Subject rhodium-catalyzed

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... 

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. 

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 trienes were then subjected to a formal Diels-Alder reaction using conditions developed by Gilbertson and others (Scheme 8.7) [34]. The propargyl tosylamide 40 and the propargyl ether 43 have both afforded the formal intramolecular Diels-Alder adducts 44 and 45 in high yield. To date, the formal cycloaddition of the siHcon-teth-ered alkyne 41 has not been affected and heating triene 42 led to a thermal Diels-Alder reaction to furnish cycloadduct 46, albeit in lower yield than the corresponding rhodium-catalyzed examples. 

The combination of allylic amination, ring-closing metathesis, and a free radical cyclization provides a convenient approach to the dihydrobenzo[b]indoline skeleton, as illustrated in Scheme 10.10. The rhodium-catalyzed aUylic amination of 43 with the lithium anion of 2-iodo-(N-4-methoxybenzenesulfonyl)arrihne furnished the corresponding N-(arylsulfonyl)aniline 44. The diene 44 was then subjected to ring-closing metathesis and subsequently treated with tris(trimethylsilyl)silane and triethylborane to afford the dihydrobenzojhjindole derivative 46a in 85% yield [14, 43]. 

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. 

The rhodium-catalyzed [m-i-n-i-1] reactions, as exemplified by the [2-t2-tl] may be accomplished with an array of substrates that have proven challenging for other metal complexes. Moreover, the ability to undertake the domino reactions significantly increases the molecular complexity and ultimately the synthetic utihty of this venerable reaction. Although the [i + 2 + 1] and [4-tl] reactions have not been as extensively studied, they will undoubtedly be the subjects of future synthetic efforts. 

Two 4-methylene-l,3-dioxane diastereoisomers, isomeric at C-6, were subjected to the rhodium-catalyzed hydro-formylation. The stereochemistry of the newly formed stereogenic carbon was guided solely by the acetal stereocenter (not by C-6) (Scheme 56) <1997JA11118, 1998TL6423>. 

The C5 aldehyde intermediate is produced from butadiene via catalytic oxidative acetoxylation followed by rhodium-catalyzed hydroformylation (see Fig. 2.30). Two variations on this theme have been described. In the Hoffmann-La-Roche process a mixture of butadiene, acetic acid and air is passed over a palladium/tellurium catalyst. The product is a mixture of cis- and frans-l,4-diacetoxy-2-butene. The latter is then subjected to hydroformylation with a conventional catalyst, RhH(CO)(Ph3P)3, that has been pretreated with sodium borohydride. When the aldehyde product is heated with a catalytic amount of p-toluenesulphonic acid, acetic acid is eliminated to form an unsaturated aldehyde. Treatment with a palladium-on-charcoal catalyst causes the double bond to isomerize, forming the desired Cs-aldehyde intermediate. 

In an example illustrating an approach to tryptamines, the aniline derivative 98, which is available by a Heck reaction involving the corresponding o-iodoaniline, was subjected to rhodium-catalyzed hydroformylation leading to the target indole 99 via the intermediate 100 (Equation 26) <1997JOC6464>. 

Padwa subsequently described some intramolecular versions of this reaction. A structurally similar aryl ester, 55, a tethered to a terminal olefin was subjected to rhodium-catalyzed diazo decomposition. Carbonyl ylide formation followed by intramolecular cycloaddition resulted in tricyclic product 56 a, Eq. 39 [66 - 69]. Cycloaddition also occurred with the amide analogue 55 b. 

The other example of a rhodium migration was discovered when 3-substituted-3-(2-hydroxyphenyl)cyclobutanones were subjected to rhodium-catalyzed conditions (Scheme 17) [76], The reaction generates dihydrocoumarin derivatives in high... 

Murakami and coworkers reported a further use for this rhodium migration [80, 81]. Instead of protonolysis, they noticed that the aryl rhodium species after the vinylic to aryl migration is nucleophilic enough to attack an ester moiety in an intramolecular fashion to afford a cyclic ketone. Thus, an internal alkyne equipped with ester groups at a specific place was subjected to the rhodium-catalyzed hydroarylation conditions (Scheme 21). Indeed, the desired ketone was obtained in an 89% yield. Not only methyl esters can serve as acylation agents ethyl esters and isopropyl esters are also suitable substrates. 

However, subsequent work by Padwa and co-workers led to a successfiil formal synthesis of vallesamidine. Thus, following the results from a closely related model study, these workers subjected diazo imide 535 to the standard rhodium-catalyzed carbenoid generation and cyclization to give 537 via isomunchnone 536 (Fig. 4.162). Conversion of 537 to a previously synthesized precursor to vallesamidine 538 was uneventful. 

Wan and coworkers reported a rhodium-catalyzed [2 -I- 2 + 2] cycloaddition of oximes and diynes for the synthesis of pyridines in 2013 [45]. In their mechanistic study, they exclude the dehydration of oxime to generate the corresponding nitrile followed by the cycloaddition of the nitrile and the alkynes to afford the pyridine as the pathway. As only a trace amount of benzonitrile was produced from oxime under the reaetion conditions. Additionally, pyridine product was detected in less than 20% yield when benzonitrile was subjected to the reaetion with diyne instead of oxime (Scheme 3.19). EtOh was tested as solvent here as well, but not produeed. Later on, they found that by using Rh (NBD)2BF4/MeO-Biphep as the eatalyst system, the reaction can be performed in EtOH [46]. 

We tested several routes to the aldehyde boronate 48. The following one turned out to be most reliable Acrolein-dimethylacetal was subjected to rhodium catalyzed hydroboration to give 49. Acetal cleavage was achieved with yet another mild reagent, cerium-montmorillonite in moist dichloromethane 24) to give 98% of the desired aldehyde 48. (25)... 

We subjected this compound to rhodium catalyzed hydroformylation using the BIPHEPHOS ligand, which guarantees the predominant formation of the unbranched aldehyde. (28, 29) Hydroformylation, allylboration and the second hydroformylation proceeded cleanly to furnish the piperidinol derivative 57 in 73% yield. The latter compound was found to exist as a mixture of the lactoi and the aldehyde form. (25)... 

Very recently, Compain proposed an approach to spirocyclic imino sugars (Scheme 41). The synthetic route started from known cyclobutanol 208, which was transformed into carbamate 209. Rhodium-catalyzed intramolecular C-H amination performed on that compound led to oxazolidinone 211. Subsequently, this compound was Al-allylated and subjected to the ring-closing metathesis with the Grubbs II cat. As a... 

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