[reaction: see text] An overall stereoconservative protection and deprotection method of amino and carboxyl groups is presented. N-Phthaloyl N'-alkyl secondary amides of alpha-amino acids can be generated from corresponding N-phthaloyl amino acids by coupling reaction of N-alkylamines using mixed anhydride method. These secondary amides can be transformed by thermal rearrangement of intermediate nitrosoamides to O-alkyl esters with retention of configuration and excellent yields.

A series of [2,6-bis(2-benzimidazolyl)pyridyl]chromium chlorides have been synthesized and characterized by elemental analysis and IR spectroscopy, along with X-ray diffraction analysis for the structures of C1 and C7. When methylaluminoxane (MAO) was employed as the cocatalyst, the chromium complexes showed high activity for ethylene oligomerization and polymerization. Oligomers were produced with high selectivity for α-olefins, and polyethylenes were generated with extremely broad molecular weight distributions. In the presence of diethylaluminum chloride (Et2AlCl), these chromium complexes showed moderate activity for ethylene polymerization and produced high-molecular-weight linear polyethylene.

The organometallic Lewis acid B(C 6 F 5 ) 3 adds to the terminal dCH 2 group of the (butadiene)metallocene complexes 5a and 5b to give the ansa-metallocene betaine systems [Me 2 Si-(C 5 H 4 ) 2 ]Zr[C 4 H 6 -B(C 6 F 5 ) 3 ] (6a) and [Me 2 Si(3-MeC 5 H 3 ) 2 ]Zr[C 4 H 6 -B(C 6 F 5 ) 3 ] (6b), respectively, in high yield. Both complexes were characterized by X-ray diffraction. They both contain a substituted η 3 -allyl ligand F of E configuration, and they show a characteristic (ortho aryl)-C-F‚‚‚Zr interaction that stabilizes the electron-deficient metal center inside the dipolar structure. B(C 6 F 5 ) 3 also adds to one butadiene terminus of (s-cis-η 4 -C 4 H 6 )[Me 2 C(C 5 H 4 )-(indenyl)]Zr to give a high yield of a single isomer of the respective ansa-metallocene [C 4 H 6 -B(C 6 F 5 ) 3 ] betaine complex 9. The X-ray crystal structure analysis of 9 has revealed that in this case a (Z)-η 3 -allyl-CH 2 B(C 6 F 5 ) 3 ligand is formed. This precluded the (aryl)C-F‚‚‚Zr coordination. Instead, the zirconium center in 9 forms a stabilizing internal ion pair interaction between the negatively polarized [B]-C(4)H 2 methylene group and the positive zirconium center. The analogously structured ansa-metallocene [(Z)-C 4 H 6 -B(C 6 F 5 ) 3 ] betaine complex 12 is obtained in high yield from B(C 6 F 5 ) 3 addition to (s-cis-η 4 -butadiene)[Me 2 C-(C 5 H 4 )(fluorenyl)]Zr. In solution the complexes 6, 9, and 12 exhibit structures that are analogous to those found in the solid state. However, treatment of (butadiene)[Me 2 Si(C 5 H 4 ) 2 ]-Zr (5a) with B(C 6 F 5 ) 3 under kinetic control (233 K in toluene-d 8 ) quantitatively yields the [Me 2 Si(C 5 H 4 ) 2 ]Zr[(Z)-C 4 H 6 -B(C 6 F 5 ) 3 ] betaine isomer 13, which contains the stabilizing [B]-C(4)H 2 ‚‚‚Zr internal ion pair interaction. Subsequent thermally induced rearrangement of the kinetic product 13 (∆G q rearr (298 K) ) 21.5 ( 0.5 kcal mol -1 ) then results in the formation of the eventually observed thermodynamic ansa-metallocene betaine product 6a, that contains the (E)-C 4 H 6 -B(C 6 F 5 ) 3 ligand and exhibits internal (aryl)C-F‚‚‚Zr coordination. A similar reaction sequence was observed during the addition of B(C 6 F 5 ) 3 to the parent (butadiene)zirconocene system 1: at 213 K the kinetic Cp 2 Zr[(Z)-(1-3η),κC 4 -C 4 H 6 -B(C 6 F 5 ) 3 ] betaine product 14 is formed, which rapidly rearranges at temperatures above 253 K to yield the previously observed stable Cp 2 Zr[(E)-C 4 H 6 -B(C 6 F 5 ) 3 ] betaine system 2, which is characterized by an internal C-F‚‚‚Zr bond. The ansa-metallocene betaines 6, 9, and 12 are all active homogeneous single-component Ziegler catalysts for ethene and propene polymerization. They are similarly effective as the usually employed ansa-metallocene dichloride/methylalumoxane catalyst systems.

