Reduction and Rearrangement of a Boron(I) Carbonyl Complex

Abstract The one‐electron reduction of a cyclic (alkyl)(amino)carbene (CAAC)‐stabilized arylborylene carbonyl complex yields a dimeric borylketyl radical anion, resulting from an intramolecular aryl migration to the CO carbon atom. Computational analyses support the existence of a [(CAAC)B(CO)Ar].− radical anion intermediate. Further reduction leads to a highly nucleophilic dianionic (boraneylidene)methanolate.

1, 2) display reactivity reminiscent of low-oxidation-state TM complexes, [19,20] including the coordination of CO. [21][22][23][24][25][26][27] Borylene carbonyl complexes (LB(CO)R) are generally obtained either by the direct addition of CO to a dicoordinate borylene (LBR) [27] or by releasing TM-bound borylenes through the addition of CO or other strong donors. [25,26] Spectroscopic and theoretical studies show a B À CO bonding pattern analogous to that of TM carbonyls, with the CO ligand donating into the empty orbital at boron and the boron lone pair backdonating into the p* orbital at CO (Figure 1 c). [24,26] Like their TM counterparts, borylene carbonyls undergo exchange reactions with other Lewis bases upon UV irradiation, [25] or photolytic intramolecular oxidative addition with C À H and C À C s bonds. [25,26] Lin and Xie also reported a cationic borylene carbonyl reacting with nucleophiles under reduction, migration, or complete cleavage of CO in a TMlike manner. [28] Inspired by this metallomimetic behavior, we report herein the one-and two-electron reduction chemistry of a carbonyl borylene, LB(CO)R, and highlight how it differs from that of TM carbonyls.
The possible resonance forms of monomeric [3]C À , which help stabilize the radical and anionic charge, are shown in Figure 4, together with the Mulliken spin densities of dimeric 3 and natural bond order (NBO) [42] calculations within the "different hybrids for different spins" approach. [43] The spin densities are delocalized throughout the BCO moieties, with the largest contribution at boron, in agreement with the experimental EPR hyperfine coupling constants. The NBO picture of the system indicates a bonding situation resembling the mesomeric structures B (a system) and D (b system). The dominant attractive contribution to the structure comes from the O(lp)!B-C(p*) donor-acceptor interaction (a system), as revealed by the second-order stabilization energies (Figure 4 c). This also indicates that delocalization through BCO plays a major role for the stabilization of 3.
The unexpected formation of 3 from the reduction of 2 can be rationalized by the one-electron reduction of 2 to an intermediate borylene radical anion [2]C À , followed by radical attack of the CO carbon C36 at the ipso-carbon of the Dip group, and subsequent migration of Dip to C36 to generate [3]C À (Scheme 2). DFT calculations revealed that [2]C À is indeed a stable minimum energy structure with a quasi-linear BCO arrangement of 174.48, a B1 À C1 bond length of 1.507 , and spin density mainly located at C1 (0.64) and C36 (0.38, Scheme 2). Radical anions of TM carbonyls and their clusters can be generated both chemically and electrochemically. [44][45][46][47] While a DFT study of group-6 [TM(CO) 4 PPh 3 ]C À radical anions showed that spin density in these species is largely

Angewandte Chemie
Communications located at the metal center, [48] [TM(CO) n ]C À complexes (TM = Fe, n = 5; TM = Cr, n = 6) also display CO-centered radical reactivity similar to that of [2]C À , undergoing facile hydrogen atom transfer with trialkyltin hydrides to yield the formyl complexes [TM(CO) nÀ1 (CHO)] À . [49] To our knowledge the radical transfer of a nitrogen-bound aryl group to CO has never been observed in TM carbonyl chemistry. In low-valent main group chemistry, however, the cleavage of N À C aryl bonds by the insertion of borylene or silylene fragments has been observed at N-heterocyclic olefin [50] or carbene ligands, [51] respectively.
The cyclic voltammogram of 3 in THF showed several irreversible oxidation waves as well as a reduction wave at À2.66 V (relative to the Fc/Fc + couple), suggesting the possibility of further chemical reduction (see Figure S36 in the SI). Indeed the reduction of 3 with 3 equiv KC 8 in THF yielded a dark red solution of an extremely air-sensitive NMR-active species, compound 4-K (Scheme 1 d), with an 11 B NMR shift at 14.5 ppm. Upon hydrolysis of 4-K the Nprotonated dimer 4-H was isolated (Scheme 1 e), with a similar 11 B NMR shift at 15.0 ppm and a characteristic 1 H NMR NH singlet at 3.82 ppm. Alternatively, 4-H could be accessed directly from 1 by reduction with 10 equiv KC 8 under CO in the presence of B(OH) 3 as the proton source (Scheme 1 f).
To conclude, despite analogies in their bonding patterns the reduction of a (CAAC)B(CO)Ar borylene carbonyl complex proceeds quite differently from that of TM carbonyls. While the latter may undergo a one-electron reduction to a metal-centered radical anion, the one-electron reduction of (CAAC)B(CO)Ar results in an unprecedented aryl migration from the CAAC nitrogen to the former carbonyl carbon atom, yielding a novel ketyl boron radical anion. Calculations show that this reaction likely proceeds via an intermediate [(CAAC)B(CO)Ar]C À radical anion, with a significant amount of spin density localized at the carbonyl carbon rather than the boron atom. This work highlights once more that borylenes display a unique reactivity quite distinct from TM analogues.