Rhodium zwitterionic complexes

Anionic polymerization of phenylacetylene to a trans-cisoid polymer in the presence of crown ether phase-transfer catalysts initiated by sodium amide has been reported.425 In contrast, the zwitterionic rhodium complex Rh+(COD)BPhJ yields a ds-transoid product in the presence of Et3SiH.426... 

In 2001, Van den Hoven and Alper reported the unexpected 2(Z)-6(ft)-47/-[l,4]-thiazepin-5-one 215, as the major product, from the reaction of acetylenic thiazole 214 with carbon monoxide and hydrogen in presence of a zwitterionic rhodium complex and triphenyl phosphite. After optimization of the reaction condition, the pressure, and the temperature, up to 90% yield is achieved with good selectivity for thiazepine 215 over thiazole side products 216-218 (Scheme 38) <2001 JA1017>. 

Hydroformylation reactions that are mediated by rhodium catalysts can also be incorporated into cascade sequences. The zwitterionic rhodium complex 694 promotes a tandem cyclohydrocarbonylation/CO insertion reaction producing pyrroli-none derivatives that contain an aldehyde functional group in good yields (01JA10214). In one example, exposure of a-imino alkyne 693 to catalytic quantities of 694 and (PhO)3P under an atmosphere of CO and H2 at 100 °C produced pyrrolinone 695 in 82% yield (Scheme 113). A variety of alkyl substitutents can be tolerated in this reaction. 

Originally discovered by Reppe and co-workers, hydroaminomethylation of olefins has attracted attention as an economical, one-pot synthesis of amines. Alper and co-workers have demonstrated that zwitterionic rhodium complex [Rh (q -cyclooctadiene)(q -CgH5BPh3)-] 74 is an excellent catalyst for hydroaminomethylation of phenylalkenes 75 to afford the corresponding branched methylated amines 76 with high regioselectivity (Scheme 22) [42]. 

However, a zwitterionic rhodium complex Rh 7P(OPh)3 mainly provides the formyl diene 200, together with the nonconjugated unsaturated aldehyde 201 as by product. This phosphite provides similar yields to BIPHEPHOS, which, however, gives a more active catalyst. 

Rhodium carbonyl complexes in the presence of amines are used as catalytic systems, although zwitterionic rhodium complex has also provided excellent yields. 

Diolefins in zwitterionic rhodium complexes can likewise be replaced by chelating phosphines. NMR studies have revealed that cationic rhodium complexes, formed with diphosphines in the first step, lose under air COD and a new zwitterionic complex is formed, as exemplarily shown in Scheme 1.26 [52]. Such complexes have been frequently screened in hydroformylation [65]. 

This ligand forms electroneutral zwitterionic rhodium complexes, which are not soluble in water but are very soluble in alcohol-water mixtures [14], which is required for reactions in biphasic systems with aqueous methanol or ethanol as the lower phase used to enhance the solubility of olefins or other moderately polar substrates. 

The zwitterionic rhodium complex [( n6-C 5BPh3)] [Rh( n -COD)2] was reported O to be an excellent regio- and stereoselective catalyst for the reactions of alkynes with hydrosilanes... 

A surprising variation on the silylformylation reaction has been reported by Zhou and Alper. A zwitterionic rhodium(I) complex, (l,5-COD)Rh+(Tj6-PhBPh3), catalyzes the silylformylation of alkynes under normal reaction conditions. However, if H2 is added to the system, the reaction may proceed to yield silylalkenals of a different structure. Interestingly, although the H2 must play a key role in the reaction, it is not incorporated in the product. At this time, the mechanistic role of the hydrogen remains unclear. The authors term this reaction a silylhydroformylation [Eq. (51)].126... 

The only report of catalytic germylformylation in the literature at this time is also effected by the zwitterionic rhodium(I) complex (l,5-COD)Rh1 (T76PhBPh3)-.127 The synthesis of (Z)-3-germylalk-2-enals in good yield is always accompanied by a minor amount of hydrogermylation [Eq. (52)]. 

Zwitterionic rhodium(I) complex, Rh (CODX -CfiHsBPtn), is also found to be an efficient catalyst for the silylformylation of 1-alkynes at 40 °C and 40 atm of CO in CH2C12 (equation 125) although no reaction occurs with internal alkynes327. However, silylhydroformylation takes place when the reaction is carried out under hydroformylation conditions, i.e. in the presence of CO and H2 (CO/H2 = 1/1), to give (E)-2-silylmethyl-2-alkenals (319) in 54-92% isolated yields (equation 128). The intermediacy of 7r-allenyl-Rh species is proposed to account for the formation of 31 9327. When 4-acetoxy-l-butyne and 4-(p-tosyloxy)-l-butyne are used as the substrates, saturated silylhydroformylation products are obtained327. 

In the first work on the PTC carbonylation of organohalides, Pd(PPh3)4 was used as a metal catalyst [110]. There are a number of other examples of the use of Pd° complexes in two-phase carbonylations. The selective formation of one of four possible products, including esters, depends mainly on the composition of the metal complex (e. g., Pd(PPh3)4 vs. Pd(dba)2) [129]. Radical intermediates may be involved in the formation of several of the products. Esters (e. g., benzyl phenylacetate from benzyl bromide) are the major products of the carbonylation of benzyl bromides using a zwitterionic rhodium catalyst [130]. 

