Tungsten rearrangement

Synthesis From Other Ring Systems. These syntheses are further classified based on the number of atoms in the starting ring. Ring expansion of dichlorocyclopropane carbaldimine (53), where R = H and R = ryl, on pyrolysis gives 2-arylpyridines. Thermal rearrangement to substituted pyridines occurs in the presence of tungsten(VI) oxide. In most instances the nonchlorinated product is the primary product obtained (63). 

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... 

Electronically rich 1,3-butadienes such as Danishefsky s diene react with chromium alkenylcarbene complexes affording seven-membered rings in a formal [4S+3C] cycloaddition process [73a, 95a]. It is important to remark on the role played by the metal in this reaction as the analogous tungsten carbene complexes lead to [4S+2C] cycloadducts (see Sect. 2.9.1.1). Formation of the seven-membered ring is explained by an initial cyclopropanation of the most electron-rich double bond of the diene followed by a Cope rearrangement of the formed divinylcyclopropane (Scheme 65). Amino-substituted 1,3-butadienes also react with chromium alkenylcarbene complexes to produce the corre-... 

Tungsten alkoxycarbene complexes underwent similar anti-syn rearrangements but were much less prone to undergo CO loss [5-10]. 

Cyclopropanation of l,3-dienes. a,0-Unsaturated carbenes can undergo [4 + 2]cycloaddition with 1,3-dienes (12, 134), but they can also transfer the carbene ligand to an isolated double bond to form cyclopropanes. Exclusive cyclopropanation of a 1,3-diene is observed in the reaction of the a,(3-unsaturated chromium carbene 1 with the diene 2, which results in a frans-divinylcyclopropane (3) and a seven-membered silyl enol ether (4), which can be formed from 3 by a Cope rearrangement. However, the tungsten carbene corresponding to 1 undergoes exclusive [4 + 2]cycIoaddition with the diene 2. 

In 1991 Fischer et al. observed the interesting phenomenon that treatment of benzyUdene tungsten carbene complex 9 with triphenylketeneimine at —70 °C in CH2CI2 gave zwitterionic intermediate 11, which was derived from the rearrangement of the initially formed zwitterioinic intermediate 10 [5]. Careful analysis of... 

A similar hydride transfer was observed by Ipaktschi in the rearrangement of a 3-dialkylaminovinylidene-tungsten derivative [55]. 

W-W distances (2.634(3) A) are significantly shorter than those in the rhombus cluster and consistent with 12 CVE. The W4S6 framework may be regarded as an adamantanoid core, but the coordination geometry around the tungsten atoms is distorted from a regular tetrahedron. This reductive core rearrangement of sulfide clusters is unique and will open a preparative pathway to similar clusters. 

Mechanistic studies (108) have suggested that the formation of the binu-clear complex (71) on acidification of the product of the reaction between methyllithium and W(CO)s[C(OEt)Me] proceeds via the intermediate formation of dimethylcarbene- and vinylidene-tungsten complexes. These then cycloadd and rearrange as shown ... 

A complicated insertion and rearrangement process was observed from reactions of CNBu with WMe6. At — 78°C in diethylether air-stable crystals of the five-coordinate tungsten(VI) compound V N(Bu )CMe2 (Me) (NBu ) N(Bu )CMeCMe2 were obtained, and this complex was shown to have the structure 87 from an X-ray diffraction study. A mechanism for... 

While the germanium-containing tungsten complex 166 rearranged to 167 on photolysis, the isomeric species 168 directly eliminated the silene Me2Si=CH2 yielding 169 (Scheme 26)80. 

CASSCF calculations.7 Treatment of substituted 5-triethylsilyloxyhexa-l,2,5-triene derivatives with catalytic amounts of W(CO)6 was found to give the products of formal Cope rearrangement 2-triethylsilyloxyhex-l-en-5-ynes. The mechanism is believed to involve 6-endo attack by the silyl enol ether on the tungsten-activated allene, followed by ring opening with simultaneous loss of W(CO)s.8... 

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