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Novel cleavage patterns of CC bonds and CSi bonds were achieved via alkyne- or ketone-induced reactions of zirconacyclobutene–silacyclobutene fused ring compounds; thermodynamically favored six-membered zirconacyclohexadiene derivatives were formed in excellent yields from the four-membered zirconacyclobutenes via this unprecedented alkyne-induced CC bond and CSi bond cleavage process.

The authors further expanded the scope of the reaction by employing lanthanum catalysis and azides to generate 19 additional products (51–96% yield).

A variety of silacycles including benzosiloles, six- and eight-membered silacyclic skeletons, were efficiently synthesized via a Pd-catalyzed intramolecular Mizoroki–Heck reaction of vinylsilanes.

The Pd-catalyzed amination is followed by C—C bond formation and C—O coupling to produce functionalized quinoline derivatives from gem-dibromoenynes and anilines in the presence of NaOtBu.

An efficient Pd-catalyzed cleavage of C(sp(3))-N bonds in secondary amines and a consequent C(sp(2))-N and C(sp(3))-N coupling process was developed. Various secondary amines could be used to react with alkenyl or aryl dibromides, affording pyrroles and indoles in high yields. Cyclopentadiene-phosphine ligands, a new type of P-olefin ligand, were found to be able to promote the efficiency of this Pd-catalyzed process remarkably. A reactive Pd complex coordinated with a cyclopentadiene-phosphine ligand was successfully isolated and structurally characterized.

Synthesis and structural characterization of half-sandwich rare-earth metal tris(trimethylsilylmethyl) anionic complexes bearing one 1-phenyl-2,3,4,5-tetrapropylcyclopentadienyl ligand are achieved. These soluble anionic compounds show good reactivity in the stoichiometric reaction with dibenzoylmethane (DBM) to give the salt-free half-sandwich complex bearing two chelate DBM ligands. More importantly, they can serve as excellent and general catalyst precursors for the addition of different types of amines including primary aromatic amines (ArNH(2)), acyclic (RR'NH, ArRNH, and ArAr'NH; R, R' = Alkyl group, Ar, Ar' = Aromatic group), or cyclic secondary aliphatic amines to carbodiimides yielding efficiently guanidines. Acyclic secondary amines of the general formula ArAr'NH and ArRNH cannot be achieved efficiently by the previous rare-earth catalysts because of their weak nucleophilicity and steric hindrance. Our results show clearly the reactivities of anionic trialkyl precursors are comparable with the corresponding neutral alkyl complex in the stoichimetric reaction but exhibit better catalytic activity than the known catalysts.

A BuLi-mediated Fritsch—Buttenberg—Wiechell rearrangement of tetrahalobutadienes gives the gem-dihaloenyne products in good yields.

Dilithiation of the alkynylbenzothiophene (I) is followed by reaction with sulfur or selenium and acid chlorides.

A variety of silacycles including benzosiloles, six- and eight-membered silacyclic skeletons, were efficiently synthesized via a Pd-catalyzed intramolecular Mizoroki–Heck reaction of vinylsilanes.

A series of 2,6-diazasemibullvalenes (NSBVs) were synthesized and isolated from the reaction of 1,4-dilithio-1,3-dienes with nitriles via oxidant-induced C–N bond formation. For the first time, the activation barrier and an X-ray crystal structure of a substituted 2,6-diazasemibullvalene were determined. All NSBVs show extremely rapid aza-Cope rearrangement in solution, but the rapid aza-Cope rearrangement is “frozen” in the solid state, as shown by solid-state NMR measurements and X-ray single-crystal structural analysis. Insertion of unsaturated compounds or a low-valent metal center into the NSBV C–N bond gave diverse and interesting ring-expansion products. Theoretical analysis showed that the localized structure is predominant and that the homoaromatic delocalized structure exists as a minor component in the equilibrium.

A highly regioselective base-mediated ring expansion of 2,4-diiminoazetidines via cleavage of C-N and C(sp(3))-H bonds is achieved for the first time to afford efficiently 2,3-dihydropyrimidinesulfonamides. The mechanism of the ring expansion via tandem 4π electrocyclic ring-opening/1,5-H shift/6π electrocyclic ring-closing is well confirmed by the trapping experiments of two key intermediates and deuterium labeling studies.

