13. Diazotization of amines and dediazoniation of diazonium ions |
647 |
(2)Via aryne intermediates: The aryne is formed in two steps, the first being a heterolytic dediazoniation, followed by a heterolytic dissociation of a substituent (H, COO
etc.) in one of the o-positions relative to the diazonio group (DN C DN0 ). The (metastable) aryne reacts rapidly by addition of a Brønsted acid (H2O, HCl etc.).
(3)By single electron transfer from an electron donor, e.g. a transition metal ion, a trivalent phosphorous derivative or a base, followed by dissociation of the intermediate diazenyl radical in an aryl radical and dinitrogen. The aryl radical reacts with the solvent or with added reagents in various ways, as shown by the relatively large number of classical named reactions (Sandmeyer, Pschorr, Gomberg Bachmann, Meerwein reactions).
In contrast to dediazoniations which follow mechanisms (1) and (3), the dediazoniation via aryne intermediates has little importance in organic synthesis. The widely used trapping technique for aryne intermediates was shown recently to be applicable also to
Buckminsterfullerene (C60)104, if 2-aminobenzoic acid is added to a refluxing benzene solution of C60 and pentyl nitrite. A general caveat with respect of conclusions to negative results of trapping experiments as ‘clear’ evidence against aryne intermediates should be repeated here: Cadogan105 reported a long time ago that (solid) benzenediazonium fluoroborate gave trapping products with tetracyclone or anthracene only if water was carefully excluded.
The dediazoniation mechanism via aryl cation (1) was originally postulated by Hammett95b. Some apparent contradictions became clear by investigations of Swain and Zollinger in the 1970s (reviews2,2a,7f). The substituent effects, which were originally not understandable on the basis of a classical Hammett treatment, became clear by using dual substituent parameters (DSP), i.e. a treatment in which mesomeric (resonance) and field (inductive) effects are handled independently of each other. These two effects have opposite signs.
This property is relatively rare in the very large number of reactions for which substituent effects were evaluated quantitatively106. It seems to be common, however, for all dediazoniations of arenediazonium ions and of related compounds, e.g. of substituted phenyl azides forming nitrenes, as well as for additions of carbenes to alkenes.
These opposite signs can be explained by considering a twofold orbital interaction between the two parts of an arenediazonium ion, namely between the -HOMO of the diazonio group and the -LUMO of the aryl residue, and between the -HOMO of the aryl residue and the -LUMO of the diazonio group. These two overlaps stabilize the C N bond and reduce the rate of dediazoniation into a phenyl cation and a nitrogen molecule. The two opposing HOMO LUMO interactions are shown in Figure 1. Thus
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FIGURE 1. HOMO LUMO interactions between N2 and an aryl cation106. Reprinted with permission from H. Zollinger, J. Org. Chem., 55, 3846 (1990). Copyright (1990) American Chemical Society
648 |
Heinrich Zollinger |
the DSP treatment provides a reasonable interpretation of substituent effects on the basis of the DN C AN mechanism, i.e. rate-limiting formation of an aryl cation in aromatic dediazoniations.
Extended kinetic measurements of dediazoniations in trifluoroethanol, water and other solvents107,108, and a statistical treatment109, demonstrated that a mechanism with two steady-state intermediates, namely initial formation of a tight ion molecule pair (35) followed by the ‘free’ (solvated) aryl cation (36), fits the experimental results significantly better than a mechanism with 36 only (Scheme 8).
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SCHEME 8
The very detailed kinetic investigation of the diazotization mechanism was, in part, necessary because the existence of a phenyl cation as its major hypothesis was doubted by theoreticians in the 1970s. Their calculated energy difference between the benzenediazonium ion and the phenyl cation was, depending on the calculation method used, 180 360 kJ mol 1, i.e. much higher than the experimental values of activation energies
(114 117 kJ mol 1110 ). As shown later, these experimental energies are very little influenced by the solvent111. For the sake of the theoreticians, the statement of Castenmiller and Buck made in their paper using MINDO/3 calculations in 1977112 should be mentioned: ‘Calculations of this kind of model appear to be beyond the scope of the present possibilities’.
