Ab initio and experimental studies on the hetero-Diels-Alder and cheletropic additions of sulfur dioxide to (E)-1-methoxybutadiene: a mechanism involving three molecules of SO(2)

Kinetics on the cheletropic addition of sulfur dioxide to (E)-1-methoxybutadiene (1) to give the corresponding sulfolene 2 (2-methoxy-2,5-dihydrothiophene-1,1-dioxide) gave the rate law d[2]/dt = k[1][SO(2)](x)() with x = 2.6 +/- 0.2 at 198 K. Under these conditions, no sultine 3 [(2RS,6RS)-6-methoxy-3,6-dihydro-1,2-oxathiin-2-oxide] resulting from a hetero-Diels-Alder addition was observed, and the cheletropic elimination 2 --> 1 + SO(2) did not occur. Ab initio and DFT quantum calculations confirmed that the cheletropic addition 1 + SO(2) --> 2 follows two parallel mechanisms, one involving two molecules of SO(2) and the transition structure with DeltaG(++) = 18.2 +/- 0.2 kcal/mol at 198 K (exptl); 22.5-22.7 kcal/mol [B3LYP/6-31G(d,p)], the other one involving three molecules of SO(2) with DeltaG(++) = 18.9 +/- 0.1 kcal/mol at 198 K (exptl); 19.7 kcal/mol [B3LYP/6-31G(d,p)]. The mechanism involving only one molecule of SO(2) in the transition structure requires a higher activation energy, DeltaG(++) = 25.2 kcal/mol [B3LYP/6-31G(d,p)]. Comparison of the geometries and energetics of the structures involved into the 1 + SO(2) --> 2, 3 and 1 + 2SO(2) --> 2, 3 + SO(2) reactions obtained by ab initio and DFT methods suggest that the latter calculation techniques can be used to study the cycloadditions of sulfur dioxide. The calculations predict that the hetero-Diels-Alder addition 1 + SO(2) --> 3 also prefers a mechanism in which three molecules of SO(2) are involved in the cycloaddition transition structure. At 198 K and in SO(2) solutions, the entropy cost (TDeltaS(++)) is overcompensated by the specific solvation by SO(2) in the transition structures of both the cheletropic and hetero-Diels-Alder reactions of (E)-1-methoxybutadiene with SO(2).     

Twin-rotor system created on a [4]radialene frame

[reaction: see text] Three extended [4]radialenes with two tricyclic rings connected with exocyclic butatriene units have been synthesized. The compounds, possessing thioxanthene and dihydroanthracene moieties as the terminal substituents, show a fast rotation around the butatriene bonds at ambient temperatures (DeltaG() = 13.7 and 14.9 kcal/mol, respectively). In contrast, the fluorene-substituted analogue shows a much higher rotational barrier (DeltaG() = 17.8 kcal/mol) of the butatriene bonds due to the reduced steric repulsion between the two fluorene moieties at the ground state.     

Synthesis and Structure of [Tp(Bu)()t()2]In, a Highly Twisted [Tris(3,5-di-tert-butylpyrazolyl)hydroborato]indium(I) Complex: Comparison with the Re-Evaluated Ordered Structure of [Tp(Bu)()t]In

The indium(I) complex [Tp(Bu)()t()2]In ([Tp(Bu)()t()2] = tris(3,5-di-tert-butylpyrazolyl)hydroborato), synthesized by the reaction of [Tp(Bu)()t()2]Na with InCl, exhibits a structure in which the [Tp(Bu)()t()2] ligand adopts a highly twisted configuration due to steric interactions of the tert-butyl substituents in the 5-positions of the pyrazolyl groups. In contrast, the absence of 5-tert-butyl substituents allows the pyrazolyl groups in [Tp(Bu)()t]In to be coplanar with their respective In-N-N-B planes. The structure of [Tp(Bu)()t]In has been previously reported but was noted to exhibit an unusual type of disorder in which a nitrogen atom of one molecule was coincident with the boron atom of its disordered configuration [Dias, H. V. R.; Huai, L.; Jin, W.; Bott, S. G. Inorg. Chem. 1995, 34, 1973-1974]. In view of the unusual nature of the disorder, which involved both a 2-fold rotation and a canting of the molecule, the disordered structure of [Tp(Bu)()t]In was re-evaluated. Significantly, an ordered structure of [Tp(Bu)()t]In was obtained. The disorder present in the previously reported structure is a consequence of adopting a space group with unnecessarily high symmetry. Thus, [Tp(Bu)()t]In provides an example where the structure is much better described as ordered in a noncentrosymmetric space group, rather than disordered in the centrosymmetric alternative. [Tp(Bu)()t()2]In is monoclinic, of space group P2(1)/c (No. 14), with a = 18.781(9) ?, b = 10.380(2) ?, c = 20.849(6) ?, beta = 112.76(3) degrees, and Z = 4. [Tp(Bu)()t]In is orthorhombic, of space group Cmc2(1) (No. 36), with a = 16.193(3) ?, b = 15.214(3) ?, c = 9.963(3) ?, and Z = 4.     

