SRN1 reaction

A similar goal can be achieved using the conditions of the SRN1 reaction. The anion of DMSO is generated by NaNH2 in DMSO andtheSRNl reaction is initiated by sunlight402 (Scheme 6). 

The sulphonyl group as activating factor in SRN1 reactions. 1039... 

The mechanistic aspects of the SRN1 reaction were discussed in Section 11.6 of Part A. The distinctive feature of the SRN1 mechanism is an electron transfer between the nucleophile and the aryl halide.181 The overall reaction is normally a chain process. 

Oxidative cross-coupling reactions of alkylated derivatives of activated CH compounds, such as malonic esters, acetylacetone, cyanoacetates, and certain ketones, with nitroalkanes promoted by silver nitrate or iodine lead to the formation of the nitroalkylated products.67 This is an alternative way of performing SRN1 reactions using a-halo-nitroalkanes. 

The reaction of a-bromo or a-iodonitroalkanes with sodium benzenesulfinate gives a-nitro sulfones in 85-95% yields (Eq. 5.73), which proceeds via SRN1 reaction (Section 5.4).117... 

Since the publication of the review on Single Electron Transfer and Nucleophilic Substitution in this same series,1 reviews or research accounts have appeared concerning several particular points among those addressed here, namely, dynamics of dissociative electron transfer,2-6 single electron transfer and Sn2 reactions,2,7 9 and SRN1 reactions.10,11... 

It is a pleasure to acknowledge the essential contribution of Dr. C. P. Andrieux to most of the work reported above as well as that of Dr. D. Lexa in the field of porphyrins, Professor Moiroux and Dr. A. Anne to cation radical reactivity, Dr. M. Robert to photoinduced dissociative electron transfer and to the stepwise/concerted competition and Drs. P. Hapiot and Medebielle to recent work on thermal SRn1 reactions. Many students from our group have also contributed effectively to the work, namely, C. Costentin, G. Delgado, V. Grass, A. Le Gorande, C. Tardy and D. L. Wang. Fruitful and pleasant... 

FIGURE 2.30. Redox catalysis induction of Srn1 reactions. Cyclic voltammetry in liquid ammonia + 0.1 M KC1 at —40°C of (a) redox catalyis of the reductive cleavage of 2-chlorobenzonitrile, RX, by 4-cyanopyridine, P. The dotted reversible cyclic voltammogram corresponds to P in the absence of RX. The solid line shows the catalytic increase of the current, (b) Transformation of the voltammogram upon addition of the nucleophile PhS. Adapted from Figure 1 in reference 23, with permisison from the American Chemical Society. 

The base may deprotonate either C3 or C4. Deprotonation of C3 makes it nucleophilic. We need to form a new bond from C3 to C8 via substitution. The mechanism of this aromatic substitution reaction could be addition-elimination or Sr I. The requirement of light strongly suggests SRN1. See Chap. 2, section C.2, for the details of drawing an SRN1 reaction mechanism. 

Nitrobenzenes react with potassium cyanide in the presence of cetyltrimethylammo-nium bromide to yield benzonitriles [71], The reaction also requires the presence of chloro substituents on the ring and at least two nitro groups (Table 2.9). Diazosulphides, ArN=NSPh, are converted into the benzonitriles, ArCN, by a photochemically induced SRN1 reaction with tetra-n-butylammonium cyanide [72, 73], Yields vary from <20% to >70%. Photocyanation of aromatic hydrocarbons has been achieved using tetra-n-butylammonium cyanide in acetonitrile or dichloromethane [74, 75]. 

There are not many successful examples of arylation of carbanions by nucleophilic aromatic substitution. A major limitation is the fact that aromatic nitro compounds often react with carbanions by electron-transfer processes.111 However, such substitution can be carried out under the conditions of the SRN1 reaction (see Section 11.4). 

The most common electron transfer process in organic chemistry, the SRN1 reaction, takes place somewhat differently. This particular reaction [e.g. (90)]... 

The lack of reactivity of 3-halo substituents under non-radical nucleophilic substitution conditions allows differential functionalization of pyri-dines by 3-umpolung and 2-nucIeophilic substitution processes. Thus, treatment of 2-fluoro-3-iodopyridine (189) with oxygen or amine nucleophiles affords products 191 which, upon subjection of SRN1 reactions with carbon, phosphorus, and sulfur systems, give 2,3-difunctionalized pyri-dines 192 (Scheme 56) (88JOC2740). 

Abstraction of a hydrogen atom from the solvent HS [Eq. (66)] makes it desirable to run the reaction in a solvent that is a poor hydrogen-atom donor liquid ammonia is a preferred solvent. Inorganic salts, such as potassium iodide or bromide, may be employed as supporting electrolyte in the SRN1 reaction in NH3. 

