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2 + 2 Photocycloadditions are idealized, convergent construction approaches of 4-membered heterocyclic rings, including azetidines. However, methods of direct excitation are limited by the unfavorable photophysical properties of imines and electronically unbiased alkenes. Here, we report copper-catalyzed photocycloadditions of non-conjugated imines and alkenes to produce a variety of substituted azetidines. Design principles allow this base metal-catalyzed method to achieve 2 + 2 imine-olefin photocycloaddition via selective alkene activation through a coordination-MLCT pathway supported by combined experimental and computational mechanistic studies.

 

Eine erste konsistente Serie einkerniger 17‐Elektronenkomplexe von drei Elementen der Gruppe 7 wurde in kristalliner Form hergestellt und durch Röntgenkristallstrukturanalyse sowie spektroskopische Methoden untersucht. Die paramagnetischen Verbindungen der Zusammensetzung [M⁰(CO)(CNp‐F‐Ar^DArF2)₄] (M=Mn, Tc, Re; Ar^DArF2=2,6‐(3,5‐(CF₃)₂C₆H₃)₂C₆H₂F)) werden durch jeweils vier sterisch anspruchsvolle Isocyanid‐Liganden stabilisiert, wodurch auch eine Dimerisierung der Metalloradikale verhindert wird. Die Verbindungen besitzen eine quadratisch‐pyramidale Struktur mit jeweils einem Carbonyl‐Liganden als Apex. EPR‐Spektren der Technetium‐ und Rheniumverbindungen zeigen eine deutliche Anisotropie mit großen⁹⁹Tc und^185,187Re‐Hyperfeinstrukturkopplungen für jeweils eine der Komponenten. Für die Bestimmung der EPR‐Parameter des entsprechenden Mn⁰‐Komplexes wurde Hochfeld‐EPR (Q‐Band und W‐Band) verwendet.

 

The first consistent series of mononuclear 17‐electron complexes of three Group 7 elements has been isolated in crystalline form and studied by X‐ray diffraction and spectroscopic methods. The paramagnetic compounds have a composition of [M⁰(CO)(CNp‐F‐Ar^DArF2)₄] (M=Mn, Tc, Re; Ar^DArF2=2,6‐(3,5‐(CF₃)₂C₆H₃)₂C₆H₂F) and are stabilized by four sterically encumbering isocyanides, which prevent the metalloradicals from dimerization. They have a square pyramidal structure with the carbonyl ligands as apexes. The frozen‐solution EPR spectra of the rhenium and technetium compounds are clearly anisotropic with large⁹⁹Tc and^185,187Re hyperfine interactions for one component. High‐field EPR (Q band and W band) has been applied for the elucidation of the EPR parameters of the manganese(0) complex.

 

The mixed isocyanide/carbonyl complexes cis - and trans -[Re(CO) 3 Br(CNAr Dipp2 ) 2 ] (Ar Dipp2 = 2,6-(2,6-(i-Pr) 2 C 6 H 3 ) 2 C 6 H 3 ) can be synthesized from reactions of [Re(CO) 5 Br] and CNAr Dipp2 depending on the conditions applied. Reduction of the neutral Re( i ) species gives the monoanionic complex [Re(CO) 3 (CNAr Dipp2 ) 2 ] − or the neutral [Re(CO) 3 (CNAr Dipp2 ) 2 ], which contain rhenium in the formal oxidation states “−1” and “0”, respectively. 

Relative to other cyclic poly‐phosphorus species (that is,cyclo‐P_n), the planarcyclo‐P₄group is unique in its requirement of two additional electrons to achieve aromaticity. These electrons are supplied from one or more metal centers. However, the degree of charge transfer is dependent on the nature of the metal fragment. Unique examples of dianionic mononuclear η⁴‐P₄complexes are presented that can be viewed as the simple coordination of the [cyclo‐P₄]²⁻dianion to a neutral metal fragment. Treatment of the neutral, molybdenumcyclo‐P₄complexes Mo(η⁴‐P₄)I₂(CO)(CNAr^Dipp2)₂and Mo(η⁴‐P₄)(CO)₂(CNAr^Dipp2)₂with KC₈produces the dianionic, three‐legged piano stool complexes, [Mo(η⁴‐P₄)(CO)(CNAr^Dipp2)₂]²⁻and [Mo(η⁴‐P₄)(CO)₂(CNAr^Dipp2)]²⁻, respectively. Structural, spectroscopic, and computational studies reveal a similarity to the classic η⁶‐benzene complex (η⁶‐C₆H₆)Mo(CO)₃regarding the metal‐center valence state and electronic population of the planar‐cyclic ligand π system.

 

Relative to other cyclic poly‐phosphorus species (that is,cyclo‐P_n), the planarcyclo‐P₄group is unique in its requirement of two additional electrons to achieve aromaticity. These electrons are supplied from one or more metal centers. However, the degree of charge transfer is dependent on the nature of the metal fragment. Unique examples of dianionic mononuclear η⁴‐P₄complexes are presented that can be viewed as the simple coordination of the [cyclo‐P₄]²⁻dianion to a neutral metal fragment. Treatment of the neutral, molybdenumcyclo‐P₄complexes Mo(η⁴‐P₄)I₂(CO)(CNAr^Dipp2)₂and Mo(η⁴‐P₄)(CO)₂(CNAr^Dipp2)₂with KC₈produces the dianionic, three‐legged piano stool complexes, [Mo(η⁴‐P₄)(CO)(CNAr^Dipp2)₂]²⁻and [Mo(η⁴‐P₄)(CO)₂(CNAr^Dipp2)]²⁻, respectively. Structural, spectroscopic, and computational studies reveal a similarity to the classic η⁶‐benzene complex (η⁶‐C₆H₆)Mo(CO)₃regarding the metal‐center valence state and electronic population of the planar‐cyclic ligand π system.