Alkynes catalysts, rhodium complexes

Secondary amines can be added to certain nonactivated alkenes if palladium(II) complexes are used as catalysts The complexation lowers the electron density of the double bond, facilitating nucleophilic attack. Markovnikov orientation is observed and the addition is anti An intramolecular addition to an alkyne unit in the presence of a palladium compound, generated a tetrahydropyridine, and a related addition to an allene is known.Amines add to allenes in the presence of a catalytic amount of CuBr " or palladium compounds.Molybdenum complexes have also been used in the addition of aniline to alkenes. Reduction of nitro compounds in the presence of rhodium catalysts, in the presence of alkenes, CO and H2, leads to an amine unit adding to the alkene moiety. An intramolecular addition of an amine unit to an alkene to form a pyrrolidine was reported using a lanthanide reagent. 

Similar reactions have been carried out on acetylene. Aldehydes add to alkynes in the presence of a rhodium catalyst to give conjugated ketones. In a cyclic version of the addition of aldehydes, 4-pentenal was converted to cyclopen-tanone with a rhodium-complex catalyst. In the presence of a palladium catalyst, a tosylamide group added to an alkene unit to generate A-tosylpyrrolidine derivatives. ... 

Rhodium complexes facilitate the reductive cydization of diyne species in good yield, although the product olefin geometry depends on the catalysts used. Moderate yields of -dialkylideneclopentane 169 resulted if a mixture of diyne 146 and trialkylsilane was added to Wilkinson s catalyst ClRh[PPh3]3 (Eq. 33) [101]. If, however, the diyne followed by silane were added to the catalyst, a Diels-Alder derived indane 170 was produced (Eq. 34). Cationic Rh complex, (S-BINAP)Rh(cod) BF4, provides good yields of the Z-dialkylidenecyclopentane derivatives, although in this case, terminal alkynes are not tolerated (Eq. 35) [102]. 

Efforts to tune the reactivity of rhodium catalysts by altering structure, solvent, and other factors have been pursued.49,493 50 Although there is (justifiably) much attention given to catalysts which provide /raor-addition processes, it is probably underappreciated that appropriate rhodium complexes, especially cationic phosphine complexes, can be very good and reliable catalysts for the formation of ( )-/3-silane products from a air-addition process. The possibilities and range of substrate tolerance are demonstrated by the two examples in Scheme 9. A very bulky tertiary propargylic alcohol as well as a simple linear alkyne provide excellent access to the CE)-/3-vinylsilane products.4 a 1 In order to achieve clean air-addition, cationic complexes have provided consistent results, since vinylmetal isomerization becomes less competitive for a cationic intermediate. Thus, halide-free systems with... 

The synthesis of cationic rhodium complexes constitutes another important contribution of the late 1960s. The preparation of cationic complexes of formula [Rh(diene)(PR3)2]+ was reported by several laboratories in the period 1968-1970 [17, 18]. Osborn and coworkers made the important discovery that these complexes, when treated with molecular hydrogen, yield [RhH2(PR3)2(S)2]+ (S = sol-vent). These rhodium(III) complexes function as homogeneous hydrogenation catalysts under mild conditions for the reduction of alkenes, dienes, alkynes, and ketones [17, 19]. Related complexes with chiral diphosphines have been very important in modern enantioselective catalytic hydrogenations (see Section 1.1.6). 

In less-coordinating solvents such as dichloromethane or benzene, most of the cationic rhodium catalysts [Rh(nbd)(PR3)n]+A (19) are less effective as alkyne hydrogenation catalysts [21, 27]. However, in such solvents, a few related cationic and neutral rhodium complexes can efficiently hydrogenate 1-alkynes to the corresponding alkene [27-29]. A kinetic study revealed that a different mechanism operates in dichloromethane, since the rate law for the hydrogenation of phenyl acetylene by [Rh(nbd)(PPh3)2]+BF4 is given by r=k[catalyst][alkyne][pH2]2 [29]. 

Alkynes react with the bulky germanium hydride (MejSdjGeH to selectively yield (Z)-alkenes (Equation (105)).67 The hydrogermylation of alkynols or alkynes can be catalyzed by a rhodium complex (Equation (106), Table 18) and some of the intermediates were identified (Scheme 16).132 Similar rhodium species react with alkynes to yield alkenyl complexes,133 and other transition metal complexes have been employed as hydrogermylation catalysts including those containing palladium.134,135... 

We (79TH1 81GEP3117363 84USP4588815) and others (87MI1) have studied acetylacetonato and rj -cp-rhodium complexes as catalysts in the pyridine formation [Eq.(l)]. Resin-attached cp-rhodium complexes are also active in the cocyclization of alkynes and nitriles, and the activity is... 

