Transition methyl derivatives

Quantum chemistry methods allow the prediction of the ultraviolet transitions in good agreement with the experimental values in the case of thiazole and its three methyl derivatives (Table 1-18). A very weak absorption has been indicated at 269.5 nm that could correspond to an n- TT transition given by calculation at 281.5 nm (133). Ultraviolet absorption spectroscopy has been investigated in connection with steric interactions in the A-4-thiazoline-2-thione (74) series (181). It was earlier demonstrated by NMR technique that 4-alkyl-3 isopropyl-A-4-thiazoline-2-thiones exist in solution as equilibrium mixtures of two conformers (75 and 76), the relative populations of which vary with the size of R4 (182) for R4 = rBu the population of rotamer A is 100%, whereas for R4 = Me it is only 28%. Starting from the observed absorption wavelength for... 

COMPARISON OF ULTRAVIOLET ABSORPTION SPECTRA OF THIAZOLE AND HS METHYL DERIVATIVES WITH CALCULATED VALUES OF TRANSITIONS BY DIFFERENT METHODS... 

Pyrazole and its C-methyl derivatives acting as 2-monohaptopyrazoles in a neutral or slightly acidic medium give M(HPz) X, complexes where M is a transition metal, X is the counterion and m is the valence of the transition metal, usually 2. The number of pyrazole molecules, n, for a given metal depends on the nature of X and on the steric effects of the pyrazole substituents, especially those at position 3. Complexes of 3(5)-methylpyrazole with salts of a number of divalent metals involve the less hindered tautomer, the 5-methylpyrazole (209). With pyrazole and 4- or 5-monosubstituted pyrazoles M(HPz)6X2... 

In terms of free enthalpies, the SN2 transition state is more stable than the ET transition state for the methyl derivative. As steric hindrance increases, the difference becomes smaller and smaller. The free enthalpies of the two transition states are practically equal for the isopropyl derivative, while the balance turns in favor of the ET transition state for the t-butyl derivative. 

It is reasonable to relate the values of k+fk at least qualitatively to the extents of bond making and breaking in the transition state. Bond making is all important in hydrolyses of carboxylic anhydrides, diaryl carbonates and methyl arenesulfonates. Bond breaking will be important in hydrolyses of alkyl halides and sulfonates, except for methyl derivatives, and especially so in water which can effectively solvate the leaving anion. 

In Section 3 (p. 148) we saw that both early and late transition states for SN2 reactions of methyl derivatives are likely to have progressed about 50% in terms of their charge development. This proposal is quite contrary to the generally accepted view and in this section we wish to discuss how general this disparity between charge and geometric progression is likely to be. 

Transition metal compounds with covalent carbon-metal bonds include organo-zinc, organo-cadmium, and organo-mercury compounds. Carbon-13 shifts of the methyl derivatives (Table 4.71) indicate a heavy atom deshielding. Diphenylmercury displays carbon shifts similar to those of phenyllithium and phenylmagnesium bromide (Table 4.53). 

The kinetic stability of group 14 metalloles towards [4 + 2] dimerization is dependent on the nature of the heteroatom and on the substituents bonded to it and the ring carbons. Like cyclopentadiene, the silole 2 is unstable as a monomer. The silicon methylated derivatives (3 and 4) are also unstable, but two mesityl groups provide stabilization by steric hindrance of the [4 + 2] dimerization transition state15. The same effect has been observed for 1,1-dimesitylgermole24. 

Green, M.L.H., Hughes, A.K., Popham, N.A., Stephens, A.H.H. and Wong, L.L. (1992) Nuclear magnetic resonance studies on partially deuteriated transition metal—methyl derivatives. /. Chem. Soc., Dalton Trans., 3077. 

Immediately upon their isolation, trifluoromethyl-containing derivatives of (low valent) transition metals were universally found to be much more robust, both thermally and oxidatively, than their methylated counterparts. Many perfluoromethyl complexes, for example, have been found to be unreactive in air whereas the corresponding methyl derivatives are air sensitive. Under anaerobic conditions a number of trifluoromethylated species decompose at temperatures approximately 100°C higher than the analogous methylated compounds. 

Aroyl esters of anthracene-9-methanol are photolysed in methanol to give products consistent with the anthracene-9-methyl cation as an intermediate.41 Rate constants for the solvolyses of secondary alkyl tosylates in fluorinated solvents were analysed in terms of the possible involvement of very short-lived carbocation-tosylate ion pair intermediates.42 The effect of added electrolytes on the rate of solvolysis of cumyl chloride and its -methyl derivative was studied in 90% aqueous acetone and 80% aqueous DMSO, with the results revealing a combination of a special salt effect and a mass law effect.43 Kinetic parameters obtained for the solvolysis of (8) (R1 = R2 = Me and R1 = Ar, R2 = H) show that there is substantial n, n participation in the transition state [e.g. (9). 44... 

In this respect it is the PPPCI results on the ir-electronic system which a priori should be considered. They enable one to predict that the N(l)H and N(3)H tautomers should be bathochromic by about 10 my with respect to the N 1)H and N(9)H ones. In fact, the comparison of the theoretical evaluation of the energy of the first singlet transition (Si) with the experimental values of Amax in the methylated derivatives of the tautomers indicates an extremely satisfactory agreement both in the ordering of all four tautomers and in the absolute values. For a more detailed study of the spectrum of the most common N(9)H tautomer see Pullman and Pullman.63,74... 

Table IV. Cotton effects for the n - t carbonyl transition of 7-keto steroids and their 6-methylated derivatives.

Interesting results came from the study of the reaction rates of these flexibly capped cyclodextrins with ra-f-butylphenyl acetate and m-nitrophenyl acetate (9). The ra-f-butylphenyl acetate was bound more weakly to theN-methyl derivative by a factor of 2.3, even with a new hydrophobic floor. This was reasonable if the geometry of the system were such that the substrate could not penetrate now as deeply into the cavity as it normally does. The result is in contrast with the situation of adamantanecarboxylate, which does not penetrate completely the cavity in the first place. On the other hand, the reaction rate for acetyl transfer within the complex was increased by a factor of 9. This was also expected, since now the new, more shallow, binding geometry of the substrate was closer to the geometry required in the transition state for acetyl transfer, so there was a smaller potential well from which the substrate had to climb. 

The 1,3-dipolar cycloadditions of ethyl 4//-furo[3,2-/>]pyrrole-5-carboxylate (8a) or its 4-methyl derivative (8f) (Equation (2)) with C-benzoyl-A-phenylnitrone and ACV-diphenylnitrone proceeded regiospecifically at positions 2 and 3 of the furan ring. During these reactions, exclusively endo cycloadducts were formed, because their transition states are stabilized by secondary orbital interactions <81CCC2421>. 

The Chemical Shifts of Methyl Derivatives of Transition Metals... 

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