Alkynyl Carbene Research Articles

Rhodium(II)-Catalyzed Alkynyl Carbene Insertion into N-H Bonds.

The first insertion of an alkynyl carbene into N-H bonds under Rh-catalysis is developed. Alkynyl hydrazone carboxylates are used as donor-acceptor carbene precursors and are exquisitely inserted into the N-H bonds of various amines, amides, and 1,2-diamines. A wide variety of 3-alkynyl 3,4-dihydroquinoxalin-2(1H)-ones and densely functionalized α-alkynyl α-amino esters are obtained in good to excellent yields. Further, chemoselective N-H insertion reactions, mechanistic studies, and various synthetic transformations for obtaining valuable heterocycles are demonstrated.

Microwave-Assisted Reactivity of a Fischer Alkynyl Carbene Complex with Benzylidene Anilines

The reaction between the Fischer carbene complex (CO)5Cr=C(OEt)CºCPh and various benzylidene anilines RCH=NR1 was promoted by microwave irradiation, generating stable cross-conjugated metallahexatrienes in 45-70 % yield. Compared to conventional heating, the present conditions provided shorter reaction times with moderate yields. The geometrical configuration and the most stable conformation for each of the Fischer carbene complexes and their oxidation products were established by NMR and DFT analysis. The reaction mechanism was explored by DFT calculations of the potential energy surface, suggesting a 1,4-addition/ring closure/electrocyclic opening cascade process. Resumen. Se reporta la reactividad entre el carbeno de Fischer (CO)5Cr=C(OEt)CºCPh y las bencilidén anilinas, RCH=NR1, empleando irradiación de microondas. Los resultados indican que el calentamiento por microondas generó metalohexatrienos cruzados estables en rendimientos de 45-70 %, mostrando además, que los nuevos complejos se obtienen en tiempos de reacción más cortos y rendimientos moderados en comparación con las condiciones de calentamiento convencional. La configuración geométrica y la conformación más estable para los complejos carbénicos de Fischer y de sus derivados oxidados fueron establecidos por medio de RMN y DFT. La exploración de la superficie de energía potencial por cálculos DFT mostró que el proceso consistió en una reacción en cascada incluyendo una secuencia de adición-1,4, cierre de anillo y apertura electrocíclica.

Rh-Catalyzed [2,3]-Sigmatropic Rearrangement of Alkynyl Carbenes with Allyl Sulfides to Access Sulfide-Substituted 1,5-Enynes.

This study describes the development of Rh-catalyzed [2,3]-sigmatropic rearrangement of alkynyl carbenes with allyl sulfides. The protocol exhibits equitable functional group tolerance and allows the formation of a variety of synthetically valuable sulfide-substituted 1,5-enyne products. To the best of our knowledge, this is the first example of [2,3]-sigmatropic rearrangement of alkynyl carbenes. DFT analysis supports the involvement of rhodium carbene generation, sulfonium ylides formation, and [2,3]-sigmatropic rearrangement pathway.

Synthesis of Alkynyl Hydrazones from Unprotected Hydrazine and Their Reactivity as Diazo Precursors.

Alkynyl hydrazones are synthesized conveniently from 2-oxo-3-butynoates and hydrazine by suppressing the susceptible formation of pyrazoles. The resultant hydrazones are transformed into alkynyl diazoacetates under metal-free and mild oxidative conditions in excellent yields. Further, the alkynyl cyclopropane and propargyl silane carboxylates are synthesized in good yields by developing an unprecedented copper-catalyzed alkynyl carbene transfer reaction.

Synthesis of Propargyl Boranes via Silver Catalyzed Insertion of Alkynyl Carbene into B‐H Bond

AbstractA silver‐catalyzed insertion reaction of alkynyl carbenes derived in situ by decomposition of alkynyl N‐nosylhydrazones, into a B−H bond is reported. This protocol emerges as a straightforward synthesis of various propargylic boron compounds with up to 95 % yield. The operationally simple method exhibits high functional group tolerance and produces a variety of propargyl boranes which are synthetically useful building blocks. Deuterium labelling experiment and DFT studies show that the reaction proceeds via insertion of alkynyl Ag‐carbene into the B−H bond and generation of silver carbene occurs in the rate‐determining step.

