Synthesis of β-amino acid derivatives via copper-catalyzed asymmetric 1,4-reduction of β-(acylamino)acrylates

A new set of reaction conditions has been established to facilitate the copper-catalyzed enantioselective 1,4-reduction of β-(acylamino)acrylates toward a selection of β-alkyl-β-amino acid derivatives in high yields and with uniformly high ee values (up to 99%) irrespective of the use of (E)- or (Z)-substrates.     

Highly enantioselective asymmetric hydrogenation of beta-acetamido dehydroamino acid derivatives using a three-hindered quadrant rhodium catalyst

[reaction: see text] A previously reported three-hindered quadrant chiral ligand and its corresponding rhodium complex provide high enantioselectivity for the asymmetric hydrogenation of beta-acetamido dehydroamino acid substrates. Both (E)- and (Z)-substrates are hydrogenated with high enantioselectivity in all of the reported examples. Asymmetric hydrogenation of a cyclic beta-acetamido dehydroamino acid substrate in 85% ee is also reported.     

Kinetik und stereochemischer Verlauf der thermischen Umlagerung der vier stereoisomeren Propenyl-(but-2′-enyl)-äther

The gas-phase rearrangement of (1Z, 2′E)-, (1Z, 2′Z)-, (1E, 2′E)-, and (1E, 2′Z)- propenyl but-2′-enyl ether (Z, E)-, (Z, Z)-, (E, E)-, and (E, Z)-1) into erythro- and threo-2, 3-dimethyl-pent-4-en-al (erythro- and threo-2) was investigated over a temperature range from 142,5° to 190,0° at 20–35 Torr (for kinetic data and activation parameters see table 2). All four stereoisomeric ethers 1 rearrange preferentially via a chair-like transition state C into the aldehydes 2 (ΔΔG^≠ (160°) = 2,5–2,7 kcal/mol for B – C (B = boat-like transition state). The relative rates (k_rel) for (Z, Z)-1, (Z, E)-1, (E,Z)-1, and (E,E)-1 at 160° are 1,0, 2,9, 4,3 and 9,0 respectively (see table 5). Taking into account the relative enthalpies of the ethers 1 and the steric interaction in the C transition state of the ethers 1 (see table 6), k_rel values can be estimated. They are in good agreement with those observed (see table 5).     

Highly stereoselective synthesis of (E)- and (Z)-alpha-fluoro-alpha,beta-unsaturated esters and (E)- and (Z)-alpha-fluoro-alpha,beta-unsaturated amides from 1-bromo-1-fluoroalkenes via palladium-catalyzed carbonylation reactions

The highly stereoselective synthesis of (E)- and (Z)-alpha-fluoro-alpha,beta-unsaturated esters and (E)- and (Z)-alpha-fluoro-alpha,beta-unsaturated amides is described. 1-Bromo-1-fluoroalkenes (E/Z approximately 1:1), which are readily available starting materials, have been found to isomerize to high E/Z ratios after storage at -20 degrees C for 1 week or by photolysis at 254 nm. Since the (E)-isomers have been found to react faster than the corresponding (Z)-isomers at room temperature in Pd(0)-catalyzed reactions, the palladium-catalyzed carboalkoxylation of high E/Z 1-bromo-1-fluoroalkenes lead to a high Z/E (Z/E >/= 98:2) ratio of the alpha-fluoro-alpha,beta-unsaturated esters. When 1-bromo-1-fluoroalkenes (E/Z approximately 1:1) were reacted with HCOOH/NBu(3)/Pd(II)/DMF, the (E)-isomer was selectively reduced, and the remaining (Z)-1-bromo-1-fluoroalkenes were recovered in essentially pure isomeric form. The resulting mixture of (Z)-1-bromo-1-fluoroalkenes and the reduced products underwent similar palladium-catalyzed carboalkoxylation reactions at 70 degrees C, and the (E)-alpha-fluoro-alpha,beta-unsaturated esters were stereospecifically obtained. This methodology was also successfully applied for the stereospecific synthesis of (Z)- and (E)-alpha-fluoro-alpha,beta-unsaturated amides: the palladium-catalyzed carboamidation reaction of high E/Z and (Z)-1-bromo-1-fluoroalkenes lead to pure (Z)- and (E)-alpha-fluoro-alpha,beta-unsaturated amides, respectively.     

