Transformation sequence hydrocarbons

Metha.nol-to-Ga.soline, The most significant development in synthetic fuels technology since the discovery of the Fischer-Tropsch process is the Mobil methanol-to-gasoline (MTG) process (47—49). Methanol is efftcientiy transformed into C2—C q hydrocarbons in a reaction catalyzed by the synthetic zeoHte ZSM-5 (50—52). The MTG reaction path is presented in Figure 5 (47). The reaction sequence can be summarized as... 

The reaction sequence outlined in Scheme 20.30 for the preparation of the chlorinated enyne-allenes was successfully adopted for the synthesis of the C44H26 hydrocarbon 251 having a carbon framework represented on the surface of C60 (Scheme 20.50) [83]. Condensation of the monoketal of acenaphthenequinone (243) with the lithium acetylide 101 afforded the propargylic alcohol 244. On exposure to thionyl chloride, 244 underwent a cascade sequence of reactions as described in Scheme 20.30 to furnish the chloride 248. Reduction followed by deprotection produced 250 to allow a repeat of condensation followed by the cascade transformation and reduction leading to 251. 

The same reason was given in the literature for the formation of a number of other bimetallic oxocomplexes such as Cd4Sn4(p4-O)(OCH2Bu,)10(OAc)10 [325] and [PbTi2(p4-0)(0Et)7(0Ac)]2 [320]. When the alkoxides of zirconium, hafnium, or cerium are used for this reaction, the formation of the oxocomplexes can be due to the elimination of an unsaturated hydrocarbon from the initial alkoxide [1565], leading to the formation of monometallic oxoalkox-ides as intermediates (the latter are formed already on desolvation of [M(OR)4(ROH)]2 and are always present as admixtures in the samples of des-olvated M(OR)4, where R is a primary or secondary radical [1612] (see also Section 12.12). In this case the possible sequence of transformations (taking place usually on reflux in toluene) can look as follows ... 

The synthesis of quinones from arenes is an area which demands further research, despite the number of reagents presently available for this transformation. This is highlighted by the synthesis of the naphthoquinone (3). Direct oxidation of the dibromoarene (1) was unsatisfactory, and therefore Bruce and coworkers had to resort to a multistep sequence involving nitration, reduction, diazotization, displacement by hydroxide and finally oxidation of the phenol (2) with Fremy s salt (Scheme 1). Although there are examples of the oxidation of polynuclear aromatic hydrocarbons to quinones, the direct oxidation of an arene to a quinone is a process not encountered in the synthesis of more complex mt ecules. 

A thoughtful reader would have noticed that, while plenty of methods are available for the reductive transformation of functionalized moieties into the parent saturated fragments, we have not referred to the reverse synthetic transformations, namely oxidative transformations of the C-H bond in hydrocarbons. This is not a fortuitous omission. The point is that the introduction of functional substituents in an alkane fragment (in a real sequence, not in the course of retrosynthetic analysis) is a problem of formidable complexity. The nature of the difficulty is not the lack of appropriate reactions - they do exist, like the classical homolytic processes, chlorination, nitration, or oxidation. However, as is typical for organic molecules, there are many C-H bonds capable of participating in these reactions in an indiscriminate fashion and the result is a problem of selective functionalization at a chosen site of the saturated hydrocarbon. At the same time, it is comparatively easy to introduce, selectively, an additional functionality at the saturated center, provided some function is already present in the molecule. Examples of this type of non-isohypsic (oxidative) transformation are given by the allylic oxidation of alkenes by Se02 into respective a,/3-unsaturated aldehydes, or a-bromination of ketones or carboxylic acids, as well as allylic bromination of alkenes with NBS (Scheme 2.64). 

The stereochemistry of a thermally induced 10e electrocyclization (predicted to be disrotatory) has not been firmly established and the main synthetic application is found in the formation of azulenes and ring-fused azulenes as in the transformation (452) to (453). Thermolysis of (454) with spontaneous elimination of dimethylamine from intermediate (455) afforded the fused azulene structure (456). The chemistry of even higher order (12e to 20e") pericyclic processes has been recently reviewed. An example of an unusual sequence of pericyclic processes is the transformation of heptahendecafulvadiene (457) to the pentacyclic hydrocarbons (462) and (463) in a 2 1 ratio. The pathway for this transformation can be viewed as an initial conrotatory 20e electrocyclization followed by a cascade of 10e and 6e pericyclic processes. ... 

