Chelated Diborenes and Their Inverse‐Electron‐Demand Diels–Alder Reactions with Dienes

A doubly base‐stabilised diborane based on a benzylphosphine linker was prepared by a salt elimination reaction between 2‐LiC6H4CH2PCy2⋅Et2O and B2Br4. This compound was reduced with KC8 to its corresponding diborene, with the benzylphosphine forming a five‐membered chelate. The diborene reacts with...

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Bibliographic Details
Published in:Angewandte Chemie International Edition 2018-11, Vol.57 (46), p.15276-15281
Main Authors: Stennett, Tom E., Mattock, James D., Pentecost, Leanne, Vargas, Alfredo, Braunschweig, Holger
Format: Article
Language:English
Subjects:
boron
Butadiene
Chelates
cycloaddition
DFT calculations
Diborane
Diels-Alder reactions
Dienes
Isomerization
Isomers
Isoprene
low-valent compounds
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Summary:A doubly base‐stabilised diborane based on a benzylphosphine linker was prepared by a salt elimination reaction between 2‐LiC6H4CH2PCy2⋅Et2O and B2Br4. This compound was reduced with KC8 to its corresponding diborene, with the benzylphosphine forming a five‐membered chelate. The diborene reacts with butadiene, 2‐trimethylsiloxy‐1,3‐butadiene, and isoprene to form 4,5‐diboracyclohexenes, which interconvert between their 1,1‐ (geminal) and 1,2‐ (vicinal) chelated isomers. The 1,1‐chelated diborene undergoes a halide‐catalysed isomerisation into its thermodynamically favoured 1,2‐isomer, which undergoes Diels–Alder reactions more slowly than the kinetic product. Live and let dienes: Sterically unhindered diborenes based on a chelating benzylphosphine group undergo Diels–Alder cycloaddition reactions with dienes to generate 4,5‐diboracyclohexenes.
ISSN:1433-7851
1521-3773
DOI:10.1002/anie.201809217