Fluorous chemistry: From biphasic catalysis to a parallel chemical universe and beyond

The development of the title discipline is briefly summarized, and the general topics of the articles in this issue introduced. Definitions are proposed for the terms 'fluorous', 'fluorous medium', 'fluorous separation technique', 'fluorous tag', 'fluorous reaction component', 'fluorous reaction', and 'fluorous chemistry'. © 2002 Elsevier Science Ltd. All rights reserved

An easily accessed class of recyclable hypervalent iodide reagents for functional group oxidations: bis(trifluoroacetate) adducts of fluorous alkyl iodides, CF3(CF2)(n-1)I(OCOCF3)(2)

Reactions of commercial fluorous alkyl iodides RfnI (1-R-fn; R-fn = CF3(CF2)(n-1); n = 7, 8, 10, 12) with 80% H2O2 and trifluoroacetic anhydride give RfnI(OCOCF3)(2) (2-R-fn; 89-97%). These rapidly oxidize 1,4-hydroquinones in methanol. Subsequent additions of CF3C6F11 or FC-72 give liquid/liquid biphase systems. The product quinones are generally isolated in gt = 95% yields from the methanol phases, and 1-R-fn in gt = 95% yields from the fluorous phases. Alternatively, the very low solubilities of 1-R-f10 in 10:1 v/v methanol : water or 1-R-f12 in methanol allow efficient recovery via solid/liquid phase separations without recourse to fluorous solvents. The recovered 1-R-fn may be reoxidized to 2-R-fn and reused

Synthesis, structure, and reactivity of fluorous phosphorus/carbon/phosphorus pincer ligands and metal complexes

Reactions of the diphosphine 1,3-C6H4(CH2PH2)(2) and fluorous alkenes H2C=CHRfn (R-fn = (CF2)(n-1)CF3; n = 6, 8) at 75 degrees C in the presence of AIBN give the title ligands 1,3-C6H4(CH2P(CH2CH2Rfn)(2))(2) (3-R-fn) and byproducts 1,3-C6H4(CH3)(CH2P(CH2CH2Rfn)(2)) (4-R-fn) in 1:3 to 1:5 ratios. Workups give 3-R-fn in 4-17% yields. Similar results lre obtained photochemically. Reaction of 1,3-C6H4(CH2Br)(2) and HP(CH2CH2Rf8)(2) (5) at 80 degrees C(neat, 1:2 mol ratio) gives instead of simple substitution the metacyclophane [1,3-C6H4(CH2P(CH2CH2Rf8)(2)CH2-1,3-C6H4CH2P(CH2CH2Rf8)(2)CH2](2+) 2Br(-), which upon treatment with LiAlH4 yields 3-R-f8 (20%), 4-R-f8, and other products. Efforts to better access 3-R-f8, either by altering stoichiometry or using various combinations of the phosphine borane (H3B) PH(CH2CH2Rf8)(2) and base, are unsuccessful. Reactions of 3-R-fn with Pd(O2CCF3)(2) and [IrCl(COE)(2)](2) (COE = cyclooctene) give the palladium and iridium pincer complexes (2,6,1-C6H3(CH2P(CH2CH2Rfn)(2))(2)) Pd(O2CCF3) (10-R-fn; 80-90%) and (2,6,1-C6H3(CH2P(CH2CH2Rf8)(2))(2)) Ir(Cl)(H)(11-R-f8; 29%), which exhibit CF(3)C(6)F(11)3toluene partition coefficients of gt 96 : lt 4. The crystal structure of 10-R-f8 shows CH2CH2Rf8 groups with all-anti conformations that extend in parallel above and below the palladium square plane to create fluorous lattice domains. NMR monitoring shows a precursor to 11-R-f8 that is believed to be a COE adduct

Synthesis and oxidation of chiral rhenium phosphine methyl complexes of the formula (η5-C5Me5)Re(NO)(PR3)(CH3): in search of radical cations with enhanced kinetic stabilities

