Controlled reductive C–C coupling of isocyanides promoted by an aluminyl anion

We report the reaction of the potassium aluminyl, K[Al(NON)] ([NON]2− = [O(SiMe2NDipp)2]2−, Dipp = 2,6-iPr2C6H3) with a series of isocyanide substrates (R-NC). In the case of tBu-NC, degradation of the isocyanide was observed generating an isomeric mixture of the corresponding aluminium cyanido-κC and -κN compounds, K[Al(NON)(H)(CN)]/K[Al(NON)(H)(NC)]. The reaction with 2,6-dimethylphenyl isocyanide (Dmp-NC), gave a C3-homologation product, which in addition to C–C bond formation showed dearomatisation of one of the aromatic substituents. In contrast, using adamantyl isocyanide Ad-NC allowed both the C2- and C3-homologation products to be isolated, allowing a degree of control to be exercised over the chain growth process. These data also show that the reaction proceeds through a stepwise addition, supported in this study by the synthesis of the mixed [(Ad-NC)2(Dmp-NC)]2− product. Computational analysis of the bonding within the homologised products confirm a high degree of multiple bond character in the exocyclic ketenimine units of the C2- and C3-products. In addition, the mechanism of chain growth was investigated, identifying different possible pathways leading to the observed products, and highlighting the importance of the potassium cation in formation of the initial C2-chain.


Controlled Reductive C-C Coupling of Isocyanides Promoted by an Aluminyl Anion
Crystallographic Details S35 Table S1 Crystal structure and refinement data for 1a, 1-crypt, 2·Et2O and 2·toluene S36 Table S2 Crystal structure and refinement data for 3·Et2O, 4·THF and 5·toluene S37 Computational Methodology S37 Figure S26 Representation of the HOMO of 5·toluene S39 Table S3 Relative energies for computed structures.
An isomeric mixture exists in solution. Overlap in the 1 H NMR spectrum causes most of the peaks to be indistinguishable between the two isomers. However, a noticeable shift is observed between the SiMe2 signals that has been used to calculate the relative ratio between each isomer as 5:3.

Crystallographic Details
Crystals were covered in inert oil and suitable single crystals were selected under a microscope and mounted on an Agilent SuperNova diffractometer fitted with an EOS S2 detector. Data were collected at 120 K (unless indicated otherwise) using focused microsource Cu Ka radiation at 1.54184 Å. Intensities were corrected for Lorentz and polarisation effects and for absorption using multi-scan methods. [2] Space groups were determined from systematic absences and checked for higher symmetry. All structures were solved using direct methods with SHELXS, [3] refined on F 2 using all data by full matrix least-squares procedures with SHELXL-97, [4] within the WinGX [5] program. Non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were placed in calculated positions or manually assigned from residual electron density where appropriate, unless otherwise stated.

Computational Methodology
DFT calculations were run with Gaussian 16 (C.01). [6] The Al, Si and K centres were described with the Stuttgart RECPs and associated basis sets, [7] and the 6-31G** basis set was used for all other atoms (BS1).
[8] A polarization function was also added to Al (ζd = 0.190), Si (ζd = 0.284) and K (ζd = 1.000). Initial BP86 optimizations were performed using the 'grid = ultrafine' option, [9] with all stationary points being fully characterized via analytical frequency calculations as minima or transition states (all positive eigenvalues or one imaginary eigenvalue respectively), and with intrinsic reaction coordinate calculations confirming the connectivity of the reaction pathways. All energies were recomputed with a larger basis set featuring 6-311++G** basis sets on all atoms (BS2). Corrections for the effect of toluene (ε = 2.3741) solvent were run using the polarizable continuum model and BS1, [10] using the keyword "scrf=toluene" within Gaussian. Single-point dispersion corrections to the BP86 results employed Grimme's D3 parameter set with Becke-Johnson damping as implemented in Gaussian. [11] Natural Bonding Orbital (NBO7) [12] analyses were performed on the BP86-optmised geometries at the BP86/6-311++G** level, within Gaussian 16 (C.01).

Breakdown of Energy Contributions
The following tables detail the evolution of the relative energies as the successive corrections to the initial SCF energy are included.