T-Butyloxirane

Tsuruta observed in the cyclooligomerization of t-butyloxirane that both (R)- and (S)-t-birtyloxiranes predominantly yield cyclic tetramers however, all the other macrocycles (from dimer to octamer) have also been observed in smaller concentrations ... 

This stereoselectivity in cyclooligomerization has not been observed for (RS)-t-butyloxirane. The presence of two enanthiomers, (R) and (S), apparently decreases the probability of the formation of the preferred conformation of the linear chain which leads to the G G T sequence. 

This phenomenon may be illustrated by the example of polymerization of t-butyl-oxirane 21). The cyclic tetramer of t-butyloxirane (formed at a yield up to 60 %) is exclusively in the gauche+-gauche+-trans (G+G+T) conformation ... 

A unique and significant effect of the bulky substituent has recently been found also in a cationic oligomerization of (5)-t-butyloxirane with boron trifluoride etherate as initiator, where a cyclic tetramer was formed in an excellent yield... 

It was previously reported (5) that the bulk polymerization of t-butyloxirane initiated with potassium t-butoxide (t-BuOK) proceeded according to the living mechanism with the initiator efficiency being 100 K. 

The curve for t-butyloxirane In Fig. 3 was analyzed In more detail by deviding the curve Into j-stages. From experimental data at every stage. It was possible to calculate a parameter, o.j, which Is defined as follows ... 

C.C. Price (10) reported the IsoSyn mechanism for the stereochemistry of polymerization of 55-t-butyloxirane initiated with -BuOK. For the formation of IsoSyn poly(t-butyloxirane), -RRSSRRSSRRSSRRSS-, the following conditions should be established... 

In order to elucidate the nature of the growing chain control mechanism, a series of NMR studies was carried out with poly(t-butyloxirane) which was prepared by the polymerization of racemic monomer initiated by t-BuOK (11). Pulsed FT C-NMR spectra of the polder are shown in Fig. 4. By comparison with NMR spectraof poly[(5)-t-buty1oxirane] shown in Fig. 2, it was possible to... 

At the very initial stage of the 5,fl-copolymerization (R/S 76/24) of t-butyloxirane, any ordered secondary structure has not yet been established in the growing polymer chain, so that it is reasonable to use the value sr/ SR 1° [obtained... 

Specific Formation of Cyclic Tetramer by Cationic Oligomerization of (i )-t-Butyloxirane... 

Scheme 65. Reagents a, EtMgBr, Et20, -10°C to 0°C b, Swem oxidation c, HOCHjCHjOH, p-TsOH, PhMe, reflux d, BuLi, THE, -IS C e, (2S)-2-butyloxirane, DMPU, -50°C to 0 C t Hj (50 atm), 10% Pd/C, EtOH g, p-TsCl, DMAP, pyridine, -10 C to 0°C h, chromatography 2 i, 2M HCI, M62CO, reflux j, chromatography on neutral alumina (activity grade III) k, SKb, RjO, rt, then NEtj.

In order to get information on the site of bond cleavage during the polymerization process, (i )- -butyloxirane was polymerized in bulk with the t-BuOK initiator. The C NMR spectrum of poly[(i )- -butyloxirane] is given in Fig. 2. 

The NMR studies carried out on the living polymerization system of (i )-f-butyloxirane led us to conclude the 8-opening to be operative in the propagation process of t-butyl oxirane. This conclusion was drawn on the basis of the following observations. 

Figure 4. FT C-NMR spectra (25.03 MHz) of P((RS)-tett-butyloxirane) initiated by t-BuOK in bulk. Solvent, CDCl (cone. 30 u>/v%) Temp.,...

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