Before I take a couple of days off with the Family, I got to thinking about all that stored reaction knowledge as an organic chemist, and terms that enter the field that influence how we develop our strategies. A quick glance at the bookshelf — Umpolung strategies, Retrosynthetic and Synthon Disconnection Approaches, Tandem Reactions, New Transition Metal Catalysed Processes. It does seem like a lot to develop a concise approach to one’s work. So let’s mix it up and not have a particular approach but include some terms and see if there is some work out there: allenes, umpolung, synthon and microwave — so I want to find allenes where there is going to be some reversal of reactivity — that piece will provide us a useful synthon and will be used in a microwave reaction……no, I haven’t moved from espresso to bourbon quite yet….one more day, :).

So it is pretty well established that electon-deficient allenes undergo nucleophilic attack the alpha, beta electron-deficient double-bond to the EWG.

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Normal addition to electron-deficient allene

However, these particular allenes can also undergo a reversal of reactivity in the presence of phosphines to provide attack at the gamma position.

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New reactivity pattern for electron-deficient allenes

In a recent paper, a group takes advantage of the pattern and recognizes that an intermediate formed from addition of phosphines to butadienoates can be used as a 3-carbon synthon [3+2 synthon] in a cycloaddition reaction with electron-deficient alkenes and imines.

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Formation of the 3-carbon synthon from allene

The scheme below illustrates the allene and N-Tosyl imine used for the method. If you read the through the method development, PPh3 was typically used for the formation of the intermediate for the cycloaddition — normal heating times were reduced from hours to minutes. A more nucleophilic PBu3 was used in place of Ph3 to obtain the desired reaction in one of the examples. Optimal conditions included 100C mw 5 min. It is possible that a different solvent could be used but overall a pretty good process. The cis selectivity was found in all of the examples utilizing this sequence.

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[3+2] cycloaddition with allene and N-Tosyl imine

Additional examples and the same process was used for the formation of subst. cyclopentenes as well as some preliminary work on the starting allene in a Lewis acid allenoate-Claisen rearrangment, opening up additional difficult substitution patterns in a simple process. Happy reading!

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