Conformational Studies by Dynamic NMR. 57.(1) Stereodynamics of Syn-Anti Interconversion of Disubstituted Acyl Durenes

The orthogonal syn and anti isomers, originated by the restricted rotation about the Ar-C(O)Bu(t) single bonds in 1,4-bis(2,2-dimethylpropanoyl)durene (2e), have been separated by preparative thin layer chromatography. In solution they reach an equilibrium where the syn-anti ratio depends upon the polarity of the solvent. This allowed us to assign the anti structure, which has a null dipole moment, to the least retained isomer. The free energy of activation (DeltaG) for the interconversion was found to be 22.5 kcal mol(-)(1), a value high enough for identifying these species as configurational isomers. When less hindered derivatives, also having two RCO (R = Pr(i), Et, Me) substituents in the positions 1,4 of the durene moiety, were examined, the syn and anti forms could be detected only at low temperature by means of NMR spectroscopy. The corresponding interconversion barriers (DeltaG = 13.4, 11.7, 10.9 kcal mol(-)(1), respectively) are, in fact, much lower than for R = Bu(t), indicating that in these cases we are dealing with conformational rather than with configurational isomers.     

Conformational studies by dynamic NMR. 74.(1) stereomutations of the conformational enantiomers in peri-substituted 1-acylnaphthalenes

Naphthalenes bearing an acyl and a phenyl group in a peri relationship give rise to a pair of enantiomers in the temperature range where the rotations of the acyl group are slow. Such enantiomers were observed by means of low temperature NMR spectra in chiral environments. The barrier to rotation for the acyl substituents, that causes the interconversion of the enantiomers, was demonstrated to be lower than that for the phenyl group. In an appropriately synthesized derivative it was possible to measure the two barriers that were found equal to 10.4 and 15.9 kcal mol(-)(1), respectively. The barriers for the acyl group rotation increase regularly (from 9.5 to 13.2 kcal mol(-)(1)) with the increasing dimension of the RCO groups (R = Me, Et, Pr(i), Bu(t)). When a bromine atom replaces the phenyl group, the enantiomerization barrier for the corresponding acyl derivatives increases significantly.     

Intramolecular Photocycloaddition of Unsaturated Isoquinuclidines. Synthesis of 2-Azatetracyclo[4.0.0.(4,9)0(7,10)]decanes and 3-Azatetracyclo[6.1.1.0.(2,7)0(5,9)]decanes

2-Azabicyclo[2.2.2]oct-5-enes bearing an endo alkenyl substituent were synthesized by Diels-Alder addition of methyl vinyl ketone to a 1,6-dihydropyridine derived from methyl nicotinate. Although 1,5-dienes with this skeleton were unreactive under thermal conditions, they were photochemically reactive. Irradiation of these dienes through a Corex filter resulted in intramolecular [2 + 2] cycloaddition to give "parallel" and "crossed" photoadducts along with small amounts of a hexahydroisoquinoline. The latter is thought to represent leakage of a diradical intermediate responsible for the parallel photoadduct. The new 2-azatetracyclo[4.4.0.0.(4,9)0(7,10)]decane and 3-azatetracyclo[6.1.1.0.(2,7)0(5,9)]decane structures formed in the photochemical reactions are thermally stable.     

