Indenyl Effect Due to Metal Slippage? Computational Exploration of Rhodium‐Catalyzed Acetylene [2+2+2] Cyclotrimerization

The mechanism of CpRh (Cp=cyclopentadienyl) and IndRh (Ind=indenyl)‐catalyzed acetylene [2+2+2] cyclotrimerization has been revisited aiming at finding an explanation for the better performance of the latter catalyst found experimentally. The hypothesis that an ancillary ligand of the precatalyst remains bonded to the metal center throughout the whole catalytic cycle, based on the experimental evidence that the nature of this ligand can exert some control in cocyclotrimerization of different alkynes, is considered. Strong hapticity variations occur in both the CpRh‐ and IndRh‐catalyzed processes. As the Ind ligand undergoes a more facile slippage than Cp, the energy profile is far smoother in the IndRh‐catalyzed cyclotrimerization. This difference in the energetics of the process translates into an enhanced activity of the IndRh catalyst, in nice agreement with experiment.     

Asymmetric Induction in [3 + 2] Dipolar Cycloaddition Reactions of Nitrile Oxides with Chiral (α-Oxyallyl)silanes

The dipolar cycloaddition reactions of(α-oxyallyl)silanes 12a-g with 2,2-dimethylpropanenitrile oxide and benzonitrile oxide have been studied. Mixtures of anti (14a-g and 16a-g) and syn (15a-g and 17a-g) Δ²-isoxazolines are formed. The direction and magnitude of asymmetric induction depends on the allylie oxygen substituent: a free hydroxy provides a modest excess of the syn diastereomer, a silyl ether shows good selectivity for the anti diastereomer, and various acyl derivatives show low diastereoselectivity. The significance of these results is discussed in terms of two current models for asymmetric induction.     

Asymmetric [3+2]-Cycloaddition of Morita–Baylis–Hillman Carbonates with Maleimides Catalyzed by Chiral Ferrocenylphosphines

New chiral ferrocenylphosphines LB1–LB9 were designed and prepared through simple synthetic approaches. These air-stable ferrocenylphosphines were applied to promote asymmetric [3+2]-cycloaddition of Morita–Baylis–Hillman carbonates with maleimides, among which LB7 was shown to have good catalytic activity to afford the corresponding multifunctional cyclopentenes in up to 59% yield and up to 53% ee under mild reaction conditions. A plausible reaction mechanism was proposed.     

Rhodium‐Catalyzed Enantioselective Intramolecular C?H Silylation for the Syntheses of Planar‐Chiral Metallocene Siloles

Reported herein is the rhodium‐catalyzed enantioselective C H bond silylation of the cyclopentadiene rings in Fe and Ru metallocenes. Thus, in the presence of (S)‐TMS‐Segphos, the reactions took place under very mild conditions to afford metallocene‐fused siloles in good to excellent yields and with ee values of up to 97 %. During this study it was observed that the steric hindrance of chiral ligands had a profound influence on the reactivity and enantioselectivity of the reaction, and might hold the key to accomplishing conventionally challenging asymmetric C H silylations.     

Counterion Effects on the Columnar Mesophases of Triphenylene‐Substituted [18]Crown‐6 Ethers: Is Flatter Better?

