Rhodium complexes axial chiral

The cationic rhodium(l)/axially chiral biaryl bisphosphine complex-catalyzed [2+2+2] cycloaddition of biaryl-linked tetraynes with dialkynylketones or dialkynylphosphine oxides afforded helically chiral l,l -bitriphenylenes, containing adensely substituted fluorenone or phosphafluorene core (Scheme 21.23) [27]. 

The cationic rhodium(I)/axially chiral biaryl diphosphine complex catalyzed the enantioselective desymmetrization of symmetric dialkynylphosphine oxides via rhodium-catalyzed [2- -2-1-2] cycloaddition with 1,6-diynes to give / -stereogenic alkynylphosphine oxides (Scheme 4.50) [53]. This method was also applied to... 

As described in this chapter, a number of rhodium-based catalysts have been developed and employed widely for the [2 -I- 2 -I- 2] cycloaddition reactions of alkynes. Among these rhodium catalysts, Wilkinson s complex [RhCl(PPh3)3] and cationic rhodium(I)/biaryl bisphosphine complexes are the most frequently employed. As Wilkinson s complex is a stable single-component catalyst, this complex is the most easily usable catalyst in organic synthesis. On the other hand, cationic rhodium(I)/ (axially chiral) biaryl bisphosphine complexes exhibit excellent catalytic activity and selectivity under mild conditions. These chiral complexes, especially, enable the catalytic enantioselective syntheses of various chiral arenes with high enantioselectivity. 

Importantly, asymmetric variants of the foregoing reactions have been developed by using the cationic or neutral rhodium(I)/axially chiral biaryl bisphosphine complexes as catalysts. Although the rhodium-based catalysts are expensive, these are highly stable and can be handled readily using conventional laboratory equipment. Therefore, I believe that rhodium-catalyzed [2 - - 2 - - 2] cycloaddition reactions wiU be employed continuously for the synthesis of complex aromatic compounds. 

The catalytic reaction giving allenes by the addition of a hydrosilane twice to 1,3-diynes65 has been applied to the asymmetric synthesis of axially chiral allenylsilanes although the selectivity and scope of this reaction are relatively low. A chiral rhodium complex coordinated with (23, 43 )-PPM is the best catalyst for the addition of phenyldimethyl-silane to diyne 52 giving allene 53 with 22% ee (Scheme 14).66 663... 

Figure 5.26 Synthesis of a carbene precursor with an axially chiral backbone made from ben-zannulated 1,1 -bipiperidine and its rhodium(l) complexes.

Figure 5,27 Axial chirality in a square planar rhodium(l) carbene complex caused by an...

Note The rhodium(I) carbene complex in Figure 5.27 is not only planar chiral (in the ferrocene scaffold), but also axially chiral (in the rhodium coordination plane). 

The first place in catalytic hydrogenation nowadays is taken by Rh or Ru complexes of BINAP. This ligand has axial chirality as the naphthalene rings cannot rotate past each other. These compounds were developed by Noyori, who with Knowles and Sharpless received the 2001 Nobel prize for their contributions to asymmetric synthesis. BINAP 20 is usually made from BINOL 19 and either 19 or 20 can be resolved. Rhodium complexes similar to those we have met include a molecule of cyclooctadiene and, as these are Rh(I) compounds, a counterion, often triflate 21. Both enantiomers of BINAP are available commercially.8... 

Axially chiral N-arylsuccinimides were prepared by the addition of arylboronic acids to l-(2-tert-butylphenyl)maleimide (103) (Scheme 8.25), and excellent diastereo- and enantioselectivities were obtained upon catalysis with the rhodium complex of diene 51 [90]. 

Enantioselective synthesis of chiral biphenols from dienone (175) is catalysed by a chiral rhodium(I)-BINAP complex, involving a highly selective but traceless tfansfer of central-to-axial chirality. (g)... 

Axially chiral biaryls are an important class of molecules for both biologically active compounds and chiral ligands (78-80). The most common approach to obtain biaryls is by aryl coupling followed by resolution of the racemic product to afford enantiopure biaryls. Even though enantioselective partial intramolecular cyclotrimerization of diyne with alkynes (81,82) or nitriles (83) were developed with various transitional metals, it was difficult to carry out complete intermolecular reaction. Using a cationic chiral rhodium complex as catalyst, a regioselective intermolecular cross-cyclotrimerization of alkynes 72 and 73 for... 

Unfortunately, thus far there has been only one example of the asymmetric version of rhodium-catalyzed asymmetric arylation of aldehydes. In this report, by Miyaura [44], a rhodium complex coordinated with axially chiral monodentate phosphine Ug-and, (S)-MeO-mop, catalyzed the addition of phenylboronic acid (2m) to 1-naph-... 

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]. 

The [2 -I- 2 -I- 2] cycloaddition reaction can give rise to chiral compounds, especially biaryls [3q]. Control of the enantioselectivity in such transformations is of prime importance, notably because biaryls can be used as ligands in asymmetric catalysis. This topic is covered in detail in Chapter 9. Nowadays, cobalt still looks like a poor relation in this field, which is largely dominated by rhodium. Nevertheless, a report from Heller et al. shows for the first time that phosphorus-bearing axially chiral biaryls 9 can be formed by enantioselective benzene formation using the neomenthyl-indenyl cobalt complex II as a catalyst (Scheme 1.3) [7]. Good yields... 

The enantioselective synthesis of axially chiral hydroxy carboxylic acid derivatives 42 was accomplished by the cationic rhodium(I)/BINAP complex-catalyzed [2 + 2 + 2] cycloaddition of a,w-diynes 40 with 2-alkoxynaphthalene-derived alkynyl esters 41 with high yields and ee values (Scheme 9.15) [17],... 

For the synthesis of C2-symmetric axially chiral biaryls, Tanaka et al. using the cationic rhodium(I)/Segphos complex as a catalyst, reported various enantioselective double [2-I-2-I-2] cycloaddition reactions. The double [2-1-2-1-2] cycloaddition of readily prepared ether-linked tetraynes 49 with two molecules of monoynes 50 proceeded to give Ca-symmetric axially chiral biaryls 51 in good to high ee values, although the product yields were low to moderate (Scheme 9.18) [19]. 

As shown in Sections 9.4.1 and 9.4.2, electron-deficient and coordinating alkynyl carbonyl compounds showed high reactivity in cationic rhodium(I)/biaryl bispho-sphine complex-catalyzed enantioselective [2- -2-1-2] cycloaddition. Therefore, Tanaka et al. investigated the use of alkynylphosphonates or alkynylphosphine oxides instead of alkynyl carbonyl compounds for the practical synthesis of axially chiral biaryl phosphorus compounds. The enantioselective [2- -2-1-2] cycloadditon... 

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