Hydrometallation Of alkynes

Alkenylzirconium compounds 34 obtained by the hydrometallation of alkynes were transformed to 1-alkenylboron compounds via transmetallation with B-halo-9-BBN or B-chlorocatecholborane62,63 (Scheme 5). Mercuration followed by transmetallation to BH3 was advantageous over the lithiation route in the synthesis of indole-3-boronic acid 35.64... 

A transmetallation sequence from the products of hydrometallation of alkynes provides ac[Pg.205]

The hydrometallation of alkynes is one of the most widely used routes to vinyl organometallics,131 which in turn are valuable in the construction of molecular structures containing carbon-carbon double bonds of defined stereochemistry. 

A number of cycloisomerization reactions of enynes to construct five-membered car-bocycles with a variety of transition metal catalysts have been reported thus far [24]. The mechanisms that have been proposed for the cycloisomerization of enynes include 1) hydrometallation of alkyne followed by carbometallation of the olefin ... 

For hydrometallation of alkynes, similar analysis shows that D(M—H) - D(M—vinyl) need only be less than ca. 37 kcal mol" metal-vinyl bonds are generally stronger than metal-alkyl bonds, so this should be easy to achieve. Wayland has concluded that on thermodynamic grounds ...virtually any transition metal should be capable of accomplishing this reaction . ... 

Aluminum-alkyne interactions were invoked when hydrosilylation of alkynes proceeded in the presence of catalytic amounts of AICI3 or EtAlCl2 [31]. Although most hydrometalations of alkynes occur in a cis configuration, trans-selective hydrosilylation was consistently observed (Scheme 6.14). 

Vinyl-Lithiums from Ketones The Shapiro Reaction The Aliphatic Friedel-Crafts Reaction Hydrometallation of Alkynes... 

Among transition metals, zirconium is the most important for hydrometallation of alkynes. The available 16 electron complex Cp2ZrHCl (Cp = cyclopentadienyl) 122 (zirconocene hydrochloride) adds stereoselectively syn to alkynes to give the -vinyl zirconium species 126. The initial interaction is the donation of two electrons by the triple bond 123 to make the reasonably stable 18-electron n-complex (or r 2-complex) 124. Typically for a transition metal Ji-complex, a ligand is now transferred from the metal to one end of the tr-bond 125 while the metal itself forms a stable 16-electron o-complex at the other 126. The H transfer is intramolecular so the metal and H atoms must add to the same side of the triple bond. The zirconium atom transfers the least stable anion among its ligands. This is obviously II as Cl is much more stable. The metal prefers to take the terminal position because the resulting o-complex is more stable as it is a o-complex of the less-substituted carbanion.27... 

Hydrometalation of alkynes is one of the most direct and powerful synthetic tools for the formation of alkenyl metal species, which could be simply hydrolyzed to yield the corresponding alkenes or readily utilized as building blocks for further transformations such as cross coupling and electrophilic substitution reactions. 

Other frans-hydrometalation of alkynes includes the use of a combination of InCls and DIBAL to produce alkenyl indium species [43], and a Lewis acid catalyzed hydrogermylation of terminal/intemal alkynes (Fig. 19) [44]. In both cases, frans-hydrometalation was believed to account for the stereoselectivity of the reactions. 

Similar reactivity is observed in the cyclization of enynes in the presence of the yttrium-based catalyst 70 and a silane reductant [53,54]. The 1,6- and 1,7-enynes 90 and 91 provide -E-alkylidene-cyclopentancs 92 and -cyclohexanes 93 in very good yield (Eq. 15, Scheme 20) [55]. These transformations likely proceed by syn hydrometallation of the 7r-basic alkyne, followed by insertion of the alkene and a-bond metathesis. The reaction of 1,6-enynes tolerated... 

Palladium complexes are effective catalysts for the reductive cydization of enyne substrates [53,54], The first report of catalytic cydization of 1,6- and 1,7-enynes 115a,b to cyclopentane 116a and cyclohexane 116b derivatives appeared in 1987 (Eq. 19) [70]. The authors proposed that the Pd(II) species 117 forms by oxidative addition of acetic acid to Pd(0) (Scheme 25). Complex 117 hydrometallates the alkyne to give 118, which cyclizes to provide... 

