DIAD analogs

In an effort to correlate the conformational features of polysilane derivatives with their properties, calculations are performed to determine the relative stabilities of the conformational states of the meso and racemic diads of polysilapropylene. Energy maps are constructed in terms of internal rotation angles to calculate the average properties of the chain. The calculations show that the difference In energy between the various states of the meso and racemic dlad Is small. Hence, PSP can be considered to be more flexible than the analogous carbon polymer, PP. The characteristic ratios of the unperturbed end-to-end distances for the /so- and syndiotaclic PSP are less than those for the PP of corresponding tacticity. 

The C-2 epimer of 1 was synthesized from 57 using Mitsunobu conditions of PPh3, DIAD, and acetic acid in a 38% yield (65) [34]. However, the C-2 acetyl and benzyl C-2 epimers of 1 also have been prepared using modified Mitsunobu conditions [50]. Diphenyl-2-pyridylphosphine and either acetic acid or benzoic acid were dissolved in THF followed by the addition of di-terf-butylazodicarbox-ylate (DBAD) at 60 °C to yield analogs 65 and 66 in 80% and 75% yield, respectively [50]. 

To rigorously confirm the microstructure of the oligomers, and by analogy the polymer, crystals of dimer 5 were grown from cold methylene chloride and subjected to single crystal X-ray diffraction analysis. An ORTEP plot of 5 is presented in Fig. 4.21 along with appropriate labehng. The tacticity of this diad was determined to be erythro-diisotactic. 

The structures of stereoregular and tactic polymers described above are ideal. Real polymers are always irregular they do not have perfect steric or tactic structures. Thus, the mean sequence and composition of steric and configurational diads, triads, etc. must be given by suitable statistical parameters, in analogy to the case for constitutional diads, triads, etc. 

Since the two hydrogen atoms of a methylene group in an isotactic diad are not nmr-spectroscopically equivalent, they have also been called (and also called heterosteric or diastereotopic). In analogy to this, the methylene group of a syndiotactic diad has been called racemic (and also called homosteric or enantiotopic). For these reasons the composition fractions of isotactic and syndiotactic diads are often given in the literature as (m) or (r) instead of asXi or The names racemic and meso are not equivalent... 

About the nomenclature of like and unlike The meso and racemo nomenclature commonly used for vinyl monomer diads, are not applicable to the head-to-tail polyketone because the junchon unit between the two stereocenters, -CH2-C(=0)-, is not symmetric. Poly(a-amino acid)s and poly(propylene oxide) are other examples of polymers with asymmetric (W. V. Metanomski, Compendium of Macromolecular Nomenclature, lUPAC Macromolecular division). Accordingly, the words like and unlike, which are used in organic chemistry, are applied like (/) is used for the diad consisting of the same configuration (analogous to meso) and unlike ( ) for the opposite (analogous to racemo). 

The commercially available diethyl azadicarboxylate like reagents are listed below. DEAD (4) and DIAD (5) are by far the most frequently used. The methyl (6), benzyl (7) and terr-butyl (8) analogs of the most commonly used reagents are also known, but used much less frequently. Bis-(2,2,2-trichloroethyl)azadicarboxylate, another commercially available compound, has also been reported, but applications thereof appear to be very limited. ADDP, l,r-(azodicarbonyl)-dipiperidine (10), was first reported by Tsunoda. This reagent appears be useful for more difficult Mitsunobu reactions related reagents in which the piperidine moiety has been replaced by morpholine or A-methyl piperazine are also known. These reagents and the reduced hydrazine products thereof can often be precipitated out by the addition of hexanes to Ae reaction mixture additionally, treatment with mild acid can be useful in the removal of the A-methyl piperidine reagent. 

The product of reactivity ratios rjTj can be related to other quantities for which more direa physical evidence is available, e.p. the parameter introduced by Chuj6 (48) to describe deviations from randomness in terms of the number and weight averages of isotactic (or syndiotactic) diads. The analogy with the true copolymerization is remarkable here a similar parameter, the index of sequential homogeneity (49), characterizes the distribution of sequence length of the two monomers. 

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