Rhodium l Catalysts

Very recently, Wiedenhoefer272 has devised the first asymmetric 1,6-enyne hydrosilylation/cyclization tandem process using a rhodium(l) catalyst with (R)-276 as chiral ligand where rhodium-BINAP complexes were not effective (Scheme 70). More developments on this reaction are covered in Chapter 11.13. 

The [5 + 2]-cycloadditions of tethered alkyne-VCPs that are 1,2-disubstituted on the cyclopropane ring 5j—1 have been studied and a mechanism has been advanced to explain the regio- and stereoselectivities of the reactions.37 In most cases, the product resulting from cleavage of the less-substituted (sterically less encumbered) carbon-carbon bond is obtained. The [5 + 2]-reaction is stereospecific in that a /ram-rclationship of the substituents on the cyclopropane leads to a m-relationship of the substituents in the product and vice versa (Equations (4) and (5)). For some tethered alkyne-VCPs which contain a functional group that weakens the carbon-carbon bond of the cyclopropane system, the more substituted (weaker) carbon-carbon bond can be cleaved selectively depending on the choice of catalyst. Thus far, the rhodium(l)-catalysts are more selective catalysts than the mthenium(0)-catalysts in the [5 + 2]-reaction of these substituted alkyne-VCPs (Scheme 7).38... 

The first example of an enantioselective [5 + 2]-cycloaddition was reported for the tethered alkene-VCP 7a, which upon treatment with a chiral rhodium complex afforded the m-fused bicyclo[5.3.0]decene 8a in 80% yield and 63% enantiomeric excess (ee) (Equation (6)).39 A later study found that when a 2,2-bis(diphenyl-phosphanyl)-l,l-binaphthyl (BINAP)-modified rhodium(l) catalyst is used, good to excellent ee s and yields are achieved with a variety of substrates (Equation (7)).40... 

Alkenylstannanes are usually reported to be resistant to hydrogenation with typical metal catalysts, though a rhodium(l) catalyst has been used at 100atm to reduce 3-hydroxy-l-alkenylstannanes.220 Diimine has been used to reduce some acyclic and cyclic methylstannylalkene compounds, but lower yields were obtained with tributylstan-nylalkenes (Equation (72)).221,198... 

Figure 6.46 Carbene ligands employed in rhodium(l) catalyst hydroformylation reactions.

The Application of DuPHOS Rhodium(l) Catalysts for Commercial Scale Asymmetric Hydrogenation... 

DuPHOS Rhodium(l) Catalysts A Comparison of COD Versus NBD Precatalysts Under Conditions of Industrial Application... 

Since the above methodology provides easy access to a variety of a-amino acid derivatives, many applications for the synthesis of natural products have been reported [23-25]. The HWE reaction of the stericaUy hindered aldehyde 81 with phosphonate 82 using TMG (3) proceeded to give (Z)-enamide 83 in 80% yield from the alcohol (2-step yield) [26]. The resulting enamide 83 was submitted to the asymmetric hydrogenation reaction using Burk s rhodium(l) catalyst [27] to give 84 in 85% yield as the sole product (Scheme 7.17). The a-amino acid ester 84 was successfully converted to neodysiherbaine A (85). 

Figure 8.2 The chiral(phosphine)rhodium(l) catalyst used for asymmetric catalytic hydrogenation of an amino acid precursor.

Full accounts have appeared concerning the asymmetric reduction of ketones " and keto-esters " through hydrosilylation employing a rhodium(l) catalyst possessing chiral phosphine ligands. 

Figure 7 7.13 Enantioselective reduction of labeled/unlabeled N-acyl A -amino acid deriva-or H2 at 1-5 atm mediated by chiral rhodium(l) catalysts...

Tethering of analogues of homogeneous rhodium(l) catalysts can be achieved by either of the two routes indicated in Fig. 10 [101]. In principle, each method involves competition reactions. If the surface is prefunctionalised then the complex has both the hydroxylated surface and the pendant ligand available for reaction. (The concentration of the former may be reduced by subsequent silylation with, e.g., Me3SiCl). When the reverse sequence is adopted then the precursor complex may react with the surface either at the silicon centre or at the metal coordination sphere. The chemistry of such systems however... 

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