Applications of bismuth(III) compounds in organic synthesis

Tetrahedron 58 (2002) 8373–8397

Tetrahedron report number 618

*

Nicholas M. Leonard, Laura C. Wieland and Ram S. Mohan

Department of Chemistry, Illinois Wesleyan University, Bloomington, IL 61701, USA

Received 29 July 2002

Contents

1. Introduction

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1.1. Availability of bismuth(III) compounds

2. Oxidations using bismuth(III) compounds

2.1. Oxidation of alcohols

2.2. Oxidation of acyloins to diketones

2.3. Oxidative cleavage of oleﬁns

2.4. Conversion of alkenes to diols

2.5. Oxidation of epoxides

2.6. Oxidation of sulﬁdes to sulfoxides

2.7. Nitration of aromatic hydrocarbons

2.8. Oxidation of Hantzsch 1,4-dihydropyridines to pyridines

3. Removal of common protecting groups

3.1. Deprotection of S,S-acetals to carbonyl compounds

3.2. Deprotection of O,O-acetals

3.3. Deprotection of a tert-butyldimethylsilyl (TBDMS) group

3.4. Deprotection of oximes to carbonyl compounds

3.5. Conversion of hydrazones and semicarbazones to carbonyl compounds

3.6. Deprotection of 1,1-diacetates (acylals)

3.7. Cleavage of tert-butyl ethers

4. Carbon–carbon bond forming reactions

4.1. Friedel–Craft’s acylation

4.2. Allylation catalyzed by bismuth(III) salts

4.2.1. Allylation of acid chlorides (acylation of allyltrimethylsilane)

4.2.2. ‘Barbier type’ allylation of imines

4.2.3. Addition of allyl groups to aldehydes

4.3. Allylation of acetals

4.4. Aldol condensations

4.5. Knoevenagel condensation

4.6. Reformatsky-type reactions

4.7. Asymmetric trimethylsilylcyanation of aldehydes using chiral bismuth compounds

4.8. Diels–Alder and the aza-Diels–Alder reactions

5. Carbon–heteroatom bond forming reaction

5.1. Carbon–nitrogen bond formation

5.1.1. N-Derivatization of L-proline esters

5.1.2. Formation of azlactones

5.1.3. Formation of dihydropyrimidinones (the Biginelli reaction)

5.1.4. One-pot synthesis of nucleosides using bismuth(III) bromide as a catalyst

5.2. Carbon–oxygen bond formation

5.2.1. Formation of ethers

5.2.2. Benzylation of alcohols promoted by BiCl₃

Keywords: non-toxic; bismuth; bismuth(III) compounds; Green Chemistry; Lewis acids; environment friendly; organic synthesis.

*

Corresponding author. Fax: þ1-309-556-3864; e-mail: rmohan@iwu.edu

PII: S0040-4020(02)01000-1

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5.2.3. Acylation of alcohols catalyzed by bismuth(III) triﬂate

5.2.4. Selective esteriﬁcation using bismuth(III) carboxylates

5.3. Formation of acetals catalyzed by bismuth(III) triﬂate

5.4. Formation of 1,1-diacetates (acylals)

5.5. Cleavage of epoxides catalyzed by bismuth(III) chloride

5.6. Glycosylation promoted by bismuth(III) chloride

5.7. Selective hydrolysis of aryl esters using bismuth(III) mandelate

5.8. Carbon–sulfur bond formation

5.8.1. Synthesis of thioacetals

5.8.2. Conversion of epoxides to thiiranes

5.9. Sulfonylation of aromatics

5.10. Synthesis of sulﬁdes and selenides promoted by Sm–BiCl₃

5.11. Synthesis of allyl sulﬁdes

5.12. Formation of carbon–halogen bonds

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5.12.1. Synthesis of b-haloketones and esters

5.12.2. Chlorination of alcohols catalyzed by bismuth(III) salts

5.12.3. Synthesis of peracetylated glycosyl halides

6. Reductions with bismuth compounds

6.1. Selective reduction of nitro compounds to azoxy compounds

6.2. Reduction of nitro compounds to amines

6.3. Chemoselective reduction of a,b-unsaturated ethyl esters using BiCl₃–NaBH₄

6.4. Reduction of aromatic a-haloketones

6.5. Miscellaneous reductions

7. Rearrangements promoted by bismuth(III) salts

7.1. Rearrangement of epoxides

7.2. Allylic rearrangement of glycols (the Ferrier rearrangement)

8. Conclusions

1. Introduction

1.1. Availability of bismuth(III) compounds

The word bismuth is derived from the German word

Weissmuth or white substance. Bismuth is the 83rd element

in the periodic table with an atomic mass of 208.980 and is

the heaviest stable element on the periodic table. Although

bismuth is a relatively rare element (ranking 64th in

abundance in the Earth’s crust), large quantities of bismuth

are produced annually as a by-product of copper and tin

reﬁning. In spite of its heavy metal status, bismuth is

considered to be safe, as it is non-toxic and non-

carcinogenic.¹This is in sharp contrast to closely located

elements in the periodic table such as arsenic, antimony,

lead and tin which are highly toxic and their use poses

environmental hazards. Bismuth and its compounds have

been used in medicinal preparations for over four hundred

years. Several bismuth compounds have application as

treatment for gastric disorders. Bismuth has an electron

conﬁguration of [Xe]4f ¹⁴5d ¹⁰6s ²6p ³, and due to the weak

shielding of the 4f electrons (Lanthanide contraction),

bismuth(III) compounds exhibit Lewis acidity. Bismuth

also exhibits a þ5 oxidation state and several organobis-

muth compounds with bismuth in the þ5 are known. Most

bismuth compounds are relatively non-toxic, easy to handle

and can tolerate small amounts of moisture. With increasing

environmental concerns and the need for ‘green reagents’,

the interest in bismuth and its compounds has increased

tremendously in the last decade. Several review articles and

a monograph has focused on the applications of bismuth and

its compounds in organic synthesis.^2–5This report is limited

to a review of the recent literature on applications of

inorganic bismuth(III) compounds in organic synthesis. The

report is organized by reaction type.

Several bismuth(III) compounds are now commercially

available at relatively low costs. These include bismuth(III)

acetate Bi(OAc)₃(CAS # 22306-37-2), bismuth(III) bro-

mide BiBr₃(CAS # 7787-58-8), bismuth chloride BiCl₃

(CAS # 7787-60-2), bismuth(III) ﬂuoride BiF₃(CAS #

7787-61-3), bismuth(III) iodide BiI₃(CAS # 7787-64-6),

bismuth nitrate pentahydrate Bi(NO₃)·5H₂O (CAS # 10035-

06-0), bismuth(III) oxide Bi₂O₃(CAS # 1304-76-3),

bismuth oxide perchlorate BiOClO₄, bismuth(III) chloride

oxide BiOCl (CAS # 7787-59-9), bismuth(III) iodide oxide

BiOI (CAS # 7787-63-5), bismuth(III) phosphate BiPO₄

(CAS # 10049-01-1), bismuth(III) selenide Bi₂Se₃(CAS #

12068-69-8), bismuth(III) subnitrate BiONO₃(CAS #

10361-46-3), bismuth(III) sulﬁde Bi₂S₃(CAS # 1345-07-

9), bismuth(III) telluride Bi₂Te₃(CAS # 1304-82-1),

bismuth(III) titanate Bi₂O₃·2TiO₂(CAS # 12048-51-0),

and bismuth(III) tungstate Bi₂(WO4)₃(CAS # 13595-86-3).

A versatile catalyst, bismuth triﬂate, Bi(OSOCF₃)₃is not yet

commercially available, but can be easily synthesized in lab

following a literature procedure.^6–9Most bismuth com-

pounds are crystalline solids and are relatively stable in air.

2. Oxidations using bismuth(III) compounds

2.1. Oxidation of alcohols

The oxidation of 28 alcohols to ketones using bismuth

nitrate, Bi(NO₃)₃, impregnated on montmorillonite has been

reported (Scheme 1).¹⁰The oxidation is very rapid at room

temperature and proceeds well with out the need for any

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precipitate of elemental bismuth is obtained. Since the

oxidation works well with bismuth acetate also, it is

proposed that bismuth triacetate formed under the reaction

conditions is the actual oxidant.

This methodology has been used in the synthesis of several

natural products including the conversion of 2-hydroxy-

pulegone to diophenolene. Several oxidative transform-

ations of compounds containing a steroid ring are also

known.

