Intramolecular N-coordination in Ketiminoboranes † ‡

Treatment of the imine PhC(vNSiMe 3)py with Et 2 BOMe or BF 3 ·Et 2 O afforded bicyclic ketiminoboranes 4a and 4b via intra-molecular N-coordination. The basicity of the imine N is evidenced by their reactivity towards Brønsted and Lewis acids and the structures of 4a·HCl and 4b·BF 3 are reported as well as the dipyridyl imine derivative 4c·HCl. The use of intramolecular coordination of a pyridyl group has been exploited in recent years as a route to novel main group heterocycles. For example Dyer and co-workers investigated the intramolecular N-coordination of 2-pyridyl-N-phosphino-imines (1) and found that an equilibrium existed between open and closed forms 1a and 1b. 1 The 2-coordinate nitrogen in 1b is found to be sufficiently basic to form the adduct 1c with Lewis acidic AlCl 3 , 1 whilst oxidation with [(i Pr 2 N) 2 P][OTf ] led to an unusual π-conjugated coupled products (1d and 1e) (Scheme 1) via an oxidative radical coupling process. 2 Studies on the chemistry of related group 16 compounds revealed similar behaviour between ring-open and ring-closed products (Scheme 2). For example, when X = Ar (E = S) the open-form 2a is favoured with a short intramolecular S⋯N contact whereas when X = Cl (E = S, Se) then the ring-closed form 2b was favoured. 3 Work by Brusso and co-workers has revealed that at elevated temperatures ring-opening of these N-bridgehead thiadiazoles can occur. 4 Similar intramolecular N-coordination has been implemented to generate hypervalent Si IV (3). 5 In these compounds the group 14/15/16 heteroatoms are all formally electron precise centres and intramolecular N-coordination makes them hypervalent affording some degree of labi-lity between open and closed forms. Conversely group 13 elements are Lewis acidic and ring closure is expected to be strongly favoured. Ketiminoboranes, R 2 CvN-BR 2 were reported by Hawthorne, Wade and Lappert in the 1960's and are variously monomeric or dimeric depending upon substitu-ents, with the monomeric ketimines R 2 CvNBR 2 formally iso-electronic with allene. 6 In the current manuscript we describe the synthesis of ketiminoboranes in which the R group is capable of intramolecular coordination forming novel C/N/B heterocycles (4a–c). These heterocycles are similar to Scheme 1 Scheme 2 † In memory of Ken Wade: Teacher, oft-times mentor, colleague and friend. His contributions in the field of structure and bonding in main group chemistry will continue into the future, but his guidance, encouragement and …

The use of intramolecular coordination of a pyridyl group has been exploited in recent years as a route to novel main group heterocycles.For example Dyer and co-workers investigated the intramolecular N-coordination of 2-pyridyl-N-phosphinoimines (1) and found that an equilibrium existed between open and closed forms 1a and 1b. 1 The 2-coordinate nitrogen in 1b is found to be sufficiently basic to form the adduct 1c with Lewis acidic AlCl 3 , 1 whilst oxidation with [( i Pr 2 N) 2 P][OTf ] led to an unusual π-conjugated coupled products (1d and 1e) (Scheme 1) via an oxidative radical coupling process. 2 Studies on the chemistry of related group 16 compounds revealed similar behaviour between ring-open and ring-closed products (Scheme 2).For example, when X = Ar (E = S) the open-form 2a is favoured with a short intramolecular S⋯N contact whereas when X = Cl (E = S, Se) then the ring-closed form 2b was favoured. 3Work by Brusso and co-workers has revealed that at elevated temperatures ring-opening of these N-bridgehead thiadiazoles can occur. 4Similar intramolecular N-coordination has been implemented to generate hypervalent Si IV (3). 