1,3-Pentadiene bromination

It is noteworthy that, at variance with bromination and chlorination which generally occur without isomerization of the disubstituted double bond, fluorine addition to the 1,2-bond of cis- and trans-1,3-pentadienes gives mainly the trans-adduct 13, besides smaller amounts of compounds 14-16 (equation 24). 

Thus, fluorination of 1,3-dienes proceeds through an allylic ion, while weakly bridged halonium ions are the intermediates in chlorination and bromination of dienes (vide infra). Furthermore, starting from the experimental evidence that 13 is produced under kinetic conditions and not from subsequent rearrangement of the 1,2- and 1,4-adducts, the authors suggested that 13 arose from rearrangement of the allyl cation intermediate, 17. Consistent with an open ion pair intermediate is also the stereoselective formation of the threo isomer from both 1,3-pentadienes, as well as the preference for the addition to the 1,2-bond observed in the reaction of both isomeric pentadienes. This selectivity may indeed... 

Later on, product distribution studies15 of the ionic addition of chlorine to conjugated dienes, and in particular to cyclopentadiene, 1,3-cyclohexadiene, cis,cis-, trans,trans-and c ,fraws-2,4-hexadienes, and cis- and trans-1,3-pentadienes have supplied the first stereochemical data, showing that the stereochemistry of 1,4-addition is predominantly syn, although to an extent smaller than that of bromine addition. Moreover, the 1,2-addition is generally non stereoselective, except for the addition to the 3,4-bond of cis-and trans-1,3-pentadienes where the attack is 89-95% anti. Finally, appreciable amounts of cis- 1,2-dichlorides were obtained from the two cyclic dienes, whereas 2,4-hexadienes showed a preference for anti 1,2-addition, at least in the less polar solvents (carbon tetrachloride and pentane). On the basis of all these results the mechanism shown in equation 29 was proposed. 

Furthermore, in the addition to the 3,4-bond of 1,3-pentadienes, the anti stereoselectivity observed with both bromine and chlorine has been attributed to a tightly bridged bromonium ion intermediate involving less charge dispersal in the vinyl group. In support of this hypothesis, it has been noted that bromine addition to the terminal double bond of the 1,3-pentadienes occurs without isomerization of the internal cis or trans double bond15. 

The addition of bromine chloride (BrCl) and amine-bromine chloride complexes to cyclopentadiene, isoprene and cis- and fraws-l,3-pentadienes has been also investigated74,77. The amine-bromine chloride complexes react with these dienes to give mixtures of bromochlorides in ratios markedly different from those obtained with BrCl. In particular, in analogy with Br2, BrCl gives significantly more 1,4-addition and the complexes give more anti 1,2-addition. Only Markovnikov 1,2-adducts have been reported for BrCl addition to these dienes. Furthermore, in the case of cyclopentadiene, 1,2-addition proceeds completely anti whereas 1,4-addition gives predominantly the cis adducts (equation 45). 

No similar reaction takes place during bromination. Transformation of the optically active (—)-2,3-pentadiene, the most studied compound, gave a mixture of dibromides in inert solvents through a stereoselective anti addition 255... 

Chiral crystals generated from non-chiral molecules have served as reactants for the performance of so-called absolute asymmetric synthesis. The chiral environments of such crystals exert asymmetric induction in photochemical, thermal and heterogeneous reactions [41]. Early reports on successful absolute asymmetric synthesis include the y-ray-induced isotactic polymerization of frans-frans-l,3-pentadiene in an all-frans perhydropheny-lene crystal by Farina et al. [42] and the gas-solid asymmetric bromination ofpjp -chmethyl chalcone, yielding the chiral dibromo compound, by Penzien and Schmidt [43]. These studies were followed by the 2n + 2n photodimerization reactions of non-chiral dienes, resulting in the formation of chiral cyclobutanes [44-48]. In recent years more than a dozen such syntheses have been reported. They include unimolecular di- r-methane rearrangements and the Nourish Type II photoreactions [49] of an achiral oxo- [50] and athio-amide [51] into optically active /Mactams, photo-isomerization of alkyl-cobalt complexes [52], asymmetric synthesis of two-component molecular crystals composed from achiral molecules [53] and, more recently, the conversion of non-chiral aldehydes into homochiral alcohols [54,55]. 

HexabromocyclopentaeEeiie (2,90), An improved procedure for the West preparation of hexabromocyclopentadiene (2) by an exchange reaction of hexachlorocyclo-pentadiene (1, Hooker Chemical Company) with boron tribromide in the presence of aluminum bromide and bromine has been published. ... 

Key to method (A) exchange reaction with tin heterocycle (B) hydride addition to diyne (C) oxidation of saturated ketone (D) bromination-dehydrobromination by pyrolysis (E) reaction of RLi or ArLi with exocyclic M-Cl of preformed diene (F) ring expansion reaction from cyclopentadiene derivative (G) LiAlHi reduction of exocyclic M-Cl (H) carbene insertion into five-membered cyclo-pentadiene derivative. Doering-Hoffman method (I) 1,6-cycloaddition of GeCU. 

