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Scheme 20 Two pathways for the isomerization of ene-oxide 74

Figure 22 Two pathways for the isomerization of ene-oxide 74

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Despite of the presence of the hydroxyl group, fluorinated alcohols are poor hydrogen-bond acceptors. Therefore Synthesis 2007, No. 19, 2925-2943 © Thieme Stuttgart - New York Because of their strong negative inductive effect, fluorine substituents increase the acidity of the hydroxyl group. The acidity of alcohols increases with the number of fluorine atoms in the molecule [pKa cethanot = 15.17, PK..(2-fuoroethanol) = 14.42, PKacrrpy = 12.4]. Perfluoro-tert- butanol exhibits a high acidity (pK, = 5.2), similar to that of acetic acid. Fluorinated alcohols are highly polar sol- vents, as shown in solvatochromic investigations.>>° HFIP was the most polar solvent among 360 compounds investigated. Both high ionizing power and lower nucleo- philicity were attributed to HFIP and TFE in comparison o their parent analogues (Table 2).!°1!13
Despite of the presence of the hydroxyl group, fluorinated alcohols are poor hydrogen-bond acceptors. Therefore Synthesis 2007, No. 19, 2925-2943 © Thieme Stuttgart - New York Because of their strong negative inductive effect, fluorine substituents increase the acidity of the hydroxyl group. The acidity of alcohols increases with the number of fluorine atoms in the molecule [pKa cethanot = 15.17, PK..(2-fuoroethanol) = 14.42, PKacrrpy = 12.4]. Perfluoro-tert- butanol exhibits a high acidity (pK, = 5.2), similar to that of acetic acid. Fluorinated alcohols are highly polar sol- vents, as shown in solvatochromic investigations.>>° HFIP was the most polar solvent among 360 compounds investigated. Both high ionizing power and lower nucleo- philicity were attributed to HFIP and TFE in comparison o their parent analogues (Table 2).!°1!13
Table 4 Hydrogenation of Esters in Fluorinated Alcohols
Table 4 Hydrogenation of Esters in Fluorinated Alcohols
Table 2 pK, and Polarity of Fluorinated and Common Solvents?!" Table 3 Hydrogen-Bonding Properties of Fluorinated and Commonly Used Solvents***!+”
Table 2 pK, and Polarity of Fluorinated and Common Solvents?!" Table 3 Hydrogen-Bonding Properties of Fluorinated and Commonly Used Solvents***!+”
solvents.”° HFIP was the most effective solvent in the hy- drogenation of benzyl benzoate acetylacetonate and 1,1,1-tris(dip these conditions, conversion of the corresponding saturated alcoho with ruthenium(IID) henylphosphino-meth- yl)ethane [MeC(CH,PPh,)3] at 120 °C and 85 bar. Under benzoyl benzoate oc- curred with turnover numbers (TON) of >2000. Dimethyl maleate and methyl palmitate were also hydrogenated to s using the same cata- lytic system. The authors speculated that the remarkable activity found in TFE and HFIP (Table 4, entries 2 and 3) compared to that in isopropyl alco attributed to the high acidity of the hol (entry 1) might be fluorinated alcohols. reaction rates and almost perfect chemoselectivity in the hydrogenation of a mixture of 1-hexyne and 1-octene at 1 bar and 22°C with Wilkinson’s catalyst [tris(triphe- nylphosphine)rhodium(]) chloride; Rh(PPh;);Cl]. The relative rate of the hydrogenation of 1-hexyne increased from | to 12 when the solvent, benzene—ethanol (1:1), was replaced by benzene—TFE, whilst the rate of the 1-octene hydrogenation remained constant.
