Selectivity during the cleavage of polymer supported aryl alkyl ethers

Reactive & Functional Polymers 50 (2001) 85–93 www.elsevier.com / locate / react

Selectivity during the cleavage of polymer supported aryl alkyl ethers ´ Tetenyi ´´ Peter Jr.* ˜ Department of Organic Chemistry, Semmelweis University, Budapest, Hogyes E. u. 5 -7, H-1092 Budapest, Hungary Received 15 August 2000; received in revised form 20 June 2001; accepted 10 September 2001

Abstract Polymer supported carboxylic acids were cleaved quantitatively by boron triiodide, since the linker was attached to the substrate by an aryl alkyl ether bond. Various reagent to substrate ratios as well as ratios of the reagents (sodium tetrahydrido borate to iodine) were tested. Partial reduction also happened during the cleavage, due to the unreacted sodium tetrahydrido borate present in the reaction mixture and because the new NaBH 4 / BX 3 system generated in the reaction mixture proved to be a very powerful reducing agent. Yields were compared to the similar cleavage reaction of ethers by hydriodic acid. Yields were the function of the initial concentration of boron triiodide. Structures of the cleaved substrates were evaluated by infrared and 1 H NMR spectroscopy. The cleaved polymer could be reused with 26.7% loss of its original activity.  2001 Elsevier Science B.V. All rights reserved. Keywords: Aryl alkyl ethers; Boron triiodide; Cleavage; Partial reduction, polymer supported; Selectivity as function of concentration of BI 3

1. Introduction Substrates supported to 2% crosslinked polystyrene microbeads by an aryl alkyl ether bond were prepared during my PhD research [1,2]. A new linker [3] was developed for the syntheses of these substrates, like of a C 4 -substrate-support: (P)–CHOH–C 6 H 4 –O–CH 2 CH 2 CH 2COOH prepared in 10 steps (Reaction Eq. (1)) as well as of a C 6 -substrate-support: (P)– CHOH–C 6 H 4 –O–CH 2 CH 2 CH 2 CH 2 CH 2 COOH prepared in 17 steps.

*Fax: 136-1-2170-851. ´´ E-mail address: [email protected] (P. Tetenyi Jr.).

There are not too many choices for the proper linker, and the attachment through phenolic oxygen was found to have the necessary stability during the whole synthesis. The use of other attaching functional groups (e.g., SH, NHCH 3 , CH 2 –OH, etc.) would limit strongly the kind of reactions applicable for this polymer-supported synthesis. On the other hand, the substrate must be removed easily and at once during the final cleavage step. It was important to find the right cleavage method, because the success of the whole synthesis depended on the final step and, since the used attachment / linker proved to be an excellent one during the whole synthesis. However, a cleavage method for such aryl alkyl ethers, where the oxygen is in a non-activated

1381-5148 / 01 / $ – see front matter  2001 Elsevier Science B.V. All rights reserved. PII: S1381-5148( 01 )00101-8

86

´´ / Reactive & Functional Polymers 50 (2001) 85 – 93 P. Tetenyi

(1) position, has not known been reported in the literature. Moreover, I was attempting to find a mild cleavage method that did not change the structure of any existing C=C double bonds. That was why my research interest in the cleavage reaction was directed to the field of application of boron halides, instead of mineral acids.

2. Literature background (1) Cleavage of ether bonds (in general organic chemistry): usually strong, concentrated mineral acids are used for this purpose: HBr, HBr in glacial acetic acid, HI, etc. [4–6]. However, the raw product is usually rather impure, and many side reactions (elimination,

