1,3-Diethynylallenes: Carbon-Rich Modules for Three-Dimensional Acetylenic Scaffolding
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0-catalyzed cross-coupling of substrates, which bear bispropargylic leaving groups with silyl-protected alkynes, has provided access to a variety of 1,3-diethynylallenes, a new family of modules for three-dimensional acetylenic scaffolding. In enantiomerically pure form, these C-rich building blocks could provide access ± by oxidative oligomerization ± to a fascinating new class of helical oligomers and polymers with all-carbon backbones (Fig. 2). In the first of two routes, a bispropargylic epoxide underwent ring opening during S2-type cross-coupling, and the resulting alkoxide was silyl-protected, providing 1,3-diethynylallenes ()-8, ()-12 (Scheme 3), and ()-15 (Scheme 5). A more general approach involved bispropargylic carbonates or esters as substrates (Scheme 6 ± 8), and this route was applied to the preparation of a series of 1,3diethynylallenes to investigate how their overall stability against undesirable [2 2] cycloaddition is affected by the nature of the substituents at the allene moiety. The investigation showed that the 1,3-diethynylallene chromophore is stable against [2 2] cycloaddition only when protected by steric bulk and when additional electron delocalization is avoided. The regioselectivity of the cross-coupling to the bispropargylic substrates is entirely controlled by steric factors: attack occurs at the alkyne moiety bearing the smaller substituent (Schemes 9 and 10). Oxidative Hay coupling of the terminally mono-deprotected 1,3-diethynylallene ()-49 afforded the first dimer 50, probably as a mixture of two diastereoisomers (Scheme 12). Attempts to prepare a silyl-protected tetraethynylallene by the new methodology failed (Scheme 13). Control experiments (Schemes 14 ± 16) showed that the Pd 0 -catalyzed cross-coupling to butadiyne moieties in the synthesis of this still-elusive chromophore requires forcing conditions under which rapid [2 2] cycloaddition of the initial product cannot be avoided.
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