Nature of the Rh-H(2) Bond in a Dihydrogen Complex Stabilized Only by Nitrogen Donors. Inelastic Neutron Scattering Study of Tp(Me)2RhH(2)(eta(2)-H(2)) (Tp(Me)2 = Hydrotris(3,5-dimethylpyrazolyl)borate)

The tunnel splitting of the librational ground state and the torsional frequencies of the dihydrogen ligand in Tp(Me)()2RhH(2)(eta(2)-H(2)) (Tp(Me)()2 = hydrotris(3,5-dimethylpyrazolyl)borate) were measured using inelastic neutron scattering spectroscopy. The barrier for the rotation of the H(2) ligand and its H-H separation, calculated from these data, are 0.56(2) kcal/mol and 0.94 ?, respectively. These values indicate that pi-back-donation from the Tp(Me)()2RhH(2) fragment to H(2) is relatively weak and/or the interaction between the coordinated dihydrogen molecule and the two cis-hydride ligands significantly lowers the barrier for H(2) rotation.     

Mechanism of homogeneous Ir(III) catalyzed regioselective arylation of olefins

The mechanism of hydroarylation of olefins by a homogeneous Ph-Ir(acac)(2)(L) catalyst is elucidated by first principles quantum mechanical methods (DFT), with particular emphasis on activation of the catalyst, catalytic cycle, and interpretation of experimental observations. On the basis of this mechanism, we suggest new catalysts expected to have improved activity. Initiation of the catalyst from the inert trans-form into the active cis-form occurs through a dissociative pathway with a calculated DeltaH(0 K)() = 35.1 kcal/mol and DeltaG(298 K)() = 26.1 kcal/mol. The catalytic cycle features two key steps, 1,2-olefin insertion and C-H activation via a novel mechanism, oxidative hydrogen migration. The olefin insertion is found to be rate determining, with a calculated DeltaH(0 K)() = 27.0 kcal/mol and DeltaG(298 K)() = 29.3 kcal/mol. The activation energy increases with increased electron density on the coordinating olefin, as well as increased electron-donating character in the ligand system. The regioselectivity is shown to depend on the electronic and steric characteristics of the olefin, with steric bulk and electron withdrawing character favoring linear product formation. Activation of the C-H bond occurs in a concerted fashion through a novel transition structure best described as an oxidative hydrogen migration. The character of the transition structure is seven coordinate Ir(V), with a full bond formed between the migrating hydrogen and iridium. Several experimental observations are investigated and explained: (a) The nature of L influences the rate of the reaction through a ground-state effect. (b) The lack of beta-hydride products is due to kinetic factors, although beta-hydride elimination is calculated to be facile, all further reactions are kinetically inaccessible. (c) Inhibition by excess olefin is caused by competitive binding of olefin and aryl starting materials during the catalytic cycle in a statistical fashion. On the basis of this insertion-oxidative hydrogen transfer mechanism we suggest that electron-withdrawing substituents on the acac ligands, such as trifluoromethyl groups, are good modifications for catalysts with higher activity.     

Thermodynamics of Inter- and Intramolecular Hydrogen Bonding in (Diphosphine)platinum(II) Diolate Complexes and Its Role in Carbohydrate Complexation Regioselectivity

Thermodynamic data are reported for intermolecular hydrogen-bonding association of 1 and 2 equiv of phenol with [1,3-bis(diphenylphosphino)propane](phenylethane-1,2-diolato)platinum(II) ((dppp)Pt(Ped)) in dichloromethane solution: = -7.0 +/- 0.1 kcal/mol, = -7.7 +/- 0.4 kcal/mol, = -11.3 +/- 0.4 eu, and = -17.8 +/- 1.2 eu. For comparison, the thermodynamics for hydrogen bonding of phenol to triphenylphosphine oxide in dichloromethane were also determined: DeltaH degrees = -5.1 +/- 0.3 kcal/mol; DeltaS degrees = -8.8 +/- 1.0 eu. Competitive coordination exchange reactions have been used to determine the apparent intramolecular hydrogen bond strengths in (dppp)Pt(1,2-O,O'-glycerolate) and (dppp)Pt(1,2-O,O'-butane-1,2,4-triolate) in both dichloromethane (DeltaG(313) = -2.05 +/- 0.05 and -2.52 +/- 0.06 kcal/mol, respectively) and pyridine (DeltaG(313) = -0.62 +/- 0.03 and -0.82 +/- 0.03 kcal/mol, respectively). Based on these findings, the role of hydrogen-bonding interactions in determining the regioselectivities of complexation of carbohydrates to diphosphine Pt(II) is discussed.     

