Alkyl conversion

Although the 3a,4,5,6a-tetrahydrofuro[3,2-fiQisoxazoles are unstable in the presence of Bronsted acids (e.g., Equation (58)), they are relatively stable to mild Lewis acids. As exemplified in Scheme 35, the combination of an organolithium and lithium bromide facilitates alkylative conversion of the isoxazoline nucleus (207) to the bicyclic isoxazolidines (208) and (209) <89CL1079>. A similar transformation was also reported for the tetrahydrofuro[3,4-fiQisoxazole (210) (Equation (59)). 

B-ZSM-5 samples (A and C) were observed to be inactive for the alkylation, even at 600°C, with a very low methanol conversion yielding light olefins. The SAPO-11 sample (H) gave very low alkylation conversion with thermodynamic para-xylene equilibrium selectivity. The latter result obviously arises from the pore dimensions (6 x 6.2 A), larger than for MFI samples. 

Bifunctional with metal hydrides (or alkyls) (conversion of butenes to... 

C-Alkylation. (Chloromethyl)dimethylphenylsilane (1) can be utilized to install a silylmethyl group on carbon via base promoted C-alkylation of terminal alkynes, dihydropyrazines, mal-onic esters, phenylacetonitriles, sulfoxides, and imines. Although 1 has been used directly in the alkylation, conversion to the corresponding iodide 2 via Finkelstein displacement (eq 1) prior to alkylation is sometimes warranted. Except for the malonic esters (eq 2), strongly basic conditions and low temperatures (with slow warming) are generally employed in the transformation (eqs 3 and 4). 

The nature of the achiral R OH affects chiral selectivity, and by using 3,5-dimethyl-phenol high optical yields of optically active secondary alcohols are obtained from ketones. A similar principle is seen in the alkylative conversion of aldehydes and ketones to alcohols by alkyl transfer from the aluminate (19), and in the reduction of ketones by a-aminoester boranes (20) in the presence of Lewis acids. 

As a complementary process to reforming, isomerization converts normal paraffins to iso-paraffins, either to prepare streams for other conversions nCi —> /C4 destined for alkylation or to increase the motor and research octane numbers of iight components in the gasoiine pooi, i.e., the C5 or Cs-Ce fractions from primary distillation of the crude, or light gasoline from conversion processes, having low octane numbers. 

We cite isomerization of Cs-Ce paraffinic cuts, aliphatic alkylation making isoparaffinic gasoline from C3-C5 olefins and isobutane, and etherification of C4-C5 olefins with the C1-C2 alcohols. This type of refinery can need more hydrogen than is available from naphtha reforming. Flexibility is greatly improved over the simple conventional refinery. Nonetheless some products are not eliminated, for example, the heavy fuel of marginal quality, and the conversion product qualities may not be adequate, even after severe treatment, to meet certain specifications such as the gasoline octane number, diesel cetane number, and allowable levels of certain components. 

In the conversion of alcohols into alkyl halides, the mechanism is probably ... 

The process whereby aldehydes are produced from arylmethyl (also alkyl and other) halides by the action of hexamine is known as the Sommelet reaction. The reaction is essentially the conversion of an amine into an aldehyde the hexamine serves the dual role of converting the halide into the amine and the amine into the aldehyde, but its function is different in the two steps. When starting from a halide, the reaction proceeds in three stages —... 

There are, however, two useful alkylating-redudng methods. One is the methylenation of the ester carbonyl group with Tebbe s reagent, the other is the conversion of thionolactones to cyclic thioketals and subsequent reduction. 

Amines are powerful nucleophiles which react under neutral or slightly basic conditions with several electron-accepting carbon reagents. The reaction of alkyl halides with amines is useful for the preparation of tertiary amines or quaternary ammonium salts. The conversion of primary amines into secondary amines is usually not feasible since the secondary amine tends towards further alkylation. 

An important method for construction of functionalized 3-alkyl substituents involves introduction of a nucleophilic carbon synthon by displacement of an a-substituent. This corresponds to formation of a benzylic bond but the ability of the indole ring to act as an electron donor strongly influences the reaction pattern. Under many conditions displacement takes place by an elimination-addition sequence[l]. Substituents that are normally poor leaving groups, e.g. alkoxy or dialkylamino, exhibit a convenient level of reactivity. Conversely, the 3-(halomethyl)indoles are too reactive to be synthetically useful unless stabilized by a ring EW substituent. 3-(Dimethylaminomethyl)indoles (gramine derivatives) prepared by Mannich reactions or the derived quaternary salts are often the preferred starting material for the nucleophilic substitution reactions. 

One effective method for synthesis of tryptophan derivatives involves alkylation of formamido- or acetamido- malonate diesters by gramine[l,2]. Conversion to tryptophans is completed by hydrolysis and decarboxylation. These reactions were discussed in Chapter 12. An enolate of an a-nitro ester is an alternative nucleophile. The products can be converted to tryptophans by rcduction[3,4],... 

The major portion of the present chapter concerns the conversion of alcohols to alkyl halides by reaction with hydrogen halides... 

Selectivity is not an issue m the conversion of alcohols to alkyl halides Except for certain limitations to be discussed m Section 8 15 the location of the halogen sub stituent m the product corresponds to that of the hydroxyl group m the starting alcohol... 

Chemical reactivity and functional group transformations involving the preparation of alkyl halides from alcohols and from alkanes are the mam themes of this chapter Although the conversions of an alcohol or an alkane to an alkyl halide are both classi tied as substitutions they proceed by very different mechanisms... 

This concludes discussion of our second functional group transformation mvolv mg alcohols the first was the conversion of alcohols to alkyl halides (Chapter 4) and the second the conversion of alcohols to alkenes In the remaining sections of the chap ter the conversion of alkyl halides to alkenes by dehydrohalogenation is described... 

We have seen this situation before m the reaction of alcohols with hydrogen halides (8ection 4 11) m the acid catalyzed dehydration of alcohols (8ection 5 12) and m the conversion of alkyl halides to alkenes by the El mechanism (8ection 5 17) As m these other reactions an electronic effect specifically the stabilization of the carbocation intermediate by alkyl substituents is the decisive factor The more stable the carbo cation the faster it is formed... 

Nitriles contain the —C=N functional group We have already discussed the two mam procedures by which they are prepared namely the nucleophilic substitution of alkyl halides by cyanide and the conversion of aldehydes and ketones to cyanohydrins Table 20 6 reviews aspects of these reactions Neither of the reactions m Table 20 6 is suitable for aryl nitriles (ArC=N) these compounds are readily prepared by a reaction to be dis cussed m Chapter 22... 

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