Boron and aluminum complexes of sterically demanding phosphinimines and phosphinimides

Reactions of sterically demanding phosphinimines R3PNH [R=i-Pr (1), t-Bu (2)] were examined. Reactions with B(C6F5)3 formed the adducts (R3PNH)B(C6F5)3 [R=i-Pr (3), t-Bu (4)] in high yield. On the other hand, 2 reacts with HB(OBu)2, evolving H2 to give t-Bu3PNB(OBu)2 (5). The reaction of 2 equiv of 2 with BH3.SMe2 affords the species (t-Bu3PN)2BH (6). In contrast, the reaction of n-Bu(t-Bu)2PNH with BH3.SMe2 results in the formation of the robust adduct n-Bu(t-Bu)2PNH.BH3 (8). An alternative route to borane-phosphinimide complexes involves Me3SiCl elimination, as exemplified by the reaction of BCl2Ph with n-Bu3PNSiMe3, which gives the product n-Bu3PNBCl(Ph) (9). The corresponding reactions of the parent phosphinimines 1 and 2 with AlH3.NMe2Et give the dimers [(mu-i-Pr3PN)AlH2]2 (10) and [(mu-t-Bu3PN)AlH2]2 (11). Species 11 reacts further with Me3SiO3SCF3 to provide [(mu-t-Bu3PN)AlH(OSO2CF3)]2 (12). The reaction of the lithium salt [t-Bu3PNLi]4 (13) with BCl3 proceeds smoothly to give t-Bu3PNBCl2 (14), which is readily alkylated to give t-Bu3PNBMe2 (15). Subsequent reaction of 15 with B(C6F5)3 results in methyl abstraction and the formation of [(mu-t-Bu3PN)BMe]2[MeB(C6F5)3]2 (16). The reaction of 13 in a 2:1 ratio with BCl3 gives the salt [(t-Bu3PN)2B]Cl (17). This species can be methylated to give (t-Bu3PN)2BMe (18), which undergoes subsequent reaction with [Ph3C][X] (X=[B(C6F5)4], [PF6]) to form the related salts [(t-Bu3PN)2B][B(C6F5)4] (19) and [(t-Bu3PN)2B][PF6] (20), respectively. Analogous reactions with [Ph3C][BF4] afforded [t-Bu3PNBF2]2 (21). Compounds 3, 4, 6, 8, 11, 12, 17, 19, and 21 were characterized by X-ray crystallography.     

Competition between nonclassical hydrogen-bonded acceptor sites in complexes of neutral AH2 Radicals (A = B, Al, and Ga): A theoretical investigation

An ab initio computational study of the properties of the neutral AH2 radicals (A = B, Al, Ga) as hydrogen-bond (HB) acceptors, with H-X (X = F, Cl, Br, CN, and CCH) as HB donors, is carried out at the UMP2/6-311++G(2d,2p) level. Two different minima have been found for each of the 15 possible dimers. One structure corresponds to a single-electron hydrogen-bonded complex (SEHB), with the A atom acting as an HB acceptor. The second corresponds to a dihydrogen bond complex between one of the hydrogen atoms of AH2 and the H-X molecule. Thus, all the atoms of the neutral AH2 molecule can act as HB acceptors and none as donors. The stability of the SEHB complexes decreases as BH2 > AlH2 > GaH2, while for the dihydrogen-bonded complexes the order is AlH2 > GaH2 > BH2. For the BH2 radical the SEHB complexes are stronger than the dihydrogen bonded ones, while the opposite is found for the AlH2 and GaH2 systems. Regarding the HB donors, the order found for the binding energy in the two types of complexes is H2A...HF > H2A...HCl > H2A...HBr > H2A...HCN > H2A...HCCH.     