solvents.”° HFIP was the most effective solvent in the hy- drogenation of benzyl benzoate acetylacetonate and 1,1,1-tris(dip these conditions, conversion of the corresponding saturated alcoho with ruthenium(IID) henylphosphino-meth- yl)ethane [MeC(CH,PPh,)3] at 120 °C and 85 bar. Under benzoyl benzoate oc- curred with turnover numbers (TON) of >2000. Dimethyl maleate and methyl palmitate were also hydrogenated to s using the same cata- lytic system. The authors speculated that the remarkable activity found in TFE and HFIP (Table 4, entries 2 and 3) compared to that in isopropyl alco attributed to the high acidity of the hol (entry 1) might be fluorinated alcohols. reaction rates and almost perfect chemoselectivity in the hydrogenation of a mixture of 1-hexyne and 1-octene at 1 bar and 22°C with Wilkinson’s catalyst [tris(triphe- nylphosphine)rhodium(]) chloride; Rh(PPh;);Cl]. The relative rate of the hydrogenation of 1-hexyne increased from | to 12 when the solvent, benzene—ethanol (1:1), was replaced by benzene—TFE, whilst the rate of the 1-octene hydrogenation remained constant.
Scheme 1 Three possible effects of an alcohol in the hydrogenation of carbon dioxide In situ spectroscopic investigations gave evidence that the fluorinated alcohol assists in the conversion of the catalyst precursor into [Ru(OAc)(PMe;),]Cl. However, other ben- eficial effects may come into play during the catalysis. These effects, until now not fully understood, may con- cern the activation of carbon dioxide by a concerted acti- vation mechanism between the fluorinated alcohol and the Ru-H moiety, followed by a stepwise transfer of hydro- gen (Scheme 1, ‘a’), or the protonation of an organometal- lic intermediate may facilitate the dissociation of formic acid from the metal center in the course of an ionic hydro- genation mechanism (Scheme |, ‘b’), or finally, in a con- certed ionic mechanism the fluorinated alcohol may serve as a hydrogen source together with the hydrido metal complex (Scheme 1, ‘c’).
Scheme 1 Three possible effects of an alcohol in the hydrogenation of carbon dioxide In situ spectroscopic investigations gave evidence that the fluorinated alcohol assists in the conversion of the catalyst precursor into [Ru(OAc)(PMe;),]Cl. However, other ben- eficial effects may come into play during the catalysis. These effects, until now not fully understood, may con- cern the activation of carbon dioxide by a concerted acti- vation mechanism between the fluorinated alcohol and the Ru-H moiety, followed by a stepwise transfer of hydro- gen (Scheme 1, ‘a’), or the protonation of an organometal- lic intermediate may facilitate the dissociation of formic acid from the metal center in the course of an ionic hydro- genation mechanism (Scheme |, ‘b’), or finally, in a con- certed ionic mechanism the fluorinated alcohol may serve as a hydrogen source together with the hydrido metal complex (Scheme 1, ‘c’).
Table 6 Palladium-Catalyzed Asymmetric Hydrogenation of a-Phthalimido Acetophenone in Different Solvents A related palladium catalyst based on the chiral ligand (S)-SEGPHOS was found to be effective in the asymmet- ric hydrogenation of a variety of N-phosphinyl imines in TFE as solvent. Chiral products obtained in 87-99% ee are valuable precursors of chiral amines.*4
Table 6 Palladium-Catalyzed Asymmetric Hydrogenation of a-Phthalimido Acetophenone in Different Solvents A related palladium catalyst based on the chiral ligand (S)-SEGPHOS was found to be effective in the asymmet- ric hydrogenation of a variety of N-phosphinyl imines in TFE as solvent. Chiral products obtained in 87-99% ee are valuable precursors of chiral amines.*4
® With TpRu(PPh;)(MeCN)H at 50 bar and 100 °C. Table 5 Hydrogenation of Carbon Dioxide to Formic Acid*
® With TpRu(PPh;)(MeCN)H at 50 bar and 100 °C. Table 5 Hydrogenation of Carbon Dioxide to Formic Acid*
Synthesis 2007, No. 19, 2925-2943 © Thieme Stuttgart - New York A fluorinated alcohol also showed promising results in the synthesis of B-amino acids through asymmetric hydroge- nation of their unsaturated precursors. Thus, as solvent, TFE gave bet er results in terms of stereoselectivity and reaction rates than did methanol (Table 8).°738 Moreover, when [Rh{(R,R)-4a}(cod)]BF, was employed as a precat- alyst in all tria were obtained and enantiose in the hydrogenation of acyla ectivities was observed in the sin TFE, the same or even better enantiose- ectivities were noted in comparison to dichloromethane. Enantioselectivities higher he use of than 90% ed3738 (en- ries 1-12) and non-acylated enamines (entries 13—16).*° In general, a beneficial effect of polar solven s on yields hydrogena- be noted that ion of unsaturated B-amino acids.*® It should, however, he hydrogenation of correspond ing amides with precatalysts based on ligand 6 gave higher enantiose- ectivities in methanol than in TFE (entries 17 and 18).°? “A 56% yield was reported for a two-step sequence.