C-C coupling reactions in the coordination sphere of rhodium(I) and rhodium(III): New routes for the di- and trimerization of terminal alkynes

The alkynyl(vinylidene)rhodium(I) complexes trans-[Rh(C[triple bond, length as m-dash]CR)(=C=CHR)(PiPr3)2] 2, 5, 6 react with CO by migratory insertion to give stereoselectively the butenynyl compounds trans-[Rh{eta1-(Z)-C(=CHR)C[triple bond, length as m-dash]CR}(CO)(PiPr3)2](Z)-7-9, of which (Z)-7 (R=Ph) and (Z)-8 (R=tBu) rearrange upon heating or UV irradiation to the (E) isomers. Similarly, trans-[Rh{eta1-C(=CH2)C[triple bond, length as m-dash]CPh}(CO)(PiPr3)2] 12 and trans-[Rh{eta1-(Z)-C(=CHCO2Me)C[triple bond, length as m-dash]CR}(CO)(PiPr3)2](Z)-15, (Z)-16 have been prepared. At room temperature, the corresponding "non-substituted" derivative trans-[Rh{eta1-C(=CH2)C[triple bond, length as m-dash]CH}(CO)(PiPr3)2] 18 is in equilibrium with the butatrienyl isomer trans-[Rh(eta1-CH=]C=C=CH2)(CO)(PiPr3)2] 19 that rearranges photochemically to the alkynyl complex trans-[Rh(C[triple bond, length as m-dash]CCH=CH2)(CO)(PiPr3)2] 20. Reactions of (Z)-7, (E)-7, (Z)-8 and (E)-8 with carboxylic acids R'CO2H (R'=CH3, CF3) yield either the butenyne (Z)- and/or (E)-RC[triple bond, length as m-dash]CCH=CHR or a mixture of the butenyne and the isomeric butatriene, the ratio of which depends on both R and R'. Treatment of 2 (R=Ph) with HCl at -40 degrees C affords five-coordinate [RhCl(C[triple bond, length as m-dash]CPh){(Z)-CH=CHPh}(PiPr3)2] 23, which at room temperature reacts by C-C coupling to give trans-[RhCl{eta2-(Z)-PhC[triple bond, length as m-dash]CCH=CHPh}(PiPr3)2](Z)-21. The related compound trans-[RhCl(eta2-HC[triple bond, length as m-dash]CCH=CH2)(PiPr3)2] 27, prepared from trans-[Rh(C[triple bond, length as m-dash]CH)(=C=CH2)(PiPr3)2] 17 and HCl, rearranges to the vinylvinylidene isomer trans-[RhCl(=C=CHCH=CH2)(PiPr3)2] 28. While stepwise reaction of 2with CF3CO2H yields, via alkynyl(vinyl)rhodium(III) intermediates (Z)-29 and (E)-29, the alkyne complexes trans-[Rh(kappa1-O2CCF3)(eta2-PhC[triple bond, length as m-dash]CCH=CHPh)(PiPr3)2](Z)-30 and (E)-30, from 2 and CH3CO2H the acetato derivative [Rh(kappa2-O2CCH3)(PiPr3)2] 33 and (Z)-PhC[triple bond, length as m-dash]CCH=]CHPh are obtained. From 6 (R=CO2Me) and HCl or HC[triple bond, length as m-dash]CCO2Me the chelate complexes [RhX(C[triple bond, length as m-dash]CCO2Me){kappa2(C,O)-CH=CHC(OMe)=O}(PiPr3)2] 34 (X=Cl) and 35 (X=C[triple bond, length as m-dash]CCO2Me) have been prepared. In contrast to the reactions of [Rh(kappa2-O2CCH3)(C[triple bond, length as m-dash]CE)(CH=CHE)(PiPr3)2] 37(E=CO2Me) with chloride sources which give, via intramolecular C-C coupling, four-coordinate trans-[RhCl{eta2-(E)-EC[triple bond, length as m-dash]CCH=CHE}(PiPr3)2](E)-36, treatment of 37with HC[triple bond, length as m-dash]CE affords, via insertion of the alkyne into the rhodium-vinyl bond, six-coordinate [Rh(kappa2-O2CCH3)(C[triple bond, length as m-dash]CE){eta1-(E,E)-C(=CHE)CH=CHE}(PiPr3)2] 38. The latter reacts with MgCl2 to yield trans-[RhCl{eta2-(E,E)-EC[triple bond, length as m-dash]CC(=CHE)CH=CHE}(PiPr3)2] 39, which, in the presence of CO, generates the substituted hexadienyne (E,E)-EC[triple bond, length as m-dash]CC(=CHE)CH=CHE 40.     

