Rhodium sequential reactions

Based on previous success in the Pauson-Khand reaction [43], Evans demonstrated a sequential approach to the synthesis of eight-membered rings, which involved a rhodium-catalyzed aUyhc amination reaction followed by carbocyclization, to effect a three-component couphng (Scheme 12.11). To date, this transformation is only the second example of a sequential rhodium-catalyzed reaction in which only temperature is used to modulate catalytic activity. 

The reduction of acetaldehyde to ethanol could be explained by its chemisorption near rhodium particles and the action of spill over hydrogen. On Rh/La 0, Bell has observed that at low residence times acetaldehyde is the primary product whereas at longer residence times the formation of ethanol becomes the dominant process. They concluded that this pattern is characteristic of the sequential reaction process ... 

RhClCO(dppp) 2] for the sequential construction of an enyne precursor, starting from a malonic acid derivative and allylic acetate, which was converted in situ to the cycloaddition product with excellent yields. Obviously, the Pd complex catalyzes the allylic substitution reaction, while the rhodium catalyst is responsible for the PKR (Eq. 6). 

The catalyst used is typically platinum, palladium, rhodium, or ruthenium, or sometimes an appropriate derivative. Precise details of the reaction remain vague, but we believe the catalyst surface binds to both the substrate, e.g. an alkene, and hydrogen, weakening or breaking the jr bond of the alkene and the a bond of hydrogen. Sequential addition of hydrogen atoms to the alkene carbons then occurs and generates the alkane, which is then released from the surface. 

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. 

The regio- and diastereoselective rhodium-catalyzed sequential process, involving allylic alkylation of a stabilized carbon or heteroatom nucleophile 51, followed by a PK reaction, utilizing a single catalyst was also described (Scheme 11.14). Alkylation of an allylic carbonate 53 was accomplished in a regioselective manner at 30 °C using a j-acidic rhodium(I) catalyst under 1 atm CO. The resulting product 54 was then subjected in situ to an elevated reaction temperature to facilitate the PK transformation. 

Disappointingly, the trimethylphosphite-modified Wilkinson catalyst, which had proven effective for the allylic substitution reaction [30], furnished only a trace amount of the PK product. By screening various rhodium catalysts for both reactions, it was determined that [RhCl(CO)dppp]2 was the optimum complex for the sequential pro-... 

Evans and co-workers simultaneously developed of the rhodium-catalyzed [4-+2 + 2] car-bocychzation reaction through the identification of a novel mode of reactivity for the five-membered metallacycle intermediate [14, 37]. Evans proposed that 1,3 dienes should be amenable to addition to this intermediate, which would provide a new reaction pathway. The tethered enyne 80 (Scheme 12.9), was expected to afford the metallacycle I, which, upon sequential incorporation of a butadiene and reductive ehmination, would afford the desired 5,8-fused product 81. Additionally it was rationahzed that this transformation should be highly diastereoselective, as outlined in Scheme 12.10. Treatment of the heteroatom-tethered enyne I under standard carbocyclization (Pauson-... 

Rhodium carbonyl complexes also catalyze the cascade cyclization/hydrosilylation of 6-dodecene-l,l 1-diynes to form silylated tethered 2,2 -dimethylenebicyclopentanes. For example, reaction of ( )-85 with dimethylphenylsilane catalyzed by Rh(acac)(CO)2 in toluene at 50 °G under GO (1 atm) gave 86a in 55% yield as a single diastereomer (Equation (56)). Rhodium-catalyzed caseade cyclization/hydrosilylation of enediynes was stereospecific, and reaction of (Z)-85 under the conditions noted above gave 86b in 50% yield as a single diastereomer (Equation (57)). Rhodium(i)-catalyzed cascade cyclization/hydrosilylation of 6-dodecene-1,11-diynes was proposed to occur via silyl-metallation of one of the terminal G=G bonds of the enediyne with a silyl-Rh(iii) hydride complex, followed by two sequential intramolecular carbometallations and G-H reductive elimination. ... 

To a 10-mL round-bottomed flask fitted with a nitrogen balloon was added sulfide catalyst (0.2 equiv), anhydrous dioxane (4.0 mL), rhodium(II) acetate dimer (2 mg, 0.01 equiv), substrate (0.5 mmol), benzyl triethylammonium chloride (23 mg, 0.2 equiv), and tosylhydrazone sodium salt (1.5 equiv). The reaction mixture was stirred vigorously at 40 °C for 48 h. Work-up consisted of the sequential addition to the reaction mixture of water (5 mL) and ethyl acetate (5 mL). The aqueous layer was washed with ethyl acetate (2.5 mL) and the combined organic phases dried (MgSC ), filtered, and concentrated in vacuo. The crude products were analyzed by aH NMR to determine the diastereomeric ratio, and then purified by FC to afford the corresponding cyclopropane. 

A few homometallic HNCC have also been produced by the condensation reactions of anionic carbonyl clusters with cationic complexes. For instance, sequential buildup of rhodium HNCC via incorporation of Rh(CO)2 fragments has been achieved by reacting [Rh(CO)2(MeCN)2] in acetonitrile with a series of anionic rhodium clusters as illustrated by the reactions shown in Eqs. (11) and (12) (223). 

An alternative to sequential reductive elimination and hydrolysis of acetyl iodide is direct reaction of water with a rhodium acetyl complex to give acetic acid. The relative importance of these two alternative pathways has not yet been fully determined, although the catalytic mechanism is normally depicted as proceeding via reductive elimination of acetyl iodide from the rhodium center. 

There are two reports of an alkyl to aryl rhodium migration process. The first example was reported in 2000 by Miura [75], It was discovered that, upon reaction of phenyl boronic acid with norbomene under rhodium-catalyzed conditions, a merry-go-around type sequential alkylation occurred up to four times on the aromatic ring, resulting in a 1,2,3,4-tetranorbomylated benzene as the final product (Scheme 16). Mechanistically, this reaction involves an alkyl to aryl migration of rhodium. Thus, aryl rhodium intermediate 29, generated via an initial transmetalation step,... 

Finally, recent work has examined in detail the kinetics of the fundamental B-H oxidative addition step which leads to the formation of rhodium(III) [and ruthenium(II)] boryl hydrides. Conversion of fac-(triphos)Rh(H)3 into fac-(triphos)Rh(H)2(Bpin) via sequential H2 reductive elimination/HBpin oxidative addition was induced by laser flash photolysis and kinetic data determined from UV measurements. Thus, an extremely high second-order rate constant was determined for the reaction of the 16-electron intermediate... 

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