Ethers and formals

As far as heterocyclic compounds are concerned (cyclic ethers and formals and some others) the situation is rather happier, since on general chemical grounds there is virtually no species other than -onium ions which need be considered as propagating species. 

Polymerization Procedure and Characterization. Cyclic ethers and formals were polymerized by adding a measured amount of monomer into the initiator solution at 0°C. The polymer was precipitated with methanol or ethyl ether and freeze-dried from benzene or fractionated by chloroform. The block copolymer of styrene and tetrahydrofuran was dissolved in 1-butanol and refluxed for 12 hours with sodium metal. The solution was washed with water, and the 1-butanol was distilled off. The residual polymer was freeze-dried from benzene, and poly-THF was extracted with 2-propanol in a Soxhlet apparatus. 

Polymerization of Cyclic Ethers and Formats by Poly-THF Dioxolenium Salt. The polymerization of cyclic ethers and formals by PTHF-dioxolenium salt was carried out to clarify the presence of termination or transfer reactions. The results are shown in Table IV. In the polymerization of 3,3-bischloromethyloxetane (BCMO), block copolymer soluble in chloroform and having the expected molecular weight was formed the homopolymer of BCMO insoluble in chloroform was not observed. The block copolymer showed crystalline bands of BCMO at 700, 860, and 890 cm 1, suggesting the formation of ABA block. 

Table 8.5 Metathesis reactions of diene ethers and formals...

Stabilised sulphur ylides react with alkenylcarbene complexes to form a mixture of different products depending on the reaction conditions. However, at -40 °C the reaction results in the formation of almost equimolecular amounts of vinyl ethers and diastereomeric cyclopropane derivatives. These cyclopropane products are derived from a formal [2C+1S] cycloaddition reaction and the mechanism that explains its formation implies an initial 1,4-addition to form a zwitterionic intermediate followed by cyclisation. Oxidation of the formed complex renders the final products [30] (Scheme 8). 

Studies on the cationic polymerization of cyclic ethers, cyclic formals, lactones and other heterocyclic compounds have proliferated so greatly in the last few years that a detailed review of the evidence concerning participation of oxonium and analogous ions in these reactions cannot be given here. Suffice it to say that there is firm evidence for a few, and circumstantial evidence for many such systems, that the reactive species are indeed ions and there appears to be no evidence to the contrary. A few systems will be discussed in sub-sections 3.2 and 4.4. 

On the other hand, in cyclic ethers (alkene oxides, oxetans, tetrahydrofuran) and formals the reaction site is a carbon-oxygen bond, the oxygen atom is the most basic point, and, hence, cationic polymerization is possible. The same considerations apply to the polymerization of lactones Cherdron, Ohse and Korte showed that with very pure monomers polyesters of high molecular weight could be obtained with various cationic catalysts and syncatalysts, and proposed a very reasonable mechanism involving acyl fission of the ring [89]. 

As far as the polymerisation of heterocyclic monomers is concerned, the situation is qualitatively similar, but quantitatively different. As a model for the active species in oxonium polymerisations, Jones and Plesch [10] took Et30+PF6 and found its K in methylene dichloride at 0 °C to be 8.3 x 10"6 M however, in the presence of an excess of diethyl ether it was approximately doubled, to about 1.7 x 10 5 M. This effect was shown to be due to solvation of the cation by the ether. Therefore, in a polymerising solution of a cyclic ether or formal in methylene dichloride or similar solvents, in which the oxonium ion is solvated by monomer, the ion-pair dissociation equilibrium takes the form... 

Cyclic formals react with dinitrogen pentoxide in chlorinated solvent to yield unstable but interesting ring-opened products, including hemiformal nitrates 1,3-dioxolane (44) reacts to yield a mixture of hemiformal nitrate (45) and formal ether (46) products. Similar products are formed from acyclic formals and dinitrogen pentoxide. ... 