Scheme 1.

pre-treatment of the catalyst or microwave irradiation. All

attempts to oxidize 18 alcohols using this reagent failed.

The oxidation of benzoins to benzils using bismuth nitrate in

aqueous acetic acid has also been recently reported (Scheme

3).¹²While the bismuth oxide method requires the use of a

slight excess of the oxidant, 0.40 equiv. of bismuth nitrate is

sufﬁcient for oxidation. The reaction is accelerated by the

presence of 0.10 equiv. of copper(II) acetate. However, the

oxidation of pyridoin and furoin (Scheme 3, entries 5 and 6)

was not successful. When commercially available furil or

pyridil were subjected to the reaction conditions, their

disappearance could be followed by TLC and aqueous

work-up did not yield any organic product. Hence, it is

proposed that the expected products, pyridil and furil, are

2.2. Oxidation of acyloins to diketones

One of the ﬁrst examples of the use of bismuth compounds

in organic synthesis was the use of bismuth(III) oxide,

Bi₂O₃for oxidation of acyloins to diketones.¹¹The reaction

can be carried out in acetic acid alone as the solvent or with

2-ethoxyethanol as a co-solvent (Scheme 2). For sensitive

substrates such as pyridoin and furoin, this method was

found to be superior to other oxidation methods such as

copper catalyzed-ammonium nitrate oxidation. This reac-

tion can also be used as a qualitative test for acyloins, since

upon completion of the rapid oxidation at 1008C, a black

Scheme 2.

Scheme 3.

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Scheme 4.

Scheme 5.

Scheme 6.

dependent upon the solvent used. In benzene, the main

products were cyclohexenol 2, and cyclohexenone 3. The

oxidation of several other cyclooleﬁns was also examined.

4-Methylcyclohexene was oxidized to the corresponding

aldehyde in high selectivity while the presence of an alkyl

group at 1, 2 or 3-positions retarded the reaction. A

mechanism involving isomerization of peroxy radical

catalyzed by the acidic sites of bismuth sulfate has been

proposed. The observation that addition of small amounts of

pyridine inhibited the reaction and changed the product

distribution supports the role of acidic catalysis.

formed in good yields but they undergo subsequent C–C

bond cleavage to give the corresponding water-soluble

aldehydes.

The oxidation of a-ketols to carboxylic acids has been

achieved with molecular oxygen as the oxidant and bismuth

mandelate as a catalyst (Scheme 4).¹³This oxidation is

accelerated in the presence of DMSO. The catalyst itself is

prepared from Bi₂O₃and mandelic acid. When the reaction

was carried out under a nitrogen atmosphere, very little of

the expected carboxylic acid was obtained suggesting that

oxygen is essential in the oxidation.

2.4. Conversion of alkenes to diols

The Prevost (dry) and the Woodward–Prevost (wet)

reactions for the conversion of alkenes to diols are typically

carried out with silver(I) salts.¹⁵Modiﬁcations of these

reactions have also led to the use of mercury(II), copper(II),

thallium(I) and lead(IV) salts. cis- and trans-diol derivatives

can be prepared from alkenes by reaction with bismuth(III)

acetate in ‘wet’ and ‘dry’ acetic acid, respectively (Schemes

6 and 7).¹⁶Bismuth acetate is commercially available but

was also prepared in situ from the inexpensive and readily

available bismuth oxycarbonate (BiO)₂CO₃. For preparation

of cis-diols, it was found that maximum selectivity is

obtained when equimolar amounts of alkene and water are

used.

There are many advantages to this system. The oxidant is

cheap and clean. The reaction is catalytic in bismuth(III) in

contrast to several other reagents known to achieve the same

transformation such as sodium periodate. The functional

group selectivity exhibited is also valuable since alcohols,

geminal-diols and oleﬁns are not affected under the reaction

conditions.

2.3. Oxidative cleavage of oleﬁns

The advantages of using bismuth acetate include its low

cost, favorable stoichiometry (all three acetate groups can

be transferred) and above all, its low toxicity especially

compared to lead, mercury and thallium salts.

The autoxidation of cyclohexene catalyzed by insoluble

bismuth sulfate in acidic solvents has been reported

(Scheme 5).¹⁴

2.5. Oxidation of epoxides

The major product of this oxidation is 1-cyclopentene-1-

carboxaldehyde. The product distribution was found to be

The oxidative cleavage of substituted styrene oxides to the

Scheme 7.

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Scheme 8.

in acetic acid as well as BiBr₃–Bi(NO₃)₃·5H₂O in benzene

has been described (Schemes 11 and 12).^20,21

In benzene as the solvent, the use of bismuth nitrate alone

did not result in complete oxidation. However, the addition

of BiBr₃enhanced the catalytic activity signiﬁcantly. By

carrying out the reaction under an oxygen atmosphere, it

was demonstrated that the reaction consumed a stoichio-

metric amount of oxygen. The method is particularly

attractive since several other functional groups such as

aldehyde and alcohol did not suffer oxidation. The

formation of NO₂as evidenced by a brown gas suggests

that Bi(NO₃)₃and BiBr₃serve as a source of NO₂and a

promoter of NO₂release, respectively.

Scheme 9.

Scheme 10.

2.7. Nitration of aromatic hydrocarbons

Aromatic hydrocarbons are nitrated by Bi(NO₃)₃·5H₂O

impregnated on K10 montmorillonite in the presence of

acetic anhydride (Scheme 13).²²It was found that the choice

of solvent signiﬁcantly altered the product distribution. For

example, the nitration of toluene gave the highest proportion

(80%) of the para isomer in CCl₄as the solvent. No

nitration product was obtained in acetonitrile as the solvent.

While nitration occurred in acetone, cyclohexane and 1,2-

dichloroethane, none of these solvents gave para isomer

(.50%) as the main product.

Scheme 11.

Scheme 12.

All these reactions display an induction period manifested

by a color change that has been attributed to drying of the

reaction centers by acetic anhydride at the solvent–clay

interface. The best yields were obtained when the

hydrocarbon was added at the end of this induction period.

Scheme 13.

2.8. Oxidation of Hantzsch 1,4-dihydropyridines to

pyridines

corresponding carboxylic acid in DMSO is catalyzed by

bismuth(III) mandelate (Scheme 8).^17,18The reaction is

accelerated by the presence of electron-donating groups on

the phenyl ring. This is consistent with a Hammet r value of

21.08.

Hantzsch 1,4-dihydropyridines have been used as analogs of

NAD(P)H coenzymes to study the mechanism and synthetic

potential of various redox reactions. During the redox

processes and in the course of drug metabolism, they are

oxidized to the corresponding pyridine derivatives. Several

of the existing methods for this oxidation require the use of

corrosive and toxic reagents like CrO₃, PCC or ceric

ammonium nitrate as well as harsh reaction conditions. A

less harsh method that utilizes Bi(NO₃)₃·5H₂O for the

oxidation works well in acetic acid at room temperature has

been reported (Scheme 14).²³

The oxidation of epoxides to cyclic carbonates using

molecular oxygen as the oxidant and BiBr₃as the catalyst

has been reported (Scheme 9).¹⁹This reaction is both

catalyst and solvent speciﬁc. When the oxidation was

carried out in DMSO, only the carboxylic acid was obtained

(Scheme 10). When BiCl₃was used in place of BiBr₃, no

carbonate was obtained. Increasing the amount of BiCl₃

resulted in formation of the corresponding chlorohydrins as

a mixture of isomers. However, no bromohydrin formation

was noticed with BiBr₃as the catalyst. It is proposed that the

reaction involves the incorporation and oxidation of the

carbonyl group of DMF.

3. Removal of common protecting groups

3.1. Deprotection of S,S-acetals to carbonyl compounds

The oxidative deprotection of S,S-acetals using Bi(NO₃)₃·5-

H₂O as a catalyst proceeds smoothly at room temperature to

give the corresponding carbonyl compounds in good yields

(Scheme 15).²⁴

2.6. Oxidation of sulﬁdes to sulfoxides

The oxidation of sulﬁdes to sulfoxides using Bi(NO₃)·5H₂O

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N. M. Leonard et al. / Tetrahedron 58 (2002) 8373–8397

Scheme 17.

Scheme 14.

Scheme 18.

Scheme 15.

Most of the common methods for deprotection of S,S-

acetals require the use of heavy metals such as copper(II),

mercury(II), silver(I) and an oxidizing agent such as

molecular halogen, N-halosuccinimdes or MCPBA. Thus,

the use of air and a relatively non-toxic bismuth salt

offers signiﬁcant improvement over these existing

methods. The addition of 5.0 mol% BiCl₃accelerated the

reaction rate in some cases. Benzene was found to be the

best solvent though toluene and acetonitrile also worked

well.

