Novel pyridine‐bis(ferrocene‐isoxazole) ligand: synthesis and application to palladium‐catalyzed Sonogashira cross‐coupling reactions under copper‐ and phosphine‐free conditions

We present here the first synthesis and application to Sonogashira reaction of pyridine‐bis(ferrocene‐isoxazole) Pd(II) complex 5 , prepared from 2,6‐bis‐(5‐ferrocenylisoxazole‐3‐yl)pyridine. Under copper‐ and phosphine‐free conditions, the stable complex 5 efficiently catalyzed the cross‐coupling of aryl halides with terminal alkynes in DMF–H₂O with TBAB as an additive, hexahydropyridine as base and affording internal arylated alkynes in moderate to excellent yields. Copyright © 2008 John Wiley & Sons, Ltd.     

Palladium‐Catalyzed Asymmetric Decarboxylative Cycloaddition of Vinylethylene Carbonates with Michael Acceptors: Construction of Vicinal Quaternary Stereocenters

An efficient method for the diastereo‐ and enantioselective construction of vicinal all‐carbon quaternary stereocenters through palladium‐catalyzed decarboxylative cycloaddition of vinylethylene carbonates with activated Michael acceptors was developed. By using a palladium complex generated in situ from [Pd₂(dab)₃]⋅CHCl₃ and a phosphoramidite ligand as a catalyst under mild reaction conditions, the process provides multifunctionalized tetrahydrofurans bearing vicinal all‐carbon quaternary stereocenters in high yields with a high level of absolute and relative stereocontrol.     

Intramolecular [2+2] Photocycloaddition of 3‐ and 4‐(But‐3‐enyl)oxyquinolones: Influence of the Alkene Substitution Pattern,Photophysical Studies,and Enantioselective Catalysis by a Chiral Sensitizer

The intramolecular [2+2] photocycloaddition of four 4‐(but‐3‐enyl)oxyquinolones (substitution pattern at the terminal alkene carbon atom: CH₂, Z‐CHEt, E‐CHEt, CMe₂) and two 3‐(but‐3‐enyl)oxyquinolones (substitution pattern: CH₂, CMe₂) was studied. Upon direct irradiation at λ=300 nm, the respective cyclobutane products were formed in high yields (83–95 %) and for symmetrically substituted substrates with complete diastereoselectivity. Substrates with a Z‐ or E‐substituted terminal double bond showed a stereoconvergent reaction course leading to mixtures of regio‐ and diastereomers with almost identical composition. The mechanistic course of the photocycloaddition was elucidated by transient absorption spectroscopy. A triplet intermediate was detected for the title compounds, which–in contrast to simple alkoxyquinolones such as 3‐butyloxyquinolone and 4‐methoxyquinolone–decayed rapidly (τ≈1 ns) through cyclization to a triplet 1,4‐diradical. The diradical can evolve through two reaction channels, one leading to the photoproduct and the other leading back to the starting material. When the photocycloaddition was performed in the presence of a chiral sensitizer (10 mol %) upon irradiation at λ=366 nm in trifluorotoluene as the solvent, moderate to high enantioselectivities were achieved. The two 3‐(but‐3‐enyl)oxyquinolones gave enantiomeric excesses (ees) of 60 and 64 % at ?25 °C, presumably because a significant racemic background reaction occurred. The 4‐substituted quinolones showed higher enantioselectivities (92–96 % ee at ?25 °C) and, for the terminally Z‐ and E‐substituted substrates, an improved regio‐ and diastereoselectivity.     

Hydroalumination of a Chlorotrialkynylsilane: Spontaneous Stepwise 1,3‐Dyotropic Rearrangement via an Intermediate Silyl Cation

A new functionalised alkynylsilane, Cl‐Si(C?C‐CMe₃)₃ ( 3 ), was obtained by a facile multistep synthesis. Treatment of 3 with equimolar quantities of the hydrides H‐M(CMe₃)₂ (M=Al, Ga) gave the mixed alkenyl‐di(alkynyl)silanes, in which the chlorine atom adopts a bridging position between the aluminium and silicon atoms. Dual hydrogallation of 3 resulted in the formation of a di(alkenyl)‐alkynylsilane containing two gallium atoms, one of which is coordinated to the chlorine atom, and the second is bonded to the α‐carbon atom of the remaining alkynyl group. A tert‐butylsilane was unexpectedly formed by a unique 1,3‐dyotropic chlorine–tert‐butyl exchange for the corresponding dialuminium compound. One aluminium atom is bonded to a tert‐butyl group, a terminal chlorine atom and the α‐carbon atom of the ethynyl moiety; the second is coordinatively unsaturated, with two terminal tert‐butyl substituents. High‐level quantum‐chemical calculations favour a stepwise dyotropic rearrangement with an intermediate cationic silicon species over a simultaneous tert‐butyl–chlorine migration via a five‐coordinate silicon atom in the transition state.     

