Synthesis of porphyrins bearing trans-thiols

Article abstract of DOI:10.1021/ol9911728

(matrix presented) A route to porphyrins bearing frans-thiols is described using a thioacetyl-containing aldehyde or a thioacetyl-containing dipyrromethane in the presence of catalytic BF₃·OEt₂ followed by oxidation. Metal complexation and ammonium hydroxide induced acetyl removal provides a route to these important molecular systems for future electronics experiments in which the thiols would serve as the adhesion points to gold probes.

Full text of DOI:10.1021/ol9911728

ORGANIC
LETTERS
2000
Vol. 2, No. 2
111-113
Synthesis of Porphyrins Bearing
trans-Thiols
,†
Raymond C. Jagessar and James M. Tour*
Department of Chemistry and Biochemistry, UniVersity of South Carolina,
Columbia, South Carolina 29208, and Department of Chemistry and Center for
Nanoscale Science and Technology, Rice UniVersity, MS 222, 6100 Main Street,
Houston, Texas 77005
tour@rice.edu
Received October 21, 1999
ABSTRACT
A route to porphyrins bearing trans-thiols is described using a thioacetyl-containing aldehyde or a thioacetyl-containing dipyrromethane in
the presence of catalytic BF3‚OEt2 followed by oxidation. Metal complexation and ammonium hydroxide induced acetyl removal provides a
route to these important molecular systems for future electronics experiments in which the thiols would serve as the adhesion points to gold
probes.
Future computational systems will likely consist of logic
devices that are ultradense, ultrafast, and molecular-sized.1
With the ultimate goal of constructing a molecular computer
in which the fundamental switching elements and wires are
fabricated from single, or very small packets of, molecules,
we have been studying the responses of several classes of
compounds in nanometer-sized arrays (nanopores) consisting
of ∼1000 molecules per array.2 Fundamental to their
construction is the self-assembly of conjugated molecules
in a 30-nm-diameter cavity; a pore size that is smaller than
the defect density of the self-assembled monolayer or the
underlying metal substrate. The oligomers are generally thiol-
functionalized at the extremities for sandwiched adhesion
between the proximal gold contacts. The first contact is
formed by self-assembly from solution, and the second, by
evaporation of gold to the juxtaposed end. To date, we have
focused primarily on oligo(phenylene ethynylene)s and oligo-
(thiophene ethynylene)s.3 Due to the plethora of interesting
electrical properties that have been reported on porphyrins,4
and to the modifications of their electronic properties through
various meso-substituted patterns and the use of porphyrin/
metal complexation, it behooves us to study this class of
compounds in the nanopore embodiment. We describe here
(3) (a) Jones, L., II; Schumm, J. S.; Tour, J. M. J. Org. Chem. 1997, 62,
1388. (b) Pearson, D. L.; Tour, J. M. J. Org. Chem. 1997, 62, 1376.
(4) Littler, B. J.; Ciringh, Y.; Lindsay, J. S. J. Org. Chem. 1999, 64,
2864, and references therein.
(5) (a) Chen, J.; Reed, M. A.; Asplund, C. L.; Cassell, A. M.; Myrick,
M. L.; Rawlett, A. M.; Tour, J. M.; Van Patten, P. G. App. Phys. Lett.
1999, 75, 624. (b) Rawlett, A.; Chen, J.; Reed, M. A.; Tour, J. M. Polym.
Mater., Sci. Eng. (Am. Chem. Soc., DiV. Polym. Mater.) 1999, 81, 140. (c)
Seminario, J. M.; Zacarias, A. G.; Tour, J. M. J. Am. Chem. Soc. 1999,
121, 411. (d) Allara, D. L.; Dunbar, T. D.; Weiss, P. S.; Bumm, L. A.;
Cygan, M. T. Tour, J. M.; Reinerth, W. A.; Yao, Y.; Kozaki, M.; Jones,
L., II. Molecular Electronics: Science and Technology; Aviram, A.; Ratner,
M., Eds. Ann. N. Y. Acad. Sci. 1998, 852, 349-370. (e) Cygan, M. T.;
Dunbar, T. D.; Arnold, J. J.; Bumm, L. A.; Shedlock, N. F.; Burgin, T. P.;
Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. J. Am. Chem. Soc.
1998, 120, 2721. (f) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T.
D.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S.
Science 1996, 271, 1705.
† Address correspondence to Rice University, tour@rice.edu.
(1) (a) Molecular Electronics-Science and Technology; Aviran, A.,
Ratner, R., Eds. Ann. N. Y. Acad. Sci. 1998, 852. (b) Joachim, C.; Roth,
S. Atomic and Molecular Wires; Kluwer: London, 1997. (c) Goldhaber-
Gordon, D.; Montemerlo, M. S.; Love, J. C.; Opiteck, G. J.; Ellenbogen, J.
C. Proc. IEEE 1997, 85, 521. (d) Nanostructures and Mesoscopic Systems;
Kirk, W. P., Reed, M. A., Eds.; Academic: New York, NY, 1992. (e) Tour,
J. M.; Kozaki, M.; Seminario, J. M. J. Am. Chem. Soc. 1998, 120, 8486.
(2) Zhou, C.; Deshpande, M. R.; Reed, M. A.; Jones, L., II; Tour, J. M.
Appl. Phys. Lett. 1997, 71, 611.
10.1021/ol9911728 CCC: $19.00 © 2000 American Chemical Society
Published on Web 12/29/1999