A quarter of a century later the situation is different. A very remarkable advance in the theoretical understanding of the dediazoniation reaction is Glaser’s work113,114. Glaser and coworkers used for the calculation of changes in the stabilities of fragments of a molecule a new method, which is based on Bader’s theory of atoms in molecules115, and used ab initio methods on high levels (MP3 in the later calculations). The result for the activation energy of dissociation of the C N bond in the benzenediazonium was calculated to be 105.9 kJ mol 1 and, if electron correlation and the vibrational zero-point energy are included, 115 kJ mol 1 clearly an excellent result in comparison to the experimental values!
In addition to this very important result for the understanding of the dediazoniation reaction, Glaser’s work revealed a completely new finding on the structure of arenediazonium ions: The calculations did not give a total charge of C1.0e for the diazonio group as we write it in the Blomstrand Kekule´ formula, but only C0.018e, namely 0.540e for N˛ and C0.558e for Nˇ ! The diazonio group in the benzenediazonium ion is therefore highly polarized. Glaser113 proposed the formula 37 to represent its properties.
13. Diazotization of amines and dediazoniation of diazonium ions |
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This dative bond description is therefore appropriate for the benzenediazonium ion. Glaser and coworkers found, however, that other diazonium ions (X N2C ), e.g. fluorodiazonium ion X D F , have practically uncharged residues X, but highly charged diazonio groups, and that there are intermediate cases with a roughly even distribution of the positive charge on X and on N2. In analogy to other problems, e.g. mesomeric structures or enol keto equilibria, we think3c that it is appropriate to keep for naming the expression ‘diazonium ion’ for all compounds (X N2C ), irrespective as to whether the positive charge is dominantly on X or N2.
Fundamental knowledge on dediazoniations following mechanism (3), i.e. those initiated by an electron transfer, increased also significantly in the 1980s, but was based more on experimental than on theoretical work. Galli’s experimental investigations116 and his masterly review of 1988117 demonstrated clearly that all homolytic dediazoniations, irrespective of whether they are catalyzed by metal ions or by light, whether they are carried out in the presence of stoichiometric or excess amounts of nonmetallic, inorganic or organic compounds, or whether electrochemical or radiolytic electron sources are used, are characterized by an initiation by electron transfer.
In these electron transfers a phenyldiazenyl radical is formed. At ambient temperature its lifetime was estimated to be 10 7 s118; it is consistent with estimates of its decay rate constants119. Some accurate information on the lifetime of this radical was obtained by Suehiro’s group, in part together with Rieker, at lower temperatures ( 48 to 117 °C120, review121). Results indicate that the diazo group rotates about the C N˛ axis and that the C N˛ Nˇ angle is about 40°.
Redox potentials of the halide ions explain117 that direct electron release to the benzene-
diazonium ion takes place only with iodide (and astatide, 211At 122 . This corresponds well with experience in organic synthesis: iodo-de-diazoniations are possible without catalysts, light or other special procedures. For bromoand chloro-de-diazoniations, catalysis by cuprous salts (Sandmeyer reaction) is necessary. For fluorination, the Balz Schiemann reaction of arenediazonium tetrafluoroborates in the solid state (thermolysis) or in special solvents must be chosen, i.e. a heterolytic dediazoniation without electron transfer. Galli116,117 demonstrated that in chloro-de-diazoniations the yield is strongly dependent on the redox potential of electron transfer catalysts (highest yields with CuC and Sn2C ), but that the rate of electron transfer influences the yield also. Electron transfer is likely to be the rate-limiting step of aryl radical formation in dediazoniations catalyzed by transition metal salts.
An informative result for the competition between homolytic and heterolytic dediazoniations was found by Kuokkanen123: in DMSO the dediazoniation of 4- nitrobenzenediazonium ion is homolytic, as shown by product analyses, and has an activation volume V6D D C 6.4 š 0.4 ð 10 3 M 1, whereas for the heterolytic reaction of benzenediazonium ion in DMSO V6D D C 10.4 š 0.4 ð 10 3 M 1.
