Reliable low-cost theoretical procedures for studying addition-fragmentation in RAFT polymerization

Enthalpies for the beta-scission reactions, R'SC(*)(Z)SR --> R'SC(Z)=S + (*)R (for R, R' = CH(3), CH(2)CH(3), CH(2)CN, C(CH(3))(2)CN, CH(2)COOCH(3), CH(CH(3))COOCH(3), CH(2)OCOCH(3), CH(2)Ph, C(CH(3))(2)Ph, and CH(CH(3))Ph and Z = CH(3), H, Cl, CN, CF(3), NH(2), Ph, CH(2)Ph, OCH(3), OCH(2)CH(3), OCH(CH(3))(2), OC(CH(3))(3), and F) have been calculated using a variety of DFT, MP2, and ONIOM-based methods, as well as G3(MP2)-RAD, with a view to identifying an accurate method that can be practically applied to larger systems. None of the DFT methods examined can reproduce the quantitative, nor qualitative, values of the fragmentation enthalpy; in most cases the relative errors are over 20 kJ mol(-1) and in some cases as much as 55 kJ mol(-1). The ROMP2 methods fare much better, but fail when the leaving group radical (R(*)) is substituted with a group (such as phenyl or CN) that delocalizes the unpaired electron. However, provided the primary substituents on the leaving group radical are included in the core system, an ONIOM-based approach in which the full system is studied via ROMP2 (or SCS- or SOS-MP2) calculations with the 6-311+G(3df,2p) basis set and the core system is studied at G3(MP2)-RAD can reproduce the corresponding G3(MP2)-RAD values of the full systems within 5 kJ mol(-1) and is a practical method for use on larger systems.     

Should dithiophosphinate esters function as RAFT agents?

[graph: see text] High-level ab initio calculations indicate that *CH3 addition to the sulfur center of S=P(Z)(Z')SCH3 (Z,Z' = CH3, CN, OCH3, Ph) is considerably less exothermic than addition to the corresponding RAFT agents, S=C(Z)SCH3. This suggests that dithiophosphinate esters may have only limited use in controlling free-radical polymerization, but should make excellent radical chain carriers in organic synthesis. The results cast doubt on the notion that phosphoranyl radicals are more "intrinsically" stabilized than carbon-centered radicals.     

Syntheses of ketonated disulfide-bridged diruthenium complexes via C-H bond activation and C-S bond formation

The alpha-C-H bonds of 3-methyl-2-butanone, 3-pentanone, and 2-methyl-3-pentanone were activated on the sulfur center of the disulfide-bridged ruthenium dinuclear complex [(RuCl(P(OCH3)3)2)2(mu-S2)(mu-Cl)2] (1) in the presence of AgX (X = PF6, SbF6) with concomitant formation of C-S bonds to give the corresponding ketonated complexes [(Ru(CH3CN)2(P(OCH3)3)2)(mu-SSCHR1COR2)(Ru(CH3CN)3(P(OCH3)3)2)]X3 ([5](PF6)3, R1 = H, R2 = CH(CH3)2, X = PF6; [6](PF6)3, R1 = CH3, R2 = CH2CH3, X = PF6; [7](SbF6)3, R1 = CH3, R2 = CH(CH3)2, X = SbF6). For unsymmetric ketones, the primary or the secondary carbon of the alpha-C-H bond, rather than the tertiary carbon, is preferentially bound to one of the two bridging sulfur atoms. The alpha-C-H bond of the cyclic ketone cyclohexanone was cleaved to give the complex [(Ru(CH3CN)2(P(OCH3)3)2)(mu-SS-1- cyclohexanon-2-yl)(Ru(CH3CN)3(P(OCH3)3)2)](SbF6)3 ([8](SbF6)3). And the reactions of acetophenone and p-methoxyacetophenone, respectively, with the chloride-free complex [(Ru(CH3CN)3(P(OCH3)3)2)2(mu-S2)]4+ (3) gave [(Ru(CH3CN)2(P(OCH3)3)2)(mu-SSCH2COAr)(Ru(CH3CN)3(P(OCH3)3)2)](CF3SO3)3 ([9](CF3SO3)3, Ar = Ph; [10](CF3SO3)3, Ar = p-CH3OC6H4). The relative reactivities of a primary and a secondary C-H bond were clearly observed in the reaction of butanone with complex 3, which gave a mixture of two complexes, i.e., [(Ru(CH3CN)2(P(OCH3)3)20(mu-SSCH2COCH2CH3)(Ru(CH3CN)3(P(OCH3)3)2)](CF3SO3)3 ([11](CF3SO3)3) and [(Ru(CH3CN)2(P(OCH3)3)2)(mu-SSCHCH3COCH3)(Ru(CH3CN)3(P(OCH3)2)](CF3SO3)3 ([12](CF3SO3)3), in a molar ratio of 1:1.8. Complex 12 was converted to 11 at room temperature if the reaction time was prolonged. The relative reactivities of the alpha-C-H bonds of the ketones were deduced to be in the order 2 degrees > 1 degree > 3 degrees, on the basis of the consideration of contributions from both electronic and steric effects. Additionally, the C-S bonds in the ketonated complexes were found to be cleaved easily by protonation at room temperature. The mechanism for the formation of the ketonated disulfide-bridged ruthenium dinuclear complexes is as follows: initial coordination of the oxygen atom of the carbonyl group to the ruthenium center, followed by addition of an alpha-C-H bond to the disulfide bridging ligand, having S=S double-bond character, to form a C-S-S-H moiety, and finally completion of the reaction by deprotonation of the S-H bond.     