´´ / Reactive & Functional Polymers 50 (2001) 85 – 93 P. Tetenyi

addition, isomerisation or substitution) can take place. There is a much better and more selective method for cleavage of ethers by boron trihalogenides (BBr 3 or BI 3 ) [6–10]. This process has many advantages [6,11], and moreover, the ex tempore preparation of boron trihalogenides does not need storing and measuring in these aggressive and slowly decomposing electrophiles. However, there is a disadvantage of boron trihalogenides that these must be used in excess if polyfunctional substrates are applied, since BX 3 is co-ordinated to each atom having lone electron pairs (like O, S, N, P; but not halogens). Excess of BX 3 means that mineral acid, in large amounts, is formed during the isolation and this acid can cause some of the above-mentioned side reactions. That is why careful, gradual neutralisation is needed during the working-up of the mother liquor. On the other hand, the ex tempore formation of boron trihalogenides is always partial, causing generation of a new, very powerful reducing system NaBH 4 / BX 3 . Similar examples introduced just for the reduction of carboxylic acid derivatives are known from the research of Brown and Subba Rao [12,13]. (2) In the field of cleavage reactions of polymer-supported ethers, many examples are known, where the ether is either in activated position and / or is acid-labile. These substrates might contain a trityl ether bond [14–20], or an acetal bond [21], or a benzyl ether bond [22], as well as ethers of 1,2-disubstituted ethane derivatives [23,24]. However, none of the abovementioned ethers could be used for our research due to their high lability. Other examples on the cleavage reactions of polymer-supported ethers, where the ether is in non-activated position, are not known.

3. Experimental conditions The reactions were carried out by magnetic stirring at about 300–600 r.p.m. The reaction mixtures were heated in a paraffin oil bath

87

electrically regulated within 628C. The IR spectra were taken using a Pye Unicam SP 3–200 spectrophotometer, the NMR spectra were taken using a Bruker 200-MHz spectrometer. In the followings, (P) stands for 2% crosslinked styrene–divinylbenzene copolymer in bead form. A substrate-support with carboxylic acid functional group was chosen because in that way the course as well as the ratio of the cleavage and the reduction reaction could be checked and, moreover, I got a direct experimental proof on the presence of an electrophile (boron triiodide); previously, the actual presence of boron triiodide could not have been proven [6,11]. The cleavage reactions run in air, at boiling temperatures. Quantity and quality of the isolated raw substrate as well as of the isolated polymer support were determined instead of taking samples from the reaction mixture. The mass changes of the polymer were measured, and IR and 1 H NMR spectra were taken. Before cleavage, the substrate-support was dried in a drying apparatus at 808C until achieving permanent mass. Sodium tetrahydrido borate of good quality, and water-free, distilled 1,2-dichloroethane were used as solvent.

3.1. Method (a) A one-necked, round-bottomed flask of 25 cm 3 equipped with Liebig-condenser was used: 0.10 g (P)–CHOH–C 6 H 4 OCH 2 CH 2 CH 2 COOH and 0.05 g of red phosphorus was added to 3 cm 3 concentrated (57%) hydriodic acid, and stirred at 1058C for 3 h. The reaction mixture was diluted with 10 cm 3 of water, and was filtered. The polymer was washed with 5 cm 3 of water, 233 cm 3 of methanol, 2 cm 3 of toluene– methanol 1:1 mixture, 2 cm 3 of methanol, 2 cm 3 of toluene–methanol 1:1 mixture, and 232 cm 3 of methanol, respectively. The polymer was dried in a drying apparatus at 608C until achieving constant mass. The filtrate was neutralised with solid sodium hydrogen carbonate until

88

´´ / Reactive & Functional Polymers 50 (2001) 85 – 93 P. Tetenyi

reaching pH 5, the solution was decolorised by adding sodium hydrogen sulphite, and it was 3 extracted by 235 cm of dichloromethane. The organic phase was saved, and the aqueous phase was evaporated by using a rotational vacuum apparatus at maximum 408C until the complete removal of methanol. Then the remaining aqueous phase was extracted with 235 cm 3 of dichloromethane, and the organic phase was poured to the previous organic phase. The combined organic phase was dried over anhydrous sodium sulphate, filtered, and evaporated using a rotational vacuum apparatus at maximum 358C. Mass of the distillation residue was measured.

3.2. Method ( b) A two-necked round-bottomed flask of 25 cm 3 was equipped with a Liebig-condenser. 2.8 g (511.2 mmol) crystalline iodine were added to 2.0 cm 3 of absolute 1,2-dichloroethane, with stirring at room temperature, 0.40 g (510.5 mmol) sodium tetrahydrido borate was gradually added to it. When addition was completed, 0.20 g (P)–CHOH–C 6 H 4 OCH 2 –CH 2 CH 2COOH was added to the reaction mixture at once, and the flask was heated and stirred in an oil bath of 808C for 3 h. The reaction mixture was cooled down by placing it in an iced water bath, and 5 cm 3 of water were added from a dropping funnel, with vigorous stirring. Then the reaction mixture was filtered and isolated by the method used for method (a). The combined raw substrate (0.15 g) was dissolved in 2 cm 3 of methanol in a round-bottomed flask, and 0.4 g (510 mmol) sodium hydroxide dissolved in 1 cm 3 of water was added. The mixture was stirred in an oil bath at 708C for 1 h, using a Liebig condenser. Then the reaction mixture was cooled down to room temperature, 5 cm 3 of water as well as 5 cm 3 of dichloromethane were added, and it was extracted; the aqueous phase was extracted again with 5 cm 3 of dichloromethane. Then the organic phase was saved; the aqueous phase was made free of methanol using a rotational vacuum apparatus at a temperature

