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1,3‐Diethynylallenes: Carbon‐Rich Modules for Three‐Dimensional Acetylenic Scaffolding
Author(s) -
Livingston Robert,
Cox Liam R.,
Odermatt Severin,
Diederich François
Publication year - 2002
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/1522-2675(200210)85:10<3052::aid-hlca3052>3.0.co;2-4
Subject(s) - chemistry , regioselectivity , moiety , allene , steric effects , cycloaddition , silylation , substituent , stereochemistry , cycloisomerization , combinatorial chemistry , organic chemistry , catalysis
Regioselective Pd 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 S n 2′‐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 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.