Norbomenes, hydrosilylation

Norbomanol with 96% ee was obtained by the asymmetric hydrosilylation of norbomene. Monofunctionalization of norbomadiene (565) was achieved with high chemo- and enantioselectivities to give uro-2-trich 1 orosilyl-5-norbornene (566) with one equivalent of trichlorosilane and converted to the alcohol 567 with 95% ee. With 2.5 equivalents, the chiral disilylnorbomane 568 was obtained rather than the me so isomer 569 (18 1). The disilylated product 568 was converted to the diol 570 with 99% ee [218],... 

Recently, much better results have been obtained using the palladium catalyst D [with ( + )-(/ )-MOP], Hydrosilylation of norbomene with trichlorosilane in the presence of this catalyst gives a quantitative yield of e.vo-2-trichlorosilylbornane which upon oxidation produces (l.S .2.S.4f )- .w-2-norbornanol (25) with 96% ee30. 

Pd-phosphine complexes with chiral phosphines (MDPP, e. g., menthyldiphe-nylphosphine) may also be used as catalysts for the asymmetric hydrosilylation of C=C bonds (alkenes, arylalkenes, norbomene, cycloalkadienes, and dienes) (e. g., [75, 76]), affording a relatively high yield and optical purity. 

Ligands of type 1 have also led to interesting results in kinetic resolutions during Cu-catalyzed reactions of dialkyIzinc with cycloalkene oxides (eq. (1)) [18] and cyclohexenones [19]. On the other hand, monophosphonites bearing a fused 1,4-dioxane ring behave moderately in the Rh-catalyzed hydrosilylation of ketones (up to ca. 56 % ee) [20] cf. Section 2.6 Finally, a phosphapallada-cyclic complex has been reported as an exceptionally fast catalyst for the hydroarylation of norbomene (TONs up to 10 °) but with low ees (< 25 %) [21]. 

Asymmetric hydrosilylations of terminal alkenes, 1-arylalkenes, norbomenes and dihydrofurans with HSiCIj have been successfully performed by Hayashi and coworkers [914, 915, 916, 1340, 1341]. These reactions take place at 40°C when catalyzed by chiral palladium complexes, and the most efficient ligand is monophosphine 3.51 (R = Me) (Figure 7.19). The regioselectivity of the hydrosilylation of terminal olefins is opposite to that usually observed after treatment with H2O2/KF, secondary alcohols are obtained as major products [752, 855, 1340], The regioisomeric primary alcohols are typically formed in only about 10% yield in these reactions. 

Asymmetric hydrosilylation of norbomene (2). This reaction can be effected in high regio- and enantioselectivity by reaction of 2 with CliSiH catalyzed by (allyl)chloropalladium dimer complexed with (R)-l. The product can be converted to (lS,2S,4R)-norbornanol (4) in 96% ee. 

With the exception of norbomene, internal olefins do not undergo hydrosilylation. Hydrosilylation of 3-phenylpropene with PhSiDj forms a unique product and the process tolerates a variety of fianctional groups, halides, ethers and acetals, despite the well known strong Lewis acidity of the catalysts. Cyclisation/silylation of 1,5-dienes or 1,6-enynes has been reported to give a single product (Scheme 14) [26]. In the case of metallocene complexes bearing a menthyl substituent, ee values near 70% were obtained for the asymmetric hydrosilylation of 2-phenyl-but-l-ene [31]. 

It is used for preparing cationic COD-Rh complexes with phosphine hgands for enantioselective [2+2+2] cycloaddition of unsymmetrical diynes with styrene and norbomene derivatives to yield bi- and tetra-cyclic products with good ( 50%) to very good (>90%) enantiomeric eruichment [Shibata et al. Tetrahedron 63 12853 2007], for hydrogenation [Nagel et al. Chem Ber 119 3326 1986, Ojima et al. Tetrahedron 45 6901 1989, Sawamura et al. JAm Chem Soc 117 9602 1995], and for hydrosilylation [Takeuchi et al. J Org Chem 60 3045 1995]. 

For the selectivity of the cross-metathesis reaction with ethylene (ethenolysis) the choice of a suitable catalyst and a cycloolefin for which ring-opening is energetically favorable as well as the optimization of the reaction conditions are crucial. Cycloolefins with a norbomene skeleton were chosen for their high ring strain. The silicon-containing educts can be obtained by hydrosilylation of norbomadiene with the corresponding silanes. 

Stereoselective intramolecular alkene hydrosilylation followed by Si-C cleavage is a valuable route to diols both relative - and absolute stereochemistries may be controlled. The rates of the fundamental steps in the [Rh(diphosphine)] catalysed reactions are controlled to some extent by the nature of the diphosphine. From deuterium-labelling studies a silyl insertion mechanism becomes apparent. Whether such mechanisms are applicable to C=0 hydrosilylation versions of this reaction is not yet known. Other highly enantioselective (83-96% ee) C=0 silylations use 13,162 but attempts to use readily available (-)-sparteine as a rhodium ligand are much less successful (5-8% ee).i Further details of the spectacularly effective MOP ligand 14 have appeared.i Optical purities of 93-96% ee are realised using 0.01 mol% Pd2( x-Cl)2( n-C3H5)2 and 0.02 mol% 14 for terminal alkenes and norbomene. 

Although the MOP ligands induce excellent enantioselectivity in the asymmetric hydrosilylation of 1-alkenes, norbomenes and dihydrofurans as described above, the reaction of styrene catalyzed by (l )-MeO-MOP-Pd under the same conditions, i.e. neat at 0°C, gives Ph(Me)CHSiCl3 with only 14% ee. The enantioselectivity can be improved up to 71% ee by using benzene as the solvent at 5 and the reactions... 

Hydrosilylation of norbornadiene (nbd) by chloro- or alkyl-hydrosilanes in the presence of Pd- or Mo-catalysts [178, 179] led to the corresponding silylnor-bornenes. Norbornenes with ethane spacer between bicyclic nuclear and silicon atom can be synthesized by hydrosilylation of 5-vinyl-2-norbomene [180],... 

---