Succinimidyl radical

NBS can also be used to brominate alkanes. For example, cyclopropane, cyclopentane, and cyclohexane give the corresponding bromides when irradiated in a solution of NBS in dichloromethane. Under these conditions, the succinimidyl radical appears to be involved as the hydrogen-abstracting intermediate ... 

Whereas bromine radicals (133) and succinimidyl radicals (134) react by the Sh2 mechanism at the tin center in tetraalkyltins, but not in alkyltin halides, alkoxyl radicals (135) and ketone triplets (136) react with alkyltin halides, but not tetraalkyltins this may reflect the conflicting, electronic demands of the radical reagents which, as electrophilic species, should be more reactive towards tetraalkyltins than alkyltin halides, but which would also tend to make use of a 5d orbital... 

The relative reactivities of alkyltin compounds towards tert-butoxyl radicals, ketone triplets, and succinimidyl radicals are dominated by the steric effect of the alkyl ligands (R" > R"), but that towards bromine atoms follows the reverse sequence (R" < R ). 

When NBS is used to brominate non-alkenyl substrates such as alkanes, another mechanism, involving abstraction of the hydrogen of the substrate by the succinimidyl radical " 14 can operate. " This mechanism is facilitated by solvents (such as CH2CI2, CHCI3, or MeCN) in which NBS is more soluble, and by the presence of small amounts of an alkene that lacks an allylic hydrogen (e.g., ethene). 

When the perester [31] was used as a source of succinimidyl radicals in the presence of MNP, the 27-line spectrum of nitroxide [32] was recorded (Chalfont et al., 1970). This nitroxide was the only species detected when a chlorobenzene solution of [31] and MNP was heated and then examined spectroscopically only after it had been allowed to cool. When, instead, a reaction mixture was heated in the spectrometer cavity, overlapping spectra of... 

Although it proved possible to conclude from the results of further experiments with the perester that succinimidyl radicals from this source could abstract benzylic hydrogen from toluene, the reaction system presented further difficulties which are still unresolved. For example, when solutions of NBS and MBN are mixed in the dark, a high concentration of [32] is immediately produced. Whilst this helped to establish the origin of the 27-line spectrum, it constitutes a fresh mechanistic puzzle. 

The trapping of the succinimidyl radical and its congeners is a classical problem (Lagercrantz and Forshult, 1969 Chalfont and Perkins, 1970 ... 

The succinimidyl radical is now available for the next cycle of the radical chain reaction. 

Wohl in 1919 reported that A -bromoacetamide (CH CONHBr) induced allylic bromination. " Then iV-bromosuccinimide (30) was described in 1942 by Ziegler and co-workers to be useful in such free radical bromination reactions (equation 41), " and this widely utilized procedure is known as the Wohl-Ziegler reaction. In 1963 the mechanism of the reaction was proposed to involve halogen atoms in the hydrogen abstraction step " " " instead of succinimidyl radicals as had been commonly supposed. The halogen atom mechanism had previously been proposed by Gosselain et al. for reactions of yV-chlorosuccinimide. " ... 

EPR experiments on carbon-centred radicals with either a- or /J-boronic ester substituents have been reported.168 While the a-substituted radicals were modestly thermodynamically stable, the /J-substituted radicals underwent easy /J-climination. An EPR experiment on the photo-oxidation of phenolic compounds containing at least one free ortho position has indicated the formation of persistent secondary radicals derived from dimerization or polymerization from C-0 coupling.169 The structure of the succinimidyl radical has been re-examined using density functional theory with a variety of basis sets. The electronic ground state was found to be of cr-symmetry allowing for facile -scission. These conclusions were also predicted using MP2 but... 

The Ziegler reaction is an important method for the synthesis of benzylic or allylic bromides. /V-bromosuccinimide (NBS) is the reagent used in this reaction, which proceeds via the mechanism outlined in Scheme 4.13. The role of NBS is to provide a low, steady-state concentration of Br2 in solution so as to avoid competing electrophilic processes. The choice of solvent is critical for this reaction. It is important to use a solvent in which NBS is insoluble in order to prevent Br abstraction from NBS, which results in formation of the succinimidyl radical. Previously, CC14 was the solvent of choice. However, this study found that the reaction could be conducted in sc C02 reaction yields and selectivities were identical to those found in CC14. 

