Issue in Honor of Prof. Heinz Heimgartner ARKIVOC 2011 (vi) 176-198
reported NHCs and the other two known structures in the literature, namely SIMes,4 and SItBu.8a The average length of the C2–N1(3) bonds in these free carbenes increases only very slightly from the average values found in the imidazolinium salts and indicate a partial double bond character of these bonds. The remaining bond lengths in the five-membered N-heterocycle in both the salts and the free carbenes identify these as single bonds. The most apparent change in bond angles between imidazolinium salts and free carbenes can be found when looking at the N–C–N angles, which are about 9° larger in the salts relative to those in the free carbene species. The opening of the N–C–N angle and the slight shortening of the C2–N1(3) bonds in the salts may relieve some of the strain inherent in five-membered cycloalkane rings, which results in the familiar puckering of the ring into a half-chair or envelope conformation. Of the NHC salts in Table 2, all but one have quite planar five-membered heterocyclic rings, where the deviation of any atom from the mean plane through the ring is less than 0.03 Å. The exception is syn-(2,7)-SIPrNap·HCl in which atoms C4 and C5 lie about 0.08 Å from the mean plane, resulting in the ring having a shallow half-chair conformation twisted on C4–C5. Two of the free carbenes listed in Table 2, syn-(2,7)- SIMeNap and anti-(2,7)-SIPrNap, have planar five-membered rings, while the corresponding rings in syn-(2)-SIPrNap and 5b have distinct half-chair conformations again twisted on C4–C5. These two atoms lie between 0.10 and 0.12 Å from the mean plane through the ring.
Table 2. Selected bond lengths of NHC salts with naphthyl wingtips and of known free carbenes
Imidazolinium salt C2—N1(3) C4—C5 N1(3)—C5(4) N1(3)—CAr syn-(2)-SIMeNap.HBF4 a 1.311(3), 1.311(3) 1.522(5) 1.485(3), 1.485(3) 1.440(3), 1.440(3) anti-(2)-SICyNap.HBF4 a 1.313(5), 1.306(5) 1.536(6) 1.479(5), 1.470(5) 1.448(5), 1.448(5) anti-(2)-SICyheptNap.HBF4 (4c) 1.308(3), 1.315(3) 1.536(3) 1.484(3), 1.479(3) 1.444(3), 1.454(3) syn-(2,7)-SIPrNap.HCla 1.318(4), 1.307(4) 1.528(4) 1.483(4), 1.481(4) 1.442(4), 1.442(4) anti-(2,7)-SICyNap.HBF4 (4f) 1.304(3), 1.310(3) 1.542(3) 1.487(2), 1.491(3) 1.440(2), 1.439(3) anti-(2,7)-SICyheptNap.HBF4 (4g) 1.308(4), 1.301(4) 1.544(4) 1.496(4), 1.490(4) 1.439(4), 1.455(4)
Free carbenes
syn-(2)-SIPrNapa 1.351(2), 1.3498(19) 1.514(2) 1.4814(19), 1.486(2) 1.4266(19), 1.433(2) syn-(2)-SICypentNap (5b) 1.348(2), 1.351(2) 1.520(2) 1.483(2), 1.484(2) 1.423(2), 1.432(2) syn-(2,7)-SIMeNapa 1.344(4), 1.348(4) 1.516(4) 1.480(4), 1.480(4) 1.435(4), 1.433(4) anti-(2,7)-SIPrNapa 1.337(3), 1.354(3) 1.519(4) 1.471(3), 1.473(3) 1.432(4), 1.430(3) SIMesb 1.352(5), 1.345(5) 1.505(6) 1.475(5), 1.487(5) 1.427(5), 1.437(5) SItBuc 1.348(1), 1.347(1) 1.512(2) 1.475(1), 1.476(1) 1.480(1), 1.480(1)
a For details, see ref. 10. b For details, see ref. 4. c For details, see ref. 8a.
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