The hetero-Diels-Alder addition of sulfur dioxide to 1-fluorobuta-1,3-dienes: the sofa conformations preferred by 6-fluorosultines (6-fluoro-3,6-dihydro-1,2-oxathiin-2-oxides) enjoy enthalpic and conformational Anomeric effects

The reactivity of (E)- and (Z)-1-fluorobuta-1,3-diene ((E)- and (Z)-11), 2-fluorobutadiene (12), (E)- and (Z)-1-(fluoromethylidene)-2-methylidenecyclohexane ((E)- and (Z)-13) toward SO(2) has been explored and compared with that of (Z)- and (E)-1-(fluoromethylidene)-2-methylidene-3,4-dihydronaphthalene ((Z)-8 and (E)-8). In agreement with quantum calculations, 12 is unreactive toward SO(2) (no cycloaddition, only polymerization), whereas (E)-1-fluoro-1,3-dienes react more rapidly than their (Z)-isomers to give the corresponding 6-fluorosultines following the endo (Alder rule) mode of hetero-Diels-Alder addition. No sulfolene has been observed following the cheletropic mode of addition with the fluorodienes, in contrast to other substituted dienes. In agreement with the calculations, cis-2-fluoro-3,4-oxathiabenzobicyclo[4.4.0]dec-1(6),9-diene-4-oxide (cis-9, the sultine obtained by SO(2) addition to (Z)-8 under conditions of kinetic control) adopts a sofa conformation with the oxygen atom of the ring lying in the average plane of the four carbon atoms of its sultine moiety when it is in the crystalline state at -100 degrees C. A similar sofa conformation was found for its trans-isomer, trans-9, obtained by isomerization of cis-9 or by hetero-Diels-Alder addition of SO(2) to (E)-8. Experiments (equilibrium constant for hetero-Diels-Alder additions, bond lengths, and bond angles in crystalline fluorosultines cis-9 and trans-9) and high-level quantum calculations on cis- and trans-6-fluoro-3,6-dihydro-1,2-oxathiin-2-oxide (cis- and trans-20) confirm the existence of a stabilizing, enthalpic, anomeric (gem-disubstitution by sulfinyloxy and fluoro groups) effect, which is interpreted in terms of (lone pair) n(O1)-->sigma*(C-F) hyperconjugative interactions. This effect is strongest in the sofa conformers with a gauche arrangement of the sigma(O1,S2) and sigma(C6,F) bonds. The calculations suggest also that n(O1)-->sigma*(S2,O2'), pi*(S=O), and n(S2)-->sigma*(O1,C6) interactions intervene and affect the relative stability of the conformers (sofa, boat, pseudo-chair) found for 6-fluorosultines cis- and trans-20.     

