A radical thia-Brook rearrangement

Article abstract of DOI:10.1039/c4cc01683a

Geminal mercapto trialkyl- and trialkoxy-silanes undergo an efficient radical chain rearrangement, whereby the silyl group migrates from carbon to sulfur; the starting materials are readily obtained by exploiting the peroxide initiated radical addition of dithiocarbonates (xanthates) to trialkyl- or trialkoxy-vinylsilanes. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc01683a

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
A radical thia-Brook rearrangement†‡
Beatrice Quiclet-Sire* and Samir Z. Zard*
´
Cite this: Chem. Commun., 2014,
50, 5990
Received 5th March 2014,
Accepted 14th April 2014
DOI: 10.1039/c4cc01683a
www.rsc.org/chemcomm
Geminal mercapto trialkyl- and trialkoxy-silanes undergo an effi-
cient radical chain rearrangement, whereby the silyl group migrates
from carbon to sulfur; the starting materials are readily obtained by
exploiting the peroxide initiated radical addition of dithiocarbonates
(xanthates) to trialkyl- or trialkoxy-vinylsilanes.
We recently found that various xanthates readily add to vinyl
trialkoxysilanes allowing access to numerous functional tri-
alkoxysilane derivatives 3 (Scheme 1).1 This peroxide initiated
radical chain addition is flexible, modular, and experimentally
Scheme 1 A possible route to geminal mercapto trialkoxysilanes.
very simple to implement.2 Trialkoxysilanes are of key importance turned out to be rearranged trialkoxysilane 5a, as determined
in numerous areas: material sciences, sol gels and organogelators, by analysis of the NMR spectrum of the mixture. We noticed,
surface modification and monolayer formation, especially on furthermore, that, while the combined yield of 4a and 5a was
metal oxides and silica surfaces, supported catalysts etc.3 In the generally good, their relative yield varied significantly with the
course of this work we stumbled upon an unexpected and very exact experimental conditions. In one experiment, the ratio of
efficient radical thia-Brook rearrangement we now describe.
4a : 5a by NMR was approximately 2 : 1 in a combined yield of
While the xanthate group in adduct 3 may be reductively 82% (reaction time 1 h).
removed if needed, it is in fact a protected form of the corre-
The unexpected formation of rearranged trialkoxysilane 5a
sponding thiol 4.4 The presence of the thiol would indeed offer may be rationalised by the mechanism displayed in Scheme 2
some further interesting possibilities, either by itself as a cross- and proceeding through a radical thia-Brook rearrangement.8
linking handle (via the disulfide) or as a springboard for a host In one experiment, we succeeded in isolating a pure sample of
of transformations through ionic (e.g. alkylation) or radical thiol 4a in 53% yield and, indeed, exposing it to di-t-butyl
(e.g. addition to alkenes, the so-called ‘‘thiol click reaction’’5) peroxide (DTBP) in refluxing chlorobenzene for 1 h afforded a
processes. With these considerations in mind, we attempted to quantitative yield of rearranged silane 5a. This mechanism also
generate the thiol through the Chugaev reaction6,7 by simply accounts for the variability in the relative yield of 4a and 5a
heating adduct 3a in diphenyl ether at 200 1C. The thermolysis initially observed, since the efficiency of the chain reaction
takes place under neutral conditions that should not affect leading to the latter depends on the presence of adventitious
groups sensitive to nucleophilic attack, such as the phthali- radical initiators (oxygen, traces of metallic salts, etc.).
mido group present in 3a.
Whereas the radical Brook rearrangement involving silicon
In the event, heating xanthate 3a in diphenyl ether at 200 1C migration from carbon to oxygen is well documented,9 only one
for 1 h furnished after purification two inseparable products, instance of a radical thia-Brook rearrangement has been reported as
one of which was indeed the expected thiol 4a but the other far as we know.10 It involves the cleavage of a silicon–silicon bond in
going from sulfur radical 10 to silicon radical 11 (Scheme 3). This
`
Laboratoire de Synthese Organique, CNRS UMR 7652, Ecole Polytechnique,
step was incorporated in a radical sequence allowing the use of thiol
9 in ingenious tin-free reductive dehalogenations and Barton–
McCombie deoxygenations.
F-91128 Palaiseau, France. E-mail: beatrice.sire@polytechnique.edu,
samir.zard@polytechnique.edu; Fax: +33 169335972; Tel: +33 169335971
† This paper is dedicated with respect to the memory of Professor Adrian G. Brook
(University of Toronto).
Rupture of a carbon–silicon bond in a thia-Brook rearrange-
‡ Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cc01683a ment, as in the present case, appears to be unprecedented.
5990 | Chem. Commun., 2014, 50, 5990--5992
This journal is ©The Royal Society of Chemistry 2014