This catalyst system was the first to utilize both terminal alkynes and olefins in the intramolecular reaction. Although a mechanistic rationale for the observed stereoselectivity was not offered, the formation of the single stereoisomer 26 may be rationalized through the diastereotopic binding of the rhodium complex to the diene moiety (Scheme 12.3). This facial selective binding of the initial ene-diene would then lead to the formation the metallacycle III, which ultimately isomerizes and reductively eliminates to afford the product [14]. 

In our initial studies on the [5+2] cycloaddition, several metal catalysts were screened. Rhodium(I) systems were found to provide the optimum yields and generality [26]. Since the introduction of this new reaction in 1995, our group and others have reported other catalyst systems that can effect the cycloaddition of tethered VCPs and systems. These new catalysts thus far include chlororhodium dicarbonyl dimer ( [RhCl(CO)2]2 ) [27], bidentate phosphine chlororhodium dimers such as [RhCl(dppb)]2 [28] and [RhCl(dppe)]2 [29], and arene-rhodium complexes [(arene)Rh(cod)] SbFs [30]. [Cp Ru(NCCH3)3] PFg has also been demonstrated to be effective in the case of tethered alkyne-VCPs [31], but has not yet been extended to intermolecular systems or other 2n -components. 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

Rhodium complexes with l,3-bis(di-fert-butylphosphino)methane (dtbpm), [(dtbpm) RhCl]2/PPh3 (89), (dtbpm)RhS i(OEt)3 (PMe3) (90) and (dtbpm)RhMe(PMe3) (91) are found to be effective catalysts for the hydrosilylation of an internal alkyne, 2-butyne, with HSi(OEt)3 at ambient temperature without solvent to yield (E)-2-triethoxysilyl-2-butene with complete stereoselectivity in quantitative yield using a proper concentration of the catalysts, i.e. >0.05 mol% for 89, >0.4 mol% for 90 and 91114. When the reaction is carried out at lower catalyst concentrations, i.e. 0.1 mol% for 90 or 91, (Z)-product is formed via frans-addition in 7-13% yield. 

Research on intermolecular hydroacylation has also attracted considerable attention. The transition-metal-catalyzed addition of a formyl C-H bond to C-C multiple bonds gives the corresponding unsymmetrically substituted ketones. For the intermolecular hydroacylation of C-C multiple bonds, ruthenium complexes, as well as rhodium complexes, are effective [76-84]. In this section, intermolecular hydroacylation reactions of alkenes and alkynes using ruthenium catalysts are described. 

Rhodium complexes are effective catalysts for the PKR and are receiving much attention. In addition to the studies by Narasaka with [RhCl (CO)2]2 [90], Jeong has introduced several species as new catalysts. Some of these rhodium complexes need activation with AgOTf. The reaction works well with non-terminal alkynes (36) and the scope and efficiency is dependent on the catalyst used. In the case of chiral species, a careful choice of conditions, including CO pressure, activation, solvent and ligands, is essential to obtain 37 with high enantioselectivity (Scheme 12) [91]. 

During our investigation, PPhs and CH3SO3H were found to add to unactivated alkynes in the presence of a palladium or rhodium complex. The regiochemistry and stereochemistry could be controlled by a judicious selection of the metal catalyst. Alkenylphosphonium salts have various applications in synthetic chemistry,and the present method enables an easy access to the organophosphorous compounds using readily available starting materials. 

Reactions of soluble metal complexes, whose mechanisms of catalysis appear to be reasonably well known, can serve as a guide to the main reaction paths followed on heterogeneous catalysts. Mononuclear complexes catalyze syn addition of H2 to alkynes to yield initially only cis isomers, as in equation (25). 5 More recently, Muetterties and coworkers showed that the dinuclear rhodium hydride complex ( yi-H)Rh[P(OPr )3]2 2 (38) converts alkynes to trans isomers as initial products (equation 26). The alkyne addition compound (39) was isolated its structure shows the vinyl group bonded to one rhodium atom by a a-bond and to the other by a ir-bond, while the substituents on the vinyl group are trans to one another. This structure resembles ones hypothesized earlier to explain the formation of trans isomers and alkanes. Hydrogenations of alkynes which are catalyzed by the dinuclear rhodium hydride are much slower than the hydrogenation of an alkene catalyzed by the dinuclear tetrahydride (40), which is formed rapidly from (38) in the presence of H2 (equation 11). ... 

Rhodium complexes of various types, e.g. IRhCI(PPh3)3], [RhCl(CO)(PPh,3)2], [lRhCl(CO)2 2l, [RhH(CO)(PPh3)3], [(RhCl(CgHi4)212] and [ RhCl(C2H4)2 2], are available for the hydrosilylation of alkenes and alkynes as well as enones and ketones. Under strictly deoxygenated conditions with the pure rhodium complex, the reaction is extremely slow, and a trace amount of oxygen or peroxide is necessary to activate the catalyst. ... 

