Rhodium directed alkylation

Oxidative addition and reductive elimination were studied in a range of solvents and different CH3I concentrations at three different temperatures using different kinetic techniques (IR and UV-vis). The rate of rhodium(iii)-alkyl formation shows a direct relationship of the pseudo-first-order rate constant on [CH3I], as illustrated in Figure 4. 

Scheme 62 A direct alkylation of quinoline A/-oxides catalyzed by rhodium.

Brookhart et a. [99] were the first to report the rhodium-catalyzed directed alkylation of arylketones with olefins (Scheme 19.68). 

Subsequent to Brookhart s findings, Jun el al. [101] developed a rhodium-catalyzed alkylation of a,P-unsaturated carbonyl compounds in the presence of diethylamine (Scheme 19.70). The proposed mechanism involves the in silu formation of an enamine, followed by C-H activation directed by this enam-ine, olefin hydrometallation, and reductive ehmination. In situ enamine hydrolysis regenerates the carbonyl group. The authors demonstrated that the formation of product 32 originates from the isomerization of initially formed compound 31 with the rhodium catalyst. 

Nakao, Hiyama, et al. [141] could also demonstrate that a Ni/P(Cyp)3 catalyst was highly effective for the direct alkylation of polyfluoroarenes with vinylarenes or 1,3-dienes, again occurring with the Markovnikov regioselectivity (Scheme 19.98). This method nicely complements the anti-Markovnikov rhodium-catalyzed process (Scheme 19.90) [132]. 

Van der Boom ME, Higgitt CL, Milstein D. Directly observed p-H elimination of unsaturated PCP-based rhodium(III)-alkyl complexes. Organometallics. 1999 18 2413-2419. 

Other functionalizations described include the direct alkylation of the title compound. For example, it was demonstrated that applying similar alkenes describe vide supra under rhodium catalysis leads to a direct hydroalkenylation, furnishing the corresponding 2-alkyl pyridine -oxides. Conversely, benzyl chlorides were effectively coupled to the title compound under similar conditions described for direct sp arylation (eq 30). ... 

Rhodium-catalyzed allylic alkylation provides an expeditious entry into a variety of useful synthons for asymmetric synthesis. For example, the application of this reaction to a range of enantiomerically enriched allylic carbonates with the sodium salt of methyl phenylsulfonylacetate provides products that represent important synthons for target-directed synthesis (Tab. 10.1) [17]. 

The ability to harness alkynes as effective precursors of reactive metal vinylidenes in catalysis depends on rapid alkyne-to-vinylidene interconversion [1]. This process has been studied experimentally and computationally for [MC1(PR3)2] (M = Rh, Ir, Scheme 9.1) [2]. Starting from the 7t-alkyne complex 1, oxidative addition is proposed to give a transient hydridoacetylide complex (3) vhich can undergo intramolecular 1,3-H-shift to provide a vinylidene complex (S). Main-group atoms presumably migrate via a similar mechanism. For iridium, intermediates of type 3 have been directly observed [3]. Section 9.3 describes the use of an alternate alkylative approach for the formation of rhodium vinylidene intermediates bearing two carbon-substituents (alkenylidenes). 

In intramolecular reactions, direct interaction with the thiophene ring is possible by appropriate positioning of the tether as shown for the 3-substituted thiophene (229), which resulted in alkylation to form (230) and (231) (equation 46).)6la In contrast, the rhodium(II) acetate catalyzed decomposition of the 2-... 

The regio- and stereo-selective rhodium-catalysed allylic alkylations of chelated enolates have been investigated.25 It has been found that the Rh-catalysed allylic alkylation is as efficient and versatile as the Pd-catalysed version. In reactions of chelated enolates with suitable protecting groups, high yields and selectivities were obtained, and the regioselectivity can be directed by the reaction parameters. 

The directing group promoted C-H activation reaction is applicable to sp C-H bonds adjacent to the nitrogen in alkylamines, as shown in Scheme 5. Alkylation occurred when reaction of 25 with CO and ethylene was conducted in the presence of Ru3(CO)12 as catalyst [11], On the other hand, the use of a rhodium complex as catalyst resulted in C-H carbonylation [12],... 

Direct palladium-catalyzed GG bond formation can be achieved between A -methy 1 imidazo 1 e and an aryl halide with selectivity for the more electron-rich C(5) position (Scheme 162) if C(5) is already substituted, then C(2) is preferred. iV-Methylbenzimidazole can be similarly arylated by various aryl iodides at the 2-position when catalyzed by copper(I) iodide to give excellent yields <1998BCJ467>. Rhodium can also catalyze direct C(2) arylation of N-alkyl benzimidazoles <20040L35>. 

Schatzschneider U, Barton JK. Bifunctional rhodium intercalator conjugates as mismatch-directing DNA alkylating agents. J. Am. Chem. Soc. 2004 126 8630-8631. 

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