Bulky Alkynes Research Articles

Compatibility Assessment of Unactivated Internal Alkynes in ­Rhodium-Catalyzed [2+2+2] Cycloadditions

AbstractFunctionalized 1,2,4,5-tetrasubstituted benzenes are synthetically difficult or laborious to access. The Rh-catalyzed [2+2+2] cycloaddition of a diyne and internal alkyne offers a seemingly straightforward route to these scaffolds; however, this has been largely restricted to alkynes bearing activating (coordinating) functional groups, with very few examples of unactivated alkynes. In this work, we disclose an assessment of Rh-catalyzed [2+2+2] cycloadditions employing unactivated internal alkynes, focusing on the structural diversity and compatibility of both alkyne and diyne components. The limitations of this method are disclosed, with exceptionally bulky alkynes and specific functional groups undergoing side reactions. Furthermore, the practicalities of gram-scale reactions and catalyst recovery/reuse are demonstrated.

Three-component regioselective carboamidation of 1,3-enynes via rhodium(III)-catalyzed C-H activation.

Rhodium-catalyzed regio- and stereoselective three-component carboamidation of 1,3-enynes has been realized using indoles and dioxazolones as the functionalizing reagents. A wide range of multi-substituted skipped 1,4-dienes have been constructed in good yields and excellent stereoselectivity. The stereoselectivity is under substrate control. 1,3-Enynes bearing a relatively bulky alkyne terminus reacted with Z-selectivity. In contrast, a sterically less hindered alkyne terminus tends to predominantly give the E-configured skipped diene.

Terminal Alkyne Coupling Reactions Through a Ring: Effect of Ring Size on Rate and Regioselectivity.

Terminal alkyne coupling reactions promoted by rhodium(I) complexes of macrocyclic NHC‐based pincer ligands—which feature dodecamethylene, tetradecamethylene or hexadecamethylene wingtip linkers viz. [Rh(CNC‐n)(C2H4)][BArF 4] (n=12, 14, 16; ArF=3,5‐(CF3)2C6H3)—have been investigated, using the bulky alkynes HC≡CtBu and HC≡CAr’ (Ar’=3,5‐tBu2C6H3) as substrates. These stoichiometric reactions proceed with formation of rhodium(III) alkynyl alkenyl derivatives and produce rhodium(I) complexes of conjugated 1,3‐enynes by C−C bond reductive elimination through the annulus of the ancillary ligand. The intermediates are formed with orthogonal regioselectivity, with E‐alkenyl complexes derived from HC≡CtBu and gem‐alkenyl complexes derived from HC≡CAr’, and the reductive elimination step is appreciably affected by the ring size of the macrocycle. For the homocoupling of HC≡CtBu, E‐tBuC≡CCH=CHtBu is produced via direct reductive elimination from the corresponding rhodium(III) alkynyl E‐alkenyl derivatives with increasing efficacy as the ring is expanded. In contrast, direct reductive elimination of Ar'C≡CC(=CH2)Ar’ is encumbered relative to head‐to‐head coupling of HC≡CAr’ and it is only with the largest macrocyclic ligand studied that the two processes are competitive. These results showcase how macrocyclic ligands can be used to interrogate the mechanism and tune the outcome of terminal alkyne coupling reactions, and are discussed with reference to catalytic reactions mediated by the acyclic homologue [Rh(CNC‐Me)(C2H4)][BArF 4] and solvent effects.

Formal Lossen Rearrangement/Alkenylation or Annulation Cascade of Heterole Carboxamides with Alkynes Catalyzed by CpRhIII Complexes with Pendant Amides.

It has been established that a cyclopentadienyl (Cp) RhIII complex with two aryl groups and a pendant amide moiety catalyzes the formal Lossen rearrangement/alkenylation cascade of N-pivaloyl heterole carboxamides with internal alkynes, leading to alkenylheteroles. Interestingly, the use of sterically demanding internal alkynes afforded not the alkenylation but the [3+2] annulation products ([5,5]-fused heteroles). In these reactions, the pendant amide moiety of the CpRhIII complex may accelerate the formal Lossen rearrangement. The use of five-membered heteroles may deter reductive elimination to form strained [5,5]-fused heteroles; instead, protonation proceeds to give the alkenylation products. Bulky alkyne substituents accelerate the reductive elimination to allow the formation of the [5,5]-fused heteroles.

