Preferential α-hydrosilylation of terminal alkynes by bis-N-heterocyclic carbene rhodium(III) catalysts

We describe a bis-N-heterocyclic carbene rhodium(III) complex, featuring two trifluoroacetato ligands, that affords a variety of a-vinylsilanes in good yields by hydrosilylation of terminal alkynes. Selectivities around 7:1 α/β-(E) were reached, while the β-(Z) product was only marginally obtained. This example sharply contrasts with the β-(Z)-selectivity observed for its parent diiodido complexFunded by: the Spanish Ministry of Economy and Competitiveness (MINECO/FEDER). Grant Number: CONSOLIDER INGENIO CSD2009–0050, CTQ2011–27593 projects, and “Juan de la Cierva” (M.I.) and Ramón y Cajal (P. J. S. M.) programs DGA. Grant Number: FSE-E07. King Fahd University of Petroleum & Minerals–University of Zaragoza research agreement. The Center of Research Excellence in Petroleum Refining & Petrochemicals at King Fahd University of Petroleum & Minerals.Peer Reviewe

Removal and Reconstitution of the Carotenoid Antenna of Xanthorhodopsin

Salinixanthin, a C40-carotenoid acyl glycoside, serves as a light-harvesting antenna in the retinal-based proton pump xanthorhodopsin of Salinibacter ruber. In the crystallographic structure of this protein, the conjugated chain of salinixanthin is located at the protein–lipid boundary and interacts with residues of helices E and F. Its ring, with a 4-keto group, is rotated relative to the plane of the π-system of the carotenoid polyene chain and immobilized in a binding site near the β-ionone retinal ring. We show here that the carotenoid can be removed by oxidation with ammonium persulfate, with little effect on the other chromophore, retinal. The characteristic CD bands attributed to bound salinixanthin are now absent. The kinetics of the photocycle is only slightly perturbed, showing a 1.5-fold decrease in the overall turnover rate. The carotenoid-free protein can be reconstituted with salinixanthin extracted from the cell membrane of S. ruber. Reconstitution is accompanied by restoration of the characteristic vibronic structure of the absorption spectrum of the antenna carotenoid, its chirality, and the excited-state energy transfer to the retinal. Minor modification of salinixanthin, by reducing the carbonyl C=O double bond in the ring to a C-OH, suppresses its binding to the protein and eliminates the antenna function. This indicates that the presence of the 4-keto group is critical for carotenoid binding and efficient energy transfer

Photoreactions and Structural Changes of Anabaena Sensory Rhodopsin

Anabaena sensory rhodopsin (ASR) is an archaeal-type rhodopsin found in eubacteria. The gene encoding ASR forms a single operon with ASRT (ASR transducer) which is a 14 kDa soluble protein, suggesting that ASR functions as a photochromic sensor by activating the soluble transducer. This article reviews the detailed photoreaction processes of ASR, which were studied by low-temperature Fourier-transform infrared (FTIR) and UV-visible spectroscopy. The former research reveals that the retinal isomerization is similar to bacteriorhodopsin (BR), but the hydrogen-bonding network around the Schiff base and cytoplasmic region is different. The latter study shows the stable photoproduct of the all-trans form is 100% 13-cis, and that of the 13-cis form is 100% all-trans. These results suggest that the structural changes of ASR in the cytoplasmic domain play important roles in the activation of the transducer protein, and photochromic reaction is optimized for its sensor function

Nucleophilicity of Neutral versus Cationic Magnesium Silyl Compounds

Charge and ancillary ligands affect the reactivity of monomeric tris(trimethylsilyl)silyl magnesium compounds. Diamine-coordinated (tmeda)Mg{Si(SiMe3)3}Me (tmeda = tetramethylethylenediamine; 2-tmeda) and (dpe)Mg{Si(SiMe3)3}Me (dpe =1,2-N,N-dipyrrolidenylethane; 2-dpe) are synthesized by salt elimination reactions of L2MgMeBr and KSi(SiMe3)3. Compounds 2-tmeda or 2-dpe react with MeI or MeOTf to give MeSi(SiMe3)3 as the product of Si–C bond formation. In contrast, 2-tmeda and 2-dpe undergo exclusively reaction at the magnesium methyl group with electrophiles such as Me3SiI, B(C6F5)3, HB(C6F5)2, and [Ph3C][B(C6F5)4]. These reactions provide a series of neutral, zwitterionic, and cationic magnesium silyl compounds, and from this series we have found that silyl group transfer is less effective with cationic magnesium compounds than neutral complexes