Structure, Conformation, and Stereodynamics of N-Nitroso-2,4-diaryl-3-azabicyclo[3.3.1]nonanes and N-Nitroso-2,4-diaryl-3-azabicyclo[3.3.1]nonan-9-ones(1)

The variable temperature (1)H, (13)C, and (19)F NMR spectra were measured for the title N-nitrosamines. The observed unusually low N-N rotation barriers (12-15 kcal/mol) result from a significant deviation of the nitrosamino system from planarity. A pyramidal character of the amino nitrogen was confirmed by the X-ray crystal structures of two compounds and by bathochromic shifts of the n-pi absorption bands in the UV spectra. The nonplanarity of the nitrosamino moiety is due to the strong pseudoallylic A((1,3)) strain caused by the steric interaction of the NNO group with the neighboring aryl substituents fixed in the equatorial positions of the bicyclic skeleton. In addition, the barriers to the C-C rotation of aryl groups were examined at temperatures lower than required to "freeze" the N-N rotation and different DeltaG() values were observed for the aryls oriented syn and anti to the nitroso oxygen.     

On the viability of small endohedral hydrocarbon cage complexes: X@C4H4, X@C8H8, X@C8H14, X@C10H16, X@C12H12, AND X@C16H16

Small hydrocarbon complexes (X@cage) incorporating cage-centered endohedral atoms and ions (X = H(+), H, He, Ne, Ar, Li(0,+), Be(0,+,2+), Na(0,+), Mg(0,+,2+)) have been studied at the B3LYP/6-31G(d) hybrid HF/DFT level of theory. No tetrahedrane (C(4)H(4), T(d)()) endohedral complexes are minima, not even with the very small hydrogen atom or beryllium dication. Cubane (C(8)H(8), O(h)()) and bicyclo[2.2.2]octane (C(8)H(14), D(3)(h)()) minima are limited to encapsulating species smaller than Ne and Na(+). Despite its intermediate size, adamantane (C(10)H(16), T(d)()) can enclose a wide variety of endohedral atoms and ions including H, He, Ne, Li(0,+), Be(0,+,2+), Na(0,+), and Mg(2+). In contrast, the truncated tetrahedrane (C(12)H(12), T(d)()) encapsulates fewer species, while the D(4)(d)() symmetric C(16)H(16) hydrocarbon cage (see Table of Contents graphic) encapsulates all but the larger Be, Mg, and Mg(+) species. The host cages have more compact geometries when metal atoms, rather than cations, are inside. This is due to electron donation from the endohedral metals into C-C bonding and C-H antibonding cage molecular orbitals. The relative stabilities of endohedral minima are evaluated by comparing their energies (E(endo)) to the sum of their isolated components (E(inc) = E(endo) - E(cage) - E(x)) and to their exohedral isomer energies (E(isom) = E(endo) - E(exo)). Although exohedral binding is preferred to endohedral encapsulation without exception (i.e., E(isom) is always exothermic), Be(2+)@C(10)H(16) (T(d)(); -235.5 kcal/mol), Li(+)@C(12)H(12) (T(d)(); 50.2 kcal/mol), Be(2+)@C(12)H(12) (T(d)(); -181.2 kcal/mol), Mg(2+)@C(12)H(12) (T(d)(); -45.0 kcal/mol), Li(+)@C(16)H(16) (D(4)(d)(); 13.3 kcal/mol), Be(+)@C(16)H(16) (C(4)(v)(); 31.8 kcal/mol), Be(2+)@C(16)H(16) (D(4)(d)(); -239.2 kcal/mol), and Mg(2+)@C(16)H(16) (D(4)(d)(); -37.7 kcal/mol) are relatively stable as compared to experimentally known He@C(20)H(20) (I(h)()), which has an E(inc) = 37.9 kcal/mol and E(isom) = -35.4 kcal/mol. Overall, endohedral cage complexes with low parent cage strain energies, large cage internal cavity volumes, and a small, highly charged guest species are the most viable synthetic targets.     