The homolytic dediazoniations in alkaline aqueous solutions, in methanol and in highly nucleophilic solvents are extremely complex. CIDNP spectra were considered for many years as a useful source for the understanding of these processes, as shown by the relatively large number of such investigations before 1981 (16, see Zollinger7g), but the potential of this method for these problems seems to be exhausted (5 publications 1982 1992).
650 |
Heinrich Zollinger |
For the dyestuff and pigment industry, a better knowledge of dediazoniation under such conditions would be useful because we estimate the loss in yields of industrially produced azo compounds due to competitive (unwanted) dediazoniations to be at least 10% of the production. These 10% are mainly diazo tars which have been investigated systematically in only three papers since the 1950s124.
The stability of arenediazonium ions in solution and of their salts in the solid state against dediazoniation is increased by complexation with crown ethers2b. Harada and Sugita124a showed recently that the shelf life of photosensitive diazonium salts for diazo imaging processes can be improved by this complexation.
B. Hydro-de-diazoniations
As indicated by Kornblum in 1944125 the classical method of hydro-de-diazoniation by treating a diazonium salt in boiling acidic ethanol often leads to ethoxy-de-diazoniation. Kornblum replaced that method by dediazoniation in an aqueous solution of hypophosphorous acid (H3PO2), in some cases in the presence of a catalyst, e.g. 0.05 0.10 mol% Cu. Experimental evidence indicates that aryl radicals are involved.
Another method using trivalent phosphorous compounds as single electron donors was developed recently by joint work of Yasui and Ohno with coworkers126. It is known that trivalent phosphorous compounds, which are easily converted into compounds of a higher oxidation state, readily form a fourth covalent bond when treated with an electrophile. Examples are the Arbuzov and the Wittig reactions. These authors demonstrated that arenediazonium salts react with triphenylphosphine or with trialkylphosphites (alkyl D methyl or ethyl) in methanol, ethanol, propanol and cyclohexanol at room temperature in the dark under N2 to give the corresponding arenes, i.e. the products of hydro-de- diazoniation, together with triphenylphosphine oxide or with trialkyl phosphates. The yields of the arenes are dependent on the ratio of the reagents (optimum at diazonium salt: phosphorous compound D ca 2). In the presence of O2 yields are much lower. This and other evidence indicate a radical mechanism. The authors propose a radicalchain mechanism initiated by electron transfer from the phosphorous compound to the diazonium ion and formation of the aryl radical and the cation radical of the phosphorous reagent, e.g. (C6H5)3PCž .
Earlier already ‘one-pot’ methods for the diazotization and hydro-de-diazoniation of aromatic amines were developed127 129. Doyle128 showed that diazotization of 17 representative aromatic amines with one to three electron-withdrawing and-releasing substituents using tert-butyl nitrite in N,N-dimethylformamide yields the corresponding deamination products in yields of 41 85%. Threadgill and Gledhill129 used formamide as solvent. It is obvious that these two relatively good oxidizing solvents react in a similar way as the trivalent phosphorous compounds used by the Japanese groups126. In a mixture of equal volumes of acrylonitrile and acetonitrile, however, Doyle obtained only traces of the expected arene. Polyacrylonitrile was formed, indicating that acrylonitrile traps a radical intermediate.
Threadgill and Gledhill reported yields in the range 60 80% if the aniline derivative contains a nitro group. With weaker electron-withdrawing groups (COR, COOR, CN, Br) the yields are lower. The method does not work if the aniline contains only electron donor groups (CH3, OCH3). With regard to general applicability it is therefore inferior to Doyle’s procedure, although it has the advantage that NaNO2 can be used as nitrosation reagent.