Substituent effects on pyrid-2-yl ureas toward intramolecular hydrogen bonding and cytosine complexation

Equilibria between two conformational isomers of pyrid-2-yl ureas, the (E,Z) and (Z,Z) forms, have been studied in DMF-d(7) at -70 degrees C. Most of them show a small preference for the (E,Z) form with an equilibrium constant K(i) around 1-2. However, the K(i) value for 1-methyl-2-(3-(pyrid-2-yl)ureido)pyridinium iodide (12) was found to be 14.2 +/- 1.2. That is 1 order of magnitude larger than those of the others, which indicates that the positively charged 1-methylpyridinium-2-yl substituent would facilitate the (E,Z) form formation. Pyrid-2-yl ureas bind cytosine in DMF-d(7) with binding constants K(B) ranging from 30 to 1700 M(-1). Electron withdrawing substituents, such as the 4-O(2)NC(6)H(4)- or 1-methylpyridinium-4-yl substituent, preferentially facilitate the intermolecular cytosine complexation with large binding constants.     

1,4-取代-2-丁炔共二聚反应中溶剂效应和盐效应对反应选择性的影响

PdCl_2(PhCN)_2催化的1,4-取代-2-丁炔与烯丙基氯的共二聚反应中,考察了不同的溶剂、不同的盐组分对生成双键的顺反异构体的影响。实验结果发现,1,4-二氯-2-丁炔与烯丙基氯的反应中,不加溶剂和氯化物,产物的双键以E式为主,加入溶剂和氯化物,产物以Z式为主。     

Synthesis, structure, and reactivity of novel 3,4-diphosphacyclobutenes bearing silyl groups

[reaction: see text] Copper-mediated homocoupling of sterically hindered 2-(2,4,6-tri-tert-butylphenyl)-1-trialkylsilyl-2-phosphaethenyllithiums afforded 1,2-bis(trialkylsilyl)-3,4-diphosphacyclobutenes (1,2-dihydrodiphosphetenes) through a formal electrocyclic [2+2] cyclization in the P=C-C=P skeleton as well as 2-trimethylsilyl-1,4-diphosphabuta-1,3-diene. Reduction of 1,2-bis(trimethylsilyl)-3,4-diphosphacyclobutenes followed by quenching with electrophiles afforded ring-opened products, (E)-1,2-bis(phosphino)-1,2-bis(trimethylsilyl)ethene and (Z)-2,3-bis(trimethylsilyl)-1,4-diphosphabut-1-ene. The structures of the ring-opened products indicated E/Z isomerization around the C=C bond after P-P bond cleavage of 5, and the isomerization of the P-C=C skeleton. Ring opening of 1,2-bis(trimethylsilyl)-3,4-diphosphacyclobutenes affording (E,E)- and (Z,Z)-1,4-diphosphabuta-1,3-dienes was observed upon desilylation.     

Enantioselective syntheses of syn- and anti-β-hydroxyallylsilanes via allene hydroboration-aldehyde allylboration reactions

The kinetic hydroboration of allenylsilane 5 with ((d)Ipc)(2)BH at -40 °C provides allylborane 9Z with ≥12:1 selectivity. When the hydroboration is performed at temperatures above -40 °C, 9Z isomerizes to the thermodynamically more stable allylborane 9E with >20:1 selectivity. Subsequent treatment of 9Z or 9E with aldehydes at -78 °C provides syn- or anti-β-hydroxyallylsilanes, 7 or 8, respectively.     

