Alkynes rhodium catalyst

Hydrosilylation in the presence of a carbon electrophile is often accompanied by C-C bond formation. For example, three-component coupling of hydrosilane, alkyne, and y unsaturated aldehyde is suggested to proceed via oxanickelacycle intermediate to give (Z)-enol silyl ether (Scheme 3-28). Hydrosilylation of alkenes under a carbon monoxide atmosphere allows carbonyl incorporation, giving silyl enol ethers by using a cobalt or iridium catalyst (Schemes 3-29 and 3-30). Under similar reaction conditions in the presence of a rhodium catalyst, alkynes are converted to y silyl-substituted acroleins (Scheme 3-31). ... 

Recently, Aumann et al. reported that rhodium catalysts enhance the reactivity of 3-dialkylamino-substituted Fischer carbene complexes 72 to undergo insertion with enynes 73 and subsequent formation of 4-alkenyl-substituted 5-dialkylamino-2-ethoxycyclopentadienes 75 via the transmetallated carbene intermediate 74 (Scheme 15, Table 2) [73]. It is not obvious whether this transformation is also applicable to complexes of type 72 with substituents other than phenyl in the 3-position. One alkyne 73, with a methoxymethyl group instead of the alkenyl or phenyl, i.e., propargyl methyl ether, was also successfully applied [73]. 

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. ... 

Subsequently, cationic rhodium catalysts are also found to be effective for the regio- and stereoselective hydrosilation of alkynes in aqueous media. Recently, Oshima et al. reported a rhodium-catalyzed hydrosilylation of alkynes in an aqueous micellar system. A combination of [RhCl(nbd)]2 and bis-(diphenylphosphi no)propanc (dppp) were shown to be effective for the ( >selective hydrosilation in the presence of sodium dodecylsulfate (SDS), an anionic surfactant, in water.86 An anionic surfactant is essential for this ( )-selective hydrosilation, possibly because anionic micelles are helpful for the formation of a cationic rhodium species via dissociation of the Rh-Cl bond. For example, Triton X-100, a neutral surfactant, gave nonstereoselective hydrosilation whereas methyltrioctylammonium chloride, a cationic surfactant, resulted in none of the hydrosilation products. It was also found that the selectivity can be switched from E to Z in the presence of sodium iodide (Eq. 4.47). 

Arylation of alkynes via addition of arylboronic acids to alkynes represents an attractive strategy in organic synthesis. The first addition of arylboronic acids to alkynes in aqueous media catalyzed by rhodium was reported by Hayashi et al.89 They found that rhodium catalysts associated with chelating bisphosphine ligands, such as 1,4-Ws(diphenyl-phosphino)butane (dppb) and 1,1 -/ E(diphenylphospliino)fcrroccnc... 

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 coordination of the alkyne to the rhodium catalyst allows the carborhodation of the triple bond to afford the vinylrhodium intermediate 47 (Scheme 14). The rearrangement of this organometallic compound into the 2-(alkenyl)phenylrhodium intermediate 48 is evidenced by one deuterium incorporation resulting from the deuter-iolysis of the Rh-C bond. The addition of the phenylrhodium intermediate 45 must occur before its hydrolysis with water. The 2-(alkenyl)phenylrhodium intermediate 45, generated by the phenylrhodation of an alkyne followed by... 

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... 

Asymmetric cyclization-hydrosilylation of 1,6-enyne 91 has been reported with a cationic rhodium catalyst of chiral bisphosphine ligand, biphemp (Scheme 30).85 The reaction gave silylated alkylidenecyclopentanes with up to 92% ee. A mechanism involving silylrhodation of alkyne followed by insertion of alkene into the resulting alkenyl-rhodium bond was proposed for this cyclization. 

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]. 

Phosphonium salts can be synthesized by the transition-metal-catalyzed addition reaction of triaryphosphines and acids to unsaturated compounds. The reaction of PPh3, CH3SO3H, and alkynes in the presence of a palladium or rhodium catalyst gave alkenylphosphonium salts. Although Pd(PPh3)4 directed the C-P bond formation at the internal carbon atom of aliphatic 1-alkynes (Markovnikov mode), [RhCl(cod)]2... 

