sign in sign up
<<
Home , Diamantane, Formyl fluoride, Onium ion

Proc. Indian Acad. Sci. (Chem. Sci.), Vol. 100, Nos 2 & 3, April 1988, pp. 143-185. t~ Printed in India.

Synthetic methods and reactions*

G K SURYA PRAKASH and GEORGE A OLAH* Donald P and Katherine B Loker Research Institute, and Department of , University of Southern California, Los Angeles, California 90089, USA

Abstract. This review deals with the development of a series of reagents and reactions for organic synthesis using simple starting materials. A wide array of carbocationic and onium reagents, haiogenating agents, FriedeI-Crafts catalysts, metal-induced oxidation- reductions and silicon reagents were utilized in basic (unit) reactions, which serve as building blocks for general synthetic transformations.

Keywords. ; onium ; halogenating agents; Friedel-Crafts reactions; oxidizing agents; reducing agents; silicon reagents.

Introduction

Synthetic chemistry in general is directed either towards the preparation of specific molecules or towards the development of new reactions and reagents as well as improvement of existing methods. Over the years our group has been involved in developing selective reagents and methods for organic synthetic transformations using simple and inexpensive starting materials. The development of basic (unit) reactions are important as they serve as building blocks for all syntheses including those of complex target molecules such as natural products. In this review we give an account of our synthetic studies of recent years. In a short review it is not possible to give sufficient background for such a diverse and broad field or comparison with other existing methods and reactions. The reader should be aware of the large diversity of existing synthetic methods to which we hope to have made some useful contributions.

Nitronium and nitrosonium salts and related reagents as nitrating agents

Conventional electrophilic nitration of aromatic compounds (Hoggett et al 1971) using mixtures of sulfuric and nitric acids has several inherent difficulties. In particular, the water produced in the reaction dilutes the acid and therefore reduces its strength. Furthermore, the strong oxidizing ability of the mineral acids makes them unsuitable for nitrating many acid-sensitive compounds. Disposal of the spent acid also poses a significant environmental problem. In order to circumvent these difficulties we developed nitronium salts (such as NO~ BF~- and NO~-PF6) and related reagents as effective and efficient nitrating agents.

* For correspondence *Considered synthetic methods and reactions. Part 131. For part 130 see: M B Sassaman, K D Kotian, G K S Prakash and G A Olah. 1987 J. Org. Chem. 52 4314. 143 144 G K Surya Prakash and George A Olah

Nitronium ion salts are readily prepared from the reaction of nitric acid (or organic and inorgarric nitrates) with HF and BF3 (Olah and Kuhn 1973) and other Lewis acids such as PFs, SbF~, etc. When nitric acid is used in the preparation NO+BF4 is formed as by-product because of the presence of nitrous acid as an impurity. This can be avoided by purifying the nitric acid with urea. These nitronium ion salts nitrate aromatics in organic solvents in close-to-quantitative yields (Olah et a11961, 1962; Olah and Kuhn 1962). As HF and BF3(PFs) can be easily recovered and recycled, the method can be considered as a nitric acid nitration using a superacidic catalyst (Olah et al 1985a).

HNO3 + HF + 2BF3, " NO~-BFs + BF3 : OH2 RONO2 + HF + 2BF3. : NO+BFz + BF3ORH Ar-H + NO~-BF4 ArNO2 + HF + BF3 (PFs (PF.s) Nitronium ion salts are such powerful nitrating agents that they can effect even trinitration of to trinitrobenzene (Olah and Lin 1974a). Nitronium ion salts enable nitration of any conceivable aromatic substrate. It is, however, unfortunate that till recently controversial mechanistic studies of aromatic nitration (Eberson and Radner 1987) overshadowed the broad preparative utility of the nitronium salts. Nitronium salts have also been found to nitrate aliphatic . In particular, gives moderate yield of 1-nitroadaman- tane (Olah and Lin 1971; Olah and Olah 1987). More recently desililative nitration of alkyl and allyi has been achieved using nitronium salts (Olah and Rochin 1987). Nitronium ions salts have been found to react with bistrimethylsilylacetylene to produce nitrotrimethylsilyl (Schmitt and Bedford 1986). The reaction is catalyzed by external fluoride ion and in all probability proceeds through an addition elimination path. Electrophilic nitration of with nitronium salts has been carried out in pyridinium polyhydrogen fluoride medium (vide infra) to give nitrofluorinated . More selective nitronium ion salts, such as N-nitropyridinium salts, which are prepared from the corresponding , act as convenient transfer nitrating agents (Cupas and Pearson 1968; Olah et al 1980a). Transfer nitrations are applicable to both carbon and heteroatom nitrations. For example, they allow safe, acid-free preparation of alkyl nitrates and polynitrates from (polyols) in nearly quantitative yield (Olah et al 1978a). N-Nitroamines have also been used as convenient transfer nitrating agents using Lewis or Bronsted acid catalysis (Olah et al 1981a). With BF3 as catalyst, alkyl nitrates such as CH3ONO 2 and n-BuONO2 (Olah and Lin 1973, 1974b) or acetone cyanohydrin nitrate (Olah et al 1978b) or silver nitrate (Olah et al 1981b) were found to be efficient selective nitrating agents. Nitrations with alkyl nitrates were also carried out by using superacidic solid perfluorinated sulfonic acid catalyst (including Nation-H | to be discussed later) instead of liquid protic or Friedel-Crafts Lewis acid catalysts. Further the azeotropic nitration of aromatics with nitric acid was also developed over these superacid catalysts. Furthermore mercury(II)-promoted azeotropic nitration of aromatics with Nation- H (vide infra) works rather well (Olah et al 1982a). Synthetic methods and reactions 145

NO2

FSO3H 02N NO2 H

CH3NO2

I Si --CH3 NO;BF; .~ ~i--F + CH3NO2 I I BF3 CH _ I 3 NO;BF4 I CH2"- CH~-CH2._.?t_CH 3 ~CH2"-- CH=CH2 NO2 I CH3 + BF3 i I .O;Bq 1 SI--~C--Si-- 02N--C,,=C--Si~ l I F- I \, /

R

1.3,5- (CH3)3C6H 3 + N~ PF6-( BF4- )

NO2 R

1,3,5-(CH3)3C6HzNO z + PF 6 (BF4) I* H

We have also found that nitrosonium salts (Cook 1963) in dimethylsulfoxide (Olah et al 1978c) can act as a good nitrating agent. The S-nitro to S-nitritoonium ion transformation was directly observed by 13C and 15N NMR spectroscopy (Olah et al 1979a). 146 G K Surya Prakash and George A Olah R

CHz--CH--CH 2 Jr ~ ~(BF4-) ----- CH2--CH--CH 2 +

OHI OHI OHI "I ONOI 2 L 0 z ONOI 2 NO;~ R 3 ~ (BF4-) R ~ .. ell,, (CH,),, (CH,), i" H

I

NOz

R

H

BF3 ArH + H3CONO2 or BuONO2 or (H30)2C(ONO2)Cn ArNO2 or AgNO3

CnF2n+iSO3H CH3C6H5 + HNO3 CH3C6H4NO2 + H20 or Nafion-H

R R R

Hg(N03) 2 > or Hg-Nationate Hg(NO3) NO2 (or Hg-Nafionate) ] .o;

H3C~;_O_ + H3C~+ + NO . S--ONO ArH ArNO2 + (CH3)2S J H3C H3C/ Synthetic methods and reactions 147

Oxidative reactions with NO + and NO~ salts

Stable nitronium (NO~-) and related nitrosonium (NO +) salts (Cook 1963), particularly with PF6 and BFz counterions, can act as selective mild oxidizing agents. Nitrosonium tetrafluoroborate in CH2CI2 has been employed to oxidize secondary tributylstannyl and trimethylsilyl to the corresponding ketones. Under similar conditions benzylic alcohols are oxidized to their respective carbonyls (Olah and Ho 1976a). Regioselective oxidation of alkyl(cycloalkyl)- methyl ethers has been achieved using nitronium tetrafluoroboratc (Ho and Olah 1977a). Both nitronium and nitrosonium ions oxidatively cleave oximes and N,N-dimethyl-hydrazones to their corresponding carbonyl compounds (Olah and Ho 1976b). Nitrosonium hexafluorophosphate has been used for the regioselective oxidative cleavage of benzylic esters (Ho and Oiah 1977b). Thiocarbonyls have been transformed to carbonyls using nitrosonium tetrafluoroborate (Olah et al 1984). Nitronium and nitrosonium ions also cleave thioketals (Olah et al 1979).

2 NO+BF4 R2 0 R\CH--O--MR3 > RI/ CH2CI2 Rl MR3=Sn(n-C4H9)3, Si(CH3)3

~I CH2Cl2 R2

Nitronium ion, besides being a good nitrating agent (reacting at the end as a polarizable electrophile), can also react at the oxygen end (by rearrangement). Dialkyl(aryl) sulfides and selenides as well as trialkyi-(aryi-), triaryl and triarylstibines react with nitronium salts to give the corresponding oxides (Olah et al 1979a). Furthermore, sulfoxides are oxidized to sulfones with nitronium ion (Olah and Gupta 1983). Both nitronium and nitrosonium ions are good - and halide-abstracting agents. Fluorination of haloadamantanes and diamantane has been achieved with nitronium ion in pyridinium polyhydrogen fluoride medium (Oiah et al 1983a). The reaction has been extended to polyhaloadamantanes and diamantanes (Olah et al 1984b). Recently, under similar conditions, 1,4,9-tribromotriamantane unex- pectedly gave 1,4,7,9-tetrafluorodiamantane (Krishnamurthy et al 1986), a reaction which involves hydride abstraction. More recently, unusual /3-fluorination of secondary alkyl and cycioalkyl bromides in their reaction with NO_,+ ion in pyridinium polyhydrogen fluoride medium was observed. The reaction should involve a bromonium ion intermediate (Hashimoto et al 1987). Direct ionic fluorination of alkanes such as adamantane, diamantane and triphenylmethane has been achieved with NO § ion in pyridinium polyhydrogen fluoride (Olah et al 1983b). 148 G K Surya Prakash and George A Olah

R2CHOCH3 > R2C-O CH2Cl2

1 NO+ + R1 R\C.N~oH/ or 1102 ) ~C----'O R2 CH2CI2 R2 /

R1 "%fC~N.../CH3 C=O R2'' \CH 3 R2/ 1•2 NO+BF; RI"'.I/O--CHC6H5 CH2C12 , RL~-o. 0 ~ S 1) NO+BF; CH2C|2 2) H20

0 CH2CI2

+ ~0 2 ONO 0 NO2 I II R~S~R 1, R~S~R + R~S R > R~S~R + NO+ + +

NO2+ R~Se~R > R~Se~R + NO+

§ NO2 R3X > R3P----O + NO+

X=P, As, or Sb, R=alkyl, aryl .0;% R~-S---R R~ S~R + NO+PF6- II 0 Synthetic methods and reactions " 149

X F No2 ~"~ (HF)nF- IU X=CI, Br. I :~§

It Br F

Br r A >

Br F

Br F

Br ,;S- So ~162 ~;- F

Br F

Br _NOz*BF4~ ~Br _ ~3 ~ (HF)n"PY ~"~H - 150 G K Surya Prakash and George A Olah ,• NO'~BF~ (HF)nF"

Nitrosonium ion acts as a mild initiator for the condensation of alkyl or arylalkyl halides with to give the Ritter reaction product (i.e., the amide) upon hydrolysis (Olah et al 1979c).

