Reactions of diborenes with terminal alkynes: mechanisms of ligand-controlled anti-selective hydroalkynylation, cycloaddition and CC triple bond scission

Abstract

The reactions of terminal acetylenes with doubly Lewis base-stabilised diborenes resulted in different outcomes depending on the nature of the ligands at boron and the conformation of the diborene (cyclic...

Ein unsymmetrisches, cyclisches Diboren basierend auf einem chelatisierenden CAAC‐Liganden sowie dessen Aktivierung kleiner Moleküle und Umlagerungsreaktionen

“…[2c] Außerdem wurde gezeigt, dass die Anwesenheit labiler stabilisierender Liganden ein Diboren zur Reaktion mit 2-Butin unter photolytischen Bedingungen bringt, woraus eine Monophosphan-stabilisierte,h omoaromatische 1,3-Dihydro-1,3diboret-Spezies resultierte.J üngst haben wir über die Reaktionen von NHC-und Phosphan-stabilisierten Diborenen mit terminalen Alkinen berichtet, von denen Hydroalkinylierungs-und [2+ +2]-Cycloadditionsprodukte sowie Verbindungen unter Spaltung der C-C-Dreifachbindung isoliert wurden; das Reaktionsprodukt war von der Natur der B = B-Doppelbindung und den Reaktionsbedingungen abhängig. [25] Inspiriert von den eben beschrieben Fortschritten haben wir die Reaktivitätvon 2 gegenüber Alkinen untersucht.…”

Hybrid Inorganic–Organic Cross-Metathesis between Diborenes and Acetylene

“…Doubly phosphine-and N-heterocyclic carbene (NHC)stabilized diborenes are known to undergo cycloaddition and hydroalkynylation reactions with alkynes. 30,31 In contrast, the reactions of the diborenes [(CAAC)BH] 2 (1 H ) 34 and [(CAAC)B(CN)] 2 (1 CN ) 35 with acetylene (1 atm) in benzene at 80 °C for several days resulted in complete and selective conversion (as determined by 11 with those of the isoelectronic (3E,5E)-octa-1,3,5,7-tetraene confirms the higher degree of delocalization in 2 Y deduced from the structural data (see Figure S23).…”

“…For the doubly NHC-or phosphine-stabilized 1,2dihydro-1,2-diborete analogues of Int3 Y , ring strain is relieved by ring expansion through NHC ligand activation or rearrangement to the more stable 1,3-diboretes, respectively, all via closed-shell intermediates. 31 The presence of the CAAC ligands, which enable the stabilization of diradical intermediates, seems to be the key for changing the outcome of these reactions to the homolytic cleavage of the B−B bond. Finally, rotation of the central C−C bond of Int4 Y yields the more stable trans isomer 2 Y .…”

“…Doubly phosphine- and N-heterocyclic carbene (NHC)-stabilized diborenes are known to undergo cycloaddition and hydroalkynylation reactions with alkynes. , In contrast, the reactions of the diborenes [(CAAC)­BH] 2 ( 1 H ) and [(CAAC)­B­(CN)] 2 ( 1 CN ) with acetylene (1 atm) in benzene at 80 °C for several days resulted in complete and selective conversion (as determined by 11 B NMR spectroscopy) to the blue compounds [(CAAC)­BHCH] 2 ( 2 H ) and [(CAAC)­B­(CN)­CH] 2 ( 2 CN ), respectively, in which the C 2 H 2 unit has inserted into and split the BB double bond (Figure ). Formally, this reaction is analogous to the cross-metathesis between an alkene and an alkyne (Scheme a,e).…”

“…While heavier homodinuclear main-group double bonds undergo spontaneous [2 + 2] cycloaddition reactions with alkynes, these do not proceed further to the cycloreversion step, which would generate the corresponding metathesis product. − In fact, hybrid inorganic–organic cross-metathesis is limited to a couple of recent publications by Dielmann and co-workers, showing the reversible cycloaddition/metathesis between terminal alkynes and the oxo- and thiophosphonium species [Y 2 PE] + (E = O, S; Y = NIDip, IDip =1,3-bis­(2,6-diisopropyl­phenyl)­imidazol-2-ylidene), a heavy congener of the alkyne–carbonyl metathesis (Scheme d). , However, there have been, to our knowledge, no reports of cross-metathesis reactions between homodinuclear organic and inorganic main-group multiple bonds.…”