Alkynes group 4 metal catalysts

Organometallic compounds which have main group metal-metal bonds, such as S—B, Si—Mg,- Si—Al, Si—Zn, Si—Sn, Si—Si, Sn—Al, and Sn—Sn bonds, undergo 1,2-dimetallation of alkynes. Pd complexes are good catalysts for the addition of these compounds to alkynes. The 1,2-dimetallation products still have reactive metal-carbon bonds and are used for further transformations. 

Concerning consecutive reactions, a typical example is the hydrogenation of alkynes through alkenes to alkanes. Alkenes are more reactive alkynes, however, are much more strongly adsorbed, particularly on some group VIII noble metal catalysts. This situation is illustrated in Fig. 2 for a platinum catalyst, which was taken from the studies by Bond and Wells (45, 46) on hydrogenation of acetylene. The figure shows the decrease of... 

In hydrogenation, early transition-metal catalysts are mainly based on metallocene complexes, and particularly the Group IV metallocenes. Nonetheless, Group III, lanthanide and even actinide complexes as well as later metals (Groups V-VII) have also been used. The active species can be stabilized by other bulky ligands such as those derived from 2,6-disubstituted phenols (aryl-oxy) or silica (siloxy) (vide infra). Moreover, the catalytic activity of these systems is not limited to the hydrogenation of alkenes, but can be used for the hydrogenation of aromatics, alkynes and imines. These systems have also been developed very successfully into their enantioselective versions. 

As mentioned above, MPVO catalysts are very selective towards carbonyl compounds. Alkenes, alkynes or other heteroatom-containing double bonds are not affected by these catalysts, while they can be reduced by transition-metal catalysts. Examples of the reduction of a,/ -unsaturated ketones and other multifunctional group compounds are compiled in Table 20.3. 

Silylformylation, defined as the addition of RsSi- and -CHO across various types of bonds using a silane R3SiH, CO, and a transition metal catalyst, was discovered by Murai and co-workers, who developed the Co2(CO)8-catalyzed silylformylation of aldehydes, epoxides, and cyclic ethers [26]. More recently, as described in detail in Section 5.3.1, below, alkynes and alkenes have been successfully developed as silylformylation substrates. These reactions represent a powerful variation on hydroformylation, in that a C-Si bond is produced instead of a C-H bond. Given that C-Si groups are subject to, among other reactions, oxidation to C-OH groups, silylformylation could represent an oxidative carbonylation of the type described in Scheme 5.1. 

In our initial studies on the [5+2] cycloaddition, several metal catalysts were screened. Rhodium(I) systems were found to provide the optimum yields and generality [26]. Since the introduction of this new reaction in 1995, our group and others have reported other catalyst systems that can effect the cycloaddition of tethered VCPs and systems. These new catalysts thus far include chlororhodium dicarbonyl dimer ( [RhCl(CO)2]2 ) [27], bidentate phosphine chlororhodium dimers such as [RhCl(dppb)]2 [28] and [RhCl(dppe)]2 [29], and arene-rhodium complexes [(arene)Rh(cod)] SbFs [30]. [Cp Ru(NCCH3)3] PFg has also been demonstrated to be effective in the case of tethered alkyne-VCPs [31], but has not yet been extended to intermolecular systems or other 2n -components. 

When desired vinylidene-mediated pathways are not sufficiently favorable. Group 9 metal catalysts can access a set of typical side-reaction pathways. Alkyne dimerization to give conjugated enynes or higher oligomers is often observed. Polysubstituted benzenes resulting from [2 + 2 + 2] alkyne cyclotrimerization are also common coproducts. Fortunately, the selectivity of rhodium and iridium catalysts can often be modulated by the variation of spectator ligands. 

Although ruthenium and Group 6 metal catalysts are commonly employed for anti-Markovnikov alkyne hydrofunctionalization (Chapter 10), some interesting rhodium- and iridium-catalyzed methods have also been reported. These can be divided into three groups based on the nature of the incoming functional group ... 

The key success of these metal-catalyzed processes lies in the replacement of an unachievable carbozincation by an alternative carbometallation involving the transition metal catalyst, or another pathway such as an alkene-alkene (or alkyne) oxidative coupling promoted by a group IV transition metal complex, followed by transmetallation. An organozinc is ultimately produced and the latter can be functionalized by reaction with electrophiles. 

For other , 1-disubstituted alkynes lacking an activating group, transition metal catalysts or promotors could be used to achieve allylzincation. 

The polymerization of substituted alkynes is postulated to proceed either by the metathesis mechanism or by an insertion mechanism (18). Numerous alkyne derivates have been shown to polymerize in the presence of group V, VI, and VIII transition metal catalysts. 

