Sigmatropic transformations hydrogen shifts

In many cases the transformations may be more complex than indicated by Eqs. (9.89)-(9.100). An example of this is the photochemistry of cis,cis-1,3-cyclooctadiene [Eq. (9.94)].<169) A close examination of this reaction indicates that bicyclo[4.2.0]oct-7-ene is formed but in low relative yields during the initial reaction (see Table 9.9). In addition, the cis,trans-1,3-cyclooctadiene is formed and then consumed as the reaction proceeds. Fonken showed that the bicyclooctene initially formed, however, was not from thermal isomerization of the cis,trans-diene. Still a third reaction was the 1,3 sigmatropic hydrogen shift to form the cis, cis-1,4-cyclooctadiene ... 

A third symmetry-allowed transformation in which vinylallenes partidpate involves sigmatropic hydrogen shifts. As shown in Scheme 5.45, two types of these concerted processes have so far been observed. 

Comparing the [l,5]-sigmatropic hydrogen shifts in pyrrole and phosphole, at the MP2/6-31G level, Bachrach has shown that the more stable tautomeric form is, respectively, IH -pyrrole and 2//-phosphole2d. The activation enthalpies were computed to be 186 kJmoG1 for 1H-pyrrole to 2H-pyrrole transformation and only 67 kJmoG1... 

The aromaticities of symmetry-allowed and -forbidden transition states for electrocyclic reactions and sigmatropic rearrangements involving two, four, and six r-electrons, and Diels-Alder cycloadditions, have been investigated by ab initio CASSCF calculations and analysis based on an index of deviation from aromaticity. The order of the aromaticity levels was found to correspond to the energy barriers for some of the reactions studied, and also to the allowed or forbidden nature of the transition states.2 The uses of catalytic metal vinylidene complexes in electrocycliza-tion, [l,5]-hydrogen shift reactions, and 2 + 2-cycloadditions, and the mechanisms of these transformations, have been reviewed.3... 

A cyclopentadecanone (V/74) and a ( )-muscone (V/70) synthesis were carried out by another method related to the reactions discussed in this Chapter. Scheme V/12 shows the thermal transformation of the cyclododecanone derivative, V/71, to V/74 [39], The proposed mechanism for this conversion presumably includes a [1.5] hydrogen shift, followed by a [3.3] sigmatropic rearrangement. 

Three applications of this reaction are possible (1) isomerization of sterically hindered aryl radicals, (2) enol-keto transformation, and (3) sigmatropic hydrogen shift. 

Finally, no examples of carbanion-accelerated 1,3-sigmatropic rearrangements appear to have been observed to date, although Okamura and coworkers have reported a transformation which may involve a sulfinyl carbanion accelerated 1,5-hydrogen shift. ... 

The chances are that the ester (283), analogously to (279), may be transformed by elimination of alcohol to the intermediate ketene (284) which cyclizes via (285) attended by 1,5-sigmatropic hydrogen shift to (286) ... 

The cyclopropanation of dienes with 17 is not very selective and results in the formation of a 4 1 ratio of cis- and irans-l,2-divinylcyclopropanes 18. The c/.v-isomers 18 immediately undergo a Cope rearrangement to yield 19 under the reaction conditions, whereas the more stable trans-isomers can be isolated. However, the Pww-compounds 18 (R1 = CH3) arc rapidly transformed to ring-opened trienes 20 via a [1,5] sigmatropic hydrogen shift on distillation or attempted chromatography. 

Sol 10. (b) Vitamin D3 (cholecalciferol) is produced through the action of ultraviolet irradiation (UV) on its precursor 7-dehydrocholesterol. The transformation occurs in two steps. In the first step, 7-dehydrocholesterol is photo-lyzed by ultraviolet light in a brr-electron conrotatory electrocyclic reaction to give previtamin D3. In the second step, previtamin D3 spontaneously isomerizes to vitamin D3 in a thermal antarafacial sigmatropic [1,7] hydrogen shift. 

By using the allenylborane 61 to form the organoborate complex 63, the trimethyltin chloride-induced transformation led to the enediallene 64 (Scheme 15) (25). After an electrocyclic reaction, the o-quinodimethane 65 was produced, which then underwent a [1,5] sigmatropic hydrogen shift to furnish 66. Oxidation with alkaline hydrogen peroxide followed by protonolysis with acetic acid then produced the phenol 67. 

Intermediate 5 is not isolated. It is transformed into the more stable dihydronaphthalene 2 by means of a thermal [1,5] sigmatropic hydrogen shift. This type of sigmatropic rearrangement can be understood in terms of the frontier molecular orbital theory considering the interaction between the H(ls) orbital and the LUMO ( l/3 ) of the diene component in the transition state (Fig. 14.2). A positive overlap between the orbitals where bond breaking and bond making takes place is produced when the H atom slides across the top face of the planar transition state. This kind of shift is called suprafacial. 

Irradiation of 3,3-diphenylthiooxindole gives 9-phenylthioxanthene (72T597). It is likely that this transformation involves successive cheletropic extrusion of CO, electrocyclization and a photolytic 1,3-sigmatropic shift of hydrogen (Scheme 152). The last step can also be brought about by base catalysis. 

Woodward and Hoffmann define a sigmatropic change of order [i, j] the migration of a cr-bond, flanked by one or more ir-electron systems, to a new position whose termini are i-1 and j-1 atoms removed from the original loci, in an uncatalyzed intramolecular process (8). The transformation XXIX-> XXX, for example, involving the shift of a hydrogen atom from c-j to c-1 is described [1, j]. 

Disubstituted cycloheptatrienes are of particular interest because they do not possess hydrogens available for [1,7] sigmatropic shifts as a primary reaction. As a result, other photochemical transformations should be observable. Irradiation of 3,7,7-trimethylcycloheptatriene (21) 26> in benzene (or cyclohexane) solution gives a mixture consisting of 2,2,6-trimethylbicyclo[3.2.0]hepta-3,6-diene (22), 1,5,7-trimethylcyc-loheptatriene (23), 1,3,7-trimethylcycloheptatriene (24), and unreacted... 

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