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.