Rhodium BINAP

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. 

Nitroalkenes are good candidates for the rhodium-catalyzed asymmetric 1,4-addition of organoboronic acids. Hayashi et al. reported that the reaction of 1-nitrocyclohexene with phenylboronic acid in the presence of rhodium/ -BINAP catalyst gave 99% ee of 2-phenyl-1-nitrocyclohexane (Scheme 38).117... 

Asymmetric cyclization was also successful in the rhodium-catalyzed hydrosilylation of silyl ethers 81 derived from allyl alcohols. High enantioselectivity (up to 97% ee) was observed in the reaction of silyl ethers containing a bulky group on the silicon atom in the presence of a rhodium-BINAP catalyst (Scheme 23).78 The cyclization products 82 were readily converted into 1,3-diols 83 by the oxidation. During studies on this asymmetric hydrosilylation, silylrhodation pathway in the catalytic cycle was demonstrated by a deuterium-labeling experiment.79... 

Under a pressure (20 bar) of carbon monoxide, carbonylative silylcarbocyclization of enyne 92 was examined in the presence of a cationic rhodium-BINAP catalyst (Scheme 31).86 Although the enantioselectivity is low, the five-membered carbocycle functionalized with an alkenylsilane moiety and a formyl group was obtained with high selectivity. 

Diastereoselectivehydrogenation.3 The 8-hydroxy-a,p-unsaturated ester (3) undergoes hydrogenation in the presence of rhodium [(+ )-BINAP]-norbornadiene tetrafluoroborate (1) (10, 36) almost exclusively to give 4. This catalyst (1) is more effective than rhodium(DIPHOS)norbornadiene tetrafluoroborate (2) (12, 426). 

Asymmetric 1,4-addition of arylzinc chlorides to ( >3-arylpropenals has proceeded with high enantioselectivity in the presence of a rhodium-(/ )-binap catalyst and chlorotrimethylsilane.102 The corresponding 3,3-diarylpropanals were obtained in high yields and excellent enantiomeric excess (98-99% ee). 

Arylmetallic reagents other than arylboronic acid were shown to be applicable in the rhodium-BINAP-catalyzed arylation. Aryltitanium reagent 93 con-... 

Rhodium BINAP complexes catalyze enantioselective 1,3-H shifts 7... 

One additional favourable feature of the use of rhodium-BINAP catalysts is that they are stereospecific the ( )-enamine gives the (i )-amine and the Z)-enamine gives the (5)-amine with catalysts containing (5)-BINAP. Obviously, the use of (i )-BINAP catalysts affords the opposite enantiomers of the amine (Figure 21). Thus to obtain a high enantiomeric purity, it is essential to start from isomerically pure amine. Almost perfect enantioselectivity (> 96% ee) and quantitative yields were obtained by all routes. Initially, [Rh(BINAP)(-COD)]" (COD = 1,5-cyclooctadiene) was used as catalyst precursor, and TON up to 8000 were achieved. Further improvements in TON were achieved with... 

The use of chiral rhodium-BINAP complexes for the asymmetric isomerization of alkenes has been utilized in the industrial synthesis of menthol by Ryoji Noyori (winner of the 2001 Nobel Prize in Chemistry). This synthetic method was industrialized by Takasago International Corporation and provides (—)-menthol to pharmaceutical and food companies worldwide. In this case the catalyst [(S-BINAP)-Rh(COD)] or [(S-BINAP)2-RuC104 ] is used for the asymmetric isomerization of diethylgeranylamine (1.62) to 3-(R)-citronellalenamine (1.63) (Scheme 1.13). 

For the transfer of arj l and alkenyl groups to enones, Hayashi s procedure, employing the corresponding boronic adds and a rhodium-BINAP catalyst, is the method of choice at present [24, 25]. For the transfer of alkyl groups to cydic enones the use of dialkylzinc reagents in the presence of copper-phosphoramidite catalysts is superior. Although the first examples of hi ly enantiosdective 1,4-ad-ditions of R Zn reagents to nitroalkenes have been reported, similar catalytic methods for numerous other dasses of a, -unsaturated compounds still need to be devdoped. 

