Alkenes, cyclic, addition

The stereochemistry of radical addition of hydrogen bromide to alkenes has been studied with both acyclic and cyclic alkenes. Anti addition is favored.This is contrary to what would be expected if the s[p- carbon of the radical were rapidly rotating or inverting with respect to the remainder of the molecule ... 

The simple diastereoselectivity is dependent on the structure of the alkene syn addition is normally observed with acyclic (Tfj-alkenes, whereas anti addition occurs with acyclic (Z)-alkenes and cyclic alkenes (Table 5). The isomerization of (Z)- to (Ej-alkenes prior to reaction can explain the prevalent or exclusive formation of the syn //-nitro acetate from either isomeric alkene. 

Dihydroxylation of alkenes, the addition of two OH groups, can occur cis when either 0s04 or Mn04 is used cyclic esters are implicated. 

Addition reactions of the 4-tm-butylcyclohexyl radical have been studied with a variety of alkenes and also in atom-abstraction reactions (see Section D.2.2.)3 58. While hydrogen or halide abstraction reactions yield preferentially the axial product, unselective addition occurs with terminally unsubstituted alkenes. In addition reactions of alkenes substituted by alkyl groups at the attacked olefinic center, the preference for axial attack decreases further and the equatorial addition product is formed. This influence of the size of the reaction partner on the selectivity is rationalized on the basis of the simultaneous presence of steric effects (hindering axial attack) and torsional effects (hindering equatorial attack), very similar to those discussed for nucleophilic addition reactions to cyclic ketones59. 

A great variety of sulfones are available from the addition of RSO2X to alkenes, alkynes, and allenes [6, 8, 18, 27-30] (Eqs. 11-16). In the case of cyclic alkenes, the addition is highly stereoselective in favor of the trans adduct (Eq. 11). 

A wide range of cyclopropane derivatives can be prepared by this carbene addition to alkenes.294 There is a general preference for the less congested exo product in reactions with cyclic alkenes. Initial addition of the... 

The Diels-Alder reaction is an addition reaction involving an alkene and a diene. Let s look at a representative Diels-Alder reaction involving 1,3-butadiene and an alkene (dienophile means diene-lover). The reaction involves a cyclic transition state. The product is usually a cyclic addition product. Study the representative reaction given below. Pay close attention to how the new bonds are formed in relation to the reactants. 

The previously accepted pathway consisted of P-H oxidative addition to Pt(0) to form 19 followed by coordination and insertion of the alkene in the Pt-P bond to form 20 and a final reductive elimination to furnish the product and regenerate the catalyst. Another possibility is the nucleophilic attack of phosphido complex 19 to the alkene ( Michael addition mechanism, as in anionic polymerisation) to generate the zwitterionic intermediate 21. This complex can yield the hydrophosphination product 11 via one of two complementary pathways. Carbanion attack at the cationic platinum hydride i.e. intramolecular hydrogen transfer) would yield the final phosphine complexed to Pt(0) that would be displaced by an equivalent of PHR R to furnish, after oxidative addition, starting complex 19. Alternatively, the anionic carbon atom in 21 could attack the platinum centre directly, forming the cyclic intermediate 22. From here Pt-P bond dissociation would generate 20, which would furnish the product after reductive elimination. 

The reaction of bromine with (E)-stilbene (47) to give meso-stilbene dibromide (48) as the major product (Eq. 10.21) is another example of an electrophilic addition reaction of alkenes. The addition of bromine to many alkenes is a stereospecific reaction that proceeds by anti addition to the double bond. However, the addition of bromine to 47 is not stereospecific because small amounts of dl-stilbene dibromide (49) are also formed in this reaction. The formation of wreso-stilbene dibromide presumably occurs via the nucleophilic attack of bromide on the intermediate cyclic bromonium ion, 50. The possible interconversion of 50 and the acyclic carbocation 51 (Eq. 10.22) is one possible way to account for the presence of dl-stilbene dibromide in the product. 

The classic analysis that implicates the formation of a cyclic halonium ion emphasizes the stereochemistry of the products. With unconjugated alkenes, anti addition is consistently seen. This requires backside attack of the nucleophile, indicating that the "frontside" has been blocked. For example, the addition of bromine to 3-t-butylcyclohexene gives trans-dibromo products (Eq. 10.15). 

The initial reaction of O3 and alkenes are known to be the formation of carbonyl compounds and carbonyl oxide via primary ozonide formed by cyclic addition of O3 to double bond. In the case of ethylene, the reaction formula can be represented as. 

