Electrophilic metal carbenoids, readily available from diazo compounds, prove to be good reagents for the preparation of ylides under neutral conditions. We have extended the strategy to synthesize allenes from diazoacetates with both stable and unstable ketenes in the presence of PPh3 and Fe(TCP)Cl (0.5 mol %), which provided an easy access to optically active 4,4-disubstituted allenic esters by employing enantiopure phosphines. The mechanism involving the generation of ylide through catalytic transfer of an iron(II) carbene to phosphine was confirmed. A tandem Wittig–Nazarov cyclization reaction was further developed based on these results, culminating in a straightforward method for the construction of β-methylenecyclopentenones in a multistep one-pot manner without isolation of inter-mediates. By combining AsPh3 and iron carbenoids, a catalytic Wittig reaction in toluene/water biphasic system was accomplished, affording α,β-unsaturated esters in high yields with E/Z above 50/1. Surprisingly, crotonate-derived phosphorus ylide reacted with electrophilic iron carbenoid, affording 1,1,4-trisubstituted 1,3-butadienes with high regio- and stereo-selectivities, where a new ylide generated via formal intermolecular carbenoid insertion into olefinic C–H bonds of the crotonate-derived phosphorus ylide was proposed.
Palladium-catalyzed regio- and enantioselective migratory allylic C(sp3)-H functionalization