Mori–Hiyama versus Hay Coupling for Higher Polyynes

Abstract

Abstract Dimerizations of C 4 precursors RC≡CC≡CTMS [R = C 6 H 5 ( 1 ), p ‐CH 3 C 6 H 5 , ( 2 ), 4‐ n ‐C 5 H 11 C 6 H 4 ( 3 ), C 6 H 9 ( 4 ), p ‐CH 3 OC 6 H 4 ( 5 ), p ‐C 6 H 5 COC 6 H 4 ( 6 ), p ‐NO 2 C 6 H 4 ( 7 ), and p ‐NCC 6 H 4 ( 8 )] by Hay coupling with in situ deprotection gave stable octatetraynes R(C≡C) 4 R ( 1 – 8 ‐C 8 ‐ 1 – 8 ) in 42–80 % yields as light yellow to brown powders. The same tetraynes were next synthesized from the same C 4 precursors by an alternative Mori–Hiyama method that typically utilizes trimethylsilyl‐substituted alkynes without prior deprotection. The latter protocol proved useful and gave the expected products but with lower yields of 8–74 %. The resulting tetraynes were characterized by spectroscopic methods and in some cases by X‐ray crystallography. In the cases of 1 ‐C 8 ‐ 1 and 3 ‐C 12 ‐ 3 , close analysis of the crystal data showed high degrees of carbon chain linearity, with % contraction indexes of 0.13 % and 0.19 %, respectively. The chains were more strained in the cases of 6 ‐C 8 ‐ 6 (0.34 %) and 3 ‐C 8 ‐ 3 (0.68 %). Deeper analysis of packing motifs revealed 3 ‐C 12 ‐ 3 to be a very promising candidate for topochemical 1,4‐polymerization.