Abstract:Synthesis of tetrakis(cyclopropylmethyl)methane, a new symmetric product has been described using the radical mediated gem-diallylation of cyclopropylmethyl xanthate as a key step and its single crystal X-ray analysis established its C2-symmetry.
“…Such a feature indicates the existence of a single conformer having diastereotopic methylene hydrogens, due to the CH 2 −C q rotation being slow in the NMR time scale at such a low temperature. This also implies that the conformer has S 4 symmetry, in agreement with the crystal structure shape, whose C 2 static symmetry becomes S 4 with fast cyclopropyl−CH 2 bond rotation in solution. Indeed our ab initio calculations (see further) predict that the T3 -type conformer of Scheme is more stable than all the other five possible conformers of 1e , and the corresponding computed structure reported in Figure appears almost identical with the experimental one.…”
Section: Resultssupporting
confidence: 76%
“…On the right is displayed the simulation obtained with the rate constants shown. 4 X-ray diffraction (top) and ab initio computed (bottom) structures of the most stable conformer T3 of compounds 1e and 1f . The crystallographic data of 1f are reported in the Experimental Section, for those of 1e see ref .…”
Section: Resultsmentioning
confidence: 99%
“…Those adopting a T1 conformation are 1 , X = Br, , Cl, , CH 2 CH 2 Br, or CHCH 2 . Those adopting a T3 conformation are 1 , X = CH 2 Br, cyclopropyl, and COOH . There are in addition eight examples of Pentaerythritol derivatives 1 , X = OR, of which six have a T1 structure and two have a T3 structure …”
Variable-temperature NMR studies of tetraethylmethane (1a), tetrapropylmethane (1b), tetrachloromethylmethane (1c), tetrabromomethylmethane (1d), tetracyclopropylmethylmethane (1e), and tetrabenzylmethane (1f) show a range of dynamic behavior. Separate signals for two types of conformation are observed for 1a, 1c, and 1d at low temperatures, with more than 95% of the molecules in a time-averaged D2d conformation, and the S4 conformation as the minor populated alternative. Compound 1e populates only S4-type conformations but equilibrates slowly between degenerate versions of these at low temperatures. Compounds 1b and 1f show a temperature-dependent spectrum but the low-temperature limit spectrum could not be observed. Ab initio calculations agree well with experiment on the conformational equilibria and suggest in particular that compounds 1b and 1f behave similarly to compounds 1a and 1e, respectively. A crystal structure of compound 1f is reported.
“…Such a feature indicates the existence of a single conformer having diastereotopic methylene hydrogens, due to the CH 2 −C q rotation being slow in the NMR time scale at such a low temperature. This also implies that the conformer has S 4 symmetry, in agreement with the crystal structure shape, whose C 2 static symmetry becomes S 4 with fast cyclopropyl−CH 2 bond rotation in solution. Indeed our ab initio calculations (see further) predict that the T3 -type conformer of Scheme is more stable than all the other five possible conformers of 1e , and the corresponding computed structure reported in Figure appears almost identical with the experimental one.…”
Section: Resultssupporting
confidence: 76%
“…On the right is displayed the simulation obtained with the rate constants shown. 4 X-ray diffraction (top) and ab initio computed (bottom) structures of the most stable conformer T3 of compounds 1e and 1f . The crystallographic data of 1f are reported in the Experimental Section, for those of 1e see ref .…”
Section: Resultsmentioning
confidence: 99%
“…Those adopting a T1 conformation are 1 , X = Br, , Cl, , CH 2 CH 2 Br, or CHCH 2 . Those adopting a T3 conformation are 1 , X = CH 2 Br, cyclopropyl, and COOH . There are in addition eight examples of Pentaerythritol derivatives 1 , X = OR, of which six have a T1 structure and two have a T3 structure …”
Variable-temperature NMR studies of tetraethylmethane (1a), tetrapropylmethane (1b), tetrachloromethylmethane (1c), tetrabromomethylmethane (1d), tetracyclopropylmethylmethane (1e), and tetrabenzylmethane (1f) show a range of dynamic behavior. Separate signals for two types of conformation are observed for 1a, 1c, and 1d at low temperatures, with more than 95% of the molecules in a time-averaged D2d conformation, and the S4 conformation as the minor populated alternative. Compound 1e populates only S4-type conformations but equilibrates slowly between degenerate versions of these at low temperatures. Compounds 1b and 1f show a temperature-dependent spectrum but the low-temperature limit spectrum could not be observed. Ab initio calculations agree well with experiment on the conformational equilibria and suggest in particular that compounds 1b and 1f behave similarly to compounds 1a and 1e, respectively. A crystal structure of compound 1f is reported.
“…Unlike in the cases of 14-17, only a single conformer appears to be populated, a result consistent with ab initio [RHF 6-31G(d) level] computations predicting that the energy difference with respect to the second most stable conformer T1 is as large as 1.34 kcal mol -1 . The structure derived from the NMR spectroscopic data and from calculations very closely matches that determined by X-ray diffraction in the solid state [88] (see Figure S- Variable-temperature NMR spectroscopy was capable of detecting restricted rotation about the sp 2 -sp 3 bonds in hindered aryl carbinols [89] and of measuring the corresponding barriers. Pairs of stereolabile atropisomers with different populations were observed with aryl ring lacking twofold symmetry axes.…”
Dynamic NMR spectroscopy can determine energy barriers due to internal motion over the range of about 4.5-23 kcal mol -1 . Conformational analysis of the frozen conformations can be simulated and interpreted by reliable theoretical calculations based mainly on density functional theory (DFT). The same calculations can identify transition states
“…The dynamics of the conformational changes of these highly congested hydrocarbons have been studied by NMR spectroscopy . Tetrakis(cyclopropylmethyl)methane ( 498 ) was synthesized along the same lines as tetracyclopropylmethane ( 496 ) 79 …”
Section: Branched Aggregates Of Three-membered
Ringsmentioning
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