Reaction path ethers

Fig. 5. Methanol-to-hydrocarbons reaction path at 371°C, where (A ) is methanol ( ), dimethyl ether (), water (D), paraffins (and Cg...

Under the catalytic action of Rh2(OAc)4, formation of a propargylic ether from a terminal alkyne (229, R1=H) is preferred as long as no steric hindrance by the adjacent group is felt162,218>. Otherwise, cyclopropenation may become the dominant reaction path [e.g. 229 (R1 = H, R2 = R3 = Me) and methyl diazoacetate 56% of cyclopropene, 36% of propargylic ether162)], in contrast to the situation with allylic alcohols, where O/H insertion is rather insensitive to steric influences. 

In a recent publication, Chang and Silvestri have discussed this reaction in detail (109). They reported that under conditions of low (ca. 10%) conversion substantial amounts of dimethyl ether, formed by the reversible dehydration of methanol, are present and 78% of the primary hydrocarbon product consists of C2-C4 olefins. Also, if dimethyl ether, in the absence of water, is used instead of methanol, essentially the same hydrocarbon product distribution is obtained. On the basis of these observations, Chang and Silvestri suggest the reaction path shown below ... 

The reaction path was not recalculated for the methyl vinyl ether, H2C=C(H)(OCH3) which was chosen as a model for the experimental H2C=C(H)(OEt). It is assumed that the path is not modified this hypothesis has been validated with H2C=C(H)F [8]. The calculations are thus limited to the products and key intermediates. 

The presence of organolithium compounds in etheric solvents at temperatures above 0°C may lead to extensive decomposition of the solvent and solute a slow electron transfer side reaction of lithium naphthalene or sodium naphthalene with the THE solvent (equation 5) has been reported . The three isomeric forms of BuLi were shown to induce extensive decomposition of THE. The main path for this process is metallation at position 2 of THE, leading to ring opening and elimination of ethylene. An alternative path is proton abstraction at position 3, followed by ring opening. The presence of additives such as (—)-sparteine (24), DMPU (25), TMEDA and especially HMPA does not prevent decomposition but strongly affects the reaction path. ... 

While many researchers have used the 1,3-APT process to generate cyclic nitrones, it is clear that the operating reaction pathway in the oxime to isoxazolidine conversion may not always be predicted. In the work of Aurich and co-workers (317,318), the polycyclic isoxazohdine 292 was isolated as the major product from thermolysis of oxime 293 and may have been formed via two separate reaction paths (Scheme 1.61). In the proven route, initial 1,3-APT of 293 formed a 1,4-oxazine nitrone (294), which acted as the acceptor for the second 1,3-APT with the remaining oxime function. The cyclic nitrone 295 so formed underwent a 1,3-dipolar cycloaddition with the allyl ether forming the isolated polycyclic isoxazolidine adduct 292. 

Main reaction paths of the thermal desorption of ethanol are proposed in Scheme 5. The species observed directly by NMR spectrscopy are surrounded by broken lines. At temperatures less than 323 K, the dehydration did not proceed, and only reversible desorption took place. The protonated ethanol dimer is transformed into protonated ether at temperatures exceeding 323 K. Diethyl ether is formed only in the gas phase by replacement with ethanol. Protonated ethanol monomer probably gives ethylene via the ethoxide at temperatures exceeding 333 K (169). 

The available rate data for the substitution reactions of phenol, diphenyl ether, and anisole are summarized in Table 5. The elucidation of the reactivity of phenol is hindered by its partial conversion in basic media into the more reactive phenoxide anion. Because of the high reaction velocity of phenol and the even greater reactivity of phenoxide ion the relative rates are difficult to evaluate. Study of the bromination of substituted phenols (Bell and Spencer, 1959 Bell and Rawlinson, 1961) by electrochemical techniques suitable for fast reactions indicates the significance of both reaction paths even under acidic conditions. 

The use of model compounds is a convenient starting point to determine the reaction path, particularly for stepwise polymerizations. For epoxy-amine systems, a monofunctional epoxide such as phenyl glycidyl ether (PGE) is often used for these studies (Verchere et al., 1990 Mijovic and Wijaya, 1994). Figure 5.10 shows the reaction scheme for the curing of a monoepoxide with a diamine. 

The initial dehydration reaction is sufficiently fast to form an equilibrium mixture of methanol, dimethyl ether, and water. These oxygenates dehydrate further to give light olefins. They in turn polymerize and cyclize to form a variety of paraffins, aromatics, and cycloparaffins. The above reaction path is illustrated further by Figure 3 in terms of product selectivity measured in an isothermal laboratory reactor over a wide range of space velocities. ( 3) The rate limiting step is the conversion of oxygenates to olefins, a reaction step that appears to be autocatalytic. In the absence of olefins, this rate is slow but it is accelerated as the concentration of olefins increases. 

Characteristic of such dienes, a-keto-p,7-unsaturated esters such as (3 see also Table 3 and Table 5) exhibit good thermal reactivity toward simple vinyl ethers in [4 + 2] cycloaddition reactions that proceed with exclusive regiocontrol predominately through an erulo transition state. The endo selectivity increases as the reaction temperature is decreased and both the reaction rate and the endo selectivity increase as the reaction pressure is increased (Figure 3, Table 3). The substantial increase in the diastereoselectivity of the pressure-promoted [4 + 2] cycloaddition reaction of (3) with a cis 1,2-disub-stituted dienophile has been attributed to the additional differences in the volume of activation between the reaction paths leading to the endo and exo diastereomers due to the additional cis C-2 dienophile sub-... 

Ethereal solvents, principally THF, either with or without sonication, have been reported to give results similar to those obtained on reductions in NH3 with no added proton donor, and pinacol formation as a major reaction path. a potentially useful selective reduction of unhindered cyclohexanones in the presence of other ketones using A1 amalgam in aqueous THF has been described and will be discussed in detail subsequently (Section 1.4.3.3.2).2 in this procedure aliphatic ketones give no pinacols however, aromatic ketones give only the corresponding pinacol.2 ... 

Treatment of a,p-unsaturated tosylhydrazones with NaBH4 in MeOH affords principally allylic ethers from cyclic derivatives and pyrazoles with most noncyclic examples. This divergent behavior compared to saturated tosylhydrazones has been attributedto a lessening of the electrophilicity of conjugated imine ir-bonds, which allows initial abstraction of the acidic N—H proton by BH4 to compete with reduction, and gives alternative reactions related to the Bamford-Stevens process as depicted in Scheme 4. An exception to this may be the deoxygenation of conjugated vinyl triflates (entry 11, Table 6). The cyclopropanation and elimination products produced in entry 4, Table 6 also probably arise from similar, alternative reaction paths. ... 

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