Chemical surface modification via radical C-C bond-forming reactions

Article abstract of DOI:10.1021/ja0686716

Radical C-C bond-forming reactions at self-assembled monolayers (SAMs) can be used for the modification of alkene terminated Si wafers under neutral conditions. Many functional groups are tolerated under radical conditions and biologically interesting mol

Full text of DOI:10.1021/ja0686716

Published on Web 04/14/2007
Chemical Surface Modification via Radical C-C Bond-Forming Reactions
Kai Oliver Siegenthaler,§ Andreas Scha¨fer,‡ and Armido Studer*,§
Fachbereich Chemie, Organisch-Chemisches Institut, Westfa¨lische Wilhelms-UniVersita¨t, Corrensstrasse 40,
48149 Mu¨nster, Germany, and nanoAnalytics GmbH, Heisenbergstrasse 11, 48149 Mu¨nster, Germany
Received December 4, 2006; E-mail: studer@uni-muenster.de
Table 1. Radical Carboaminoxylation of 7-Octenyl SAMs Using
HO-TEMPO-malonate at 125 °C in ClCH2CH2Cl under Different
Conditionsa
Self-assembled monolayers (SAMs) bearing specific functional
groups have gained widespread interest.1 Functional groups at the
surface allow careful tuning of surface properties such as wettability,
biocompatibility and corrosion resistance. Many standard organic
reactions such as alkene oxidations, amino acylations, imine bond
formations, alcohol acylations, and epoxide openings with alcohols
and amines have been successfully conducted at SAMs.1c,e All these
reactions comprise carbon-heteroatom bond formations. However,
C-C bond-forming reactions at SAMs are far more difficult to
achieve, and only very few reports have appeared to date.2,3
Recently, we reported successful thermal radical carboaminoxyl-
ations of unactivated olefins (eq 1).4,5 Radical generation occurs
via thermal C-O bond homolysis of the starting alkoxyamine.
C-radical addition onto the olefin with subsequent irreversible
nitroxide trapping affords the corresponding carboaminoxylation
product.6 Importantly, C-O bond homolysis in the starting
alkoxyamine is reversible, and therefore formal long-lived C-
radicals result. Generation of radicals can be achieved over days
without destroying the C-radical precursor. Therefore, this approach
should be particularly suited for radical C-C bond formations at
olefin-terminated SAMs which have not been reported to date (eq
2).7
1
t [h]
c [mol L-
]
CA(adv) [deg]
CA(rec) [deg]
1
2
3
4
5
6
7
8
0
5
0
95 ( 4
90 ( 3
88 ( 2
76 ( 2
77 ( 1
90 ( 3
85 ( 2
78 ( 5
63 ( 9
60 ( 3
58 ( 3
46 ( 5
43 ( 1
61 ( 5
54 ( 4
41 ( 2
1.0
1.0
1.0
1.0
0.5
0.7
1.5
10
20
40
20
20
10
a CA(adv) ) advancing contact angle; CA(rec) ) receding contact angle.
Figure 1. Chemically modified Si-wafers A, B, C, and D.
to a further decrease of the CA (entries 3, 4). However, reaction
for 40 h did not further increase the polarity of the surface (entry
5). We found that the surface reaction was less efficient under higher
dilution (entries 6 and 7). Decreasing the reaction time to 10 h and
increasing the concentration (1.5 molar) did not improve the result
(entry 8).
Importantly, our method operates under neutral conditions and
many functional groups are tolerated. Moreover, functionalized
alkoxyamines are readily prepared (see SI). In Figure 1 wafers
A-D, which were successfully chemically modified by using
functionalized alkoxyamines, are depicted. Surface polarity can
readily be altered upon changing the substituents at the malonate
moiety. Carboaminoxylation using an alkoxyamine bearing short
polar tetraethyleneglycol tails leads to a surface with a smaller CA
(A, CA(adv) ) 79 ( 1°), whereas di-tert-butyl malonyl radical
additions provide more hydrophobic surfaces (B, CA(adv) ) 103
( 1°). Importantly, the desired functionality can also be introduced
via the nitroxide component of the alkoxyamine as documented
for wafer C.
