Rhodium complex main group

Rhodium complexes catalyze 1,2-addition of main group metal compounds to aldimines as well. Table 5 summarizes the reported methods. Electron-withdrawing substituents such as sulfonyl and acyl groups on the imino nitrogen atom are important to obtain sufficiently high reactivity. Asymmetric synthesis (diastereoselective and enantioselective) has also been accomplished. 

The catalytic H—B addition of organoboron compounds to unsaturated molecules is synthetically interesting in main-group chemistry, as a wide range of functional groups can be transformed from the selective new C—B bond [1]. Although considerable energy has been expended to use rhodium] I) complexes to catalyze the... 

The ability to harness alkynes as effective precursors of reactive metal vinylidenes in catalysis depends on rapid alkyne-to-vinylidene interconversion [1]. This process has been studied experimentally and computationally for [MC1(PR3)2] (M = Rh, Ir, Scheme 9.1) [2]. Starting from the 7t-alkyne complex 1, oxidative addition is proposed to give a transient hydridoacetylide complex (3) vhich can undergo intramolecular 1,3-H-shift to provide a vinylidene complex (S). Main-group atoms presumably migrate via a similar mechanism. For iridium, intermediates of type 3 have been directly observed [3]. Section 9.3 describes the use of an alternate alkylative approach for the formation of rhodium vinylidene intermediates bearing two carbon-substituents (alkenylidenes). 

Few complexes are known containing interstitial main-group atoms that also fall within the scope of this review (see 79-81), but one example containing antimony has been reported by Vidal (143), in which this element resides in the center of an icosahedron of rhodium atoms, [SbRhI2(CO)27]3 (143) (Fig. 40). 

Some use has been made of severe reaction conditions in which a metal salt or complex is reduced in the presence of a main group element compound [Eqs. (229)-(233)]83 375,378,384,385 to give extremely robust compounds. Unfortunately, little of the subsequent chemistry of these high-nuclearity species has been reported, except for spectroscopic investigations of the rhodium compounds that have shown the metal frameworks to be fluxional as well as the ligand complement. 

Two kinds of chiral tertiary phosphine ligands have been used in asymmetric hydrogenation experiments involving rhodium complexes the Horner and Monsanto groups have concentrated on ligands whose chirality is centered at an asymmetric phosphorus atom, and the New Hampshire and Paris groups have focused their attention mainly on phosphides that carry chiral carbon moieties. 

The bis( -methylene)rhodium complex (Structure 8) undergoes thermal decomposition at 300-350 °C to yield mainly propene and methane H-labeling experiments showed that the propene originates rather selectively (>75 %) from two CH2 groups and one CH3 unit a //-methylidene (CH) intermediate 9 (from H abstraction) with consecutive isomerizations is assumed to yield the propene (Scheme 3). 

A phosphite-modified calixarene with unsubstituted hydroxyl groups was used as a ligand in 1-hexene hydroformylation catalyzed by rhodium complexes [224], The reaction was carried out at a synthesis gas pressure of 6.0 MPa and 160 °C. Rh(acac)(CO)2 was a catalyst precursor. In 3 h, the conversion of the initial alkene virtually reached its theoretically predicted value the yield of aldehydes was 80-85%, and the normal-to-isomeric aldehyde ratio was approximately 1 1. Some similar phosphites 83 were also studied as components of catalytic systems for 1-octene hydroformylation [225]. It was shown that the nature and steric volume of substituent R have no essential effect on the main laws of the process. For example, the conversion was 80-90% at a selectivity with respect to nonanal of about 60% in all cases. The regioselectivity with respect to nonanal was considerably increased to 90-92% by using the chelate biphosphite 84 [220]. 

The Rh(III) organometallic complex 39 (Scheme 11.7) conjugated to biotin through the carboxyl group present on the bipyridine ligand, exhibited emission similar to that of complex 38 (i.e., low emission at 298 K), and also accumulated mainly in the cytoplasm of HeLa cells [110]. This small number of examples is obviously not enough to establish the potential interest of rhodium complexes for cell imaging. 

