Catalysis Chemoselectivity

Chemoselective NHC catalysis Chemoselective reduction SCHEME 11 Proposed strategy for the synthesis of (—)-7-deoxyloganin (24). 

Stereoselective and chemoselective semihydrogenation of the internal alkyne 208 to the ew-alkene 210 is achieved by the Pd-catalyzed reaction of some hydride sources. Tetramethyldihydrosiloxane (TMDHS) (209) i.s used in the presence of AcOH[116]. (EtO)3SiH in aqueous THF is also effective for the reduction of alkynes to di-alkenes[l 17], Semihydrogenation to the d.v-alkene 211 is possible also with triethylammonium formate with Pd on carbon[118]. Good yields and high cis selectivity are obtained by catalysis with Pd2fdba)3-Bu3P[119],... 

The chemoselective desilylation of one of the two different silyi enoi ethers in 10 to give the monosilyl enol ether II is realized by the Pd-catalyzed reaction of Bu3SnF. The chemoselectivity is controlled by steric congestion and the relative amount of the reagent[7,8]. An interesting transformation of the 6-alkoxy-2,3-dihydro-6//-pyran-3-one 12 into the cyclopentenone derivative 13 proceeds smoothly with catalysis by Pd(OAc)2 (10 mol%)[9]. 

It turned out that the Friedel-Crafts reaction and the chlorination can be done in the same pot. The vhlorination needs to be chemoselective as reaction on -.he methyl group or next to the carbonyl group could ccur. Lewis acid catalysis Is the answer. 

Gyorgy S, Istvan K, Bela T, Mihaly B (2000) Ultrasonics in chemoselective heterogeneous metal catalysis. Sonochemical hydrogenation of unsaturated carhonyl compounds over platinum catalysts. Ultrason Sonochem 7(4) 173-176... 

Borabenzene complexes of cobalt such as Co(C5H5BPh)(COD) (51) and its 5-ethyl analog show the same type of catalysis but improved activity and chemoselectivity (77). Thus, 51 as the catalyst precursor gave the hitherto best results in the catalytic synthesis of the valuable 2-vinylpyridine from C2H2 and CH2=CHCN (120°C, 51 bar, 2 hours, turnover number 2164) (77,101). Furthermore, this catalyst for the first time allowed the synthesis of pyridine from C2H2 and HCN under mild conditions (110°C, 23 bar, 60 minutes, turnover number 103) (77). 

High chemoselectivity is observed in this ruthenium-catalyzed isomerization of allylic alcohols. Simple primary and secondary alcohols and isolated double bonds are not affected by these catalysts. Furthermore, free hydroxy group is essential for this catalysis. The reaction of l-acetoxycyclododec-2-ene-4-ol furnished 4-acetoxycyclododecanone in high yield (Scheme 14).37... 

The current research areas with ruthenium chemistry include the effective asymmetric hydrogenation of other substrates such as imines and epoxides, the synthesis of more chemoselective and enantioselective catalysts, COz hydrogenation and utilization, new methods for recovering and recycling homogeneous catalysts, new solvent systems, catalysis in two or three phases, and the replace-... 

Although various substrates that have two alkene linkages or both alkene and alkyne linkages conform to the catalysis, the chemoselectivity is rather low and mixtures of products are formed. However, when bulky Me3Si is bound to the alkynyl bond in an en-yne substrate, the alkene moiety is selectively hydrophosphorylated (Scheme 34). 

The modular synthetic approach has allowed the preparation of many QUINAPHOS derivatives in both diastereomeric forms. Testing of the single diastereo-mers in catalysis has been revealed to be of crucial importance. Thus, a pronounced interplay between the axial and central chirality has been observed in all applications of QUINAPHOS in catalysis, whereby the choice of the diastereomer may affect not only the enantioselectivity but also the activity and the chemoselectivity of a catalyzed reaction. 

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