Oxygen insertion reactions of mixed N-heterocyclic carbene–oxazolinylborato zinc alkyl complexes

Abstract

We report the synthesis of a new mixed oxazoline-carbene scorpionate ligand, bis(4,4-dimethyl-2-oxazolinyl)(1-mesitylimidazolyl)phenylborate (PhB(Ox(Me2))2Im(Mes)). Reactions of the protonated form PhB(Ox(Me2))2(Im(Mes)H) with dialkylzinc compounds provide four-coordinate zinc alkyl complexes, and X-ray diffraction studies of the {PhB(Ox(Me2))2Im(Mes)}ZnR (R = Me, Et) compounds show significant structural distortions involving the R groups shifting away from the carbene donor. The reaction of {PhB(Ox(Me2))2Im(Mes)}ZnEt (3) and O2 provides an isolable mononuclear zinc alkylperoxide {PhB(Ox(Me2))2Im(Mes)}ZnOOEt (4), which has been characterized by single crystal X-ray diffraction and (17)O NMR spectroscopy.

Oxygenation Chemistry of Magnesium Alkyls Incorporating β‐Diketiminate Ligands Revisited

“…Am ajor breakthrough in the oxygenationc hemistry of main-group metal alkyls came with the isolationa nd structure characterization of the first Group 13 metal alkylperoxides [2] and zinc alkylperoxides. [3][4][5][6] For example, we demonstrated for the first time the essential role of the coordination state of the organometallics pecies as wella sc onvincingly provedt he formation of oxo [3d, 5a,b] hydroxide, [5d] and carboxylate [5c] species as the resulto ft he homolytic cleavageo ft he alkylperoxide OÀO bond.B ased on these systematic studies, we have proposed an inner-sphere electron transfer (ISET) mechanism for the oxygenationo fo rganometallicsw ith nonredox active centers, whichs trongly contradicts the commonly accepted radicalchain mechanism. [3][4][5][6] For example, we demonstrated for the first time the essential role of the coordination state of the organometallics pecies as wella sc onvincingly provedt he formation of oxo [3d, 5a,b] hydroxide, [5d] and carboxylate [5c] species as the resulto ft he homolytic cleavageo ft he alkylperoxide OÀO bond.B ased on these systematic studies, we have proposed an inner-sphere electron transfer (ISET) mechanism for the oxygenationo fo rganometallicsw ith nonredox active centers, whichs trongly contradicts the commonly accepted radicalchain mechanism.…”

“…[3] Since then systematic studies by our group have been focusedo nt he factorst hat control the facile activationo fd ioxygen by organometallics with nonredox active centers. [3][4][5][6] For example, we demonstrated for the first time the essential role of the coordination state of the organometallics pecies as wella sc onvincingly provedt he formation of oxo [3d, 5a,b] hydroxide, [5d] and carboxylate [5c] species as the resulto ft he homolytic cleavageo ft he alkylperoxide OÀO bond.B ased on these systematic studies, we have proposed an inner-sphere electron transfer (ISET) mechanism for the oxygenationo fo rganometallicsw ith nonredox active centers, whichs trongly contradicts the commonly accepted radicalchain mechanism. [6c, 7] In addition, we 1) developedanovel [(L)ZnR]/O 2 initiating systemf or radical organic reactions (whichi se asier to handle than the case of the widely used ZnMe 2 /O 2 system);2 )marked out ac ourse for the future discovery,i ncluding the functionalization of aliphatic CÀHb onds in associationw ith ZnOÀOR bond activation, [5e] and 3) developed av ery efficientc atalytic systemf or asymmetric epoxidation of electron-deficient olefins based on well-defined zinc alkylperoxides supported by chiral auxiliary N,N-ligands.…”

“…[18] The molecular structure of 2a is essentially analogous to that reportedp reviously by Bailey [18] and consists of ac entrosymmetric dimer with magnesium centers bridged by benzylperoxide groups with m-h 2 :h 1 binding mode (Figure 1a). In our case, however,t he better quality crystallographic data allows for am ore detailed discussion of the geometric parameters of 2a.T he alkylperoxide oxygen atomsg roup are off the Mg 2 (m-O) 2 plane with the torsion angle between the Mg 2 (m-O) 2 plane and the OÀOl inkage equal to 109.928 and the Mg-O-O angle equal to 72.328.T he MgÀOd istances fall in an arrow range of 2.026-2.078 ,w hile the Mg····O2 distance is 2.118 (6) .T he O1ÀO2 bond distance in the bridging alkylperoxidegroups equals 1.383 (6) .…”

Boron‐Centered Scorpionate‐Type NHC‐Based Ligands and Their Metal Complexes

“…The first example of a modified monoanionic tripodal ligand in which one oxazoline ring was replaced with an N‐heterocyclic carbene, generated from a N ‐substituted imidazolium moiety, was reported by Sadow et al They synthesized the pre‐carbene ligand PhB(ox c Me₂ ) 2 (HIm Mes ) (L 24 ) through the reaction between PhB(ox c Me₂ ) 2 and 1‐mesitylimidazole. The pre‐carbene ligand L 24 was crystallized as {PhB(ox c Me₂ ) 2 (HIm Mes )(LiCl)(THF)} 2 , a dimer with bridging chloride ligands . Analogously, PhB(ox c Me₂ ) 2 (HIm t Bu ) (L 25 ) was prepared by combination of PhB(ox c Me₂ ) 2 and 1‐ tert ‐butylimidazole (Figure ) …”

Hydroelementation of Unsaturated C–C Bonds Catalyzed by Metal Scorpionate Complexes

“…L 1 ≠ L 2 ) (Figure ). The field of heteroscorpionate ligands has developed considerably in recent years, with several variations in donor arms, including carbenes, acetoxy, amine, amidinate, and thiol donors . The definition of scorpionate requires that the ligand: 1) be tridentate; 2) coordinate fac to the metal center; 3) contain a central atom bearing the three donor arms which cannot coordinate.…”