THF exchange and molecular dynamics in the series (BDI)MgX(THF), where X = Bu(n), NEt2, and OBu(t) and BDI = 2-[(2,6-diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-2-ene.

The complexes (BDI)MgX(THF), where X = Bu(n), NEt2, and OBu(t), are shown to undergo THF exchange at low added concentrations of THF by a dissociative mechanism: X = Bu(n), ΔH(#) (kcal mol(-1)) = 13.4 ± 0.4 and ΔS(#) (cal mol(-1) K(-1)) = 6.3 ± 1.6; X = NEt2, ΔH(#) (kcal mol(-1)) = 15.2 ± 0.5 and ΔS(#) (cal mol(-1) K(-1)) = 11.4 ± 2.3; X = OBu(t), ΔH(#) (kcal mol(-1)) = 16.4 ± 0.3 and ΔS(#) (cal mol(-1) K(-1)) = 9.5 ± 1.3. The apparent aryl group rotations involving the BDI ligands have, within experimental error, the same activation parameters as the THF dissociation, which suggests that the two are correlated involving a three coordinate reactive intermediate akin to what is well-known for related (BDI)ZnR compounds involving three-coordinate trigonal planar Zn(2+). At higher concentrations of THF for X = Bu(n) and OBu(t), but not for X = NEt2, the coalescence temperatures for apparent aryl group rotation are depressed from those of the pure compounds in toluene-d8, and evidence is presented that this correlates with an associative interchange process which becomes dominant in neat THF. We estimate the Ia mechanism to have activation parameters: ΔH(#) (kcal mol(-1)) = 5.4 ± 0.1 and ΔS(#) (cal mol(-1) K(-1)) = -20.9 ± 0.3 for X = Bu(n) and ΔH(#) (kcal mol(-1)) = 8.3 ± 0.8 and ΔS(#) (cal mol(-1) K(-1)) = -19.8 ± 3.0 for X = OBu(t). For the complex (BDI)MgBu(n)(2-MeTHF), the dissociative exchange with added 2-MeTHF occurs more readily than for its THF analogue, as expected for the more sterically demanding Lewis base O-donor: ΔH(#) (kcal mol(-1)) = 12.8 ± 0.5 and ΔS(#) (cal mol(-1) K(-1)) = 8.6 ± 1.8. At very low temperatures in toluene-d8, restricted rotation about the Mg-O(THF) bond is observed for the complexes where X = NEt2 and OBu(t) but not for the complex where X = Bu(n). These observations, which have been determined from dynamic NMR studies, are correlated with the reactivities of these complexes in solution.

Internally versus externally solvated derivatives of doubly bridged 1,4-dilithio-2-butene: structures and dynamic behavior. A "T" shaped dimeric cluster in the solid state.

X-ray crystallographic NMR and calculational modeling studies using B3LYP/6-311G* of selected dilithium derivatives of the 1,3-butadiene dianion including cis-dilithio-1,4-bis(TMS)-2-butene·(TMEDA)(2)2, internally solvated cis-dilithio-1,4-bis[bis(2-methoxyethyl)aminomethyldimethylsilyl]-2-butene 5, and using only modeling, 1,4-dilithio-2-butene·(TMEDA)(2)9 reveal remarkably similar structural and NMR parameters. In the solid, 5 consists of unusual "T" shaped dynamic clusters. In all three bridging lithiums are sited between 1.8 and 1.9 Å normal to the centroids of opposite faces of the near coplanar of the 2-butene component. Typical bond lengths of the latter are 1.458 ± 0.004, 1.385 ± 0.006, and 1.459 ± 0.003 Å, for C1-C2, C2-C3, and C3-C4, respectively. The (13)C chemical shifts lie within the ranges δ 21 ± 0.5, 99 ± 0.7, 99 ± 0.7 and 21 ± 0.5 for C1, C2 and C3 together, and C4, respectively. Dynamic (13)C NMR provides activation parameters for nitrogen inversion in 2 and 5, overall molecular inversion of 5, and conformational interconversion of 2.

Comparison of an internally coordinated 2-pentenyllithium with its 4-sila analog. Structure and dynamic behavior: unexpected 13C7Li spin coupling.

