Rhodium(III)‐Catalyzed Alkenylation–Annulation of closo‐Dodecaborate Anions through Double B−H Activation at Room Temperature

Abstract

1,2,3-Trisubstituted closo-dodecaborates with B-O, B-N, and B-C bonds as well as a fused borane oxazole ring have been synthesized by rhodium-catalyzed direct cage B-H alkenylation and annulation of ureido boranes in the first reported example of regioselective B-H bond functionalization of the [B H ] cage by transition-metal catalysis. This reaction proceeded at room temperature under ambient conditions and exhibited excellent selectivity for efficient monoalkenylation with good functional-group tolerance. The urea moiety enabled B-H activation by acting as a directing group, was incorporated in the oxazole ring in situ, and also avoided multiple alkenylation. A possible mechanism is proposed on the basis of the isolation of a rhodium agostic intermediate and control experiments.

The chemical bonding and spectral assignments of rhodium(III)-catalyzedcloso-dodecaborate complexes: Ab initio study

“…The strongest IR peak at 1532 cm −1 (experimentally at 1537 cm −1 ) arises from the involvement of C‐O stretching and N‐H bending, combined with the CH 2 wagging vibration, while the neighboring medium band at 1557 cm −1 (experimentally at 1573 cm −1 ) is mainly dominated by the C‐N stretching vibration. It is noteworthy that, the B‐H‐Rh vibrations (involving the B‐H stretching and Rh‐H bending) can be theoretically found at 2031 cm −1 and experimentally at 2084 cm −1 , and the deviation of 53 cm −1 should ascribe to the influence of measure conditions, for example, experimentally as a KBr pellet and dry film . Conversely, the strong IR band at 2504 cm −1 corresponds to the B‐H stretching vibrations, which is consistent with previously experimental study, located at 2514 cm −1 .…”

“…The harmonic vibrational frequencies of the RhCD complex were evaluated using theoretical calculations at the B3LYP/def‐SVP level of theory, and a scaling factor of 0.96 was applied to assign well the experimental IR spectrum. In Figure , the calculated and experimental IR spectra are compared, and it is found that the calculated IR spectrum is in good agreement with the experimental observation, from the previous report …”

“…The geometry of the X-ray crystal RhCD structure from previous report [25] was fully optimized in the framework of DFT using the B3LYP/def-SVP level of theory. The final relaxed geometry is shown in Figure 1, in which the B2H2Rh bridged bonds play an important role in the structural FIG URE 1 Optimized geometry of the RhCD structure, computed using the hybrid B3LYP functional stabilization, and are consistent with the other complexes with the agnostic interactions.…”

“…In addition, some aromatic complexes involving the closo-[B 12 H 12 ] 22 anion were also reported. [21][22][23][24] In this concern, Zhang et al [25] synthesized the rhodium(III)-catalyzed closo-dodecaborate complexes (labeled as RhCD, Figure 1) using several spectroscopy methods (NMR, Mass, IR), and gained insight into the reaction mechanism of catalytic procedure, which involves the adduct with the alkyne coordination, and it was deduced that the RhCD complex is a key intermediate in the catalytic mechanism from two reasons. First, the B-H activation weakens the interaction between B 12 H 12 cage and RhCp* in acetic acid environment.…”

Synthesis and Applications of Perfunctionalized Boron Clusters

“…Metal-catalyzed transformations will likely emerge in the future targeting synthesis of these molecules. 172 Significantly, many opportunities can be envisioned that stem directly from the ability to generate complex structures that contain multiple functional groups appended to its vertices ( vide supra ). Beyond metal-based catalysis, several studies indicate that perhalogenated carboranes and boron clusters can undergo substitution at B–Cl and B–F vertices, 173,174 suggesting it should be possible to design more well-behaved reaction protocols for creating vertex-differentiated systems via this relatively unexplored pathway.…”