Bismuth reagents

Barton and co workers have explored the aryladon of various nucleophiles inclndmg nitroalkanes using bismuth reagents Reacdon of 2-nitropropane v/ith triphenylbismnth carbonate gives 2-nitro-2-phenylpropane in 80% yield Recently, this aryladon has been used for the synthesis of unusual amino acids Aryladon of ct-nitro esters v/ith triphenylbismnth dichloride followed by redncdon gives unique ct-amino acids fEq 5 68 ... 

Allyltantalum-mediated allylation of aldimines was reported by Bhuyan et al. in 1993 these workers compared tantalum and bismuth organometallic reagents over a series of transformations. Up to 60% yield was obtained for tantalum-mediated allylation (benzylidene aniline as electrophile). Bismuth reagents offered better yields in the cases compared but, in all cases, a remarkable effect for the addition of Bu NBr was observed.186... 

A recent report has shown that tricyclopropylbismuth can be utilized in the cross-coupling reaction with aryl halides and triflates [57]. Representative examples are shown in Scheme 38. Addition of a base activator and excess of the bismuth reagents are necessary to obtain the coupling products in good yields, although more than one cyclopropyl group was transferred when 0.5 equivalents of... 

For a review of acylations with bismuth reagents, see Finet Chem. Rev. 1989,89, 1487-1501. 

Fig. 14 a. Schematic arrangement for Flow Injection Analysis for a simple spectrophotometric determination, b A typical experimental output from such a system, showing lack of carry over even with large samples. (Metal ion Bismuth Reagent Pyrocatechol violet, 5 x 10 3 M pH 2-4 Sample size 200 pi Analysis rate 80 hr-1)... 

A catalytic bismuth system (Ph3Bi-NBS-K2C03-MeCN with 1% water) has been repotted to cleave a range of 1,2-glycols efficiently and is shown to have a different mechanism from the cyclic process observed with the stoichiometric bismuth reagent (2). The catalytic system cleaves cis- and rra/t -decalin-... 

Vinyl epoxides can be cross-coupled with vinylstannanes <2001JOC589>. This reaction proceeds through an (if-allyOpalladium complex (Equation 57) <2001JOC589>. Similar reactivity can be observed using bismuth reagents (Equation 58) <2001SC2365>. Vinyl oxiranes react with substituted allenes to form functionalized allyl alcohols <2004JOC4686>. 

In this case, the second A-arylation was slow and required 1.5 equivalents of bismuth reagent and 1 equivalent of copper diacetate. For example, the first arylation of (5)-valine took place in 1 day at room temperature to yield 85% product, but the second A-arylation required 14 days at room temperature to yield 69% product. However, no racemization of the amino acid ester was detected, and for this reason, this procedure offers an advantage over the palladium-mediated arylation of amines that racemize amino acids342. 

The remainder of the mechanistic information presented in this review draws from the investigations and proposals by Lockhardt391 and Barton and coworkers343 on the intermediacy of hypervalent copper(III) species during the reactions of diaryliodonium salts and pentavalent bismuth reagents. Most of the reactions utilize Cu(I) or Cu(II) salts as the starting copper source. Radical mechanisms are ruled out because the reactions are not inhibited when radical scavengers (such as 1,1-diphenylethylene) are added. It appears that Cu(H) is not the major catalytic species in the reaction. 

In the case of 2,6-disubstituted phenols, the nature of the bismuth reagent, the nature of the alkyl substituents and the reaction conditions determined the outcome of the reactions. 2 Thus, in the reactions of 2,6-dimethylphenol (19) with pentaphenylbismuth (4) or with tetraphenylbismuthonium derivatives under basic conditions, ortho C-phenylation resulted in the formation of 6-phenylcyclo-hexadienone (20) in good yield. On the other hand, oxidative dimerisation took place in the reaction of 2,6-dimethylphenol with triphenylbismuth carbonate to afford the diphenoquinone (21) quantitatively 25... 

Therefore, the observation of ESR signals due to phenyl free-radicals resulted from a minor competing decomposition pathway of the bismuth reagent, and the arylation reaction itself does not proceed by a free-radical pathway. 

The arylation of lower aliphatic alcohols was observed when the copper-catalysed decomposition of triarylbismuth diacetate was carried out in simple alcohols, used as solvent (20 mL per mmole of bismuth reagent). The yields of alkyl aryl ethers (based on the bismuth reagent) ranged from 60 to 95% for primary and secondary alcohols, but only 9% were obtained in the case of t rt-butyl alcohol.Under stoichiometric conditions, the 0-phenylation of 3-p-cholestanol by triphenylbismuth diacetate (1 equiv.) was not significantly improved upon addition of copper diacetate. 5 Different copper compounds, such as Cu(OAc)2, CuCl2, CuCl or metallic copper, can be used as effective catalysts. 

Although there is no reaction between triphenylbismuthane and copper diacetate triphenyl-bismuthane can transfer a phenyl group to alcohols and phenols when a stoichiometric amount of copper diacylate is used. When primary or secondary alcohols, used in large excess, were treated with triphenylbismuthane in the presence of copper diacetate in the ratio Ph3Bi Cu(OAc)2 =1 2, without solvent in sealed ampoules, the 0-phenyl ethers were formed in 43-91% yields (based on the bismuth reagent), at a very slow rate (several days at room temperature). No reaction with phenol was described under these conditions. ... 

The reaction of simple aliphatic amines with triphenylbismuth diacetate and copper diacetate (the ratio used being 5-20 1 0.01-0.02) in tetrahydrofuran led to good yields of the derived arylamine (60-85% based on the bismuth reagent). In the case of the phenylation of iVJV-diphenylamine, a very poor yield was (< 3%) of triphenylamine was realized.99... 

In the phenylation reaction of substituted anilines, the electronic nature of the substituents plays a role only on the reaction rate, not on the overall yield (4-methoxyphenyl 91% after 15 minutes and 4-nitrophenyl 90% after 16 hours). The steric hindrance is also a factor which influences the reaction in slowing down the reaction rate. To obtain good to excellent yields, an excess of bismuth reagent is then required with hindered substrates. For example, mesitylamine required 2.2 equivalents of triphenylbismuth diacetate to afford the AT-phenyl derivative in 92% after 24 hours. Moreover, triphenylamine was obtained in 23% yield after 48 hours by arylation of, iV-diphenylamine. A variety of aliphatic, alicyclic, heterocyclic and aromatic amines as well as hydrazines were N-arylated by this system. No reaction took place with a-amino acids but their esters were mono-iV-phenylated under... 

The reaction of tributylbismuthane or tribenzylbismuthane with alcohols or phenol in the presence of copper diacetate led to very poor yields of the alkylbutyl or alkylbenzyl ethers (10-20% based on the bismuth reagent), l ... 

Phenylation of phenols, enols and other anions by a pentavalent organo-bismuth reagent under neutral, acidic or basic conditions. 

Table 4.13 TMG (1) assisted a- -alkenylation of fi-keto esters with bismuth reagents ...

TMG (1) assisted alkene transfers from alkenyl bismuth reagents to reactive electrophiles have been reported [83]. Treatment of p-keto esters, p-diketones and phenols with alkenyltriarylbismuthonium salts in the presence of TMG (1) smoothly affords a-alkeny-lated products [83b] (Table 4.13). 

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