Rhodium alkoxides

Although detailed mechanistic studies are not reported, the postulated mechanism for the reductive cyclization of allenic carbonyl compounds involves entry into the catalytic cycle via silane oxidative addition. Allene silylrhodation then provides the cr-allylrhodium hydride A-18, which upon carbometallation of the appendant aldehyde gives rise to rhodium alkoxide B-14. Oxygen-hydrogen reductive elimination furnishes the hydrosilylation-cyclization product... 

It seems logical that these reactions are mechanistically related. In every case reported to date, the relative stereochemistry between the nucleophile and alcohol moiety is trans, indicating anti addition in the ring-opening event. Lautens and coworkers suggest a syn insertion of the rhodium to form the allyl rhodium alkoxide. Anti addition of the soft nucleophile, with possible initial protonation of the... 

The current working model for the beneficial role of protic and halide additives in reactions with aliphatic amines is outlined in Scheme 9.10 [11]. In these reactions, the rhodium dihalide dimer 30 is proposed to enter two different catalytic cycles. In the productive catalytic cycle, the dimer is cleaved by solvation or binding with the substrate to give 31. Oxidative insertion followed by pro to nation of the rhodium alkoxide... 

A few years earlier, Herrmann et al. published a carboxylic ester functionalised imi-dazolium salt that was synthesised directly from imidazole and bromoacetic acid ethyl ester [216]. Owing to its method of synthesis the imidazolium salt is C -symmetric with two ester functional wingUp groups. Generation of the rhodium(I) and palladium(II) carbene complexes was realised by reaction of the imidazolium salt with a rhodium alkoxide precursor or with palladium(II) acetate in the presence of NaOEt and Nal (see Figure 3.76). The silver(I) oxide method had not been discussed in the literature at the time [11]. 

Krug and Hartwig recently reported that arylrhodium(I) complex 317 imderwent insertion of benzaldehyde into the aryl-Rh bond to give a rhodium alkoxide intermediate, which led to the formation of either ketone 318 or diarylmethanol 319, depending on the reaction conditions. Ketone 318 was formed under nonaqueous conditions, whereas diarylmethanol 319 was formed under aqueous conditions. This type of insertion reaction is limited to aryl aldehydes. 

This concept was extended to an asymmetric conjugate aUcynylation of enones [59]. In this case, -alkynyl ehmination from the rhodium-alkoxide complex 101 (Scheme 22), delivers not only alkynyl-rhodium species 103 but simultaneously also reveals the reacting substrate, a, -unsaturated ketones 102. Subsequent conjugate addition gives rise to -alkynylketones 105 in high yields and enantioselec-tivities. This on-demand release and generation of the alkynyl-rhodium and the enone substrate bypasses the common dimerization issues associated with the formation of alkynyl-rhodium species from terminal alkynes. 

Alkoxide-chloride exchange followed by retro-allylation would initially provide o-crotylrhodium, which would be in fast equilibrium with it-crotylrhodium (Scheme 5.38). The crotylrhodium would react with benzaldehyde to yield rhodium alkoxide 30. With trimethylphosphine as a ligand, protonation of 30 would proceed smoothly to yield the alcohol product. When the ligand is bulky, the protonation of 30 would be so slow that P-hydride elimination would prevail. The subsequent iterative hydrorhodation/p-elimination would generate oxa-Jt-allylrhodium, which ends up with smooth protonation to yield the saturated ketone. 

Rhodium-catalyzed allyl transfer to a,P-unsaturated ester proceeds in a conjugate manner (Scheme 5.39) [27]. Initially formed rhodium enolate would undergo P-hydride elimination followed by isomerization to afford a,p,y,5-unsaturated ester. 2-tert-Butylphenol is essential for smooth protonation to regenerate catalytically active rhodium alkoxide. 

Rhodium catalysts have also been used. Benzylic halides were converted to carboxylic esters with CO in the presence of a rhodium complex. In this case, the R could come from an ether R20, a borate ester B(OR )3, or an Al, Ti, or Zr alkoxide. Reaction with an a,co-diiodide, BU4NF and Mo(CO)e gave the corresponding lactone. ... 

