Domino rhodium-catalyzed

There are two important rhodium-catalyzed transformations that are broadly used in domino processes as the primary step. The first route is the formation of keto carbenoids by treatment of diazo keto compounds with Rh11 salts. This is then followed by the generation of a 1,3-dipole by an intramolecular cyclization of the keto carbenoid onto an oxygen atom of a neighboring keto group and an inter- or intramolecular 1,3-dipolar cycloaddition. A noteworthy point here is that the insertion can also take place onto carbonyl groups of aldehydes, esters, and amides. Moreover, cycloadditions of Rh-carbenes and ring chain isomerizations will also be discussed in this section. 

The second rhodium-catalyzed route which is widely used in connection with domino processes is that of hydroformylation. This by itself is a very important industrial process for the formation of aldehydes using an alkene and carbon monoxide. Finally, rhodium catalysts have also been used in this respect. 

In a similar way as described for the hydroformylation, the rhodium-catalyzed silaformylation can also be used in a domino process. The elementary step is the formation of an alkenyl-rhodium species by insertion of an alkyne into a Rh-Si bond (silylrhodation), which provides the trigger for a carbocyclization, followed by an insertion of CO. Thus, when Matsuda and coworkers [216] treated a solution of the 1,6-enyne 6/2-87 in benzene with the dimethylphenylsilane under CO pressure (36 kg cm"2) in the presence of catalytic amounts of Rh4(CO)12, the cyclopentane derivative 6/2-88 was obtained in 85 % yield. The procedure is not restricted to the formation of carbocycles rather, heterocycles can also be synthesized using 1,6-enynes as 6/2-89 and 6/2-90 with a heteroatom in the tether (Scheme 6/2.19). Interestingly, 6/2-91 did not lead to the domino product neither could 1,7-enynes be used as substrates, while the Thorpe-Ingold effect (geminal substitution) seems important in achieving good yields. 

Scheme 6/2.23. Rhodium-catalyzed domino allylation/Pause-Khand process...

Wittig yhdes have been shown to be compatible with hydroformylation conditions, and may thus be used in a domino reaction sequence such as from 16a to 38 (Scheme 5.15) [20]. When an a-unsubstituted ylide is employed, the resulting alkene undergoes in-situ rhodium-catalyzed hydrogenation in a triple tandem reaction to convert 10 a to 39. Several other examples were reported establishing the generality of this domino reaction sequence. 

Rhodium-catalyzed three-component domino coupling of 1,6-diyne 41, hydrosilane, and Cgo proceeds to give fullerene adduct 42 in good yield (Eq. 10) [29]. In this case, dienophile Cgo does not interfere with the silylcarbocyclization process and traps the 1,2-bis-exo-methylidenecyclopentane intermediates to furnish the corresponding Cgo-linked carbo- or heterocycle 42. 

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. 

Scheme 8.17 Intermolecular rhodium-catalyzed domino reaction by Hayashi et al. [28].

Scheme 8.25 Rhodium-catalyzed domino isomerization-aldol reaction.

Recently, extensive efforts have been made in the development of domino conjugate reduction/inter and intramolecular aldol reaction catalyzed by various metal complexes based on rhodium, iridium, cobalt, indium, nickel, and copper [22]. In 1999, Morken and Taylor [23] reported a rhodium-catalyzed conjugative reductive/aldol reaction between aldehyde and acrylate using QjMeSiH as reducing agent with a maximum diastereoselectivity of 23 1. 

Scheme 12.55 Rhodium-catalyzed domino synthesis of 1.3-dienes.

The first total synthesis of this natural product was achieved by Chiu and Lam [139]. Key step of the synthesis is a rhodium-catalyzed domino cychza-tion/cycloaddition reaction to form the tricyclic core of the diterpenoid from hnear a-diazoketone 337. Concerning the mechanism of the reaction, it is hkely that the rhodium catalyst, when reacted with 337 at 0 °C, formed a carbenoid species which immediately cyclized to 341 (Scheme 14.53). This 1,3-dipole then underwent an intramolecular cycloaddition with the aUcene to give a mixture of two cycloadducts in 81% yield with 339 as the major product (dr= 1 3.1 338 339). The minor diastereomer 338 was probably formed via a less stable boat conformation of the tether in contrast to the chair conformation shown in 341, leading to the desired product Decreasing the temperature from 0 to —15 °C did not increase the dr but lowered the yield. It is also remarkable that the reaction afforded no more than 0.5 mol% of the rhodium(II)octanoate dimer ([Rh2(Oct)4]). Further transformation of 339 finally furnished (—)-indicol (340) in an overall yield of 10% over 21 steps. 

