Propeller conformation

An early application was to the pathway for conformational isomerization of molecules Ar3Z, with three aromatic rings on the same centre (Mislow, 1976). Typically the system is pyramidal (tetrahedral overall where there is a fourth substituent on Z), and the rings are close enough in space that they cannot rotate independently about the Z-Ar bond. Triphenylphosphine oxide, to take a specific example, crystallizes in a propeller conformation [4 Z = P=OJ which is chiral, with all three benzene rings rotated in the same sense from the relevant C-P-O plane. A study (Bye et al., 1982) of deformations from this geometry for more than 1000 related structures in various environments allowed a detailed description of the pathway for... 

Simple enols stabilized by bulky aryl groups have been reviewed.131 Amide enols, tip2C=C(OH)NR1R2 (tip = 2,4,6-triisopropylphenyl), can be generated by reaction of amines with ditipyl ketene, are observable by NMR, and slowly tautomerize. Vinyl alcohols with two or three bulky aryls have propeller conformations and are chiral, but are not easily resolved. 

Figure 4.76 depicts an intramolecular dynamic equilibrium of a dendrimer with four blade dendrons, in the course of which the (helical) propeller conformation is inverted, i.e. the sense of the chirality changes [37a]. In such cases it was observed that all four dendrons undergo correlated motion, rather than just one dendron changing its conformation without the other three dendrons rotat-... 

One molecule of this type is tris-l-(2-methylnaphthyl)borane (6), 32> for which four isomeric propeller conformations are possible (Table 1). These isomers make up two diastereomeric cW-pairs (Fig. 8). One set of enantiomers (A and A, hereafter A A) are of C3 symmetry and consequently each enantiomer has three equivalent methyl groups. The other two (B and B, hereafter BB) have Ci symmetry and each enantiomer has three diastereotopic methyl groups. These facts are reflected in the TH-nmr spectrum of the methyl region at — 70 °C (Fig. 9). The three resonances of equal intensity are those arising from BB whereas the more intense upheld singlet derives from the methyl groups of AA. At this temperature, the ratio of BB to AA is 3.0 to 2.7. As the sample is warmed, the population of BB increases relative to that of AA, indicating a positive entropy for the equilibrium AA BB, and a crossover temperature (zl( 0=0) below which AA is more stable. A plot of AGt vs T yields, for the equilibrium AA BB, AH0 0.61 0.05 kcal/mol and zJS° 3.1 0.2 eu. The crossover temperature is therefore ca. — 76 °C. The major part of this entropy difference is accounted for by the differ ence in symmetry (C3 vs Ci) of the two diastereomers (R In 3=2.18 eu, for the equilibrium as shown). 

Trimesitylmethane is an example of isomerism and isomerization in ArgZX systems with C3 symmetry. Pioneering studies on related triaryl-methanes provided valuable observations concerning structure and mechanism of stereoisomerization. 39-41) jn particular, an X-ray diffraction study 41> of dimesityl-(2,4,6-trimethoxyphenyl)methane showed that this compound adopts a propeller conformation in the solid state. A similar conformation was found for triphenylmethane in the gas phase by electron diffraction.42) Nmr evidence is also consistent with such a conformation for triarylmethanes in solution. 39-41) jn the following, we shall briefly describe our experience with trimesitylmethane (7). 43)... 

Triarylsilanes have also been shown to adopt propeller conformations in the solid state 45 and in solution. 46> Trimesitylsilane (8) exhibits a temperature dependent -H-nmr spectrum which indicates rapid stereoisomerization at ambient temperature. Thus, at 40 °C the spectrum features two singlets in the methyl region in a ratio of 1 2, assigned to the para and ortho methyl substituents, respectively. Upon cooling the sample, the ortho methyl proton signal broadens and splits into two signals of equal intensity, a result consistent with a propeller conforma-... 

By this analysis, plots representing both angles and the corresponding energies can be obtained. As an example, Fig. 2 shows one of these plots for the case of 2,2-bis(4-phthalimidophenyl) ether. As can be seen, because of the symmetry of the aromatic rings, the plot has two planes of symmetry along the diagonals. Due to this symmetry, all the minima correspond to the same propeller conformation that has been observed several times in this kind of structure. 

