Structure and electronic nature of the benzaldehyde/boron trifluoride adduct

Combined Theoretical and Experimental Investigation of Lewis Acid-Carbonyl Interactions for Metathesis

The coordination of a carbonyl to a Lewis acid represents the first step in a wide range of catalytic transformations. In many reactions it is necessary for the Lewis acid to discriminate between starting material and product, and as a result, how these structures behave in solution must be characterized. Herein, we report the application of computational modeling to calculate properties of the solution interactions of acetone and benzaldehyde with FeCl₃. Using these chemical models, we can predict spectral features in the carbonyl region of infrared (IR) spectroscopy. These simulated spectra are then directly compared to experimental spectra generated via titration-IR. We observe good agreement between theory and experiment, in that, between 0 and 1 equiv carbonyl with respect to FeCl₃, a pairwise interaction dominates the spectra. When >1 equiv carbonyl is present, our theoretical model predicts two possible structures composed of 4:1 carbonyl to FeCl₃, for acetone as well as benzaldehyde. When these predicted spectra are compared with titration-IR data, both structures contribute to the observed solution interactions. These findings suggest that the resting state of FeCl₃-catalyzed carbonyl-based reactions employing simple substrates starts as a Lewis pair, but this structure is gradually consumed and becomes a highly ligated, catalytically less active Fe-centered complex as the reaction proceeds. An analytical model is proposed to quantify catalyst inhibition due to equilibrium between 1:1 and 4:1 carbonyl:Fe complexes.

Transition metal catalysis—a unique road map in the stereoselective synthesis of 1,3-polyols

The present review summarizes recent diverse reactions employed in the formation of 1,3-polyols providing an overview of the mechanistic pathway and the enantioselectivity obtained, in terms of the properties of transition metals directly involved in the catalytic transformations and their interaction with various ligands.

A detailed NMR- and DFT-based study of the Sakurai-Hosomi-Yamamoto asymmetric allylation reaction

A Lewis acid complex between benzaldehyde and the silver catalyst was detected by (31)P NMR and shown to be the direct precursor to allylation within the Sakurai-Hosomi-Yamamoto reaction. Structural and thermochemical hybrid-DFT calculations indicated that benzaldehyde predominantly formed an η(1)-σ-complex with the catalyst; however, two other competing conformers involving different coordination modes were found, including an activated μ(2)-bound complex. The differences in (31)P NMR shifts upon complexation were calculated by the gauge-independent atomic orbital (GIAO-DFT) method for each conformer. The minimum energy conformer was found to correlate well with chemical shift trends observed experimentally, and an analysis of Mullikan charge populations revealed that the carbonyl carbon of the highest-energy conformer was the most electron-deficient. Furthermore, one minor and three major silicon intermediates were detected by (29)Si NMR and, with the aid of (1)H-(29)Si HSQC, were assigned by comparison with parent compounds and GIAO-DFT calculations. Finally, a tentative mechanism was proposed based on these findings.

Catalytic enantioselective Diels--Alder reactions: methods, mechanistic fundamentals, pathways, and applications

One hundred years after the birth of Kurt Alder and seventy-five years after the discovery of his famous reaction, one of the most important and fascinating transformations in chemistry, research on that process continues to surprise, excite, delight, and inform the chemical community. This article is based on presentations given first at the University of Cologne, Germany (Kurt Alder lecture, 1992), then at the Roger Adams Award Symposium (1993), and later at the Bürgenstock Conference of 2001, and describes research by our group on the development and understanding of enantioselective versions of the Diels-Alder reactions. The elements of this review include (1) development of new chiral Lewis acid catalysts for highly enantioselective (>25:1) [4+2] cycloadditions; (2) the fine mechanistic details and pre-transition-state assemblies of these reactions; (3) the fundamental understanding of catalytic activity and enantioselectivity for highly enantioselective Diels-Alder processes; and (4) applications to the synthesis of complex molecules. The range and power of the Diels-Alder reaction have steadily increased over seven decades. The end of this remarkable development is not in sight, a high compliment to this field of Science and to its great inventor.

An investigation of (C5H5)Fe(C5H4–C(OBF3)–CH3),

Acetylferrocene readily forms an adduct with BF3. The product is a neutral model for protonated acetylferrocene, a species that is a crucial intermediate in the formation of 1,3,5-trisferrocenyl-benzene and other conjugated heterometallic derivatives by cyclocondensation. The title compound’s structure was determined in the solid state by X-ray diffraction and its spectroscopic properties were examined in detail, particularly the electronic excitation spectrum, which confirms the previous identification of the in situ generated protonated ketone.

A Large Scale Synthesis of a Natural Antibiotic, 2,4-Diacetylophloroglucinol (DAPG)

A natural antibiotic, 2,4-diacetylophloroglucinol (DAPG), has been prepared by the acetylation of phloroglucinol (PhL) with acetic anhydride in the presence of boron trifluoride etherate complex. The crude DAPG is efficiently purified using chromatographic filtration with boiling eluent.