Keumi and coworkers130 describe hydro-de-diazoniations of arenediazonium tetrafluoroborates using chlorotrimethylsilane, Me3SiCl, in tetrahydrofuran or tetrahydrofuran/ N,N-dimethylformamide mixtures. Excellent yields were obtained with polycyclic arene derivatives such as 2-fluorene-, 2-fluorenone- and 1-pyrenediazonium tetrafluoroborate and
13. Diazotization of amines and dediazoniation of diazonium ions |
651 |
other similar diazonium salts. In a modification of this method 2-halogenofluorenones can be synthesized. In the presence of azidotrimethylsilane in DMF solution the corresponding aryl azides are obtained.
C. Halo-de-diazoniations and Related Reactions
Halo-de-diazoniations are a series of reactions in which the replacement of the diazonio group changes from a heterolytic de-diazoniation in the case of the fluorination (Balz Schiemann reaction) to transition metal-catalyzed chlorination and bromination (Sandmeyer reaction) and finally to iodination and astatination where no catalyst is necessary due to the favorable redox potentials of I and At (I : E° D 1.3 V).
The reactions of CN and N3 , i.e. cyanoand azido-de-diazoniations, are closely related to the Sandmeyer type of halo-de-diazoniation, as similar reaction conditions are applied.
For the classical Balz Schiemann reaction, i.e. heating solid arenediazonium tetrafluoroborates without solvent, success with respect to yield is still rather difficult to predict.
In recent years, however, two fluoro-de-diazoniation procedures were published which seem to be better, at least for the specific cases investigated.
In Milner’s method132 the amine is diazotized with solid nitrosonium tetrafluoroborate in CH2Cl2 and, without isolation, the diazonium salt is heated and yields the fluoroarene in good yield. The method is also applicable to aniline derivatives bearing carboxy and hydroxy substituents, compounds which give poor yields in the classical procedure.
The new method of Yoneda’s group132 is also a ‘one-pot’ diazotization fluoro- de-diazoniation in a liquid liquid two-phase mixture of pyridine and hydrogen fluoride. Yields for 25 aromatic amines and diamines are 50 100%, except for 2- and 3-fluorobenzoic acid, the three nitroanilines, 3- and 4-diaminobenzene and 4,40 - diaminodiphenyl-oxide (10 50%). In their 1994 paper the authors demonstrate that, in the same system, the photochemical decomposition gives in many cases significantly higher yields than the thermal reaction. The most spectacular increase in yield was found for the fluorination of 2-fluoroaniline where o-difluorobenzene was obtained photochemically in 80.2% yield, but thermally only in 0.6%!
Yoneda’s method was applied to the synthesis of 4-fluoro-3-(trifluoromethyl)phenol, a useful herbicide. It was prepared in 84% yield133.
Fluoro-de-diazoniation was reviewed by Olah, Chgambers and Prakash in their monograph on fluorinations134.
The most important finding for the synthetic applications of the Sandmeyer reaction is the clear experimental evidence of Galli116a that both oxidation states of copper ion are necessary for high yields. This claim is understandable on the basis of the reaction mechanism; cupric ions are a ligand transfer reagent (see reviews7h,117). The fact that the presence of CuII ions was not realized much earlier is understandable, because cuprous salts are rarely completely free of cupric impurities. In aqueous systems they form cupric ions by equilibration135 as well as by air oxidation. The following comparative experiments of Galli116a in the chloro-de-diazoniation of benzenediazonium sulfate in water at room temperature are instructive. Yields of chlorobenzene are: with 0.25 M CuCl 45%; with 0.25 M CuCl C 0.25 M Cu(NO3)2 63%; with 0.25 M Cu(NO3)2 <0.1%.
Recently Dassbjerg and Lund136 found a new modification of catalyzed chloro-de- diazoniation. The treatment of arenediazonium or 3-pyridinediazonium fluoroborates with ferrous chloride in a 3:1 mixture of tetrachloromethane and acetonitrile (but not acetonitrile alone) yields the corresponding aryl chlorides or 3-chloropyridine respectively, in nearly quantitative yield.
