Traceless Protection for More Broadly Applicable Olefin Metathesis

An operationally simple in situ protection/deprotection strategy that significantly expands the scope of kinetically controlled catalytic Z‐ and E‐selective olefin metathesis is introduced. Prior to the addition of a sensitive Mo‐ or Ru‐based complex, treatment of a hydroxy‐ or a carboxylic‐acid‐containing olefin with commercially available HB(pin) or readily accessible HB(trip)₂ (pin=pinacolato, trip=2,4,6‐tri(isopropyl)phenyl) for 15 min is sufficient for efficient generation of a desired product. Routine workup leads to quantitative deprotection. A range of stereochemically defined Z‐ and E‐alkenyl chlorides, bromides, fluorides, and boronates or Z‐trifluoromethyl‐substituted alkenes with a hydroxy or carboxylic acid group were thus prepared in 51–97 % yield with 93 to >98 % stereoselectivity. We also show that, regardless of whether a polar functional unit is present or not, a small amount of HB(pin) may be used to remove residual water, significantly enhancing efficiency.     

Dichotomy of Manganese Catalysis via Organometallic or Radical Mechanism: Stereodivergent Hydrosilylation of Alkynes

Herein, we disclose the first manganese‐catalyzed hydrosilylation of alkynes featuring diverse selectivities. The highly selective formation of E‐products was achieved by using mononuclear MnBr(CO)₅ with the arsenic ligand, AsPh₃. Whereas using the dinuclear catalyst Mn₂(CO)₁₀ and LPO (dilauroyl peroxide) enabled the reversed generation of Z‐products in good to excellent stereo‐ and regioselectivity. Such a way of controlling the reaction stereoselectivity is unprecedented. Mechanistic experiments revealed the dichotomy of manganese catalysis via organometallic and radical pathways operating in the E‐ and Z‐selective routes, respectively.     

Stereoselective Synthesis of Vinylboronates by Rh‐Catalyzed Borylation of Stereoisomeric Mixtures

The stereoselective preparation of vinylboronates via rhodium‐catalyzed borylation of E/Z mixtures of vinyl actetates is described, and this method was also extended to synthesis of vinyldiboronates. These transformations feature high functional group compatibility and mild reaction conditions. Control experiments support a mechanism that involved a Rh‐catalyzed borylation‐isomerization sequence. The isomerization of (Z)‐vinylboronates to (E)‐isomers was also demonstrated.     

Photocontrol of Polar Aromatic Interactions by a Bis‐Hemithioindigo Based Helical Receptor

The first example of a bis‐hemithioindigo (bis‐HTI)‐based molecular receptor was realized. Its folding and selective binding affinity for aromatic guest molecules can be precisely controlled by visible light and heat. The thermodynamically stable state of the bis‐HTI is the s‐shaped planar Z,Z‐configuration. After irradiation with 420 nm light only the E,Z‐configuration is formed in a highly selective photoisomerization. The E,Z‐isomer adopts a helical conformation because of the implementation of repulsive steric interactions. The E,Z‐configured helix is able to recognize electron‐poor aromatic guests exclusively through polar aromatic interactions and also distinguishes between regioisomers. After heating, the Z,Z‐configuration is completely restored and the aromatic guest molecule is efficiently released.     

Efficient and Selective Formation of Macrocyclic Disubstituted Z Alkenes by Ring‐Closing Metathesis (RCM) Reactions Catalyzed by Mo‐ or W‐Based Monoaryloxide Pyrrolide (MAP) Complexes: Applications to Total Syntheses of Epilachnene,Yuzu Lactone,Ambrettolide, Epothilone C,and Nakadomarin A

