1,3-Diethynylallenes: Carbon-Rich Modules for Three-Dimensional Acetylenic Scaffolding

Regioselective Pd0-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 Sn2′-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 1,3-diethynylallenes 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 π-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 Pd0-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.