Showing papers in "Journal of Organic Chemistry in 2000"

Simple, Efficient Catalyst System for the Palladium-Catalyzed Amination of Aryl Chlorides, Bromides, and Triflates

Abstract: Palladium complexes supported by (o-biphenyl)P(t-Bu)2 (3) or (o-biphenyl)PCy2 (4) are efficient catalysts for the catalytic amination of a wide variety of aryl halides and triflates. Use of ligand 3 allows for the room-temperature catalytic amination of many aryl chloride, bromide, and triflate substrates, while ligand 4 is effective for the amination of functionalized substrates or reactions of acyclic secondary amines. The catalysts perform well for a large number of different substrate combinations at 80−110 °C, including chloropyridines and functionalized aryl halides and triflates using 0.5−1.0 mol % Pd; some reactions proceed efficiently at low catalyst levels (0.05 mol % Pd). These ligands are effective for almost all substrate combinations that have been previously reported with various other ligands, and they represent the most generally effective catalyst system reported to date. Ligands 3 and 4 are air-stable, crystalline solids that are commercially available. Their effectiveness is believed t...

Studies on the Mechanism of B(C6F5)3-Catalyzed Hydrosilation of Carbonyl Functions

Abstract: The strong organoborane Lewis acid B(C6F5)3 catalyzes the hydrosilation (using R3SiH) of aromatic and aliphatic carbonyl functions at convenient rates with loadings of 1−4%. For aldehydes and ketones, the product silyl ethers are isolated in 75−96% yield; for esters, the aldehydes produced upon workup of the silyl acetal products can be obtained in 45−70% yield. Extensive mechanistic studies point to an unusual silane activation mechanism rather than one involving borane activation of the carbonyl function. Quantitative kinetic studies show that the least basic substrates are hydrosilated at the fastest rates; furthermore, increased concentrations of substrate have an inhibitory effect on the observed reaction rate. Paradoxically, the most basic substrates are reduced selectively, albeit at a slower rate, in competition experiments. The borane thus must dissociate from the carbonyl to activate the silane via hydride abstraction; the incipient silylium species then coordinates the most basic function, whic...

Scope and Limitations of the Pd/BINAP-Catalyzed Amination of Aryl Bromides

Abstract: Mixtures of Pd2(dba)3 or Pd(OAc)2 and BINAP catalyze the cross-coupling of amines with a variety of aryl bromides. Primary amines are arylated in high yield, and certain classes of secondary amines are also effectively transformed. The process tolerates the presence of several functional groups including methyl and ethyl esters, enolizable ketones, and nitro groups provided that cesium carbonate is employed as the base. Most reactions proceed to completion with 0.5−1.0 mol % of the palladium catalyst; in some cases, catalyst levels as low as 0.05 mol % Pd may be employed. Reactions are considerably faster if Pd(OAc)2 is employed as the precatalyst, and the order in which reagents are added to the reaction has a substantial effect on reaction rate. It is likely that the catalytic process proceeds via bis(phosphine)palladium complexes as intermediates. These complexes are less prone to undergo undesirable side reactions which lead to diminished yields or catalyst deactivation than complexes of the correspon...

Ruthenium Carbene Complexes with N,N‘-Bis(mesityl)imidazol-2-ylidene Ligands: RCM Catalysts of Extended Scope

Abstract: The ruthenium carbene complexes 3a,b bearing imidazol-2-ylidene ligands constitute excellent precatalysts for ring-closing metathesis (RCM) reactions allowing the formation of tri- and tetrasubstituted cycloalkenes. They also apply to annulations that are beyond the scope of the standard Grubbs carbene 1 as well as to ring-closing reactions of acrylic acid derivatives even if the resulting α,β-unsaturated lactones (or lactams) are tri- or tetrasubstituted. The reactivity of 3a was found to be highly dependent on the reaction medium: particularly high reaction rates are observed in toluene, although this solvent also leads to an increased tendency of the catalyst to isomerize the double bonds of the substrates.