Neutral and cationic aluminium complexes of a sterically demanding N-imidoylamidine ligand

The N-imidoylamidine ligand i-Pr2C6H3N(C(Me)NC6H3i-Pr2)2 2 was prepared. Direct reactions with AlI3 or AlMe3 afforded [(i-Pr2C6H3N(C(Me)NC6H3i-Pr2)2)AlI2][AlI4] 3 and [i-Pr2C6H3N(C(Me)NC6H3i-Pr2)2)AlMe2][AlMe4].AlMe3, 4 respectively. Thermolysis of 4 gave (i-Pr2C6H3NC(=CH2)(NC6H3i-Pr2)(C(Me)NC6H3i-Pr2)AlMe2 6. Subsequent reaction with B(C6F5)3 gave the zwitterionic species [(i-Pr2C6H3)N(C(=CH2)NC6H3i-Pr2)(C(Me)NC6H3i-Pr2)AlMe(mu-MeB(C6F5)3)] 7. In a related reactions of 2, [Ph3C][B(C6F5)4] and AlMe3, AlH3.NEtMe2 or AlD3.NMe3, the complexes [(i-Pr2C6H3N(C(Me)NC6H3i-Pr2)2)AlR2][B(C6F5)4] (R = Me 5, H 8, D 9) and [(i-Pr2C6H3)N(C(=CH2)NC6H3i-Pr2)(C(Me)NC6H3i-Pr2)AlH][B(C6F5)4] 10 are formed. Single-crystal X-ray data for 2, 3, 5 and 10 are reported.     

Thermodynamic properties of molecular borane phosphines, alane amines, and phosphine alanes and the [BH(4)(-)][PH(4)(+)], [AlH(4)(-)][NH(4)(+)], and [AlH(4)(-)][PH(4)(+)] salts for chemical hydrogen storage systems from ab initio electronic structure theory

The heats of formation for the molecules BH(3)PH(3), BH(2)PH(2), HBPH, AlH(3)NH(3), AlH(2)NH(2), HAlNH, AlH(3)PH(3), AlH(2)PH(2), HAlPH, AlH(4)(-), PH(3), PH(4), and PH(4)(+), as well as the diatomics BP, AlN, and AlP, have been calculated by using ab initio molecular orbital theory. The coupled cluster with single and double excitations and perturbative triples method (CCSD(T)) was employed for the total valence electronic energies. Correlation consistent basis sets were used, up through the augmented quadruple-zeta, to extrapolate to the complete basis set limit. Additional d core functions were used for Al and P. Core/valence, scalar relativistic, and spin-orbit corrections were included in an additive fashion to predict the atomization energies. Geometries were calculated at the CCSD(T) level up through at least aug-cc-pVTZ and frequencies were calculated at the CCSD(T)/aug-cc-pVDZ level. The heats of formation of the salts [BH(4)(-)][PH(4)(+)](s), [AlH(4)(-)][NH(4)(+)](s), and [AlH(4)(-)][PH(4)(+)](s) have been estimated by using an empirical expression for the lattice energy and the calculated heats of formation of the two component ions. The calculations show that both AlH(3)NH(3)(g) and [AlH(4)(-)][NH(4)(+)](s) can serve as good hydrogen storage systems that release H(2) in a slightly exothermic process. In addition, AlH(3)PH(3) and the salts [AlH(4)(-)][PH(4)(+)] and [BH(4)(-)][PH(4)(+)] have the potential to serve as H(2) storage systems. The hydride affinity of AlH(3) is calculated to be -70.4 kcal/mol at 298 K. The proton affinity of PH(3) is calculated to be 187.8 kcal/mol at 298 K in excellent agreement with the experimental value of 188 kcal/mol. PH(4) is calculated to be barely stable with respect to loss of a hydrogen to form PH(3).     

Complexes of guanidinium ion (NH2)3C+ with super Lewis acidic XH4+(X = B and Al): comparison with XH3 complexes and protonated and methylated guanidinium dications

Structures of the complexes (1 and 8) of the guanidinium ion (H(2)N)(3)C(+) with super Lewis acidic BH(4)(+) and AlH(4)(+) were calculated using the DFT method at the B3LYP/6-311+G** level. (13)C NMR chemical shifts were also calculated by the GIAO-MP2 method. Each of the dicationic complexes contains a hypercoordinate boron or aluminum atom with a two-electron three-center (2e-3c) bond. Guanidinium ion was found to form a strong complex with BH(4)(+) but a relatively weak one with AlH(4)(+). On the other hand, complexations of guanidinium ion with neutral BH(3) and AlH(3) lead only to very weak complexes (5 and 9). The structures of mono- and dicationic complexes were compared with the structures of protonated and methylated guanidinium dications.     

