Low-temperature rearrangement

Interestingly, it was possible to probe the spin-forbidden component of the tunneling reaction with internal and external heavy atom effects. Such effects are well known to enhance the rates of intersystem crossing of electronically excited triplets to ground singlet states, where the presence of heavier nuclei increases spin-orbit coupling. Relative rates for the low-temperature rearrangements of 12 to 13 were... 

Spirocyclic oxindole 60 was synthesized by [3,3]-sigmatropic rearrangement of the Af-phenyl-O-acylhydroxamic acid 58 (equation 19). The potassium enolate formed by treatment of 58 with potassium hexamethyldisilazide at low temperature rearranged to 59, which easily cyclized to the spirocyclic oxindole 60. Spirooxindoles were previously synthesized by Wolff and Taddei. The spirooxindole 60 was formed in 51% yield from cyclohexanecarboxylic acid after heating the preformed lithium salts of phenyl hydrazide 61 to 205-210 °C. 

Low-temperature rearrangements of vinylcyclopropanes have been reported and reviewed. Remote-charge acceleration has been invoked, in some cases, to account for the decreased temperatures which are required to bring about the bond reorganization. Bicyclic cyclopen-tenols of type 1 have been prepared in excellent yields in a rearrangement that may constitute the only case of a concerted vinylcyclopropane-cyclopentene rearrangement. ... 

Some of the low-temperature rearrangements listed in this section take place under Lewis acid catalysis (iron(III) chloride, iodotrimethylsilane, zinc(II) bromide, 010.]. °... 

However, the equivalent of 49 is known from the reaction of diphenyl ketene with cyclopentadiene. The adduct 50, formed at low temperature, rearranges through [3,3] sigmatropic shift to form what appears to be the [2 + 2] adduct 51. The reaction of 1-methoxybutadiene with diphenyl ketene to form 52 at low temperature is akin to [2 + 2] reaction involving nc=c of the ketene. However, this species also rearranges further by [3,3] sigmatropic shift to form 53, which is akin to [4 + 2] cycloaddition of the butadiene with 7rc=o °f the ketene, just as in 49 [ 18]. 

The pinacol-like rearrangement of halohydrins typically require elevated temperatures or extended reaction times. One notable exception is the low temperature rearrangement of vinyl alcohols 12. derived from the corresponding a-halo ketones by addition of a metalloalkyne and direct reduction of the resulting adduct with lithium aluminum hydride, providing an efficient and stereocontrolled access to a-alkenyl alcohols 1364-6 The intermediacy of vinyl alcohols 12, as a necessary precedent to rearrangement has been inferred from the observation that metal alkoxides 11 (M = Li, Mg) do not rearrange under the reaction conditions and are stable even at elevated temperatures. 

Et0)2P=0 radicals, obtained from t-BuO and (Et0)2P(H)=0, abstract halogen from alkyl halides, with competition studies being used to establish relative rates. Absolute second-order rate constants for reaction with butyl halides are 10 times slower for the phosphonyl radical compared with Et3Si. Phosphinyl-radical intermediates are implicated from ESR and P cidnp experiments when ketoximato-phosphorus(III) intermediates (formed from ketoximes and X2PCI at low temperatures) rearrange... 

Dimerization.—The lithio-salts of thiazoles[e.g. (58)], though stable at low temperatures, rearrange and dimerize to (60) at room temperature, by way of ketenimines (59), in 90% yield. The dimers (60) are reconverted into the starting material (57) above 150 °C. The reaction is general to heteroaryl systems of type (61), and has also been performed using 2-methyl-5-phenyl-l,3,4-thiadiazole. ... 

Nickel(O) forms a n-complex with three butadiene molecules at low temperature. This complex rearranges spontaneously at 0 °C to afford a bisallylic system, from which a large number of interesting olefins can be obtained. The scheme given below and the example of the synthesis of the odorous compound muscone (R. Baker, 1972, 1974 A.P. Kozikowski, 1976) indicate the variability of such rearrangements (P. Heimbach, 1970). Nowadays many rather complicated cycloolefins are synthesized on a large scale by such reactions and should be kept in mind as possible starting materials, e.g. after ozonolysis. 