Radical cyclization of polyfunctional 5-hexenyl halides mediated by Et2Zn and catalyzed by nickel or palladium salts has been demonstrated to produce stereoselectively polyfunctional 5-membered carbo- and heterocycles [56, 57]. Based on this strategy a formal synthesis of methylenolactocin (11) was achieved (Scheme 20). The acetal 130, readily being built up by asymmetric alkylation of aldehyde 127 followed by reaction with butyl vinyl ether and NBS, served as the key intermediate for the construction of the lactone ring. Nickel(II)-catalyzed carbometallation was initiated with diethylzinc to yield exclusively the frans-disubstituted lactol 132, which could be oxidized directly by air to 134. Final oxidation under more forcing conditions then yielded the lactone (-)-75 as a known intermediate in the synthesis of (-)-methylenolactocin (11) [47aj. 

Another rhodium vinylidene-mediated reaction for the preparation of substituted naphthalenes was discovered by Dankwardt in the course of studies on 6-endo-dig cyclizations ofenynes [6]. The majority ofhis substrates (not shown), including those bearing internal alkynes, reacted via a typical cationic cycloisomerization mechanism in the presence of alkynophilic metal complexes. In the case of silylalkynes, however, the use of [Rh(CO)2Cl]2 as a catalyst unexpectedly led to the formation of predominantly 4-silyl-l-silyloxy naphthalenes (12, Scheme 9.3). Clearly, a distinct mechanism is operative. The author s proposed catalytic cycle involves the formation of Rh(I) vinylidene intermediate 14 via 1,2-silyl-migration. A nucleophilic addition reaction is thought to occur between the enol-ether and the electrophilic vinylidene a-position of 14. Subsequent H-migration would be expected to provide the observed product. Formally a 67t-electrocyclization process, this type of reaction is promoted by W(0)-and Ru(II)-catalysts (Chapters 5 and 6). 

The formal isomerization enthalpies of methyl n-butyl ether to methyl terf-butyl ether and of di-n-butyl ether to n-butyl ferf-butyl ether are about —24kJmoU (Iq) and —26.5 kJmol (g), respectively. From these, and the experimental enthalpy of formation of di-ferf-butyl peroxide, the enthalpies of formation of di-n-butyl peroxide are ca —333 kJmol (Iq) and —288 kJmol (g), wildly divergent from the reported values. 

For the comparison of hydroperoxides with methyl ethers (equation 2), we find there is enthalpy of formation data only for dimethyl ether, isopropyl methyl ether and t-butyl methyl ether (again ignoring the ethyl and propyl hydroperoxides). The enthalpies of formal reaction 2 for R = Me, i-Pr and f-Bu (two gas phase enthalpies of formation for f-BuOOH) are —53.1, —54.9 and —37.6 or —48.6 kJmoU, respectively, in the gas phase. In the liquid phase, the enthalpies of reaction are —7.4, —35 (from the estimated enthalpy of formation of isopropyl hydroperoxide) and —20.0 kJmoU, respectively. Because the enthalpy of formation deviations from linearity for dimethyl ether and methyl hydroperoxide might not be identical, the reaction enthalpy might not be consistent with those... 

Thus 4-chlorophenyl 2,4,5-trichlorophenyl ether (48, Scheme 7) produced 4% of a mixture of the dibenzofurans 49 and 50. Only in the case of 2,3,4-trichlorophenyl 2,3,4,5,6-pentai hlorophenyl ether was production of dibenzofurans by formal loss of o,o -chlorine detected. Neither product was identified, but one is presumably the expected product, 1,2,3,4,8,9-hexachloro-dibenzofuran, and the other must be due to a rearrangement. Chlorination of diphenyl ether in the gas phase is unusual. At 300°C the major product is 4-chlorophenyl phenyl ether, as in the liquid phase, but as the temperature is increased (400-500°C), the amount of 4-chlorophenyl phenyl ether decreases at the expense of 3-chlorophenyl phenyl ether, and dibenzofuran is also produced. ... 

The effect of lithiating various unsaturated ethers, including 2,3-dihydrofuran, has been examined by means of 13C NMR spectroscopy and from the results (Table 24) the degree of s character in the unsaturated carbon atoms has been estimated. It differs but little amongst vinyl ethers and is somewhat more than the 33.3% of a formal sp2 hybrid (80JOC4959). 

