Rhodium -catalyzed reactions

Under catalysis by an Rh(l)OH complex, alkenyl- and arylsilanes add to aldehydes56,59 and internal alkynes.60 The addition to alkynes proceeds in a cA-mode, which is indicative of m-addition of an alkenyl- or arylrhodium intermediate (Equation (11)). 

Acylation of alkenylsilanes with acid anhydrides is efficiently catalyzed by [RhCl(CO)2]2-61 Based on the fact that stoichiometric reaction of an alkenylsilane with the Rh complex gives the corresponding alkenylrhodium species, the proposed mechanism involves alkenyl transfer from Si to Rh as the initial step. 

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Rhodium catalyzed reaction of A -butenyl-l,3-propanediamines 397 with a mixture of H2 and CO gave usually a mixture of hydroformylated 398 and 399 and carbonylated products 400 and 401 in the presence of a phosphite [PPha, PBu3, PCCgHiOa, P(o-tol)3] (97TL4315, 97T17449). When the hindered biphosphite, BIPEPHOS, and a 9 1 or 1 1 mixture of H2 and... 

Approximately 2.5 million tons of acetic acid is produced each year in the United States for a variety of purposes, including preparation of the vinyl acetate polymer used in paints and adhesives. About 20% of the acetic acid synthesized industrially is obtained by oxidation of acetaldehyde. Much of the remaining 80% is prepared by the rhodium-catalyzed reaction of methanol with carbon monoxide. 

When the rhodium-catalyzed reaction is performed under a high pressure of CO in the presence of phosphite ligands, aldehyde products (159) are formed by insertion of CO into the rhodium-alkyl bond followed by reductive elimination (Eq. 31) [90]. The bimetallic catalysts were immobilized as nanoparticles, giving the same products and functional group tolerance, with the advantage that the catalyst could be recovered and reused without loss of... 

The reaction of crotyl bromide with ethyl diazoacetate once again reveals distinct differences between rhodium and copper catalysis. Whereas with copper catalysts, the products 125 and 126, expected from a [2,3] and a [1,2] rearrangement of an intermediary halonium ylide, are obtained by analogy with the crotyl chloride reaction 152a), the latter product is absent in the rhodium-catalyzed reaction at or below room temperature. Only when the temperature is raised to ca. 40 °C, 126 is found as well, together with a substantial amount of bromoacetate 128. It was assured that only a minor part of 126 arose from [2,3] rearrangement of an ylide derived from 3-bromo-l-butene which is in equilibrium with the isomeric crotyl bromide at 40 °C. 

A small amount of formate esters (4%) is formed in the cobalt hydroformylation cycle (46). The amount is undetectable in the rhodium-catalyzed reaction. 

Fell and Bari (89) also studied the rhodium-catalyzed reaction. A rho-dium-N-methylpyrrolidine-water catalyst system was very effective for producing the propane-1,2-diol acetate directly. The best yields (>90%) of product of about 9 1 alcohol aldehyde ratio were obtained in the region of 95°-l 10°C. This range was very critical, as were other reaction parameters. Rhodium alone gave the best yield of aldehyde (83%) at 60°C. Triphenylphosphine as cocatalyst induced the decomposition of the aldehyde product. 

It has been recently reported that an organic pseudohalide can function as a promoter in the methanol carbonylation reaction. Webber et al. (21) have shown that pentachlorobenzenethiol will promote the rhodium-catalyzed reaction but at less than 5% of the rate of the iodide system. 

Ruthenium- and rhodium-catalyzed reactions lead to aryl ketones, and in some cases these processes are reversible (Equations (112) and (113)).103... 

However, the decarbonylation reaction can be suppressed by the use of specially tailored chelating groups. Intermolecular processes involving dienes and salicylaldehydes are now known, and are thought to proceed via a double chelation mechanism, akin to the Jun-type system. Rhodium-catalyzed reactions lead to hydroacylated products, under relatively mild conditions (Equation (134)).117... 

In addition to palladium catalysts, Co(OAc)2 shows a catalytic activity for the arylation of heterocycles, including thiazole, oxazole, imidazole, benzothiazole, benzoxazole, and benzimidazole.78 As shown in Scheme 6, the catalytic system Co(OAc)2/9/Cs2C03 gives G5 phenylated thiazole, while the bimetallic system Co(OAc)2/CuI/9/Cs2C03 furnishes the G2 phenylated thiazole. The rhodium-catalyzed reaction of heterocycles such as benzimidazoles, benzoxazole, dihydroquinazoline, and oxazoline provides the arylation product with the aid of [RhCl(coe)]2/PCy3 catalyst.79 The intermediacy of an isolable A-heterocyle carbene complex is proposed. 

