Electronic effects in polyarylvinyl propellers. Solid state structures and dynamic behaviour in solution of several crowded enol derivatives

Highly Regioselective, Sequential, and Multiple Palladium-Catalyzed Arylations of Vinyl Ethers Carrying a Coordinating Auxiliary: An Example of a Heck Triarylation Process

“…We believe that the reluctance of the β,β-diarylated olefins 9 , even in the presence of bidentate ligands, to undergo subsequent α-arylation is a consequence of the fact that the two β-arylsubstituents impose a significant steric blockade of the double bond. An X-ray structure of N,N -dimethyl-2-[(2,2-diphenyl)ethenyloxy]ethanamine 9h with palladium dichloride revealed a palladium-chelating dimeric structure 16 and showed that the phenyl rings in fact were locked in a “propeller” conformation (Figure ), strongly disfavoring π-complex formation/insertion.

3 X-ray structure of complex 16 .

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Artificial Molecular Rotors

“…Similarly, Biali, Rappoport, and co-workers have extensively studied geared rotations in polyarylated ethenes and ethenones, ,, and Schlögl et al found that 1,1‘,2,2‘-tetraarylethenes ( 33 , Figure ) could be separated into enantiomers (Δ G 293 ⧧ = 22.2 kcal mol -1 ; t 1/2 = 38.3 min at 21 °C). Isomerization was suggested to occur by a two-ring flip mechanism, while, in 1,1,2,2-tetra- o- tolylethane ( 34 ) and tetra- o -tolylethene ( 35 ), Willem et al found an interconversion of the isomers to proceed via a four-ring flip.…”

“…If Z ) CH, a stereogenic center is introduced and 2 5 or 16 diastereomeric dl pairs will result (but flip mechanisms only lead to interconversion of diastereomers, not enantiomers). Mislow defines the structure where a * b * c as maximally labeled 200 and has tabulated all the possible stereoisomers for different combinations a, b, and c. 199 Similarly, Biali, Rappoport, and co-workers have extensively studied geared rotations in polyarylated ethenes and ethenones, 356,358,359 and Schlo ¨gl et al 353 found that 1,1′,2,2′-tetraarylethenes (33, Figure 20) could be separated into enantiomers (∆G 293 q ) 22.2 kcal mol -1 ; t 1/2 ) 38.3 min at 21 °C). Isomerization was suggested to occur by a two-ring flip mechanism, while, in 1,1,2,2-tetra-o-tolylethane (34) and tetrao-tolylethene (35), Willem et al 354 found an interconversion of the isomers to proceed via a four-ring flip.…”

Radical Cation Ester Cleavage in Solution. Mechanism of the Mesolytic O−CO Bond Scission

“…The steric hindrance in the model compounds is apparent from the 1 H NMR spectra that exhibit significant peak broadening because of coalescence. Such a behavior has been observed in many related systems caused by a two- or three blade propeller conformation in which rotation of the mesityl groups is hindered. 19b, In the bisenol carbonate 3a , the rotation is completely frozen on the time scale of the NMR experiment, and thus, a double set of signals is obtained for the o -mes-CH 3 and the m -mes-H protons.…”

“…Synthesis and Characterization of the Model Compounds. To properly measure the kinetics of O−CO mesolytic cleavage, we synthesized ester systems derived from 2,2-dimesitylenols, for reasons outlined above. 8b,19a For comparison, we have included data for 1g , h and 2g , h . , All systems offer decisive advantages from a practical point of view. They are oxidized at rather low potentials because of the electron-rich dimesitylenol moiety, and the products formed after O−C bond cleavage are well-known.…”