Arene chemoselectivity

Surprisingly, the organometallic catalyst shows good stability under the reaction conditions (CFjCOOH/CH Cl ). In the absence of 107 (Fig. 2.18), Pd(OAc)j under the same conditions catalyses the same reaction with reduced activity (ca. 50% conversion in 24 h) and different chemoselectivity. Arenes, substituted by electron-withdrawing snbstitnents react slower. Both internal and terminal alkynes undergo the reaction, however, the former require more forcing conditions [90],... 

Highly diasteroselective and chemoselective reductions may be performed on the hydroxy functions of (r/6-arene)-tricarbonylchromium complexes. Treatment of the chromium-complexed benzylic alcohol 29 with triethylsilane and boron trifluoride etherate in dichloromethane at —78° to 0° gives only diastereomer 30 in 75% yield (Eq. 40).181 In a similar fashion, treatment of the complexed exo-allyl-endo-benzylic alcohol 31 with an excess of Et3SiH/TFA in dichloromethane at room temperature under nitrogen produces only the endo-aflyl product 32 in 92% yield after 1.5 hours (Eq. 41). It is noteworthy that no reduction of the isolated double bond occurs.182... 

The asymmetric hydroformylation of functionalized aliphatic alkenes is generally more difficult than the hydroformylation of vinyl arenes. The rhodium-catalyzed hydroformylation of vinyl acetate (36) yields 2- and 3-acetoxypropanals, 37 and 38, with high chemoselectivity. Ethyl acetate and acetic acid can also be found as by-products. One of the potential applications of vinyl acetate hydroformylation is the production of enantiopure propane 1,2-diol (Scheme 6). 

Although allyl-arenes are prone to olefin isomerization, several successful reactions have been performed, for example in the chemoselective oxygenation of 22 to aryl-acetone 23 (Table 2) [38]. Allyl alcohols sometimes react sluggishly, but examples with high ketone selectivity are known, for example the oxidation of tertiary alcohol 24 to a-hydroxyketone 25 [39]. 

The observed chemoselectivity is unique. Anodic treatment on BDD of 3,4,5-trimethoxy toluene results in the exclusive formation of the mixed biaryl 56. This method can be further performed with benzo[l,3]dioxole-containing arenes as reaction partners, giving biaryls 57 and 58 in acceptable yields. Furthermore, naphthalene moieties can be directly located onto 4-methyl guaiacol as the products 59 and 60 reveal. This novel cross-coupling can be expanded to other phenolic reaction partners as well [28]. The displayed selection of mixed biaryls 53-60 is accessible in a single step. In the workup protocol, HFIP is almost quantitatively recovered since it represents the most volatile component in the electrolyte. In addition, nonconverted starting materials can be recycled by short path distillation with approximately 80% efficiency (Scheme 23). 

A major breakthrough was achieved with transfer hydrogenation catalysts generated from [Ru(Cl2(arene)] and a semi-AT-sulfonated chiral diamine (e.g., AT-to-syl-l,2-diphenyl-l,2-diaminoethane, N N) [19]. With HCOOH/NEtj as donor, enantioselectivities up to 97% were obtained for substrates of types 7 and 8 (s/c 200-1000 tof 83 h ), providing a new general route to alkaloids (Tables 4 and 5). Furthermore, this catalyst exhibited a high chemoselectivity towards the reduction of C=N vs. C=0 functions. As a consequence, imines of type 7 can even be reduced in acetone with <5% production of 2-propanol. 

Use of Lithium Naphthalenide. Lithium arene radical anion complexes are mild and highly effective reagents for the reductive desulfonylation process of functionalized sulfones. These reagents have only rarely been used with vinylic and allylic sulfones. In addition to high yields and their operational simplicity, metal arene radical anion complexes demonstrate high chemoselectivity (Eq. 67).123... 

The most common anchors for traceless linkage of arenes are based on silyl linkers due to the well-known protodesilylation of the Si-aryl bond. The first traceless silyl linkers were described independently by Ellman et al. and Veber et al. in 1995 [104, 106], Starting from resin-bound arylstannane 113, Ellman has shown the advantages of this type of detachment in the synthesis of a diverse benzodiazepine library (Scheme 16.27). Improvements in the chemoselectivity of the cleav-... 

While intramolecular oxidative arylations and also intermolecular homocouplings can be performed in a highly selective fashion, a significant challenge is to achieve chemoselectivities in intermolecular oxidative arylations between two different (hetero)arenes. 

Notably, in the presence of (arene)Cr(CO)3 complexes, dienes are exclusively converted into cz5-olefins [2, 7-9]. These catalysts are superior in stereo- and chemoselectivity to ordinary catalysts including the Lindlar catalyst, cationic... 

Cyclodimerization of 1,3-butadiene via arene metal-mediation has received relatively little attention. To date, the only example of such a dimerization has been described by Zenneck and co-workers [46]. This group has shown that (r -arene)(r[ -diazadiene)iron complexes catalyze 1,3-butadiene dimerization to afford a mixture of 3-vinylcyclohexene and 1,5-cyclooctadiene. The initial observations showed a strong dependence of [4-1-4]/[4-1-2] product ratio on the nature of the arene coordinated to the metal center. These early results offer good opportunities for the further improving on the chemoselectivity of this cyclization. 

These nanoheterogeneous systems stabilized with various methylated cyclodextrins have shown efficient activity for the hydrogenation of olefins, and more particularly of aromatic compounds, under biphasic conditions at room temperature and atmospheric hydrogen pressure. Moreover, interesting chemoselectivities have been observed in the hydrogenation of various substituted arene derivatives (Table 11.8). The hydrogenation was easily controlled by the relevant choice of cavity and degree of methylation of the cyclodextrin. 

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