Asymmetric Catalysis with Silicon‐Based Cuprates: Enantio‐ and Regioselective Allylic Substitution of Linear Precursors

An enantio‐ and regioselective allylic silylation of linear allylic phosphates that makes use of catalytically generated cuprate‐type silicon nucleophiles is reported. The method relies on soft bis(triorganosilyl) zincs as silicon pronucleophiles that are prepared in situ from the corresponding hard lithium reagents by transmetalation with ZnCl₂. With a preformed chiral N‐heterocyclic carbene–copper(I) complex as catalyst, exceedingly high enantiomeric excesses are achieved. The new method is superior to existing ones using a silicon–boron reagent as the source of the silicon nucleophile.     

Cobalt‐Catalyzed,Aminoquinoline‐Directed C(sp2)H Bond Alkenylation by Alkynes

A method for cobalt‐catalyzed, aminoquinoline‐ and picolinamide‐directed C(sp²)? H bond alkenylation by alkynes was developed. The method shows excellent functional‐group tolerance and both internal and terminal alkynes are competent substrates for the coupling. The reaction employs a Co(OAc)₂?4 H₂O catalyst, Mn(OAc)₂ co‐catalyst, and oxygen (from air) as a terminal oxidant.     

Computational study of 4‐fluoro‐4‐chloro‐ and 4‐fluoro‐4‐bromo‐4‐silathiacyclohexane S‐oxides: Effect of halogen on the SO→Si intramolecular coordination in the boat and twist conformers

Chair, twist, and boat conformations of axial and equatorial 4‐fluoro‐4‐chloro‐ and 4‐fluoro‐4‐bromo‐4‐silathiacyclohexane S‐oxides have been calculated at the second‐order Møller–Plesset (MP2) and density functional theory (DFT) levels of theory. The 2,5‐twist and 1,4‐boat conformers of the axial sulfoxides are stabilized by transannular interaction between the sulfinyl oxygen and silicon to give trigonal bipyramidal geometry at silicon. The stability of conformers containing pentacoordinate silicon is governed by the balance of the transannular sulfinyl oxygen–silicon coordination, nonbonded H…H and O…Hlg_ax interactions, electronegativity of Hlg_ax and nucleofugality of Hlg_eq, and may approach the stability of the lowest‐energy chair conformers. The effect of pentacoordination on the energy and geometry of Si‐halogenated 4‐silathiane S‐oxides is analyzed as a function of the halogen nature and position. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Cobalt‐Catalyzed,Aminoquinoline‐Directed C(sp2)?H Bond Alkenylation by Alkynes

A method for cobalt‐catalyzed, aminoquinoline‐ and picolinamide‐directed C(sp²) H bond alkenylation by alkynes was developed. The method shows excellent functional‐group tolerance and both internal and terminal alkynes are competent substrates for the coupling. The reaction employs a Co(OAc)₂⋅4 H₂O catalyst, Mn(OAc)₂ co‐catalyst, and oxygen (from air) as a terminal oxidant.     

Enantiospecific Alkynylation of Alkylboronic Esters

Enantioenriched secondary and tertiary alkyl pinacolboronic esters undergo enantiospecific deborylative alkynylation through a Zweifel‐type alkenylation followed by a 1,2‐elimination reaction. The process involves use of α‐lithio vinyl bromide or vinyl carbamate species, for which application to Zweifel‐type reactions has not previously been explored. The resulting functionalized 1,1‐disubstituted alkenes undergo facile base‐mediated elimination to generate terminal alkyne products in high yield and excellent levels of enantiospecificity over a wide range of pinacolboronic ester substrates. Furthermore, along with terminal alkynes, internal and silyl‐protected alkynes can be formed by simply introducing a suitable carbon‐ or silicon‐based electrophile after the base‐mediated 1,2‐elimination reaction.     

