Bromotrichloromethane, radical addition reactions

Bicyclobutanes as a structural subunit of complex hydrocarbon frameworks also undergo radical addition reactions with benzenethiol and bromotrichloromethane. In compounds such as 23, where the azo functionality can also be involved in a radical reaction, a half-cage compound 24 was produced in 64% yield.38 The generation of other half-cage compounds from 23 and its analogs are shown in Table 10.38... 

Fluorinated adducts can be obtained either by addition of polyfluoroalkyl halides to various alkenes or by condensation of halides, such as bromotrichloromethane or carbon tetrachloride, with fluorinated alkenes. Factors which control the regiospecificity and relative rate of these free-radical addition reactions have been studied. 

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.]... 

The Re(III) complex Re(PPh3)2(MeCN)Cl3 (2 mol%) catalyzes the ATRA of tetrachloromethane or bromotrichloromethane to terminal alkenes in 39-76% yield [303]. p-Pinene suffered a cyclobutylcarbinyl radical ring opening, thus supporting the free radical mechanism. With l, -dienes double addition was found, while 1,3-dienes gave the 1,4-addition product. Internal alkenes were almost inert under the reaction conditions. 1,6-Dienes 158 underwent a tandem radical addition/ cyclization reaction to cycles 159 in 64—87% yield with 3-6 1 c/s-diastereos-electivity (cf. Fig 43). This compares well to the results obtained with the most frequently used catalyst Ru(PPh3)3Cl2 (see Part 2, Sects. 3.3.1 and 3.3.2). 

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. 

In the radical addition of bromotrichloromethane or carbon tetrachloride to norbornadiene the product ratio showed a considerable dependency on the reaction temperature. At 100 °C the tricyclo[2.2.1.0 ]heptyl derivatives 33a and 34a predominated with 81% yield, whereas at 0 C the norbornenyl adduct 32a was the main product (64%). A change in the bromotrichloromethane concentration from 9.69mol L to 0.95 mol caused the ratio 32a/33a- -34 a to change from 64 36 to 21 79. Kinetic measurements at temperatures between 0 and... 

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. 

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. ... 

The initial ruthenium(II) catalyst 66 abstracts a halogen (either chlorine or bromine) from the substrate forming a ruthenium(III) species 67. This is followed by pi complexation (68), radical addition (69) and halogen atom transfer to form the desired product (70). Starting from 65a, enantioselectivities of the resulting product 70a ranged from 20 to 40% ee with excellent chemical yields [28]. Reactions with a slightly different substrate bromotrichloromethane (65b) provided 70b in 32% ee, and a poor yield of 26% [29]. 

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. 

A group transfer tandem addition of bromotrichloromethane to diallyl amine 157 has been reported [95SC3529]. The radical reaction can be initiated using either azobisisobutyronitrile (AIBN) or manganese(III) acetate electrochemically. It should be noted that the cis diastereomer is formed as the major product. 

Radical reaction mechanisms of and at organic germanium, tin and lead 2. Homolytic addition of bromotrichloromethane to allyltriorganostannanes... 

Allyl radicals substituted at only one of the terminal carbon centers usually react predominantly at the unsubstituted terminus in reactions with nonradicals. This has been shown in reactions of simple dienes such as butadiene, which react with hydrogen bromide, tetrachloromethane or bromotrichloromethane to yield overall 1,4-addition products . The reaction of allyl radicals with hydrogen donors such as thiols or tin hydrides has been investigated and reviewed repeatedly. In most cases, the thermodynamically more favorable product is formed predominantly. This accords with formation of either the higher substituted alkene or the formation of conjugated tt-systems. Not in all cases, however, is the formation of the thermodynamically more favorable product identical to overall 1,4-addition to the diene. In those cases in which allyl radicals are formed through reaction of dienes with tin hydrides or thiols, the... 

In a similar manner, (acetonitrile)trichlorobis(triphenylphosphane)rhenium(lll), as well as dichlorotris(triphenylphosphane)ruthenium(II), catalyze the addition of carbon tetrachloride and bromotrichloromethane to 1,6-dienes to give 1.2-disubstituted cyclopentanes or analogous heterocycles. The cyclization products, obtained in good yield, show preferred cis geometry of the neighboring substituents33. The reactions are interpreted to proceed via a radical mechanism with the metal acting as simple initiator. 

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). 

No product derived from the transannular hydrogen abstraction is observed in the addition of bromotrichloromethane, because bromine atom abstraction is sufficiently rapid to prevent effective competition by the hydrogen-abstraction process. Another example of transannular cyclization of unsaturated radicals is found in the reaction of 1,4-cyclooctadiene with acetaldehyde in the presence of benzoyl peroxide, which gives a cyclized ketone by a process involving an intramolecular addi-... 

Fortunately, Bertrand was able to obtain nitrite esters of ethylenic alcohols as pure, characterized products. She assumed that photolysis of the nitrites would lead to the alkoxyl radical, as in the Barton reaction, with subsequent intramolecular addition. Isolation of tetrahydrofurfural oxime (62 R = H) starting from the nitrite of 4-pentene-l-ol (61 R = H) supported this hypothesis (Scheme 33). This mechanism was later confirmed by esr spin-trapping techniques and, independently by Rieke, who scavenged the cyclized radical with bromotrichloromethane. 

The first discovered reactions at the double bond of alkenylboronates were additions of free radicals [21]. Bromotrichloromethane gave, after initiation with light or in the presence of azobisisobutyronitrile (AIBN), the corresponding a-bromoboronate in excellent yield (Scheme 9.9). These products were the first compounds of this class of boronic esters and the starting point of a rich chemistry [22]. Hydrogen bromide, under irradiation with ultraviolet light, led to 2-bromoethaneboronate [23, 24]. 

Functionalized alkenyl diamino- and dialkoxyboranes have been produced regio-and stereoselectively through addition of carbon- or heteroatomgenerated from bromotrichloromethane, thiols, phosphines and tributyltin hydride) to ethynylbis(diisopropylamino)boranes. The synthetic utility of these reactions was illustrated by the preparation of stereodefmed (Z)- or ( )-alkenylboronic esters via palladium-catalyzed crossStille reaction and a Suzuki coupling under basic conditions can be further conducted (Scheme 9.14) [33]. 

---