Titanocene complex

In stoichiometric applications of titanocene complexes, no attempt to use ligands other than unsubstituted cyclopentadienyl have been reported. The use of more complex titanocenes [57,58] in catalytic reactions is, however, very promising for controlling the regio- and stereochemical course of the reaction. 

A rather new concept in the context of domino radical cydizations has been developed by Gansauer and coworkers utilizing titanocene-complexes for the radical opening of unsaturated epoxides. The titanocene-catalyzed reactions [61] of 3-145 primarily led to radical 3-146, which underwent a subsequent intermolecular addition to a present a,(3-unsaturated carbonyl compound to form bicyclic carbocy-cles of type 3-148 via the intermediate 3-147 after aqueous work-up (Scheme 3.38) [62]. From a kinetic point of view, the reaction is remarkable since the intermolecular addition of simple radicals to a,(3-unsaturated carbonyl compounds is not an easy task, as highlighted above. 

Brintzinger and co-workers, while investigating the formation and chemical reactivity of various titanocenes, prepared a novel triphenyl-phosphine-titanocene complex of empirical formula [C10H10TiP(C6H5)3]2 (22) whose structure to date has been undetermined. The purple complex 22 can be prepared by the four routes illustrated below 24). 

In addition to phosphine ligands, a variety of other monodentate and chelating ligands have been introduced to functionalized polymers [1-5]. For example, cyclo-pentadiene was immobilized to Merrifield resins to obtain titanocene complexes (Fig. 42.13) [102]. The immobilization of anionic cyclopentadiene ligands represents a transition between chemisorption and the presently discussed coordinative attachment of ligands. The depicted immobilization method can also be adopted for other metallocenes. The titanocene derivatives are mostly known for their high hydrogenation and isomerization activity (see also Section 42.3.6.1) [103]. 

Fig. 42.13 Immobilization of cyclopentadiene for the preparation of polymer-supported titanocene complex. 

We are currently employing substituted titanocene complexes to achieve reagent-con-trolled cyclizations. 

The results of investigations of a number of titanocene complexes shown in Scheme 12.24 are summarized in Table 12.2. 

Table 12.2. Reductive opening of epoxide 1 with various titanocene complexes.

Brintzinger s complex 2 [42] (10 mol%) performed poorly in terms of both the enantio-selectivity of the epoxide opening (56 %) and the product yield (55 %). The titanocene complex 3 [42] obtained from (1R,2S,5R) menthol gave variable results due to its sensitivity to traces of moisture (ee = 20—52 %). Clearly, the axially positioned cyclopentadienyl group is not ideal. 

A series of titanocene complexes 100-104 has been prepared according to Eq. (29). Complex 100 turned out to be highly unstable and readily decomposes in... 

Bis-titanocene complex 184 reacted with one equivalent of sulfur monochloride to give intermediate mono-titanocene 185 which, upon treatment with ethane-1,2-disulfenyl chloride, transformed to bicyclic sulfur-carbon heterocycle 186 (1991CB2141 Scheme 93). 

Hexathiaspirododecane 237 containing six sulfur atoms in the ring was prepared from titanocene complex 238 with sulfur monochloride (1996ZAAC1594). The structure of 237 had a chair conformation as identified by X-ray analysis (Scheme 126). 

A new sulfurimide heterocycle with six sulfurs - octyl hexathiazepane 239 - was obtained from titanocene complex 240 in high yield (1998AG(E)492 Scheme 127). 

Sulfur monochloride reacted with another titanocene complex 241 to give nonasulfur imide 242. The reaction proceeded smoothly at room temperature (1996AG(E)2537 Scheme 128). 

Gansauer, and a titanocene complex, TiCl3(THF)3, is also applicable as disclosed by Nelson and co-workers Catalytic pinacol coupling reactions occur with TiCU and zinc (or aluminum) in the presence of an acylating reagent such as acetic anhydride or acetyl chloride. ... 

It was 1996 when Buchwald and Hicks reported the first example of an asymmetric PKR involving a catalytic amount of a chiral titanocene complex. The titanium catalyst (6 ,6 )-(EBTHI)Ti(GO)2 (EBTHI = ethylene-1,2-bis( 7 -4,5,6,7-tetrahydro-l-indenyl)) obtained in situ by treatment of (6 ,6 )-(EBTHI)TiMe2 under CO pressure was efficient for the formation of enantiomerically enriched carbocyclization adducts. ... 

When (PhMe4C5)2TiCl2 is reduced with magnesium in THF three main products are formed. They are the diamagnetic doubly tucked-in titanocene complex (PhMe4C5)... 

A chiral titanocene complex catalyzes enantioselective hydrogenation of imines in a moderate to high optical yield (Scheme 78) (117). 

The protonation of the analogous titanocene complex 55 with trifluoro-acetic acid led to similar products methanethiol and bis(trifluoroacetato) titanocene. When 55 was treated with Mel, rather than the corresponding iodide complex, the cationic Me+ adduct 123 was obtained with iodide as a counterion. However, the acylation of 55 with acetyl chloride afforded the chloride 124 related to 122 (Scheme 30).67... 

In contrast, the related T -thioformaldehyde titanocene complexes 55 [see Eq. (11)] did not give clean insertion products with various triple (C=C and C=N) and double (C=0, C=N, C=S) bonds.67... 

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