Aromatic hydrocarbons, such as benzene add to alkenes using a ruthenium catalyst a catalytic mixture of AuCVAgSbFs, or a rhodium catalyst, and ruthenium complexes catalyze the addition of heteroaromatic compounds, such as pyridine, to alkynes. Such alkylation reactions are clearly reminiscent of the Friedel-Crafts reaction (11-11). Palladium catalysts can also be used to for the addition of aromatic compounds to alkynes, and rhodium catalysts for addition to alkenes (with microwave irradiation). " Note that vinyhdene cyclopropanes react with furans and a palladium catalyst to give aUylically substituted furans. ... 

Cationic rhodium complexes, e. g., [Rh(cod)2] BF [48] and [Rh(coe)] C104 [49], have recently appeared as regio- and stereoselective catalysts for hydrosily-lation of alkynes and exclusive formation of vinylsilanes instead alkylsilanes in the hydrosilylation of alkenes. 

In the presence of a nickel catalyst, aroyl-, alkanoyl, and aminocarbonyl-stannanes add to alkynes to give (3-acylvinylstannanes (Scheme 5.7.12). " Compared with other nickel-catalyzed carbostannylations of alkynes, the regioselectivity of the acylstannylation is not sufficient. The aminocarbonylstannylation of alkynes is also catalyzed by a rhodium complex. ... 

Usually, initiation of chain growth wifh rhodium catalysts does not require an alkylating agent. Kern has postulated fhe formation of an active species via oxidative addition of the terminal alkyne to rhodium, so-called hydroethynylation [138], This reaction has also been reported by Werner et al. [141] when contacting [RhCl(P Pr3)2] with phenylacetylene in fhe presence of pyridine, the alkyne hydride complex 23 is generated (Scheme 7.10). In fhe presence of a base such as cyclopentadienyl sodium, a tetracoordinated Rh complex 24 was obtained and isolated. 

Joo et al. utilized the highly water-soluble rhodium complex [RhCl(CO)(TPPTS)2] for the polymerization of terminal alkynes (phenylacetylene and (4-methylphe-nyl)acetylene for the structure of TPPTS cf. Section 7.2.2.3) [150]. This catalyst selectively produces cis-transoid polymers at room temperature in homogeneous solution in water/methanol mixtures, as well as in biphasic mixtures of water and chloro-... 

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. 

Pathways corresponding to those of Scheme 3 are applicable for hydrogenation of alkynes by monohydride catalysts, the key intermediate now is a vinyl rather than an alkyl (10). Some rhodium complexes effective for hydrogenation of alkynes to alkenes were mentioned earlier and are listed in Table 3 with other rhodium catalysts. 

Table 3. Rhodium Catalysts That Effect Hydrogenation of Alkynes to Monoenes Complex (Ref.)...

Rhodium complexes generated from A-functionalized (S)-proline 3.60 [933, 934, 935] or from methyl 2-pyrrolidone-5-carboxylates 3.61 [936, 937, 938] catalyze the cyclopropanation of alkenes by diazoesters or -ketones. Diastereoisomeric mixtures of Z- and E-cydopropylesters or -ketones are usually formed, but only the Z-esters exhibit an interesting enantioselectivity. However, if intramolecular cyclopropanation of allyl diazoacetates is performed with ligand 3.61, a single isomer is formed with an excellent enantiomeric excess [936,937], The same catalyst also provides satisfactory results in the cyclopropanation of alkynes by menthyl diazoacetate [937, 939] or in the intramolecular insertion of diazoesters into C-H bonds [940]. 

Hydrogenation of alkynes. This cationic complex of rhodium has been found to be superior to other catalysts, including the Lindlar catalyst, for hydrogenation of alkynes to c/y-olefins. Thus CaH5CsCCOOC2Ha is hydrogenated in acetone with this catalyst to ethyl c/i-cinnamate in essentially quantitative yield with no trace of the franj-isomer or of the completely reduced acid. In this particular case, use of Lindlar catalyst results primarily in complete reduction. 

A brief review of the cotrimerization of alkynes with nitriles to give pyridines has appeared. Pyridine itself is produced from C2H2 and HCN in benzene at 110 C/60 min under 23 bar pressure. The borabenzene complex (54) as the catalyst gives 103 turnovers. The catalytic formation of substituted thiophenes from alkynes and elemental sulfur in the presence of [CPC0L2] catalysts is also mentioned. Analogous rhodium complexes [Cp RhL2] also catalyze the formation of pyridines. 

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