Palladium-catalyzed borylation of aryl halides or triflates with dialkoxyborane: A novel and facile synthetic route to arylboronates

Abstract: A direct borylation of aryl halides or triflates with dialkoxyborane was investigated. The coupling reaction of pinacolborane with aryl halides or triflates in the presence of a catalytic amount of PdCl2(dppf) together with a base provided arylboronates in high yields. The product distributions were strongly dependent on the base employed, and the tertiary amine, especially Et3N, was effective for the selective formation of the boron−carbon bond. The reaction conditions were so mild that arylboronates having a variety of functional groups such as carbonyl, cyano, and nitro groups were readily prepared.

A Novel B(C6F5)3-Catalyzed Reduction of Alcohols and Cleavage of Aryl and Alkyl Ethers with Hydrosilanes†

Abstract: The primary alcohols 1a−e and ethers 4a−d were effectively reduced to the corresponding hydrocarbons 2 by HSiEt3 in the presence of catalytic amounts of B(C6F5)3. To the best of our knowledge, this is the first example of catalytic use of Lewis acid in the reduction of alcohols and ethers with hydrosilanes. The secondary alkyl ethers 4j,k enabled cleavage and/or reduction under similar reaction conditions to produce either the silyl ethers 3m−n or the corresponding alcohol 5a upon subsequent deprotection with TBAF. It was found that the secondary alcohols 1g−i and tertiary alcohol 1j, as well as the tertiary alkyl ether 4l, did not react with HSiEt3/(B(C6F5)3 reducing reagent at all. The following relative reactivity order of substrates was found: primary ≫ secondary > tertiary. A plausible mechanism for this nontraditional Lewis acid catalyzed reaction is proposed.

4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) Dyes Modified for Extended Conjugation and Restricted Bond Rotations

Abstract: Five new, constrained, aryl-substituted 4,4-difluoro-4-bora-3a, 4a-diaza-s-indacene (BODIPY) dyes (3f,g and 4h-j) were prepared and investigated to see if they have more favorable fluorescence characteristics than the unconstrained systems 2 that were prepared in previous studies. Dye types 3 and 4 have relatively rigid conformations caused by the heteroatom (3f and 3g) or ethylene bridge (4h-j) linkers that preclude free rotation of the substituted-benzene molecular fragments. In the event, the new dye types 3 and 4 have longer lambda(max abs) (620-660 nm) and lambda(max)(fluor) (630-680 nm) values than compounds 2. They also exhibit higher extinction coefficients (>100 000 M(-1) cm(-1), except for 3g). Their fluorescent quantum yields are high (up to 0.72 for 4j), with the exception of compound 3g, which has a quantum yield of only 0.05. The redox properties of dyes 3 and 4 have also been examined.

Fast Microwave-Assisted Preparation of Aryl and Vinyl Nitriles and the Corresponding Tetrazoles from Organo-halides

Abstract: Aryl and vinyl nitriles have been prepared in very high yields from the corresponding bromides using palladium-catalyzed reactions with microwave irradiation employed as the energy source. Furthermore, flash heating was used successfully for the conversion of these nitriles into aryl and vinyl tetrazoles by cycloaddition reactions. One-pot transformation of aryl halides directly to the aryl tetrazoles could be accomplished both in solution and on solid support. All reactions were completed in minutes rather than in hours or days as previously reported with the standard thermal heating technique. A very potent HIV-1 protease inhibitor (Ki = 0.56 nM), comprising two tetrazole heterocycles as carboxyl group bioisosteres, was prepared in one pot by microwave-promoted cyanation of a bromo precursor and a subsequent cycloaddition reaction. The temperature−time profiles at 13, 20, and 60 W magnetron input power in DMF are presented.

Structures of TMC-95A-D: novel proteasome inhibitors from Apiospora montagnei sacc. TC 1093.

Abstract: Four novel proteasome inhibitors, TMC-95A-D (1-4) have been isolated from the fermentation broth of Apiospora montagnei Sacc. TC 1093, isolated from a soil sample. All of the molecular formulas of 1-4 were established as C(33)H(38)N(6)O(10) by high-resolution FAB-MS. Their planar structures were determined on the basis of extensive analyses of 1D and 2D NMR, and degradation studies. Compounds 1-4 have the same planar structures to each other, and are unique highly modified cyclic peptides containing L-tyrosine, L-aspargine, highly oxidized L-tryptophan, (Z)-1-propenylamine, and 3-methyl-2-oxopentanoic acid units. The absolute configuration at C-11 and C-36 of 1-4 was determined based on chiral TLC and HPLC analyses of their chemical degradation products. The ROESY analysis along with (1)H-(1)H coupling constants clarified the absolute stereochemistry at C-6, -7, -8, and -14 of the cyclic moieties. These studies revealed the relationships of 1-4 to be diastereomers at C-7 and C-36.