Cationic ring‐opening polymerization of novel 1,3‐dehydroadamantanes with various alkyl substituents: Synthesis of thermally stable poly(1,3‐adamantane)s

Cationic ring‐opening polymerizations of 5‐alkyl‐ or 5,7‐dialkyl‐1,3‐dehydroadamantanes, such as 5‐hexyl‐ ( 4 ), 5‐octyl‐ ( 5 ), 5‐butyl‐7‐isobutyl‐ ( 6 ), 5‐ethyl‐7‐hexyl‐ ( 7 ), and 5‐butyl‐7‐hexyl‐1,3‐dehydroadamantane ( 8 ), were carried out with super Brønsted acids, such as trifluoromethanesulfonic acid or trifluoromethanesulfonimide in CH₂Cl₂ or n‐heptane. The ring‐opening polymerizations of inverted carbon–carbon bonds in 4–8 proceeded to afford corresponding poly(1,3‐adamantane)s in good to quantitative yields. Poly( 4–8 )s possessing alkyl substituents were soluble in 1,2‐dichlorobenzene, although a nonsubstituted poly(1,3‐adamantane) was not soluble in any organic solvent. In particular, poly( 8 ) exhibited the highest molecular weight at around 7500 g mol^?1 and showed excellent solubility in common organic solvents, such as THF, CHCl₃, benzene, and hexane. The resulting poly( 4–8 )s containing adamantane‐1,3‐diyl linkages showed good thermal stability, and 10% weight loss temperatures (T₁₀) were observed over 400 °C. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4111–4124     

Formation of Substituted Oxa‐ and Azarhodacyclobutanes

The preparation of substituted oxa‐ and azarhodacyclobutanes is reported. After exchange of ethylene with a variety of unsymmetrically and symmetrically substituted alkenes, the corresponding rhodium–olefin complexes were oxidized with H₂O₂ and PhINTs (Ts=p‐toluenesulfonyl) to yield the substituted oxa‐ and azarhodacyclobutanes, respectively. Oxarhodacyclobutanes could be prepared with excellent selectivity for incorporation of the oxygen atom on the more substituted carbon atom of the alkene. At the same time, azarhodacyclobutanes showed good‐to‐excellent selectivity for heteroatom incorporation on the less substituted carbon. Furthermore, it was shown that steric modifications of the ancillary ligand have a significant influence on the selectivity of Rh–olefin complex formation as well as formation of the substituted azametallacycles.     

Asymmetric Synthesis of Silicon‐Stereogenic Silanes by Copper‐Catalyzed Desymmetrizing Protoboration of Vinylsilanes

The catalytic asymmetric creation of silanes with silicon stereocenters is a long‐sought but underdeveloped topic, and only a handful of examples have been reported. Moreover, the construction of chiral silanes containing (more than) two stereocenters is a more arduous task and remains unexploited. We herein report an unprecedented copper‐catalyzed desymmetrizing protoboration of divinyl‐substituted silanes with bis(pinacolato)diboron (B₂pin₂). This method enables the facile preparation of an array of enantiomerically enriched boronate‐substituted organosilanes bearing contiguous silicon and carbon stereocenters with exclusive regioselectivity and generally excellent diastereo‐ and enantioselectivity.     

N‐(tert‐Butoxycarbonyl)‐α‐aminoisobutyryl‐α‐aminoisobutyric acid methyl ester: two polymorphic forms in the space group P21/n

The title compound (systematic name: methyl 2‐{2‐[(tert‐butoxycarbonyl)amino]‐2‐methylpropanamido}‐2‐methylpropanoate), C₁₄H₂₆N₂O₅, (I), crystallizes in the monoclinic space group P2₁/n in two polymorphic forms, each with one molecule in the asymmetric unit. The molecular conformation is essentially the same in both polymorphs, with the α‐aminoisobutyric acid (Aib) residues adopting ϕ and ψ values characteristic of α‐helical and mixed 3₁₀‐ and α‐helical conformations. The helical handedness of the C‐terminal residue (Aib2) is opposite to that of the N‐terminal residue (Aib1). In contrast to (I), the closely related peptide Boc‐Aib‐Aib‐OBn (Boc is tert‐butoxycarbonyl and Bn is benzyl) adopts an α_L‐P_II backbone conformation (or the mirror image conformation). Compound (I) forms hydrogen‐bonded parallel β‐sheet‐like tapes, with the carbonyl groups of Aib1 and Aib2 acting as hydrogen‐bond acceptors. This seems to represent an unusual packing for a protected dipeptide containing at least one α,α‐disubstituted residue.     