Synthesis 2007, No. 19, 2925-2943 © Thieme Stuttgart - New York A fluorinated alcohol also showed promising results in the synthesis of B-amino acids through asymmetric hydroge- nation of their unsaturated precursors. Thus, as solvent, TFE gave bet er results in terms of stereoselectivity and reaction rates than did methanol (Table 8).°738 Moreover, when [Rh{(R,R)-4a}(cod)]BF, was employed as a precat- alyst in all tria were obtained and enantiose in the hydrogenation of acyla ectivities was observed in the sin TFE, the same or even better enantiose- ectivities were noted in comparison to dichloromethane. Enantioselectivities higher he use of than 90% ed3738 (en- ries 1-12) and non-acylated enamines (entries 13—16).*° In general, a beneficial effect of polar solven s on yields hydrogena- be noted that ion of unsaturated B-amino acids.*® It should, however, he hydrogenation of correspond ing amides with precatalysts based on ligand 6 gave higher enantiose- ectivities in methanol than in TFE (entries 17 and 18).°? “A 56% yield was reported for a two-step sequence.
Table 7 Asymmetric Hydrogenation of o-Fluorinated Iminoesters
Table 7 Asymmetric Hydrogenation of o-Fluorinated Iminoesters
* Conditions: X = BF,, 1 atm, 25 °C, 2 mol% ligand. > Not determined. © Conditions: X = Cl, 7 atm, 50 °C, 0.3 mol% ligand. Table 8 Enantioselective Hydrogenation (continued)
* Conditions: X = BF,, 1 atm, 25 °C, 2 mol% ligand. > Not determined. © Conditions: X = Cl, 7 atm, 50 °C, 0.3 mol% ligand. Table 8 Enantioselective Hydrogenation (continued)
Scheme 2 Catalytic epoxidation with the arylselenic acid 8 and hy- drogen peroxide system Fluorinated arylselenic acids were found to be effective organocatalysts in the epoxidation of olefins.*’ Several polar and non-coordinating solvents that were inert to ox- idizing conditions were studied in initial attempts at the epoxidation of cyclohexene in the presence of diselenide 7, and TFE was found to be the best medium in terms of reaction rates. In the first step, diselenide 7 is oxidized to selenic acid 8 (Scheme 2). In turn, the latter acts as a real catalyst.
Scheme 2 Catalytic epoxidation with the arylselenic acid 8 and hy- drogen peroxide system Fluorinated arylselenic acids were found to be effective organocatalysts in the epoxidation of olefins.*’ Several polar and non-coordinating solvents that were inert to ox- idizing conditions were studied in initial attempts at the epoxidation of cyclohexene in the presence of diselenide 7, and TFE was found to be the best medium in terms of reaction rates. In the first step, diselenide 7 is oxidized to selenic acid 8 (Scheme 2). In turn, the latter acts as a real catalyst.