Treatment of 6-(dimethylamino)-6-methylfulvene (1) with methyllithium followed by the reaction of the resulting [C 5 H 4 -CMe 2 NMe 2 ]Li reagent with CpZrCl 3 gave the complex [(η 5 -C 5 H 4 -CMe 2 NMe 2 )CpZrCl 2 ] (2). Its treatment with 2 molar equiv of methyllithium furnished [(η 5 -C 5 H 4 -CMe 2 NMe 2 )CpZr(CH 3 ) 2 ] (3). Complex 3 reacted with B(C 6 F 5 ) 3 by methyl group transfer. The in situ [Zr] + -CH 3 cation system generated in this manner proved to be unstable under the reaction conditions and instantaneously eliminated methane with formation of 4. In the course of this reaction a N-CH 3 hydrogen atom was abstracted. Complex 4 was stabilized by the addition of 1 equiv of the alkyl isocyanide RNtC (R ) CMe 3 , n-C 4 H 9 , -CMe 2 -CH 2 CMe 3 ) to yield the respective adducts 5. The complex [(η 5 :η 2 (C,N)-C 5 H 4 -CMe 2 N(dCH 2 )-CH 3 )CpZr(κC-CtN-CMe 3 )] + [CH 3 B(C 6 F 5 ) 3 ] -(5a) was characterized by an X-ray crystal structure analysis. It exhibits an η 2 -coordination of the pendant formaldiminium moiety to zirconium (d(Zr-N) ) 2.318(7) Å, d(Zr-C) ) 2.272(7) Å). The (η 2 -R 2 NCH 2 )Zr moiety shows a characteristic 15 N NMR chemical shift (δ -376 ppm), deshielded by ca. ∆δ ≈ -40 ppm relative to the 15 NMe 2 NMR resonance found for 3. Complex 4 reacts with butadiene or isoprene by insertion into the Zr-CH 2 bond of the (η 2 -formaldiminium)Zr moiety to form the metallacyclic (π-allyl)metallocene complexes 6a,b. The X-ray crystal structure analysis of 6b shows a close Zr-N contact at 2.491(4) Å. A corresponding 15 N NMR feature was found at δ -357 ppm. The complex [(η 5 -C 5 H 4 -CMe 2 NMe 2 ) 2 Zr(CH 3 ) 2 ] reacts analogously: upon treatment with B(C 6 F 5 ) 3 in a 1:1 ratio, CH 3 is transferred from zirconium to boron, and 1 equiv of methane is liberated to give complex 8 ( 15 N NMR signals at δ -357 and -372 ppm). Complex 8 was also characterized by X-ray diffraction. It shows coordination of both nitrogen atoms to zirconium: i.e., the presence of a (η 2 -R 2 NCH 2 )Zr + three-membered-ring system, formed by C-H activation, and a κN-coordinated intact pendant -CMe 2 -NMe 2 group. The latter is displaced upon the addition of tert-butyl isocyanide to yield the (κC-isonitrile)(η 2formaldiminium)metallocene cation complex 9.