To distinguish between dissociation of a B-N coordination bond by S N 1-and S N 2-type mechanisms, two series of 1,3,2-dioxaborolanes (boronates) and BEt 2 (borane) complexes carrying a 2,6-bis((dimethylamino)methyl)phenyl group as a third substituent were synthesized by the reaction of the corresponding organolithium compound with an appropriate boron reagent. In the solid state, the boronate complex exhibits a structure in which only one NMe 2 group is coordinated to a tetracoordinated boron atom according to the X-ray analysis and the solid-state NMR. In solution there is a rapid exchange between the coordinated and uncoordinated amine ligands. The barriers to B-N dissociation in the boronate and borane complexes are lower by >3.4 and 6.6 kcal/mol than in the corresponding monoamino complexes, respectively, which is due to electronic assistance in an S N 2-type mechanism. This observation is supported by ab initio calculations for the system of NH 3 and BH 3 . The dynamic process observed in the boronate complex with 4,4-diphenyl substituents is also discussed.
Facile exchange between the intramolecularly coordinated and uncoordinated amine ligands in the title compounds is evidenced by NMR spectroscopy in solution, whereas the dynamic process is frozen in the solid phase as shown by X-ray crystallography.Dynamic NMR studies of 2-(N,N-dimethylaminomethyl)phenylboronates 1 revealed that the dissociation of the intramolecular N-B coordination bond was significantly accelerated in nucleophilic solvents such as diethyl ether and acetone, and this rate enhancement was attributed to the assistance of solvent molecules by approaching from the back-side of the leaving amine ligand (SN2-type mechanism).l Although this mechanistic explanation was indirectly supported by introducing a bulky moiety around the boron atom,2 further studies to gain direct evidence for this explanation were limited because of the structural constraints of the system. Introduction of another amine ligand at the remaining o-position is another approach to the verification of the mechanism, where the second amine ligand instead of solvent molecules can strongly interact with the boron atom from the back-side of the coordinated ligand. Therefore, we synthesized the title organoboron complexes 2 with a 2,6-bis(N,N-dimethylaminomethyl)phenyl group.3 This communication reports the interesting dynamic behaviour of these organoboron compounds, involving facile exchange of the two amine ligands as well as the X-ray crystallographic structure.Compounds 2a and b-f were synthesized from 2,6-bis(N,N-dimethylaminomethy1)phenylboronic acid4 with an appropriate 1,2-diol in 79 and 97% yield, respectively.' The lH NMR spectrum of 2a gave three singlets in the aliphatic region at 6 2.41 (NCH3), 3.69 (NCH2) and 4.10 (OCH2) at room temperature in CD2C12, these signals showing no line shape changes even though the sample was cooled to -100°C. Compound 2b also gave three singlets for the aliphatic groups at room temperature. Among them, only the benzylic methylene protons exhibited line shape changes from a singlet to an AB quartet as the temperature was lowered, whereas the signals due to the N-methyl and 0-methylene protons were singlets throughout the temperature range examined.
Rates of the dissociation of the intramolecular B–N coordination bond in two series of phenylborane derivatives, the boronate and diethylborane complexes, with –CHMeNMe2 or –CMe2NMe2 group at the o-position were determined by the NMR lineshape analysis or saturation transfer method. The new organoboron compounds were synthesized from the corresponding organolithium compounds with appropriate boron reagents. Comparison of the kinetic data with those of the –CH2NMe2 compounds reveals that the barrier height to the dissociation, namely the kinetic basicity of the amine ligand, is increased as the molecule possesses more methyl groups at the benzylic position for both of the series of boron compounds. The X-ray structure of one of the boronate complexes and the NMR titration measurements of model amines indicate that the basicity of the amine ligand is not affected much by the methyl substitution in the coordinated form. Therefore, the substituent effect on the kinetic basicity is mainly ascribed to the destabilization of the transition state by the geminal dimethyl groups rather than to any inductive or steric effects at the initial state, especially for the –CMe2NMe2 compounds.
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