Thermally allowed conversions

Within the isolobal formalism, the conversion of 47 to 48 is a symmetry-allowed process, if it were to proceed as a concerted reaction (50). Structure 47 represents a transoid-2-meta.Wa-1,3-butadiene. In the bonding description, complex 48 represents formally a 1-metalla-bicyclo[1.1.0]butane. Therefore, the conversion of 47 to 48 represents a thermally allowed, concerted [ 2a + 2S] ring closure, in analogy to the pericyclic ring opening of bicyclo[1.1.0]butanes to give trans,trans-, 3-butadienes. 

A pericyclic reaction is allowed if orbital symmetry is conserved. In such reactions there is conversion of the ground (electronic) state of reactant into the ground state of the product. Such reactions are said to be thermally allowed, or there is the conversion of the first excited state of the reactant into the first excited state of the product. There are photo-chemically allowed reactions. 

This bonding overlap in the HOMO when the rotation occurs makes the formation of the new sigma bond favorable. The conversion of a diene to a cyclobutene by a conro-tatory motion is thermally allowed. 

Once the theory of pericyclic reactions was developed, it was recognized that the conversion of Dewar benzene to benzene is an electrocyclic reaction. This conversion involves two pairs of electrons one pair of pi electrons and one pair of sigma electrons of the Dewar benzene. (The third pair of electrons is located in exactly the same place in both the reactant and the product and so is not involved in the reaction.) An electrocyclic reaction involving two pairs of electrons must occur by a conrotatory motion if it is to be thermally allowed. However, the conrotatory opening of Dewar benzene is geometrically impossible, because it would result in a benzene with a trans double bond, a compound with too much angle strain to exist. 

The conversion of chorismate to prephenate, catalyzed by the enzyme choris-mate mutase, is involved in the biosynthesis of the amino acids phenylalanine and tyrosine. Classify this pericyclic reaction and explain whether it is thermally allowed or not. 

On a strict symmetry basis, conversion of 1,2-dimethylenecyclobutane to the planar biallylene with preservation of a plane of symmetry is thermally allowed with a disrotatory opening 85>. Direct conversion to the perpendicular Dza species maintained aCa axis gives a thermally... 

The thermally activated conversion of cyclobutenes to 1,3-butadienes is controlled by orbital symmetry. Woodward and Hoffman S and Fukui described the allowed thermal process as conrotatory, i.e. one in which the substituents at C-3 and C-4 both move in the same direction, either in a clockwise (equation 8) or counter-clockwise fashion (equation 9). In the photochemical process the motion of the same substituents is in the opposite direction, resulting in the formation of either a trans,trans or a cis,cis 1,3-diene (equations 10 and 11). 

On the other hand MINDO/3 calculations reveal that the conversion of benzvalene to benzene (equation 126) is thermally allowed. It is worth noting that according to the calculations, the transition state of this reaction is highly asymmetric. For example, the two side bonds which are cleaved during the course of the reaction have lengths of 1.601 and 2.100 A. Thus the term concerted in this case does not imply an identical degree of progress for all of the processes in the transition state. 

Only recently was a new caibothermal reduction process developed in which the WC is synthesized by a rapid carbothermal reduction of tungsten oxides in a vertical graphite transport reactor (RCR entrainment process) [3.49]. Rapid heating of the WO3/C mixture driven by thermal radiation allows conversion of the mixture into a carbide precursor (WCi. c) within very short reaction times (a matter of seconds). In a second step, additional carbon is added to the carbide precursor to form a mixture, which then imdergoes a second heat treatment to convert the precursor into substantially pure WC. 

A more detailed consideration of the Woodward-Hofimann postu-ulates for olefinic systems in the presence of a transition metal indicates that the thermally forbidden dimerization of two ethylene molecules to cyclobutane becomes allowed if the orbitals of the olefins can interact symmetrically with the dxt and dyz orbitals of the transition metal catalyst (53). One would consequently also expect transition metal complexes to catalyze the conversion of quadricyclene (IV) back to norbornadiene. This has been reported to be the case (54). The reactions leading to the formation of VI, XXX, and XXXI are examples of processes in which thermally allowed sigmatropic reactions become subject to catalysis by transition metal complexes. The catalysts thus display the dual role of removing symmetry restrictions and of generally lowering activation energies. 

