Titanocene catalyzed

Using the titanocene-catalyzed co-hydrogenation of cyclohexene, we have studied the kinetics of the polymerization of a number of primary silanes ( 20 ). The rate law was found to be ... 

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. 

Scheme 3.38. General concept for the titanocene-catalyzed domino reaction.

Scheme 3.39. Examples of titanocene-catalyzed domino reactions.

Recently, the Cuerva group also presented a titanocene-catalyzed domino cycliza-tion of an aryl epoxypolyene such as 3-151, which led to the formation of a trans/ anti/trans-fused tricyclic compound 3-152, though in only moderate yield. Nevertheless, six stereogenic centers are formed in this domino process [64]. 3-152 could be transformed into the natural terpenoid stypoldione (3-153) (Scheme 3.40) [65]. 

Scheme 3.40. Titanocene-catalyzed synthesis of a tricyclic terpenoid.

The inter- and intramolecular catalytic reductive couplings of alkynes and aldehydes recently have experienced rapid growth and are the topic of several recent reviews.5 h-8k 107 With respect to early transition metal catalysts, there exists a single example of the catalytic reductive cyclization of an acetylenic aldehyde, which involves the titanocene-catalyzed conversion of 77a to ethylidene cyclopentane 77b mediated by (EtO)3SiH.80 This process is restricted to terminally substituted alkyne partners (Scheme 53). 

The very first example of the catalytic reductive cyclization of an acetylenic aldehyde involves the use of a late transition metal catalyst. Exposure of alkynal 78a to a catalytic amount of Rh2Co2(CO)12 in the presence of Et3SiH induces highly stereoselective hydrosilylation-cyclization to provide the allylic alcohol 78b.1 8 This rhodium-based catalytic system is applicable to the cyclization of terminal alkynes to form five-membered rings, thus complementing the scope of the titanocene-catalyzed reaction (Scheme 54). 

TABLE 6-3. Chiral Titanocene-Catalyzed Asymmetric Hydrogenation of Unfunctionalized Trisubstituted Olefins... 

Scheme 6-46. Titanocene-catalyzed asymmetric hydrosilylation of imines.

Generally, amine products can be obtained with 91-99% ee. It is noteworthy that the ee of the product does not correspond to the E Z ratio of the starting imines. Imines 103 and 104 exist as 2.5 1 and 1.8 1 (E)/(Z) isomers, but titanocene-catalyzed reduction produces amines with 93% and 97% ee, respectively.105... 

Titanocene-Catalyzed Reductive Epoxide Opening to Alcohols... 

Scheme 12.12. Planned titanocene-catalyzed reductive epoxide opening.

Titanocene-Catalyzed Additions to a,ft-Unsaturated Carbonyl Compounds... 

Scheme 12.16. Titanocene-catalyzed addition to a, 3-unsaturated carbonyl compounds.

As a first step towards this goal, we managed to develop a titanocene-catalyzed 5 -exo cyclization of radicals derived from suitably unsaturated epoxides. The mechanism of the cyclization is depicted in Scheme 12.18 [31],... 

Table 12.1. Titanocene-catalyzed formation of [3.3.0] systems. For the sake of clarity, only the major isomer is shown.

Scheme 12.20. Titanocene-catalyzed radical tandem reactions.

Using the catalytic system described above, the enantioselective opening of meso epoxides could also be pursued. Although many excellent examples of ring-opening of meso epoxides by Sn2 reactions have recently been reported, the reaction planned here is conceptually different [40]. In the SN2 reaction, the path of the incoming nucleophile has to be controlled. In the titanocene-catalyzed reaction, the intermediate radical has to be formed selectively. If an intermediate similar to that invoked in the Bartmann ring-open-... 

Table 5 Optimization of the titanocene-catalyzed transformation of 10 to cis-16 at room temperature. All reactions are carried out at 0.1 M concentration...

Scheme 60 Mechanism for titanocene-catalyzed epoxide ring opening and tandem reaction...

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