Inversion of Configuration at the Phosphorus Nucleophile in the Diastereoselective and Enantioselective Synthesis of P-Stereogenic syn-Phosphiranes from Chiral Epoxides.

Nucleophilic substitution results in inversion of configuration at the electrophilic carbon center (SN 2) or racemization (SN 1). The stereochemistry of the nucleophile is rarely considered, but phosphines, which have a high barrier to pyramidal inversion, attack electrophiles with retention of configuration at P. Surprisingly, cyclization of bifunctional secondary phosphine alkyl tosylates proceeded under mild conditions with inversion of configuration at the nucleophile to yield P-stereogenic syn-phosphiranes. DFT studies suggested that the novel stereochemistry results from acid-promoted tosylate dissociation to yield an intermediate phosphenium-bridged cation, which undergoes syn-selective cyclization.

Trimerization and cyclization of reactive P-functionalities confined within OCO pincers.

In order to stabilize a 10-P-3 species with C 2v symmetry and two lone pairs on the central phosphorus atom, a specialized ligand is required. Using an NCN pincer, previous efforts to enforce this planarized geometry at P resulted in the formation of a C s-symmetric, 10π-electron benzazaphosphole that existed as a dynamic "bell-clapper" in solution. Here, OCO pincers 1 and 2 were synthesized, operating under the hypothesis that the more electron-withdrawing oxygen donors would better stabilize the 3-center, 4-electron O-P-O bond of the 10-P-3 target and the sp3-hybridized benzylic carbon atoms would prevent the formation of aromatic P-heterocycles. However, subjecting 1 to a metalation/phosphination/reduction sequence afforded cyclotriphosphane 3, resulting from trimerization of the P(i) center unbound by its oxygen donors. Pincer 2 featuring four benzylic CF3 groups was expected to strengthen the O-P-O bond of the target, but after metal-halogen exchange and quenching with PCl3, unexpected cyclization with loss of CH3Cl was observed to give monochlorinated 5. Treatment of 5 with (p-CH3)C6H4MgBr generated crystalline P-(p-Tol) derivative 6, which was characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography. The complex 19F NMR spectra of 5 and 6 observed experimentally, were reproduced by simulations with MestreNova.

Synthesis of gold phosphido complexes derived from bis(secondary) phosphines. structure of tetrameric [Au(MesP(CH(2))(3)PMes)Au](4).

Treatment of 2 equiv of Au(THT)Cl (THT = tetrahydrothiophene) with the bis(secondary) phosphines HP(R) approximately PH(R) (linker approximately = (CH(2))(3), R = Mes = 2,4,6-Me(3)C(6)H(2) (1), R = Is = 2,4,6-(i-Pr)(3)C(6)H(2) (2), R = Ph (4); approximately = (CH(2))(2), R = Is (3); HP(R) approximately PH(R) = 1,1'-(eta(5)-C(5)H(4)PHPh)(2)Fe (5)), gave the dinuclear complexes (AuCl)(2)(mu-HP(R) approximately PH(R)) (6-10). Dehydrohalogenation with aqueous ammonia gave the phosphido complexes [(Au)(2)(mu-P(R) approximately P(R))](n) (11-15). Ferrocenyl- and phenylphosphido derivatives 15 and 14 were insoluble; the latter was characterized by solid-state (31)P NMR spectroscopy. Isitylphosphido complexes 12 and 13 gave rise to broad, ill-defined NMR spectra. However, mesitylphosphido complex 11 was formed as a single product, which was characterized by multinuclear solution NMR spectroscopy, solid-state (31)P NMR spectroscopy, and elemental analyses. Mass spectrometry suggested that this material contained eight gold atoms (n = 4). A structure proposed on the basis of the (1)H NMR spectra, containing a distorted cube of phosphorus atoms, was confirmed by X-ray crystallographic structure determination. NMR spectroscopy, including measurement of the hydrodynamic radius of 11 by (1)H NMR DOSY, suggested that this structure was maintained in solution. Density functional theory (DFT) structural calculations on 11 were also in good agreement with the solid-state structure.

Optical spectroscopic studies of light-harvesting by pigment-reconstituted peridinin-chlorophyll-proteins at cryogenic temperatures.

Low temperature, steady-state, optical spectroscopic methods were used to study the spectral features of peridinin-chlorophyll-protein (PCP) complexes in which recombinant apoprotein has been refolded in the presence of peridinin and either chlorophyll a (Chl a), chlorophyll b (Chl b), chlorophyll d (Chl d), 3-acetyl-chlorophyll a (3-acetyl-Chl a) or bacteriochlorophyll a (BChl a). Absorption spectra taken at 10 K provide better resolution of the spectroscopic bands than seen at room temperature and reveal specific pigment-protein interactions responsible for the positions of the Qy bands of the chlorophylls. The study reveals that the functional groups attached to Ring I of the two protein-bound chlorophylls modulate the Qy and Soret transition energies. Fluorescence excitation spectra were used to compute energy transfer efficiencies of the various complexes at room temperature and these were correlated with previously reported ultrafast, time-resolved optical spectroscopic dynamics data. The results illustrate the robust nature and value of the PCP complex, which maintains a high efficiency of antenna function even in the presence of non-native chlorophyll species, as an effective tool for elucidating the molecular details of photosynthetic light-harvesting.

Femtosecond time-resolved absorption spectroscopy of astaxanthin in solution and in alpha-crustacyanin.

Steady-state absorption and femtosecond time-resolved spectroscopic studies have been carried out on astaxanthin dissolved in CS2, methanol, and acetonitrile, and in purified alpha-crustacyanin. The spectra of the S0 --> S2 and S1 --> S(n) transitions were found to be similarly dependent on solvent environment. The dynamics of the excited-state decay processes were analyzed with both single wavelength and global fitting procedures. In solution, the S1 lifetime of astaxanthin was found to be approximately 5 ps and independent of solvent. In alpha-crustacyanin, the lifetime was noticeably shorter at approximately 1.8 ps. Both fitting procedures led to the conclusion that the lifetime of the S2 state was either comparable to or shorter than the instrument response time. The data support the idea that dimerization of astaxanthin in alpha-crustacyanin is the primary molecular basis for the bathochromic shift of the S0 --> S2 and S1 --> S(n) transitions. Planarization of the astaxanthin molecule, which leads to a longer effective pi-electron conjugated chain and a lower S1 energy, accounts for the shorter tau1 in the protein.