Synthesis of Allenes

The synthesis of allenes from organocopper compounds and acetylenic acetates was... 

Synthesis of allenic alcohols hy 1., 2-substitution of chlorine on ether groups by hydride... 

The discovery of carbene and carbenoid additions to olefins was the major breakthrough that initiated the tapping of this structural resource for synthetic purposes. Even so, designed applications of cyclopropane chemistry in total syntheses remain limited. Most revolve around electrophilic type reactions such as acid induced ring opening or solvolysis of cyclopropyl carbinyl alcohol derivatives. One notable application apart from these electrophilic reactions is the excellent synthesis of allenes from dibromocyclopropanes 2). 

A closer survey revealed that in the period elapsed since 1984 an average of 400 publications on allenes have appeared each year, amounting to close to 8000 in total. This made it obvious that one mortal author could not cover the whole field alone. As a consequence, Norbert Krause, both a renowned expert in allene chemistry and experienced in editing books, was commissioned as an additional editor. The crucial part then was to identify those fields of allene chemistry displaying the new and significant developments since 1984. We decided to divide the project into four parts, Synthesis of Allenes , Special Classes of Allenes , Reactions of Allenes , and Applications . 

Twenty chapters cover such new and exciting developments as metal-catalyzed synthesis of allenes, strained cyclic allenes, the numerous applications of different metallated allenes in organic synthesis, as well as the many addition and rearrangement reactions of allenes and allene units in natural products like the remarkable enyne-allenes. 

As mentioned in the Introduction, this chapter focuses on reactions that deliver allenes as the product. The principles discussed in Sections 1.2.1-1.2.9, of course, also allow the synthesis of allenes as reactive intermediates, which due to other functional groups that are present, undergo further reactions in situ. The most important examples here are base-catalyzed isomerizations to furans [347, 348] ring transfer reactions of propargylic ethers or amines [216, 349-371] and enyneallene cycliza-tion reactions starting from propargylic sulfones [372-375] and related substrates [376, 377]. Details are discussed, for example, in Chapters 16 and 20. 

Isomerization reactions are an excellent method for the synthesis of allenes. Depending on the method, numerous functional groups are tolerated, hence these reactions match the demands of modern synthesis. In the other chapters of this book we will encounter these reactions again, embedded in a wider chemical context. Not the preparation of the allene but its use in organic synthesis and the benefits of its high reactivity will then be the major focus. 

Scheme 2.1 Important reaction types in the metal-mediated synthesis of allenes. X= leaving group Acc = acceptor substituent.

Scheme 2.14 Synthesis of allenes 38 and 40 by reduction of propargyl acetates with lithium dimethylcuprate. THP = tetrahydropyranyl.

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