5 In these compounds the group 14/15/16 heteroatoms are all formally electron precise centres and intramolecular N-coordination makes them hypervalent affording some degree of lability between open and closed forms.Conversely group 13 elements are Lewis acidic and ring closure is expected to be strongly favoured.Ketiminoboranes, R 2 CvN-BR 2 were reported by Hawthorne, Wade and Lappert in the 1960's and are variously monomeric or dimeric depending upon substituents, with the monomeric ketimines R 2 CvNBR 2 formally isoelectronic with allene. 6In the current manuscript we describe the synthesis of ketiminoboranes in which the R group is capable of intramolecular coordination forming novel C/N/B heterocycles (4a-c).These heterocycles are similar to Scheme 1 Scheme 2 † In memory of Ken Wade: Teacher, oft-times mentor, colleague and friend.His contributions in the field of structure and bonding in main group chemistry will continue into the future, but his guidance, encouragement and support for so many of the young academics he came in contact with will be sorely missed.‡ Electronic supplementary information (ESI) available: Full experimental N,N′-boron chelate complexes, particularly derivatives of BODIPY, which have attracted considerable attention for their fluorescent properties, 7 as dyes in photodynamic therapy, 8 as well as photo-induced electron and energy transfer 9 and as optical switches 10 inter alia.In the current paper we describe the generation of 4a-4c and find that the 2-coordinate imine nitrogen is strongly basic, permitting us to isolate and structurally characterise 4a•HCl, and 4b•BF 3 and 4c•HCl.§ Compounds 4a-4c were prepared using a similar condensation reaction to that employed by Wade 6e to prepare Ph 2 CvNBPh 2 i.e. via the condensation of the imine Ar 2 CvNSiMe 3 with either Et 2 BOMe or BF 3 OEt 2 .Crystals of 4a•HCl and 4c•HCl appeared over 3 days and were isolated by filtration (27-37% unoptimised isolated yield).The HCl presumably arises from adventitious hydrolysis of Me 3 SiCl.Crystals of 4b•BF 3 were initially recovered in low yield from the reaction of PhC(vNSiMe 3 )py with BF 3 •Et 2 O in a 1 : 1 ratio but substantially improved yields (61%) were achieved using a 1 : 2 ratio.This suggests that the low solubility of the adduct favours crystallisation of the 1 : 2 product.
The 1 H NMR spectrum of 4a•HCl clearly reveals a broad singlet at 16.3 ppm consistent with N-protonation whilst the 11 B NMR spectrum revealed a singlet at +8 ppm consistent with a tetrahedral B centre and a molecular ion peak at m/z = 251 with an isotope distribution pattern consistent with 4a•H + .The structure of 4a•HCl was determined by X-ray diffraction (Fig. 1) and found to crystallise as a THF solvate.The B-C bonds are unexceptional but the B-N bond lengths are slightly different (within 3 esd's) with the B1-N1 bond (1.561(5) Å) somewhat shorter than the formally dative pyridyl B-N bond (1.595(5) Å).Both are consistent with B-N single bond character (1.57-1.60Å). 11 The C10-N1 bond at 1.285(4) is short, consistent with significant imine character.At 3.058(3) Å the N1⋯Cl1 distance is consistent with a conventional N-H⋯Cl hydrogen-bonded contact. 12he 11 B and 19 F spectra of 4b•BF Crystals of 4b•BF 3 were grown from the mother liquor on standing for 24-48 h.Single crystal structure determination revealed one molecule per asymmetric unit (Fig. 3).The heterocyclic C 2 N 2 B ring exhibits a similar geometry to the ethyl derivative with a longer B-N bond to the pyridyl nitrogen (1.600(2) Å) than to the imine nitrogen (1.574(2) Å) and a short imine-like CvN bond (1.286(2) Å).These distances fall at the extremes of those reported previously for other C 2 N 2 B heterocycles with a pyridyl nitrogen atom coordinated to a BF 2 group in which the dative bonds fall in the range 1.60-1.63Å and the covalent B-N bonds fall in the range 1.50-1.57Å. 9,12 The exo  B-N dative bond length to the BF 3 group, at 1.600(2) Å, is identical to the dative pyridyl-N-B bond.