Substitute Effect on the Structure,Stability of Valence Isomers and Aromaticity of 1‐H‐Boratabenzene

Density functional theory (B3LYP) calculations were performed on the Me and F substituted valence isomeric forms of 1‐H‐boratabenzene. The calculations revealed that the planar benzene analog is the lowest energy isomer. Its aromaticity is analyzed in the light of the nucleus‐independent chemical shift (NICS) and shows that aromaticity increases in F substituted, but decreases in Me substituted. These calculations indicate substitution of BH with BMe and BF doesn't cause significant variation in bond length.     

Aluminium complexes containing bidentate and symmetrical tridentate pincer type pyrrolyl ligands: synthesis, reactions and ring opening polymerization

A series of aluminium derivatives containing substituted bidentate and symmetrical tridentate pyrrolyl ligands, [C(4)H(3)NH(2-CH(2)NH(t)Bu)] and [C(4)H(2)NH(2,5-CH(2)NH(t)Bu)(2)], in toluene or diethyl ether were synthesized. Their reactivity and application for the ring opening polymerization of ε-caprolactone have been investigated. The reaction of AlMe(3) with one equiv. of [C(4)H(3)NH(2-CH(2)NH(t)Bu)] in toluene at room temperature affords [C(4)H(3)N(2-CH(2)NH(t)Bu)]AlMe(2) (1) in 70% yield by elimination of one equiv. of methane. Interestingly, while reacting AlMe(3) with one equiv. of [C(4)H(3)NH(2-CH(2)NH(t)Bu)] in toluene at 0 °C followed by refluxing at 100 °C, [{C(4)H(3)N(2-CH(2)N(t)Bu)}AlMe](2) (2) has been isolated via fractional recrystalliztion in 30% yield. Similarly, reacting AlMe(3) with two equiv. of C(4)H(3)NH(2-CH(2)NH(t)Bu) generates [C(4)H(3)N(2-CH(2)NH(t)Bu)](2)AlMe (3) in a moderate yield. Furthermore, complex 1 can be transformed to an aluminium alkoxide derivative, [C(4)H(3)N(2-CH(2)NH(t)Bu)][OC(6)H(2)(-2,6-(t)Bu(2)-4-Me)]AlMe (4) by reacting 1 with one equiv. of HOC(6)H(2)(-2,6-(t)Bu(2)-4-Me) in toluene via the elimination of one equiv. of methane. The reaction of AlR(3) with one equiv. of [C(4)H(2)NH(2,5-CH(2)NH(t)Bu)(2)] in toluene at room temperature affords [C(4)H(2)N(2,5-CH(2)NH(t)Bu)(2)]AlR(2) (5, R = Me; 6, R = Et) in moderate yield. Surprisingly, from the reaction of two equiv. of [C(4)H(2)NH(2,5-CH(2)NH(t)Bu)(2)] with LiAlH(4) in diethyl ether at 0 °C, a novel complex, [C(4)H(2)N(2-CH(2)N(t)Bu)(5-CH(2)NH(t)Bu)](2)AlLi (7) has been isolated after repeating re-crystallization. Furthermore, reacting one equiv. of C(4)H(2)NH(2,5-CH(2)NH(t)Bu)(2) with AlH(3)·NMe(3) in diethyl ether generates an aluminium dihydride complex, [C(4)H(2)N(2,5-CH(2)NH(t)Bu)(2)]AlH(2) (8), in high yield. Additionally, treating 8 with one equiv. of HOC(6)H(2)(-2,6-(t)Bu(2)-4-Me) in methylene chloride produces [C(4)H(2)N(2,5-CH(2)NH(t)Bu)(2)][OC(6)H(2)(-2,6-(t)Bu(2)-4-Me)]AlH (9) with the elimination of one equiv. of H(2). The aluminium alkoxide complex 4 shows moderate reactivity toward the ring opening polymerization of ε-caprolatone in toluene.     

