Free rotation axial

In ethylene poljmerizations by Ni(II)-based a-diimine catalysts, the aryl groups are roughly perpendicular to the coordination plane so the bulky substituaits on the aryls are positioned at the axial directions to retard associative chain transfer ructions [6,7]. At elevated temperatures, the aryl groups may freely rotate away firm the perpoidicular orientation, resulting in increased associative chain transfes and a resulting decrease in MW of the PE. In addition such free rotation makes the sfructote of the cationic active species more unstable, resulting in fast decrease of activity. 

As shown in Scheme 8-11, nucleophilic entry from the a-face (24a) may be hindered by the sterically bulky substituent R2 on the oxazoline moiety therefore entry from the / -face 24/ predominates. Free rotation of the magnesium methoxy bromide may be responsible for the sense of the axial chirality formed in the biaryl product. If the azaenolate intermediate 25 is re-aromatized with a 2 -methoxy substituent complexed to Mg, (iS )-biphenyl product is obtained. Upon re-aromatization of azaenolate 25B, (R)-product is obtained. 

Axially chiral phosphoric acid 3 was chosen as a potential catalyst due to its unique characteristics (Fig. 2). (1) The phosphorus atom and its optically active ligand form a seven-membered ring which prevents free rotation around the P-0 bond and therefore fixes the conformation of Brpnsted acid 3. This structural feature cannot be found in analogous carboxylic or sulfonic acids. (2) Phosphate 3 with the appropriate acid ity should activate potential substrates via protonation and hence increase their electrophilicity. Subsequent attack of a nucleophile and related processes could result in the formation of enantioenriched products via steren-chemical communication between the cationic protonated substrate and the chiral phosphate anion. (3) Since the phosphoryl oxygen atom of Brpnsted acid 3 provides an additional Lewis basic site, chiral BINOL phosphate 3 might act as bifunctional catalyst. 

Figure 15.3 gives pressure profiles at three cross-machine locations. Thus, a complex three-dimensional flow held is set up with an a priori, unspecified free boundary. Axial how (cross-machine direction) continues throughout the nip zone all the way to the exit, but the rate varies because of the varying gap size. That is, in the narrow region of the nip, drag how in the direction of rotation is predominant as compared to cross-machine pressure how. 

Contrary to most allocolchicinoids which lack a C-ll substituent, compounds 78 and 80 show stable axial chirality. The above study constitutes the first example in the allocolchicinoid series where both configurations at the biaryl axis can be obtained from a given stereochemistry at C-7. In the natural alio series, free rotation around the biaryl axis is often possible at room temperature, and the configuration of the biaryl axis is controlled by the stereochemistry at C-7 due to conformational constraints in the C-ring [14]. While several total enantioselective syntheses of colchicine address the control of the stereochemistry at C-7 [106], no direct enantioselective synthesis of natural allocolchicines has been reported to date. ... 

In practical terms, axial chirality often occurs when free rotation along an axis in the molecule is sufficiently hindered. A well known example is the binaphthyl system where free rotation around the common bond can be prevented by introducing bulky substituents in the 2,2 -positions. If these substituents are NHC, then an axial chiral bis-carbene results... 

Van t Hoff postulated free rotation round a single bond in order to explain the lack of cis and Irons isomers in molecules of the type of di-chlorethane. In the light of the quantum mechanical theory of the chemical bond, the free rotation is explained by the axial symmetiy of the a bond between the two carbon atoms. Thus the a bond is not in itself a hindrance to free rotation, but as the rotation occurs the relative configurations of the atoms will be changed, so that the distances between the non-bonded atoms and consequently their energies of interaction will alter. 

For basic studies the most suitable type is the rotating disc which is either placed in the end face of a rotating cylinder. Fig. 3.2, or is used as a free rotating system with an integral shaft. Fig. 3.3 [7]. Further models are the rotating cylinder, free or co-axial [11,12J, and pipe and channel flow [7,14]. 

The spectra of NF2 in Ar matrices at 4.2 K were compared for two different mole ratios of matrix-to-radical precursor, M/R = 300 and 1200 (deposition of room-temperature equilibrium mixtures of N2F4/NF2 and matrix gas onto a liquid helium cooled sapphire rod). The M/R = 300 sample gave an approximately axially symmetric g tensor indicating nearly free rotation of the NF2 radical about an axis perpendicular to the molecular plane (x axis, see below). The more diluted sample (M/R = 1200) unexpectedly exhibited the spectrum of a randomly oriented radical with three different principal values for the g tensor this may be due to a stronger crystalline field effect. The spectrum of NF2 in Kr at 4.2 K (M/R = 300) indicated freer rotation than in Ar with M/R = 300. At about 30 K the completely resolved isotropic triplet set of triplets of an almost freely rotating NF2 radical was observed in the Ar and Kr matrices [6, 7]. With CCI4 as the matrix material, somewhat distorted spectra were observed at 4.2 to 30 K [6]. 

An improved degree of orientation was achieved by use of a large gradient between the temperature of the initial radical-matrix mixture (Ne/NF2 = 3000, Ar/NF2 = 2500, Kr/NF2 not given) and the final temperature at the time of deposition and by use of a uniformly controlled rate of sample spray. In Ar and Kr better orientation was observed than in the softer Ne matrix. Anisotropic g tensors and axially symmetric N- and F-hf tensors were obtained at 4.2 K. As the sample was warmed to 30 K rotation was observed about the x axis followed by rotation about the x and the C2 axes above 30 K almost free rotation was indicated by the nine-line spectra giving isotropic g factors and isotropic hf coupling constants. The ESR spectrum of NF2 in an N2 matrix (M/R = 2000) at 4.2 K was not informative [9]. 

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