Catalyst Control of Selectivity in the C–O Bond Alumination of Biomass Derived Furans

Non-catalysed and catalysed reactions of aluminium reagents with furans, dihydrofurans and dihydropyrans were investigated and lead to the ring-expanded products due to the formal insertion of the aluminium reagent into a C–O bond of the heterocycle. Specifically, the reaction of [{(ArNCMe)2CH}Al] (Ar = 2,6-di-iso-propylphenyl, 1) with furan, 2-methylfuran, 2,3-dimethylfuran and 2-methoxyfuran proceeded between 25 and 80 ºC leading to ring-expanded and dearomatised products due to the net transformation of a sp2 C–O bond into a sp2 C–Al bond. The kinetics of the reaction of 1 with furan were found to be 1st order with respect to 1 with activation parameters ΔH‡ = +19.7 (± 2.7) kcal mol-1, ΔS‡ = –18.8 (± 7.8) cal K-1 mol-1 and ΔG‡298 K = +25.3 (± 0.5) kcal mol-1 and a KIE of 1.0 ± 0.1. DFT calculations support a stepwise mechanism involving an initial (4+1) cycloaddition of 1 with furan to form a bicyclic intermediate that rearranges by an a-migration. The selectivity of ring-expansion is influenced by factors that weaken the sp2 C–O bond through population of the s*-orbital. Inclusion of [Pd(PCy3)2] as a catalyst in these reactions results in expansion of the substrate scope to include 2,3-dihydrofurans and 3,4-dihydropyrans but also improves the selectivity. Under catalysed conditions, the C–O bond that breaks is that adjacent to C–H bond. The aluminium(III) dihydride reagent [{(MesNCMe)2CH}AlH2] (Mes = 2,4,6-trimethylphenyl, 2) can also be used under catalytic conditions to effect a dehydrogenative ring-expansion of furans. Further mechanistic analysis of the Pd-catalysed reaction of 1 with furan shows that C–O bond functionalisation occurs via an initial C–H bond alumination. Kinetic products can be isolated that are derived from installation of the aluminium reagent at the 2-position of the heterocycle. C–H alumination proceeds with a strong primary KIE of 4.8 ± 0.3 consistent with a turnover limiting step involving oxidative addition of the C–H bond to a palladium catalyst. Isomerisation of the kinetic C–H aluminated product to the thermodynamic C–O ring expansion product is an intramolecular process that is again catalysed by [Pd(PCy3)2]. DFT calculations suggest that the key C–O bond breaking step involves attack of an aluminium based metalloligand on the 2-palladated heterocycle. The new methodology has been applied to the upgrading of molecules derived from furfuraldehyde, an important platform chemical from biomass.

Catalyst Control of Selectivity in the C–O Bond Alumination of Biomass Derived Furans

Non-catalysed and catalysed reactions of aluminium reagents with furans, dihydrofurans and dihydropyrans were investigated and lead to the ring-expanded products due to the formal insertion of the aluminium reagent into a C–O bond of the heterocycle. Specifically, the reaction of [{(ArNCMe)2CH}Al] (Ar = 2,6-di-iso-propylphenyl, 1) with furan, 2-methylfuran, 2,3-dimethylfuran and 2-methoxyfuran proceeded between 25 and 80 ºC leading to ring-expanded and dearomatised products due to the net transformation of a sp2 C–O bond into a sp2 C–Al bond. The kinetics of the reaction of 1 with furan were found to be 1st order with respect to 1 with activation parameters ΔH‡ = +19.7 (± 2.7) kcal mol-1, ΔS‡ = –18.8 (± 7.8) cal K-1 mol-1 and ΔG‡298 K = +25.3 (± 0.5) kcal mol-1 and a KIE of 1.0 ± 0.1. DFT calculations support a stepwise mechanism involving an initial (4+1) cycloaddition of 1 with furan to form a bicyclic intermediate that rearranges by an a-migration. The selectivity of ring-expansion is influenced by factors that weaken the sp2 C–O bond through population of the s*-orbital. Inclusion of [Pd(PCy3)2] as a catalyst in these reactions results in expansion of the substrate scope to include 2,3-dihydrofurans and 3,4-dihydropyrans but also improves the selectivity. Under catalysed conditions, the C–O bond that breaks is that adjacent to C–H bond. The aluminium(III) dihydride reagent [{(MesNCMe)2CH}AlH2] (Mes = 2,4,6-trimethylphenyl, 2) can also be used under catalytic conditions to effect a dehydrogenative ring-expansion of furans. Further mechanistic analysis of the Pd-catalysed reaction of 1 with furan shows that C–O bond functionalisation occurs via an initial C–H bond alumination. Kinetic products can be isolated that are derived from installation of the aluminium reagent at the 2-position of the heterocycle. C–H alumination proceeds with a strong primary KIE of 4.8 ± 0.3 consistent with a turnover limiting step involving oxidative addition of the C–H bond to a palladium catalyst. Isomerisation of the kinetic C–H aluminated product to the thermodynamic C–O ring expansion product is an intramolecular process that is again catalysed by [Pd(PCy3)2]. DFT calculations suggest that the key C–O bond breaking step involves attack of an aluminium based metalloligand on the 2-palladated heterocycle. The new methodology has been applied to the upgrading of molecules derived from furfuraldehyde, an important platform chemical from biomass.