The twisted lateral tetraalkyloxy ortho‐terphenyl units in dibenzo[18]crown‐6 ethers 1 a – f were readily converted into the flat tetraalkyloxytriphenylene systems 2 a – f by oxidative cyclization with FeCl₃ in nitromethane. Reactions of the latter with potassium salts gave complexes KX ?2 , which displayed mesomorphic properties. The aromatization increased both the clearing and melting points; the mesophase stabilities, however, were mainly influenced by the respective anions upon complexation with various potassium salts. In contrast, the alkyl chain lengths played only a secondary role. Among the potassium complexes of triphenylene‐substituted crown ethers KX ?2 , only those with the soft anions I^? and SCN^? displayed mesophases with expanded phase temperature ranges of 93 °C and 132 °C (for KX ?2 e ), respectively, as compared to the corresponding o‐terphenyl‐substituted crown ether complexes KI ?1 e (ΔT=51 °C) and KSCN ?1 e (plastic crystal phase). Anions such as Br^?, Cl^?, and F^? decreased the mesophase stability, and PF₆^? led to complete loss of the mesomorphic properties of KPF₆ ?2 although not for KPF₆ ?1 . For crown ether complexes KX ?2 (X=F, Cl, Br, I, BF₄, and SCN), columnar rectangular mesophases of different symmetries (c2 mm, p2 mg, and p2 gg) were detected. In contrast to findings for the twisted o‐terphenyl crown ether complexes KX ?1 , the complexation of the flat triphenylene crown ethers 2 with KX resulted in the formation of organogels. Characterization of the organogel of KI ?2 e in CH₂Cl₂ revealed a network of fibers.     

Asymmetric Synthesis of Isoindolones by Chiral Cyclopentadienyl‐Rhodium(III)‐Catalyzed C?H Functionalizations

Directed Cp*Rh^III‐catalyzed carbon–hydrogen (C H) bond functionalizations have evolved as a powerful strategy for the construction of heterocycles. Despite their high value, the development of related asymmetric reactions is largely lagging behind due to a limited availability of robust and tunable chiral cyclopentadienyl ligands. Rhodium complexes comprising a chiral Cp ligand with an atropchiral biaryl backbone enables an asymmetric synthesis of isoindolones from arylhydroxamates and weakly alkyl donor/acceptor diazo derivatives as one‐carbon component under mild conditions. The complex guides the substrates with a high double facial selectivity yielding the chiral isoindolones in good yields and excellent enantioselectivities.     

Enantioselective Synthesis of Distorted π-Extended Chiral Triptycenes Consisting of Three Distinct Aromatic Rings by Rhodium-Catalyzed [2+2+2] Cycloaddition

The enantioselective synthesis of distorted π-extended chiral triptycenes, consisting of three distinct aromatic rings, has been achieved with high ee value of 87 % by the cationic rhodium(I)/segphos complex-catalyzed enantioselective [2+2+2] cycloaddition of 2,2′-di(prop-1-yn-1-yl)-5,5′-bis(trifluoromethyl)-1,1′-biphenyl with 6-methoxy-1,2-dihydronaphthalene followed by the diastereoselective Diels–Alder reaction and aromatization. Demethoxy derivatives were also synthesized by the C−O bond cleavage. In this synthesis, the use of the electron-deficient diyne and the electron-rich alkene is crucial to suppress the undesired strain-relieving carbocation rearrangement and stabilize the distorted triptycene structure.     

Dynamic Equilibria in Solvent‐Mediated Anion,Cation and Ligand Exchange in Transition‐Metal Coordination Polymers: Solid‐State Transfer or Recrystallisation?

