2.3- Dibromopentane, stereoisomers

These two compounds are constitutional isomers (skeletal isomers), (b) The two dibromopentane stereoisomers are diastereomers. 

A third type of stereoisomer occurs when a molecule with several stere-ogenic centers contains an internal plane of symmetry. This usually happens when two of the stereogenic centers are attached to the same four different valences. For example, 2,4-dibromopentane has two stereogenic centers and thus four stereoisomers, 5-8. It is easily seen that 6 and 7 are enantiomers, 5 and 6 are diastereomers, and so on. 

In this case, the atoms of A and B do not align, making A and B nonsuperimposable mirror images—enantiomers. A and B are two of the four possible stereoisomers for 2,3-dibromopentane. 

Switching the positions of H and Br (or any two groups) on one stereogenic center of either A or B forms a new stereoisomer (labeled C in this example), which is different from both A and B. Then draw the mirror image of C, labeled D. C and D are nonsuperimposable mirror images—enantiomers. We have now drawn four stereoisomers for 2,3-dibromopentane, the maximum number possible. 

There are four stereoisomers for 2,3-dibromopentane enantiomers A and B, and enantiomers C and D. What is the relationship between two stereoisomers like A and C A and C represent the second broad class of stereoisomers, called diastereomers. Diastereomers are stereoisomers that are not mirror images of each other. A and B are diastereomers of C and D, and vice versa. Figure 5.8 summarizes the relationships between the stereoisomers of 2,3-dibromopentane. 

Whereas 2,3-dibromopentane has two stereogenic centers and the maximum of four stereoisomers, 2 -dibromobutane has two stereogenic centers but fewer than the maximum number of stereoisomers. 

When a compound has more than one stereogenic center, the and 5 configuration must be assigned to each of them. In the. stereoisomer of 2,3-dibromopentane drawn here. C2 has the S configuration and C3 has the R, so the complete name of the compound is (25,3I )-2.3-dibromopentane. 

Reaction of styrene with the commercially available 2,4-dibromopentan-3-one (52) in the presence of a stoichiometric amount of Fc2(CO)9 gives a 65% yield of cyclopentanone (53) as a mixture of two dia-stereoisomers (equation 44). The cycloadduct is formed in a stepwise fashion via electrophilic attack of the incipient (oxyallyl)Fe complex (54) on the alkene to form the zwitterionic species (55). Ring closure of this intermediate affords the cyclopentanone product (equation 45). In light of this mechanism, it is not surprising tl.at the reaction is highly chemoselective, i.e. only electron-rich alkenes will participate in the cycloaddition. One is also limited to alkyl-substituted oxyallyl systems since both a,a -dibromo- and a,a,a, a -tetrabromo-acetone fail to yield any cycloadduct. Fe2(CO)9 is the most efficient reagent, although Fe(CO)s can also be used. The dibromo ketones are either commercially available or easily prepared by simple procedures. ... 

For 2,3-dibromopentane we should not expect more than four stereoisomers (2 = 4). Our next task is to write three-dimensional bond-line formulas for the possible stereoisomers. 

Since structures 1 and 2 differ only in the arrangement of their atoms in space, they represent stereoisomers. Structures 1 and 2 are also mirror images of each other thus 1 and 2 represent a pair of enantiomers. Structures 3 and 4 correspond to another pair of enantiomers. Structures 1-4 are all different, so there are, in total, four stereoisomers of 2,3-dibromopentane. 

Last, we check that none of these formulas is identical to another by testing the superposability of each one with the others. We should not expect any to be identical because none of the formulas has an internal plane of symmetry. The case would have been different for 2,4-dibromopentane, however, in which case there would have been one meso stereoisomer (a type of stereoisomer that we shall study in the next section). 

For 2,3-dibromopentane we should not expect more than four stereoisomers (2 = 4). 

Build a model of each of the four stoeoisomo-s of 2,3-dibromopentane. Why does 2,3-dibromopentane have four stereoisomers, wha-eas 2,3-dibromobutane has only three ... 

The two stereoisomers of 3-methyl-2,4-dibromopentane shown below were cyclized by treatment with zinc in 1-propanol-water. The products were analyzed for the ratio of cis- to trans- nvae hy -cyclopropanes the results are as follows ... 

The treatment of the ethylene ketal of acetylfuran 24 with 2,4-dibromopentan-3-one 25 in the presence of zinc-copper couple gave oxabicyclo[3.2.1]octane 26 as a single dia-stereoisomer in 53% yield (Scheme 19.12). Either a single configuration of the oxyallylic cation derived from 25 is formed (i.e., W-shaped) or the cation with that configuration... 

How many chiral centers are there in 2,3-dibromopentane How many different stereoisomers are there ... 

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