Theoretical calculations (DFT B3LYP/6-311G*) on the reaction of 4b with BF 3 indicate adduct formation in the gas phase is favoured by 75 kJ mol −1 (see ESI †).Additional calculations along the B⋯N bond forming pathway reveal no significant activation energy barrier to formation of 4b•BF 3 .However stabilisation of the 'free' BF 3 in coordinating solvents through adduct formation such as MeCN•BF 3 or THF•BF 3 is expected to destabilise 4b•BF 3 with respect to loss of BF 3 .An NBO analysis revealed a bonding pattern best represented by the figure shown for 4b•BF 3 (eqn (1)) (see ESI †).Notably the reaction of py 2 CvO with Li[N(SiMe 3 ) 2 ]/Me 3 SiCl, followed by 1 equivalent of Et 2 BOMe afforded the pyridyl analogue, 4c•HCl in which the diazaborole nitrogen is protonated rather than the pyridyl nitrogen atom, reflecting the strongly basic nature of the diazaborole nitrogen atom ( pK b = 5.6, calculated using DFT methods), cf.pyridine ( pK b = 8.8). 14Synthetic details and crystallographic data for 4c•HCl are available as ESI.† The current studies reflect the diversity of heterocyclic ring systems accessible by intramolecular N-coordination.Unlike the later p-block elements in which intramolecular coordination generates a hypervalent multi-centre bonding interaction, the electron poor boron centre adopts a 4-coordinate electron-precise centre upon intramolecular coordination.The resultant heterocycle offers a strongly basic nitrogen atom which affords similar acid-base chemistry to the N-pyridyl phosphine-imines.
Intramolecular N-coordination in ketiminoboranes † ‡ Catherine E. Bacon, a Liz Mansour, b John J. Hayward b and Jeremy M. Rawson* b Treatment of the imine PhC(vNSiMe 3 )py with Et 2 BOMe or BF 3 •Et 2 O afforded bicyclic ketiminoboranes 4a and 4b via intramolecular N-coordination.The basicity of the imine N is evidenced by their reactivity towards Brønsted and Lewis acids and the structures of 4a•HCl and 4b•BF 3 are reported as well as the dipyridyl imine derivative 4c•HCl.
3 revealed four 19 F and four 11 B NMR resonances, the intensities of which varied depending upon solvent.In the 11 B NMR in MeCN two triplet resonances are observed in the 4-8 ppm range corresponding to two chemically distinct BF 2 environments, comparable with other 4-coordinate BN 2 F 2 centres. 10,13In addition, a quartet at 0 ppm and a singlet at −1 ppm are observed (see ESI †).The quartet we tentatively assign to the N-coordinated BF 3 in 4b•BF 3 and the singlet as BF 3 MeCN, based on chemical shift.These observations suggest a dynamic equilibrium (eqn (1)) in which the coordinated BF 3 is labile in the presence of coordinating solvents.In the 19 F NMR three resonances exhibit 11 B hyperfine coupling (see ESI †) and the HMQC 2D NMR spectrum (Fig. 2) along with coupling constants confirms the assignments of the corresponding BF 2 and BF 3 groups.In the 19 F NMR spectrum in MeCN the BF 2 fluorine atoms in both 4b and 4b•BF 3 appear around −159 ppm, reflecting very similar chemical environments whereas the BF 3 resonances appear at −152 ppm (BF 3 •MeCN) and −141 ppm (4b•BF 3 ).The resonance at −152 ppm appears as two signals in an approximate 4 : 1 ratio separated by 0.3 ppm and reflects the 11 B and 10 B isotopomers (∼80 : 20 natural abundance).In non-coordinating solvents such as benzene just two 11 B resonances are detected suggesting displacement of BF 3 in non-coordinating solvents is unfavourable and the structure of 4b•BF 3 appears fully retained in solution.