Scheme 19.

(Scheme 17).²⁶Under these conditions, cyclic acetals are

not deprotected. Acetals derived from non-conjugated

aldehydes are also resistant to these reaction conditions.

The chemoselectivity of the method has been demonstrated

by selective deprotection of a bi-functional compound

(Scheme 18). The tert-butyldimethylsilyl group is not

affected while the diethyl acetal group is removed at room

temperature.

3.2. Deprotection of O,O-acetals

Several bismuth(III) compounds have been used to

deprotect O,O-acetals. One chemoselective method for

deprotection of acetals makes use of the catalytic activity

of BiCl₃in CH₃OH as the solvent (Scheme 16).²⁵The

method however requires the use of 0.5 equiv. of BiCl₃.

Under these conditions, THP ethers, TBDMS groups and

benzyl groups are not affected. The deprotection of 1,3-

dioxolanes was slow at room temperatures but was

accelerated under reﬂux conditions.

While the above methods for O,O-deprotection illustrate the

versatility of bismuth(III) compounds as useful catalysts,

these methods cannot be classiﬁed as highly catalytic. A

highly catalytic, mild and efﬁcient method for the

deprotection of acetals, which takes advantage of the

catalytic activity of bismuth(III) triﬂate has recently been

reported (Scheme 19).²⁷The added advantage of this

method is that the solvent system (aqueous THF) is

relatively non-toxic.

Another mild and chemoselective method utilizes

Bi(NO₃)₃·5H₂O in CH₂Cl₂for the deprotection of acetals

Scheme 16.

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Scheme 23.

Scheme 20.

3.3. Deprotection of a tert-butyldimethylsilyl (TBDMS)

group

Scheme 24.

While several methods exist in the literature for the

deprotection of a TBDMS group, few are selective in

nature. A highly efﬁcient procedure for the selective

cleavage of alkyl TBDMS groups in the presence of aryl

TBDMS groups using BiBr₃in wet CH₃CN as a catalyst has

been reported (Scheme 20).²⁸

3.4. Deprotection of oximes to carbonyl compounds

The regeneration of carbonyl compounds from oximes is an

important synthetic transformation, especially in light of the

fact that oximes can be synthesized from non-carbonyl

compounds. Several methods in the literature utilize

chromium-based compounds for deprotection of oximes.

However, the carcinogenic nature of chromium compounds

makes their use less attractive. An efﬁcient method that

utilizes BiCl₃and microwave irradiation has been used for

oxime deprotection (Scheme 23).³¹This method works well

with both aldoximes and ketoximes, though deprotection of

oximes from a,b-unsaturated carbonyl compounds gave

only modest yields.

It is proposed that the hydrolysis is possibly catalyzed

by HBr, that is generated in situ from the reaction of

BiBr₃with water. Since BiBr₃is a stable solid, it is

much easier to handle than other reagents such as I₂/TMSCl

typically used to effect the same transformation.

Another method that utilizes excess BiCl₃/NaI in CH₃CN

has been developed (Scheme 21). However, it is not clear

whether this method requires the use of wet CH₃CN or

not.²⁹

The selective deprotection of ketoximes has been achieved

using Bi(NO₃)₃·5H₂O in a relatively non-toxic solvent

system acetone–H₂O (Scheme 24).³²Under these con-

ditions, aldoximes are not affected. The use of mont-

morillonite K 10 served to accelerate the reaction rate. In the

absence of copper(II) acetate, the product was found to

contain a small amount of acetone oxime. The formation of

acetone oxime is not surprising in light of the fact that

oximes have been synthesized from aldehydes and ketones

using acid catalyzed transoximations by acetone oxime.³³

Since a suspension of Bi(NO₃)₃·5H₂O in water is acidic, it is

proposed that deprotection could be catalyzed by the nitric

acid released under the reaction conditions. It is also

The selective removal of an alkyl TBDMS group using this

method has also been achieved in the presence of other

functional groups such as –OTs, NHBOC, –OMe, and –

OBn. A variety of trimethylsilyl ethers of alcohols and

phenols are easily removed in the presence of catalytic

amounts of bismuth(III) salts such as BiCl₃, Bi(CF₃COO)₃,

and Bi(OTf)₃in methanol at room temperature (Scheme

22).³⁰TBDMS groups are not affected by BiCl₃or

Bi(CF₃COO)₃under these conditions but they are depro-

tected by Bi(OTf)₃. Thus it is clear that the solvent plays a

crucial role in determining selectivity.

Scheme 21.

Scheme 22.

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Scheme 25.

Scheme 28.

of 2-tert-butoxy derivatives of thiophenes and furans

(Scheme 27).³⁷Bismuth(III) chloride was found to be a

superior catalyst to ZnCl₂and FeCl₃.

Scheme 26.

In cleavage reactions catalyzed by bismuth(III) triﬂate, the

intermediacy of a tert-butyl cation was demonstrated by the

formation of tert-butyl methyl ether and isobutylene upon

addition of methanol.

possible that owing to the Lewis acidity of bismuth(III)

compounds, coordination of bismuth to the oxime nitrogen

increases the susceptibility of the oxime carbon to

nucleophilic attack by water.

3.5. Conversion of hydrazones and semicarbazones to

carbonyl compounds

4. Carbon–carbon bond forming reactions

4.1. Friedel–Craft’s acylation

An efﬁcient method for the hydrolytic cleavage of CvN

bond of dimethyl and tosylhydrazones to yield the

corresponding carbonyl compounds using BiCl₃in wet

THF under microwave irradiation has been reported

(Scheme 25).³⁴The method works well with hydrazones

derived from both aldehydes and ketones. The mild nature

of this method is illustrated by the fact that dimethyl-

hydrazones from a,b-unsaturated ketone were hydrolyzed

without rearrangement of the a,b-double bond. Under

similar conditions, the deprotection of semicarbazones to

the corresponding carbonyl compound has also been

reported. Benzyltriphenylphosphonium peroxymonosulfate

has been found to be an efﬁcient reagent for the conversion

of oximes, phenylhydrazones, 2,4-dinitrophenylhydrazones

and semicarbazones to the corresponding carbonyl com-

pounds.³⁵This reaction is catalyzed by BiCl₃.

One of the most important reactions of aromatic compounds

is the Friedel–Craft’s acylation. Traditionally, this reaction

is carried out in the presence of stoichiometric amounts of

AlCl₃. The use of AlCl₃, however, leads to undesirable and

corrosive gaseous efﬂuents and mineral wastes and hence is

not very attractive from an industrial viewpoint. Metal salts

such as FeCl₃and ZnCl₂work in catalytic amounts, but their

use is restricted to the acylation of activated aromatics.

Triﬂic acid works well as a catalyst, however, its corrosive

and hygroscopic nature makes handling and its subsequent

recovery in anhydrous form rather difﬁcult.

The use of bismuth(III) chloride and bismuth(III) triﬂuoro-

methanesulfonate (triﬂate), Bi(CF₃SO₃)·4H₂O for the acyl-

ation of aromatics has been reported. (Scheme 28).³⁸

Bismuth(III) triﬂate has been reported to be an efﬁcient

catalyst for the Friedel–Craft’s acylation of a variety of

substituted benzenes.^39,40

3.6. Deprotection of 1,1-diacetates (acylals)

1,1-Diacetates (acylals) have attracted recent attention since

they are easily synthesized and can serve as selective

protecting groups for aldehydes. Bismuth(III) chloride in

CHCl₃is a mild reagent for the deprotection of 1,1-

diacetates (Scheme 26).³⁶Acylals from aryl aldehydes are

deprotected at a signiﬁcantly faster rate than those from

aliphatic aldehydes and hence the method can be used for

selective deprotection of the former.

Preferential formation of the para isomer was observed in

all cases. The Friedel–Craft’s acylation of anisole catalyzed

by BiCl₃as well as Bi₂O₃has been reported.⁴¹Bismuth(III)

chloride is not a strong enough catalyst for the acylation of

ketones. Detailed NMR studies suggest that the success of

bismuth(III) salts in acting as catalysts for the Friedel–

Craft’s acylation lies in the fact that they preferentially

complex to the acyl chloride relative to the ketones.

Bismuth(III) compounds, in particular, BiCl₃forms p

complexes with aromatics and it has been suggested that

this might facilitate solubility of BiCl₃and hence lead to

reaction in a homogeneous condition.