Dehydration reaction of hydroxyl substituted alkenes and alkynes on the Ru(2)S(2) complex

A variety of inter- and intramolecular dehydration was found in the reactions of [[Ru(P(OCH(3))(3))(2)(CH(3)CN)(3)](2)(mu-S(2))](CF(3)SO(3))(4) (1) with hydroxyl substituted alkenes and alkynes. Treatment of 1 with allyl alcohol gave a C(3)S(2) five-membered ring complex, [[Ru(P(OCH(3))(3))(2)(CH(3)CN)(3)](2)[mu-SCH(2)CH(2)CH(OCH(2)CH=CH(2))S]](CF(3)SO(3))(4) (2), via C-S bond formation after C-H bond activation and intermolecular dehydration. On the other hand, intramolecular dehydration was observed in the reaction of 1 with 3-buten-1-ol giving a C(4)S(2) six-membered ring complex, [[Ru(P(OCH(3))(3))(2)(CH(3)CN)(3)](2) [mu-SCH(2)CH=CHCH(2)S]](CF(3)SO(3))(4) (3). Complex 1 reacts with 2-propyn-1-ol or 2-butyn-1-ol to give homocoupling products, [[Ru(P(OCH(3))(3))(2)(CH(3)CN)(3)](2)[mu-SCR=CHCH(OCH(2)C triple bond CR)S]](CF(3)SO(3))(4) (4: R = H, 5: R = CH(3)), via intermolecular dehydration. In the reaction with 2-propyn-1-ol, the intermediate complex having a hydroxyl group, [[Ru(P(OCH(3))(3))(2)(CH(3)CN)(3)](2)[mu-SCH=CHCH(OH)S]](CF(3)SO(3))(4) (6), was isolated, which further reacted with 2-propyn-1-ol and 2-butyn-1-ol to give 4 and a cross-coupling product, [[Ru(P(OCH(3))(3))(2)(CH(3)CN)(3)](2)[mu-SCH=CHCH(OCH(2)C triple bond CCH(3))S]](CF(3)SO(3))(4) (7), respectively. The reaction of 1 with diols, (HO)CHRC triple bond CCHR(OH), gave furyl complexes, [[Ru(P(OCH(3))(3))(2)(CH(3)CN)(3)](2)[mu-SSC=CROCR=CH]](CF(3)SO(3))(3) (8: R = H, 9: R = CH(3)) via intramolecular elimination of a H(2)O molecule and a H(+). Even though (HO)(H(3)C)(2)CC triple bond CC(CH(3))(2)(OH) does not have any propargylic C-H bond, it also reacts with 1 to give [[Ru(P(OCH(3))(3))(2)(CH(3)CN)(3)](2)[mu-SCH(2)C(=CH(2))C(=C=C(CH(3))(2))]S](CF(3)SO(3))(4) (10). In addition, the reaction of 1 with (CH(3)O)(H(3)C)(2)CC triple bond CC(CH(3))(2)(OCH(3)) gives [[Ru(P(OCH(3))(3))(2)(CH(3)CN)(2)][mu-S=C(C(CH(3))(2)OCH(3))C=CC(CH(3))CH(2)S][Ru(P(OCH(3))(3))(2)(CH(3)CN)(3)]](CF(3)SO(3))(4) (11), in which one molecule of CH(3)OH is eliminated, and the S-S bond is cleaved.     

Radical hydrogen bonding: origin of stability of radical-molecule complexes

Natural bond orbital analysis is used to investigate the nature of hydrogen bonding in a series of binary open-shell complexes involving hydroperoxy radical HO(2)...X and analogous closed-shell H(2)O(2)...X complexes (where X = H(2)O, H(2)O(2), HONO, HONO(2), CH(3)OH, HCOOH, CH(3)COOH, and H(2)SO(4)) in order to elucidate and identify the electronic factors responsible for the strength of radical hydrogen bonds. Results from this study suggest that the radical species strongly alters the strength of the characteristic n --> sigma(*) donor-acceptor interaction in the hydrogen bonding. This interaction is found to contribute to the unusually strong binding in radical-molecule complexes. These findings have important new ramifications for our fundamental understanding of radical hydrogen bonds.     

