Am. Chem. Soc. 1985, 107,
4234. (d) Kim, S. S.; Seo, J . S.; Yoon, M. H. J . Org. Chem. 1987, 52,
3691. (e) Kim, S. S.; Lee, C. S.; Kim, C. C.; Kim, H. J . J . Phys. Org.
Chem. 1990, 3, 803. (f) Kim, S. S.; Kim, S. Y.; Ryou, S. S.; Lee, C. S.;
Yoo, K. H. J . Org. Chem. 1993, 58, 192. (g) Kim, S. S.; Kim, H. R.;
Kim, H. B.; Youn, S. J .; Kim, C. J . J . Am. Chem. Soc. 1994, 116, 2754.
(15) Davico, G. E.; Bierbaum, V. M.; DePuy, C. H.; Ellison, G. B.;
Squires, R. R. J . Am. Chem. Soc. 1995, 117, 2590.
(16) Ades, H. F.; Companion, A. L.; Subbaswamy, K. R. J . Phy.
Chem. 1991, 95, 6502.
(17) Fisher, I. P.; Palmer, T. F.; Lossing, F. P. J . Am. Chem. Soc.
1964, 86, 2741.
(18) Thornton, E. R. J . Am. Chem. Soc. 1967, 89, 2915.
(19) Marcus, R. A. J . Phys. Chem. 1968, 72, 891.

Notes
J . Org. Chem., Vol. 61, No. 14, 1996 4829
of the attacking radicals. Since the cyanide group is
univalent and the reactions are concerted, the cyanide
abstractions can be classified as the SH2 reactions.
Exp er im en ta l Section
Ma t er ia ls. The reagents are commercially available from
major suppliers. Liquids were distilled with center-cut collec-
tion, and solids recrystallized according to standard procedures.20
Ben zyl isocya n id es (YC6H4CH2NC; Y ) p-OCH3, p-CH3, H,
m-OCH3, p-Cl, m-CN, and p-CN) were prepared according to a
known method21 and characterized by NMR and IR data.
Kin etic P r oced u r e a n d Ga s Ch r om a togr a p h ic An a lyses.
A reaction mixture consisted of YC6H4CH2NC (0.5-1.0 M), CCl4
(1.5-2.0 M), C6H5Br (3 mM, internal standard), benzoyl peroxide
(BP, 5 mM), and C6H6 (solvent). This was placed and degassed
in Pyrex ampoules by the freeze-pump-thaw method. The
mixture was then heated at 100 °C for 3.5 h. The products were
analyzed on DB-1 capillary column (30 m) with Varian 3300 gas
chromatography equipped with FID. The resolutions were
optimum with temperature programming from 80-230 °C.
A reaction mixture consisted of YC6H4CH2NC (1 M), C6H5-
CH2NC (1 M), n-Bu3SnH (2.5 M), naphthalene (3 mM, internal
standard), AIBN (3 mM), and C6H6 (solvent). This was placed
and degassed in Pyrex ampoules by freeze-pump-thaw method
for the thermal reactions at 80 °C for 1 h. The reaction time
was adjusted to consume 10-15% of starting benzyl isocyanides.
The products were analysed on carbowax 20M capillary column
(70 m) with a Varian 3300 gas chromatograph equipped with
FID. The resolutions were optimum with temperature program-
ming from 50-180 °C. A reaction mixture was of C6H5CH2NC
(0.5 M) or C6H5CD2NC (0.5 M), p-CH3C6H4CH2NC (0.5 M), n-Bu3-
SnH (0.5 M), naphthalene (3 mM, internal standard), AIBN (3
mM), and C6H6 (solvent). This was similarly treated as above
for the thermolyses and analyses when otherwise stated. DB-1
capillary column (30 m) was instead employed.
Deu ter a ted ben zyl isocya n id e (C6H5CD2NC)22 was ob-
tained via dehydrations of C6H5CD2NDCHO, which could be
derived from reactions of C6H5CD2ND2 with HCO2H. Reductions
of C6H5CN with LiAlD4 gave C6H5CD2ND2.23 The percent of
deuteration of the isonitrile was 98%, according to the NMR
spectrum.
(20) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. F. Purification of
Laboratory Chemicals, 2nd Ed.; Pergamon Press: Oxford, 1980.
(21) Ho¨fle, G.; Lange, B. Organic Syntheses; Wiley: New York, 1990;
Coll. Vol. VII, p 27.
(22) Ugi, I.; Meyr, R.; Lipinskz, M.; Bodeshein, F.; Rosendahl, F.
Organic Syntheses; Wiley: New York, 1973; Collect. Vol. V, p 300.
(23) Amundsen, L. H.; Nelson, L. S. J . Am. Chem. Soc. 1951, 73,
242.
Ack n ow led gm en t. We warmly thank the Ministry
of Education(BSRI-95) and OCRC/KOSEF for their
financial supports.
J O940292K