Synthesis of Cyclopentadienyl-Based Trioxo-rhenium Complexes and Their Use as Deoxydehydration Catalysts

The cyclopentadienyl-based trioxo-rhenium
complexes CptttReO3 (Cpttt = 1,2,4-tritert-butylcyclopentadienyl) 1b and Cp*ReO3 (Cp*
= pentamethylcyclopentadienyl) 4b) are known to be active
catalysts for the deoxydehydration (DODH) of vicinal diols to olefins.
Here, we report on the preparation of a series of complexes of the
general formula Cp′ReO3 (Cp′ = 1,3-di-tert-butylcyclopentadienyl (Cptt, 2b, 18%), 1,2,3-triisopropylcyclopentadienyl (3b, 4%),
1,2,3-trimethyl-4,5,6,7-tetrahydroindenyl (6b, 36%),
and tetramethylcyclopentadienyl (7b, 33%)) in which the
electronic and steric properties of the Cp′ ligand are varied.
These complexes were synthesized via oxidative decarbonylation from
the corresponding Cp′Re­(CO)3 complexes with either
H2O2 or tBuOOH. An in situ NMR investigation revealed that complexes 2b, 3b, and 6b are unstable under
the oxidizing reaction conditions. This information was used to determine
the optimal reaction time to isolate the complexes 2b, 3b, and 6b. These “piano-stool”
configurated Cp′ReO3 complexes were characterized
spectroscopically and by single crystal X-ray diffraction. The new
complexes 2b, 3b, 6b, and 7b were found to be generally less thermally stable than CptttReO3 (1b) or Cp*ReO3 (4b). It appeared that 2b and 6b were
better catalysts for the DODH of 1,2-octanediol to octene with PPh3 as an oxygen-atom acceptor. Remarkably, the less substituted
Cp′ReO3 complexes (1b, 2b, and 3b) afforded significantly less 2-octene (presumably
from isomerization of the primary 1-octene product) in comparison
to those containing per-alkylated Cp-moieties (4b and 6b).