Heterolytic formation

FIGURE 6.9 Confirmed heterolytic formation pathway for 5a-a-tocopheryl benzoate (11) without involvement of 5a-C-centered radicals and its proof by trapping of ortho-quinone methide 3 with ethyl vinyl ether to pyranochroman 13. Shown are the major products of the reaction of a-tocopherol (1) with dihenzoyl peroxide. 

Substituent effects on rates (Nishida, 1967) and equilibria (Mindl and Vecera, 1972) of the heterolytic formation of benzhydryl cations, analogous to those obtained in the bromination of 1,1-diphenylethylenes, can be analysed in terms of selectivity relationships (50) and (51). Here ak is... 

Figure 3. Heterolytic formation of glycol and formaldehyde from 1,3 dioxolane in SCW (in collaboration with M. Jones).

Photochromism Based on Dissociation Processes. Both heterolytic and homolytic dissociation processes can result in the generation of a photochromic system. An example of an heterolytic process is the reversible formation of triphenylmethyl cation, by photolysis of... 

A free-radical reaction is a chemical process which involves molecules having unpaired electrons. The radical species could be a starting compound or a product, but the most common cases are reactions that involve radicals as intermediates. Most of the reactions discussed to this point have been heterolytic processes involving polar intermediates and/or transition states in which all electrons remained paired throughout the course of the reaction. In radical reactions, homolytic bond cleavages occur. The generalized reactions shown below illustrate the formation of alkyl, vinyl, and aryl free radicals by hypothetical homolytic processes. 

First, let us consider the formation of ions from covalently bound species, i.e., the heterolytic cleavage of the covalent (or partially covalent) bond. Charge separation under the influence of the solvent generates an ion pair in a process called ionization this ion pair may then separate into free ions in a dissociation step (Eq. 8-18). 

Compound 104 could not be obtained from 103, and a hypothesis about its formation considered the (homolytic or heterolytic) cleavage of the O—N bond (Scheme 43) (68TL2417). The sensitized reaction didnotgive adifferentresult the author supposed that the reaction involved the excited triplet state of the molecule. When the reaction was carried out in methanol, 104 was obtained in 8% yield... 

A number of mechanisms for thermal decomposition of persulfate in neutral aqueous solution have been proposed.232 They include unimolccular decomposition (Scheme 3.40) and various bimolecular pathways for the disappearance of persulfate involving a water molecule and concomitant formation of hydroxy radicals (Scheme 3.41). The formation of polymers with negligible hydroxy end groups is evidence that the unimolecular process dominates in neutral solution. Heterolytic pathways for persulfate decomposition can he important in acidic media. 

In conclusion, it is very likely that the influence of solvents on the change from the heterolytic mechanism of dissociation of the C —N bond in aromatic diazonium ions to homolytic dissociation can be accounted for by a mechanism in which a solvent molecule acts as a nucleophile or an electron donor to the P-nitrogen atom. This process is followed by a one- or a two-step homolytic dissociation to an aryl radical, a solvent radical, and a nitrogen molecule. In this way the unfavorable formation of a dinitrogen radical cation 8.3 as mentioned in Section 8.2, is eliminated. 

In Section 8.3 the mechanism of heterolytic dediazoniation of arenediazonium ions was discussed, and it was shown that the hypothesis of Crossley et al. (1940) that the aryl cation is the characteristic metastable intermediate in those reactions was not consistent with some experimental facts found in 1952 by Lewis and Hinds. Nevertheless, these facts did not have significant influence on the scientific community, which continued to accept the original and apparently convincing hypothesis of the rate-limiting formation of an aryl cation as an intermediate as correct . The incom-patabilities of various mechanistic hypotheses with experimental facts were, however, discussed in some detail only two decades later (Zollinger, 1973 a). Another year passed before I performed a crucial experiment that refuted a number of hypotheses (Bergstrom et al., 1974, 1976). ... 

