Propargylic compounds insertion

Acylzirconocene chlorides 78, which are easily available through the hydrozirco-nation of alkenes or alkynes with Cp2Zr(H)Cl and subsequent CO insertion, can be used as acyl anion equivalents Cu(I)-catalyzed reactions with propargyl compounds 77 afford allenyl ketones 79 (Scheme 3.40) [86]. The use of an excess of 77 (2 equiv. to 78) is important for the selective preparation of 79, which prevents an undesirable side reaction of the allenic products 79 with 78. 

From a mechanistic viewpoint, the Pd(0)-eatalysed reactions of propargylic compounds so far discovered can be classified into four types I IV The allenyl complexes 5 undergo three types of transformations depending on reactants. Type I reactions proceed by insertion of unsaturated bonds to the n-bond between Pd and the sp2 carbon in 5. Type la is the insertion of alkenes to the palladium-carbon n-bond, and the 1,2,4-alkatrienes are formed by /f-elimination. Alkynes insert to form the alkenylpalladium 6, which undergoes various transformations such as insertion of unsaturated bonds and anion captures. 

The Pd(0)-catalyzed reactions of propargylic compounds so far discovered can be classified into four types, I, II, III, and TV, from a mechanistic viewpoint. The allenyl intermediate complex 8 undergoes three types of transformation, depending on reactants. The reactions of Type I proceed by insertion of unsaturated bonds into the a-bond between palladium and sp carbon in 8. This a-bond has a reactivity similar to the a-bond formed by the oxidative addition of alkenyl halides to Pd(0) in the Heck reaction [3]. Therefore, reactions similar to those observed in the Heck reaction are expected to occur witli the intermediate 8. Alkenes and carbon monoxide are known to insert into the palladium-carbon a-bond. The allene derivatives 9 are formed by these reactions (Scheme 11.3). 

VI.3 MIGRATORY INSERTION REACTIONS OF ALLYL, PROPARGYL, AND ALLENYL 2509 C. PROPARGYLIC COMPOUNDS... 

Although superficially similar, propargyl compounds do not form t -complex intermediates, but give t -allenic complexes. As part of a catalytic cycle, these can undergo typical reactions, such as coupling (Schemes 9.82 and 9.83), ° ° reduction by formate, alkene insertion and carbonylation (Scheme 9.84). 

Among several propargylic derivatives, the propargylic carbonates 3 were found to be the most reactive and they have been used most extensively because of their high reactivity[2,2a]. The allenylpalladium methoxide 4, formed as an intermediate in catalytic reactions of the methyl propargylic carbonate 3, undergoes two types of transformations. One is substitution of cr-bonded Pd. which proceeds by either insertion or transmetallation. The insertion of an alkene, for example, into the Pd—C cr-bond and elimination of/i-hydrogen affords the allenyl compound 5 (1.2,4-triene). Alkene and CO insertions are typical. The substitution of Pd methoxide with hard carbon nucleophiles or terminal alkynes in the presence of Cul takes place via transmetallation to yield the allenyl compound 6. By these reactions, various allenyl derivatives can be prepared. 

There are several reports in the literature dealing with the bimolecular [3 + 2] cycloaddition reactions of alkynyl-substituted diazo compounds. Propargyl diazoacetate 212, when stored for 2 weeks at 0 °C, was transformed into an oligomer to which the constitution 213 was assigned (273) (Scheme 8.50). The alkynyl-diazoketone 214 requires a much higher temperature and is transformed into pyrazole 215, which probably arises from intermolecular cycloaddition, pyrazole tautomerization, and carbenic N/H insertion (274). The inter-intramolecular... 

Rh complexes are examples of the most effective catalysts for the polymerization of monosubstituted acetylenes, whose mechanism is proposed as insertion type. Since Rh catalysts and their active species for polymerization have tolerance toward polar functional groups, they can widely be applied to the polymerization of both non-polar and polar monomers such as phenylacetylenes, propiolic acid esters, A-propargyl amides, and other acetylenic compounds involving amino, hydroxy, azo, radical groups (see Table 3). It should be noted that, in the case of phenylacetylene as monomer, Rh catalysts generally achieve quantitative yield of the polymer and almost perfect stereoregularity of the polymer main chain (m-transoidal). Some of Rh catalysts can achieve living polymerization of certain acetylenic monomers. The only one defect of Rh catalysts is that they are usually inapplicable to the polymerization of disubstituted acetylenes. Only one exception has been reported which is described below. 

Bacteria, antimicrobials against, 12, 456 Baeyer-Villiger oxidation, via tin amides, 9, 370 Barbier-Grignard-type reactions, and sonochemical metal insertions, 1, 315 Barbier-type reactions allenyl and propargyl tins, 9, 358 with allylic tins, 9, 357 with antimony(III) compounds, 9, 426 with bismuth(III) compounds, 9, 433 with cerium reagents, 10, 409 with indium compounds, 9, 685... 

Insertion into C-Zr bonds. Lithiated propargylic chlorides (by reaction with LTMP) are capable of inserting into zirconacycles. Reaction of the ring-expanded species with electrophiles leads to open-chain compounds. Stereodefmed 1-lithio-l-chloro-1,3-butadienes are prepared and each isomer reacts with alkylzirconocenes to afford (E)- or (Z)-dienes. ... 

Sulfur dioxide inserts into alkyltin bonds to give stannylsulfinates, R3Sn0-S(0)R,46 but the reactivity falls off in the sequence R = allyl allenyl propargyl > phenyl > benzyl > methyl. Insertion into a vinyl-tin bond occurs with retention of configuration. The reaction with aryl and benzyl compounds is cleanly first order in each of the two reactants, and is rather insensitive to the nature of any substituents in the benzyl ring. 

The catalytic [2 + 2 + 1]-cycloaddition reaction of two carbon—carbon multiple bonds with carbon monoxide has become a general synthetic method for five-membered cyclic carbonyl compounds. In particular, the Pauson-Khand reaction has been widely investigated and established as a powerful tool to synthesize cyclopentenone derivatives.110 Various kinds of transition metals, such as cobalt, titanium, ruthenium, rhodium, and iridium, are used as a catalyst for the Pauson-Khand reaction. The intramolecular Pauson-Khand reaction of the allyl propargyl ether and amine 91 produces the bicyclic ketones 93, which bear a heterocyclic ring as shown in Scheme 31. The reaction proceeds through formation of the bicyclic metallacyclopentene intermediate 92, which subsequently undergoes insertion of CO to give 93. 

The synthesis of a-benzofuranylacetamides and indolylacetamides has been achieved through a palladium-catalyzed process (Scheme 3.61 and Example 3.10) [67]. The overall reaction consisted of an intramolecular oxy/amino-palladation/isocyanide insertion process starting from functionalized propargyl alcohols. Although a vast array of compounds were successfully generated using this approach, the conversion of nitrated anilines was unsuccessful under the reaction conditions. 

Weak base such as cesium carbonate has been utilized in this reaction to generate diazo compounds in situ from tosylhydrazones through the Bamford-Stevens reaction. The reaction is initiated by palladium-promoted decarboxylation of propargylic carbonate to form propargylpalladium complex A, which then tautomerizes to afford allenylpalladium intermediate B. Subsequently, the common carbene formation-migratory insertion-p-hydride elimination occurs to afford various vinylallenes (Fig. 30). 

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