Mesityl substituents

The classic disilene 1 is unusual in that it exists in at least three crystalline modifications orange and yellow unsolvated forms and a yellow toluene solvate (Fig. 2). The orange polymorph has a helical conformation in which all of the mesityl substituents are twisted in the same direction thus molecules of 1 in this form are chiral.51 The toluene solvate has an unusual conformation in which two mesityl rings cis to each other are nearly coplanar with the Si=Si bond, while the other two cis mesityl groups are nearly orthogonal.41 The structure of the yellow unsolvated form is not yet known. Because of the flat potential surface for the Si=Si... 

The carbonyl complex [Ag(L9)(C0)] (12), also of quite remarkable stability, is obtained by reaction of [Ag(L9)(C2H4)] (11) with CO in hexane. Nevertheless, the CO can be easily removed by increasing the temperature of the solution or by purging with an inert gas. Hence, such a reversible guest encapsulation within a molecular container might find applications for gas separation and storage. Again, one likely reason for the stability of the complexes is the protection offered by the bulky mesityl substituents that surround the ethylene or CO unit. 

Facial selectivity in 1,3-dipolar cycloadditions to cis-3,4-dimethylcyclobutene (73) (Scheme 1.21) was studied. Only phenylglyoxylo- and pyruvonitrile oxides lacked facial selectivities (anti syn = 1 1). All other nitrile oxides formed preferably anti-74. The anti/syn ratio increased from 60 40 (R = P-O2NC6K4) and 65 35 (R = Ph) to 87 13 and 92 8 for bulky ten-Bu and mesityl substituents, respectively. The transition-state structure of the cycloaddition of formonitrile oxide was determined using both HF/6-31G and B3LYP/6-31G methods. The... 

Figure 16. Optimized structures of the endo (lOa-anf and exo (IQb-anti) r -silyll allyl complexes with 4-(dimethylamino)-styrene as a substrate. Highlighted in (a) is the steric contact between the hydrogen of the benzylic carbon of the substrate and a hydrogen of the mesityl substituent 12 63 A) for the endo-anti intermediate, lOa-on/i. Highlighted in (b) is the steric contact between on hydrogen of the methyl group of the substrate and a hydrogen of the mesityl suhstituent 12 47 A) for the exo-anti silyl-allyl intermediate, Wb-anti. Hydrogen atoms are not shown except those whose interactions are highlighted.

The Schneider group independently reported an asymmetric vinylogous Mannich reaction (Scheme 27) [47]. Addition of silyl dienolates 73 to A-PMP-protected imines 74 was promoted by phosphoric acid (R)-3g (5 mol%, R = Mes) with mesityl substituents to afford tra i -a,p-nnsatnrated 8-amino esters 75 in high yields (66-94%) together with good enantioselectivities (80-92% ee). 

Chiral dicarboxylic acid (R)-5g (5 mol%, R = Mes) bearing simpler mesityl-substituents at the 3,3 -positions was found to catalyze efficiently the trans-selective asymmetric aziridination of iV-aryl-monosubstituted diazoacetamides 177 and aromatic (V-Boc imines 11 (Scheme 75) [94], In sharp contrast to previous reports on this generally dx-selective sort of aziridination, this method exhibited unique fran -selectivity and afforded exclusively the fran -aziridines 178 in moderate to good yields along with excellent enantioselectivities (<20-71%, 89-99% ee). The 1,2-aryl shift products 179 were observed as side products in varying ratios (178 179= 56 44-90 10). Diazoacetamides were chosen instead of diazoesters. Due... 

A semiquantitative study by Nazzal and Mueller-Westerhoff131 of the extent of cyclic electron delocalization ( aromaticity ) within each ligand ring in both classes was based on the NMR chemical shift differences observable in mesityl derivatives of these compounds. If a delocalized cyclic structure carries a mesityl substituent, the methyl groups in the ortho position will experience an upheld shift, while those in the para position do not. Their chemical shift difference A<5 can be used as a measure of delocalization. Compounds with Ad > 0.30 Hz are considered to be aromatic , i.e. highly delocalized. 

Fig. 7 Solid state structures of 59, 62 and 63 (methyl groups on the mesityl substituents in 62 and 63 omitted for clarity)...

Open-chain y -keto carboxylic esters with two mesityl substituents, such as methyl 3-hydroxy-2,3-dimesityl-2-propenoate, exist in solution only as the (Z)-isomer (4b) and the (ii)-isomer (4c) no keto form (4a) has been observed [223]. The (Z)-form predominates in nonpolar solvents such as cyclohexane (90 cmol/mol) and benzene (87 cmol/mol). Increasing solvent polarity shifts this Z)( E) equilibrium in favour of the more polar (7i)-isomer, up to 76 cmol/mol ( )-form in ethanol [223], The introduction of mesityl substituents stabiUzes enols of simple monocarbonyl compounds such as 2,2-dimesitylethenol, Mes2C=CH—OH [42, 224]. 

The description of the steric properties of phosphanes using the Tolman cone angle [113] proved to be an excellent concept capable of explaining many phenomena in the coordination chemistry of phosphanes and their applications, especially in homogenous catalysis. That there is a steric influence connected with NHC was noticed very early, in fact it was thought that the steric hindrance introduced by the N-mesityl substituents was a contributing factor in the isolation of the first stable carbene in 1991 as opposed to dimerisation to the known tetraaminoethylenes [1],... 

It is interesting to note that C-H activation on ruthenium NHC complexes is not limited to intramolecular protons located in the N-sidechain of the carbene, but occurs inter-molecularly as well. Leimer et al. reacted [MesIRuH PCyj] with toluene-dg at ambient temperature and observed a rapid H/D exchange reaction involving the four hydride hydrogen atoms on ruthenium, the methyl protons of the mesityl substituents of the carbene ligand and the deuterium atoms on the meta positions of toluene-dg. The ortho-, para- and methyl-deuterium atoms of the solvent did not participate [145]. 

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