Diazaphospholene Precatalysts for Imine and Conjugate Reductions

Reductive Asymmetric Aza-Mislow-Evans Rearrangement by 1,3,2-Diazaphospholene Catalysis

1,3,2-diazaphospholene hydrides (DAP-H) enable smooth conjugate reduction of polarized double bonds. The transiently formed phosphorus-enolate provides a potential platform for reductive α-functionalizations. In this respect, asymmetric C-heteroatom bond forming processes are synthetically appealing but remain elusive. We report a 1,3,2-diazaphospholene-catalyzed three-step cascade reaction of N-sulfinyl acrylamides comprised of conjugate reduction, [2,3]-sigmatropic aza-Mislow-Evans rearrangement and subsequent S-O bond cleavage. The obtained enantio-enriched α-hydroxy amides are formed in good yields and excellent enantiospecificity. The stereo-defined P-bound N,O-ketene aminal ensures an excellent transfer of chirality from the sulfur stereocenter to α-carbon. The transformation operates under mild conditions at ambient temperature. Moreover, DAP-H is a competent reductant for the cleavage of formed sulfenate ester, eliminating the extra step in traditional Mislow-Evans processes.

Catalytic Asymmetric Conjugate Reduction

Enantioselective reduction reactions are privileged transformations for the construction of trisubstituted stereogenic centers. While these include established synthetic strategies, such as asymmetric hydrogenation, methods based on the enantioselective addition of hydridic reagents to electrophilic prochiral substrates have also gained importance. In this context, the asymmetric conjugate reduction (ACR) of α,β-unsaturated compounds has become a convenient approach for the synthesis of chiral compounds with trisubstituted stereocenters in α-, β-, or γ-position to electron-withdrawing functional groups. Because such activating groups are diverse and amenable of further derivatizations, ACRs provide a general and powerful synthetic entry towards a variety of valuable chiral building blocks. This Review provides a comprehensive collection of catalytic ACR methods involving transition-metal, organic, and enzymatic catalysis since its first versions dating back to the late 1970s.

1,3,2‐Diazaphospholene‐Catalyzed Reductive Cyclizations of Organohalides**

1,3,2‐diazaphospholenes hydrides (DAP‐Hs) are highly nucleophilic organic hydrides serving as main‐group catalysts for a range of attractive transformations. DAP hydrides can act as stoichiometric hydrogen atom transfer agents in radical reactions. Herein, we report a DAP‐catalyzed reductive radical cyclization of a broad range of aryl and alkyl halides under mild conditions. The pivotal DAP catalyst turnover was achieved by a DBU‐assisted σ‐bond metathesis between the formed DAP halide and HBpin, which rapidly regenerates DAP‐H. The transformation is significantly accelerated by irradiation with visible light. Mechanistic investigations indicate that visible light irradiation leads to the formation of DAP dimers, which are in equilibrium with DAP radicals and accelerate the cyclization. The direct use of (DAP)₂ enabled a catalytic protocol in the absence of light.

Alkoxydiaminophosphine Ligands as Surrogates of NHCs in Copper Catalysis

A family of phosphine ligands containing a five-membered ring similar to the popular N-heterocyclic carbene ligands and an alkoxy third substituent has been developed. These alkoxydiaminophosphine ligands (ADAP) can be generated in one pot and reacted with a copper(I) source leading to the high yield isolation of complexes [(ADAP)CuX]₂ (X=Cl, Br). The dinuclear nature of these compounds has been established by means of X-ray studies and DOSY experiments. A screening of the catalytic properties of these complexes toward carbene-transfer reactions from diazocompounds to C-H bonds (alkane, arene), olefins or N-H bonds, as well as in CuAAC or nitrene transfer reactions have shown a performance at least similar, if not better, than their (NHC)CuCl analogues, opening a new window in copper catalysis with these readily tunable ADAP ligands.

Diazaphospholene and Diazaarsolene Derived Homogeneous Catalysis

The past 20 years has seen significant advances in main group chemistry and their use in catalysis. This Minireview showcases the recent emergence of phosphorus and arsenic containing heterocycles as catalysts. With that, we discuss how the Group 15 compounds diazaphospholenes, diazaarsolenes, and their cationic counterparts have proven to be highly effective catalysts for a wide range of reduction transformations. This Minireview highlights how the initial discovery by Gudat of the hydridic nature of the P−H bond in these systems led to these compounds being used as catalysts and discusses the wide range of examples currently present in the literature.

