Vinylidene hydride

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... 

The reaction of an alkyne, HCsCR, with a metal hydride can proceed in several ways. It may proceed by insertion into the M—H bond to form M—CH=CHR, or transfer of an H to the hydride to give (Hj)M—CsCR or formation of a vinylidene hydride, (H)M=C=CHR. A study by Crabtree and co-workers shows the variation with the substituent R for the reaction of one metal hydride with different alkynes, as shown in the following Scheme ... 

There have a number of computational studies of hypothetical RMMR species [10-13, 40, 411. The simplest compounds are the hydrides HMMH. Some calculated structural parameters and energies of the linear and trans-bent metal-metal bonded forms of the hydrides are given in Table 1. It can be seen that in each case the frans-bent structure is lower in energy than the linear configuration. However, these structures represent stationary points on the potential energy surface, and are not the most stable forms. There also exist mono-bridged, vinylidene or doubly bridged isomers as shown in Fig. 2... 

V. Formation of Hydride-Vinylidene Complexes by Addition of Terminal... 

FORMATION OF HYDRIDE-VINYLIDENE COMPLEXES BY ADDITION OF TERMINAL ALKYNES TO OsHCI(CO)(P Pr3)2... 

In contrast to the reactions shown in Scheme 3, the complex OsHCl(CO)(P Pr3)2 reacts with cyclohexylacetylene at room temperature to give the hydride-vinylidene derivative OsHCl(C=CHCy)(CO)(P Pr3)2 in 70% yield (Scheme 8).37 Kinetic measurements yield a second-order rate constant of (6.0 0.2) x 10 3 at... 

The metalloalkyne complex Ru ( )-CH=CH(CH2)4C CH Cl(CO)(P,Pr3)2 exhibits behavior similar to that of cyclohexylacetylene (Scheme 10).40 Thus, it reacts with OsHCl(CO)(P Pr3)2 to give the hydride-vinylidene derivative (P Pr3)2 (CO)ClRu ( )-CH=CH(CH2)4CH=C OsHCl(CO)(P,Pr3)2, which evolves in toluene into the heterodinuclear-pi-bisalkenyl complex (P Pr3)2(CO)ClRu (is)-CH=CH(CH2)4CH=CH-( ) OsCl(CO)(P,Pr3)2. Kinetic measurements between 303 and 343 K yield first-order rate constants, which afford activation parameters ofAH = 22.1 1.5, kcal-mol-1 andAS = -6.1 2.3 cal-K 1-mol 1. The slightly negative value of the activation entropy suggests that the insertion of the vinylidene ligand into the Os—H bond is an intramolecular process, which occurs by a concerted mechanism with a geometrically highly oriented transition state. 

Hydride addition to the cationic Os(O) carbyne complex 10 occurs at the para position of the aryl ring rather than at the carbyne carbon, affording the vinylidene complex 11 (33) ... 

The most electrophilic site in the cationic dH Ru and Os arylcarbyne complexes is not the carbyne carbon, but the para position of the aryl ring. As noted in Section II, B,2, hydride reduction of these compounds affords zero-valent vinylidene species ... 

Already 20 years ago, Antonova et al. proposed a different mechanism, with a more active role of the transition metal fragment [3], The tautomerization takes place via an alkynyl(hydrido) metal intermediate, formed by oxidative addition of a coordinated terminal alkyne. Subsequent 1,3-shift of the hydride ligand from the metal to the P-carbon of the alkynyl gives the vinylidene complex (Figure 2, pathway b). 

In the transformation of a 1-alkyne to a vinylidene in the coordination sphere of a transition metal, the migrating hydrogen atom plays a key role. Usually, ancillary ligands on the metal are only spectators and do contribute to small modifications of the bonding properties of the metal fragment. However, if a hydride is present as a ligand to the transition metal center, it may interfere with the alkyne to vinylidene transformation. This may open up new selective and efficient routes to vinylidene complexes. 

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. 

SCHEME 13. Conversion of aldehydes into alkynes under homologation. Application of hydride shifts in vinylidene carbenoids... 

Both P-hydride transfers result in polypropene molecules with one vinylidene and one n-propyl end group. The two transfers are zero- and first-order, respectively, in monomer. P-Hydride transfer yields vinyl end groups in ethylene polymerization. 

