Alkyl compounds spectroscopy

The potentially tautomeric side-chain thiol systems exist mainly in the thiol form in liquid solution and in the gas phase, as found by IR and NMR spectroscopy and by a study of ionization potentials.126 Upon alkylation using the ion-pair extraction method, only the S-alkylated compounds were obtained. The synthesis, reactions, and properties of some selenides of thiophene, furan, and selenophene have been reviewed.127... 

As mentioned at the beginning of this chapter, unsubstituted 1,4-thiazine 3 prefers the 2/7-form <1948JA684> but oxidation of sulfur as well as electron-withdrawing substituents make the compound adopt the 477-form. This can be seen in IR spectroscopy by the appearance of an N-H band and in the ease of N-alkylation. IR spectroscopy helped also to prove the structure 149 to be the lactam <1958JA5198> as opposed to the aza-enol first reported (Scheme 5) <1948JA3517>. 

We have already seen in Section 2.2.2 that metal-alkyl compounds are prone to undergo /3-hydride elimination or, in short, /3-elimination reactions (see Fig. 2.5). In fact, hydride abstraction can occur from carbon atoms in other positions also, but elimination from the /8-carbon is more common. As seen earlier, insertion of an alkene into a metal-hydrogen bond and a /8-elimination reaction have a reversible relationship. This is obvious in Reaction 2.8. For certain metal complexes it has been possible to study this reversible equilibrium by NMR spectroscopy. A hydrido-ethylene complex of rhodium, as shown in Fig. 2.8, is an example. In metal-catalyzed alkene polymerization, termination of the polymer chain growth often follows the /8-hydride elimination pathway. This also is schematically shown in Fig. 2.8. 

Besides dihomooxacalix[4]arenes bearing carbonyl groups at their lower rims (28-32) reported before 2000 [26, 27], 9a has been used to prepare the inherently chiral triethyl ester 33 [28] and the tetra(2-pyridyl) 34 [29] derivatives. Depending on the reaction conditions (use of 80 % NaH instead of 95 % NaH and less reaction time), partial 0-alkylation of 9a with 2-(chloromethyl)pyridine hydrochloride was also achieved by Marcos and coworkers [29]. Five of the eight possible partially alkylated compounds 35 (mono-substituted 35b and di-substituted 35c-f) were obtained in the cone conformation. Recently, the same group reported the preparation of a series of bidentate urea (36-41) and thiourea (42) derivatives in a four step synthesis from the parent compound 9a [30, 31]. Their structure determination in solution by NMR spectroscopy (and in the solid state for compounds 38 and 41 by single crystal X-ray crystallography) indicated the cone conformation for all the compounds. The cone tetraethyl ester 43 was also described [11]. 

The importance of ring size holds also for tautomerism of -pyrrol-5-ones and. d -dihydro-6-pyridones. While the former compounds behave as cyclic 1-methyl-2-alkyl-2-hydroxy-5-pyrrolidones 179) (76) [or, on distillation, as the dehydrated l-methyl-2-alkyl-J -pyrrolones (77)], the latter compounds exist as acyclic N-methylamides of 8-oxo-acids (78) [as shown by infrared spectroscopy (/80)j. The dehydration of 78 during distillation to form l-methyl-2-alkyl-. -dihydro-6-pyridones (79) is achieved only with difficulty. 

Activation of the catalyst is usually performed by exposure to a co-catalyst, namely an aluminum alkyl. The model catalysts were successfully activated by trimethylalumimun (TMA) and triethylaluminum (TEA), commonly used for this purpose. The compounds were dosed from the gas phase either at room temperature for a prolonged time or for a much shorter time at a surface temperature of 40 K. Nominal 3400 L of TMA or TEA were exposed at room temperature. The chemical integrity of the co-catalyst was verified by IR spectroscopy of condensed films grown at low temperature on the substrates. The spectra were typical for condensed and matrix isolated species [119]. 

There are several examples of alkyl halides reacting with 1,2,3-thiadiazoles at nitrogen to yield either salts or mesoionic compounds <1996CHEC-II(4)289>. Similarly, with Meerwein s reagent, several substituted thiadiazoles yielded various 2- and 3-methylated 1,2,3-thiadiazoles (Scheme 4 Table 8) <1993JHC301>. The isomer ratios were determined by integrating the methyl singlets in the H NMR spectra and the compounds were further studied by 1SN NMR spectroscopy (Section 5.07.3.4). 

The first silicon-organophosphorus betaine with a thiolate center (15a) was synthesized by the reaction of stable silanethione (14) with trimethyl-methylenephosphorane (Scheme 8) and characterized by multinuclear NMR spectroscopy.14 Compound 15a is formed under kinetic control and is transformed, under the thermodynamically controlled conditions, into the silaacenaphthene salt (16). The processes presented in this scheme reflect the competition of the basicity and nucleophilicity of phosphorus ylides. Betaine 15b prepared from less nucleophilic and less basic ylide with phenyl substituents at the phosphorus atom is much less resistant toward retro-decomposition compared to the alkyl analog. Its equilibrium concentration does not exceed 6%. 

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