Platinum complexes rearrangement

A quantitative solid-state ene addition between two ligands of the platinum complex 397 gives the rearranged platinum complex 398 upon extensive heating to 140 °C [123]. This rearrangement reaction (cf. Sect. 24) is treated here, as it is the only known quantitative solid-state ene addition to date. Further quantitative solid-state [4-1-4] additions and higher vinylogs, as well as ene additions also of the intermolecular type, await detection both as thermal and photochemical reactions. 

An iron tetracarbonyl complex (295) ° and a platinum bis(triphenylphosphine) complex of thiete 1,1-dioxide have been prepared. Platinum complexes of 3-phenyl- and 3-(p-bromophenyl) thiete 1,1-dioxide also have been prepared. No complex was obtained with the 3-t-butyl derivative. The pale-yellow, crystalline iron complex decomposes in refluxing hexane in the presence of excess sulfone to Fe2S2(CO)9, indicating a drastic structural rearrangement. Other carbon-containing fragments were not observed. The bis(triphenylarsine)platinum complex of 3-02-bromophenyl) thiete sulfone is decomposed photochemically to the thiete sulfone. The same result is achieved on treatment of the complex with tetra-cyanoethylene. ... 

Previously, it had been shown that pyrolysis of (86) at 620°C in a vertical nitrogen-flow system gave a complex mixture of products, including (88) (3.5%), (64 R, R = H) (0.8%), 1,1,3,3-tetramethyl-1,3-disila-2-methylidenecyclopentane (2.1 %) and 3,3,4,5,5-pentamethyl-3,5-disilacyclo-pentene (2%) <79JOM(168)23>. Formation of disilacyclopentanes by platinum-catalyzed rearrangement of vinyl disilanes also has been reported <86JAP61207389). 

In these syntheses, Peppard had to rely on the very weak trans effect in the cobalt complex [Co(NH3) i (SO3) 2 ]-. The cis-dichloro-cis-diammineethylenediaminecobalt(III) ion is asymmetric, and Peppard partially resolved it into its enantiomeric forms by adsorption on quartz ground to 100 mesh. This method of resolution is frequently, but not always, effective. Before it can be fully utilized, we will need to learn a great deal more about the principles of adsorption. The preparation of Peppard s isomer s is particularly interesting because cobalt(III) complexes rearrange easily, whereas the platinum compounds do not. 

The mechanism of catalytic hydrosilylation is not well understood. Study of these reactions is hampered by their complexity induction periods are often involved, reaction conditions such as the nature of the catalyst and reacting groups are critical factors, and side-reactions, such as alkene rearrangements, are common. A widely accepted mechanism for homogeneous catalysis by platinum complexes is based on the work of Chalk and Harrod (Scheme 10)5,8,262. With Speier s catalyst, it appears that initially, and probably during the induction period, silane reduces the platinum to a Pt(0) or Pt(II) complex that is the active catalytic species265. 

Deuterium labeling has been used to detect a cyclobutylmethylplatinum formation and ring cleavage by -alkyl elimination that occurs in a reversible way for the platinum complex in Scheme 6.51 [150]. A similar degenerate rearrangement occurs for the pentenyl derivative [Y(Cp )2() -) -CH2(CH2)2CH=CH2 ] [151]. 

Transition-metal-catalyzed processes often work nicely for the aromatic Claisen rearrangement. Mechanistic aspects might be different from those under usual thermal conditions. Platinum complexes catalyzed the reaction of allyl 2-naphthyl ether 41 to afford l-allyl-2-naphthol 42 regioselectively in excellent yield [40]. Molybdenum hexacarbonyl also catalyzed the one-pot conversion of allyl aryl ethers to coumaran derivatives such as 43 from 17 [41]. 

A free radical initiator azobisisobutyronitrile or an inhibitor, hydroquinone, have no effect on the rate of oxygen consumption. In any case a dissociative mechanism is precluded since autoxidation of uncoordinated phosphines is a radical process which gives mixed R P(OXOR)3 products. The mechanism suggested involves the formation and rearrangement of a cobalt dioxygen complex, equations (81) and (82), possibly via a dissociative oxygen insertion step of the type proposed by Halpem for platinum complexes. 

As a result of an unexpected metal-induced phosphoryl migration reaction from carbon to nitrogen, ligand 47 can display different isomeric forms that lead to the formation of a cationic bis-chelated platinum complex containing a rearranged and an intact ligand 47 as part of a six-membered and a four-membered chelate (Scheme 14.26). 

A rhodium complex similar to the platinum complex described in (4) also showed an ability to catalyze formation of phenanthrenes from biphenylene and alkynes [22]. Cyclotrimerization was observed as a side reaction, and with silyl substituted alkynes some [1,2]-silyl rearrangements were seen, leading to [1,1] addition of the alkyne across the C-C bond (13). 

Mochida and coworkers studied the oxidative addition and rearrangement chemistry of hydrogenated di- and trisilanes with Pt(0) complexes. Dinuclear platinum complexes with silylene or silanylene bridges were frequently formed in the course of these reactions (Scheme 37) [435-438]. 

Scheme 37 Rearrangement of a disilanyl platinum complex to a disilyl complex and further to a dinuclear disilylene complex...

Since the whole catalytic cycle involves a single metal-substrate bond, changing from n- to cr-bonds in the successive elementary steps of Fig. 10.41, simultaneous coordination of the alkyne and the alkene to the metal centre is not required. Thus, it was anticipated that metal complexes with even a single vacant coordination site might afford suitable catalysts for these rearrangements. Platinum complexes with a... 

Reactions of Pt(PPhj)2(R)X (R = CH3, C Hj X = Cl, Br, I) and methyl isocyanide 144) and analogous reactions of Pd(phos)2(CH3)I (phos = PPh3, PPhMc2) complexes with cyclohexyl isocyanide 169, 170) were reported about the same time. As might perhaps be anticipated, the platinum reactions were slower and one can isolate the intermediate species and observe their rearrangement to the inserted products [Eq. (8)]. The isolation of the... 

The rearrangement of platinacyclobutanes to alkene complexes or ylide complexes is shown to involve an initial 1,3-hydride shift (a-elimina-tion), which may be preceded by skeletal isomerization. This isomerization can be used as a model for the bond shift mechanism of isomerization of alkanes by platinum metal, while the a-elimination also suggests a possible new mechanism for alkene polymerisation. New platinacyclobutanes with -CH2 0SC>2Me substituents undergo solvolysis with ring expansion to platinacyclopentane derivatives, the first examples of metallacyclobutane to metallacyclopentane ring expansion. The mechanism, which may also involve preliminary skeletal isomerization, has been elucidated by use of isotopic labelling and kinetic studies. 

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