Phenol triflate selectivity

The synthetic potential of reductions by formate has been extended considerably by the use of ammonium formate with transition metal catalysts like palladium and rhodium. This forms a safe alternative to use of hydrogen. In this fashion it is possible to reduce hydrazones to hydrazines, azides and nitro groups to amines, to dehalogenate chloro-substituted aromatics, and to carry out various reductive removals of functional groups. For example, phenol triflates are selectively deoxygenated to the aromatic derivatives using triethylammonium formate as reductant and a palladium catalyst. - These recent af li-cations have been reviewed. 

With intermediate 60 in hand, a key photoinduced HBr elimination furnished the second 12-membered ring and provided intermediate 61. After reductive removal of the spectator phenol through its triflate, selective acylation, indole C-2 chlorination, and treatment with tris(dimethylamino)sulfur trimethyldifluorosilicate, afforded the desbromo diazonamide B (62). Finally, phosphoryl cyanide-mediated condensation with commercial (5)-ot-hydroxyisovaleric acid delivered (-)-diazona-mide A (47) (Scheme 9). 

A potentially useful variant of the synthesis of some Mo(CHR)(NAr)(OR )2 complexes (R=CMe3 or CMe2Ph) consists of addition of two equivalents of a relatively acidic alcohol (R OH=a fluorinated alcohol or phenol) to Mo(NAr)(N-t-Bu)(CH2R)2 [78,79], a variation of the reaction of that type that was first reported in 1989 [80]. The more basic t-butylimido ligand is protonated selectively. This synthesis avoids the addition of triflic acid to Mo(NAr)2(CH2R)2 to give Mo(CHR)(N-t-Bu)(triflate)2(dimethoxyethane), the universal precursor to any Mo(CHR)(NAr)(OR )2 complex. Unfortunately, the method does not appear to succeed when R OH does not have a relatively high pK,. 

For the mono-C-glycosylphenol, the commercially available 2,4,6-trihydroxyacetophe-none was chosen and selectively methylated at C-2 and C-4. The partially protected phenol was glycosylated with the C-benzyl-protected glucosyl trichloroacetimidate in the presence of trimethylsilyl triflate as promoter to give directly a C-(benzyl protected)glycosylphenol. The unprotected hydroxyl group of this compound was converted with benzoyl chloride into a fully protected C-glycoside phenol. Treatment of the benzoate derivative with sodium hydroxide in... 

Data for the use of alcohols as alkylating agents in superacids are scarce. A study of the alkylation of phenol and naphthols with ferf-butyl alcohol has shown198 that triflic acid adsorbed on aminopropyl-modified silica is the most selective to yield monoalkylated products compared to solid acids (triflates immobilized in silica). 

Achiral ytterbium Lewis acid was prepared from ytterbium triflate [Yb(OTf)3], ( )-(+)-l,l -bis(2-naphthol) (BINOL) and DBU (1), and subjected to aza Diels-Alder reactions of achiral imines (A-benzylidene-2-hydroxyanilines) and achiral dienophiles [79]. In this reaction the use of a chiral Lewis acid containing 1,3,5-trimethylpiperidine instead of 1 resulted in a lowering of the enantiomeric excess of adduct. Thus, the phenolic hydrogen of the imine interacts with DBU (1) in transition state, as shown in Figure 3.7, to increase the selectivity. 

Thienyl(phenyl)iodonium salts and other heteroaryl(phenyl)iodonium salts can be used as the selective heteroaryl transfer agents in reactions with phenol ethers. These heteroarylations occur at room temperature in the hexafluoroisopropanol solution in the presence of trimethylsilyl triflate via a SET mechanism [876]. 

The use of aryl tosylates as electrophiles is attractive, because they can be prepared from readily available phenols with less-expensive reagents than those required for synthesis of the corresponding triflates. Importantly, tosylates are more stable towards hydrolysis than triflates, yet significantly less reactive as electrophiles. As a result, protocols for traditional cross-coupHng reactions were only recently developed (see Chapter 2). In contrast, catalytic direct arylations with aryl tosylates were not reported until recently. Interestingly, a rathenium complex derived from heteroatom-substituted secondary phosphine oxide (HASPO) preligand 78 [40] allowed for direct arylations with both electron-deficient, as well as electron-rich aryl tosylates [41]. As pronucleophiles, pyridine, oxazoline and pyrazole derivatives could be efficiently functionalized. Selective mono- or diarylation reactions could be accomplished through the judicious choice of the... 

Scheme 11.7 illustrates that gold and silver triflate-catalyzed intramolecular hydroarylation of allenic anilines and phenols offers an efficient route to dihydroquinoline and chromene derivatives under mild reaction conditions (Watanabe et al. 2007). The hydroarylation takes place at the terminal or central allenic carbon, depending on the substrate structure, leading to a highly selective formation of six-membered rings. 

Triflate anions are omitted for clarity.) Yb(OTf)3, (i )-(+)-BINOL and DBU form a complex with two hydrogen bonds, and the axial chirality of (/ )-(+)-BINOL is transferred via the hydrogen bonds to the amine parts. The additive would interact with the phenolic hydrogen of the imine, which is fixed by bidentate coordination to Yb(lll). Since the top face of the imine is shielded by the amine, the dienophiles approach from the bottom face to achieve high levels of selectivity. 

Given a broad selection of different halogen compounds and additionally the triflates of numerous phenols, the majority of substitution and coupling problems can be solved. 

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