Rhodium-catalyzed synthesis

As mentioned in the previous section, the carbonylation of methanol to acetic acid is an important industrial process. Whereas the [Co2(CO)s]-catalyzed, iodide-promoted reaction developed by BASF requires pressures of the order of 50 MPa, the Monsanto rhodium-catalyzed synthesis, which is also iodide promoted and which was discovered by Roth and co-workers, can be operated even at normal pressure, though somewhat higher pressures are used in the production units.4,1-413 The rhodium-catalyzed process gives a methanol conversion to acetic acid of 99%, against 90% for the cobalt reaction. The mechanism of the Monsanto process has been studied by Forster.414 The anionic complex m-[RhI2(CO)2]- (95) initiates the catalytic cycle, which is shown in Scheme 26. 

Takahashi [31]. To get good yields, nitrobenzene was added as a hydrogen acceptor (Scheme 1.11a). But on running the carbonylation of azobenzene via cobalt catalysis, quinazoUne was obtained as the terminal product Furthermore, Chatani and coworkers described a rhodium-catalyzed synthesis of maleimides starting from a combination of alkynes and pyridine-2-yhnethylamine in the presence of CO (Scheme 1.11b) [53]. 

Park and coworkers developed a rhodium-catalyzed synthesis of pyri-dines from 2i/-azirines and carbenoids [72]. In the presence of catalytic amount of [Rh2(esp)2], highly substituted pyridines were produced in good to excellent yields (Scheme 3.34). This procedure was applied by Liang and CO workers in the synthesis of CFs-containing pyridines recently [73]. 

Joh, T. Nagata, H. Takahashi, S. (1994) Rhodium-catalyzed synthesis of 2 5H)-furanones from terminal alkynes and non-substituted alkynes under water-gas shift reaction conditions, Inorg. Chim. Acta, 220,45-53. 

Scheme 7.1 Rhodium-catalyzed synthesis of an atropoisomeric diphosphine.

Scheme 7.12 Rhodium-catalyzed synthesis of tricychc angular 5/5/6 scaffolds.

Scheme 7.15 Rhodium-catalyzed synthesis of a chiral bicyclic cyclohexadiene.

SCHEME 2.108 Rhodium-catalyzed synthesis of Z-vinyl acetates from enamides [ 155],... 

SCHEME 3.10 Rhodium-catalyzed synthesis of cyclic sulfonamides [9]. 

The rhodium-catalyzed synthesis of 2-picolylamines and imidazo[l,5-a]pyridines from pyridotriazoles has been achieved (Scheme 3.16) [16], For the preparation of 2-picolylamines (Example 3.4), the reaction was operationally simple and consisted of stirring a dinuclear rhodium catalyst with the two reagents at 120 °C. This approach tolerated a broad spectrum of preexisting functional groups, and even 3(2-/0-pyridazinone was successfully used. For the preparation of the imidazo[l,5-fl]pyridines, a two-stage approach was used for the preparation of the heterocycles that included an initial N-H insertion followed by cyclization. 

SCHEME 3.141 Rhodium-catalyzed synthesis of azomethine yUdes and their use in multicomponent reactions [150]. 

SCHEME 5.16 Rhodium-catalyzed synthesis of allylic sulfones [17]. 

Carbery et al. reported the novel design and rhodium-catalyzed synthesis of a heli-cenoidal DMAP analog bearing a 4-dialkylaminopyridine unit and a helical scaffold (Scheme 10.9) [11]. This compound could be employed successfully as a chiral Lewis base organocatalyst in the kinetic resolution of secondary alcohols with carboxylic acid anhydrides (Scheme 10.10) [11]. 

SCHEME 17.22 Rhodium-catalyzed synthesis of cyclohexe-nones from cyclopropyl propargyl esters. 

Since 1960, the Hquid-phase oxidation of ethylene has been the process of choice for the manufacture of acetaldehyde. There is, however, stiU some commercial production by the partial oxidation of ethyl alcohol and hydration of acetylene. The economics of the various processes are strongly dependent on the prices of the feedstocks. Acetaldehyde is also formed as a coproduct in the high temperature oxidation of butane. A more recently developed rhodium catalyzed process produces acetaldehyde from synthesis gas as a coproduct with ethyl alcohol and acetic acid (83—94). 

