Trans bromination

The influence that a possible chelation has on the stereochemistry of the bromine-lithium exchange in dibromoaUcenes was first studied in the chiral MEM (methoxyethoxymethyl) ether 40. When this compound is treated with 1.2 equivalents of n-butyllithium in tetrahydrofnran, the acids E- and Z-42 are obtained in a ratio of 32 68 after carboxylation. Obvionsly, a kinetically favored substitution of the more easily accessible trans bromine atom occurs. When, however, slightly less than one equivalent (0.95-0.98 equiv) of -bntyllithinm is slowly added to a solution of 40 in diethyl ether at — 105°C, the -confignrated carbenoid 41 forms almost exclusively, and the carboxylic acids E- and Z-42 are obtained after reaction with dry ice in a ratio of over 99 1 (Scheme 9) ,i93 ... 

There are many = 4 species here, most of which appear to have a trans octahedral [MnX2L4] structure. Indications of this come from one available X-ray structure91 [MnBr2(5-Me-pyrazole)4j, which has trans bromines at 2.73 A and Mn—N of 2.25 A, from the ESR data and perhaps also from some of the IR data. Two crystalline forms of Mn(NCS)2(5-Me-pyr)4, both of which are isomorphous with Ni species, have been interpreted as examples of cis and trans isomers, but the data do not absolutely require the acceptance of the view that one is a cis isomer. 

Mixtures of CF3S03H and the triflates of B, A1 or Ga form a new superacid system, i.e. CF3S03H2 + [E(0S02CF3)4]- (E = B, A1 or Ga), which show superior catalytic activity in isomerization of alkanes, in trans-bromination and trans-alkylation of aromatics and in other related Friedel-Crafts reactions as compared with CF3S03H alone30-33. The relative reactivity sequence is B > Ga > Al. The triflates E(0S02CF3)3 were prepared from the reaction of EX3 (X = Br, Cl) with CF3S03H33. 

The final product of this reaction could in fact be either of two compounds as the two bromine atoms may be cis or trans. Bromination in chloroform at -20 °C gives mostly a liquid cis dibromide while reaction in hydrocarbon solvents gives the crystalline trans isomer. On standing the cis isomer slowly turns into the trans. 

Table 8.5 lists data for acetolysis of 2-substituted cyclohexyl brosylates having the general structure 25. The data confirm that a trans acetate reacts faster than does a cis acetate. The activation entropy data suggest a more ordered transition structure for the acetolysis of the trans isomer, consistent with the model of anchimeric assistance. A trans bromine is nearly as effective as a trans acetoxy, but a trans chloro group is far less effective—apparently due to the low stability of a chloronium ion in comparison with a bromonium ion. 

Anti additions to alkenes typically result in a stepwise mechanism formation of a cationic cyclic intermediate such as a bromonium ion, followed by backside attack by a nucleophile to open up the ring. Such is the mechanism for the anti addition of Br2. The bromination reaction results in trans bromines since the second bromine (as Br ) has to come in from the opposite face as the first bromine (as Br+) in order to do an Sn2 attack on the bromonium ion. Other mechanisms involving the bromonium ion include reaction of an aikene with Br2/H20 (adds -Br and -OH anti) and Br2/ROH (adds -Br and -OR flnfi)-Treatment of an aikene with a peroxyacid in water forms an epoxide that undergoes a ring opening in situ to give a trans diol product. Trans stereoselectivity is also seen in the dissolving metal reduction of alkynes. 

Int-A leads to the product A, when the bromonium ion is attacked by bromide at the more substituted, and therefore more electrophilic carbon with an Sn2 mechanism (backside attack results in trans bromines). The same mechanism gives rise to product B from Int-B. Since the intermediates are formed in unequal amounts, the diastereomeric products are also expected to be formed in unequal amounts. 

This chapter continues our discussion of addition reactions, and many of the mechanistic ideas of Chapter 9 apply. For example, many polar addition reactions start with the formation of the more stable carbocation.The most important new concept in this chapter is the requirement for overall anti addition introduced by three-membered ring intermediates. This concept appears most obviously in the trans bromination and chlorination of alkenes. 

Table 3.4 summarizes Weiss results on the temperature effect.As can be seen, a higher temperature and not a lower temperature favors the racemic product. When fumarate ion was brominated at a lower temperature, the same distribution of products was obtained. Thus, it appears that the reaction is steroselective for the meso product rather than being a stereospecific trans addition. At a higher temperature, trans bromination predominates. 