Synthesis of Polysubstituted Symmetrical BODIPYs via Fischer Carbene Complexes: Theoretical, Photophysical and Electrochemical Evaluation.

A series of new symmetrical highly substituted BODIPYs 6 a-l was synthesized through a prefunctionalization approach in 35 %-89 % yields from the pyrrole core. This strategy allowed modulation of the substituents at the different positions based on the choice of Fischer's alkynyl carbenes, oxazolones and aldehydes used as precursors. The substituent variation at positions 2, 6, 3 and 5 had the greatest effect on the modulation of their photophysical properties such as absorption (λabs ) and emission (λem ) wavelengths, extinction coefficient (ϵ), quantum yields (ϕ), Stokes shifts (Δν), fluorescence decay, radiative (krad ) and non-radiative (knr ) constants and the CIE 1931 coordinates. Theoretical calculations allowed to corroborate the effect of the substituents of meso-position on the modification of the dihedral angles. Cyclic voltammetry studies revealed that the BODIPY series presents similar redox potential behavior, being electrochemically active even in successive cycles, which suggests that transport by diffusion is the dominant process.

Ag-Catalyzed Insertion of Alkynyl Carbenes into C-C Bonds of β-Ketocarbonyls: A Formal C(sp2) Insertion.

Here we report a silver-catalyzed alkynyl carbene insertion into β-ketocarbonyls using alkynyl N-nosylhydrazones as alkynyl carbene precursors, which provides access to trisubstituted allenyl ketones. This reaction represents the first example of an alkynyl carbene insertion into a C-C σ bond, affording products homologated with an sp2 carbon center. The products are useful substrates for further transformations. Experimental investigations and theoretical calculations suggest the reaction proceeds through a stepwise enol cyclopropanation/retro-aldol pathway.

Silver-catalyzed cyclopropanation of alkenes with alkynyl N-nosylhydrazones leading to alkynyl cyclopropanes.

Here, a novel method for the stereoselective synthesis of alkynyl cyclopropanes, by the silver-catalyzed alkynylcyclopropanation of alkenes using alkynyl N-nosylhydrazones as alkynyl carbene precursors, is reported. This method provides a straightforward and powerful approach for preparing various alkynyl cyclopropanes in high yield with excellent stereoselectivities. In addition, the practicality of this method was demonstrated by gram-scale synthesis and late-stage modification of bioactive molecules.

Enantioselective Insertion of Alkynyl Carbenes into Si-H Bonds: An Efficient Access to Chiral Propargylsilanes and Allenylsilanes.

Chiral propargylsilanes and chiral allenylsilanes have emerged as versatile building blocks for organic synthesis. However, efficient methods for preparing these organosilicon compounds are lacking. We herein report a highly enantioselective method for synthesis of chiral propargylsilanes and chiral allenylsilanes from readily available alkynyl sulfonylhydrazones. Specifically, chiral spiro phosphate dirhodium complexes were used to catalyze asymmetric insertion of alkynyl carbenes into the Si-H bonds of silanes to afford a variety of chiral propargylsilanes with excellent enantioselectivity. Subsequently, a platinum catalyst was used for stereospecific isomerization of the chiral propargylsilanes to the corresponding chiral allenylsilanes.

Selective synthesis of trisubstituted pyrroles through the reactions of alkynyl Fischer carbene complexes with oxazolones.