NO+BF~ RI--X , [R1-N-C-R2] + H20) R 1- NH--C--R 2 R2.--.C m N II O

,A combination of hydride abstraction and Ritter-type reaction has been employed to synthesize N-(1-adamantyl) amides from (Olah and Gupta 1980a). NHCOR

,~ l ) RCN NO+PF6 Z) H20

Halogenating agents and related reagents

Pyridinium polyhydrogen fluoride Anhydrous fluoride (AHF) is one of the most widely used and inexpensive fluorinating agents. However, due to its low boiling point (19.5~ and corrosive nature, reactions involving its use are generally carried out in special equipment and under pressure. Efforts prior to ours were made to effect fluorinations at atmospheric pressure by complexing with various nonbonded electron-pair donors. Hirschmann et al (1956) first reported the use of the tetrahydrofuran/AHF system. Subsequently, stable solutions of AHF in (Bergstrom et al 1963; Aranda et al 1965, 1966) amides (French Patent 1964a; Balicheva et al 1969), carbamic acids and esters (French Patent 1964b), trialkyl phosphines (van der Akken and Jellinek 1967) and alcohols (Thomas 1971; Politanskii et al 1974) were used. Basically, however, use of these complexes remained limited to fluorination of specific organic compounds (mostly steroids), and none of them achieved wider acceptance. Our interest in the fluorination of organic compounds in general prompted us to undertake detailed examination of several -AHF complexes. Synthetic methods and reactions 151

AHF forms remarkably stable solutions in up to a molar composition of 1 CsHsN:9 HF (30% CsHsN, 70% HF, w/w). Such material was first used by Bergstrom et al (1963) to fluorinate steroids. Our studies showed it to be pyridinium polyhydrogen fluoride (PPHF; CsHsNH(HF)• in which the complex anion exists in equilibrium with a small amount of free hydrogen fluoride. This material was found to possess a relatively low vapour pressure at temperatures up to 60~ (Olah and Nojima 1973) and to be a convenient and effective replacement (at atmospheric pressure) for AHF in numerous applications. Now known as Olah's reagent, 30/70 PPHF is available commercially as a liquid which can be stored in poly() bottles in a refrigerator (or even at room temperature). Thus, AHF has been 'tamed' to allow chemists to use it under greatly simplified conditions easily adaptable to general laboratory syntheses. Note that PPHF solutions containing a lower concentration of HF can be used as well, and that other amine-poly(hydrogen fluoride) complexes, such as those derived from , , substituted pyridines and triethanolamine, are also useful as sources of hydrogen fluoride. Synthetic transformations that have been achieved with 30/70 PPHF are discussed below. The topic has been recently reviewed (Olah et al 1986a). Although alkenes and are generally insoluble in pyridinium poly(hy- drogen fluoride), addition of a tetrahydrofuran solution of these compounds to the reagent yields mono- and difluoroalkanes, respectively, in typical Markownikoffo type additions. When alkenes are added in a similar manner to a solution of PPHF containing N-halosuccinimides, the corresponding halofluoro compounds are isolated. Only vicinal fluoroiodoalkenes are formed upon addition of alkynes to a solution of PPHF containing N-iodosuccinimide. Iodofluorination or bromof- iuorination of alkenes can also be effected using the appropriate plus an equivalent amount of silver nitrate in the PPHF solution. The halofluorination method can also be modified to prepare vicinal difluorides from the corresponding alkenes without isolation of the intermediates. This is simply achieved by adding silver(I) fluoride to the solution of a-halo-/3-fluoroalkanes to effect the halogen- exchange reaction in situ (Olah et al 1979d).

__C~=C__ - -- CH2---CF2-- X \ / x2 Ag"o3 X-Br o | - PPHF F x R_CH.CHR 1 PPHF + N-halosucctnamtde or X?\AgNO3

RCHFCHFR1 152 G K Surya Prakash and George A Olah

Secondary and tertiary alcohols are readily converted to the corresponding fluorides by pyridinium poly(hydrogen fluoride). With tertiary alcohols, the reaction proceeds conveniently even at low temperatures. Secondary alcohols react at room temperature. Primary alcohols are unaffected by pyridinium poly(hy- drogen fluoride) itself, but their conversion to alkyl fluorides (30-88% yield) proceeds smoothly in the presence of added sodium fluoride (Olah et al 1979d). Recently, good results have also been reported in reactions of carbohydrates with pyridinium poly(hydrogen fluoride) to afford fluorides (Hayashi et al 1984; Szarek et al 19821). Deaminative halogenations, whereby an amino group is displaced by fluoride or other halide, have been studied in the case of a-amino acids (Olah and Welch 1974a; Olah et al 1981c, 1983c), carbamates (Olah and Welch 1974b), and aminoarenes (Olah et al 1979d). The reaction of a-amino acids in pyridinium poly(hydrogen fluoride) solution with an excess of sodium nitrite leads, via diazotization followed by in situ nucleophilic dediazonization, to the formation of a-fluorocarboxylic acids. However, difficulties were experienced (Keck and R6tey 1980) in the case of fluorinating some a-amino acids, e.g., valine, isoleucine, phenylalanine, tyrosine, and threonine, because total or partial rearrangement to /3-fluorocarboxylic acids occurred owing to anchimeric assistance by alkyl, aryl, or hydroxy groups during the diazotization reactions. Such rearrangements can be suppressed (Olah et al 1981c) by carrying out the reaction in reagent media of lower HF concentration (pyridine/hydrogen fluoride, 52:48 w/w).

py/HF (52:48) F- RCHCO2H ~ [RCHCOEH] ~ RCHCOEH I aNo, I -N2 1 NH2 N~" F O O II ' PPH II R-O-C-NH2 ~ R--O--c- F NaNO 2

Treatment of alkyl carbamates dissolved in pyridinium poly(hydrogen fluoride) with an excess of sodium nitrite at room temperature gives the corresponding fluoroformates. The reaction is considered to proceed via diazotization followed by in situ dediazonization. The deaminative introduction of a halogen into an aromatic nucleus is commonly accomplished via diazotization of the corresponding amine and decomposition of the resultant diazonium salts in the presence of a suitable halide donor. For the preparation of specifically fluorinated aromatic compounds, the Balz-Schiemann reaction has been the most widely used method (Sushchitsky 1965). When aminoarenes are diazotized in pyridinium poly(hydrogen fluoride) solution with. sodium nitrite, subsequent fluorinative dediazonization results in the formation of the corresponding fluoroarenes. The fluoroarenes formed are frequently isomeri- cally pure compounds. The conversions proceed smoothly at room (or slightly elevated) temperature and do not require the isolation of the diazonium salt intermediates. Mechanistic implications have been discussed elsewhere (Olah and Welch 1975). The method has recently been'further improved (Fukuhara et al 1987). Synthetic methods and reactions 153

PPHF ArNH2 ~ Ar--F NaNO 2 Pyridinium poly(hydrogen fluoride) has also been used (Rosenfeld and Widdowson 1979) to effect Wallach-type conversion of aryltriazenes to aromatic fluorides, the yields often being high. Diazo compounds can be converted to monofluorides via treatment with PPHF; in the presence of N-halosuccinimides geminal fluorohalo compounds result (Olah and Welch 1974c).

PPHF Ar-N = NNRE[R2 = (CHE)5,Me2] ) ArN~- --~ ArF

PPHF RC(O)CH2F RC(O)CHN2 PPHF-NXS RC(O)CHXF (X=CI,Br,I)

Several inorganic fluorinations have been carried out using pyridinium polyhy- drogen fluoride. Sulfur tetrafluoride is an extremely useful reagent for the preparation of organofluorine compounds (Sheppard and Sharts 1969; Chambers 1973; Boswell et al 1974; Wang 1985). Although a variety of procedures for its synthesis are known (Naumann and Padma 1973; Becher and Massone 1974), the most convenient laboratory procedure to date is that of Tullock et al (1960), in which sulfur dichloride is heated in a solvent of high dielectric constant, such as acetonitrile, with metal fluorides such as sodium fluoride. Pyridinium poly(hy- drogen fluoride), being a polar solvent as well as containing a large reservoir of fluoride ion, is also a good reagent for this conversion which proceeds smoothly at 45~ and atmospheric pressure (Olah et al 1977a). Another important reagent that has been prepared using PPHF is sulfuryl chloride fluoride (SOECIF). Due to its low nucleophilicity (Dean and Gillespie 1969) and wide liquid temperature range (- 124.7~ to + 7.1~ SO2CIF is extensively used as a solvent for the generation of stable carbocations (Olah 1974); it is also useful as a synthetic reagent. Existing syntheses of the compound involve the action of fluoride salts on sulfuryl chloride (Woyski 1950; Tullock and Coffman 1964), treatment of potassium fluorosulfite with chlorine (Seel 1967), the reaction of hydrogen fluoride, chlorine and sulfur dioxide with potassium bifluoride supported on charcoal(British Patent 1966), or the reaction of nitrogen trifluoride with sulfuryl chloride (Glemser and Biermann 1967). Another preparation of SOaCIF involves treating chlorine monofluoride and sulfur dioxide at low temperature (Schack and Wilson 1970). None of these methods is as convenient as the reaction of pyridinium poly(hydrogen fluoride) with sulfuryl chloride which proceeds efficiently and smoothly under mild conditions (Olah et al 1977a).

PPHF 3 SCI 2 - ~ SF4 + $2C12($2CIF)

PPHF 502C12 ) SO2C1F 154 G K Surya Prakash and George A Olah

In situ sulfur tetrafluoride reagent Sulfur tetrafluoride prepared in situ from SC12 and PPHF (vide supra) can be conveniently used to convert carbonyl compounds into geminal difluorides (Olah et al 1977a). This is definitely an improvement over the use of gaseous SF4 which requires superatmospheric conditions.

5C12 PPHF R2CO R~ /F ~, SF4 C + S02F2 R/ \e

Cyanuric fluoride During our studies on fluorinating agents, we found that cyanuric fluoride is a convenient fluorinating agent (Olah et al 1973a). Carboxylic acids, including , are easily converted into their acyl fluorides. This method gives a 40% yield of the elusive formyl fluoride and 80-90% yields of the higher homologous acyl fluorides.

3H~C(o H + F ~ ; 3H~C + HI)

OH

Selenium tetrafluoride In our efforts to overcome the necessity of using pressure equipment for fluorinations, we have found selenium tetrafluoride to be a simple alternative reagent to sulfur tetrafluoride (Olah et al 1974a). Selenium tetrafluoride, SeF4, which has an atmospheric boiling point of 106~ is a very effective fluorinating agent of broad scope. Fluorination of ketones, aldehydes, etc., generally proceeds in high yield, frequently even higher than those with SF4. However, selenium compounds are generally toxic and must be handled with great care.

SeF4 R2C ---- O ) R2CF2

Reactions of sulfuryl chloride fluoride Sulfuryl chloride fluoride, a well utilized solvent for the generation of carbocations (Olah 1974), is also a good chlorinating agent for certain substrates. Enol silyl ethers react with SO2CIF rather readily to provide the corresponding a-chloro ketones in good to excellent yields (Olah et al 1984c). SO2CIF is also a good dehydrating agent. Alcloximes have been dehydrated to the corresponding nitriles in the presence of triethylamine (Olah et al 1980b). SO2CIF has also been used to condense carboxylic acids with primary amines in the presence of triethylamine to the respective amides (Olah et al 1980c). Furthermore, SO2CIF has been effectively Synthetic methods and reactions 155 used to regenerate carbonyl compounds from thioketals (Olah et al 1981d). SO2CIF is also a good oxidizing agent for the conversion of tertiary phosphines and arsines to the corresponding oxides (Olah et al 1983d). Thiols have been oxidized to disulfides with SO2CIF (Olah et al 1983e).

