Domino rhodium catalysts

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 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. 

Hayashi and coworkers [28] established an intermolecular domino 1,4-addition/aldol reaction. The reaction of vinyl ketones with aldehydes and several arylboranes in the presence of a rhodium catalyst was carried out smoothly, which eventually furnished aldol products 60 in generally very good yields and syn selectivities (Scheme 8.17). 

Shinokubo and coworkers [79] performed a domino coupling reaction of aryl boronic acids 139 with internal alkynes 140 and acrylates 141 by using a rhodium catalyst in water to give nine examples of the 1,3-diene derivatives 142 in 37-81% yields (Scheme 12.55). The reaction indicates that the use of water offers the possibility to optimize known reactions, leading to novel transformations. 

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 5.131 Domino hydroformylation-cross aldol addition with a chiral rhodium catalyst and a chiral aldol condensation catalyst.

Scheme 5.133 Domino hydroformylation-(aldol condensation)-hydrogenation reaction with a rhodium catalyst based on NAPHOS for both hydroformylation and hydrogenation.

The successful completion of this domino reaction encouraged us to check some further hydroformylation-allylboration-hydroformylation sequences. For instance, the homoallylic alcohol 58 was converted to the ynol ether 59. To convert the latter to the alkenyl boronate (55%) 60 a zirconium catalyst (30) had to be chosen for the hydroboration. With rhodium catalysts chemoselectivity between the alkyne and the alkene moieties was... 

Hydroformylation of olefins has been established as an important industrial tool for the production of aldehydes. In recent years, novel asymmetric tandem reactions have included a rhodium-catalysed enantioselective hydroformylation. In this context, in 2007 Abillard and Breit ° and Chercheja and Eilbracht independently reported a novel domino hydroformylation-aldol reaction catalysed by an achiral rhodium catalyst and L-proline catalyst (Scheme 7.49). Possibly owing to the fact that proline is hard but the rhodium catalyst is soft, the proline can be compatible with the rhodium catalyst to allow this domino reaction to be achieved. By fine adjustment of the hydroformylation rate to that of the L-proline-catalysed aldol addition, the undesired homodimerisation of the aldehyde could be avoided. As a result, by in situ hydroformylation reaction, the donor aldehyde of a... 

Earlier, the same authors employed the same strategy to the synthesis of optically active amine compounds from alkenes through a three-component domino hydroformylation-Mannich reaction. The combination of rhodium catalyst hydroformylation with L-proline-catalysed asymmetric Mannich reaction worked well to afford the domino product in moderate yield and good enantioselectivity of 74% ee, as shown in Scheme 7.51. 

Interestingly, cycloisomerizations of 1,6-enynes with a hydroxyl group at the allyhc terminus (R = OH) generate aldehyde functionalities by virtue of the enol-aldehyde tautomerism in the final product (Fig. 10.18) [29]. The method has been extended recently to a range of 1,6-enynes with various aryl and heteroaryl substituents on the alkyne function, and applied then to a sequential cycloisomerization/isomerisation/ hydrogenation/acetalization domino reaction leading to the bicychc acetals displayed in Fig. 10.18. The rhodium catalyst is assumed to promote the initial cycloisomerization step as well as the isomerisation/hydrogenation process [31]. 

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-... 

More recently, during research aimed at supporting the highly linear selective hydroformylation catalyst [Rh(H)(Xantphos)(CO)2] onto a silica support, the presence of a cationic rhodium precursor in equilibrium with the desired rhodium hydride hydroformylation catalyst was observed. The presence of this complex gave the resulting catalyst considerable hydrogenation activity such that high yields of linear nonanol could be obtained from oct-1-ene by domino hy-droformylation-reduction reaction [75]. 

Given the previous discussion on reductive amination, it is surprising that the potentially more complicated domino hydroformylation-reductive amination reactions have been more thoroughly developed. The first example of hydroaminomethylation was reported as early as 1943 [83]. The most synthetically useful procedures utilize rhodium [84-87], ruthenium [88], or dual-metal (Rh/Ir) catalysts [87, 89, 90]. This area was reviewed extensively by one of the leading research groups in 1999 [91], and so is only briefly outlined here as the second step in the domino process is reductive amination of aldehydes. Eilbrachfs group have shown that linear selective hydroaminomethylation of 1,2-disubstituted alkenes... 

Abstract The purpose of this chapter is to present a survey of the organometallic chemistry and catalysis of rhodium and iridium related to the oxidation of organic substrates that has been developed over the last 5 years, placing special emphasis on reactions or processes involving environmentally friendly oxidants. Iridium-based catalysts appear to be promising candidates for the oxidation of alcohols to aldehydes/ketones as products or as intermediates for heterocyclic compounds or domino reactions. Rhodium complexes seem to be more appropriate for the oxygenation of alkenes. In addition to catalytic allylic and benzylic oxidation of alkenes, recent advances in vinylic oxygenations have been focused on stoichiometric reactions. This review offers an overview of these reactions... 

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). 

The [5+2] cycloaddition has been included in several domino processes. For example, Feng and Zhang have recentiy developed a novel, regiospecific, and diastereoselective domino intramolecular hetero-[5+2] cycloaddition-Claisen rearrangement of vinylic oxirane-alkyne substrates 90, which employed the rhodium Af-heterocyclic carbene complex, RhCl(/-Pr)(cod), as catalyst [95]. The process provided the corresponding [3.1.0] bicyclic products 91 in moderate to high yields (47-92%), as shown in Scheme 20.39. The authors have demonstrated the complete chirality transfer by performing the reaction with an enantiomerically enriched... 

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