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 diisopropyidipropargylmalonate, [CH2[b(x)][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 ' = 2,6-Me2C6H3 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][b(3)]CH2]. Treatment of [CH2][b(3)][CH2] with 2 equiv of [Mo][CHCMe2Ph] gives [Mo][b(3)][Mo], which upon treatment with 2 equiv of [CH2][b][O] gives the pentamer, [CH2][b(5)][CH2] which contains 11 double bonds in conjugation in a molecule in which the repeat units are connected by trans C=C bonds. The oligoenes are not always obtained in high yield in pure form because (inter alia) C=C metathesis reactions compete with the Mo-Wittig reaction of the aldehyde (e.g., to give [CH2][b(2)][CH2] also in the synthesis of [CH2][b(3)][CH2]). These complications could be minimized by employing adducts (L)[Mo][b(x)[Mo](L), where L = PMe3 or quinuclidine, instead of base-free [Mo][b(x)][Mo] species in the Mo-Wittig reaction. X-ray structures are reported for [CH2][b(2)][CH2], [CH2)][b(3)][CH2], (THF)[Mo][b(2)][Mo](THF), (Et2O)[Mo][b(3)][Mo](Et2O), and a mixture of homochiral and heterochiral [quin][Mo][b][Mo][quin].