less than 358C until the complete removal of methanol. Then the aqueous phase was extracted again with 5 cm 3 of dichloromethane; the organic phase was poured into the previous organic phase. The aqueous phase was saved (see below). The combined organic phase was dried over anhydrous sodium sulphate, filtered and evaporated at a temperature less than 358C, resulting in sample A. The aqueous phase was acidified with concentrated hydrochloric acid until pH 3, and then extracted with 335 cm 3 of dichloromethane. The organic phase was dried over anhydrous sodium sulphate, filtered, and evaporated at a temperature less than 358C, resulting in sample C. The soluble fraction of sample A in diethyl ether or the soluble fraction of sample C in the mixture of dichloromethane and diisopropyl ether in the ratio of 1–4 were used only for the structure determinations, after evaporation. 4. Mass balance For yields and mass balance, see Fig. 1. These data show that cleavage really did take place, not only reduction of the polymer-supported substrate. Compound A (infrared in cm 21 , film): 3600– 3100 (nO–H), 3000–2820 (nC–H), 1455 (dCH 2 ), 935 and 875 (gO–H). 1 H NMR (CDCl 3 ): 0.89 asym d (2H); 1.27 s (6H). Compound C (infrared in cm 21 , film): 3600– 2500 (nOH(COOH)), 3000–2820 (nC–H), 1720 (nC5O(COOH)), 1460 (dCH 2 ), 1180–1130 (nC–O). 1 H NMR (CDCl 3 ): 0.92 tr (2H); 1.30 s (4H); 1.47 s (0.4 H) The heteroprotons in both samples could be proven only indirectly. 5. Results and discussion The reaction conditions mentioned in Section 3 as (a) and (b) were tested with many changes. The reactions were evaluated concerning the next points:

´´ / Reactive & Functional Polymers 50 (2001) 85 – 93 P. Tetenyi

89

Fig. 1. Mass balance of the cleavage reaction.

1. the measured dry mass, IR spectrum of the isolated polymer; 2. dry mass, IR spectrum and 1 H NMR spectrum of the isolated (raw) substrate.

properties of the substrate-support in aqueous medium. These facts indicated the unsuitability of this method, even knowing that the substrate did not contain any C=C bonds.

Point 1 was compared to the appropriate data of the starting substrate-support.

5.2. Method ( b)

5.1. Method (a) With this method, no other solvents were used. After a reaction time of 3 h, the polymer was isolated; but the relative absorbance of the carbonyl stretching band of the substrate-support at A 1725 /A 1440 decreased by only 5.1%. In the same time, the mass of the isolated polymer increased (!) by 50%. A small amount of cleaved (raw) substrate could be isolated from these experiments, but its IR spectrum showed that this compound was not carboxylic acid (there was a sharp stretching band at 1690 cm 21 , and there was no n O–H (COOH)-associated stretching band between 2500 and 3500 cm 21 ). It was concluded that the desired reaction ran poorly, but the cleaved substrate underwent other elimination / addition reactions within the reaction mixture. This poor yield was partially caused by the poor solvation / swelling

This reaction is carried out by dissolving sodium tetrahydrido borate and iodine in 3:1 ratio, respectively, in dichloromethane, and doing the reaction at room temperature for 3–18 h [11]. However, these conditions had to be changed for a polymer-supported synthesis, that was why 1,2-dichloroethane at 808C was applied for 3 h. The reaction equation for the cleavage is shown in Reaction Eq. (2).