Alkene terminated SAMs were prepared using oxidized Si wafers
and 7-octenyl or 10-undecenyl trimethoxysilane, respectively (see
Supporting Information (SI)). Surface carboaminoxylations were
studied using HO-TEMPO-malonate as C-radical precursor (see eq
2, R1 ) R2 ) CO2Me, n ) 6). Addition reactions were performed
by dipping a 7-octenyl-terminated wafer into a solution of the
alkoxyamine in ClCH2CH2Cl with subsequent heating at 125 °C.
Concentration and reaction time were systematically varied, and
analysis of the chemically modified surface was performed via
contact angle (CA) measurements providing information on the
surface polarity (Table 1, 7-octenyl-terminated surface CA(adv) )
95 ( 4°, entry 1). Reaction for 5 h provided a slightly lower CA
as one would expect for a successful reaction using HO-TEMPO-
malonate (entry 2). Increase of the reaction time to 10 or 20 h led
Even complex molecules with biologically interesting function-
alities like TEMPO-biotin conjugates can be added to alkene
terminated SAMs (D, CA(adv) ) 72 ( 3°). Carboaminoxylations
§ Westfa¨lische Wilhelms-Universita¨t.
‡ nanoAnalytics GmbH.
9
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J. AM. CHEM. SOC. 2007, 129, 5826-5827
10.1021/ja0686716 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
By going from alkenyl terminated wafers (7-octenyl: d ) 2.08
nm, ellipsometry; 10-undecenyl: d ) 2.10 nm, XPS) to the addition
products, a significant increase of SAM thickness was measured
(E, d ) 2.70, ellipsometry; J, d ) 2.95 nm, XPS; L, d ) 3.62 nm,
XPS; see SI). This indicates that the added functional groups are
probably not buried within the monolayer as also supported by the
changes of the CAs.
In conclusion we reported first radical C-C-bond forming
reactions at SAMs. These reactions can be conducted under neutral
conditions and many functional groups are tolerated. This is a major
advantage over most of the carbon-heteroatom bond-forming
processes at SAMs which are ionic reactions and require often
protecting group strategies. We believe that the radical surface
chemistry will become an important alternative to the established
ionic chemistry for chemical surface modification. The new method
can also be used for the immobilization of biologically interesting
molecules.
Acknowledgment. We thank the Fonds der Chemischen In-
dustrie for a stipend (K.O.S.) and the Deutsche Forschungsgemein-
schaft (STU 280/5-1) for supporting our work. Dr. H. Wormeester,
MESA+ Research Institute (Enschede, NL) is acknowledged for
performing ellipsometry. Wacker Siltronics AG is acknowledged
for donation of Si wafers. An anonymous referee is acknowledged
for helpful comments. This work is dedicated to Professor Hans J.
Scha¨fer on the occasion of his 70th birthday.
Figure 2. Carboaminoxylation using bulky alkoxyamines.
using alkoxyamines derived from sterically highly hindered nitrox-
ides are far more efficient as compared to the TEMPO-mediated
processes.4c This is also true for the surface radical chemistry as
reaction with the bulky alkoxyamine 1 at 125 °C is completed within
2 h (ClCH2CH2Cl, 1 M, method A, CA ) 81 ( 3°; wafer E).8,9
Surface reaction using 1 can even be achieved at 90 °C (20 h, CA-
(adv) ) 77 ( 3°; wafer E, method B). With oily alkoxyamines,
radical chemistry at SAMs can be conducted by simply covering
the wafer with a film of the alkoxyamine and subsequent heating
without using any solvent (90 °C, 15 h, CA(adv) ) 74 ( 3°; wafer
F, method C). Using methods A and B wafers G-I were
successfully prepared showing that various functional groups can
readily be introduced. It is known that the alkyl chain length in
alkyl trialkoxysilanes influences SAM formation.1a,10 Therefore, 10-
undecenyl covered wafers were modified by radical carboaminoxyl-
ations providing wafers J-L. Generally larger contact angle
changes were observed upon C-C bond formation indicating that
a higher density of the functional groups at the surface is achieved
starting from longer and more densely packed alkene-terminated
wafers (see Figure 2).
To further study the surface chemistry, X-ray photoelectron
spectroscopy (XPS) was performed. SAM modification with
TEMPO-alkoxyamines leading to wafers C and D was quantified
revealing that about 30% (wafer C) or 44% (wafer D) of the surface
bound alkenes underwent reaction (Figure 1). With bulkier
alkoxyamines, SAMs can be subjected to radical carboaminoxyl-
ation under milder conditions (see above). XPS results show that
under these conditions SAM modification proceeds more efficiently.
