Synthetic Routes and Methods Towards Achieving a Terminal Borylene
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Carbenes were studied as early as the 19th century where the initial goal primarily focused on the possible avenues that could utilize the species. Initial attempts in obtaining and isolating the carbene moiety was through trapping methods, under extreme conditions. Upon further characterizations, the “trapped” carbene had a triplet ground state configuration precluding its ability to be stable and isolated. Bertrand and Arduego, in the 1990’s were able to isolate the first stable carbene species. The divalent species, with a delocalized lone pair at the carbene carbon could undergo catalysis and be used as an ancillary ligand to electron deficient elements. James Hedrick and Robert Grubbs provided the first examples of carbenes to be used in catalysis through Ring Open Polymerization (ROP) and Ring Open Metathesis Polymerization (ROMP). As carbenes grew in prominence, the ability of carbenes to stabilize main group elements had become evident. Robinson, Bertrand, and Braunschweig with co-workers pioneered the field of carbene stabilized boron species that studied the reactivity of starting with a carbene stabilized B(III) and reducing it to B(I). this illuminated the reactive nature of the reduction of B(III) to B(I), as it afford boracycles, when not properly stabilized, or dimerization, when B(I) compensates for its vacant orbitals. We sought out to explore the chemistry of carbene-stabilized boron main group species by observing the reaction CAAC-B(N(iPr)2Cl2) when treated with KC8 as a reducing agent. With our best efforts to obtain full characterization of the CAAC-aminoborylene, we were unsuccessful in achieving a crystal structure of 29. The treatment of CAAC-Cp*BCl2 with KC8 led to a boracycle 32, as seen in 11 which was attributed to the reactive nature of generating B(I) in situ, leading to further reaction to fulfill its coordination environment. Treatment of 30 with AlCl3 adopts its η5-Cp*B orientation 33, where it could be reduced subsequently to obtain the CAAC-CP*B(I) terminal. Unfortunately, the reaction affords 32. Going from 33 to 34, we report isolating the first example of a CAAC stabilized boryl radical cation that is 2-coordinate and B(II). DAC is a more π-acidic carbene (reference Figure 1.5) than CAAC. The formation of DAC-B(N(iPr)2Cl2) was unobserved due to the withdrawing properties of the carbene’s carbonyl backbone. However, with the abstraction agent TMSOTf, the coordination of the aminoborane could be facilitated, generating a borenium cation with a triflate anion 35. This adduct could be reduced once to form the 36. Contrary to what literature suggest, this radical spends most of its time on the carbene carbon, accentuating the withdrawing character of the carbene (DAC). The radical could be further reduced to generate the borylene 37, which is confirmed by NMR. In spite of our best efforts, X-Ray data was left to be obtained.