(CH3)3St~O~ /H SOzC1F ~ R~ C1 RI~'---'~R2 CH2C1z Rz

R~ S02C1F -C~----fl~ ) R--C~N H/ OH NEt3

R.LO H _1)S02C1F/NEt 3 Rll_ NH_R 2 2) R2NH2/NEt3 R1 S"----. R

~C/ I - SO2C1F ~ ~C----O e~" \sJ "2o/ethe~ e'

r + i~ H20 R3P = 0 + HC1 + HF + SO2

R--SH S02CIF ) R--S~S--R

Friedel-Crafts and related superacid-catalyzed reactions

Alkylation and acylation over solid superacid catalysts In Friedel-Crafts alkylations using or related metal halide catalysts, generally complex mixtures of products are formed due to polyalkylation, isomerization, and transalkylation processes. The reactions are accompanied by extensive complex formations necessitating the use of large amounts of the catalyst, i.e., the formation of red oils (Olah 1973). The use of high-acidity solid acid catalysts is gaining popularity because they minimize side reactions resulting in clean heterogeneous reaction mixtures without \ . . the usual messy workup problems. Conventional sohd acid catalysts such as the sulfonated polystyrene resins are of limited utility, and chalcogenides like silica-alumina require high temperatures. We have found (Olah et al 1975) a series of solid or supported perfluorinated alkanesulfonic acids capable of effecting a wide variety of Friedel-Crafts-type reactions. These perfluorinated acids include the acid form of Du Pont's commercially available Nation | ion membrane resin (Nation-H) and the longer-chain (C6 to C~8) perfluorinated alkanesulfonic acids, representative of which is perfluorodecanesulfonic acid (PDSA) 156 G K Surya Prakash and George A Olah

CF3 I -- (CF2- CF2)m--(OCFz---CF), CF3(CFa)9SO3H I PDSA Nation - H O(CF2)2SOaH

If needed, the acidity of these solid acids can be further increased by complexing with higher valency metal fluorides, such as SbFs, TaFs, and NbF5 (Olah and Kaspi 1977). Alkylation of aromatics with olefins, alkyl halides, alcohols (including methyl ), esters, and other alkylating agents readily occurs over these catalysts (Kaspi and Olah 1978; Kaspi et al 1978; Olah and Meidar 1979; Olah et al 1980d). Transalkylation of aromatics with di- or polyalkylbenzenes can also be carried out with equal ease (Olah and Kaspi 1978a,b). RO'1= CHz~ ROH~ Nafion-H ArH + R~X ArR (cozR)/' CGHG + R'IR2cGH4..----). CGHsR1 + CGH5R2

Not only alkylations but various other reactions (Olah et al 1978d), including acylations, sulfonylations, and halogenations, are effectively catalyzed. Interesting- ly, attempted acylation with preferentially gives ketene under the reaction conditions. Nafion-H ArH + ArCOCI ) ArCOAr Nafion-H is an excellent catalyst for the Fries rearrangement (Olah et al 1983f). More recently it has been used for de-tert-butylation of aromatic substrates (Olah et al 1987a). ~~R O~ Nafion-H

CH3 --~ HCOCH3 Nafton-H > NHCOCH3 toluene reflux, 12 hr Synthetic methods and reactions 157

New chloromethytating agents In the course of our work on chloromethylation we have developed new chloromethylating agents of decreased volatility, which greatly reduces health hazards. 1-Chloro-4-chloromethoxybutane is a particularly effective reagent (Olah et al 1974b, 1976a) as it reacts via oxygen participation, forming tetrahydrofuran as the byproduct. This complexes the acid catalyst and decreases secondary side reactions (e.g., formation of diarylmethanes).

ZnCl or A~ + ClCH20(CHz)3CH2CI 2 > ArCH2C1 + SnCl4 -HCI

Formylation with formyl fluoride and Formyl fluoride, the only stable acyl halide of formic acid, reacts with aromatics in the presence of catalyst to yield aldehydes (Olah and Kuhn 1960). Improvbd preparations of formyl fluoride, such as the discussed use of cyanuric fluoride or the reaction of benzoyl fluoride with , allowed further improvement of the reaction.

BF3 ArH + FCHO ) ArCHO Formyl fluoride was also used in the preparation of the elusive formic anhydride (Olah et al 1979e). Formic anhydride was further prepared by three different dehydration methods.

_~0 .coo. H C~"~ H--C~: 4- NoO--(J~H -- >

occ H C~O

2 H --C,,~O ethe, ~0 H C~SOzNCO,I IrtsN ~* ether

Nation-H-catalyzed reactions

Nation-H | is a convenient superacidic catalyst for carrying out a variety of synthetic transformations. The subject has been recently reviewed (Olah et al 1986b). Nafion-H is an excellent catalyst to prepare methoxy methyl ethers from alcohols and dimethoxymethane (Olah et al 1981e). Similarly high yields of 158 G K Surya Prakash and George A Olah

O-tetrahydropyranyl ethers are obtained by reacting alcc,hols with dihydro-4H- pyran in the presence of Nation-H. On the other hand, the O-tetrahydropyranyl ethers can also be cleaved with Nation-H in the presence of (Olah et al 1983g). Thus, both protection and deprotection of alcohols can be effected with Nation-H.

Nafion-H R-OH + HsCOCH2OCH s ) R-OCH2OCHa -CHsOH .o.. ,

Aliphatic alcohols generally dehydrate to the corresponding ethers in the presence of Nafion-H. Low temperatures (~ 100~ favor ether production while at higher temperatures only alkenes are obtained (Olah et al 1977b). In the liquid phase diols are converted to cyclic ethers (Olah et al 19810.

HO_~_(CH2)7~_O H Nafion-H ,> I I -n20

Nafion-H is also a convenient acid catalyst for esteritication reactions. Primary and secondary alcohols give good yields of the esters in both liquid- and gas-phase reactions. Tertiary alcohols gave poor results due to the predominance of dehydration reaction to alkenes followed by polymerization (Olah et al 1978e).

Nafion-H R t - COOH + R 2- OH > RI-COOR 2 18--100% R 1 = alkyl, aryl R 2 = alkyl, aralkyl

When ketones or aldehydes are treated with trimethyl orthoformate in the presence of Nation-H, the corresponding dimethyl acetals are formed in excellent yields (Olah et al 1981g). In the absence of water, the acetals decompose sluggishly to yield the corresponding aldehyde or ketone. In the Nation-H-catalyzed hydrolysis, formation of the corresponding carbonyl compounds is instantaneous and these are obtained in excellent yields.

R1 Naflon-HlCCl 4 . ~C-O + HC(OCH3)3 R2,,,, 89-100~ " R1 R2"~C/(OCH3)2 + H-COOCH3 RI, RE = H, alkyl, aryl Synthetic methods and reactions 159

Ethylenedithioacetals (1,3-dithiolanes) can also be prepared in nearly quantita- tive yields by Nafion-H catalysis. In this case, heating solution of the corresponding carbonyl compounds, 1,2-ethanedithiol, and Nafion-H in benzene under reflux with azeotropic removal of water from the reaction mixture enables the isolat'on of pure products by simple filtration followed by crystallization or distillation. Similar preparatiGns of cyclic acetals using 1,3-butanediol or glycidiol have been patented (Japanese patent 1981; Hughes 1977).

R1 R2/Xc'o + CHz-SH 7g-loos R:,/ \s_l

The 1,1-diacetates can also be prepared by vigorously stirring equivalent amounts of aldehyde and freshly distilled acetic anhydride, and a catalytic amount of Nafion-H (Olah and Mehrotra 1982) at ambient temperature. Most of the reactions were carried out in the absence of any solvent but dry tetrachloromethane was used in some cases without noticeable change in yields of diacetates.

Nafion-H R-CH=O + (H3C-CO)20 , R-CH (OCO-CH3)2 5O-99% R = alkyl, aryl

Nafion-H catalyzes the hydration and methanolysis of epoxides under very mild conditions generally affording high yields of the product (Olah et al 1981h). The catalysts can be readily regenerated for further use without loss of activity. Only trans products are observed in the reaction with cyclohexene oxide and cyclopentene oxide, indicating back-side attack. The conversion of ethylene oxide to ethylene glycol is patented (Kim 1979).

Nor1on~4 "'"IS '.' $7-8t| ,,-.., ~ ?" ,,.,,, ?" ,., ether = "'~f, ~/ r "'~. L/" Rt/~ \R' R'/iH UN'R.

Nafion-H has been reported to catalyze the pinacol-pinacolone rearrangement (Olah and Meidar 1978a). The 1,2-diols rearrange to the corresponding ketones.

HO OH R v2 O

R 1 - C - C- R 1 ) R 1- C - C- R v2 I I 82- % I R 2 R 2 R E 160 G K Surya Prakash and George A Olah

The Rupe rearrangement of alkynyl tertiary alcohols is one of the most feasible routes into a-/3-unsaturated carbonyl compounds. The major drawback of this reaction, however, is that the unsaturated product can subsequently undergo acid-catalyzed polymerization (Ansell et al 1956) and form side products such as vinyl and aldehydes (Hasbrouck and Kiessling 1973).

0 hf Ion-H/H* CH 3 C=-CH e4l =

0 H3C',, Neflon-H/CCl 4. ,x'eflux H3C~. II CH-CH-C=_CH -_ /C=CH-C-CH 3 / I H3C OH H3C

Significant improvement of the Rupe rearrangement is observed with the use of Nafion-H as a catalyst (Olah and Fung 1981), with yields better than those obtained using mercury-impregnated Dowex-50 resin (Newman 1953). The gas-phase rearrangement of allyl alcohols to the corresponding aldehydes has been reported to proceed by Nafion-H catalysis at 170-190~ (Olah et al 1978f), while the isomerization of styrene oxide proceeds at room temperature in 1/2 h to give 47% yield of phenylacetaldehyde (Japanese patent 1982). Hydration of alkynes is usually carried out in dilute sulphuric acid solutions to afford carbonyl compounds. Except for reactive alkynes, the reaction rates in the absence of mercury(II) salts are low, even when higher acid concentrations are used. One of the serious problems arising from using mercury(II) salts is the formation of a precipitate of an inactive sludge consisting of finely divided metallic mercury mixed with insoluble mercury(II) organic compounds. Apart from difficulties in the workup stage, this also causes loss of catalytic activity and environmental problems. Use of mercury(II)-impregnated Nafion-H (replacing about 25% of the acidic protons by mercury(II)) as a catalyst alleviates all these problems affording the correspond!ng ketones very cleanly and easily (Olah and Meidar 1978b).