Unexpected acidity enhancement triggered by AlH3 association to phosphines

The complexes formed by the interaction between a series of phosphines R-PH(2) (R = H, CH(3), c-C(3)H(5), C(6)H(5)) and AlH(3) have been investigated through the use of high-level G4 ab initio calculations. These very stable complexes behave as much stronger acids than the isolated phosphines. This dramatic acidity enhancement, which can be as high as 174 kJ mol(-1), results from a much greater stabilization of the anionic deprotonated species with respect to the neutral one, upon AlH(3) association. This effect depends quantitatively on the nature of the substituent R and is smaller for R = C(6)H(5) because of the conjugation of the P lone pair with the aromatic system. More unexpectedly, however, the phosphine-alane complexes, RPH(2):AlH(3), are more acidic than the corresponding phosphine-borane RPH(2):BH(3) analogues. This unexpected result is due to the enhanced stability of the anionic deprotonated species for complexes involving AlH(3), because the delocalization of the newly created P lone pair with the P-Al bonding density is more favorable when the Lewis acid is aluminum trihydride than when it is borane.     

Heterobuckybowls: a theoretical study on the structure, bowl-to-bowl inversion barrier, bond length alternation, structure-inversion barrier relationship, stability, and synthetic feasibility

Hybrid density functional theory (DFT) calculations at the B3LYP/cc-pVDZ level have been performed on a series of heterobuckybowls, 3X, C(18)X(3)H(6) (X = O, NH, CH(2), BH, S, PH, PH(3), Si, SiH(2), and AlH). The minimum energy conformations and the transition states for bowl-to-bowl inversion, where the geometry is bowl shaped, are computed and characterized by frequency calculations. The geometries of heterotrindenes, 2X, C(12)X(3)H(6) (X = O, NH, CH(2), BH, S, PH, PH(3), Si, SiH(2), and AlH), were obtained, and the bond length alternation (Delta) in the central benzenoid ring shows remarkable sensitivity as a function of substituent with a wide range of fluctuations (-0.014 to +0.092 A). The Delta computed in 2BH was found to be comparable with the highest bond alternation reported to date in benzenoid frameworks. The inversion dynamics of these heterobowls and their bowl depths were fit to a mixed quartic/quadratic function. The size of the heteroatom seems to exclusively control the bowl depth and rigidity as well as the synthetic feasibility. In contrast, the bond length alternation seems to be controlled by electronic factors and not by the size of the substituted atom either in trindenes or in heterosumanenes. The thermodynamic stability of this class of compounds is very much comparable with trithiasumanene (3S), which has been synthesized recently. The chemical hardness (eta) was measured to assess the stability of the heterosumanenes. The strain energy buildup in a sequential ring closure strategy along two synthetic routes, namely a triphenylene route and a trindene route, were explored, and the trindene route was found to be highly favorable for making such compounds compared to the triphenylene route. However, in both routes the ease of the synthetic feasibility increases as the size of the heteroatom increases.     

Metal-induced coordination inversion and carbon-nitrogen bond rearrangement. Structurally characterized phenyl isocyanate inserted into aluminum methyl compounds and O- and N-bound aluminum compounds

[C(4)H(3)N(CH(2)NMe(2))-2]AlMe(2) (1) is prepared in 88% yield by the reaction of substituted pyrrole [C(4)H(4)N(CH(2)NMe(2))-2] with 1 equiv of AlMe(3) in methylene chloride. Reaction of compound 1 with 1 equiv of phenyl isocyanate in toluene generates a seven-membered cycloaluminum compound [C(4)H(3)N[CH(2)NPh(CONMe(2))]-2] AlMe(2) (2). The phenyl isocyanate was inserted into the aluminum and dimethylamino nitrogen bond and induced an unusual rearrangement which results in C-N bond breaking and formation. A control experiment shows that the reaction of substituted pyrrole [C(4)H(4)N(CH(2)NMe(2))-2] with 1 equiv of phenyl isocyanate in diethyl ether yields a pyrrolyl attached urea derivative [C(4)H(3)N(CH(2)NMe(2))-2-[C(=O)NHPh]-1] (3). The demethanation reaction of AlMe(3) with 1 equiv of 3 in methylene chloride at 0 degrees C afforded O-bounded and N-bounded aluminum dimethyl compounds [C(4)H(3)N(CH(2)NMe(2))-2-[C(=O)NPh]-1]AlMe(2) (4a) and [C(4)H(3)N(CH(2)NMe(2))-2-[CO(=NPh)]-1]AlMe(2) (4b) in a total 78% yield after recrystallization. Both 4a and 4b are observed in (1)H NMR spectra; however, the relative ratio of 4a and 4b depends on the solvent used. Two equivalents of AlMe(3) was reacted with 3 in methylene chloride to yield a dinuclear aluminum compound AlMe(3)[C(4)H(3)N(CH(2)NMe(2))-2-[C(=O)NPh]-1] AlMe(2) (5). Reaction of 5 with another equivalent of ligand 3 results in the re-formation of compounds 4a and 4b.     