Synthesis, structure, electrochemistry, and spectroelectrochemistry of hypervalent phosphorus(V) octaethylporphyrins and theoretical analysis of the nature of the PO bond in P(OEP)(CH(2)CH(3))(O)

A variety of phosphorus(V) octaethylporphyrin derivatives of the type [P(OEP)(X)(Y)](+)Z(-) (OEP: octaethylporphyrin) (X = CH(3), CH(2)CH(3), C(6)H(5), F; Y = CH(3), CH(2)CH(3), OH, OCH(3), OCH(2)CH(3), On-Pr, Oi-Pr, Osec-Bu, NHBu, NEt(2), Cl, F, O(-); Z = ClO(4), PF(6)) were prepared. X-ray crystallographic analysis of eleven compounds reveals that the degree of ruffling of the porphyrin core becomes greater and the average P-N bond distance becomes shorter as the axial ligands become more electronegative. Therefore, the electronic effect of the axial substituents plays a major role in determining the degree of ruffling although the steric effect of the substituents plays some role. A comparison of the (1)H NMR chemical shifts for the series of [P(OEP)(CH(2)CH(3))(Y)](+)Z(-) complexes with those of the corresponding arsenic porphyrins, which possess a planar core, indicates a much smaller ring current effect of the porphyrin core in the severely ruffled phosphorus porphyrins. The electrochemistry, spectroelectrochemistry and ESR spectroscopy of the singly reduced compounds are also discussed. The OH protons of [P(OEP)(X)(OH)](+) are acidic enough to generate P(OEP)(X)(O) by treatment with aq dilute NaOH. X-ray analysis of P(OEP)(CH(2)CH(3))(O) reveals that the PO bond length is very short (1.475(7) A) and is comparable to that in triphenylphosphine oxide (1.483 A). The features of the quite unique hexacoordinate hypervalent compounds are investigated by density functional calculation of a model (Por)P(CH(2)CH(3))(O) and (Por)P(F)(O) (Por: unsubstituted porphyrin).     

Facile allylic C-H bond activation on the bridging disulfide ligand in the Ru(III) dinuclear complex having a conjugated RuSSRu core

Treatment of [[Ru(P(OCH(3))(3))(2)(CH(3)CN)(3)](2)(mu-S(2))](CF(3)SO(3))(4) (1), which is prepared by the reaction of [[RuCl(P(OCH(3))(3))(2)](2)(mu-S(2))(mu-Cl)(2)] (2) with 4 equiv of AgCF(3)SO(3), with terminal alkenes such as 1-pentene, allyl ethyl ether, allyl phenyl ether, 1,4-hexadiene, and 3-methyl-1-butene, resulted in the formation of complexes carrying a C(3)S(2) five-membered ring, [[Ru(P(OCH(3))(3))(2)(CH(3)CN)(3)](2)[mu-SCH(2)CH(2)CR(1)R(2)S]](CF(3)SO(3))(4) (3, R(1) = CH(2)CH(3), R(2) = H, 40%; 4, R(1) = OCH(2)CH(3), R(2) = H, 60%; 5, R(1) = OC(6)H(5), R(2) = H, 73%; 6, R(1) = CH=CHCH(3), R(2) = H, 48%; 7, R(1) = R(2) = CH(3), 40%). Reaction of 1 with methylenecycloalkanes was found to give several different types of products, depending on the ring size of the substrates. A trace of [[Ru(P(OCH(3))(3))(2)(CH(3)CN)(3)](2)[mu-SCH(CH(2)CH(2))CH(CH(3))S]](CF(3)SO(3))(4) (9) having a C(2)S(2) four-membered ring to bridge the two Ru atoms was obtained by the reaction of 1 with methylenecyclobutane, whereas the reaction with methylenecyclohexane gave [[Ru(P(OCH(3))(3))(2)(CH(3)CN)(3)](2)[mu-S(CH(2)(C=CHCH(2)CH(2)CH(2)CH(2))S)](CF(3)SO(3))(3) (10) in 69% yield via C-S bond formation and elimination of a proton. Throughout these reactions with alkenes giving a variety of products, the activation of the allylic C-H bond is always the essential and initial key step.     