Rational syntheses of porphyrins bearing up to four different meso substituents.

Abstract: Porphyrins bearing specific patterns of substituents are crucial building blocks in biomimetic and materials chemistry. We have developed methodology that avoids statistical reactions, employs minimal chromatography, and affords up to gram quantities of regioisomerically pure porphyrins bearing predesignated patterns of up to four different meso substituents. The methodology is based upon the availability of multigram quantities of dipyrromethanes. A procedure for the diacylation of dipyrromethanes using EtMgBr and an acid chloride has been refined. A new procedure for the preparation of unsymmetrical diacyl dipyrromethanes has been developed that involves (1) monoacylation with EtMgBr and a pyridyl benzothioate followed by (2) introduction of the second acyl unit upon reaction with EtMgBr and an acid chloride. The scope of these acylation methods has been examined by preparing multigram quantities of diacyl dipyrromethanes bearing a variety of substituents. Reduction of the diacyl dipyrromethane to the c...

Flash Vacuum Pyrolysis of Methoxy-Substituted Lignin Model Compounds

Abstract: The flash vacuum pyrolysis (FVP) of methoxy-substituted β-O-4 lignin model compounds has been studied at 500 °C to provide mechanistic insight into the primary reaction pathways that occur under conditions of fast pyrolysis. FVP of PhCH2CH2OPh (PPE), a model of the dominant β-O-4 linkage in lignin, proceeds by C−O and C−C cleavage, in a 37:1 ratio, to produce styrene plus phenol as the dominant products and minor amounts of toluene, bibenzyl, and benzaldehyde. From the deuterium isotope effect in the FVP of PhCD2CH2OPh, it was shown that C−O cleavage occurs by homolysis and by 1,2-elimination in a ratio of 1.4:1, respectively. Methoxy substituents enhance the homolysis of the β-O-4 linkage, relative to PPE, in o-CH3O-C6H4OCH2CH2Ph (o-CH3O-PPE) and (o-CH3O)2-C6H3OCH2CH2Ph ((o-CH3O)2-PPE) by a factor of 7.4 and 21, respectively. The methoxy-substituted phenoxy radicals undergo a complex series of reactions, which are dominated by 1,5-, 1,6-, and 1,4-intramolecular hydrogen abstraction, rearrangement, and β−...

Peloruside A: A Potent Cytotoxic Macrolide Isolated from the New Zealand Marine Sponge Mycale sp.

Abstract: A novel, polyoxygenated, pyranose ring containing 16-membered macrolide peloruside A (1) exhibiting cytotoxic activity in the nanomolar range was isolated from the New Zealand marine sponge Mycale sp. The structure of 1 and relative stereochemistry of the 10 stereogenic centers were determined on a 3 mg sample using a variety of spectroscopic methods. Compound 1 was isolated along with the previously reported cytotoxins mycalamide A (2) and pateamine (3) from a single specimen of this sponge.

Coupling Reactions of Alkynylsilanes Mediated by a Cu(I) Salt: Novel Syntheses of Conjugate Diynes and Disubstituted Ethynes

Abstract: Reaction of 1-trimethylsilylalkyne with copper(I) chloride in a polar solvent, DMF, at 60 degrees C under an aerobic conditions smoothly undergoes homo-coupling to give the corresponding symmetrical 1,3-butadiynes in 70-99% yields. In addition, (arylethynyl)trimethylsilanes are found to couple with aryl triflates and chlorides in the presence of Cu(I)/Pd(0) (10 mol %/5 or 10 mol %) cocatalyst system to give the corresponding diarylethynes in 49-99% yields. The cross-coupling reaction is applied to a one-pot synthesis of the corresponding unsymmetrical diarylethynes from (trimethylsilyl)ethyne via sequential Sonogashira-Hagihara and the present cross-coupling reactions using two different aryl triflates. The reactions of (arylethynyl)trimethylsilanes with aryl(chloro)ethynes in the presence of 10 mol % of CuCl also yield the corresponding unsymmetrical 1,3-butadiynes in 43-97% yields.