Scheme 3 Epoxidation of functionalized cyclohexenes in HFIP A broadening of these investigations included a series of 4-substituted cyclohexenes that were subjected to epoxi- dation with hexafluoroacetone in HFIP (Scheme 3).*? The corresponding epoxides were isolated in yields ranging from 63% to 95% after 20 hours. Full conversion of oct- 1-ene was reported only after 48 hours.
Scheme 3 Epoxidation of functionalized cyclohexenes in HFIP A broadening of these investigations included a series of 4-substituted cyclohexenes that were subjected to epoxi- dation with hexafluoroacetone in HFIP (Scheme 3).*? The corresponding epoxides were isolated in yields ranging from 63% to 95% after 20 hours. Full conversion of oct- 1-ene was reported only after 48 hours.
* Conditions A: DCE-EtOAc (10:1), anhyd H,O, (2 equiv), perfluoroheptadecan-9-one (5 mol%). Conditions B: TFE, 60% H,O, (2 equiv perfluoroheptadecan-9-one (5 mol%). Table 9 Epoxidation of Olefins Catalyzed by Perfluoroheptadecan-9-one
* Conditions A: DCE-EtOAc (10:1), anhyd H,O, (2 equiv), perfluoroheptadecan-9-one (5 mol%). Conditions B: TFE, 60% H,O, (2 equiv perfluoroheptadecan-9-one (5 mol%). Table 9 Epoxidation of Olefins Catalyzed by Perfluoroheptadecan-9-one
Table 10 Solvent Effect in the Hexafluoroacetone-Catalyzed Epoxidation of 3-Carene (10)
Table 10 Solvent Effect in the Hexafluoroacetone-Catalyzed Epoxidation of 3-Carene (10)
Scheme 4 Catalytic epoxidation of olefins with perfluoroketone 12 and Oxone® as co-oxidant
Scheme 4 Catalytic epoxidation of olefins with perfluoroketone 12 and Oxone® as co-oxidant
Scheme 5 Postulated mechanism for the Baeyer—Villiger epoxidation with hydrogen peroxide in HFIP in the presence of Brgnsted acids The flavin catalyst 19 displays a high stability in TFE. In the oxidation of 17, a very high turnover number of 19400 was observed after 1650 hours by application of only 0.005 mol% catalyst. With this catalytic system and mo- lecular oxygen, sulfides could be oxidized to sulfoxides in yields ranging from 95% to 97%, tertiary amines to amine Baeyer—Villiger oxidations with hydrogen peroxide can proceed smoothly in fluorinated alcohols. Both
Scheme 5 Postulated mechanism for the Baeyer—Villiger epoxidation with hydrogen peroxide in HFIP in the presence of Brgnsted acids The flavin catalyst 19 displays a high stability in TFE. In the oxidation of 17, a very high turnover number of 19400 was observed after 1650 hours by application of only 0.005 mol% catalyst. With this catalytic system and mo- lecular oxygen, sulfides could be oxidized to sulfoxides in yields ranging from 95% to 97%, tertiary amines to amine Baeyer—Villiger oxidations with hydrogen peroxide can proceed smoothly in fluorinated alcohols. Both
Scheme 6 Allylic oxidation of cyclohexene The reactivity of homogeneous copper catalysts in allylic oxidations can be considerably improved by running the reaction in HFIP.© The relevant copper catalyst was pre- pared from perfluorolauric acid and copper(I) oxide, and was easily recycled. The oxidation was performed in the presence of tert-butyl perbenzoate. In addition, an enantio- selective version, known as the Kharasch—Sosnovsky re- action, was investigated by application of the fluorinated proline ligand 22 in HFIP (Scheme 6).°! Benzoate 21 was isolated in 77% yield and with 20% ee, which is, however, inferior to the results obtained in benzene when proline was used as ligand (59% yield and 45% ee). The use of HFIP allowed for recycling of the catalyst, but this was accompanied by losses in both activity and selectivity.
