Stereochemistry, ionic addition

The stereochemistry of acid-catalyzed hydration is very similar to the stereochemistry of ionic addition of HX (this should make sense, as we have already seen that the mechanisms for each of these reactions are identical). If only one stereocenter is formed, then we expect a pair of enantiomers (racemic mixture), regardless of whether the reaction was anti or syn. You will probably not see an example where two new stereocenters are formed, becanse the stereochemical ontcome in such a case is complex and is beyond the scope of our conversation. 

Later on, product distribution studies15 of the ionic addition of chlorine to conjugated dienes, and in particular to cyclopentadiene, 1,3-cyclohexadiene, cis,cis-, trans,trans-and c ,fraws-2,4-hexadienes, and cis- and trans-1,3-pentadienes have supplied the first stereochemical data, showing that the stereochemistry of 1,4-addition is predominantly syn, although to an extent smaller than that of bromine addition. Moreover, the 1,2-addition is generally non stereoselective, except for the addition to the 3,4-bond of cis-and trans-1,3-pentadienes where the attack is 89-95% anti. Finally, appreciable amounts of cis- 1,2-dichlorides were obtained from the two cyclic dienes, whereas 2,4-hexadienes showed a preference for anti 1,2-addition, at least in the less polar solvents (carbon tetrachloride and pentane). On the basis of all these results the mechanism shown in equation 29 was proposed. 

The stereochemistry of the ionic addition was investigated in the reaction shown in... 

One further point. We have encountered the two-step addition of unsym-metrical reagents in which the first step is attack by positive halogen formation of halohydrins (Sec. 6.14), and ionic addition of IN3 and BrN3 (Problem 7, p. 247). The orientation is what would be expected if a carbonium ion were the intermediate. Propylene chlorohydrin, for example, is CH3CHOHCH2CI IN3 adds to terminal alkenes to yield RCH(N3)CH2l. Yet the exclusively anti stereochemistry... 

Nevertheless, in many cases the stereochemistry of the products indicates a concerted reaction. According to Enders et al. the stereochemistry of the final products can be explained equally plausibly by means of a concerted cycloaddition or a sequential series of ionic addition steps, and the authors leave the question of the actual mechanism open. 

Since the ionic addition of hydrogen bromide and the acid-catalyzed addition of water to an alkene proceed via planar carbocations, the formation of products derived from rearranged carbocations is relatively common. In contrast, the bromi-nation of alkenes, which is illustrated by the addition of bromine to cyclopentene (Eq. 10.18), occurs via a cyclic bromonium ion 38 related to 33, so skeletal rearrangements of such cations are not generally observed. Moreover, the intervention of this cyclic ion dictates that the stereochemistry of the addition is completely in the anti sense, as shown. The reddish color of bromine is discharged upon addition to an alkene, making this reaction a useful qualitative test for unsaturation (Sec. 25.8A). 

For a review of some earlier studies on the stereochemistry of photoprotonation, see Marshall, J.A., Photosensitized ionic additions to cyclohexenes, Acc. Chem. Res., 2,33, 1969. 

The effects of increasing the concentration of initiator (i.e., increased conversion, decreased M , and broader PDi) and of reducing the reaction temperature (i.e., decreased conversion, increased M , and narrower PDi) for the polymerizations in ambient-temperature ionic liquids are the same as observed in conventional solvents. May et al. have reported similar results and in addition used NMR to investigate the stereochemistry of the PMMA produced in [BMIM][PFgj. They found that the stereochemistry was almost identical to that for PMMA produced by free radical polymerization in conventional solvents [43]. The homopolymerization and copolymerization of several other monomers were also reported. Similarly to the findings of Noda and Watanabe, the polymer was in many cases not soluble in the ionic liquid and thus phase-separated [43, 44]. 

In agreement with the involvement of ionic intermediates for electrophilic halogenation of alkenes, an important role is also exerted by the solvent. Not only the reaction rate is strongly solvent-dependent, but also the stereochemical course of the addition process may be affected by the polarity of the medium. Solvent properties determine the reaction rate the overall kinetic order the nature of the products the stereochemistry of the products... 

Catalytic hydrogenation delivers hydrogen to one face of the alkene the consequence is syn addition of hydrogen. This is a departure from our usual observations with ionic mechanisms, where the groups typically add to a double bond with anti stereochemistry (see Section 8.1.2). 

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