Mechanism of the

Abstract: Stereochemical studies on [2 + 2] photoaddition of cis-/trans-4-propenylanisole (cis-1 and trans-1) and cis-1-(p-methoxyphenyl)ethylene-2-d(1) (cis-3-d(1)) to C(60) exhibit stereospecificity in favor of the trans-2 cycloadduct in the former case and nonstereoselectivity in the latter. The observed stereoselectivity in favor of the cis-6-d(3) [2 + 2] diastereomer by 12% in the case of the photochemical addition of (E)-1-(p-methoxyphenyl)-2-methyl-prop-1-ene-3,3,3-d(3) (trans-5-d(3)) to C(60) is attributed to a steric kinetic isotope effect (k(H)/k(D) = 0.78). The loss of stereochemistry in the cyclobutane ring excludes a concerted addition and is consistent with a stepwise mechanism. Intermolecular secondary kinetic isotope effects of the [2 + 2] photocycloaddition of 3-d(0) vs 3-d(1), and 3-d(6) as well as 5-d(0) vs 5-d(1), and 5-d(6) to C(60) were also measured. The intermolecular competition due to deuterium substitution of both vinylic hydrogens at the beta-carbon of 3 exhibits a substantial inverse alpha-secondary isotope effect k(H)/k(D) = 0.83 (per deuterium). Substitution with deuterium at both vinylic methyl groups of 5 yields a small inverse k(H)/k(D) = 0. 94. These results are consistent with the formation of an open intermediate in the rate-determining step.

An Improved Synthesis of Functionalized Biphenyl-Based Phosphine Ligands

Abstract: Functionalized dicyclohexyl- and di-tert-butylphosphinobiphenyl ligands are prepared by the reaction of arylmagnesium halides with benzyne, followed by the addition of a chlorodialkylphosphine. This one-pot procedure is considerably less expensive and time-consuming than the method used previously to prepare such ligands. The cost of introducing the dicyclohexylphosphine group can be decreased by preparing chlorodicyclohexylphosphine from PCl3 and cyclohexylmagnesium chloride, and using the reagent without further purification. The new method is significant, as a variety of ligands can be produced in useful amounts by a procedure that is simple, with starting materials that are relatively inexpensive, and, in most cases, without chromatographic purification.

New method for preparation of coumarins and quinolinones via Pd-catalyzed intramolecular hydroarylation of C-C triple bonds.

Abstract: A new and general method has been developed for preparation of coumarins and quinolinones by intramolecular hydroarylation of alkynes. Various aryl alkynoates and alkynanilides undergo fast intramolecular reaction at room temperature in the presence of a catalytic amount of Pd(OAc)(2) in a mixed solvent containing trifluoroacetic acid (TFA), affording coumarins and quinolinones in moderate to excellent yields with more than 1000 turnover numbers (TON) to Pd. The methodology proved to tolerate a number of functional groups such as Br and CHO. On the basis of isotope experiments, a possible mechanism involving ethynyl chelation-assisted electrophilic metalation of aromatic C-H bonds by in-situ generated cationic Pd(II) species has been discussed. Also the involvement of vinylcationic species has been suggested.

Synthesis of chiral bisphosphines with tunable bite angles and their applications in asymmetric hydrogenation of beta-ketoesters.

