Rh-catalyzed stereoselective

Scheme 11.11 Rh-catalyzed stereoselective synthesis of VCPs and their Et2AlCl-promoted rearrangement.

Morken has recently disclosed a Rh-catalyzed stereoselective enolization technique for the transformation of the conjugated ester 140 into the -silylketene acetal utilizing dichloromethylsilane as a reducing agent. Intermediate 141 undergoes the Ireland-Claisen rearrangement to yield 142. ... 

Rh-catalyzed stereoselective isomerization of tert-cyclobutanols into chiral indanoles 2.2.1 Overview... 

Figure 2.6 Computed catalytic cycle for Rh-Catalyzed Stereoselective Isomerizatioit of fert-Cyclobutanol tcb imder DFT/B3LYP/LANL2DZ(Rh)/6-31G (all other) level of fheoiy in the gas-phase. (Data from Yu, H. et al., Chem. Fur.., 20,3839-3848,2014.)...

A Rh-catalyzed Reformatsky reaction of chiral imine (24) led to the stereoselective preparation of the a,a-difluoro-jS-amino acid (25). 25 was converted to difluor-oalkene (26), and subsequently L-Val-i/r[(Z)CF=CH]Gly derivative (23) in greater than 82% for both steps. The samarium diiodide-mediated reductive transformation of the y,y-difluoro-a, S-enoates proceeded via successive two-electron transfers to form a dienolate species which upon kinetically controlled trapping with fert-BuOH formed 23 (Scheme 6). 

Hydroxymethylated cyclopropenes, which can be readily prepared by Rh-catalyzed reaction of diazoesters and alkynes, are good snbstrates for nncatalyzed and Cu-catalyzed carbomagnesiation. For example, a range of substitnted cyclopropanes 82 can be synthesized in a regio- and stereoselective fashion by the Cu-catalyzed addition of Grignard reagents to (3-hydroxymethyl)cyclopropenes 81 (Scheme 57) . 

A highly stereoselective kinetic resolution process for Rh-catalyzed enyne cycloisomerization has also been developed by Zhang et al. [41]. This transformation has enabled the highly enantioselective synthesis of polyfunctionalized tetrahydrofurans and lactones with two or three adjacent stereocenters and it is regarded as a major breakthrough in enynes cycloisomerization and in kinetic resolution (Scheme 7). 

On the Mechanism of Stereoselection in Rh-Catalyzed Asymmetric Hydrogenation A General Approach for Predicting the Sense of Enantioselectivity ... 

Comparing the sense of enantioselection obtained with P-chiral ligands generating a clearly defined asymmetric environment with that observed with ligands bearing backbone chirality, - one can conclude that the stereoselection in Rh-catalyzed asymmetric hydrogenation takes place in octahedral Rh(III) complexes. ... 

Gridnev, I. D. Imamoto, T. On the mechanism of stereoselection in Rh-catalyzed Asymmetric hydrogenation A general approach for predicting the sense of enantioselectivity. Acc. Chem. Res. 2004,37,633-644. 

Deoxygenation of Epoxides. Rh -catalyzed deoxygenation of epoxides with dimethyl diazomalonate was shown to occur stereoselectively. This reaction feature was used for efficient inversion of double bond configuration in azacycloundecene fragment in the total s)mthesis of manzamine C (eq 32). ... 

Scheme 42 Reverse stereoselectivity in the Rh-catalyzed intramolecular C—H ami-nation of sulfamate 42.

Tanaka et al. examined chiral Rh-catalyzed intramolecular cycloaddition using triynes with substituents at both ends and obtained [7] to [lOJmeto-cyclophanes with high enantioselectivity. This is the first example of catalytic and enantioselective synthesis of planar-chiral cyclophanes using the [2+ 2+ 2]-cycloaddition approach (Scheme 8.14) [11a]. The tether structure of the 1,6-diyene moiety of the triynes affected the stereoselectivity, and the reaction of ester-tethered triynes realized almost perfect enantioselectivity (Scheme 8.15) [11a]. 

Recently, a large number of novel transformations that involve rhodium (Rh)-catalyzed C-N bond-forming reactions have been developed for the construction of different Af-heterocyclic rings. Such transformations frequently occur with high stereoselectivity and a broad array of substrates under mild conditions. Selection of a suitable catalyst for a preferred substrate can seem intimidating. However, a rational solution to this problem can be delivered considering the possible mechanistic cycles for the individual transformations. 

The first example of Rh-catalyzed C-H coupling of 2-phenylpyridines with 7-azabenzonorbornadienes to provide czs-fused dihydrocarbazoles 39 was reported by Li and coworkers (Eq. (5.38)) [23]. The C-N bond forms via a double C-H activation, in which a seven-membered rhodacyclic complex is a key intermediate as a result of stereoselective insertion of the olefin into the Rh-C(aryl) bond. This strategy has also been employed for the synthesis of dihydrocarbazoles by using various directing groups. 

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