Tetracyclic silaheterocycle formed through a pericyclic reaction cascade including a two-fold intramolecular C–C bond activation

Reductive debromination of the tribromoamidosilane 2 gave the tetracyclic silaheterocycle 3 through a unique reaction cascade involving unprecedented two-fold intramolecular cycloaddition by transient silylenes. Experimental and computational analyses of the reaction mechanism allowed the identification of the key intermediates that lead to the silaheterocycle 3 or, alternatively, to the cyclotrisilene 19.


Synthesis and Characterization of 1
A solution of n-BuLi (1.6 M in n-hexane; 26 mL, 42 mmol, 1.0 equiv.) was added dropwise to a solution of DippNH2 (8.0 mL, 42 mmol, 1.0 equiv.) in pentane (50 mL) at 0 °C. The reaction mixture was allowed to warm to room temperature and was stirred for 16 h. The colourless precipitate, LiDippNH, was filtered off, dried in vacuo, and used without further purification. LiDippNH (7.0 g, 32 mmol, 1.0 eq.) was dissolved in Et2O (150 mL) and cooled to 0 °C. Chloro(dimethyl)phenylsilane (6.4 mL, 32 mmol, 1.0 eq.) was added dropwise and the mixture was stirred overnight at 0 °C. The colourless precipitate was filtered off and the volatiles were removed in vacuo to give {N(SiMe2Ph)Dipp}H as an orange oil that was used without further purification.

Synthesis and Characterization of 9
[N{SiMe2Ph}Dipp]SiBr3] 2 (2.000 g, 3.5 mmol, 1.0 equiv.) was dissolved in THF (30 ml       Comment to Figure S80: Signals labeled with * in Figure S80 could not clearly assigned to 19 due to line broadening of 19 at 210 K. Figure S77 shows the 29 Si NMR spectrum at 300 K were both signals of 3 and 19 were identified.

Details of Single Crystal X-ray Diffraction Analyses
Crystals of 1, 2, 3•(1.5 PhMe), 8, and 19' were removed from a Schlenk tube under an argon atmosphere and covered with a layer of hydrocarbon oil. A suitable crystal was selected, attached to a glass fiber, and quickly placed in a low temperature argon stream. The data were collected S4 at 100 K on a Bruker Venture with Mo Kα radiation (λ = 0.71073 Å) and corrected for Lorentz and polarization effects with SAINT S4 and absorption using Blessing's method S5 as incorporated into the program SADABS. S6,S7 Crystal structures were solved by using the SHELXTL program package to determine the space group based upon intensity statistics. S8 The structure was determined by direct methods with a majority of the non-hydrogen atoms from the molecule of interest being located directly using the program SHELXS. S9 Refinement of the structure was achieved using the program SHELXL with all non-hydrogen atoms refined anisotropically. S10 Difference-Fourier least-squares refinement cycles were required to locate all non-hydrogen atoms.

Refinement Details
For compounds 1, 2, 8, and 19', no constraints or restraints were used in the refinement. However, compound 3 co-crystallises with the solvent toluene (PhMe). One of the solvent molecules is located next to a symmetry element and was treated with PART-1, SIMU, and DELU instructions. In addition, three reflexes were omitted because they were affected by the beam stopper.

Computational Details
The geometries of compounds 5-7 and 10-19 along with those of the associated transition states, were optimised in the gas phase with density functional theory using the PBE1PBE functional, S11-S14 def2-TZVP basis sets, S15 and Grimme's D3 dispersion correction with Becke-Johnson damping, S16,S17 and employing the Gaussian 16-C.01 program suite. S18 The structures were confirmed to be either minima or first order transition states on the potential energy hypersurface via calculation of the associated vibrational frequencies (all positive or one imaginary, respectively). The nature of all transition states found was further confirmed by following the reaction coordinate to both forward and reverse directions. Table S3 includes the Gibbs free energies of all investigated species, while their structures are provided in a separate coordinate file in standard xyz-format.