Scheme 6 Allylic oxidation of cyclohexene The reactivity of homogeneous copper catalysts in allylic oxidations can be considerably improved by running the reaction in HFIP.© The relevant copper catalyst was pre- pared from perfluorolauric acid and copper(I) oxide, and was easily recycled. The oxidation was performed in the presence of tert-butyl perbenzoate. In addition, an enantio- selective version, known as the Kharasch—Sosnovsky re- action, was investigated by application of the fluorinated proline ligand 22 in HFIP (Scheme 6).°! Benzoate 21 was isolated in 77% yield and with 20% ee, which is, however, inferior to the results obtained in benzene when proline was used as ligand (59% yield and 45% ee). The use of HFIP allowed for recycling of the catalyst, but this was accompanied by losses in both activity and selectivity.
The use of chiral oxazoline ligands 24—26 (Figure 1) with HFIP as additive was studied in copper-catalyzed enantio- selective Mukayama—Michael addition reactions.°*” In 1997, Katsuki discovered the highly enantioselective copper(J) trifluoromethanesulfonate/(S,S)-t-Bu-box (24a) catalyzed addition of 2-(trimethylsilyloxy)furans 27a,b to acrylate 28b with HFIP as additive (Table 12).°® It was shown that HFIP is essential to achieving high chemical yield.
The use of chiral oxazoline ligands 24—26 (Figure 1) with HFIP as additive was studied in copper-catalyzed enantio- selective Mukayama—Michael addition reactions.°*” In 1997, Katsuki discovered the highly enantioselective copper(J) trifluoromethanesulfonate/(S,S)-t-Bu-box (24a) catalyzed addition of 2-(trimethylsilyloxy)furans 27a,b to acrylate 28b with HFIP as additive (Table 12).°® It was shown that HFIP is essential to achieving high chemical yield.
Table 11 Solvent Effect in the Catalytic Aerobic Oxidation of Methyl 4-Tolyl Sulfide (17) oxides (yields of 94-97%), and secondary amines to ni- trones (yields of 77-85%).
Table 11 Solvent Effect in the Catalytic Aerobic Oxidation of Methyl 4-Tolyl Sulfide (17) oxides (yields of 94-97%), and secondary amines to ni- trones (yields of 77-85%).
Table 13 Effect of Additives on the Catalyzed Michael Addition to Acryloyl Oxazolidone
Table 13 Effect of Additives on the Catalyzed Michael Addition to Acryloyl Oxazolidone
Scheme 8 Asymmetric Michael addition catalyzed by a lanthanum— linked-BINOL complex
Scheme 8 Asymmetric Michael addition catalyzed by a lanthanum— linked-BINOL complex
Scheme 9 Enantioselective alkynylation of fluorinated ketone 39
Scheme 9 Enantioselective alkynylation of fluorinated ketone 39
Scheme 10 Mukaiyama—Mannich reaction HFIP was effective as an additive in the Mukaiyama— Mannich reaction of 42 with silylether 43 catalyzed by copper(II) trifluoromethanesulfonate and diamine 45 (Scheme 10).*! Addition of two equivalents increased the enantioselectivity from 49% ee to 93% ee. The authors suggested that HFIP is also necessary to promote the turn- over of the catalyst.
Scheme 10 Mukaiyama—Mannich reaction HFIP was effective as an additive in the Mukaiyama— Mannich reaction of 42 with silylether 43 catalyzed by copper(II) trifluoromethanesulfonate and diamine 45 (Scheme 10).*! Addition of two equivalents increased the enantioselectivity from 49% ee to 93% ee. The authors suggested that HFIP is also necessary to promote the turn- over of the catalyst.
Table 14 Solvent Effect on the Mannich Reaction of 46 *[bmim]BF, = 1-butyl-3-methylimidazolium tetrafluoroborate.
Table 14 Solvent Effect on the Mannich Reaction of 46 *[bmim]BF, = 1-butyl-3-methylimidazolium tetrafluoroborate.
