Alcohol elimination processes

Addition and elimination processes are the reverse of one another in a formal sense. There is also a close mechanistic relationship between the two reactions, and in many systems reaction can occur in either direction. For example, hydration of alkenes and dehydration of alcohols are both familiar reactions that are related as an addition-elimination pair. 

In comparison to the N- and S-counterparts, alkoxides possess lower nucleophilicity. Therefore, the reductive elimination process to form the C—O bond is much slower than those to form C— N and C—S bonds [103]. Palucki, Wolfe and Buchwald developed the first intramolecular Pd-catalyzed synthesis of cyclic aryl ethers from o-haloaryl-substituted alcohols [104]. For example, 3-(2-bromophenyl)-2-methyl-2-butanol (91) was converted to 2,2-dimethylchroman (92) under the agency of catalytic Pd(OAc)2 in the presence (S)-(-)-2,2 -bis(di-p-tolylphosphino)-l,r-binaphthyl (Tol-BINAP) as the ligand and K2CO3 as the base. The method worked well for the tertiary alcohols, moderately weE for cychc secondary alcohols, but not for acyclic secondary alcohols. 

Yields of chlorides are good to excellent for primary and secondary alcohols, but a competing olefin-forming elimination process renders the method of limited value for preparing tertiary chlorides.12 An adaptation of the procedure using carbon tetrabromide allows the synthesis of alkyl bromides. Some examples are the preparation of rt-C5H11Br (97%) and C H6CH2Br (96%).12 Farncsyl bromide has been prepared in 90% yield from fame sol.23... 

Toluene is used more commonly than the other BTXs as a commercial solvent. There are scores of solvent applications, though environmental constraints and health concerns diminish the enthusiasm for these uses. Toluene also is used to make toluene diisocyanate, the precursor to polyurethane foams. Other derivatives include phenol, benzyl alcohol, and benzoic acid. Research continues on ways to use toluene in applications that now require benzene. The hope is that the dealkylation-to-benzene or disproportionation steps can be eliminated. Processes for manufacturing styrene and terephthalic acid—the precursor to polyester fiber—are good, commercial prospects. 

An instructive example on how stereochemical features influence the stereochemical outcome of the elimination is the pyrolysis of xanthates from erythro-and f/treo-1,2-diphenyl-1-propanol. The erythro-alcohol 8 is converted into E-methylstilbene 9 only, and the f/ireo-alcohol 10 is converted into the corresponding Z-isomer 11 only. These results support the assumption of a syn-elimination process through a cyclic transition state ... 

Arylation of allylic alcohols leads to l-aryl-3-alkanones. The orientation for this reaction is in compliance with the general trend indicated above. Furthermore, the accepted reaction mechanism points to a Pd-H elimination process in which H... 

Comparison of kh with the calculated k,ot = 0.15 d 1 shows that abiotic hydrolysis is the most important removal mechanism for BzC in the pond ( 75%>) thus, you have to worry about the transformation product benzyl alcohol (Fig. 12.1). About 7% is removed by flushing (kw = VIQ = 0.01 d"1), and the rest by other processes. Considering the properties of benzyl chloride (e.g., Kj0Vi, Ajaw, see Appendix C), the most likely additional elimination processes are gas exchange and biotransformation (see later chapters). 

In organic chemistry, elimination processes are those decompositions of molecules whereby two fragments are split off and the multiplicity of the bonds between two carbon atoms or a carbon atom and a hetero atom is increased. Such a broad definition also embraces the dehydrogenation of hydrocarbons and alcohols which is dealt with in Chap. 2. Here we shall restrict our review to the olefin-forming eliminations of the type... 

Interestingly, when a fi-substituted alcohol is used in the Barton-McCombie reaction and if a [3-elimination process occurs faster than the hydrogen transfer step, then the formation of a double bond is observed. We have just seen such an example with a dixanthate (see Section 3.1.3). Many others are known as in [3-hydroxy sulfides [231] and [3-hydroxysulfones [232,233] in a modified Julia synthesis of olefins. 