Asymmetric hydrogenation was boosted towards synthetic applications with the preparation of binap 15 by Noyori et al. [55] (Scheme 10). This diphosphine is a good ligand of rhodium, but it was some ruthenium/binap complexes which have found spectacular applications (from 1986 up to now) in asymmetric hydrogenation of many types of unsaturated substrates (C=C or C=0 double bonds). Some examples are listed in Scheme 10. Another important development generated by binap was the isomerization of allylamines into enamines catalyzed by cationic rhodium/binap complexes [57]. This reaction has been applied since 1985 in Japan at the Takasago Company for the synthesis of (-)-menthol (Scheme 10). 

A further interesting contrast between rhodium and ruthenium hydrogenation catalysts in kinetic resolution is provided. Most of the published work for the latter relates to ruthenium (BINAP) chemistry but a wider spectrum of allylic alcohols is reduced with satisfactory selectivity the need for an electron-withdrawing group at the a -position is no longer evident. Where a direct comparison can be drawn between rhodium(BINAP) and ruthenium(BlNAP) (Table 6, entry 1), the reduction with a given enantiomer of catalyst gives the opposite enantiomer of a... 

The only catalytic system which rivals [rhodium(BINAP)]+ in its efficiency for asymmetric isomerization is the closely related BlPHEMP ligand complex with the reaction being carried out under similar conditions8. Again, very high enantiomeric excesses are obtained and the rigidity of the biphenyl ligand backbone is a critical feature. 

The isomerisation of allyhc amines into the corresponding enamines is an excellent example of asymmetric catalysis, which has been exploited on a commercial basis. The isomerisation of the aUylamine (12.01) with a rhodium/BINAP complex occurs with excellent yield and enantioselectivity to give the enamine (12.02) as the initial product. ... 

In the next step, diethylgeranylamine is treated with an enantiomericaUy pure rhodium-BINAP catalyst. The product, an enamine of citroneUal, is obtained in quantitative yield and in high optical purity. 

During an examination of the use of substituted maleimides 100, Hayashi discovered that the regioselectivity in the addition was a function of the ligand employed (Scheme 8.24) [89]. Whereas, rhodium/BINAPquaternary stereocenter, rhodium/ diene-catalysis led to ds/trans-iraxtaies of 102. 

Most of the currently applied protocols for rhodium-catalyzed conjugate addition chemistry involve the use of aqueous solvent systems which ensure catalytic turnover by protonation of the intermediate rhodium enolate. Consequently, tandem reaction sequences with electrophiles other than a proton are troublesome. In early investigations, Hayashi reported a rhodium/BINAP-catalyzed conjugate addition-aldol reaction under anhydrous conditions by use of 9-aryl-9-borabicyclo[3.3.1]no nanes (9-Ar-9-BBN) as aryl sources [117]. The reaction between tert-butyl vinyl ketone (145) with 9-(4-fluorophenyl)-9-BBN (146) and propionaldehyde (147) led to the formation of a syn/anti-mixiuve of 148 in a 0.8 to 1 ratio (Scheme 8.39). 

Ally lie amines are isomerised by transition metal catalysts to enamines. The isomerisation can be rendered asymmetric with rhodium-BINAP complexes. 

In later work, Zhang also examined the reactivity of ether substrates with additional substituents at the allylic position (Fig. 10.19) [32] and demonstrated that an highly effective kinetic resolution process takes place in the presence of rhodium/BINAP catalysts. In these cycloisomerization processes, the initial kinetic resolution coupled with a diastereoselective cyclization step allow the synthesis of tetrahydrofurans with two adjacent stereogenic centres, with excellent e.e. and conversion rates. In the representative reaction shown in Fig. 10.19, the 5-configured BINAP matches the 2/ -configured enyne substrate which is thus quantitatively converted into the expected tetrahydrofurane. The 25 -configured substrate remains unchanged. 

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