The syntheses of pyrazino-containing sultines and their application in diels-alder reactions with electron-poor olefins and

The Diels-Alder reactions of heterocyclic o-quinodimethanes, generated in situ from 6,7-disubstituted quinoxalino[2,3-d]-[1, 2lambda(4)]oxathiine 2-oxides (6a-c), 2,3-disubstituted-8, 9-dihydro-6H-8lambda(4)-[1,2]oxathiino[4,5-g]quinoxalin-8-one (7a-c) (sultines), and pyrazinosultine (22), with electron-poor olefins and [60]fullerene are described. The heterocyclic-fused sultines 7a-c and 22 are readily prepared from the corresponding dibromides 9a-c and 24 with the commercially available Rongalite (sodium formaldehyde sulfoxylate). When heated in the presence of electron-poor dienophiles and [60]fullerene, all of the sultines underwent extrusion of SO(2), and the resulting heterocyclic o-quinodimethanes (3a-d, 4a-c, and 25) were intercepted as the 1:1 adducts in good to excellent yields. The temperature-dependent (1)H NMR spectra of fullerene derivatives 31-38 show a dynamic process for the methylene protons. The activation free energies (DeltaG(c)()) determined for the boat-to-boat inversion of these pyrazino-containing C(60) compounds (31-34 and 38) are found to be in the range of 14.1-14.8 kcal/mol, but they are in the range of 15. 2 to >17.1 kcal/mol for adducts 35-37. The activation free energies (DeltaG(c)()) are significantly affected by (1) the orientations and (2) the substituents of the quinoxaline rings and (3) the extended benzannulation in the arenes of C(60) adducts (see Table 2), which implies that both electronic interactions and steric effects between the aromatic addends and C(60) are important. Tautomerization of methylquinoxaline to its enamine is invoked as a rationalization for the lowering of DeltaG(c)() in some of the fulleroadducts.     

Addition of Pt(PBu(t)(3)) groups to Ru(5)(CO)(12)(eta(6)-C(6)H(6))(mu(5)-C). Synthesis, structures, and dynamical activity

The reaction of Ru(5)(CO)(12)(eta(6)-C(6)H(6))(mu(5)-C), 7, with Pt(PBu(t)(3))(2) yielded two products Ru(5)(CO)(12)(eta(6)-C(6)H(6))(mu(6)-C)[Pt(PBu(t)(3))], 8, and Ru(5)(CO)(12)(eta(6)-C(6)H(6))(mu(6)-C)[Pt(PBu(t)(3))](2), 9. Compound 8 contains a Ru(5)Pt metal core in an open octahedral structure. In solution, 8 exists as a mixture of two isomers that interconvert rapidly on the NMR time scale at 20 degrees C, DeltaH() = 7.1(1) kcal mol(-1), DeltaS() = -5.1(6) cal mol(-)(1) K(-)(1), and DeltaG(298)(#) = 8.6(3) kcal mol(-1). Compound 9 is structurally similar to 8, but has an additional Pt(PBu(t)(3)) group bridging an Ru-Ru edge of the cluster. The two Pt(PBu(t)(3)) groups in 9 rapidly exchange on the NMR time scale at 70 degrees C, DeltaH(#) = 9.2(3) kcal mol(-)(1), DeltaS(#) = -5(1) cal mol(-)(1) K(-)(1), and DeltaG(298)(#) = 10.7(7) kcal mol(-1). Compound 8 reacts with hydrogen to give the dihydrido complex Ru(5)(CO)(11)(eta(6)-C(6)H(6))(mu(6)-C)[Pt(PBu(t)(3))](mu-H)(2), 10, in 59% yield. This compound consists of a closed Ru(5)Pt octahedron with two hydride ligands bridging two of the four Pt-Ru bonds.     

