Rhenium complexes vinylidenes

Vinyl-substituted benzo[c]furans can be prepared by reaction of n-alkynylbenzaldehydes with chromium Fischer carbene complexes. Initially a benzo[c]furan chromiumtricarbonyl complex is believed to be formed which is converted into an alkylidenephthalan derivative or can be trapped with electron-deficient dienophiles with excellent ( 3 ti-selectivity (Equation 128) <20000L1267>. More elaborate vinylidene Fischer carbene complexes yield dienyl benzo[c]furans that undergo [8-1-2] cycloaddition with DMAD to furnish furanophane derivatives <2003JA12720>. An equilibrium between 7] -(o-ethynylbenzoyl)rhenium complexes and rhenium benzo[f]furyl carbene complexes has been observed. These species behave like other benzo[< ]furans in the reaction with DMAD <20040M4121>. 

Finally, insertion of alkynes into dihydrogen mononuclear cationic rhenium complexes, stabilized by the triphos (= l,l,l-tris(diphenylphosphinomethyl)ethane) ligand, leads to vinylidene derivatives which react with water or ethanol eliminating silanols or methane, Fig. 18. Solvents were acetone or THF no chromatography was used. 

Synthesis and characterisation of the first organometallic Tc(VIII) complex [MeTcOs] is described. Its reactions with alkenes and with aromatic amine bases are different to those of the analogous rhenium complex. 25 Terminal alkynes react with [Tc(Cl)(dppe)2] to form vinylidene complexes [Tc(=C=CHR)(Cl)(dppe)2] (R = Ph, Me, Bu ). Treatment of these complexes with acid yields the respective carbyne complexes [Tc( CH2R)(Cl)(dppe)2]. The Tc-C bond lengths in these complexes vary form 1.861(9) to 1.724(7) A, 26... 

Rapid development of this area followed the discovery of routes to these complexes, either by ready conversion of terminal alkynes to vinylidene complexes in reactions with manganese, rhenium, and the iron-group metal complexes (11-14) or by protonation or alkylation of some metal Recent work has demonstrated the importance of vinylidene complexes in the metabolism of some chlorinated hydrocarbons (DDT) using iron porphyrin-based enzymes (15). Interconversions of alkyne and vinylidene ligands occur readily on multimetal centers. Several reactions involving organometallic reagents may proceed via intermediate vinylidene complexes. 

The synthesis of alkylidyne complexes by y -addition of electrophiles to vinylidene and acetylide ligands is now well established (5,6). Pombeiro and co-workers synthesized several new rhenium alkylidyne complexes by protonation of the electron-rich vinylidene complexes 13 [Eq. (18)] (55). The mechanism of formation of the benzylcarbyne complex 14 (R = Ph)... 

One of several complexes formed by photochemically reacting CpRe(CO>3 and PhC CH in THF, was the binuclear complex Cp-(CO)2Re M-T T)--(=C=C(Ph)C(Ph)==CH2) ReCp(CO)2 (65) (129). The formation of 65 involves a photoinduced coupling of two acetylene units. The structure of 65 reveals that the vinyl substituent of a vinylidene complex is Tj -bonded to a second rhenium center. 

The basicity at the 3-carbon is illustrated by the reactions in Equations 13.26 and 13.27. Reaction of the octahedral rhenium vinylidene in Equation 13.26 with HBF generates the cationic carbyne complex from addition of a proton to the basic 3-carbon. Because low-valent metals are often basic, addition of a proton to the vinylidene 3-carbon is likely to occur by a multi-step process initiated by protonation of the metal center. This initial protonation at the metal center would then be followed by migration of the proton to the 3-carbon. The reaction of acid with the iridium vinylidene in Equation 13.27 illustrates this mechanism. In this case, protonation first generates an iridium-hydride complex. The hydride in this complex tlien migrates to the p-carbon to generate an alkylidyne complex. ... 

We have shown that carbyne complexes of rhenium can be conveniently prepared by 3-protonation at vinylidene ligands [5], a route which has been described for other systems [6]. The vinylidene species have been obtained by 1,2-hydrogen shift at 1-alkynes (HCeCR) when activated by an electron-rich d Re centre, typically trans- ReCl (dppe) 2 (dppe=Ph2PCH2CH2PPh2) provided by a parent dinitrogen complex [5]. 

The rhenium-carbon distance, 1.772(7) A, is, as expected, considerably shorter than those in the related vinylidene and q aminocarbyne complexes trans-[ReCl(=C=CHPh)(dppe)2] 2.046(8) A [5], and traT -[ReCl(=C-NHMe)(dppe)2][BF4], 1.80(3) 1 [8] (see below), respectively, in which there is a 7r-electron delocalisation along the... 

Some of the vinylidene complexes include cobalt, rhodium and rhenium in halfsandwich complexes, which are synthesized from acetylene complexes". This reaction involves an intermediate alkinyl(hydrido) complex, which can sometimes be isolated. The bonding between the metal and the a-carbon atom in vinylidene rhodium complexes is shorter than in carbene rhodium complexes, which indicates a high electron density on the center atom. 

Also, reaction of the rhenium vinylidene complexes 16, generated in situ, with Cp(CO)2Re, affords the [2-1-1] cycloadduct 17. ... 

Vinylidene complexes of manganese and rhenium, generated in situ, undergo [2+2] cycloaddition reactions with imines to form 52. ... 

Rhenium or manganese vinylidene complexes 58 react with benzalazines to give the criss-cross [3+2] cycloadducts 59 in good yields. ... 

From titanaallenes and carbodiimides, [2+2] cycloadducts are also obtained. The aza-titanacyclubutanes are thermally stable up to 150 Some rhenium and manganese vinylidene complexes react with carbodiimides to give isocyanate complexes rather than the expected [2+2] cycloadducts... 

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