Rhodium alkyne complex

A study of dimerization of alkynes catalysed by RhCl(PMe3)3 showed the reaction to yield a mixture of straight-chain and branched products. The authors were able to identify a number of rhodium/alkyne complexes along the reaction route, and conclude that the reaction may be summarized by the scheme presented here. Extrusion of the metal then gave rise to products. 

Nucleophilic attack by a carbanion on a rhodium-acetylene complex has been postulated in the reaction of tolane with MeMgBr in the presence of (Ph3P)3 RhBr (Michman and Balog, 1971). This interpretation is supported by the known formation of rhodium-alkyne complexes under these conditions (Muller and Segnitz, 1973) and by the predominant formation of trans-a-methyl-stilbene. Additional study of the scope of this reaction would seem worthwhile since recycling of the rhodium-containing by-product (Ph3P)3 RhOH, should be possible. 

Scheme 15 Formation of 4-alkenyl(phenyl)-substituted 5-dialkylamino-2-ethoxycyclopen-tadienes 75 via transmetallated alkyne-inserted rhodium-carbene complexes 74 [73]. For further details see Table 2...

The reaction of alkenes with alkenes or alkynes does not always produce an aromatic ring. An important variation of this reaction reacts dienes, diynes, or en-ynes with transition metals to form organometallic coordination complexes. In the presence of carbon monoxide, cyclopentenone derivatives are formed in what is known as the Pauson-Khand reaction The reaction involves (1) formation of a hexacarbonyldicobalt-alkyne complex and (2) decomposition of the complex in the presence of an alkene. A typical example Rhodium and tungsten ... 

Rhodium also has been reported as a catalyst for [2+2+2] alkyne cycloaddition in water. Uozumi et al. explored the use of an amphiphilic resin-supported rhodium-phosphine complex as catalyst (Eq. 4.60). The immobilized rhodium catalyst was effective for the [2+2+2] cycloaddition of internal alkynes in water,113 although the yields of products were not satisfactory. 

The rhodium-catalyzed cyclization/hydrosilylation of internal diyne proceeds efficiently with high stereoselectivity (Scheme 106). However, terminal diynes show low reactivity to rhodium cationic complexes. Tolerance of functionalities seems to be equivalent between the rhodium and platinum catalysts. The bulkiness of the hydrosilane used is very important for the regioselectivity of the rhodium-catalyzed cyclization/hydrosilylation. For example, less-hindered dimethylethylsilane gives disilylated diene without cyclization (resulting in the double hydrosilylation of the two alkynes), and /-butyldimethylsilane leads to the formation of cyclotrimerization compound. 

Arylative cyclization of alkynals with arylboronic acids is catalyzed by rhodium-diene complexes and even proceeds enantioselectively in the presence of a chiral diene (Equation (48)).399... 

Thus far, rhodium(i) complexes are the most general, efficient, and selective catalysts, uniquely enabling [5 + 21-cycloadditions of tethered alkyne-VCPs, alkene-VCPs, and allene-VCPs. For example, when tethered alkene-VCP 7a (Equation (2)) is treated with [(cod)Rh(CioH8)]SbF6, the bicyclo[5.3.0]decene is produced in 96% yield. 

The mechanistic and synthetic puzzle of alkyne hydrosilylation opened more fully with the discovery that rhodium will catalyze the /r.mr-hydrosilylation of terminal alkynes.22 There is much work extant in this area, and good summaries of the various catalytic systems exist.11 A trans-addition process to give (Z)-j3-silane products G is well precedented with trialkylsilanes (Table 3), for both rhodium and mixed rhodium-cobalt complexes (entry 4).22,26 However, the selectivity erodes significantly upon switching to Me2PhSiH (entry 5), and, due to the mechanistic requirements for equilibration of the /3-silyl vinylrhodium intermediate, electron-poor silanes react exclusively to give CE)-/3-silane products B (see entries 6 and 7). 

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). 

Rhodium(I) and ruthenium(II) complexes containing NHCs have been applied in hydrosilylation reactions with alkenes, alkynes, and ketones. Rhodium(I) complexes with imidazolidin-2-ylidene ligands such as [RhCl( j -cod)(NHC)], [RhCl(PPh3)2(NHC)], and [RhCl(CO)(PPh3)(NHC)] have been reported to lead to highly selective anti-Markovnikov addition of silanes to terminal olefins [Eq. 