The first broadly applicable set of protocols for efficient Z‐selective formation of macrocyclic disubstituted alkenes through catalytic ring‐closing metathesis (RCM) is described. Cyclizations are performed with 1.2–7.5 mol % of a Mo‐ or W‐based monoaryloxide pyrrolide (MAP) complex at 22 °C and proceed to complete conversion typically within two hours. Utility is demonstrated by synthesis of representative macrocyclic alkenes, such as natural products yuzu lactone (13‐membered ring: 73 % Z) epilachnene (15‐membered ring: 91 % Z), ambrettolide (17‐membered ring: 91 % Z), an advanced precursor to epothilones C and A (16‐membered ring: up to 97 % Z), and nakadomarin A (15‐membered ring: up to 97 % Z). We show that catalytic Z‐selective cyclizations can be performed efficiently on gram‐scale with complex molecule starting materials and catalysts that can be handled in air. We elucidate several critical principles of the catalytic protocol: 1) The complementary nature of the Mo catalysts, which deliver high activity but can be more prone towards engendering post‐RCM stereoisomerization, versus W variants, which furnish lower activity but are less inclined to cause loss of kinetic Z selectivity. 2) Reaction time is critical to retaining kinetic Z selectivity not only with MAP species but with the widely used Mo bis(hexafluoro‐tert‐butoxide) complex as well. 3) Polycyclic structures can be accessed without significant isomerization at the existing Z alkenes within the molecule.     

Rh(I)/(III)-N-Heterocyclic Carbene Complexes: Effect of Steric Confinement Upon Immobilization on Regio- and Stereoselectivity in the Hydrosilylation of Alkynes

Rh(I) NHC and Rh(III) Cp* NHC complexes (Cp*=pentamethylcyclopentadienyl, NHC=N-heterocyclic carbene=pyrid-2-ylimidazol-2-ylidene (Py−Im), thiophen-2-ylimidazol-2-ylidene) are presented. Selected catalysts were selectively immobilized inside the mesopores of SBA-15 with average pore diameters of 5.0 and 6.2 nm. Together with their homogenous progenitors, the immobilized catalysts were used in the hydrosilylation of terminal alkynes. For aromatic alkynes, both the neutral and cationic Rh(I) complexes showed excellent reactivity with exclusive formation of the β(E)-isomer. For aliphatic alkynes, however, selectivity of the Rh(I) complexes was low. By contrast, the neutral and cationic Rh(III) Cp* NHC complexes proved to be highly regio- and stereoselective catalysts, allowing for the formation of the thermodynamically less stable β-(Z)-vinylsilane isomers at room temperature. Notably, the SBA-15 immobilized Rh(I) catalysts, in which the pore walls provide an additional confinement, showed excellent β-(Z)-selectivity in the hydrosilylation of aliphatic alkynes, too. Also, in the case of 4-aminophenylacetylene, selective formation of the β(Z)-isomer was observed with a neutral SBA-15 supported Rh(III) Cp* NHC complex but not with its homogenous counterpart. These are the first examples of high β(Z)-selectivity in the hydrosilylation of alkynes by confinement generated upon immobilization inside mesoporous silica.     

A New Monomer for π‐Conjugated Polymers — Synthesis and Characterization of Bis((Z)‐5‐phenyl‐2‐phenylmethylidene‐1, 3‐dithiole‐4‐yl)monosulfane

Bis((Z)‐5‐phenyl‐2‐phenylmethylidene‐1, 3‐dithiole‐4‐yl)monosulfane ( 6 ), a molecule consisting of two diphenyldithiafulvene units connected by a sulfur bridge, was synthesized by the selective lithiation of (Z)‐4‐phenyl‐2‐phenylmethylidene‐1, 3‐dithiole ( 7a ) at the endocyclic double bond and by subsequent reaction of the lithiated intermediate with bis(phenylsulfonyl)sulfane. Since this reaction sequence proceeded with retention of configuration, of three possible isomers (E, E, Z, E, and Z, Z) only the Z, Z form was obtained. On the basis of the X‐ray structure analysis and the NMR‐spectroscopic characterization of 6 supplemented by the NMR parameters of (E)‐ and (Z)‐4‐phenyl‐2‐phenylmethylidene‐1, 3‐dithiole, it was demonstrated that two characteristic ⁵J coupling constants of the proton at the exocyclic double bond indicate the configuration (Z or E) of disubstituted dithiafuvene derivatives.     

Synthesis of a Photoaffinity‐Labeled (11Z)‐Retinal: Identification of Retinal/Rhodopsin Cross‐Linked Sites along the Visual‐Transduction Path

The retinal chromophore (11Z)‐3‐diazo‐4‐oxoretinal ( 1 ) with two photo‐labile moieties has been synthesized by semi‐hydrogenation of an 11‐yne precursor with activated Zn in aqueous media. Incorporation of 1 into opsin yielded diazoketo rhodopsin (DK‐Rh), which, upon bleaching, gave rise to intermediates batho‐Rh, lumi‐Rh, meta‐Rh, and meta‐II‐Rh corresponding to those of native Rh but at lower temperatures. Photoaffinity labeling of DK‐Rh and these bleaching intermediates showed that the ionone ring cross‐linked to Trp265 of helix F in DK‐Rh and batho intermediate, and to Ala169 of helix D in lumi, meta‐I, and meta‐II intermediates. These results demonstrate the occurrence of large conformational changes along the visual transduction path, which, in turn, is responsible for activation of the G‐protein.     

