Catalytic reactions involving alkynes

Figure 2 The observed intermediates in the catalytic reaction involving the production of substituted benzenes by trimeriza-tion of alkynes with Ru2(CO)6 (R-DAB) as starting complex...

Group 4 metal based catalysts have been studied intensively in hydroamination reactions involving alkynes and allenes [77 81], but (achiral) hydroamination reac tions involving aminoalkenes were only recently reported [82 84]. The reactivity of these catalysts is significantly lower than that of rare earth, alkali, and alkaline earth metal based catalysts. In most instances, gem dialkyl activation [37] of the aminoalk ene substrate is required for catalytic turnover. 

Hydroamination reactions involving alkynes and enantiomerically pure chiral amines can produce novel chiral amine moieties after single pot reduction of the Schiffbase intermediate 82 (Scheme 11.27) [123]. Unfortunately, partial racemiza tion ofthe amine stereocenter was observed with many titanium based hydroamina tion catalysts, even in the absence of an alkyne substrate. No racemization was observed when the sterically hindered Cp 2TiMe2 or the constrained geometry catalyst Me2Si(C5Me4)(tBuN)Ti(NMe2)2 was used in the catalytic reaction. Also, the addition of pyridine suppressed the racemization mostly. 

We had established in previous catalytic reactions involving complex 24 that this precatalyst was activated by the removal of the cod (1,5-cyclooctadiene) from the ruthenium by its reaction with the alkyne substrate via a [2 + 2 + 2] cydization as illustrated in Equation 1.64 [57]. Thus, not only does this reaction constitute an activation of the Ru complex 24 by reacting off the cod, it also serves as a novel atom economic reaction in its own right. Both internal and terminal alkynes participate. The overall atom economy of this process is outstanding since cod itself is simply available by the nickel-catalyzed dimerization of butadiene. Thus, the tricyclic product is available by the simple addition to two molecules of butadiene and an alkyne with anything else only needed catalytically. 

Ni(0) complex are formed upon heating this complex. Hoberg and co-workers fiirther studied the C-C coupling reactions of CO2 with alkynes, alkenes (including cycloalkenes), and 1,2- or 1,3-dienes (4-10). Unfortunately, most of these reactions produce stable five-membered metallacycle complexes and the catalytic reactions, involving insertion of activated allies (or other reagents) into the five-membered metallacycle followed by reductive elimination, have not been realized. 

Another interesting catalytic transformation involving alkynes is the hydro phosphinylation, which affords alkenylphosphine oxides [62]. The formation of hydride-phosphinito compounds is one of the key steps of the reaction. These species are formed by oxidative addition of the P-H bond of diphenylphosphine oxide to platinum(O) and palladium(O) complexes, which act as catalytic precursors. In this context, it should be mentioned that a novel method to prepare hydride-phosphinito compounds has been recently reported. The new strategy starts from 133 and involves the oxidative addition of the P-H bond of... 

Further exploration of the higher activity of the Ni complexes compared to Pd analogs led to the discovery of a novel nano-sized catalytic system with superior performance for hydrothiolation and hydroselenation reactions of alkynes [ 152,153]. Furthermore, it was found that with a simple catalyst precursor - Ni(acac)2 - the reaction was carried out with excellent yields and excellent selectivity, even at room temperature. Both terminal and internal alkynes were successfully involved in the addition reaction. This catalytic system was tolerant to various functional groups in alkynes and was easily scaleable for the synthesis of 1-50 g of product (Scheme 3.85) [152, 153]. The proposed mechanism of the catalytic reaction involved (i) catalyst self organization with nano-sized particles formation, (ii) alkyne insertion into the Ni—Z bond and (iii) protonolysis with RZH (Scheme 3.86). 

Stoichiometric reaction of the above mentioned Pd complex with 1-octyne gave a mixture of a- and j -isomers in 65% total yield. Different ratio of the a- and )S-isomers was observed in the stoichiometric reaction, which could be attributed to the changes in reaction conditions (temperature, concentration, etc.). It was proposed that catalytic cycle involves alkyne insertion into the Pd-H bond—i.e. hydropalladation [86]. 

Using NiCl2 without ligand resulted in development of efficient catalytic system for addition of alkylphosphinates not only to terminal, but also to internal alkynes (Scheme 8.38) [100]. For terminal alkynes a mixture of a and p isomers was obtained with different substituents. Addition reaction to alkenes was more difii-cult compared to the same reaction involving alkynes [100]. For example in reaction with 1-octene 100% yield was found with Pd catalyst, but only 25% with NiCli. In the presence of phosphine ligand the yield was increased to 69% (L = dppf). Some other reactions of phosphinates addition to the multiple bonds have been also studied [101-104]. 

Muetterties has suggested that the dimeric hydride [RhH(P OiPr 3)2]2 catalyzes alkene and alkyne hydrogenation via dinuclear intermediates [91]. However, no kinetic evidence has been reported to prove the integrity of the catalysts during the reactions. On the other hand, studies of the kinetics of the hydrogenation of cyclohexene catalyzed by the heterodinuclear complexes [H(CO) (PPh3)2Ru((u-bim)M(diene)] (M = Rh, Ir bim=2,2 -biimidazolate) suggested that the full catalytic cycle involves dinuclear intermediates [92]. 

A recent study has, however, unraveled a most intricate four-step catalytic cycle involving a bimetallic cyclic carbozirconation (Scheme 1.60) [150]. The stoichiometric conversion of EtZrCp2Cl and Et3Al into the five-membered bimetallic complex and its subsequent stoichiometric reaction with an alkyne to give an aluminacydopentene and EtZrCp2Cl were the two key experimental findings. 

Vinyl acetate is one of many compounds where classical organic chemistry has been replaced by a catalytic process. It is also an example of older acetylene chemistry becoming outdated by newer processes involving other basic organic building blocks. Up to 1975 the preferred manufacture of this important monomer was based on the addition of acetic acid to the triple bond of acetylene using zinc amalgam as the catalyst, a universal reaction of alkynes. 

An efficient synthesis of functionalized carbazoles was developed by the palladium-catalyzed annulation of a variety of internal alkynes. This reaction involves arylpalladation of the alkyne, followed by intramolecular Heck olefination, and double bond isomerization. The iodoindole 588 reacts with the alkyne 589 in the presence of a catalytic amount of palladium(O) to give substituted carbazoles 590. In this reaction two new C-C bonds are formed in a single step. Higher reaction temperatures were necessary due to the low reactivity of the iodoindole (566) (Scheme 5.29). 

Another rhodium vinylidene-mediated reaction for the preparation of substituted naphthalenes was discovered by Dankwardt in the course of studies on 6-endo-dig cyclizations ofenynes [6]. The majority ofhis substrates (not shown), including those bearing internal alkynes, reacted via a typical cationic cycloisomerization mechanism in the presence of alkynophilic metal complexes. In the case of silylalkynes, however, the use of [Rh(CO)2Cl]2 as a catalyst unexpectedly led to the formation of predominantly 4-silyl-l-silyloxy naphthalenes (12, Scheme 9.3). Clearly, a distinct mechanism is operative. The author s proposed catalytic cycle involves the formation of Rh(I) vinylidene intermediate 14 via 1,2-silyl-migration. A nucleophilic addition reaction is thought to occur between the enol-ether and the electrophilic vinylidene a-position of 14. Subsequent H-migration would be expected to provide the observed product. Formally a 67t-electrocyclization process, this type of reaction is promoted by W(0)-and Ru(II)-catalysts (Chapters 5 and 6). 

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