Biomedical Engineering Reference
In-Depth Information
Scheme 18
Homopolycyclotrimerizations of terminal and internal diynes and synthesis of
hb
-PAPs with functional end groups
dominant 1,2,4-benzenetriyl core structures were obtained in high yields
(up to 93%). Internal diynes could also be polymerized into hexasubsti-
tuted
hb
-PAPs [67]. Copolycyclotrimerization of the aliphatic diynes with
monoynes enabled the incorporation of functional groups into the
hb
-PAP
structure at the molecular level. This was demonstrated by the synthesis of
hb
-P[
32
(5)-IV]: the
hb
-PAP was decorated by redox-active ferrocene units on
its periphery [68].
The alkyne cyclotrimerizations catalyzed by the complexes of late tran-
sitionmetalssuchasNi,Zn,Rh,Pd,Ru,Co,Ir,etc.havebeenexten-
sively studied and the involved reaction mechanisms have been well estab-
lished [69-76]. Although the acetylene cyclotrimerizations catalyzed by the
complexes of early transition metals have been less studied, highly regioselec-
tive systems have been developed [77-79]. For example, some titanium com-
plexes have been found to catalyze the regioselective cyclotrimerizations of
terminal alkynes in high efficiencies, giving 1,2,4-trisubstituted benzenes in
excellent regioselectivities (
95%) [77]. Group
V transition metal complexes such as niobium and tantalum halides usually
work very well and produce
hb
-PAPs with predominately 1,2,4-substituted
benzenes. It has been proposed that tantallacyclic intermediates are involved
in tantalum-catalyzed alkyne cyclotrimerizations [80-83]. The diyne poly-
cyclotrimerization may have followed similar paths with the metallacyclic
intermediates serving as the initiating and propagating species. An in-situ
generated Ta(III) species [84, 85] may oxidatively add to a diyne (
a
)toform
≥
97%) and isolation yields (
≥
