Efficient Zirconocene-Coupling of Silicon-Substituted Diynes to Polymers and Macrocycles

The zirconocene-coupling of diynes with internal silicon substituents, MeC⋮CMe2SiArSiMe2C⋮CMe (1: Ar = 1,4-C6H4; 2: Ar = 1,3-C6H4; 3: Ar = 4,4'-C6H4C6H4), generates regiospecific polymers containing zirconacyclopentadiene in the main chain (5−7). These organometallic polymers hydrolyze cleanly to butadienediyl polymers of the type [Me2SiArSiMe2CHCMeCMeCH]n (11−13), and polymer 5 reacts with iodine to give the iodine-containing polymer [1,4-Me2SiC6H4SiMe2C(I)CMeCMeC(I)]n (14). The organometallic polymers undergo facile and high-yield degradations to macrocycles under mild conditions (refluxing tetrahydrofuran solution). The size and shape of the resulting macrocycles depend upon the nature of the diyne spacer group. Thus, polymers 5 and 7 containing parallel diyne units convert to the trimeric macrocycles [Me2SiArSiMe2C4Me2ZrCp2]3 (15: Ar = 1,4-C6H4; 24: Ar = 4,4'-C6H4C6H4), while polymer 6 gives the dimeric macrocycle [1,3-Me2SiC6H4SiMe2C4Me2ZrCp2]2 (18). The dimeric macrocycle [Me2SiC6H4SiMe2C6H4SiMe2C4Me2ZrCp2]2 (20) was obtained directly from the zirconocene coupling of Me2Si[(1,4-C6H4)SiMe2(C⋮CMe)]2 (4) by heating the reaction mixture to reflux. In a similar manner, the diyne Me2Si(C⋮CMe)2 was converted in high yield to the hexameric macrocycle [Me2SiC4Me2ZrCp2]6 (22). The macrocycles 15, [1,4-Me2SiC6H4SiMe2C4Me2H2]3 (16), and 18 were characterized by single-crystal X-ray crystallography. Molecules of 15 adopt a nearly planar C3 macrocyclic structure with a cavity described by an average transannular Si···Si distance of 13.2 Å, while the hydrolyzed macrocycle 16 has a chair conformation. This conformation change results from conversion of cis diene groups in the zirconacyclopendiene fragments to trans diene groups in 16. The high-yield formation of macrocycles apparently results from the reversible nature of the alkyne-coupling reaction, which allows for a low-energy pathway to the smallest macrocycle possessing minimal ring strain.