Rhodium, l

Halpern J, Okamoto T and Zakhariev A 1976 Mechanism of the chlorotris(triphenylphosphine)rhodium(l)-catalyzed hydrogenation of alkenes J. Mol. Catal. 2 65-9... 

Rhodium-catalysed addition (10) of hydridosilanes (Chapter 17) to a/3-unsaturated carbonyl compounds can be performed regioselectively, to afford either the product of 1,2-addition, or, perhaps more usefully, that of 1,4-addition, i.e. the corresponding silyl enol ether this latter process is an excellent method for the regiospecific generation of silyl enol ethers. Of all catalyst systems investigated, tris(triphenylphosphine)rhodium(l) chloride proved to be the best. 

Rhodium i s a hard metal with high reflectance and good resi stance to corrosion. Ruthenium has properties which are similar to those of rhodium.l" The C VD ofboth metals is similar. 

Recently a novel chiral ferrocene-based amidinato ligand and its rhodium complexes have been described. The chiral N,N -bis(ferrocenyl)-substituted formamidine (N,N -bis[(S)-2- (lR)-l-(diphenylphosphino)ethyl ferrocen-l-yl]for-mamidine was prepared from commercially available (IR)-l-(dimethylamino) ethyl ferrocene by a multistep procedure in an overall yield of 29%. Deprotonation of the ligand with -butyllithium followed by addition of [RhCl2(COD)2] as illustrated in Scheme 167 yielded the corresponding (formamidinato)rhodium(l)... 

A very extensive and detailed study of the cationic rhodium(i)-catalyzed isomerization of allylic alcohols demonstrated that mono- and disubstituted allylic alcohols can be efficiently isomerized to the corresponding carbonyl compounds through the corresponding enol compounds (Scheme 20).45 The isomerization using cationic rhodium(l)... 

Although the asymmetric isomerization of allylamines has been successfully accomplished by the use of a cationic rhodium(l)/BINAP complex, the corresponding reaction starting from allylic alcohols has had a limited success. In principle, the enantioselective isomerization of allylic alcohols to optically active aldehydes is more advantageous because of its high atom economy, which can eliminate the hydrolysis step of the corresponding enamines obtained by the isomerization of allylamines (Scheme 26). 

The enantioselective isomerization of allylic alcohols using cationic rhodium(l)/BINAP complex was reported.9,11 Although the enantioselectivities were lower than those achieved by the isomerization of the corresponding enamines, 3,3 -disubstituted allylic alcohols were isomerized to the corresponding aldehydes in moderate yield and enantioselectivity (Scheme 27). 

A similar desymmetrization of the dienyl ethers was accomplished using cationic rhodium(l)/BIPNOR complex. In this reaction, an oxygen-containing co-solvent was necessary, and the best result was obtained with a 3 1 mixture of toluene and 1,2-dimethoxyethane (glyme) (DME) (Equation (21)). 

Tandem hydroacylation-isomerization of 5-alkynals catalyzed by a cationic rhodium(l)/BINAP complex was applied to the short synthesis of dihydrojasmone (Scheme 49).88... 

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. 

In order to probe the mechanism, this transformation was conducted under molecular deuterium atmosphere with cationic rhodium(l) complex (Scheme 110). The final compound 440 showed the incorporation of two deuterium atoms in each double bond. This is in agreement with a heterolytic activation of D2. Two different pathways are proposed. The first one involves the formation of a rhodacycle 438 followed by reductive elimination. The second one consists of a deuteriorhodation/carborhodation sequence, affording the same intermediate 437. A vinylrhodium... 

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... 

Almost all stable carbenes behave as 2-electron-CT-donating ligands with a few exceptions. In particular, in almost all cases corresponding Rh(I) complexes were targeted due to the easy synthetic method. An exception is the cyclopropenylidene carbene, with an extremely acute carbene angle. In this case, a second equivalent of carbene squeezes into the rhodium center, eliminating a chloride anion, giving the cationic dicarbenic rhodium(l) complex [51] (Scheme 5). 

A series of rhodium-modified heterogeneous materials has been prepared [36a] using the silicon oxides Aerosil 200 and SBA-15 and dinuclear rhodium(l) siloxide precursors [ Rh( x-OSiMe3)(cod) 2], [ Rh( x-OSiMe3)(nbd) 2], [ Rh( x-OSiMe3)(tfb) 2]... 

The key step employed a rhodium(l)-catalysed allenic Pauson-Khand reaction to generate the tricyclic ring system 276 from the allenyne 275. However, all attempts to introduced the missing methyl group by a 1,4-addition protocol failed to provide the completed neodolastane framework. Allenyne 275 was synthesized from the enone 274 by a multistep procedure. The quaternary atom was constructed by sequential enolate alkylations. 

Rhodium(l)-Catalyzed Asymmetric Addition of Organometallic Reagents to Electron-Deficient Olefins... 

I 3 Rhodium(l)-CatalYzed Asymmetric Addition of OrganometaUic Reagents to Electron-Deficient Oiefns... 

Recent Advances in Rhodium(l)-Catalyzed Asymmetric Olefin Isomerization and Hydroacylation Reactions... 

Rhodium (l)-Cata yzed Asymmetric Isomerization of Olefins 83 Tab. 4.1 Rh(l)/PF-P(o-Tol)2-catalyzed isomerization of E-allylic alcohols. 

Rhodium(l)-Catalyzed Asymmetric Hydroacylation of Olefins and Alkynes with Aldehydes... 

Stereoselective Rhodium(l)-Catalyzed Hydroformylatlon and Silylformylation Reactions and their Application to Organic Synthesis... 

I 5 Stereoselective Rhodium(l)-Catalyzed Hydroformylation and Silylformylation Reactions... 

---