Hetero Diels−Alder Reaction of Vinyl Allenes and Aldehydes. An Experimental and Computational Study

The hetero Diels-Alder reaction of vinyl allenes and aldehydes in the presence of a Lewis acid has been studied both experimentally and theoretically. Differently substituted vinyl allenes and aldehydes were used to obtain information on the structural requirements of the reaction. Theoretical calculations using the density functional theory indicate that the reaction proceeds through a highly asynchronous polar transition state.

A New Perspective in the Lewis Acid Catalyzed Ring Opening of Epoxides. Theoretical Study of Some Complexes of Methanol, Acetic Acid, Dimethyl Ether, Diethyl Ether, and Ethylene Oxide with Boron Trifluoride

Several 1:1, 1:2, and 2:2 complexes between BF3 and CH3OH (Met), CH3COOH (AcA), (CH3)2O (DME), (CH3CH2)2O (DEE), and (CH2)2O (EOX) have been studied using ab initio (MP2) and density functional theory (DFT) (PBE, B3LYP) methods and the 6-311++G(3df,2pd) basis set. Geometrical structures and vibrational frequencies are reported, in most cases, for the first time. A detailed comparison of the vibrational frequencies for the O...BF3 vibrational modes, as well as for the nu(OH) band in the methanol and acetic acid complexes with BF3, is performed, and the theoretical frequency shifts are compared with the available experimental information. Thermochemical properties are calculated by employing counterpoise correction to alleviate the basis set superposition error. The DFT enthalpy of complexation of the 1:1 complexes results in the order of stability (AcA)2>AcA:BF3>DEE:BF3>DME:BF3>Met:BF3>EOX:BF3>(Met)2; in contrast, MP2 shows the noticeable difference that the AcA:BF3 complex is much less stable (similar to Met:BF3). The order of stability shows that, even though acetic acid prefers dimerization to complexation with BF3, the case is exactly the opposite for methanol. In both cases, the interaction of BF3 with the dimer gives rise to very stable trimers. However, in contrast to the interaction of BF3 with the methanol dimer being stronger than that with the monomer, the interaction of BF3 with the acetic acid dimer is weaker than that with the monomer. The relative strength of the complexes, discussed in the context of BF3-catalyzed ring opening of epoxides, suggests that the effect of the catalyst in a nonprotogenic solvent should be more properly ascribed to activation of the nucleophile instead of activation of the epoxide.

Intramolecular Hetero-Michael Addition of β-Hydroxyenones for the Preparation of Highly Substituted Tetrahydropyranones

Structurally diverse beta-hydroxyenones are shown to undergo nonoxidative 6-endo-trig ring closure to form highly substituted tetrahydropyranones. Amberlyst-15, Al(ClO(4))(3) x 9 H(2)O and [Pd(MeCN)(4)](BF(4))(2) were found to be suitable catalysts for these intramolecular conjugate additions, preventing side reactions, such as dehydration or retroaldolisation. The use of [Pd(MeCN)(4)](BF(4))(2) is particularly effective, as this palladium-mediated reaction is under kinetic control and generates tri- and tetrasubstituted tetrahydropyranones with high levels of diastereocontrol. In the presence of the Lewis acid Al(ClO(4))(3) x 9 H(2)O, the reaction proceeded with a similar level of diastereocontrol; however, in contrast to [Pd(MeCN)(4)](BF(4))(2), this catalyst can promote enolisation. The palladium-mediated reaction was also found to be compatible with an enantioenriched beta-hydroxyenone substrate, giving no loss of enantiopurity upon ring closure. The most distinctive synthetic development to emerge from this new chemistry is the possibility to access tri- and tetrasubstituted 2,6-anti-tetrahydropyranones from anti-aldol precursors. These compounds are particularly difficult to access by using alternative methodologies. Two modes of activation were envisaged for the ring closure, involving metal coordination to either the C=C or C=O functional groups. Experimental results suggest that C=O coordination was the preferred mode of activation for reactions performed in the presence of Al(ClO(4))(3) x 9 H(2)O or [Pd(MeCN)(4)](BF(4))(2).

Molecular recognition of alpha,beta-unsaturated carbonyl compounds using aluminum tris(2,6-diphenylphenoxide) (ATPH): structural and conformational analysis of ATPH complexes and application to the selective vinylogous aldol reaction

Various alpha,beta-unsaturated carbonyl compounds were coordinated with aluminum tris(2,6-diphenylphenoxide) (ATPH) to give the corresponding Lewis acid-base complexes in a distinctive coordination fashion (selective coordination). ATPH recognizes carbonyl substrates and subsequently orients itself as it forms a stable complex through selective coordination with the carbonyl oxygen. Selective coordination also confers a conformational preference to each carbonyl compound under the steric and electronic influence of ATPH, which enables the vinylogous aldol reaction of alpha,beta-unsaturated carbonyl compounds to give the corresponding gamma-aldol products with different regio- and stereoselectivities.