View Article Online
Communication
ChemComm
Scheme 2 An unexpected radical thia-Brook rearrangement.
Scheme 4 Further examples of the radical thia-Brook rearrangement.
An unexpected problem was encountered with substrate 3d,
prepared using xanthate 1d (Scheme 5). The thermolysis step
gave, after chromatographic purification, thiolactone 12 (39%)
and a polar mixture of compounds. This mixture was simply
Scheme 3 Cleavage of a silicon–silicon bond by a thia-Brook rearrangement.
The driving force may be the formation of a carbon radical 7a
stabilised by a sulfur atom, with the possible equilibrium between
intermediates 6a and 7a being finally driven by the irreversible
hydrogen abstraction from another thiol molecule 4a.
A similar sequence could be accomplished starting from the
addition product of xanthate 1a to vinyl trimethylsilane 2a0
(Scheme 2). Thus, thermolysis of 3a0 indeed furnished the
corresponding rearranged derivative 5a0, but this compound
was too labile to chromatographic purification and decom-
posed to the free thiol 8, which was isolated in 60% yield.
A small amount of un-rearranged thiol 4a0 (ca. 10%) was also
observed by NMR of the crude reaction mixture. In view of the
hydrolytic lability of trimethylsilyl derivatives, the remainder of
the study was conducted with the tri(t-butoxy)silyl derivatives.
Our next task was to examine the scope of the reaction.
Addition of chloropyridinyl xanthates 1b to vinyl tri(t-butoxy)-
silane 2a afforded adduct 3b in 68% yield (Scheme 4). Thermo-
lysis for 1 h gave a mixture of thiol 4b (42%) and thia-Brook
product 5b (28%). Repetition of the experiment and exposure of
the crude product from the thermolysis to DTBP in refluxing
chlorobenzene for 2 h furnished 5b in a better yield (78%).
In the case of adduct 3c, derived from xanthate 1c, no attempt
was made to separate the intermediate thiol. The crude mixture
was treated directly with DTBP in refluxing chlorobenzene to
furnish the rearranged material 5c in 62% yield.
Scheme 5 Unexpected formation of a thialactone.
Chem. Commun., 2014, 50, 5990--5992 | 5991
This journal is ©The Royal Society of Chemistry 2014