A Versatile Synthesis of β‐Lactam‐Fused Oxacycles through the Palladium‐Catalyzed Chemo‐, Regio‐, and Diastereoselective Cyclization of Allenic Diols

Chemo‐, regio‐ and stereocontrolled palladium‐catalyzed preparations of enantiopure morpholines, oxocines, and dioxonines have been developed starting from 2‐azetidinone‐tethered γ,δ‐, δ,ε‐, and ε,ζ‐allendiols. The palladium‐catalyzed cyclizative coupling reaction of γ,δ‐allendiols 2 with allyl bromide or lithium bromide was effective as 8‐endo cyclization by attack of the primary hydroxy group to the terminal allene carbon to afford enantiopure functionalized oxocines; whereas the palladium‐catalyzed cyclizative coupling reaction of 2‐azetidinone‐tethered ε,ζ‐allendiols 4 furnished dioxonines 16 through a totally chemo‐ and regioselective 9‐endo oxycyclization. By contrast, the palladium‐catalyzed cyclizative coupling reaction of 2‐azetidinone‐tethered δ,ε‐allendiols 3 with aryl and alkenyl halides exclusively generated six‐membered‐ring compounds 14 a and 15 a . These results could be explained through a 6‐exo cyclization by chemo‐ and regiospecific attack of the secondary hydroxy group to the internal allene carbon. Chemo‐ and regiocontrol issues are mainly influenced by the length of the tether rather than by the nature of the metal catalysts and substituents. This reactivity can be rationalized by means of density functional theory calculations.     

Silicon‐ and Tin‐Based Cuprates: Now Catalytic in Copper!

Silicon‐ and tin‐containing molecules are versatile building blocks in organic synthesis. A stalwart method for their preparation relies on the stoichiometric use of silicon‐ and tin‐based cuprates, although a few copper(I)‐catalyzed or even copper‐free protocols have been known for decades. In this Concept, we describe our efforts towards copper(I)‐catalyzed carbon? silicon and also carbon? tin bond formations using soft bis(triorganosilyl) and bis(triorganostannyl) zinc reagents as powerful sources of nucleophilic silicon and tin. Conjugate addition, allylic substitution, and carbon? carbon multiple bond functionalization is now catalytic in copper!     

Wide Band Gap Non‐Fullerene Small Molecular Acceptors Containing Spirobifluorene and Benzotriazole with Three Different End‐Capped Groups for P3HT‐Based Organic Solar Cells

Spirofluorene (SF) and benzo[d][1,2,3]triazole (BTA) have been considered as promising building blocks to construct n‐type photovoltaic materials. Herein, three new small molecule acceptors (SMAs) named BTA21 , BTA23 and BTA27 with the structure of A₂ = A₁‐D‐A₁ = A₂ have been designed, in which SF and BTA were used as a central unit of D and bridged acceptor unit of A₁, respectively. In addition, 3‐ethylrhodanine, 2‐(3‐ethyl‐4‐ oxothiazolidin‐2‐ylidene)malononitrile and malononitrile were chosen as terminal acceptor units to modulate the properties of the final SMAs. Three SMAs show wide optical band gaps (E_g) of 2.19, 2.15 and 2.21 eV, respectively, with gradually down‐shift of the lowest unoccupied molecular orbital (LUMO) levels in the order of BTA21 , BTA23 and BTA27 depending on the electron‐withdrawing capability of terminal acceptor units. BTA21 shows great advantages with respect to donor poly(3‐hexylthiophene) (P3HT) over BTA23 and BTA27 , such as well energy‐level matching, complementary absorption and proper morphology. Concequently, P3HT: BTA21 shows the best power conversion efficiency (PCE) value of 3.28% with an open‐circuit voltage (V_OC) of 1.02 V, a short‐circuit current (J_SC) of 5.45 mA·cm^–2 and a fill factor (FF) of 0.59. These results indicate that the terminal acceptor group end‐capped in SMAs plays a significant role in controlling their optical, electronic, and photovoltaic properties.     