~ Hg/Nafion-H/ O CzHsOH or AcOH I[ R 1 - C-C - R 2 + H20 ~ R1-CH2-C-R 2 65--94% No loss in catalytic activity is observed if the hydration is done at room temperature, in which case the reaction is complete in 90 min. While refluxing in completes the reaction in 5 min, the catalyst is unsuitable for reuse. A related reaction of considerable interest is the construction of 3(2H)-furanone ring skeleton, a key intermediate in the manufacture of certain anti-tumor drugs, muscarine alkaloid synthesis, etc. The transformation of butyne-l,4-diol deriva- tives into 4,5-dihydro-3(2H)-furanones is by far the most promising in a practical sense because of the simple synthetic pathway. Mercury-impregnated Nafion-H has been reported to be extremely efficient in catalyzing this transformation (Saimoto et al 1983). Synthetic methods and reactions 161

R ~ 0 I Nil i on-H/NO ~-~ .R~ R t -CH-C=C-C-R 3 _-_- I I 4~Dsz R! OH OH

Recently, Baeyer-Villiger oxidation of ketones to lactones has been observed to proceed smoothly over Nafion-H (Olah et al 1987b) with a minimum of workup procedure using a single equivalent of the oxidizing agent hydrogen peroxide. Nafion-H also catalyzes Ritter reaction. Alcohols react with nitriles to provide the corresponding amides (Olah et al 1987b). 0 o ,~~ Nafion-H H202 0 R~OH + R]C~-N Nafion-H > R_NH_~_R1

Nafion-H also catalyzes Diels-Alder reactions. The uncatalyzed reaction of 1,3-cyclohexadiene and acrolein gives 25% of the adduct upon heating to 100~ for 3.5 h, Nafion-H catalysis gives 88% after stirring for 40 h at 25~ This is definitely an improvement (Olah et al 1979f). + ~CH=O N.fIO~-H/CH~CI~~CH=O

CH 2

There are a host of other acid-catalyzed reactions that have been reported by other researchers in the field. For details the readers are referred to a recent review (Olah et al 1986b).

Onium ion and salt reagents

Dialkylhalonium salts The well-known Meerwein salts (Meerwein et al 1940; Meerwein 1966), trialkylox- onium tetrafluoroborates and hexachloroantimonates (as well as the hexaf- luorophosphate salts used in our work (Olah and Svoboda 1973a; Olah et al 1973b)), are widely used as transfer alkylating agents. However, they lack selectivity, as the onium oxygen atom cannot be substituted by sulfur, selenium, or tellurium, because these onium ions show very little reactivity. 162 G K Surya Prakash and George A Olah

Dialkylhalonium salts are very effective alkylating agents. The salts such as dimethylbromonium and dimethyliodonium fluoroantlmonates can be readily prepared from excess alkyl halide with antimony pentafluoride or fluoroantimonic acid and isolated as stable solids. The less stable chloronium salt can be obtained in solution. As the nature of the halogen atom can be readily varied, from I + to Br + to CI § the use of halonium ions provides useful selectivity in their alkylation reactions (Olah and Svoboda 1973b; Olah and Mo 1974).

HF/SbFs 2RX ~ RXR+SbF6 or SbF s R = CH3,C2Hs, etc.; X--I, Br, C1 R-X+-R + Nu---~ RNu + RX (Nu- = nucleophile)

A great variety of other halonium ions were also prepared, in our laboratory and by Peterson and other investigators, including alkyl-, cycloalkyl-, vinyl and arylhalonium ions, and their alkylating ability was studied (Olah 1975; Olah et al 1979g; Prakash et al 1985).

R X R R X Ar R X C~ +/R x

Carbocation Salts On the basis of our methods developed for preparing solutions of stable carbocations in superacidic media, we were recently successful in isolating a series of stable crystalline carbocations by evaporation of Freon-type solvents (Olah 1974). Typically, isolated carbocation fluoroantimonate salts include such tertiary ions as the tert-butyl and adamantyl cation (Olah et al 1973c), but also stabilized secondary ions such as the norbornyl cation.

(H3C)3C§ SbF "

Isolated acylium salts and similarly isolated sulfonyl halide-antimony pentaf- luoride complexes are effective acylating and sulfonylating agents (Olah and Lin 1974c; Olah et al 1974c) respectively. R-C=O+'SbF6,+ ArH ~ ArCOR RSO2F.SbF5 + ArH ~ ArSO2R Methyl and ethyl fluoride form stable addition complexes with antimony pentafluoride. These complexes are powerful alkylating agents (Olah et al 1969; 1972) and have been used, for example, to alkylate carbonyl sulfide and Synthetic methods and reactions 163 carbon disulfide to provide effective thio- and dithio-carboxylation agents for aromatics (Olah et a11981i).

SbF5 in SO2 + COX S + C RF ) RO=S=O > R =X -78~ (X=O,S) SbFg SbF6 l(i)Arn (R = Me, Et) |-40 to -60"C (ii) H20

X ArC SR

Miscellaneous onium ion reagents Bromodimethylsulfonium bromide (crystalline solid, decomposes at 80~ pre- pared by reacting and bromine (Furukawa et al 1973), is a convenient source of electrophilic bromine. We have found the reagent very useful in the cleavage of thioketals (Olah et al t979h). Oxidation of thiols (Olah et al 1979i) readily gives disulfides. Epoxides, upon treatment with bromo (BDMS) or chlorodimethylsulfonium chloride in the presence of triethylamine, are readily converted into a-bromo and a-chloro ketones respectively. Enamines also react with BDMS to give ~-bromo ketones (Olah et al 1979j). BDMS intermediate generated by reacting t-butylbromide and dimethylsulfoxide has been used to cleave thioketals (Oiah et al 1982b). Similarly bromo(iodo)trimethylsilane reacts with dimethylsulfoxide to provide, in situ BDMS, which has again been used to cleave thioketals (Olah et al 1982c). Recently, regioselective halogenation of phenols, phenol ethers and has been achieved using bromo- and chlorodimethylsulfonium halides (Oiah et al 1986c). Apart from sulfonium ions novel substituted diazonium ions have been prepared and used to effect interesting electrophilic reactions. Protonation of leads to amino diazonium ion which very effectively aminates aromatics (Mertens et al 1983). Cyanodiazonium ion prepared in situ by the diazotization of cyanamide with nitrosonium ion has been used to cyanate aromatic compounds. Even nitramide is diazotized to nitrodiazonium ion, which acts as a good nitrating agent. On the other hand reactions of fluorodiazonium with aromatic compounds gave little evidence of electrophilic fluorination (Olah et al 1985b).

Oxidizing and oxygenating agents

Higher valency metal fluorides In the course of our studies we have investigated a series of higher valency metal fluorides such as UF6, WF6, IFs, MoF6, and CoF 3 as oxidizing agents. In spite of the ready availability of uranium hexafluoride depleted of fissionable 235U, the study of the reactions of UF 6 with organic compounds remained virtually unexplored. The highly covalent nature of UF 6 makes it particularly suitable for reactions (Olah et al 1976b; Olah and Welch 1978a) in nonaqueous solvents. Stable 164 G K Surya Prakash and George A Olah

I) BDHS CH2CI2 2) H20

I XS{CH~,)2"eX'.._ [~X 2 EI~jN X Br,CI

0 0

2 H~O4

ORl OR

CH2Cl 2 (CH3)2S-+X X~ X=Br, Cl X

R R

XN2 + X X = H2N, CN, NO2

solutions of UF6 in chlorofluorocarbons (Freons) or chlorohydrocarbons ( chloride or chloroform) can be conveniently used in the usual glass apparatus. Ethers undergo oxidative cleavage (Olah et al 1976b; Olah and Welch 1978a) with UF6 to form carbonyl compounds and alcohols. Furthermore, the direction of cleavage is predictable and thus the utility of ethers (such as benzyl or benzhydryl) as protecting groups of alcohols can be broadened. The cleavage of methyl ethers also takes place in high yields and is regiospecific. Trapping experiments with phenyllithium suggest the intermediacy of methoxycarbenium ions in the reaction. Benzyl and benzhydryl ethers are cleaved to the corresponding alcohols and or benzophenone, respectively. Benzylic alcohols are further readily oxidized to the corresponding carbonyl compounds. Oxidative cleavage of protected carbonyl compounds such as tosylhydrazones and N,N-dimethylhydra- Synthetic methods and reactions 165

+ -UF4, -HF RR'CHOMe + UF6 --* RR'CHOMeF- RR'C=OMe+F - I ~, H20 UF5 RR'C=O RR' = alkyl, aryl zones takes place with ease upon aqueous quenching of the initially formed UF6 adducts. N,N-Dimethylalkyl-(cycloalkyl-)aminesare also oxidized by UF6, yielding the corresponding carbonyl compounds upon aqueous workup.

/CH3 -HF +/CH3 H20 RR'CHN + UF6 .UF4 ) RR'C=N RR'CO \CH 3 F-\CH3

Tungsten hexafluoride, WE6, similar to uranium hexafluoride, has been found to be a convenient oxidizing agent (Olah and Welch 1976). Typically, N,N- dimethylhydrazones and tosylhydrazones are cleaved to the carbonyl compounds under mild conditions. WF 6 is easily handled, does not attack glass, and is readily soluble in a variety of solvents including chloroform and 1,1,2-trichlorotrif- luoroethane (Freon 113). Iodine pentafluoride, IFs, is also a versatile oxidizing agent (Olah and Welch 1977) capable of oxidizing a variety of functional groups such as alkyl iodides, tertiary amines, alkyl methyl ethers, and alcohols. Although it has been previously reported that reactions with iodine pentafluoride are difficult to control, we have found that its solution in 1,1,2-trichlorotrifluoroethane is easy to handle.

R~ /H IF5 ~, R~ R/C~oH R/c=O

/ I. IF 5 ) RCHO RCH2N~ 2. H20

R = alkyl, aryl

Similarly we have found (Olah et al 1976c) that molybdenum hexafluoride, MoF6, oxidatively cleaves dimethylhydrazones and tosylhydrazones in tetrahydro- furan solution in high yield under moderate conditions. In certain cases cobalt trifluoride, CoF3, has also been found to be a suitable reagent for the oxidative cleavage of hydrazones and oximes to their parent carbonyl compounds (Olah et a11977). This reagent is also relatively easy to handle and shows significant selectivity, giving the highest yields with N,N-diethylhydra- zones. 166 G K Surya Prakash and George A Olah

Ozonium and peroxonium ion reagents During the course of our work on the utility of superacids, we have found that hydrogen peroxide and ozone protonate re+adily to give the reactive electrophilic oxygenating agents, H20-OH and O --- O--OH, respectively. Protonated ozone (ozonium ion, OaH +) (Olah et al 1978g), upon reaction with a tertiary such as , gives a very unstable trioxide, which immediately undergoes cleavage-rearrangement, to yield acetone and methyl alcohol. The reaction can be considered as the aliphatic equivalent of the well-known cumene hydroperoxide route for preparing phenol and acetone.

I . ,o oo 0--0 - I CH~ H~ / ,o2 CHs~ 4. ~C--'-OC H5 (CHI)20=O 4- CH30H CH3 CH3 J

Aromatic hydrocarbons such as benzene and alkylbenzenes are also readily hydroxylated to phenols with hydrogen peroxide in superacidic media with high selectivity (Olah and Ohnishi 1978). As the phenolic products are protonated in the media, they are thus protected from further oxidation.

H202 ArH ) ArOH + H20 H*

The above reaction has been further improved using HF: pyridine as solvent (Olah et al 1979k). HF: BF3 in conjunction with H202 has also been found to be a good system for electrophilic hydroxylation of aromatics (Olah et al 1981j).

Miscellaneous oxidations

Aqueous bromine with NaHCO3 in CH2CI 2 and HMPT has been used for the oxidative cleavage of ketoximes and tosylhydrazones (Olah et al 1979). Hydrogen peroxide in conjunction with hydrochloric acid cleaves thioketals (Olah et al 1980e). Hydrogen peroxide and potassium carbonate in aqueous methanol is a good reagent for the improved transformation of nitro into carbonyl compounds (Olah et al 1980e). Oxidative cleavage of tosylhydrazones has been achieved with sodium peroxide in aqueous benzene (Ho and Olah 1976a). Aldoximes are directly oxidized to the carboxylic acids with sodium peroxide in aqueous ethariol (Ho and Synthetic methods and reactions 167

Olah 1976b). Chlorosulfonyl isocyanate along with dimethylsulfoxide has been used as a mild oxidizing agent for the conversion of alcohols to ketones (Olah et al 1980g).

