Lawrence R. Sita

Self-assembled molecular rectifiers

Self-assembled monolayers (SAMs) of C6H5–C≡C–C6H4–C≡C–C6H4SH (1) on Au(111) and Ag(111) exhibit electrical rectifying behavior when probed by the scanning tunneling microscope whereas SAMs derived from decanethiol, benzenethiol and 4-phenylethynylbenzenethiol do not. We suggest that rectification arises from the conjugated length of the molecular framework of 1 and an increase of the donor character of the sulfur atom to this system upon adsorption to the metal. This is supported by surface second-harmonic generation measurements which show a significantly larger second order optical nonlinearity of SAMs of 1 compared to the other systems.

Thiophenol protecting groups for the palladium-catalyzed heck reaction: Efficient syntheses of conjugated arylthoils

Abstract A variety of S -thiophenol protecting groups have been evaluated for the Heck reaction. Of these, the S -acetyl group appears to be the best suited for facile removal under mild conditions to provide the corresponding free thiols in high yields.

Structure/Property Relationships of Polystannanes

Publisher Summary This chapter focuses on the structure or property relationships of polystannanes. Distannane, which can be considered a heavy-atom analog of ethane, is the simplest chemical structure that falls under the preceding definition for the class of compounds known as polystannanes. It has long been known that the electronic spectrum of hexaphenyldistannane exhibits a strong absorption maximum, which has been attributed to the presence of the Sn–Sn bond in this compound. Sulfur and selenium can similarly be inserted into the Sn–Sn bond, and while peralkyl distannanes are typically more reactive than peraryl ones, bulky substituents can retard these reactions by raising energy barriers for bimolecular processes. The use of sterically demanding substituents is, in fact, the basis for a strategy known as kinetic stabilization, which has been successfully employed for the synthesis of polystannane frameworks that would normally have highly reactive Sn–Sn bond. Propellanes are a class of nonclassical structures, since they possess bridgehead atoms having formally inverted tetrahedral geometries that force the four bonds from each of the bridgehead atoms to lie within the same hemisphere. Given this configuration, theoretical investigations have long sought to determine the stability and reactivity of these molecular frameworks prior to the availability of representative synthetic derivatives.

Mechanistic details for metathetical exchange between XCO (X = O and RN) and the tin(II) dimer, {Sn[N(SiMe3)2] (μ-OBut)}2

Abstract Metathetical exchange between carbon dioxide and the tin(II) dimer, {Sn[N(SiMe 3 ) 2 ]( μ -OBu 1 )} 2 ( 3 ) has been observed to cleanly produce the two new heteroleptic tin(II) dimers, Sn[N(SiMe 3 ) 2 ]( μ -OBu t ) 2 Sn(OSiMe 3 ) ( 6 ) and [Sn(OSiMe 3 )]( μ -OBu t )] 2 ( 7 ]). In addition, reaction of 3 with I equiv, of tert-butylisocyanate ( 8 ), at 25°C, quantitatively provides 6 , and with 2 equiv., quantitatively provides 7 . Likewise 6 reacts with 1 equiv, of 8 to quantitatively provide 7 . The mechanism for these latter processes has been investigated by low temperature 1 H NMR spectroscopy which reveals that metathetical exchange does not involve the tri-coordinate tin(II) centers of the dimeric structures, but rather, it occurs, in each case, via the transient monomeric tin(II) species, Sn[N(SiMe 3 ) 2 ]( μ -OBu t ) ( 4 ), that undergoes metathesis to produce, initially the open dimer intermediate, Sn(OCNBu t )(OSiMe 3 )( μ -OBu t )Sn(OBu t ) (OSiMe 3 ) ( 12 ), that is observed at −10°C. Subsequent redistribution reactions then generate the final products that are observed. Together, these mechanistic details provide additional support for the ‘monomeric tin(II)’ hypothesis proposed earlier for metathetical exchange between XCO and Sn[N (SiMe 3 ) 2 ] 2 ( 1 ).