Activator, biphenyl

Although the sulfone activated biphenyl and the ketone activated naphthalene moiety for the displacement polymerization have been reported by Attwood et al. [11], these were rediscovered by Cummings et al. [12] and Hergenrother et al. [13], respectively, for the synthesis of poly(aryl ethers). Recently, Singh and Hay [14] reported polymers containing 0-dibenzoyl benzene (1,2,3) moiety by reaction between bis(O-fluorobenzoyl) benzene or substituted benzene with bisphenates of alkali metal salt in DMAC as follows ... 

The products 4 are formed as racemic mixtures, but can be resolved by recrystallization of diastereomeric salts.23 - 25 Syntheses starting from optically active biphenyl compounds are also known.26 -28 l,l -Binaphthyl-2,2 -diamine(5) can be transformed to the dinaphtho[1,4]di-azocine 6 by melting with benzil.29... 

Berson, J. A., and M. A. Greenbaum Asymmetric induction studies with optically active biphenyls. V. On the unreliability of absolute configurational assignments based on hydride reductions ofphenylglyoxylatcs. J. Amer. chem. Soc. 81, 6456 (1959). 

Optically active biphenyl derivatives (Section 13-5 A) are racemized if the two aromatic rings at any time pass through a coplanar configuration by rotation about the central bond. This can be brought about more or less easily by heat, unless the 2,2 -ortho substituents are very large. 

In spite of the uncertainty in the equilibrium conformation of biphenyl in the liquid phase, it is reasonable to anticipate an energy barrier to free rotation and coplanarity. Adoption of the highly successful Westheimer-Mayer model (Westheimer, 1956) for the racemization of optically active biphenyls leads to a calculated value of 3.9 kcal mole-1 for the barrier to free rotation (Howlett, 1960). The calculation was made on the basis of a large stabilization energy, 7 kcal mole-1, for the coplanar molecule (Guy, 1949). 

The classical theories of steric hindrance related largely to restrictions on rotation in rigid molecules containing double bonds or aromatic rings, and restrictions on rotation above single bonds caused by the collision of bulky groups, as in optically active biphenyls. It is now known" - that steric effects are equally important in c es where rotation about single bonds had previously been considered to be unrestricted. This was first proved... 

History and Concepts. A complementary approach for molecular structure calculations is available, and it is referred to as the molecular methanics or force field method it is also known as the Westheimer method. In 1946, twenty years after the impressive development of quantum theory, three papers appeared in the literature which applied classifical mechanical concepts to problems of chemical interest. Westheimer investigated the racemization of some optically active biphenyl derivatives. His work demonstrated the potential usefulness of molecular mechanics. The other two papers were attempts to tackle more complex problems. 

Q2. An optically active biphenyl was synthesized in two isotopic modifications, the normal version 55 and the deuterio analogue 56. These compounds undergo racemization in ethanol, and at -19.8 °C the first-order rate constants for racemization were for 55, A h = 6.48 x 10 5 s-1, and for 56, kD = 7,71 x 10 5 s1. This gives a rate constant ratio H/ D of 0.84 (see Melander and Carter 12 for related work see Mislow et aiP). What conclusions can be drawn concerning the relative sizes of deuterium and hydrogen ... 

Steric isotope effects are less clear cut, possibly because many small effects considered to be steric are in fact a mix of steric and stereoelectronic. Early work on the racemisation of optically active biphenyls gave a value of kne/ De of 0.85 for a 2,2 -dimethylbiphenyl also containing a 6,6 ethylene bridge (4,5-dimethyl 9,10 dihydrophenanthrene), and 1-deuteriocyclohexane prefers the deuterium-axial conformation by 25 J mol V but the preference decreases next to a heteroatom. Effects of deuterium substitution of carbon-bound protons in glucose on the anomeric equilibrium in water cannot be simply rationalised by a single effect the equilibrium isotope effect (defined as [P]H[a]D/[p]D[a]H) being 1.043 for HI, 1.027 for H2, 1.027 for H3, 1.001 for H4, 1.036 for H5 and 0.998 for H6,6. ... 

Joe was an outstanding physical chemist he and his wife Maria [Goeppert Mayer] wrote the outstanding text in statistical mechanics. During the war, he had been working at Aberdeen, Maryland, using the world s first digital computer to calculate artillery trajectories. Perhaps Joe could have access to that computer, and could show me how to solve my determinant on it. So I went to him and asked him to help me. He didn t know about optically active biphenyls, so I made some molecular models and explained the stereochemistry to him, and... 

A more promising test of the validity of steric secondary isotope effects is to set up an experiment in which such effects should have a maximum chance to manifest themselves. Racemizations of optically active biphenyls and related compounds might be expected to provide excellent tests, for the activation energies of these reactions are determined mainly by the extent of steric crowding in the transition states. 

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