Four allylic lithium compounds have been prepared with a tethered ligand, (CH(3)OCH(2)CH(2))(2)NCH(2)C(CH(3))(2)-L, attached to a terminal allyl carbon. They are equimolar equilibrium mixtures of 3-endo-L-allyllithium with 3-exo-L-allyllithium, 9en and 9ex, respectively, and (separately) 1-exo-TMS-3-endo-L-allyllithium with 1-exo-TMS-3-exo-L-allyllithium, 13en and 13ex, respectively. Carbon-13 NMR analysis shows that all four compounds are monomeric and internally coordinated, the allyl moiety in each one is partially localized, and (7)Li is spin coupled to both (31)C(1) and (13)C(2) of allyl. NMR line shape changes show that 9en, 9ex, 13en, and 13ex all undergo fast transfer of the lithium-coordinated ligand between faces of the allyl planes at low temperatures. Within each equilibrium system the compound with the exo-tethered ligand inverts faster than its endo analog. All four compounds invert faster in THF-d(10) compared to in diethyl ether-d(10) as solvent. Corresponding Eyring activation parameters are reported. Also, inversion in THF-d(8) for all four compounds is characterized by large negative DeltaS(double dagger) values, whereas in diethyl ether-d(10) DeltaS(double dagger) values for inversion are near neutral. NMR line shape changes due to the dynamics of bimolecular C,Li exchange and rotation around the C(2)-C(3) bonds are just detected via selective line broadening effects above 290 K. These processes are much slower than inversion.

Molecular encapsulation via metal-to-ligand coordination in a Cu(I)-folded molecular basket.

A molecular basket, composed of a semirigid C3v symmetric tris-norbornadiene framework and three pyridine flaps at the rim, has been shown to coordinate to a Cu(I) cation and thereby fold in a multivalent fashion. The assembly was effective (Ka = 1.73 +/- 0.08 x 10(5) M(-1)) and driven by enthalpy (DeltaH(o) = -7.2 +/- 0.1 kcal/mol, DeltaS(o) = -0.25 eu). Variable temperature (1)H NMR studies, assisted with 2D COSY and ROESY investigations, revealed the existence of Cu(I)-folded basket 10b with a molecule of acetonitrile occupying its interior and coordinated to the metal. Interestingly, 10b is in equilibrium with Cu(I)-folded 10a , whose inner space is solvated by acetone or chloroform. The incorporation of a molecule of acetonitrile inside 10a was found to be driven by enthalpy (DeltaH(o) = -3.3 +/- 0.1 kcal/mol), with an apparent loss in entropy (DeltaS(o) = -9.4 +/- 0.4 eu); this is congruent with a complete immobilization of acetonitrile and release of a "loosely" encapsulated solvent molecule during 10a/b interconversion. From an Eyring plot, the activation enthalpy for incorporating acetonitrile into 10a was found to be positive (DeltaH(double dagger) = 6.5 +/- 0.5 kcal/mol), while the activation entropy was negative (DeltaS(double dagger) = -20 +/- 2 eu). The results are in agreement with an exchange mechanism whereby acetonitrile "slips" into an "empty" basket through its side aperture. In fact, DFT (BP86) calculations are in favor of such a mechanistic scenario; the calculations suggest that opening of the basket's rim to exchange guests is energetically demanding and therefore less feasible.

Arylallylic lithium compounds, internally versus externally coordinated: comparison of their structures and dynamic behavior via X-ray crystallography and NMR.

A comparison of externally coordinated arylallyllithiums with internally coordinated arylallyllithiums using a combination of X-ray and NMR studies shows that 2-bis(2-methoxyethyl)aminomethyl-1-phenylallyllithium 7, and 2-bis(2-methoxyethyl)aminomethyl-1,3-diphenylallyllithi um 8, are indeed fully internally coordinated with all phenyls endo, and lithium is close to one terminal allyl carbon. By contrast, among the externally coordinated analogs 1-phenylallyl-lithium.(HMPT)4 5, and 1-phenylallyllithium.(THF) 6, both phenyls are exo and the proximity of lithium to anion was only detected in compound 6. Lithium-7 NMR clearly identifies the Li(HMPT)4+ complex in 5, whereas a 7Li{1H} HOESY experiment reveals 7Li to be coordinated to THF in 6 and close to the PhC terminal allyl carbon. Carbon-13 NMR shows that all of the above compounds are fully delocalized despite differences among the sites of lithium and their separations from the anions. Changes in the 13C NMR line shapes show that the diphenyl compound 8 undergoes a very fast 1,3-lithium sigmatropic shift, and all of the phenyls in the above compounds undergo fast rotation around their phenyl Cipso-Callyl bonds. Barriers to rotation, DeltaH from NMR line shapes for 5, 6, 7, and 8 respectively, are 19.8, 14.6, 10.2, and 8.9 kcal x mol-1. The decrease in barriers is clearly correlated with a decrease in separation of lithium from an allyl terminal carbon and implies that especially among the latter three, 6, 7, and 8, lithium is involved in the mechanism for phenyl rotation. This question is discussed.

Structural distortions and dynamic behavior of the elusive "L"-shaped cis-bicyclo[3.3.0]octenyllithium: X-ray crystallographic and NMR studies.

