NBS reaction

This light-initiated NBS reaction is similar to the photolysis of o-nitrobenzylidene acetals but "is a one-step process instead of the following two-step procedure. 

The NBS reaction is carried out in a clever way. The allylic compound is dissolved in carbon tetrachloride, and one equivalent of NBS is added. NBS is denser than CCI4 and not very soluble in it, so it sinks to the bottom of the CCI4 solution. The reaction is initiated using a sunlamp for illumination or a radical initiator such as a peroxide. The NBS gradually appears to rise to the top of the CCI4 layer. It is actually converted to succinimide, which is less dense than CCI4. Once all the solid succinimide has risen to the top, the sunlamp is turned off, the solution is filtered to remove the succinimide, and the CC14 is evaporated to recover the product. 

Investigations in four laboratories indicate that NBS and Br reactions show such similarity that a bromine atom chain is the most plausible mechanism for the NBS reaction. 

If the sample solution contains Nb separated from other metals, no masking agents, including EDTA and tartrate, are necessary. In this case the sensitivity of the Nb reaction with BPR is higher (e = 6.0-10 ) [42],... 

An NBS reaction can be induced thermally, by UV irradiation,359 360 by addition of radical-formers (dibenzoyl peroxide, 2,2 -azoisobutyronitrile, or dimethyl 2,2 -azoisobutyrate),361,362 or by increasing the NBS suface (depositing it on neutral Si02).354... 

Trimethylsilylmethyl ethers. After radical bromination of MejSiCH20Me with NBS, reaction with an alcohol results in the mixed acetal of trimethylsilylacetaldehyde. Reduction with Dibal-H selectively removes the methoxy group. 

O-Deprotection. Free-radical-initiated bromination followed by treatment with water removes a benzyl ester. A similar method is also effective to cleave allyl ethers. Regioselective oxidations. An alkane-1,2-diol can be selectively oxidized to give the ketol (instead of the hydroxy aldehyde) via NBS reaction of the dibutylstan-nylene acetal. ... 

New Reaction Techniques Involving NBS. Several new reaction techniques have been applied to NBS reactions to develop potentially useful new synthetic methods, a selection of these are outlined below. 

The initiation step provides a radical source by thermal or photochemical dissociation of initiators, which then provides bromine radicals by reaction with Br2. Initiators are sometimes present in the alkene as allyl hydroperoxides which may be present due to inadvertent, prior autooxidation. Bromine or HBr may be present in trace amounts in NBS. Reaction of the bromine radical 20 with the substrate 1 proves selective for allylic or benzylic hydrogens due to the near thermoneutral nature of the reaction which breaks the C-H bond and forms the H-Br bond. Reaction of the formed carbon-centered radical 21 with Br2 provides the desired bromide 3 and Br 20. Hydrogen bromide 17 reacts with NBS to form succinimide 4 and resupplies the required low concentration of Br2. Alternatively, reaction of substrate radical 21 with NBS 2 provides product 3 and succinimidyl radical 22 (S ). Due to energy and kinetics considerations, abstraction of the allylic hydrogen by the S should be slower than abstraction of bromine from NBS by an allyl radical. In using solvents in which NBS, succinimide 4 or it s radical 22 are not very soluble, S is not the key chain-carrier. Byproducts and side-reactions can occur with S. ... 

The chemistry of the NBS reaction with indoles has been extensively investigated. It seems relevant to discuss this here together with some aspects related to the bromination of tryptophan and related 3-substi-tuted indoles 175, 218, 295, 443). 

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