Hydrides vinylidene complexes

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) ... 

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. 

Regio- and stereo-selective dimerization of alk-l-ynes catalysed by classical and non-classical hydride complexes of Ru(II) and Os(II) stabilized by the tripodal polydentate ligand (Pl PCH CH P has been reported to produce the corresponding (Z)-1,4-disubstituted butenynes. Irrespective of the nature of the hydride ligand (classical or non-classical), vinylidene complexes appear to be the immediate precursors to the C—C bond-forming step.67... 

Acetylene-vinylidene rearrangements of silylacetylene-iron carbonyl complexes have been observed,537 while iron-acetylide hydride complexes of the type [Fe(H)(C=CR)(dmpe)2], where dmpe=l,2-bis(dimethylphosphino)ethane, have been found to react with anions to afford substituted alkenyl complexes. It has been proposed538 that a likely reaction course for this latter rearrangement involves initial protonation of the cr-bound acetylide ligand at the carbon (I to the metal centre to form a vinylidene complex. Metal-to-carbon hydride migration in this vinylidene complex with attack by the anion would then lead to the neutral complex (see Scheme 106). A detailed mechanistic investigation has been carried out539 on the novel metathetical... 

Most efforts to explore the reactivity of ruthenium carbene complexes have employed the alkoxycarbene species so readily synthesized from the inter- or intramolecular reaction of vinylidene complexes with alcohols. These electrophilic alkoxycarbene complexes exhibit only limited reactivity at Ca, primarily with hydride reagents. For example, treatment of the 2-oxacyclopentylidene complex 97 with NaAlH2(OCH2CH2OMe)2 affords the neutral 2-tetrahydrofuranyl complex (98) [Eq. (89)] (55), as was anticipated from similar reductions of iron carbene complexes (87). 

An extension of this synthetic methodology leads to /x-vinylidene complexes. Reaction of H2C=CBr2 with 4 leads to complex 106 (Fig. 19) (253,254), containing only terminal carbonyls, together with 4. The vinylic hydrogens are nearly coplanar with the Co—Co bond in the solid state. Successive reactions of 106 with acid and hydride give the alkylidene 107. 

The chemistry of iron vinylidene complexes is dominated by the electrophilicity of the carbon atom adjacent to the iron organometallic unit. While addition of water leads to an acyl complex (i.e., the reverse of the dehydration shown in equation 10), addition of an alcohol leads to a vinyl ether complex. Similarly, other iron vinyl complexes can be prepared by the addition of thiolate, hydride, or an organocuprate (Scheme 33). " The nucleophilic addition of imines gave enaminoiron intermediates that could be further elaborated into cyclic aminocarbenes. This methodology has been used to provide access to /3-lactams and ultimately penicillin analogs, and good diastereoselectivities were observed (6 1-15 1) (Scheme 34). 04 Iso, vinylidene complexes are intermediates in cyclizations of alkynyl irons with substituted ketenes, acid chlorides, and related electrophiles an example is shown (equation 11). These cyclizations led to the formation of a series of isolable and characterizable cyclic vinyl iron complexes. 

The unsaturated hydride Mn2(li-H)2(CO)6(lt-dppm) reacts with RCMHH (R = H, Ph, Bu , COOMe) to form either hydrido-alkenyls Mn2(M.-H)(ji2. n. l -CR=CH2XCO)6(ii-dppm) (R = H, Ph), or alkenylidene species 2(tt-H)(il2,tl3,Ti2-CR=CHBuO(CO)6(M.- pm) and (OC)3Mn(ji-dppm) (i-C=CHCOOMe Mn(CO)3. The photochemical reactions of Mn2(CO)8(p.-dppm) with alkynes results in vinylidene complexes. 35 The photochemical reaction between Mn2(CO)io and MeCsCNEt2 affords the ynamine complex 86. This isomeiizes into an aminoallene complex on warming. 36 The corresponding riienium complex Re2(CXD)8(p.-MeC2NMe2) is transformed into three complexes on thermolysis these are the (dimethylamino)allenyl Re2(CO)7(p-T 3-... 

Shortly after Finn s work came to light a catalytic rhodium(I) system was reported. An acyclic enediyne 40 was heated to 50 °C in the presence of just 0.05 equiv of RhCl(/-Pr2P)2 and EtjN in benzene to provide substituted arene 41 in 58% yield. The latter reaction is presumed to involve Myers-Saito cyclization of an in situ formed vinylidene complex. A catalytic cycle becomes possible due to steps involving /3-hydride elimination and reductive elimination. ... 

Alternately, vinylidene complexes can be produced either from Ru hydride complexes, by insertion of the triple bond to form a vinyl Ru intermediate which further undergoes a-hydrogen migration, or from a Ru carbene, by a... 

The vinyl complexes are accessible by transmetallation, oxidative addition of a vinyl halide, addition of an acid on a neutral alkyne complex, insertion of an alkyne in a metal hydride, reduction of a vinylidene complex or nucleophilic attack of a... 

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... 

Vinylidene complexes are also suitable precursors via protonation of carbynes [Os =GGH(R)Ph (77 -G5H4SiPh3)(GePh3)(PPp3)][BF4] (R = H, Me) and the first isolated hydride-carbyne derivative... 

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