Allenylidene complexes mononuclear

Table 1 Structural data of selected mononuclear allenylidene complexes... 

The regioselectivity of the nucleophilic additions on allenylidene complexes (C vs Cy) is subtly controlled by the electronic and steric properties both of the substituents on the unsaturated hydrocarbon chain and the ancillary ligands on the metal atom, as well as by the nucleophile employed. In this section we will summarize the nucleophilic reactivity of mononuclear Group 6-9 allenylidenes. 

Species (A) and (B) constitute the main class of unsaturated carbenes and play important roles as reactive intermediates due to the very electron-deficient carbon Cl [1]. Once they are coordinated with an electron-rich transition metal, metal vinylidene (C) and allenylidene (D) complexes are formed (Scheme 4.1). Since the first example of mononuclear vinylidene complexes was reported by King and Saran in 1972 [2] and isolated and structurally characterized by Ibers and Kirchner in 1974 [3], transition metal vinylidene and allenylidene complexes have attracted considerable interest because of their role in carbon-heteroatom and carbon-carbon bond-forming reactions as well as alkene and enyne metathesis [4]. Over the last three decades, many reviews [4—18] have been contributed on various aspects of the chemistry of metal vinylidene and allenylidene complexes. A number of theoretical studies have also been carried out [19-43]. However, a review of the theoretical aspects of the metal vinylidene and allenylidene complexes is very limited [44]. This chapter will cover theoretical aspects of metal vinylidene and allenylidene complexes. The following aspects vdll be reviewed ... 

Quite recently, some mononuclear ruthenium complexes such as [(p-cymene)RuX-(CO)(PR3)]OTf (X = Cl, OTf, R = Ph, Cy) have been found to work as catalysts for the propargylation of aromatic compounds such as furans, where some ruthenium complexes were isolated as catalytically active species from the stoichiometric reactions of propargylic alcohols (Scheme 7.27) [31]. The produced active species promoted the propargylation of furans vdth propargylic alcohols bearing not only a terminal alkyne moiety but also an internal alkyne moiety, indicating that this propargylation does not proceed via allenylidene complexes as key intermediates. 

The allenylidene [M=C=C=CR R ] unit represents a member of the class of carbon-rich unsaturated hydrocarbyl fragments, and it is capable of binding one to four metal centers. In sharp contrast to the rapidly developing chemistry of mononuclear allenylidene complexes, the scarcity of bi- and polynuclear species of this type has hindered the development of an extensive chemistry. Selected examples from the allenylidene literature are cited in this article owing to the close structural analogy with allenyl derivatives. The reader is referred to a review article by Bruce covering the relevant literature of this metallacumulene up to 1989 (la). 

A. Preparation of Mononuclear Propargyl, Allenyl, and Allenylidene Complexes... 

Mononuclear allenylidene complexes have been prepared by either (i) reaction of a deprotonated propargyl alcohol with a binary metal carbonyl or (ii) dehydration of the acetylenic alcohol HC=CCR20H with an appro-... 

The formation of allenylidene derivatives from ethynyl-hexanol and alkenyl-vinylidene mononuclear complexes (9), the formation of mononuclear ruthenium allenyl complexes from terminal alkynes (10), the intermediacy of ruthenium-allenylidene complexes in forming propargylic alcohols (II), and in the cyclization of propargyl alcohols (12), and the use of mononuclear ruthenium compounds in allylic alkylation catalysis (13) have also been reported. 

Reductively induced alterations from cumulenic to alkynyl resonance structures have been observed for mononuclear and dinuclear ruthenium allenylidene complexes. The half-wave potentials for the one-electron reduction of allenylidene complexes [ Ru = C = C = C(ER )(R )] ( Ru = trun.s-Cl(L2)2Ru L2 = chelating diphosphine ER = NR2, SR, SeR, aryl, alkyl R = aryl, alkyl) strongly depends on the nature of the ER substituent. Amino- and aryl-substituted congeners with reduction potentials of ca. -2.2 V and -1.0 V, respectively, constitute the two extremes within this series. These sizable potential... 

Water-soluble ruthenium vinylidene and allenylidene complexes were also synthetized in the reaction of [ RuC12(TPPMS)2 2] and phenylacetylene or diphenylpropargyl alcohol [29], The mononuclear Ru-vinylidene complex [RuCl2 C=C(H)Ph (TPPMS)2] and the dinuclear Ru-allylidene derivative [ RuCl( x-Cl)(C=C=CPh2)(TPPMS)2 2] both catalyzed the cross-olefin metathesis of cyclopentene with methyl acrylate to give polyunsaturated esters under mild conditions (Scheme 7.10). 

The homobimetallic, ethylene-ruthenium complex 15, which contains three chloro bridges, was readily obtained from the reaction of [RuCl2(/ -cymene)]2 with 1 atm of ethylene [34]. In 2009, Demonceau and Delaude [34] showed that complex 15 could be a useful precursor to allow subsequent access to the diruthenium vinylidene complex 16, allenylidene complex 17, and indenylidene complex 18 (Scheme 14.8). Upon reaction with propargylic alcohol, complex 15 afforded vinylidene complex 16, which converted into the allenylidene complex 17 in the presence of molecular sieves [34]. As shown in the acid-promoted intramolecular rearrangement of mononuclear ruthenium allenylidene complexes [19, 20, 32], the addition of a stoichiometric amount of TsOH to complex 17 at -50 °C led to the indenylidene binuclear complex 18 [34]. Complex 18 has been well... 

Structural parameters of selected mononuclear complexes containing heteroatomic 7i-donor-substituted allenylidene ligands are summarized in Table 1. When these data are compared to those reported for related complexes bearing conventional alkyl or aryl substituents (see Table 1 of chapter All-Carbon-Substituted Allenylidene and Related Cumulenylidene Ligands ), a lengthening of the M=C bond... 

Mononuclear osmium half-sandwiches, with rf-cyclopentadienyls and 7]5-indenyls alkenyls and allenyls with t/-M-C bonds, 6, 558 alkenyl vinylidenes, 6, 593 alkyl, aryl, acyl complexes, 6, 552 with alkylidyne complexes, 6, 599 alkynyl and enynyl complexes, 6, 567 allenylidene and cumulenylidene complexes,... 

The mononuclear complex [TiCp2(=C=C=CPh2)(PMe, ) (2), generated by reacting l,l-dilithio-3,3-diphenylallenylidene with dichlorotitanocene in the presence of trimethylphosphine (Scheme 3) [87], and the dinuclear derivative [(ZrCp2Et)2(/t-77 z/2-C=C=CMe2)] [52] are the only Group 4 allenylidenes described to date in the literature. 

Reverse processes, that is reactions of propargyl or allenylidene ligands coordinated to mononuclear centers with water, methanol, amines, and other ligands are known. Tungsten l and platinum complexes give a series of reactions (some of which are mediated by surfaces) comparable with those discussed for clusters. 

Different reactivity trends on M3(CO)i2 clusters (M = Fe, Ru, Os) have been observed, depending on the nature of the cluster metals. Two dehydration routes have been observed for cluster-bound propargyl alcohols - the less common is similar to the main process observed for mononuclear complexes and involves loss of the terminal alkynic hydrogen and of the alcoholic OH (Route A). The more common, when CH3 groups are available, involves loss of one methyl hydrogen and of the adjacent OH (Route B). The former process leads to allenylidene and the second to ene-yne (or vinylacetylide) derivatives. Examples are given in Fig. 21. 

---