Intermolecular [4+2]-cycloaddition of vinylallenes with alkynes is efficiently mediated by means of an electronically tuned rhodium catalyst (Scheme 16.81) [91]. A five-membered rhodacycle is formed from the vinylallene. Coordination followed by insertion of an alkyne to the rhodacycle generates a seven-membered rhodacycle, from which rhodium(I) is eliminated reductively to produce a cyclohexatriene, leading to the aromatic compound. 

Oligomerization and polymerization of terminal alkynes may provide materials with interesting conductivity and (nonlinear) optical properties. Phenylacetylene and 4-ethynyltoluene were polymerized in water/methanol homogeneous solutions and in water/chloroform biphasic systems using [RhCl(CO)(TPPTS)2] and [IrCl(CO)(TPPTS)2] as catalysts [37], The complexes themselves were rather inefficient, however, the catalytic activity could be substantially increased by addition of MesNO in order to remove the carbonyl ligand from the coordination sphere of the metals. The polymers obtained had an average molecular mass of = 3150-16300. The rhodium catalyst worked at room temperature providing polymers with cis-transoid structure, while [IrCl(CO)(TPPTS)2] required 80 °C and led to the formation of frani -polymers. 

As would be expected, catalytic hydroboration is effective for alkynes as well as al-kenes, and prior examples have been reviewed [6]. An interesting development has been the diversion of the normal syn- to the anti-addition pathway for a terminal alkyne, with 99% (catechoborane) and 91% (pinacolborane) respectively (Fig. 2.5) [20]. The new pathway arises when basic alkylphosphines are employed in combination with [Rh(COD)Cl]2 as the catalyst in the presence of Et3N. Current thinking implies that this is driven by the initial addition of the rhodium catalyst into the alkynyl C-H bond, followed by [1,3]-migration of hydride and formal 1,1-addition of B-H to the resulting alkylidene complex. The reaction is general for terminal alkynes. 

Since the PK reaction with electron-defident alkynes was also problematic, even when stoichiometric Co2(CO)g was employed, promoters such as trialkylamine N-oxide were required for the reaction to proceed [14]. Alternatively, W(CO)5-THF may be employed semi-catalytically for this class of substrates [9cj. The rhodium catalyst [RhCl(CO)2]2, has shown great versatility for electron-deficient alkynes (Scheme 11.4) the reaction times are much shorter (1-3 h) than those of the usual examples (Tab. 11.3). This rhodium-catalyzed PK reaction may be extended to the synthesis of 6,5-fused ring analogs, as exemplified in the synthesis of bicyclo[4.3.0]nonenone 2o from the 1,7-enyne lo (Eq.4) [13 bj. 

Alternatively, rhodium catalysts have been revealed to be effective for the coupling of two alkynes with an isocyanide to afford the iminocyclopentadienes 62 and 62 in high yield (Tab. 11.9) [36bj. The coordinating solvent dibutyl ether, in combination with por-tionwise addition of the isocyanides, is key to the success of this transformation. 

Substitution of the VCP is tolerated both on and adjacent to the cyclopropane ring. Diester-substituted and heteroatom (O, NTs) tethers are well tolerated. Reactions were conducted with 2-10 mol% catalyst at up to 0.20 M, as illustrated. Most importantly, reactions with the naphthalene catalyst were found to be more rapid than those with other catalysts. For example substrate 54 is readily converted in >99% yield to cycloadduct 55 in only 15 min at room temperature (entry 1). Complex 93 efficiently catalyzes the reactions of both alkynes and alkenes with VCPs, offering greater generahty than thus far observed with non-rhodium catalysts. This catalyst is particularly advantageous in the cases of substrates 100 and 102, for which the desired product is not formed cleanly with Wilkinson s catalyst due to product isomerization. 

Witulski and Alayrac reported the synthesis of clausine C (clauszoline-L) (101) by a rhodium-catalyzed alkyne cyclotrimerization of diyne 1014 and propiolic ester 635 (561). Analogous to the hyellazole (245) synthesis (see Scheme 5.75), the diyne precursor 1014 required for this key cyclotrimerization reaction was obtained starting from readily available 2-iodo-5-methoxyaniline. Using Wilkinson s catalyst, [RhClfPPhsls], crossed-alkyne cyclotrimerization of 1014 and 635 led to N-tosylclausine C (1015) in 78% yield in an isomeric ratio of 3.8 1. Finally, deprotection of the tosyl group with TBAF in refluxing TFIF afforded clausine C (clauszoline-L) (101) (561) (Scheme 5.147). 