It has been suggested that the initial formation of iodine on addition of iodide to a diazonium salt solution is caused by oxidation of the iodide by excess nitrite from the preceding diazotization. Packer and Taylor (1985) demonstrated that, if urea was added as a nitrite scavenger (see Sec. 2.1) to a diazotization solution, that solution produced iodine much more rapidly than a portion of the same diazonium salt solution not containing urea, but eventually the latter reaction too appeared to follow the same course. This confirms the role of excess nitrite, and suggests that the iodo-de-diazoniation steps only occur in the presence of iodine or triiodide (I -). The same authors also found that iodo-de-diazoniation is much slower under nitrogen. All these observations are consistent with radical-chain processes, but not with a heterolytic iodo-de-diazoniation. 

With regard to the mechanism of these Pd°-catalyzed reactions, little is known in addition to what is shown in Scheme 10-62. In our opinion, the much higher yields with diazonium tetrafluoroborates compared with the chlorides and bromides, and the low yields and diazo tar formation in the one-pot method using arylamines and tert-butyl nitrites (Kikukawa et al., 1981 a) indicate a heterolytic mechanism for reactions under optimal conditions. The arylpalladium compound is probably a tetra-fluoroborate salt of the cation Ar-Pd+, which dissociates into Ar+ +Pd° before or after addition to the alkene. An aryldiazenido complex of Pd(PPh3)3 (10.25) was obtained together with its dediazoniation product, the corresponding arylpalladium complex 10.26, in the reaction of Scheme 10-64 by Yamashita et al. (1980). Aryldiazenido complexes with compounds of transition metals other than Pd are discussed in the context of metal complexes with diazo compounds (Zollinger, 1995, Sec. 10.1). 

The results of dediazoniations in aqueous acid without copper evidently show very little selectivity between sites with quite different electron densities. This is seen, for example, in the products from the reactions of the triarylmethanol derivatives 10.50 with R = C1 and R = CH3 (R" = R" = H). The yield ratios for ring closure to rings B and C (products 10.53 and 10.52 respectively) are 35/39 for R =C1 and 43/45 for R = CH3. The significantly higher yield of phenols (10.54) in the case of R = C1 (39%) relative to that of R = CH3 (10%) indicates that, as expected for a significant contribution by a heterolytic mechanism, the compound 10.50 (R = CH3) has a lower heterolytic, but not a lower homolytic, reactivity. For the same two reagents, but in the presence of copper powder, the ratios of 10.53 to 10.52 are 26/41 for R = C1 and 40/57 for R = CH3, with very little formation of phenols (3%). 

In a recent continuation of the work on dediazoniation of 2-(2 -propenyloxy)ben-zenediazonium salts (10.55, Z = 0, n= 1, R=H) in the presence of ferrocene, Beckwith et al. (1992) found that 3-ferrocenylmethyl-2,3-dihydrobenzofuran (10.65) is formed. The results are consistent with a mechanism involving electron transfer and dediazoniation followed by homolytic attack on the ferrocenium ion. This investigation resolved a long-lasting dispute regarding the heterolytic or homolytic character of the formation of arylferrocenes from arenediazonium ions (for literature since 1955 see Beckwith et al., 1992, references 1-7). 

The Ir complexes 83 or [lr(lMes)Cl2Cp ], in the presence of NaOAc and excess of (Bcat), catalyse the diboration of styrene, at high conversions and selectivities for the diborated species, under mild conditions. Other terminal alkenes react similarly. The base is believed to assist the heterolytic cleavage of the (cat)B-B(cat) bond and the formation of Ir-B(cat) species, without the need of B-B oxidative addition [66]. 

The enantioselective P-borylation of a,P-unsaturated esters with (Bpin) was studied in the presence of various [CuCl(NHC)] or [Cu(MeCN)(NHC)] (NHC = chiral imidazol-2-ylidene or imidazolidin-2-ylidene) complexes. The reaction proceeds by heterolytic cleavage of the B-B bond of the (Bpin), followed by formation of Cu-boryl complexes which insert across the C=C bond of the unsaturated ester. Best yields and ee were observed with complex 144, featuring a non-C2 symmetric NHC ligand (Scheme 2.31) [114]. 

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