Because of a smaller rotation of 16° in the propeller conformation (E-conformation), the conformation dependence of the substituent effect correlations is not very serious in this system. Nevertheless, the non-linear behaviour should be similar to that in the trityl carbocation system. From comparison of the plots in Figs 8 and 15, the plot for Y = p-MeO for the benzhydryl cation should be related to the correlation for the T-conformation and that for Y = P-NO2 should be related to the P-conformer correlation. The difference in the slopes gives no clue as to the intrinsic selectivity (p) of this system. 

The twist angles of the aryl groups from coplanarity in the propeller conformation of a,a-diarylethyl cation [31C ] are comparable with those of [29C (X,Y)]. The dependence of the selectivity parameters upon conformation therefore should be as significant as in the above [29C (X,Y)] system. The substituent effects on the protonation equilibrium of diarylethylenes [31(X,Y)] can be interpreted by assigning a preferred conformation to the cations. It also seems evident in this system that the p value for the P-conformers does not vary so significantly with a series of Y substituents. Any processes involving a.a-diarylethyl cation [31C ] as an intermediate should reflect the dependence of selectivity upon conformations. Small p values for the solvolyses of 1,1-diphenylethyl-OPNB [13(X,Y)] cannot be interpreted by the tool of increasing electron demand . 

The correlation results for the bromination of diarylethylenes [31(X,Y)] summarized in Table 14 also involve the same serious problem. The p value increases significantly as the fixed substituent Y becomes more EW. This behaviour is indeed what is expected for the quantitative reactivity-selectivity relationship. However, in Table 14, the range of variable substituents X involved in the correlation of the respective Y sets is evidently different from set to set. The correlation for the Y = p-MeO set giving p = -2.3 should be referred to as the correlation for the T-conformation where X is more EW than Y, correlations for Y = p-Me, H and p-Br sets giving p = -3.6 may be referred to the E-conformation, and those for Y = m-Hal, especially P-NO2, refer without doubts to the P-conformation. The variation of p value cited in Table 14 demonstrates nothing other than the dependence of the selectivity p upon the propeller conformation of the diaryl carbocations. While there is no doubt regarding the importance of RSR in the mechanistic studies, these results lead to the conclusion that the RSR, or most of the non-additivity behaviour of a,a-diarylcarbocation systems which have been cited as best examples of quantitative RSR, may indeed be only an artifact. 

Trimesitylmethane (12a) is an example of an AtjZX propeller possessing C3 symmetry. The X-ray analysis of dimesityl(2,4,6-trimethoxyphenyl)methane (12b) proves the propeller conformation for the solid state For the interconverting exo- and endomethoxyl groups of 12 in a temperature range of 118-176 °C, AH = 74 kJ/mol, AS = 9.8 eu are found, whereas for 13 a lower entropy of activation... 

The tri-o-thymotids 206-209 spontaneously crystallize as solvent adducts and form enantiomorphic crystals, which contain the propeller conformation of the host molecule The(-(-)-CHa3 adduct of206, e.g., shows a rotation value of[a]o = —83, the (—)-206 CHCI3 adduct = —77. The half-lifes are t,y2 = 34 min at 274.6 K,... 

The calculated [40] low-energy form of benzophenone has a propeller conformation with 01 = 02 = 30°, which agrees well with the crystal structure (29.4°, 30.9°) [41]. Figure 9.17 shows the possible mechanisms for the interconversion of the molecule to the enantiomer with opposite helicity. Each phenyl ring can choose between two ways to attain its new position. One is rotation through the plane of the... 

X-ray data of indolizine (2) indicate contributions of C=S -bonds in the molecular structure <85IZV2724>. An X-ray structural investigation of (3) reveals that it occupies a two-bladed propeller conformation with an anti relation between the methylene protons and the four methyl groups. In the crystal, the molecule lacks a C2 symmetry and is asymmetric rather than helical. Since the indolizine ring carries no substituents which conjugate, the structural results supply information about the geometry of indolizine itself <87CB23i>. 

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