Transition Metal-Catalyzed Synthesis of Allenes

This chapter will cover reactions in which allenyl C=C=C skeletons are constructed using transition metal catalysts. Thus, functionalization or derivatization of compounds having cumulated C=C=C moieties will not be covered in this chapter. In addition, reactions using stoichiometric transition metal reagents (see Chapter 2) will be excluded from the discussion. 

Modem Allene Chemistry. Edited by N. Krause and A.S.K. Hashmi Copyright 2004 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30671-4 

On the other hand, several isolable ()73-allenyl)palladium complexes have been prepared recently [17-21] and their role in catalytic reactions has been discussed [22-26], 

In the following sections, the palladium-catalyzed conversion of the propargyl electrophiles into allenes will be briefly summarized with some representative examples and some recent reports. Although selectivity between the allenic and the pro-pargylic products has been one of the central topics in Pd-catalyzed reactions of the propargyl electrophiles, reactions giving allenes as main products will be considered. For more comprehensive reviews on these topics, previous publications should be consulted [7-14], 

Palladium-Catalyzed Coupling Reactions of Propargyl Electrophiles with Hard Carbon Nucleophiles 

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This chapter has discussed the transition metal-catalyzed synthesis of allenes. Because allenes have attracted considerable attention as useful synthons for synthetic organic chemistry, effective synthetic methods for their preparation are desirable. Some recent reports have demonstrated the potential usefulness of optically active axially chiral allenes as chiral synthons however, methods for supplying the enantiomerically enriched allenes are still limited. Apparently, transition metal-catalyzed reactions can provide solutions to these problems. From the economics point of view, the enantioselective synthesis of axially chiral allenes from achiral precursors using catalytic amounts of chiral transition metal catalysts is especially attractive. Considering these facts, further novel metal-catalyzed reactions for the preparation of allenes will certainly be developed in the future. 

Several examples are known of the transition metal-catalyzed synthesis of 1,2,3-buta-trienes, which possess one more cumulated C=C double bond than allenes. Most of the reported examples of the butatriene synthesis involve dimerization of terminal alkynes and conjugated enynes are typical side products of the reactions. 

ArisawaM, Suwa A, Fujimoto K, Yamaguchi M. Transition metal-catalyzed synthesis of ( )-2-(alkylthio)alka-l, 3-dienes from allenes and dialkyl disulfides with concomitant hydride transfer. Synth. Catal. 2003 345 560-563. 

The transition metal-catalyzed addition of alcohols to unsaturated systems has not been widely investigated. Reports on addition of alcohols to 1,3-diene [24] or allene [25] have appeared but have very limited scope. We recently reported the palladium/benzoic acid-catalyzed inter- and intramolecular addition of alcohols to alkynes in which various acyclic and cyclic allylic ethers are produced [26], The Pd-catalyzed addition of alcohols to alkylidenecyclopropanes proceeds smoothly providing a powerful tool for synthesis of allylic ethers [27a]. An intramolecular version of the hydroalkoxylation has been demonstrated in which the phenol-tethered alkylidenecyclopropanes undergo facile cyclization to give exomethylene products [27b],... 

Allenes, while arguably underused in synthesis as a whole, have become popular functionalities in cycloisomerization chemistry and provide access to a wide variety of products. Ruthenium, cobalt, platinum, palladium, rhodium, and iridium catalysts are efficient in the transition metal-catalyzed Alder-ene reactions of allenes. 

As evidenced by the many examples in Section 19.2, [2 + 2]- and [4 + 2]-cydoaddi-tion reactions are by far the most common application of allenes in natural product synthesis. Far less common is the use of allenes in transition metal-catalyzed carbon-carbon bond-forming reactions, the topic of Section 19.3. 

Catalytic enantioselective synthesis of 4,4-dimethyl-l-phenyl-l,2-pentadiene from 4,4-dimethyl-1,2-pentadiene and iodobenzene using 0.4 to 1 mol % of palladium complexes containing chiral phosphane ligands as the catalyst for the enantioselective cross coupling134 is the only example of substoichiometric transition metal catalyzed enantioselective allene synthesis. 

Allenes are versatile synthetic precursors in organic chemistry and the synthesis of many natural products involves the use of allenic compounds [27]. However, they have received much less attention than alkenes or alkynes for transition metal catalyzed reactions. The explanation lies in the problems of selectivity that these substrates display as a result of their reactivity and their inherent chirality [28]. 

Synthesis of aromatic compounds with variations in the metal-catalyzed Bergman cycloaromatization is an alternative to the catalysis by transition metals of [2-f2-f2] cyclotrimerization of alkynes and [4-1-2] benzannulation of enynes [261-263]. On the other hand, much less attention has been paid to the use of transition metal catalysis in enyne-allene cycloaromatization [264]. 

T. Takahashi, T. Doi, and K. Yamamoto, in Transition Metals for Organic Synthesis, Vol. 1, M. Beller and C. Bohn, Eds., Wiley-VCH, Weinheim, 1998, 265-274. Palladium-Catalyzed Cyclization of Allylic Acetates with Alkenes and Allenes. 

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