A series of internal alkynes has been investigated, revealing that the presence of an alkyl or aryl group does not appear to change the course of the reaction. Internal alkynes, however, do not undergo germylformylation in this system. In the case of tin, two reports exist of the stannylformyla-tion of unsaturated carbon substrates, but both proceed by a free radical mechanism initiated by AIBN and do not require a transition metal catalyst.128... 

Addition reactions of three kinds of main group metal compounds, namely R—M X (carbometallation, when R are alkyl, alkenyl, aryl or allyl groups), H—M X (hydrometallation with metal hydrides) and R—M —M"—R (dimetallation with dimetal compounds) to alkenes and alkynes, are important synthetic routes to useful organometallic compounds. Some reactions proceed without a catalyst, but many are catalysed by transition metal complexes. 

Addition of organometallic compounds of main group metals R—M —X (M = B, Al, Zn, Mg, Sn) to alkenes and alkynes is called carbometallation. Some reactions proceed without a catalyst, but they are promoted or accelerated by transition metal... 

Addition of hydride bonds of main group metals such as B—H, Mg—H, Al—H, Si—H and Sn—H to alkenes and alkynes to give 513 and 514 is called hydrometallation and is an important synthetic route to compounds of the main group metals. Further transformation of the addition product of alkenes 513 and alkynes 514 to 515,516 and 517 is possible. Addition of B—H, Mg—H, Al—H and Sn—H bonds proceeds without catalysis, but their hydrometallations are accelerated or proceed with higher stereoselectivity in the presence of transition metal catalysts. Hydrometallation with some hydrides proceeds only in the presence of transition metal catalysts. Hydrometallation starts by the oxidative addition of metal hydride to the transition metal to generate transition metal hydrides 510. Subsequent insertion of alkene or alkyne to the M—H bonds gives 511 or 512. The final step is reductive elimination. Only catalysed hydrometallations are treated in this section. 

Considering the mechanistic rationales of the transition metal-catalyzed enyne cycloisomerization, different catalytic pathways have been proposed, depending on the reaction conditions and the choice of metal catalyst [3-5, 45], Complexation of the transition metal to alkene or alkyne moieties can activate one or both of them. Depending on the manner of formation of the intermediates, three major mechanisms have been proposed. The simultaneous coordination of both unsaturated bonds to the transition metal led to the formation of metallacydes, which is the most common pathway in transition metal-catalyzed cycloisomerization reactions. Hydrometalation of the alkyne led to the corresponding vinylmetal species, which reacts in turn with olefins via carbometalation. The last possible pathway involves the formation of a Jt-allyl complex which could further react with the alkyne moiety. The Jt-allyl complex could be formed either with a functional group at the allylic position or via direct C-H activation. Here the three major pathways will be discussed in a generalized form to illustrate the mechanisms (Scheme 8). 

Many organometallic compounds that have main group metal-hydrogen or metal-metal bonds undergo 1,2-hydrometallation or 1,2-dimetallation of alkynes. Pd complexes are good catalysts for these processes [118]. Since the resulting products contain one or two reactive carbon-metal bonds they are well suited for further transformations, particularly in a sequential fashion. 

Among acetylenes with heteroatoms other than silicon, only l-(pentafluorophenyl)-1-alkynes have been known so far to polymerize with group 5 transition metal catalysts. These acetylenes produce insoluble polymers in the presence of TaCl5—n-Bu Sn 65 >. 

To obtain vinylsilanes from alkynes, transition metal complexes of Group VIII combined with a main group metal chloride are particularly effective." In the presence of a heterogeneous catalyst like Pd/y-alumina, Rh/carbon and polymer bound Pt, trichlorosilane gives trichlorovinylsilane with atmospheric pressure of acetylene." Platinum supported on sulfur-containing silica gel is a practical catalyst for 1,2-dihydrosilylation, as exemplified in equation (10). ... 

Intramolecular [3-I-2]-cycloaddition reactions have also been performed using alkyne moieties as reactants. An early example of this methodology is the high-yield formation of 27 from alkyne 26. As in the case of the (Z)-3-benzyloxybut-2-enoate derivatives (vide supra), the presence of the 3-siloxy group is essential for the reaction to proceed. A diastereomeric mixture of 26 leads to the formation of the two diastereomeric bicyclic adducts 27 in a combined yield of 88%. ° Interestingly, albeit with only 60 /o conversion after 100 hours, the same products are also formed at room temperature under the influence of ultrasound and 6 mol /o of tetrakis(triphenylphosphane)palladium(0) as catalyst. In a purely thermal reaction in the absence of a metal catalyst, a totally different reaction, presumably involving a Diels —Alder type addition of the alkyne to a vinylbenzene moiety, takes place. ... 

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