Substituted cis-bicyclo[3.3.0]octenyllithium prepared by addition of t-BuLi to 3-methylene-1,4-cyclooctadiene in the presence of TMEDA crystallizes as a dimer with one unsolvated Li(+) sandwiched between the external faces of two allyl anions in a triple ion, and external to it the second Li(+) is bidentately complexed to TMEDA, 8. Within each allyl unit, the allyl bonds have different lengths, and all four rings deviate from coplanarity which relieves strain in the rings despite introducing partial localization of the allyl anions. A similar structure prevails in solution as shown by (7)Li NMR and the results of (7)Li{(1)H} HOESY and (1)H, (1)H NOESY experiments. Carbon-13 NMR line shape changes indicate that the system undergoes a fast allyl bond shift concerted with conformation shifts of the out of plane carbons, ca. DeltaG = 9 kcal x mol(-1). Cyclopentyllithium prepared by CH(3)Li cleavage of the trimethylstannyl derivative slowly undergoes an allowed ring opening to pentadienyllithium as well as deprotonating the solvent. The different behavior of dienylic lithium species is attributed to the relative separation of their termini.

Perturbation of conjugation in internally solvated allylic lithium compounds: variation of ligand structure. NMR and X-ray crystallography.

Several allyic lithium compounds were prepared with different potential ligands tethered at C2. These are with CH3OCH2CH2NCH3CH2-, 5 and 1-TMS 6, with (CH3)2NCH2CH2NCH3CH2-, 1-TMS 7, and with ((CH3)2NCH2CH2)2NCH2-, 8 and 1-TMS 9. In all these compounds Li is fully coordinated to the pendant ligand and is sited off the axis perpendicular to the allyl plane at one of the allyl termini as indicated by a combination of X-ray crystallography and NMR spectra. Compounds 5 and 8 are Li-bridged dimers as shown by X-ray crystallography and also dimeric in benzene solution as determined from freezing point determinations. Compounds 6, 7, and 9 are monomeric in THF-d8 or diethyl ether-d10 solution and exhibit one bond 13C1, 6Li scalar coupling at low temperature. Taken together the crystallographic and NMR data indicate that all of these compounds incorporate partially delocalized allylic moieties. Compounds 5 and 8 undergo fast 1,3-Li-sigmatropic shifts that are proposed to take place within low concentrations of monomers in fast equilibrium with prevalent dimers. Averaging with increasing temperature of the one-bond 13C, 6Li coupling constant in 6, 7, and 13 provided the dynamics of bimolecular C-Li exchange with Delta H++ values of 6.7, 12, and 13 kcal x mol(-1), respectively. Averaging of the diastereotopic N(CH3)2 13C resonances of 7 is indicative of fast transfer of coordinated ligand between faces of the allyl plane Delta H++ = 5.3 kcal x mol(-1) combined with slower inversion at nitrogen. Compound 8 exhibits similar effects. It is concluded that variation of the ligand structure changes dynamic behavior of the compounds but has little influence of their degrees of delocalization.

Internally solvated cyclopentadienyllithium compounds: structural integrity of the Cp-Li+ moiety. NMR, dynamics, X-ray crystallography, and calculations.

[Structure: see text]. N(CH2CH2OCH3)2 are as follows: T = CHCH(CH3)2, 6; T = (CH2)2, 10; T = (CH2)3, 14. The results of NOE NMR experiments for 6, 10, and 14 together with X-ray crystallography of 14 support internally coordinated monomeric structures for all three compounds. Models have been constructed for 6, 10, and 14 from modifications of an internally solvated allylic lithium compound at the B3LYP level of theory using basis set 6-311G*. The resulting structural features are very similar to those obtained from the NMR and crystallographic data. In addition, 13C NMR shifts obtained with the GIAO procedure using the results of the B3LYP/6-311G* calculations are closely similar to the experimental shifts, which validate B3LYP as a suitable model for these compounds. The Li+ centroid distance of ca. 1.9 A to 2.0 A obtained for 6, 10, and 14 is common to most crystallographic data for externally solvated Cp-Li+ compounds as well as one which incorporates a (CpLiCp)- triple ion. It is concluded that the ligand tether and the stereochemistry around Li+ accommodate to maintain the structural integrity of Cp-Li+. NMR and crystallography show 14 to be chiral. Carbon-13 NMR line shape changes are attributed to inversion via a lateral wobble mechanism with DeltaH++ = 6 kcal x mol(-1) and DeltaS++ = -2 eu. It is also shown that a 6,6-dimethylfulvene is deprotonated at methyl by LiN(CH2CH2OCH3)3 as well as by butyllithium in the presence of PMDTA producing isopropenyl Cp-Li+ compounds 24 and 25, respectively. NMR line shape changes of the sample containing 24 have been qualitatively interpreted to result from a combination of fast transfer of coordinated ligand between faces of the carbanion plane as well as a lithium-exchange process.

NMR studies of structure and reorganization dynamics of an [6Li]-allylic lithium compound complexed to [14N,15N]-N,N,N',N'-tetramethylethylenediamine: inversion and ligand lithium exchange.