Current Processes. The development of superactive third-generation supported catalysts enabled the introduction of simplified processes, without sections for catalyst deactivation or removal of atactic polymer. By eliminating the waste streams associated with the neutralization of catalyst residues and purification of the recycled diluent and alcohol, these processes minimize any potential environmental impact. Investment costs arc reduced by approximately one-third over slurry process plants. Energy consumption is minimized by elimination of the distillation of recycled diluent and alcohol. The total plant cost for the production of polymer is less than 130% of the monomer price, when a modem process is used, compared to 175% for a slurry process. 

It could then be argued that the electron rapidly returns to the primary ion, affording an excited state (reaction 195). However, the alcohol models (liquid and gas phase) show that at least the lowest, electronically excited state predominantly splits the O-H bond or eliminates H2, to give the corresponding carbonyl compounds. With O-deuterated alcohols, these processes lead80 to HD (>75%). Obviously, the mechanistic concepts developed for the liquid alcohols cannot be applied directly to solid carbohydrates. 

Allylsilanes 12 react with a-ketoesters 174 to give [3 + 2] cycloadducts 175 in 50-85% yields through 1,2-silyl migration (equation 73)59,228. Use of an allylic trimethylsilane produces an allyl alcohol by-product through a competing silyl elimination process. It can be circumvented by use of a bulkier silicon-containing reagent, such as the... 

This result dictated that any alkene-generating elimination process has to proceed via conditions not basic enough to enolize the tertiary amide. Using a procedure developed by Ochiai, the tetraalkylstannane unit of 119 was converted into the chlorotrialkylstannane of 121 in excellent yield. The formation of a halotin species enabled the use of the Tamao-Fleming oxidation36 for formation of the primary alcohol within 122. 

The removal of a molecule of a hydrogen halide from an alkyl halide to yield an alkene is effected under strongly basic conditions, e.g. a concentrated alcoholic solution of sodium or potassium hydroxide or alkoxide. This overall reaction has been submitted to most rigorous mechanistic studies. Most of the factors (temperature, nature of base, structure of substrate, solvent, etc.) which control product composition have been evaluated. It thus appears that under the conditions noted above, an E2 process, in which the participating sites adopt an ann -periplanar conformation leading to an anti-elimination process, is generally favoured. 

These oxidations are complicated, but it is probably best to remember that elimination step. It is common in many alcohol oxidation processes. 

Now for some of the reactions you have seen in the last few chapters. Starting with carbonyl substitution reactions, the first example is the conversion of acid chlorides into esters. The simplest mechanism to understand is that involved when the anion of an alcohol (a metal alkoxide RO ) reacts with an acid chloride. The kinetics are bimolecular rate = fc[MeCOCl] [RO ]. The mechanism is the simple addition elimination process with a tetrahedral intermediate. 

Thermal elimination reactions of molecules in the gas phase and in solution usually occur by 1,2-processes by a variety of mechanisms (Banthorpe, 1963) some 1,4-elimination processes have also been suggested (Harding et al., 1967). In mass spectrometry, isotopic labelling shows that elimination of water from primary aliphatic alcohols occurred mainly by 1,4-processes, the remainder being 1,3- and 1,5-reactions, provided the aliphatic chain was sufficiently long (Benz and Biemann, 1964 S. Meyerson and Leitch, 1964). Similar studies showed that dehydrochlorination of 1-chloropentane and 1-chlorobutane luider electron impact proceeded preferentially by 1,3-elimination processes (Duffield et at., 1966). 

The low Sn2 reactivity of 1°-alkyl bromide, 2,2-dimethyl-1-bromopropane (neopentyl bromide, 2.5), is explained by steric hindrance to the required 180° alignment of reacting orbitals. However, under Sn 1 conditions, neopentyl bromide (2.5) reacts at roughly the same rate as other 1°-alkyl halides such as ethyl bromide. Ionization of alkyl halides to carbocation in SnI is the rate-determining step. Although the product from ethyl bromide is ethanol as expected, neopentyl bromide (2.5) yields 2-methyl-2-butanol (2.6) instead of the expected 2,2-dimethyl-1-propanol (neopentyl alcohol) (2.7). This is because once formed the ethyl carbocation can only be transformed by a substitution or elimination process. In the case of the neopentyl carbocation, however, the initially formed l°-carbocation may be converted... 

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