Handbook of Functionalized Organometallics Applications in S

420
10 Functional Organonickel Reagents
10.6
Intramolecular Coupling of Enynes or Alkynes
Intramolecular carbometallations have been shown to be a useful approach for
the construction of cyclic systems under very mild conditions. The nickel(1) catalyst
generated in situ from the reduction of NiCl 2(PPh 3) 2 with sodium naphthalenide
in THF has been shown to be active for the cyclization of enynes. The reducing
agent can be replaced by CrCl 2 and similar results are obtained. Either five- or
six-membered rings can be constructed conveniently under these conditions [148].
EtO2C NiCl2(PPh3) 2/CrCl2 EtO2C EtO2C 82% EtO2C HO
NiCl2(PPh3) 2/CrCl2 59%
HO
Within a similar context,nickel(0) complex has been shown to promote cyclization
of enynes and isocyanides to form bicyclooctenone derivatives in good yields
[149]. Phosphine or diimine ligands are required in these reactions [150].
TBSO
Ph
TBSO
Ph
Ni(COD) 2,PBu3 ArNC
NAr
CSA
83% (10:1)
H
(9-Anth)
N N
(9-Anth)
TBSO
It is interesting to note that,in the absence of phosphine ligands,dienes are
obtained as the major products [151].
n-Pr
O
n-Bu Ni(COD) 2,ArNC
n-Pr
O
60% (+ iminocyclopentene, 28%)
n-Bu
The couplings of 1,6- and 1,7-internal diynes or oxygen-linkage mixed terminal/internal
diyne with isocyanides provide useful entries for constructing bicyclic
cyclopentenone skeletons [152]. The cyclizations of symmetric or unsymmetric
diynes bearing amine moiety at the tethered chain proceeded with modest to good
regioselectivity in acceptable isolated yield [151,153].
H
Ph
O