Triamidoamine-Supported Zirconium Compounds in Main Group Bond-Formation Catalysis

Abstract

Conspectus The rationale to pursue long-term study of any system must be sound. Quick discoveries and emergent fields are more than temptations. They remind us to ask what are we gaining through continued study of any system. For triamidoamine-supported zirconium, there has been a great deal gained with yet more ahead. Initial study of the system taught much that is applied to catalysis. Cyclometalation of a trimethylsilyl substituent of the ancillary ligand, abbreviated (N3N) when not metalated for simplicity, via C–H bond activation is facile and highly reversible. It has allowed for the synthesis of a range of Zr–E bonds, which are of fundamental interest. More germane, cyclometalation has emerged as our primary product liberation step in catalysis. Cyclometalation also appears to be a catalyst resting state, despite how cyclometalation is a known deactivation step for many a compound in other circumstances. Catalysis with triamidoamine-supported zirconium has been rich. Rather than summarizing the breadth of reactions, a more detailed report on the dehydrocoupling of phosphines and hydrophosphination is provided. Both reactions demonstrate the outward impact that the study of (N3N)­Zr-based catalysis has afforded. Dehydrocoupling catalysis, or bond formation via loss of hydrogen, is particular to 3p and heavier main group elements. The reaction has been important in the formation of E–E and E–E′ bonds in the main group for molecular species and materials. While study of this reaction at (N3N)Zr compounds provides key insights into mechanism, discoveries in the area of P–P and Si–Si bond formation with (N3N)Zr derivatives as catalysts have greater reach than merely the synthesis of main group element containing products. For example, that work has informed design principles for the identification of catalysts that transfer low-valent fragments. The successful application of these principles was evident in the discovery of a catalyst that transfers phosphinidene (“PR”) to unsaturated substrates. Hydrophosphination exhibits perfect atom economy in the formation of P–C bonds. The reaction can proceed without a catalyst, but the purpose of a catalyst is enhanced reactivity and selectivity. Nevertheless, significant challenges in this reaction remain. In particular, (N3N)Zr compounds have demonstrated high activity in hydrophosphination and readily utilize unactivated unsaturated organic molecules, challenging substrates for any heterofunctionalization reaction. This activity has led to not only impressive metrics in the catalysis but access to previously untouched substrates and formation of unique products. The particular properties of the (N3N)Zr system that engage in this reactivity may influence other heterofunctionalization reactions. The recently discovered photocatalytic hydrophosphination with (N3N)­ZrPRR′ compounds already appears to be general rather than unique and may drive additional bond formation catalysis among early transition-metal compounds.

A Commercially Available Ruthenium Compound for Catalytic Hydrophosphination

“…Indeed, our main objective in investigating ruthenium was the potential for observing α‐phosphinidene elimination with a heavier metal that may promote singlet‐like phosphinidene chemistry rather than the triplet‐like phopshinidene transfer promoted by 1 . The absence of phosphinidene transfer with 2 and the observation of hydrophosphination instead is consistent with our developing hypothesis that greater reactivity with unsaturated substrates diminishes transfer of low‐valent fragments . In any case, comparison of the same phosphine and unsaturated substrates for both catalysts is important for benchmarking hydrophosphination catalysis …”

“…[11] The absence of phosphinidene transfer with 2 and the observation of hydrophosphination instead is consistent with our developing hypothesis that greater reactivity with unsaturated substrates diminishes transfer of low-valent fragments. [12] In any case, comparison of the same phosphine and unsaturated substrates for both catalysts is important for benchmarking hydrophosphination catalysis. [9b] Other substrates, however, were not amenable to hydrophosphination using 2.…”

Photocatalytic Hydrophosphination with Air-Stable and Commercially Available Bis(acetylacetonato)copper(II) (Cu(acac)2)

“…12 We have identified that zirconium compounds (Chart 1, compound D) are capable for intermolecular hydrophosphination with unactivated alkenes, which initially suffered from sluggish reactivity. 15 In further investigations of this system, acceleration under photocatalytic conditions was observed, which was the critical feature for the high reactivity of triamidoamine-supported zirconium. 13 Though we started with primary phosphines as substrates, 13,16 photocatalytic conditions availed a broader swath of substrates including secondary phosphines at reasonable reaction times at ambient temperature.…”

Effect of Photolysis on Zirconium Amino Phenoxides for the Hydrophosphination of Alkenes: Improving Catalysis

“…A wide range of phosphine chemistry has been developed, with metal catalyzed hydrophosphination being one of the most economic avenues for P-C bond formation. Though significant progress has occurred [1,7,[15][16][17][18][19][20][21][22][23][24], challenges remain for this transformation, with catalyst and substrate scope being two key avenues for improvement [1,15,[25][26][27][28][29][30].…”