The solution properties of a series of transition‐metal–ligand coordination polymers [ML(X)_n]_∞ [M=Ag^I, Zn^II, Hg^II and Cd^II; L=4,4′‐bipyridine (4,4′‐bipy), pyrazine (pyz), 3,4′‐bipyridine (3,4′‐bipy), 4‐(10‐(pyridin‐4‐yl)anthracen‐9‐yl)pyridine (anbp); X=NO₃^?, CH₃COO^?, CF₃SO₃^?, Cl^?, BF₄^?; n=1 or 2] in the presence of competing anions, metal cations and ligands have been investigated systematically. Providing that the solubility of the starting complex is sufficiently high, all the components of the coordination polymer, namely the anion, the cation and the ligand, can be exchanged on contact with a solution phase of a competing component. The solubility of coordination polymers is a key factor in the analysis of their reactivity and this solubility depends strongly on the physical properties of the solvent and on its ability to bind metal cations constituting the backbone of the coordination polymer. The degree of reversibility of these solvent‐induced anion‐exchange transformations is determined by the ratio of the solubility product constants for the starting and resultant complexes, which in turn depend upon the choice of solvent and the temperature. The extent of anion exchange is controlled effectively by the ratio of the concentrations of incoming ions to outgoing ions in the liquid phase and the solvation of various constituent components comprising the coordination polymer. These observations can be rationalised in terms of a dynamic equilibrium of ion exchange reactions coupled with Ostwald ripening of crystalline products. The single‐crystal X‐ray structures of [Ag(pyz)ClO₄]_∞ ( 1 ), {[Ag(4,4′‐bipy)(CF₃SO₃)] ? CH₃CN}_∞ ( 2 ), {[Ag(4,4′‐bipy)(CH₃CN)]ClO₄ ? 0.5 CH₃CN}_∞ ( 3 ), metal‐free anbp ( 4 ), [Ag(anbp)NO₃(H₂O)]_∞ ( 5 ), {[Cd(4,4′‐bipy)₂(H₂O)₂](NO₃)₂ ? 4 H₂O}_∞ ( 6 ) and {[Zn(4,4′‐bipy)SO₄(H₂O)₃] ? 2 H₂O}_∞ ( 7 ) are reported.     

Evolution of Routes for Asymmetric Total Synthesis of Cyclocitrinol Enabled by Type Ⅱ [5+2] Cycloaddition

Main observation and conclusion The asymmetric total synthesis of an unusual C25 steroid containing a unique bicyclo[4.4.1]undecene A/B ring system,resulting in...     

Enantioselective Palladium‐Catalyzed C?H Functionalization of Indoles Using an Axially Chiral 2,2′‐Bipyridine Ligand

A palladium‐catalyzed enantioselective C H functionalization of indoles was achieved with an axially chiral 2,2′‐bipyridine ligand, thus providing the desired indol‐3‐acetate derivatives with up to 98 % ee. Moreover, the reaction protocol was also effective for asymmetric O H insertion reaction of phenols using α‐aryl‐α‐diazoacetates. This represents the first successful application of bipyridine ligands with axial chirality in palladium‐catalyzed carbene migratory insertion reactions.     

Enantioselective Palladium‐Catalyzed C?H Functionalization of Indoles Using an Axially Chiral 2,2′‐Bipyridine Ligand

A palladium‐catalyzed enantioselective C H functionalization of indoles was achieved with an axially chiral 2,2′‐bipyridine ligand, thus providing the desired indol‐3‐acetate derivatives with up to 98 % ee. Moreover, the reaction protocol was also effective for asymmetric O H insertion reaction of phenols using α‐aryl‐α‐diazoacetates. This represents the first successful application of bipyridine ligands with axial chirality in palladium‐catalyzed carbene migratory insertion reactions.     

Thermal conversion of 15,9-triynes : (2+2+2]cycloadditions or [3.3]sigmatropic shifts?