Syntheses of highly unsaturated isocyanides via organometallic pathways

The carbon carbon coupling reaction by nucleophilic attack of (CO)(5)Cr(CN-CF=CF(2)) 1 by lithium or Grignard compounds 2a-i yields the isocyanide complexes (CO)(5)Cr(CN-CF=CF-R) 3a-i (a R = CH=CH(2), b R = CH=CF(2), c R = C≡CH, d R = C≡C-SiMe(3), e R = C≡C-Ph, f R = C≡C-C(6)F(4)OMe, g R = C≡C-C(6)H(3)(CF(3))(2), h R = C(6)F(5), i R = C(6)H(3)(CF(3))(2)) as mixtures of E and Z isomers. The dinuclear complexes 5a-c are obtained from the reaction of 1 with the dilithio or dimagnesium compound 4a-c as the Z,Z-, E,Z- and E,E-isomers, respectively. (CO)(5)Cr(CN-CF=CF-C≡C-C≡C-CF=CF-NC)Cr(CO)(5)7 is obtained as a mixture of Z,Z-, Z,E- and E,E-isomers from (CO)(5)Cr(CN-CF=CF-C≡C-H 3d by Eglington-Glaser coupling. (CO)(5)Cr(CN-CF=CF-C≡C-CF=CF-NC)Cr(CO)(5)6 and (CO)(5)Cr(CN-CF=CF-C=C-C≡C-CF=CF-NC)Cr(CO)(5)7 react with octacarbonyldicobalt forming the cluster compounds Z,Z-[{η(2)-μ(2)-(CO)(5)Cr(CN-CF=CF-C≡C-CF=CF-NC)Cr(CO)(5)}Co(2)(CO)(6)] Z,Z-8, E,Z-[{η(2)-μ(2)-(CO)(5)Cr(CN-CF=CF-C≡C-CF=CF-NC)Cr(CO)(5)}Co(2)(CO)(6)] E,Z-8 and E,E-[{η(2)-μ(2)-(CO)(5)Cr(CN-CF=CF-C≡C-CF=CF-NC)Cr(CO)(5)}Co(2)(CO)(6)] E,E-8 and Z,Z-[{η(2)-μ(2)-(CO)(5)Cr(CN-CF=CF-C≡C-C≡C-CF=CF-NC)Cr(CO)(5)}{Co(2)(CO)(6)}(2)] Z,Z-9, E,Z-[{η(2)-μ(2)-(CO)(5)Cr(CN-CF=CF-C≡C-C≡C-CF=CF-NC)Cr(CO)(5)}{Co(2)(CO)(6)}(2)] E,Z-9 and E,E-[{η(2)-μ(2)-(CO)(5)Cr(CN-CF=CF-C≡C-C≡C-CF=CF-NC)Cr(CO)(5)}{Co(2)(CO)(6)}(2)] Z,Z-9, respectively. The crystal and molecular structures of E-3d, Z-3h, Z,Z-8, E,Z-8 and Z,Z-9 were elucidated by single-crystal X-ray crystallography.     

Stereoselective preparation of (E)- and (Z)-alpha-fluorostilbenes via palladium-catalyzed cross-coupling reaction of high E/Z ratio and (Z)-1-bromo-1-fluoroalkenes

A highly stereoselective method to prepare both (E)- and (Z)-alpha-fluorostilbenes is described. 1-Bromo-1-fluoroalkenes (E/Z approximately 1:1), a readily available starting material, isomerizes to high E/Z ratios by storage at -20 degrees C or by photolysis at 254 nm. Stille coupling between these high E/Z 1-bromo-1-fluoroalkenes and aryl stannanes gave (Z)-alpha-fluorostilbenes in high stereoselectivity. (Z)-1-Bromo-1-fluoroalkenes, which were kinetically separated from 1-bromo-1-fluoroalkenes (E/Z approximately 1:1), can participate in Suzuki coupling reactions to give (E)-alpha-fluorostilbenes stereoselectively.     

Novel Aplysinopsin-Type Alkaloids from Scleractinian Corals of the Family Dendrophylliidae of the Mediterranean and the Philippines. Configurational-assignment criteria,stereospecific synthesis,and photoisomerization