An efficient and simple synthesis of novel trisubstituted 1H-pyrroles 4a-qvia 1,3-dipolar cycloaddition of Δ3-trifluoromethyloxazolones 2a-d with both chromium and tungsten alkynyl Fischer carbene complexes (1a-h) is described. An unexpected and unreported -CF3 group elimination process was observed in the pyrrole structure. Our experimental and theoretical data suggested that the metal fragment may be responsible for this phenomenon. The dipolar cycloaddition proceeded efficiently to produce a single regioisomer, which was unambiguously established through NMR and single-crystal X-ray diffraction studies. Nevertheless, the reaction of alkynyl carbenes bearing an α,β,γ,δ-unsaturated moiety with excess oxazolone 2a produced a polycyclic compound 6 speculatively formed through a cascade reaction involving 1,6-, 1,4- and 1,2-nucleophilic addition steps.

Two Types of σ-Allenyl Complexes from Reactions of Silylenes and Germylenes with Chromium Fischer Alkynyl(alkoxy)carbenes.

The reactivity of amidinatotetrylenes of the type E(tBu2 bzm)R1 (E=Si, Ge; tBu2 bzm=N,N'-bis(tertbutyl)benzamidinate; R1 =alkyl or aryl) with the chromium Fischer alkynylcarbene complexes [Cr{C(OEt)C2 R2 }(CO)5 ] (R2 =Ph; ferrocenyl, Fc) has been studied. At room temperature, two different reaction pathways have been identified: (a) attack of the amidinatotetrylene to the alkynyl C2 atom (γ-attack), which leads to σ-allenyl complexes in which the original Ccarbene atom maintains its attachment to the Cr(CO)5 and OEt groups (compounds 3 ), and (b) attack of the amidinatotetrylene to the Ccarbene atom (α-attack), which ends in σ-allenyl complexes in which the original Ccarbene atom is not attached to the metal atom and has been inserted into an E-N bond of the amidinatotetrylene forming an E-C-N-C-N five-membered ring (compounds 4 ). It has been found that compounds 3 are thermodynamically less stable than their corresponding 4 isomers and that some of the former (E=Ge; R1 =CH2 SiMe3 ) can be transformed into the latter upon heating. At high temperatures (>70 °C) the reactions involving bulky amidinatotetrylenes (R1 =Mes, tBu) end in the carbene-substitution products [Cr{E(tBu2 bzm)R1 }(CO)5 ].

Unexpected Zwitterionic Allenyls from Silylenes and a Fischer Alkynylcarbene: A Remarkable Silylene-Promoted Rearrangement.

The silylenes Si(tBu2 bzam)R (tBu2 bzam=N,N'-bis(tertbutyl)benzamidinate; R=mesityl, CH2 SiMe3 ) attack the Ccarbene atom of the Fischer alkynyl(ethoxy)carbene complex [W(CO)5 {C(OEt)C2 Ph}] to give, after a striking rearrangement, zwitterionic σ-allenyl complexes in which the original carbene C atom forms part of the allene C3 fragment and also of a Si-C-N-C-N five-membered ring after insertion into a Si-N bond of the original amidinatosilylene. These remarkable allenyl products, which contain two stereogenic groups, are selectively formed as single diastereomers.

Understanding the Reactivity of Group 6 Metal (M = Cr, W) Alkynyl Fischer Carbene Complexes with Multi‐Reactive Masked Dienes

Group 6 alkynyl Fischer carbene complexes behave as electrophiles with highly reactive masked dienes such as anthrone and 3‐hydroxy‐2‐pyrone, which also can react as bidentate nucleophiles. Both Cr0 and W0 alkynyl carbene complexes require tertiary amines to promote the formation of 1,4‐C‐addition products, but a marked dependence of the reactivity on the metal was observed. Although W0 complexes form exclusively the 1,4‐C‐adducts in the presence of a catalytic amount of base, Cr0 complexes lead to new anthrone‐derived bis‐carbene complexes, obtained by a sequential 1,4‐C‐ and 1,4‐O‐addition process with high diastereoselectivity and only in the presence of an excess of base. DFT computational studies show that the gap between the LUMO of the complex and the HOMO of the anthrone determines the differences in reactivity and that the stability of the intermediate metal allenyls formed after the initial 1,4‐addition is decisive for the outcome of the reaction.