A DFT study of the mechanisms and regio- and stereochemistry of the lewis acid-catalyzed reactions of 5-alkoxyoxazoles with aldehydes: aryl substitution at the 2-position of 5-alkoxyoxazole is critical to the formation of 4-alkoxycarbonyl-2-oxazoline

Density functional theory at the B3LYP/6-31G level has been used to study the mechanisms and regio- and stereochemistry of the Lewis acid-catalyzed reactions of aldehydes with 5-alkoxyoxazoles. Similar to the uncatalyzed reaction between aldehyde and 5-methoxyoxazole, which has an activation energy of 30.5 kcal/mol and intrinsically favors production of 2-alkoxycarbonyl-3-oxazoline, the Lewis acid-catalyzed reaction also prefers to generate 2-alkoxycarbonyl-3-oxazoline in a more efficient way with an activation energy of about 3 kcal/mol (with respect to separated acetaldehyde-AlCl3 complex and 5-methoxyoxazole) in the gas phase. Only when an aryl group is introduced to the 2 position of 5-alkoxyoxazoles can the Lewis acid-catalyzed reactions furnish 4-alkoxycarbonyl-2-oxazolines. The reasons for this switch of regiochemistry and the factors affecting the stereochemistry are discussed.

Synthetic studies on the marine natural product halichondrins

In connection with the de elopment of a practical synthesis of the right half, and its analog E7389, of halichondrin B, an efficient and scalable synthesis of the two major building blocks is reported. In addition, a new synthesis of the C20–C26 segment via a regiospecific and stereoselecti e SN2' process is presented. A sulfonamide class of ligands is shown to be effective for asymmetric Ni/Cr-mediated reactions under both stoichiometric and catalytic conditions, and the X-ray structure reveals this class of ligands to be tridentate. On the basis of three X-ray structures, a possible mechanism is suggested for this process. Stable and crystalline Cr(III)/sulfonamide complexes are shown to be effective for catalytic Cr-mediated coupling reactions of allyl, alkenyl, and alkyl halides with aldehydes, and some examples for application of the stoichiometric and catalytic asymmetric processes are presented.

Increased Felkin-Anh selectivity using AlMe(3) in the addition of vinyllithiums to alpha-chiral aldehydes: do "ate" complexes play any role?

[reaction: see text] AlMe(3) dramatically increases the diastereoselectivity of addition of vinyllithiums to alpha-chiral aldehydes but decreases that of methyllithium. Our results are explained in terms of an addition of the free vinyllithium on the Me(3)Al-aldehyde complex.

Asymmetric Ni(II)/Cr(II)-mediated coupling reaction: stoichiometric process

[reaction: see text] Via an X-ray analysis, the sulfonamide bearing R(1) = i-Pr, R(2) = Me, and R(3) = Me is shown to be a tridentate ligand to a Cr(III) salt. This class of ligands, represented by R(1) = t-Bu, R(2) = 2-naphthyl, and R(3) = Me, is effective to achieve an asymmetric Ni/Cr-mediated coupling reaction and, with the C14-C38 segment of halichondrins, its synthetic potential has been demonstrated. A possible mechanism is suggested for the process.

Equilibrium constants between boron trifluoride etherate and carbonyl compounds in chloroform solution

[reaction: see text] Equilibrium constants for boron trifluoride complexation with carbonyl compounds relative to diethyl ether were determined in CDCl(3). With benzaldehyde the equilibrium constant is 0.208. There is a 1.28-fold preference for BF(3) bonding to benzaldehyde-D over benzaldehyde-H. The rho(+) value for complexation with substituted benzaldehydes is -2.0. Equilibrium constants with cyclohexanone and isobutyraldehyde were found to be not very different from those predicted from the heats of addition of BF(3) relative to that for diethyl ether. The heats of addition can be correlated with Taft's beta values and with beta' values.

Alkali salts of C3-symmetric, linked aryloxides: selective binding of substrates with metal aggregates

The lithium, sodium, and potassium salts of tris(3,5-dialkyl-2-hydroxyphenyl)methanes (tert-butyl, tert-pentyl, methyl) have been prepared by reaction of the triarylmethane with n-butyllithium, sodium hydride, and potassium hydride, respectively. These compounds are all hexanuclear aggregates composed of two triarylmethane units. Whereas the lithium salt is compact and cannot bind oxygen-donor solvent molecules, the sodium and potassium systems have vacant coordination sites that can interact with solvents. For the sodium compounds, the solvent can be subsequently removed, and the resulting coordinatively unsaturated compounds have been shown to selectively bind oxygen-donor substrates (ethers, aldehydes, and ketones) of suitable size and shape. The paper reports the synthesis and characterization of these novel compounds, including thirteen crystal structures of the salts and their adducts.