Abstract: Although many effective chiral bisphosphines have been developed, there is no general solution in dealing with the many challenging transition metal-catalyzed asymmetric transformations since enantioselectivities are often substrate-dependent. Subtle changes in geometric, steric, and/or electronic properties of chiral ligands can lead to dramatic variations of reactivity and enantioselectivity. Conformationally rigid and yet tunable chiral ligands offer a great advantage in optimizing the enantioselectivity of a reaction by maximizing the possibility of a low energy enantiotopic approach of substrates in the stereochemistry defining step. Recently, we have developed two conformationally rigid chiral bisphosphines1 (BICP and PennPhos) which have been shown to be effective for several asymmetric reactions. We envision that a strategy to restrict sp2-sp2 rotation in chiral biaryl ligands such as BINAP,2 BIPHEMP,3 and MeO-BIPHEP3 (Figure 1) will be useful in generating new chiral ligands with tunable bite angles. A closely related idea has been applied to generate chiral biaryl compounds with a range of dihedral angles.4 Extensive studies by several research groups have demonstrated that changing bite angles of chelating bisphosphines have a dramatic effect on the reactivity and selectivity of reactions.5 The correlation of bite angles of chiral chelating phosphines with the enantioselectivity of a reaction may provide significant insights for future ligand design and therefore is of fundamental importance. Chiral atropisomeric biaryl diphosphines such as BINAP, BIPHEMP, and MeO-BIPHEP are very effective ligands for many asymmetric reactions.2,3 The sp2-sp2 rotation in these chiral biaryl ligands causes only a small energy change within a wide range of bite angles with transition metals. While these ligands have been proven effective, sometimes they are not efficient for certain substrates due to the lack of ligand rigidity. To overcome this drawback, we proposed to introduce a bridge with variable length to link the diaryl groups so that the new ligands are rigid with tunable bite angles.4 Ideally, a change in the bite angle of the metal-ligand complex will allow highly enantioselective transformation with certain substrates. We chose MeO-BIPHEP as the starting compound to make these type of chiral bisphosphines, which are called TunaPhos ligands (Scheme 1). Enantiomerically pure MeO-BIPHEP was made according to a reported procedure3 and demethylated to provide HO-BIPHEP (7) in high yield. Reaction of 7 with alkyl dihalides in the presence of excess anhydrous K2CO3 in DMF formed C1-C6TunaPhos ligands (1-6) (Scheme 1). The dihedral angles of TunaPhos, MeO-BIPHEP, and BINAP based on a CAChe MM2 calculation are listed in Table 1. The bite angle (P-metal-P) of CnTunaPhos with transition metals should increase with an increase in the dihedral angle. Furthermore, the TunaPhos ligands should be less flexible compared with BINAP and (1) For PennPhos, see (a) Jiang, Q.; Jiang, Y.; Xiao, D.; Cao, P.; Zhang, X. Angew. Chem., Int. Ed. Engl. 1998, 37, 1100-1103. (b) Jiang, Q.; Xiao, D,; Zhang, Z.; Cao, P.; Zhang, X. Angew. Chem., Int. Ed. Engl. 1999, 38, 516. (c) Zhang, Z.; Zhu, G.; Jiang, Q.; Xiao, D.; Zhang, X. J. Org. Chem. 1999, 64, 1774. For BICP, see: (d) Zhu, G.; Cao, P.; Jiang, Q.; Zhang, X. J. Am. Chem. Soc. 1997, 119, 1799. (e) Zhu, G.; Casalnuovo, A. L.; Zhang, X. J. Org. Chem. 1998, 63, 8100. (f) Zhu, G.; Zhang, X. J. Org. Chem. 1998, 63, 9590. (g) Cao, P.; Zhang, X. J. Org. Chem. 1999, 64, 2127. (h) Cao, P.; Zhang, X. J. Am. Chem. Soc. 1999, 121, 7708. (2) (a) Noyori, R.; Takaya, H. Acc. Chem. Res. 1990, 23, 345. (b) Takaya, H.; Ohta, T.; Noyori, R. In Catalytic Asymmetric Synthesis; Ojima, I., Ed.; VCH: New York, 1993. (c) Noyori, R. Asymmetric Catalysis in Organic Synthesis; Wiley: New York, 1994. (3) (a) Schmid, R.; Cereghetti, M.; Heiser, B.; Schonholzer, P.; Hansen, H.-J. Helv. Chim. Acta 1988, 71, 897. (b) Schmid, R.; Foricher, J.; Cereghetti, M.; Schonhoizer, P. Helv. Chim. Acta 1991, 74, 370. (c) Schmid, R.; Broger, E. A.; Cereghetti, M.; Crameri, Y.; Foricehr, J.; Lalonde, M.; Muller, R. K.; Scalone, M.; Schoettel, G.; Zutter, U. Pure Appl. Chem. 1996, 68, 131. (4) (a) Lustenberger, P.; Martinborough, E.; Denti, T. M.; Diederich, F. J. Chem. Soc., Perkin. Trans. II 1998, 747. (b) Harada, T.; Takeuchi, M.; Hatsuda, M.; Ueda, S.; Oku, A. Tetrahedron: Asymmetry 1996, 7, 2479. (c) Lipshutz, B. H.; Shin, Y.-J. Tetrahedron Lett. 1998, 7017. (5) (a) Davies, I. W.; Deeth, R. J.; Larsen, R. D.; Reider, P. J. Tetrahedron Lett. 1999, 40, 1233. (b) Kamer, P. C. J.; Reek, J. N. H.; van Leeuwen, P. W. N. M. CHEMTECH 1998, 28(9), 27. (c) van der Veen, L. A.; Boele, M. D. K.; Bregman, F. R.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.; Goubitz, K.; Fraanje, J.; Schenk, H.; Bo, C. J. Am. Chem. Soc. 1998, 120, 11616. (d) Meessen, P.; Vogt, D.; Keim, W. J. Organomet. Chem. 1998, 551, 165. (e) Sakaki, S.; Takeuchi, K.; Sugimoto, M.; Kurosawa, H. Organometallics 1997, 16, 2995. (f) Harada, T.; Takeuchi, M.; Hatsuda, M.; Ueda, S.; Oku, A. Tetrahedron: Asymmetry 1996, 7, 2479. (g) Davies, I. W.; Gerenda, L.; Castonguay, L.; Senanayake, C. H.; Larsen, R. D.;. Verhoeven, T. R.; Reider, P. J. Chem. Commun. 1996, 1753. (h) Kranenburg, M.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.; Vogt, D.; Wilhelm, K. J. Chem. Soc., Chem. Commun. 1995, 2177. (i) Kranenburg, M.; van der Burgt, Y. E. M.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.;. Goubitz, K.; Fraanje, J. Organometallics 1995, 14, 3081. (j) Brown, J. M.; Guiry, P. J. Inorg. Chem. Acta 1994, 220, 249. (k) Yamamoto, K.; Momose, S.; Fanahashi, M.; Ebata, S.; Ohmura, H.; Kamatsu, H.; Miyazawa, M. Chem. Lett. 1994, 189. (l) Casey, C. P.; Whiteker, G. T. Israel J. Chem. 1990, 30, 299. (m) Casey, C. P.; Whiteker, G. T.; Melville, V.; Petrovich, L. M.; Gavney, J. A.; Powell, D. R. J. Am. Chem. Soc. 1992, 114, 5535. Figure 1. Chiral atropisomeric bisphosphines. 6223 J. Org. Chem. 2000, 65, 6223-6226

