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.

Nucleophilic substitution results in inversion of configuration at the electrophilic carbon center (S 2) or racemization (S 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.

Alkylation of the bis(secondary) phosphine IsHP(CH 2 ) 2 PHIs (1; Is = isityl = 2,4,6-(i-Pr) 3 C 6 H 2 ) with 2-(bromomethyl)naphthalene using 10 mol % of the catalyst precursor Pt((R,R)-Me-DuPhos)-(Ph)(Cl) and the base NaOSiMe 3 selectively yielded meso-IsP(CH 2 Ar)(CH 2 ) 2 P(CH 2 Ar)(Is) (2; Ar = 2-naphthyl; dr = meso/rac ratio = 3.4:1). Half-alkylated IsP(CH 2 Ar)(CH 2 ) 2 PH(Is) (3), an intermediate in this reaction, was prepared from 1 by deprotonation (s-BuLi) and alkylation with 2-(chloromethyl)naphthalene. Analysis of the observed diastereo-and enantioselectivity in the Pt-catalyzed alkylations of 1 and 3 yielded quantitative information on the stereoselectivity of both P-C bond-forming steps. The first alkylation (1 f 3) resulted in diastereoselective formation of a tertiary phosphine stereocenter (∼2:1 ratio). In the second alkylation (3 f 2), however, both (R P )-3 and (S P )-3 (the label refers to the configuration of the tertiary phosphine) selectively formed meso-2, instead of (R,R)-2 or (S,S)-2, respectively (the ratios were ca. 3:1 and 7:1). Thus, the tertiary phosphine in 3 favored alternation of stereochemistry in the alkylation of the secondary phosphine (substrate control with negative cooperativity). Platinum-catalyzed alkylation of IsPH(CH 2 ) 2 OSi(iPr) 3 (6) gave IsP(CH 2 Ar)(CH 2 ) 2 OSi(i-Pr) 3 (9) in a 1.5:1 enantiomeric ratio (er). A related reaction of IsPH(CH 2 ) 2 OSiMe 3 (4) gave a mixture of IsP(CH 2 Ar)(CH 2 ) 2 OR (R = SiMe 3 (7); R = H (8)), while alkylation of IsPH(CH 2 ) 2 OH (5) gave 8 in about 2:1 er. Thus, the nature, and even the absolute configuration, of the pendant group X three bonds from the reactive phosphorus center in the substrates IsHP(CH 2 ) 2 X (X = PHIs (1), P(CH 2 Ar)(Is) (3), OSiMe 3 (4), OH (5), OSi(i-Pr) 3 (6)) had a strong influence on the selectivity of Pt-catalyzed phosphorus alkylation. Possible mechanistic explanations for this substrate control are discussed.

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 chlo-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 Q y bands of the chlorophylls. The study reveals that the functional groups attached to Ring I of the two protein-bound chlorophylls modulate the Q y 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.

The complexes Pt((R,R)-Me-DuPhos)(Ph)(Cl) (1) and Pt((R,R)i-Pr-DuPhos)(Ph)(Cl) (2) have been used as catalyst precursors in Pt-catalyzed asymmetric alkylation of secondary phosphines. To investigate structure− reactivity−selectivity relationships in these reactions, analogous complexes with different bis(phospholane) ligands and/or Pt-hydrocarbyl groups were prepared. Treatment of Pt(COD)(R)(Cl) (R = Me, Ph) with BPE or DuPhos ligands gave Pt((R,R)-Me-BPE)(Me)(Cl) (3), Pt((R,R)-Ph-BPE)(Me)(Cl) (5), Pt((R,R)-Ph-BPE)(Ph)(Cl) (6), and Pt((R,R)-i-Pr-DuPhos)(Me)(Cl) (7). However, treatment of Pt(COD)(Me)(Cl) with (R,R)-Me-FerroLANE gave a mixture of products, which were converted upon heating to Pt((R,R)-Me-FerroLANE)(Me)(Cl) (8). A related mixture formed from Pt(COD)(Ph)(Cl) precipitated trans-[Pt((R,R)-Me-FerroLANE)(Ph)(Cl)] n (9T), which on treatment with AgOTf followed by LiCl gave cis-Pt((R,R)-Me-FerroLANE)(Ph)(Cl) (9) as the major product. The reaction of Pt(COD)(Ph)(Cl) with (R,R)-Me-BPE gave the dinuclear dication [(Pt((R,R)-Me-BPE)(Ph)) 2 (μ-(R,R)-Me-BPE))][Cl] 2 (10) instead of the expected Pt((R,R)-Me-BPE)(Ph)(Cl) (4). The iodide Pt((R,R)-Me-BPE)(Ph)(I) (11) was formed from Pt(COD)(Ph)(I) and BPE but decomposed readily. Treatment of Pt(COD)X 2 with (R,R)-Me-BPE gave Pt((R,R)-Me-BPE)X 2 (X = Cl (12), I (13)). Reaction of Pt(COD)Ph 2 with (R,R)-Me-BPE gave Pt((R,R)-Me-BPE)Ph 2 (14), which was protonated with HCl to yield 4. Treatment of Pt((R,R)-Me-DuPhos)Cl 2 with excess (9-phenanthryl) magnesium bromide gave Pt((R,R)-Me-DuPhos)(9-phenanthryl)(Br) (15), while a similar reaction with excess (6-methoxy-2naphthyl)magnesium bromide gave Pt((R,R)-Me-DuPhos)Ar 2 (16). Complexes 3, 4, 6−10, and 12−14 were structurally characterized by X-ray crystallography. Structure−reactivity−selectivity relationships in this series of Pt catalyst precursors were investigated in the catalytic alkylation of the bis(secondary phosphine) PhHP(CH 2 ) 3 PHPh with benzyl bromide.

