Cationic Terminal Borylene Complexes: A Synthetic and Mechanistic Investigation of MB Metathesis Chemistry

Abstract

Metathesis reactions constitute a key component of modern synthetic chemistry; olefin metathesis, for example, provides a versatile and widely exploited carbon-carbon bond-forming methodology.[1] Such reactions are typically catalyzed by organometallic complexes that contain M=C bonds.[2] The synthesis of analogous complexes that contain M=Si bonds, for example, has led to an in-depth investigation of their reactivity towards unsaturated substrates. [3] Synthetic approaches that lead to the isolation of related systems with M = B bonds have been developed only recently: [4][5][6][7] for example, halide-abstraction chemistry gives access to cationic terminal borylene complexes, [L n M= BX][7] Consequently, reports of the fundamental chemistry of M=B bonds are somewhat limited (predominantly to metal-metal transfer reactions and addition/substitution reactivity towards nucleophiles). [4,6,7] Thus, the chemistry of [Cp*Fe(CO) 2 (BMes)] + (Mes = mesityl = 2,4,6-Me 3 C 6 H 2 , Cp* = pentamethylcyclopentadienyl), for example, is dominated by the electrophilic character at both the Fe and B centers. [7] In an attempt to tune the reactivity of these highly unsaturated complexes we investigated the synthesis of cationic aminoborylene systems, [L n M = BNR 2 ]+ .[5] Chloride abstraction from [CpFe(CO) 2 {B(NiPr 2 )Cl}] (2) (Cp = cyclopentadienyl) by Na(BAr À . The steric bulk of the amino substituents is a key point: the use of the smaller NMe 2 group results in the formation of a thermally fragile borylene product in the subsequent halide-abstraction step, [7b, 8] whereas [CpFe(CO) 2 {B(tmp)Cl}] (tmp = tetramethylpiperamino) was inaccessible from tmpBCl 2 . Compound 2 is a pale yellow sublimable crystalline solid, which was characterized by multinuclear NMR and IR spectroscopy, mass spectrometry, and X-ray crystallographic analysis (Figure 1).The reaction of 2 with Na(BAr f 4 ) in dichloromethane results in a quantitative conversion (determined by 1 H and 11 B NMR spectroscopy) into 3. The latter product (along with the C 5 H 4 Me analogue) is a colourless oil at (or close to) room temperature, but its formulation can be definitively established from spectroscopic and reactivity data. The measured 11 B chemical shift for 3 (d B = 93.5 ppm) is very close to that reported by Braunschweig et al. for neutral terminal-aminoborylene systems of the type L n M=BN(SiMe 3 ) 2 (d B = 86.6-98.3 ppm).[5] The downfield shift upon chloride abstraction (Dd B = 38.1 between 2 and 3) mirrors that found for and 88.0 ppm, respectively). [7,8] The 1 H and 13 C NMR data are

Reductive Borylene–CO Coupling with a Bulky Arylborylene Complex

“…The susceptibility of the boron center in borylene complexes to nucleophilic attack is well‐documented and can result in the cleavage of the MB bond 10a. 11a,b, 14 Similar reactivity has been observed in isoelectronic metal carbyne chemistry, with the other possibility being an initial attack at the metal center 15. With this in mind, we decided to investigate the reactivity of the title compound toward the addition of electrons and compare its reduction chemistry with the aminoborylene [(OC) 5 Cr{BN(SiMe 3 ) 2 }].…”

Transition‐Metal Complexes of Boron—New Insights and Novel Coordination Modes

“…Experimental evidence suggests that steric shielding at both sides of the boron atom is required to ensure stability of the resulting borylene complexes 59b. However, by analogy with the successful use of amino groups to stabilize the borylene complexes of Group 6 metals, 77 and 78 , use of the π‐donating diisopropylamino substituent in place of the mesityl group at boron recently allowed for the isolation of the new cationic complex [(η 5 ‐C 5 H 5 )Fe(CO) 2 (BN i Pr 2 )][BAr F 4 ] ( 83 ) as a thermally stable colorless oil 60. Interestingly, treatment of the asymmetric mesityloxyboryl compounds [(η 5 ‐C 5 R 5 )Fe(CO) 2 {B(Cl)OMes}] (R=H, 58 ; R=Me, 60 ) with Na[BAr F 4 ] led to the isolation of the corresponding fluoroboryl species [(η 5 ‐C 5 R 5 )Fe(CO) 2 {B(F)OMes}] (R=H, Me) as well as the difluoroboryl complex [(η 5 ‐C 5 Me 5 )Fe(CO) 2 (BF 2 )] ( 16 ) 28.…”

“…A broader scope of reactivity appears to be possible for the cationic aminoborylene complex [(η 5 ‐C 5 H 5 )Fe(CO) 2 (BN i Pr 2 )][BAr F 4 ] ( 83 ) 60. Hence, reaction of 83 with Ph 3 PS or Ph 3 AsO resulted in the products of net FeB bond metathesis, [ i Pr 2 NB(μ‐S) 2 BN i Pr 2 ] and ( i Pr 2 NBO) 3 ,63 respectively, and formation of the organometallic compounds [(η 5 ‐C 5 H 5 )Fe(CO) 2 (L)][BAr F 4 ] (L=PPh 3 , AsPh 3 ) (Scheme ) 60…”

Synthese von 1‐Aza‐2‐borabutatrien‐Rhodiumkomplexen durch thermischen Borylentransfer von [(OC)5MoBN(SiMe3)2]

“…[12] Der Borylentransfer hat sich über die letzte Dekade rasch entwickelt und wurde auf die Synthese von Borirenen [13] und neuen Borylenkomplexen [12,14] sowie auf Metathesereaktionen angewendet. [15] Des weiteren ist die Insertion des DB-R-Fragments in olefinische C-H-Bindungen beschrieben worden. [16] Ein 7 kristallisiert in der monoklinen Raumgruppe P2 1 /c.…”

“…[12] Der Borylentransfer hat sich über die letzte Dekade rasch entwickelt und wurde auf die Synthese von Borirenen [13] und neuen Borylenkomplexen [12,14] sowie auf Metathesereaktionen angewendet. [15] Des weiteren ist die Insertion des DB-R-Fragments in olefinische C-H-Bindungen beschrieben worden. [16] Ein besonders interessanter, jedoch unerforschter Bereich der Chemie von Übergangsmetall-Borylen-Komplexen ist deren Reaktivität gegenüber (oder Übertragung auf) Metall-Kohlenstoff-Mehrfachbindungen.…”