From Synthesis Gas. A rhodium-catalyzed process capable of converting synthesis gas directly into acetaldehyde in a single step has been reported (83,84). 

Homogeneous rhodium-catalyzed hydroformylation (135,136) of propene to -butyraldehyde (qv) was commercialized in 1976. -Butyraldehyde is a key intermediate in the synthesis of 2-ethyIhexanol, an important plasticizer alcohol. Hydroformylation is carried out at <2 MPa (<290 psi) at 100°C. A large excess of triphenyl phosphine contributes to catalyst life and high selectivity for -butyraldehyde (>10 1) yielding few side products (137). Normally, product separation from the catalyst [Rh(P(C2H2)3)3(CO)H] [17185-29-4] is achieved by distillation. 

Abstract Recent advances in synthetic aspects of the rhodium-catalyzed hydroformylation of alkenes are reviewed. Emphasis is given to practical improvements, efficient new catalysts for regioselective and enantioselective hydroformylation, and to applications of the reaction in organic synthesis. Furthermore, new developments in directed hydroformylation are covered as well as new approaches toward efficient hydroformylation catalysts employing the concept of self-assembly. 

Hydroamination of olefins under most catalytic conditions proceed with Markovnikov addition of the N-H bond across the olefin. Shown below is a rhodium-catalyzed intramolecular, anti-Markovnikov, hydroamination developed for the synthesis of 3-arylpiperidines 167 <06JA6042>. Further evaluation of this reaction as a synthesis of multisubstituted piperidines revealed that substrates with substituents a or y to the amino group did not produce the expected piperidine, however, substrates with a substituent (1 to the amino group produce piperidines in high yield. 

Another important reaction principle in modem organic synthesis is carbon-hydrogen bond activation [159]. Bergman, Ellman, and coworkers have introduced a protocol that allows otherwise extremely sluggish inter- and intramolecular rhodium-catalyzed C-H bond activation to occur efficiently under microwave heating conditions. In their investigations, these authors found that heating of alkene-tethered benzimidazoles in a mixture of 1,2-dichlorobenzene and acetone in the presence of di-//-... 

A rhodium-catalyzed intramolecular C-H functionalization has been employed for the synthesis of bicyclic imidazoles. The alkene acts as an anchor to the metal, directing the C-H functionalization process, which involves the formation of an Rh(l) carbene intermediate (Equation (118)).107... 

The signature application for the G-H insertion in synthesis is probably the total synthesis of (—)-tetrodotoxin 126 by Du Bois and Hinman.233 Two stereospecific G-H activation steps, rhodium-catalyzed carbene G-H insertion and carbamate-based nitrene C-H insertion, have been used to install the two tetrasubstituted centers C6 and C8a (Scheme 12). Diazoketone 122 was treated with 1.5mol% Rh2(HNCOCPh3)4, and cyclic ketone 123 was selectively formed in high yield without purification. The reaction of carbamate 124 with 10mol% Rh2(HNCOCF3)4, PhI(OAc)4, and MgO in C6H6 solvent furnished the insertion product 125 in 77% yield. 

Similarly, ketimines (benzylimines of aromatic ketones) undergo the rhodium-catalyzed ortho-alkenylation with alkynes to give or/ o-alkenylated aromatic ketones after hydrolysis.61 This method is applied to an efficient one-pot synthesis of isoquinoline derivatives by using aromatic ketones, benzylamine, and alkynes under Rh catalysis (Equation (55)). 

Grieco in the total synthesis of (—)-epothilone B 134 used a rhodium-catalyzed hydroboration as a key step in the synthesis of the macrocyclic ring (Figure 15).141 Completion of the synthesis of the C(3)-C(12) fragment was carried out using a rhodium-catalyzed hydroboration as the key step. 

In conclusion it is evident from the foregoing examples that transition metal-catalyzed hydroborations and hydroaluminations occupy an important role in organic synthesis. While rhodium-catalyzed hydroboration has been extensively developed, the hydroalumination is just starting to emerge as a useful reaction in organic synthesis. 

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