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Chemistry &Industry, Volume 87, Issue 9, Page 38-41, September 2023.
Are Monophospha(III)amidines and ‐guanidines with Ionizable Hydrogens Tautomeric? Towards a Deeper Understanding of Two Related Hetero‐element Functional Groups
Which is preferred, C-amino-(σ2,λ3)-phosphaalkene P=C or C-(σ3,λ3)-phosphinoimine N=C? The answer seems to depend on the backbone substituent R. When R is an aryl group (or hydrogen) P=C is consistently preferred over N=C, though both isomers exist in solution. When R is a t Bu or amino group, N=C is preferred, with no evidence for the other isomers in solution.
Abstract
This paper presents definitive structural evidence for N,P(III)-monophosphaamidines in P=C and N=C isomeric forms from a combination of new syntheses, single-crystal X-ray diffraction (SC-XRD), solid-state NMR and FTIR. Evidence is also provided for C-amino-(σ2,λ3)-phosphaalkene and C-(σ3,λ3)-phosphinoimine tautomerism in solution using multi-nuclear NMR methods. Synthesis and SC-XRD structure determination of a trisubstituted N,N’,P(III)-monophosphaguanidine is presented, the first structure of a phospha(III)guanidine with two ionizable H atoms. The structural evidence is convincing for an N=C geometry, resulting in both N−H and P−H in the molecule. A detailed computational investigation using DFT methods is presented, with the goal of understanding the tautomeric structure preferences both at the fundamental level (parent molecules with all substituents on the heteroatoms being hydrogen) and using the full structures containing the very bulky 2,6-diisopropylphenyl (Dipp) substituents employed in this study. Arguments are espoused for treating phospha(III)amidines and -guanidines as new types of functional groups, similar to but distinct from the familiar organic analogues. Limited reactivity studies and a voltammetry study of one phospha(III)amidine are included with the supporting information.
Photoinduced Pyridine N‐oxides Catalyzed Carbon Radical Generation from Alkylboronic Acids
Photoexcited 4-nitropyridine N-oxide biradical was found to catalyze the carbon radical generation from alkylboronic acids with blue LED irradiation without exogenous photocatalysts. With a wide range of readily available aliphatic boronic acids, including methyl boronic acid, the developed catalytic system allowed simple and robust applicability for alkylation, amination, and cyanation reactions.
Abstract
Herein we report a protocol for the generation of alkyl carbon radicals from alkylboronic acids wherein photoexcited 4-nitropyridine N-oxide biradical features a catalyst to promote the nucleo-homolytic substitution of boronic acids. With a wide range of readily available aliphatic boronic acids, including methyl boronic acid, the developed catalytic system demonstrates broad applicability for alkylation, amination, and cyanation.
Chiral Lewis Base‐Catalysed Asymmetric Syntheses of Benzo‐fused ϵ‐Lactones
An asymmetric protocol for the synthesis of novel benzofused ϵ-lactones starting from quinone methides and activated acetic acid esters using chiral isothiourea Lewis base catalysts has been developed.
Abstract
We herein report a two-step protocol for the asymmetric synthesis of novel chiral benzofused ϵ-lactones starting from O-protected hydroxymethyl-para-quinone methides and activated aryl esters. By using chiral isothiourea Lewis base catalysts a broad variety of differently substituted products could be obtained in yields of around 50 % over both steps with high levels of enantioselectivities, albeit low diastereoselectivities only.
First Asymmetric Total Synthesis of Salinosporamides D and I Using Memory of Chirality and Dynamic Kinetic Resolution
Described herein are the total syntheses of salinosporamides via a strategy involving rapid construction of the pyrrolidinone core of the natural products from the simple amino acid serine. The key to the success of this synthesis was the stereoselective installation of three contiguous stereocenters on the pyrrolidinone core via a single aldol cyclization step, which employed the principles of ′memory of chirality” and dynamic kinetic resolution.
Abstract
The first total synthesis of (−)-salinosporamide D was achieved using d-serine as the sole chiral source. This approach stands out for its application of the principles of memory of chirality and dynamic kinetic resolution in the intramolecular aldol reaction of serine-derived oxazolidines. These strategies enabled the rapid construction of a pyrrolidinone core and installation of contiguous stereocenters. The key intermediate was readily obtained, culminating the asymmetric total synthesis of salinosporamide D. The total synthesis of salinosporamides A and I was also achieved using the same chemistry.
One‐Pot Catalytic Synthesis of Optically Active Drug (S)‐Clopidogrel
A three-step four reactions innovative asymmetric synthesis of popular antiplatelet drug (S)-clopidogrel, marketed as Plavix, has been developed. The telescoped approach relies on the use of a sole organocatalyst, commercially available reagents and a single solvent. Fine optimization of the reaction conditions enabled isolation of the drug in 61 % overall yield and 62 % ee.
Abstract
A first catalytic preparation of the popular antiplatelet drug (S)-clopidogrel, known with the tradename of Plavix, has been developed in a one-pot and innovative approach. The synthesis can be performed using commercially available ortho-chlorobenzaldehyde, 1-naphthylsulfonyl acetonitrile, tert-butyl hydroperoxide (TBHP), 4,5,6,7-tetrahydrothieno[3,2-c]pyridine and 20 mol % of a quinidine derived organocatalyst in a single solvent. A Knoevenagel/asymmetric epoxidation/domino ring-opening esterification (DROE) sequence enabled to obtain (S)-clopidogrel in 61 % overall yield and 62 % ee. Fine optimization of the reaction conditions proved to be crucial to set up a selective and efficient process.
Synthesis and Characterization of Platinum(IV) Complexes Containing 1‐Phenyl‐1H‐pyrazole and α‐Diimine Ligands
Analogous PtIV and IrIII complexes containing α-diimines as N^N ligands exhibit distinctly different photophysical behavior. This arises from the metal-induced differences in the energy gaps between the excited ligand-centered 3*LC and metal-to-ligand 3*MLCT or ligand-to-metal 3*LMCT charge transfer states.
Abstract
The paper deals with the synthesis of a series of cationic [Pt(ppz)2(N^N)]2+ complexes containing deprotonated 1-phenyl-1H-pyrazole as cyclometallating (C^N) ligands and α-diimines (N^N) in the form of water-soluble salts with OTf− (trifluoromethanesulfonate) counter ions. These complexes were obtained from cis-[Pt(ppz)2Cl2] through chloride ligands substitution with α-diimines. The complexes were identified by means of NMR spectroscopy and their molecular structures were confirmed by X-ray crystallography. The photophysical properties of these complexes were studied in detail. These complexes showed strong luminescence in MeOH/EtOH 1 : 1 glasses at 77 K but were almost non-emitting in MeCN solutions at room temperature. Their emission properties were compared to analogues IrIII complexes.