Neopentyl complexes

Osborn s discovery (14) that aluminum halides bimTto oxo ligands in tungsten oxo neopentyl complexes, and that these complexes decompose to give systems which will efficiently metathesize olefins, raised more questions concerning the role of the Lewis acid. A subsequent communication (20) answered some of the questions the aluminum halide removes We oxo ligand and replaces it with two halides to yield neopentylidene complexes (equation 8). 

Reaction of tris(neopentyl) complexes of titanium, zirconium and hafnium with molecular oxygen furnishes the corresponding tris(neopentoxy) complexes [42, 43, 51]. A peroxo complex is an intermediate in this reaction, being relatively stable in the case of titanium [42]. The alkoxide species can also be formed upon reaction with alcohols under mild conditions [42, 52]. The alcoholysis reaction is fast, with a low dependence on the steric hindrance of the alkyl chain [42]. Hydrolysis leads to ](=SiO)M(OH)3] or ](=SiO)2M(OH)2], depending on the precursor species. Deu-... 

The stepwise reduction of the ethynyl complex Fe(C=CH)(dppe)(fj-C5H5) to the neopentyl derivative has been achieved by two sequences of methylation and hydride additions, illustrating the propensity for nucleophilic addition to C , and electrophilic addition to Cfi (25). The initial conversion to [Fe(=C=CMe2)(dppe)(i/-C5H5)]+ (5) has been described above (Section III,A,2) further reactions of this complex with Na[HB(OMe)3], Me30+, and NaBH4 afford the neopentyl complex ... 

The /3-methyl elimination reactions are rare by comparison, examples being the decomposition of zirconium neopentyl complexes 109... 

Bis(pentametiiylcyclopentadienyl)actinide(IV) mono- and dialkyl complexes are readily prepared from Cp 2 Cl2. A rather large number of alkyl groups have been employed, and compounds of this class are generally prepared by metathesis (equation 59). In the solid state, complexes of formula Cp 2" n 2 adopt a pseudotetrahedral coordination geometry about the actinide metal. A neutron diffraction study of the thorium neopentyl complex Cp 2Th(CH2-t-Bu)2 also revealed agostic Th-H-C interactions with the q -CH bonds. ... 

When the methyl complex Pd(PPh3)2(I)(Me) is treated with Ag in CeDe at 85 °C, the only product is PPh3Me+. No benzene alkylation takes place. Heating the cationic Pd(PPh3)2(CD3CN)(Me)+ also produces only PPh3Me+. The relative instability of methyl radical probably accounts for the different reaction paths of the benzyl and methyl cationic complexes. When phosphonium salt formation is not possible, for example, in the neopentyl complex... 

Direct (3-Mo elimination was observed when activating zirconocene methyl neopentyl complexes with B(C6F5)3.555 With sterically bulky Cp ligands, instantaneous isobutylene elimination is observed at — 75 °G however, for the bis-(Cp) compound, the zwitterionic neopentyl complex species is stable at 0 °C but undergoes clean and reversible f3-Me elimination at 25 °C (Scheme 177). This finding is consistent with (3-Me elimination as the major chain-transfer pathway in propylene polymerizations using a sterically encumbered metallocene catalyst. 

Dialkylplatinum(II) complexes bearing P-hydrogens undergo exclusive P-hydrogen elimination on thermolysis [179,180,182]. On the other hand, the di-neopentyl complex, which has no P-hydrogens. undergoes y-C—H bond activation to provide a platinacyclobutane (eq (140)) [183]. 

Hortonobserved direct /1-Me elimination in activating zirconocene methyl neopentyl complexes with FAB. Depending on the cyclopentadienyl ligand steric bulk, instantaneous isobutylene elimination is observed at —75 °C for bis-Cp ligation however for a bis-Cp ligation environment, the cationic species formed is stable at 0 °C yet undergoes clean, revers-... 

Ibers, DiCosimo and Whitesides showed that the bis-neopentyl platinum complex of equation 16 gave facile cyclometalation. They were surprised that the reverse reaction, which would of course be an alkane activation with neopentane as the substrate, did not take place because, thermodynamically, the bonds formed should compensate for the bonds lost. Presumably, it is the unfavorable entropic term which is the major factor in preventing the reverse reaction, but the substantial steric bulk of the neopentyl group may tend to reduce the Pt—neopentyl bond strength in a bis-neopentyl complex. 

Careful design of the substrate allows multiple bond formation in a single reaction. In the multiply unsaturated iodide 5.167, after oxidative addition, two alkyne insertions occur, followed by an alkene insertion (Scheme 5.47). This insertion generates a neopentyl complex, which undergoes a second alkene insertion. The sequence is terminated by an eventual -hydride elimination giving the steroid-like 5.168. 

A series of alkylidenetantalum(V) complexes has provided considerable insight into another type of activation process in metathesis catalysis. Thus benzyl and neopentyl complexes undergo a abstraction, the more hindered neopentyl complex the more readily. Treatment with an excess of phosphine [L = P(CH3)3] gives a abstraction in a very hindered seven-coordinate species [TaL2(CH2C(CH3)3)2Cl3]. Nmr studies show a complicated series of dissociation and dimerization equilibria among carbene complexes of the type [TaL (CHC(CH3)3)Cl3] with n = and a full account of metathesis catalysis... 

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