Proton, 13C, 6Li, and 15N NMR line-shape studies of exo,exo-1-trimethylsilyl-3-(dimethylethylsilyl)allyllithium-6Li complexed to [14N,15N]-N,N,N',N'-tetramethylethylenediamine (TMEDA) 2 as a function of temperature and of added diamine reveal the dynamics of three fast equilibrium reorganization processes. These are (with DeltaH values in kilocalories per mole and DeltaS values in entropic units): mutual exchange of lithium between two 2 molecules (6.3, -21), exchange of TMEDA between its free and complexed states (5.0 and -22), and first-order transfer of complexed ligand between the allyl faces (7.0 and -20). Intermediates that are dimeric in TMEDA are proposed for the first two of these reorganization processes.

Perturbation of conjugation in allylic lithium compounds due to stereochemical control of internal lithium coordination: crystallography, NMR, and calculational studies.

Several allylic lithium compounds have been prepared with ligands tethered at C(2). These are with (CH(3)OCH(2)CH(2))(2)NCH(2)-, 6, 1-TMS 5, 1,3-bis(TMS) 8, and 1,1,3-tris(TMS) 9. Allylic lithiums with (CH(3)OCH(2)CH(2))(2)NCH(2)C(CH(3))(2)-, are 10, 1-TMS 11, and 1,3-bis(TMS), 12 compounds with -C(CH(3))(2)CH(2)N-((S)-(2-methoxymethyl)-pyrrolidino) at C(2) 13, 1-TMS 14, and 1,3-bis(TMS) 15. In the solid state, 8-10 and 12 are monomers, 6 and 13 are Li-bridged dimers, and 5 and 7 are polymers. In solution (NMR data), 5, 7-12, 14, and 15 are monmeric, and 6 is a dimer. All samples show lithium to be closest to one of the terminal allyl carbons in the crystal structures and to exhibit one-bond (13)C-(7)Li or (13)C(1)-(7)Li spin coupling, for the former typically ca. 3 Hz and for the latter 6-8 Hz. In every structure, the C(1)-C(2) allyl bond is longer than the C(2)-C(3) bond, and both lie between those for solvated delocalized and unsolvated localized allylic lithium compounds, respectively, as is also the case for the terminal allyl (13)C NMR shifts. Lithium lies 40-70 degrees off the axis perpendicular to the allyl plane at C(1). These effects are variable, so the trend is that the differences between the C(1)-C(2) and C(2)-C(3) bond lengths, (13)delta(3)-(13)delta(1) values, and the (13)C(1)-(7)Li or (13)C-(6)Li coupling constants all increase with decreasing values of the torsional angle that C(1)-Li makes with respect to the allyl plane.

Axial chirality in 1,4-disubstituted (ZZ)-1,3-dienes. Surprisingly low energies of activation for the enantiomerization in synthetically useful fluxional molecules.

Trialkylsilyltrialkylstannes (R(3)Si-SnR'(3)) add to 1,6-diynes in the presence of Pd(0) and tris-pentaflurophenylphosphine to give 1,2-dialkylidenecyclopentanes with terminal silicon and tin substituents. The (ZZ)-geometry of these s-cis-1,3-dienes, resulting from the organometallic reaction mechanisms involved, forces the silicon and tin groups to be nonplanar, thus making the molecules axially chiral. There is rapid equilibration between the two helical forms at room-temperature irrespective of the size of the Si and Sn substituents. However, the two forms can be observed by 1H, 13C, and 119Sn NMR spectroscopy at low temperature. The rates of enantiomerization, which depend on the Si and Sn substituents, and the substitution pattern of the cylopentane ring can be studied by dynamic NMR spectroscopy using line shape analysis. The surprisingly low energies of activation (DeltaG++ = 52-57 kJ mol(-1)) for even the bulky Si and Sn derivatives may be attributed to a widening of the exo-cyclic bond-angles of the diene carbons.

Dynamic helical chirality of an intramolecularly hydrogen-bonded bisoxazoline.

The synthesis and conformational properties of 2,6-bis-[2-((4S)-4-methyl-4,5-dihydro-1,3-oxazol-2-yl)phenyl]carbam oylpyridines, 2, have been described. Bisoxazoline 2a was prepared in five steps from 2-nitrobenzoyl chloride in an overall yield of 71%. In contrast to related structures such as 1, bisoxazoline 2a exhibits a highly biased P-type helical conformation in solution and in the solid state. In the crystal lattice, 2a further assembles into a left-handed helical superstructure aligned along the crystallographic c axis. The barrier to helical interconversion, as measured by line-shape analysis of the temperature-dependent (1)H NMR spectra of thiobenzyl derivative 2b, was determined to be quite low ((Delta)G(++) = 12.3 kcal/mol), indicating the presence of a highly dynamic helical chirality.