The bromo-de-diazoniation of 3-carboxy-2-naphthalenediazonium bromide hydrate (38, X D Br) is an interesting exception to the rule that bromo-de-diazoniations are successful
652 |
Heinrich Zollinger |
only in the presence of catalysts. Gougoutas137 found that this diazonium salt forms 3- bromo-2-naphthoic acid (39, X D Br) in the solid state by thermolysis at 100 °C (70% yield). The analogous iodo-de-diazoniation (X D I in equation 30) gives an 83% yield already by reaction at 30 60 °C138. Gougoutas determined also the X-ray structure of the two diazonium salts 38 (X D Br and Cl, respectively) and used that information for explaining their dediazoniations in comparison to Balz Schiemann fluorinations. His conclusions were, however, in part criticized7i.
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Cyano-, azidoand iodo-de-diazoniations proceed in general in good yields by classical methods. Some modified procedures were reported, however, in the 1980s and 1990s. Keumi and coworkers130 varied their method of treating diazonium salts with chlorotrimethylsilane (see Section III.B) for the formation of aryl azide by using azidotrimethylsilane as reagent with diazonium tetrafluoroborates in DMF solution.
1-Aryl-3,3-dialkyltriazenes are precursors for diazonium ions because they form diazonium ions in acid-catalyzed hydrolyses. Treatment of such triazenes with trimethylsilyl halides in acetonitrile at 60 °C resulted in the rapid evolution of nitrogen and in the formation of aryl halides139 without an electron transfer reagent or another catalyst. Yields with silyl bromide and with silyl iodide were 60 95%. The authors explain the reaction as shown in equation 31. The formation of the intermediate is indicated by higher yields if electron-withdrawing substituents (X D CN, COCH3) are present. In the opinion of the present author, it is likely that the dissociation of this intermediate is not a concerted reaction, but that the dissociation of the N aryl bond to form an aryl cation is followed by the addition of the halide. The reaction is therefore mechanistically not related to the homolytic halo-de-diazoniations.
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X = H, CH3 , CH3 CO, CN
1-Aryl-3,3-dialkyltriazenes were used for other types of halo-de-diazoniations140, namely triazenes in the presence of methanesulfonic acid, trifluoroacetic acid or a cation exchange resin (BioRad AG 50W-X12) in dry acetonitrile with NaI, LiBr, n- butylammonium fluoride or CsF at 75 °C. Brominations and iodinations yielded 75 99%
13. Diazotization of amines and dediazoniation of diazonium ions |
653 |
aryl halides, but florinations were not successful (4 10%; 20 24% in chlorinated solvents such as CCl4 or CCl3CN).
Triazenes can also be used to investigate the Sandmeyer mechanism proper, as shown in these and other papers from the same group140. However, the investigations demonstrate that their modifications allow bromo-de-diazoniations to be performed without an electrontransfer catalyst.
D. Substitutions of the Diazonio Group by Reagents with Reacting C-Atoms
In this section we include the intramolecular arylation of the Pschorr type, the intermolecular arylation (Gomberg Bachmann reaction), the arylation of alkenes and alkynes (Meerwein reaction) and related processes.
Pschorr’s synthesis of phenanthrene (1893) in five steps with the essential dediazoniation and ring closure of 2-diazonio-˛-phenylcinnamic acid giving, on addition of copper powder, phenanthrene-9-carboxylic acid, is today still the highest yielding one of all the reactions discussed in this section, Pschorr was able to get 93% yield, and today electrochemically induced Pschorr and related reactions141 give almost quantitative yields in several cases.
Instead of the ethene bridge between the two phenyl rings in the phenathrene synthesis, a large number of other oneor two-atom bridges were tested after Pschorr in 1893. Thus, Graebe and Ullmann found, only one year later, that 2-diazoniobenzophenone (40, X D CO ) can be converted into fluorenone in an analogous manner, and also in excellent yield. With more than a dozen other bridging groups X the yield is, however, much lower (see compilation7j).
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It is therefore not astonishing that in the Gomberg Bachmann reaction, the intermolecular counterpart of the Pschorr synthesis, yields are generally low. The homolytic part of the Gomberg Bachmann reaction is, in the opinion of March142 and of the present author, not sufficiently well understood on the basis of the experimental data. Galli117a does not discuss this problem in his review.