Aryl acetylenes undergo dimerization to give 1-aryl naphthalenes at 180 °C in the presence of ruthenium and rhodium porphyrin complexes. The reaction proceeds via a metal vinylidene intermediate, which undergoes [4 + 2]-cycloaddition vdth the same terminal alkyne or another internal alkyne, and then H migration and aromatization furnish naphthalene products [28] (Scheme 6.29). 

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). 

Chatani s proposed mechanism bears some similarity to that of Jun s reaction (Scheme 9.12). They both begin with hydroamination of the C=C 7t-bond of a rhodium vinylidene. The resultant aminocarbene complexes (71 and 62) are each in equilibrium with two tautomers. The conversion of 71 to imidoyl-alkyne complex 74 involves an intramolecular olefin hydroalkynylation. Intramolecular syn-carbome-tallation of intermediate 74 is thought to be responsible for ring closure and the apparent stereospecificity of the overall reaction. In the light of the complexity of Chatani and coworkers mechanism, the levels of chemoselectivity that they achieved should be considered remarkable. For example, 5 -endo-cyclization of intermediate 72 was not observed, though it has been for more stabilized rhodium aminocarbenes bearing pendant olefins [27]. 

Gyclization/hydrosilylation of enynes catalyzed by rhodium carbonyl complexes tolerated a number of functional groups, including acetate esters, benzyl ethers, acetals, tosylamides, and allyl- and benzylamines (Table 3, entries 6-14). The reaction of diallyl-2-propynylamine is noteworthy as this transformation displayed high selectivity for cyclization of the enyne moiety rather than the diene moiety (Table 3, entry 9). Rhodium-catalyzed enyne cyclization/hydrosilylation tolerated substitution at the alkyne carbon (Table 3, entry 5) and, in some cases, at both the allylic and terminal alkenyl carbon atoms (Equation (7)). 

Shibata and co-workers have reported an effective protocol for the cyclization/hydrosilylation of functionalized eneallenes catalyzed by mononuclear rhodium carbonyl complexes.For example, reaction of tosylamide 13 (X = NTs, R = Me) with triethoxysilane catalyzed by Rh(acac)(GO)2 in toluene at 60 °G gave protected pyrrolidine 14 in 82% yield with >20 1 diastereoselectivity and with exclusive delivery of the silane to the G=G bond of the eneallene (Equation (10)). Whereas trimethoxysilane gave results comparable to those obtained with triethoxysilane, employment of dimethylphenylsilane or a trialkylsilane led to significantly diminished yields of 14. Although effective rhodium-catalyzed cyclization/hydrosilylation was restricted to eneallenes that possessed terminal disubstitution of the allene moiety, the protocol tolerated both alkyl and aryl substitution on the terminal alkyne carbon atom and was applicable to the synthesis of cyclopentanes, pyrrolidines, and tetrahydrofurans (Equation (10)). 

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 rhodium(II) complex [Rh2(OAc)4] reduces terminal and cyclic alkenes, activated alkenes and alkynes.148 Various polar solvents could be used, but DMF was preferred. Following a kinetic investigation of the hydrogenation of 1-decene, the mechanism shown in equations (32)-(35) was proposed. 

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. 

Indenyl)rhodium(I) complexes, preparation, 7, 184—185 Indium complexes acid halide reactions, 9, 683 in alkene and alkyne allyindations, 9, 693 alkyl, aryl, alkynyls, 3, 288 in alkynylations, 9, 720... 

Enynes (77) can react with arylboronic acids in the presence of a catalytic amount of a rhodium(I) complex under mild conditions to give (Z)-l-(l-arylethyl- (g) idene)-2-vinylcyclopentanes (78).100 In analogy, the rhodium-catalysed cyclization of the cyano-substituted alkynes (79) with arylboronic acids has been reported to produce the cyclic ketone (80) as the first example of a nucleophilic addition of an Rh(I) species to a cyano group.101... 

These rhodium carbene complexes were successfully employed as catalysts in the hydrosilylation of terminal alkynes. It was found that selectivities improve when the remaining wingtip group on the imidazole ring is small. 

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). ... 

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