Addition to triple bonds

Fieser et al. have already found that bromination of trans-stilbene with pyridinium hydrobromide perbromide in acetic acid gave exclusively meso-stilbene dibromide, and have further shown that the agent possesses far greater stereoselectivity than free bromine (ref. 26). Fournier et al. have reported the bromo-addition to double-bond of several alkenes by use of TBA Br3 (ref. 27). Moreover, Bethelot et al. described the bromo-addition to triple-bond of alkynes with TBA Br3 (ref. 28). 

Of course, the trans isomer will give the opposite results the threo pair if the addition is syn and the erythro pair if it is anti. The threo and erythro isomers have different physical properties. In the special case where Y=W (as in the addition of Br2), the erythro pair is a meso compound. In addition to triple-bond compounds of the type ACsCA, syn addition results in a cis alkene and anti addition in a trans alkene. By the definition given on page 166 addition to triple bonds cannot be stereospecific, though it can be, and often is, stereoselective. 

If the carbanion has even a short lifetime, 6 and 7 will assume the most favorable conformation before the attack of W. This is of course the same for both, and when W attacks, the same product will result from each. This will be one of two possible diastereomers, so the reaction will be stereoselective but since the cis and trans isomers do not give rise to different isomers, it will not be stereospecific. Unfortunately, this prediction has not been tested on open-chain alkenes. Except for Michael-type substrates, the stereochemistry of nucleophilic addition to double bonds has been studied only in cyclic systems, where only the cis isomer exists. In these cases, the reaction has been shown to be stereoselective with syn addition reported in some cases and anti addition in others." When the reaction is performed on a Michael-type substrate, C=C—Z, the hydrogen does not arrive at the carbon directly but only through a tautomeric equilibrium. The product naturally assumes the most thermodynamically stable configuration, without relation to the direction of original attack of Y. In one such case (the addition of EtOD and of Me3CSD to tra -MeCH=CHCOOEt) predominant anti addition was found there is evidence that the stereoselectivity here results from the final protonation of the enolate, and not from the initial attack. For obvious reasons, additions to triple bonds cannot be stereospecific. As with electrophilic additions, nucleophilic additions to triple bonds are usually stereoselective and anti, though syn addition and nonstereoselective addition have also been reported. 

For those substrates more susceptible to nucleophilic attack (e.g., polyhalo alkenes and alkenes of the type C=C—Z), it is better to carry out the reaction in basic solution, where the attacking species is RO . The reactions with C=C—Z are of the Michael type, and OR goes to the side away from the Z. Since triple bonds are more susceptible to nucleophilic attack than double bonds, it might be expected that bases would catalyze addition to triple bonds particularly well. This is the case, and enol ethers and acetals can be produced by this reaction. Because enol ethers are more susceptible than triple bonds to electrophilic attack, the addition of alcohols to enol ethers can also be catalyzed by acids. " One utilization of this reaction involves the compound dihydropyran... 

In base-catalyzed addition to triple bonds, the rate falls in going from a primary to a tertiary alcohol, and phenols require more severe conditions. Other catalysts, namely, BF3 and mercuric salts, have also been used in addition of ROH to triple bonds. 

A summary of electrophilic additions to triple bonds and allenes involving a vinyl cation is given in Table IV. 

All of the above mentioned examples of vinyl cation intermediates have involved electrophilic additions to triple bonds or allenes or participation in solvolyses of such multiple bonds. In a sense, these reactions derive from analogies in normal... 

Instability, functional polymerisation, additions to triple bond... 

For reviews of ionic additions to triple bonds, see, in Patai The Chemistry of the Carbon-Carbon Triple Bond Wiley New York, 1978, the articles by Schmid, pt. 1. pp. 275-341. and by Dickstcin Miller, pt. 2, pp. 813-955 Miller Tanaka Sel. Org. Transform. 1970, /, 143-238 Wintcrfeldl, in Viche, Ref. 49. pp. 267-334. For comparisons of double and triple bond reactivity, see Mclloni Modena Toncllato Acc. Chem. Res. 1981,14, 227-233 Allen C hiang Kresge Tidwell J. Org. Chem. 1982, 47. 775. 

Nevertheless, bridged ions 14 have been implicated in some additions to triple bonds. See. for example. Pincock Yales. Ref. 14 Mauger Berliner J. Am. Chem. Soc. 1972, 94. 194 Bassi Tonellato J. Chem. Soc.. Perkin Trans. 1 1973, 669 Schmid Modro Lenz Garratt Yates J. Org. Chem. 1976, 41. 2331. 

The principles of radical addition reactions of alkenes appear to apply equally to alkynes, although there are fewer documented examples of radical additions to triple bonds. Two molecules of hydrogen bromide can add to propyne first to give cis-1 -bromopropene (by antarafacial addition) and then 1,2-dibromopropane ... 

Double cyclizations to butenolides and furanes. Radicals can undergo intramolecular addition to triple bonds when separated by three carbons. This strategy can be used for synthesis of butenolides (equation 1) and -substituted furanes (equation II). Cyclization of vinyl bromides." Fused and bridged ring systems can be prepared... 

Polyynes have served as starting materials for the synthesis of a wide variety of heterocyclic ring systems. The reactions used involve addition to triple bonds, and any of the common mechanistic pathways may be followed, i.e. nucleophilic, electrophilic or free radical attack as well as concerted cycloadditions. Although the evidence does not permit unequivocal classification of many of the reactions into one of these categories, the ones considered here are those which most likely involve nucleophilic attack at some stage. In a formal sense the reactions amount to successive additions of a divalent nucleophile to two triple bonds the first involves intermolecular and the second intramolecular attack, as illustrated in equation (19) for the addition of HoS to a diyne. 

We can start with a Michael addition. This sounds crazy as we have just explained that Michael addition leads to rapid equilibration. However, Michael addition to triple bonds may not. One remarkably simple case is the addition of LiBr in weakly acidic solution to a triple bond conjugated to an ester 52. 

Catalysts for transesterification of vinyl acetate as well as addition to triple bonds Thermometers and leveling bulbs Mercury vapor and fluorescent lamps Electrical switches... 

Hydrogen Abstraction Reaction with N, S and -OH = Addition to Triple Bonds = Addition to Olefinic Bonds = Addition to Aromatic Rings = Addition to Fused Rings =... 

In type 3 cyclization, the aryl halides 136 having an alkyne side chain at ortho position undergo cis carbopalladation of 137 to generate 138, which is trapped by a nucleophile YH to give the cyclized product 139. In this cyclization, cis addition to triple bonds occurs (Scheme 3.13). 

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