Catalytic asymmetric alkylation of the bis(secondary phosphines) IsHP(CH 2 ) n PHIs (1a-e, n = 1-5, Is = isityl = 2,4,6-(i-Pr) 3 C 6 H 2 ) with benzyl bromide using the base NaOSiMe 3 and the catalyst precursor Pt((R,R)-Me-DuPhos)(Ph)(Cl) gave the bis(tertiary phosphines) Is(PhCH 2 )P(CH 2 ) n P(CH 2 Ph)Is (2a-e, n = 1-5) via the intermediates Is(PhCH 2 )P(CH 2 ) n PHIs (4a-e, n = 1-5). The rates of these reactions depended strongly on n, in the order 1a < 1b < 1c ≈ 1d ≈ 1e. The bulkier bis(secondary phosphine) Mes*HP(CH 2 ) 2 PHMes* (5, Mes* = 2,4,6-(t-Bu) 3 C 6 H 2 ) did not undergo catalytic alkylation under these conditions. The alkylation selectivity also depended on n. Alkylation of 1b was meso-selective, while alkylation of 1a,c-e was rac-selective, occurring with similar diastereoselectivity and enantioselectivity for the longer linkers (1c-e). The product ratios suggested that the catalyst controlled the selectivity for 1d,e, while substrate control operated for ethano-bridged 1b, with negative cooperativity. Substrate control also likely occurred for 1a, for which competition from the background alkylation was significant. Analysis of the observed diastereo-and enantioselectivity for Pt-catalyzed alkylation of 1c and the mixed secondary/tertiary phosphine IsHP(CH 2 ) 3 P(CH 2 Ph)Is (4c) yielded quantitative information on the selectivity of both P-C bond-forming steps, which was consistent with predominant catalyst control, altered slightly by the influence of the substrate.

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.

Additional Information on Synthesis of IsHP(BH 3 )CH 2 PHIs(BH 3 ) (1-BH 3 )As mentioned in the experimental section, use of a slight excess of IsMgBr (2.4 equiv) ensured complete 1,2diarylation of Cl 2 PCH 2 PCl 2 , but subsequent formation of IsH during the workup hindered crystallization of 1-BH 3 . In that case (starting with 2.24 g (10.3 mmol) of Cl 2 PCH 2 PCl 2 ), the following procedure was used to obtain the product as a solid.Removal of the solvent from crude 1-BH 3 (formed on addition of BH 3 -THF to crude 1) gave an oily mass. 31 P NMR analysis showed two peaks corresponding to the diastereomers of the diphosphine diborane (δ -31.8, -33.9, 1:1). The mass was diluted with ca. 100 mL of pentane and allowed to stand for one week. A white precipitate (106 mg) was collected by filtration. 31 P NMR analysis of both the solid and solution showed the presence of 1-BH 3 and minor impurities. After removing the solvent from the solution, the oily mass was dissolved in 5 mL of pentane and then the solvent was removed under reduced pressure. This process was repeated until the oily mass took on a chalky appearance and was no longer translucent. At this point the mass was redissolved in 5 mL of pentane and a small amount of white precipitate appeared. The slurry was passed through a 0.5 x 2.0 cm Celite pipet column, which collected the white precipitate while the brown solution passed through. The column was washed with 5 mL more pentane and the pentane washings were collected -some of the white solid seemed to dissolve with this wash. Finally the Celite column was washed with 5 mL of Et 2 O, which was then removed in vacuo to afford 0.185 g of a white solid. A sample of 5 mg of the white solid was dissolved in 0.7 mL of Et 2 O, and 31 P NMR analysis of the material showed two peaks corresponding to the two diastereomers of the diphosphine diborane (δ -32.1 (minor), -33.9 (major), 1:10) and a minor species corresponding to a diphosphine monoborane (δ -29.5, -103.3 (< 5%)). After 24 h, this sample had reverted to a spectrum very similar in appearance to the original reaction solution, now with a 7:10 ratio of diastereomers. The pentane was removed S7 from the washings in vacuo and the above protocol (addition/removal of pentane followed by a Celite column) was repeated to afford another 0.554 g of a white solid (0.739 g total (1.44 mmol, 14.0% yield from Cl 2 PCH 2 PCl 2 )). Further repetitions proved unfruitful. Additional Information on Synthesis of rac-and meso-MiniPhos 5, IsMePCH 2 PMeIsPt-Catalyzed Synthesis of MiniPhos 5: Background Reactions Coupling of PHMeIs and CH 2 I 2 in the presence of NaOSiMe 3 but without a Pt catalyst was monitored by 31 P NMR spectroscopy for 96 h. After this time, signals were identified at δ -53.4 (1%), -53.9 (<1%), -56.6 (<1%), -61.4 (<1%), -107.4 (<1%). The low conversion without the catalyst suggests that the background is not significant in the catalytic reaction. Similarly, the reaction of MeI with IsHPCH 2 PHIs (1) and NaOSiMe 3 in the absence of a Pt catalyst was monitored...