Low-Coordinate W-W Quadruple Bond as an Alkene Surrogate: Divergent [2 + 2 + 2] and [2 + 2 + 1] Cycloadditions with Alkynes
Related Experiment Video
Updated: Oct 31, 2026

Preparation of a Corannulene-functionalized Hexahelicene by Copper(I)-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units
Published on: September 18, 2016
Wen-Chi Chen1, Cian-Wei Yang1, Bo-An Liao1
1Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan.
Abstract:
Although quadruply bonded metal-metal complexes constitute a large and well-established family, their cycloaddition chemistry remains underdeveloped because the quadruple bond is typically confined within a tetragonal, highly ligand-protected coordination environment. Herein, we demonstrate that a low-coordinate amidinate-supported W-W quadruple bond in W2(μ-Cl)-(μ-κ2-Cl2Li-(THF))-[μ-κ2-HC-(N-2,6- i Pr2C6H3)2]2 behaves as an alkene surrogate in cycloaddition chemistry. Depending on ligand coordination, this W-W quadruple bond engages alkynes in either formal [2 + 2 + 2] or [2 + 2 + 1] pathways. In the absence of ancillary donor ligands, direct reaction with two alkynes proceeds through tail-to-tail [2 + 2 + 2] cycloaddition to afford C4W2 metallacycles, W2Cl2[μ-η2:η2-1,4-R2C4H2]-[μ-κ2-HC-(N-2,6- i Pr2C6H3)2]2 (R = C6H5, 2-MeC6H4, 4-MeOC6H4, and n-C3H7), and a corresponding [2 + 2] intermediate, W2Cl2[μ-η2-2-MeC6H4CCH]-[μ-κ2-HC-(N-2,6- i Pr2C6H3)2]2, could be isolated and structurally characterized. In contrast, coordination of σ-donating ligands to the W-W core diverts the reaction pathway by stabilizing vinylidene intermediates, W2Cl2(C5H5N)-[μ-η1:η2-CC-(H)-R1]-[μ-κ2-HC-(N-2,6- i Pr2C6H3)2]2 (R1 = 2-MeC6H4, t Bu, and SiMe3), which subsequently react with a second alkyne (R2CCH) to furnish C3W2 ring products of the type W2Cl2[μ-η2:η2-1-R2-4-R1H-C4H]-[μ-κ2-HC-(N-2,6- i Pr2C6H3)2]2 bearing an exocyclic alkene fragment. In both [2 + 2 + 2] and [2 + 2 + 1] manifolds, while the initial cycloadducts adopt nonplanar conformations, sequential reduction populates the W-W δ manifold and drives planarization via δ-π conjugation between the W2 unit and the organic fragment. Ultimately, the C4W2 systems are transformed into aromatic bimetallic metallabenzenes, [W2(μ-κ2-1,4-R2C4H2)-(μ-κ2-HC-(N-2,6- i Pr2C6H3)2)2] (R = C6H5, 2-MeC6H4, 3-MeC6H4, 4-MeC6H4, 4-MeOC6H4, 4-FC6H4, and n-C3H7), whereas the C3W2 system evolves into a bimetallic fulvene analogue, W2Cl-[μ-κ2-1-C6H5-4- t BuH-C4H]-[μ-κ2-HC-(N-2,6- i Pr2C6H3)2]2 and W2(μ-H)-[μ-κ2-1-C6H5-4- t BuH-C4H]-[μ-κ3-HC-(N-2,6- i Pr2C6H3)-(N-2- i Pr-6-(C3H6)-C6H3)]-[μ-κ2-HC-(N-2,6- i Pr2C6H3)2]. Taken together, these results expand the scope of cycloaddition partners to include a low-coordinate W-W quadruple bond.
More Related Videos
07:50Efficient Synthesis of All-Carbon Quaternary Centers via the Conjugate Addition of Functionalized Monoorganozinc Bromides
Published on: May 26, 2019
10:17Efficient Construction of Drug-like Bispirocyclic Scaffolds Via Organocatalytic Cycloadditions of α-Imino γ-Lactones and Alkylidene Pyrazolones
Published on: February 7, 2019
Related Concept Videos
Electrophilic Addition to Alkynes: Halogenation
Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a π bond. In alkynes, the presence of two π bonds allows for the addition of two equivalents of halogens (bromine or chlorine). The addition of the first halogen molecule forms a trans-dihaloalkene as the major product and the cis isomer as the minor product. Subsequent addition of the second equivalent yields the tetrahalide.
Cycloaddition Reactions: Overview
Cycloaddition Reactions: MO Requirements for Thermal Activation
[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction
Electrophilic 1,2- and 1,4-Addition of X2 to 1,3-Butadiene
Preparation of Alkynes: Alkylation Reaction
Alkylation of terminal alkynes with primary alkyl halides in the presence of a strong base like sodium amide is one of the common methods for the synthesis of longer carbon-chain alkynes. For example, treatment of 1-propyne with sodium amide followed by reaction with ethyl bromide yields 2-pentyne.