3.7. Cleavage of tert-butyl ethers

Bismuth(III) chloride, BiCl₃and bismuth(III) triﬂate,

Bi(OTf)₃were found to be good catalysts for the cleavage

Scheme 27.

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8381

Scheme 29.

4.2.2. ‘Barbier type’ allylation of imines. The electro-

reductive Barbier type allylation of carbonyl compounds

and imines has attracted much attention from both

mechanistic and synthetic viewpoints. The electroreductive

allylation of imines with allyl bromides had been carried out

in a Bi⁰/BiCl₃/Bu₄NBr/THF–(Al anode)–(Pt cathode)

system (Scheme 31).⁴⁴Lead(II) chloride also worked well

in place of BiCl₃but owing to the toxicity of lead salts,

BiCl₃is a better alternative. Zinc(II) chloride and tin(II)

chloride were less effective catalysts. Ether solvents, such as

THF and dimethoxyethane were found to be the most

effective.

Scheme 30.

Scheme 31.

The alkylation of immonium cations to amines promoted by

bismuth(III) chloride in aqueous media has been reported

(Scheme 32).⁴⁵A two step procedure for the alkylation of

amines involving a Mannich reaction with benzotriazole

and an aldehyde to yield the 1-(aminoalkyl)benzotriazole

followed by alkylation with a Grignard reagent to give the

N-alkylated amine on elimination of the benzotriazole has

been developed. The alkylation described in Scheme 32

offers an advantage over the use of Grignard reagent.

Substrates that are incompatible with a Grignard reagent,

such as those containing an acidic hydrogen or soluble only

in aqueous solutions can also be alkylated.

The acylation of aromatic ethers under microwave

irradiation catalyzed by bismuth(III) salts has also been

reported (Scheme 29).⁴²The ease of handling of bis-

muth(III) salts and the preferential formation of para isomer

makes them attractive catalysts for Friedel–Craft’s

acylation.

4.2. Allylation catalyzed by bismuth(III) salts

4.2.1. Allylation of acid chlorides (acylation of allyltri-

methylsilane). In the presence of catalytic amounts of

BiCl₃–3 NaI or BiCl₃–1.5 ZnI₂, the acylation of allyl-

trimethylsilane with a variety of acyl chlorides readily

occurred at room temperature (Scheme 30).⁴³This pro-

cedure is one of the ﬁrst examples of a method that uses

relatively non-toxic and inexpensive reagents for allylation

of carbonyl compounds.

4.2.3. Addition of allyl groups to aldehydes. The addition

of an allyl group to an aldehyde to give the corresponding

homoallyl alcohol is a useful synthetic transformation. The

allylation of aldehydes mediated by BiCl₃–metallic Zn or

Fe proceeds under mild conditions to afford the correspond-

ing homoallyl alcohol in good yield (Scheme 32).^46,47The

reaction is chemoselective and is speciﬁc for aldehydes.

Carboxylic acids, nitriles and esters were recovered

unchanged under the allylation conditions. The procedure

suffers from the fact that BiCl₃is required in stoichiometric

amounts (Scheme 33).

The acylation of allylsilanes catalyzed by GaCl₃and InCl₃

has been studied. However, under these conditions, the

isomerization of the product allylketones to 1-propenyl

ketones could not be avoided. The advantage of the method

shown in Scheme 30 is that compounds like 1-adamantane-

carbonyl chloride, which undergo decarbonylation during

Friedel–Craft’s synthesis reacted readily to give 1-ada-

mantylallyl ketone in 80% yield. Several control experi-

ments suggest that BiCl₃is indeed the active species in these

reactions. In a few cases, a small amount of the conjugated

ketone product was also obtained. Again, control studies

ruled out isomerization of the allylsilane. Thus the

conjugated ketone must be forming by isomerization of

the allylketone.

A two-phase electroreductive system has also been devel-

oped for chemoselective allylation of aldehydes (Scheme

34).⁴⁸The best results were obtained in acidic solutions.

The bismuth(III) chloride mediated allyation of carbonyl

compounds containing a carboxyl group has also been

studied (Scheme 35).⁴⁹With pyruvic acid, 2-oxobutyric

Scheme 32.

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N. M. Leonard et al. / Tetrahedron 58 (2002) 8373–8397

of homoallyl ethers and hence this reaction has received

much attention. In an early example, BiBr₃(20 mol%) was

shown to catalyze the allylation of dimethyl acetals derived

from benzaldehyde and heptanal.⁵¹More recently, a highly

catalytic procedure employing bismuth(III) triﬂate for the

allylation of a variety of acetals has been developed

(Scheme 37).⁵²The corresponding homoallyl ethers are

obtained in good yields under mild reaction conditions. It

has been reported that with TiCl₄as the activator, the

reaction of cinnamaldehyde dimethyl acetal with allyl-

trimethylsilane gave only the diallylated product.⁵³Even at

low temperatures (2788C) the monoallylated product was

not formed. Similar results were obtained when allylation of

cinnamaldehyde dimethylacetal was carried out using allyl

bromide in the presence of AlBr₃.⁵⁴In contrast, no

diallylated product was observed using bismuth triﬂate as

the catalyst.

Scheme 33.

Scheme 34.

acid and acetoacetic acid lithium salts, the corresponding

homoallyl alcohol was obtained in good yields. When 4-

oxopentanoic acid, 5-oxohexanoic acid, 3-benzoylpropionic

acid and phthaldehydic acid were used, the corresponding

lactone derived from dehydration of the homoallyl alcohol

was obtained in good yield.

4.4. Aldol condensations

One of the most important carbon–carbon bond forming

reactions in organic chemistry is the Mukaiyama-aldol

condensation. Bismuth(III) chloride has been well studied

as a catalyst for carbon–carbon forming reactions by Wada

and co-workers as well as Dubac and co-workers and has

proven to be an efﬁcient and mild catalyst for such reactions.

Bismuth(III) chloride (5 mol%) has been shown to be an

efﬁcient catalyst for the reaction of silyl enol ethers with

aldehydes at room temperatures (Scheme 38).^55,56Both

aliphatic and aromatic aldehydes gave good yields. Not

surprisingly, ketones gave lower yields than aldehydes

especially with more hindered silyl enol ethers. With

The intramolecular Sakurai cyclization of homoallylic

alcohols to yield polysubstituted tetrahydropyrans is

efﬁciently catalyzed by Bi(OTf)₃·H₂O (Scheme 36).⁵⁰The

products were obtained in good yields and none of the

desilylated by-products were obtained.

4.3. Allylation of acetals

Since carbonyl groups are frequently protected as acetals,

the direct allylation of acetals constitutes a useful synthesis

Scheme 35.

Scheme 36.

Scheme 37.

Scheme 38.

N. M. Leonard et al. / Tetrahedron 58 (2002) 8373–8397

8383

Scheme 39.

Scheme 40.

Scheme 41.

crotonaldehyde the 1,2 and 1,4 products were obtained in a

2.2:1 ratio while the reaction with chalcone gave the 1,4

product alone. The drawback of this method is the low

diastereoselectivity observed.

Pioneering work by Dubac and co-workers has led to the

development of a very versatile bismuth(III) catalyst viz.

bismuth triﬂuormethanesulfonate. Bismuth triﬂate, Bi(CF₃

SO₃)₃·4H₂O, is a very efﬁcient catalyst for the Mukaiyama

aldol-type reactions (Scheme 41).⁶⁰

Le Roux et al. have demonstrated that the catalytic power of

BiCl₃can be increased by the addition of several different

metal iodides (Scheme 39).⁵⁷Detailed optimization studies

showed that with NaI, the optimal yields were obtained

when the ratio of BiCl₃/NaI was 1:3. Interestingly,

ultrasound had a remarkable accelerating effect on the rate

of the reaction at room temperature but was ineffective at

2308C. The catalyst was typically prepared by mixing the

two anhydrous halides BiCl₃and MI_nin CH₂Cl₂at room

temperature. This resulted in an almost instantaneous

formation of black crystals. Analysis of the crystals by

X-ray powder pattern techniques indicated that BiI₃was

formed in all cases in addition to mixed halides. However,

BiI₃alone proved to be an ineffective catalyst for the

reaction. Besides being relatively non-toxic, the use of

bismuth salts for this reaction offers another advantage. The

silylated intermediates can be isolated thus avoiding the

hydrolysis in the presence of the catalyst, which can lead to

undesirable side reactions. The mild reaction conditions

allow a catalytic cross aldol reaction between aldehydes and

silyl enol ethers derived from furfural, compounds normally

prone to polymerization under strongly acidic conditions

(Scheme 40).^58,59

The reactions work well with both aliphatic and aromatic

aldehydes. Bismuth triﬂate was found to be a superior

catalyst in comparison to other metal triﬂates such as

Ce(OTf)₄, Sc(OTf)₃and Yb(OTf)₃. It is proposed that the

mechanism of catalysis by bismuth triﬂate involves a

transmetallation reaction between Bi(OTf)₃·4H₂O and the

silyl enol ether resulting in the formation of trimethylsilyl

triﬂate which could be the true catalyst for the reaction.

