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diastereomer.
Next, allenyne 328 was subjected to the reaction conditions to test whether the double
bond selectivity can be retained in the presence of a methyl group at the proximal allenic
position. Unfortunately, the sluggish reaction resulted in 48% yield of the 4-alkylidene
cyclopentenone 329, while the desired 330 was not observed (Scheme 4.54).
Scheme 4.54 Attempted cyclocarbonylation reaction of 328.
BzN
MeO 2C
•
328
H
H
5 mol % [Rh(CO) 2dppb]BF 4
DCE, CO (0.2 atm), 80 o C, 6h
Bz
MeO 2C
N
O
BzN O
MeO 2C
329 (48%) 330 not observed
Finally, an attempt was made to extend the scope of the new catalyst to substrates other than
amino-acid derived allenynes. However, when the diester-tethered allenyne 331 was subjected to
10 mol % of catalyst, cyclocarbonylation did not occur at 100 °C.
Scheme 4.55 Attempted cyclocarbonylation reaction of 331.
EtO 2C
EtO 2C
331
•
H
H
10 mol % [Rh(CO) 2dppb]BF 4
DCE, CO (0.2 atm), 100 o C, 6h
No reaction
recovered starting material
In summary, we have demonstrated a novel Rh(I)-catalyzed cyclocarbonylation reaction
of amino-ester tethered allenynes selective for the proximal double bond of the allene. Selectivity
was accomplished under partial pressure of CO using a freshly prepared Rh(I)-catalyst
containing a dppb ligand. The origin of the observed selectivity is subject to speculation, and
further studies are required to asses this issue. One potential explanation is based on recent
computational studies by Brummond and Bayden addressing the selectivity observed in the
“normal” Rh(I)-catalyzed and Mo-mediated allenic cyclocarbonylation reactions. 231 The
calculations support that the product-determining step in both reactions is the oxidative addition
137