Reduction of Bis[η5-(ω-alkenyl)tetramethylcyclopentadienyl]titanium Dichlorides: An Efficient Synthesis of Long-Chainansa-Bridged Titanocene Dichlorides by Acidolysis of Cyclopentadienyl-Ring- Tethered Titanacyclopentanes

Abstract

The reduction of symmetric, fully-substituted titanocene dichlorides bearing two pendant omega-alkenyl groups, [TiCl2(eta5-C5Me4R)2], R = CH(Me)CH= CH2 (1a). (CH2)2CH=CH2 (1b) and (CH2)3CH=CH2 (1c), by magnesium in tetrahydrofuran affords bis(cyclopentadienyl)titanacyclopentanes [Ti(IV)[eta1:eta1: eta5:eta5-C5Me4CH(Me)CH(Ti)CH2CH(CH2(Ti))CH(Me)C5Me4]] (2a), [Ti(IV)[eta1:eta1:eta5: eta5-C5Me4(CH2)2CH(Ti)(CH2)2CH(Ti)(CH2)2C5Me4]] (2b) and [Ti(IV)[eta1:eta1:eta5:eta5-C5Me4(CH2)2CH(Ti)CH(Me)CH(Me)CH(Ti)(CH2)2C5Me4]](2c), respectively, as the products of oxidative coupling of the double bonds across a titanocene intermediate. For the case of complex 1c, a product of a double bond isomerisation is obtained owing to a preferred formation of five-membered titanacycles. The reaction of the titanacyclopentanes with PbCl2 recovers starting materials 1a from 2a and 1b from 2b, but complex 2c affords, under the same conditions, an isomer of 1c with a shifted carbon - carbon double bond, [TiCl[eta5-C5Me4(CH2CH2CH=CHMe)]2] (1c'). The titanacycles 2a-c can be opened by HCl to give ansa-titanocene dichlorides ansa-[[eta5:eta5-C5Me4CH(Me)CH2CH2CH(Me)CH(Me)C5Me4]TiCl2] (3a), ansa-[[eta5:eta5-C5Me4(CH2)8C5Me4]TiCl2] (3b), along with a minor product ansa-[[eta5:eta5-C5Me4CH2CH=CH(CH2)5C5Me4]TiCl2] (3b'), and ansa-[[eta5:eta5-CsMe4(CH2)3CH(Me)CH(Me)CH=CHCH2C5Me4]TiCl2] (3c), respectively, with the bridging aliphatic chain consisting of five (3a) and eight (3b, 3b' and 3c) carbon atoms. The course of the acidolysis changes with the nature of the pendant group; while the cyclopentadienyl ring-linking carbon chains in 3a and 3b are fully saturated, compounds 3c and 3b' contain one asymetrically placed carbon-carbon double bond, which evidently arises from the beta-hydrogen elimination that follows the HCl addition.

Conformational Features of Group-4ansa-Metallocenes with Long −(CH2)n− Chains Connecting Their Cyclopentadienyl Ligands

“…Mach et al have described the structure of [(CH 2 ) 8 Cp* 2 ]TiCl 2 (1, see Figure 1) which has the bridging octamethylene chain located at the narrow back side, almost like a ''small'' ansa-metallocene. [7] In contrast, in rac-[(CH 2 ) 12 (1-indenyl) 2 ]ZrCl 2 (rac-2c), the other previously described example, the bridging hydrocarbyl chain is C 2 -symmetrically located at the front of the bent metallocene wedge. [8] This bisects the ClϪZrϪCl angle as shown in the bottom projection in Figure 1.…”

“…Sect.). In addition, a similar series of rac-and meso-[(CH 2 ) n (1-indenyl) 2 ]ZrCl 2 complexes (rac-/meso-2aϪc, n ϭ 8, 10, and 12) was investigated by this combination of force-field/DFT calculations, and the Ti-based system 1 recently described by K. Mach et al (see above), [7] was included in the study for comparison. Scheme 1.…”

“…[6,7] The metallacyclopentane formation is not problematic, but the necessary subsequent cleavage by e.g. HCl, appears to lead to the expected ''large'' ansa-metallocene product only in a limited number of cases.…”

“…Mach's peralkylated ansa-titanocene complex 1 was actually synthesized in that way. [7] Even in the titanium systems, competing pathways leading to ringopened acyclic products were observed. This completely dominated the protolytic cleavage in the case of the corresponding zirconocene examples, as we had recently shown.…”

“…These results were complemented experimentally by the synthesis of two representative ''large'' ansa-zirconocene dichlorides, the investigation of their respective Figure 1. SCHAKAL plots of the ''large'' ansa-metallocenes 1 (top) and rac-2c (bottom), that were previously described in the literature [7,8] dynamic conformational interconversion in solution, and the structural characterization of one example by X-ray diffraction. This has provided us with a much more detailed picture of the conformational properties of the interesting group-4 ''large'' ansa-metallocene complexes, relative to the selected views derived from previous descriptions based only on a limited number of experimental observations.…”