From the scleractinian coral Tubastraea sp. (Dendrophylliidae) collected at Palawan, Philippines, 3′-deimino-3′-oxoaplysinopsin ( 4 ) and 6-bromo-3′-deimino-3′-oxoaplysinopsin ( 6 ) are now isolated as 5:2 mixtures of (E/Z) stereoisomers. The 3′-deimino-2′,4′-bis(demethyl)-3′-oxoaplysinopsin ( 7 ) and 6-bromo-3′-demino-2′,4-bis(demethyl)-3′-oxoaplysinopsin ( 5 ) are isolated as 2:3 and 1:1 (E/Z) mixtures, respectively, from another dendrophylliid, Leptopsammia pruvoti, collected near Marseille, Mediterranean coast of France. Larger amounts of these and related compounds, needed for a full structural determination, are obtained by synthesis. Thus, condensations of indol-3-carboxaldehyde (9) or of its 6-bromo derivative 14 with hydantoin (15) , 3-methylhydantoin (11) , or 1,3dimethylhydantoin (10) give the prevalent natural aplysinopsins with high stereospecificity. The minor stereoisomers (Z)- 4 , (Z)- 6 , (E)- 7 , and (E)- 5 are obtained by (E/Z) photoisomerization under UV light of the condensation mixtures. The configuration is assigned from larger H? C(8)/C(5′) ¹H, ¹³C couplings in the (E) than in the (Z) isomer, and, in the case of 4 and 6 , from NOE enhancement at Me? N(2′) on irradiation at H? C(8). The stereospecificity of the condensations is attributed to steric inhibition to planarity in the rate-limiting transition states, due to N(2′)/H? C(2) repulsion with (Z)- 4 and (Z)- 6 , or to C(5′)?O/H? C(2) repulsion with (E)–7 or (E)- 5 . As the aplysinopsins undergo (E/Z ) phostoisomerization also under the daylight conditions of the laboratory, their isomeric composition in nature can not be presently assessed.     

Intramolecularly sensitized precipitons: a model system for application to metal sequestration

We have investigated light-triggered or catalytically activated precipitation agents and have proposed the name "precipiton" for such molecules or molecular fragments. A phase separation is induced when the precipiton isomerizes to a low-solubility form. In this paper we describe the first intramolecularly activated precipitons. The isomerization process is induced by intramolecular triplet energy transfer from a covalently attached metal complex. As expected, intramolecular sensitization leads to a more rapid isomerization than can be achieved by intermolecular sensitization at accessible concentrations. Two isomeric bichromophoric precipiton species, each containing [Ru(bpy)(3)](2+) and 1,2-bis(biphenyl)ethene units covalently linked together by an ether tether, have been synthesized and characterized, and their photochemical properties have been investigated. The rates of photoisomerization of these complexes, [((Z)-1,2-bis(biphenyl)ethene-bpy)Ru(bpy)(2)](PF(6))(2) (2Z) and [((E)-1,2-bis(biphenyl)ethene-bpy)Ru(bpy)(2)](PF(6))(2) (2E), were compared to those of their untethered analogues, (Z)-1,2-bis(biphenyl)ethene-OTBS (1Z) and (E)-1,2-bis(biphenyl)ethene-OTBS (1E), where ruthenium sensitization occurred through an intermolecular pathway. Upon irradiation with visible light (lambda > or = 400 nm) in degassed solution, 2Z/E and 1Z/E obeyed reversible first-order rate kinetics. The intramolecularly sensitized precipiton 2Z isomerized 250 times faster (k(2Z-->2E) = 1.0 x 10(-3) s(-1) with a 51% neutral density filter) than the intermolecular case 1Z (k(1Z-->1E) = 0.80 x 10(-5) s(-1)). For 1E and 2E, the isomerization rates were k(1E-->1Z) = 11.0 x 10(-5) s(-1) and k(2E-->2Z) = 1.6 x 10(-3) s(-1), respectively. The average Z/E mole ratio at the photostationary state was 62/38 for 2Z/E and 93/7 for 1Z/E. The impetus for this study was our desire to evaluate the possibility of using metal-binding precipitons that would precipitate only upon metal-to-precipiton binding and would be inert to visible light in the absence of metals.     

