A Frustrated and Confused Lewis Pair

Abstract

We report a new class of frustrated Lewis pairs (FLPs) by the hydroboration of bulky isocyanates ArNCO ( Ar=2,6-iPr C H ) and ArNCO ( Ar=2,6-Ph -4-tBuC H ) with Piers' borane (HB(C F ) ). While hydroboration of smaller isocyanates such as ArNCO leads to isocyanate-N/B FLP adducts, hydroboration of the bulkier ArNCO allows isolation of the substrate-free aminoborane with a short, covalent N-B bond. This confused FLP reversibly binds unsaturated substrates such as isocyanates and isocyanides, suggesting the intermediacy of a "normal" FLP along the reaction pathway, supported by high-level DFT studies and variable-temperature NMR spectroscopy. These results underscore the possibility of FLP behavior in systems that possess no obvious frustrated Lewis acid-base interaction.

α‐Borylated Phosphorus Ylides (α‐BCPs): Electronic Frustration within a C−B π‐Bond Arising from the Competition for a Lone Pair of Electrons

“…In the system reported here, however, a reaction with one substrate molecule leads to cleavage of the B−C bond, in return furnishing a boron center with a free coordination site. In case of addition of PhNCO to 1 , the resulting molecule 6 even has a very similar structural motif to the postulated FLP structure of the “confused” Lewis pair reported by Grimme and Warren et al . It also has to be noted that the reactivity of 1 with the (hetero)cumulenes X=C=Y is similar to known P/B‐based FLP systems and systems with adjacent p‐orbitals like the phosphinoboronate ester Ph 2 PBpin (pin=1,2‐O 2 C 2 Me 4 ) reported by Stephan and Westcott et al …”

α‐Borylated Phosphorus Ylides (α‐BCPs): Electronic Frustration within a C−B π‐Bond Arising from the Competition for a Lone Pair of Electrons

“…In the system reported here, however, a reaction with one substrate molecule leads to cleavage of the B−C bond, in return furnishing a boron center with a free coordination site. In case of addition of PhNCO to 1 , the resulting molecule 6 even has a very similar structural motif to the postulated FLP structure of the “confused” Lewis pair reported by Grimme and Warren et al . It also has to be noted that the reactivity of 1 with the (hetero)cumulenes X=C=Y is similar to known P/B‐based FLP systems and systems with adjacent p‐orbitals like the phosphinoboronate ester Ph 2 PBpin (pin=1,2‐O 2 C 2 Me 4 ) reported by Stephan and Westcott et al …”

Hydro-coupling of isocyanates promoted by 1,2-bis(arylimino)acenaphthene aluminum hydrides

“…They consist of two six-membered metallacycles that resulted from the hydrogenation-induced carbon-to-nitrogen coupling of two isocyanate moieties. According to a previous study of the coupling of isocyanates by Piers’ borane, 22 the first two molecules of isocyanate probably react with a hydride through CN bonds, whereas the second two molecules of PhNCO undergo insertion via CO bonds.…”

“…These are Cp 2 Ti(μ-H) 2 (H)Al{C(TMS) 3 } 21 and Piers’ borane (HB(C 6 F 5 ) 2 ). 22 The former promotes the hydro-coupling of t BuCNO, while Piers’ borane is reactive even towards isocyanates bearing really bulky substituents, ArNCO (Ar = 2,6-Ph 2 -4- t BuC 6 H 2 or 2,6-iPr 2 C 6 H 3 ). All the reported products of hydro-coupling of isocyanates show structures of type B (Fig.…”

Hydride oxidation from a titanium–aluminum bimetallic complex: insertion, thermal and electrochemical reactivity

“…Such isocyanate insertion products are uncommon: a related example was recently reported with a boron-based Lewis acid. 62 Despite isocyanate being a weak oxidizing agent, the product retains the Ti( iii ) center; one hydride reduced the backbone carbon and two hydrides were transferred to an isocyanate fragment which was presumably the source of the oxo moiety, however no other products could be identified. Bulky isocyanates such as bis(2,6-diisopropylphenyl)isocyanate did not lead to isolable products.…”

α‐Borylated Phosphorus Ylides (α‐BCPs): Electronic Frustration within a C−B π‐Bond Arising from the Competition for a Lone Pair of Electrons

