Carbon-bromide bond

In the case of o-deuterio/>/ owobenzenc, on the other hand, breaking of the weaker carbon-bromide bond (step 2) is much faster than the protonation by ammonia (reverse of step 1) as fast as a carbanion is formed, it loses bromide ion. In this case, isotopic exchange is not important. (It may even be that here steps (I) and (2) are concerted.)... 

During their study on 6-exo-trig radical cyclizations of axially chiral a-halo-ortho-alkenyl anihdes, Curran and coworkers [40] prepared the tricyclic amide 93/94 in moderate yield (49-51%) and excellent chirality transfer (98-99%) via a 6-exo-trig/5-exo-trig sequence (Scheme 5.20). The reaction was proposed to be initiated by an attack at the carbon-bromide bond with BujSnH, providing a-amidyl radical 97 (Scheme 5.21). 

When the 1,2-dibromoethylene complex Fe(CO)4(c/ -CHBr=CHBr) is subjected to photolysis, the insertion of iron(O) into a carbon bromide bond is observed, leading to the formation of the vinyl complex Fe2(CO)6Br(CH=CHBr) (Ref. 124) ... 

Under these conditions, several 5-halofuranosides and 6-halopyranosides ring-opened to the corresponding pent-4-enals and hex-5-enals, respectively [16,17]. Mechanistically, the Bernet-Vasella reaction proceeds via oxidative insertion of zinc into the carbon-bromide bond, followed by reductive elimination with the loss of methoxide to give the aldehyde product (Scheme 3.2) [18]. 

TT-Allylpalladium chloride (36) reacts with the nucleophiles, generating Pd(0). whereas tr-allylnickel chloride (37) and allylmagnesium bromide (38) reacts with electrophiles (carbonyl), generating Ni(II) and Mg(II). Therefore, it is understandable that the Grignard reaction cannot be carried out with a catalytic amount of Mg, whereas the catalytic reaction is possible with the regeneration of an active Pd(0) catalyst, Pd is a noble metal and Pd(0) is more stable than Pd(II). The carbon-metal bonds of some transition metals such as Ni and Co react with nucleophiles and their reactions can be carried out catalytic ally, but not always. In this respect, Pd is very unique. 

Olefins react with bromine by addition of the latter to the carbon-carbon double bond. In contrast the Wohl-Ziegler bromination reaction using N-bromosuccinimide (NBS) permits the selective substitution of an allylic hydrogen of an olefinic substrate 1 by a bromine atom to yield an allylic bromide 2. 

Allylic bromides can also serve as progenitors for nucleophilic organochromium reagents. An elegant example is found in Still and Mobilio s synthesis of the 14-membered cembranoid asperdiol (4) (see Scheme 2).7 In the key step, reduction of the carbon-bromine bond in 2 with chromium(n) chloride in THF is attended by intramolecular carbonyl addition, affording a 4 1 mixture of cembranoid diastereoisomers in favor of the desired isomer 3. Reductive cleav-... 

Finally, obtaining olefin 93 from the reaction of thiirene oxide 18a with two equivalents of phenylmagnesium bromide may be a consequence of the initial nucleophilic Michael-type addition of the latter across the carbon carbon double bond of the cyclic sulfone22 (see equation 31). 

There are almost no studies of substituent effects on additions to carbon-carbon triple bonds extant in the literature. Bowden and Price (208) have reported a correlation of rates of addition of hydrogen bromide to 3-substituted propiolic acids with the Hammett equation using the Op constants. Unfortunately, there are only three substituents in the set. Sufficient data are available for a single set of 1,3-dipolar cycloaddition. The set studied is shown in Table XXXIII, and the results of the correlation are in Table XXXIV. The correlation was significant the delocalized effect is predominant in this set. 

Chiang, Y. Kresge, J. Zhu, Y. Flash photolytic generation and study of /j-quinone methide in aqueous solution. An estimate of rate and equilibrium constants for heterolysis of the carbon-bromine bond in p-hydroxybenzyl bromide. J. Am. Chem. Soc. 2002,124, 6349-6356. 

There is on the other hand a great deal of evidence showing that the electrochemical reduction of 1,2-dihalides to olefins can occur via a concerted pathway, i.e., via a transition state (39) in which both carbon-halogen bonds are partially broken and the carbon-carbon double bond is partially formed. An important, indeed critical, point of evidence supporting the conclusion that reduction is concerted lies in the remarkable ease with which vicinal dihalides are reduced. For example, the half-wave potentials of ethyl bromide and 1,2-dibromoethane are -2.08 V and -1.52 V (vs. s.c.e.), respectively 15 >46) those of ethyl iodide and /J-chloroethyl iodide are -1.6 V and -0.9 V, respectively 47). These very large differences must reflect the lower energy of delocalized transition state 39 relative to the transition state for reduction of an alkyl monohalide. 

Similar reactions occur with all aliphatic halides and the rates of substitution are related to the degree of ionic character of the carbon-halogen bond. For preparation purposes, trityl bromide or propargyl bromide are more convenient than allyl bromide. The compounds obtained are listed in Table XI. They were obtained pure and characterized fully. Zr (allyl) 3Br and Zr(allyl)2Br2 are sufficiently soluble in toluene for polymerizations to be initially homogeneous. Their relative reactivities are listed in Table XI. In all cases hydrogen was used to reduce the molecular weight of the polymer formed. In this respect the polymer derived from Zr (allyl )3Br was more readily modified than that from Zr (allyl) 4, but in order to avoid... 

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