An orbital phase theory for the torquoselectivity of the ring-opening reactions of 3-substituted cyclobutenes: geminal bond participation

We apply an orbital phase theory to the torquoselectivity of the electrocyclic reactions of 3-substituted (X) cyclobutenes. The torquoselectivity is shown to be controlled by the orbital-phase relation of the reacting pi(CC) and sigma(CC) bonds with the sigma(CX) bond geminal to the sigma(CC) bond to be cleaved. The inward rotation of electron-donating sigma(CX) bonds and outward rotation of electron-withdrawing sigma(CX) bonds have been deduced from the orbital-phase theory. Enhancement of the inward rotation by the electron-donating capability of the sigma(CX) bonds is confirmed by the correlation between the torquoselectivity and sigma(CX) orbital energy. The orbital overlaps between the geminal sigma(CX) (sigma(CH)) and sigma(CC) bonds are found to be important as well. Unsaturated substituents with low-lying unoccupied pi orbitals also promote the inward rotation.     

An examination of hyperconjugative and electrostatic effects in the hydride reductions of 2-substituted-4-tert-butylcyclohexanones

To better understand electronic effects on the diastereoselectivity of nucleophilic additions to the carbonyl group, a series of 2-X-4-tert-butylcyclohexanones (X = H, CH(3), OCH(3), F, Cl, Br) were reacted with LiAlH(4). Reduction of ketones with equatorial substituents yields increasing amounts of axial alcohol in the series for X [H < CH(3) < Br < Cl < F < OCH(3)]. These data cannot be explained by steric or chelation effects or by the theories of Felkin-Anh or Cieplak. Instead, an electrostatic argument is introduced: due to repulsion between the nucleophile and the X group, axial approach becomes energetically less favorable with an increase in the component of the dipole moment anti to the hydride approach trajectory. The ab initio calculated diastereoselectivities were close to the experimental values but did not reproduce the relative selectivity ordering among substituents. For reduction of ketones with axial substituents, increasing amounts of axial alcohol are seen in the series for X [Cl < Br < CH(3) < OCH(3) < H < F]. After some minor adjustments are made, this ordering is consistent with both the electrostatic model and Felkin-Anh theory. Cieplak theory cannot account for these data regardless of adjustments. Ab initio calculated diastereoselectivities were reasonably accurate for the nonpolar substituents but were poor for the polar substituents.     

Disproportionation of O-methylhydroxylamine catalyzed by aquapentacyanoferrate(II)

The aquapentacyanoferrate(II) ion, [Fe(II)(CN)(5)H(2)O](3-), catalyzes the disproportionation reaction of O-methylhydroxylamine, NH(2)OCH(3), with stoichiometry 3NH(2)OCH(3) → NH(3) + N(2) + 3CH(3)OH. Kinetic and spectroscopic evidence support an initial N coordination of NH(2)OCH(3) to [Fe(II)(CN)(5)H(2)O](3-) followed by a homolytic scission leading to radicals [Fe(II)(CN)(5)(?)NH(2)](3-) (a precursor of Fe(III) centers and bound NH(3)) and free methoxyl, CH(3)O(?), thus establishing a radical path leading to N-methoxyamino ((?)NHOCH(3)) and 1,2-dimethoxyhydrazine, (NHOCH(3))(2). The latter species is moderately stable and proposed to be the precursor of N(2) and most of the generated CH(3)OH. Intermediate [Fe(III)(CN)(5)L](2-) complexes (L = NH(3), H(2)O) form dinuclear cyano-bridged mixed-valent species, affording a catalytic substitution of the L ligands promoted by [Fe(II)(CN)(5)L](3-). Free or bound NH(2)OCH(3) may act as reductants of [Fe(III)(CN)(5)L](2-), thus regenerating active sites. At increasing concentrations of NH(2)OCH(3) a coordinated diazene species emerges, [Fe(II)(CN)(5)N(2)H(2)](3-), which is consumed by the oxidizing CH(3)O(?), giving N(2) and CH(3)OH. Another side reaction forms [Fe(II)(CN)(5)N(O)CH(3)](3-), an intermediate containing the nitrosomethane ligand, which is further oxidized to the nitroprusside ion, [Fe(II)(CN)(5)NO](2-). The latter is a final oxidation product with a significant conversion of the initial [Fe(II)(CN)(5)H(2)O](3-) complex. The side reaction partially blocks the Fe(II)-aqua active site, though complete inhibition is not achieved because the radical path evolves faster than the formation rates of the Fe(II)-NO(+) bonds.     