Hence, reaction with 1 leads to very high conversion with respect
to the surface bound octene (89%, wafer E, method A). Even with
sterically hindered alkoxyamines surface modification proceeds
smoothly under the described conditions (46%, wafer H, method
B; 69%, wafer I, method B; 40%, wafer K, method A). Also
fluorination was readily achieved (77%, method C; wafer L).
Analysis of the SAM thickness by ellipsometry and angle-
resolved XPS further supports the success of the surface reaction.
Supporting Information Available: Experimental procedures and
compound characterization data. This material is available free of charge
via the Internet at http://pubs.acs.org.
References
(1) (a) Ulman, A. Chem. ReV. 1996, 96, 1533. Mrksich, M. Chem. Soc. ReV.
2000, 29, 267. (b) Chechik, V.; Crooks, R. M.; Stirling, C. J. M. AdV.
Mater. 2000, 12, 1161. (c) Sullivan, T. P.; Huck, W. T. S. Eur. J. Org.
Chem. 2003, 17. (d) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R.
G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (e) Onclin, S.; Ravoo,
B. J.; Reinhoudt, D. N. Angew. Chem., Int. Ed. 2005, 44, 6282.
(2) (a) Yousaf, M. N.; Mrksich, M. J. Am. Chem. Soc. 1999, 121, 4286. (b)
Yousaf, M. N.; Chan, E. W. L.; Mrksich, M. Angew. Chem., Int. Ed. 2000,
39, 1943. (c) Dutta, S.; Perring, M.; Barrett, S.; Mitchell, M.; Kenis, P. J.
A.; Bowden, N. B. Langmuir 2006, 22, 2146.
(3) Fryxell, G. E.; Rieke, P. C.; Wood, L. L.; Engelhard, M. H.; Williford,
R. E.; Graff, G. L.; Campbell, A. A.; Wiacek, R. J.; Lee, L.; Halverson,
A. Langmuir 1996, 12, 5064.
(4) (a) Wetter, C.; Jantos, K.; Woithe, K.; Studer, A. Org. Lett. 2003, 5, 2899.
(b) Wetter, C.; Studer, A. Chem. Commun. 2004, 174. Herrera, A. J.;
Studer, A. Synthesis 2005, 1389. (c) Molawi, K.; Schulte, T.; Siegenthaler,
K. O.; Wetter, C.; Studer, A. Chem.sEur. J. 2005, 11, 2335.
(5) Cyclization reactions: (a) Studer, A. Angew. Chem., Int. Ed. 2000, 39,
1108. (b) Teichert, A.; Jantos, K.; Harms, K.; Studer, A. Org. Lett. 2004,
6, 3477. (c) Uenoyama, Y.; Tsukida, M.; Doi, T.; Ryu, I.; Studer, A. Org.
Lett. 2005, 7, 2985. (d) Janza, B.; Studer, A. Org. Lett. 2006, 8, 1875.
(6) These reactions are controlled by the persistent radical effect: (a) Fischer,
H. Chem. ReV. 2001, 101, 3581. (b) Studer, A. Chem.sEur. J. 2001, 7,
1159. (c) Studer, A. Chem. Soc. ReV. 2004, 33, 267. (d) Studer, A.; Schulte,
T. Chem. Rec. 2005, 5, 27.
(7) Radical polymerizations from surfaces are known and are not considered
as selective controlled C-C-bond formations in the present context: (a)
Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 592. (b) Prucker, O.;
Ru¨he, J. Macromolecules 1998, 31, 602.
(8) The same contact angle was obtained if the reaction was run for 20 h.
(9) Reaction of 1 with 7-octenyl trimethoxysilane in solution provides the
corresponding carboaminoxylation product which can be covalently bound
to the surface by standard SAM formation. A CA(adv) of 81 ( 3° was
measured (see SI). The same value is obtained for wafer E prepared by
surface radical carboaminoxylation showing that the radical C-C bond
formation at the surface is indeed a highly efficient process.
(10) Bierbaum, K.; Kinzler, M.; Wo¨ll, C.; Grunze, M.; Ha¨hner, G.; Heid, S.;
Effenberger, F. Langmuir 1995, 11, 512.
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