Russell J. Wakeham

Iodide as an Activating Agent for Acid Chlorides in Acylation Reactions

Acid chlorides can be activated using a simple iodide source to undergo nucleophilic attack from a variety of relatively weak nucleophiles. These include Friedel-Crafts acylation of N-methylpyrroles, N-acylation of sulfonamides, and acylation reactions of hindered phenol derivatives. The reaction is believed to proceed through a transient acid iodide intermediate.

Synthesis, Characterization, and Some Properties of Cp*W(NO)(H)(η3-allyl) Complexes

Sequential treatment at low temperatures of Cp*W(NO)Cl2 in THF with 1 equiv of a binary magnesium allyl reagent, followed by an excess of LiBH4, affords three new Cp*W(NO)(H)(η(3)-allyl) complexes, namely, Cp*W(NO)(H)(η(3)-CH2CHCMe2) (1), Cp*W(NO)(H)(η(3)-CH2CHCHPh) (2), and Cp*W(NO)(H)(η(3)-CH2CHCHMe) (3). Complexes 1-3 are isolable as air-stable, analytically pure yellow solids in good to moderate yields by chromatography or fractional crystallization. In solutions, complex 1 exists as two coordination isomers in an 83:17 ratio differing with respect to the endo/exo orientation of the allyl ligand. In contrast, complexes 2 and 3 each exist as four coordination isomers, all differing by the orientation of their allyl ligands which can have either an endo or an exo orientation with the phenyl or methyl groups being either proximal or distal to the nitrosyl ligand. A DFT computational analysis using the major isomer of Cp*W(NO)(H)(η(3)-CH2CHCHMe) (3a) as the model complex has revealed that its lowest-energy thermal-decomposition pathway involves the intramolecular isomerization of 3a to the 16e η(2)-alkene complex, Cp*W(NO)(η(2)-CH2═CHCH2Me). Such η(2)-alkene complexes are isolable as their 18e PMe3 adducts when compounds 1-3 are thermolyzed in neat PMe3, the other organometallic products formed during these thermolyses being Cp*W(NO)(PMe3)2 (5) and, occasionally, Cp*W(NO)(H)(η(1)-allyl)(PMe3). All new complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of most of them have been established by single-crystal X-ray crystallographic analyses.

Alternative Hydrogen Source for Asymmetric Transfer Hydrogenation in the Reduction of Ketones

cis‐1,4‐Butenediol is shown to be a highly active hydrogen source for asymmetric transfer hydrogenation in the reduction of ketones. With the use of a ruthenium catalyst, cis‐1,4‐butenediol is isomerised and subsequently oxidised to a lactone as an irreversible step, which provides the driving force for the asymmetric reduction of ketones.

Cationic and Neutral Cp*M(NO)(κ2-Ph2PCH2CH2PPh2) Complexes of Molybdenum and Tungsten: Lewis-Acid-Induced Intramolecular C–H Activation

Treatment of CH2Cl2 solutions of Cp*M(NO)Cl2 (Cp* = η5-C5(CH3)5; M = Mo, W) first with 2 equiv of AgSbF6 in the presence of PhCN and then with 1 equiv of Ph2PCH2CH2PPh2 affords the yellow-orange salts [Cp*M(NO)(PhCN)(κ2-Ph2PCH2CH2PPh2)](SbF6)2 in good yields (M = Mo, W). Reduction of [Cp*M(NO)(PhCN)(κ2-Ph2PCH2CH2PPh2)](SbF6)2 with 2 equiv of Cp2Co in C6H6 at 80 °C produces the corresponding 18e neutral compounds, Cp*M(NO)(κ2-Ph2PCH2CH2PPh2) which have been isolated as analytically pure orange-red solids. The addition of 1 equiv of the Lewis acid, Sc(OTf)3, to solutions of Cp*M(NO)(κ2-Ph2PCH2CH2PPh2) at room temperature results in the immediate formation of thermally stable Cp*M(NO→Sc(OTf)3)(H)(κ3-(C6H4)PhPCH2CH2PPh2) complexes in which one of the phenyl substituents of the Ph2PCH2CH2PPh2 ligands has undergone intramolecular orthometalation. In a similar manner, addition of BF3 produces the analogous Cp*M(NO→BF3)(H)(κ3-(C6H4)PhPCH2CH2PPh2) complexes. In contrast, B(C6F5)3 forms the 1:1 Lewis acid-base adducts, Cp*M(NO→B(C6F5)3)(κ2-Ph2PCH2CH2PPh2) in CH2Cl2 at room temperature. Upon warming to 80 °C, Cp*Mo(NO→B(C6F5)3)(κ2-Ph2PCH2CH2PPh2) converts cleanly to the orthometalated product Cp*Mo(NO→B(C6F5)3)(H)(κ3-(C6H4)PhPCH2CH2PPh2), but Cp*W(NO→B(C6F5)3)(κ2-Ph2PCH2CH2PPh2) generates a mixture of products whose identities remain to be ascertained. Attempts to extend this chemistry to include related Ph2PCH2PPh2 compounds have had only limited success. All new complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of most of them have been established by single-crystal X-ray crystallographic analyses.