The four most common methods for the synthesis of late transition metal enolates are oxidative addition to halocarbonyl compoxmds, ligand metathesis with main group enolates or silyl enol ethers, nucleophilic addition of anionic metal complexes to halocarbonyl electrophiles, and insertion of an a,3-imsaturated carbonyl compoimd into a metal hydride. Examples of these synthetic routes are shown in Equation 3.47-Equation 3.50. Equation 3.47 shows the synthesis of a palladium enolate complex by oxidative addition of ClCHjC(0)CHj to Pd(PPh3), Equation 3.48 shows the synthesis of a palladium enolate complex by the addition of a potassium enolate to an aryl Pd(II) halide complex, and Equation 3.49 shows the synthesis of the C-bound W(II) enolate complex in Figure 3.7 by the addition of Na[( n -C5R5)(CO)jW] to the a-halocarbonyl compound. Finally, Equation 3.50 shows the synthesis of a rhodium enolate complex by insertion of but-l-en-3-one into a rhodium hydride. This last route has also been used to prepare enolates as intermediates in reductive aldol processes. - ... 

In most cases of hydrosilylation in polymer systems, the role of catalysts is played by transition metal complexes, those of platinum in particular, mainly Speier s and Karstedt s catalysts (3,4,6,12,18). Rhodium complexes also play an important role. They show higher resistance to poisoning than by platinum complexes (7). In this group of catalysts, one of the most efficient catalysts in... 

Supramolecular chemistry has been a very popular research topic for three decades now. Most applications are foreseen in sensors and opto-electronical devices. Supramolecular catalysis often refers to the combination of a catalyst with a synthetic receptor molecule that preorganizes the substrate-catalyst complex and has also been proposed as an important possible application. The concept, which has proven to be powerful in enzymes, has mainly been demonstrated by chemists that investigated hydrolysis reactions. Zinc and copper in combination with cyclodextrins as the receptor dramatically enhance the rate ofhydrolysis. So far, the ample research devoted to transition metal catalysis has not been extended to supramolecular transition metal catalysis. A rare example of such a supramolecular transition metal catalyst was the results of the joined efforts of the groups of Nolte and Van Leeuwen [SO], They reported a basket-shaped molecule functionalized with a catalytically active rhodium complex that catalyzed hydrogenation reactions according to the principles of enzymes. The system showed substrate selectivity, Michaelis Menten kinetics and rate enhancement by cooperative binding of substrate molecules. The hydroformylation of allyl catachol substrates resulted in a complex mixture of products. 

Calixarene phosphite 30 with unsubstituted hydroxyl group was used as a ligand in the rhodium complex-catalyzed hydroformylation of 1-hexene (Scheme 4.19) [78]. The almost quantitative conversion of initial alkene was observed in 3 hours with aldehyde yields of 80-85%. Unfortunately, the selectivity was low because the aldehydes with normal and isomerized chains were formed in similar amounts. A number of structurally similar phosphites 31 as components of catalytic systems for the hydroformylation of 1-octene are described (Scheme 4.19) [79], It was shown that the nature and steric volume of R did not significantly affect main processes. Thus, the depth of conversion was 80-90% with about 60% selectivity in all cases. 

In 1978, J. K. Stille and his group proposed an interesting extension of the concept of asymmetric synthesis via rhodium complexation by attaching the metallic site to an insoluble polymer (15). The main advantage of this modification is the possibility of recovering the optically active phosphine-rhodium complex catalyst. 

The rhodium-catalyzed conversion of aryl pyridyl ethers into arylboronates has been achieved using an NHC-supported rhodium catalyst (Scheme 6.31) [62]. The main theme of this work was the use of rhodium complexes to promote the cleavage of the pyridyl ether fragment and borylation of the arene. The reaction was carried out at elevated temperatures and afforded moderate to good yields of the arylboronates. One of the most impressive aspects of this chemistry was its tolerance to a wide range of functional groups. Heteroaryl ethers as well as substrates bearing esters, amides, and even a free amine were successfully converted into arylboronates. If the substrate is appropriately functionalized, this would be a reasonable approach to the formation of arylboronates. 

As invented by Wender,196,197 a variant of the second transformation can take place if the alkene partner is substituted by a participating group such as a strained cyclopropyl or a cyclobutanone (in the case of a 1,6-diene).198 The whole process, which mainly relies on the use of rhodium or ruthenium complexes,1 9 results in the formal... 

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