Alkene also addition

There have also been relatively few mechanistic studies of the addition of iodine. One significant feature of iodination is that it is easily reversible, even in the presence of excess alkene. The addition is stereospecifically anti, but it is not entirely clear whether a polar or a radical mechanism is involved. ... 

The addition reactions discussed in Sections 4.1.1 and 4.1.2 are initiated by the interaction of a proton with the alkene. Electron density is drawn toward the proton and this causes nucleophilic attack on the double bond. The role of the electrophile can also be played by metal cations, and the mercuric ion is the electrophile in several synthetically valuable procedures.13 The most commonly used reagent is mercuric acetate, but the trifluoroacetate, trifluoromethanesulfonate, or nitrate salts are more reactive and preferable in some applications. A general mechanism depicts a mercurinium ion as an intermediate.14 Such species can be detected by physical measurements when alkenes react with mercuric ions in nonnucleophilic solvents.15 The cation may be predominantly bridged or open, depending on the structure of the particular alkene. The addition is completed by attack of a nucleophile at the more-substituted carbon. The nucleophilic capture is usually the rate- and product-controlling step.13,16... 

Dipolar addition to alkenes also occurs with species other than ozone, often to give products much more stable than the labile molozonides (54), e.g. addition of azides (61) to give dihydrotriazoles (62) ... 

The orientation of addition of an unsymmetrical adduct, HY or XY, to an unsymmetrically substituted alkene will be defined by the preferential formation of the more stabilised carbanion, as seen above (cf. preferential formation of the more stabilised carbocation in electrophilic addition, p. 184). There is little evidence available about stereoselectivity in such nucleophilic additions to acyclic alkenes. Nucleophilic addition also occurs with suitable alkynes, generally more readily than with the corresponding alkenes. 

In 1994, Thomas reported146,147 that alkenes also underwent an addition reaction with vinylketene complexes that differed crucially in the loss of the ketene carbonyl fragment. Complexes 252.a-252.d were isolated as yellow crystalline solids. Clearly this suggests that the process occurs by a mechanism different from the alkyne insertion, and this will be discussed... 

As with alkenes, the addition of silyl radicals to a carbon-carbon triple bond (Reaction 5.16) is also the key step in the hydrosilylation of alkynes [9,10]. 

When alkenes like 242a-d are treated with metallic lithium, not only the reduction leading to doubly lithium bridged compounds of type 243 can occur, but also addition reactions under the formation of 1,4-dilithiobutanes of type 244". The impact of various substituents on this well-known phenomenon and further reaction steps was examined intensively by Maercker and coworkers (Scheme 85). On this basis, an insight into the mechanisms of controlling these highly sensitive reactions can be gained. 

The addition of singlet oxygen to alkenes also gives dioxetanes. A number of mechanisms have been proposed and the literature abounds with theoretical and experimental results supporting one or more possible intermediates (a) 1,4-diradicals, (b) 1,4-dipolar, (c) perepoxides, or (d) concerted (Scheme 95). Both ab initio and semi-empirical calculations have been done and to date the controversy is still not resolved. These mechanisms have been reviewed extensively (77AHC(21)437, 80JA439, 81MI51500 and references therein) and will not be discussed here, except to point out that any one mechanism does not satisfactorily account for the stereospecificity, solvent effects, isotope effects and trapped intermediates observed. The reaction is undoubtedly substrate-dependent and what holds for one system does not always hold for another. 

Of the isomeric aldehydes indicated in Eq. (7.1), the linear aldehyde corresponding to anti-Markovnikov addition is always the main product. The isomeric branched aldehyde may arise from an alternative alkene insertion step to produce the [RCH(Me)Co(CO)3] or [RCH(Me)Rh(CO)(PPh3)2] complexes, which are isomeric to 2 and 8, respectively. Alternatively, hydroformylation of isomerized internal alkenes also give branched aldehydes. The ratio of the linear and branched aldehydes, called linearity, may be affected by reaction conditions, and it strongly depends on the catalyst used. Unmodified cobalt and rhodium carbonyls yield about 3-5 1 mixtures of the normal and iso products. 