Chemistry of some cation-like silyl zirconocene complexes

The aim of this study was to explore applied and mechanistic aspects of the silane dehydropolymerization reaction catalyzed by cationic early transition metallocene compounds. This problem was dealt with by a combination of spectroscopic techniques, kinetic studies, and trapping of reaction intermediates. The results of this integrated approach allow the proposal of a new redox mechanism for the dehydropolymerization reaction which explains and predicts the reactivity and selectivity of cationic early transition metal catalysts.The novel zirconocene complexes $rm lbrack Cp sp prime sb2 Zr( mu$-H)(SiHR) rbrack$sb2 sp{2+}$ lbrack BR$sp prime sb{n}$(C$sb6$F$sb5) sb{4-n} rbrack sb2 sp{2-}$(Cp$sp prime$ = Cp, MeCp, Me$sb5$Cp; R = Ph or PhCH$sb2;$ R$sp prime$=Bu or H) were isolated from silane dehydropolymerization reaction mixtures catalyzed by Cp$sp prime sb2$ZrCl$sb2$/2BuLi/B(C$sb6$F$sb5) sb3$ or by $rm lbrack Cp sp prime sb2Zr( mu$-H)H rbrack$sb2$/2B(C$rm sb6 F sb5) sb3$ combination catalysts. A total structure analysis of these compounds, performed by multinuclear, multidimensional NMR spectroscopy, shows that each zirconocene fragment bears a positive charge, which is delocalized between the metal center and the hydrosilane ligand. Reactions and intermediates leading to zirconocene complexes $rm lbrack Cp sp prime sb2Zr( mu$-H)(SiHr) rbrack$sb2 sp{2+}$ lbrack BR$sp prime sb{n}$(C$sb6$F$sb5) sb{4-n} rbrack sb2 sp{2-}$ (Cp$sp prime$ = Cp, MeCp, Me$sb5$Cp; R = Ph or PhCH$sb2;$ R$sp prime$ = Bu or H.) were investigated { it in situ}, by NMR and EPR studies, and by trapping unstable intermediates with PMe$sb3.$ Two novel Zr(III) complexes, $rm lbrack Cp sb2Zr sp{III} rbrack sp+ lbrack$BBu$sb{n}$(C$sb6{ rm F} sb5) sb{4-n} rbrack sp-$ and $rm lbrack Cp sb2Zr sp{III}(PMe sb3) sb2 rbrack sp+ lbrack$BBu$sb{n}$(C$sb6{ rm F} sb5) sb{4-n} rbrack sp-$ were identified by EPR spectroscopy. The unstable, diamagnetic compound $rm lbrack Cp sb2Zr sp{IV}Bu rbrack sp+ lbrack$BBu$sb{n}$(C$sb6{ rm F} sb5) sb{4-n} rbrack sp-$ was identified by $sp1$H-$sp1$H COSY and $sp1$H-$sp{13}$C HMQC experiments at ${-}20 sp circ$C. Redistribution of the borate butyl and pentafluorophenyl ligands was found to occur by a direct boron to boron migration and not by a metal assisted mechanism. The latter was ruled out by an independent synthesis of expected reaction intermediates for both reaction pathways $rm( lbrack Ph sb3C rbrack lbrack BBu(C sb6F sb5) sb3 rbrack, lbrack Cp sb2Zr(C sbA key step in the synthesis of the active dehydrocoupling catalyst is the thermal decomposition of$ rm Cp sb2ZrBu sb2,$which at room temperature affords paramagnetic Cp$ sb2$ZrR (R = Bu, crotyl and H) and other minor paramagnetic compounds. The pathway and the trends of this reductive decomposition were investigated in detail by EPR and multinuclear, multidimensional NMR spectroscopy. A number of intermediates were characterized. Reactions leading to the above compounds are rationalized from mechanistic and thermochemical points of view. Although it has not been possible to identify all of the decomposition products, the very fact that the$ rm Cp sb2ZrCl sb2$/2BuLi reagent contains substantial amounts of Zr(III) explains the origin of the unusual reactivity of the$ rm Cp sb2ZrCl sb2$/2BuLi reagent in dehydropolymerization of silanes and sheds new light on the reaction mechanism.A kinetic study of the photochemical reaction of$ rm lbrack Cp sb2Zr(SiHPh)( mu$-H) rbrack$ sb2 sp{2+} lbrack$BBu$ sb{n}$(C$ sb6$F$ sb5) sb{4-n} rbrack sb2 sp{2-}$with benzylsilane was carried out with a combination of NMR and EPR spectroscopy. The role of the paramagnetic intermediate,$ rm lbrack Cp sb2Zr sp{III} rbrack sp+ lbrack$BBu$ sb{n}$(C$ sb6$F$ sb5) sb{4-n} rbrack sp-,$was investigated. The rate law and qualitative observations support a new Zr(IV)/Zr(III) redox mechanism.The use of ``cation-like'' metallocene combination catalysts$ rm(Cp sp prime sb2MCl sb2$- 2BuLi -$ rm B(C sb6F sb5) sb3; Cp sp prime{=} eta sp5$-cyclopentadienyl, or substituted$ eta sp5$-cyclopentadienyl; M=Ti, Zr, Hf, U) for dehydropolymerization of silane significantly improves the polymer molecular weight. (Abstract shortened by UMI.

A large photolysis-induced pKa increase of the chromophore counterion in bacteriorhodopsin: implications for ion transport mechanisms of retinal proteins.

The proton-pumping mechanism of bacteriorhodopsin is dependent on a photolysis-induced transfer of a proton from the retinylidene Schiff base chromophore to the aspartate-85 counterion. Up until now, this transfer was ascribed to a > 7-unit decrease in the pKa of the protonated Schiff base caused by photoisomerization of the retinal. However, a comparably large increase in the pKa of the Asp-85 acceptor also plays a role, as we show here with infrared measurements. Furthermore, the shifted vibrational frequency of the Asp-85 COOH group indicates a transient drop in the effective dielectric constant around Asp-85 to approximately 2 in the M photointermediate. This dielectric decrease would cause a > 40 kJ-mol-1 increase in free energy of the anionic form of Asp-85, fully explaining the observed pK alpha increase. An analogous photolysis-induced destabilization of the Schiff base counterion could initiate anion transport in the related protein, halorhodopsin, in which aspartate-85 is replaced by Cl- and the Schiff base proton is consequently never transferred