Theoretical study on the regioselectivity of the B80 buckyball in electrophilic and nucleophilic reactions using DFT-based reactivity indices

Density functional theory calculations at the B3LYP/SVP and B3LYP/6-311G(d) levels were carried out for a series of XH(3)B(80) complexes with X = {N, P, As, B, Al}. To probe the regioselectivity of B(80), the electronic Fukui function, the molecular electrostatic potential (MEP), and the natural bond orbital (NBO) were determined. These indices were shown to provide reliable guides to predict the relative reactivities of the boron buckyball sites. Thermodynamic stabilities of the complexes formed by the reaction of B(80) with nucleophiles (NH(3), PH(3), AsH(3)) and electrophiles (BH(3), AlH(3)) are in good agreement with the prediction of regioselectivity indicated on the basis of Fukui and MEP indices. The qualitative results suggest the boron buckyball to be an amphoteric and hard molecule. It has two distinct reactive sites localized on caps and frame, which act as acids and bases, respectively. Most of the complexes are stable with formation energies comparable to that of the analogous complexes of the borane molecule, BH(3)BH(3), BH(3)NH(3), and BH(3)AlH(3). The B-H-B bond characteristics of diborane are recovered in B(80)BH(3). Exohedral complexes are more stable than endohedral complexes. The most stable complexes are those with NH(3) on the caps and BH(3) on the pentagonal ring of B(80).     

Neutral homoaromaticity in some heterocyclic systems

[reaction: see text] Neutral homoaromaticity has been evaluated in heterocyclic systems related to the bicyclo[3.2.1]octane skeleton with replacement of CH(2) at C-2 in bicyclo[3.2.1]octa-3,6-diene with X = BH, AlH, Be, Mg, O, S, PH, NH (12); replacement of CH at C-3 in bicyclo[3.2.1]octa-3,6-dien-2-yl anion with PH, S, NH, O (13); and replacement at C-2 and C-3 with N and O (14). Stabilization energies (SE) are evaluated using density functional theory and homodesmotic equations at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) level for series 12-14. Stabilization energies are compared with diamagnetic susceptibility exaltations, Lambda, CSGT-B3LYP/6-31G(d)//B3LYP/6-31G(d), and nucleus-independent chemical shifts (NICS), GIAO-B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d). Analysis of frontier orbitals and geometries, B3LYP/6-31G(d)//B3LYP/6-31G(d), and proton affinities of 2-azabicyclo[3.2.1]octa-3,6-diene, pyrrole, and related model systems, B3LYP/6-311+G(2d,2p)//B3LYP/6-31G(d), provide complementary evidence supporting the division of the substrates evaluated into antihomoaromatic (12, X = BH, AlH, and Be), nonhomoaromatic (12, X = O, S, NH, PH), and homoaromatic (13, X = S, PH, NH, O and 14 X = ON), with 12 (X = Mg) appearing as transitional between anti- and nonhomoaromatic.     

Kinetic study of the phthalimide N-oxyl radical in acetic acid. Hydrogen abstraction from substituted toluenes, benzaldehydes, and benzyl alcohols