When 1,4-pentadiene is treated with bromine under conditions (what are they ) that favor formation of the dihalide, there is obtained the expected product, 4,5-dibromo-l-pentene. Addition of more bromine yields the 1,2,4,5-tetrabromo-... 

Preparation (b). The reaction sequence is shown in the formulation. Treatment of paraldehyde and absolute ethanol with hydrogen chloride gas at —5° produces a-chloroethyl ethyl ether (1), which on bromination affords a,j8-dibromoethyl ethyl ether (2). Coupling with allylmagnesium bromide gives (3), which on reaction with zinc dust in n-butanol generates 1,4-pentadiene. Thus one double bond is that of allyl bromide, and the other is generated by elimination of BrOCjH, from the bromohydrin ethyl ether (3). 

Generally, in bromine addition to carbon-carbon double bonds, bromine bridging, solvent dependent dissociation of the ionic intermediates, steric interactions between the counteranion and the first bonded halogen during the nucleophilic step, the possibility of carbon-carbon rotation in the carbenium ion intermediate, preassociation phenomena and nucleophilic assistance determine the stereochemical behavior of the reaction . Several of these factors have been invoked to explain the stereochemistry of bromine addition to dienes, although others have been completely ignored or neglected. Bromine addition to cyclopentadiene, 1,3-cyclohexadiene, 2,4-hexadienes and 1,3-pentadienes has been examined repeatedly by Heasley and coworkers and the product distribution has been... 

Note that the illustrated conformation has the acrolein oriented in an 5-cis conformation. This is in contrast to the usual 5-trans conformation of acroleins coordinated to a Lewis acid (Figure 6.13a), but it is supported by the fact that cyclo-pentadiene adds to the opposite face of acrolein itself [216]. It is likely that both s-cis and 5-trans dienophile conformers are present, and that the -cis conformer is more reactive. In other words, Curtin-Hammett kinetics [235] are operative. The rationale for this increased reactivity is as follows the j-trans conformation of 2-bromoacrolein would place the bromine above the indene ring. Cycloaddition to the top (Si) face of the 5-trans conformer would force the bromine into closer proximity to the indene as C2 rehybridizes from sp2 to sp3, a situation that is avoided in cycloaddition to the top (Re) face of the 5-cis conformer. 

PENTADIENE or PENTA-1,3-DIENE (504-60-9) Forms explosive mixture with air (flash point -20°F/-29°C). Violent reaction with strong oxidizers, bromine, perchlorates, perchromates, pyridinium perchlorate, peroxyacetic acid, peroxybenzoic acid. Attacks some plastics, rubber, and coatings. Flow or agitation of substance may generate electrostatic charges due to low conductivity. 

CH2=CH—CH—CH=CH2 1,4-Pentadiene has the weakest C—H bond (arrow), a bond that is doubly allylic (DH° 77 kcal mol-1) this isomer will therefore be brominated fastest. Because only a very... 

Electrophilic additions also occur to cumulated dienes (allenes). Caserio and co-workers established that the mechanisms of both the oxy-mercuration reaction and the electrophilic addition of bromine to allenes are similar to the corresponding additions to olefins. The oxymercuration of ( )-(—)-2,3-pentadiene (82) in methanol produced 83% of (S)-traMS-3-acetoxy-mercuri-4-methoxy-2-pentene (83), confirming the anti pathway for the addition. Also formed was 17% of cis-3-acetoxymercuri-4-methoxy-2-pentene (84), which was presumed to have the R configuration. As shown in Figure 9.48, the product ratios can be explained on the basis of preequilibrium formation of mercurinium ions resulting from attachment of mercury to either the top or the bottom of one of the double bonds. Subsequent rate-limiting attack of methanol on the mercurinium ions is easier for the pathway... 

Draw all of the contributing resonance structures and the resonance hybrid for the carbocation that would result from ionization of bromine from 5-bromo-1,3-pentadiene. Open the computer molecular model at the book s website depicting a map of electrostatic potential for the pentadienyl carbocation. Based on the model, which is the most important contributing resonance structure for this cation Is this consistent with what you would have predicted based on your knowledge of relative carbocation stabilities ... 

PROBLEM 13.45 The major product in the bromination (1 equivalent ofBra in CH2CI2) of ( , )-l-phenyl-l,3-pentadiene is ( )-3,4-dibromo-l-phenyl-l-pentene regardless of the reaction conditions (thermodynamic or kinetic control). Show the mechanism of the reaction and account for this selectivity. 

Compare the ally lie bromination reactions of 1,3-pentadiene and 1,4-pentadiene. Which should be faster Which is more energetically favorable How do the product mixtures compare ... 

Which compound should undergo allyhc bromination at the faster rate, 1,3-pentadiene or 1,4-pentadiene How would the... 

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