The rhodium-catalyzed reaction of pyrrolidine with iodobenzene gives 2-phenylpyrroline in a high yield (Equation (72)). This reaction involves the formation of an imine rhodium hydride complex and phenylation (Equation (73)). 

Rhodium-catalyzed reactions of diynes and an isonitrile give rise to iminocyclopentadienes (Equation (68)).421 Portionwise addition of the isonitrile (5 x0.2equiv.) was found to increase the yield. The reaction may proceed through formation of metallacyclopentadienes followed by insertion of an isonitrile molecule. 

The first rhodium-catalyzed reductive cyclization of enynes was reported in I992.61,61a As demonstrated by the cyclization of 1,6-enyne 37a to vinylsilane 37b, the rhodium-catalyzed reaction is a hydrosilylative transformation and, hence, complements its palladium-catalyzed counterpart, which is a formal hydrogenative process mediated by silane. Following this seminal report, improved catalyst systems were developed enabling cyclization at progressively lower temperatures and shorter reaction times. For example, it was found that A-heterocyclic carbene complexes of rhodium catalyze the reaction at 40°C,62 and through the use of immobilized cobalt-rhodium bimetallic nanoparticle catalysts, the hydrosilylative cyclization proceeds at ambient temperature.6... 

Introduction of two different chalcogen elements to the C-C unsaturated bond is of particular interest from both synthetic and mechanistic viewpoints. Therefore, extensive studies have been carried out on the development of the (RE)2/(R E )2 binary system without using RE-E R compounds, which are difficult to prepare. (Z)-l-Seleno-2-thio-1-alkenes are produced regio- and stereoselectively when a mixture of diaryl disulfides and diaryl diselenides is subjected to a rhodium-catalyzed reaction with alkynes (Equation (68)).193... 

The thioselenation is achieved also by a radical mechanism under photo-irradiation. The adduct has the regio-chemistry opposite to that observed in the rhodium-catalyzed reaction (Scheme 39).194 196... 

From the results presented here, one could get the impression that such allenes with hydroxyl groups in the substituents will always form heterocydes in the presence of transition metal catalysts, but in the presence of other substrates even allenylcarbinols can react to give different products. Examples are the rhodium-catalyzed reaction of allenylcarbinol 78 and phenylacetylene 79 to 80 [42], the palladium-catalyzed reaction of 81 and pyrrolidine 82 to 83 [43] and the ruthenium-catalyzed reaction of 78 and 79 to 84, an isomer of the rhodium-catalyzed reaction of the same substrates mentioned above [44] (Scheme 15.19). 

For adoption by the synthetic-organic community, a new method must pass certain tests. If it is catalytic, then the turnover must be efficient so that the quantity of (presumably expensive) catalyst employed is small. The reaction needs to be selective in all the desirable ways, where appropriate including chemo-, regio-, and stereospecificity. Much of the work on hydroboration has been aimed at progress toward those goals. The rhodium-catalyzed reaction of catecholborane with a simple styrene frequently forms part of the standard catalytic screening procedures for a novel ligand. 

Scheme 3.17 Rhodium-catalyzed reaction of arylsilane 46 with acrylate 45 [33].

Rhodium-catalyzed asymmetric conjugate addition has enjoyed uninterrupted prosperity since the first report by Hayashi and Miyaura [6]. Its high enantioselectivity and wide applicability are truly remarkable. However, some problems still remain, since the carbon atoms that can be successfully introduced by this rhodium-catalyzed reaction have been limited to sp carbons and the substrates employed have been limited mostly to the electron-deficient olefins free from sterically bulky substituents at a- and / -positions. These issues will be the subject of increasing attention in the future. 

In sharp contrast to the unique pattern for the incorporation of carbon monoxide into the 1,6-diyne 63, aldehyde 77 was obtained as the sole product in the rhodium-catalyzed reaction of 1,6-enyne 76 with a molar equivalent of Me2PhSiH under CO (Scheme 6.15, mode 1) [22]. This result can be explained by the stepwise insertion of the acetylenic and vinylic moieties into the Rh-Si bond, the formyl group being generated by the reductive elimination to afford 77. The fact that a formyl group can be introduced to the ole-finic moiety of 76 under mild conditions should be stressed, since enoxysilanes are isolated in the rhodium-catalyzed silylformylation of simple alkenes under forcing conditions. The 1,6-enyne 76 is used as a typical model for Pauson-Khand reactions (Scheme 6.15, mode 2) [23], whereas formation of the corresponding product was completely suppressed in the presence of a hydrosilane. The selective formation of 79 in the absence of CO (Scheme 6.15, mode 3) supports the stepwise insertion of the acetylenic and olefmic moieties in the same molecules into the Rh-Si bond. 