Spontaneous generation of stable pnictinyl radicals from "jack-in-the-box" dipnictines: a solid-state, gas-phase, and theoretical investigation of the origins of steric stabilization

The molecular structures of the stable phosphinyl and arsinyl radicals, .PnR(2) [Pn = P (2); As (4); R = CH(SiMe(3))(2)], have been determined by gas-phase electron diffraction (GED) in conjunction with ab initio molecular orbital calculations. The X-ray crystal structures of the corresponding dipnictines, the "dimers", R(2)PnPnR(2) [Pn = P (1), As (3)], and the chloro derivatives R(2)PnCl [Pn = P (5), As (6)] have also been determined. Collectively, these structural investigations demonstrate that large distortions of the ligands attached to Pn occur when the pnictinyl radicals unite to form the corresponding dipnictine dimers. Principally, it is the shape and flexibility of the CH(SiMe(3))(2) ligands that permit the formation of the P-P and As-As bonds in 1and 3, respectively. However, theoretical studies indicate that in the process of pnictinyl radical dimerization to form 1 and 3, both molecules accumulate substantial amounts of potential energy and are thus primed to spring apart upon release from the solid state by melting, dissolution, or evaporation. The insights gleaned from these unusual systems have permitted a deeper understanding of the functioning of sterically demanding substituents.     

Synthesis and spectroscopy of N3P3X5OCH=CH2 (X = Cl, F, OCH3, OCH2CF3, N(CH3)2) and N3P3X4(OCH=CH2)2 (X = Cl, N(CH3)2). Correlations of ultraviolet photoelectron spectroscopy and nuclear magnetic resonance data to electronic and geometrical structure

The syntheses of the vinyloxycyclotriphosphazene derivatives N3P3X5OCH=CH2 (X = OMe, OCH2CF3) and the N3P3(NMe2)4(OCH=CH2)2 isomeric mixture along with improved preparations of N3P3X5OCH=CH2 (X = F, NMe2) are reported. The interactions between the vinyloxy function and the cyclophosphazene in these and the previously reported N3P3Cl5 (OCH=CH2) and N3P3F6-n(OCH=CH2)n (n = 1-4) have been examined by ultraviolet photoelectron spectroscopy (UPS) and NMR spectroscopy. The UPS data for the chloro and fluoro derivatives show a strong electron-withdrawing effect of the phosphazene on the olefin that is mediated with decreasing halogen substitution. The 1H and 13C NMR data for N3P3X5OCH=CH2 (X = F, Cl, OMe, OCH2CF3, NMe2) show significant changes as a function of the phosphazene substituent. There is a linear correlation between the beta-carbon chemical shift on the vinyloxy unit and the phosphorus chemical shift at the vinyloxyphosphorus centers. The chemical shifts of the different phosphorus centers on each ring are also related in a linear fashion. These relationships may be understood in terms of the relative electron donor-acceptor abilities of the substituents on the phosphazene ring. The 1H NMR spectra of the N3P3(NMe2)4(OCH-CH2)2 isomeric mixture allow for assignment of the relative amounts of cis and trans isomers. A model for the observed cis preference in the formation of N3P3Cl4(OCH=CH)2 is presented.     

Effect of substituents on the strength of N-X (X = H, F, and Cl) bond dissociation energies: a high-level quantum chemical study

The effect of substituents on the strength of N-X (X = H, F, and Cl) bonds has been investigated using the high-level W2w thermochemical protocol. The substituents have been selected to be representative of the key functional groups that are likely to be of biological, synthetic, or industrial importance for these systems. We interpreted the effects through the calculation of relative N-X bond dissociation energies (BDE) or radical stabilization energies (RSE(NX)). The BDE and RSE(NX) values depend on stabilizing/destabilizing effects in both the reactant molecule and the product radical of the dissociation reactions. To assist us in the analysis of the substituent effects, a number of additional thermochemical quantities have been introduced, including molecule stabilization energies (MSE(NX)). We find that the RSE(NH) values are (a) increased by electron-donating alkyl substituents or the vinyl substituent, (b) increased in imines, and (c) decreased by electron-withdrawing substituents such as CF(3) and carbonyl moieties or through protonation. A different picture emerges when considering the RSE(NF) and RSE(NCl) values because of the electronegativities of the halogen atoms. The RSE(NX)s differ from the RSE(NH) values by an amount related to the stabilization of the N-halogenated molecules and given by MSE(NX). We find that substituents that stabilize/destabilize the radicals also tend to stabilize/destabilize the N-halogenated molecules. As a result, N-F- and N-Cl-containing molecules that include alkyl substituents or correspond to imines are generally associated with RSE(NF) and RSE(NCl) values that are less positive or more negative than the corresponding RSE(NH). In contrast, N-F- and N-Cl-containing molecules that include electron-withdrawing substituents or are protonated are generally associated with RSE(NF) and RSE(NCl) values that are more positive or less negative than the corresponding RSE(NH).     