Nucleophilic trifluoromethylation of carbonyl compounds and disulfides with trifluoromethane and silicon-containing bases

Abstract: Provided that DMF (or another N,N-dialkylformamide) is present in the reaction medium, at least in a catalytic amount, fluoroform trifluoromethylates efficiently carbonyl compounds, even enolizable ones, when opposed to (TMS)2N- M+, generated in situ from N(TMS)3 and M+ F- or RO- Na+. When F- is used in a catalytic amount, silylated α-(trifluoromethyl)carbinols are obtained: in this case, the four-component system HCF3/N(TMS)3/catalytic F-/catalytic DMF behaves like the Ruppert's reagent, especially as far as nonenolizable carbonyl compounds are concerned (CF3SiMe3 remains more efficient for enolizable carbonyl compounds). This process involves an adduct between DMF and -CF3 which is the true trifluoromethylating agent. In the same way, fluoroform efficiently trifluoromethylates disulfides and diselenides when deprotonated with a strong base selected from t-BuOK or N(SiMe3)3/Me4NF (or TBAT). t-BuOK is more adapted to the trifluoromethylation of aryl disulfides whereas N(SiMe3)3/F- is well suited to that o...

Synthesis of new two-photon absorbing fluorene derivatives via Cu-mediated Ullmann condensations

Abstract: The Ullmann amination reaction was utilized to provide access to a number of fluorene analogues from common intermediates, via facile functionalization at positions 2, 7, and 9 of the fluorene ring. Through variation of amine or iodofluorene derivative, analogues bearing substitutents with varying electron-donating and electron-withdrawing ability, e.g., diphenylamino, bis-(4-methoxyphenyl)amine, nitro, and benzothiazole, were synthesized in good yield. The novel fluorene derivatives were fully characterized, including absorption and emission spectra. Didecylation at the 9-position afforded remarkably soluble derivatives. Target compounds 4, 5, and 9 are potentially useful as fluorophores in two-photon fluorescence microscopy. Their UV-vis spectra display desirable absorption in the range of interest suitable for two-photon excitation by near-IR femtosecond lasers. Preliminary measurements of two-photon absorption indicate the derivatives exhibit high two-photon absorptivity, affirming their potential as two-photon fluorophores. For example, using a 1,210 nm femtosecond pump beam, diphenylaminobenzothiazolylfluorene 4 exhibited nondegenerate two-photon absorption, with two-photon absorptivity (delta) of ca. 820 x 10(-50) cm(4) s photon(-1) molecule(-1) at the femtosecond white light continuum probe wavelength of 615 nm.

Stereoelectronic Interactions in Cyclohexane, 1,3-Dioxane, 1,3-Oxathiane, and 1,3-Dithiane: W-Effect, σC-X ↔ σ*C-H Interactions, Anomeric EffectWhat Is Really Important?

Abstract: Stereoelectronic effects proposed for C−H bonds in cyclohexane, 1,3-dioxane, 1,3-oxathiane, and 1,3-dithiane were studied computationally. The balance of three effects, namely, σC-X → σ*C-Heq, σC-Heq → σ*C-X, and np(X) → σ*C-Heq interactions, was necessary to explain the relative elongation of equatorial C(5)−H bonds. The role of homoanomeric np → σ*C(5)-Heq interaction is especially important in dioxane. In dithiane, distortion of the ring by long C−S bonds dramatically increases overlap of σC(5)-Heq and σ*C-S orbitals and energy of the corresponding hyperconjugative interaction. Anomeric np(X) → σ*C-Hax interactions with participation of axial C−H bonds dominate at C(2), C(4), and C(6). The balance of hyperconjugative interactions involving C−Hax and C−Heq bonds agrees well with the relative bond lengths for all C−Hax/C−Heq pairs in all studied compounds. At the same time, the order of one-bond spin−spin coupling constants does not correlate with the balance of stereoelectronic effects in dithiane and o...

Efficient Oxidation of Alcohols to Carbonyl Compounds with Molecular Oxygen Catalyzed by N-Hydroxyphthalimide Combined with a Co Species

Abstract: Highly efficient catalytic oxidation of alcohols with molecular oxygen by N-hydroxyphthalimide (NHPI) combined with a Co species was developed. The oxidation of 2-octanol in the presence of catalytic amounts of NHPI and Co(OAc)2 under atmospheric dioxygen in AcOEt at 70 °C gave 2-octanone in 93% yield. The oxidation was significantly enhanced by adding a small amount of benzoic acid to proceed smoothly even at room temperature. Primary alcohols were oxidized by NHPI in the absence of any metal catalyst to form the corresponding carboxylic acids in good yields. In the oxidation of terminal vic-diols such as 1,2-butanediol, carbon−carbon bond cleavage was induced to give one carbon less carboxylic acids such as propionic acid, while internal vic-diols were selectively oxidized to 1,2-diketones.

Di-tert-butyl dicarbonate and 4-(dimethylamino)pyridine revisited. Their reactions with amines and alcohols

Abstract: The reaction of BOC2O in the presence and absence of DMAP was examined with some amines, alcohols, diols, amino alcohols, and aminothiols. Often, unusual products were observed depending on the ratio of reagents, reaction time, polarity of solvent, pKa of alcohols, or type of amine (primary or secondary). In reactions of aliphatic alcohols with BOC2O/DMAP, we isolated for the first time carbonic-carbonic anhydride intermediates; this helps explain the formation of symmetrical carbonates in addition to the O-BOC products. In the case of secondary amines, we succeeded to isolate unstable carbamic-carbonic anhydride intermediates that in the presence of DMAP led to the final N-BOC product. The effect of N-methylimidazole in place of DMAP was also examined.

