Reactions head-to-tail

Problem 8.44 (a) Write a schematic structure for the mer of the polymer from head-to-tail reaction of 2-methyl-1,3-butadiene, (b) Account for this orientation in polymerization, (c) Show how the structure is deduced from the product... 

Propagation of the polymerization occurs nearly exclusively by head-to-tail reactions, with only a small fraction of head-to-head reactions. The relative ratio of these two reactions is only a function of temperature and has been found to be independent of molecular weight, polymerization solvent, and method of polymerization. The head-to-head addition yields a 1,2-glycol structure in the resulting poly(vinyl alcohol), which in turn influences the degree of crystallinity, strength, solubility, and thermal stability. 

A head-to-tail reaction of derivatives of iminosulfenic acid gives the corresponding 1,4,2,5-dithiadiazines.4... 

As a general rule, natural products derived from isoprenoid units arise from head-to-tail reactions of that synthon. The majority of the structures of such products built up by condensation of IPP will as a result display a branched methyl on every fifth atom in the chain. Thus, reaction of IPP (1-5) with the isomer with the internal double bond (1-6) proceeds by head-to-tail coupling with expulsion of a pyrophosphate ion. The free alcohol from the product 2-1 (Scheme 2.2) is the fragrant terpene geraniol. Reaction of 2-1 with a second isoprene unit in this case again takes place by head-to-tail reaction to afford 2-2. The free alcohol from this 15-carbon triene is known as farnesol and is generally classed as a sesquiterpene (Latin sesqui-, one and a half). The molecule is displayed in the unlikely conformation 2-2b, in anticipation of the next reaction. 

The head-to-tail configuration is preferred almost to the exclusion of the other two. An important reason for this is steric hindrance, which favors head-to-tail reaction, especially if R is bulky. 

Biosynthesis A. is one of the few monoterpenes that is not formed by the known biosynthetic routes for ter-penes since there are two methyl groups on C-3 of A. A. prepared biosynthetically from [2- C]-mevalonic acid contains 80% of the radioactivity at C-9/C-10 and only 13% at C-7/C-8. This difference in activity means that the C5 precursors of the two halves of A. undergo different enzymatic reactions, i.e., a head-to-tail reaction of the C5 precursors does not occur. 

Among the different resonance forms shown in Figure 1.6, form (c) is the more reactive one due to its important substituent inductive effect and its absence of steric hindrance. The next step is the dimer formation between the radical cation and its resonant form (most probably form (c)) by the so-called head-to-tail reaction in acidic medium. 

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