Alkyne to vinylidene isomerization

Figure 2. Proposed mechanisms for the alkyne to vinylidene isomerization coordinated to a transition metal fragment...

These isomerization reactions are of great interest to theoreticians because the role of many factors (metal, substituents on the organic fragment, ancillary ligands) on the outcome of the reaction can be studied through computations. The purpose of this chapter is to describe the theoretical studies carried out on the isomerization of alkyne to vinylidene and alkene to carbene in the presence of transition metal fragments. 

C(sp2)-H energies are close. Likewise, the transformation of an alkene to a substituted carbene corresponds mostly to the loss of the it component of a C=C double bond. The loss of these bonds is clearly costly in energy but a key point to the present story is that the energy cost is different for the two systems. Loss of one of the it bonds of an alkyne corresponds to 40-50 kcal.mol 1, whereas the loss of the it bond in an alkene amounts to 70-80 kcal.mol 1. Therefore the isomerization of primary alkyne to vinylidene and of alkene to substituted carbene are both endothermic with the latter having the larger endothermicity. 

Several mechanisms have been currently proposed for isomerizing primary alkyne to vinylidene in presence of transition metal fragments [2, 3],... 

DFT calculations confirmed the similarities with the alkyne/vinylidene transformation but have revealed that additional parameters were essential to achieve the isomerization [8, 20-23]. The hydride ligand on the 14-electron fragment RuHC1L2 opens up a pathway for the transformation similar to that obtained for the acetylene to vinylidene isomerization. However, thermodynamics is not in favor of the carbene isomer for unsubstituted olefins and the tautomerization is observed only when a re electron donor group is present on the alkene. Finally the nature of the X ligand on the RuHXL2+q (X = Cl, q=0 X = CO, q=l) 14-electron complex alters the relative energy of the various intermediates and enables to stop the reaction on route to carbene. 

Since Bruce s pioneering work in the area of ruthenium vinylidene chemistry (1), it has been well known that isomerization of a terminal alkyne to a vinylidene on a metal center is not only favorable but also effects a reversal in the reactivity of the carbon atoms. However, hydration catalysis was not possible, because alkyl migration from a proposed acyl intermediate led to an... 

This study supports rate-determining H-OH bond breaking, which constrasts with previous reports that identified vinylidene isomerization as the key step in catalytic alkyne activation. The results indicate an enzyme-like mechanism is operative involving cooperative substrate activation by a metal center and proximal hydrogen bond donor/acceptors. In the future we will apply these principles to the activation of additional species. 

Alkynes react readily with a variety of transition metal complexes under thermal or photochemical conditions to form the corresponding 7t-complexes. With terminal alkynes the corresponding 7t-complexes can undergo thermal or chemically-induced isomerization to vinylidene complexes [128,130,132,133,547,556-569]. With mononuclear rj -alkyne complexes two possible mechanisms for the isomerization to carbene complexes have been considered, namely (a) oxidative insertion of the metal into the terminal C-Fl bond to yield a hydrido alkynyl eomplex, followed by 1,3-hydrogen shift from the metal to Cn [570,571], or (b) eoneerted formation of the M-C bond and 1,2-shift of H to Cp [572]. 

The reaction between acetylene and RhfCOXi CjH i -QH,) [which acts as a source of the Rh(COXf/5-C9H7) fragment] affords 33 in 50% yield (61). The reaction is supposed to proceed via oxidative addition of the alkyne to the rhodium fragment, followed by isomerization to the vinylidene complex which then interacts with a second rhodium fragment ... 

Addition of internal alkynes to (t)5-C5H5)(PR3)2RuCI does not lead to the formation of the corresponding disubstituted vinylidene (68). The failure of this reaction could reflect the relative difficulty of a 1,2-alkyl shift for internal alkynes as compared to the 1,2-proton shift for the successful rearrangement of terminal alkynes (Scheme 9). Alternatively, if the deprotonation-reprotonation route is important in the rearrangement of terminal alkynes (vide supra), then clearly internal alkynes would not undergo a similar isomerization. 

The formation of reactive intermediates provides possible opportunities for new reaction design. An attractive highly reactive intermediate, carbenes, which demonstrate numerous useful synthetic pathways, most notably by addition to alkenes and alkynes and also insertion into X-H bonds, where X is both carbon and heteroatoms, suffers from problems associated with their accessibility. Undoubtedly, the most useful class of precursor is the diazo compounds, whose safety problems restrict their use. For the specific case of vinylidenes, an attractive possibility is a terminal alkyne which is isomeric with a vinylidene. Although the thermolysis appears to effect this transformation (Equation 1.1, path a), the extraordinarily high temperatures required make the prospect of a transition metal-catalyzed version (Equation 1.1, path b) attractive. The early studies of Werner [6] using Rh and Bruce and co-workers [7] using Ru proved the facility with which such species would form however, the studies focused on the formation and isolation of the vinylidene-metal complexes and their stoichiometric reactions. 

For alkynes, an electrophilic activation/insertion pathway leads to the Markovnikov product, whereas an alkyne/vinylidene isomerization pathway is specific towards anti-Markovnikov addition products [4—6]. Some insightful studies on elementary steps of the above mechanisms will now be presented. 

Recently, an analogous Rh(I)-catalyzed transformation of terminal o-alkynylani-lines 109 into indoles 111 was reported by Trost (Scheme 9.42) [93]. Mechanistically, this cycloisomerization proceeded via an alkyne-vinylidene isomerization of a terminal alkyne 109, leading to the Rh-vinylidene 110, similar to that proposed by McDonald (Scheme 9.41). Consequently, internal o-alkynylaniBnes were shown to be completely unreactive in this transformation. 

Once generated, one way to trap a vinylidene complex could be in an electrocyclic reaction. Indeed, treatment of an alkyne 8.143, having a dienyl unit attached, with a ruthenium catalyst yields a tricyclic product 8.144 (Scheme 8.40). Deuterium-labeling experiments are consistent with the alkyne-vinylidene isomerization. A tungsten catalyst, W(CO)5.THF, may also be used. ... 

Direct isomerization q -alkyne to q -vinylidene complexes by a bimolecular mechanism has also been considered. " ... 

The most general route to generate vinylidene complexes [M]=C=C(H)R is the direct activation of terminal alkynes HC=CR by a coordinatively unsaturated transition metal complex, via the generation of unstable 77 -alkyne or hydride-alkynyl intermediates which tautomerize into the thermodynamically more stable vinylidene isomers (Scheme 15) ia,ic,207 theoretical and kinetic studies on the metal-mediated alkyne-vinylidene isomerization... 

In addition to alcohols, some other nucleophiles such as amines and carbon nucleophiles can be used to trap the acylpalladium intermediates. The o-viny-lidene-/j-lactam 30 is prepared by the carbonylation of the 4-benzylamino-2-alkynyl methyl carbonate derivative 29[16]. The reaction proceeds using TMPP, a cyclic phosphite, as a ligand. When the amino group is protected as the p-toluenesulfonamide, the reaction proceeds in the presence of potassium carbonate, and the f>-alkynyl-/J-lactam 31 is obtained by the isomerization of the allenyl (vinylidene) group to the less strained alkyne. 

Wakatsuki et al. (4) proposed vinyl complex, 5, and presented DFT results supporting isomerization to a vinylidene hydride as the rate determining step. Our results indicate that the rate determining step involves H-OH bond breaking and that protonation of a bound alkyne is the rate determining step in this... 

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