Cyclopentene bromine addition

There are only a few studies of the bromination products of congested alkenes. Such products generally consist of the corresponding allylic bromo-derivatives, which are consistent with /5-proton elimination by the counter-ion from the bromonium ion. For example, the ionic bromination of octamethyl-cyclopentene in CC14 leads exclusively to l,2-di(bromomethyl)hexamethyl-cyclopentene as in Scheme 12 (Mayr et al, 1986). Bromine addition (30) to... 

Most studies of bromine addition to alkenes have presumed that the intermediate proceeds to product and does not revert to alkene and bromine. Brown and co-workers determined that bromonium ions generated from the solvolysis of the fra s-2-bromo-l-brosylates of wclohexene or cyclopentene could react with added Br to produce Br2- Furthermore, erythro-2-hTomo-l, 2-diphenylethanol was foimd to react with anhydrous HBr (in 1,2-dichloroethane or chloroform) to produce both frans-stilbene and meso-l,2-dibromo-l,2-diphenylethane. The reaction of the erythro diastereomer can be explained by a mechanism involving anchimeric assistance in departure of water, which leads to a bromonium ion that reverts to the stilbene, as shown in Figure 9.10. ... 

Acid-Catalyzed Hydration of 2-Methylpropene 227 Hydroboration of 1-Methylcyclopentene 233 Oxidation of an Organoborane 235 Bromine Addition to Cyclopentene 237 Epoxidation of Bicyclo[2.2.1]-2-heptene 240 Free-Radical Addition of Hydrogen Bromide to 1-Butene 243... 

When the halogenation reaction is carried out on a cycloalkene, such as cyclopentene, only the trews stereoisomer of the dihalide addition product is formed rather than the mixture of cis and trans isomers that might have been expected if a planar carbocation intermediate were involved. We say that the reaction occurs with anti stereochemistry, meaning that the two bromine atoms come from opposite faces of the double bond—one from the top face and one from the bottom face. 

How does the formation of a bromonium ion account for the observed anti stereochemistry of addition to cyclopentene If a bromonium ion is formed as an intermediate, we can imagine that the large bromine atom might "shield" one side of the molecule. Reaction with Br ion in the second step could then occur only from the opposite, unshielded side to give trans product. 

Cyclopentadiene adducts (mono-, tetra- and hexa-adducts) of were stabilized against retro-reaction by selective hydrogenation and bromination of the pendant groups [21]. Utilization of Adam s catalyst and dilute bromine solutions exclusively leads to an addition to the cyclopentene double bonds, because itself is inert towards these reagents. The increased stability of the reduced cycloadducts can be demonstrated by mass spectrometry [21]. 

LiBr and in the presence of cyclopentene as a scavenger olefin. The kinetics, determined by monitoring the formation of strong acids (TfOH or HBr), show that the rate of solvolysis of 65 is dependent on [Br-] (at a constant ionic strength). In the presence of Br-, the products are trans- 1,2-dibromides and bromo-solvates of both cyclohexene and cyclopentene. The cyclopentenyl products have been shown to arise from the electrophilic addition of Br2/Br3 to cyclopentene, while trans-l, 2-dibromocyciohexane 67 is formed by Br- capture of the bromonium ion 66 on carbon. The Br2 required for bromination of cyclopentene results from attack by Br- on the bromonium ion 66 on Br+. On the basis of the ratio of the cyclopentyl products to 67, Br- capture of the solvolytically produced bromonium ion 66 (by attack on Br+) is 4-5 times more prevalent than attack on carbon in AcOH, and ca 25 times more preferred in MeOH123. 

The addition of bromine to cyclopentene is a stereospecific anti addition. 

Stereochemistry of Halohydrin Formation Because the mechanism involves a halonium ion, the stereochemistry of addition is anti, as in halogenation. For example, the addition of bromine water to cyclopentene gives fran.v-2-bromocyclopentanol, the product of anti addition across the double bond. 

Propose a mechanism for the addition of bromine water to cyclopentene, being careful to show why the trans product results and how both enantiomers are formed. 

Make a model of cyclopentene and the product of trans addition of bromine. 

A general method for the cyclization of an unactivated 1,1-disubs-tituted alkene to the corresponding cyclopentene has been described. Bromination followed by the addition of KHMDS in the same pot gave the vinyl bromide intermediate, which reacted further in situ to give the alkylidene carbene. This is then inserted in a 1,5-fashion into a C-H bond to yield the corresponding cyclopentene (eq 44). KHMDS proved to be superior in this process as compared to LiHMDS and NaHMDS. 

It is prepared by addition of bromine to cyclopentene at -20° followed by fractional distillation. The H NMR... 

The addition of bromine to cyclopentene, for example, produces trans-, 2-dibromocyclopentane, not ar-l,2-dibromocyclopentane. 

---