Alkenes dibromo, reaction with

Based on what we ve seen thus far, a possible mechanism for the reaction of bromine with alkenes might involve electrophilic addition of Br+ to the alkene, giving a carbocation that could undergo further reaction with Br- to yield the dibromo addition product. 

The thermo- and photocycloaddition of alkenes will be discussed in Chapter 12, on pericyclic reactions. On the other hand, transition-metals have effectively catalyzed some synthetically useful cycloaddition reactions in water. For example, Lubineau and co-worker reported a [4 + 3] cycloaddition by reacting a,a-dibromo ketones with furan or cyclopen-tadiene mediated by iron or copper, or a-chloro ketones in the presence of triethylamine (Eq. 3.48).185... 

A systematic investigation of the frans-selective monosubstitution of a variety of 1,1-dibromo-1-alkenes has established that their reaction with alkenyl-87, alkynyl-85 and arylzincs86 in the presence of 1-5 mol% Pd(DPEphos)Cl2 provides a generally applicable and satisfactory procedure (Scheme 37). For most cases, THF is a reasonably satisfactory... 

Boron trifluoride and boron trifluoride-diethyl ether complex can be used as a source of fluoride ions in the presence of hypobromites and hypochlorites, e.g. methyl hypobromitc, tert-butyl hypobromite, methyl hypochlorite in carbon tetrachloride at 25 C. The addition of bromine monofluoride" and chlorine monofluoride" to various alkenes is accompanied by the formation of the corresponding alkoxybromides and alkoxychlorides which hinder the isolation of the halofluorinated products.57 jV-Bromo- and A -chloro-substiluted alkyl- and arylamines. -amides, and -imides, A -chloro-A,-methylamine, A -bromo-A -methylamine, A -chloro-A, /V-dimethylamine, A-bromo-A.A-dimethylamine, ACV-dichloro-A -methylamine, V,fV-dibromo-,V-mcthylaminc, A -bromosuccinimide, -V-chlorosuccinimide, Af-bromoacct-amide, A.A -dichlorourethane, can be used in the reaction instead of the hypohalites. The reactions with various alkenes conducted in dichloromethane at room temperature in the presence of boron trifluoride-diethyl ether complex produce bromofluoro and chlorofluoro addition products in 40-80 % yield. However, the reactions are complicated by the addition of A -halo-succinimides and Af.A-dichlorourcthane to the C = C bonds.58... 

Aliphatic, aromatic, and alicyclic alkenes have been converted into the corresponding bromo fluorides 2 by reaction with l,3-dibromo-5,5-dimethylhydantoin (DBH) and silicon tetrafluoride in 1,4-dioxane at room temperature in a highly regio-, stereo- and chemoselective manner.9... 

Transition structures for the lithium-bromine exchange reaction of 1,1-dibromo-alkenes with methyllithium have been located by both the B3LYP and the MP2 levels of theory with the 6-31+G basis set.67 The reaction with methyllithium dimer gave similar results with lower activation energies. These calculations predict both the kinetic and the thermodynamic stereoselectivity correctly. It has been found that predominantly the sterically more constrained bromine atom of 1,1-dibromoalkenes reacted with alkyllithium (dimer) in the kinetic condition. 

Fluorinatioa.1 Alkyl bromides are converted to alkyl fluorides by reaction with this reagent in CFI3CN (35-92% yield). Bromofluorination of alkenes can be effected with AgF/CaF2 and l,3-dibromo-5,5-dimethylhydantoin (DBH). Iodo-... 

Reactions with dihalomethanes, 1,3-dibromopropane, 2,3-dibromobutane, 2,3-dibromo-2-methylpentane, 1,2-dibromocyclohexane, and 2-iodo-2-methylbutane did not produce telluronium salts. The halides were converted to alkenes ... 

Dibenzo[2.2]paracyclophane-1,9-diene (2), a unique molecule with two parallel sets of mutually orthogonal aromatic rings, was first synthesized in 1985 by Wong et al. [18]. The reported five-step synthesis gave the strained hydrocarbon in only 0.5% yield. Later a more versatile and economic approach to dibenzoannelated [2.2]paracyclophanedienes 11 including the parent compound 2 was developed by de Meijere et al. [19]. The dibromo[2.2]paracyclo-phane-l-ene (6) and the tetrabromo[2.2]paracyclophane-1,9-diene (7) were obtained from commercially available [2.2]paracyclophane (1) in a bromination-dehydrobromination reaction sequence. With compounds 6 and 7 available in large quantities, the palladium catalyzed coupling with alkenes (Heck reaction)... 

Displacement reactions. 1,3-Dibromo-l-trimethylsilylpropene is available from allyltrimethylsilane by reaction with NBS. The dibromo compound couples with Grignard reagents to effect chain elongation. With further displacement of the vinylic bromine and protodesilyation, it constitutes an intriguing approach to (Z)-alkenes. ... 

