Synthesis of Vinylidene Complexes

Several hundred examples of vinylidene complexes have been prepared. Vinylidene complexes have been prepared by rearrangement of alkyne complexes, additions of acid or base to acetylide complexes, by deprotonation of carbyne complexes, by dehydration of acyl complexes, and by ot-hydrogen shifts from vinyl complexes. Syntheses from alkjme and from acetylide complexes are most common. A complex of a terminal alkyne and a transition metal can exist as an alkyne complex or as a vinylidene complex. Although the free vinylidene is much higher in energy than the free alkyne, the vinylidene complex is often more stable tlnan the alkyne complex. Vinylidene complexes are most often obtained with late transition metals because this tautomer possesses less repulsion between the filled (i-orbitals of the metal and the filled ir-orbitals of the ligand. 

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. 

Anionic vinylidene complexes can also be prepared by deprotonation of acetylide complexes because acetylides are slightly acidic at the carbon 7 to the metal. For example, the 

Schrock carbenes are typically generated by a-hydrogen elimination from a high-valent, early metal, dialkyl complex concomitant with or followed by elimination of alkane. The first S5mthesis of an alkylidene by Schrock is summarized in Equation 13.13. Addition of neopentyl zinc to tantalum pentachloride generated tris(neopentyl)dichlorotantalum. Subsequent addition of neopentyllithium did not generate the homoleptic neopentyltantalum compound. Instead, this reaction generated the first alkylidene complex, tris(neopentyl) tantalum neopentylidene.  

---

Most of the chemistry performed on orthometallated ylides has been carried out with Pd and Pt as metal centers. Few examples dealing with other metals (Co, Ru, and Os mainly, see 62 and 63) have been reported. Complex (74) has been prepared by reaction of a Ru-vinylidene with PPh3 [153] while Os derivative (75) has been obtained after treatment of the methylimido complex with PPh3 [154]. Orthoruthe-nated indenyl complexes [155] have been synthesized by reaction of the halomethyl precursors with PPh3, and the oxidative addition of the yUde Ph3P=CHC(0)H to clusters of Ru and/or Os also allows the synthesis of orthometallated complexes... 

Complexes with a bridging vinylidene ligand were also used as the starting compound for the synthesis of thioaldehyde complexes. Sequential treatment of 104 with cyclohexene sulfide, HBF4, and Li[BHEt3] gave the thioaldehyde complexes 105 [Eq. (24)].209... 

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 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)... 

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,... 

Finally, Hill et al reported the synthesis of vinylidene and allenylidene complexes of the types [TpRu( = C = CHR)(L)2] (L = PPh3 or L2 = 1,1 -bis(diphenylphosphino)-ferrocene R = Ph, C6H4Me), [TpRu( = C = C = CR2)(L)2] and TpRuCl( = C = C = CPh2)(PPh3). The eationie complexes underwent an unprecedented eoupling with dithiocarbamates, whieh led to metallacyclic vinyl and allenyl complexes (Seheme 34). Similar allenylidene eomplexes but featuring redox-active substituents and ligands based on ferrocene were reported by Hartmann et al ... 

The activation of adequate small unsaturated-C molecules, typically isocyanides or alkyne-derived vinylidenes and alkynyls, towards 3-electrophilic attack by electron-rich group 6 (Mo or W) or 7 (Re) transition metal centres constitutes a convenient and established route for the synthesis of carbyne complexes, which has been discussed in this paper. 

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). 

Following the synthesis of metal carbyne complexes, the first metalladiyne derivative was prepared by treatment of W =C(OEt)C=CPh (CO>5 with BX3 (X = C1, Br, I) (pentane, -45°C) to give rranj-W(=CC=CPh)(X)(CO)4 (334 Scheme 77) in good yields (30-60%). Subsequent reactions with NHMea give W sCCH=CPh(NMc2) (X)(CO)4 by addition to the C=C triple bond, the structure of which indicates a contribution from the vinylidene resonance form. ... 

The metal-ligand fragment L M, the number of carbon atoms x, and the substituents at the terminal sp -carbon may vary considerably and, correspondingly, the properties and reactivities. The early members of the series of cumulenylidene complexes (x=l, 2, 3 carbene, vinylidene and allenylidene complexes) have established themselves as invaluable building blocks in stoichiometric synthesis and as highly potent catalyst precursors. The higher members might potentially be very useful candidates for application as one-dimensional wires and in opto-electronic devices. 

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

The content of this book gathers in the same volume all aspects of vinylidene- and allenylidene-metal complexes, including the preparation of these organometallic carbon-rich systems with a metal-carbon double bond, their stoichiometric reactivity and theoretical aspects, and their applications in catalysis for the production of fine chemicals, mainly in the field of selective transformations of functional terminal alkynes. It provides essential general information on catalytic transformations of alkynes and their use in synthesis. 

The synthesis of the thioaldehyde complex 63 somewhat paralleled the synthesis outlined in Scheme 17. In this case, sulfur and hydrogens were transferred to the vinylidene ligand in a single step [Eq. (14)].187... 

The majority of the chemistry of vinylidene and propadienylidene complexes is concerned with their synthesis and reactions, which have been... 

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

---