Strained molecules: crystal structure of 2,2,3-triphenyl-1-azabicyclo[1.1.0]butane

Abstract

The first X-ray crystal structure determination of a I-azabicyclo[l .I .O]butane derivative is reported.

1-Aza-2,4-disilabicyclo[1.1.0]butanes with Superelongated C–N σ-Bonds

“…These distances substantially exceed the standard C–N single bond length of 1.48 Å and are also longer than those observed in silaaziridines, (NHC)­(ArN)­SiC­(Ar′) 2 N­(SiMe 3 ) (NHC = N-heterocyclic carbene, Ar = 2,6- i Pr 2 C 6 H 3 , Ar′ = 4-MeC 6 H 4 1.553(3) Å), (L)­(R)­SiC­(H)­(Ph)­N­(Ph) (R = NPh 2 1.499(2) Å, t Bu 1.488(2) Å, Cl 1.510(5) Å), and (alkyl) 2 SiC­(H)­( p -RC 6 H 4 )­N­(Ph) ((alkyl) 2 = C­(Me 3 Si) 2 CH 2 CH 2 ­(SiMe 3 ) 2 C, R = H 1.488(3) Å, OMe 1.482(3) Å, Cl 1.489(3) Å) . It is interesting to note that the bridging C–N single bond within a similarly folded, carbon-based aziridine ring in 2,2,3-triphenyl-1-azabicyclo­[l.1.0]­butane exhibits a much shorter bond length of 1.467(8) Å compared to the disila-congeners studied here (19–23% longer). Also notably, the bridgehead carbon atoms in 2 – 5 exhibit a pronounced inverted tetrahedral geometry, with much smaller bond angles than that in an ideal tetrahedron (109.5°; e.g.…”

“…All species feature a central SiCNSi bicyclic core folded along the C–N axis (Figures S2-1–S2-6 in the Supporting Information (SI)). The flap-angles between the two silaaziridine planes are 133.2° ( 2 ), 131.5° ( 3 ), 128.3° ( 4 ), and 130.9° ( 5 ), respectively, slightly larger than that of 2,2,3-triphenyl-1-azabicyclo­[l.1.0]­butane (118.7°) . Strikingly, the bridging C–N interatomic distances are 1.782(2) Å for 2 , 1.801(1) Å for 3 , 1.745(2) and 1.769(1) Å for 4 , and 1.757(1) and 1.750(2) Å for 5 (Table S2-2, Figures and S2-1–S2-6 in the SI).…”

“…4), and 130.9°( 5), respectively, slightly larger than that of 2,2,3-triphenyl-1azabicyclo[l.1.0]butane (118.7°). 15 Strikingly, the bridging C−N interatomic distances are 1.782(2) Å for 2, 1.801(1) Å for 3, 1.745 (2) and 1.769(1) Å for 4, and 1.757(1) and 1.750(2) Å for 5 (Table S2-2, Figures 2 and S2 Stretching the CN bond of HCN by 51% from its equilibrium geometry to the bond length computed for 2 actually reproduces these peculiar properties of the electron density in the bonding region, namely a positive value of ∇ 2 ρ(r b ) at the bcp, a larger ratio Ξ, and, most notably, a decomposition of the Laplacian along the bond path into two separated polarized atomic fragments (see Figure S4-25 in SI). Similar results are obtained upon elongation of the C−N bond in heptamethyldisilazane by 51% (cf.…”

Synthesis and Applications of Bicyclo[1.1.0]butyl and Azabicyclo[1.1.0]butyl Organometallics

“…On the other hand, the nitrogen‐containing analogue of BCB, 1‐azabicyclo[1.1.0]butane (ABB), has received much less attention, with the structural parameters often assumed to be equivalent to those of BCB [33] . This is not an unreasonable prediction as x‐ray analysis has demonstrated the similarities between the bond angles and lengths of ABB and BCB which would suggest that the same hybridization models are valid for the aza‐analogue [34–35] . Interestingly, recent work from Anderson and Duarte calculated the strain‐release energy (SRE) of the central bond of ABB to be −31.4 kcal mol −1 , whereas the SRE for BCB was determined to be −40.2 kcal mol −1 [36] .…”

“…[33] This is not an unreasonable prediction as x-ray analysis has demonstrated the similarities between the bond angles and lengths of ABB and BCB which would suggest that the same hybridization models are valid for the aza-analogue. [34][35] Interestingly, recent work from Anderson and Duarte calculated the strain-release energy (SRE) of the central bond of ABB to be À 31.4 kcal mol À 1 , whereas the SRE for BCB was determined to be À 40.2 kcal mol À 1 . [36] In this report the authors state that strain-release alone is an insufficient predictor of reactivity and a larger contribution to the kinetic barrier in strained systems comes from the extent of bond delocalization.…”