Acetylenes. Markovnikov

The addition of (TMS)3SiH to a number of monosubstituted acetylenes has also been studied in some detail. These reactions are highly regioselective (anti-Markovnikov) and give terminal (TMSlsSi-substituted alkenes in good yields. High cis or trans stereoselectivity is also observed, depending on the nature of the substituents at the acetylenic moiety. For example, the reaction of the alkynes 23 and 24 with (TMSlsSiH, initiated either by EtsB at room temperature (method or by thermal decomposition of di-ferf-butyl peroxide at 160 °C... 

The hydration of triple bonds is generally carried out with mercuric ion salts (often the sulfate or acetate) as catalysts. Mercuric oxide in the presence of an acid is also a common reagent. Since the addition follows Markovnikov s rule, only acetylene gives an aldehyde. All other triple-bond compounds give ketones (for a method of reversing the orientation for terminal alkynes, see 15-16). With allqmes of the form RC=CH methyl ketones are formed almost exclusively, but with RC=CR both possible products are usually obtained. The reaction can be conveniently carried out with a catalyst prepared by impregnating mercuric oxide onto Nafion-H (a superacidic perfluorinated resinsulfonic acid). ... 

The authors proposed the complex [PdCl(SPh)(PhSH)]n 15 as an active catalyst, with thiol acting as a ligand of Pd(II) (Eq. 7.11). On the other hand, the reaction using RhCl(PPh3)3 as a catalyst precursor furnished the anti-Markovnikov ds-adducts 16 when a terminal acetylene was employed as a substrate (Eq. 7.12). 

By the example of 34 different alkynes, it was convincingly demonstrated that the product of the treatment of [PtCLJ with CO at 40-110 °C is a very powerful alkyne hydration catalyst some of the reactions are shown on Scheme 9.7 [25], The best medium for this transformation is THF containing 5 % H2O. The reaction can also be performed in a water-organic solvent two-phase system (e.g. with 1,2-dichloroethane), however in this case addition of a tetralkylammonium salt, such as Aliquat 336, is required to facilitate mass transfer between the phases. After the reaction with CO, the major part of platinum is present as H2[ Pt3(CO)6 n], but the catalytic effect was assigned to a putative mononuclear Pt-hydride, [PtHCl(CO)2], presumably formed from the cluster and some HCl (supplied by the reduction of [PtCU]). The hydration of terminal acetylenes follows Markovnikov s mle leading exclusively to aldehyde-free ketones. 

The first anti-Markovnikov hydration of terminal acetylenes, catalyzed by mthenium(ll)-phosphine complexes, has been described in 1998 [27]. As shown on Scheme 9.8, the major products were aldehydes, accompanied by some ketone and alcohol. In addition to TPPTS, the fluorinated phosphine, PPh2(C6Fs) also formed catalytically active Ru-complexes in reaction with [ RUC12(C6H6) 2]. 

A highly regioselective, efficient, and clean anti-Markovnikov hydration of terminal acetylenes has been realized through the use of catalytic amounts of Ru complexes.561 Typically, [CpRu(dppm)Cl] catalyzes the reaction at 100°C to give aldehydes in high yields (81-94%). Triflic acid or trifluoromethanesulfonimide effectively catalyzes the hydration of alkynes without a metal catalyst to afford Markovnikov products (ketones).562... 

Substituted acetylenes react more slowly than does acetylene. The Markovnikov rule is usually followed, that is, terminal acetylenes form 2-substituted acrylic acids122,124,161 [Eq. (7.19)162] ... 

Stereoselective Markovnikov addition of difluoro(aryl)-k3-bromane (25) to terminal acetylenes (24) has been reported, which gives rise to ( )-/3-fluoroalkenyl-k3-bromanes (26).41... 

In the alkyne dimerization catalyzed by palladium systems, all proposed mechanisms account for an alkynyl/alkyne intermediate with cis addition of the alkynyl C-Pd bond to the alkyne in a Markovnikov fashion, in which the palladium is placed at the less-substituted carbon, both to minimize steric hindrance and to provide the most stable C-Pd bond (Scheme 2a). The reverse regioselectivity in the palladium-catalyzed dimerization of aryl acetylenes has been attributed to an agostic interaction between the transition metal and ortho protons of the aromatic ring in the substrate (Scheme 2b) [7, 8],... 

This is an indirect method to obtain an anti Markovnikov addition of HI to terminal acetylenes. Chlorination929 and bromination930 (with bromine or with sodium bromide931) of vinylboronic acids results in inverted vinyl halides. This allows the three-step stereospecific inversion of ( )vinyl bromide into the [Z] isomer (equation 137)932. 