Cyclooctatetraene can be obtained on an industrial scale by metal carbonyl catalyzed thermal tetramerization of acetylene. If cyclooctatetraene is UV-irradiated at low temperature in the presence of acetone, it is reversibly rearranged to form semibullvalene (H.E. Zimmerman, 1968, 1970). 

Grignard reagent comes from the substitution products it gives with various reactive substrates. When the low-temperature adduct is heated in an autoclave at 90 to 170 C for 3 to 6 hr, it does not rearrange to 2-ethylthiazole (12) as is the case in the pyridine series (436). 

Thiazole-N-oxides are prepared by the action at low temperature (-10°C) of hydrogen peroxide in acetic acid (474). 4-MethyIthiazole and 2,4-dimethylthiazole afforded the corresponding N-oxides with yields of 27 and 58%, respectively (Scheme 88). Thiazole-N-oxides without a methyl group in the 2-position are so unstable that they have a tendency to form 2-hydroxythiazoles and are decomposed by oxidation, whereas a 2-methyl group would prevent such rearrangement (474). 

The labile hydroxyl group is easily replaced by treatment with thionyl chloride, phosphorous chlorides, or even aqueous hydrogen haUdes. At low temperatures aqueous hydrochloric (186) or hydrobromic (187) acids give good yields of 3-halo-3-methyl-l-butynes. At higher temperatures these rearrange, first to l-halo-3-methyl-1,2-butadienes, then to the corresponding 1,3-butadienes (188,189). 

Perfluoroepoxides have also been prepared by anodic oxidation of fluoroalkenes (39), the low temperature oxidation of fluoroalkenes with potassium permanganate (40), by addition of difluorocarbene to perfluoroacetyl fluoride (41) or hexafluoroacetone (42), epoxidation of fluoroalkenes with oxygen difluoride (43) or peracids (44), the photolysis of substituted l,3-dioxolan-4-ones (45), and the thermal rearrangement of perfluorodioxoles (46). 

A number of catalysts of Pd(II), Pt(II), Rh(I), and Ir(I) induce rearrangements of 0-a11y1ic-.9-methy1 dithiocarbonates at 25°C (45). In a relatively low temperature procedure, olefins readily form from certain classes of xanthate esters (46) ... 

Photochemical studies on the ring degradation of 3-hydroxy-l,2-benzisoxazole also yielded benzoxazolone, and (40), (41) and (42) (Scheme 14) were believed to be potential intermediates. Low temperature IR measurements indicated the presence of (42) during the photochemical reaction (73JA919, 71DIS(B)4483, 71JOC1088). Sensitization studies indicate that the rearrangement is predominantly a triplet reaction, and the keto tautomer is believed to be the key orientation for the photolysis. 

A dramatic diflference in reactivity is evident when cb-divinylcyclopropane is compared wifli vinylcyclopropane. ciy-Divinylcyclopropane can only be isolated at low temperature because it very rapidly imdeigoes Cope rearrangement to 1,4-cycloh ta-... 

Hydrogen fluoride adds to more complex molecules, such as unsaturated steroids, to give fluorinated derivatives [/, 8] Low temperatures and inert diluents, such as tetrahydrofuran or methylene chloride, are generally employed. With bicyclic unsaturated terpenes, rearrangements often accompany addition to the double bond [/]. 

Cyclopropyl methanols when treated with a combination of hydrogen fluoride, pyridine, potassium hydrogen fluoride, and diisopropylamine undergo fluonnation and rearrangement to give excellent yields of homoallylic fluorides Chlorobenzene substituted cyclopropyl methanols at low temperatures leads to ring expansion to give... 

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