NaHCO3/H20/CH2C12 R2",~, /C===O RI/C-~--IT'~OH HMPT, Br2 RI R1 RI..,.. /s- l 1) HC1 ) ~- 0 R2/C ~S 3 2) H202 R2/

RI RI~H-..NO 2 HzOzlKzC03L CH3OH/Hz0 - R2~ r R2 R1 C6H6JH20 > "~C=O Na202 R2/ 0 Na202 R~ CH-----N~'OH 1) > R-- ~---OH CzH5PH 2) H* R1 R1 CISOzNC ~CH --OH ~ ~C=O RZ/ DMSO RZ/ iv Ceric ion generated by treating catalytic amount of ceric nitrate and sodium bromate has been employed to oxidize alkyl and silyl ethers to the corresponding carbonyls (Olah et al 1980h). iv Ce has also been used to oxidize silylnitronates to their respective carbonyl compounds (Olah and Gupta 1980b). Olefins have been oxidized to epoxides and carbonyl compounds with peroxoura- nium oxide, UOn.4H20 (Olah and Welch 1978b).

R1 1) Ce4+ / BrO3" R1 ~CH--'OR3 21 HE0 , Rz/ R

R1 / +/OSt-- Ce(NH4)z(N03)6,25~ 5h RI~ "C==N \ > C~----'O R2/ \0e RZ/

Q + (U04* 4 H20) " ~10 "+ ~CHO 168 G K Surya Prakash and George A Olah

Low valency metal ions as reducing agents

Our interest in metal ion reagents led us to investigate the aqueous vanadium(II) ion as a useful reducing agent. V(II) reduces a-halo ketones to parent ketones (Ho and Olah 1976c). Benzils and quinones are readily reduced to benzoins and hydroquinones (Ho and Olah 1976d). Aryl azides are reduced to aryl amines (Ho et al 1976). Vanadium(II) ion also cleaves semipolar heterooxygen bonds. For example, sulfoxides are reduced to sulphides in good yields (Olah et al 1976d). V(II) ion is an excellent electron donor and facilitates the reductive dimerization of tropylium salts to ditropyls (Olah and Ho 1976c). Oximes have been reductively cleaved (Olah et al 1980i). Even 2,4-dinitrophenylhydrazones undergo reductive cleavage to the parent carbonyls (Olah et al 1981k).

2 2

-- H20/TH F RI--- __ R3 ~3 I 0 H20/THF H

~_N 3 V(]I) ~ ~._NH 2

R1/I~R 2 V(II) > RI""S~ R2 or HO(III)

~ BF4- VC~/HzO/I.r R1 RI~= V(II) ~ ~C=O N~"OH THF H20 R2 or R2 Ho(III)

R1 NO2 R1 "~N~NH --~N02 v(Ii) R2 - H20/THF R Synthetic methods and reactions 169

Vanadium(II) ion reductions can also be carried out in nonaqueous media and are of interest because nonacidic conditions can be employed and difficulties of limited substrate can be eliminated. Thus we have found that the coupling of benzylic and allylic halides and debromination of vic-dibromides proceeds smoothly in tetrahydrofuran solutions (Ho and Olah 1977c). The vanadium(II) species is conveniently generated in situ by the lithium aluminum hydride reduction of vanadium(III) chloride in anhydrous tetrahvdrofuran. Besides the vanadium(II) ion, low-valent titanium (Olah and Prakash 1976, 1977; Olah et al 1976e) and molybdenum ions (Olah et al 1976d) are also useful reducing agents. Thus molybdenum(Ill) reagent, prepared by treating molybdenyl chloride (MoOCI3) with zinc dust in tetrahydrofuran solution, reduces sulfoxides to sulfides at room temperature in excellent yields. Oximes too are reductively cleaved to the corresponding carbonyl compounds in high yields (Olah et a11976c).

VC13/LtA1H4

THF

Miscellaneous reducing systems Azo- and hydrazoarenes undergo reductive cleavage to aminoarenes by palladium- catalyzed hydrogen transfer from cyclohexene (Ho and Olah 1977d). In the presence of catalytic amount of AIC13, Pd/cyclohexene system is also very efficient in reducing benzylic alcohols to the corresponding hydrocarbons ((Olah and Prakash 1978). This reagent system also reductively cleaves benzylic and benzhydrilic ethers and acetals (Olah et al 1979m). ""k Ar-~N=N--Ar ) ArNH2 RI 0 R1 I / c. Ar-- C---OH

R1 I 3 P./c 11 At--C--OR Ar--CH + R30H

Magnesium metal in methanol in the presence of catalytic amount of palladium is a good reducing system for the reduction of double bonds (Olah et al 19811). The method is a significant improvement over conventional catalytic hydrogenation procedures since cyclopropyl groups are not reduced by the system. Replacement of methanol by heavy methanol (i.e., CH3OD ) provides a good method for direct reductive deuteration of double bonds. 170 G K Surya Prakash and George A Olah

CH30 fl )

.,l.Ic H ,•• CH~'OD Mg/Pd/C ) D

Trialkylsilanes such as triethylsilane have been used extensively as reducing agents (Colvin 198!; Weber 1983). During the course of our studies we have found that triethylsilane in conjunction with trifluoromethanesulfonic and is a good reducing agent for the reduction of diaryl and alkylaryl ketones to their respective hydrocarbons (Olah et al 1986d). Under Nafion-H catalysis triethylsilane reduces acetals and ketals to the corresponding ethers (Olah et al 1986e). Furthermore, triethylsilane in the presence of either trimethylsilyl iodide or triflate (vide infra) reductively couples carbonyl compounds to symmetrical ethers. The method has also been extended to the preparation of unsymmetrical ethers by condensing carbonyl compounds with trimethylsilyl ethers in the presence of triethylsilane and trimethylsilyl iodide. The method also works very well with trimethylsilane (Sassaman et al 1987).

R2 1) CF3SO3H/CH2C12 R2 2) Et3SiH

R1 .OR RI \c/ Nafion-H ) ~ C___OR3 Rz/oR3 " Et3SiH/CH2C12 R/

R1 Et3SiH/Clt2C12 ) R~ 0 "~ R1 R2/ --~i-I or --~iOTf R2 R2 I I R1 ~C=O + R~/CH--OTMS' --SI--IEt3SiHICH2C121 ~ RIR2H~C~O"CH~4R3R RZ7 R4 I

Trimethyl-(ethyl-)amine-sulfur dioxide and related reagents

Sulfur dioxide, due to its low boiling point (- 10~ and obnoxious nature, gained little use as a reagent in organic synthesis. To overcome these difficulties we have Synthetic methods and reactions 171 studied various stable complexes of sulfur dioxide with different tertiary amines. This led to the findings that trimethylamine-sulfur dioxide, a white solid crystalline complex (m.p. 77~ and triethylamine-sulfur dioxide, a liquid complex stable at room temperature, are convenient substitutes for sulfur dioxide. These complexes can be utilized in various synthetic reactions. Trimethylamine:SO2 complex dehydrates aldoximes to the corresponding nitriles in the presence of triethylamine (Olah and Vankar 1978). The trimethyl(ethyl)amine : SO2 complex or pyridine : SO3 complex readily deoxygenates sulfoxide to sulfides in the presence of sodium iodide and iodine (Olah et al 1979n). The trimethyl(ethyl)amine:SO2 complex also deoxygenates pyridine-N-oxides (Olah et al 1980j). a-Haloketones undergo dehalogenation in the presence of either trimethyl(ethyl)amine:SO2 or pyridine : SO2 complex and sodium iodide (Olah et al 19790). Primary aliphatic or arylaliphatic nitrocompounds undergo dehydration to nitriles in the presence of either trimethyl(ethyl)amine : SO2 or hexamethylphosphorous triamide (Olah et al 1979p).

(CH3)3N--SO2 R--CH ~ N~OH R--C~----N N(C2Hs)3

4. - R/lXR0 (CH3)3N-S02 ) R--S--R

R (RlsN'$02 ~1

0

i (R)3N:SO2INal R-- --~--R ] -'or "~ R-- CHi-- ~--RI ~3 + NaI "1-%"~ ' R~C~N P[N(CH3)2]3 C1CH2CH2C1

Formylation with N-formyl amines

Comins and Meyers (1978) have reported the use of 2-(N-methyl-N-formylami- no)pyridine as a formylating agent for Grignard reagents in good yields. However, the reagent 2-(N-methyl-N-formylamino)pyridine is not readily available and has to 172 G K Surya Prakash and George A Olah be prepared by reacting 2-aminopyridine with phenylformate at room temperature followed by methylation of 2-(N-formylamino)pyridine with methyl iodide. Because of these drawbacks we developed N-formylpyridine as a good formylating agent for both Grignard and organolithium reagents in either ether or hydrocarbon solvents at room temperature. After the reaction quenching the reaction mixture with aqueous acid releases the aldehyde and piperidine (Olah and Arvanaghi 1981).

+ IPl ethers~ ~N ~ H30+ > RCH0 + ~--~ + N(OH)2 I H H/C,~o p~,,CHR H=NgX = L'i

Similarly, N-formylmorpholine can also be used as an efficient formylating agent for Grignard and organolithium reagents (Olah et al 1984d). Treatment of this reagent in ether at 0~ with a wide variety of organolithium or Grignard reagents results in formation, upon acidic workup, of the corresponding pure aldehyde. a-Lithioalkanephosphonates are also formylated by N-formylmorpholine as well as N-formylpiperidine in THF at -78~ to furnish, upon acidic workup, the corresponding aldehydes (Olah et al 1984d). O

H/(~ %0 O 2 O R~ , II R ,,-c,~w mO~'PC'~Rs (R O)2PCH~R 3 i~e,~ne/TH-F

_I_ R$ ~_,,,'~~:,~ O R2 _P CHiN O I O~, ,O

Contrary to earlier reports (Sharefkin and Forschirm 1963; Fauvarque et al 1972), Grignard reagents as first described by Bouveault (1904) react with N,N- dimethylformamide in usual etheral solvents to give, upon acidic workup, the corresponding aldehydes in good yields (Olah et al 1984e). The reaction, however, must be carried out under mild conditions (0-20~ and with avoidance of excess Synthetic methods and reactions 173

(CH3)2NCHO ~OMgX(Li) H~O+ RMgX or RLi RCH ~ RCHO ~N(CH3) 2

HN(CH3)2 + ti(MgX)OH

Grignard reagent. Otherwise secondary reactions, particularly reduction and electron-transfer reactions, take place. In contrast, reactions of alkyl- and aryllithium with N-N-dimethylformamide, in some cases, did not give satisfactory results. This can be attributed to competing one-electron-transfer processes (Olah et al 1984e). The reaction has been further extended (Amaratunga and Fr6chet 1983). lodotrimethylsilane and related "hard-soft" silicon reagents