Water-solubility does not always depend solely upon the water-solubility of the functionalized ligand itself it can also arise from specific water-metal interactions. The literature reports cationic as well as zwitterionic complexes [21-24], These systems are not yet well known and some complexes can behave in an unexpected way. Thus, it has been reported that the SULPHOS-rhodium complex 14... 

Rhodium complexes with a bis(diphenylmethyl)norbornane a bis(dioxa-phospholane) and a,a-TREDlP (30) ° as well as a zwitterionic rhodium-... 

HDS catalysts generally consist of (heterogeneous) Mo or W sulfides on alumina supports. However, Bianchini et al. described a two-step procedure for HDS of thiophenes by the hydrogenolysis of thiols, followed by the desulfurization of the thiols by applying their zwitterionic rhodium(I) complex, [Rh(sulphos((cod)] (see previous section) [17]. This complex is soluble in polar solvents, such as methanol and methanol-water mixtures, but not in hydrocarbons. Benzo[b]thiophene was chosen as substrate since it is one of the most difficult thiophene derivatives to degrade. Under the mild reaction conditions of the two-step process, the benzene rings of the (di)benzothiophenes were not affected. In the absence of a base, the double bond of benzo[b]thiophene was hydrogenated, while in the presence of a base (NaOH) 2-ethylthiophenolate was the major product (Scheme 1). 

Several studies in this field involve the use of SULPHOS (Scheme 45), a tripodal phosphine ligand having a para-sulfonated benzylic substituent on its bridgehead carbon atom. The rhodium complexes [(COD)Rh(SULPHOS)] and [Rh(CO)2(SULPHOS)] are zwitterionic, that is, the sulfonate group acts as the... 

A family of neutral cationic and zwitterionic Rh(I) and Ir(I) complexes with P,N-, P,0-, P(S),0-, and P(S),N-substituted indene ligands have been tested in the hydrosilylation and the dehydrogenative silylation of st3rrene. Cationic rhodium complexes, such as [Rh(cod)2]+BF4, have recently appeared as regio- and stereoselective catalysts for hydrosilylation of alkynes and exclusive formation of vinylsilanes instead of alkylsilanes in the hydrosilylation of alkenes (73). 

Alper utilized in several investigations zwitterionic Rh complexes (Scheme 1.22). They can be simply prepared by the reaction of rhodium chloride with sodium tetraphenylborate and a cyclic diene in aqueous methanol [52]. Upon the effect of syngas, the diene (COD or NBD) is replaced by CO [53]. NBD is superseded already at room temperature, whereas the substitution of COD required gentle heating. Especially, the COD-based precatalyst was tested In a large variety of hydroformylation reactions [54]. 

Organometallic chemistry of pyrrole is characterized by a delicate balance of the ti N)- and -coordination modes. Azacymantrene is an illustration of the considerable nucleophilicity of the heteroatom. However, azaferrocene can be alkylated at C2 and C3 sites. Ruthenium and osmium, rhodium, and iridium chemistry revealed the bridging function of pyrroles, including zwitterionic and pyrrolyne complex formation. The ti (CC) coordination of osmium(2- -) allows versatile derivatizations of the heteroring. 

Several additional studies were carried out to obtain information about the precise behavior of the various components in the model system. The interplay between the manganese porphyrin and the rhodium cofactor was found to be crucial for an efficient catalytic performance of the whole assembly and, hence, their properties were studied in detail at different pH values in vesicle bilayers composed of various types of amphiphiles, viz. cationic (DODAC), anionic (DHP), and zwitterionic (DPPC) [30]. At pH values where the reduced rhodium species is expected to be present as Rh only, the rate of the reduction of 13 by formate increased in the series DPPC < DHP < DODAC, which is in line with an expected higher concentration of formate ions at the surface of the cationic vesicles. The reduction rates of 12 incorporated in the vesicle bilayers catalyzed by 13-formate increased in the same order, because formation of the Rh-formate complex is the rate-determining step in this reduction. When the rates of epoxidation of styrene were studied at pH 7, however, the relative rates were found to be reversed DODAC DPPC < DHP. Apparently, for epoxidation to occur, an efficient supply of protons to the vesicle surface is essential, probably for the step in which the Mn -02 complex breaks down into the active epoxidizing Mn =0 species and water. Using a-pinene as the substrate in the DHP-based system, a turnover number of 360 was observed, which is comparable to the turnover numbers observed for cytochrome P450 itself. 

Fig. 3.17. Two reactions that demonstrate the stereospecificity of Rh-catalyzed cis-cyclo-propanations of electron-rich alkenes. — The zwitterionic resonance form A turns out to be a better presentation of the electrophilic character of rhodium-carbene complexes than the (formally) charge-free resonance form B or the zwit-ter-ionic resonance form (not shown here) with the opposite charge distribution ( adjacent to the C02Me groups, on Rh) rhodium-carbene complexes preferentially react with electron-rich alkenes.

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