Catalyst Control of Selectivity in the C–O Bond Alumination of Biomass Derived Furans

Non-catalysed and catalysed reactions of aluminium reagents with furans, dihydrofurans and dihydropyrans were investigated and lead to the ring-expanded products due to the formal insertion of the aluminium reagent into a C–O bond of the heterocycle. Specifically, the reaction of [{(ArNCMe)2CH}Al] (Ar = 2,6-di-iso-propylphenyl, 1) with furan, 2-methylfuran, 2,3-dimethylfuran and 2-methoxyfuran proceeded between 25 and 80 ºC leading to ring-expanded and dearomatised products due to the net transformation of a sp2 C–O bond into a sp2 C–Al bond. The kinetics of the reaction of 1 with furan were found to be 1st order with respect to 1 with activation parameters ΔH‡ = +19.7 (± 2.7) kcal mol-1, ΔS‡ = –18.8 (± 7.8) cal K-1 mol-1 and ΔG‡298 K = +25.3 (± 0.5) kcal mol-1 and a KIE of 1.0 ± 0.1. DFT calculations support a stepwise mechanism involving an initial (4+1) cycloaddition of 1 with furan to form a bicyclic intermediate that rearranges by an a-migration. The selectivity of ring-expansion is influenced by factors that weaken the sp2 C–O bond through population of the s*-orbital. Inclusion of [Pd(PCy3)2] as a catalyst in these reactions results in expansion of the substrate scope to include 2,3-dihydrofurans and 3,4-dihydropyrans but also improves the selectivity. Under catalysed conditions, the C–O bond that breaks is that adjacent to C–H bond. The aluminium(III) dihydride reagent [{(MesNCMe)2CH}AlH2] (Mes = 2,4,6-trimethylphenyl, 2) can also be used under catalytic conditions to effect a dehydrogenative ring-expansion of furans. Further mechanistic analysis of the Pd-catalysed reaction of 1 with furan shows that C–O bond functionalisation occurs via an initial C–H bond alumination. Kinetic products can be isolated that are derived from installation of the aluminium reagent at the 2-position of the heterocycle. C–H alumination proceeds with a strong primary KIE of 4.8 ± 0.3 consistent with a turnover limiting step involving oxidative addition of the C–H bond to a palladium catalyst. Isomerisation of the kinetic C–H aluminated product to the thermodynamic C–O ring expansion product is an intramolecular process that is again catalysed by [Pd(PCy3)2]. DFT calculations suggest that the key C–O bond breaking step involves attack of an aluminium based metalloligand on the 2-palladated heterocycle. The new methodology has been applied to the upgrading of molecules derived from furfuraldehyde, an important platform chemical from biomass.

