Chemistry Reference
In-Depth Information
In the case of the 2,6-diisopropylphenoxide ligand, cyclometallation by low valent
metal centres can lead to overall dehydrogenation of the alkyl side chain. The low
valent intermediate can be generated by a number of pathways. In the case of niobium,
reduction of the dichloride [Nb(OC
6
H
3
Pr
i
2
-2,6)
3
Cl
2
] with sodium amalgam (2 Na per
Nb) in THF leads to the dehydrogenation product shown in Eq. (6.61).
217
,
218
Pr
i
[Nb(OAr)
3
Cl
2
]
O
ArO
ArO
Me
Me
+
2Na/Hg
−
2NaCl
Nb
Me
Pr
i
ArO
H
+ THF
− H
2
Nb
OAr
O
Nb
OAr
ArO
Pr
i
OAr
THF
O
Me
ArO
H
Nb
H
ArO
H
H
(6.61)
In the case of tantalum, the addition of olefins to the dihydride [Ta(OC
6
H
3
Pr
i
2
-
2,6)
2
(H)
2
Cl
2
(L)
2
](LD tertiary phosphine) generates one equivalent of alkane along
with a dehydrogenation product (Eq. 6.62).
219
Pr
i
Pr
i
O
Ta
H
PMe
2
Ph
Me
(6.62)
Cl
+
H
2
+
Pr
i
H
PMe
2
Ph
Cl
O
O
Ta
Pr
i
Pr
i
PhMe
2
P
PMe
2
Ph
OAr
It appears that addition of either the methyne or a methyl CH bond to the low
valent metal centre precedes
ˇ
-hydrogen abstraction and elimination of H
2
,whichcan
sometimes be detected spectroscopically. The resulting aryloxide contains an ortho-
˛
-methylvinyl group strongly
2
-bound, resulting in a metallacyclopropane ring. This
ring can undergo protonation to yield a five-membered metallacycle or undergo ring
expansion reactions.
218
The ligands 2,6-di-
tert
-butylphenoxide and 2,6-diphenylphenoxide undergo
cyclometallation at numerous high valent early d-block metal centres. The aliphatic
or aromatic CH bonds of these ligands have been activated by metal hydride,
220
alkyl,
221 - 225
alkylidene (Eq. 6.63),
11
,
226
alkylidyne (Eq. 6.64)
227
,
228
and benzyne (o-
phenylene)
229
functional groups to produce six-membered oxa-metallacycles.