Reactions of racemic [(η5-C5Me5)Re(NO)(NCCH3)(CO)]+ BF4− and phosphines PR3 (R=C6H5a; 4-C6H4CH3b; 4-C6H4-t-C4H9c; 4-C6H4C6H5d; 4-C6H4OCH3e; c-C6H11f) give the phosphine carbonyl complexes [(η5-C5Me5)Re(NO)(PR3)(CO)]+ BF4− (5a–5f+ BF4−; 55–95%). These are treated with LiEt3BH and then BH3·THF to give the phosphine methyl complexes (η5-C5Me5)Re(NO)(PR3)(CH3) (2a–2f, 50–86%). Cyclic voltammetry shows that the new compounds 2b–2f undergo chemically reversible one-electron oxidations that are thermodynamically more favorable than that of 2a (ΔE°=0.07, 0.07, 0.01, 0.09, 0.22 V; CH2Cl2). The radical cations 2+ X− can be generated with Ag+ X− or (η5-C5H5)2Fe+ X− (X−=PF6−, SbF6−), as evidenced by IR and ESR spectra, but are labile and efforts to isolate pure salts fail. Reaction of 2a and TCNE give (η5-C5Me5)Re(NO)(η2-TCNE)(CH3), which is crystallographically characterized and proposed to form by initial electron transfer followed by radical chain substitution

Synthesis and oxidation of chiral rhenium phosphine methyl complexes of the formula (η5-C5Me5)Re(NO)(PR3)(CH3): in search of radical cations with enhanced kinetic stabilities

Reactions of racemic [(η5-C5Me5)Re(NO)(NCCH3)(CO)]+ BF4− and phosphines PR3 (R=C6H5a; 4-C6H4CH3b; 4-C6H4-t-C4H9c; 4-C6H4C6H5d; 4-C6H4OCH3e; c-C6H11f) give the phosphine carbonyl complexes [(η5-C5Me5)Re(NO)(PR3)(CO)]+ BF4− (5a–5f+ BF4−; 55–95%). These are treated with LiEt3BH and then BH3·THF to give the phosphine methyl complexes (η5-C5Me5)Re(NO)(PR3)(CH3) (2a–2f, 50–86%). Cyclic voltammetry shows that the new compounds 2b–2f undergo chemically reversible one-electron oxidations that are thermodynamically more favorable than that of 2a (ΔE°=0.07, 0.07, 0.01, 0.09, 0.22 V; CH2Cl2). The radical cations 2+ X− can be generated with Ag+ X− or (η5-C5H5)2Fe+ X− (X−=PF6−, SbF6−), as evidenced by IR and ESR spectra, but are labile and efforts to isolate pure salts fail. Reaction of 2a and TCNE give (η5-C5Me5)Re(NO)(η2-TCNE)(CH3), which is crystallographically characterized and proposed to form by initial electron transfer followed by radical chain substitution

How to insulate a reactive site from a perfluoroalkyl group : photoelectron spectroscopy, calorimetric, and computational studies of long-range electronic effects in fluorous phosphines P((CH2)m(CF2)7CF3)3

This study advances strategy and design in catalysts and reagents for fluorous and supercritical CO2 chemistry by defining the structural requirements for insulating a typical active site from a perfluoroalkyl segment. The vertical ionization potentials of the phosphines P((CH2)(m)R-f8)(3) (m = 2 (2) to 5 (5)) are measured by photoelectron spectroscopy, and the enthalpies of protonation by calorimetry (CF3SO3H, CF3C6H5). They undergo progressively more facile (energetically) ionization and protonation (P(CH2CH3)(3) > 5 > 4 approximate to P(CH3)(3) > 3 > 2), as expected from inductive effects. Equilibrations of trans-Rh(CO)(Cl)(L)(2) complexes (L = 2, 3) establish analogous Lewis basicities. Density functional theory is used to calculate the structures, energies, ionization potentials, and gas-phase proton affinities (PA) of the model phosphines P((CH2)(m)CF3)(3) (2'-9'). The ionization potentials of 2'-5' are in good agreement with those of 2-5, and together with PA values and analyses of homodesmotic relationships are used to address the title question. Between 8 and 10 methylene groups are needed to effectively insulate a perfluoroalkyl segment from a phosphorus lone pair, depending upon the criterion employed. Computations also show that the first carbon of a perfluoroalkyl segment exhibits a much greater inductive effect than the second, and that ionization potentials of nonfluorinated phosphines P((CH2)(m)CH3)(3) reach a limit at approximately nine carbons (m = 8)