High refractive index hyperbranched polymers with different naphthalene contents prepared through thiol-yne click reaction using di-substituted asymmetric bulky alkynes

In the present study, different aromatic moieties, naphthalene and benzene, were introduced into alkynes to constitute 1,3,5-tris(naphthylethynyl)benzene as B3 monomer. This monomer could be reacted with different ratios of hexane-1,6-dithiol (A2 monomer) via mono-selective Thiol-Yne reaction (TYR) to form hyperbranched polymers PI. TYR yielded in both, model reactions and polymerizations, mono-adducts with high regioselectivity and proved the absence of any bis-adducts. Polymers of very high molecular weights of up to 625,000 g/mol could be achieved without any sign of gel formation. All the polymers exhibit high transparency in the visible wavelength range and also show high refractive indices of up to 1.745 at 589.7 nm due to the high incorporation of naphthalene and phenyl groups. Besides, all polymers prove excellent solubility and processability together with excellent thermal properties.

Copper-catalyzed Z-selective semihydrogenation of alkynes with hydrosilane: a convenient approach to cis-alkenes

A copper catalyst generated in situ from widely available copper salt and imidazolium salt in the presence of t-BuOK showed high efficiency for the semihydrogenation of a wide range of internal and terminal alkynes to their corresponding alkenes without obvious over-reduction. Functional groups, such as hydroxyl, nitro, halides, and amino, etc. were tolerated. The Z/E ratios of the obtained alkenes were generally >99%. Finally, semireduction of bulky alkynes also went smoothly.

An Improved Protocol for the Synthesis of [(η4-C4R4)Co(η5-C5H5)] Complexes

The reaction of bulky alkynes C2R2 with (η5-C5H5)Co(CO)(dimethyl fumarate) under microwave irradiation provides complexes of the type [(η4-C4R4)Co(η5-C5H5)] in good to excellent yields. This protocol represents a significant improvement over those reported previously. In particular, the formation of insertion products such as cyclopentadienones or cyclohexadienes can be avoided. In addition, because of the exceptional stability of (η5-C5H5)Co(CO)(dimethyl fumarate), the reactions can be carried out in crude solvents. The easy access to [(η4-C4R4)Co(η5-C5H5)] complexes stimulated a study of their reactivity, notably under cross-coupling conditions.

Effects of a Flexible Alkyl Chain on an Imidazole Ligand for Copper-Catalyzed Mannich Reactions of Terminal Alkynes

Copper-catalyzed Mannich reactions of terminal alkynes and secondary amines with aqueous formaldehyde can be accelerated by the use of a catalytic amount of an imidazole ligand carrying a long alkyl chain. The alkyl chain shows an efficient steric effect and helps the reaction. This imidazole ligand is efficient for various substrates, including even bulky alkynes.

Effects of a Flexible Alkyl Chain on a Ligand for CuAAC Reaction

Imidazole derivatives substituted by a normal alkyl group are shown to be efficient as a ligand for the copper(Ι)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. An alkyl chain on the imidazole ligands shows an efficient steric effect and benefits the reaction. Such functionalities of an alkyl chain allow a rapid CuAAC reaction of even a bulky alkyne, which has been difficult to perform under conventional conditions.

Mesityl Alkyne Substituents for Control of Regiochemistry and Reversibility in Zirconocene Couplings: New Synthetic Strategies for Unsymmetrical Zirconacyclopentadienes and Conjugated Polymers

Reaction of 2 equivs of MesC[triple bond]CPh with Cp(2)Zr(eta(2)-Me(3)SiC[triple bond]CSiMe(3))(pyr) afforded the zirconacyclopentadiene Cp(2)Zr[2,5-Ph(2)-3,4-Mes(2)C(4)]. The regiochemistry of this isomer (betabeta with respect to the mesityl substituents) was determined through single-crystal X-ray analysis and 2D (NOESY, HSQC, HMBC) NMR experiments. This selectivity is attributed largely to a steric-based directing effect of the o-methyl ring substituents since coupling of 1,3-dimethyl-2-(phenylethynyl)benzene with zirconocene gave a single regioisomer (o-xylyl groups in both beta-positions) while coupling of 1,3-dimethyl-5-(phenylethynl)benzene gave a statistical distribution of zirconacyclopentadiene regioisomers. The coupling reaction of 2 equivs of MeC[triple bond]CMes or PrC[triple bond]CMes with Cp(2)Zr(eta(2)-Me(3)SiC[triple bond]CSiMe(3))(pyr) at ambient temperature gave the betabeta regioisomers, Cp(2)Zr[2,5-Me(2)-3,4-Mes(2)C(4)] and Cp(2)Zr[2,5-Pr(2)-3,4-Mes(2)C(4)], respectively, as the major products. Heating solutions of these zirconacycles at 80 degrees C for several hours resulted in an increase in the amount of the unsymmetrical product. For reaction mixtures of PrC[triple bond]CMes and Cp(2)Zr(eta(2)-Me(3)SiC[triple bond]CSiMe(3))(pyr) the major (and apparently thermodynamic) product under these reaction conditions was Cp(2)Zr[2,4-Mes(2)-3,5-Pr(2)C(4)]. The steric strain in the mesityl-substituted zirconacycles allowed for facile substitution reactions of MesC[triple bond]CPh or PrC[triple bond]CMes by less bulky alkynes (i.e., tolan and 3-hexyne) to give the unsymmetrical ziconacyclopentadienes Cp(2)Zr[2,4,5-Ph(3)-3-MesC(4)], Cp(2)Zr[2-Ph-3-Mes-4,5-Et(2)C(4)], and Cp(2)Zr[2-Pr-3-Mes-4,5-Ph(2)C(4)]. Reaction of a mesityl-terminated diyne containing a rigid dihexylfluorenylene spacer with zirconocene afforded poly(p-fluorenylenedienylene) after demetalation with benzoic acid.