“…In the system reported here, however, a reaction with one substrate molecule leads to cleavage of the B−C bond, in return furnishing a boron center with a free coordination site. In case of addition of PhNCO to 1 , the resulting molecule 6 even has a very similar structural motif to the postulated FLP structure of the “confused” Lewis pair reported by Grimme and Warren et al . It also has to be noted that the reactivity of 1 with the (hetero)cumulenes X=C=Y is similar to known P/B‐based FLP systems and systems with adjacent p‐orbitals like the phosphinoboronate ester Ph 2 PBpin (pin=1,2‐O 2 C 2 Me 4 ) reported by Stephan and Westcott et al …”

Hydro-coupling of isocyanates promoted by 1,2-bis(arylimino)acenaphthene aluminum hydrides

“…They consist of two six-membered metallacycles that resulted from the hydrogenation-induced carbon-to-nitrogen coupling of two isocyanate moieties. According to a previous study of the coupling of isocyanates by Piers’ borane, 22 the first two molecules of isocyanate probably react with a hydride through CN bonds, whereas the second two molecules of PhNCO undergo insertion via CO bonds.…”

“…These are Cp 2 Ti(μ-H) 2 (H)Al{C(TMS) 3 } 21 and Piers’ borane (HB(C 6 F 5 ) 2 ). 22 The former promotes the hydro-coupling of t BuCNO, while Piers’ borane is reactive even towards isocyanates bearing really bulky substituents, ArNCO (Ar = 2,6-Ph 2 -4- t BuC 6 H 2 or 2,6-iPr 2 C 6 H 3 ). All the reported products of hydro-coupling of isocyanates show structures of type B (Fig.…”

Hydride oxidation from a titanium–aluminum bimetallic complex: insertion, thermal and electrochemical reactivity

“…Such isocyanate insertion products are uncommon: a related example was recently reported with a boron-based Lewis acid. 62 Despite isocyanate being a weak oxidizing agent, the product retains the Ti( iii ) center; one hydride reduced the backbone carbon and two hydrides were transferred to an isocyanate fragment which was presumably the source of the oxo moiety, however no other products could be identified. Bulky isocyanates such as bis(2,6-diisopropylphenyl)isocyanate did not lead to isolable products.…”

α‐Borylated Phosphorus Ylides (α‐BCPs): Electronic Frustration within a C−B π‐Bond Arising from the Competition for a Lone Pair of Electrons

“…In the system reported here, however, a reaction with one substrate molecule leads to cleavage of the B−C bond, in return furnishing a boron center with a free coordination site. In case of addition of PhNCO to 1 , the resulting molecule 6 even has a very similar structural motif to the postulated FLP structure of the “confused” Lewis pair reported by Grimme and Warren et al . It also has to be noted that the reactivity of 1 with the (hetero)cumulenes X=C=Y is similar to known P/B‐based FLP systems and systems with adjacent p‐orbitals like the phosphinoboronate ester Ph 2 PBpin (pin=1,2‐O 2 C 2 Me 4 ) reported by Stephan and Westcott et al …”

Hydro-coupling of isocyanates promoted by 1,2-bis(arylimino)acenaphthene aluminum hydrides

“…They consist of two six-membered metallacycles that resulted from the hydrogenation-induced carbon-to-nitrogen coupling of two isocyanate moieties. According to a previous study of the coupling of isocyanates by Piers’ borane, 22 the first two molecules of isocyanate probably react with a hydride through CN bonds, whereas the second two molecules of PhNCO undergo insertion via CO bonds.…”

“…These are Cp 2 Ti(μ-H) 2 (H)Al{C(TMS) 3 } 21 and Piers’ borane (HB(C 6 F 5 ) 2 ). 22 The former promotes the hydro-coupling of t BuCNO, while Piers’ borane is reactive even towards isocyanates bearing really bulky substituents, ArNCO (Ar = 2,6-Ph 2 -4- t BuC 6 H 2 or 2,6-iPr 2 C 6 H 3 ). All the reported products of hydro-coupling of isocyanates show structures of type B (Fig.…”

Hydride oxidation from a titanium–aluminum bimetallic complex: insertion, thermal and electrochemical reactivity

“…Such isocyanate insertion products are uncommon: a related example was recently reported with a boron-based Lewis acid. 62 Despite isocyanate being a weak oxidizing agent, the product retains the Ti( iii ) center; one hydride reduced the backbone carbon and two hydrides were transferred to an isocyanate fragment which was presumably the source of the oxo moiety, however no other products could be identified. Bulky isocyanates such as bis(2,6-diisopropylphenyl)isocyanate did not lead to isolable products.…”