Metastable ion study of organosilicon compounds. Part XIII: dimethoxydimethylsilane, (CH(3))(2)Si(OCH(3))(2), and dimethoxymethylsilane, CH(3)SiH(OCH(3))(2)

Unimolecular metastable fragmentations of dimethoxydimethylsilane, (CH(3))(2)Si(OCH(3))(2) (MW 120, 1), and dimethoxymethylsilane, CH(3)SiH(OCH(3))(2) (MW 106, 2), upon electron impact ionization have been studied by means of mass-analyzed ion kinetic energy (MIKE) spectrometry and the D-labeling technique in conjunction with thermochemistry. The results have been compared with those of the corresponding carbon analogues, 2,2-dimethoxypropane, (CH(3))(2)C(OCH(3))(2) (MW 104, 3) and 1,1-dimethoxyethane, CH(3)CH(OCH(3))(2) (MW 90, 4). In analogy with the cases of 3 and 4, both molecular ions from 1 and 2 are formed at very low abundance at 70 eV, and begin to decompose by the expulsion of the substituents (H, CH(3) or OCH(3)) on the central silicon atom. These decompositions are followed by the loss of a formaldehyde molecule (CH(2)O), as commonly observed in the mass spectra of methoxysilanes. Further, an ethylene (C(2)H(4)) or a dimethyl ether (CH(3)OCH(3)) molecule loss is observed in the fragmentation of some intermediate ions generated from 1(+)* and 2(+)*, but the mechanisms are different than those in the cases of 3 and 4. Some of these fragmentations are also different than those reported previously. The relative abundance of the ions in many MIKE spectra is explained by the extension of the Stevenson-Audier rule. The reaction, which is in contrast to the rule, however, is rationalized by the energy of the transition state for the reaction, estimated by semi-empirical molecular orbital calculation. The peak at m/z 59 from 2(+)* consists only of CH(3)OSi(+) ion, whereas the peak from 1(+)* consists of two different ions, CH(3)OSi(+) and (CH(3))(2)Si(+)H. The ions CH(3)OSi(+) from 1(+)* and 2(+)* are generated by at least two and three separate routes respectively.     

Substituent effects on the bond dissociation enthalpies of aromatic amines

Bond dissociation enthalpy differences, Z-X DeltaBDE = BDE(4-YC(6)H(4)Z-X) - BDE(C(6)H(5)Z-X), for Z = CH(2) and O are largely independent of X and are determined mainly by the stabilization/destabilization effect of Y on the 4-YC(6)H(4)Z(*) radicals. The effects of Y are small (< or =2 kcal/mol for all Y) for Z = CH(2), but they are large for Z = O, where good correlations with sigma(p)(+)(Y) yield rho(+) = 6.5 kcal/mol. For Z = NH, two sets of electrochemically measured N-H DeltaBDEs correlate with sigma(p)(+)(Y), yielding rho(+) = 3.9 and 3.0 kcal/mol. However, in contrast to the situation with phenols, these data indicate that the strengthening effect on N-H BDEs of electron-withdrawing (EW) Y's is greater than the weakening effect of electron-donating (ED) Y's. Attempts to measure N-H DeltaBDEs in anilines using two nonelectrochemical techniques were unsuccessful; therefore, we turned to density functional theory. Calculations on 15 4-YC(6)H(4)NH(2) gave N-H DeltaBDEs correlating with sigma(p)(+) (rho(+) = 4.6 kcal/mol) and indicated that EW and ED Y's had comparable strengthening and weakening effects, respectively, on the N-H bonds. To validate theory by connecting it to experiment, the N-H DeltaBDEs of four 4,4'-disubstituted diphenylamines and five 3,7-disubstituted phenothiazines were both calculated and measured by the radical equilibration EPR technique. For all compounds, theory and experiment agreed to better than 1 kcal/mol. Dissection of N-H DeltaBDEs in 4-substituted anilines and O-H DeltaBDEs in 4-substituted phenols into interaction enthalpies between Y and NH(2)/OH (molecule stabilization/destabilization enthalpy, MSE) and NH*/O* (radical stabilization/destabilization enthalpy, RSE) reveals that for both groups of compounds, ED Y's destabilize the molecule and stabilize the radical, while the opposite holds true for EW Y's. However, in the phenols the effects of substituents on the radical are roughly 3 times as great as those in the molecule, whereas in the anilines the two effects are of comparable magnitudes. These differences arise from the stronger ED character of NH(2) vs OH and the weaker EW character of NH* vs O*. The relatively large contributions to N-H BDEs in anilines arising from interactions in the molecules suggested that N-X DeltaBDEs in 4-YC(6)H(4)NH-X would depend on X, in contrast to the lack of effect of X on O-X and CH(2)-X DeltaBDEs in 4-YC(6)H(4)O-X and 4-YC(6)H(4)CH(2)-X. This suggestion was confirmed for X = CH(3), H, OH, and F, for which the calculated NH-X DeltaBDEs yielded rho(+) = 5.0, 4.6, 4.0, and 3.0 kcal/mol, respectively.     