Preparation and reactivity of [D3d]-octahedrane: the most stable (CH)12 hydrocarbon

The synthesis of the (CH)12 hydrocarbon [D(3d)]-octahedrane (heptacyclo[6.4.0.0(2,4).0(3,7).0(5,12).0(6,10).0(9,11)]dodecane) 1 and its selective functionalization retaining the hydrocarbon cage is described. The B3LYP/6-311+G* strain energy of 1 is 83.7 kcal mol(-1) (4.7 kcal mol(-1) per C-C bond) which is significantly higher than that of the structurally related (CH)16 [D(4d)]-decahedrane 2 (75.4 kcal mol(-1); 3.1 kcal mol(-1) per C-C bond) and (CH)20 [I(h)]-dodecahedrane 3 (51.5 kcal mol(-1); 1.7 kcal mol(-1) per C-C bond); the heats of formation for 1-3 computed according to homodesmotic equations are 52, 35, and 4 kcal mol(-1). Catalytic hydrogenation of 1 leads to consecutive opening of the two cyclopropane rings to give C2-bisseco-octahedrane (pentacyclo[6.4.0.0(2,6).0(3,11).0(4,9)]dodecane) 16 as the major product. Although 1 is highly strained, its carbon skeleton is kinetically quite stable: Upon heating, 1 does not decompose until above 180 degrees C. The B3LYP/6-31G* barriers for the S(R)2 attack of the tBuO. and Br3C. radicals on a carbon atom of one of the cyclopropane fragments (Delta(298) = 27-28 kcal mol(-1)) are higher than those for hydrogen atom abstraction. The latter barriers are virtually identical for the abstraction from the C1-H and C2-H positions with the tBuO. radical (DeltaG(298) = 17.4 and 17.9 kcal mol(-1), respectively), but significantly different for the reaction at these positions with the Br3C. radical (DeltaG(298) = 18.8 and 21.0 kcal mol(-1)). These computational results agree well with experiments, in which the chlorination of 1 with tert-butyl hypochlorite gave a mixture of 1- and 2-chlorooctahedranes (ratio 3:2). The bromination with carbon tetrabromide under phase-transfer catalytic (PTC) conditions (nBu4NBr/NaOH) selectively gave 1-bromooctahedrane in 43 % isolated yield. For comparison, the PTC bromination was also applied to 2,4-dehydroadamantane yielding 54 % 7-bromo-2,4-dehydroadamantane.     

Conformational equilibria of trans -3-X-cyclohexanols (X = Cl, Br, CH3 and OCH3). A low temperature NMR study and theoretical calculations

The integration of 1H and 13C NMR spectra, at - 90 degrees C in CS2/CD2Cl2 (9:1), for the trans-3-chlorocyclohexanol (1), trans-3-bromocyclohexanol (2), and trans-3-methoxycyclohexanol (4) showed that the equatorial-axial (ea) conformer occurs as ca 63, 63, and 69% in the conformational equilibrium, respectively. This corresponds to the following DeltaG(ea-ae) values (from (1)H spectrum): - 0.32 +/- 0.01, - 0.32 +/- 0.04, - 0.48 +/- 0.05 kcal mol(-1); and to (from 13C spectrum): - 0.31 +/- 0.04, - 0.35 +/- 0.05, and - 0.44 +/- 0.01 kcal mol(-1), respectively, in very good agreement within both series. Thus, although bromine is bulkier than chlorine, the 1,3-diaxial steric effects are similar in these equilibria. However, the integration of (1)H NMR spectrum for the trans-3-methylcyclohexanol (3) gave 90% of the 3ae conformer in the equilibrium, at - 90 degrees C on CS2/CD2Cl2 (9:1), corresponding to a DeltaG(ea-ae) value of 1.31 +/- 0.02 kcal mol(-1). The values obtained through the additivity rule, with data from monosubstituted cyclohexanes (DeltaG(Ad) = DeltaG(X) + DeltaG(OH)), for compounds 1, 2, and 4 (-0.37 +/- 0.15, - 0.34 +/- 0.09, and - 0.46 +/- 0.04 kcal mol(-1), respectively) are in very good agreement with the experimental values, but it is significantly smaller for compound 3 (0.79 +/- 0.02 kcal mol(-1)). Theoretical calculations through different levels of theory (HF/6-311 + g**, B3LYP/6-311 + g**, MP2/6-31 + g**, and CBS-4M) showed that CBS-4M is the best method for the study of conformational equilibria for these systems, since it provides DeltaG(ea-ae) values similar to the experimental values.     

