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species onto the proximal double bond of the allene assembles the azepine core 203 resulting in a
Rh(III)-hydride species which undergoes a reductive elimination to give the product 189.
Recently, a cyclization of a (π-allyl)-Pd-OAc species onto the internal double bond of the allene
in all-carbon tethered ene-allenes leading to 5-membered ring formation has been reported. 138 In
addition, Trost reported a cycloisomerization of 1,6-enynes catalyzed by CpRu(CH3CN)2PF6,
which was rationalized by a similar C-H insertion-carbocyclization mechanism. 139
Exclusive formation of the exocyclic olefin in azepine 189 with E geometry can be
rationalized by coordination of the crotyl moiety away from the terminal allenyl methyl group to
avoid unfavorable steric interactions, as shown in intermediate 199 (Scheme 3.45). To test
whether the methyl group on the alkene moiety influences the resulting stereochemistry of the
exocyclic olefin, ene-allene 75b was subjected to the Rh(I)-catalyzed reaction conditions. The
reaction resulted in the formation of azepine 211 as a mixture of exocyclic olefin isomers in 2.5 :
1 ratio (Scheme 3.47), clearly indicating a decrease in selectivity.
Scheme 3.47 Rh(I)-catalyzed ene reaction of 75b.
Bz
N
•
MeO2C Bn
1%
HaHa' Hb 75b
2.9%
N
Bz
Hc Hc MeO2C Bn Hc 211a
major diastereomer (Z)
10 mol %
[Rh(CO) 2Cl] 2
DCE, 90 o C, 4h, 66%
0%
211a : 211b = 2.5 : 1
Bz
N
MeO2C Bn
Bz
MeO 2C
Ha Ha'
N
H b
Bn
211
1.8%
H c
H c
H c
211b
minor diastereomer (E)
The two isomers were separated by normal phase HPLC and the major product 211a was
assigned Z-geometry based on nOe data (it should be noted that the E/Z notation is changed from
77
5
0%