The regioselective anti-Markovnikov addition of benzoic acid to phenyl-acetylene has also been carried out with success at 111 °C in the presence of ruthenium complexes containing a tris(pyrazolyl)borate (Tp) ligand [RuCl(Tp)(cod), RuCl(Tp)(pyridine), RuCl(Tp)(N,N,Ar,AT-tetramethylethyl-enediamine )] with a stereoselectivity in favour of the (E)-enol ester isomer [22]. The o-enynyl complex Ru(Tp)[PhC=C(Ph)C=CPh](PMe/-Pr2) (C) efficiently catalyses the regioselective cyclization of a,cu-alkynoic acids to give en-docyclic enol lactones [23] (Eq. 2). 

We are currently investigating the reactions of l with other protic acids and the interactions of the resulting products with olefins and acetylenes. The object of this work is to establish if dinuclear complexes can function as catalysts for the anti-Markovnikov addition of HX to terminal olefins or acetylenes. Preliminary results show that the HI adduct (17) reacts slowly with 1-hexene at 80° (products not yet definitely established). The reaction of 17 with ethylene (45 psi, 75°) yields 3-pentanone as the primary organic product. Acetylene (1 atm., 25°) reacts rapidly at room temperature with 17 to give as yet unidentified products. 

The addition of hydrogen fluoride to acetylene homologs can be carried out without a catalyst to yield mainly difluorinatcd alkanes.The direction of the addition is in accordance with the Markovnikov rule. For examples, see Table 40. 

Selenenyl chlorides add to alkenes, often via an AdE2 mechanism involving a bridged seleniranium ion intermediate (19) (equation 14). These reactions are therefore highly stereospecitic, resulting in anti addition. The regiochemistry of the process can be under either kinetic or thermodynamic control. In some cases, initial anti-Markovnikov products were observed at low temperature and Markovnikov adducts dominated after further equilibration. Analogous electrophilic additions to acetylenes and aUenes (Scheme 9) have also been reported. When selenenyl hahdes react with alkenes in the presence of other nucleophiles such... 

Vinyl sulfides have been prepared by the catalytic addition of the S—H bond of thiols (85) to terminal alkynes (86) under solvent-free conditions using the nickel complex Ni(acac)2 (47). High alkyne conversions (up to 99%) were achieved after 30 min at 40 °C in favor of the corresponding Markovnikov products (87) (equation 23). Other metal acetylacetonate complexes were examined for this reaction, but none showed any improvement over the nickel catalyst. Mechanistic details suggest that alkyne insertion into the Ni—S bond is important to the catalytic cycle and that nanosized structural units comprised of [Ni(SAr)2] represent the active form of the catalyst. Isothiocyanates and vinyl sulfides have been produced in related Rh(acac)(H2C=CH2)2 (6) and VO(acac)2 (35) catalyzed sulfenylation reactions of aryl cyanides and aryl acetylenes, respectively. 

Addition of benzeneselenol to terminal acetylenes 30 is catalyzed by a palladium complex to give Markovnikov adducts 31 preferentially [55]. The... 

Phosphines add to alkenes to give alkyl phosphines and to alkynes to give vinyl phosphines. In the presence of an ytterbium (Yb) catalyst, diphenylphosphine added to diphenyl acetylene to give the corresponding vinyl phosphine. A palladium catalyst was used for the addition o-diphenylphosphine to terminal alkynes, giving the anti-Markovnikov vinyl phosphine but a nickel catalyst led to the Markovnikov vinyl phosphine.Alkenes also react with diarylphosphines... 

Haloboranes add to terminal alkynes in a cij-Markovnikov manner however, the bromoboration of acetylene itself exceptionally provides a rrans-adduct which gives the corresponding ( )-l-alkenylborates (10) by reaction with organozinc halides [24] (Scheme... 

Methylacetylene (propyne), CH3—C C—H, is the next member in the alkyne family. It undergoes reactions similar to those of acetylene. The addition reactions of propyne also obey Markovnikov s rule ... 

The bromochlorination of phenyl- and alkyl-substituted acetylenes with tetrabutylammonium di-chlorobromate gives (T)-olelins stercoselectively. Unsymmetrical disubstituted acetylenes give the preferred Markovnikov adducts, but the formation of zznZz-Markovnikov adducts is more pronounced with this reagent than with molecular bromine chloride, which is less stereoselective139. 

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