Bromotrichloromethane, radical-chain addition

Highly strained systems sueh as bicyelobutane (14) and [1.1. l]propellane (15) readily underwent addition of bromotrichloromethane across the central bond by a radical mechanism. Benzoyl peroxide eatalyzed a number of addition reactions to the extremely strained central bond of [l.l.l]propellane (15). Examples were acetaldehyde, cyanogen bromide, deuteriochloroform, diphenyl disulfide, diphenyl diselenide, iodine, and tert-butyl hypochlorite.Radical chain addition of various organic disulfides to [l.l.ljpropellanes (15), initiated by 2,2 -azobis(iso-butyronitrile) gave the normal adducts across the strained central bond and homologs that contained two or more bicyclo[l.l.l]pentane moieties. 

Exercise 10-28 Bromotrichloromethane, BrCC 3, adds to 1-octene by a radical-chain mechanism on heating in the presence of a peroxide catalyst. Use the bond-energy tables to devise a feasible mechanism for this reaction and work out the most likely structure for the product. Show your reasoning. Show the most likely product of addition of BrCCI3 to 1-octyne. [Note Radical-chain reactions involve abstraction of atoms, not abstraction of groups.]... 

Cases in which allyl radicals display sufficient reactivity to participate successfully in radical chain reactions include the addition of bromotrichloromethane to butadiene the reaction of cyclopentadiene with tosyl cyanide, the addition of thiols , stannanes " and hydrogen halides . All these reactions follow the simple two-step radical chain mechanism depicted in Scheme 1, and the low reactivity of the intermediate allyl radicals can be compensated by using the trapping agent in excess or even as the solvent. In chain reactions with three or more chain-carrying radicals, this compensation is not possible anymore, because the concentration of the reaction partners has to be chosen such that the selectivity requirements for all intermediate radicals are satisfied. Complex radical chain reactions with polyenes as one of the reactants are therefore not known. 

Allenylcobaloximes, e.g. 26, react with bromotrichloromethane, carbon tetrachloride, trichloroacetonitrile, methyl trichloroacetate and bromoform to afford functionalized terminal alkynes in synthetically useful yields (Scheme 11.10). The nature of the products formed in this transformation points to a y-specific attack of polyhaloethyl radicals to the allenyl group, with either a concerted or a stepwise formation of coba-loxime(II) 27 and the substituted alkyne [62, 63]. Cobalt(II) radical 27 abstracts a bromine atom (from BrCCl3) or a chlorine atom (e.g. from C13CCN), which leads to a regeneration of the chain-carrying radical. It is worth mentioning that the reverse reaction, i.e. the addition of alkyl radicals to stannylmethyl-substituted alkynes, has been applied in the synthesis of, e.g., allenyl-substituted thymidine derivatives [64],... 

Some radical reactions occur under the control of transition metal templates. The first example of asymmetric creation of an asymmetric carbon with a halogen atom is shown by the a DIOP-Rh(I) complex-catalyzed addition of bromotrichloromethane to styrene, which occurs with 32% enantioselectivity (Scheme 99) (233). Ru(II) complexes with DIOP or BINAP ligands promote addition of arenesulfonyl chlorides to afford the products in 25-40% ee (234). A reaction mechanism involving radical redox transfer chain process has been proposed. 

Benzocyclopropene reacts with a variety of radical reagents (for example A -bromosuccinimide carbon tetrachloride bromotrichloromethane bromoform/benzoyl peroxide alkyl sulfide and ethane-1,2-dithiol with photolysis) to afford products derived from cleavage of the cyclopropane ring. The preferential mode of reaction consists of a chain reaction initiated by radical addition at Cl a followed by opening of the cyclopropyl radical to afford a benzyl radical. Yields are generally low except for the addition of the alkylsulfanyl radical, e.g. formation of 1, and no products derived from addition to the central tt-bond are formed. Cyclopropa[A]naphthalene reacts similarly with radicals and gives 2-methylnapthalene derivatives, while no addition to the central 7i-bond is observed. ... 

Previously, examples of chiral Lewis acid coordination of either the radical or radical acceptor involved coordination to Lewis basic sites, typically oxygen or nitrogen. However, certain transition metals are also capable of coordination to al-kenes and, if complexed to a chiral ligand, can also afford chiral addition products. This has been illustrated in the enantioselective atom transfer additions of alkane and arene-sulfonyl chlorides and bromotrichloromethanes to olefins using chiral ruthenium complexes. These reactions are thought to follow a radical redox chain process detailed in Eq. (20). 

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