2 Homometallic Alkoxides

Bu t
O
Bu t
(6-I)
Bu t
Bu t
O
Metal Aryloxides 615
The chemistry of rhenium aryloxides is significant. The homoleptic rhenium aryloxides
[Re(OAr)4] (OAr D OC6H3Pr i 2 -2,6, OC6H3Me2-2,6) have been obtained from
[ReCl4(THF)2] and the corresponding lithium aryloxide. 458,459 Cyclic voltammetric
studies show these square planar d 3 -species can undergo one-electron reduction. As
with other transition metal systems (Section 4.2.3) these aryloxides are more difficult
to reduce (more negative potential) than their halide counterparts. 458 The 2,6dimethylphenoxide
compound reacts with alkynes to form adducts [(RCCR)Re(OAr)4].
Treatment of the bis(alkyne) compounds [Re(O)(I)(RC CR)2] with TlOPh leads to
the corresponding phenoxide. 460 The structure of the 2-butyne, phenoxide showed a
pseudo-tetrahedral coordination sphere. The orientation of the phenoxide ligand was
interpreted in terms of minimizing -antibonding interactions. Unlike corresponding
alkoxides, the phenoxide complex did not participate in any insertion chemistry. The
tris(alkyne) [Re(I)(RC CR)3] is converted into the corresponding phenoxide by simply
treating with phenol. 461
The importance of group 5 and 6 metal alkylidene and alkylidyne compounds in
metathesis reactions has led to an exploration of related rhenium chemistry. 462 A
series of alkylidene compounds [Re(OAr)3(DCHCMe3)(NC6H3Pr i -2,6)] were obtained
by reacting dimeric [Re(CCMe3)(NHAr)Cl3]2 with KOAr. 463 The solid-state structure
of the OC6H3Cl2-2,6 derivative showed the alkylidene unit to occupy an apical
site within an essentially square pyramidal geometry. The vacant site trans to
the alkylidene group is occupied in the adduct [Re(OC6F5)3(DCHCMe3)(NC6H3Pr i -
2,6)(THF)]. The lack of metathesis activity for these compounds was ascribed
to the binding of olefins in a similar fashion, precluding 2 C 2 cycloaddition.
Addition of pyridine to [Re(OAr)3(DCHBu t )(NC6H3Pr i -2,6)] yielded the alkylidyne
[Re(OAr)2(CCMe3)(NC6H3Pr i -2,6)(py)] with elimination of phenol. 463 The alkylidene
[Re(OAr)Cl2(DCHCMe3)(NC6H3Pr i -2,6)] undergoes a similar elimination with
pyridine, and forms an active olefin metathesis catalyst when activated with GaBr3.
Some important examples of phenyl migration to rhenium-oxo groups have been
studied in detail. 464,465 The tris(pyrazolylborate) compound [(HBpz3)Re(O)(Ph)Cl]
undergoes photolysis in the presence of donor ligands to yield the d 4 -phenoxide
[(HBpz3)Re(OPh)Cl(py)]. Mechanistic studies including using the labelled compounds
[(HBpz3)Re( 16 O)(C6H5)Cl] and [(HBpz3)Re( 18 O)(C6D5)Cl] showed the reaction
to be an intramolecular 1,2-migration. 465 In related studies the triflate (OTf)
compound [(HBpz3)ReO(Ph)(OSMe2)][OTf] undergoes phenyl to oxo migration to
yield [(HBpz3)ReO(OPh)(OSMe2)][OTf] which reversibly loses Me2S. The reactions
proceed via the key intermediate [(HBpz3)Re(O)2(Ph)][OTf] which undergoes phenyl
to oxo migration at 0 Ž C to yield [(HBpz3)Re(O)(OPh)][OTf]. 464