Metallacycles metallation

Metallacycle fomiation has also been observed in bis-Cp complexes. Heating Cp 2UR[P(Si(CH2)2)2] (R = Cl [146840-37-17, CH [146840-39-3]) results in the metaHation of the phosphido ligand. These complexes are stmcturaHy similar to the group 4 and 6 transition-metal metallacycle complexes, but show a dramatically reduced reactivity. 

Metallacycles containing metal-heteroatom bond in their ring system 98YGK171. 

On heating, the neopentyl Pt(PEt3)2(CH2CMe3)2 undergoes an intramolecular metallation elimination [108a] (Figure 3.59), which appears to involve initial phosphine loss affording a platinum(IV) metallacycle. 

Keywords Valence electron rule, Metal ring, Metal cluster, AN +2 valence electron rule, 8/V +6 valence electron rule, 6N +14 valence electron rule, Pentagon stability, Cyclopentaphosphane, Hydronitrogen, Polynitrogen, Triazene, 2-Tetrazene, Tetrazadiene, Pentazole, Hexazine, Nitrogen Oxide, Disiloxane, Disilaoxirane, 1,3-Cyclodisiloxane, Metallacycle, Inorganic heterocycle... 

There are many four-membered metallacycles containing short metal—metal nonbonded distances. Cyclodisilazanes (Scheme 12a) isoelectronic to 1,3-cyclodis-iloxanes also have short Si—Si distances [136, 137]. 

A supramolecular assembly of macromolecules bearing antenna dendron has been reported. Pyrazole-anchored PBE dendrons were synthesized to examine the coordination behavior to transition-metal cations (Cu, Au, Ag) [31]. Self-assembled metallacycles were found. The Cu-metallacycle further formed luminescent fibers about 1 pm in diameter. The luminescence (605 nm) occurred by the excitation of the dendron (280 nm) and the excitation spectrum was coincident with the absorption spectrum of the dendron, suggesting the antenna effect. Interestingly, the luminescence of the Cu-metallacycle fiber disappeared when the fiber was dissociated into the individual metallacycles in C2H2. 

From these data, some key information can be drawn in both cases, the couple methane/pentane as well as the couple ethane/butane have similar selectivities. This implies that each couple of products (ethane/butane and methane/pentane) is probably formed via a common intermediate, which is probably related to the hexyl surface intermediate D, which is formed as follows cyclohexane reacts first with the surface via C - H activation to produce a cyclohexyl intermediate A, which then undergoes a second C - H bond activation at the /-position to give the key 1,3-dimetallacyclopentane intermediate B. Concerted electron transfer (a 2+2 retrocychzation) leads to a non-cychc -alkenylidene metal surface complex, C, which under H2 can evolve towards a surface hexyl intermediate D. Then, the surface hexyl species D can lead to all the observed products via the following elementary steps (1) hydrogenolysis into hexane (2) /1-hydride elimination to form 1-hexene, followed by re-insertion to form various hexyl complexes (E and F) or (3) a second carbon-carbon bond cleavage, through a y-C - H bond activation to the metallacyclic intermediate G or H (Scheme 40). Under H2, intermediate G can lead either to pentane/methane or ethane/butane mixtures, while intermediate H would form ethane/butane or propane. 

The mechanism of [3 + 2] reductive cycloadditions clearly is more complex than other aldehyde/alkyne couplings since additional bonds are formed in the process. The catalytic reductive [3 + 2] cycloaddition process likely proceeds via the intermediacy of metallacycle 29, followed by enolate protonation to afford vinyl nickel species 30, alkenyl addition to the aldehyde to afford nickel alkoxide 31, and reduction of the Ni(II) alkoxide 31 back to the catalytically active Ni(0) species by Et3B (Scheme 23). In an intramolecular case, metallacycle 29 was isolated, fully characterized, and illustrated to undergo [3 + 2] reductive cycloaddition upon exposure to methanol [45]. Related pathways have recently been described involving cobalt-catalyzed reductive cyclo additions of enones and allenes [46], suggesting that this novel mechanism may be general for a variety of metals and substrate combinations. 