An interesting reaction in the context of Gomberg Bachmann arylation is the aryl dimerization and dediazoniation of arenediazonium salts (equation 32). Both aryl groups in the diaryl product originate from the arenediazonium ion. As two procedures in Organic Syntheses143,144 describe, diphenic acid (41, X D H) and its dl-4,40 ,6,60 -tetrachloro derivative (41, X D Cl) can be obtained by diazotization of 2-aminobenzoic acid and 3,5-dichloro-2-aminobenzoic acid respectively, followed by treatment of the diazonium salt solution with a mixture of copper(II) sulfate and bishydroxylammonium sulfate. Yields
are fairly good (72 84% with X D H, 63 84% with X D Cl). The mixture of CuSO4 and
+
(HONH3)2SO42 yields a redox equilibrium between CuC and Cu2C ions. This mixture may be useful for other homolytic arylations, but we are not aware of a corresponding
investigation†.
† In 1974 Cohen and coworkers145 studied the influence of the two oxidation states of copper on aryl dimerization, but not with the mixture mentioned above.
654 |
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Heinrich Zollinger |
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The arylation of alkenes was discovered by Meerwein146 in 1939 using ˛,ˇ-unsaturated carbonyl compounds, namely coumarin and cinnamic derivatives. Diazotizations for Meerwein reactions are made in aqueous HCl. The substitution proper may be combined with addition of HCl to the double bond. As catalyst, CuCl2 is used. Various observations (see elsewhere7k) demonstrate that in typical Meerwein systems, part of CuII is reduced to CuI.
In the period 1955 1974 the Meerwein reaction was investigated intensively in the Soviet Union (review147). Various modifications were published in the 1970s, 1980s and early 1990s. They demonstrated in several cases that yields which, with few exceptions, were low in earlier publications can be surprisingly high. This is an astonishing difference to the Gomberg Bachmann reactions.
The groups of Doyle and of Oae148 showed that the yields can be improved by the use of arylamines and alkyl nitrites in place of arenediazonium ion, i.e. by a ‘one-pot’ diazotization Meerwein procedure. A condition for good yields is, however, that the alkenes be activated by electron-withdrawing groups.
An interesting modification to the classical Meerwein reaction are palladium-catalyzed arylations, because they can be applied to alkenes bearing either electron-releasing or -withdrawing substituents (but not both). The key intermediate is an ‘arylpalladium’ species that can be generated in situ by several methods, e.g. from an aryl bromide or iodide with a palladiumtriarylphosphine or PdII acetate and base, respectively. The originally used preparation via arylmercury compounds is outdated, but the reaction can also be carried out using zero-valent Pd complexes prepared in situ from PdIICl2 and sodium formate, or with bis(dibenzylideneacetone)palladium(0) [42, Pd(dba)2]149 and arenediazonium salts, in a method developed by Matsuda’s group150. In contrast to the use of bivalent Pd compounds151, the arylation with arenediazonium salts using Pd(0) compounds has the advantage that only catalytic amounts are necessary. Yields are highly dependent on substitution of the benzenediazonium salts. In most cases 51 75% was obtained, but very small yields with the 2,4,6-trimethyl and the 2- and 4-nitro derivatives152. The reaction can be performed at temperatures up to 50 °C as a ‘one-pot’ process by using a mixture
13. Diazotization of amines and dediazoniation of diazonium ions |
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of an arylamine and t-butyl nitrite in chloroacetic acid or in a mixture of chloroacetic and acetic acid153. Styrene reacted with fourteen arylamines in the presence of 5 mol% Pd(dba)2 to give the corresponding substituted stilbenes in yields of 46 97%. It is important for good yields to carry out these reactions in an acidic system. Without acid the yield was low (11%), and diazo tars were also formed.