(2) The results of the cleavage experiments ac-

´´ / Reactive & Functional Polymers 50 (2001) 85 – 93 P. Tetenyi

90

Table 1 Cleavage experiments: parameters and results A

B

C

D

E

F

G

H

I

1 2 3 4 5 6

10.5 10.5 10.5 10.5 10.5 5.2

11.2 5.6 11.2 2.8 2.8 1.4

0.46 0.23 0.23 0.12 0.12 0.06

22.0 11.0 11.0 5.7 5.7 2.9

0.94 1.88 0.94 3.75 3.75 3.71

2 2 4 2 2 2

91.2 48.8 47.6 22.7 18.9 1.4

0.32 0.45 0.47 0.19 0.20 n.c.

Standard conditions. Starting polymer, (P)–CHOH–C 6 H 4 –O– CH 2 CH 2 CH 2 COOH; 0.20 g; concentration of the polymer, 0.50 mmol / cm 3 ; reagent 1, sodium tetrahydrido borate; reagent 2, iodine; solvent, absolute 1,2-dichloroethane; temperature, 808C; and reaction time, 3 h. Columns: (A) no. of experiment; (B) molar amount of reagent 1 (mmol); (C) molar amount of reagent 2 (mmol); (D) concentration of the reagent (BI 3 ) / number of oxygens (mmol / cm 3 ); (E) total concentration of BI 3 (v / v %); (F) molar ratio of sodium tetrahydrido borate to iodine; (G) volume of the solvent (cm 3 ); (H) yield %, calculated by the comparisons of the A 1725 /A 1440 values of the polymers; (I) degree of selectivity, calculated by the comparisons of the A 1725 /A 1440 values of the isolated raw substrate as well as of the purified substrate; and n.c., not calculated.

cording to Method (b) are summarised in Table 1. The highest yield (91.2%) was reached in Exp. No. 1 (with reagent ratio of 3.75–4), based on the decrease of the relative absorbance of the stretching vibration of the carbonyl group (A 1725 /A 1440 ) in the isolated polymer (Fig. 2). However, selectivity was only moderate in this experiment (Fig. 2), showing that a considerable part (62.4%) of the cleaved carboxylic acid was reduced to alcohol due to the new, powerful reducing agent NaBH 4 / BI 3 , and only 28.5% carboxylic acid could be isolated. In the following, I tried to modify the reaction con-

Fig. 2. Product distribution for one: ( ) carboxylic acid; ( ) alcohol; and ( ) unreacted polymer.

ditions. Dilution of the reaction mixture with 1,2-dichloroethane (Exp. No. 3) was especially advantageous to facilitate stirring, and the selectivity of the cleavage increased. In this way only 46% of the cleaved carboxylic acid was reduced to alcohol, but the yield was approximately halved (total yield, 47.6%). Similar results were achieved if the amount of iodine was halved in the solvent of the original amount (selectivity: 47.2%): total yield, 48.8% (Exp. No. 2) (Fig. 3). Further decreasing of the amount of iodine (Exp. Nos. 4 and 5) or decreasing the amount of sodium tetrahydrido borate (Exp. No. 6) were unsuccessful (Figs. 4 and 5): both the yield as well as selectivity were poor. For all of these reactions, the dry mass of the isolated substrate-support did not increase (in contrast to Method (a)), but decreased by 5– 10%; this meant that side reactions caused by the ex tempore forming hydriodic acid were negligible. It was not a trivial question to apply

Fig. 3. Product distribution for two: ( ) carboxylic acid; ( ) alcohol; and ( ) unreacted polymer.

Fig. 4. Product distribution for four: ( ) carboxylic acid; ( ) alcohol; and ( ) unreacted polymer.

´´ / Reactive & Functional Polymers 50 (2001) 85 – 93 P. Tetenyi

91

iodine of as high concentration as I did, since even in experiment No. 6, the red colour of iodine remained at the end of the reaction time, demonstrating that iodine did not react completely. These experiments clearly show that the reagent ratio published by Barton et al. [11] is not optimised. It was clear from the previous experiments, that the yield or the selectivity were not related to the molar amounts of the reagents (Table 1, columns B, C) — both reagents must be used in considerable excess to the substrate. However, direct correlation was reached when the relative initial concentration of boron triiodide / heteroatom was drawn against the yield (Fig. 6). It was also remarkable that this curve would have reached 100% at about a concentration of