Broadly Applicable Z‐ and Diastereoselective Ring‐Opening/Cross‐Metathesis Catalyzed by a Dithiolate Ru Complex

A broadly applicable Ru‐catalyzed protocol for Z‐selective ring‐opening/cross‐metathesis (ROCM) is disclosed. In addition to reactions relating to terminal alkenes of different sizes, the first examples of Z‐selective ROCM processes involving heteroaryl olefins, 1,3‐dienes, and O‐ and S‐substituted alkenes as well as allylic and homoallylic alcohols are reported. Z‐Selective transformations with an α‐substituted allylic alcohol are shown to afford congested Z alkenes with high diastereoselectivity. Transformations are performed in the presence of 2.0–5.0 mol % of a recently disclosed Ru‐based dithiolate complex that can be easily prepared in a single step from commercially available starting materials. Typically, transformations proceed at ambient temperature and are complete within eight hours; products are obtained in up to 97 % yield, >98:2 Z/E, and >98:2 diastereomeric ratio. The present investigations reveal a mechanistically significant attribute of the Ru‐based dithiolates that arises from electrostatic interactions with anionic S‐based ligands.     

Rhodium(I)‐Catalyzed [4 + 2] Cycloaddition Reactions of 2‐Alkylenecyclo‐butanols with Alkynes and (E)‐2‐Nitroethenylbenzene through C(sp2)—C(sp3) Bond Cleavage

An intermolecular [4 + 2] cycloaddition was realized through C—C bond cleavage in the presence of Rh(I) catalyst. The selective ring opening of 2‐alkylenecyclobutanols enables the generation of active alkenylrhodium species, which underwent smooth cross addition over alkynes and (E)‐2‐nitroethenylbenzene, leading to highly substituted all‐carbon six‐membered rings in a single step and in a complete atom economy.     

Cobalt‐Catalyzed Regio‐ and Stereoselective Hydroboration of Allenes

An efficient pincer‐ligand‐based cobalt‐complex‐catalyzed allene hydroboration affording Z‐allylic boronates is described. The reaction demonstrates an excellent regio‐ as well as Z‐stereoselectivity and a wide substrate scope that tolerates many functional groups. Based on solvent‐assisted electrospray ionization mass spectrometry (SAESI‐MS) studies, a rationale for the cobalt‐catalyzed hydroboration involving the highly selective insertion of an allene into the Co?H bond to form Z‐allylic cobalt intermediates is proposed.     

Xantphite: A New Family of Ligands for Catalysis. Applications in the Hydroformylation of Alkenes

The novel bulky diphosphite (P∩P) ligands ( 3 and 4 ) based on the 2,7,9,9‐tetramethyl‐9H‐xanthene‐4,5‐diol ( 2 ) backbone were investigated in the Rh‐catalyzed hydroformylation of oct‐1‐ene, styrene, and (E)‐oct‐2‐ene. These diphosphites gave rise to very active and selective catalysts for the hydroformylation of oct‐1‐ene to nonanal with average rates>10000 (mol aldehyde)(mol Rh)⁻¹h⁻¹ (P(CO/H₂)=20 bar, T=80°, [Rh]=1 mM ) and maximum selectivities of 79% for the linear product. Relatively high selectivities towards the linear aldehyde (up to 70%, linear/branched up to 2.3) but very high activities (up to 39000 (mol aldehyde)(mol Rh)⁻¹h⁻¹) were observed for the hydroformylation of styrene in the presence of these bidentate ligands (P(CO/H₂)=2 – 10 bar, T=120°, [Rh]=0.2 mM ). Remarkable activities (up to 980 (mol aldehyde)(mol Rh)⁻¹h⁻¹) were achieved with these diphosphites for the hydroformylation of (E)‐oct‐2‐ene with selectivities for the linear product of 74% (l/b up to 2.8, P(CO/H₂)=2 bar, T=120°, [Rh]=1 mM ). A detailed study of the solution structure of the catalyst under catalytic conditions was performed by NMR and high‐pressure FT‐IR. The spectroscopic data revealed that under hydroformylation conditions, the bidentate ligands rapidly formed stable, well‐defined catalysts with the structure [RhH(CO)₂(P∩P)]. All the ligands showed a preference for an equatorial‐apical ( ea ) coordination mode in the trigonal bipyramidal Rh‐complexes, indicating that a bis‐equatorial ( ee ) coordination is not a prerequisite for highly selective catalysts.     