the synthetic methods used to prepare meso-substituted
porphyrins bearing thiols in the trans positions. In all cases,
the thiol end groups were attached to the porphyrin rings
via phenylene ethynylene subunits, due to our experience in
the manipulation, surface attachment, and characterization
of these oligomers on surfaces.5 These thiol-tipped systems
may prove to be of utility in a number of approaches to self-
assembled porphyrin structures where π-conjugation to the
surface is important.
Initial efforts directed toward the porphyrin targets in-
volved the preparation of dipyrromethane or aryl-substituted
dipyrromethanes with the intent of subsequent Pd/Cu cou-
pling3 to the aryl halides for preparation of the final
compounds. The porphyrin syntheses are shown in eq 1. The
Furthermore, the thioacetyl moieties did not inhibit the
reaction nor were they affected to a significant extent; the
yields were similar to those obtained in reactions that did
not have these thioacetyl functionalities. In a less controlled
dipyrromethanes could be prepared in reasonable yield and
further condensed with the complementary benzaldehyde
component to generate the trans-(halophenyl)porphyrins.4,6
Unfortunately, further attempts to elaborate the halogenated
positions via lithium-halogen exchange and subsequent
conversion directly to thioacetyl moieties (excess BuLi,
sequential quenching with S8 and AcCl)3 were unsuccessful.
Although Pd/Cu-catalyzed cross coupling reactions have been
used successfully on porphyrins, employing the coupling
partners here, the reactions were very low-yielding in our
hands. Additionally, complexing the porphyrin with zinc did
not change the unsuccessful course of the subsequent
derivatizations of 6-9. We are unclear on the source of this
coupling difficulty and few attempts were made to remedy
the problem. We immediately considered a more convergent
route.
manner, the three-component system involving pyrrole,
benzaldehyde, and 10 could be used to prepare 11 in 8%
yield after oxidation with p-chloranil. Similarly, the tetra-
(alligator clip)-substituted system 16 could be prepared from
10 and pyrrole (eq 5).4,6
The strategy was therefore modified by preparing the
aldehyde-bearing protected thiol using Pd/Cu-catalyzed
coupling of 4-iodobenzaldehyde with trimethylsilylacetylene,
subsequent deprotection, and another Pd/Cu coupling with
4-iodo-1-thioacetylbenzene (eq 2).6 Protected thiol 10 was
then condensed with the substituted dipyrromethanes (eq 3),
or 10 was condensed with pyrrole to form 15 and then further
condensed with benzaldehyde. Subsequent oxidation afforded
the target trans-substituted products (eq 4).7 Accordingly,
no further functionalization of the porphyrin was needed.
The thioacetyl groups, after hydrolysis to the free thiols,
are slated to serve as the molecular “alligator clips” for
112
Org. Lett., Vol. 2, No. 2, 2000

adhesion to gold probes.2,3 We have previously shown that
the free thiol can be liberated from its acetyl protection,
during the self-assembly process, using ammonium hydrox-
ide. If the R,ω-free aromatic thiols are used, they tend to
undergo rapid unwanted oligomerization and precipitation
in the presence of small amounts of oxygen. Hence the acetyl
protection is particularly useful.3
Finally, we have demonstrated efficient removal of the
acetyl groups in 11 using 6 µL of concentrated ammonium
hydroxide per milligram of porphyrin.8 Metal incorporation
into 11, specifically Zn (91%), Cu (95%), and Co (90%),
using the corresponding hydrated metal acetates, followed
by ammonium hydroxide-promoted thiol generation, pro-
ceeded without metal loss as indicated by 1H NMR analysis.
Therefore, we are poised for the study of the self-assembly
and nanopore testing of these trans-alligator clip-containing
porphyrin systems. Those results will be described separately.
Acknowledgment. The Defense Advanced Research
Projects Agency, via the Office of Naval Research (N00014-
99-1-0406) funded this work. We thank FMC for the
alkyllithium reagents and Dr. I. Chester of FAR Research
Inc. for trimethylsilylacetylene.
(6) For several background procedures that were used or modified for
these studies, see: (a) Lee, C. H.; Lindsey, J. S. Tetrahedron 1994, 50,
11427. (b) Wang, Q. M.; Bruce D. W. Synlett. 1995, 5580. (c) Wagner, R.
W.; Johnson, T. E.; Li, F.; Lindsey J. S. J. Org. Chem. 1995, 60, 5266. (d)
Nierengarten, J. F.; Schall. C.; Nicoud, J. F. Angew. Chem., Int. Ed. Engl.
1998, 37, 1934. (e) Alder, A. D.; Longo, F. R.; Finarelli, J. D.; Goldmacher,
J.; Assour, J.; Korsakoff, L. J. Org. Chem. 1967, 32, 476. (f) Khoung, R.
G.; Jaquinod, L.; Smith, K. M. Chem. Comm. 1997, 1057.
(7) Austin, W. B.; Bilow, N.; Kellegham, W. J.; Lau, K. S. Y. J. Org.
Chem. 1981, 46, 2280.
Supporting Information Available: Experimental pro-
cedures for the synthesis of compounds 10-16. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL9911728
(9) Tour, J. M.; Jones, L., II; Pearson, D. L.; Lamba, J. S.; Burgin, T.
P.; Whitesides, G. W.; Allara, D. L.; Parikh, A. N.; Atre, S. J. Am. Chem.
Soc. 1995, 117, 9529.
(8) All porphyrins described here were purified by flash chromatography
on silica gel.
Org. Lett., Vol. 2, No. 2, 2000
113