0.50 mmol / cm 3 , which value was the same as the initial concentration of the polymer-supported substrate, showing that boron triiodide reagent took place in the kinetic equation, too (second-order kinetics). Product distributions for these experiments are shown for comparison in Fig. 7, where the number at the bottom of each column is the number of the experiment. Fig. 7 stresses that selectivity for the carboxylic acid was highest in Exp. Nos. 2 and 3. However, if the desired synthetic target is the iodoalcohol, selectivity could be increased if molar excess of iodine to NaBH 4 was used (No. 1). Since the reaction mixture for Method (b) was quite difficult to stir, higher concentrations for the reagents could not be reached by applying less solvent or more reagents. While 1,2dichloroethane was a good solvent for the reagents, and at the beginning could reasonably swell the substrate-support, at yields of 30% or above, precipitation of the multiple borate complex of the polymer took place. This decrease of the swelling ability was caused by the formation of the multiple borate complex of the polymer that behaved as a new crosslink. This precipitation prevented any further transport of the reagent from the outside solution to the inside of the polymer beads, and thus was an efficient tool for stopping the reaction. That is why it

Fig. 6. Concentration versus yield. (m) Concentration of BI 3 / heteroatom (mmol / cm 3 ).

Fig. 7. Comparison of the product distributions: ( ) carboxylic acid; ( ) alcohol; and ( ) unreacted polymer.

Fig. 5. Product distribution for six: ( ) carboxylic acid; ( ) alcohol; and ( ) unreacted polymer.

92

´´ / Reactive & Functional Polymers 50 (2001) 85 – 93 P. Tetenyi

was so important that concentration of the reagent should be about equal to the concentration of the substrate in that very moment, when this decrease of the swelling ability of the beads closed the channels near or within the outer surface of the polymer beads (see Exp. No. 1). On the other hand, I wanted to keep the temperature of the reaction as low as possible to prevent elimination side reactions.

6. Reusability of the cleaved polymer I have data for the reusability of the cleaved polymer (P)–CHOH–C 6 H 4 OH achieved by cleavage of (P)–CHOH–C 6 H 4 –O–(CH 2 ) 6 –OH synthesized in 18 steps. Total loss of mass of the polymer (including in the linker, too) was 11.3% (0.63% per each step), while the cleaved polymer could be alkylated (see step 3, in Reaction Eq. (1)) with a yield of 78%. That had to be compared to the similar yield of 92% of the alkylation of the ‘fresh’ polymer, showing 15.4% decrease of activity. These results are shown in Fig. 8. There was an another advantage of our polymer-linker system: the cleaved polymer

could be reused without any additional activation step.

7. Conclusions Application of Barton’s method to polymersupported synthesis was done successfully, using the new aryl alkyl ether polymer. However, during the reaction time, at least 50% of the cleaved substrate underwent a reduction side reaction; this alcohol formed ester with the cleaved carboxylic acid. The most important changes to Barton’s method under the conditions of the polymersupported synthesis were the following: (a) 1,2dichloroethane must be used as solvent (instead of dichloromethane); (b) limit the amount of solvent as much as possible (the concentration of the reagent must exceed 10%, v / v); (c) the ratio of the reagents must be around 1:1 (instead of 3:1), in that way boron triiodide produced theoretically must be in equimolar concentration to the heteroatoms of the substrate; (d) during the isolation, follow the procedure described here (it was especially important in the case of polymer-supported substrates); (e) the cleaved polymer could be reused with slight decrease of activity.

Acknowledgements The author is grateful for the valuable advice ´ ´ F. and support he got from Professor Laszlo Szabo´ during and after the experimental work.

References

Fig. 8. Comparison on the alkylation reaction of polymer-supported phenols ( ) yield for ether and ( ) unreacted polymer.

´ P. Tetenyi ´´ [1] L.F. Szabo, Jr., Chemistry on polymer supports 1. Polymer-supported Wittig reactions, React. Polym. 8 (1988) 193–199, See Chem. Abst. 110 (1989) 74924 r. ´ P. Tetenyi ´´ [2] L.F. Szabo, Jr., Chemistry on polymer supports 2. Intrapolymer interaction in alkylation of C–H acids, React. Polym. 11 (1989) 47–56, See Chem. Abst. 112 (1990) 97789 f.

