Radical-alkene/alkyne additions alkenes

Reductive Addition of Heteroatom Centered Radicals to Alkynes and Alkenes... 

Sml2-mediated radical cyclisations involving alkyl, alkenyl and aryl radical intermediates can be used to construct efficiently five-membered and, in certain cases, six-membered ring systems. This approach provides a useful alternative to trialkyltin hydride-mediated methods as toxic reagents and problematic tin byproducts are avoided. In addition, the use of Sml2 to induce radical cyclisations has led to the development of a number of powerful, radical/anionic sequential processes for the construction of complex systems. Sequential reactions involving radical-alkene/alkyne cyclisations are discussed in Chapter 6. 

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]. 

Sulfoxides and sulfones can be prepared on cross-linked polystyrene by oxidation of thioethers. The most commonly used reagent for this purpose is MCPBA in DCM [8,12,32,57,80-82] or dioxane [50,83] (Table 8.6), but other oxidants such as H2O2 in acetic acid [34], oxone (Entry 7, Table 8.6), or oxaziridines [84] have also been used. PEG-bound thioethers have been converted into sulfones by oxidation with MCPBA in DCM [52,54] or with Os04/NMO [85], The oxidation of thioethers to sulfoxides requires careful control of the reaction conditions to prevent the formation of sulfones. Sulfones have also been prepared by S-alkylation of polystyrene-bound sulfi-nates (Entries 8 and 9, Table 8.6), by a-alkylation of sulfones (BuLi, THF, alkyl halide [86]), and by addition of sulfinyl radicals to resin-bound alkenes or alkynes (Entry 11, Table 8.6). 

Although HI addition to alkenes and alkynes is faster than that of the other hydrohalides and free radical anti-Maikovnikov additions are not a problem, this reaction has received less attention than the others.173 The hydroiodination of alkenes is most commonly run using concentrated HI in water or acetic acid at or below room temperature. While the early literature suggests that simple terminal alkenes afford small amounts of anti-Markovnikov products, only Markovnikov products have been reported in the more recent literature (equations 125-129).67 176-179... 

A large number of accurate rate constants are known for addition of simple alkyl radicals to alkenes.33-33 Table 2 summarizes some substituent effects in the addition of the cyclohexyl radical to a series of monosubstituted alkenes.36 The resonance stabilization of the adduct radical is relatively unimportant (because of the early transition state) and the rate constants for additions roughly parallel the LUMO energy of the alkene. Styrene is selected as a convenient reference because it is experimentally difficult to conduct additions of nucleophilic radicals to alkenes that are much poorer acceptors than styrene. Thus, high yield additions of alkyl radicals to acceptors, such as vinyl chloride and vinyl acetate, are difficult to accomplish and it is not possible to add alkyl radicals to simple alkyl-substituted alkenes. Alkynes are slightly poorer acceptors than similarly activated alkenes but are still useful.37... 

Each of the syntheses of seychellene summarized in Scheme 20 illustrates one of the two important methods for generating vinyl radicals. In the more common method, the cyclization of vinyl bromide (34) provides tricycle (35).93 Because of the strength of sjp- bonds to carbon, the only generally useful precursors of vinyl radicals in this standard tin hydride approach are bromides and iodides. Most vinyl radicals invert rapidly, and therefore the stereochemistry of the radical precursor is not important. The second method, illustrated by the conversion of (36) to (37),94 generates vinyl radicals by the addition of the tin radical to an alkyne.95-98 The overall transformation is a hydrostannylation, but a radical cyclization occurs between the addition of the stannyl radical and the hydrogen transfer. Concentration may be important in these reactions because direct hydrostannylation of die alkyne can compete with cyclization. Stork has demonstrated that the reversibility of the stannyl radical addition step confers great power on this method.93 For example, in the conversion of (38) to (39), the stannyl radical probably adds reversibly to all of the multiple bond sites. However, the radicals that are produced by additions to the alkene, or to the internal carbon of the alkyne, have no favorable cyclization pathways. Thus, all the product (39) derives from addition to the terminal alkyne carbon. Even when cyclic products might be derived from addition to the alkene, followed by cyclization to the alkyne, they often are not found because 0-stannyl alkyl radicals revert to alkenes so rapidly that they do not close. 

Allyl sulfones offer the possibility to conduct cyclization reactions that are isomerizations, and investigations by Smith and Whitham indicate that this is a very promising technique for the preparation of functionalized products.118 The mechanism consists of reversible addition of a sulfonyl radical to a terminal alkene or alkyne, cyclization, and fragmentation of the resulting -sulfonyl radical. Two examples of these isomerizations are provided in Scheme 26. 

We consider the anti process (144) as the normal, if ideal, course of addition. Closer to reality is the stepwise process. If we begin with the schemes (138) or (142) for substitution at an alkene carbon, and divert the first intermediates as in (160), we acquire a basic scheme for addition. Now, the attacks of nucleophiles, radicals and electrophiles on alkenes and of nucleophiles and radicals on alkynes produce Jrans-oriented intermediates, in which the memory of their parents is impressed. If > lcz, and 3 s and fc4 k5, the path to product should be stereoselective (stereoelectronic axiom 2). Indeed anti attack of the gegen... 

