Mercaptobiphenyls

Several groups investigated aromatic thiol systems for the formation of rigid, conjugated SAM systems [86-94] and it soon became clear that among the discussed systems, mercaptobiphenyls and 4 -substituted-4-mercaptobiphenyls are promising candidates and currently the most intensively studied thiol-based SAM systems. 

Fig. 9.9 a) G eneral molecular structure of 4-substituted-4 -mercaptobiphenyl. b) Eulerian angles defining the orientation of the molecule with respect to the surface normal, c)... 

Copyright 2000 Elsevier Science), d) Proposed structure of 4-chloro-4 -mercaptobiphenyl SAM (Reprinted with permission from [10] J. F. Kang, A. Ulman, S. Liao, et al., Langmuir 2001, 77, 95—106. Copyright 2001 American Chemical Society). 

In conclusion, the overall picture arises that a biphenyl is a suitable if not ideal mesogen for the formation of highly ordered SAM systems if compared with ah-phatic thiols. This is also reflected by the significantly higher thermal stabihty (melting temperature, Tm 140°C for MMB as compared with Tm 100°C for n-decanethiol) of mercaptobiphenyl SAMs [95]. 

Fig. 9.11 a) Chemical structures of MMB and TFMB. b) Possibilities to reduce the total dipole moment in a SAM of mercaptobiphenyls by interactions with a polar solvent, tilt or assembly of opposite dipoles in mixed monolayers, c) Surface versus solution composition found for MMB and TFMB mixed systems in polar (ethanol) and less polar (toluene) solutions illustrating the effect of the assembling dipoles, d) The opposite molecular dipoles of MMB and TFMB proved to be sufficient to induce ligand exchange in order to reach an equilibrium situation in the surface composition (modified from ref [96]). 

As already indicated in Fig. 9.9 and 9.12, the induced molecular dipole by 4 -subtituent should have a significant impact upon the tilt angle of the bonded molecule. This was examined in great detail using ER-FTIR spectroscopy of SAMs of several 4 -subtituted-4-mercaptobiphenyls on Au(lll) and Ag(lll) [10]. 

In an all-glass reactor, a SAM of 4, 4-bromo-mercaptobiphenyls on Au(lll) surfaces was converted to biphenyUithium moieties by treatment with seoBuIi to initiate anionic polymerization of styrene on gold substrates (Fig. 9.24). 

It is noteworthy that the choice of the SAM system of rigid mercaptobiphenyls thiols avoided the effect of surface reconstruction discussed previously. Analog SAMs made of longer alkanethiols (Fig. 9.7 and 9.10) may most probably not be suitable when the polymerization medium (in this case benzene/cyclohexane) is nonpolar. 

Fig. 9.24 Reaction scheme for the surface-in- rigid self-assembled monolayer of 4 -lithio-4-itiated living anionic polymerization using the mercaptobiphenyl [194].

Finally, dibenzothiophene has been isolated in moderate yields from hydrolysis of either 40 or 41 (33% and 70%, respectively). Both of these compounds were formed in good yield from o-mercaptobiphenyl. However, the general synthetic significance of this reaction is not yet clear. 

Adlkofer, K., Eck, W., Grunze, M. and Tanaka, M. Surface engineering of gallium arsenide with 4-mercaptobiphenyl monolayers. Journal of Physical Chemistry 107, 587 (2003). 

Fig. 6.70. (a) Parsons-Zobel plots of bare Au(111), covered with decanethiol (DT), co-hydroxydecanethiol (HDT), and 4 -hydroxy-4-mercaptobiphenyl (HBT). (Reprinted with permission from R. P. Janek, W. R. Fawcett, and A. Ulman, J. Rhys. Chem. B 101 8550, Fig. 12, copyright 1997, American Chemical Society.) (b) Helmholtz capacity (CH), as a function of the electrode charge for Ag(111) in contact with an aqueous solution of ions that are not specifically adsorbed. (Reprinted with permission from W. Schmickler, Chem. Rev. 96 3177, Fig. 3, copyright 1996, American Chemical Society.)... 

Chloro-2-ethyl-1,5-naphthyridine with 4 -mercaptobiphenyl-2-carbonitrile (deprotected in situ) gave 4-(2 -cyanobiphenyl-4-ylthio)-2-ethyl-l,5-naphthyridine (75) (reactants, Et3N, MeOH, reflux, N2, 1 h 38% see original for more synthon details).614... 

A. Uhnan, Self-assembled monolayers of 4-mercaptobiphenyls, Acc. Chem. Res., 34, 855, 2001. 

Fig. 2. X-ray diffraction spectra of L alanine nucleated on functional SAMs compared with L alanine crystallized from aqueous solution (left) and crystallographic images and morphology of L-alanine crystallized on (a,b) 4 -methyl-4-mercaptobiphenyl SAMs, (c,d) 4 -hydroxy-4-mercaptobiphenyl SAMs and (e,f) 4-(4-mercaptophenyl) pyridine SAMs - (right).

A wide range of sulfoxides 78, except for the one with a 2-substituted phenyl group, were converted to the corresponding dibenzothiophenes 80 in moderate to high yields. The authors proposed that sulfoxides 78 are first transformed to 2 -mercaptobiphenyl-2-ylcarbaldehydes 79 by a palladium catalyst. Then the palladium-catalyzed C—H bond activation/intramolecular S-arylation of aldehydes 79 affords dibenzothiophene 80. Indeed, they confirmed that biphenyl-2-ylthiol (81) was converted to dibenzothiophene (82) under the palladium catalysts (Scheme 23.31). 

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