´´ / Reactive & Functional Polymers 50 (2001) 85 – 93 P. Tetenyi ´ P. Tetenyi ´´ [3] L.F. Szabo, Jr., Method for preparation of carbon chains by biomimetic sequential attachment on polymer support [Hung. Teljes HU 38,372 (1986), C08F 8 / 00, C07C 51 / 353, 28 Feb. 1990, 199516 / 89, Appl. 1271 / 84, 30 March, 1984. See Chem. Abst. 106 (1987) 195558 a] [4] K.F. Wedemeyer, Phenole durch Umwandlung funktioneller Phenol-Derivate, in: Houben-Weyl VI / 1c1 (1976) 313–339. [5] R.L. Burwell Jr., Chem. Rev. 54 (1954) 615–685. [6] M.V. Bhatt, S.U. Kulkarni, Cleavage of ethers, Synthesis 4 (1983) 249–282. [7] D.L. Manson, O.C. Musgrave, condensation of diketones with aromatic compounds. Part I. a-Diketones and veratrole, J. Chem. Soc. (1963) 1011–1013. [8] J.F.W. McOmie, M.L. Watts 2, D.E. West, Demethylation of aryl methyl ethers by boron tribromide, Tetrahedron 24 (5) (1968) 2289–2292. [9] V. Stehle, M. Brini, A. Pousse, Action du tribromure de bore ´ sur les ethers-oxydes aliphatiques et arylaliphatiques, Bull. Soc. Chim. Fr. (1966) 1100–1104. [10] J.-M. Egly, A. Pousse, M. Brini, Action du tribromure de ´ ´ bore sur les phenoxy-1-alcanes lineaires, Bull. Soc. Chim. Fr. 4 (1972) 1357–1360. [11] D.H.R. Barton, L. Bould, D.L.J. Clive, P.D. Magnus, T. Hase, Experiments on the synthesis of tetracycline. Part VI. Oxidation and reduction of potential ring A precursors, J. Chem. Soc. [C] (1971) 2204–2215. [12] H.C. Brown, B.C. Subba Rao, A new powerful reducing agent — sodium borohydride in the presence of aluminum chloride and other polyvalent metal halides, J. Am. Chem. Soc. 78 (1956) 2582–2588. [13] H.C. Brown, B.C. Subba Rao, Hydroboration III. The reduction of organic compounds by diborane, an acid-type reducing agent, J. Am. Chem. Soc. 82 (1960) 681–686. [14] H. Hayatsu, H.G. Khorana, Studies on polynucleotides LXXII. Deoxyribooligonucleotide synthesis on a polymer support, J. Am. Chem. Soc. 89 (1967) 3880–3887.

93

[15] L.R. Melby, D.R. Strobach, Oligonucleotide syntheses on insoluble polymer supports I. Stepwise synthesis of trithymidine diphosphate, J. Am. Chem. Soc. 89 (1967) 450– 453. [16] J.M.J. Frechet, K.E. Haque, Use of polymers as protecting groups in organic synthesis II. Protection of primary alcohol functional groups, Tetrahedron Lett. 35 (1975) 3055–3056. [17] C.C. Leznoff, T.M. Fyles, J. Weatherston, The use of polymer supports in organic synthesis VIII. Solid phase synthesis of insect sex attractants, Can. J. Chem. 55 (1977) 1143–1153. [18] T.M. Fyles, C.C. Leznoff, J. Weatherston, The use of polymer supports in organic synthesis XII. The total stereoselective synthesis of cis insect sex attractants on solid phases, Can. J. Chem. 55 (1977) 4135–4143. [19] C.C. Leznoff, T.M. Fyles, J. Weatherston, Can. J. Chem. 56 (1978) 1031–1041. [20] C.C. Leznoff, T.W. Hall, The synthesis of a soluble, unsymmetrical phthalocyanine on a polymer support, Tetrahedron Lett. 23 (30) (1982) 3023–3026. [21] C.C. Leznoff, J.Y. Wong, The use of polymer supports in organic synthesis III. Selective chemical reactions on one aldehyde group of symmetrical dialdehydes, Can. J. Chem. 50 (1973) 2892–2893. [22] S. Hanessian, F. Xie, Polymer-bound p-alkoxybenzyl trichloracetimidates: reagents for the protection of alcohols as benzyl ethers on solid-phase, Tetrahedron Lett. 39 (8) (1998) 733–736. [23] M.D. Corbridge, C.R. McArthur, C.C. Leznoff, React. Polym. 8 (2) (1988) 173–188. [24] Blanton, R.E. Salley, J. Org. Chem. 53 (3) (1989) 723–726.