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Review
. 2020 Apr 1;25(7):1624.
doi: 10.3390/molecules25071624.

Silacyclohexanes, Sila(hetero)cyclohexanes and Related Compounds: Structure and Conformational Analysis

Bagrat A Shainyan  1
Affiliations
Review

Silacyclohexanes, Sila(hetero)cyclohexanes and Related Compounds: Structure and Conformational Analysis

Bagrat A Shainyan. Molecules. .

Abstract

Conformational analysis of Si-mono- and Si,Si-disubstituted silacyclohexanes as well as their analogues with a heteroatom(s) in the ring is reviewed with the focus on the recent results. Experimental measurements in the gas phase (gas electron diffraction, GED) and low temperature NMR spectroscopy (LT NMR) on 1H, 13C and 29Si nuclei are described along with theoretical calculations at the DFT and MP2 levels of theory. Structural and conformational specific features are shown to be principally different from those of the carbon predecessors - the corresponding cyclohexanes, oxanes, thianes and piperidines. The role of various effects (steric, hyperconjugation, stereoelectronic, electrostatic) is demonstrated.

Keywords: GED; LT NMR; conformational analysis; silicon-containing heterocycles; structure; theoretical calculations.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

Figure 1
Figure 1
Lower degree of folding at silicon in the six-membered ring.
Figure 2
Figure 2
Conformational equilibrium of monosubstituted silacyclohexanes.
Figure 3
Figure 3
Conformational equilibrium of geminally 1,1-disubstituted silacyclohexanes.
Figure 4
Figure 4
Orientation of the N and O lone pairs with respect to the O–Si dipole in 1,3-dimethyl-3-isopropoxy-1,3-azasilinane (left) and 3-isopropoxy-3-methyl-1,3-oxasilinane (right).
Figure 5
Figure 5
Ratio of 1-methylthio-1-phenyl-1-silacyclohexane conformers by different methods.
Figure 6
Figure 6
29Si-NMR spectrum of 1-methylthio-1-phenyl-1-silacyclohexane 1 at 98 K.
Figure 7
Figure 7
1,3- and 1,4-thiasilacyclohexanes and their S-functional derivatives.
Figure 8
Figure 8
Conformational equilibria in the trans and cis isomers of 3-silathiane S-oxides.
Figure 9
Figure 9
Predominance of the SOeq conformer for 4-silathiane S-oxide.
Figure 10
Figure 10
‘Scorpionoid’ boat structure of axial 4-silathiane S-oxides bearing halogens at silicon.
Figure 11
Figure 11
Structurally and conformationally studied azasilacyclohexanes (azasilinanes).
Figure 12
Figure 12
‘Inward’ rotamer of 2,2,6,6-tetramethyl-4-triflyl-1,4,2,6-oxazadisilinane.
Figure 13
Figure 13
Synthesis of (3,3,7,7-tetramethylhexahydro-1H-[1.4.2]oxazasilino[4,5-d][1.4.2]oxaazasilin-9a-yl)-methanol by condensation/cyclization of 2-amino-2-(hydroxymethyl)propane-1,3-diol with ClCH2SiMe2OMe.
Figure 14
Figure 14
X-ray structure of 3,3,7,7-tetramethylhexahydro-1H-[1.4.2]oxazasilino[4,5-d][1.4.2]oxaaza-silin-9a-yl)methanol.
Figure 15
Figure 15
Conformational flexibility of 3,3,7,7-tetramethylhexahydro-1H-[1.4.2]oxazasilino[4,5-d][1.4.2]oxaazasilin-9a-yl)methanol due to nitrogen pyramid inversion.
Figure 16
Figure 16
Isomeric salts formed upon quaternization of 3,3,7,7-tetramethylhexahydro-1H-[1.4.2]oxazasilino[4,5-d][1.4.2]oxaazasilin-9a-yl)methanol.
Figure 17
Figure 17
Synthesis of 3-methyl-3-silatetrahydropyran.
Scheme 1
Scheme 1
Synthesis of 3-phenyl-3-silatetrahydropyran and 3-hydroxy-3-phenyl-3-sila-tetrahydropyran.
Figure 18
Figure 18
Structure of 3-methyl-3-phenyl-3-silatetrahydropyran [32] (left) and 3-phenyltetrahydropyran [82] (right).
Scheme 2
Scheme 2
Synthesis of 1-dimethylamino-1-phenylsilacyclohexane.
Figure 19
Figure 19
Correlation between dihedral angles C1-Si-Cipso-Cortho and C4-Si-N-Me in the Phax (left) and Pheq conformers (right) of 1-(dimethylamino)-1-phenylsilacyclohexane.
Figure 20
Figure 20
Optimized geometry (M062X/6-311G**) of possible conformers of (1,1′-phenyl-1,1′-silacyclohex-1-yl)-disiloxane. ‘ax’ and ‘eq’ refer to the position of the phenyl group.
Figure 21
Figure 21
29Si-NMR spectra of (1,1′-phenyl-1,1′-silacyclohex-1-yl)disiloxane at 113 K (above) and at 103 K (below).
Scheme 3
Scheme 3
Pummerer rearrangement of 4,4-dimethyl-1,4-thiasilinane S-oxide
Figure 22
Figure 22
Conformational equilibrium of 4,4-dimethyl-3,4-dihydro-2H-1,4-thiasiline
Scheme 4
Scheme 4
Synthetic sequence for preparation of 4,4-diphenyl-3,4-dihydro-2H-1,4-thiasiline.
Scheme 5
Scheme 5
Synthesis of S-functional derivatives of 4,4-diphenyl-3,4-dihydro-2H-1,4-thiasiline.
Figure 23
Figure 23
Total synthesis of 4,4-diphenyl-3,4-dihydro-2H-1,4-thiasiline.
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