Rhodium catalysis cycloadditions

Padwa and Prein presented an extensive experimental and theoretical study of the 1,3-dipolar cycloaddition reactions of isomunchnones with olefinic dipolaro-philes. The a-diazo carbonyl isomunchnone precursors were synthesized in the usual fashion from amides and diazoethylmalonyl chloride. For example, isomunchnone 457 was readily generated from 456 using rhodium catalysis to form... 

A second and quite different version of the (5 + 2) cycloaddition involves rhodium catalysis of an aUene with a vinylcyclopropane (Scheme 38). The product is ready for simple transformation into ttis-norguaiane sesquiterpenes [75]. 

Tokunaga and Wakatsuki reported a one-pot indole synthesis from anilines and propargyl alcohols using Rn3(CO)j2 [5], and Nicholas and colleagues reported a Ru-catalyzed indole synthesis via the reductive annulation of nitros-oarenes with alkynes (equation 3) [6, 7]. Saa and coworkers described the Ru-catalyzed cycloisomerization of o-alkynylanihnes (equation 4) [8, 9]. Nissen and Detert reported a total synthesis of lavendamycin that featnred a Ru-catalyzed [2-f2-f2] cycloaddition of an o-alkynyl-ynamide, a method that was superior to rhodium catalysis both in terms of efficiency and regiochemistiy [10]. 

Scheme 3.5 Domino carbonyl ylide formation-l,3-dipolar cycloaddition reaction catalysed by a combination of rhodium catalysis and chiral nickel catalysis.

Cycloadditions Several metals are known to trigger stereoselective [2-I-2-I-2] cycloaddition of polyunsaturated systems [3] and this approach has been applied to different types of unsaturated substrates. In this general overview, most cited examples will focus on cobalt, nickel, and rhodium catalysis. 

Hayashi and coworkers have developed the highly efficient enantioselective intramolecular [5+2] cycloaddition of heteroatom (O and NTs) alkyne-VCPs 31 under rhodium catalysis [29]. High enantioselectivities of up to >99% ee were achieved for the corresponding cycloadducts 32 by the use of the chiral phosphoramidite ligand 33 in combination with a catalyst such as [RhCKCaH lJi- The reactions were performed at 30 °C in dichloromethane as the solvent and in the presence of sodium tetrakis(3,5-bis(tiilluoromethyl)phe-nyl)borate (Barp) as source of anionic counterion, which produced a series of chiral heteroatom (O and NTs)-tethered cycloadducts in high yields, as shown in Scheme 20.13. 

The rhodium( 11)-catalyzed formation of 1,3-dipoles has played a major role in facilitating the use of the dipolar cycloaddition reaction in modern organic synthesis. This is apparent from the increasing number of applications of this chemistry for the construction of heterocyclic and natural product ring systems. This chapter initially focuses on those aspects of rhodium(II) catalysis that control dipole formation and reactivity, and concludes with a sampling of the myriad examples that exist in the Hterature today. 

The reaction of a-diazocarbonyl compounds with nitriles produces 1,3-oxazoles under thermal (362,363) and photochemical (363) conditions. Catalysis by Lewis acids (364,365), or copper salts (366), and rhodium complexes (367) is usually much more effective. This latter transformation can be regarded as a formal [3 + 2] cycloaddition of the ketocarbene dipole across the C=N bond. More than likely, the reaction occurs in a stepwise manner. A nitrilium ylide (319) (Scheme 8.79) that undergoes 1,5-cyclization to form the 1,3-oxazole ring has been proposed as the key intermediate. 

In contrast, the S,C-ylide (139a) prepared from tetrachlorothiophene and diazomalonic ester under rhodium acetate catalysis undergoes cycloaddition with acenaphthylene much more slowly however, after heating at 80 °C for 7 h, the reaction leads to the aromatized fluoranthene (226) in 97% yield. The formation of (226) involves a proton shift and loss of chlorine the exact mechanism and the location of the chlorine atoms are not clear <86JCS(Pl)233>. 

Rhodium is a rare white-silvery metal classified as a member of the platinum group metals. As a result, rhodium is commonly used as a catalyst in chemical reactions. It is also used in several chemical feedstock processes, including hydroformylation. Furthermore, rhodium has been used to catalyze more complex processes, such as higher order cycloaddition reactions. In addition, rhodium has been used in simpler reaction types such as hydrogenation and cycloisomerization. The fact that rhodium is effective for a wide range of chemical processes makes it an attractive metal for catalysis. 

Wender s group has also developed rhodium-catalyzed intermolecular [5-1-2] cycloadditions. At first, they found the catalysis system of Rh(PPh3)3Cl for the intramolecular reactions was not effective at all for the intermolecular reactions. To effect the intermolecular [5-1-2] cycloadditions, [Rh(CO)2Cl]2 must be used and oxygen substitution of the cyclopropane was necessary (see (17)) [37-39]. Then they successfully expanded the substrate to unactivated vinylcyclopropanes by adjusting the substituents. For monosubstituted alkynes, the substitution on the olefin terminus directs the formation of single isomer that minimized steric hindrance (see (18)) [40]. The [5-1-2] cycloadditions can also be applied to VCPs with allenes (see (19)) [41]. It should be noted that the alkyne substituent did not interfere with the reaction, indicating that allenes as reaction partners were superior to alkyne in the [5-1-2] cycloadditions. Curiously, the authors didn t report the corresponding intermolecular [5-1-2] cycloadditions of VCPs with alkenes. 

For example, access to axial chirality can be realized under cobalt catalysis using a chiral cobalt(I) complex [4], However, the use of chiral iridium and rhodium species dramatically improved the scope and enantioselectivities obtained for this cycloaddition. Tanaka and coworkers synthesized an atropoisomeric diphosphine oxide in 97% ee, by treatment of the suitable hexayne with [Rh(cod)2]BF4 in the presence of (7 )-TolBINAP as source of chirality (double [2-1-2-1-2] cycloaddition). Subsequent reduction afforded an axially chiral bidentate ligand as a single enantiomer (Scheme 7.1) [5]. 

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