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As shown in Table 4.1 (entries 3 and 4), reaction with 73f did not occur upon heating to 90 °C.
The unreactive nature of these two catalysts is attributed to very low π-acidity due to presence of
phosphine ligands. These observations are in accord with those by Jeong, who first reported that
activation of both catalysts by Ag-additives is necessary in the cyclocarbonylation reaction of
enynes. 156 Addition of Ag(I) salts with non-coordinating anions (e.g., CF3SO3 - , BF4 - , SbF6 - , PF6 - )
to a metal-halide complex replaces the strongly coordinating halide with a weakly coordinating
counterion thereby creating a vacant binding site on the metal and increasing its π-acidity. When
trans-Rh(CO)Cl(PPh3)2 was treated with equimolar amount of AgOTf, reaction of 73f occurred
at 50 °C to give a mixture of cyclocarbonylation products 270f and 274f in 66% yield, with the
desired 4-alkylidene cyclopentenone preferred by ca. 3 : 1 (entry 5, Table 4.1).
A similar result was obtained when N-benzoyl protected allenyne 73a was submitted to
the same reaction conditions (entry 1, Table 4.2). Changing the solvent from toluene to DCE,
resulted in formation of the cyclocarbonylation product 270a (55%) and triene 111a in 10% yield
(entry 2, Table 4.2). Based on this result DCE was retained as a solvent, and variation of the Ag-
salt additive was examined. The type of the counterion is known to have an effect on the
outcome of transition metal catalyzed reactions, but strict survey to our knowledge has not been
performed in the context of Rh(I)-catalyzed cyclocarbonylation reactions. 177 Interestingly, when
using AgBF4, the only isolated product was the desired 4-alkylidene cyclopentenone 270a in
76% yield (entry 3, Table 4.2). However, this reaction required 36 h at rt for completion, which
may seriously limit its broader application.
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