Transition states allyl bromide

Diterpenoids related to lambertianic acid were prepared by intramolecular cyclization of either an alkene or an alkyne with a furan ring <2005RJ01145>. On heating amine 101 with allyl bromide, the intermediate ammonium ion 102 was formed which then underwent [4+2] cycloadditions in situ to give the spiroazonium bromides 103 and 104 (Scheme 13). These isomers arose from either endo- or co-transition states. The analogous reaction was also carried out with the same amine 101 and propargyl bromide. The products 105 and 106 contain an additional double bond and were isolated in 58% yield. The product ratios of 103 104 and 105 106 were not presented. 

For the addition of ethylene, EtOAc as solvent was particularly advantageous and gave 418 in 60% yield (Scheme 6.86). The monosubstituted ethylenes 1-hexene, vinylcyclohexane, allyltrimethylsilane, allyl alcohol, ethyl vinyl ether, vinyl acetate and N-vinyl-2-pyrrolidone furnished [2 + 2]-cycloadducts of the type 419 in yields of 54—100%. Mixtures of [2 + 2]-cycloadducts of the types 419 and 420 were formed with vinylcyclopropane, styrene and derivatives substituted at the phenyl group, acrylonitrile, methyl acrylate and phenyl vinyl thioether (yields of 56-76%), in which the diastereomers 419 predominated up to a ratio of 2.5 1 except in the case of the styrenes, where this ratio was 1 1. The Hammett p value for the addition of the styrenes to 417 turned out to be -0.54, suggesting that there is little charge separation in the transition state [155]. In the case of 6, the p value was determined as +0.79 (see Section 6.3.1) and indicates a slight polarization in the opposite direction. This astounding variety of substrates for 417 is contrasted by only a few monosubstituted ethylenes whose addition products with 417 could not be observed or were formed in only small amounts phenyl vinyl ether, vinyl bromide, (perfluorobutyl)-ethylene, phenyl vinyl sulfoxide and sulfone, methyl vinyl ketone and the vinylpyri-dines. 

Interestingly, the Zr-promoted crotylzincation of 5-decyne occurred without allylic transposition (equation 75), presumably via a four-centered transition state, unlike the uncatalyzed addition of crotylzinc bromide to alkynylsilanes which proceeded with complete S 2 regioselectivity108. 

A natural bond orbital-based CI/MP through-space/bond interaction analysis of the Sn2 reaction between allyl bromide and ammonia17 showed that allyl bromide reacted faster than propyl bromide because the a - n and n — a interactions stabilize the allyl bromide transition state equally. 

Zirconium imido complexes have been used to carry out S 2 reactions of allylic chloride, bromide, iodide, and alkyl, aryl, and trimethylsilyl ethers in high yields at room temperature.12 The syn stereochemistry, an inverse secondary (k /k Oy = 0.88 obtained using the ( )-l-(r-butyldimethylsilyloxy)-3-deuterioprop-2-ene and the rate expression led the authors to suggest the reactions occurred via the mechanism in Scheme 4 with transition state (9). 

The gas-phase elimination kinetics of ethyl oxamate, ethyl N,N-dimethyloxamate and ethyl oxanilate have been determined in a static reactor system, seasoned with allyl bromide and in the presence of a free radical inhibitor.12 These reactions are homogeneous, unimolecular and appear to proceed through moderately polar cyclic transition states. 

The kinetics of the gas-phase elimination of 3-hydroxy-3-methylbutan-2-one have been investigated in a static system, seasoned with allyl bromide, and in the presence of the free chain radical inhibitor toluene.14 The reaction was found to be homogeneous, unimolecular and to follow a first-order rate law. The products of elimination are acetone and acetaldehyde. Theoretical estimations suggest a molecular mechanism involving a concerted non-synchronous four-membered cyclic transition state process. 

A /3-carboxyl group on the aldehyde also influences the diastereoselection. The high diastereoselectivity observed with the y-hydroxy lactone can be rationalized by a chelated transition state with the carboxylic acid group (Scheme 41).174 2-Oxocarboxylic acids also undergo diastereoselective allylation with cinnamyl bromide to provide the corresponding a-hydroxy acids as a single diastereomer (Equation (19)).1... 

Several indium-mediated intramolecular carbonyl allylation reactions have been investigated, and it has been found that these reactions provide an easy access to a variety of cyclic compounds. The intramolecular cyclization of 49a-c mediated by indium in aqueous media proceeds smoothly to afford carbocyclic systems containing y-hydroxy-Q -methylene esters 50a-c, which either spontaneously or readily cyclize to give fused o -methylene-y-butyrolactones 51a-c (Scheme 52). The same cyclization of 49d is too slow to compete with the side-reaction, in which the bromide is substituted by a hydroxy group. The ring junction stereochemistry of fused lactones 51 has been found to be cis in all cases. Of the two possible transition states, the one leading to the m-fused compounds is preferred, because the chair-chair conformation is favored over the chair-boat conformation.209... 

Allylic delocalization in the SN2 transition state. The transition state for the Sn2 reaction of allyl bromide with a nucleophile is stabilized by conjugation of the double bond with the p orbital that is momentarily present on the reacting carbon atom. The resulting overlap lowers the energy of the transition state, increasing the reaction rate. 

Here you see a typical Sn2 reaction of allyl bromide. We have drawn the transition state for this reaction. This is not because we want to encourage you to do this for all Sn2 reactions but so that we can explain the role of the allyl system. Allyl compounds react rapidly by the 8 2 mechanism because the double bond can stabilize the transition state by conjugation. 

---