It was reported by Rozhkov and Chaplina130 that under mild conditions perfluorinated r-alkyl bromides (r-RfBr) in nonpolar solvents can be added across the n bond of terminal alkenes, alkynes and butadiene. Slow addition to alkenes at 20 °C is accelerated in proton-donating solvents and is catalyzed by readily oxidizable nucleophiles. Bromination of the it bond and formation of reduction products of t-RfBr, according to Rozhkov and Chaplina, suggest a radical-chain mechanism initiated by electron transfer to the t-RfBr molecule. Based on their results they proposed a scheme invoking nucleophilic catalysis for the addition of r-RfBr across the n bond. The first step of the reaction consists of electron transfer from the nucleophilic anion of the catalyst (Bu4N+Br , Na+N02, K+SCN , Na+N3 ) to r-RfBr with formation of an anion-radical (f-RfBr) Dissociation of this anion radical produces a perfluorocarbanion and Br, and the latter adds to the n bond thereby initiating a radical-chain process (equation 91). 

Atom-transfer reactions encompass a broad range of radical addition processes in which carbon-heteroatom bonds are added across alkenes, alkynes, or other... 

Molander recognised the potential of the Sml2-mediated Barbier addition to esters for the initiation of sequential processes (Chapter 5, Section 5.4). Two types of cascade have been developed that involve nucleophilic acyl substitution the first type involves double intramolecular Barbier addition to an ester group (anionic-anionic sequences),17 and the second type consists of a Barbier addition to an ester followed by a carbonyl-alkene/alkyne cyclisation of the resultant ketone (anionic-radical sequences) (Scheme 6.12).18,19... 

In intermolecular reactions, neutral aminyl radicals, R2N", react with n systems preferably by HAT. N-protonation increases the electrophUicity of the radical center, and successful addition of aminium radicals, R2NH, to 7t systems, usually alkenes, has been reported in the hterature." Compared to aminium radicals, amidyl and imidyl radicals, for example, RN C(0)R and [RC(0)]2N , are less electrophilic. Although they are delocahzed 7t-allyl radicals, they react exclusively at nitrogen." Only very few examples for radical cascades that are initiated by addition of A-centered radicals to alkynes have been reported. 

The term hydrosilation (or hydrosilylation) refers to the addition of a molecule containing a Si-H bond across the multiple bond of a substrate, usually an alkene, alkyne, or carbonyl compound (equation 1). The reaction can be promoted by UV-light, radiation (y- and X rays), radical initiators, Lewis acids, nucleophiles, or, most importantly, transition metal catalysts. Hydrosilation is related to the important processes of hydrogenation (see Hydrogenation) and hydroboration (see Hydroboration), all of which belong to the general reaction class of hydroelementation. 

We expect the reactions complementary to equations (1) and (2), namely electrophilic attacks, to be faster for alkenes than for alkynes. Thus, reactivity ratios (/-ii and rj2) for corresponding alkynes and alkenes (PhC CH, PhCH=CH and BuC CH, BuCH=CH2) in radical copolymerizations favour the alkene over the alkyne . Electrophilic additions of Br, CI2, ArSCl and H3O+ to alkenes are usually much faster than those to alkynes . However, A (C=C)/A (C=C) can vary from 10 to < 1 for the different electrophilic processes and by 10 for one process (Br2 addition) when the solvent is changed from HjO to HOAc . This unexpected trend in reactivity continues undiminished in the rates of acid-catalysed hydration... 

Looking back at the data, we find A/fr = 9 less favourable for addition of H+ and probably < 20 kcal/mol more favourable for addition of H to QH as compared with C2H4. However, equilibrium figures are deceptive. We have seen that significant substituent and solvation effects can reduce the energy gap. In respect to electrophilic rates, this occurs in A (CsQ>A (C=C), although this order is admittedly unusual. As for nucleophilic attacks, cathodic reductions may occasionally turn out to be exceptional otherwise, the order, k C=C)>k C=C), seems to be followed. A revised statement of alkyne-alkene reactivity now reads nucleophiles react faster with alkynes radicals react faster with alkenes polar electrophiles usually react faster with alkenes. 

Both intermolecular and intramolecular additions of carbon radicals to alkenes and alkynes continue to be a widely investigated method for carbon-carbon bond formation and has been the subject of a number of review articles. In particular, the inter- and intra-molecular additions of vinyl, heteroatomic and metal-centred radicals to alkynes have been reported and also the factors which influence the addition reactions of carbon radicals to unsaturated carbon-carbon bonds. The stereochemical outcome of such additions continues to attract interest. The generation and use of alkoxy radicals in both asymmetric cyclizations and skeletal rearrangements has been reviewed and the use of fi ee radical reactions in the stereoselective synthesis of a-amino acid derivatives has appeared in two reports." The stereochemical features and synthetic potential of the [1,2]-Wittig rearrangement has also been reviewed. In addition, a review of some recent applications of free radical chain reactions in organic and polymer synthesis has appeared. The effect of solvent upon the reactions of neutral fi ee radicals has also recently been reviewed. ... 

From a synthetic point of view, bond forming steps are the most important reactions of radical ions [202]. Several principle possibilities have been described in Section 8.1 and are summarized in Scheme 52. Many carbo- and heterocyclic ring systems can be constructed by (inter- and intramolecular) radical addition to alkenes, alkynes, or arenes. Coupling of carbonyl radical anions leads to pinacols either intra-or inter-molecular which can be further modified to give 1,2-diols, acyloins or alkenes. Radical combination reactions with alkyl radicals afford the opportunity to synthesize macrocyclic rings. These radical ion-radical pairs can be generated most efficiently by inter- or intramolecular photoinduced electron transfer. 

Other radicals can add to alkenes, and the rate constant for the addition of methyl radicals to alkenes has been studied,and the rate of radical additions to alkenes in general has also been studied.The kinetic and thermodynamic control of a radical addition regiochemistry has also been studied. Alkynes... 

---