Seven new iridabenzenes were prepared by the addition of (Z)-1,2-diphenyl-3-(2-lithioethenyl)-1-cyclopropene to the Vaska-type complexes [IrCl(CO)(PR 3 ) 2 ], containing differing phosphine ligands. The iridabenzenes could be isolated from the reaction mixture either by direct means or after heating of the crude solution. In the case of the reactions using PMe 3 and PEt 3 , a σ-vinyl/η 2 -cyclopropene Ir(I) complex, best described as an iridabenzvalene, could be isolated and fully characterized. The metallabenzvalene intermediate could be cleanly converted to the corresponding iridabenzene by heating in solution. The PEt 3 -substituted iridabenzene and iridabenzvalene were characterized by X-ray crystallography. Plausible mechanisms for the formation of both iridacycles from the lithiated vinylcyclopropene as well as the isomerization of the benzvalene into the benzene are postulated.
The reaction of (Z)-1-phenyl-2-(trimethylsilyl)-3-(2-lithiovinyl)cyclopropene (4) with Vaska's complex generates the iridabenzvalene 8, the iridabenzene
9, and the cyclopentadienyl complex 10 in a 10:2:3 ratio.
Heating this mixture to 75 °C converts 8 and 9 to 10.
NMR studies over a 24 h period at 75 °C show that
samples containing pure 8 isomerize to 10 in high yield
and generate regioisomeric iridabenzene 11 as an intermediate.
Lithium−halogen exchange of either (Z)-1-phenyl-2-trimethylsilyl- (5a) or (Z)-1,2-bis(trimethylsilyl)-3-(2-iodovinyl)cyclopropene (5b) and addition to either Vaska's or Vaska-type complexes generated
iridabenzvalenes (9, 14, 17), iridabenzenes (10, 18), and/or cyclopentadienyl complexes (11, 15, 19),
depending on both the substituents on the C5 framework and the phosphine ligands on Ir. Specifically,
the reaction of 5a with Vaska's complex afforded a mixture of 9, 10, and 11. Heating this mixture to 75
°C converted 9 and 10 to 11. NMR studies at 75 °C showed that samples of 9 isomerize to 11 in high
yield and generate regioisomeric iridabenzene 12 as an intermediate. The reaction of 5b with Vaska's
complex produced benzvalene 14 as the sole product. Complex 14 transformed completely to
cyclopentadienyliridium complex 15 at 75 °C with no benzene intermediate detectable by NMR
spectroscopy. The reaction of cyclopropene 5a with Vaska-type complexes containing alkylphosphines
of varying cone angles yielded only benzvalene complexes, which either rearranged or decomposed
depending upon the extent of heating. A hybrid-DFT computational study was carried out to investigate
reactivity differences between phenyl and trimethylsilyl iridabenzvalenes, regioselective rearrangement
of 9, and the unexpected stability/instability of 14/16. These calculations rationalize the sometimes
contradictory experimental results.
Na(+) and sugar transport by cotransporters (symporters) is thought to occur as a series of ordered ligand-induced conformational changes. To localize these conformational changes in a bacterial Na(+)/galactose cotransporter, we have employed a combination of cysteine-scanning and fluorescence techniques. Single or pairs of cysteine residues were introduced into the external face of a cysteine-less Vibrio parahaemolyticus sodium/glucose cotransporter for expression in Escherichia coli, and each transporter was purified using affinity chromatography. All the mutant proteins retained transport activity in bacteria and proteoliposomes. Each mutant was exposed to two different fluorescence reagents, ThioGlo3 or pyrene maleimide, that are essentially nonfluorescent until they react with a thiol. Fluorescence was recorded as a function of time and ligand concentrations. The reagents specifically labeled six of the seven cysteine mutants, but only in Cysteine 423 was the fluorescence affected by ligands. The rate of labeling of Cys423 by ThioGlo3 or pyrene maleimide was reduced by D-galactose in Na(+) buffer. Furthermore, the fluorescence of Thioglo3-labeled Cys423 was quenched by D-galactose, but only in the presence of Na(+). This quench was not accompanied by a Stokes shift and was not produced by nontransported sugars, e.g., L-glucose. Reducing the sodium concentration from 200 to 10 mM decreased the apparent affinity for d-galactose without altering the maximum quench with saturating D-galactose. Reducing the galactose concentration from 20 to 0.5 mM reduced both the apparent affinity for Na(+) and the maximum quench at saturating Na(+). These results suggest an ordered reaction scheme with Na(+) binding first. The fluorescence results with ThioGlo3-labeled Cys423 indicate that conformational changes underlying Na(+)/galactose cotransport occur at or near the extracellular domain between transmembrane helices 10 and 11.
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