We have been interested in the possible preparation of trivalent silicon cations such as Me3Si § and have explored a wide variety of trimethylsilyl derivatives under ionizing conditions (Olah and Field 1982). The weak Si-I bond offered a possible advantage in attempted ionization of iodotrimethylsilane, Me3SiI. Although these expectations were not realized, the studies aroused our interest in the remarkable reactivity of iodotrimethylsilane as a hard-soft reagent. Iodotrimethylsilane contains silicon as a hard acid and iodide as a soft base. This reagent therefore reacts very readily with organic compounds containing oxygen (a hard base) forming a strong silicon-oxygen bond. The iodide then acts as a strong nucleophile in a subsequent displacement step, thus resulting in cleavage of carbon-oxygen bonds. This reactivity of iodotrimethylsilane has been exploited to carry out a large number of synthetically useful transformations both by our group (for a recent review see Olah and Narang 1982) and independently by Jung's group (Jung and Lyster 1977a; Jung et al 1977). lodotrimethylsilane can be generated in a variety of ways (Olah and Nat:rag 1982). lodotrimethylsilane was found to be an extremely efficient reagent for the dealkylation of esters under strictly neutral conditions to yield the corresponding silyl esters which were hydrolyzed upon aqueous workup to the carboxylic acids. Cleavage of esters was reported by us (Ho and Olah 1976e) and independently by others (Jung and Lyster 1977a). The reaction is general for alkyl esters. Phenyltrimethytsitane/iodine, the in situ iodotrimethylsilane equivalent, also cleaves esters (Ho and Oiah 1977e, 1978). Similarly, iodotrimethylsilane generated by treating iodine with hexamethyldisilane also cleaves esters rather efficiently (Olah et al 1979q). Dealkylation of esters also readily occurs with chlorotrimethylsi- lane/sodium iodide in acetonitrile (Olah et al 1979r). O O II II ! R-C-OR'+Me3Sil or . ~ R-C-O-Si-+R'I equivalents {. H2OI R-C-OH II 0 174 C- K Surya Prakash and George A Olah

Tertiary alkyl carboxylate esters with excess iodotrimethylsilane (hexamethyldisilane q-I2) on prolonged heating give tertiary alkyl iodides in good yields (Malhotra 1979). Chlorotrimethylsilane/NaI reagent cleaves lactones to the corresponding to-iodocarboxylic acids (Olah et al 1979r). This reagent system also cleaves alkylcarbamates into the corresponding amines (Olah et al 1979s).

(CH3)3Si'SI(CH3)3 + 12 ,L[_,J~ ~COOCH 3 & )

s 1THS/NaI ) I-CH2CH2CH2-COOH ,•0 CH3CN

R\ ~ R' R('N-C-O-R'' + NaI/C|THS-I CH3CN.~ R'f~NH + R'I

Alkyl aryl ethers are cleaved by iodotrimethylsilane (Voronkov et al 1976; Ho and Olah 1977e; Jung and Lyster 1977b). THF is also cleaved to trimethyl-(4- iodobutoxy) (Voronkov et al 1975). Ethers are also cleaved by iodotrimethyl- silane equivalents. i H20 Ar~--R + (CH3)3$ii A~i-- + RI ~- ArOH I (CH3)3SiI , I

Alcohols on treatment with iodotrimethylsilane or its equivalents give alkyl iodides in high yields (Jung and Ornstein 1977; Olah et al 1979r).

R-OH+ Me3SiI or ~ RI equivalents Chlorotrimethylsilane/sodium iodide in acetonitrile dehalogenates a-halo ketones to the parent carbonyl compounds (Olah et al 1980k).

/R' C1THS/NaI jR' ..... ~ R--~--CH R I ~ R" CH3CN ~ \R"

Iodotrimethylsilane or its equivalents deoxygenate sulfoxides to sulfides cleanly (Olah et al 1977, 1979s). Synthetic methods and reactions 175

Me3SII o t R--S--R RL _ 2 i " NaI/C1TMS

CH3CN

Aryl and arylalkylsulfonyl chlorides and bromides react rapidly with iodot- rimethylsilane or its in situ equivalents to provide disulfides in excellent yields (Olah et al 19801). O II R-S -X+Me3SiI ~ R-S-S-R II O Secondary nitro compounds undergo deoxygenation to the corresponding oximes with iodotrimethylsilane. Tertiary nitro compounds give iodides and primary nitro compounds undergo dehydration to the nitril~s. Tertiary nitroso alkanes give the corresponding iodides (Olah et al 1983h).

Me3CNO2 )- Me3CI

~J"-NO2 -- " :- ~NNo H

Ph -'CH2"--NO2 " @ CEN

Alkyl chlorides and fluorides react with iodotrimethylsilane or its equivalents to provide the iodides (Olah et al 1981m).

TMSI R - X ~R-I or its equivalent X=C1, F Iodotrimethylsilane in small amounts catalyzes many reactions. Methoxy- methylation of primary and secondary alcohols was achieved using dimethoxy- under iodotrimethylsilane catalysis (Olah et al 1983i). Dihydropyran reacts similarly to provide tetrahydropyranyl ethers (Olah et al 1985c).

R--OH + CH3OCH2OCH3 _ TMSI ) R_OCHzOCH3 CH2C12

R--OH § 0 TMSI ) 0 CH2C12 R 176 G K Surya Prakash and George A Olah

The efficacy of trichlorosilane/sodium iodide in CH3CN as a synthetic reagent has been explored. The reagent system appears to be more selective than iodotrimethylsilane in its reactivity (Olah et al 1981n, 1983j). We have also found (Olah et al 1979t) that chlorotrimethylsilane with lithium sulfide converts alcohols and ketones to silyl ethers and silyl enol ethers, respectively, at room temperature. A special advantage of the method is that by using t-butyldimethylsilyl chloride and lithium sulfide the synthetically important t-butyldimethylsilyl ethers can be more readily prepared than by other conventional methods.

CISiMe3/Li2S R-OH ~ R-O-SiMe3 or R-O-SiMe2-tBu or t-BuMe2SiCI/Li2S O O - SiMe II I R - C - CH2R' R - C = CHR' R, R'= alkyl, aryl

Chlorotrimethylsilane/lithium bromide in acetonitrile or hexamethylsilane/ pyridinium perbromide in chloroform react with alkyl, cycloalkyl, and arylalkyl alcohols to give the corresponding bromides (Olah et al 1980m). We have also used azido and cyanotrimethylsilane to carry out useful synthetic transformations. Both azido- and cyanotrimethylsilane react with acetals to give the corresponding a-azido and a-cyano alkoxy compounds (Kirchmeyer et al 1983a, b) under stannic chloride catalysis. Acid chlorides provide the corresponding aroyl azides and cyanides (Olah et al 1983k; Prakash et al 1983). Tertiary and secondary halides react with either azido- or cyanotrimethylsilane under stannic chloride catalysis to give the corresponding alkyl azide or (Olah et al 1985d, Prakash et al 1986).

yH /oR 3 _ c.3- t-x . 3 CH3 R2"~X R2/ ~OR3 CH2C12]SnC14 X=N3, CN R-I-x (cH) slx CHzC12/SnCl4 R--C--X X=N3, CN (CH3)3SiX ~~X Y CH2C12ISnC14>

Y = Br, 61 ~ CN, N3 Synthetic methods and reactions 177

Miscellaneous reagents

Cyanuric chloride or cyanuric fluoride (vide supra) deoxygenates sulfoxides to sulfides (Olah et al 1980n). Cyanuric chloride is effective for the cleavage of thioketals to the parent carbonyl compounds (Olah et al 19800) in the presence of AgNO3 in aqueous acetonitrile. Cyanuric chloride also serves as a dehydrating agent for amides to provide the corresponding nitriles (Olah et al 1980p).

F

R'-S- R * : R -S- R F F or CI N,'~N CI~I~N&CI

RI\c,S-- I .. *gNO, IC",CNI",O,,.'., R~C=O R 2/ "SJ 0 R 2/ CI~NAN/CI

I Ca O CI~N,,~LN/Cl

CI 3 R--CO--NH2 O HN'~NH 3 R-C-N + O"~"N~O § 3HCI

Triphenylphosphine/iodine/sodium iodide and tris(dimethylamino)/ sodium iodide (Olah et al 1978h) are excellent reagents for deoxygenation of sulfoxides to sulfides. The latter reagent also deoxygenates azoxides to azines. Mild fragmentative C-C bond cleavage of a-hydroxyketoximes has been achieved using trifluoromethanesulphonic acid anhydride or trifluoroacetic anhyd- ride or trifluoromethanesulfonyl chloride in the presence of triethylamine or pyridine. The reaction gives the corresponding nitrile and carbonyl compounds (Olah et al 1980q). 178 G K Surya Prakash and George A Olah

R--~--R (06Hs)3PI2/"ai R--S--R

0

(F3r or (F3C---C0)20 or OH[ .,N-.-OH F~C--SO2~CI/C5HsN or (C2Hs)3N el c_c," ) ~2 ~R 3 R1 ~C--=O + R~N R2/

Chlorosulfonyl isocyanate has been found to be a powerful dehydrating agent for the conversion of aldoximes and amides to nitriles (Olah et al 1979u).

CISO2NCO RC--N R - CH = N - OH NEt 3 0

II rr R -C - NH2 ,, RC~N

Hydroxylamine in formic acid reacts with aldehydes to form, in situ, aldoximes which readily dehydrate in formic acid to provide the respective nitriles (Olah and Keumi 1979).

H2NOH:HCI R-CHO ~ R-C-N HCOOH

Oxalyl chloride and related acid halides such as 5OC12, POCI2 and PCl5 in the presence of sodium iodide also deoxygenate sulfoxides to sulfides (Olah et al 1979v).

O O II I1 Cl- C- C-CI R-S-R ~ R-S-R NaI II CH3CN O Boron trihalides (bromo and iodo) in the presence of sodium iodide in acetonitrile are excellent reagents for the cleavage of lactones to ~iodocarboxylic acids (Olah et a! 1982d). The same reagent system also reduces sulfonic acids and sulfonyl derivatives to their respective disulfides (Olah et a! 1981o). Synthetic methods and reactions 179

o

BBr3 > I NaI/CH3CN ~'-'OH

o R--~--OHII BX3/Nal > R--S--S--R

One-step conversion of alicyclic ketones to lactones has been achieved using hydroxylamine-O-sulfonic acid and formic acid. The reaction proceeds through the Beckmann rearrangement of in situ produced keto oximes (Olah and Fung 1979).

IH2C')n~--c% I HzN_O_SO2_OH HCOOH) n 3-10 /,---CH 2 f-c~2 (H2C.). I IH2C.)r~ NH %,,.,...C+N_O_S02_OH " HaSO-~ '~...-C~o

A general ketone synthesis has been achieved by the Friedel-Crafts acylation of alkylsilanes (Olah et al 1977d). R 1 O I II A,c,J 2c,2 R 1 - Si -R 1 + R 2 - C - CI )

O II R 2 _ ~ - R 1 § (R1)3SiCI A versatile silylation of alcohols and acids to silyl ethers and esters has been carried out using allyltrimethylsilane with catalytic amount of triflic acid. The reaction proceeds through in situ generated trimethylsilyl triflate reagent system. Ketones react similarly under basic conditions with equimolar amount of triflic acid to provide enol silyl ethers (Olah et al 1981p). Finally, a new phase-transfer-promoted halogenation of alkenes to trans vicinal dihaloalkanes occurs with hydrohalic acid and hydrogen peroxide (Ho et al 1977).

R 1 - CH=CH -R 2+ 2HX + H202 X I R 1 - CH - CH - R 2 + H20 I X 180 G K Surya Prakash and George A Olah

CH2~HCHzStMe 3 t. CF3SO3H --- [CH3CHCHz..-.~SiMe31

CF3 SO3- A l-C H$ CH-"'CH NuSIMe 3 lib N.H, 25 "C ,-'.-',.~h,-,-tae3%Ct(:r'~er3"-" "..' -CF$S03H 5-10 mm

Null = RC(=O)OH or ROH.