Formation of Trinuclear Rhodium‐Hydride Complexes [{Rh(PP*)H}3‐ (μ2‐H)3(μ3‐H)][anion]2—During Asymmetric Hydrogenation?

Recently described and fully characterized trinuclear rhodium‐hydride complexes [{Rh(PP*)H}₃(μ₂‐H)₃(μ₃‐H)][anion]₂ have been investigated with respect to their formation and role under the conditions of asymmetric hydrogenation. Catalyst–substrate complexes with mac (methyl (Z)‐ N‐acetylaminocinnamate) ([Rh(tBu‐BisP*)(mac)]BF₄, [Rh(Tangphos)(mac)]BF₄, [Rh(Me‐BPE)(mac)]BF₄, [Rh(DCPE)(mac)]BF₄, [Rh(DCPB)(mac)]BF₄), as well as rhodium‐hydride species, both mono‐([Rh(Tangphos)‐ H₂(MeOH)₂]BF₄, [Rh(Me‐BPE)H₂(MeOH)₂]BF₄), and dinuclear ([{Rh(DCPE)H}₂(μ₂‐H)₃]BF₄, [{Rh(DCPB)H}₂(μ₂‐H)₃]BF₄), are described. A plausible reaction sequence for the formation of the trinuclear rhodium‐hydride complexes is discussed. Evidence is provided that the presence of multinuclear rhodium‐hydride complexes should be taken into account when discussing the mechanism of rhodium‐promoted asymmetric hydrogenation.     

Gold‐Catalyzed 1,2‐/1,2‐Bis‐acetoxy Migration of 1,4‐Bis‐propargyl Acetates: A Mechanistic Study

The late transition metal catalyzed rearrangement of propargyl acetates offers an interesting platform for the development of synthetically useful transformations. We have recently shown that gold complexes can catalyze a highly selective tandem 1,2‐/1,2‐bis‐acetoxy migration in 1,4‐bis‐propargyl acetates to form 2,3‐bis‐acetoxy‐1,3‐dienes. In this way, (1Z,3Z)‐ or (1Z,3E)‐ and (1E,3Z)‐1,3‐dienes could be obtained in a stereocontrolled manner depending on the electronic and steric features of the ancillary ligand bound to gold and the substituents at the propargylic positions. In this work, we report an experimental study on the scope of this transformation, plus a detailed theoretical examination of the reaction mechanism, which has revealed the key features responsible for the reaction stereoselectivity. Synthetic applications towards the one‐pot synthesis of quinoxaline heterocycles and tandem Diels–Alder processes have also been devised.     

Novel isolated,L‐amino acid‐ligated rhodium catalysts that induce highly helix‐sense‐selective polymerization of an achiral 3,4,5‐trisubstituted phenylacetylene

Two novel chiral well‐defined rhodium complexes, Rh(cod)(L‐Phe) (cod = 1,5‐cyclooctadiene, Phe = phenylalanine) and Rh(cod)(L‐Val) (Val = valine) were synthesized, isolated by recrystallization, and characterized. The helix‐sense‐selective polymerization (HSSP) of an achiral 3,4,5‐trisubstituted phenylacetylene, p‐dodecyloxy‐m,m‐dihydroxyphenylacetylene (DoDHPA) was examined by using the two Rh complexes as catalysts. These catalysts provided high molecular weight polymers (M_w 28 × 10⁴?45 × 10⁴) in about 40%–85% yields. The resulting polymers exhibited a bisignated CD signal at about 300 nm and a broad signal around 470 nm, indicating that they have preferential one‐handed helical structure. The present catalysts achieved larger molar ellipticity up to [θ]₃₁₀ = 13.0 × 10⁴ deg cm²/dmol than those with binary chiral catalytic systems, [Rh(cod)Cl]₂/(L‐phenylalaninol), [Rh(cod)Cl]₂/(L‐valinol), and [Rh(nbd)Cl]₂/(R)‐PEA. All these results manifest that the present, well‐defined Rh complexes serve as excellent catalysts for the HSSP of DoDHPA. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2346–2351     