Similar intramolecular hydroarylations of alkynes and alkenes, which obviate the need for a halide or triflate group on the aryl ring, are now well established. Sames group screened over 60 potential catalysts and over 200 reaction conditions, and found that Ru(m) complexes and a silver salt were optimal. This process appears to tolerate steric hindrance and halogen substrates on the arene (Equations (175)—(177)). The reaction is thought to involve alkene-Ru coordination and an electrophilic pathway rather than a formal C-H activation of the arene followed by alkene hydrometallation, and advocates the necessary cautious approach to labeling this reaction as a C-H functionalization... 

In addition to transition metals, recent work has demonstrated that strong Lewis acids will catalyze the addition of silanes to alkynes in both an intra- and an intermolecular fashion.14,14a-14c The formation of vinylsilanes from alkynes is possible by other means as well, such as the synthetically important and useful silylcupration15,15a of alkynes followed by cuprate protonation to afford vinylsilanes. These reactions provide products which can be complementary in nature to direct hydrometallation. Alternatively, modern metathesis catalysts have made possible direct vinylsilane synthesis from terminal olefins.16,16a... 

For use of alkynes as nucleophilic partners in catalytic hydrometallative reductive couplings to aldehydes, see ... 

Two mechanisms are conceivable for the reaction. One involves the hydrometalation of the metal hydride at alkyne followed by reductive elimination.15 Alternatively, phosphinometalation followed by protiode-metalation can occur.16 To gain insight into the mechanism, a carbonylation... 

Yamamoto has proposed a mechanism for the palladium-catalyzed cyclization/hydrosilylation of enynes that accounts for the selective delivery of the silane to the more substituted C=C bond. Initial conversion of [(77 -C3H5)Pd(GOD)] [PF6] to a cationic palladium hydride species followed by complexation of the diyne could form the cationic diynylpalladium hydride intermediate Ib (Scheme 2). Hydrometallation of the less-substituted alkyne would form the palladium alkenyl alkyne complex Ilb that could undergo intramolecular carbometallation to form the palladium dienyl complex Illb. Silylative cleavage of the Pd-G bond, perhaps via cr-bond metathesis, would then release the silylated diene with regeneration of a palladium hydride species (Scheme 2). 

The insertion of alkene to metal hydride (hydrometallation of alkene) affords the alkylmetal complex 34, and insertion of alkyne to an M—R (R = alkyl) bond forms the vinyl metal complex 35. The reaction can be understood as the cis carbometallation of alkenes and alkynes. 

Addition of hydrosilane to alkenes, dienes and alkynes is called hydrosilylation, or hydrosilation, and is a commercially important process for the production of many organosilicon compounds. As related reactions, silylformylation of alkynes is treated in Section 7.1.2, and the reduction of carbonyl compounds to alcohols by hydrosilylation is treated in Section 10.2. Compared with other hydrometallations discussed so far, hydrosilylation is sluggish and proceeds satisfactorily only in the presence of catalysts [214], Chloroplatinic acid is the most active catalyst and the hydrosilylation of alkenes catalysed by E PtCU is operated commercially [215]. Colloidal Pt is said to be an active catalytic species. Even the internal alkenes 558 can be hydrosilylated in the presence of a Pt catalyst with concomitant isomerization of the double bond from an internal to a terminal position to give terminal silylalkanes 559. The oxidative addition of hydrosilane to form R Si—Pt—H 560 is the first step of the hydrosilylation, and insertion of alkenes to the Pt—H bond gives 561, and the alkylsilane 562 is obtained by reductive elimination. 

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). 

Keeping in mind the easy access to vinylic tellurides (see Sections 5.2.3 and 5.2.5), and their fast transformation into widely-used reactive organometallics, we can consider this reaction sequence as a valid alternative to the known methods of generating vinyllithiums and, specially, vinyl cyanocuprates. In addition, the zz/r/r-hydrotel I u ration of alkynes is unique among the known, synthetically useful hydrometallations, since it allows the direct preparation of... 

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. 

All of the proposed mechanisms for the reduction of alkynes with metal hydride-transition metal halide combinations involve an initial hydrometallation of the ir-system by the transition metal hydride, formed by the reaction of the original metal hydride with the transition metal halide, to form the vi-nylmetallic intermediate (99 equation 38). For the reduction of alkenes, similar alkylmetallic intermediates are implied to be formed. In the case of the reduction of alkenes with NaBH4 in the presence of Co" in alcohol solution, the hydrometallation reaction appears to be reversible as evidenced by the incorporation of an excess of deuterium when NaBD4 was used in the reduction. ... 

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