2 Homometallic Alkoxides

Table 6.47 Simple aryloxides of mercury
Metal Aryloxides 635
Bond length ( ˚A) Bond angle ( Ž )
Compound Aryloxide M–O M–O–Ar Ref.
[Hg(OAr)(Ph)] OC6H3Cl-2-Br-4 2.006 (8) 129 i
[(1,4,8,11-tetrathiacyclo- OC6H2⊲NO2⊳3-2,4,6 2.532 (9) 121 ii
tetradecane)Hg(OAr) 2 ]
i L.G. Kuz’mina, N.G. Bokii, Yu.T. Struchkov, D.N. Kravtsov, and L.S. Golovchenko, Zh.
Strukt. Khim., 14, 508 (1973).
ii M. Herceg, B. Matkovic, D. Sevdic, D. Matkovic-Cologovic, and A. Nagl, Croat. Chem. Acta,
57, 609 (1984).
the corresponding phenol to AlMe3 and can be generated in situ for use. The steric bulk
of the aryloxides plays an important role in discriminating between various oxygen donor
ligands that can bind (and sometimes be activated) by the aluminium metal centre. As
an example MAD demonstrated >99:1 selectivity for the ether MeOCH2CH2CH2Ph
over EtOCH2CH2CH2Ph. 13 The reduction of a mixture of the ketones PhCOMe and
PhCOBut with [Bui 2AlH] in the presence of MAD led mainly to the more hindered
alcohol owing to selective binding (removal) of the smaller ketone. 544 MAD can also be
used to discriminate between different esters545 while ATPH can be used to selectively
bind aldehydes. 546 In the last case the less hindered aldehyde is bound to aluminium
and can be functionalized by Diels–Alder reactions. The unbound, more bulky aldehyde
will react with alkylating agents such as LiBun . 547 Hence the smaller, bound substrate
is electronically activated but sterically protected.
These bulky aluminium aryloxides will also promote a variety of carbon–carbon bond
forming reactions with a high degree of regio and stereoselectivity. Examples include
Michael addition to ˛,ˇ-unsaturated ketones, Diels–Alder additions, and Claisen rearrangements.
In the case of Diels–Alder reaction, ATPH promoted the exo-selective
condensation of ˛,ˇ-unsaturated ketones with dienophiles. 548 The Claisen rearrangement
can be catalysed by ATPH and its more Lewis acidic 4-bromo derivative. 549 A
series of chiral aluminium aryloxides were also synthesized and have been applied to
asymmetric Claisen rearrangements, 13 aldol reactions, 550 and aldehyde alkylations. 8
Compounds such as [Al(R)(OC6H3But 2-2,6)2] will carry out the polymerization of
methyl and ethyl methacrylate when activated with t-BuLi. 551 The nature of the alkyl
group was found to affect the tacticity of the polymer. The tetraphenylporphinato
aluminium phenoxide has been shown to be active for the ring opening polymerization
of epoxides and lactones. 552,553 The reaction is accelerated by the presence of reagents
such as MAD. 554 An adduct [Al(OC6H2But 2-2,6-Me-4)Me2(methylmethacrylate)] relevant
to this reactivity has been isolated and structurally characterized. 555
The synthesis of group 13 metal aryloxides can proceed directly from the metal
although use of halides and in particular alkyl precursors is much more common. The
homoleptic tris(aryloxides) can be monomeric with bulky, e.g. trigonal [Al(OC6H2But 2- 2,6-Me-4)3], 556,557 or chelating, e.g. octahedral [M(OC6H4(oxazole)-2)3] (MD Al, Ga,
In). 558 With smaller aryloxides dimeric structures such as [(ArO)2Al( 2-ArO)2Al(OAr)2]
(OAr D OC6H3Me2-2,6) are obtained. 559 The metal(I) aryloxides [M( 2-OC6H2(CF3)3-
2,4,6)2M] (M D In, 132 Tl560 ) contain two-coordinate metal centres.
A large number of mixed alkyl, aryloxides of all four metals have been obtained
by reacting [MR3]n with phenols. Monomeric bis(aryloxides) of aluminium, e.g.