Stereoselective generation of (E)- and (Z)-2-fluoroalkylidene-type carbenoids from (2-fluoro-1-alkenyl)iodonium salts and their application for stereoselective synthesis of fluoroalkenes

Alkylidene-type carbenoids, generated from (Z)- or (E)-(2-fluoro-1-alkenyl)iodonium salts by treatment with LDA, reacted with trialkylboranes to give (E)- or (Z)-(fluoroalkenyl)boranes stereoselectively. The resulting (fluoroalkenyl)borane can be used for the selective synthesis of (E)- or (Z)-fluoroalkenes, (E)- or (Z)-fluoroiodoalkenes, and alpha-fluoroketones. [reaction: see text]     

Fluorine segregation in crystalline materials: structural control and solid-state [2+2] cycloaddition in CF(3)-substituted tetrathiafulvalene derivatives

The well-known influence of long perfluorinated chains on the structures and stability of amphiphilic molecules in liquid crystalline mesophases or mesoscopic micellar arrangements is evaluated here in the realm of crystalline materials based on rigid aromatic molecules bearing only a limited number of CF(3) moieties. Tetrathiafulvalene (TTF) derivatives bearing one or two CF(3) groups, that is, (Z)- and (E)-(CF(3))(2)TTF ((Z)-1, (E)-1), EDT-TTF-CF(3) (2), and EDT-TTF(CF(3))(2) (3) (EDT=ethylenedithio) are prepared from the 1,3-dipolar reaction of methyl 4,4,4-trifluorotetrolate with ethylenetrithiocarbonate. The structures of neutral (Z)-1, (E)-1, 2, and 3 as indicated by single-crystal X-ray diffraction measurements reveal the recurrent formation of layered structures with a strong segregation of the fluorinated moieties and formation of fluorous bilayers, attributed to the amphiphilic character of those TTF derivatives upon CF(3) functionalization, and without need for longer C(n)F(2n+1) (n>1) perfluorinated chains. The short intermolecular distance between outer C==C double bonds observed in the layered structure of (E)-1 allows a solid-state [2+2] photocyclization with formation of chiral dyads incorporating the characteristic cyclobutane ring. These dyads containing two dihydrotetrathiafulvalene moieties facing each other exhibit reversible oxidation to the mixed-valence radical cation state and organize in the solid-state into the same layered structures with fluorous bilayers.     

Photoresponsive change of oxygen-binding affinity of a cobalt schiff-base complex with an axially coordinating stilbazole residue of a copolymer

The reversible binding reaction of oxygen to N,N'-ethylene bis(salicylideneiminato) cobalt(II) (CoS), to which a photoisomerizable stilbazole residue of copolymer 1 coordinated, was investigated. The E form of the stilbazole residues coordinating to CoS, (E)-1-CoS, showed photoisomerization into the Z form, (Z)-1-CoS, on direct ultraviolet irradiation and negligible reverse isomerization. The oxygen-binding equilibrium constant (K) values for (E)-1-CoS and (Z)-1-CoS in toluene were 3.3 x 10(-2) mmHg-1 and 4.8 x 10(-3) mmHg(-1), respectively, at 10 degrees C. Although the pK(a) values of the E and Z forms of the stilbazole residue were similar, the oxygen-binding affinity of (Z)-1-CoS was small in terms of the linear correlation of the logarithm of K (ln K) versus the pK(a). Steric hindrance of the polymer chain of 1 on the coordination of the stilbazole residue of (Z)-1 to CoS was thought to cause the small K. A photoresponsive change of apparent oxygen-binding affinity of 1-CoS along with the E/Z isomerization of the stilbazole residues was observed. The ratio of (Z)-1-CoS converted from (E)-1-CoS by the ultraviolet irradiation could be estimated from analysis of absorption spectra for the oxygen binding of a resulting mixture of (E)-1-CoS and (Z)-1-CoS.     