If six electrons in a ring constitutes aromaticity, then furan is aromatic. But such aromaticity is only formal and has no meaning unless it can be shown that furan has properties modified beyond what can be expected from a molecule that can be variously regarded as a divinyl ether and a cyclopentadiene derivative as circumstances demand. No such demonstration exists, with the possible exception of the NMR ring current shielding... 

The copolymerization of trioxane with cyclic ethers or formals is accomplished with cationic initiators such as boron trifluoride dibutyl etherate. Polymerization by ring opening of the six-membered ring to form high molecular weight polymer does not commence immediately upon mixing monomer and initiator. Usually, an induction period is observed during which an equilibrium concentration of formaldehyde is produced. 

The reactions of silenes with aldehydes and ketones is another area whose synthetic aspects have been particularly well-studied4,6 7 10 12. The favoured reaction pathways for reaction are generally ene-addition (in the case of enolizable ketones and aldehydes) to yield silyl enol ethers and [2 + 2]-cycloaddition to yield 1,2-siloxetanes (equation 44), but other products can also arise in special cases. For example, the reaction of aryldisilane-derived (l-sila)hexatrienes (e.g. 21a-c) with acetone yields mixtures of 1,2-siloxetanes (51a-c) and ene-adducts (52a-c) in which the carbonyl compound rather than the silene has played the role of the enophile (equation 45)47,50 52 98 99. Also, [4 + 2]-cycloadducts are frequently obtained from reaction of silenes with a,/i-unsaturated- or aryl ketones, where the silene acts as a dienophile in a formal Diels-Alder reaction6 29,100-102. 

Banerjee et al. [30] have accomplished a formal total sintesis of ( )-camosic acid dimethyl ether and this is described in Fig. (14). 

Among the carbonylative cycloaddition reactions, the Pauson-Khand (P-K) reaction, in which an alkyne, an alkene, and carbon monoxide are condensed in a formal [2+2+1] cycloaddition to form cyclopentenones, has attracted considerable attention [3]. Significant progress in this reaction has been made in this decade. In the past, a stoichiometric amount of Co2(CO)8 was used as the source of CO. Various additive promoters, such as amines, amine N-oxides, phosphanes, ethers, and sulfides, have been developed thus far for a stoichiometric P-K reaction to proceed under milder reaction conditions. Other transition-metal carbonyl complexes, such as Fe(CO)4(acetone), W(CO)5(tetrahydrofuran), W(CO)5F, Cp2Mo2(CO)4, where Cp is cyclopentadienyl, and Mo(CO)6, are also used as the source of CO in place of Co2(CO)8. There has been significant interest in developing catalytic variants of the P-K reaction. Rautenstrauch et al. [4] reported the first catalytic P-K reaction in which alkenes are limited to reactive alkenes, such as ethylene and norbornene. Since 1994 when Jeong et al. [5] reported the first catalytic intramolecular P-K reaction, most attention has been focused on the modification of the cobalt catalytic system [3]. Recently, other transition-metal complexes, such as Ti [6], Rh [7], and Ir complexes [8], have been found to be active for intramolecular P-K reactions. 

DOL (6), although we could obtain only a homopolymer mixture in the copolymerization of -PL and St. We intend to obtain a high molecular weight polymer with such a random sequence of cyclic ether and vinyl monomer by using cyclic formals as intermediates. 

Dibenzo[/>, J]pvran-6-ones are formed by the Pd-catalysed cyclisation of aryl 2-iodobenzoates, whereas 2-iodobenzyl arylacetates afford 3-aryldihydroisocoumarins under the same conditions <07T10889>. The same dibenzopyranones are formed by way of a Suzuki cross coupling involving functionalised salicylates, derived from a formal [3+3] cyclisation between l,3-bis(silyl enol ethers) and 3-silyloxy-2-en-l-ones, and subsequent lactonisation (Scheme 32). In a variant route, the pre-lactonisation biaryl is derived from the bis(silyl enol ethers) and 3-aryl-3-silyloxyenones <07JOC6255>. 

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