Asymmetric conjugate 1,4-addition of arylboronic acids to alpha, beta-unsaturated esters catalyzed by Rhodium(I)/(S)-binap

Abstract: Arylboronic acids underwent the conjugate 1,4-addition to alpha, beta-unsaturated esters to give beta-aryl esters in high yields in the presence of a rhodium(I) catalyst. The addition of arylboronic acids to isopropyl crotonate resulted in high yields and high enantioselectivity exceeding 90% ee in the presence of 3 mol % of Rh(acac)(C(2)H(4))(2) and (S)-binap at 100 degrees C. The rhodium/(S)-binap complex provided (R)-3-phenylbutanoate in the addition of phenylboronic acid to benzyl crotonate. The effects on the enantioselectivity of chiral phosphine ligands, rhodium precursors, and substituents on alpha,beta-unsaturated esters are discussed, as well as the mechanistic aspect of the catalytic cycle.

Switching of Pseudorotaxanes and Catenanes Incorporating a Tetrathiafulvalene Unit by Redox and Chemical Inputs

Abstract: An acyclic polyether 1a, incorporating a central tetrathiafulvalene (TTF) electron donor unit and two 4-tert-butylphenoxy groups at its termini, has been synthesized. Two macrocyclic polyethers containing two different electron donors, namely a TTF unit with, in one case, a 1,4-dioxybenzene ring (2a), and, in the other case (2b), a 1,5-dioxynaphthalene ring system, have also been synthesized. These two macrocyclic polyethers have been mechanically interlocked in kinetically controlled template-directed syntheses with cyclobis(paraquat-p-phenylene) cyclophane (34+) to afford the [2]catenanes 2a/34+ and 2b/34+, respectively. X-ray crystallography reveals that the [2]catenane 2b/34+ has the TTF unit of 2b located inside the cavity of 34+. The spectroscopic (UV/vis and 1H NMR) and electrochemical properties of compounds 1a, 2a, 2b, 2a/34+, and 2b/34+ and of the [2]pseudorotaxane 1a·34+ were investigated. The absorption and emission properties of the mono- and dioxidized forms of the TTF unit in these various ...

N-Acylbenzotriazoles: neutral acylating reagents for the preparation of primary, secondary, and tertiary amides

Abstract: Readily available N-acylbenzotriazoles 2a−q efficiently acylate aqueous ammonia and primary and secondary amines to give primary, secondary, and tertiary amides in good to excellent yields. The wide applicability of the procedure is illustrated by the preparation of (i) α-hydroxyamides from α-hydroxy acids and of (ii) perfluoroalkylated amides.

Self-consistent spectrophotometric basicity scale in acetonitrile covering the range between pyridine and DBU

Abstract: A self-consistent spectrophotometric basicity scale in acetonitrile, including DBU, ten (arylimino)tris(1-pyrrolidinyl)phosphoranes, two (arylimino)tris(dimethylamino)phosphoranes, 2-phenyl-1,1,3,3-tetramethylguanidine, 1-(2-tolyl)biguanide, benzylamine, two substituted benzimidazoles, pyridine, and ten substituted pyridines, has been created. The span of the scale is almost 12 pKa units. Altogether, 29 different bases were studied and 53 independent equilibrium constant measurements were carried out, each describing the relative basicity of two bases. The scale is anchored to the pKa value of pyridine of 12.33 that has been measured by Coetzee et al. Comparison of the basicity data of phenyliminophosphoranes and phenyltetramethylguanidines implies that the PN bond in the (arylimino)tris(1-pyrrolidinyl)phosphoranes involves contribution from the ylidic (zwitterionic) structure analogous to that found in phosphorus ylides.

Phenylglycine methyl ester, a useful tool for absolute configuration determination of various chiral carboxylic acids

Abstract: A new chiral anisotropic reagent, phenylglycine methyl ester (PGME), developed for the elucidation of the absolute configuration of chiral α,α-disubstituted acetic acids, has turned out to be applicable to other substituted carboxylic acids, such as chiral α-hydroxy-, α-alkoxy-, and α-acyloxy-α,α-disubstituted acetic acids, as well as to chiral β,β-disubstituted propionic acids. Because a carboxylic moiety is convertible from other functional groups, e.g., ozonolysis of an olefin and oxidative cleavage of a glycol, the present findings can expand the utility of the PGME method to the absolute configuration determination of various types of organic compounds, even those which initially lack oxygen functions. Several examples of the combination of chemical reactions and the PGME method are described.