0 &C._ ~R'" § EI3N + Me3SiOSOzCFi

H R. OS~Me3 , _ + Et3NH CF3503 R' R"

Acknowledgements

Support of our work by the National Science Foundation, the National Institutes of Health, the US Army Office of Research, and the Loker Hydrocarbon Research Institute of the University of Southern California is gratefully acknowledged. Our review was made possible by the merit and talent of our many enthusiastic coworkers (whose names are cited in the references) who contributed so ably. We are very fortunate to be associated with them.

References

Amaratunga W and Fr6chet J M J 1983 Tetrahedron Lett. 1143 Ansell M F, Hancock J W and Hickinbottom W J 1956 J. Chem. Soc. 911 Aranda G, Jullien J and Martin J A 1965 Bull. Soc. Chim. Fr. 1890 Aranda G, Jullien J and Martin J A 1966 Bull. Soc. Chim. Fr. 2850 Balicheva J G, Borodin P M and Pologikh I V 1969 Yad. Magn. Rezon. 49 Becher W and Massone G 1974 Chem.-Ztg. 98 117 Bergstrom C G, Nicholson R T and Dodson R M 1963 J. Org. Chem. 28 2633 Boswell G A Jr, Ripka W C, Scribner R M and Tullock C W 1974 Org. React. 21 1 Bouveault M L 1904 Bull. Soc. Chim. Fr. 31 1306 British Patent 1966 1017323 (Chem. Abstr. 1966 64 12248a) Chambers. R D 1973 Fluorine in organic chemistry (New York: Benjamin) Coivin E 1981 Silicon in organic synthesis (London: Butterworths) Comins D L and Meyers A I 1978 Synthesis 403 Synthetic methods and reactions 181

Cook D 1963 in Friedel-Crafis and related reactions (ed.) G A Olah (New York: Interscience) vol 1, p 269 Cupas C A and Pearson R L 1968 J. Am. Chem. Soc. 90 4742 Dean P A W and Gillespie R J 1969 J. Am. Chem. Soc. 91 7260 Eberson L and Rander F 1987 Acc. Chem. Res. 20 53 Fauvarque J, Ducom J and Fauvarque U J F 1972 C. R. Acad. Sci. C275 511 French Patent 1964 1370827 (Chem. Abstr. 1965 62 2712h) French Patent 1964 1374591 (Chem. Abstr. 1965 62 9212a). Fukuhara T, Yoneda N, Sawada T and Suzuki A 1987 Syn. Commun. 17 685 Furukawa N, Inoue T, Aida T and Oae S 1973 J. Chem. Soc., Chem. Commun. 212 Glemser O and Biermann U 1967 Chem. Ber. 100 2484 Hasbrouck R W and Kiessling A D A 1973 J. Org. Chem. 38 2103 Hashimoto T, Prakash G K S, Shih J G and Olah G A 1987 J. Org. Chem. 52 931 Hayashi M, Hashimoto S and Noyori R 1984 Chem. Lett. 10 1747 Hirschmann R F, Miller R, Wood J and Jones R E 1956 J. ,Am. Chem. Soc. 78 4956 Ho T L, Gupta B G B and Olah G A 1977 Synthesis 676 Ho T L, Henninger M and Olah G A 1976 Synthesis 815 Ho T L and Olah G A 1976a Synthesis 611 Ho T L and Olah G A 1976b Synthesis 807 Ho T L and Olah G A 1976c Synthesis 807 Ho T L and Olah G A 1976d Synthesis 815 Ho T L and Olah G A 1976e Angew. 'Chem., Int. Engl. Ed. 88 847 Ho T L and Olah G A 1977a J Org. Chem. 42 3097 Ho T L and Olah G A 1977b Synthesis 418 Ho T L and Olah G A 1977c Synthesis 170 Ho T L and Olah G A 1977d Synthesis 169 Ho T L and Olah G A 1977e Synthesis 417 Ho T L and Olah G A 1978 Proc. Natl. Acad. Sci. USA 75 4 Hoggett J A, Moodie R B and Schofield K 1971 Nitration and aromatic reactivity (Cambridge: University Press) Hughes O R 1977 US Patent 4003918 (Chem. Abstr. 1977 86 171466) Japanese Patent.1981 81166186 (Chem. Abstr. 1982 96 142832) Japanese Patent 1982 8218643 (Chem. Abstr. 1982 97 38670) Jung M E, Andrus W A and Ornstein P L 1977 Tetrahedron Lett. 4175 Jung M E and Lyster M A 1977a J. Am. Chem. Soc. 99 968 Jung M E and Lyster M A 1977b J. Org. Chem. 42 3761 Jung M E and Ornstein P L 1977 Tetrahedron Lett. 2659 Kaspi J, Montgomery D D and Olah G A 1978 J. Org. Chem. 43 3147 Kaspi J and Olah G A 1978 J. Org. Chem. 43 3142 Keck R and R~tey J 1980 Helv. Chim. Acta 63 769 Kim L 1979 US Patent 4165440 (Chem. Abstr. 1979 91 158329) Kirchmeyer S, Mertens A, Arvanaghi M and Olah G A 1983a Synthesis 498 Kirchmeyer S, Mertens A and Olah G A 1983b Synthesis 500 Krishnamurthy V V, Shih J G, Singh B P and Olah G A 1986 J. Org. Chem. 51 3154 Malhotra R 1979 Some synthetic reagents and reactions Ph.D. Thesis University of Southern California Meerwein H 1966 Org. Synth. 46 120 Meerwein H, Battenberg E, Gold H, Pfeil E, Willfang G 1940 J. Prakt. Chem. 27 154 83 Mertens A F, Lammertsma K, Arvanaghi M and Olah G A 1983 J. Am. Chem. Soc. 105 5657 Naumann D and Padma D K~ 1973 Z. Anorg. Allg. Chem. 401 53 Newman M S 1953 J. Am. Chem. Soc. 75 4740 Olah G A 197.3 Friedel-Crafts chemistry (New York: Wile)) Interscience) Olah G A 1974 Carbocations and electrophilic reactions (New York: Wiley) Olah G A 1975 Halonium ions (New York: Wiley Interscience) Olah G A and Arvanaghi M 1981 Angew. Chem Int. Ed. Engl. 20 878 Olah G A and Field L D 1982 Organometallics 1 1488 Olah G A and Fung A P 1979 Synthesis 537 Olah G A and Fung A P 1981 Synthesis 473 182 G K Surya Prakash and George A Olah

Olah G A and Gupta B G B 1980a J. Org. Chem. 45 3532 Oiah G A and Gupta B G B 1980b Synthesis 44 Olah G A and Gupta B G B 1983 J. Org. Chem. 48 3585 Oiah G A and Ho T L 1976a Synthesis 609 Olah G A and Ho T L 1976b Synthesis 610 Olah G A and Ho T L 1976c Synthesis 798 Olah G A and Kaspi J 1977 J. Org. Chem. 42 3046 Olah G A and Kaspi J 1978a Nouv. J. Chim. 2 581 Olah G A and Kaspi J 1978b Nouv. J. Chim. 2 585 Olah G A and Keumi T 1979 Synthesis 112 Oiah G A and Kuhn S J 1960 J. Am. Chem. Soc. 82 2380 Olah G A and Kuhn S J 1962 J. Am. Chem. Soc. 84 3684 Oiah G A and Kuhn S J 1973 Org. Synth. Colloq. 5 480 Olah G A and Lin H C 1971 J. Am. Chem. Soc. 93 1260 Olah G A and Lin H C 1973 Synthesis 489 Olah G A and Lin H C 1974a Synthesis 44 Olah G A and Lin H C 1974b J. Am. Chem. Soc. 96 2892 Olah G A and Lin H C 1974c Synthesis 342 Oiah G A and Mehrotra A K 1982 Synthesis 962 Olah G A and Meidar D 1978a Synthesis 358 Olah G A and Meidar D 1978b Synthesis 671 Olah G A and Meidar D 1979 Nouv. J. Chim. 3 269 Olah G A and Mo Y K 1974 J. Am Chem. Soc. 96 3460 Olah G A and Narang S C. 1982 Tetrahedron 38 2225 Olah G A and Nojima M 1973 Synthesis 783 Olah G A and Ohnishi R 1978 J. Org. Chem. 43 865 Olah G A and Prakash G K S 1976 Synthesis 607 Olah G A and Prakash G K S 1977 J. Org. Chem. 42 580 Olah G A and Prakash G K S 1978 Synthesis 397 Olah G A and Rochin C 1987 J. Org. Chem. 52 701 Olah G A and Svoboda J J 1973a Synthesis 52 Olah G A and Svoboda J J 1973b Synthesis 203 Olah G A and Vankar Y D 1978 Synthesis 702 Olah G A and Welch J T 1974a Synthesis 652 Olah G A and Welch J T 1974b Synthesis 654 Olah G A and Welch J T 1974c Synthesis 896 s G A and Welch J T 1975 J. Am. Chem. Soc. 97 208 Olah G A and Welch J T 1976 Synthesis 809 Olah G A and Welch J T 1977 Synthesis 419 Olah G A and Welch J. T. 1978a J. Am. Chem. Soc. 100 5396 Olah G A and Welch J T 1978b J. Org. Chem. 43 2830 Olah G A, Kuhn S J and Flood S H 1961 J, Am. Chem. Soc. 83 4581 Olah G A, Kuhn S J, Flood S H and Evans J C 1962 J. Am. Chem. Soc. 84 3687 Oiah G A, DeMember J R and Schlosberg R H 1969 J. Am. Chem. Soc. 91 2112 Olah G A, DeMember J R, Schlosberg R H and Halpern Y 1972 J. Am. Chem. Soc. 94 159 Olah G A, Nojima M and Kerekes I 1973a Synthesis 487 Olah G A, Olah J A and Svoboda J J 1973b Synthesis 490 Olah G A, Svoboda J J and Ku A T 1973c Synthesis 492 Olah G A, Nojima M and Kerekes I 1974a J. Am. Chem. Soc. 96 925 Olah G A, Beal D A, Yu S H and Olah J A 1974b Synthesis 560 Olah G A, Lin H C and Germain A 1974c Synthesis 895 Olah G A, Messina G, Bukala J, Olah J A and Mateescu G D 1975, Abstracts, 1st Chemical Congress of the North-American Continent, Mexico City, December, Paper PHSC-153 Olah G A, Beal D A and Olah J A 1976a J. Org. Chem. 41 1627 Olah G A, Welch J T and Ho T L 1976b J. Am. Chem. Soc. 98 6717 Olah G A, Welch J T, Prakash G K S and Ho T L 1976c Synthesis 808 Olah G A, Prakash G K S and Ho T L 1976d Synthesis 810 Synthetic methods and reactions 183

