Two types of intramolecular homolytic substitution reactions at group XIV atoms: Unusual radical 1,4-Sn shifts from Si to C and carbonylative SHi reaction at Si

Article abstract of DOI:10.1039/b301755a

4-[(Trimethylstannyl)diphenylsilyl]butanoyl radical, arising from the corresponding 3-(stannylsilyl)propyl radical and CO, undergoes an S_Hi reaction at Si with extrusion of trimethyltin radical to give silacyclopentanone. The parent 3-(stannylsilyl)propyl radical was also found to isomerize to (3-stannylpropyl)silyl radical via a 1,4-Sn shift from Si to C with a rate constant of 9.3 × 10⁴ s^-1 at 80°C. Ab initio and DFT MO calculations support a front-side attack mechanism.

Full text of DOI:10.1039/b301755a

Two types of intramolecular homolytic substitution reactions at group
XIV atoms: unusual radical 1,4-Sn shifts from Si to C and carbonylative
SHi reaction at Si†
Armido Studer,*a Stephan Amrein,a Hiroshi Matsubara,b Carl H. Schiesser,*b Takashi Doi,c
Tomonori Kawamura,c Takahide Fukuyamac and Ilhyong Ryu*c
a Fachbereich Chemie der Universität Marburg, Hans-Meerwein-Strasse, D-35032 Marburg, Germany.
E-mail: studer@Mailer.Uni-Marburg.de
b School of Chemistry, Bio21 Institute of Molecular Science and Biotechnology, The University of
Melbourne, Victoria, Australia 3010. E-mail: carlhs@unimelb.edu.au
c
Department of Chemistry, Faculty of Arts and Sciences, Osaka Prefecture University, Sakai, Osaka
599-8531, Japan. E-mail: ryu@ms.cias.osakafu-u.ac.jp
Received (in Cambridge, UK) 14th February 2003, Accepted 28th March 2003
First published as an Advance Article on the web 17th April 2003
4-[(Trimethylstannyl)diphenylsilyl]butanoyl radical, arising
from the corresponding 3-(stannylsilyl)propyl radical and
CO, undergoes an SHi reaction at Si with extrusion of
trimethyltin radical to give silacyclopentanone. The parent
3-(stannylsilyl)propyl radical was also found to isomerize to
(3-stannylpropyl)silyl radical via a 1,4-Sn shift from Si to C
with a rate constant of 9.3 3 104 s21 at 80 °C. Ab initio and
DFT MO calculations support a front-side attack mecha-
nism.
Thus, when 0.01 M of a benzene solution containing
3-[(trimethylstannyl)diphenylsilyl]propyl bromide (5) was
treated with pressurized carbon monoxide (97 atm) and
tributyltin hydride (1.1 equiv) in the presence of AIBN (2,2A-
azobisisobutyronitrile), two types of carbonylated product were
obtained after isolation by silica gel chromatography (eqn. 1):
Despite the tremendous achievements in free radical chemistry
during the last two decades,1 many basic transformations still
remain to be discovered in this field. Certainly, SH2 reactions at
hetero-atoms belong to this category.2 Intramolecular homo-
lytic substitution reactions at group XVI elements have been
introduced for the formation of heterocyclic rings.3 In addition,
we have previously reported that radical carbonylation with
subsequent intramolecular substitution of acyl radicals at sulfur
provides a useful method for the preparation of g-thiolactones.4
Encouraged by the recent work of the Studer group, who
reported on SHi reactions of alkyl and aryl radicals at silicon,5
we became curious about the similar SHi reaction behavior of
acyl radicals. Here we present two types of novel SHi reactions
at Si and Sn, both of which were discovered in the reaction
system of carbonylation of 3-(stannylsilyl)propyl radical 1
(Scheme 1). Thus, we report on the intramolecular homolytic
substitution reaction of acyl radical 2 at Si, which leads to the
formation of silacyclopentanone 3 and an unusual radical
1,4-Sn migration from Si to C in the parent radical 1. We also
report on the rate constant of the 1,4-Sn migration and the
results of ab initio and density functional (DFT) MO calcula-
tions, which predict a front-side attack of carbon radical at the
tin group with retention of configuration.
(1)
one was the envisaged SHi product silacyclopentanone 36 and
the other was uncyclized aldehyde 6. The non-polar fraction
contained an unexpected product, 1-silyl-3-stannylpropane 8
(X: H/OH = 81/19), which is supposed to be a rearranged
product from the parent radical 1. The use of a slower mediator,
(TMS)3SiH, suppressed the competitive formation of aldehyde
6 and reduced product 7, however, the formation of unusual
product 8 always competed with the initially planned carbonyla-
tion/SHi sequence.
The unusual formation of 8 led us to examine the similar
reaction in the absence of carbon monoxide. As expected, this
reaction gave the migration product 8 as the major product
together with a small amount of simple reduction product 7. The
intramolecularity of the migration reaction was supported by the
fact that the reaction at lower concentration ([5] = 0.005 M)
resulted in a significant increase of the 8/7 ratio. The large
amount of hydrosilane type product 8 (X = H) indicates that the
silyl radical formed after 1,4-Sn migration is mainly reduced by
tin hydride under the applied conditions.7 As a minor reaction
pathway, the isomerized silyl radical undergoes bromine
abstraction from the starting bromide to provide the correspond-
ing bromosilane, which is eventually converted to silanol 8 (X
= OH) during chromatographic separation on silica gel.
When aromatic bromide 9a was exposed to the tin hydride/
AIBN conditions with or without CO, 1,4-Sn migration
proceeded very smoothly to give the corresponding phenyl-
stannane 10a as a sole product (eqn. 2). Similarly, efficient
1,4-Sn migration was observed, when silyl phenyl ether 9b was
used as a substrate. Thus, the 1,4-Sn migration from Si to C is
a general process also applicable to aryl radicals.
Scheme 1 Two types of SHi reactions starting from radical 1.
† Electronic supplementary information (ESI) available: Experimental
procedure, spectral data for all compounds, and kinetic data. See http://
www.rsc.org/suppdata/cc/b3/b301755a/
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CHEM. COMMUN., 2003, 1190–1191
This journal is © The Royal Society of Chemistry 2003