The facilities used to harvest the ocean thermal energy are called OTEC plants (Ocean Thermal Energy Conversion). To achieve their maximum potential they must be located in the open ocean near the equator, far from any market. Several schemes to transmit their output to the market have been examined. The most practical appears to be use the electric power to electrolyze water to produce hydrogen and oxygen. The oxygen can be vented and the hydrogen liquefied for shipment to market. This scheme allows the OTEC plant to produce a storable fuel that can be used for most energy needs. 

The reverse of the [2+2]-cycloaddition is a thermally allowed o2s + o2a cycloreversion.323 Conversion of 385 to alkenes 387 and 388 illustrates this process. Note that the geometry of one alkene precursor is retained (suprafacial ring opening) but the other is reversed (antarafacial ring opening represented by 370).324 xhe thermal process requires interaction of a HOMO and a LUMO, which must proceed via the antarafacial inter-... 

Many furan derivatives react with dienophiles to form bicyclic compounds with an oxygen bridge." Most of the adducts obtained from furans are thermally labile and dissociate readily into their components on warming. The adduct from furan and maleic anhydride has been shown to have the exo strucmre 51, apparently violating the rule that the endo isomer predominates (see Section 3.1.4) (3.45). The reason for this is found in the related observation that the normal endo adduct formed from maleimide and furan at 20 °C dissociates at temperatures only slightly above room temperature and more rapidly on warming, allowing conversion of the endo adduct formed in the kinetically controlled reaction into the thermodynamically more stable exo isomer. With the maleic anhydride adduct, equilibration takes place below room temperature so that the endo adduct formed under kinetic control is not observed. 

Sigmatropic reactions of order [/, ] are thermally allowed to be suprafacial— suprafacial or antarafacial-antarafacial and are photochemically allowed to be antarafacial-suprafacial or suprafacial- antarafacial if i + j = 4n + 2. Conversely, they are thermally allowed to be antarafacial- suprafacial or suprafacial-antarafacial and photochemically allowed to be suprafacial-suprafacial or antarafacial-antarafacial if i + j=4n. 

Starting from a mixture of cis 75 and trans 74 2,3-divinylazmdines (derived from the thermal decomposition of azide 73), cis material 75 selectively underwent thermal rearrangement with significantly reduced reaction temperature [15ej. The additional loss of ring strain allowed conversion of the reactant aziridine 74 at 140 °C in xylenes to the product 76 even though a boat-Uke transition state had to be passed. Up to 40% overall yield of 76 was obtained (Scheme 10.18). 

Electrochemical and e.s.r. studies of the dibenzonorcaradiene anion radical indicate that neither the thermally allowed nor the photochemically forbidden disrotatory mode of cleavage is involved in the conversion of (247) into (248). In all probability a sequential electron-proton transfer process operates. E.s.r. studies have also shown anion radicals to be involved in the formation of diquinocyclo-propanones (250) employing a sodium-potassium alloy oxidation of (249) with potassium ferricyanide clearly involves monoradicals. ... 

The i j -configuration of the 6,7-double bond in pre-vitamin D is critical to its subsequent thermal rearrangement to the active vitamin. A photochemical isomerization of pre-vitamin D to yield the inactive trans-isoTnen occurs under conditions of synthesis, and is especially detrimental if there is a significant short wavelength component, eg, 254 nm, to the radiation continuum used to effect the synthesis. This side reaction reduces overall yield of the process and limits conversion yields to ca 60% (71). Photochemical reconversion of the inactive side product, tachysterol, to pre-vitamin D allows recovery of the product which would otherwise be lost, and improves economics of the overall process (70). 

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