A variety of biphenyls, vinylbenzenes, or arylindoles substituted with methyl- or ethylsilyl groups undergo under optimized conditions (palladium source, ligand, base, solvent) an intramolecular coupling reaction, resulting in the formation of six-membered silacycles.

Benzothiophene derivatives were synthesized in high yields from readily available o -(alkynyllithio)aryllithio compounds, sulfur, and 2 equiv of acid chlorides or other electrophiles. An acid chloride-induced electrophilic cyclization resulted in the formation of the thiophene ring.

A highly regioselective base-mediated ring expansion of 2,4-diiminoazetidines via cleavage of C-N and C(sp(3))-H bonds is achieved for the first time to afford efficiently 2,3-dihydropyrimidinesulfonamides. The mechanism of the ring expansion via tandem 4π electrocyclic ring-opening/1,5-H shift/6π electrocyclic ring-closing is well confirmed by the trapping experiments of two key intermediates and deuterium labeling studies.

The investigation of title reaction, which provides a new strategy of silyl protection/C-C bond formation sequence, is presented.

The first transition-metal-catalyzed activation of silyl C(sp(3))-H bond was realized and synthetically applied. A variety of organic skeletons substituted with SiMe(3) groups could undergo the Pd-catalyzed intramolecular coupling reaction, resulting in an unprecedented synthetic method for yielding six-membered silacycles. It was found that the adjacent Si atom played an essential role for the activation of the C(sp(3))-H bond of the SiMe(3) group; no activation reaction of the C(sp(3))-H bond of the CMe(3) group took place under the same reaction conditions.

3-Zircona-1-cyclopentenes and zirconacyclopentanes react with aldehydes at or below 25 °C to give the corresponding carbonyl addition products, i.e., 7-membered oxazirconacycles, which can be readily converted to the corresponding alcohols via protonolysis and 5-iodoalcohols via iodinolysis; the latter product can be further converted to 7-membered lactones via Pd-catalyzed carbonylation.

A selective synthesis of 5-acyl-2-iminothiazoles (IV) and their analogues (VI) is achieved via reaction of carbodiimides with carboxylic acid chlorides followed by treatment with lithiated chalcogenoalkynes.

Reaction of dilithiobutadienes like (I) with aroyl chlorides containing electron-withdrawing groups (III) proceeds with C—C bond formation and double silyl shift to give gem-bis(trimethylsilyl)cyclopentenones (IV) in good yields.

A variety of alkynyl indoles is obtained from polyhalo compounds (I) and (VIII), arylamines (II) and (V), and alkynes (III) and (VI).

Silyl-substituted 1,3-butadienes are useful building blocks and are readily applied in several types of reactions such as Diels–Alder reaction, ene reaction and allylation. They can also participate in different tandem reactions such as Diels–Alder/allylation, ene/allylation, ene/allylation/Diels–Alder reaction, ene/allylation/ene reaction and ene/allylation/Diels–Alder/allylation reaction. This feature article reviews the synthesis of silyl-substituted 1,3-butadienes, and their applications in the reaction types mentioned above, involving a tandem Diels–Alder/ene/allylation process. This article also introduces some reactions of alkenylsilanes and allylsilanes for comparison and discussion about the tandem reaction. The tandem reactions described in this article are a powerful tool to construct complicated multicyclic compounds with high selectivity and high efficiency.

A zirconocene-mediated multicomponent process, involving the Si-tethered diyne (I), tert-butyl cyanide (II) and one or two (either different or identical) nitriles, can be interrupted at different stages by hydrolysis with aq.

An efficient process involving Pd-catalyzed selective cleavage of a C(sp(3))-Si bond and consequent intramolecular C(sp(2))-Si coupling has been developed, affording benzosilolo[2,3-b]indoles as a new type of silicon-bridged polyheteroarene in excellent yields. Aldehyde was found for the first time to be able to promote the efficiency of the catalytic process remarkably.

Concise and selective: the procedure-controlled synthesis of the title compounds has been achieved for the first time by an organolithium-promoted convergent tandem annulation involving readily available terminal alkynes, chalcogen elements (S, Se, and Te), carbodiimides, and acid chlorides. A novel 1,5-acyl migration is considered to be essential for this useful transformation.