Olah G A, Prakash G K S and Liang G 1976e Synthesis 318 Olah G A, Bruce M R and Welch J T 1977a Inorg. Chem. 16 2637 Olah G A, Kaspi J and Bukala J 1977b J. Org. Chem. 42 4187 Olah G A, Welch J T and Henninger M 1977c Synthesis 308 Olah G A, Gupta B G B and Narang S C 1977d Synthesis 583 Olah G A, Ho T L, Prakash G K S and Gupta B G B 1977e Synthesis 677 Olah G A, Narang S C, Pearson R L and Cupas C A 1978a Synthesis 452 Olah G A, Malhotra R and Narang S C 1978b J. Org. Chem. 43 4628 Olah G A, Lin H C, Olah J A and Narang S C 1978c Proc. Natl. Acad. Sci. USA 75 1645 Olah G A, Malhotra R, Narang S C and Olah J A 1978d Synthesis 672 Olah G A, Keumi T and Meidar D 1978e Synthesis 929 Olah G A, Meidar D and Liang G 1978f J. Org. Chem. 43 3890 Olah G A, Parker D G and Yoneda N 1978g Angew. Chem., Int. Engl. Ed. 17 909 Olah G A, Gupta B G B and Narang S C 1978h J. Org. Chem. 43 4503 Olah G A, Gupta B G B and Narang S C 1979a J. Am. Chem. Soc. 101 5317 Olah G A, Narang S C, Salem G and Gupta B G B 1979b Synthesis 273 Olah G A, Gupta B G B and Narang S C 1979c Synthesis 274 Oiah G A, Welch J T, Vankar Y D, Nojima M, Kerekes I and Olah J A 1979d J. Org. Chem. 44 3872 Olah G A, Vankar Y D, Arvanaghi M and Sommer J 1979e Angew Chem., Int. Engl. Ed. 18 614 Olah G A, Meidr D and Fung A P 1979f Synthesis 270 Oiah G A, Prakash G K S and Bruce M R 1979g J. Am. Chem. Soc. 101 6463 Olah G A, Vankar Y D, Arvanaghi M and Prakash G K S 1979h Synthesis 720 Olah G A, Vankar Y D and Arvanaghi M 1979i Synthesis 720 Olah G A, Vankar Y D and Arvanaghi M 1979j Tetrahedron Lett. 3653 Oiah G A, Keumi T and Fung A P 1979k Synthesis 536 Olah G A, Vankar Y D and Prakash G K S 19791 Synthesis 113 Oiah G A, Prakash G K S and Narang S C 1979m Synthesis 825 Olah G A, Vankar Y D and Arvanaghi M 1979n Synthesis 984 Olah G A, Vankar Y D and Fung A P 1979o Synthesis 59 Olah G A, Vankar Y D and Gupta B G B 1979p Synthesis 36 Olah G A, Narang S C, Gupta B G B and Malhotra R 1979q Angew. Chem. Int. Engl. Ed. 91 648 Olah G A, Narang S C, Gupta B G B and Malhotra R 1979r J. Org. Chem. 44 1247 Olah G A, Narang S C, Gupta B G B and Malhotra R 1979s Synthesis 61 Olah G A, Gupta B G B, Narang S C and Malhotra R 1979t J. Org. Chem. 44 4272 Olah G A, Vankar Y D and Garcia-Luna A 1979u Synthesis 227 Olah G A, Malhotra R and Narang S C 1979v Synthesis 58 Olah G A, Narang S C, Olah J A, Pearson R L and Cupas C A 1980a J. Am. Chem. Soc. 102 3507 Olah G A, Narang S C and Garcia-Luna A 1980b Synthesis 659 Olah G A, Narang S C and Garcia-Luna A 1980c Synthesis 661 Olah G A, Malhotra R, Meidar D, Olah J A and Narang S C 1980d J. Catal. 61 96 Olah G A, Narang S C and Salem G F 1980e Synthesis 657 Olah G A, Arvanaghi M, Vankar Y D and Prakash G K S 1980f Synthesis 662 Olah G A, Vankar Y D and Arvanaghi M 1980g Synthesis 141 Oiah G A, Gupta B G B and Fung A P 1980h Synthesis 897 Olah G A, Arvanaghi M and Prakash G K S 1980i Synthesis 220 Olah G A, Arvanaghi M and Vankar Y D 1980j Synthesis 660 Olah G A, Arvanaghi M and Vankar Y D 1980k J. Org. Chem. 45 3531 Oiah G A, Narang S C, Field L D and Salem G F 19801 J. Org. Chem. 45 4792 Olah G A, Gupta B G B, Malhotra R and Narang S C 1980m J. Org. Chem. 45 1638 Olah G A, Fung A P, Gupta B G B and Narang S C 1980n Synthesis 221 Olah G A, Narang S C and Salem G F 1980o Synthesis 659 Olah G A, Narang S C, Fung A P and Gupta B G B 1980p Synthesis 657 Olah G A, Vankar Y D and Berrier A L 1980q Synthesis 45 Oiah G A, Narang S C and Fung A P 1981a J. Org. Chem 46 2706 Oiah G A, Fung A P, Narang S C and Olah J A 1981b J. Org. Chem. 46 3533 Olah G A, Prakash G K S and Chao Y L 1981c Helv. Chim. Acta 64 2528 Olah G A, Narang S C, Garcia-Luna A and Salem G F 1981d Synthesis 146 184 G K Surya Prakash and George A Olah

Olah (3 A, Husain A, Gupta B G B and Narang S C 1981e Synthesis 471 Oiah (3 A, Fung A P and Malhotra R 1981f Synthesis 474 Olah (3 A, Narang S C, Meidar D and Salem (3 F 1981g Synthesis 282 Olah G A, Fung A P and Meidar D 1981h Synthesis 280 Olah G A, Bruce M R and Clouet F L 1981i J. Org. Chem. 46 438 Olah (3 A, Fung A P and Keumi T 1981j J. Org. Chem. 46 4305 Olah (3 A, Chao Y L, Arvanaghi M and Prakash G K S 1981k Synthesis 476 Olah G A, Prakash G K S, Arvanaghi M and Bruce M R 19811Angew. Chem., Int. Engl. Ed. 93 107. Olah G A, Narang S C and Field L D 1981m J. Org. Chem. 46 3727 Olah G A, Husain A, Gupta B G B and Narang S C 1981n Angew. Chem., Int. Engl. Ed. 93 705 Olah G A, Narang S C, Field L D and Karpeles R 1981o J. Org. Chem. 46 2408 Olah (3 A, Husain A, Gupta B (3 B, Salem (3 F and Narang S C 1981p J. Ofg. Chem. 46 5212 Olah (3 A, Krishnamurthy V V and Narang S C 1982a J. Org. Chem. 47 596 Olah (3 A, Mehrotra A K and Narang S C 1982b Synthesis 151 Olah (3 A, Narang S C and Mehrotra A K 1982c Synthesis 965 Olah (3 A, Karpeles R and Narang S C 1982d Synthesis 963 Olah (3 A, Shih J G, Singh B P and Gupta B (3 B 1983a Synthesis 713 Olah (3 A, Shih J G, Singh B P and (3upta B (3 B 1983b J. Org. Chem. 48 3356 Olah G A, Shih J G and Prakash (3 K S 1983c Helv. Chim. Acta 66 1028 Olah (3 A, Gupta B G B, Garcia-Luna A and Narang S C 1983d J. Org. Chem. 48 1760 Olah (3 A, Karpeles R and Narang S C 1983e (unpublished results) Olah G A, Arvanaghi M and Krishnamurthy V V 1983f J. Org. Chem. 48 3359 Olah (3 A, Husain H and Singh B P 1983g Synthesis 892 Olah (3 A, Narang S C, Field L D and Fung A P 1983h J. Org. Chem. 48 2766 Olah (3 A, Narang S C and Husain A 1983i Synthesis 896 Oiah (3 A, Husain A, Singh B P and Mehrotra A K 1983j J. Org. Chem. 48 3667 Olah (3 A, Arvanaghi M and Prakash G K S 1983k Synthesis 636 Olah G A, Ohannesian L, Arvanaghi M and Prakash (3 K S 1984a Synthesis 785 Olah (3 A, Shih J G, Krishnamurthy V V and Singh B P 1984b J. Am. Chem. Soc. 106 4992 Olah (3 A, Ohannesian L, Arvanaghi M and Prakash G K S 1984c J. Org. Chem. 49 2032 Olah G A, Ohannesian L and Arvanaghi M 1984d J. Org. Chem. 49 3856 Olah G A, Prakash G K S and Arvanaghi M 1984e Synthesis 228 Olah (3 A, Prakash G K S and Sommer J 1985a Superacids (New York: Wiley Interscience) Olah G A, Laali K, Farnia M, Shih J G, Singh B P, Schack C A and Christe K O 1985b J. Org. Chem. 50 1338 Olah (3 A, Husain A and Singh B P 1985c Synthesis 704 Oiah (3 A, Farooq O and Prakash G K S 1985d Synthesis 1140 Olah G A, Shih J G and Prakash G K S 1986a J. Fluorine Chem. 33 377 Olah (3 A, lyer P S and Prakash (3 K S 1986b Synthesis 514 Olah G A, Ohannesian L and Arvanaghi M 1986c Synthesis 868 Olah G A, Arvanaghi M and Ohannesian L 1986d Synthesis 770 Olah G A, Yamato T, Iyer P S and Prakash G K S 1986e J. Org. Chem. 512826 Olah G A, Prakash (3 K S, Iyer P S , Tashiro M and Yamato T 1987a J. Org. Chem. 52 1881 Olah G A, Yamato T, Iyer P S, Trivedi N J, Singh B P and Prakash (3 K S 1987b Mater. Chem. Phys. 17 21 Politanskii S F, Ivanyk G D, Sarancha V N and Shevchuk V U 1974 Zh. Org. Khim. 10 693 Prakash G K S, Bruce M R and Olah (3 A 1985 J. Org. Chem. 50 2405 Prakash (3 K S, Iyer P S, Arvanaghi M and Olah (3 A 1983 J. Org. Chem. 48 3358 Prakash G K S, Stephenson M A, Shih J G and Oiah G A 1986 J. Org. Chem. 51 3215 Rosenfeld M N and Widdowson D A 1979 J. Chem. Soc., Chem. Commun. 914 Saimoto H, Hiyama T and Nozaki H 1983 Bull. Chem. Soc. Jpn. 56 3078 Sassaman M, Kotian K D, Prakash (3 K S and Olah G A 1987 J. Org. Chem. 52 4314 Schack C J and Wilson R D 1970 lnorg. Chem. 9 311 Schmitt R J and Bedford C D 1986 Synthesis 132 Seel F 1967 Inorg. Synth. 9 111 Sharefkin J (3 and Forschirm A 1963 Anal Chem. 35 1616 Synthetic methods and reactions 185

Sheppard W A and Sharts C M 1969 Organic fluorine chemistry (New York: Benjamin) Suschitsky H 1965 Adv. Fluorine Chem. 4 1 Szarek W A, Grynkiewicz G, Doboszewski B and Hay G W 1984 Chem. Lett. 10 1751 Thomas R K 1971 Proc. R Soc. London A322 137 Tullock G W and Coffman D D 1960 J. Org. Chem. 25 2016 Tullock C W, Fawcett F S, Smith W C and Coffman D D 1960 J. Am. Chem. Soc. 82 539 van der Akken M and Jellinek F 1967 Recl. Tray. Chim. Pays-Bas 86 275 V~ronkov M G, Dubinskaya E I, Pavlov S F and Gorokhova V G 1976 Izv. Akad. Nauk. SSSR, Ser. Khim. 2355 Voronkov M G, Puzanova V E, Palvov S F and Dubinskaya E I 1975 Izv. Akad. Nauk. SSSR, Ser. Khim. 448 Wang C J 1985 Org. React. 34 319 Weber W P 1983 Silicon reagents for organic synthesis (New York: Springer Verlag) Woyski M M 1950 J. Am. Chem. Soc. 72 919