Alkenes

We report the development of a novel hetero-Diels-Alder reaction involving the use of allenyl carbonyls as heterodienes. We have described the synthesis of a variety of allenyl carbonyls via a base promoted and manganese mediated isomerization of alkynyl carbonyls for use in methodology studies. In our model studies, a variety of enamines were utilized effectively as dienophiles to form "hydrolyzed" Diels-Alder adducts while investigating optimal reaction conditions. Initial studies towards the synthesis of an allenyl ester tethered to an aldehyde for an eventual application in an intramolecular Diels-Alder reaction were performed. Possible mechanisms were proposed while computational studies were performed in attempt to rationalize product selectivity.

The conversion of alkynyl carbonyls to allenyl carbonyls via manganese mediated coordinations followed by a based-catalyzed isomerization was carried out using a range of chiral and achiral amine bases. In this work we employed HPLC equipped with a chiral column to determine the enantiomeric excess. Deuterium labeling experiments suggested that the alkyne/allene rearrangement reaction involved an intermediate cumenolate. We also demonstrated that the reaction required a ligand on manganese for an amine base to be used catalytically. Phosphines were tested as a possible ligand because they are neutral two electron donors that binds to transition metals through their lone pairs. It was observed that the rate of the reaction decreased from 24hr to 3hr by use of phosphine as a ligand. It was also confirmed that amine base with pKa lower then DBU (pKa = 13.6) would not carry out the isomerization. Chiral amidine and chiral DBU derivatives were synthesized to carry out the isomerization enantioselectively. Alkoxy base were also used in isomerization that demonstrated enantioselectivity.