When ethylene and alkynes such as 4-octyne and diphenylacetylene were treated with Cp2ZrBu2 highly pair selective coupling products were formed in high yields. Similarly, styrene or trimethylvinylsilane also afforded cross coupling products with alkynes on zirconocene complex.

Substrates (IIIf) and (IIIg), bearing electron-withdrawing groups at the phenyl ring, can not be transformed to the desired products (IV).

Zirconocene-mediated multicomponent coupling of silicon-tethered diynes, nitriles, and isocyanides results in the formation 5-azaindoles and dihydropyrrolo[3,2-c]azepines in dependence of the amount of isocyanides used.

3-Zircona-1-cyclopentenes and zirconacyclopentanes react with aldehydes at or below 25 °C to give the corresponding carbonyl addition products, i.e., 7-membered oxazirconacycles, which can be readily converted to the corresponding alcohols via protonolysis and 5-iodoalcohols via iodinolysis; the latter product can be further converted to 7-membered lactones via Pd-catalyzed carbonylation.

Reactions of diynes RCC-X-CCR with zirconocene ethylene complex Cp2Zr(CH2=CH2) were investigated. When X was C6H4 or C4H2S, bridged zirconacyclopentenes were selectively formed. They were easily converted into bridged cyclopentenone derivatives. In the case of (CH2)n groups as X, the reaction products were strongly dependent on the R group.

Reaction of zirconacyclopentadienes with alkynes such as dimethyl acetylenedicarboxylate gives benzene derivatives in the presence of CuCl/LiCl.

Under an atmosphere of air, Zn(O-Tf)2 serves as an excellent catalyst precursor for the addition of various primary and secondary amines to carbodiimides.

1,2,3-Trisubstituted cyclopentadienes and tetrahydroindene derivatives were prepared by a one-step reaction from zirconacyclopentene complexes, which are easily prepared from alkynes and EtMgBr (or ethylene) and Cp 2 ZrCl 2 . Reaction of zirconacyclopentenes with phthaloyl chlorides afforded intramolecular Michael addition reaction products. The structure of one of the products was determined by X-ray study.

Zirconacyclopentane (t-Bu2C5H3)2Zr(C4H8) was isolated as stable yellow crystals and its structure was determined by X-ray analysis.

Treatment of Cp2ZrCl2 with 2 equiv of EtMgBr in THF, followed by addition of an internal alkyne and subsequent treatment with COI2 gave α,β-disubstituted cyclopentenone in good to high yields. When a conjugated enyne was used as an alkyne component, α-alkenyl cyclopentenone was selectively formed in 80–89% yields. In the case of 4,6-decadiyne, α-alkynylcyclopentenone was formed in 88% yield. Bridged alkynes such as 1,4-dihexynylbenzene afforded α,α′-bridged cyclopentenone compounds in good yields.

In this account we would like to present our recent developments in copper catalyzed or mediated carbon-carbon bond formation reactions of zirconacycles as well as reactions of related alkenylzirconocene compounds.

Treatment of bis(phenylethynyl)silane (PhC⋮C)2SiR2 (R = Me (2a), Et (2b) or Ph (2c)) with Cp2ZrEt2 (1) (Cp = cyclopentadienyl) and H2O/CuCl in this order afforded (1E,3E)-1,4-diphenyl-1,3-butadiene (3) in 56−88% yields after hydrolysis. On the other hand, hydrolysis of the reaction mixture of 2a−h with Cp2ZrEt2 gave silacyclobutene derivatives 4a−h in 61−87% yields. Zirconium-containing intermediate 7 was obtained as crystals suitable for X-ray analysis when t-BuC5H4 was used as the ligands of a zirconocene derivative. Structure of 7 showed that the intermediate contained the zirconacyclobutene−silacyclobutene fused ring system. Reaction of silacyclobutene 4a with CuCl selectively opened the silacyclobutene ring. The further treatment of the reaction mixture with PhI in the presence of a catalytic amount of Pd(PPh3)4 gave 1,3,4-triphenyl-1-silyl-1,3-butadiene compound 16. Zirconacyclopentadienes with an alkynylsilyl group at the α-position afforded zirconacyclohexadiene derivatives 19 in 82−98% yields. Wh...

Reactions of zirconacyclopentadienes with halogenation reagents such as NCS, NBS and I2 were highly substituent-selective. Mixed-dihalogenated 1,3-diene derivatives were obtained in one pot in good to high yields with high selectivities.