Unexpected [4 + 2] Cycloaddition through Chromium Non-Heteroatom-Stabilized Alkynyl Carbene Complexes: Regioselective Access to Substituted 6-Azaindoles

A formal [4 + 2] heterocycloaddition of non-heteroatom-stabilized alkynyl carbene complexes and iminopyrroles is described. The reaction implies a totally regioselective synthesis of 6-azaindole derivatives through the formation of the pyridine ring. The mechanism of the reaction has been explored by means of density functional theory calculations, which showed a preference for the [4 + 2] cycloaddition instead of the [2 + 2] or [3 + 3] cycloadditions observed with other imines. The structure of the products also shows an unusual connectivity pattern from carbene complexes.

Alkynyl N-Nosylhydrazones: Easy Decomposition to Alknynl Diazomethanes and Application in Allene Synthesis.

The decomposition of N-tosylhydrazones is a safe and convenient method for the generation of donor carbenes. However, alkynyl carbenes cannot be isolated by this route because they readily undergo intramolecular cyclization to pyrazoles as soon as formed from alkynyl N-tosylhydrazones. Here, the use of alkynyl N-nosylhydrazones for the in situ generation of alkynyl carbenes and their coupling reaction with boronic acids under metal-free conditions is reported, giving rise to a wide array of di- and trisubstituted allenes. Preliminary mechanistic investigations demonstrated that γ-protodeboration of propargyl boric acid was responsible for the initial allene formation. This methodology based on the nosyl group allows for novel transformations that involve an alkynylcarbene transient species.

Growing the Molecular Architecture at Alkynyl(amino)carbene Ligands in Diiron µ‐Aminocarbyne Complexes

The diiron µ‐aminocarbyne imidoyl complexes [Fe2{µ‐CN(Me)(R)}(µ‐CO)(CO){C(C≡CR′)=NXyl}Cp2] [R = Xyl, R′ = Ph (1a);[1] R = Xyl, R′ = Tol (1b);[1] R = Me, R′ = Ph (1c); R = Me, R′ = SiMe3 (1d); Xyl = 2,6‐C6H3Me2, Tol = 4‐C6H4Me] have been prepared by the addition of lithium acetylides to precursors containing a terminal isocyanide ligand. The treatment of 1a–c with CF3SO3CH3 afforded the alkynyl(amino)carbene derivatives [Fe2{µ‐CN(Me)(R)}(µ‐CO)(CO){C(C≡CR′)N(Me)(Xyl)}Cp2][SO3CF3] [R = Me, R′ = Ph (2a); R = Xyl, R′ = Ph (2b); R = Xyl, R′ = Tol (2c)] in high yields. The reactions of 2b,c with CH3OH/NaBF4, NH2R and NaBH4 led to [Fe2{µ‐CN(Me)(Xyl)}(µ‐CO)(CO){C(CH=C(Ph)OMe)N(Me)(Xyl)}Cp2][BF4] (3), [Fe2{µ‐CN(Me)(Xyl)}(µ‐CO)(CO){C(CH=C(Tol)NHR)N(Me)(Xyl)}Cp2][SO3CF3] [R = Et (4a); R = Ph (4b)], and [Fe2{µ‐CN(Me)(Xyl)}(µ‐CO)(CO){C(CH=CHTol)N(Me)(Xyl)}Cp2][SO3CF3] (5) in yields of 65–80 %. Complexes 3–5 were formed by the selective functionalization of the alkynyl group in 2b,c. Complex 2c reacted with NaCH(CN)2/CH2(CN)2 and NaCH(CO2Me)2 to yield [Fe2{µ‐CN(Me)(Xyl)}(µ‐CO)(CO){C{N(Me)(Xyl)}CH=C(Tol)C(CN)C(NH2)C(CN)2}Cp2] (6) and [Fe2{µ‐C(NXyl)C(C(Tol)=C(CO2Me)2)CN(Me)(Xyl)}(µ‐CO)2Cp2] (7) in yields of 65 and 60 %, respectively. All the products were purified by alumina chromatography, alumina being presumably involved in the formation of 3, and characterized by elemental analysis and IR and NMR spectroscopy. The structures of 2b, 3, 5, and 6 were ascertained by single‐crystal X‐ray diffraction analysis.

