Phenol triflate

Saa, J. M. Martorell, G. Garda-Raso, A. Palladium-catalyzed cross-coupling reactions of highly hindered, electron-rich phenol triflates and organostannanes./. Org. Chem. 1992, 57, 678-685. 

Benzo[a] and benzo[c] xanthene and dibenzo[a,c]xanthene have been synthesised by the Pd-catalysed cyclisation of aromatic 2-(arylmethyl)phenol triflate esters through S-0 bond cleavage <02T5927>. 

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. 

Phenol triflate Me, Ph, aryl, alkenyl, alkynyl B/L/LiCl [159]... 

Phenolic triflates are formed with 4-nitrophenyl triflate in the presence of K2CO3 in DMF or with Triflic anhydride in the presence of an amine base. It can be cleaved with Et4NOH in dioxane or with TBAF in THF (70-99% yield). Et4NOH will also cleave phenolic mesylates and tosylates. These triflates are also substrates for a variety of Pd-catalyzed coupling reactions. 

Heteroaryl halides (or phenolic triflates) take part in palladium-catalysed couplings with a wide range of organometallic and anionic reagents in contrast to the Heck reaction, the catalyst is often provided as preformed Pd(0), in a complex such as tetrakis(triphenylphosphine)palladium(0), Pd(Ph3P)4. 

Many of the earlier schemes in this section show examples of the synthesis of alkyl, alkoxy and cyano terminal groups alkyl and alkoxy groups are also conveniently generated in alternative ways from halides and phenols as shown in Scheme 8. If the alky-nyl coupling shown in Scheme 8 for 8.1 is carried out with the phenolic triflate 8.2, then subsequent reduction of the product to the alkyl group gives a way of creating an alkyl unit at a phenolic site. 

The most useful pseudo-halides are aryl triflates (trifluoromethylsulfonates) of phenols and enol triflates derived from carbonyl compounds[4,5,6]. 

Triflates of phenols are carbonylated to form aromatic esters by using PhjP[328]. The reaction is 500 times faster if dppp is used[329]. This reaction is a good preparative method for benzoates from phenols and naphthoates (473) from naphthols. Carbonylation of the bis-triflate of axially chiral 1,1 -binaphthyl-2,2 -diol (474) using dppp was claimed to give the monocarboxy-late 475(330]. However, the optically pure dicarboxylate 476 is obtained under similar conditions[331]. The use of 4.4 equiv. of a hindered amine (ethyldiisopropylamine) is crucial for the dicarbonylation. The use of more or less than 4.4 equiv. of the amine gives the monoester 475. 

Difunctional reagents, for example the very cheap dimethyldichlorosilane 48, which is produced on a large technical scale, and the much more reactive and expensive dimethylsilyl bis(O-triflate) 49 [65-67] (Scheme 2.8) convert alcohols or phenols 11 in the presence of bases, for example triethylamine or DBU, into the silylated compounds 50. Thus 48 and 49 and other bifunctional reagents such as di-tert-butyldichlorosilane [68] or di(tert-butylsilyl)-bis(0-triflate) [69] and the subsequently described 51 and 52 combine two alcohols to silicon-tethered molecules 50, which can undergo interesting intramolecular reactions [70-74]. 

Jacobi reports using a variant of method A to access the A,B,E-ring system of wortmannin.14 The sequential addition of methyl lithium and acetylenic Grignard reagent followed by triflation proceeds from 7 to the corresponding triflate 8 in 74% yield (Fig. 4.13). Subsequent carbonylation of the alkyne and the phenol produces the acyl oxazole 9, which is smoothly converted into the furanolactone 10 over three more steps. 

Benzyloxybenzylamine (BOBA) 48 is a new class of an amine support and was prepared from Merrifield resin in two steps [56]. BOBA resin was treated with an aldehyde in the presence of an acid to give an imine that subsequently reacted with Yb(OTf)3-catalyzed silyl enolates (Scheme 18). Cleavage with trimethylsilyl triflate (TMSOTf) or 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) gave either phenols or amines, respectively. 

For conversion of the more reactive l-arylsulfonyl-3-methylimidazolium triflate (trifluoromethane sulfonate) with alcohols or phenols, no base is required 175... 

For this purpose, perfluorooctanesulfonyl-tagged benzaldehydes were reacted with 1.1 equivalents of a 2-aminopyridine (or 2-aminopyrazine), 1.2 equivalents of an isonitrile, and a catalytic amount of scandium(III) triflate [Sc(OTf)3] under micro-wave irradiation in a mixture of dichloromethane and methanol (Scheme 7.85). A ramp time of 2 min was employed to achieve the pre-set temperature, and then the reaction mixture was maintained at the final temperature for a further 10 min. The fluorous tag constitutes a multifunctional tool in this reaction, protecting the phenol in the condensation step, being the phase tag for purification, and serving as an acti-... 

Several reports have been made of a successful catalyzed addition/ substitution reaction resulting in direct attachment of phosphorus to aromatic rings. The preparation of mixed triarylphosphines has been accomplished by the reaction of tin- or silicon-substituted diphe-nylphosphines with aryl halides catalyzed by palladium reagents.74 A similar transformation has also been reported using nickel catalysis.75 The addition/substitution of diphenylphosphine to triflate functionalized phenolic linkages has been of use for the preparation of substances as analogues of tyrosine-related amino acid derivatives, accomplished with catalysis by palladium acetate (Equation 4.29).76... 

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,. 

The Pd-catalyzed intermolecular C—O bond formation has also been achieved [105-108]. Novel electron-rich bulky phosphine ligands utilized by Buchwald et al. greatly facilitated the Pd-catalyzed diaryl ether formation [109], When 2-(di-tert-butylphosphino)biphenyl (95) was used as the ligand, the reaction of triflate 93 and phenol 94 elaborated diaryl ether 96 in the presence of Pd(OAc)2 and K3PO4. The methodology also worked for electron-poor, neutral and electron-rich aryl halides. 

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