1,x‐Elimination Reactions: Extending the Limits of a Classical Organic Reaction

Abstract

Alpha,omega-dibromo derivatives in which the two terminal carbon atom are separated by an unsaturated spacer unit ("pi spacer") undergo 1,x-elimination reactions (with x=6, 8, 10, and 14), using Mori's reagent (nBu3SnSiMe3/CsF). The resulting cumulenic intermediates cyclodimerize in a subsequent step yielding novel macrocyclic acetylenic and bridged aromatic compounds (cyclophanes). Thus 1,6-eliminations were carried out with dibromide 17 to yield 1,3,7,9-cyclododecatetrayne (20) and with benzylbromide 24 to provide cyclophanes 26 and 27. By 1,8-eliminations the 16-membered macrocycle 33 could be prepared from enediyne 31, the benzannelated 1,5-cyclooctadiyne 41 from dibromide 38, and a mixture of cyclophanes 45 and 46 from the precursor 43. 1,10-Eliminations were carried out successfully with dibromides 47, 50, and 53 yielding the corresponding unsaturated cyclophanes ("cyclophynes") 49, 52, and 55. The influence of the solvent on the cyclodimerization 47-->49 was investigated, with acetonitrile providing the highest yields. The heterophanes 59 a and b were obtained by 1,10-elimination of the precursor dibromides 57 a and b, and in an elimination experiment involving a 1:1 mixture of the dibromides 50 and 57 b the "mixed dimer" 60 was isolated, besides the homodimers 52 and 59 b. The method reached its limits with the 1,14-elimination of 68, 70, and 74 providing the cyclophanes 69, 71, and 75 in varying amounts. Two final debrominations with 76 and 77, which in principle could undergo 1,16- and 1,20-eliminations reactions, respectively, failed. The structures of the new cyclophanes 49, 50, 59 a, and 59 b were established by X-ray structural analysis; all other structure assignments rest on the usual spectroscopic and analytical data.

An Iterative Method for the Synthesis of Symmetric Polyynes

“…Diiodobutadiyne ( 2a ), diiodohexatriyne ( 3a ), and diiodooctatetrayne ( 4a ) were each prepared from the corresponding trimethylsilyl‐capped polyynes (that is, 2c , 3c , and 4c , respectively) according to previously described methods 19b,21. 2,4‐Hexadiyne‐1,6‐diol ( 9 ) and 1,6‐dibromo‐2,4‐hexadiyne ( 10 ) were also prepared using previously reported methods 15b,15c…”

Heuristic Chemistry—Elimination Reactions

“…The variety of synthetic methods for preparing carbon-rich compounds has been increasing significantly during the last 50 years. This vinylogy has already been shown for 1,6to 1,10-elimination reactions, [3] but the next closer homologues, 1,3-to 1,6-eliminations, are, surprisingly, either marginally described or ignored completely in organic chemistry textbooks. [2] An elimination is, by definition, a process that separates two fragments (atoms or groups of atoms) or leaving groups from each other.…”

“…[3,35,36] Eliminations at these spacer types usually follow the same rationale and give comparable reaction outcomes as elimination at the original spacers; alkynes give allenes and benzene rings give quinone derivatives, odd elimination gives carbenes. Mixed elimination reaction substrates can be enynes, diethynylbenzenes, or ethynylvinylbenzene.…”

π‐Conjugated Macrocycles Bearing Angle‐Strained Alkynes

“…Strained macrocycles with furylene and diethynylfuran moieties are rare. A few examples of alkyl‐tethered macrocycles or metal‐containing macrocycles are known . Macrocycles with a diethynylpyrrole moiety are scarcely found in the literature.…”