2.2.6.6- Tetramethyl-4-substituted

Table 16 shows various characteristics of segmented siloxane-(aryl ester) block copolymers. The effect of the variation in the polyester backbone was also studied by replacing bisphenol-A with tetramethyl substituted bisphenol-A. The major difference in these systems was an increase in the high temperature Tg to around 210 to 215 °C 193). 

Lithiation of the dibromodithienyl compound 464 and reaction with sulfur and subsequent oxidation yielded the tetramethyl substituted dithieno[3,Z-c.Z, 3 - ][l,2]dithiin 465 (Equation 126) <1997T7509>. 

Formation of various oxathiaadamantane derivatives such as tetramethyl-substituted dioxadithia- (237), monooxadithia- (238), and monooxamono-thiaadamantane (239) is observed among many products in the reaction of 2,4-pentadione with hydrogen sulfide.257,258... 

Krogh-Jespersen and Roth231 also investigated Jahn-Teller distortions in mono-, di- and tetramethyl substituted cyclopropyl radical cations. In all cases, the ground state of the corresponding radical cation can be considered to be derived from a 2 4, -type state and (the) transition state(s) of pseudorotation from (a) 2B2-type state(s). Several minima and transition states were found along the pseudorotation path where the most stable form corresponds to an asymmetrical2A, -type state. 

Complexes such as (62) can be submitted to hydrogenation and oxidative dehydrogenation processes of the type already described for related systems. The most interesting investigation has actually been carried out on complexes of a tetramethyl substituted ligand prepared from a nontemplate reaction sequence. In particular, oxidative dehydrogenation affords macrocyclic chro-mophores with six and seven double bonds (Scheme 24).118,141... 

Reaction with OH also leads to the splitting of the dimer (-30% efficiency), and there is evidence that one-electron oxidants such as S04 may also induce the splitting of the dimer (Heelis et al. 1992). The NO -radical-induced splitting of the tetramethyl-substituted Ura cyclobutane dimer has been investigated in acetonitrile (Kruger and Wille 2001). The -N03 radical has been generated photo-lytically from a Ce(VI) salt (Chap. 5.2). Under theses conditions, the 5-5 -linked intermediate is also trapped, possibly by a deprotonation or a Ce(IV)-mediated oxidation that competes with 3-fragmentation [reactions (310)—(313)]. 

Macomber, 1969). The larger anchimeric assistance effect on rate is observed in the case of em-dimethyl substitution at C2. Bly and Koock (1969) estimated rate enhancements due to homoallenic participation of 8-3-58 x 108 for the 2,2-dimethyl brosylate (5a of Table 6) and as large as 1-7 x 105 in the case of 2,2,5,5-tetramethyl substituted brosylate in acetic acid at 75°. It is noteworthy that these rate enhancements are larger by factor of at least 10 than in the case of homoallylio participation in related compounds. 

In a related investigation to that just discussed, the tetramethyl-substituted quaterpyridine 37, which was designed to promote co-ordination in a twisted configuration, yielded complexes with 1 1 and 2 1 (metal ligand) stoichiometries with manganese(II) and cobalt(II). With copper(I), a dimeric helical complex of... 

The homoaromatic nature of the novel tetramethyl-substituted homophe-nalenyl cation 166, a 12K-clcctron homoaromatic system, has been reported on the basis of NMR chemical shift data and NICS values. Ion 166 was also shown to exhibit stereochemical nonrigidity. ... 

Figure 8. Desymmetrization of spiro[4.4] nonane by 2,3,7,8-tetramethyl substitution.

Tetrabenzoperaza-crown-4 cyclophanes (biphenyl), table, lli Tetrabenzoperaza-crown-4 cyclophanes (miscellaneous), table, 734 Tetrabenzoperaza-crown-4 cyclophanes (tetrakis-p-phenylenyl), table, 726 Tetrabenzoperaza-crown-4 cyclophanes (tetrakis-p-xylyenyl), table, 731 Tetraethylene glycol bis(3-butenyl)-substituted, 38 chiral-substituted, 38 dichloride derivative, 179 diiodide derivative, 179, 184 tetramethyl-substituted, 38 Tetraethylenepentaamine, pertosylatcd, 49 Tetraglycolic acid, 112 Tetrahydro-l,4-diazepine, 93, 543 Tetrahydroazepine, 94 iV,A(Af iV -Tetrakis(2-hydroxyethyl)-ethylenediamine, 182... 

Even 2,3,4,5-tetramethyl substitution does not cause pronounced shift effects. The 1-phenyl derivative has 3 -1-12.71, compared to 3 +6.4 for 1-phenylphosphole. 

Table 1. Tetramethyl Substituted Polyaryl Ethers Synthesized via... 

Tetramethyl-substituted benzopentahelicene 90 (Fig. 27) was synthesized by the Diels-Alder reaction between a diene and benzyne followed by dehydrogenation [83]. 

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