The degree of polymerization is obtained by dividing the propagation rate by the sum of all chain-breaking (transfer) reactions. For the simple situation where the only P-hydride transfer is that described by Eqs. 8-37 and 8-38 (which produce vinylidene end groups in polypropene) and no P-alkyl transfer occurs, the degree of polymerization is... 

In the absence of H2 and other transfer agents, polymer molecular weight is limited by various P-hydride transfers—from normal (1,2-) and reverse (2,1-) propagating centers, before and after rearrangement [Lehmus et al., 2000 Resconi et al., 2000 Rossi et al., 1995, 1996 Zhou et al., 2001] (Sec. 8-4i-2). Vinylidene, vinylene, and trisubstituted double-bond end groups are formed in 1-alkene polymerizations, vinyl and vinylene in ethylene polymerization. [Vinyl groups are also produced in some 1-alkene polymerizations, not by P-hydride transfer, but by P-alkyl transfer (Sec. 8-4i-2).]... 

The double-bond composition varies in a complex manner with changes in metallocene and monomer concentrations because the orders of dependence of the various P-hydride transfer reactions on monomer and metallocene are not the same [Liu et al., 2001c Zhou et al., 2001]. Vinylidene content decreases with increasing monomer concentration, but increases with increasing metallocene. The trends for vinylene content are the opposite, while trisubstituted double-bond content is relatively unaffected by monomer and metallocene concentrations. 

The proposed mechanism involves either path a in which initially formed ruthenium vinylidene undergoes nonpolar pericyclic reaction or path b in which a polar transition state was formed (Scheme 6.9). According to Merlic s mechanism, the cyclization is followed by aromatization of the ruthenium cyclohexadienylidene intermediate, and reductive elimination of phenylruthenium hydride to form the arene derivatives (path c). A direct transformation of ruthenium cyclohexadienylidene into benzene product (path d) is more likely to occnir through a 1,2-hydride shift of a ruthenium alkylidene intermediate. A similar catalytic transformation was later reported by Iwasawa using W(CO)5THF catalyst [14]. 

The working mechanism involves a [2 + 2] cycloaddition between the Ru=C bond of ruthenium vinylidene and olefin to form the metallacyclobutane 92, which subsequently undergoes P-hydride elimination leading to the 7i-allyl hydride complex 93 and reductive elimination to furnish the conjugated trienes 89 (Scheme 6.31), and eventually to give the observed aromatic product 90. 

The proposed mechanism involves the formation of ruthenium vinylidene 97 from an active ruthenium complex and alkyne, which upon nucleophilic attack of acetic acid at the ruthenium vinylidene carbon affords the vinylruthenium species 98. A subsequent intramolecular aldol condensation gives acylruthenium hydride 99, which is expected to give the observed cyclopentene products through a sequential decarbonylation and reductive elimination reactions. 

The stoichiometric interaction of an enyne and [RuCl(PCy3)(pcymene)]B(Ar )4 XVIIIa containing a bulky non-coordinating anion B(ArF)4 showed by NMR at —30 ° C the formation of the alkenyl alkylidene ruthenium complex and acrolein. This formation could be understood by the initial formation of a vinylidene intermediate and transfer of a hydride from the oxygen a-carbon atom to the electrophilic vinylidene carbon, as a retroene reaction step (Scheme 8.13) [54]. 

The authors proposed mechanism, outlined in Scheme 9.19, was tested using a deuterium-labeling experiment. H-migration consistent with initial formation of a Pd-vinylidene was observed. The key intermediate of Buono s mechanism is a palladacyclobutane (125) resulting from [2 + 2]-cycloaddition. Direct C—C reductive elimination from intermediate 125 proceeds to give highly strained products (123), despite the apparent availability of a (l-hydride elimination pathway [39]. 

The termination of the growing polymeric chain may occur through several different processes, mostly by chain transfer. Either the process of chain transfer with the monomer, or the reaction of dissociation to hydride, leads to the formation of terminal vinylidenic groups, whose presence was noticed in the olefin polymers, obtained with the previously described catalysts (22). 

The amount of vinylidene found in the polymer exceeds that expected on a statistical basis, indicating that copolymerization encourages termination, Decreased molecular weight confirms it. Hogan (77) believes a tertiary hydride is more easily removed during termination than the usual secondary hydride. This effect is especially pronounced on some modifications of Cr/ silica. 

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