Studies on the Manganese-Mediated Isomerization of Alkynyl Carbonyls to Allenyl Carbonyls

Abstract

[reaction: see text] A study of the role of base in the isomerization of manganese-coordinated conjugated alkynyl carbonyls to the corresponding allenyl carbonyls is described. The use of phosphine additives indicates that manganese requires a ligand prior to isomerization with amine bases. A series of amine bases were also examined for their efficacy in this isomerization reaction revealing a strong dependence on pK(a). By contrast, potassium tert-butoxide led to rapid isomerization in the absence of added manganese ligand.

Transition metals in organic synthesis: Highlights for the year 2005

“…A regioselective hydrosilylative-decomplexation of cobalt-alkyne complexes was reported [1550]. The role of the base used for isomerization of alkyne-manganese to allene-manganese complexes was examined [1551]. (372) Iron tricarbonyl complexes continued to be used in organic synthesis.…”

Anion-Catalyzed Addition of γ-Silylallenyl Esters to Aldehydes: A New Entry into [3.2.1] Bicyclic Natural Products

“…We had previously disclosed our findings demonstrating the utility of allenyl carbonyls in Michael-Stork enamine additions although this method had only limited application to the synthesis of bicycles 2d. However, in our current studies towards the total synthesis of vitisinol D, we recently discovered that stabilized enolates are highly effective in double addition reactions to allenoates.…”

“…As part of our ongoing efforts in the total synthesis of vitisinol D, an anti-thrombotic natural product,1 we required a general and robust method for the preparation of γ-carbinol allenoates as a multifunctional coupling partner for maximal convergency. A review of existing methods including those developed in our lab2 revealed no efficient catalytic approach to the synthesis of these allenoates save Mukaiyama aldol-type reactions 3. Seeking a Lewis basic approach, we were intrigued by a report demonstrating an amine-catalyzed Morita-Baylis-Hillman (MBH) coupling of an allenyl ketone to aldehydes 4.…”

“…We recently reported a convenient one-pot procedure for the preparation of allenyl esters starting from inexpensive and readily available β-ketoesters 2e. However, in the case of silyl allenes, this method usually gives inseparable mixtures of silyl allenes and deconjugated alkynyl esters such as 1 and 2 (scheme of table 1).…”

Selective One-Pot Synthesis of Allenyl and Alkynyl Esters from β-Ketoesters

“…However, this method was limited to α-silylation, stannylation, and methylation with other alkylating agents leading to the deconjugated alkynyl ester product depending on reaction conditions 5. We have also described methods for the isomerization of α-alkynyl esters to their corresponding allenes involving manganese coordination chemistry 6. However, all of our previously disclosed methods to prepare β-alkynyl and α-allenyl esters require conjugated alkynyl esters starting materials which are not always readily available.…”