The anionic Rh(I) porphyrin [Rh(OEP) induced ring-opening reactions with 4- and 5-membered ring lactones to give organometallic products with the rhodium bonded to the alkoxide carbon rather than the carbonyl carbon. 

A similar type of immobilization was obtained by reacting the phosphonylated 2,2 -bipyridine ligand depicted in Figure 42.10 with excess titanium alkoxide. Rhodium and iridium complexes of this immobilized ligand showed activity for... 

For rhodium and iridium compounds alkoxo ligands take over the role of the basic anion. Using /z-alkoxo complexes of ( -cod)rhodium(I) and iridium(I)— formed in situ by adding the /r-chloro bridged analogues to a solution of sodium alkoxide in the corresponding alcohol and azolium salts—leads to the desired NHC complexes even at room temperature [Eq. (10)]. Using imidazolium ethoxyl-ates with [(r " -cod)RhCl]2 provides an alternative way to the same complexes. By this method, it is also possible to prepare benzimidazolin-2-ylidene complexes of rhodium(I). Furthermore, an extension to triazolium and tetrazolium salts was shown to be possible. ... 

The syn addition of the adducts suggests a mechanism different from that observed in rhodium-catalyzed alcoholysis and aminolysis reactions. Mechanistic investigations from the Tautens laboratory have revealed that the most likely mechanism involves an enantioselective carbopalladation followed by a (3-alkoxide elimination to afford the ring-opened product." ... 

Once 26 or 27 has been formed, the rhodium-aUcoxide complex is protonated by a nucdeophile molecule, generating the cationic rhodium complex 29 and an alkoxide or phenoxide nucdeophile. This proton transfer step is supported by kinetics experiments and has two effects [14]. Firstly, the organorhodium species is made more electrophilic as a result of the positive charge, and secondly, the nucleophile is rendered more nu-cdeophihc by becoming deprotonated. 

Tab. 10.8 summarizes the application of rhodium-catalyzed allylic etherification to a variety of racemic secondary allylic carbonates, using the copper(I) alkoxide derived from 2,4-dimethyl-3-pentanol vide intro). Although the allyhc etherification is tolerant of linear alkyl substituents (entries 1-4), branched derivatives proved more challenging in terms of selectivity and turnover, the y-position being the first point at which branching does not appear to interfere with the substitution (entry 5). The allylic etherification also proved feasible for hydroxymethyl, alkene, and aryl substituents, albeit with lower selectivity (entries 6-9). This transformation is remarkably tolerant, given that the classical alkylation of a hindered metal alkoxide with a secondary alkyl halide would undoubtedly lead to elimination. Hence, regioselective rhodium-catalyzed allylic etherification with a secondary copper(l) alkoxide provides an important method for the synthesis of allylic ethers. 

The mechanism operating in rhodium-catalyzed and iridium-catalyzed hydrogen transfer reactions involves metal hydrides as key intermediates. Complexes such as [ M(p.-C1)(L2) 2], [M(cod)L2](Bp4) (M = Rh, Ir L2 = dppp, bipy), and [RhCl(PPh3)3] are most likely to follow the well-established mechanism [44] via a metal alkoxide intermediate and elimination to generate the active hydride species, as shown in Scheme 2. 

Step 4 Conformational bias minimizes interactions with the indicated Ar-H and sets the stage for diastereofacial differentiation in the directed hydrogenation of the double bond. Base-promoted attachment of the alkoxide to rhodium gives the product with high diastereoselectivity. 

HYDROGENATION CATALYSTS Aretie-chnomium tricarbonyls. (1,5-Cycloocta-diene)(pyridine)(tricyclohexyl-phosphine)iridium(I) hexafluorophosphate. Di-fi.-chlorobis( 1,5-hexadlene)dirhodium. Lindlar catalyst. Palladuim(II) acetate-So-dium hydride-/-Amyl alkoxide. Rhodium catalysts. 

---