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allenynol derivatives 95 proceeding via a metallocyclic intermediate 96, by using stoichiometric
(η 2 -propene)Ti(i-PrO)2 (Scheme 3.6). 75 Livinghouse and Oh have also observed cross-conjugated
trienes during transition-metal catalyzed reactions of allenynes. 76
Scheme 3.6 Allenic cycloisomerization reported by Sato.
•
C 5H 11
X
X = OBn, OTBS
OAc, OCO 2Et
Ti(O-i-Pr) 2
Et 2O, -35 o C
C 5H 11
Ti(O-i-Pr) 2
95 96
97
X
In 2002, Brummond reported the first systematic study of the Rh(I)-catalyzed allenic
Alder-ene reaction. 77 It was shown that [Rh(CO)2Cl]2 serves as a very general catalyst for
converting allenynes into cross-conjugated trienes. For example, the cycloisomerization of 98
using 2 mol % catalyst, at 90 °C gives triene 99 in 72% yield (Scheme 3.7). The appending
olefin is obtained as a kinetic mixture of E : Z isomers (5 : 1), while the exocyclic olefin is
obtained as a single isomer with E-geometry.
Scheme 3.7 Rh(I)-catalyzed allenic Alder-ene reaction.
•
TMS
H
CH 2C 7H 15
2 mol % [Rh(CO) 2Cl] 2
toluene, 90 o C, 72%
TMS
C 7H 15
98 99 E : Z = 5 : 1
H 3
H2 H1 C 5H 11
4.5 % nOe
A mechanism based on a deuterium labeling study has been proposed to account for these
observations (Scheme 3.8). The first step is complexation of the Rh(I) to the alkyne and the distal
double bond of the allene, eventually leading to a metallocyclic Rh(III) intermediate 102 via
oxidative addition. The next step involves elimination of a β-deuteride (hydride) leading to a
Rh(III)-deuteride species 103, which undergoes a facile reductive elimination setting the
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