Syntheses, conformations, and basicities of bicyclic triamines

The multistep syntheses of several bicyclic triamines are described, all of which have an imbedded 1,5,9-triazacyclododecane ring. In 1,5,9-triazabicyclo[7.3.3]pentadecanes 12, 13, 15, and 16, two nitrogens are bridged by three carbons. The monoprotonated forms of these triamines are highly stabilized by a hydrogen-bonded network involving the bridge and both bridgehead nitrogens, producing a difference of more than 8 pK(a) units in acidities of their monoprotonated and diprotonated forms. The one- and zero-carbon bridges in 1,5,9-triazabicyclo[9.1.1]tridecane (23) and 7-methyl-1,5,9-triazabicyclo[5.5.0]dodecane (39) do not enhance the stabilities of their monoprotonated forms. X-ray crystal structures and computational studies of 12.HI and 16.HI reveal similar, but somewhat weaker, hydrogen-bonded networks, relative to 15.HI. The activation free energies for conformational inversion of 13.HI (14.4 +/- 0.2 kcal/mol), 16.HI (15.0 +/- 0.1 kcal/mol) and 16 (8.8 +/- 0.3 kcal/mol) were measured by variable-temperature (1)H and (13)C NMR spectroscopy. These experimental barriers give an estimate of 6.2 kcal/mol for the strength of the bifurcated hydrogen bond between the bridge nitrogen and cavity proton in 16.HI. Computational studies support the hypothesis that N-inversion occurs in an open conformation, leading to an estimate of 10.32 kcal/mol for the enthalpy of the bifurcated hydrogen bond in 16.HI in the gas phase.     

Bis(pyrazolylethyl) Ether Ligation to Zinc and Cobalt: Meridional vs Facial Coordination and the Suitability of Such Ligands in Providing a NNO Donor Set for Modeling Bioinorganic Aspects of Zinc Chemistry

The structures of bis(pyrazolylethyl) ether derivatives of zinc and cobalt, namely [eta(3)-O(CH(2)CH(2)pz(Pr)()i()2)(2)]Zn(NO(3))(2) and [eta(3)-O(CH(2)CH(2)pz(Me)()2)(2)]Co(NO(3))(2), have been determined with a view to addressing the applicability of such ligands in modeling bioinorganic aspects of zinc chemistry. Specific consideration is given to the possibility that bis(pyrazolylethyl) ether ligands may provide an NNO donor system which may model aspects of the binding of zinc to protein backbones in enzymes such as thermolysin. The structural studies demonstrate that the bis(pyrazolylethyl) ether ligands do indeed coordinate via each of their NNO functionalities but that the relationship to the enzyme is limited by the adoption of meridional rather than facial coordination geometries. [eta(3)-O(CH(2)CH(2)pz(Pr)()i()2)(2)]Zn(NO(3))(2) is monoclinic, P2(1)/c (No. 14), with a = 11.619(2) ?, b = 14.380(3) ?, c = 16.757(2) ?, beta = 90.44(2) degrees, and Z = 4. [eta(3)-O(CH(2)CH(2)pz(Me)()2)(2)]Co(NO(3))(2) is monoclinic, C2/c (No. 15), with a = 17.136(3) ?, b = 10.505(2) ?, c = 11.121(2) ?, beta = 104.62(3) degrees, and Z = 4.     

