Metallacyclic compounds

Cycloaddition refers to a process of unsaturated moieties forming a metallacyclic compound. It is sometimes categorised under oxidative additions, but we prefer this separate listing. Examples of the process are presented in Figure 2.22. Metal complexes which actually have revealed these reactions are M = L2Ni for reaction a, M = Cp2Ti for reactions b and c, M = Ta for d, and M = (RO)3W for e. The latter examples involving metal-to-carbon multiple bonds have only been observed for early transition metal complexes, the same ones mentioned under a-elimination, 2.20. 

Related ry -pentatrienyl compounds are also formed in the reactions of [OsClCp (=C=C=CPh2)(P -Pr3)] and [RuClCp (=C=C=CPh2) (P)- -Pr2PCH2C02Me ] with CH2=CHMgBr [197, 279]. In contrast, the insertion of the allenylidene ligand into the Os-C(alkenyl) bond of 69 has been reported to yield the five-membered metallacyclic compound 70 instead of the expected ry -pentatrienyl isomer (Scheme 23) [200]. 

Coupling with alkenes and alkynes.2 The reagent reacts with these substrates to form metallacyclic compounds, which are hydrolyzed to acyclic compounds. The adducts are formed with high regioselectivity. 

Cycloaurated gold halide complexes are also precursors to metallacyclic compounds with three gold-carbon bonds. The first examples of this type were the... 

Metallacycles have been claimed to play pivotal roles in many transition metal-mediated multi-component coupling reactions [1]. For example, [2 -i- 2 -i- 2] alkyne cyclo-trimerization leading to benzenes - the Reppe reaction - has been considered to proceed via metallacyclopentadiene and elusive metallacycloheptatriene intermediates ("common mechanism ), while metallacyclopentenes have been proposed as intermediates for the [2 -i- 2 -i- 1] cyclo-coupling reactions of an alkyne, an alkene, and CO leading to a cyclopentenone (the Pauson-Khand reaction). A metallacyclic compound - which is defined here as a carbocyclic system with one atom replaced by a transition metal element - can be generally formed by oxidative cyclization of two unsaturated molecules with a low-valent transition metal fragment [2-4]. Alter-... 

Carbonylselenide is ejected from the metallacyclic compound 76 when heated to give a selenoformaldehyde compound, 77 (86). An example of a... 

Synthesis of ferracycloalkanes became possible only when the corresponding alkanediylbis(trifluoromethanesulfonates) with the excellent leaving group CFjSOjO" were available. In these bistriflates two carbenium-like carbon atoms are present, a precondition for the nucleophilic attack of an appropriate metal Lewis base to form a metallacyclic compound. The action of such bistriflates on [Fe(CO)3L] yields the ferracycloalkanes... 

In the case of Ni, metallacyclic compounds containing the divalent metal may be obtained by alkene dimerization. Not only does this reaction occur with electron-deficient or strained alkenes, but under suitable conditions ethene or 1,7-octadiene reacts to give nickelacyclopentane species ... 

Treatment of (22) with CbH = carbazole (38) in hydrocarbon solvents gave [Cb2M]2(/x2-CR)2 ((39) Scheme 4). The reactivity of this species towards hydrogenation (M = Ta) and to organic isocyanides was studied.69,70 Reaction of (39) with 1 eq EtC=CEt produced the di-metallacyclic compound (40) by alkyne insertion into one of the bridging alkylidyne groups.71,72... 

The metallacyclic compounds react with protic reagents to yield the corresponding substituted 1,3-butadienes and the iodination reactions give the 1,4-diiodo 1,3-butadiene derivatives (Scheme 122). Reactions with different unsaturated organic molecules have also been investigated. They catalyze the cyclotrimerization of a range of alkynes. Terminal alkynes with small substituents produce the 1,2,4-trisubstituted benzene, preferentially, in an exothermic reaction. The more bulky substrates BuTl CH and Me3SiC=CH react more slowly and only the symmetrical 1,3,5-isomer is produced (Scheme 122).174... 

Chlorotris(triphenylphosphane)rhodium reacts with 3,3-dimethylcyclopropene in a stoichiometric way to form the metallacyclic compound 13 which, on exposure to carbon monoxide, undergoes insertion and reductive elimination to yield the hexamethyl-tris-a-homo-tropone 10 in 73% yield. A minor product of the reaction is the homo-cyclotrimer... 

Dinitrogen complexes of titanocene and zirconocene are excellent sources for generating metallocene species in situ, which are highly active and are readily trapped by organic and inorganic unsaturated compounds. Eq (13) is an example that involves carbonyl coupling in iron-carbonyl to give the metallacyclic compound [118]. 

Fig. 13.15. Schematic representation of the most important orbital interactions in TM-aUcene and alkyne complexes and description as metallacyclic compounds (a) TM ligand o-donation (aj) and TM — ligand TT-backdonation ( 2) aUcene complexes, (b) Metallacyclopropane. (c) TMligand o-donation (a ), TM — ligand in-plane Try-backdonation ( 2) TM ligand out-of-plane ir -donation ( 1) and TM — ligand 8-backdonation (a2) in alkyne complexes, (d) Metallacycloropene. The symmetry assignments 2, 2...

The cationic eight-membered metallacyclic compound [Hfo( -dppm)2(DMSO)4] (CF3SOi )4 has been prepared in high yield and characterized using and HgNMR spectroscopy. For the first time a metal-metal spin-spin coupling in [M2(ju-dppm)2] rings without a formal metal-metal bond is reported (/( Hg- Hg) = 699-1061 Hz dependii on the solvent). ... 

Attack of the oxygen atom of NO2 anion at Pd-coordinated alkene ligand afforded metallacycle compounds (Scheme 8.32) [53]. The X-ray structure determination of the product from dicylopentadiene complex of Pd(II) established the cis oxypalladation. The metallacycle thus formed can be regarded as an intermediate in Pd-catalyzed oxidation of alkenes to ketones or epoxides with the use of NO2 ligand as a mediator and O2 as an oxidant. 

Insertion reaction formation and conversion of metallacyclic compounds nucleophilic or electrophilic attack reactions on the catalyst surface... 

The EDA results for the metal-ligand interactions in the metallacyclic compounds CI4TM-C2H, (TM = Mo, W), which are shown in Table 7.7, are very different from the data for the ethylene and acetylene complexes. There are no results for the chromium compounds because of SCF problems [34]. The electron-sharing... 

Demerseman and coworkers studied recently the coupling reaction between an unsaturated alkynetitanium(II) complex and carbon dioxide [84]. The titanium-diphenylacetylene complex (n -C5H5)(n -C5Me5)Ti-(PhC=CPh) adds readily CO2 to give a five-membered metallacyclic compound via carbon-carbon bond formation (Equation 14). This reaction occurs smoothly in hexane under atmospheric pressure and is supported by the well known tendency of titanium to form stable Ti-0 bonds. 

Reactions of [Ti(pmcp)2 (C2H4)] with alkynes, nitriles, CO2, and aldehydes give the metallacyclic compounds ... 

Alkynes may react with acetylene complexes to form metallacyclic compounds, i.e., compounds containing a higher number of coordinated acetylene molecules, or compounds possessing other alkynes (substitution reaction) see scheme (6.190). 

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