Addition of X-Y Reagents to Alkenes

E. Block, A. L. Schwan, Electrophilic Addition of X-Y Reagents to Alkenes and Alkynes, in Comprehensive Organic Synthesis (B. M. Trost, I. Fleming, Eds.), Vol. 4, 329, Pergamon Press, Oxford, 1991. 

Electrophilic addition of the halogens and related X—Y reagents to alkenes and alkynes has been a standard procedure since the beginning of modem organic chemistry.1 Anti electrophilic bromination of such simple compounds as cyclohexene and ( )- and (Z)-2-butene, and variants of this reaction when water or methanol are solvents (formation of halohydrin or their methyl ethers, respectively), are frequently employed as prototype examples of stereospecific reactions in elementary courses in organic chemistry. A simple test for unsaturation involves addition of a dilute solution of bromine in CCU to the... 

The most important transformations of alkenes and alkynes are addition reactions. That is, a reagent we might write in a general way as X-Y adds to the multiple bond of the unsaturated reactant to yield a saturated product. Alkenes and alkynes react similarly, but we ll look only at alkenes because they re more common. 

There are many reagents that add to alkenes only by radical-chain mechanisms. A number of these are listed in Table 10-3. They have in common a relatively weak bond, X—Y, that can be cleaved homolytically either by light or by chemical initiators such as peroxides. In the propagation steps, the radical that attacks the double bond does so to produce the more stable carbon radical. For addition to simple alkenes and alkynes, the more stable carbon radical is the one with the fewest hydrogens or the most alkyl groups at the radical center. 

Kinetic studies of the reaction of oxo(salen)chromium(V) triflate with alkynes suggest that a metallaoxetane or a chromium carbene acts as an intermediate in the reaction.326 The bridging methylene complexes, (THF)nClxMo(0) 2( X-CH2)2. are prepared by treatment of MoCly(THF)2 (x = 1, y = 3, z = 2 X = 2, y = 4, z s= 0) with two equivalents of MeLi,322 both complexes act as chemoselective reagents for alkene addition to a carbonyl compound. 328... 

The addition of reagents X-Y to carbon-carbon ir-bonds may also proceed via a concerted mechanism in which each new a-bond is formed simultaneously on the same face of the ir-bond. The stereochemistry of such reactions is necessarily syn. For example, the reaction of potassium permanganate, which is purple, with an alkene such as cyclohexene proceeds via si/H-addition of permanganate ion across the ir-bond to give 39, which is colorless. Subsequent decomposition of 39 gives a ds-l,2-diol and manganese dioxide, the brown precipitate that is observed as the other product of the reaction (Eq. 10.19). This decoloration of potassium permanganate by alkenes forms the basis of the Baeyer qualitative test for the presence of carbon-carbon ir-bonds (Sec. 25.8B). 

Although reagents can always add to an alkene from either side, sometimes one side of the double bond is more sterically hindered, so an unequal mixture of addition products results. For example, when X is treated with H2 in the presence of a Pd catalyst, 80% of the product mixture contains the cis isomer Y and only 20% is the trans isomer Z. Thus, addition of H2 occurs predominantly on the side opposite to the bulky ferf-butyl group, resulting in a new equatorial C- H bond. Keeping this in mind, what is the major epoxidation product formed from X under each of the following reaction conditions (a) mCPBA or (b) Br2, H2O followed by NaH ... 

The two possible modes of addition of a reagent X—Y to an alkene are shown in (a). In syn addition, both groups add to the same side or face of the molecule. In anti addition, the groups add to the opposite faces of the molecule. The consequences of syn and anti additions are shown in (b). Geometric isomers can result from the addition of a reagent X—Y to the double bond of a cycloalkene. Syn addition produces a cis product, whereas anti addition produces a trans product. 

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