Reaction of dimethyl phosphonate (74) with the steroidal dienyl triflate 73 gave the dimethyl alkenylphosphonate 75 [23]. Dimethyl alkynylphosphonate (77) was produced in one step by the reaction of 1,1-dibromo-l-alkenes 76 with dimethyl phosphonate (74) in DMF using Pd(OAc)2-DPPF as a catalyst and propylene oxide as a scavenger of HBr. The expected monocoupling product, bromovinylphos-phonate, was not obtained [24]. Phosphonates 78 can be converted to arylphos-phines 79 by reaction with aryl Grignard reagents, followed by reduction with HSiCls [25]. 

Hexyne gives high conversions to the corresponding trans dibromo derivatives on reaction with an equimolar amount of bromine. The stereospecificity of this reaction can be interpreted in terms of a bromonium-ion intermediate, as is the case with alkenes. The mechanistic studies that have been reported to date, however, are not so detailed as in the case of alkenes. 

The bromonium ion in Figure 9.2 was proposed by Roberts and Kimball to explain the stereospecifidty of the addition to alkenes. Independent evidence for the existence of bromonium ions was provided by Winstein and Lucas, who found that ( )-e/yf/iro-3-bromo-2-butanol was converted to mcso-2,3-dibromobutane upon reaction with fuming HBr (equation 9.1), but the ( )-threo reactant formed the ( )-2,3-dibromo-butane. These authors suggested that the bromine provides anchimeric assistance to the departure of the protonated OFI group and forms a bromonium ion which, upon nucleophilic attack by bromide ion, gives a... 

Another common reaction of alkenes uses diatomic halogens such as bromine (Br2) to form 1,2-dibromides (see Chapter 10, Section 10.4.1). In this reaction, the alkene reacts as a Lewis base with the bromine atom to form a bromonium ion. When 1,3-butadiene (3) reacts with bromine, both 1,2 and 1,4 addition products are formed, just as with the HBr reaction. The products are 3,4-dibromo-l-butene (32) and a mixture otE- and Z-l,4-dibromo-2-butene (33 and 34). Initial reaction with bromine gives bromonium ion 29 however, when this reacts with bromide ion, there are two sites for reaction. If bromide attacks the less stericaUy hindered carbon atom, the product is 32, but the bromine ion may also attack the C=C unit to give products 33 + 34. Nucleophilic attack of this type is called an Sj reaction (nudeophilic substitution at an allylic carbon with displacement of the leaving group). 

Iron-stabilized oxallyl cations (generated in situ (eq 25) from a,o -dibromo ketones and Fe2(CO)9) react with alkenes. Noyori used this [3 + 2] cycloaddition reaction to produce cyclopentanone or cyclopentanone derivatives, as illustrated by a single-step synthesis of (i)-a-cuparenone (43) (eq 26). The reaction of a,a -dibromo ketones with enamines and Fe2(CO)9 yields substituted cyclopentenones in 50-100% yield (eq 27), as illustrated by the reaction with the a-morpholinostyrene (44). ... 

Although benzene is clearly unsaturatcd, it is much more stable than typical alkenes and fails to undergo the usual alkene reactions. Cyclohexene, for instance, reacts rapidly with Br2 and gives the addition product 1,2-dibromo-cyclohexane, but benzene reacts only slowly with Br2 and gives the substitution product CgH Br. As a result of this substitution, the cyclic conjugation of the benzene ring is retained. 

We have investigated the bromo-addition of alkenes and their related compounds with BTMA Br3. Thus, we found that the reaction of alkenes with BTMA Br3 in aprotic solvents such as dichloromethane and chloroform gave 1,2-dibromo adducts in a manner of stereospecific anti-addition, and, in such protic solvents as methanol and acetic acid, gave the corresponding dibromo adducts along with considerable amounts of solvent-incorporated products in regioselective manner (Fig. 18) (ref. 29). 

The dehalogenation of organic halides by organotin hydrides takes place in most cases with a free-radical mechanism [1, 84, 85], The stereospecific reduction of 1,1-dibromo-l-alkenes with Bu3SnH discovered by Uenishi and coworkers [86-89], however, did not occur in the absence of palladium complexes and did not involve radicals. For the synthesis of (Z)-l-bromo-l-alkenes, [(PPh3)4Pd] proved to be the most effective catalyst which could also be generated in situ. The reaction in Eq. (7) proceeded at room temperature and a wide range of solvents could be used. 

The authors determined the optimal reaction conditions and illustrated the scope of the method with 32 different starting compounds including alkenyl-, alkynyl-conjugated and 2,2-disubstituted 1,1-dibromo-l-alkenes. 

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