IR is grateful for a Grant-in-Aid for Scientific Research on
Priority Areas (A) “Exploitation of Multi-Element Cyclic
Molecules” from the Ministry of Education, Culture, Sports,
Science and Technology, Japan. AS thanks the Fonds der
Chemischen Industrie for financial support. CS and HM thank
the Melbourne Advanced Research Computing Centre for
generous computing support.
(2)
Notes and reference
1 Radicals in Organic Synthesis, ed. P. Renaud and M. P. Sibi, vols 1 and
2, Wiley-VCH, Weinheim, Germany, 2001.
2 Reviews: (a) C. H. Schiesser and L. M. Wild, Tetrahedron, 1996, 52,
13265; (b) J. C. Walton, Acc. Chem. Res., 1998, 31, 99; (c) A. L. J.
Beckwith, J. Chem. Soc. Rev., 1993, 22, 143.
Radical 1,4-migration is very rare,8,9 and to learn more about
the efficiency of the present 1,4-Sn migration, kinetic competi-
tion experiments of radical 1 using tributyltin hydride in a
classical radical clock experiment10 were performed (Scheme
2). These investigations provided a rate constant for the 1,4-Sn
migration in 1 of 9.3 3 104 s21 at 80 °C.
3 For recent work, see: (a) N. Al-Maharik, L. Engman, J. Malmström and
C. H. Schiesser, J. Org. Chem., 2001, 66, 6286; (b) M. W. Carland, R.
L. Martin and C. H. Schiesser, Tetrahedron Lett., 2001, 42, 4737; (c) C.
L. Winn and J. M. Doodman, Tetrahedron Lett., 2001, 42, 7091; (d) R.
Leardini, H. McNab, M. Minozzi and D. Nanni, J. Chem. Soc., Perkin
Trans. 1, 2001, 1072; (e) T. Ooi, M. Furuya, D. Sakai, Y. Hokke and K.
Maruoka, Synlett, 2001, 541; (f) Y. Nishiyama, Y. Hada, M. Anjiki, K.
Miyake, S. Hanita and N. Sonoda, J. Org. Chem., 2002, 67, 1520.
4 (a) I. Ryu, T. Okuda, K. Nagahara, N. Kambe, M. Komatsu and N.
Sonoda, J. Org. Chem., 1997, 62, 7550. For reviews on acyl radicals and
radical carbonylations, see (b) C. Chatgilialoglu, D. Crich, M. Komatsu
and I. Ryu, Chem. Rev., 1999, 99, 1991; (c) I. Ryu and N. Sonoda,
Angew. Chem., Int. Ed. Engl., 1996, 35, 1050; (d) I. Ryu, N. Sonoda and
D. P. Curran, Chem. Rev., 1996, 96, 177.
5 (a) A. Studer, Angew. Chem., Int. Ed., 1998, 37, 462; (b) A. Studer and
H. Steen, Chem. Eur. J., 1999, 5, 759; (c) S. Amrein, M. Bossart, T.
Vasella and A. Studer, J. Org. Chem., 2000, 65, 4281; (d) A. Studer, M.
Bossart and T. Vasella, Org. Lett., 2000, 2, 985. See also (e) C. H.
Schiesser, M. L. Styles and L. M. Wild, J. Chem. Soc., Perkin Trans. 2,
1996, 2257 and references therein.
Scheme 2 Rate constant for 1,4-Sn shift in the isomerization of 1 to 4.
It is generally understood that transition states involved in
SH2 reactions require a collinear (or nearly so) arrangement of
both attacking and leaving radicals.2 Indeed this has been
demonstrated clearly for reactions involving radical attack at the
halogens as well as sulfur and selenium.11 There is growing
evidence, however, that analogous reactions involving group
XIV elements can involve backside or front-side transition
mechanisms,12 and the present 1,4-Sn migration would appear
to fall in the latter category. Ab initio and DFT MO
calculations13 indicate that the front-side attack of an alkyl
radical at tin via a five-membered ring transition state is a
reasonably favorable reaction pathway. Indeed, energy barriers
between about 45 and 60 kJ mol21 are calculated at correlated
and DFT levels of theory for the rearrangement of the closely