View Article Online
ChemComm
Communication
Thus, adduct 3c0, obtained in 52% yield by the radical addition
11
of benzotriazole xanthate 1c0 to vinyl tri(t-butoxy)silane 2a,
was cleaved by 1,2-ethylenediamine into thiol 4c (76% yield) and
the latter rearranged quantitatively into tri(t-butoxy)silyl sulfide
5c by heating in refluxing chlorobenzene in the presence of
DTBP initiator (Scheme 6).
In the same manner, but without purification of the inter-
mediate thiols, xanthates 3e–g1 underwent conversion into the
corresponding radical thia-Brook rearrangement products 5e–g
(Scheme 6). The possibility of introducing a geminal trifluoro-
methyl acetamido motif is worthy of note.
In summary, we have described a hitherto unknown migra-
tion of a silicon group from carbon to sulfur by a radical chain
mechanism, a process that may be viewed as a formal radical
thia-Brook rearrangement. This provides a route to a plethora
of otherwise inaccessible functionalised silyl sulfides 5. The
possibility of capturing intermediate carbon radical 7 (Scheme 2)
before hydrogen atom abstraction from the thiol has occurred,
for example by cyclisation to a suitably located internal alkene,
could also be of some synthetic interest. Studies along these
lines are underway.
Notes and references
1 B. Quiclet-Sire, Y. Yanagisawa and S. Z. Zard, Chem. Commun., 2014,
50, 2324.
2 For reviews of the xanthate transfer, see: (a) S. Z. Zard, Angew. Chem.,
Int. Ed. Engl., 1997, 36, 672; (b) B. Quiclet-Sire and S. Z. Zard, Top.
Curr. Chem., 2006, 264, 201; (c) B. Quiclet-Sire and S. Z. Zard,
Chem. – Eur. J., 2006, 12, 6002; (d) B. Quiclet-Sire and S. Z.
Zard, Pure Appl. Chem., 2011, 83, 519; for an account of the discovery
of the basic process, see: (e) S. Z. Zard, Aust. J. Chem., 2006,
59, 663.
3 (a) S. Fujita and S. Inagaki, Chem. Mater., 2008, 20, 891; (b) H. Zou,
S. Wu and J. Shen, Chem. Rev., 2008, 108, 3893; (c) M. Llusar and
C. Sanchez, Chem. Mater., 2008, 20, 782; (d) D. B. Cordes, P. D.
Lickiss and F. Rataboul, Chem. Rev., 2010, 110, 2081; (e) S. Onclin,
B. J. Ravoo and D. N. Reinhoudt, Angew. Chem., Int. Ed., 2005,
44, 6282; ( f ) A. Ulman, Chem. Rev., 1996, 96, 1533; (g) A. P. Wight
and M. E. Davis, Chem. Rev., 2002, 102, 3589; (h) Z.-L. Lu, E. Lindner
and H. A. Mayer, Chem. Rev., 2002, 102, 3543.
4 (a) F. Duus, in Comprehensive Organic Chemistry, ed. D. H. R. Barton
and W. D. Ollis, Pergamon Press, Oxford, 1979, vol. 3, pp. 373–487;
(b) S. Ramachandra Rao, Xanthates and Related Compounds, Marcel
Dekker Inc., New York, 1971.
5 (a) C. E. Hoyle and C. N. Bowman, Angew. Chem., Int. Ed., 2010,
49, 1540; (b) A. K. Tucker-Schwartz, R. A. Farrell and R. L. Garrell,
J. Am. Chem. Soc., 2011, 133, 11026; (c) E. L. Tyson, M. S. Ament and
T. P. Yoon, J. Org. Chem., 2013, 78, 2046.
Scheme 6 Additional examples of the radical thia-Brook rearrangement.
subjected to the action of DTBP in refluxing chlorobenzene.
Purification then afforded aminopyridine 13 (41%) and the radical
thia-Brook rearrangement product 5d (55%). Aminopyridine 13 is
the leaving group in the formation thiolactone 12. This latter
compound would be difficult to obtain by more conventional
routes and is interesting in its own right, for example as a cross-
linking agent in material science; however, in the present
context, its formation by attack of the thiol sulfur on the
activated amide is clearly in competition with the desired
radical thia-Brook rearrangement.
To circumvent this complication, we resorted to a more
traditional cleavage of the xanthate group by aminolysis with
1,2-ethylenediamine.4 Thus, treatment with xanthate 3d with
1,2-ethylenediamine in a 1 : 1 (v/v) mixture of ethanol and ether
at room temperature gave the crude thiol 4d, which was not
purified but directly heated in refluxing chlorobenzene with
DTBP. This gave the expected rearranged product 5d in good
yield. No thiolactone 12 or the corresponding aminopyridine 13
were observed under these conditions. Because of the lower
temperature (heating in refluxing chlorobenzene at 130 1C vs.
thermolysis in diphenyl ether at 200 1C) and, especially, the
presence of the DTBP initiator, the radical chain process
overcomes the intramolecular ionic ring-closure leading to
thiolactone 12.
6 (a) L. Chugaev, Chem. Ber., 1899, 32, 3332; (b) H. R. Nace, Org. React.,
1962, 12, 57.
7 (a) K. K. K. Goh, S. Kim and S. Zard, J. Org. Chem., 2013, 78, 12274;
(b) B. Quiclet-Sire and S. Z. Zard, Org. Lett., 2013, 15, 5886.
8 (a) A. G. Brook, Acc. Chem. Res., 1974, 7, 77; for general reviews, see:
(b) M. A. Brook, Silicon in Organic, Organometallic, and Polymer
Chemistry, Wiley, New York, 1999; (c) H. Moser, Tetrahedron, 2001,
57, 2065.
9 For general reviews on organosilanes in radical chemistry, see:
(a) C. Chatgilialoglu, Acc. Chem. Res., 1992, 25, 188; (b) C. Chatgilialoglu,
Chem. – Eur. J., 2008, 14, 2310; (c) C. Chatgilialoglu, Organosilanes in Radical
Chemistry, Wiley, Chichester, 2003.
10 M. Ballestri, C. Chatgilialoglu and G. Seconi, J. Organomet. Chem.,
1991, 408, C1.
11 A. R. Katritzky, M. A. C. Button and S. N. Denisenko, Heterocylcles,
2001, 54, 301.
The O-isopropyl group in the xanthate is now not needed any
more and can be replaced by the simpler O-ethyl analogue.
5992 | Chem. Commun., 2014, 50, 5990--5992
This journal is ©The Royal Society of Chemistry 2014