Preparation and conformation of octaethylbiphenylene

Dimerization of tetraethylbenzyne (generated by reaction of 1, 2-dibromo-3,4,5,6-tetraethylbenzene (8) with 1 equiv of BuLi) afforded in low yield octaethylbiphenylene (3), together with a major product which was characterized as 2,3,4,5,3',4', 5'-heptaethyl-2'-vinylbiphenyl (9). X-ray diffraction indicates that biphenylene 3 adopts in the crystal a conformation of approximate C(2)(h )()symmetry with the ethyl groups within each phenylene ring arranged in an alternated up-down fashion. Notably, pairs of vicinal ethyl groups located at peri positions are oriented in a syn arrangement in the crystal. Low temperature NMR spectroscopy is consistent with the presence in solution of either the crystal conformation or a fully alternated conformation lacking any syn interaction. Molecular mechanics (MM3), semiempirical (AM1, PM3), and ab initio calculations indicate that the crystal conformation is a high energy form, and that the lowest energy conformation is the fully alternated form. The topomerization barrier of the methylene protons of the ethyl groups of 3 is 9.4 +/- 0.1 kcal mol(-)(1), which is between the rotational barriers of 8 and 1,2,3, 4-tetraethylbenzene 7 (9.9 +/- 0.1 and 8.2 +/- 0.1 kcal mol(-)(1), respectively). The similarity in rotational barriers suggests that a given tetraethylphenylene subunit does not markedly affect the rotational barrier of the ethyl groups of the other subunit.     

Proton transfer to nickel-thiolate complexes. 1. Protonation of [Ni(SC(6)H(4)R-4)(2)(Ph(2)PCH(2)CH(2)PPh(2))] (R = Me, MeO, H, Cl, or NO(2))

The kinetics of the equilibrium reaction between [Ni(SC(6)H(4)R-4)(2)(dppe)] (R= MeO, Me, H, Cl, or NO(2); dppe = Ph(2)PCH(2)CH(2)PPh(2)) and mixtures of [lutH](+) and lut (lut = 2,6-dimethylpyridine) in MeCN to form [Ni(SHC(6)H(4)R-4)(SC(6)H(4)R-4)(dppe)](+) have been studied using stopped-flow spectrophotometry. The kinetics for the reactions with R = MeO, Me, H, or Cl are consistent with a single-step equilibrium reaction. Investigation of the temperature dependence of the reactions shows that DeltaG = 13.6 +/- 0.3 kcal mol(-)(1) for all the derivatives but the values of DeltaH and DeltaS vary with R (R = MeO, DeltaH() = 8.5 kcal mol(-)(1), DeltaS = -16 cal K(-)(1) mol(-)(1); R = Me, DeltaH() = 10.8 kcal mol(-)(1), DeltaS = -9.5 cal K(-)(1) mol(-)(1); R = Cl, DeltaH = 23.7 kcal mol(-)(1), DeltaS = +33 cal K(-)(1) mol(-)(1)). With [Ni(SC(6)H(4)NO(2)-4)(2)(dppe)] a more complicated rate law is observed consistent with a mechanism in which initial hydrogen-bonding of [lutH](+) to the complex precedes intramolecular proton transfer. It seems likely that all the derivatives operate by this mechanism, but only with R = NO(2) (the most electron-withdrawing substituent) does the intramolecular proton transfer step become sufficiently slow to result in the change in kinetics. Studies with [lutD](+) show that the rates of proton transfer to [Ni(SC(6)H(4)R-4)(2)(dppe)] (R = Me or Cl) are associated with negligible kinetic isotope effect. The possible reasons for this are discussed. The rates of proton transfer to [Ni(SC(6)H(4)R-4)(2)(dppe)] vary with the 4-R-substituent, and the Hammett plot is markedly nonlinear. This unusual behavior is attributable to the electronic influence of R which affects the electron density at the sulfur.     

Conformational studies by dynamic NMR. 79. Dimesityl sulfine revisited: detection of the helical antipodes and determination of their enantiomerization pathways.

By means of low-temperature NMR spectra, it is demonstrated that dimesityl sulfine (Mes2C=SO) adopts in solution the same chiral propeller conformation (C1 symmetry) determined by X-ray diffraction in the crystalline state. With the help of MM calculations, it has been also shown that a correlated rotation (cog wheel effect) of the two mesityl rings reverses the molecular helicity according to an enantiomerization process entailing a one-ring flip pathway with delta G++ = 5.9 kcal mol-1 and a two-ring flip pathway with delta G++ = 13.8 kcal mol-1. On the contrary the Z- and E-isomers of mesityl phenyl sulfine (MesPhC=SO) adopt essentially achiral conformations (Cs symmetry), having the Ph-CSO rotation barriers equal to 5.2 and 5.8 kcal mol-1, respectively, and the mesityl-CSO rotation barriers equal to 21.3 and 15.1 kcal mol-1, respectively.