Catalyst Control of Selectivity in the C–O Bond Alumination of Biomass Derived Furans

Non-catalysed and catalysed reactions of aluminium reagents with furans, dihydrofurans and dihydropyrans were investigated and lead to the ring-expanded products due to the formal insertion of the aluminium reagent into a C–O bond of the heterocycle. Specifically, the reaction of [{(ArNCMe)2CH}Al] (Ar = 2,6-di-iso-propylphenyl, 1) with furan, 2-methylfuran, 2,3-dimethylfuran and 2-methoxyfuran proceeded between 25 and 80 ºC leading to ring-expanded and dearomatised products due to the net transformation of a sp2 C–O bond into a sp2 C–Al bond. The kinetics of the reaction of 1 with furan were found to be 1st order with respect to 1 with activation parameters ΔH‡ = +19.7 (± 2.7) kcal mol-1, ΔS‡ = –18.8 (± 7.8) cal K-1 mol-1 and ΔG‡298 K = +25.3 (± 0.5) kcal mol-1 and a KIE of 1.0 ± 0.1. DFT calculations support a stepwise mechanism involving an initial (4+1) cycloaddition of 1 with furan to form a bicyclic intermediate that rearranges by an a-migration. The selectivity of ring-expansion is influenced by factors that weaken the sp2 C–O bond through population of the s*-orbital. Inclusion of [Pd(PCy3)2] as a catalyst in these reactions results in expansion of the substrate scope to include 2,3-dihydrofurans and 3,4-dihydropyrans but also improves the selectivity. Under catalysed conditions, the C–O bond that breaks is that adjacent to C–H bond. The aluminium(III) dihydride reagent [{(MesNCMe)2CH}AlH2] (Mes = 2,4,6-trimethylphenyl, 2) can also be used under catalytic conditions to effect a dehydrogenative ring-expansion of furans. Further mechanistic analysis of the Pd-catalysed reaction of 1 with furan shows that C–O bond functionalisation occurs via an initial C–H bond alumination. Kinetic products can be isolated that are derived from installation of the aluminium reagent at the 2-position of the heterocycle. C–H alumination proceeds with a strong primary KIE of 4.8 ± 0.3 consistent with a turnover limiting step involving oxidative addition of the C–H bond to a palladium catalyst. Isomerisation of the kinetic C–H aluminated product to the thermodynamic C–O ring expansion product is an intramolecular process that is again catalysed by [Pd(PCy3)2]. DFT calculations suggest that the key C–O bond breaking step involves attack of an aluminium based metalloligand on the 2-palladated heterocycle. The new methodology has been applied to the upgrading of molecules derived from furfuraldehyde, an important platform chemical from biomass.

Chemo- and Regioselective Palladium(II)-Catalyzed Aminoaryl­ation of N-Allylureas Providing 4-Arylmethyl Imidazolidinones

The aminoarylation reaction of N-allylureas under oxidative palladium catalysis, in the absence of ligands and by using inexpensive H2O2 as the sole oxidant, occurs in a chemo- and regioselective fashion. This intramolecular process takes place via a 5-exo-trig cyclization, and represents an easy approach to 4-arylmethyl imidazolidinones.

Synthesis of 1,2-divinylcyclopropanes by metal-catalyzed cyclopropanation of 1,3-dienes with cyclopropenes as vinyl carbene precursors

The synthesis of 1,2-divinylcyclopropanes by the reaction of cyclopropenes with 1,3-dienes is reported. The process relies on the ability of ZnCl2 or [Rh2(OAc)4] to generate metal–vinyl carbene intermediates from cyclopropenes, which effect cyclopropanation of 1,3-dienes. Most of the reactions proceeded in reasonable yields while the diastereoselectivity strongly depends on the structure of the diene. An example of an intramolecular process as well as the use of furan and 1,4-cyclohexadiene as dienes are also reported.

Photo-redox reactivity of titanium-oxo clusters: mechanistic insight into a two-electron intramolecular process, and structural characterisation of mixed-valent Ti(iii)/Ti(iv) products

The photo-reactivity of titanium-oxo clusters is investigated, revealing an intramolecular, solvent assisted, two-electron redox process that generates blue-coloured Ti(iii)/Ti(iv) clusters.