No abstract

The ansa-zirconocene dichlorides [Me(2)Si(C(5)H(4))(3-R-C(5)H(3))]ZrCl(2) 7a-e (R = H, CH(3), cyclohexyl, -CHMe(2), -CMe(3)) were reacted with butadiene-magnesium to yield the respective (eta(4)-butadiene)metallocenes 17a-e. The chiral examples give a mixture of two s-cis and two s-trans diastereomers. The strong Lewis acid B(C(6)F(5))(3) adds selectively to a terminal butadiene carbon atom to yield the (butadiene)metallocene/B(C(6)F(5))(3) betaine complexes 18a-e. Initially, the formation of the Z-18 isomers is preferred. These consecutively rearrange to the thermodynamically favored isomers E-18. The dipolar systems 18 are active single component metallocene catalysts for the stereospecific polymerization of methyl methacrylate. With increasing steric bulk of the attached single alkyl substituent an increasingly isotactic poly(methyl methacrylate) is obtained. A similar trend is observed in the methyl methacrylate polymerization at the [Me(2)Si(C(5)H(4))(3-R-C(5)H(3))]ZrCH(3)(+) catalysts (9a-e) that were conventionally prepared by methyl abstraction from the corresponding ansa-zirconocene dimethyl complexes by treatment with B(C(6)F(5))(3). A comparison of the poly(methyl methacrylates) obtained at these two series of catalysts has revealed substantial differences in stereoselectivity that probably originate from an influence of the respective counteranions. An initial reactive intermediate of methyl methacrylate addition to the dipolar single component metallocene catalyst E-18a was experimentally observed and characterized by NMR spectroscopy at 253 K. The subsequently formed series of [PMMA-C(4)H(6)(-)B(C(6)F(5))(3)](-) anion oligomers (at the catalyst 18c) was monitored (after quenching) and characterized by electrospray mass spectrometry.

Treatment of 3-phenylcyclopent-2-enone (1) with either morpholine or pyrrolidine in the presence of 3 Å molecular sieves yielded the corresponding cyclopentadienamines 2a and 2b. Subsequent deprotonation with n-butyllithium followed by the reaction with zirconium tetrachloride led to the formation of the amino-substituted bent metallocenes 4a and 4b. Crystallization furnished the isomerically pure complex meso-bis(phenyl-3-pyrrolidinylcyclopentadienyl)zirconium dichloride (meso-4b), which was characterized by X-ray diffraction. Complex meso-4b exhibits a C(Cp)-N bond length of 1.349(3) Å. A rotational barrier of ∆G q (CN-rot) (248 K) ) 12.1 ( 0.2 kcal mol -1 was measured for meso-4b in solution by dynamic 600 MHz 1 H NMR spectroscopy. Similar treatment of 2-indanone with morpholine, followed by deprotonation and reaction with ZrCl 4 , gave bis(2-morpholinoindenyl)ZrCl 2 (7a), which was also characterized by an X-ray crystal structure analysis (the indenyl-C2/12-N bond lengths are 1.349(5)/1.381(5) Å). Two rotational processes were subsequently frozen out on the 600 MHz 1 H NMR time scale upon decreasing the temperature from ambient to give a C-N rotational barrier of ∆G q (CN-rot) (268 K) ) 12.9 ( 0.2 kcal mol -1 and an activation energy of the rotation about the zirconium-Cp(indenyl) vector of ∆G q (Ind-rot) (178 K) ) 7.6 ( 0.5 kcal mol -1 . In solution and in the solid state only a single C 2 -symmetric metallocene rotamer of 7a was detected. The same is found for bis(2-pyrrolidinoindenyl)ZrCl 2 (7b). By dynamic 1 H NMR spectroscopy the two rotational activation barriers of complex 7b were determined as ∆G q (CN-rot) (338 K) ) 16.4 ( 0.2 kcal mol -1 and ∆G q (Ind-rot) (198 K) ) 9.1 ( 0.5 kcal mol -1 .

The lithium salts of [bis(diisopropylamino)thioxophosphoranyl]diazomethane (2) and [(diisopropylamino)(dicyclohexylamino)thioxophosphoranyl]diazomethane (5) react with the (diisopropylamino)(dicyclohexylamino)chlorophosphine (3) and bis(diisopropylamino)chlorophosphine (6), leading to the nitrilimines 4 (63% yield) and 7 (69% yield), respectively. The lithium salt of 2 also reacts with the bis(dicyclohexylamino)phosphenium ion, affording nitrilimine 9 in 51% yield. Using the presence of a chiral substituent either at the carbon or at the nitrogen terminus of the CNN skeleton, variable-temperature solution NMR studies of nitrilimines 4 and 7 demonstrate that they possess a bent allenic structure. The free energy of activation for racemization is ca. 30 kJ mol-1. Ab initio/DFT studies performed on nitrilimine with C-P(S)H2 and N-PH2 substituents 10 show that the most likely pathway for the racemization process is inversion at the carbon atom, followed by rotation of the PSR2 fragment. Bis(trityl)nitrilimine 17 reacts with (R)-α-(acryloxy)-β,β-dimethyl-γ-butyrolactone, leading to diastereomeric pyrazolines 18a,b (3/1 ratio) in 60% total yield; this is the first example of a diastereoselective [3 + 2]-cycloaddition reaction involving a nitrilimine.