Scheme 11 Palladium-catalyzed carbon monoxide/olefin copoly- merization
Scheme 11 Palladium-catalyzed carbon monoxide/olefin copoly- merization
Scheme 12 Asymmetric platinum(II)-catalyzed glyoxylate-ene re- action
Scheme 12 Asymmetric platinum(II)-catalyzed glyoxylate-ene re- action
Scheme 13 Asymmetric intramolecular [4+2] cycloaddition Livinghouse and co-workers discovered the first mild rhodium-catalyzed alkyne—diene [4+2] cycloaddition.”’ It was found that Wilkinson’s catalyst [(PPh,),RhCl] was capable of catalyzing this fast intermolecular addition in TFE. Reaction rates in other solvents were much slower. An asymmetric version was developed with DIOP-related ligands [DIOP = 4,5-bis(diphenylphosphinomethy])-2,2- dimethyl-1,3-dioxolane]. TFE was used as a cosolvent to provide the same stereoselectivity, but resulted in lower conversion.”*? The application of P-chirogenic diphos- phines in TFE gave the best enantioselectivity and conver- sion (Scheme 13).!° The active catalyst was generated in situ prior to the reaction by hydrogenation of the relevant nbd-rhodium complex (nbd = norbornadiene) with mo- lecular hydrogen.
Scheme 13 Asymmetric intramolecular [4+2] cycloaddition Livinghouse and co-workers discovered the first mild rhodium-catalyzed alkyne—diene [4+2] cycloaddition.”’ It was found that Wilkinson’s catalyst [(PPh,),RhCl] was capable of catalyzing this fast intermolecular addition in TFE. Reaction rates in other solvents were much slower. An asymmetric version was developed with DIOP-related ligands [DIOP = 4,5-bis(diphenylphosphinomethy])-2,2- dimethyl-1,3-dioxolane]. TFE was used as a cosolvent to provide the same stereoselectivity, but resulted in lower conversion.”*? The application of P-chirogenic diphos- phines in TFE gave the best enantioselectivity and conver- sion (Scheme 13).!° The active catalyst was generated in situ prior to the reaction by hydrogenation of the relevant nbd-rhodium complex (nbd = norbornadiene) with mo- lecular hydrogen.
Table 17 [5+2] Cycloaddition of a Vinylcyclopropane in DCE and TFE
Table 17 [5+2] Cycloaddition of a Vinylcyclopropane in DCE and TFE
The beneficial effect that TFE had as a cosolvent in the Pauson—Khand reaction ({2+2+1] cycloaddition) was not- ed by Livinghouse and co-workers.!°-! The reaction in the mixed solvent system TFE-DME (2:1) provided the desired diastereomeric enones 57 (57a/57B = 1.6) in 80% yield to the exclusion of 56, which is usually observed in other solvents (Scheme 14). This modified protocol al- lows for the undesired cycloisomerization of terminal acetylenes to be avoided, and was used successfully in the synthesis of nakadomarin, a cytotoxic agent isolated from an Amphimedon sponge."'°
The beneficial effect that TFE had as a cosolvent in the Pauson—Khand reaction ({2+2+1] cycloaddition) was not- ed by Livinghouse and co-workers.!°-! The reaction in the mixed solvent system TFE-DME (2:1) provided the desired diastereomeric enones 57 (57a/57B = 1.6) in 80% yield to the exclusion of 56, which is usually observed in other solvents (Scheme 14). This modified protocol al- lows for the undesired cycloisomerization of terminal acetylenes to be avoided, and was used successfully in the synthesis of nakadomarin, a cytotoxic agent isolated from an Amphimedon sponge."'°
Scheme 15 [6+2] Cycloaddition of cycloheptatriene (60) with ter- minal alkynes The cobalt-catalyzed [6+2] cycloaddition of cyclohep- tatriene (60) with terminal alkynes was sensitive to the choice of solvent (Scheme 15).'!* The catalyst system of Col,(dppe), zinc, and zinc(II) iodide showed average yields in TFE with aryl and alkyl acetylenes, and TFE proved to be beneficial in promoting the cycloaddition of propargyl and homopropargyl alcohols (60% and 67% yields).
