Sterically Congested Molecules, 11. The 2,6‐Diisopropylphenyl Substituent at CC and CN Groups

Abstract

Two series of new compounds with sterically shielded, sp2-hybridized C-a atoms are prepared from bromo(chloro)-2,6-diisopropylbenzene. The a-(2,6-diisopropylphenyl) substituent common to all of them is used as an NMR-spectroscopic indicator for the ease of aryl-a single-bond rotation. The corresponding AG* values are >23.8 kcal/mol with methyl or bromine as additional a substituents at a C=C bond, but <8.5 kcal/mol with a-H. The analogous 2,6-diisopropylacetophenone imine has AG* = 18.0 kcaYmol while the N-substituted imines prefer the (Z) conformations with AG* > 23.3 kcaV mol, and AG* = 31.5 kcaYmol for the (Z)-hydrazone. Comparisons of these data with reaction conditions show that chemical transformations are not seriously impeded unless they depend on reagent attack at a K face of the C-a atom in a C = C or C = N group which rotates slowly about the bond to 2,6-diisopropylphenyl.2,6-Dialkylphenyl groups are often used to stabilize sensitive molecules by sterical shieldingI21. With isopropyl as the 2,6 substituents, they can convey additional stereochemical information if displaying diastereotopic (non-equivalent) methyl groups in their 'H-or 13C-NMR ~pectra[~-~]. Thus, the 2,6-diisopropylphenyl moiety may serve as a protective indicator of molecular symmetry; but curiously enough, its application was only rarely reportedL61. This points to synthetic difficulties, caused by steric hindrance whose degree will be exemplified in the present work by reaction conditions and spectroscopy. ResultsBromo-2,6-diisopropylbenzene (2) prepared from the cheaply available 2,6-diisopropylaniline (1) was found to be heavily contaminated by 1,4-dibrom0-2,6-diisopropylbenzene, such that we could never attain the reported['~*] yield of 44%. Treatment of the Grignard derivativeU9] of 2 with acetaldehyde gave the secondary alcohol 3 and the styrene 4. The latter showed enantiotopic (equivalent) methyl groups ('H NMR) even at low temperature and therefore rapid vinyl rotation, since a non-planar equilibrium conformation could be inferred from the weak UV absorptions. The geminal olefinic proton coupling constant 2J = 2.4 Hz is positive, as deduced from computer simulations of the vinyl 'H-NMR spectrum in combination with selective double irradiation (INDOR). The bromine adduct 5 was dehydrobrominated with complete regio-and stereoselectivity to give only the (E)-P-bromostyrene 6; therefore, the abromostyrene 8 had to be prepared from 6 via the alkyne 7 by addition of hydrogen bromide. With a total yield of ca. 1% over more than 6 steps, however, 8 could not be obtained in a suficient amount and purity to serve as a convenient precursor for further transformations.,CHMe2 1-6

Microsolvation, Dimerization, and sp2-Stereoinversion of Monomeric α-(2,6-Diisopropylphenyl)vinyllithium

“…n -Butyllithium ( n- BuLi) destroyed the known , bromoalkene 4 in pentane or benzene solution with formation of LiBr and polymerization. In the presence of small amounts (≥1 equiv) of THF, Et 2 O, or t- BuOMe ( tert -butyl methyl ether), however, the Br/Li interchange reaction (Scheme ) of 4 ( i -Pr = isopropyl) with n- BuLi in cyclopentane yielded up to 80% of the title compound 3Pr along with the “parent” olefin 7a , the lithium arylacetylide 8 , and the 2-arylhex-1-ene 6 that arose through the slow coupling reaction of 3Pr with its byproduct 1-bromobutane ( n- BuBr).…”

“…The exceptionally high barrier in THF contains the energetic effect of the monomer’s enhanced π-charge delocalization that would have to be sacrificed on rotational passage through a coplanar conformation. For comparisons of such π-electronically enhanced barriers with those that are controlled mainly by steric hindrance against this C-α/C-1 rotation, we recall that the olefin 7a rotates fast on the NMR time scale, whereas the rotational barrier of bromoalkene 4 was Δ G ⧧ >24 kcal mol –1 at 172 °C …”

“…A dry NMR tube (5 mm) was charged with the bromoalkene 4 (118 mg, 0.44 mmol), cyclopentane (0.3 mL), and anhydrous Et 2 O (0.050 mL, 0.44 mmol), then cooled to −60 °C under argon gas cover, treated with n- Bu 6 Li (0.50 mmol) in cyclopentane (0.50 mL), and warmed after 15 min. To avoid LiBr formation, the tube was immediately fixed in a nearly horizontal position, cooled to −30 °C, and opened slowly to a dry vacuum line for the cautious evaporation of all volatile compounds at 0.01 mbar with gradual warming to +50 °C during up to 4 h. The tube was then carefully vented with dry argon gas, whereupon the yellow, oily residue was cooled to −30 °C and dissolved in an appropriate solvent (0.70 mL).…”

Reactions of an Aluminum Amide with Aromatic Nitriles: Formation of Al−N−C Frameworks

“…Benzene- d 6 and toluene- d 8 were condensed from Na/benzophenone ketyl and stored in a glovebox. [Me 2 AlNH(Dipp)] 2 , [MeAlN(Dipp)] 3 , 2,6-dialkylbenzonitriles, and TbpCN were prepared using literature procedures. 1 H and 13 C spectra were recorded on Bruker 500 or 400 MHz instruments.…”