Reactions of superoxo and oxo metal complexes with aldehydes. Radical-specific pathways for cross-disproportionation of superoxometal ions and acylperoxyl radicals

The aquachromyl(IV) ion, Cr(aq)O(2+), reacts with acetaldehyde and pivaldehyde by hydrogen atom abstraction and, in the presence of O(2), produces acylperoxyl radicals, RC(O)OO(*). In the next step, the radicals react with Cr(aq)OO(2+), a species accompanying Cr(aq)O(2+) in our preparations. The rate constant for the Cr(aq)OO(2+)/CH(3)C(O)OO(*) cross reaction, k(Cr) = 1.5 x 10(8) M(-1) s(-1), was determined by laser flash photolysis. The evidence points to radical coupling at the remote oxygen of Cr(aq)OO(2+), followed by elimination of O(2) and formation of CH(3)COOH and Cr(V)(aq)O(3+). The latter disproportionates and ultimately yields Cr(aq)(3+) and HCrO(4)(-). No CO(2) was detected. The Cr(aq)OO(2+)/C(CH(3))(3)C(O)OO(*) reaction yielded isobutene, CO(2), and Cr(aq)(3+), in addition to chromate. In the suggested mechanism, the transient Cr(aq)OOOO(O)CC(CH(3))(3)(2+) branches into two sets of products. The path leading to chromate resembles the CH(3)C(O)OO(*) reaction. The other products arise from an unprecedented intramolecular hydrogen transfer from the tert-butyl group to the CrO entity and elimination of CO(2) and O(2). A portion of C(CH(3))(3)C(O)OO(*) was captured by (CH(3))(3)COO(*), which was in turn generated by decarbonylation of acyl radicals and oxygenation of tert-butyl radicals so formed.     

Catalytic transesterification of dialkyl phosphates by a bioinspired dicopper(II) macrocyclic complex

For a number of phosphoryltransfer enzymes, including the exonuclease subunit of DNA polymerase I, a mechanism involving two-metal ions and double Lewis-acid activation of the substrate, combined with leaving group stabilization, has been proposed. Inspired by the active site structure of this enzyme, we have designed as a synthetic phosphoryl transfer catalyst the dicopper(II) macrocyclic complex LCu(2). Crystal structures of complexes [(L)Cu(2)(mu-NO(3))(NO(3))](NO(3))(2) (1), [(L)Cu(2)(mu-CO(3))(CH(3)OH)](BF(4))(2) (2), and [(L)Cu(2)(mu-O(2)P(OCH(3))(2))(NO(3))](NO(3))(2) (3) illustrate various possibilities for the interaction of oxoanions with the dicopper(II) site. 1 efficiently promotes the transesterification of dimethyl phosphate (DMP) in CD(3)OD, k(cat) = 2 x 10(-)(4) s(-)(1) at 55 degrees C. 1 is the only available catalyst for the smooth transesterification of highly inert simple dialkyl phosphates. From photometric titrations and the pH dependence of reactivity, we conclude that a complex [(L)Cu(2)(DMP)(OCH(3))](2+) is the reactive species. Steric bulk at the -OR substituents of phosphodiester substrates O(2)P(OR)(2)(-) drastically reduces the reactivity of 1. This is explained with -OR leaving group stabilization by Cu coordination, an interaction which is sensitive to steric crowding at the alpha-C-atom of substituent R. A proposed reaction mechanism related to that of the exonuclease unit of DNA polymerase I is supported by DFT calculations on reaction intermediates. The complex [(L)Cu(3)(mu(3)-OH)(mu-CH(3)O)(2)(CH(3)CN)(2)](ClO(4))(3) (4) incorporates a [Cu(OH)(OCH(3))(2)(CH(3)CN)(2)](-) complex anion, which might be considered as an analogue of the [PO(2)(OCH(3))(2)(OCD(3))](2)(-) transition state (or intermediate) of DMP transesterification catalyzed by LCu(2).     

C-H cleavage and double alkyl additions to the disulfido ligand in dicyanido-bridged Ru2S2 complexes

