9 Frustrated Lewis Pairs Based on Transition Metals
327
Scheme 9.4 Catalytic
hydrogenation of alkenes by
Zr + /N FLP 15
H 2
R
R
Cp*
Zr
Cp*
O
N
i Pr 2
Cp*
Zr
Cp*
O
N
i Pr 2
H
H
R
R
Cp*
Zr
Cp*
O
N
i Pr 2
H
R
R
15
cation [Cp 2 ZrH]
+ , whose subsequent combination with a released amine facilitates
FLP-type H 2 cleavage.
The same group explored other related intramolecular Zr
+ /P architectures, some
of which are collected in Fig. 9.2. Those include a P-based version of 15 in which the
pendant amine is substituted by a PPh 2 terminus [61]. The corresponding zirconium
cation dimerizes to yield an oxygen bridged dicationic product, despite which it
undergoes the 1,4-addition of chalcone in an FLP-like manner. A richer reactivity was
reported for compound 16, whose bulkier steric profile prevented dimerization [61].
The latter compound exhibits FLP reactivity with benzaldehyde, nitrosobenzene,
and an ynone (Scheme 9.5). The similar Zr
+ /P compound 17 was described shortly
after and its FLP reactivity toward ketones, aldehydes, α,β-unsatutated carbonyl
compounds, and heterocumulenes was demonstrated [62].
Scheme 9.5 FLP reactivity
of Zr + /P compound 16
toward benzaldehyde,
nitrosobenzene, and an
ynone. Counteranions have
been omitted for clarity
Cp
Zr
Cp
O
P
Ph 2
16
Cp
Zr
Cp
O
PPh 2
N
O
Ph
Cp
Zr
Cp
O
PPh 2
O
H
Ph
Cp
Zr
Cp
O
PPh 2
O
·
Ph
Ph
H
O
Ph
O
PhNO