Anti addition reactions

This scheme represents an alkyne-bromine complex as an intermediate in all alkyne brominations. This is analogous to the case of alkenes. The complex may dissociate to a inyl cation when the cation is sufficiently stable, as is the case when there is an aryl substituent. It may collapse to a bridged bromonium ion or undergo reaction with a nucleophile. The latta is the dominant reaction for alkyl-substituted alkynes and leads to stereospecific anti addition. Reactions proceeding through vinyl cations are expected to be nonstereospecific. 

Anti stereochemistry (Section 7.2) The opposite of syn. An anti addition reaction is one in which the two ends of the double bond are attacked from different sides. An anti elimination reaction is one in which the two groups leave from opposite sides of the molecule. 

This is followed by an anti addition reaction in which water is added to the new double bond, but in the reverse sense. The hydrogen retained throughout the process is shown with an asterisk. Note that we... 

In the first step, the pyrimidine is hydrogenated at the double bond by dihydrouracil dehydrogenase (EC 1.3.1.2) to a dihydropyrimidine (Scheme 1.6.2). The enzymes obtained from rat [4] and human [5] liver have been purified and characterized. They were later subjected to molecular cloning [6], The hydrogens are added to the double bond at the Si face of C5 and C6 in an anti-addition reaction. This was deduced from NMR spectra recorded from the isolated degradation products after administration of [5-2H]- and [6-2H]uracil and 2H20 to a mammalian enzyme system [7]. 

In summary, it appears that bromination usually involves a complex that collapses to an ion pair intermediate. The ionization generates charge separation and is assisted by solvent, acids, or a second molecule of bromine. The cation can be a P-carbocation, as in the case of styrenes, or a bromonium ion. Reactions that proceed through bromonium ions are stereospecific anti additions. Reactions that proceed through open carbocations can be syn selective or nonstereospecific. 

Answer Draw a three-dimensional representation of the Fischer projection and rotate about the C-2 to C-3 bond to obtain a staggered conformation placing the deuterium and the hydroxyl group in an anti arrangement assuming that an anti addition reaction occurred. In this conformation, the two —CO H groups are arranged as if the addition reaction occurred by an anti periplanar transition state. Thus, the addition is indeed anti. 

The two modes of addition and the associated stereochemistry resemble other addition reactions we studied earlier. The suprafacial addition is a concerted syn addition to one of the ti systems. The antarafacial addition corresponds to a concerted anti addition. Although anti addition reactions are common in the chemistry of alkenes, the two groups that add to the alkene are not bonded to each other in the transition state. In cycloaddition reactions, both atoms of the molecule that bond to the terminal atoms of the second molecule are also connected to each other. Thus, only if the number of atoms in each of the two molecules is quite large can one molecule add to the other in an antarafacial process. 

It should be noted that formation of trans-product can be achieved in an anti-addition reaction through the outer-sphere mechanism. Theoretical studies have demonstrated that syn-addition and anti-addition reactions may start from the same 7i-complex, and direction of the multiple bond activation depends on the polarity of solvent [17, 18]. Relative reactivity in the inner-sphere and outer-sphere mechanisms contributes to the overall -/Z- selectivity of the addition reaction to alkynes (stereoselectivity issue). In some cases it is possible to switch the direction of C-Het bond formation by finding a suitable ligand [19]. In case of alkenes syn-addition and a f -addition processes do not necessarily result in different stereochemistry (unrestricted rotation around the single C-C bond in the product). Occurrence of these mechanisms for the N [20, 21], P [22, 23], O [24-26], S, Se [27, 28] heteroatom groups and application of different metal catalysts are discussed in detail in the other chapters of this book. Stereochemical pathways of nucleometallation and development of enantioselective catalytic procedures were reviewed [29]. In this chapter we focus our attention on the mechanism of irmer-sphere insertion reaction involving double and triple carbon-carbon bonds. 

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