The phthalimide N-oxyl (PINO) radical was generated by the oxidation of N-hydroxyphthalimide (NHPI) with Pb(OAc)4 in acetic acid. The molar absorptivity of PINO* is 1.36 x 10(3) L mol(-1) cm(-1) at lambda(max) 382 nm. The PINO radical decomposes slowly with a second-order rate constant of 0.6 +/- 0.1 L mol(-1) s(-1) at 25 degrees C. The reactions of PINO(*) with substituted toluenes, benzaldehydes, and benzyl alcohols were investigated under an argon atmosphere. The second-order rate constants were correlated by means of a Hammett analysis. The reactions with toluenes and benzyl alcohols have better correlations with sigma+ (rho = -1.3 and -0.41), and the reaction with benzaldehydes correlates better with sigma (rho = -0.91). The kinetic isotope effect was also studied and significantly large values of k(H)/k(D) were obtained: 25.0 (p-xylene), 27.1 (toluene), 27.5 (benzaldehyde), and 16.9 (benzyl alcohol) at 25 degrees C. From the Arrhenius plot for the reactions with p-xylene and p-xylene-d(10), the difference of the activation energies, E(a)(D) - E(a)(H), was 12.6 +/- 0.8 kJ mol(-1) and the ratio of preexponential factors, A(H)/A(D), was 0.17 +/- 0.05. These findings indicate that quantum mechanical tunneling plays an important role in these reactions.     

Half-sandwich bis(tetramethylaluminate) complexes of the rare-earth metals: synthesis, structural chemistry, and performance in isoprene polymerization

The protonolysis reaction of [Ln(AlMe(4))(3)] with various substituted cyclopentadienyl derivatives HCp(R) gives access to a series of half-sandwich complexes [Ln(AlMe(4))(2)(Cp(R))]. Whereas bis(tetramethylaluminate) complexes with [1,3-(Me(3)Si)(2)C(5)H(3)] and [C(5)Me(4)SiMe(3)] ancillary ligands form easily at ambient temperature for the entire Ln(III) cation size range (Ln=Lu, Y, Sm, Nd, La), exchange with the less reactive [1,2,4-(Me(3)C)(3)C(5)H(3)] was only obtained at elevated temperatures and for the larger metal centers Sm, Nd, and La. X-ray structure analyses of seven representative complexes of the type [Ln(AlMe(4))(2)(Cp(R))] reveal a similar distinct [AlMe(4)] coordination (one eta(2), one bent eta(2)). Treatment with Me(2)AlCl leads to [AlMe(4)] --> [Cl] exchange and, depending on the Al/Ln ratio and the Cp(R) ligand, varying amounts of partially and fully exchanged products [{Ln(AlMe(4))(mu-Cl)(Cp(R))}(2)] and [{Ln(mu-Cl)(2)(Cp(R))}(n)], respectively, have been identified. Complexes [{Y(AlMe(4))(mu-Cl)(C(5)Me(4)SiMe(3))}(2)] and [{Nd(AlMe(4))(mu-Cl){1,2,4-(Me(3)C)(3)C(5)H(2)}}(2)] have been characterized by X-ray structure analysis. All of the chlorinated half-sandwich complexes are inactive in isoprene polymerization. However, activation of the complexes [Ln(AlMe(4))(2)(Cp(R))] with boron-containing cocatalysts, such as [Ph(3)C][B(C(6)F(5))(4)], [PhNMe(2)H][B(C(6)F(5))(4)], or B(C(6)F(5))(3), produces initiators for the fabrication of trans-1,4-polyisoprene. The choice of rare-earth metal cation size, Cp(R) ancillary ligand, and type of boron cocatalyst crucially affects the polymerization performance, including activity, catalyst efficiency, living character, and polymer stereoregularity. The highest stereoselectivities were observed for the precatalyst/cocatalyst systems [La(AlMe(4))(2)(C(5)Me(4)SiMe(3))]/B(C(6)F(5))(3) (trans-1,4 content: 95.6 %, M(w)/M(n)=1.26) and [La(AlMe(4))(2)(C(5)Me(5))]/B(C(6)F(5))(3) (trans-1,4 content: 99.5 %, M(w)/M(n)=1.18).     

Synthesis, coupling, and condensation reactions of 1,2-diborylated benzenes: an experimental and quantum-chemical study