A Visible‐Light‐Promoted O‐Glycosylation with a Thioglycoside Donor

Visible‐light irradiation of 4‐p‐methoxyphenyl‐3‐butenylthioglucoside donors in the presence of Umemoto's reagent and alcohol acceptors serves as a mild approach to O‐glycosylation. Visible‐light photocatalysts are not required for activation, and alkyl‐ and arylthioglycosides not bearing the p‐methoxystyrene are inert to these conditions. Experimental and computational evidence for an intervening electron donor–acceptor complex, which is necessary for reactivity, is provided. Yields with primary, secondary, and tertiary alcohol acceptors range from moderate to high. Complete β‐selectivity can be attained through neighboring‐group participation.     

Catalytic CN Bond Alkynylation of N‐Benzylic Sulfonamides with Terminal Alkynes

A new cross‐coupling reaction of N‐benzylic sulfonamides with terminal alkynes for the synthesis of internal alkynes is reported. In the presence of 5 mol% of (Tf)₂NH/Bi(OTf)₃ (1:1), a broad range of N‐benzylic sulfonamides react smoothly with arylacetylenes to afford structurally diverse internal alkynes in moderate to excellent yields. We reasoned that vinyl cations could be formed by the regioselective attack of terminal alkynes with benzyl cations generated in situ from N‐benzylic sulfonamides under acidic conditions, which then eliminated to form a carbon‐carbon triple bond.     

Selective organic synthesis through generation and reactivity control of hyper‐coordinate metal species

This paper is a review of my 40 years of research at Kyoto, Sagamihara, and Yokohama, all based on the generation of hyper‐coordinate metal species such as ate complexes and pentacoordinate silicates. The topics are: (i) carbenoid reagents for carbon–carbon bond‐forming reactions, (ii) nucleophilic substitution at acetal carbons using aluminate reagents, (iii) preparation of magnesium enolates and its reaction with nitriles, (iv) Cr(II) reagents for reduction of organic halides and highly selective carbon–carbon bond formation, (v) organic synthesis with organosilion reagents/fluoride ions, (vi) cross‐coupling reaction of organosilicon compounds, and (vii) silicon‐based conjugate addition to α,β‐unsaturated carbonyl acceptors. © 2008 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 8: 337–350; 2008: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.20162     

Exceptionally E‐ and β‐Selective NHC–Cu‐Catalyzed Proto‐Silyl Additions to Terminal Alkynes and Site‐ and Enantioselective Proto‐Boryl Additions to the Resulting Vinylsilanes: Synthesis of Enantiomerically Enriched Vicinal and Geminal Borosilanes

An exceptionally site‐ and E‐selective catalytic method for preparation of Si‐containing alkenes through protosilylation of terminal alkynes is presented. Furthermore, the vinylsilanes obtained are used as substrates to generate vicinal or geminal borosilanes by another catalytic process; such products are derived from enantioselective protoborations of the Si‐substituted alkenes. All transformations are catalyzed by N‐heterocyclic carbene (NHC) copper complexes. Specifically, a commercially available imidazolinium salt, cheap CuCl (1.0 mol %) and Me₂PhSi–B(pin), readily and inexpensively prepared in one vessel, are used to convert terminal alkynes to (E)‐β‐vinylsilanes efficiently (79–98 % yield) and in >98 % E and >98 % β‐selectivity. Vinylsilanes are converted to borosilanes with 5.0 mol % of a chiral NHC–Cu complex in 33–94 % yield and up to 98.5:1.5 enantiomeric ratio (e.r.). Alkyl‐substituted substrates afford vicinal borosilanes exclusively; aryl‐ and heteroaryl‐substituted alkenes deliver the geminal isomers preferentially. Different classes of chiral NHCs give rise to high enantioselectivities in the two sets of transformations: C₁‐symmetric monodentate Cu complexes are most suitable for reactions of alkyl‐containing vinylsilanes and bidentate sulfonate‐bridged variants furnish the highest e.r. for substrates with an aryl substituent. Working models that account for the observed trends in selectivity are provided. Utility is demonstrated through application towards a formal enantioselective total synthesis of naturally occurring antibacterial agent bruguierol A.