Alkynes, nucleophilic attack upon

The simple hexaalkylditins, RsSnSnRs, do not disproportionate on heating, but, in oxolane (tetrahydrofuran) or acetonitrile in the presence of a base such as a Grignard reagent, or in the more strongly basic solvent hexamethylphosphoric triamide (HMPT), disproportionation readily occurs at room temperature, and, in HMPT, addition occurs to such alkynes as phenylacetylene and diphenylbutadiyne. The disproportionation is considered to proceed by nucleophilic attack upon tin (259, 260), e.g.,... 

The proposed mechanism involves the formation of ruthenium vinylidene 97 from an active ruthenium complex and alkyne, which upon nucleophilic attack of acetic acid at the ruthenium vinylidene carbon affords the vinylruthenium species 98. A subsequent intramolecular aldol condensation gives acylruthenium hydride 99, which is expected to give the observed cyclopentene products through a sequential decarbonylation and reductive elimination reactions. 

The fact that the w orbital drops drastically on bending is, of course, related to the fact that the vinyl anion (sp2 lone pair) is more stable than the ethyl anion (higher energy sp3 lone pair). That is, upon bending, more s character is mixed into the LUMO of the alkyne, which ultimately becomes the sp2 anion lone pair upon addition of a nucleophile. However, our treatment also explains why nucleophilic attack on alkynes is preferred even for early transition states or for those reactions, such as cycloadditions and thermal rearrangements, in which anions are not formed. 

Contrary to the previous pathway of P-H addition to alkyne - that is, via alkyne insertion into the M-P bonds - this reaction has been shown to proceed via the nucleophilic attack of the phosphine to a ruthenium-vinylidene intermediate to yield the anti-Markovnikov product with a predominant (Z -stereoisomer (Scheme 8.36). Indeed, it has been shown that [Cp RuL2] X intermediate gives vinylidene species with propargyl alcohols. The (Z)-isomer is formed as the major product, but iso-merizes easily into the ( )-isomer upon isolation by chromatography over silica gel. 

Scheme 26. Exo-endo dig cyclization modes involving nucleophilic attack of the arene upon the metal-alkyne n complex.

Hydroarylation can also be mediated by Au(I) and Au(III) (Scheme 33) (384). In the case of aryl substituted alkynes, the Au(III) Ji complex undergoes electrophilic aromatic substitution with the electron-rich arene to give aLkenyl-Au(III) complex, which is immediately protonated by the H generated upon C C bond formation. For the Au(I)-catalyzed hydroarylation, the cationic gold complex k coordinates the alkyne, with subsequent nucleophilic attack by the arene from the opposite face leading to an alkenyl-gold complex, which is protonated to the desired products. The nature of the reaction causes the regioselectivity of this reaction to be sensitive to electronic rather than steric factors. 

The reverse reaction will be discussed in section 8.3. An a-carbon of vinyli-dene ligand, generated from 1,2-hydrogen shift of terminal alkyne upon contact with coordinatively unsaturated metal fragment, is also susceptible to the nucleophilic attack (Scheme 8.17). This attack is believed to constitute a catalytic cycle for anti-Markownikov addition of NuH (Nu = RO, RCOO, etc.) to alkynes [4c]. Schrock carbenes are sensitive to electrophiles (see 8.3.1), but not to nucleophiles. 

A plausible mechanism has been suggested showing condensation of aldehyde and amine to generate the imine, which upon activation by AgNOs results in the nucleophilic attack from the C-2 position of the pyrrole ring onto the imine carbon followed by aromatization. n-Complexation between the alkyne and Ag(l) renders a regjoselective second intramolecular attack of nucleophilic NH onto the alkyne, which on subsequent deprotonation leads to the formation of cyclized product. 

Unactivated, unsaturated hydrocarbons are not routinely subject to attack by nucleophiles. Complexation of alkenes, alkynes, or arenes to a positively charged metal center may lead to activation of the hydrocarbyl group towards nucleophilic attack and, in particular, to addition of hydride ion. The effect of the [LnM] fragment on the reactivity of the unsaturated ligand is largely due to the transfer of electron density from the unsaturated hydrocarbyl to the metal center upon coordination, a process that is most effective when the metal center is positively charged. Figure 10.5 ... 

The calculations also suggest why tram addition of nucleophiles to alkynes is invariably observed, and why the apparently unusual trajectories proposed by Baldwin48 for such attacks occur. Table 3 shows the energy changes which occur upon various distortions of acetylene and ethylene, as well as energies of interaction of the various geometries of these species with hydride ion, a model nucleophile. 

Alkyne ligands are susceptible to attack by nucleophiles, which typically leads to the formation of -vinyl complexes " this is particularly true in the case of electron-deficient alkynes. Noteworthy in this regard are the reactions of [Tp W(C0)2( ] -RC=CR )]+ cations with hydride reagents that yield )) -vinyl, ) -allene, or ry -alXyX complexes depending upon the nature of the alkyne substituents." ... 

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