Alkynes Lewis acid catalyzed

Alkenylsilanes can be made by Lewis acid-catalyzed hydrosilation of alkynes. Both AlClj and C2H5A1C12 are effective catalysts.73 The reaction proceeds by net anti addition, giving the Z-alkenylsilane. The reaction is regioselective for silylation of the terminal carbon. 

Lewis acid-catalyzed hydrostannylation has been observed using ZrCl4. With terminal alkynes the Z-alkenylstannane is formed.142 These reactions are probably similar in mechanism to Lewis acid-catalyzed additions of silanes (see p. 811). 

Mascarenas developed a synthetic method to 1,5-oxygen-bridged medium-sized carbocycles through a sequential ruthenium-catalyzed alkyne-alkene coupling and a Lewis-acid-catalyzed Prins-type reaction (Eq. 3.45). The ruthenium-catalyzed reaction can be carried out in aqueous media (DMF/H20 = 10 1).181... 

The skeletal rearrangements are cycloisomerization processes which involve carbon-carbon bond cleavage. These reactions have witnessed a tremendous development in the last decade, and this chemistry has been recently reviewed.283 This section will be devoted to 7T-Lewis acid-catalyzed processes and will not deal, for instance, with genuine enyne metathesis processes involving carbene complex-catalyzed processes pioneered by Katz284 and intensely used nowadays with Ru-based catalysts.285 By the catalysis of 7r-Lewis acids, all these reactions generally start with a metal-promoted electrophilic activation of the alkyne moiety, a process well known for organoplatinum... 

Burger2 has shown that alkynes undergo both Lewis acid-catalyzed and thermal carbonyl-yne reactions with 3,3,3-trifluoropyruvates to give allenes. Reaction of 1 (Equation (2)) occurs to give a 1 1 mixture of diastereomeric allenyl carbinols 2. Alternatively, reaction of hexyne 1 and methyl trifluoropyruvate with MgBr2-OEt2 at low temperature afforded 2 as an 8 1 mixture of diastereomers. The thermal reaction does not suffer from allylic alcohol byproducts arising from reaction of the substrate with the Lewis acid.3... 

Particularly interesting is the reaction of enynes with catalytic amounts of carbene complexes (Figure 3.50). If the chain-length between olefin and alkyne enables the formation of a five-membered or larger ring, then RCM can lead to the formation of vinyl-substituted cycloalkenes [866] or heterocycles. Examples of such reactions are given in Tables 3.18-3.20. It should, though, be taken into account that this reaction can also proceed by non-carbene-mediated pathways. Also Fischer-type carbene complexes and other complexes [867] can catalyze enyne cyclizations [267]. Trost [868] proposed that palladium-catalyzed enyne cyclizations proceed via metallacyclopentenes, which upon reductive elimination yield an intermediate cyclobutene. Also a Lewis acid-catalyzed, intramolecular [2 + 2] cycloaddition of, e.g., acceptor-substituted alkynes to an alkene to yield a cyclobutene can be considered as a possible mechanism of enyne cyclization. 

Other Lewis acid-catalyzed hydrosilylations of alkenes and alkynes on hydride-terminated silicon surfaces result in a surface modified with alkyl and alkenyl functionalities. ... 

Intramolecular radical cyclization of an aryl bromide and an alkyne can be used to produce dihydroquinolines (Equation 57) <1998TL2965>. An analogous reaction setup utilizes a Lewis acid-catalyzed novel one-pot domino pathway using silver catalysis in high regioselectivity (Scheme 26) <2005OL2675>. Three mole equivalents of the alkyne are used with the final cyclization step arising from alkynic addition. 

An alternate route to formation of alkyl monolayers is via Lewis acid catalyzed reactions of alkenes with the hydrogen terminated surface. In this approach, a catalyst such as ethyl aluminum dichloride is used to mediate the hydrosilylation reaction of an alkene (or alkyne), resulting in the same type of product as in the case of the photochemical or thermal reactions. This type of reaction is well known based on molecular organosilane chemistry and has also been used successfully to alkylate porous silicon [31]. Although this route has been shown to work on H/Si(lll), the resulting monolayers are found to have lower coverages than those achieved using the photochemical or thermal approach [29], Another concern with this approach is the possibility of trace metal residues from the catalyst that could adversely affect the electronic properties of these surfaces (even when present at levels below the detection limit of most common surface analysis techniques). 

The Lewis acid-catalyzed reactions of allylsilanes with simple alkenes and alkynes introduce both allyl and silyl groups into the unsaturated bonds (Equations (53) and (54)).209 210b These allylsilylations are 7-regiospecific with respect to allylsilanes. The /raor-addition of allylsilanes occurs in the reaction with alkynes. There are two possible mechanisms for the Lewis acid-promoted allylsilylations, that is, trimethylsilyl cation- and Lewis acid-mediated pathways. 

As noted in Section 14.19.2.1.1, the Lewis acid-catalyzed hydrosilylation of unactivated acetylenes provided a general method for the synthesis of alkenylsilanes. Hence, intramolecular /ra .r-hydrosilylation of TMS-substituted alkyne 30 bearing a benzene ring spacer led to the formation of rfb-cycliz.ation product 31 (Equation 10) <2000JOC8919>. 

Lewis acid-catalyzed reactions of readily ionizable alkyl halides with alkynes yield vinyl halides [207]. The regioselectivities of these additions can be rationalized by the relative stabilities of the intermediate vinyl cations. Unlike the situation described for additions to alkenes, there is no preference for anti-additions, and the stereoselectivities can be explained by the intermediacy of nonbridged species [208]. The site of nu-... 