A variety of alternate methods for the reductive coupling of aldehydes and alkynes have been developed. A number of important hydrometallative strategies have been developed, although most of these methods require the stoichiometric formation of a vinyl metal species or metallacycle. A very attractive hydrogenative coupling method has recently been developed, and its scope is largely complementary to the nickel-catalyzed methods. A very brief overview of these methods is provided below. 

Scheme 8 displays reactions where 43 behaves as a source of a Zr(II) derivative.30 They can be formally viewed as oxidative additions to the [p-Bu -calix[4]-(0Me)2(0)2Zr] fragment. The main driving force in the case of ketones is the high oxophilicity of the metal, which induces the reductive coupling of benzophenone leading to 45, or the addition of dibenzoyl causing the formation of the dioxo-metallacycle in 46, which contain a C-C double bond. It has to be mentioned that... 

Low valent transition metal centers preferentially coordinate to the phosphorus in diazaphospholes. Accordingly, P-coordinated complexes of [l,2,3]diazapho-spholes with Cr, W, Fe, and Mn carbonyls were obtained as early as 1980 [1, 2,4], Later, Kraaijkamp et al. observed [108] both P- or -coordination modes in complexes of [l,2,3]diazaphospholes with MX2(PEt3) (M = Pt, Pd X = C1, Br). Methanolysis of these complexes led to the diazaphosphole ring opening and formation of five membered metallacyclic P,/V-chelates (103), incorporating P-bonded phosphonite and /V-coordinated hydrazone functionalities (Scheme 32) [109],... 

Finally, the possibility of building the M=C bond into an unsaturated metallacycle where there is the possibility for electron delocalization has been realized for the first time with the characterization of osmabenzene derivatives. For these reasons then, it seemed worthwhile to review the carbene and carbyne chemistry of these Group 8 elements, and for completeness we have included discussion of other heteroatom-substituted carbene complexes as well. We begin by general consideration of the bonding in molecules with multiple metal-carbon bonds. 

Similar metallacyclic species have been proposed as intermediates in other reactions of metal compounds with diazoalkanes (57,55). The presence of unfavorable steric interactions in an intermediate such as 53 could well explain the failure to observe any reaction of diaryldiazoalkane with 45 (55). 

For very electrophilic carbene ligands bound to a metal center which also has coordinated an aromatic phosphine ligand,there is the possibility of the following intramolecular substitution reaction leading to a metallacycle ... 

The transition-metal induced rearrangement of strained cyclopropanes is mostly caused by inserting metal atoms into a three-membered ring, thus producing metallacycles and/or rf- allyl metal complexes. Tipper reported the first example of the metallacycles obtained from [Pt(C2H4)Cl2]2 [3]. The stereospecific addition of cyclopropanes has been investigated from both mechanistic and synthetic view points [4],... 

Substitution of cyclopropane rings with the alkenyl group permits unique ring transformations based on metal coordination interaction with four -electrons. The transition-metal-induced ring-opening rearrangement also results in the formation of metallacycles. Further elaboration is attained by insertion and reductive elimination. 

Strained ring compounds undergo insertion of a low-valence metal complex to give metallacycles and the cycloaddition of metallacycles has a potential in synthesis, as described above. This method is useful in ring transformations of cyclobutenediones and cyclobutenones. 

In view of the extensive and fruitful results described above, redox reactions of small ring compounds provide a variety of versatile synthetic methods. In particular, transition metal-induced redox reactions play an important role in this area. Transition metal intermediates such as metallacycles, carbene complexes, 71-allyl complexes, transition metal enolates are involved, allowing further transformations, for example, insertion of olefins and carbon monoxide. Two-electron- and one-electron-mediated transformations are complementary to each other although the latter radical reactions have been less thoroughly investigated. 

Examples of heavier alkali metal complexes include [ GH(SiMe3)(SiMe2OMe) M] (M = Na 59, K 60) as well as the polymeric etherate [CH(SiMe3)(SiMe2OMe)K(OEt2)]oo 61.69 All these examples demonstrate the potency of intramolecular coordination, since methoxide-metal interactions under formation of metallacycles are observed in all cases. 

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