1,2-Bis(pinacolboryl)benzene (1,2-C(6) H(4) (Bpin)(2) , 2) was synthesized in preparatively useful yields from 1,2-C(6) H(4) Br(2) , iPrO?Bpin, and Mg turnings in the presence of 1,2-C(2) H(4) Br(2) as an entrainer. Compound 2 is a versatile starting material for the synthesis of (un)symmetrically substituted benzenes (i.e., 1,2-C(6) H(4) (Ar(1) )(Ar(2) )) through sequential Suzuki-Miyaura coupling reactions. Alternatively, it can be transformed into bis-borate Li(2) [1,2-C(6) H(4) (BH(3) )(2) ] (3) through reduction with Li[AlH(4) ]. In the crystal lattice, the diethyl ether solvate 3?OEt(2) establishes a columnar structure that is reinforced by an intricate network of B?(μ-H)?Li interactions. Hydride-abstraction from compound 3 with Me(3) SiCl leads to the transient ditopic borane 1,2-C(6) H(4) (BH(2) )(2) , which can either be used in situ for subsequent hydroboration reactions or trapped as its stable NMe(2) Et diadduct (6). In SMe(2) solution, the putative diadduct 1,2-C(6) H(4) (BH(2) ?SMe(2) )(2) is not long-term stable but rather undergoes a condensation reaction to give 9,10-dihydro-9,10-diboraanthracene, HB(μ-C(6) H(4) )(2) BH, and BH(3) . 9,10-Dihydro-9,10-diboraanthracene was isolated from the reaction mixture as its SMe(2) monoadduct (7), which dimerizes in the solid state through two B?H?B bridges ((7)(2) , elucidated by X-ray crystallography). In contrast, hydride-abstraction from compound 3 in THF or CH(2) Cl(2) provides the unique exo-adduct H(2) B(μ-H)(2) B(μ-C(6) H(4) )(2) B(μ-H)(2) BH(2) (8, elucidated by X-ray crystallography). Quantum-chemical calculations on various conceivable isomers of [1,2-C(6) H(4) (BH(2) )(2) ](2) revealed that compound 8 was the most stable of these species. Moreover, the calculations confirmed the experimental findings that the NMe(2) Et diadduct of 1,2-C(6) H(4) (BH(2) )(2) is significantly more stable than the corresponding SMe(2) complex and that the latter complex is not able to compete successfully with borane-dimerization and -condensation. The reaction cascade in SMe(2) , which proceeds from 1,2-C(6) H(4) (BH(2) )(2) to the observed adducts of HB(μ-C(6) H(4) )(2) BH, has been elucidated in detail and the important role of B?C?B-bridged intermediates has been firmly established.     

Detection and Reaction of Oxaphosphetanes Derived from Benzaldehyde and 1-Adamantylmethylidene Ylide

The Wittig reaction of (1-adamantylmethylidene)triphenylphosphorane (Ph(3)P=CH(1-Ad)) with benzaldehyde was investigated, and the results were compared with those of other ylides. The substituent effect in the reaction of the ylide with benzaldehydes was determined by competition experiments, which gave a Hammett rho value of 3.2. The rho value is much larger than those reported for analogous reactions of Ph(3)P=CH(CH(2))(2)CH(3) (rho = 0.20) and Ph(3)P=CH(CH(3))(2) (rho = 0.59), indicating that the reaction mechanism differs for Ph(3)P=CH(1-Ad) and the other ylides. The cis/trans ratio of the product alkene is 74/26 for the reaction with the parent benzaldehyde and highly depends on the position of the substituent; ortho substituted benzaldehydes gave the trans alkenes up to 90%. Monitoring the reaction by means of (31)P NMR revealed that both cis and trans oxaphosphetane intermediates were formed and that the formation and decomposition of the cis oxaphosphetane are 7-12 times faster than those of the trans oxaphosphetane. From the comparison of the reaction of Ph(3)P=CH(1-Ad) + benzaldehyde with those of Ph(3)P=CH(CH(2))(2)CH(3) + benzaldehyde and benzophenone, and Ph(3)P=CH(CH(3))(2) + benzophenone, it was concluded that all the reactions with these nonstabilized ylides proceed via an electron-transfer mechanism and that the rate-determining step changes from the electron transfer step to that of radical combination when the substrate or ylide becomes more sterically demanding.     

Reaction Ergodography and Transition State Analyses of the Addition of AIH_3 to Acetylene

The present paper covers the reaction ergodography for the addition of AlH3 to acetylene investigated by ab initio calculation with RHF/3-21G basis set. The changes in some physical properties along the reaction path (IRC) are presented. The formations of FMOs of the transition state have been analyzed according to the Fukui' s method. On the basis of the perturbation theory and Wolfsborg Helmholtz formula, we calculated the FMO' s energy orders of TS and its formation that shows the HOMO of TS consists of LUMO, HOMO of AlH3 and HOMO of HCCH.     

