Oxacyclobutane reactions

Clearly, much more information is needed about the behaviour of these two monomers and oxacyclobutane is perhaps the more favorable for further study because of its clearer catalyst-co-catalyst relationship and the absence of vinyl ether formation. The particular information needed about oxacyclobutane reactions at the present time are (a) knowledge of the fate of the catalyst, (b) viscosity-molecular weight relationship, (c) much more information about the variation of molecular weight with the reaction variables, and (d) information about the reaction of both monomer and polymer with oxonium ions and about the ease of formation of oxonium ions by oxacyclobutane. 

A number of new resist materials which provide very high sensitivities have been developed in recent years [1-3]. In general, these systems owe their high sensitivity to the achievement of chemical amplification, a process which ensures that each photoevent is used in a multiplicative fashion to generate a cascade of successive reactions. Examples of such systems include the electron-beam induced [4] ringopening polymerization of oxacyclobutanes, the acid-catalyzed thermolysis of polymer side-chains [5-6] or the acid-catalyzed thermolytic fragmentation of polymer main-chains [7], Other important examples of the chemical amplification process are found in resist systems based on the free-radical photocrosslinking of acrylated polyols [8]. 

The lesser known four-membered cyclic ether, oxacyclobutane (oxe-tane), (CH2)30, also is cleaved readily, but less so than oxacyclopropane. Oxacyclopentane (oxolane, tetrahydrofuran) is a relatively unreactive water-miscible compound with desirable properties as an organic solvent. It often is used in place of diethyl ether in Grignard reactions and reductions with lithium aluminum hydride. 

A related reaction, which has no precedent in thermal chemistry, is the cycloaddition of an alkene and an aldehyde or ketone to form an oxacyclobutane ... 

The molecular weight of the polyoxacyclobutane through any one polymerization may rise to a maximum and then decrease (19) but in similar experiments with ethylene oxide the observed maximum was no greater than the experimental error of the weight measurements. This point is of considerable interest and deserves more attention than it has received, for it raises once again the question of monomer-polymer equilibrium and suggests that such an equilibrium may be obscured in the epoxides by the non-equilibrium depolymerization to dioxane. It could also mean however that oxonium ion formation is much slower with oxacyclobutane than with epoxides so that depolymerization becomes important only towards the end of the reaction. 

In a related manner, disilacyclopropane (disilirane) derivatives can be synthesized by the reaction of disilene (322) with other reactive compounds. Thus, via diazomethane one can add a methylene component to 322 to form the corresponding 1,2-disilacyclopropanes (323). These are versatile intermediates which give rise to a large variety of compounds (vide infra). One type of application, the conversion of 323 with m-CPBA to a l,3-disila-2-oxacyclobutane (disilaoxetane) derivative (324), is shown in equation 147171. 

Two side reactions can occur during ring closure a spiro diether can be formed from the trichloropentaerythritol monoacetate in a simultaneously occurring reaction or a vinylidene compound, CH2= (CH2C1)2, can form from the monomer 3,3-bis(chloromethyl) oxetane [2,2-bis(chloro-methyl)oxacyclobutane] by a consecutive reaction. 

Oxetanes (oxacyclobutanes) react much like oxiranes (epoxides, oxacyclopro-panes),but much more vigorous conditions are required.Thus, oxetane (oxacyclobu-tane) reacts with hydrogen bromide (HBr) under ionic conditions to provide 1,3-dibromopropane. Presumably, the ring-opening reaction occurs first to yield 3-bromo-l-propanol and then, under the more vigorous conditions, protonation and Sn2 displacement of water follows (Equation 8.53) to produce the 1,3-dibromide. 

Alternatively, polymerization of 3,3-bis(chloromethyl)oxacyclobutane may be effected by aluminium compounds such as alkoxides, amalgam and hydride at elevated temperatures (150—200°C). The mode of operation of these initiators is unknown they are usually associated with anionic reactions whereas the polymerization of cyclic ethers (other than epoxides) generally involves homogeneous cationic mechanisms. It may be that at high temperatures either the monomer is activated and anionic polymerization can occur or there is reaction between the initiator and monomer to form cationic species. 

Preparation 2-7 A solution of 10.34 g (0.156 mol) of potassium hydroxide dissolved in 200 ml of ethanol was added dropwise into a solution of 50 g (0.154 mol) of pentaerythritol tribromide dissolved in 200 ml of ethanol, and the resulting mixture was refluxed for about 30 min. The resulting reaction product was cooled down to room temperature, and then filtered to remove KBr, followed by evaporating the ethanol. The residue was distilled in vacuum to obtain 28 g (0.115 mol) of 3,3-bis(bromomethyl)oxacyclobutane. 

Give the major product(s) of each of the following reactions. (Hint The strained oxacyclobutanes react like oxacyclopropanes.)... 

The reactivity of the four-membered heterocycloaUcanes bears out expectations based on ring strain They undergo ring opening, as do their three-membered cyclic counterparts, but more stringent reaction conditions are usually required. The reaction of oxacyclobutane with CH3NH2 is typical. 

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