The oxidation of simple internal alkenes is very slow. The clean selectiv oxidation of a terminal double bond in 40, even in the presence of an internt double bond, is possible under normal conditions[89,90]. The oxidation c cyclic alkenes is difficult, but can be carried out under selected condition Addition of strong mineral acids such as HCIO4, H2S04 and HBF4 accelerate the oxidation of cyclohexene and cyclopentene[48,91], A catalyst system 0 PdSO4-H3PM06W6Oii(j [92] or PdCF-CuCF m EtOH is used for the oxidatioi of cyclopentene and cyclohexene[93]. 

Stereochemical features in the oxidative addition and the elimination of /3-hydrogen of cyclic and acyclic alkenes are different. The insertion (palladation) is syn addition. The syn addition (carbopalladation) of R—Pd—X to an acyclic alkene is followed by the syn elimination of 3-hydrogen to give the trans-a ksne 6, because free rotation of 5 is possible with the acyclic alkene. On the other hand, no rotation of the intermediate 7 is possible with a cyclic alkene and the syn elimination of /3-hydrogen gives the allylic compound 8 rather than a substituted alkene. 

Addition of a hydroxy group to alkynes to form enol ethers is possible with Pd(II). Enol ether formation and its hydrolysis mean the hydration of alkynes to ketones. The 5-hydroxyalkyne 249 was converted into the cyclic enol ether 250[124], Stereoselective enol ether formation was applied to the synthesis of prostacyclin[131]. Treatment of the 4-alkynol 251 with a stoichiometric amount of PdCl2, followed by hydrogenolysis with formic acid, gives the cyclic enol ether 253. Alkoxypalladation to give 252 is trans addition, because the Z E ratio of the alkene 253 was 33 1. 

Addition of nucleophiles to both activated and unactivated alkenes is catalyzed by Pd(II). Addition of alcohols or AcOH to alkenes bearing EWGs is catalyzed by PdCl2(PhCN)2 to give the corresponding ethers and esters. The addition of an alcohol to the cyclic acetal of acrolein 82 to give the ether 83 is also possible with the same catalyst[64]. Amines add to the vinylic ether 84 to give 85, but not to simple alkenes[65]. 

Overall the reaction leads to addition of two hydroxyl groups to the double bond and IS referred to as hydroxylation Both oxygens of the diol come from osmium tetraox ide via the cyclic osmate ester The reaction of OSO4 with the alkene is a syn addition and the conversion of the cyclic osmate to the diol involves cleavage of the bonds between oxygen and osmium Thus both hydroxyl groups of the diol become attached to the same face of the double bond syn hydroxylation of the alkene is observed... 

The bond highlighted m yellow is the peptide bond ) Pencyclic reaction (Section 10 12) A reaction that proceeds through a cyclic transition state Period (Section 1 1) A honzontal row of the penodic table Peroxide (Section 6 8) A compound of the type ROOR Peroxide effect (Section 6 8) Reversal of regioselectivity oh served m the addition of hydrogen bromide to alkenes brought about by the presence of peroxides m the reaction mixture... 

Intramolecular alkylnitrene addition to an alkenic moiety situated S,e to the electron deficient center has been utilized for the preparation of bi- and tri-cyclic aziridines (Scheme 11) (68JA1650). Oxidation of the primary alkylamine can be effected cleanly with NCS, LTA or mercury(II) oxide. 

The preferred stereochemistry of addition to cyclic alkenes is anti The additions are not as highly stereoselective as hydrogen bromide addition, however. 

The above cycloaddition process consists of two separate [3-1-2] cycloaddition steps and represents a 1,3-2,4 addition of a multiple bond system to a hetero-1,3-diene [7S7]. The structure ot the azomethine imine intermediate has been proved unequivocally by X-ray analysis [195] Ethylene [194], acetylene [/iS2] . many alkyl- and aryl- as well sgemmal dialkyl- and diaryl-substituted alkenes [196,197, 198, 199], dienes [200], and alkynes [182, 201], certain cyclic alkenes [198, 199,... 

Addition of halogens (Sections 6.14-6.16) Bromine and chlorine add to alkenes to form vicinal dihalides. A cyclic halonium ion is an intermediate. Stereospecific anti addition is observed. 

We saw in the previous section that when Br2 reacts with an alkene, th cyclic bromonium ion intermediate reacts with the only nucleophile presen Br- ion. If the reaction is carried out in the presence of an additional nuclec phile, however, the intermediate bromonium ion can be intercepted by th added nucleophile and diverted to a different product. In the presence of watei for instance, water competes with Br- ion as nucleophile and reacts with th bromonium ion intermediate to yield a broinohydrin. The net effect is additioi of HO-Br to the alkene by the pathway shown in Figure 7.1. 

Acid-catalyzed epoxide opening takes place by protonation of the epoxide to increase its reactivity, followed by nucleophilic addition of water. This nucleophilic addition is analogous to the final step of alkene bromination, in which a cyclic bromonium ion is opened by a nucleophile (Section 7.2). That is,... 

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