Silicon NMR studies as well as the fact that the reaction of

acetals is faster than that of aldehydes lends support to this

mechanism. It has been previously shown that TMS triﬂate

is a better activator of acetals than aldehydes. An advantage

of Bi(OTf)₃·4H₂O over TMSOTf is that the former is easier

to handle and is less moisture sensitive.

4.5. Knoevenagel condensation

The condensation of an aldehyde or ketone with an active

methylene compound, the Knoevenagel reaction, is an

important method to synthesize a,b-unsaturated carboxylic

acids, and is typically catalyzed by organic amines. A

solvent free procedure for the Knoevenagel reaction,

catalyzed by BiCl₃has been developed (Scheme 42).⁶¹

When the same reaction was carried out in benzene, longer

reaction times were required and the isolated yields were

also unacceptably low.

4.6. Reformatsky-type reactions

The Reformatsky reaction has traditionally been carried out

Scheme 42.

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N. M. Leonard et al. / Tetrahedron 58 (2002) 8373–8397

BiCl₃was used along with a chiral co-catalyst. This is

attributed to a lowering of the Lewis acidity of BiCl₃up on

complexation with the co-catalyst. The enantioselectivity

was inﬂuenced considerably by the reaction temperature,

with the best results obtained at 2238C. Interestingly, best

results were obtained when the catalyst was prepared at 08C

for 3 h with a slight excess of BiCl₃. Asymmetric induction

was observed in CH₂Cl₂and ClCH₂CH₂Cl but not in polar

solvents although the reaction did proceed in reasonable

yields in the latter solvents.

Scheme 43.

using zinc metal in an organic solvent. More recently, there

have been reports of Reformatsky-type reactions in water

using zinc powder or tin–aluminum couple. The reaction of

a-halocarbonyl compounds with aldehydes in the presence

of BiCl₃–Al occurs readily in water to yield the b-hydroxy

compounds in good yields (Scheme 43).⁶²The reaction,

which can also be carried out with catalytic amounts of

BiCl₃–Al, occurs readily with aliphatic and aromatic

aldehydes. While detailed mechanistic studies were not

carried out, based on the preferential formation of erythro

products in case of reaction between 2-bromocyclo-

hexanone and benzaldehyde, an acyclic transition state has

been proposed. A cyclic transition state and single electron

transfer (SET) mechanisms have been ruled out.

While the observed enantioselectivities are not sufﬁcient to

make this procedure practical, the future for chiral bismuth

catalysts does hold promise. The catalyst structure, which

has not yet been established, will need to be determined for

further improvements in the observed enantioselectivities.

4.8. Diels–Alder and the aza-Diels–Alder reactions

The Diels–Alder reaction still remains one of the best ways

to construct a six-membered ring. Lewis acid catalysis of the

Diels–Alder reaction has been a subject of rigorous study.

Aluminum(III) chloride is one commonly employed Lewis

acid catalyst for the Diels–Alder reaction but its use poses

some disadvantages: it is rather corrosive and often leads to

polymerization of the diene substrates. Dubac and co-

workers discovered that BiCl₃and Bi(OTf)₃are mild and

efﬁcient catalysts for the Diels–Alder reaction (Scheme

46).⁶³The catalytic activity exhibited by these bismuth(III)

salts exceeds that of scandium, titanium, samarium and

ytterbium based efﬁcient catalysts.

4.7. Asymmetric trimethylsilylcyanation of aldehydes

using chiral bismuth compounds

Bismuth(III) chloride was found to be an efﬁcient catalyst

for the cyanation of aldehydes with TMSCN to afford the

corresponding cyanohydrin in good yields (Scheme 44).

While aldehydes reacted faster than ketones, both gave very

high yields of the corresponding cyanohydrin.

The same authors have developed the ﬁrst known asym-

metric reaction with a chiral bismuth catalyst, opening new

doors for bismuth chemistry. Moderate enantioselectivities

were obtained by use of a chiral bismuth(III) catalyst

prepared in situ from BiCl₃and (2R,3R)-(þ)-diethyl tartrate

(Scheme 45). No asymmetric induction was observed when

One advantage of both bismuth chloride and bismuth triﬂate

over scandium triﬂate is that they are much less expensive.

Dubac and co-workers also found that in the BiCl₃catalyzed

Scheme 44.

Scheme 45.

Scheme 46.

N. M. Leonard et al. / Tetrahedron 58 (2002) 8373–8397

8385

Scheme 47.

Scheme 48.

Scheme 49.

Diels–Alder reaction of dienes with electrophiles such as

ethyl mesoxalate, in addition to the expected [4þ2] product,

the 2H-pyran, there was competitive (15–20%) formation

of the ene reaction product (Scheme 47).⁶⁴The two

products, were however, easily separable by chromato-

graphy and thus this method allows relatively easy access to

the 2H-pyrans.

48).⁶⁵In contrast, with zinc chloride as a promoter, a full

equivalent of ZnCl₂is required and the reaction times are

considerably longer (36 h instead of 0.5–1 h). The advan-

tage of using the relatively water stable catalyst bismuth(III)

triﬂate is that the imine can be generated and subjected to

the Diels–Alder reaction in a one pot procedure (Scheme

49).

The hetero Diels–Alder reaction involving imino-dienes

(the aza-Diels–Alder reaction) is a useful method for the

synthesis of nitrogen containing heterocycles and has found

application in the synthesis of several natural products. Both

bismuth(III) chloride, BiCl₃and bismuth triﬂate, Bi(CF₃

SO₃)₃, are very efﬁcient catalysts for the hetero Diels–Alder

reactions of imines with Danishefsky’s diene (Scheme

A bismuth(III) catalyzed intramolecular hetero Diels–Alder

reaction of 2-cyano-1-azadienes to yield oxazinopiperidines

has been reported (Scheme 50).⁶⁶Bismuth(III) chloride–

copper(II) triﬂate was found to be one of the most effective

catalysts for this reaction while silver salts such as AgOTf

and AbSbPF₆were found to be totally inactive. The use of

TiCl₄led to partial degradation of the product, presumably

due to the presence of trace amounts of HCl. The reaction

was found to be very sensitive to the solvent used. While

the use of toluene and benzene gave good yields of the

cycloaddition product, considerable decomposition of the

azadiene occurred in more polar solvents such as CH₂Cl₂

and THF. No trace of the isomerized compound or the cyclic

enamine, observed under thermal conditions, was detected

using BiCl₃as the catalyst. The slow reaction times led to

some loss in catalyst activity due to hydrolysis but this was

˚

avoided by the addition of 3 A sieves.

An efﬁcient synthesis of pyrano quinolines derivatives

also takes advantage of the ability of BiCl₃to catalyze the

aza-Diels–Alder reaction.⁶⁷In this case, reaction of N-aryl

aldimines with nucleophilic oleﬁns affords the quinoline

derivatives in high yields (Scheme 51).

Scheme 50.

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N. M. Leonard et al. / Tetrahedron 58 (2002) 8373–8397

of the cis/trans isomers is found to be highly temperature

dependent. While the trans-isomer was obtained almost

exclusively at 08C, a 1:1 mixture of cis and trans-isomers

resulted at room temperature.

5. Carbon–heteroatom bond forming reaction

5.1. Carbon–nitrogen bond formation

Scheme 51.

5.1.1. N-Derivatization of ^L-proline esters. Katritzky and

co-workers have developed an efﬁcient BiCl₃mediated

method for the N-derivatization, in aqueous media, of ^L-

proline and pipecolinic esters (Scheme 55).⁷¹

The advantages of this method include mild reaction

conditions, and recoverability of the catalyst.

A novel synthesis of hexahydrodibenzo[b,h] naphthapyri-

dine derivatives using a BiCl₃catalyzed intramolecular

hetero Diels–Alder reaction has been reported by

Sabitha and co-workers (Scheme 52).⁶⁸The products are

obtained as mixtures of cis and trans-diastereomers in a 1:1

ratio.