Reactions of nickel-group 6 complexes with a bulky terminal alkyne to afford alkyne–carbonyl coupled products: X-ray structure of the heterobimetallic nickelacyclobutenone complex [(η 5-C 5Me 5) Mo(CO) 2(η 5-C 5H 4Me)] (Ni–Mo)

The nickel–molybdenum complex [(η 5-C 5Me 5)Ni Mo(CO) 3(η 5-C 5H 4Me)] can be considered to contain a partially dative nickel–molybdenum double bond. This complex reacts with the bulky terminal alkyne HC CCPh 2(OMe) (DPMP) to afford the alkyne–carbonyl coupled metallacyclic product ▪ ( 3c, R = CPh 2(OMe), Ni–Mo) regioselectively and exclusively. No traces of a nickel–molybdenum μ-alkyne complex, analogous to similar complexes isolated with less bulky alkynes, were observed. The structure of complex 3c was established via a single crystal X-ray diffraction study. It exhibits the same connectivity as that observed with a related complex formed with the smaller but-2-yne, but some significant differences are observed between the two structures. Reactions of the nickel–molybdenum and -tungsten species [(η 5-C 5Me 5)Ni M(CO) 3(η 5-C 5H 5)] (M = Mo, W) with DPMP proceeded analogously and afforded similar products.

Diverse catalytic activity of the cationic actinide complex [(Et 2N) 3U][BPh 4] in the dimerization and hydrosilylation of terminal alkynes. Characterization of the first f-element alkyne π-complex [(Et 2N) 2U(CC tBu)(η 2-HCC tBu)][BPh 4

The cationic actinide complex [(Et 2N) 3U][BPh 4] is an active catalytic precursor for the selective dimerization of terminal alkynes. The regioselectivity is mainly towards the geminal dimer but for bulky alkyne substituents, the unexpected cis-dimer is also obtained. Mechanistic studies show that the first step in the catalytic cycle is the formation of the acetylide complex [(Et 2N) 2UC CR][BPh 4] with the concomitant reversible elimination of Et 2NH, followed by the formation of the alkyne π-complex [(Et 2N) 2UC CR(RC CH)][BPh 4]. This latter complex (R= t Bu) has been characterized spectroscopically. The kinetic rate law is first order in organoactinide and exhibits a two domain behavior as a function of alkyne concentration. At low alkyne concentrations, the reaction follows an inverse order whereas at high alkyne concentrations, a zero order is observed. The turnover-limiting step is the C C bond insertion of the terminal alkyne into the actinideacetylide bond to give the corresponding alkenyl complex with Δ H ‡=15.6(3) kcal mol −1 and Δ S ‡=−11.4(6) eu. The following step, protonolysis of the uraniumcarbon bond of the alkenyl intermediate by the terminal alkyne, is much faster but can be retarded by using CH 3C CD, allowing the formation of trimers. The unexpected cis-isomer is presumably obtained by the isomerization of the trans-alkenyl intermediate via an envelope mechanism. A plausible mechanistic scenario is proposed for the oligomerization of terminal alkynes. The cationic complex [(Et 2N) 3U][BPh 4] has been found to be also an efficient catalyst for the hydrosilylation of terminal alkynes. The chemoselectivity and regiospecificity of the reaction depend strongly on the nature of the alkyne, the solvent and the reaction temperature. The hydrosilylation reaction of the terminal alkynes with PhSiH 3 at room temperature produced a myriad of products among which the cis- and trans-vinylsilanes, the alkene and the silylalkyne are the major components. At higher temperatures, besides the products obtained at room temperature, the double hydrosilylated alkene, in which the two silicon moieties are connected at the same carbon atom, is obtained. The catalytic hydrosilylation of (TMS)C CH and PhSiH 3 with [(Et 2N) 3U][BPh 4] was found to proceed only at higher temperatures. Mechanistically, the key intermediate seems to be the uranium–hydride complex [(Et 2N) 2UH][BPh 4], as evidenced by the lack of the dehydrogenative coupling of silanes. A plausible mechanistic scenario is proposed for the hydrosilylation of terminal alkynes taking into account the formation of all products.

