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3.4 Rhodium(I)-Catalyzed Cycloisomerization of Ene-Allenes
The Rh(I)-catalyzed allenic Alder-ene reactions of amino ester tethered allenynes
discussed earlier in this chapter (Section 3.2), generally proceed in 10 min to give the
corresponding cross-conjugated trienes in high yield (Table 3.1). The success of this
transformation prompted us to examine the effect of replacing the alkyne moiety with an alkene.
It is well known that in the related Rh(I)-catalyzed cyclocarbonylation reaction of 1,n-enynes,
replacing the alkyne component with an alkene completely diminishes the reactivity. 126 This can
be attributed to lower propensity of the diene to undergo the first oxidative addition step forming
a five-membered metallocycle, which is a presumed intermediate in the reaction. In the case of
ene-allenes, however, the increased reactivity of the allene may compensate for this issue. At the
time this study was initiated, there were only few examples utilizing ene-allenes in
cycloisomerization reactions. For example, Trost first reported the cycloisomerization of 1,6-
ene-allenes 187 to five-membered dienes 188 by using bimetallic Ni-Cr catalyst (Scheme
3.41). 127
Scheme 3.41 Cycloisomerization of ene-allenes.
conditions
X
•
R
R
187
X = Me, -CO2Et, NTs, O etc.
conditions: A. 10 mol% (p-diphenylphosphinopolystyrene)NiCl 2 and 30 mol% CrCl 2.
B. 5 mol% RuClH(CO)(PPh 3) 3.
C. 5 mol% [RhCl(cod)] 2, 10 mol% P(OPh) 3.
Subsequently, the same transformation was reported by Kang using RuClH(CO)(PPh3)3 as a
catalyst, 128 and Itoh, using [RhCl(COD)]2 as catalyst with a triphenylphosphite ligand. 129
To examine the effect of replacing the alkyne with an alkene on the Rh(I)-catalyzed
72
X
188
R
R