This method constitutes a useful synthesis of N-substituted

a-amino acids, which are important due to their use in the

preparation and conformation studies of peptide analogs as

well as biological activity. The presence of multiple bonds

in the newly introduced groups allows the potential for

further synthetic manipulation. The methodology has also

been extended to the N-derivatization of racemic N-

pipecolinic acid ethyl ester.

Another application of this method can be seen in the

synthesis of tetrahydrochromano quinolines in good yields

starting from aromatic amines and O-allylated derivatives of

salicylaldehydes (Scheme 53).⁶⁹By preparing the imine in

situ, the problems associated with handling the moisture

sensitive imines are avoided.

5.1.2. Formation of azlactones. Bismuth(III) acetate has

been used as a catalyst for the synthesis of azlactones from

aromatic aldehydes (Scheme 56).⁷²

Due to their interesting biological properties, the synthesis

of octahydroacridines has received much attention recently.

A one-pot procedure for the stereocontrolled synthesis of

octahydroacridines has been developed using an aza-Diels–

Alder reaction catalyzed by BiCl₃(Scheme 54).⁷⁰The ratio

While the standard procedure for azlactone synthesis

consists of using a stoichiometric amount of fused

anhydrous sodium acetate, 10 mol% of Bi(OAc)₃is

sufﬁcient to catalyze the reaction and the crude product is

found to be .98% pure.

Scheme 52.

Scheme 53.

Scheme 54.

N. M. Leonard et al. / Tetrahedron 58 (2002) 8373–8397

8387

Scheme 55.

Scheme 56.

Scheme 57.

Scheme 58.

5.1.3. Formation of dihydropyrimidinones (the Biginelli

reaction). Dihydropyrimidinones have attracted recent

attention due to their interesting biological properties.

They hold promise as calcium channel blockers and

antihypertensive agents. The classical method for their

synthesis, as developed by Biginelli involves one-pot

condensation of ethyl acetoacetate, benzaldehyde and urea

under strongly acidic conditions. This method however

suffers from low yields. Bismuth(III) chloride has been

found to be an attractive catalyst for the Biginelli reaction

(Scheme 57).⁷³

Bismuth(III) chloride efﬁciently catalyzes the reaction of

silylated heterocyclic bases with a-^D-ribofuranosyl bromide

to yield the corresponding b-^D-glucoside in good yields

(Scheme 58).⁷⁴This method has the advantage of being a

one-pot procedure and utilizes the relatively non-toxic salt

BiCl₃.

5.2. Carbon–oxygen bond formation

5.2.1. Formation of ethers. The bismuth(III) catalyzed

reductive homocoupling of carbonyl compounds and

heterocoupling of a carbonyl compound with an alkoxysi-

lane to afford the corresponding symmetrical and unsym-

metrical ethers has been reported. This methodology has

been utilized in the single-step preparation of novel

crownophanes with oleﬁnic and acetylenic linkages

(Schemes 59 and 60).⁷⁵

The method works even with aliphatic aldehydes which

normally give poor yields in the Biginelli reaction.

5.1.4. One-pot synthesis of nucleosides using bis-

muth(III) bromide as a catalyst. One common method

for synthesis of nucleosides is the condensation of a

silylated base with a peracetylated ribofuranosyl halide in

the presence of mercuric salts. Owing to the high toxicity of

mercury, this method is not very environment friendly.

The reaction does not work with amino-substituted

benzaldehydes, acetophenone or benzophenone. Similar

results were observed in the homocoupling reactions.

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N. M. Leonard et al. / Tetrahedron 58 (2002) 8373–8397

triﬂate. The acylation of alcohols has traditionally been

carried out in the presence of tertiary amines. However,

tertiary amines such as pyridine and 4-(dimethylamino)pyri-

dine (DMAP) are highly toxic and unpleasant to handle.

Hence, attention has focused on development of Lewis acid

catalysts for acylation of alcohols. Among the most efﬁcient

is scandium triﬂate reported by Yamamoto and co-

workers.⁸²Recently, bismuth(III) triﬂate has been reported

to be an efﬁcient catalyst for the acylation of 18, 28 and 38

alcohols.⁸³The benzoylation and pivalation of 18 alcohols is

also efﬁciently catalyzed by bismuth(III) triﬂate but 28 and

38 alcohols fail to react. The comparatively lower cost and

ease of handling of bismuth triﬂate makes it an attractive

alternative to scandium triﬂate for acylation of alcohols. A

practical method suitable for large scale acetylation of

alcohols has been developed using bismuth triﬂate (Scheme

62).⁸⁴

Scheme 59.

5.2.4. Selective esteriﬁcation using bismuth(III) car-

boxylates. Bismuth(III) carboxylates, obtained by treatment

of triphenylbismuth with various carboxylic acids, are

efﬁcient reagents for the selective esteriﬁcation of acids,

using benzylic and tertiary alkyl bromides (Scheme 63).⁸⁵

Scheme 60.

The methodology has also been extended to formation of

tosylates (Scheme 63). Since the bismuth carboxylates are

almost colorless while BiBr₃is a bright yellow solid, these

reactions can be followed by a color change. The procedure

does not work with primary bromides and hence esters of

primary alcohols cannot be synthesized by this method. The

authors have also carried out detailed investigations into the

reactivity of bismuth(III) halides towards alcohols (Scheme

64).

Scheme 61.

Scheme 62.

5.3. Formation of acetals catalyzed by bismuth(III)

triﬂate

5.2.2. Benzylation of alcohols promoted by BiCl₃.

Arylation reactions with trivalent organobismuth com-

pounds have limited scope. Triarylbismuthine reacts with

metal halides to give arylation products. Arylation with

pentavalent bismuth is better studied and such reactions

have been extensively reviewed and are not considered

here.^76–78The benzylation of optically active aliphatic

alcohols as well as cis-and trans-2-methylcyclohexanols

catalyzed by bismuth(III) bromide in the presence of sec-

phenethylalcohol has been studied (Scheme 61).^79–81It is

proposed that the reaction involves an intermediate with a

hexa-coordinated bismuth atom. The use of a full equivalent

of BiBr₃and CCl₄as the solvent detract from the synthetic

utility of this procedure.

Acetals are obtained in good yields by treatment of

aldehydes and ketones with trialkyl orthoformate and the

corresponding alcohol in the presence of 0.1 mol%

Bi(OTf)₃·4H₂O (Scheme 65).⁸⁶The procedure works well

for the synthesis of both dimethyl and diethyl acetals.

The synthesis of dialkyl acetals from diaryl ketones is more

difﬁcult, and the standard acetalization conditions generally

do not work with diaryl ketones. Triﬂic acid (20 mol%) has

been shown to be a catalyst for the synthesis of diaryl

ketones.⁸⁷Bismuth triﬂate (1.0 mol%) was found to be a

very efﬁcient catalyst for the acetalization of diaryl ketones

as well (Scheme 66). The ease of handling of bismuth

triﬂate compared to triﬂic acid coupled with its catalytic

5.2.3. Acylation of alcohols catalyzed by bismuth(III)

Scheme 63.

Scheme 64.

N. M. Leonard et al. / Tetrahedron 58 (2002) 8373–8397

8389

Scheme 68.

Scheme 65.

Scheme 66.

Scheme 69.

Scheme 70.

alcohols and diols by catalytic ring opening of epoxides, is

efﬁciently catalyzed by BiCl₃(Scheme 70).⁸⁹

Scheme 67.

This method works with both aliphatic and cyclic epoxides.

With terminal epoxides, attack by the solvent occurs, as

expected, on the less substituted carbon. The use of BiCl₃,

which is relatively non-hygroscopic and inexpensive, offers

some advantages over existing methods which make use of

FeCl₃, FeCl₃–SiO₂, ceric ammonium nitrate and lanthanide

triﬂates. Ferric chloride is very hygroscopic while ceric

ammonium nitrate is not very catalytic in nature (80 mol%).

Lanthanide triﬂates suffer from high cost.

efﬁciency makes this procedure an attractive method for the

synthesis of acetals of diaryl ketones.

1,3-dioxolanes or cyclic acetals are typically made from

ethylene glycol and the equilibrium is shifted in favor of the

product by azeotropic removal of water with benzene. Two

methods for the synthesis of 1,3-dioxolanes catalyzed by

bismuth triﬂate, both of which avoid the use of benzene,

have been developed (Scheme 67). In reﬂuxing toluene, not

surprisingly while the reaction does not go to completion,

high enough conversion is achieved to allow isolation of the

pure dioxolane by chromatography in moderate to good

yields.