Scheme 15 [6+2] Cycloaddition of cycloheptatriene (60) with ter- minal alkynes The cobalt-catalyzed [6+2] cycloaddition of cyclohep- tatriene (60) with terminal alkynes was sensitive to the choice of solvent (Scheme 15).'!* The catalyst system of Col,(dppe), zinc, and zinc(II) iodide showed average yields in TFE with aryl and alkyl acetylenes, and TFE proved to be beneficial in promoting the cycloaddition of propargyl and homopropargyl alcohols (60% and 67% yields).
The addition of TFE to the self-metathesis reaction of (Z)- 5-tert-butyldimethylsiloxy-2-pentenoate catalyzed by 63 led to the isomerization of the olefin in 89% yield, but did not result in the desired self-metathesis taking place.''®
The addition of TFE to the self-metathesis reaction of (Z)- 5-tert-butyldimethylsiloxy-2-pentenoate catalyzed by 63 led to the isomerization of the olefin in 89% yield, but did not result in the desired self-metathesis taking place.''®
Table 19 Catalytic Asymmetric Fluorination of tert-Butyl Ester 64
Table 19 Catalytic Asymmetric Fluorination of tert-Butyl Ester 64
Scheme 16 Catalytic ring opening of oxabenzonorbornadiene with alcohols Lautens et al. investigated the catalytic addition of differ- ent nucleophiles to oxabenzonorbornadiene 66 (Scheme 16).!”° Surprisingly, it was found that TFE was a good solvent for the reaction that used methanol as nu- cleophile. The enantioselectivity of the addition with diphosphine 6 as organocatalyst increased in a TFE- methanol (1:1) mixture (96% ee) compared to the reaction in neat methanol (45% ee). The reaction using other ali- phatic alcohols was also advantageously performed in TFE solution. The use of tetrahydrofuran led to better yields, but did not affect the enantioselectivity. TFE itself was also reacted with oxabenzonorbornadiene in tetrahy- drofuran solution in the presence of catalyst 6. Reactions in which mixtures of HFIP and isopropyl alcohol were used led exclusively to the addition of HFIP.
Scheme 16 Catalytic ring opening of oxabenzonorbornadiene with alcohols Lautens et al. investigated the catalytic addition of differ- ent nucleophiles to oxabenzonorbornadiene 66 (Scheme 16).!”° Surprisingly, it was found that TFE was a good solvent for the reaction that used methanol as nu- cleophile. The enantioselectivity of the addition with diphosphine 6 as organocatalyst increased in a TFE- methanol (1:1) mixture (96% ee) compared to the reaction in neat methanol (45% ee). The reaction using other ali- phatic alcohols was also advantageously performed in TFE solution. The use of tetrahydrofuran led to better yields, but did not affect the enantioselectivity. TFE itself was also reacted with oxabenzonorbornadiene in tetrahy- drofuran solution in the presence of catalyst 6. Reactions in which mixtures of HFIP and isopropyl alcohol were used led exclusively to the addition of HFIP.
Scheme 17 Asymmetric copolymerization of meso-cyclohexene oxide (68) and carbon dioxide The asymmetric copolymerization of meso-cyclohexene oxide 68 and carbon dioxide with proline-derived catalyst 70 in the presence of various alcohols was investigated by Nozaki and co-workers (Scheme 17).'?! In general, the presence of alcohols improved both the chemical yield and the enantioselectivity. A lower activity (86% yield) and enantioselectivity (60% ee) were observed with TFE as an additive in comparison with non-fluorinated aliphat- ic alcohols (ethanol: 91% yield and 75% ee).
