Bisaryl formation

Scheme 5. Bisaryl formation via electro-generated Ni(O) phosphane complexes...

The electrochemical allylation of carbonyl compounds by electroreductivc regeneration of a diallyltin reagent from allyl bromide and a Sn species leads to formation of homoallylic alcohols in yields of 70-90 % even in methanol or methanol/water (Table 7, No. 12) Bisaryl formation is possible also from aryl iodides or bromides in the presence of electro-generated Pd phosphane complexes (Table 7, No. 13) In the presence of styrenes, 1,3-butadienes, or phenyl acetylene the products of ArH addition are formed in this way (Table 7, No. 14) . The electroreductivc cleavage of allylic acetates is also possible by catalysis of an Pd°-complex (Table K No. 15)... 

Many other reactions have been used to construct macrocycles, and many are related to specific bond formations and therefore not very widely applicable. Since bisaryl ethers and biphenyl units are relatively common in naturally occurring macrocycles [4] (both units can be found in the important antibiotic vancomycin, 20, Fig. 7), some approaches to their synthesis will... 

Very similar to the Co(I)-complexes, Ni(O)-complexes can be used as redox catalysts. Thus, the formation of bisaryls and poly(l,4-phenylene) from aryl halides has been studied intensively (Scheme 5). 

Reductive Etherifications and Acetal Reductions. Additional applications of triethylsilane in the reduction of C-0 bonds also continue to surface. The Kusanov-Pames dehydrative reduction of hemiacetals and acetals with trifluorosulfonic acid/EtsSiH has proven especially valuable. Under such conditions, 4,6-O-benzyli-dene acetal glucose derivatives can be asymmetrically deprotected to 6-0-benzyl-4-hydroxy derivatives (eq 28) and thioketone derivatives can be converted to syn-2,3-bisaryl (or heteroaryl) di-hydrobenzoxanthins with excellent stereo- and chemoselectivity (eq 29). Triethylsilane is also useful in a number of related acetal reductions, including those used for the formation of C-glycosides. For example, EtsSiH reductively opens 1,3-dioxolan-4-ones to 2-alkoxy carboxylic acids when catalyzed by HCU. Furthermore, functionalized tetrahydrofurans are generated in good yield from 1,2-0-isopropylidenefuranose derivatives with boron trifluoride etherate and EtsSiH (eq 30). These same conditions lead to 1,4- or 1,5-anhydroalditols when applied to methyl furanosides or pyranosides. ... 

Having stitched together the four aryl building blocks, all that essentially remained from 37 to complete the AB/C-O-D fragment were two macrocyclization events. The first of these closures, the formation of the C-O-D bisaryl ether system, was mildly effeeted through treatment of this compound with CuBr Me2S and K2CO3 in a mixture of pyridine and MeCN at 82 °C. Unfortunately, this macrocyclization failed to afford any atropselectivity in that both 36 and 84 were obtained in a 1 1 ratio in 67 % combined yield. 

The first critical objective in this campaign was finding a method to convert the aryl triazene into a phenol, since its mission in directing the formation of both bisaryl ether systems was now complete. Although model studies indicated its relatively facile removal, this motif unfortunately proved far more resilient within the context of 93 as direct techniques to effect its transformation into a phenol under various acidic conditions universally failed, leading only to reduced product (i. e. a hydrogen atom instead of the triazene... 

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