Transition metal rings

FIGURE16.8 The valence molecular orbitals of four-member transition metal ring-like molecules. The atomic components of the each of the four molecular orbitals of each of the six sets, (a) the Oj set, and (h) the set 8 arising from the orbitals, are shown at the bottom. At the top row, (c) the arising from the orbitals, (d) the o, arising from the d y orbitals, (e) the Jt, arising from the d orbitals, and (f) the Jt, arising from the d y orbitals, are displayed. 

Not all ligands use just two electrons to bond to transition metals Chromium has the electron configuration [Ar]4s 3rf (6 valence electrons) and needs 12 more to satisfy the 18 electron rule In the compound (benzene)tricarbonylchromium 6 of these 12 are the tt elec Irons of the benzene ring the remammg 6 are from the three carbonyl ligands... 

Section 14 14 Transition metal complexes that contain one or more organic ligands offer a rich variety of structural types and reactivity Organic ligands can be bonded to a metal by a ct bond or through its it system Metallocenes are transition metal complexes m which one or more of the ligands is a cyclopentadienyl ring Ferrocene was the first metallocene synthesized Its electrostatic potential map opens this chapter... 

Reactions with Transition-Metal Compounds. The numerous pubhshed products of reactions of transition-metal compounds with a2iridines can be divided into complexes in which the a2iridine ring is intact, compounds formed by reaction of a2iridine with the ligands of a complex, and complexes in which the a2iridine molecule is fragmented (imido complexes). 

Ring-Opening Polymerization. As with most other inorganic polymers, ring-opening polymerization of cyclotetrasilanes has been used to make polysilanes (109,110). This method, however, has so far only been used for polymethylphenylsilane (eq. 12). Molecular weights (up to 100,000) are higher than from transition-metal catalyzed polymerization of primary silanes. 

As with other hydroperoxides, hydroxyaLkyl hydroperoxides are decomposed by transition-metal ions in an electron-transfer process. This is tme even for those hydroxyaLkyl hydroperoxides that only exist in equiUbrium. For example, those hydroperoxides from cycHc ketones (R, R = alkylene) form an oxygen-centered radical initially which then undergoes ring-opening -scission forming an intermediate carboxyalkyl radical (124) ... 

Transition metals have also been inserted into the aziridine ring affording derivatives (295). Stereochemical studies suggest that transfer of a proton is followed by bimolecular attack on the ring with subsequent closure on the carbonyl group (76AG(E)495). 

Schmidt reaction of ketones, 7, 530 from thienylnitrenes, 4, 820 tautomers, 7, 492 thermal reactions, 7, 503 transition metal complexes reactivity, 7, 28 tungsten complexes, 7, 523 UV spectra, 7, 501 X-ray analysis, 7, 494 1 H-Azepines conformation, 7, 492 cycloaddition reactions, 7, 520, 522 dimerization, 7, 508 H NMR, 7, 495 isomerization, 7, 519 metal complexes, 7, 512 photoaddition reactions with oxygen, 7, 523 protonation, 7, 509 ring contractions, 7, 506 sigmatropic rearrangements, 7, 506 stability, 7, 492 N-substituted mass spectra, 7, 501 rearrangements, 7, 504 synthesis, 7, 536-537... 

The use of supported transition metal oxide and Ziegler-Natta-type catalysts for polymerising aliphatic olefins (alkenes) was extended in the 1960s and 1970s to the ring-opening polymerisation of cyclo-olefins. 

Although most of the macrocycles that contain phosphorus or arsenic which have thus far been prepared, are primarily transition metals binders, two compounds have been prepared which are essentially crown ethers containing phosphorus. Kudrya, Shtepanek and Kirsanovhave prepared two compounds which are essentially polyoxygen macrocycles but which contain one or two methylphosphonic acid esters as part of the ring. These two macrocycles are shown below as 7d and 17 and are both prepared by the reaction of 2,2 [oxybis(ethyleneoxy)] bisphenolate with methylphosphonic dichloride in a mixture of acetonitrile and benzene. The crystalline monomer 16) and dimer 17) were isolated in 17% and 11% yields respectively as indicated in Eq. (6.13). 

The solvated sulfenamides [Li2( BuNSC6H4Me-4)2(THF)n] (n = 2,4) have dimeric structures with a central Li2N2 ring. The coordination mode is determined by the extent of solvation of the Li" ions monosolvation allows for rj -N,S coordination whereas disolvation restricts the coordination mode to // -M Variable temperature NMR studies indicated that a dynamic exchange between these two structural types occurs in THF solution (Scheme 10.10). The dihapto coordination mode is observed exclusively in transition-metal complexes and the... 

The combination of hard (A) and soft (5) coordination in the 1,5-P2N4S2 ring system leads to a diversity of coordination modes in complexes with transition metals (Lig. 13.1). In some cases these complexes may be prepared by the reaction of the dianion [Ph4P2N4S2] with a metal halide complex, but these reactions frequently result in redox to regenerate 13.3 (L = S, R = Ph). A more versatile approach is the oxidative addition of the neutral ligand 13.3 (L = S) to the metal centre. 

E. Hengge and H. Stuger, in H. W. Roesky (ed.). Rings, Clusters and Polymers of Main Group and Transition Metals, Elsevier, Amsterdam, 1989, pp. 107-38. 

A unique method to generate the pyridine ring employed a transition metal-mediated 6-endo-dig cyclization of A-propargylamine derivative 120. The reaction proceeds in 5-12 h with yields of 22-74%. Gold (HI) salts are required to catalyze the reaction, but copper salts are sufficient with reactive ketones. A proposed reaction mechanism involves activation of the alkyne by transition metal complexation. This lowers the activation energy for the enamine addition to the alkyne that generates 121. The transition metal also behaves as a Lewis acid and facilitates formation of 120 from 118 and 119. Subsequent aromatization of 121 affords pyridine 122. 

---