Highly Stereoselective Conjugate Addition and α‐Alkynylation Reaction with Electron‐Deficient Alkynes Catalyzed by Chiral Scandium(III) Complexes

The highly Z‐selective asymmetric conjugate addition of 3‐substituted oxindoles to alkynyl carbonyl compounds has been developed by using scandium complexes of chiral N,N′‐dioxides under mild conditions. The thermodynamically unstable Z‐olefin derivatives were obtained in excellent yields and high enantiomeric and geometric control. The catalyst was also found to be effective in the asymmetric acetylenic substitution reaction of 3‐substituted oxindoles, giving excellent enantioselectivities.     

Selective Synthesis of Z‐Cinnamyl Ethers and Cinnamyl Alcohols through Visible Light‐Promoted Photocatalytic E to Z Isomerization

A photocatalytic E to Z isomerization of alkenes using an iridium photosensitizer under mild reaction conditions is disclosed. This method provides scalable and efficient access to Z‐cinnamyl ether and allylic alcohol derivatives in high yields with excellent stereoselectivity. Importantly, this method also provides a powerful strategy for the selective synthesis of Z‐magnolol and honokiol derivatives possessing potential biological activity.     

Light‐Mediated Total Synthesis of 17‐Deoxyprovidencin

An asymmetric synthesis of the diterpenoid 17‐deoxyprovidencin is described. Key steps include an aldol addition, a base‐catalyzed Wipf‐type furan formation, a Z‐selective ring‐closing metathesis for macrocyclization, a photochemical E/Z isomerization to a highly strained and conformationally restricted ring system, and the stereoselective formation of two epoxides on the ring.     

Preparation,Crystal Structure and Spectroscopic Studies of Mixed valence Compound [(Cp4i Rh)2 (μ‐Cl)3] [Rh(CO)2Cl2]

[(Cp⁴ⁱ Rh)₂(μ‐Cl)₃] [Rh(CO)₂Cl₂] (Cp⁴ⁱ = tetraisopropyl‐cyclopenta‐dienyl) has been prepared and its crystal is in the space group of Pbar with a= 0.9417 (8), b = 1.4806 (3), c = 1.5062 (2) nm, a = 92.980(10), β = 97.42(3), γ = 93.98 (3)°, V = 2.0735(18) nm³ and Z = 2. The crystal structure consists of a cation of [(η⁵‐Cp⁴ⁱ) Rh (III)(μ‐Cl)₃ Rh (III) (η⁵‐Cp⁴ⁱ)]⁺ and an anion of [Rh (I) (CO)₂ Cl₂]. The two bulky tetraisopropylcyclopentadienyl ligands are in the ecliptic conformation with angle of 10.19° between two cyclopentadienyl ring planes.     

Spatial Separation of Covalent,Ionic, and Metallic Interactions in Mg11Rh18B8 and Mg3Rh5B3

The crystal structures of Mg₁₁Rh₁₈B₈ and Mg₃Rh₅B₃ have been investigated by using single‐crystal X‐ray diffraction. Mg₁₁Rh₁₈B₈: space group P4/mbm; a=17.9949(7), c=2.9271(1) Å; Z=2. Mg₃Rh₅B₃: space group Pmma; a=8.450(2), b=2.8644(6), c=11.602(2) Å; Z=2. Both crystal structures are characterized by trigonal prismatic coordination of the boron atoms by rhodium atoms. The [BRh₆] trigonal prisms form arrangements with different connectivity patterns. Analysis of the chemical bonding by means of the electron‐localizability/electron‐density approach reveals covalent B? Rh interactions in these arrangements and the formation of B? Rh polyanions. The magnesium atoms that are located inside the polyanions interact ionically with their environment, whereas, in the structure parts, which are mainly formed by Mg and Rh atoms, multicenter (metallic) interactions are observed. Diamagnetic behavior and metallic electron transport of the Mg₁₁Rh₁₈B₈ and Mg₃Rh₅B₃ phases are in agreement with the bonding picture and the band structure.