Nuclear magnetic resonance spectra of organogermanium compounds. Part 4. Nuclear magnetic resonance spectra and molecular mechanics calculations of germacyclohexane, methylgermacyclohexanes, and dimethylgermacyclohexanes

Abstract

Molecular mechanics calculations in germacyclohexane and methyl-substituted germacyclohexanes reveals that (i) the ring is flattened around the germanium and that (ii) a methyl group on germanium has a slight preference for the axial conformation rather than the equatorial, which is in line with the results for the corresponding silacyclohexanes. The 13C and 73Ge chemical shifts of these germacyclohexanes are consistent with the calculations. Thus, both 13C and 73Ge data indicate that I -m e t h y l -Igermacyclohexane is a ca. 60: 40 mixture of axial and equatorial isomers. Attempts to observe directly the two conformers b y freezing the inversion on the n.m.r. time scale failed because of the very lower barrier t o inversion.Though the chemistry of organogermanium compounds with tetravalent germanium has been described,24 the structural details of these compounds are much less investigated. We have initiated a study to cast light upon this unexplored field chiefly by n.m.r. (including 73Ge n.m.r.) spectroscopy ' 9 ' y 6 and molecular mechanics calculations. Germacyclohexane (1G) is a key compound in organogermanium chemistry and its role is exactly the same as that of R pJ\R 1 1 4 6 M e X = C c y c l o h e x a n e s ( C ) X = Si s i l a c y c l o h e x a n e s ( S ) X = Ge germacyclohexanes (GI cyclohexane (1C) in the chemistry of carbocyclic compounds.Crucial features of the chemistry of (IC) are associated with the stereochemistry of the ring, e.g., ring reversal and its barrier, 1,3-diaxial interactions or relative stabilities of equatorial and axial conformers of monosubstituted species. Although these are fully documented for (lC),' the equivalent has not been reported for (W.The situation is slightly better for silacyclohexane (1s) and derivatives. Thus, Ouellette 13 reported molecular mechanics calculations of (lS), 1-methyl-(2S), 4-methyl-(4S), and 1,ldimethyl-1-silacyclohexane (5S), and found that the axial isomer of (2s) is slightly more favourable than the equatorial (by 0.20 kcal mol-l), while Jensen and Bushweller attempted to determine the barrier height of ring reversal of (5S), which was found in the range of 5.25-5.75 kcal mol-'.Carleer and Anteunis l o reported that the conformational energy, i.e., the free energy difference between the axial and equatorial conformers, as determined from the spin-spin coupling constant for the methyl group of (2S), is 0.34 kcal mol-' in favour of the axial position, in agreement with the prediction of molecular mechanics calculations.*The purpose of this study is to obtain stereochemical information on the hitherto unexplored (1G) and its methyl derivatives by means of n.m.r. spectroscopy and molecular mechanics calculations. Combined with the corresponding information for (1s) and (lC), it is expected to open a new field of chemistry of the heteracyclohexanes of Group IVB elements.For this purpose (IG), 1-, 3-, and 4-methyl-1-germacyclohexane (2G)--(4G), and 1,l-, 1,3-, and 1,4-dimethyl-l-germacyclohexane (5G)-(7G)were prepared, and their I3C and 73Ge n.m.r. s...

Structure of tetraspiro[1,3,5,7-tetraoxa-2,4,6,8-tetragermacyclooctane-2,1':4,1'':6,1''':8,1''''-tetrakisgerminane]

“…The C---Ge bond lengths [1.929 (5)-1.942 (7) A] are within the range of C(sp3)--Ge bond lengths [1.933 (6)-1.967 (3)A] previously reported for germacyclanes (Faucher, Mazerolles, Jaud & Galy, 1978;Preut, Krcher & Neumann, 1985;Sommese, Cremer, Campbell & Thompson, 1990). All the C---C--C angles (2) (Takeuchi, Shimoda, Tanaka, Tomoda, Ogawa & Suzuki, 1988), except for the bond angle Cring-----Gc ¢-~ for "~ring, which the calculation gave a significantly smaller value (101.9°). This disagreement seems to be reasonable, because the calculation was not carried out for (1) but for (2).…”

“…1 and 2. There are only relatively minor differences between the two crystal- solution (Takeuchi, Shimoda, Tanaka, Tomoda, Ogawa & Suzuki, 1988). To obtain the precise molecular geometry of the germacyclohexane ring, which is not contained in the Cambridge Structural Database files, we carried out an X-ray crystallographic analysis of the title compound (1) at 142 K.…”

G73e Nuclear Magnetic Resonance Spectroscopy of Germanium Compounds

“…Takeuchi et al published a series of thirteen papers in the 1980s that focused on the 73 Ge NMR of numerous germanes and correlated their spectral data with those of the analogous silicon and tin compounds [48][49][50][60][61][62][63][64][65][66][67][68][69]. Molecular mechanics calculations were also used to further explain the experimental data.…”

Stereochemistry and Conformation of Organogermanium, Organotin and Organolead Compounds

“…Takeuchi and coworkers prepared a series of methyl and phenyl substituted germacyclohexanes to evaluate conformational preferences of these substituents (Scheme 17) 35,36 . Based on analysis of 13 C NMR spectra and molecular mechanics calculations, they concluded that a C-methyl prefers the equatorial orientation by ca 1.4kcal mol −1 but the Ge-methyl substituent actually shows a slight intrinsic preference for the axial orientation ( Figure 9).…”

“…Enantioenriched α-alkoxyorganolead compounds have been prepared through lithiation of stannane precursors and trapping of the lithiated species with a triorganolead halide (equation 35) 75 . In the presence of TiCl 4 , these plumbanes add to aldehydes to afford monoprotected syn-1,2-diols.…”