related 3-(stannasilyl)propyl radical (Scheme 3).14
6 Silacyclopentanones belong to an exotic class of silaheterocycles. For a
previous synthesis, see: (a) J. A. Soderquist and A. Hassner, J. Org.
Chem., 1980, 45, 541; (b) A. Hassner and J. A. Soderquist, Tetrahedron
Lett., 1980, 21, 429.
7 D. Dakternieks, D. J. Henry and C. H. Schiesser, Organometallics,
1998, 17, 1079.
8 For a rare example of 1,4-Si migration, see ref 5d.
9 For 1,3-Sn migration, see: (a) S.-Y. Chang, Y.-F. Shao, S.-F. Chu, G.-T.
Fan and Y.-M. Tsai, Org. Lett., 1999, 1, 945. For 1,5-Sn migrations (b)
S. Kim, S. Lee and J. S. Koh, J. Am. Chem. Soc., 1991, 113, 5106; (c)
S. Kim and J. S. Koh, Chem. Commun., 1992, 1377; (d) E. Hasegawa,
K. Ishiyama, T. Kato, T. Horaguchi, T. Shimizu, S. Tanaka and Y.
Yamashita, J. Org. Chem., 1992, 57, 5352; (e) S. Kim, K. M. Yeon and
K. S. Yoon, Tetrahedron Lett., 1997, 38, 3919; (f) S. Kim, M. S. Jung,
C. H. Cho and C. H. Schiesser, Tetrahedron Lett., 2001, 42, 943; (g) H.
Sano, D. Asanuma and M. Kosugi, Chem. Commun., 1999, 1559. For
1,6-Sn migration see (h) S. Kim and K. M. Lim, Chem. Commun., 1993,
1152.
10 C. Chatgilialoglu, K. U. Ingold and J. C. Scaiano, J. Am. Chem. Soc.,
1981, 103, 7739.
11 (a) C. H. Schiesser and L. M. Wild, J. Org. Chem., 1998, 63, 670; (b)
C. H. Schiesser and L. M. Wild, J. Org. Chem., 1999, 64, 1131.
12 (a) S. M. Horvat, C. H. Schiesser and L. M. Wild, Organometallics,
2000, 19, 1239; (b) S. Kim, S. M. Horvat and C. H. Schiesser, Aust. J.
Chem., 2002, 55, 753.
Scheme 3 MP2/DZP optimized structure and activation energy (kJ mol21
for 1,4-radical migration of Sn from Si to C.
)
13 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery Jr., R. E.
Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N.
Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R.
Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski,
G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick, A.
D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz,
A. G. Baboul, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.
Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-
Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P.
M. W. Gill, B. Johnson, W. Chen, M. W. Wong, J. L. Andres, C.
Gonzalez, M. Head-Gordon, E. S. Replogle and J. A. Pople, Gaussian
98, Revision A.7, Gaussian Inc., Pittsburgh, PA, 1998.
In summary, we present two types of novel homolytic
substitution reactions at group XIV atoms: (i) an SHi-type
reaction of acyl radical at silicon and (ii) a 1,4-Sn migration
from silicon to alkyl and aryl radicals, comprising an unusual
SHi-type reaction at tin. A kinetic study indicates that the
isomerization of 1 to 4 in benzene via 1,4-Sn migration takes
place with a rate constant of 9.3 3 104 s21 at 80 °C.
Furthermore, ab initio and DFT MO calculations for the tin
migration reaction reasonably predict a transition state involv-
ing front-side attack at tin. We are currently looking at other
substrates and conditions for efficient carbonylative SHi
cyclization reactions as well as further examples of radical
1,4-migration reactions.
14 Preliminary experiments provide values of 46 kJ mol21 and 11.8 for the
energy barrier and log (A/s21) for the migration in 1, in good agreement
with these caluculated data.
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