Cationic rhodium /BINAP complex catalyst

The catalyst system employed was the cationic rhodium solvent complex [Rh(P-P)S2]+ (P-P = BINAP, CHIRAPHOS, S = solvent). The BINAP ligand enhances the activity of the complex (Table 10), although additional studies have revealed that both the solvent and the substituents on Si influence the levels of enantioselectivity (Scheme 29).131,132... 

Aryl ethynyl ethers can also be employed in cationic rhodium(I) complex-catalyzed intermolecular [2- -2-1-2] cycloaddition. The [2- -2-1-2] cycloaddition of aryl ethynyl ethers proceeded by using the cationic rhodium(I)/Hg-BINAP catalyst (Scheme 4.13) [24], The same rhodium(I) complex catalyzed cross-[2 - - 2 - - 2] cycloaddition of two molecules of aryl ethynyl ethers with electron-deficient internal alkynes (Scheme 4.14) [24]. 

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... 

The first catalysts used were Co(I)-DIOP (DIOP, see Figure 11b) based systems. Up to 32% ee was achieved with 39% yield in the isomerization of (A, AO-diethylgeranylamine to (i )-citronellal. Subsequently, some rhodium (I)-DIOP or BINAP (BINAP = 2,2 -bis(diphenylphosphino)-1,1 -binaphthyl) systems proved to be very active. In particular, cationic rhodium(I)-BINAP complexes show very high selectivities and catalytic activities for this isomerization. BINAP is an atropoisomeric diphosphine (Figure 20) which was first synthesized by Noyori and Takaya and since then its metal complexes have been extensively used as catalysts in a variety of asymmetric syntheses. 

In our asymmetric process development, the discovery of the thermally stable rhodium bis-BINAP complex was outstanding as it enabled the repeated use of catalyst [10]. In Scheme 3, the synthesis of the cationic rhodium bis-Tol-BINAP complex 13 is illustrated. Its precursor 12 can be prepared quantitatively using cheap sodium perchlorate in a binary system in the presence of a phase transfer catalyst. It is possible to convert 12 to 13 by monitoring the reaction either by the volume of hydrogen absorbed or the color change from orange (12) to deep red (13). 

The reversal in the regioselectivity of the hydroboration of styrenes using a cationic rhodium complex (see Scheme 5.14), to provide the secondary (rather than primary) organoborane, allows the study of the asymmetric hydroboration of mono-Caryl) substituted alkenes (styrenes). A variety of chiral phosphine ligands can be used to good effect, a popular choice being (7 )-2,2 -bis(diphenylphosphino)-l, T-binaphthyl (BINAP). Just 0.02 molar equivalents of the rhodium catalyst and... 

Allylamines are isomerized by cationic rhodium complexes to the corresponding -enamines.Using the catalyst [Rh( -binap)(COD) containing the optically pure biphosphine ligand -binap, high asymmetric induction in this isomerization can be achieved (Scheme 9). 

An alkene isomerization/enantioselective intramolecular Alder-ene reaction cascade occurs using a cationic rhodium(I)/(7 )-BINAP complex as catalyst (Scheme 150). 0... 

In 2003, Tanaka et al. discovered that cationic rhodium(I)/biaryl bisphosphine complexes are highly active and selective catalysts for 1,2,4-selective intermolecular [2 + 2 + 2] cycloaddition of terminal alkynes (Scheme 4.9) [4], Furthermore, a cationic rhodium(I)/Hg-BINAP complex catalyzed chemo- and regioselective intermolecular cross-[2 + 2 + 2] cycloaddition of dialkyl acetylenedicarboxylate with two... 

Cationic rhodium(I)/biaryl bisphosphine complexes are an effective catalyst for intermolecular [2 + 2 + 2] cycloaddition of alkynes with nitriles under mild conditions. The [2 + 2 + 2] cycloaddition of 1-dodecyne with ethyl cyanoacetate proceeded at 60 °C in the presence of the cationic rhodium(I)/BINAP catalyst to give trisubstituted pyridines in high yield with moderate regioselectivity (Scheme 4.53) [55]. 

While testing two different catalysts, Tanaka found that cationic rhodium in a binary system (cationic Rh(I)/H8-binap) is effective in chemo- and regioselective addition reactions of terminal alkynes with acetylenedicarboxylate to form 1,2,3,4-tetra-substituted benzenes with excellent yield of 99% [9, 44, 45]. It is also important to note that this reaction is tolerant to a large number of functional groups, including alkenes, alkyl halides, and esters. Although cationic iridium complex Ir(I) did not give a positive result in the cycloaddition reactions, the authors showed that the catalytic system with neutral Ir(I) can facilitate cycloaromatization of dimethyl acetylenedicarboxylate and terminal alkynes [45]. 

Tanaka, K. (2007) Cationic rhodium (I)/BINAP-type bisphosphine complexes versatile new catalysts for highly chemo-, regio-, and enantioselective [2-I-2-I-2] cycloadditions. Synlett, (13), 1977-1993. 

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]. 

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