The gas phase pyrolyses of variously labeled 1,5,9-decatriynes (1) and 1,5,9-cyclododecatriynes (2) were investigated to determine possible modes of thermal isomerizations. Conditions iocluded temperatures in the range 400–600°, pressures of 40–10^-4 Torr,and contact times of ca 1 ms to to 15s.The labeling patterns in 1 and 2 were chosen such as to be able to distinguish direct intramolecular [2 +2+2]cycloadditions of the alkyna units to form an aromatic ring (perbaps with subsequent rearrangements), and [3.3]sigmatropic shifts of the 1,5-diyne moieties. Methods for synthesizing the isotopically (particularly ¹³C) labeled triynes were devised and unplemented. The route to 5,6-¹³C₂-1,5,9-decatriyne (1c) made use of a new procedure for the synthesis of symmetrically diaubstituted alkynes involing coupling between two equivalents of an alkl copper reagent and diiodoacetylene-¹³C₂. The synthesis of 1,10-¹³C₂-1,5,9-cyclododecatriyne (2b) was accomplished starting with K¹³CN, elaboration to labeled diethylsuccinate, a crucial bis-Wittig condensation to labeled l,5,9-cyclododecatriene 10, and bromination-dehydrobromination of the latter (NaOH-ethylene glycol). Products from the pyrolysis of unlabeled 1a included [1,2:4,5] dicyclobutabenzene, naphthalene, and 3,4-dimethylidene-1-(but-3-ynyl)cyclobutene. Pyrolysis of 1b gave 3,6-dideuterio[l,2:4,5]dicyclobutabenzene and partially deuterated naphthalene, that of 1c produced 1,2-¹³C₂-[1,2,:4,5]dicyclobutabenzene and 9,10-¹³C₂-naphthalene. While the pyrolysis of 2a resulted in hexamethylidenecyclohexane (hexaradialene), 2b furnished 1,4-¹³C₂-hexaradialene. The results rule out the occurrence of [2+2+2]cycloadditions of the alkyne units, but are consistent with the intervention of a series of [3.3]sigmatropic shifts which connect starting materials with products.     

Ketene-acetylene [2 + 2] cycloadditions: cyclobutenone and/or oxete formation?

The [2 + 2] cycloaddition of monosubstituted acetylenes to ketene has been studied by ab initio(G2(MP2,SVP) and DFT (B3LYP/6-31Gd)) methods. The activation barrier decreases with increasing electron-donating ability of the acetylene substituent, and it can be roughly correlated with the energy of the acetylene HOMO. The addition to the C[double bond, length as m-dash]C bond of ketene (giving cyclobutenones) is preferred for the less electron-rich acetylenes, but for the most electron rich ones (X = NH(2) and NMe(2)) the addition to the C[double bond, length as m-dash]O bond (giving oxetes) becomes competitive, with activation barriers as low as ca. 45 (30) kJ mol(-1) for the two computational methods used. The cyclobutenones and oxetes can undergo ring opening to vinylketenes and acylallenes, respectively. Furthermore, the latter two compounds can interconvert by a 1,3-shift of the substituent X. The acylallenes become thermodynamically more stable than the vinylketenes for [small pi]-(lone pair) donating substituents X, and the 1,3-shift barrier also decreases, to ca. 130 kJ mol(-1) for X = NMe(2). In contrast, the 1,3-shifts of CH(3) and H have very high barriers.     

Asymmetric Diels—Alder Reaction Catalyzed by Novel Chiral Dibenzo[a,c] cycloheptadiene Bis（oxazoline） Metal Complexes

A series of novel chiral C2-symmetric bis (oxazoline) ligands have been synthesized.The copper and magnesium complexes,prepared in situ from copper(Ⅱ)-triflate of magnesium triflate with the new enantiopure oxazoline ligands,were evaluated as chiral catalysts in the enantioselective Diels-Alder reaction of cyclopentadiene with N-crotenoyl-oxazolidin-2-one.Primary results showed that diastereoselectivity up to 94% and enantioselectivity up to 68% ee for endo products were observed respectively with these ligands.     

Rhodium-Catalyzed Chemo-, Regio-, Diastereo-, and Enantioselective Intermolecular [2+2+2] Cycloaddition of Three Unsymmetric 2π Components

We have developed the Rh⁺/H₈-binap-catalyzed chemo-, regio-, diastereo-, and enantioselective intermolecular [2+2+2] cycloaddition of three unsymmetric 2π components. Thus, two arylacetylenes react with a cis-enamide to yield a protected chiral cyclohexadienylamine. Moreover, replacing one arylacetylene with a silylacetylene enables the [2+2+2] cycloaddition of three distinct unsymmetric 2π components. These transformations proceed with excellent selectivity (complete regio- and diastereoselectivity and up to >99 % yield and >99 % ee). Mechanistic studies suggest the chemo- and regioselective formation of a rhodacyclopentadiene intermediate from the two terminal alkynes.