Rhodium complexes catalyst, cyclization

Very recently, Wiedenhoefer272 has devised the first asymmetric 1,6-enyne hydrosilylation/cyclization tandem process using a rhodium(l) catalyst with (R)-276 as chiral ligand where rhodium-BINAP complexes were not effective (Scheme 70). More developments on this reaction are covered in Chapter 11.13. 

The first rhodium-catalyzed reductive cyclization of enynes was reported in I992.61,61a As demonstrated by the cyclization of 1,6-enyne 37a to vinylsilane 37b, the rhodium-catalyzed reaction is a hydrosilylative transformation and, hence, complements its palladium-catalyzed counterpart, which is a formal hydrogenative process mediated by silane. Following this seminal report, improved catalyst systems were developed enabling cyclization at progressively lower temperatures and shorter reaction times. For example, it was found that A-heterocyclic carbene complexes of rhodium catalyze the reaction at 40°C,62 and through the use of immobilized cobalt-rhodium bimetallic nanoparticle catalysts, the hydrosilylative cyclization proceeds at ambient temperature.6... 

The stoichiometric reaction of low-valent rhodium salts with l, -diynes to afford rhodacyclopentadiene complexes is well established and has been reviewed.73 733 The first rhodium-catalyzed reductive cyclization of a non-conjugated diyne has been reported only recently.74 743 The stereochemical outcome of the rhodium-catalyzed hydrosilylation-cyclization is dependent upon the choice of catalyst. Whereas reductive cyclization of 1,6-diyne 54a catalyzed by Rh4(CO)i2 provides modest yields of the Z-vinylsilane 54c, exposure of 54a to Wilkinson s catalyst... 

An intramolecular C-H/olefin coupling reaction can provide a cyclization product. Rhodium complexes involving [RhCl(coe)2]2-PR3 and ( -C5Me5)Rh(C2H3SiMe3)2 complexes are superior as catalysts. Some ruthenium complexes are also reasonably effective for cyclization reactions. Intramolecular olefmic C-H/olefm coupling with the aid of Ru(CO)2(PPh3)3, which is also effective for the reaction of aromatic ketones with olefins, yields the carbocyclic compounds in high yield (Eq. 9.11) [25]. 

Application of this cyclization reaction to a large variety of 4-pentenals with the aid of the rhodium complex has been reported. The first example of an asymmetric cyclization of 4-pentenals via hydro acylation using a chiral rhodium diphosphine catalyst was published by Sakaki et al. in 1989 [ 104]. The diphosphine ligand ((lS,2S)-rraws-l,2-bis(diphenylphosphinomethyl)cyclohexane) having a cyclohexane backbone in the chiral center shows the better asymmetric induction than DIOP ligand. Various types of enals are applicable to this asymmetric intramolecular hydro acylation reaction [105,106]. The use of BINAP ligand as the chiral auxiliary improves the optical yield to >99% ee when 4-substituted 4-pentenals are used as the substrate (Eq. 49) [106]. Steric repulsion between the substituent at the 4-position and the substituent on the phosphine atom controls the enantiofacial selection. 

Rhodium complexes facilitate the reductive cyclization 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]. 

Doyle s rhodium(n) carboxamidate complexes are undisputedly the best catalysts for enantioselective cyclizations of acceptor-substituted carbenoids derived from diazo esters and diazoacetamides, displaying outstanding regio- and stereocontrol.4 These carboxamidate catalysts consist of four classes of complexes pyrrolidinones... 

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. 

A hydrosilylation/cyclization process forming a vinylsilane product need not begin with a diyne, and other unsaturation has been examined in a similar reaction. Alkynyl olefins and dienes have been employed,97 and since unlike diynes, enyne substrates generally produce a chiral center, these substrates have recently proved amenable to asymmetric synthesis (Scheme 27). The BINAP-based catalyst employed in the diyne work did not function in enyne systems, but the close relative 6,6 -dimethylbiphenyl-2,2 -diyl-bis(diphenylphosphine) (BIPHEMP) afforded modest yields of enantio-enriched methylene cyclopentane products.104 Other reported catalysts for silylative cyclization include cationic palladium complexes.105 10511 A report has also appeared employing cobalt-rhodium nanoparticles for a similar reaction to produce racemic product.46... 

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