Transition-Metal-Like Catalysis with a Main-Group Element: Bismuth-Catalyzed C-F Coupling of Aryl Boronic Esters

The main-group age: The past decade has revealed main-group-element compounds that display transition-metal-like reactivity in stoichiometric and even catalytic transformations. Cornella and co-workers have now reported a bismuth complex that catalyzes fluoroarene formation from aryl boronate esters, a reaction that is almost unprecedented in transition-metal catalysis.

Hydrostibination

A rigid naphthalenediamine framework has been used to prepare antimony hydrides that feature LUMO shapes and energies similar to those found in secondary boranes. By exploiting this feature, we introduce the first examples of uncatalyzed hydrostibination reactions of robust C≡C, C=C, C=O, and N=N bonds as new elementary hydrometalation reactions analogous to hydroboration. These results endorse the notion of a diagonal relationship between the lightest p-block element and the heaviest Group 15 elements and may lead to the conception of novel reaction chemistry.

Catalyst-Free and Solvent-Free Facile Hydroboration of Imines

A facile process for the catalyst-free and solvent-free hydroboration of aromatic as well as heteroaromatic imines is reported. This atom-economic methodology is scalable, compatible with sterically and electronically diverse imines, displaying excellent tolerance towards various functional groups, and works efficiently at ambient temperature in most of the cases, affording secondary amines in good to excellent yield after hydrolysis.

A 1,3,2-Diazaphospholene-Catalyzed Reductive Claisen Rearrangement

1,3,2-Diazaphospholenes (DAPs) are an emerging class of organic hydrides. In this work, we exploited them as efficient catalysts for very mild reductive Claisen rearrangements. The method is tolerant towards a wide variety of functional groups and operates at ambient temperature. Besides being enantiospecific for substrates with existing stereogenic centers, the diastereoselectivity can be switched by varying solvents and DAP catalysts. The reaction kinetics show direct rearrangements of O-bound phospholene enolates and provide a proof-of-principle for catalytic enantioselective reactions.

A Nucleophilicity Scale for the Reactivity of Diazaphospholenium Hydrides: Structural Insights and Synthetic Applications

Nucleophilicity parameters (N, s_N ) of a group of representative diazaphospholenium hydrides were derived by kinetic investigations of their hydride transfer to a series of reference electrophiles with known electrophilicity (E) values, using the Mayr equation log k₂ =s_N (N+E). The N scale covers over ten N units, ranging from the most reactive hydride donor (N=25.5) to the least of the scale (N=13.5). This discloses the highest N value ever quantified in terms of Mayr's nucleophilicity scales reported for neutral transition-metal-free hydride donors and implies an exceptional reactivity of this reagent. Even the least reactive hydride donor of this series is still a better hydride donor than those of many other nucleophiles such as the C-H, B-H, Si-H and transition-metal M-H hydride donors. Structure-reactivity analysis reveals that the outstanding hydricity of 2-H-1,3,2-diazaphospholene benefits from the unsaturated skeleton.

Chiral 1,3,2-Diazaphospholenes as Catalytic Molecular Hydrides for Enantioselective Conjugate Reductions

Secondary 1,3,2-diazaphospholenes have a polarized P-H bond and are emerging as molecular hydrides. Herein, a class of chiral, conformationally restricted methoxy-1,3,2-diazaphospholene catalysts is reported. We demonstrate their catalytic potential in asymmetric 1,4-reductions of α,β-unsaturated carbonyl derivatives, including enones, acyl pyrroles, and amides, which proceeded in enantioselectivities of up to 95.5:4.5 e.r.

Tris(2,4,6-trifluorophenyl)borane: An Efficient Hydroboration Catalyst

The metal-free catalyst tris(2,4,6-trifluorophenyl)borane has demonstrated its extensive applications in the 1,2-hydroboration of numerous unsaturated reagents, namely alkynes, aldehydes and imines, consisting of a wide array of electron-withdrawing and donating functionalities. A range of over 50 borylated products are reported, with many reactions proceeding with low catalyst loading under ambient conditions. These pinacol boronate esters, in the case of aldehydes and imines, can be readily hydrolyzed to leave the respective alcohol and amine, whereas alkynyl substrates result in vinyl boranes. This is of great synthetic use to the organic chemist.