U nder thermal or Lewis acid catalyzed conditions, reactions of fluoroalkyl carbonyl compounds with alkenes usually provide the ene adduct. " One example of a [2 + 2] cycloaddition between hexafluoroacetone and an alkyne has been reported. ... 

Lewis acid catalyzed reactions of allenes with alkenes generally give cyclobutanes rather than ene adducts. AlCb catalyzed reactions of alkylallenes with alkenes give alkylidenecyclobutanes. Similarly, AICI3 catalyzed reactions of alkynes with alkenes give cyclobutenes. These reactions are believed to occur by stereospecific cycloaddition of the alkene with the vinyl cation formed by complexation of AICI3 to the allene or alkyne. 

F. Zipoli, M. Bernasconi, and A. Laio (2005) Ab initio simulations of Lewis-acid-catalyzed hydrosilylation of alkynes. Chem,. Phys. Chem,. 6, p. 1772... 

Lewis Acid-catalyzed Carbosilylation of Unactivated Alkynes and Alkenes... 

The Lewis acid-catalyzed aUylsilylation of alkynes and alkenes with allylsilanes was introduced by Jung [511] and Yamamoto [512]. Jung and co-workers found fhat AICI3 catalyzes the aUylsilylation of simple alkenes and phenyl-substituted alkynes wifh allylsilanes (Scheme 10.191) [513]. The allylation occurs regiospecifically at the y-position of the allylsilanes. When cycloalkenes are used as substrates, trans adducts are obtained exclusively. Jung et al. initially reported [513 a] that the aUylsilylation of alkynes proceeded in a cis-addition mode they later noted, however, fhat fhe stereochemical assignment of fhe aUylsilylation products was incorrect and the products were not cis but trans adducts [511]. It has been proposed that the mechanism of fhe AlCls-catalyzed aUylsilylation involves addition of a silyl cation or its equivalent to fhe carbon-carbon multiple bond and subsequent aUylation of the resulting -silylcarbenium ion [511]. 

Yamamoto et al. have systematically studied the Lewis acid-catalyzed carbosilylation of alkynes with allylsilanes and related compounds [512]. They have disclosed that the EtAlCl2 (cat.J-MeaSiCl (excess) catalyst system or a sub-stoichiometric amount of HfC effectively induces the trans-aUylsilylation of the alkynes (Scheme 10.192) [514]. The HfCLj-promoted method, in particular, is quite useful for stereo- and regioselective aUylsilylation of a variety of aUylsilanes under mild conditions. In sharp contrast to the mechanism proposed by Jung et al., Yamamoto et al. have proposed that EtAlCh or HfCLi adds to an alkyne, the activated alkyne is allylated by an allylsilane, and the resulting vinylmetal becomes an aUylsilylation product by transmetalation. 

Lewis acid catalyzed versions of [4 4- 2] cycloadditions are restricted to functionalized dieno-philes. Nonfunetionalized alkenes and alkynes cannot be activated with Lewis acids and in thermal [4 + 2] cycloadditions these suhstrates usually show low reactivity. It has been reported that intcrmolecular cycloaddition of unactivated alkynes to dienes can be accelerated with low-va-lent titanium, iron or rhodium catalysts via metal-mediated - -complex formation and subsequent reductive elimination39 44. Usually, however, low product selectivities are observed due to side reactions, such as aromatization, isomerization or oligomerization. More effective are nickel-catalyzed intramolecular [4 4- 2]-dienyne cycloadditions which were developed for the synthesis of polycycles containing 1.4-cyclohexadienes45. Thus, treatment of dienyne 1, derived from sorbic acid, with 10mol% of Ni(cod)2 and 30 mol % of tris(o-biphenyl) phosphite in tetrahydrofuran at room temperature affords bicyclic 1,4-dienes 2, via intramolecular [4 + 2] cycloaddition, with excellent yield and moderate to complete diastereocontrol by substituents attached to the substrate. The reaction is sensitive towards variation in the catalyst and the ligand. 

Abstract Progress in the field of metal-catalyzed redox-neutral additions of oxygen nucleophiles (water, alcohols, carboxylic acids, and others) to alkenes, alkynes, and allenes between 2001 and 2009 is critically reviewed. Major advances in reaction chemistry include development of chiral Lewis acid catalyzed asymmetric oxa-Michael additions and Lewis-acid catalyzed hydro-alkoxylations of nonacti-vated olefins, as well as further development of Markovnikov-selective cationic gold complex-catalyzed additions of alcohols or water to alkynes and allenes. 

Shirakawa, E. et al., Nickel-catalyzed carbostannylation of alkynes with allyl-, acyl-, and aUcynylstan-nanes Stereoselective synthesis of trisubstituted vinylstannanes, J. Am. Chem. Soc., 121, 9907, 1999. Matsukawa, Y. et al., Lewis acid catalyzed allylstannylation of unactivated alkynes. Tetrahedron, 155, 3779, 1999. 

Lewis Acid-catalyzed [3+2] Cycloaddition. The title compound efficiently catalyzed an efficient and highly regiose-lective 1,3-dipolar cycloaddition reaction of alkynes and N-tosylazomethine ylides (eq 36). Cleavage of the C-C bond of iV-tosyl aziridines under mild conditions was catalyzed by Sc(OTf)3, affording the Af-tosylazomethine ylides. The reaction proceeds smoothly with various internal alkynes to afford the desired cycloadducts in moderate to good yields. 

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