ChemInform Abstract: Regioselective Multicomponent Synthesis of 2,4,6‐Trisubstituted Phenols from Fischer Alkynyl Carbene Complexes.

AbstractThe Fischer alkynyl carbenes react with terminal alkynes to give phenolic products in a cascade process via cycloaddition of alkynylethoxy carbene complexes with pentamethylcyclopentadiene, treatment with terminal alkynes to form functionalized cyclohexadienones regioselectively, reduction to tricyclic alcohols and retro‐Diels—Alder reaction.

Regioselective Multicomponent Synthesis of 2,4,6‐Trisubstituted Phenols from Fischer Alkynyl Carbene Complexes

AbstractA series of 2,4,6‐trisubstituted phenols 7a–7p has been prepared in moderate to good yields (23–77 %) by a retro‐Diels–Alder reaction from hindered tricyclic alcohols 6a–6q, using mild acidic conditions. The tricyclic alcohols were obtained by reduction of highly functionalized cyclohexadienones 5a–5q, which in turn were regioselectively prepared under mild conditions in high yields by treatment of a series of stable chromium(0) and tungsten(0) Fischer dienyl carbenes 4a–4g with different terminal alkynes 3a–3l. Carbenes 4a–4g were prepared by a [4+2] cycloaddition of alkynylethoxy carbene complexes 1a–1g with 1,2,3,4,5‐pentamethylcyclopentadiene (2). In a complementary route, compounds 5a–5q were also obtained by a multicomponent reaction in yields similar to those obtained in the multistep process (25–84 %), starting from 1a–1g, 2, and 3a–3l. The phenol products have a wide range of potential applications, and may be useful intermediates for further transformations.

ChemInform Abstract: Intermolecular and Regioselective Access to Polysubstituted Benzo‐ and Dihydrobenzo[c]azepine Derivatives: Modulating the Reactivity of Group 6 Non‐Heteroatom‐Stabilized Alkynyl Carbene Complexes.

AbstractA three‐component reaction of Fischer carbene complexes (I), lithium acetylides and benzaldehyde imines offers an efficient approach to stable tungsten containing benzazepinium zwitterions.

Intermolecular and regioselective access to polysubstituted benzo- and dihydrobenzo[c]azepine derivatives: modulating the reactivity of group 6 non-heteroatom-stabilized alkynyl carbene complexes.

We highlight the versatility of non-heteroatom-stabilized tungsten-carbene complexes 3 synthesized in situ, which have been used in a modular approach to access 2-benzazepinium isolable intermediates 5. By employing very mild conditions, benzazepinium derivatives 5 have been obtained in high yield from simple compounds, such as acetylides 2, Fischer-type alkoxycarbenes 1, and phenylimines 4. The process, involving a formal [4+3] heterocycloaddition, occurs in a totally regioselective manner, which differs from the approach previously observed in similar procedures for other carbene analogues. This work, which involves three components, reveals a control of the reactivity of non-heteroatom-stabilized carbene complexes 3 ([4+3] vs. [2+2]-heterocycloaddition reactions) depending on the acetylide substitution pattern. The influence of the substitution pattern in the behavior of the complexes has been computationally analyzed and rationalized. Finally, elaboration of the 2-benzazepinium intermediates allows access to 3H-benzo[c]azepines 6 and 3H-1,2-dihydrobenzo[c]azepines 7-9 with high control of the substitution of the nine positions of the heterocycle.