In contrast to eq. 2.29, eq. 2.30 shows the oxidative conversion of aldehydes (62) to amides (63) via acyl bromides with NBS/AIBN/R2NH under refluxing conditions in CC14 [74]. The reaction comprises of the abstraction of the formyl hydrogen atom by the succinimidyl radical, bromine atom abstraction from NBS by the acyl radical, and lastly,... 

Homolytic cleavage of a tin carbon bond in a tetraorgan-otin compound is less frequently encountered than is heterolytic cleavage. Such radicals as Bn, , and the suc-cinimidyl radical can take part in Sn2 displacement with tetraorganotins (Scheme 8). The reactivity of ITiSn in MezCO towards the succinimidyl radical is in the sequence of R = Me > Et > Pr > Bn this is the reverse of that for radical reactions of iodine and bromine. 

Togo, H., Hirai, T. Environmentally-friendly Wohl-Ziegler bromination Ionic-liquid reaction and solvent-free reaction. Synlett 200Z, 702-704. Greenwood, J. R., Vaccarella, G., Capper, H. R., Mewett, K. N., Allan, R. D., Johnston, G. A. R. Theoretical studies on the free-radical bromination of methylpyridazines in the synthesis of novel heterocyclic analogs of neutro-transmitters. THEOCHEM1996, 368, 235-243. Gainsforth, J. L., Klobukowski, M., Tanner, D. D. Structure and Reactions of the Succinimidyl Radical A Density Functional Study. J. Am. Chem. Soc. 1997,119, 3339-3346. 

The Sh2 reaction at the tin centre with direct cleavage of the SnC bond in acyclic tetraalkyltins is clearly recognisable only in the reaction of halogens under free radical conditions and in the reaction of the succinimidyl radical from /V-halogcnosuccinimidcs. 

Use of other halogenating agents, such as A -bromosuccinimide, under conditions where the chain-carrying radical was succinimidyl rather than bromine, resulted in a majority of attack in the side chain. However, the final abstraction of a bromine atom from 7V-bromosuccinimide was so rapid that j8-scission could not compete and unrearranged products accumulated. Similarly, in the reaction of A -bromo-3,3-dimethylglutarimide, or of 7V-bromophthaIimide (10), with methylcyclopropane (11), only unrearranged (bromomethyl)cyclopropane (12) could be isolated. ... 

The heterogeneous A-bromosuccinimide (NBS) bromination (Ziegler bromination Scheme 4) of toluene in SC-CO2 was initiated photochemically with azobis(isobutyronitrile) (AIBN) (40 °C, 170 bar, 4 h). [23] In CCI4, this reaction proceeds without succinimidyl radicals, with the same selectivity as the direct bromination. The yield is quantitative on the 4 mmol scale no 4-bromotoluene is observed. 

The mechanism of the Ziegler bromination is depicted in Scheme 7 (27). The reason that CCI4 is the ideal solvent for this reaction arises from the fact that NBS is insoluble in CCI4. As such, there is insufficient NBS in solution to allow the propagation of a succinimidyl radical chain. The role of NBS in this reaction is to maintain a low, steady-state concentration of Br2 in solution. Because of the very... 

The N-haloimide halogenations are also controlled partially by the fact that Cl or the iV-succinimidyl radical are much more reactive than Br in hydrogen abstracting reactions and, towards a hydrocarbon of low reactivity such as neopentane, a bromine atom chain would be quite ineffective. With mixtures of NBS and CI2, halogenation occurs to form the alkyl bromides but with the selectivity expected for chlorine atom attack Apparently ClBr is formed and reacts with the alkyl radical to form RBr and a chlorine atom. A similar situation exists for the bromination of alkanes using a mixture... 

Finally, even though the succinimidyl radical is much larger, the triatomic skeleton still plays a role. Essentially analogous curves are found (Fig. 11) for the a MOs, but due to the presence of the nitrogen the odd electron can also be placed in a jr MO localized at nitrogen (b, symmetry), and this situation is actually found to be preferred. Un other words, the succinimidyl radical also possesses at least two low-energy states separated by not more than lOkcalmol", but in this case the lower corresponds to a Jt radical. 

Fig. II. Calculated angular potential energy curve for the lowest four states in the succinimidyl radical.

Whereas photosensitized bromination of cyclopropane affords 1,3-dibromo-propane, bromination with NBS gives cyclopropyl bromide. Thus, whereas Br reacts exclusively by attack on carbon to give ring-opening, the succinimidyl radical abstracts hydrogen to give a cycloprop radical, which rapidly combines with a bromine atom. ... 

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