Polyfunctional fluoronitro alcohols are provided by die SRN1 reaction of a perfluoroalkyl iodide or alkylene duodides with the anhydrous lithium salt of 2-nitropropane-l,3-diol acetonide. Hydrolysis of the resulting perfluoroalkyl-... 

The mechanism of the reaction depicted in Scheme 4-6 differs from the SN1 or SN2 mechanism in that it involves the stage of one-electron oxidation reduction. The impetus of this stage may be the easy detachment of the bromine anion followed by the formation of the fluorenyl radical. The latter is unsaturated at position 9, near three benzene rings that stabilize the radical center. The radical formed is intercepted by the phenylthio anion. That leads to the anion radical of the substitution product. Further electron exchange produces the substrate anion radical and the final product in its neutral state. The reaction takes place and consists of radical (R) nucleophilic (N) monomolecular (1) substitution (S), with the combined symbol of SRN1. Reactions of SrnI type may have both branch-chain and nonchain character. 

Cleavage of a C—S bond in the initially formed anion radical has been shown to be the first chemical step in the electrochemically induced rearrangement of 5,5-diarylbenzene-1,2-dicarbothioates to 3,3-bis(arylthio)phthalides in dimethylformamide. The reaction can be effected with 0.1 F/mol and is considered a kind of internal SRN1 reaction (Praefke et al. 1980). 

The results obtained in the photostimulated SRN1 reaction between carbanions from 2,4,4-trimethyl-2-oxazoline or 2,4-dimethylthiazole and 2-bromopyridine are also consistent with the incomplete formation of the carbanions in the KNH2-NH3(iiq ) system. In these cases, 2-aminopyridine is formed alongside the corresponding pyridyl-2-methylene oxa-zolinyl or thiazolyl substitution products (Wong et al. 1997). When the SRN1 pathway is impeded by conducting the reaction in the dark or in presence of di(tert-butyl) nitroxide, the ionic animation reaction dominates. 

Unlike conventional nucleophile substitution, the cyclopropane ring does not cleave during a SRN1 reaction. 

There are two important requirements regarding reagents for SRN1 reactions They have to be electronodonors with respect to the substrates or active interceptors of intermediary radicals. For example, the phenyl thiolate and diphenylphosphite ions are active in such interception the phenolate ion is inactive. The introducing group sometimes exerts an influence on chain branching. All of these peculiarities have already been detailed in Chapters 3,4, and 5. 

Photostimulation increases the number of reagents and substrates that can be involved in SRN1 reactions. However, it complicates their performance see the review by Ivanov (2001). Therefore, the dark reactions are of special interest. 

Some SRN1 reactions can take place in the dark and with no catalyst. For example, the interaction of freons with nucleophiles in dimethylformamide at 20°C proceeds without photoirradiation. The chain process begins when the system pressure reaches 2 atm, in other words, when the concentration of the gaseous reagent becomes sufficient (Waksel-man Tordeux 1984) (Scheme 8-7). 

They carried this reaction out under pseudo-first-order conditions (excess of 2-nitro-propanate ions) in acetonitrile at 25°C, under argon atmosphere in a light-protecting vessel. The 2-nitropropanate ion was introduced as the tetramethylammonium salt. Two products were formed (Scheme 8-10). One of the products was the expected C-substituted compound. The other was an unstable species, which decomposed into the 4-nitrocumyl alcohol during workup and was ascribed to O-substitution. In 1975, Komblum had obtained the same products. He considered the C-substitution as an SRN1 reaction and the O-substi-... 

The SrnI character of the reaction was ascertained by the effect of light irradiation and the addition of a radical trap. Namely, under light irradiation, the half-reaction time was considerably shortened (3 instead of 41 min). Addition of di-tert-butyl nitroxide completely quenched the reaction Neither the C-substitution nor the O-substitution was observed after 4 hr. The radical trap may only react with the R radicals that escaped the solvent cage where R, Nu, and X- have been formed. This means that, in the absence of the trap, the R radical does react with Nu before diffusing out of the cage. The fact that the radical trap quenches the formation of both the C- and O-substitution products confirms that both species result from an SRN1 reaction. 

SRN1 reactions are, in general, eased up sterically. This feature is employed for synthetic purposes. For instance, the cyanoacetate substituent was inserted in the sterically shielded position of a benzene ring (Suzuki et al. 1983) (Scheme 8-14). The reaction proceeds in hexamethylphosphorotriamide. Photoirradiation results in the formation of undesirable by-products, but initiation with cuprous iodide leads to the target substance, at more than 60% yield. 

In what has become a classic example of an SRN1 reaction, the 2-propylnitronate anion undergoes efficient perfluoroalkylation in its reaction with perfluoroalkyl iodides [302] ... 

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