Tab. 8.1 summarizes the various substrates that were subjected to the rhodium-catalyzed reaction using a Rh-dppb catalyst system. Only ds-alkenes were cycloisomerized under these conditions, because the trans-alkenes simply did not react. Moreover, the formation of the y-butyrolactone (Tab. 8.1, entry 8) is significant, because the corresponding palladium-, ruthenium-, and titanium-catalyzed Alder-ene versions of this reaction have not been reported. In each of the precursors shown in Tab. 8.1 (excluding entry 7), a methyl group is attached to the alkene. This leads to cycloisomerization products possessing a terminal alkene, thus avoiding any stereochemical issues. Also,... 

The rhodium-catalyzed PK reaction has many unique features as compared to the original cobalt-mediated version. For example, most of the rhodium-catalyzed reactions could be carried out under only 1 atm CO. Initially, these reactions were expected to proceed more effectively under higher CO pressures. However, experimental results demonstrated that the reaction was retarded by elevated CO pressure (>1 atm.), while the reaction at a lower CO pressure (0.1 atm.) proved to be superior [13b]. For example, the reaction of la with [RhCl(CO)2]2 under a high pressure of CO (3 atm.) afforded 2a in 70% yield, but was not complete after 36 h. The analogous reaction under a mixed CO/Ar atmosphere (1 atm. CO/Ar=l 9) was complete within an hour to afford 2a in 90% yield. 

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. 

Numerous studies have been directed toward expanding the chemistry of the donor/ac-ceptor-substituted carbenoids to reactions that form new carbon-heteroatom bonds. It is well established that traditional carbenoids will react with heteroatoms to form ylide intermediates [5]. Similar reactions are possible in the rhodium-catalyzed reactions of methyl phenyldiazoacetate (Scheme 14.20). Several examples of O-H insertions to form ethers 158 [109, 110] and S-H insertions to form thioethers 159 [111] have been reported, while reactions with aldehydes and imines lead to the stereoselective formation of epoxides 160 [112, 113] and aziridines 161 [113]. The use of chiral catalysts and pantolactone as a chiral auxiliary has been explored in many of these reactions but overall the results have been rather moderate. Presumably after ylide formation, the rhodium complex disengages before product formation, causing degradation of any initial asymmetric induction. 

Cyclohexadiene derivatives are less reactive than butadiene derivatives, thus only a few examples of cycloadditions with these compoimds are known (Figure 4.3) [37 0]. The cyclohexadiene bicychc derivative 32 was synthesized by rhodium-catalyzed reaction of toluene with tert-butyldiazoacetate and cycloadds in about 40% yield to Cjq [39]. The product has anti-cyclopropane orientation relative to the entering dienophile Cjq. Valence isomerization of 33 (Scheme 4.4) leads to the cyclobutene-fused cyclohexene 35 that adds in good yields (50%) at moderate temperatures (110 °C) to Cjq [40]. The reaction of with the electron-deficient cyclohexene 34 is also possible in moderate yields [38]. 

The synthesis of thiiranes with subsequent elimination of sulfur is an important procedure for the creation of C=C bonds, especially for sterically crowded systems (47,48), in analogy to the Eschenmoser-sulfide-contraction reaction (116). The spontaneous elimination of sulfur was observed in the rhodium-catalyzed reaction of diazo compound 62, which gave rise to the formation of cyclopentenone derivative 63 (117) (Scheme 5.24). A synthesis of indolizomycin was published by Danishefsky and co-workers (118) and involved a similar annulation step. In this case, however, the desulfurization reaction was achieved by treatment with Raney Ni. 

When diallyl diazomalonate or allyl aryldiazoacetates are used as substrates in the rhodium-catalyzed reaction with di(tert-butyl)thioketene, 4-allyl-l,3-oxathiolan-5-one derivatives of type 158 are formed (82). After the initial 1,5-dipolar electro-cyclization, 157a undergoes a subsequent Claisen rearrangement to give the thermodynamically more stable compound 158 (Scheme 5.47). 

The first example of the [3 + 2] cycloaddition of ketocarbenes with a phosphorus-carbon triple bond has also been reported. Thus, the rhodium-catalyzed reaction of 2-diazocyclohexane-l,3-diones with tert-butylphosphaethyne gave 1,3-oxaphosphole 320 (377) (Scheme 8.80) in moderate yield. 

Padwa et al. (38) also explored the rhodium-catalyzed reaction of diazo imides to form isomtinchnones (Scheme 10.10). Thus, 70 smoothly forms isomtinchnones 71 that can be intercepted in high yield with DM AD to give furans 73, following loss of methyl isocyanate from the cycloadducts 72. 

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