Vinylidene complexes synthesis

Scheme 2.54 Synthesis of allenes with titanocene vinylidene complexes.

Although terminal alkynes can easily be converted into vinylidene complexes, vinylidene complexes have not yet been extensively used as intermediates in organic synthesis [150,546,576-578,944]. Some cyclization reactions, which might proceed by transient formation of vinylidenes, are listed in Table 3.23 (see also Sections 2.1.5.1 and 2.2.2). 

Scheme 3.10 Synthesis of [Cp (dmpe) Mn=(C)4H2] by reaction of a stannyl (silylethynyl)vinylidene complex with tetrabutylammonium fluoride.

Scheme 5. Synthesis and reactions of iron-porphyrin vinylidene complexes.

Addition of the acetylenic alcohols HC=C(CH2) OH (x = 3,4) to 1 affords a one-pot synthesis of the cyclic carbene complexes (88). The reaction proceeds via initial formation of the vinylidene complexes, followed by an intramolecular attack of the terminal alcohol function on the a carbon [Eq. (84)] (85). Combining the nucleophilicity at the /3 carbon of... 

Unusual bridging (/z-cyclopropylidene)diiron complexes having a tetrahedral carbene carbon have been studied as model intermediates in carbon-carbon bond formation in the Fischer-Tropsch synthesis The cyclopropylidene complexes cis- and trans-[Cp(Co)Fe]2(iU-Co)(/z-C3H4) were readily prepared by cyclopropanation in ether, of the corresponding cis- and /ra 5-vinylidene complexes [CpCoFe](jU-CO)(jU-CH2) with diazomethane in the presence of CuCl (equation 181). Both isomers are air stable in the solid state. Solutions of the complexes are air stable for several hours, provided they are kept in the dark. The pure //-cyclopropylidene isomers slowly interconvert in solution, like their parent //-vinylidene and other alkylidene complexes. The final equilibrium ratio cis .trans = 4.8 1 is reached after two weeks. 

A considerable amount of work has been done in this area, which has been spurred in part by interest in the synthesis of multimetallic complexes in which the metal atoms are connected by unsaturated chains of several carbon atoms that may serve as molecular wires. The initial and still most widely employed synthetic route to vinylidene complexes involves addition of four equivalents of a triaUcylphosphine (T = PPrj hereafter unless otherwise... 

The incorporation of the second equivalent of imine can be prevented in these reactions if a vinylidene complex, such as (31), is employed which is 3optimized conditions indicated, the reaction of (31) with A -meAyl benzaldehyde imine will provide the cycloadduct (32) in quite good yield. A rather unc odox oxidation procedure (BU4NNO2, 6.5 kbar 1 bar = 10 Pa) is required for the effective cleavage of the cationic complex (32) to the 3-lactam (33). This reaction was shown to involve a two-step process, since the salt (37) could be isolated in 80% yield by column chromatography if the reaction was stO[q)ed shortly after the reaction mixture was Ivought to room tenq>erature. The reaction with the cyclic thioimidate (34) indicates that vinylidene complexes can be useful in the synthesis of functionalized 3-lactams in good yields with high stereoselectivity. 

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)... 

Protonation of vinylidene and acetylide ligands was also found to be useful for the synthesis of high-oxidation state molybdenum alkylidyne complexes. Green and co-workers demonstrated protonation of the vinylidene complex 20 as shown in Eq. (21) (64). Selegue and co-workers... 

Transition metal carbyne complexes are described by the general formula L M=CR where the carbyne ligand (=CR) is bonded to the metal by a metal-carbon triple bond. Transition metal carbene complexes have found numerous applications in synthetic organic chemistry through a variety of carbene transfer and cycloaddition reactions [17]. In contrast, carbyne (L M=CR) and vinylidene (L M=C=CRR ) complexes have far fewer applications, in part because their overall chemistry is significantly less developed [18]. Addition reactions to transition metal vinylidene complexes will be discussed in Chapter 21. The first successful synthesis of a carbyne complex was reported by Fischer and co-workers in 1973 [Eq. (8) 19]. Subsequently, many other carbyne complexes have been synthesized by the classic route of Fischer or by new synthetic methods [20]. 

A. G. (1980) Cyclopentadienyl-ruthenium and -osmium chemistry X. Reactions of vinylidene complexes with alcohols and water synthesis of alkoxy (alkyl) carbene, aryl and alkyl complexes. Australian Journal of Chemistry, 33, 1471-1483 (c) Bruce, M.I., Swincer, A.G., Thompson,... 

Quite recently, a platinum(II) vinylidene complex was reported. Its synthesis was accomplished by reaction of Pt(PPh3)2(CH3)(CCR) with triflic acid and tetrafluoroboric acid, respectively, to give the resulting cationic Pt(II) complex. (CH3)(PPh3)2Pt(=C= CHR)]+ X- (X = BF4-. Cp3S03-). ° No data on the catalytic activity of this complex, which has not been isolated but whose structure has been proven by NMR experiments, have been reported so far. 

Scheme 55. Synthesis and reactivity of anionic vinylidene complexes.

The reductive electrochemistry of iron-carbene porphyrins has been investigated in aprotic solvents [115]. With the vinylidene complex, there is a 2e -I- H reduction of the ligand leading to the formation of the corresponding iron(II) vinyl complex. The energies required to reduce by two electrons the other carbene complexes are quite similar [115]. The dichlorocarbene complex is an exception because the reduction is facihtated by the extreme instability of the one-electron intermediate. Formation of cr-alkyl iron(III) porphyrins has been confirmed by independent synthesis [116]. The a-alkyl iron(III) porphyrins can then be obtained by a one-electron reoxidation re-... 

Equations 13.10-13.12 show three examples of the synthesis of vinylidene complexes by reactions of metal-acetylide complexes with acid or base. The molybdenum(II) acetylide complex in Equation 13.10 reacts with acid to protonate the p-carbon and generate a cationic vinylidene complex. In this case, the vinylidene complex is thermodynamically unstable. Warming to 0 °C leads to rearrangement of this species to the tautomeric alkyne complex. In contrast, the more electron-rich molybdenum-acetylide complex in Equation 13.11 containing three phosphite donors generates a vinylidene complex upon addition of a proton from alumina to the 3-carbon of the acetylide. The vinylidene form of the complex is apparently more stable than the alkyne complex in this case. 

---