5.6. Glycosylation promoted by bismuth(III) chloride

Glycosylations using 1-glycosyl dimethyl phosphite are

promoted by bismuth(III) chloride to afford the correspond-

ing glucoside as a mixture of anomers (Scheme 71).⁹⁰

Bismuth(III) chloride presumably acts as an activator by

complexing to the soft base, phosphorous(III).

5.4. Formation of 1,1-diacetates (acylals)

Acylals have often been used as protecting groups for

carbonyl compounds because they are stable to neutral and

basic conditions. Bismuth triﬂate, Bi(OTf)₃·4H₂O, is a

highly efﬁcient catalyst (0.1 mol%) for the formation of

acylals from aromatic aldehydes (Scheme 68).⁸⁸Ketones

are not affected under the reaction conditions and hence the

chemoselective protection of aldehydes in the presence of

ketones can be achieved (Scheme 69). The reaction can be

carried out in solvent or under solvent-free conditions.

Aliphatic aldehdyes reacted faster under solvent-free

conditions.

5.7. Selective hydrolysis of aryl esters using bismuth(III)

mandelate

Bismuth(III) mandelate which is easily prepared from Bi₂O₃

and (L)-mandelic acid, is an efﬁcient catalyst for the

hydrolysis of aryl esters (Scheme 72).⁹¹The presence of

molecular oxygen slightly increases the reaction rate, while

the introduction of water decreases the catalytic activity.

5.8. Carbon–sulfur bond formation

5.5. Cleavage of epoxides catalyzed by bismuth(III)

chloride

5.8.1. Synthesis of thioacetals. Several bismuth(III) salts,

such as BiBr₃, BiCl₃, BiI₃and Bi₂(SO₄)₃act as catalysts for

the conversion of carbonyl compounds to the corresponding

thioacetals (Scheme 73).⁹²The reaction works well with a

variety of aldehydes and ketones, with the best turnover

value reaching 2300.

The cleavage of epoxides in a regio- and stereoselective

manner to yield a variety of 1,2-difunctional compounds is a

useful step in several synthetic schemes, and has received

much attention. The synthesis of b-alkoxy and b-acetoxy

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N. M. Leonard et al. / Tetrahedron 58 (2002) 8373–8397

Scheme 71.

NH₄SCN and catalytic amounts of BiCl₃has been

developed (Scheme 75).⁹³The short reaction times, high

yields and the use of easy to handle reagents make this

procedure an attractive way to synthesize thiiranes from

epoxides.

5.9. Sulfonylation of aromatics

Scheme 72.

The sulfonylation of aromatics is efﬁciently catalyzed by

bismuth(III) chloride and bismuth(III) triﬂate (Scheme

76).^38,94This work is the ﬁrst report of the metal triﬂate

catalyzed sulfonylation. Bismuth(III) chloride works well

with activated substrates but not with deactivated aromatics

such as chlorobenzene.

By adjusting the amount of the catalyst, preferential

protection of one carbonyl compound over the other is

possible. Thus, benzaldehyde is preferentially protected

over acetophenone when reacted with 1,2-ethanethiol in the

presence of 0.2 mol% BiCl₃while cyclohexanone is

preferentially protected over 2-heptanone in the presence

of 0.5 mol% BiCl₃.

Detailed mechanistic studies have been conducted to

elucidate the catalytic cycle with bismuth(III) triﬂate.

These studies suggest a mechanism dependant on the

electrophilic reagent used. With acid chlorides and sulfonyl

chlorides, the proposed mechanism involves a TfO²/Cl²

ligand exchange, leading to BiCl₃and a mixed anhydride,

RCOOTf or RSO₂OTf. The acylation or sulfonylation of the

The conjugate addition of thiophenol to a,b-unsaturated

carbonyl compounds is also catalyzed by BiCl₃(Scheme

74).

5.8.2. Conversion of epoxides to thiiranes. A simple

method for the conversion of epoxides to thiiranes using

Scheme 73.

Scheme 74.

Scheme 75.

Scheme 76.

N. M. Leonard et al. / Tetrahedron 58 (2002) 8373–8397

8391

Scheme 77.

(Scheme 78).⁹⁷An attractive feature of this method is

that the reaction is carried out in relatively non-toxic

solvent systems such as aqueous THF or aqueous DMF.

The yields were signiﬁcantly lowered when the reaction

was carried out under air instead of a nitrogen

atmosphere.

Scheme 78.

Scheme 79.

Scheme 80.

5.12. Formation of carbon–halogen bonds

5.12.1. Synthesis of b-haloketones and esters. b-Halo-

carbonyl compounds are useful intermediates in organic

synthesis. However, in contrast to a-halocarbonyl com-

pounds, which are easy to synthesize, there are fewer

methods to obtain b-halocarbonyl compounds. b-Halo-

carbonyl compounds undergo dehydrohalogenation under

drastic reaction conditions. A mild one-pot method for their

synthesis couples the aldol condensation with halogenation

(Scheme 79).⁹⁸The method takes advantage of the

activation of the silicon–chlorine bond of chlorotrimethyl-

silane by a Bi(III) salt.⁹⁹

aromatic compound with these active electrophiles would

also result in the formation of triﬂic acid, TfOH. Of the

various other metal triﬂates, only In(OTf)₃has shown

catalytic activity comparable to Bi(OTf)₃. The mixed

anhydride can then be regenerated by one of two possible

pathways: reaction of TfOH with acid chloride, or with

BiCl₃leading to recovery of the triﬂate group on the

bismuth atom. When the electrophile is an anhydride, it has

been suggested that the mechanism involves a coordination

complex. Triﬂic acid has also been used as a catalyst for the

sulfonylation of arenes. Remarkably, the catalytic activity

of triﬂic acid is remarkably enhanced by the addition of

small amounts of bismuth(III) chloride.⁹⁵

5.12.2. Chlorination of alcohols catalyzed by bis-

muth(III) salts. Bismuth(III) chloride has been shown to

be an effective reagent for the chlorination of 28 and 38

alcohols (Scheme 80).¹⁰⁰Benzyl alcohols gave only about a

30% yield of benzyl chloride while the main product was

dibenzyl ether. Control experiments suggest that the ether is

formed directly from the alcohol and not by way of the

corresponding chloride.

5.10. Synthesis of sulﬁdes and selenides promoted by

Sm–BiCl₃

The reaction of benzyl bromide with disulﬁdes and

diselenides to give benzyl sulﬁdes and benzyl selenides is

promoted by Sm–BiCl₃in aqueous THF or aqueous DMF

as the solvent (Scheme 77).⁹⁶

The chlorination of a variety of 28, 38, allylic and benzylic

alcohols by chlorosilanes is also catalyzed by BiCl₃

(Scheme 81).¹⁰¹Alkoxysilanes have been detected as

intermediates in this reaction. With the exception of

cinnamyl alcohol, the chlorination of allylic alcohols by

this method was accompanied by rearrangement of the

carbon skeleton to give a mixture of chlorides.

5.11. Synthesis of allyl sulﬁdes

A synthesis of allyl sulﬁdes and allyl selenides via the

reaction of allyl bromide with disulﬁdes and diselenides

promoted by the Sm–BiCl₃system has been reported

Efﬁcient halogen exchange reactions catalyzed by BiBr₃

and BiCl₃have also been developed (Scheme 82).¹⁰²

A

Scheme 81.

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N. M. Leonard et al. / Tetrahedron 58 (2002) 8373–8397

bismuth(III) halides as catalysts (Scheme 83).¹⁰³Again,

as in the chlorination of alcohols, advantage is taken of

the fact that bismuth(III) halides are good activators of

silicon–halide bond.

Scheme 82.

Scheme 83.

6. Reductions with bismuth compounds

6.1. Selective reduction of nitro compounds to azoxy

compounds

The selective reduction of nitro compounds to azoxy

compounds has received considerable attention. However,

many of these reactions suffer from accompanying side

Scheme 84.

