Synthesis of Oligo(1,6-heptadiynes) with a Single Structure and Terminal Methylene Groups Using Molybdenum-Based Wittig and Metathesis Chemistry. 1. 2,6-Dimethylphenylimido Systems

In this paper we demonstrate a method of synthesizing oligoenes of diisopropyldipropargylmalonate, [CH2][bx][CH2] (x ) 2, 3, 4, 5), in which the “b” repeat unit is identical to the five-membered ring formed in a tail-to-tail cyclopolymerization of diisopropyldipropargylmalonate, and methylene groups are capping the ends. For example, a “degenerate” metathesis reaction between the “monomer”, [CH2][b][CH2], and 2 equiv of Mo(NAr′)(CHCMe2Ph)(ORF6)2 ([Mo][CHCMe2Ph] where Ar′ ) 2,6Me2C6H3 and ORF6 ) OCMe(CF3)2) gives [Mo][b][Mo]. An “Mo-Wittig” reaction between [Mo][b][Mo] and 2 equiv of the aldehyde, [CH2][b][O], then gives the “trimer”, [CH2][b3][CH2]. Treatment of [CH2][b3][CH2] with 2 equiv of [Mo][CHCMe2Ph] gives [Mo][b3][Mo], which upon treatment with 2 equiv of [CH2][b][O] gives the “pentamer”, [CH2][b5][CH2], which contains 11 double bonds in conjugation in a molecule in which the repeat units are connected by trans CdC bonds. The oligoenes are not always obtained in high yield in pure form because (inter alia) CdC metathesis reactions compete with the Mo-Wittig reaction of the aldehyde (e.g., to give [CH2][b2][CH2] also in the synthesis of [CH2][b3][CH2]). These complications could be minimized by employing adducts (L)[Mo][bx][Mo](L), where L ) PMe3 or quinuclidine, instead of base-free [Mo][bx][Mo] species in the Mo-Wittig reaction. X-ray structures are reported for [CH2][b2][CH2], [CH2][b3][CH2], (THF)[Mo][b2][Mo](THF), (Et2O)[Mo][b3][Mo](Et2O), and a mixture of homochiral and heterochiral [quin][Mo][b][Mo][quin].