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Scheme 3.49 Rh(I)-catalyzed ene reaction of 75c-e.
N
MeO2C R1 P •
R 2
10 mol %
[Rh(CO) 2Cl] 2
DCE or tol, 90 o C
N
R1 P
MeO2C 75c P = Cbz, R 1 = Me, R 2 = Me 212, 63%
75d P = Cbz, R 1 = Me, R 2 = i-Pr 213, 51%
75e P = Bz, R 1 = CH 2OTBS, R 2 = Me 214, 72%
These, and additional examples from the Brummond group, demonstrate the broad scope and
good functional group compatibility of the Rh(I) catalyzed reaction. It was demonstrated that
highest rates of conversion and yields up to 95% can be obtained by using a bulky group on the
terminus of the allene (e.g., t-butyl). Additionally, replacing the nitrogen tether with oxygen,
leads to formation of oxepines in moderate yields (Scheme 3.50). A valuable feature of these
cycloisomerizations is the generation of two stereodefined olefins, one of which is an enamide
(or vinylether) and can potentially be exploited in further transformations aimed at target- or
diversity-oriented synthesis. 140
Scheme 3.50 Formation of oxepine 216 via cycloisomerization of ene-allene 215. n
O
215
•
t-Bu
10 mol %
[Rh(CO) 2Cl] 2
DCE , 90 o C, 55%
In summary, a novel Rh(I)-catalyzed cycloisomerization of ene-allenes leading to
functionalized azepines was discovered. The application of these unique compounds to DOS is
subject of current investigation. Finally, it should be noted that in 2004, Itoh reported a similar
cycloisomerization reaction of a carbon tethered ene-allene affording a 4-alkylidenecycloheptene
under Rh(I) catalysis. 141
n Reactions performed by Dr. Hongfeng Chen (Brummond group)
79
O
216
t-Bu
R 2