Concise preparation of 2,2-difluorohomopropargyl carbonyl derivatives. Application to the synthesis of 4,4-difluoroisoquinolinone congeners

A scalable synthesis of 2,2-difluorohomopropargyl esters was achieved using a magnesium-promoted Barbier reaction of substituted difluoropropargyl bromides with alkyl chloroformates. These 2,2-difluorohomopropargyl esters were effective precursors in the synthesis of homopropargylic amides-by aminolysis using AlMe3, as well as of ketones-through the reaction of the corresponding Weinreb amides with Grignard reagents. Ring closing metathesis using difluorinated 1,7-enyne carbonyl compounds furnished six-membered diene products, which were used as susbstrates in a Diels-Alder reaction to afford 4,4-difluoroisoquinolin-3-ones. The [2 + 2 + 2] cycloaddition of alkynes with fluorinated 1,7-diyne amides gave 4,4-difluoro-1,4-dihydro-3(2H)-isoquinolinone derivatives regioselectively.     

Effect of transition state aromaticity and antiaromaticity on intrinsic barriers of proton transfers in aromatic and antiaromatic heterocyclic systems; an ab initio study

An ab initio study of two series of carbon-to-carbon proton transfer reactions is reported. The first series refers to the heterocyclic C(4)H(5)X(+)/C(4)H(4)X (X = CH(-), NH, S, O, PH, CH(2), AlH, BH) systems, and the second to the linear [Formula: see text] (X = CH(-), NH, S, PH, O, CH(2), AlH, BH) reference systems . The major objective of this study was to examine to what degree the aromaticity of C(4)H(4)X (X = CH(-), NH, S, O, PH) and the antiaromaticity of C(4)H(4)X (X = AlH, BH) is expressed at the transition state of the proton transfer and how this affects the respective intrinsic barriers. From the differences in the barriers between a given cyclic system and the corresponding linear reference system , ΔΔH(++) = ΔH(++)(cyclic) - ΔH(++)(linear), it was inferred that in the cyclic systems both aromaticity and antiaromaticity lower ΔH(++)(cyclic). This conclusion was based on the assumption that the factors not associated with aromaticity or antiaromaticity such as resonance, inductive and polarizability effects in the protonated species, and charge delocalization occurring along the reaction coordinate affect ΔH(++) for the cyclic and linear systems in a similar way and hence offset each other in ΔΔH(++). The extent by which ΔH(++)(cyclic) is lowered in the aromatic systems correlates quite well with the degree of aromaticity of C(4)H(4)X as measured by aromatic stabilization energies as well as the NICS(1) values of the respective C(4)H(4)X. According to the rules of the principle of nonperfect synchronization (PNS), these results imply a disproportionately large degree of aromaticity at the transition state for the aromatic systems and a disproportionately small degree of transition state antiaromaticity for the antiaromatic systems. These conclusions are consistent with the changes in the NICS(1) values along the reaction coordinate. Other points discussed in the paper include the complex interplay of resonance, inductive, and polarizability effects, along with aromaticity and antiaromaticity on the proton affinities of C(4)H(4)X.     

Sigma- and pi-bond strengths in main group 3-5 compounds

The sigma- and pi-bond strengths for the molecules BH2NH2, BH2PH2, AlH2NH2, and AlH2PH2 have been calculated by using ab initio molecular electronic structure theory at the CCSD(T)/CBS level. The adiabatic pi-bond energy is defined as the rotation barrier between the equilibrium ground-state configuration and the C(s)symmetry transition state for torsion about the A-X bond. We also report intrinsic pi-bond energies corresponding to the adiabatic rotation barrier corrected for the inversion barrier at N or P. The adiabatic sigma-bond energy is defined as the dissociation energy of AH2XH2 to AH2 + XH2 in their ground states minus the adiabatic pi-bond energy. The adiabatic sigma-bond strengths for the molecules BH2NH2, BH2PH2, AlH2NH2, and AlH2PH2 are 109.8, 98.8, 77.6, and 68.3 kcal/mol, respectively, and the corresponding adiabatic pi-bond strengths are 29.9, 10.5, 9.2, and 2.7 kcal/mol, respectively.