Rhodium-catalyzed synthesis functionalization

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. 

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... 

When (R,R)-Me-Duphos, (R,R,R,R)-BICPO [9] or BINAP [10] is used, the rhodium-catalyzed asymmetric cycloisomerization of 3 affords 4 with up to >99.5% enantiomeric excess (Scheme 7 2). This methodology was applied to the synthesis of functionalized a-methylene-y-butyrolactone derivatives 6 such as (-i-)-pilocarpine 7 (Scheme 7.3) [11]. 

Shibata and co-workers have reported an effective protocol for the cyclization/hydrosilylation of functionalized eneallenes catalyzed by mononuclear rhodium carbonyl complexes.For example, reaction of tosylamide 13 (X = NTs, R = Me) with triethoxysilane catalyzed by Rh(acac)(GO)2 in toluene at 60 °G gave protected pyrrolidine 14 in 82% yield with >20 1 diastereoselectivity and with exclusive delivery of the silane to the G=G bond of the eneallene (Equation (10)). Whereas trimethoxysilane gave results comparable to those obtained with triethoxysilane, employment of dimethylphenylsilane or a trialkylsilane led to significantly diminished yields of 14. Although effective rhodium-catalyzed cyclization/hydrosilylation was restricted to eneallenes that possessed terminal disubstitution of the allene moiety, the protocol tolerated both alkyl and aryl substitution on the terminal alkyne carbon atom and was applicable to the synthesis of cyclopentanes, pyrrolidines, and tetrahydrofurans (Equation (10)). 

The group of Van Leeuwen has reported the synthesis of a series of functionalized diphenylphosphines using carbosilane dendrimers as supports. These were applied as ligands for palladium-catalyzed allylic substitution and amination, as well as for rhodium-catalyzed hydroformylation reactions [20,21,44,45]. Carbosilane dendrimers containing two and three carbon atoms between the silicon branching points were used as models in order to investigate the effect of compactness and flexibility of the dendritic ligands on the catalytic performance of their metal complexes. Peripherally phosphine-functionalized carbosilane dendrimers (with both monodentate... 

Recently, Reek et al. published the synthesis of a 9H,9 H- [4,4 ]bicarbazole-3,3r-diol (BICOL)-based chiral monodentate phosphoramidite ligand, which was functionalized with two different third-generation carbosilane dendritic wedges (Fig. 26) [57]. As reference reaction in the catalytic study, the rhodium-catalyzed asymmetric hydrogenation of Z-methyl-a-acetamido-cinnamate was chosen. Using a ligand-to-rhodium ratio of 2.2 led to enantio-selectivities which were comparable to the results obtained using the parent BINOL-derived monodentate phosphoramidite MonoPhos. 

An important aspect of the metal catalyzed hydroboration reaction is its ability to selectively reduce certain functionalities within a molecule. For instance, a key step in the synthesis of a tripeptide derivative containing the Phe-Arg hydroxyethy-lene dipeptide iosostere is the selective rhodium-catalyzed hydroboration of a lactone. The use of disiamylborane, 9-H-BBN, dicyclohexylborane, and (.9)-alpmeborane, however, gave only low to variable yields of the alcohol due to competitive reduction of the y-lactone to the hemiacetal (equation 8). In another example, hydroboration of the diene illustrated in equation (9) with HBcat and RhCl(PPh3)3 gave exclusive formation of the terminal alcohol derived from reaction of the less substituted alkene. Interestingly, uncatalyzed reactions failed to hydroborate this substrate selectively. ... 

Davies has further exploiled his previously reporied approach to (he tropanc skeleton related to cocaine based on the rhodium catalyzed decomposition of the vinyidiazomethane 81 in the presence of A/-Boc-pyrroIe (82) <01BMCL487>. Reduction of the non-conjugated double bond followed by A -deprotection and N-alkylation provided substrate 83 which was susceptible to conjugate addition of nucleophiles such as 84 in the presence of CuBr to afford 3-p-aryl tropanes which exhibited potent binding affinity for both the dopamine and serotonin transporters. Additionally, this author described the synthesis of various methyl heteroaryldiazoacetate analogues of 81, (me of which possessed an indole function, for use in catalytic asymmetric cyclopropanations . 

Commercial applicahons have advanced to the pilot plant scale so far. It can be speculated that in the overall scheme the aforementioned issues of catalyst stability and membrane stability and performance are critical issues. In the particular case of rhodium-catalyzed hydroformylation (for higher aUcenes or functional olefins) for the synthesis of fine chemicals it can be assumed that, as a nonscientific and nontechnical driver, the price of rhodium will contribute to the commercial success. 

Synthesis and functionalization of indoles through rhodium catalyzed... 

Abstract Carbon d ylide dipoles are important intermediates with great application in heterocyclic chemistry. Here, we show how the rhodium-catalyzed a-diazocarbonyl compounds are employed in the generation of carbonyl ylides and their effective use for the synthesis, as well as functionalization, of heterocycles. Herein we discuss recent advancements in this field mainly describing the synthesis and importance of various oxygen-and nitrogen-containing heterocyclic systems and natural products from a-diazocarbonyl compounds. 

A rhodium-catalyzed transannulation of tosyl-triazoles 9 with silyl or alkyl enol ethers 10 was developed that allows for the synthesis of substituted pyrroles 11 with regiocontrol.The addition ofTsOH promotes the final dehydration step to afford pyrroles with different functionality. The method can also be adjusted to allow for the synthesis of 3-pyrrolin-2-ones by using silyl ketene acetals as one of the coupling partners (14TL6455). 

Rhodium-catalyzed chelation-assisted C—H bond functionalization reactions (enantioselective annulation of aryl imines, dihydropyridine synthesis from imines and ahcynes, one-pot synthesis of pyridines from imines and alkynes, 2-arylpyridine alkylation with imines) 12ACR814. Synthesis of pyridine and dihydropyridine derivatives by regjo- and stereoselective addition to N-activated pyridines 12CRV2642. 

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