2 Homometallic Alkoxides

614 Alkoxo and Aryloxo Derivatives of Metals
phenoxide ring was also found to lead to considerable rate enhancement. 444 Electrochemical
studies of the precursors have also been carried out. 201 Later studies on the
ring-opening polymerization of dicyclopentadiene by precursors [W⊲O⊳⊲OAr⊳x Cl4-x ]
activated with tin hydrides showed similar electronic effects. The activity of the precursors
was correlated with the W(IV)–W(V) reduction potentials. 448 The combination of
the precursor trans-[W(O)(OC6H3Br2-2,6)2Cl2] activated by [Et4Pb] has been shown
to be a catalyst for the stereoselective ring closing of chiral dienes. 449
The alkylation of the bis(aryloxide) [W(OC6H3Ph2-2,6)2Cl4] with LiCH2CMe3
leads to cyclometallated alkylidene compounds [W(OC6H3Ph- 1-C6H4)(OC6H3Ph2- 2,6)(DCHCMe3)(OR2)]. 227 These compounds will catalyse the metathesis of 2-pentene
with high stereoselectivity, polymerize 1-methyl-norbornene to 100% cis, 100% headto-tail,
syndiotactic polymer, and cyclize a variety of functionalized dienes.
The most important development in this area has been the careful mechanistic
studies of well-defined, d0-alkylidene(aryloxide) and alkylidyne(aryloxide) catalysts. 450
The isolated alkylidyne compound [W( CCMe3)(OC6H3Pri 2-2,6)3] reacts with internal
alkynes to generate stable tungstabutadiene compounds, e.g. structurally characterized
[W(OC6H3Pri 2-2,6)3(C3Et3)]. 228 This complex will metathesize alkynes at rates dependent
on the rate of fragmentation of the intermediate metallacycles. The corresponding
2,6-dimethylphenoxide species can be generated from dinuclear [W2(OC6H3Me2-2,6)6]
by treatment with alkynes, effectively triple bond metathesis involving a W W bond. 451
The aryloxide in this case was not bulky enough to generate an active metathesis
catalyst, whereas the bulkier 2,6-di-tert-butylphenoxide led to a cyclometallated, alkylidene
compound. The metallacyclic compound [W(OC6H3Pri 2-2,6)3(C3HBut 2 )] (obtained
from ButC CH⊳ undergoes elimination of phenol (deprotonation of the ˇ-CH bond)
andformationof[W(OC6H3Pri 2-2,6)2(C3But 2 )] which forms a pyridine adduct.452 An
important class of alkene metathesis catalysts are the imido-alkylidene complexes
[M(DNR)(DCHR0 )(OR)2] (MDMo, W) typically referred to as “Schrock-type” catalysts.
The activity of these catalysts is strongly dependent on the electronic and
steric properties of the alkoxide/aryloxide ancillary ligands. 266,453 The discrete catalyst
[W(O)(OC6H3Ph2-2,6)2(DCHBut )(PMePh2)] has been structurally characterized. 454
6.2.8 Group 7 Metal Aryloxides
The vast majority of the studies of manganese aryloxides have involved delineating how
ligand architecture affects the aggregation and coordination geometry about the metal.
Synthetic strategies for coordination compounds, e.g. [MnfOC6H4⊲20-py⊳-2g3] 455 which
contains a mer-MnO3N3 core typically utilize starting materials such as manganese
acetate in protic solvents. For simple aryloxides, extensive use of the bis(amido)
compound [MnfN⊲SiMe3⊳2g2] has been made. Early work showed that a variety
of extremely oxygen sensitive bis(aryloxides) could be obtained. 456 The complex
[Mn(OC6H3But 2-2,6)2] was reported to form the compound (6-I) when exposed to O2.
Later work showed that the OC6H2But 3-2,4,6 derivative was dimeric, [(ArO)Mn( -
OAr)2Mn(OAr)] with three-coordinate Mn. 457 The tetrahedral adduct [Mn(OC6H2But 3- 2,4,6)2(NCMe)2] was obtained directly from the corresponding chloride (Eq. 6.30). 108