Polyhalogenated alkenes also undergo HBr addition (equations 84-87).61,63,131 Even tetrahaloethylenes undergo HBr addition in a continuous flow reactor in the presence of a catalyst.132... 

The addition of hydrazoic acid to numerous acyclic and cyclic alkenes also proceeds well in the presence of titanium tetrachloride or aluminum trichloride.262 Dichloromethane or chloroform are ideal solvents for these catalysts. The addition is regiospecific and tolerates the presence of primary and secondary alcohols, as well as esters (equation 184). 

Thiiranes can be formed directly and stereospecifically from 1,2-disubstituted alkenes by addition of trimethylsilylsulfenyl bromide, formed at -78 C from reaction of bromine with bis(trimethylsilyl) sulfide (Scheme 7).12 A two-step synthesis of thiiranes can be achieved by addition of succinimide-A/-sulfe-nyl chloride or phthalimide-A -sulfenyl chloride to alkenes followed by lithium aluminum hydride cleavage of the adducts (Scheme 8).13 Thiaheterocycles can also be formed by intramolecular electrophilic addition of sulfenyl chlorides to alkenes, e.g. as seen in Schemes 914 and 10.13 Related examples involving sulfur dichloride are shown in Schemes 1116 and 12.17 In the former case addition of sulfur dichloride to 1,5-cyclooctadiene affords a bicyclic dichloro sulfide via regio- and stereo-specific intramolecular addition of an intermediate sulfenyl chloride. Removal of chlorine by lithium aluminum hydride reduction affords 9-thiabicyclo[3.3.1]nonane, which can be further transformed into bicyclo[3.3.0]oct-1,5-ene.16... 

The cycloaddition of a variety of sulfur heterocycles to alkenes has also been reported. Dibromomaleic anhydride undergoes [ 2 + 2] cycloaddition to thiophen295 and to benzo[b]thiophen.296 The photoaddition of benzo-[(>]thiophen 1,1-dioxides to alkenes also affords cyclobutane derivatives.297 Of particular interest is the addition of 3-acetoxybenzo[6]thiophen (350) to cyclohexene to give the adduct (351), which on treatment with base, undergoes a retro-aldol ring opening to benzo[6]thiepinone (352).298 A mixture of stereoisomeric cyclobutane derivatives were obtained on irradiation of 2,6-diphenyl-4//-thiopyran-4-one 1,1-dioxide in cyclohexene.299 The [ 2 + 2] cycloaddition of sulfolen to maleic anhydride and to maleimide has been reported.300... 

Alkenes also undergo a variety of other addition reactions in which a reagent is added across the double bond. Hydration and hydrohalogenation are classic examples. 

The insertion reaction is stereospecific and syn. Moreover the /S-hydride elimination is also syn. For acyclic alkenes there is free rotation in the organopalladium intermediate so that the more stable /ra .v-alkene is formed. Electron-withdrawing groups in the alkene also increase the rate of the insertion reaction and give higher yields generally, but the reaction is limited to relatively sterically unhindered alkenes. In general, polar solvents such as DMF or acetonitrile are most commonly used. There are several common additives which aid in the reaction. These include lithium or tetraalkylammonium chlorides and bromide, silver salts, or cuprous iodide, but exactly how they function is unknown at present. 

All one-step additions to C=C double bonds are mechanistically required to take place cis-selectively (ds > 99 1) (Sections 3.3.1-3.3.3). In addition, the heterogeneously catalyzed hydrogenation of alkenes also usually takes place with very high cw-selectivity, in spite of its being a multistep reaction (Section 3.3.4). 

The hydroboration/oxidation/hydrolysis of trisubstituted alkenes also takes place as a cis-addition. The reaction equation from Figure 3.25 shows this using 1-methylcyclo-hexene as... 

Earlier the chapter we discussed how to make single diastereoisomers by stereospecific additions to double bonds of fixed geometry. But if the alkene also contains a chiral centre there will be a stereoselective aspect to its reactions too its faces will be diastereotopic, and there will be two possible outcomes even if the reaction is fully stereo specific. Here is an example where the reaction is an... 

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