Selective Dimerization of Terminal Alkynes Promoted by the Cationic Actinide Compound [(Et2N)3U][BPh4]. Formation of the Alkyne π-Complex [(Et2N)2U(C⋮CtBu)(η2-HC⋮CtBu)][BPh4

The cationic actinide complex [(Et2N)3U][BPh4] is an active catalytic precursor for the selective dimerization of terminal alkynes. The regioselectivity is mainly toward the geminal dimer, but for bulky alkyne substituents the unexpected cis dimer is also obtained. Mechanistic NMR studies show that a η2-π-complex is formed between a terminal alkyne and an uranium−acetylide intermediate. This latter complex has been characterized spectroscopically. A plausible mechanistic scenario is proposed for the dimerization process.

Reactions of complex ligands 74. Alkyne carbene chromium chelates: Synthesis, structure and carbene dimerization

An alkyne-carbene chromium chelate 2, a model complex for the first intermediate of the benzannulation reaction, is generated by low-temperature photodecarbonylation of pentacarbonyl[(tert-butylethynylphenyl)methoxycarbene]chromium 1. X-ray structure determinations and 13C NMR studies on both compounds indicate only a weak coordination of the alkyne both in the solid state and in solution. Whereas thermal dimerization of the carbene ligand is directed by less bulky alkyne substituents to give chrysenes, pentacarbonyl complex 1 affords a mixture of stilbene diastereomers.

Steric factors in neutral and anionic alkyne complexes of tungsten(0)

The labile W(0) source [W(CO) 3 (NCC 2 H 5 ) 3 ] reacts with the bulky alkyne bis(trimethylsilyl) acetylene (BTMSA) under reduced pressure to give the tris-alkyne) complex [W(Me 3 SiC≡CSiMe 3 ) 3 (CO)], which has a low barrier to alkyne rotation

Cross-coupling reactions of d2 tantalum alkyne complexes: selective 1,3-diene syntheses and their relevance to alkyne cyclization chemistry

Ta(DIPP){sub 3}Cl{sub 2}(OEt{sub 2}) (DIPP = 2,6-diisopropylphenoxide) can be reduced by two electrons in the presence of the bulky alkynes PhC{triple bond}CPh and Me{sub 3}SiC{triple bond}CMe to provide the pale yellow adducts (DIPP){sub 3}Ta(PhC{triple bond}CPh) (1) and (DIPP){sub 3}Ta(Me{sub 3}SiC{triple bond}CMe) (2). The reduction of Ta(DIPP){sub 3}Cl{sub 2}(OEt{sub 2}) in the presence of smaller internal alkynes (viz. EtC{triple bond}CEt) or the terminal alkynes Me{sub 3}SiC{triple bond}CH or Me{sub 3}CC{triple bond}CH affords metallacyclopentadienes directly. The molecular structure of the PhC{triple bond}CPh adduct 1 is approximately tetrahedral (L-Ta-L angles average 109.4{degree}) and features very short Ta-C{sub alkyne} distances (2.070 (3) and 2.076 (3) {angstrom}, respectively) and an elongated C{triple bond}C' bond (1.346 (5) {angstrom}), which indicate a strongly bound and substantially reduced alkyne ligand. The molecular structure of metallacycle 3 reveals a trigonal bipyramidal geometry (L{sub ax}-Ta-L{sub ax} = 164.9 (3){degree}) with the metallacyclic {alpha} carbons occupying one axial and one equatorial site. The alkyne complex (DIPP){sub 3}Ta(PhC{triple bond}CPh) (1) reacts with MeC{triple bond}CMe, EtC{triple bond}CEt, Me{sub 3}CC{triple bond}CH, Me{sub 3}SiC{triple bond}CH, or PhC{triple bond}CH to afford high yields of the metallacyclization products. The alkyne adduct (DIPP){sub 3}Ta(Me{sub 3}SiC{triple bond}CMe) (2) also engages in metallacyclization chemistry. All of the metallacyclopentadiene complexes canmore » be hydrolyzed with H{sub 2}O/acetone solutions to afford the corresponding 1,3-dienes in essentially quantitative yields.« less