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Matsuda154 also investigated the arylation of styrene derivatives containing leaving groups other than hydrogen. Both E- and Z-alkenylsilanes (Ar0CHDCHSiMe3) were easily aryldesilylated in the presence of Pd(dba)2 in acetonitrile using arenediazonium tetrafluoroborates (ArN2C BF4). The reactions gave mixtures of E-arylation products containing the aryl group at the ˛- and ˇ-carbon atoms, E-Ar0 CHDCHAr and E- Ar0 C(Ar)DCH2, as main products. This loss of regioselectivity disqualifies this reaction for synthetic purposes. Surprisingly, however, ˛-styrylstannanes (43, R D CH3, C2H5, n- C4H9) selectively gave the Z- rather than the E-stilbene derivatives (equation 33155).
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Besides two older examples of classical Meerwein reactions156, there is a more recent description of the reaction of 4-chlorobenzenediazonium chloride with but-3-en-2-one,
catalyzed by TiIII157 in stoichiometric proportions. This method was developed by Citterio and gives 4-(40 -chlorophenyl)but-3-en-2-one in 65 75% yield.
Another new catalyst was described by Leardini and coworkers158, namely FeSO4 in DMSO. It was applied to a Meerwein reaction of phenylethyne and substituted phenylethynes with arenediazonium salts containing a thioether group in the 2-position. 2-Phenylbenzothiophens are obtained in 55 95% yield.
There are two recent summaries of Meerwein reactions71,159a. Intramolecular Meerwein reactions were studied in the late 1980s, first of all by Beckwith and coworkers160. The majority of their investigations (Scheme 9) were made with 2-(20 -propenyloxy)- and 2[(20 -methyl-20-propenyl)oxy]benzenediazonium tetrafluoroborate (44, Z D O , n D 1, R D H and CH3, respectively). Other reagents were 2-(30-butenyl)benzenediazonium tetrafluoroborate (44, Z D CH2 , n D 1, R D H), and the compounds with an acety-
lamido and a sulfamoyl group Z D SO2NH . The intramolecular
Meerwein product 45 was obtained in DMSO, pyridine or acetone as solvents and with CuX2 (X D Cl, Br), CuCN, NaI, NaI C I2 and C6H5SNa as co-reagents. Most reactions gave good yields (60 89%). Lower yields of the cyclization product 46, but significant amounts of the not cyclized product 45, were found for compounds with longer side chains (e.g. 44, Z D O , n D 2 or 3).
The homolytic character of these cyclizations was corroborated in the late 1980s in theoretical investigations161; and in the early 1990s by additional experimental work162. Beckwith163 found also that the diazonium salt 44, Z D O , n D 1, R D H) forms 3-ferrocenylmethyl-2,3-dihydrobenzofuran (47) in the presence of ferrocene.
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SCHEME 9 |
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Fe
O CH2
(47)
E. Hydroxyand Mercapto-de-diazoniations
Hydroxy-de-diazoniations are well-known standard reactions for the replacement of aromatic amino groups by hydroxy groups. An aqueous solution of the diazonium salt is added slowly to boiling 5 35% (v/v) aqueous sulfuric acid. The hydroxy-de-diazoniation may fail in the presence of reactive substituents in the o-position to the diazonio group (24 references mentioned by Cohen164 in 1977!). Cohen’s method of adding large amounts of Cu(NO3)2 (15 100-fold excess) and Cu2O in equimolar amounts, relative to diazonium ions, is today hardly recommendable anymore due to environmental reasons.
Better yields are claimed by Satyamurthy and coworkers165 for a process which consists of the hydrolysis of 1-aryl-3,3-diethyltriazenes in acetonitrile using a boiling mixture of the sulfonic acid resin BioRad AG 50W-X12 and water. For nine monosubstituted diethylaryltriazenes, phenols were obtained in yields of 65 95%.
A reaction which is related to hydroxy-de-diazoniations is the formation of aryl trifluoromethylsulfonic esters (aryl triflates, ArOSO2CF3) which became widely used reagents because of their leaving-group properties. The classical method of synthesis by esterification of phenols with trifluoromethane-sulfonic anhydride or -sulfonyl halide is, however, not applicable for the preparation of aryltriflates bearing a (free)