The title compound was initially synthesized by the pyrolysis of Re3H3(CO)i2 at 190° in hydrocarbon solution. Treatment of the complex with carbon monoxide at atmospheric pressure gradually converts it into higher carbonyls, as indicated in the reaction sequence below. At slightly elevated temperatures, the reaction is much faster and hydrogen is evolved. The suggestion that the reverse transformation might be possible led to the current synthesis. The direct hydro-... 

Naphthalene was transformed into another hydrocarbon by the following sequence of reactions ... 

Broad are the implications and application of the principles of polystep reactions on polyfunctional catalyst combinations (P. B. Weisz). Here we deal with reaction sequences in which two catalyst species X and Y (or more) participate in one set of reaction sequences. Some of the general principles combine thermodynamics and physical parameters to yield important information and criteria for such rate processes, generally whether they occur in hydrocarbon transformations, organic chemistry, in a petroleum plant or in a living cell. 

The alterations in chemical composition of naturally weathered spilled oils are generally resulted from the combined effects of abiotic and biotic weathering, as Figure 27.8 and Figure 27.9 shows. The transformations of oil hydrocarbons by biodegradation are likely to occur stepwise, producing alcohols, phenols, aldehydes, and carboxylic acids in sequence. 

Fig. 4.12 Diagenetic and catagenetic transformation of steroids. The precursor sterols are gradually transformed during diagenesis into saturated hydrocarbons by dehydration (elimination of water) and hydrogenation of the double bonds. At higher temperatures, during catagenesis, the thermodynamically most stable stereoisomers are formed. Alternatively, dehydration leads to aromatic steroid hydrocarbons which are stable enough to occur in crude oils (after Rullkotter 2001). See text for detailed description of the reaction sequences.

Sinha and Keinan have reported a second synthesis of 100, which makes use not of building blocks, but of auxiliaries from the chiral C pool (AX as well as ent-AX). The synthesis begins with a reaction sequence that follows the Ci3 + C2 + Cj pattern to convert the C13 aldehyde into the hydrocarbon. This is then transformed by a threefold Sharpless asymmetric dihydrox-ylation (and partial acetalization) into the highly functionalized Cjg intermediate. The latter compound, after another chain elongation by means of the same C2 building block as used earlier, furnishes the Cig intermediate, and this then provides 100 by a three-step route (Scheme 49). 

Oxidative addition and reductive elimination reactions play key roles in C—H activation reactions, where a strong C—bond is cleaved by a transition-metal complex. These are important reactions because they permit unfunctionalized hydrocarbons to be transformed into complex molecules. Bergman reported the following classic C—H reductive elimination/oxidative addition sequence. ... 

The reaction of a late-transition-metal-alkyl complex containing d-electrons with hydrogen or a hydrocarbon to extrude alkane or arene and generate a new transition-metal-alkyl, -aryl, or -hydride complex usually occurs by a sequence of oxidative addition and reductive elimination. However, this overall transformation could also occur by a CT-bond metathesis-(Sclieme 6.5). It is difficult to determine by experiment if the reaction of a- late-transition-metal-alkyl complex occurs by the sequence of oxidative addition and reductive elimination or by o-bond metathesis. Therefore, theoretical studies have addressed this issue. In most cases, even when the process would involve a relatively high oxidation state, the process has been calculated to occur by oxidative addition and reductive elimination. ... 

Non-fluxional vinylic boranes do not react with carbonyl compounds. Nevertheless, the vinylic borane 29 reacts with acetone (2 h under reflux) yielding the Z-isomer of the homoallylic borinic ester 30b. The cw-configuration of the reaction product 30b corresponds to the following sequence of transformations (Scheme 2.11). The [1,7]-H shift in the vinylic borane 29 gives the allylic Z, Z-isomer 28d which immediately reacts with acetone before the equilibrium among the allylic isomers is established. On the other hand, cyclopentanone reacts directly with 29 under mild conditions yielding Z, Z-1,3,5-heptatriene 31 and borinic ester 32 (Scheme 2.12). Apparently 29 reacts with the enol form of cyclopentanone, and a direct splitting of the B-Q ,2 bond takes place. Similar reaction of 29 with acetic acid was used for the preparative synthesis of previously unknown hydrocarbon 31 (Scheme 2.12) [35]. 

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