Scheme 14.53 Chiu s synthesis of (-)-indicol (340) by a rhodium-catalyzed domino carbene cyclization/cycloaddition reaction.

Scheme 10.135 Rhodium-catalyzed domino carbometallation followed by Heck-type crosscoupling reaction in water [113].

Scheme 10.136 Proposed mechanism for the rhodium-catalyzed domino reaction of aryl-boronic acids with disubstituted alkynes and methyl acrylate [113].

Rh-catalyzed domino reactions have been published during the past decade, such as rhodium-catalyzed hydroallylation of activated alkenes (Scheme 12.93). This was achieved by a one-pot operation of neutral components, an allylic carbonate 188, an a,P-unsaturated ketone or ester 187, and a hydrosilane, which are activated in a specific order by [Rh(cod)(P(OPh)3)2]OTf under almost neutral conditions. The products 189 are obtained in high yields, but the regioselectivity is strongly dependent on the nature of the allylic substrate [197]. 

Rhodium-catalyzed hydroformylation of terminal alkenes in the presence of stabilized phosphorus yhdes was found to initiate a domino hydroformylation-Wittig olefination process. When monosuhstituted acceptor-stabilized phosphorus ylides were employed, a hydrogenation step follows the Wittig olefination to give a domino hydroformylation-Wittig olefination hydrogenation process (Equation 7.19) [166]. 

Within this chapter, two sections are devoted to rhodium and ruthenium. The two main procedures using rhodium are first, the formation of 1,3-dipoles from diazocompounds followed by a 1,3-dipolar cycloaddition [10] and second, hy-droformylation [11], The ruthenium-catalyzed domino reactions are mostly based on metathesis [12], with the overwhelming use of Grubbs I and Grubbs 11 catalysts. 

Besides the formation of carbenes from diazo compounds and the hydroformyla-tion, rhodium (as described previously for palladium) has also been used as catalyst in domino processes involving cycloadditions. Thus, Evans and coworkers developed a new Rh(I)-catalyzed [4+2+2] cycloaddition for the synthesis of eight-membered rings as 6/2-105 using a lithium salt of N-tosylpropargylamines as 6/2-104, allyl carbonates and 1,3-butadiene (Scheme 6/2.22) [221]. The first step is an al-... 

A broad range of olefins, acetals, epoxides, alcohols, and chlorides were demonstrated effective alternative starting materials. Cobalt and rhodium carbonyls and bimetallic complexes were shown to catalyze the domino hydro-... 

In summary, we have summarized representative examples of transition-metal-catalyzed carbonylative domino reactions. In the area of carbonylations, palladium, rhodium, and cobalt are still the main actors. The abihty of palladium catalysts in carbonylative cross-coupling, rhodium catalysts in carbonylative C-H activation, and cobalt catalyst in carbonylative reactions with unsaturated bonds is impressive. 

A broad range of olefins, acetals, epoxides, alcohols, and chlorides were demonstrated effective alternative starting materials. Cobalt and rhodium carbonyls and bimetallic complexes catalyzed the domino hydroformylation-amidocarbonylation of olefins (17-22). Addition of 0.1 mol% RheCCOie to the cobalt catalyst gave branched AT-acetyltrifluorovaline, which indicated that the hydroformylation step governs the regioselectivity of the domino process (Scheme 4) (22). 

A new enantioselective Rh-catalyzed domino transformation of boronic acids with a cyclohexadienone-tethered alkyne gave access to fused heterocycles by desymmetrization of alkyne-tethered cyclohexadienones via the formation of two new C-C bonds and two new stereocenters with good enantioselectivities, syn-addition of the rhodium-aryl species onto the alkyne (130L1148). 

According to mechanistic studies, the hydrogenation of the aldehyde precedes the double-bond isomerization to the cyclic enolether, which is rapidly transformed by a Lewis acid-catalyzed acetalization. In this sequence, the cationic rhodium complexes obviously catalyze four steps. The three last steps all proceed in a domino manner and a rhodium-hydride complex is the presumed catalytically active species. 

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