Scheme 85.

reactions such as polymerization and dehalogenation of

halogenated substrates. The selective reduction of nitro

compounds to azoxy compounds using BiCl₃–metallic zinc

has been reported (Scheme 84).¹⁰⁴

When the reduction was applied to o,o⁰-dinitrobiphenyl, the

corresponding benzo[c]cinnoline N-oxide was obtained in

good yield (Scheme 85). Several other reagents such as

Na₂S in aqueous ethanol, Ph₃P in alkaline ethanol, H₂with

Raney-Ni, Na/Hg in CH₃OH have been employed for the

same reaction. However, in these cases, the product was a

mixture of the N-oxide and the benzo[c]cinnoline.

Scheme 86.

6.2. Reduction of nitro compounds to amines

variety of 28 and 38 alkyl iodides and bromides are

quantitatively converted into the corresponding bromide

and/or chloride by exposure to BiBr₃or BiCl₃in

dichloroethane. A mechanism involving the formation of

an ion-pair R^þ/YBiX₃has been proposed for this

reaction.

The bismuth(III) chloride promoted NaBH₄reduction of a

variety of substituted nitro compounds to amines has been

reported (Scheme 86).¹⁰⁵The procedure also works

moderately well for aliphatic nitro compounds. However,

benzonitrile, benzamide and nitrobenzene itself are not

affected under these conditions. The reduction of azo-

methines to the corresponding amines has also been

achieved using this method.

5.12.3. Synthesis of peracetylated glycosyl halides.

Glycosyl halides are of interest in carbohydrate chemistry

because they have been used for the formation of the

glycosidic bond, as well as for generation of anomeric

carbocations, radicals or carbanions. While the standard

conditions for the preparation of peracetylated glycosyl

chlorides involves the use of harsh reagents such as HCl

gas or corrosive Lewis acids such as AlCl₃, ZnCl₂or

6.3. Chemoselective reduction of a,b-unsaturated ethyl

esters using BiCl₃–NaBH₄

The chemoselective reduction of carbon–carbon double

bonds in a,b-unsaturated esters using BiCl₃–NaBH₄has

been reported (Scheme 87).¹⁰⁶The optimal conditions were

found to be a combination of 4.0 mol of NaBH₄and 0.5 mol

TiCl₄,

a mild method has been developed using

N. M. Leonard et al. / Tetrahedron 58 (2002) 8373–8397

8393

Scheme 90.

Scheme 87.

Scheme 91.

Scheme 88.

of BiCl₃per mole of the ester. The reduction of the ester

moiety was found to occur when a large excess of NaBH₄

over BiCl₃was used. This method suffers from the

limitation that only ethanol worked as a solvent, limiting

the reduction to ethyl esters. Attempted reduction of methyl

esters in ethanol gave a signiﬁcant amount of the saturated

ethyl ester, resulting from transesteriﬁcation.

Scheme 92.

important synthetic transformation that has received much

attention. Despite the number of methods that have been

developed for epoxide rearrangement, only a few are both

regioselective and catalytic in nature. Bismuth triﬂate is a

highly efﬁcient catalyst for rearrangement of aromatic

epoxides to carbonyl compounds (Scheme 90).¹⁰⁹One main

advantage of this catalyst is that the use of an anhydrous

solvent is not necessary since the catalyst is fairly

insensitive to small amounts of moisture.

6.4. Reduction of aromatic a-haloketones

The reduction of aromatic a-bromo ketones and a-

iodoketones in aqueous THF has been achieved using

BiCl₃–NaBH₄(Scheme 88).¹⁰⁷The combination of NaBH₄

and BiCl₃generates bismuth metal in a highly activated and

divided state. a-Chloroketones can also be reduced using

this method, but they must be ﬁrst converted in situ to the

iodo derivative by addition of NaI.

The catalyst is remarkably efﬁcient and with trans-stilbene

oxide, 0.01 mol% of Bi(OTf)₃·4H₂O was sufﬁcient to cause

rearrangement. When a solution of trans-stilbene oxide in

dichloromethane was treated with 0.1 mol% of triﬂuoro-

methanesulfonic acid, the solution turned red and the

resulting diphenylacetaldehyde was found to be consider-

ably impure. This observation suggests that bismuth triﬂate

is acting as a Lewis acid and not simply releasing triﬂic acid

into the solution. Aliphatic epoxides bearing a tertiary

epoxide carbon also underwent rearrangement readily. For

example, 1-methylcyclohexene oxide underwent ready

rearrangement to give an 89:11 mixture of 2-methylcyclo-

hexanone (migration of methyl group) and 1-methyl-1-

cyclopentanecarboxaldehyde (migration of C–C bond),

respectively. Acyl-substituted epoxides underwent smooth

rearrangement with exclusive migration of the acyl group

(Scheme 91).

6.5. Miscellaneous reductions

A key step in a novel synthesis of 3-hydroxycephems

involves a reductive cyclization of chlorinated 4-(phenyl-

sulfonylthio)-2-azetidinones using a BiCl₃/Sn redox couple

in DMF containing pyridine (Scheme 89).¹⁰⁸The reduction

has also been carried out using the more corrosive Lewis

acid, TiCl₄/Sn couple.

The yield was signiﬁcantly lower without the addition of

pyridine. While the role of the metal salt is not clear, it is

speculated that the reduction of Bi(III) by Sn to lower valent

bismuth works as a promoter of the reaction. Other

mechanistic studies suggest that the cyclization occurs via

the reduction of the terminal chloro group rather than the

reduction of the thiosulfonate moiety.

Thus, this method provides easy access to b-oxoaldehydes

via rearrangement of epoxides. Epoxides bearing a cyano

group and nitro group proved resistant to the reaction

conditions and the starting epoxide was recovered in good

yield in all cases. In contrast, when b-methylnitrostyrene

oxide is treated with BF₃·Et₂O, a mixture comprising of ﬁve

different products is obtained.¹¹⁰In the presence of InCl₃,

7. Rearrangements promoted by bismuth(III) salts

7.1. Rearrangement of epoxides

The rearrangement of epoxides to carbonyl compounds is an

Scheme 89.

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N. M. Leonard et al. / Tetrahedron 58 (2002) 8373–8397

Scheme 93.

rearrangement accompanied by loss of the nitro group to

give 1-chloro-1-phenyl acetone is observed.¹¹¹

agement and Dr Christopher Le Roux (University Paul

Sabatier) for many useful discussions.

The rearrangement of epoxides to carbonyl compounds is

also efﬁciently promoted by bismuth(III) oxide perchlorate

(Scheme 92).¹¹²While the authors do not report any

problems with the use of this reagent, as with any

perchlorate, due caution must be exercised while handling

this reagent.

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`

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Bismuth compounds offer the added advantage of being

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8397

Biographical sketch

Nicholas Leonard, born in 1980 is a native of Des Plaines Illinois and he

attended Maine Township West high school. He graduated with a BS

degree in Chemistry from Illinois Wesleyan University in 2002. As an

undergraduate, Nick did research for 2 1/2 years with Dr Ram Mohan,

developing applications of bismuth(III) compounds in organic synthesis.

Nick has published in the Journal of Organic chemistry and Tetrahedron

Letters. Nick is currently a ﬁrst year graduate student at University of

California, Irvine. Besides chemistry, Nick enjoys volleyball, mountain

biking, rollerblading, and playing the guitar.

Laura Wieland, born in 1980 is a native of Freeport, Illinois and she

attended Freeport high school. She graduated with a BS degree in

Chemistry from Illinois Wesleyan University in 2002. As an undergraduate,

Laura did research for 3 years with Dr Ram Mohan on developing

applications of bismuth(III) compounds in organic synthesis. Laura has

published in the Journal of Organic chemistry and Tetrahedron Letters.

Laura is currently a I year graduate student at Boston College. Besides

chemistry, Laura enjoys traveling, vegetarian cooking, rock climbing and

skydiving. Laura is also an accomplished clarinet player.

Ram Mohan, born in 1965, received his undergraduate degree in chemistry

from Hansraj College in Delhi, India and Master’s degree in organic

chemistry from Delhi University. He received his PhD in 1992 from the

University of Maryland Baltimore and did his postdoctoral work at the

University of Illinois Urbana-Champaign. He is currently an Associate

Professor of Chemistry at Illinois Wesleyan University, a four-year liberal

arts school. His current research interests include applications of

bismuth(III) compounds in organic chemistry, green chemistry, Lewis

acids and chemistry of epoxides. Besides chemistry, Ram enjoys traveling

by train, cooking, classical music and origami.

Article Doi

Applications of bismuth(III) compounds in organic synthesis

DOI: 10.1016/S0040-4020(02)01000-1

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Article abstract of DOI:10.1016/S0040-4020(02)01000-1

Full text of DOI:10.1016/S0040-4020(02)01000-1

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