Rhodium catalysts demonstrate some additional unique features, effective for electron-deficient alkenes and alkynes, the previously formidable substrates (Equation (10)). Moreover, the reaction rate increases when pressure of CO is decreased down to 0.1 atm without loss of chemical yield. 

Wender and co-workers focused their attention on the selectivity of the reaction with dienynes mediated by rhodium catalysts. Disubstituted alkynes give excellent yields of PK products, whereas terminal alkynes react less efficiently. Substituents at 2- and 3-positions in the diene moiety tolerate well. The stereochemistry of alkenes is well conserved during the transformation. ... 

This appears to be the first report of the addition of H2 to the silylformyla-tion reaction mixture. Good yields are obtained when Et3SiH or PhjSiH is used in the reaction of 1-hexyne or 4-phenyl-l-butyne. Although a variety of functionally substitued terminal alkynes have been studied, most lead only to the silylformylation product and do not appear to be affected by the presence of H2 in the system. Other rhodium catalysts investigated, such as [Rh(COD)(dppb)]+BPh4 and Rh6(CO)16, catalyze the silylformylation reaction even under H2 pressure and do not lead to any of the silylhydrofor-mylated products. 

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... 

Phosphine-modified rhodium catalysts hydroformylate alkynes to saturated aldehydes.1 The reaction most likely proceeds by a rapid hydrogenation to yield the alkene, followed by hydroformylation. 

Hayashi et al. and Miyaura et al. have reported that far less nucleophilic aryl- and alkenyl-boronic acids can react with a variety of enones in the presence of a BINAP-rhodium catalyst to give adducts with high enantiopurity in general (Scheme 8D.5) [13], The one pot procedure, involving the hydroboration of alkynes as the first step (R = alkenyl), was achieved in the presence of amines without affecting the enantioselectivity [13]. 

Baidossi, W., Goren, N. and Blum, J. (1993) Homogeneous and biphasic oligomerization of terminal alkynes by some water soluble rhodium catalysts. J. Mol. Catal. A Chem., 85, 153. 

In the presence of a ruthenium catalyst, 3-diazochroman-2,4-dione 716 undergoes insertion into the O-H bond of alcohols to yield 3-alkyloxy-4-hydroxycoumarins 717 (Equation 285) <2002TL3637>. In the presence of a rhodium catalyst, 3-diazochroman-2,4-dione 716 can undergo insertion into the C-H bond of arenes to yield 3-aryl-4-hydroxy-coumarins (Equation 286) <2005SL927>. In the presence of [Rh(OAc)2]2, 3-diazochroman-2,4-dione 716 can react with acyl or benzyl halides to afford to 3-halo-4-substituted coumarins (Equation 287) <2003T9333> and also with terminal alkynes to give a mixture of 477-furo[3,2-f]chromen-4-ones and 4/7-furo[2,3-3]chromen-4-ones (Equation 288) <2001S735>. 

Hydrostannation of 1 -alkynes.1 Tributyltin hydride adds spontaneously to terminal alkynes to form a 1 1 mixture of ds- and f/ww-l-alkenylstannanes. However, use of this rhodium catalyst and a radical inhibitor (2, galvinoxyl) provides 2-alkenylstannanes as the major product. 

Potential difference in reactivity between two G-B bonds allowed the transformation of l,2-bis(boryl)-l-alkenes to 1-alkenylboranes via a cross-coupling with the aryl, 1-alkenyl, benzyl, and cinnamyl halides (Equation (23)).211-213 This tandem procedure synthetically equivalent to a yy/z-carboboration of alkynes was used for synthesizing Tamoxifen derivatives via stepwise double coupling with two of the G-B bonds.212,213 Hydrogenation of the resulting bisborylalk-enes with a chiral rhodium catalyst is synthetically equivalent to an asymmetric diboration of alkenes (Equation (24)).214... 

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