Alkynes with metal ions

A. Chemical Ionization of Alkynes with Metal Ions... 

Complexes in Which the Acceptor Is a Metal Ion and the Donor is an Alkene or an Aromatic Ring. Note that n donors do not give EDA complexes with metal ions but form covalent bonds instead. Many metal ions form complexes, which are often stable solids, with alkenes, dienes (usually conjugated, but not always), alkynes, and aromatic rings. The generally accepted picture... 

Hydration of unactivated alkynes is an important method for functionalizing this plentiful hydrocarbon source. Therefore, a variety of metal ions have been proposed as catalysts for this reaction, and almost all of the reported additions of water to terminal alkynes follow the Markonikov rule. The hydration of l-aUcynes with Hg(II) salts in sulfuric acid [85], RuCh/aq.HCl [86, 87], K[Ru (edta-H)Cl] 2H20 [88], RhCl,.3H20/aq. HCl [89], RhCl3/NR4 [90], Zeise-type Pt(II) complexes [91-93], and NaAuCl4 [94] produced exclusively methyl ketones (Eq. 6.46). 

The addition of organodichalcogenides to alkynes does not necessarily require the aid of transition metal catalysts. Indeed, the reaction proceeds via different mechanisms under various conditions (radical, In, CsOH,183 SnCl4,184 and phase-transfer catalysts185). Treatment of a mixture of (PhS)2 and terminal alkynes with GaCl3 affords (E)-products (E Z=>20 1).186 The reaction is assumed to involve a thiirenium ion as the intermediate (Scheme 38). 

The position of the triple bond can alter the reactivity of the alkynes. Acidic alkynes react with certain heavy metal ions, e.g. Ag+ and Cu+, to form precipitation. Addition of an alkyne to a solution of AgNOs in alcohol forms a precipitate, which is an indication of hydrogen attached to the triple bonded carbon. Thus, this reaction can be used to differentiate terminal alkynes from internal alkynes. 

Pyrrole and indole rings can also be constructed by intramolecular addition of nitrogen to a multiple bond activated by metal ion complexation. Thus, 1-aminomethyl-l-alkynyl carbinols (obtained by reduction of cyanohydrins of acetylenic ketones) are cyclized to pyrroles by palladium(II) salts. In this reaction the palladium(II)-complexed alkyne functions as the electrophile with aromatization involving elimination of palladium(II) and water (Scheme 42) (81TL4277). 

Several cyclofunctionalization reactions of alkynic alcohols are synthetically useful. Metal ion-promoted cyclofunctionalization of ris-2-propargylcyclopentanol systems proceeds by the 5-exo mode (equation 77 and Table 23).197 Protiodemetallation or reductive demetallation provides the cyclic enol ether in high yields. This method has been used by Noyori in the synthesis of prostacyclin (PGh).197b,197c Reactions with catalytic amounts of mercury(II) or palladium(II) salts gave the endocyclic enol ether as the major product.197 -198 A related cyclization with Ag2C03 has been reported by Chuche.191 Schwartz... 

The isolation of stable vinylidene complexes and elucidation of many of their reactions have given substance to speculations concerning their intermediacy in many reactions. Indeed, the reactions of many alkynes with a series of platinum(II) complexes were explained several years ago by considering the formation of metal-stabilized carbonium ions as nonisolable intermediates (10). Summarized below are several reactions that may reasonably be assumed to proceed via vinylidene complexes. 

A central metal ion usually has a pronounced effect on the reactivity of a coordinated ligand at the coordinated atom or atoms. An important reaction of this type which has synthetic value is the reaction of alkenes and alkynes with hydrogen and carbon monoxide in the presence of a metal carbonyl. This is actually the catalytic process of hydroformylation and, although catalysis is beyond the scope of this work, it is nevertheless of interest from the standpoint of ligand reactivity. The reaction of ethylene with hydrogen and carbon monoxide in the presence of HCo(CO)4 as a catalyst is proposed to proceed (at least formally) through the steps shown 1U13) ... 

Triple bonds can also be selectively reduced to double bonds with diisobutyla-luminum hydride (Dibal-H), ° with activated zinc (see 12-38), with hydrogen and Bi2B-borohydride exchange resin, or (internal triple bonds only) with alkali metals (Na, Li) in hquid ammonia or a low-molecular-weight amine. Terminal alkynes are not reduced by the Na NH3 procedure because they are converted to acetylide ions under these conditions. However, terminal triple bonds can be reduced to double bonds by the addition to the Na—NH3 solution of (NH4)2S04, which liberates the free ethynyl group. The reaction of a terminal alkyne with... 

The highly electronegative metal ion affords the appropriate empty orbitals for the stabilization of the complex anion in the transition state. Both late and early transition metal alkyls are prone to this reaction, but its occurrence had to be particularly, invoked in the case of the early transition metals. Many similar reactions, such as the reaction of metal alkyls with other FIX compounds, could be described as if they followed this pathway, but the use of the term a-bond metathesis is restricted to those reactions in which one reacting species is a metal hydrocarbyl or metal hydride and the other reactant is a hydrocarbon or dihydrogen. Two reactions have been depicted in Fig. 4.34. There are, of course, borderline cases when the reacting hydrocarbon is acidic, as in the case of 1-alkynes, a direct attack of the proton at the carbanion can be envisaged. It has been proposed that acyl metal complexes of the late transition metals may also... 

A theoretical study of the intermediates involved in the formation of phospha-propyne from pyrolysis of vinylphosphirane has led to a new route to phospha-alkynes. Thus, pyrolysis of trimethylsilyl(l-phosphiranyl)diazomethane has yielded MeaSiC = P, via an intermediate 1-phosphiranylmethylene . Regioselec-tivity in the [3 + 2] cycloaddition reaction between phosphaethyne and diazomethane has been studied by theoretical techniques , and further examples of reactions of this type described . Cycloaddition of phospha-alkynes with silylenes has also been reported. The primary phosphine 324 has been isolated from the addition of diethylphosphite to t-butylphosphaethyne. The chemistry of phospha-alkyne cyclotetramer systems has been reviewed and the first examples of platinum(II) complexes of such cage systems described. Aspects of the reactivity of coordinated phospha-alkynes have received further study, and a remarkable metal-mediated double reduction of t-butylphosphaethyne to the complexed fluorophosphine 325 described Phosphorus-carbon-aluminium cage structures have been isolated from the reactions of kinetically stable phospha-alkynes with trialkylaluminium compounds and new phosphaborane systems have been obtained from the reactions of phospha-alkynes with polyhedral boranes . Further studies of wo-phospha-alkyne coordination chemistry have appeared . The reactivity of the ion 326 has been explored. ... 

Other alkenyl complexes have been formed by displacement of fluoride or chloride ion from fluoro-olefins by metal carbonyl anions, by reactions of the perfluoroalkenylsilver derivative with metal halide compounds, and by insertion of an alkyne into a metal-X bond, and parameters are given with structures [168] to [187]. (8, 86-91) As... 

It is well known that alkynes are less reactive than alkenes in electrophilic reactions 1117). The theoretical explanation is not obvious [118]. Although some theoretical calculations were also reported on the reactions of acetylene with [119], Si [120] and rare metal ions (Sc", , ... 

Alkynes carrying unpaired electron centers are classified into three groups depending on in which part of the molecule the unpaired electron resides in a substituent remote from the alkyne moiety, in a transition metal ion Ugated with the alkyne, or a to the C=C triple bond. 

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