Thermal Chemistry of Cp*W(NO)(CH2CMe3(H)(L) Complexes (L=Lewis Base)

The complexes trans-Cp*W(NO)(CH2CMe3)(H)(L) (Cp* = η5-C5Me5) result from the treatment of Cp*W(NO)(CH2CMe3)2 in n-pentane with H2 (∼1 atm) in the presence of a Lewis base, L. The designation of a particular geometrical isomer as cis or trans indicates the relative positions of the alkyl and hydrido ligands in the base of a four-legged piano-stool molecular structure. The thermal behavior of these complexes is markedly dependent on the nature of L. Some of them can be isolated at ambient temperatures [e.g., L = P(OMe)3, P(OPh)3, or P(OCH2)3CMe]. Others undergo reductive elimination of CMe4 via trans to cis isomerization to generate the 16e reactive intermediates Cp*W(NO)(L). These intermediates can intramolecularly activate a C-H bond of L to form 18e cis complexes that may convert to the thermodynamically more stable trans isomers [e.g., Cp*W(NO)(PPh3) initially forms cis-Cp*W(NO)(H)(κ2-PPh2C6H4) that upon being warmed in n-pentane at 80 °C isomerizes to trans-Cp*W(NO)(H)(κ2-PPh2C6H4)]. Alternatively, the Cp*W(NO)(L) intermediates can effect the intermolecular activation of a substrate R-H to form trans-Cp*W(NO)(R)(H)(L) complexes [e.g., L = P(OMe)3 or P(OCH2)3CMe; R-H = C6H6 or Me4Si] probably via their cis isomers. These latter activations are also accompanied by the formation of some Cp*W(NO)(L)2 disproportionation products. An added complication in the L = P(OMe)3 system is that thermolysis of trans-Cp*W(NO)(CH2CMe3)(H)(P(OMe)3) results in it undergoing an Arbuzov-like rearrangement and being converted mainly into [Cp*W(NO)(Me)(PO(OMe)2)]2, which exists as a mixture of two isomers. All new complexes have been characterized by conventional and spectroscopic methods, and the solid-state molecular structures of most of them have been established by single-crystal X-ray crystallographic analyses.

Post-Synthetic Ligand Exchange in Zirconium-Based Metal-Organic Frameworks: Beware of The Defects!

Post-synthetic ligand exchange in the prototypical zirconium-based metal-organic framework (MOF) UiO-66 was investigated by in situ solution 1 H NMR spectroscopy. Samples of UiO-66 having different degrees of defectivity were exchanged using solutions of several terephthalic acid analogues in a range of conditions. Linker exchange only occurred in defect-free UiO-66, whereas monocarboxylates grafted at defect sites were found to be preferentially exchanged with respect to terephthalic acid over the whole range of conditions investigated. A 1:1 exchange ratio between the terephthalic acid analogue and modulator was observed, providing evidence that the defects had missing-cluster nature. Ex situ characterisation of the MOF powders after exchange corroborated these findings and showed that the physical-chemical properties of the MOF depend on whether the functionalisation occurs at defective sites or on the framework.