Scheme 17 Asymmetric copolymerization of meso-cyclohexene oxide (68) and carbon dioxide The asymmetric copolymerization of meso-cyclohexene oxide 68 and carbon dioxide with proline-derived catalyst 70 in the presence of various alcohols was investigated by Nozaki and co-workers (Scheme 17).'?! In general, the presence of alcohols improved both the chemical yield and the enantioselectivity. A lower activity (86% yield) and enantioselectivity (60% ee) were observed with TFE as an additive in comparison with non-fluorinated aliphat- ic alcohols (ethanol: 91% yield and 75% ee).
Scheme 18 Jodoarene-catalyzed carbon—nitrogen bond-forming re- action The first carbon—nitrogen bond-forming reaction that used iodoarene as catalyst was developed by Kita and co- workers (Scheme 18).'”” The catalytic system consisting of 4-iodotoluene and a stoichiometric amount of m-chlo- roperoxybenzoic acid as a co-oxidant was used success- fully with TFE as the reaction medium. Under these conditions, an enhancement of the spirocyclization rate, and the best turnover number, were observed. Replace- ment of TFE by HFIP led to a decrease in the turnover number, and to low yields. Poor results were observed in other solvents such as dichloromethane, methanol, and acetonitrile.
Scheme 18 Jodoarene-catalyzed carbon—nitrogen bond-forming re- action The first carbon—nitrogen bond-forming reaction that used iodoarene as catalyst was developed by Kita and co- workers (Scheme 18).'”” The catalytic system consisting of 4-iodotoluene and a stoichiometric amount of m-chlo- roperoxybenzoic acid as a co-oxidant was used success- fully with TFE as the reaction medium. Under these conditions, an enhancement of the spirocyclization rate, and the best turnover number, were observed. Replace- ment of TFE by HFIP led to a decrease in the turnover number, and to low yields. Poor results were observed in other solvents such as dichloromethane, methanol, and acetonitrile.
Scheme 19 Equilibrium of rhodium complexes in HFIP solution Several catalytic isomerization reactions were investigat- ed in the presence of fluorinated alcohols. Among these, the isomerization of olefins by a ruthenium phosphine cat- alyst was investigated in various protic and aprotic sol- vents.'*° The nature of the solvent greatly affected the isomerization rate of hex-1-ene with Ru(CO),(PPh;), (71) (Scheme 19). The application of HFIP as a solvent led to low conversion (9%), whereas 88% conversion was ob- tained in isopropyl alcohol. In HFIP, the intermediate cat- alytic ruthenium complex 73 was detected with IR and NMR spectroscopy, whereas the related intermediate [Ru(H)(CO)3(PPh,),][Oi-Pr] was not observed. The re- moval of the alcoholic solvent led to the regeneration o starting complex 71. The lower catalytic activity in the isomerization of hex-1-ene using 71 as a catalytic precur- sor observed in HFIP as compared to isopropyl alcoho was attributed to the higher stability of perfluoroalkoxy complexes 72 and 73 over their non-fluorinated ana- logues.
Scheme 19 Equilibrium of rhodium complexes in HFIP solution Several catalytic isomerization reactions were investigat- ed in the presence of fluorinated alcohols. Among these, the isomerization of olefins by a ruthenium phosphine cat- alyst was investigated in various protic and aprotic sol- vents.'*° The nature of the solvent greatly affected the isomerization rate of hex-1-ene with Ru(CO),(PPh;), (71) (Scheme 19). The application of HFIP as a solvent led to low conversion (9%), whereas 88% conversion was ob- tained in isopropyl alcohol. In HFIP, the intermediate cat- alytic ruthenium complex 73 was detected with IR and NMR spectroscopy, whereas the related intermediate [Ru(H)(CO)3(PPh,),][Oi-Pr] was not observed. The re- moval of the alcoholic solvent led to the regeneration o starting complex 71. The lower catalytic activity in the isomerization of hex-1-ene using 71 as a catalytic precur- sor observed in HFIP as compared to isopropyl alcoho was attributed to the higher stability of perfluoroalkoxy complexes 72 and 73 over their non-fluorinated ana- logues.
Related topics:
Organic ChemistryCatalysisAsymmetric catalysisHomogeneous Catalysis
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