Rhodium complexes carbon bond activation reactions

The authors established directly the time scale for activation of C-H bonds in solutions at room temperature by monitoring the C-H bond activation reaction in the nanosecond regime with infrared detection. In the first stage of the process, loss of one carbon monoxide ligand (reaction VI-7 —- VI-8 in Scheme VI.6) substantially reduces back-bonding from the rhodium ion and increases the electron density at the metal center. Formed after the solvation stage, complex VI-9 traverses a 4.2 kcai nriol barrier (A = 5.0 x lo s ) and forms the -pCTp complex VI-10 which is more reactive toward C-H oxidative addition. 

Rhodium and nickel have been by far the most common metals aside from palladium employed in Suzuki-Miyaura carbon-carbon bond-forming reactions. Platinum has been used on several occasions, for example Bedford and Hazelwood showed that platinum complexes with n-acidic, ortfto-metalated triaryl phosphite and phosphinite ligands exhibited what they termed unexpectedly good activity in Suzuki biaryl coupling reactions with aryl bromide substrates (Scheme 13.22). Application to aryl chlorides resulted in low conversion to the desired biaryl products. 

Rhodium complexes provide some of the most attractive catalysts for carbon manipulation with high reactivity, regioselectivity, scope, and functional group tolerance. In particular, rhodium complexes have displayed potential for the synthesis of various heterocyclic and carbocyclic compounds through the C—H bond activation reactions. Rhodium complexes have been shown to catalyze sp C—H bond insertion into several pi bonds including alkenes, alkynes, aldehydes, and imines. ... 

In general, carbonylation proceeds via activation of a C-H or a C-X bond in the olefins and halides or alcohols, respectively, followed by CO-insertion into the metal-carbon bond. In order to form the final product there is a need for a nucleophile, Nu". Reaction of an R-X compound leads to production of equivalent amounts of X", the accumulation of which can be a serious problem in case of halides. In many cases the catalyst is based on palladium but cobalt, nickel, rhodium and mthenium complexes are also widely used. 

Chen et al. [20], for example, reported on chelation-assisted reactions in an article entitled Chelation-Assisted Carbon-Halogen Bond Activation by a Rhodium(I) Complex in 2009. These reactions proceed by C-Br bond activation via an oxidative addition mechanism. They take place in reactions of [Rh(PPh3)2(acetone)2] PFg" with 2-(2-bromophenyl)pyridine at room temperature to give the cyclometa-lated rhodium bromide shown in Eq. (6.4). 

Scheme 6.3 Easy cyclometalation reactions with a benzylamine proceed via agostic interaction as shown in agostic intermediate 6.6 Equation (6.4) Chelation-Assisted Carbon-Halogen Bond Activation by a Rhodium(I) Complex ...

Carbene complexes, generated by the reaction between metal salts and diazo compounds can insert into C-H bonds in a form of CH activation (see Chapter 3 for other CH activation reactions). While early reactions involved the use of copper salts as catalysts (Schemes 8.143 and 8.144), rhodium complexes are now more widely used. In molecules such as cyclohexane, there is no issue of regioselectivity, but this issue is critical for the use of the reaction in synthesis. Both steric and electronic factors influence selectivity. Carbon atoms where a build up of some positive charge can be stabilized are favoured. Hence, allylic positions and positions a- to a heteroatom such as oxygen or nitrogen, are favoured. The reaction at tertiary C-H bonds, rather than primary C-H bonds is also favoured for the same reason, but, in this case, are also disfavoured by steric effects. Reactivity and selectivity are also influenced by both the structure of the catalyst, and the... 

To present, silica-supported rhodium catalysts have been successfully used for hydrogenation, hydroformylation, and hydrosilylation reactions. Zhang and coworkers" developed a heterogeneous rhodium complexes 23 catalyzed carbon-heteroatom bond formation. The reaction couples disulfides 21 or diselenides with an alkyl or acyl halide to generate unsymmetrical sulfides (24) and selenides in good yields. The catalyst could be easily recovered and recycled by filtration of the reaction solution and re-used for five cycles without significant loss of activity (maintains over 90% yield)." ... 

Hydroformylation of PhCH=CHMe in the presence of RhCl(PPh3)3 produces PhCH(CHO)-CHa-CH3 and PhCH3-CH(CHO)-CH3, as well as some PhCH2 CH2 CH3. Both Rh4(CO)i2 and mixed carbonyl-phosphine complexes derived from Rh4(CO)i2 and from Rh,(CO)ie are active catalysts for hydroformylation of alkenes. Acyl-rhodium intermediates may be important when Rh4(COX2 is the catalyst, for [Rh-(C0)2(02CR)]2 dimers have been isolated from alkene hydroformylations in which this polynuclear carbonyl has been the catalyst. Rh4(CO)i2 also catalyses the reaction between ethylene and carbon monoxide, which produces several products, including octane-3,6-dione, undecane-3,6,9-trione, and tetradecane-3,6,9,12-tetraone. The products obtained indicate a mechanism in which addition of ethylene to rhodium and insertion of carbon monoxide into a rhodium-carbon bond alternate. ... 

Many rhodium(II) complexes are excellent catalysts for metal-carbenoid-mediated enantioselective C-H insertion reactions [101]. In 2002, computational studies by Nakamura and co-workers suggested the dirhodium tetracarboxylate catalyzed diazo compounds insertion reaction to alkanes C-H bonds proceed through a three-centered hydride-transfer-like transition state (Fig. 25) [102]. Only one rhodium atom of the catalyst is involved in the formation of rhodium carbene intermediate, while the other rhodium atom served as a mobile ligand, which enhanced the electrophilicity of the first one and facilitate the cleavage of rhodium-carbon bond. In this case, the metal-metal bond constitutes a special example of Lewis acid activation of Lewis acidic transition-metal catalyst. 

In this chapter, we have successfully developed bifunctional chiral rhodium complexes bearing chiral phebox ligands that can be used in catalytic asymmetric reactions. The N,C,N meridional geometry with the rhodium-carbon covalent bond is the key character in the phebox complexes. The metal-phebox cooperative bifunctionality significantly contributes reactivity and selectivity in the catalytic asymmetric reactions. Furthermore, the prototype of the bifunctional catalyst can be explained to a wide range of asymmetric catalytic reactions promoted by the Lewis acids, hydrides, enolates, and bory active species. Their diversity further broadens the range of opportunities for asymmetric catalysis. 

A model proposed for rationalizing the stereochemical outcome is also shown in Scheme 5.112. It seems plausible that the skewed structure 441, typical for transition metal BINAP complexes, has just one open space that is filled by a coordination of rhodium to the carbon - carbon bond of cyclohexenone. As a consequence, the insertion of the phenyl group then occurs from the Sf-face to the enone to give the / -rhodium complex 442 that subsequently tautomerizes to the more stable oxallyl-type enolate [205a]. It seems that a transmetallation at the stage of the enolate - from rhodium to boron enolate, as indicated in cycle B (Scheme 5.111) - doesnot occur in all these rhodium-mediated domino reactions without exception. Thus, Hayashi and coworkers produced evidence in support of a rhodium enolate as an active nucleophile in the aldol step when phenyl-9-BBN 438 was reacted with acyclic enones in the presence of [Rh(OH)-(S)-BINAP]2. In this case, the catalytic cycle is maintained by a transmetallation of the rhodium to the boron aldolate [221]. 

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