The stereospedfic and regioselective hydrobromination of alkynes with chlorobis(T -cyclopentadienyl)hydrozirconium and NBS produces ( )-vinylic bromides in good yields. The bromine atom usually adds regioselectively to the carbon atom that bears the smaller substituent and stereoselectively trans to the larger substituent (D.W. Hart, 1975 M. Nakatsuka,... 

FIGURE 6 13 Mechanism of bromohydrin formation from cyclopentene A bridged bromonium ion is formed and is attacked by a water molecule from the side opposite the carbon-bromine bond The bromine and the hydroxyl group are trans to each other in the product... 

Figures 7 13 and 7 14 depict the stereochemical relationships associated with anti addition of bromine to (E) and (Z) 2 butene respectively The trans alkene (E) 2 butene yields only meso 2 3 dibromobutane but the cis alkene (Z) 2 butene gives a racemic mixture of 2R 3R) and 2S 3S) 2 3 dibromobutane...

Halogenation (Section 9 13) Addition of 1 mole of chlorine or bromine to an alkyne yields a trans dihaloalkene Atetrahalide is formed on addition of a second equivalent of the halogen... 

Bromination of sulfolane by BrCl under uv irradiation gives 2-bromosulfolane [29325-66 ] which reacts further to give i7j -2,5-dibroniosulfolane [30186-52-8] (5). Continued irradiation converts the <7j -isomer to /n j -2,5-dibroniosulfolane [30186-54-0] which yields first the trans-2 4 isomer [30186-53-9] and then the trans-2 4 isomer [15091 -30-2] upon further irradiation. 

Bromine monofluoride [13863-59-7], BrF, can be prepared by the direct reaction of Br2 and F2, but because it readily disproportionates it has never been prepared in pure form (57). However, BrF can be prepared in situ by the reaction of Br2 with AgF in benzene (58) or by the reaction of A/-bromoacetamide and HF in ether (59). BrF adds to simple alkenes at room temperature to give products of trans-addition. Bromine trifluoride [7787-71-5], BrF, can be formed from gaseous fluorine and Hquid bromine (60). Bromine pentafluoride [7789-30-2], BrF, is formed from the reaction of BrF vapor with gaseous fluorine at 200°C (60). The tri- and pentafluorides are commercially available. As strong fluorinating agents they are useful in... 

Carbonyl bromide/595-95-iy M 187.8, b 64.5°/760mm. Purified by distn from Hg and from powdered Sb to remove free bromine, then vacuum distd to remove volatile SO2 (the major impurity) [Carpenter et al. y Chem Soc. Faraday Trans 2 384 1977]. TOXIC... 

An interpretation of activation parameters has led to the conclusion that the bromination transition state resembles a three-membered ring, even in the case of alkenes that eventually react via open carbocation intermediates. It was foimd that for cis trans pairs of alkenes tiie difference in enthalpy at the transition state for bromination was greater than the enthalpy difference for the isomeric alkenes, as shown in Fig. 6.2. This... 

The /rans-fiised decalin system is conformationally rigid, and the stereochemistry of the product indicates that the initial addition of the trichloromethyl radical is from the axial direction. This would be expected on stereoelectronic grounds, because the radical should initially interact with the n orbital. The axial trichloromethyl group then shields the adjacent radical position enough to direct the bromine abstraction in the trans sense. 

The properties of chlorine azide resemble those of bromine azide. Pon-sold has taken advantage of the stronger carbon-chlorine bond, i.e., the resistance to elimination, in the chloro azide adducts and thus synthesized several steroidal aziridines. 5a-Chloro-6 -azidocholestan-3 -ol (101) can be converted into 5, 6 -iminocholestan-3l -ol (102) in almost quantitative yield with lithium aluminum hydride. It is noteworthy that this aziridine cannot be synthesized by the more general mesyloxyazide route. Addition of chlorine azide to testosterone followed by acetylation gives both a cis- and a trans-2iddMct from which 4/S-chloro-17/S-hydroxy-5a-azidoandrostan-3-one acetate (104) is obtained by fractional crystallization. In this case, sodium borohydride is used for the stereoselective reduction of the 3-ketone... 

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