Rhodium/chiral diene complexes

A variety of electrophiles are employed for carbonyl alkenylation and arylation that proceed through transmetalation from silicon to transition metal catalysts. For example, addition of alkenylsilanes and aryl(trimethoxy)si lanes to aldehydes is catalyzed by copper/DTBM-SEGPHOS, which mediates transmetalation from silicon to copper to give a wide variety of chiral alcohols with high degree of % ee (Scheme 3-144). The enantioselective alkenylation of imines using 2-(hydroxylmethyl)phenyl-substituted propen-2-ylsilane is catalyzed by a rhodium/chiral diene complex. ... 

A chiral rhodium(I)-diene complex catalyses the addition of dimethylzinc to N-tosylarylimines, ArCH=NTs, with ees up to 98%.40... 

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... 

Complexes of this type in which chiral diphosphines are present have become most important in asymmetric catalytic hydrogenation (see Section 61.2.3). Under hydrogenation conditions the diene is reduced to give cationic solvated rhodium(I)-phosphine complexes which initiate the... 

As mentioned, in most copper and zirconium catalyzed alkylations of imines, the addition of less reactive dimethylzinc (relative to diethylzinc) usually requires a large excess of the organometallic reagent. Hayashi reported that the methylation of N tosyl imines could be achieved using only 1.5 equiv of dimethylzinc in the presence of a chiral rhodium complex that is coordinated with chiral diene 54 [86]... 

Most rhodium catalysts for the enantioselective reduction of the C=N group are prepared in situ from a dimeric Rh-diene complex and a chiral diphosphine. Only few of the tested diphosphine ligands exhibit enantioselectivities >70% bdpp, cycphos, and phephos for imines and duphos for acylhydrazones. The activity of most Rh-diphosphine complexes for imine hydrogenation is low and therefore most of them are of limited practical use. Although some catalysts work already at ambient reaction conditions, most Rh-diphosphine complexes show low tof s even at elevated hydrogen pressures (>60 bar). 

O-Methylation of mandelic acid leads to the enantiomers of a-methoxy-M-phcnylacetic acid (10), which are also commercially available. This methylation without noticeable racemiza-tion was achieved with diazomethane, using aluminum tris(tert-butanoate) as catalyst8. Alternatively, dimethyl sulfate/ sodium hydroxide has been used15, as described in detail for the racemic compound10. The acids have been used for the construction of quite sophisticated chiral auxiliaries, e.g., a rhodium cyclopentadienyl complex (Section 7.2.2.), and for chiral dienes applied in both normal and inverse Diels-Alder reactions (Section D.1.6.1.1.1.). Chiral dienes, e.g., 1, for normal Diels -Alder reactions were prepared by pyrolysis (460 C) of a tricyclic precursor cstcrified with (S)-O-methylmandeloyl chloride or with the free acid and dicyclohexylcarbodiimide/4-dimethylaminopyridine11 -13. 

A fortunate feature of asymmetric catalysis is that chiral biphosphine rhodium diene complexes, especially with norbornadiene, crystallize well... 

Both, Hayashi and Carreira developed, independently, chiral dienes as novel ligands in asymmetric catalysis (Figure 8.6) [66]. Early investigations by Miyaura revealed that the rhodium-catalyzed conjugate addition of arylboronic acids to a,P-unsaturated ketones could be very efficiently catalyzed by a rhodium(I) complex of cyclooctadiene [67]. As a consequence, the development of chiral cod-analogues... 

Remarkable chemoselectivity was also observed when alkyne-tethered electron-deficient olefins 158 were employed as substrates (Scheme 8.44) [123]. Rhodium/ diene catalysis led preferentially to carborhodation of the alkyne, followed by 1,4-addition to the a,P-unsaturated moiety to provide adducts 159 in high yield and excellent enantioselectivity. The difference between phosphines and chiral dienes as ligands in this reaction setting was striking. Whereas, rhodium-bisphosphines complexes catalyzed the 1,4-addition to a,P-enoates more effectively than the aryla-tion of alkynes, rhodium/diene catalysts favored the arylation of alkynes over the 1,4-addition. 

Asymmetric cycloisomerization of nitrogen-bridged 1,6-enynes occurs in the presence of a cationic rhodium complex coordinated with a chiral diene/phosphine tridentate ligand to give chiral 3-azabicyclo[4.1.0]heptenes with high enantioselectivity (Scheme 149). " ... 

Novel chiral diene ligands were reported to form highly active complexes with rhodium for asymmetric conjugate addition (236). Feng s ligand of a wedge structure provided a chiral environment around rhodium, giving excellent enan-tiocontrol in the asymmetric arylation of N-tosylarylimines (237) and in asymmetric 1,4-addition of aryl boronic acids (238). 

As mentioned in the previous update, the as)unmetric cyclopropanation reaction was generally ineffective when dimethyl diazomalonate was used as carbenoid source the best ee value (50%) was reported in the cyclopropanation of styrene catalyzed by Rh2(45-MEAZ)4 (eq IS). Yet, recently the reaction of styrene with dimethyl diazomalonate in the presence of a chiral diene-rhodium catalyst was demonstrated. A catalytic amount (4 mol %) of NaBAr 4 [Ar = 3,5-bis(trilluoromethyl)phenyl] was employed for the generation of a cationic complex, and the reaction exhibited high enantioselectivity (eq 43). 

The synthesis of phenethyltetraphenylcyclopentadiene allowed the preparation of the chiral complex Rh C5Ph4CH(Me)Ph (cod). Chiral-annulated cyclopentadienyl ligands afforded the preparation of complexes such as ( -cycloocta-l,5-diene)[ j -l,2,3-triphenyl-4-methyl-7-isopropyl-lH-indenyl]rhodium, as a mixture of the exo- 424 and endo-AT ) isomers, together with a chiral indenyl complex 426, which did not show any optical activity. ... 

Hayashi and coworkers used chiral diene-rhodium complex 218 for the cyclopropanation of diazomalonate (Scheme l.KX)) [152]. The optical purity of the product 219 was more than 80% ee. 

More recent developments of this reaction include the use of chiral metal diene complexes [186]. In this context, Hayashi developed the Cj-symmetric diene ligand 293 and demonstrated its application in asymmetric arylations of N-tosyl arylimines, such as 291, to give optically active diaryl carbamines as exemplified by 294 (99%, 99% ee. Equation 31) [187], Hayashi has proffered an explanation of the observed enantioselectivity based on steric considerations. The C2-symmetric ligand 293 enforces preferential coordination of the si-face of the imine to the rhodium center, which subsequently delivers the arene to give 294 as depicted in the suggested transition state assembly 295. 

Chelating dienes have recently emerged as a promising class of chiral ligands for transition-metal-catalyzed processes as a consequence of independent concurrent studies by Hayashi [148,149] and by Carreira [150-153]. The first example with a chiral rhodium-diene complex for conjugate addition reactions employed chiral C2-symmetric diene ligand 193 (Equation 36) [148, 149]. Addition of boronic acid 192 to cyclohexenone catalyzed by the putative complex formed in situ from 193 and Rh(I) afforded adduct 194 in 90% yield and 99% ee [148]. 

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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