Novel dicyanido-bridged dicationic RuIIISSRuIII complexes [{Ru(P(OCH3)3)2}2(mu-S2)(mu-X)2{mu-m-C6H4(CH2CN)2}](CF3SO3)2 (4, X=Cl, Br) were synthesized by the abstraction of the two terminal halide ions of [{RuX(P(OCH3)3)2}2(mu-S2)(mu-X)2] (1, X=Cl, Br) followed by treatment with m-xylylenedicyanide. 4 reacted with 2,3-dimethylbutadiene to give the C4S2 ring-bridged complex [{Ru(P(OCH3)3)2}2{mu-SCH2C(CH3)=C(CH3)CH2S}(mu-X)2{mu-m-C6H4(CH2CN)2}](CF3SO3)2 (6, X=Cl, Br). In addition, 4 reacted with 1-alkenes in CH3OH to give alkenyl disulfide complexes [{Ru(P(OCH3)3)2}2{mu-SS(CH2C=CHR)}(mu-Cl)2{mu-m-C6H4(CH2CN)2}](CF3SO3) (7: R=CH2CH3, 9: R=CH2CH2CH3) and alkenyl methyl disulfide complexes [{Ru(P(OCH3)3)2}2{mu-S(CH3)S(CH2C=HR)}(mu-Cl)2{mu-m-C6H4(CH2CN)2}](CF3SO3)2 (8: R=CH2CH3, 10: R=CH2CH2CH3) via the activation of an allylic C-H bond followed by the elimination of H+ or condensation with CH3OH. Additionally, the reaction of 4 with 3-penten-1-ol gave [{Ru(P(OCH3)3)2}2{mu-SS(CH2C=CHCH2OH)}(mu-Cl)2{mu-m-C6H4(CH2CN)2}](CF3SO3) (11) via the elimination of H+ and [{Ru(P(OCH3)3)2}2(mu-SCH2CH=CHCH2S)(mu-Cl)2{mu-m-C6H4(CH2CN)2}](CF3SO3)2 (12) via the intramolecular elimination of a H2O molecule. 12 was exclusively obtained from the reaction of 4 with 4-bromo-1-butene.     

Ring-closing metathesis reactions of terminal alkene-derived cyclic phosphazenes

The first examples of ring-closing metathesis (RCM) reactions of a series of terminal alkene-derived cyclic phosphazenes have been carried out. The tetrakis-, hexakis-, and octakis(allyloxy)cyclophosphazenes (NPPh(2))(NP(OCH(2)CH=CH(2))(2))(2) (1), N(3)P(3)(OCH(2)CH=CH(2))(6) (2), and N(4)P(4)(OCH(2)CH=CH(2))(8) (3) and the tetrakis(allyloxy)-S-phenylthionylphosphazene (NS(O)Ph)[NP(OCH(2)CH=CH(2))(2)](2) (4) were prepared by the reactions of CH(2)=CHCH(2)ONa with the cyclophosphazenes (NPPh(2))(NPCl(2))(2), N(3)P(3)Cl(6), and N(4)P(4)Cl(8) and the S-phenylthionylphosphazene (NS(O)Ph)(NPCl(2))(2). The reactions of 1-4 with Grubbs first-generation olefin metathesis catalyst Cl(2)Ru=CHPh(PCy(3))(2) resulted in the selective formation of seven-membered di-, tri-, and tetraspirocyclic phosphazene compounds (NPPh(2))[NP(OCH(2)CH=CHCH(2)O)](2) (5), N(3)P(3)(OCH(2)CH=CHCH(2)O)(3) (6), and N(4)P(4)(OCH(2)CH=CHCH(2)O)(4) (7) and the dispirocyclic S-phenylthionylphosphazene compound (NS(O)Ph)[NP(OCH(2)CH=CHCH(2)O)](2) (8). X-ray structural studies of 5-8 indicated that the double bond of the spiro-substituted cycloalkene units is in the cis orientation in these compounds. In contrast to the reactions of 1-4, RCM reactions of the homoallyloxy-derived cyclophosphazene and thionylphosphazene (NPPh(2))[NP(OCH(2)CH(2)CH=CH(2))(2)](2) (9) and (NS(O)Ph)[NP(OCH(2)CH(2)CH=CH(2))(2)](2) (10) with the same catalyst resulted in the formation of 11-membered diansa compounds NPPh(2)[NP(OCH(2)CH(2)CH=CHCH(2)CH(2)O)](2) (11) and (NS(O)Ph)[NP(OCH(2)CH(2)CH=CHCH(2)CH(2)O)](2) (13) and the intermolecular doubly bridged ansa-dibino-ansa compounds 12 and 14. The X-ray structural studies of compounds 11 and 13 indicated that the double bonds of the ansa-substituted cycloalkene units are in the trans orientation in these compounds. The geminal bis(homoallyloxy)tetraphenylcyclotriphosphazene [NPPh(2)](2)[NP(OCH(2)CH(2)CH=CH(2))(2)] (15) upon RCM with Grubbs first- and second-generation catalysts gave the spirocyclic product [NPPh(2)](2)[NP(OCH(2)CH(2)CH=CHCH(2)CH(2)O)] (16) along with the geminal dibino-substituted dimeric compound [NPPh(2)](2)[NP(OCH(2)CH(2)CH=CHCH(2)CH(2)O)(2)PN][NPPh(2)](2) (17) as the major product. The dibino compound 17, upon reaction with the Grubbs second-generation catalyst, was found to undergo a unique ring-opening metathesis reaction, opening up the bino bridges and partially converting to the spirocyclic compound 16.