Disulfuration and hydrosulfuration of an alkyne at a 1,2-dicarba-closo-dodecaborane-thiolate ligand

The reaction of the dinuclear cobalt compound [(CpCoS(2)C(2)B(10)H(10))(CpCoSC(2)B(10)H(11))(n-C(4)H(9)S)] (1) with HC≡CC(O)Fc leads to the cobalt-free products (C(2)B(10)H(10))(SCH=CHCOFc)(2) (4-6), (S(2)C(2)B(10)H(10))(HC=CCOFc) (7), and (C(2)B(10)H(11))(SCH=CHCOFc) (8, 9). 4-6 are produced by hydrosulfuration of the alkyne at the 1,2-dicarba-closo-dodecaborane-dithiolate ligand with the generated vinyl groups in Z/Z, Z/E and E/E configurations, respectively. In 7, the alkyne is added to 1,2-dicarba-closo-dodecaborane-dithiolate at the two sulfur sites. 8 and 9 are the products of alkyne hydrosulfuration at the 1,2-dicarba-closo-dodecaborane-1-monothiolate ligand with the generated vinyl group in either Z or E configuration. The treatment of 1 with HC≡CCO(2)Me gives rise to the parallel products (C(2)B(10)H(10))(SCH=CHCO(2)Me)(2) (10-12) and (C(2)B(10)H(11))(SCH=CHCO(2)Me) (13, 14). All of the new compounds have been characterized by IR, NMR, elemental analysis and mass spectroscopy. The structures of compounds 4, 7, and 8 have also been determined by single-crystal X-ray diffraction analysis.     

Synthesis of (10Z)- and (10E)-19-fluoro-1alpha,25-dihydroxyvitamin D3: compounds to probe vitamin D conformation in receptor complex by 19F-NMR

To study the interaction of vitamin D with its receptor by 19F-NMR, (5Z,10Z)- and (5Z,10E)-19-fluoro-1alpha,25-dihydroxyvitamin D3 were synthesized starting from vitamin D2 via electrophilic fluorination of vitamin D-SO2 adducts as the key step. Regio- and stereoselective electrophilic fluorination at C(19) of vitamin D-SO2 adducts was achieved under the conditions using (PhSO2)2NF and bulky bases. The stereochemistry of the addition and elimination of SO2 of various vitamin D derivatives was studied in detail. SO2 causes Z-E isomerization of the 5,6-double bond of vitamin D and adds to the resulting (5E)-isomer from the sterically less hindered side opposite to the substituent at C(1). Elimination of SO2 from 19-substituted vitamin D-SO2 adducts proceeded exclusively in a suprafacial manner with respect to the diene part under either thermal or reductive conditions. Dye-sensitized photochemical isomerization of 19-fluorovitamin D derivatives was studied in detail. The rapid isomerization at the 5,6-double bond was followed by the slow isomerization at the 10,19-double bond to yield the (5E,10Z)-isomer (by nomenclature of the 1-OH derivatives) as the major product. (10Z)- and (10E)-19-Fluorovitamin Ds were also interconverted thermally probably via the corresponding previtamin D by 1,7-sigmatropic isomerization.     

Stereospecific preparation of (Z)- and (E)-2,3-difluoro-3-stannylacrylic ester synthons and a new, efficient stereospecific route to (Z)- and (E)-2,3-difluoroacrylic esters

[reaction: see text] The (Z)-2,3-difluoro-3-stannylacrylic ester is readily prepared from (Z)-1,2-difluorovinyltriethylsilane via stereospecific stannyl/silyl exchange with KF/(Bu3Sn)2O or Bu3SnCl in DMF at 70 degrees C. The corresponding (E)-2,3-difluoro-3-stannylacrylate is prepared by stereospecific carbonylation of (E)-1,2-difluorovinyl iodide followed by low temperature/in situ stannylation of the resultant (Z)-2,3-difluoroacrylic ester. With Cu(I) iodide and Pd(PPh3)4 catalysis, the (Z)- and (E)-stannylacrylate esters readily couple with aryl iodides and vinyl bromides, as well as 2-iodothiophene, at room temperature to stereospecifically produce the respective (E)- and (Z)-2,3-difluoro-3-aryl substituted acrylic esters or conjugated dienes in high yields.