Thermally promoted addition

Essentially the same cyclic transition structures were proposed for the reaction performed under high pressure. 

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An alternative approach to 6H-1,3-thiazines requires the thermally promoted addition of a,/3-unsaturated carbonyl derivatives to thionoimines. Dihydrothiazines are the initial products but they readily decompose by elimination of the 4-substituent (Scheme 66) <79BSF(2)347>. 

The stereochemical course of the thermally promoted addition of allylic trialkyl-stannanes to aldehydes is dependent upon the geometry of the 2-butenyl unit [67, 69], The reaction is believed to proceed via a cyclic, six-membered, chair-like transition structure. Reaction of an -2-butenylstannane provides the anti homoallylic alcohol, while an Z-2-butenylstannane affords the corresponding syn homoallylic alcohol (Scheme 10-37). The allylation of aldehydes with allylic stannanes has also been performed under high pressure and neutral conditions [70]. The stereochemical outcome of the reaction of E- and Z-2-butenylstannanes with aldehydes under high pressure was almost identical to the results obtained thermally. 

Since the suggested conversion process does not include a thermally promoted bond-scission step, the question arises of how the addition of hydrogen results in the bond breaking necessary for significant reduction in molecular weight. We have already noted that the nucleophilic action of the basic methanol system was sufficient to cleave diphenyl ether, and a similar route is available in the basic i-PrOH and C0/H20 systems. On the other hand, we showed in control experiments that strongly basic conditions alone were not sufficient for significant conversion of coal. 

Vanadium precursors tend to promote addition polymerization of olefins on activation with aluminum co-catalysts. Nomura and coworkers recently reported the first example of catalytic olefin metathesis by a vanadium alkylidene complex.The well-defined, thermally stable V(v) alkylidene complex 7 effected ROMP of NBE at 80 °C. Cettain alkylvanadium complexes, including V(CH2Ph)2(=NAr )(OAr ) (Ar = 2,6-Pr 2C6H3 Ar = 2,6-Me2C6H3) and... 

Cyclopentenes behave differently and often act through radical mechanisms this can lead to photoreduction to cyclopentanes, or photoaddition of the kind exemplified by norborneneand propan-2-ol 12.57). The photoadduct in this process is linked through the carbon atom of the alcohol, and not the oxygen atom. A related addition to acetonitrile 12.58) takes place when norbornene is irradiated in the presence of a silver(i) compound. It is likely thal a metal complex of the alkene is the real irradiation substrate, and the same may be true for copper(i)-promoted additions of haloalkanes to electron-deficient alkenes (2.59). When dichloromelhane is used in such a reaction the product can be reduced electrochemically to a cyclopropane (2.60), which is of value because the related thermal addition of CH.I, to alkenes in the presence of copper does not succeed with electron-poor compounds. 

Amidinobenzothiazoles 298 are prepared in high yield by a thermally-promoted rearrangement of thiadiazolium salts 296 or thiadiazolines 297. Addition of base to the rearrangement of the thiadiazolium salts 296 can improve the yield by the prior conversion of... 

In all examples examined the syn adduct was strongly favored. It should be noted that the thermal reaction affords the syn adduct which must be formed via a synclinal arrangement because of the cyclic nature of the transition state. Two arguments were advanced to explain the preference for a synclinal transition state in Lewis acid-promoted additions. The first of these entailed Coulombic attractive forces between the developing positive charge at the /3-position of the allylic stannane and the electron-... 

Energy. Thermal. The addition of heat promotes the treatment process. First, it reduces the viscosity of the oil. This effect is especially instrumental in the handling and treatment of heavy crude oils or bitumen emulsions. Second, it weakens or ruptures the film between the oil and water droplets by expanding the water present. Last, heat increases the difference in densities of the fluids and thereby tends to reduce the settling time. 

A brief description of the mechanistic rationale for thermal reactions of aldehydes and allyfic stannanes is distinguished from Lewis acid-promoted additions by the supposition of closed cyclic transition states.i" The Lewis acidity of tin provides for pre-organization and the E- or Z-geometry of the starting allyfic stannane is reflected in the formation of anti- and iy -afiylation products 26 and 29 (Scheme 5.2.5). 

Since the discovery of thermally promoted allylation of aldehydes [9], allylstannanes have been widely used in organic synthesis as stable and stereodefined reagents for C-C bond formation. Although it had been reported that activated aldehydes [10] or allylstannanes with chloride on the tin [11] could be used for allylation, remarkably innovative technology for allylation was advanced by Naruta and by Sakurai and Hosomi [12]. They disclosed that allylation was promoted by addition of a Lewis acid this substantially expanded the versatihty of the aUylstaimane procedure. Because many allylation reactions have already been documented [1], the most recent progress in this field will be described after brief description of fundamental aspects. 

The quantum tunnelling relaxation pathway is expected to be dominant at very low temperatures (when there are very few phonons). At higher temperatures the thermally activated regime, up-and-over the barrier, is dominant. At intermediate temperatures both mechanisms are important. This is easily understood from Figure 5.24 at zero field Ms = +9 and -9, 4-8 and -8, and so on are in resonance in addition to Ms = 10. Hence, the spin can be thermally promoted to, for example, -9... 

Since mixed conducting membranes operate at elevated temperatures, this additionally results in the thermal promotion of electrons from the perovskite valence band (oxygen 2p orbitals) to the conduction band (t2g orbitals of the B-site cation). This electronic conduction has been characterized as p-related [34]. One would anticipate that electronic conductivity would increase with progressively decreasing perovskite band gaps [35]. 

Clay modification with thermally stable additives that would promote deagglomeration, dispersion, and exfoliation of agglomerated nanoclays, particularly in high-temperature thermoplastic matrices. 

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