Ir-catalysed allylic substitution is supplementing the traditional Pd-catalysed variant. With simple, easily available monosubstituted allylic acetates and carbonates as substrates, Ir catalysts generally favour chiral, branched products, while Pd catalysts typically give rise to linear, achiral products. With phosphorus amidites as ligands, regioselectivities >10 : 1 and enantiomeric excess in the range 95-99 %ee are currently routinely achieved. A broad range of nucleophiles can be employed: for example stabilised carbanions, amines including their sulfonyl- and diacyl-derivatives, phenolates and alkoxides. A few applications, based on combinations of the allylic substitution and ring closing metathesis, indicate considerable potential of the method for the synthesis of biologically active compounds.
The mechanistic course of the amination of alcohols with ammonia catalyzed by a structurally modified congener of Milstein's well-defined acridine-based PNP-pincer Ru complex has been investigated both experimentally and by DFT calculations. Several key Ru intermediates have been isolated and characterized. The detailed analysis of a series of possible catalytic pathways (e.g., with and without metal-ligand cooperation, inner- and outer-sphere mechanisms) leads us to conclude that the most favorable pathway for this catalyst does not require metal-ligand cooperation.
[Cp*Ir(Pro)Cl] (Pro = prolinato) was identified among a series of Cp*-iridium half-sandwich complexes as a highly reactive and selective catalyst for the alkylation of amines with alcohols. It is active under mild conditions in either toluene or water without the need for base or other additives, tolerates a wide range of alcohols and amines, and gives secondary amines in good to excellent isolated yields.
Gold rush: Gold catalysis remains a highly attractive field of research for the discovery of new reaction types. A novel intermolecular addition of aldehydes and ketones to enynes leads to the diastereoselective synthesis of 2‐oxabicyclo[3.1.0]hexanes (see scheme).
The complex Ru-MACHO ( 1) is a widely used precatalyst for hydrogenation and dehydrogenation reactions under basic conditions. In an attempt to identify the active catalyst form, 1 was reacted with a strong base. The formation of previously unreported species was observed by NMR and mass spectrometry. This observation indicated that complex 1 quickly degraded under basic conditions when no substrate was present. X-ray crystallography enabled the identification of three complexes as products of this degradation of complex 1. These complexes suggested degradation pathways which included ligand cleavage and reassembly, along with reduction of the ruthenium atom. One of the decomposition products, the Ru 0 complex [Ru(N(CH 2 CH 2 PPh 2 ) 3 )CO] (5), was prepared independently and studied. 5 was found to be active, entirely additivefree, in the acceptorless dehydrogenation of aliphatic alcohols to esters. The hydrogenation of esters catalyzed by 5 was also demonstrated under base-free conditions with methanol as an additive. Protic substrates were shown to add reversibly to complex 5, generating Ru II −hydrido species, thus presenting a rare example of reversible oxidative addition from Ru 0 to Ru II and reductive elimination from Ru II to Ru 0 .
G=160 (182) G=44 G=66
X6 X18 X7Scheme S2. Alternative alcohol oxidation pathway via protonation of the alcoxide complex cation X6 (values in kJ/mol; R=H; in parentheses: R=Me). The protonated dicationic species X18 is predicted to be much higher in energy than the alternative transition state TS-X6-X7, which is in contrast to the analogous alternative route in the imine reduction (see main article). This is in agreement with expected trends (higher acidity and lower donor ability of alcohols compared to amines).Scheme S3. Nitrile formation through coordination of imine and subsequent oxidation analogous to the alcohol oxidation step in the proposed catalytic cycle (values in kJ/mol; R=H; in parentheses: R=Me).
S4Scheme S4. Alternative pathway for the catalytic formation of aldehyde and hydrogen. Computed free enthalpies of reaction and activation are given in kJ/mol for R=H, values in parentheses correspond to R=Me.
Computational methodologyStructures were optimized at the BP86/def2-SV(P) level of theory. i,ii,iii Absolute electronic energies, E el , were computed with the random phase approximation iv,v (RPA) using Kohn-Sham orbitals from the PBE functional vi,vii and the def2-TZVPP iii or def2-TZVPPD viii basis sets, respectively. Calculations based on only def2-TZVPPD orbitals could not be afforded for the largest molecules in this study. To reduce computational cost, the smaller basis set def2-TZVP was therefore used for the C and H atoms on the triphos ligand. The deviations in relative energies resulting from this simplification are below 14 kJ/mol in all cases tested (see below).1 The RPA method was selected due to its reliable performance in applications to transition metal chemistry, including Ru compounds, ix,x and its ability to account for noncovalent interactions. Electronic structure calculations were carried out with the TURBOMOLE program package,xi employing effective core potentials for rutheniumxii and the resolution of the identity approximation (RI) with corresponding auxiliary basis functions.xiii In RPA calculations, orbitals with energy eigenvalues below -3 E h were not correlated, and appropriate auxiliary basis sets were used.xiv,xv No exchange fitting was done.
A full account of a recently discovered gold(I)-catalyzed reaction, a cycloaddition of carbonyl compounds to enynes yielding 2-oxabicyclo[3.1.0]hexanes with four stereogenic centers, is presented. The reaction proceeds with very high diastereoselectivity. The scope of the reaction has been investigated. In addition, experiments and DFT calculations concerning mechanistic aspects were carried out. The reaction course varies with the substitution pattern of the alkene moiety of the starting enyne. Branched olefins led to 2-oxabicyclo[3.1.0]hexanes; terminally substituted olefins proceeded with the incorporation of two carbonyl components to give hexahydrocyclopenta[d][1,3]dioxines.
Carbocycles with > 90% ee were prepared via Ir-catalysed asymmetric allylic alkylation/ring closing metathesis sequences or enantioselective Ir-catalysed intramolecular allylic alkylations.
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