Vinylation 1-alkynes

When a cyclization sequence is terminated by an alkyne, vinyl cations are formed. Capture of water leads to formation of a ketone.15... 

The reaction of aryl halides with copper(I) alkynides is known as the Castro reaction (equation 3). The reaction has proved to be particularly important in the synthesis of a wide range of tolan and hetero-aromatic alkynes. Vinyl and allenic halides"" can also be used and several reviews of the reaction have been published. " ... 

However, during the addition of -keto esters onto enals, ytterbium triflate proved to be an efficient catalyst [171] as in the Michael additions of a-nitro esters [172]. Several water-soluble phosphines gave the corresponding phos-phonium salts in good yields when added to a,j9-unsaturated acids [173] or activated alkynes [174]. With alkynes, vinyl phosphine oxides or alkenes were formed depending on the pH of the aqueous solution. Significantly, the reaction of nitroalkanes with buten-2-one is considerably accelerated when going from... 

Another mode of activation of o-alkynylanilines 105, involving an alkyne-vinyl-idene isomerization, was reported by Mc-Donald (Scheme 9.40) [191]. High yields of 1-monosubstituted indoles 106 were obtained upon cydoisomerization of terminal o-alkynylanUines 105 using an in situ generated Et3N Mo(CO)5 catalyst. The authors proposed a mechanism involving the initial alkyne-vinylidene isomerization [192-194] of a terminal acetylene 105 into the reactive carbenoid 107. The... 

Scheme 46) to both isomers of vinyl halides (X = Cl,Br, or I) from alkynes. Vinyl bromides can also be obtained from alkynes via hydroboration to vinyl boronic acids followed by treatment with ICl (Scheme 47)/ and a recent alternative approach to vinyl bromides by condensation of carbonyl compounds with dibromomethane is outlined in Scheme 48. ... 

There also exists an acidregioselective condensation of the aldol type, namely the Mannich reaction (B. Reichert, 1959 H. Hellmann, 1960 see also p. 291f.). The condensation of secondary amines with aldehydes yields Immonium salts, which react with ketones to give 3-amino ketones (=Mannich bases). Ketones with two enolizable CHj-groupings may form 1,5-diamino-3-pentanones, but monosubstitution products can always be obtained in high yield. Unsymmetrical ketones react preferentially at the most highly substituted carbon atom. Sterical hindrance can reverse this regioselectivity. Thermal elimination of amines leads to the a,)3-unsaturated ketone. Another efficient pathway to vinyl ketones starts with the addition of terminal alkynes to immonium salts. On mercury(ll) catalyzed hydration the product is converted to the Mannich base (H. Smith, 1964). 

The stereospedfic and regioselective hydrobromination of alkynes with chlorobis(T -cyclopentadienyl)hydrozirconium and NBS produces ( )-vinylic bromides in good yields. The bromine atom usually adds regioselectively to the carbon atom that bears the smaller substituent and stereoselectively trans to the larger substituent (D.W. Hart, 1975 M. Nakatsuka,... 

The coupling of alkenylboranes with alkenyl halides is particularly useful for the stereoselective synthesis of conjugated dienes of the four possible double bond isomers[499]. The E and Z forms of vinylboron compounds can be prepared by hydroboration of alkynes and haloalkynes, and their reaction with ( ) or (Z)-vinyl iodides or bromides proceeds without isomerization, and the conjugated dienes of four possible isomeric forms can be prepared in high purity. 

No intennolecular reaction of malonate or /3-keto esters with halides has been reported, but the intramolecular reaction of /3-diketones such as 790 and malonates proceeds smoothly[652,653]. Even the simple ketone 791 can be arylated or alkenylated intramolecularly. In this reaction, slow addition of a base is important to prevent alkyne formation from the vinyl iodide by elim-ination[654]. 

The cis thioboration of terminal alkynes with 9-(arylthio)-9-BBN is catalyzed by Pd(Pli3P)4 in the presence of styrene. The product 136 is converted into the vinyl sulfides 137 and 138 by the treatment with MeOH or by Pd-catalyzed cross-coupling with aryl or alkenyl halides using K3PO4 in DMF[68]. No thioboration takes place with internal alkynes. 

Evidence from a variety of sources however indicates that alkenyl cations (also called vinylic cations) are much less stable than simple alkyl cations and their involve ment m these additions has been questioned Eor example although electrophilic addi tion of hydrogen halides to alkynes occurs more slowly than the corresponding additions... 

The addition proceeds in three discrete steps and the intermediates can be isolated. Simple alkenes are less reactive than alkynes and do not undergo the addition to aHylic boranes, but electron-rich alkyl vinyl ethers react at moderate temperatures to give 1,4-dienes or dienyl alcohols (440). 

Formation of Sulfides. Thiols react readily with alkenes under the same types of conditions used to manufacture thiols. In this way, dialkyl sulfides and mixed alkyl sulfides can be produced. Sulfides are a principal by-product of thiol production. Mixed sulfides can be formed by the reaction of the thiol using a suitable starting material, as shown in equations 21, 22, and 23. Vinyl sulfides can be produced by the reaction of alkynes with thiols (38). 

The three basic mechanisms that have been considered to be involved in electrophilic additions to alkynes are shown below. The first involves a discrete vinyl cation. In general, it can lead to either of the two stereoisomeric addition products. The second mechanism is a termolecular process which would be expected to lead to stereospecific anti addition. The... 

Alkynes react when heated with trifluoroacetic acid to give addition products. Mixtures of syn and anti addition products are obtained. Similar addition reactions occur with trifluoromethanesulfonic acid. These reactions are analogous to acid-catalyzed hydration and proceed through a vinyl cation intermediate. 

The stereochemistry of addition is usually anti for alkyl-substituted alkynes, whereas die addition to aryl-substituted compounds is not stereospecific. This suggests a termo-iecular mechanism in the alkyl case, as opposed to an aryl-stabilized vinyl cation mtermediate in the aryl case. Aryl-substituted alkynes can be shifted toward anti addition by including bromide salts in the reaction medium. Under these conditions, a species preceding the vinyl cation must be intercepted by bromide ion. This species can be presented as a complex of molecular bromine with the alkyne. An overall mechanistic summary is shown in the following scheme. 

This scheme represents an alkyne-bromine complex as an intermediate in all alkyne brominations. This is analogous to the case of alkenes. The complex may dissociate to a inyl cation when the cation is sufficiently stable, as is the case when there is an aryl substituent. It may collapse to a bridged bromonium ion or undergo reaction with a nucleophile. The latta is the dominant reaction for alkyl-substituted alkynes and leads to stereospecific anti addition. Reactions proceeding through vinyl cations are expected to be nonstereospecific. 

The first step in the addition of an electrophile such as HBr to an alkyne involves protonation and subsequent formation of an intermediate vinyl cation. Where does propyne protonate Compare energies of 1-methylvinyl and 2-methylvinyl cations. Which is more stable Why Measure CC bond distance in the more stable cation. Does the cation incorporate a full triple bond (as in propyne) or a double bond (as in propene). Examine atomic charges and electrostatic potential maps to locate the positive charge in the two cations. Is the more stable ion the one in which the charge is better delocalized Use the charges together with information about the ions geometry to draw Lewis structures (or a series of Lewis structures) for 1-methylvinyl and 2-methylvinyl cations. 

Several intermediates are involved in the latter reaction. The first is a radical anion resulting from electron transfer from sodium to the alkyne. This then deprotonates ammonia leading to a vinyl radical. The process repeats (electron transfer and deprotonation), and involves a vinyl anion intermediate. 

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