Synthesis of Electrophiles Derived from Dimeric Aminoboranes and Assessing Their Utility in the Borylation of π Nucleophiles.

Dimeric aminoboranes, [H2BNR2]2 (R = Me or CH2CH2) containing B2N2 cores, can be activated by I2, HNTf2 (NTf2 = [N(SO2CF3)2]), or [Ph3C][B(C6F5)4] to form isolable H2B(µ-NR2)2BHX (for X = I or NTf2). For X = [B(C6F5)4]- further reactivity, presumably between [H2B(µ-NMe2)2BH][B(C6F5)4] and aminoborane, forms a B3N3-based monocation containing a three-center two electron B-(µ-H)-B moiety. The structures of H2B(µ-NMe2)2BH(I) and [(µ-NMe2)BH(NTf2)]2 indicated a sterically crowded environment around boron, and this leads to the less common O-bound mode of NTf2 binding. While the iodide congener reacted very slowly with alkynes, the NTf2 analogues were more reactive, with hydroboration of internal alkynes forming (vinyl)2BNR2 species and R2NBH(NTf2) as the major products. Further studies indicated that the B2N2 core is maintained during the first hydroboration, and that it is during subsequent steps that B2N2 dissociation occurs. In the mono-boron systems, for example, i Pr2NBH(NTf2), NTf2 is N-bound; thus, they have less steric crowding around boron relative to the B2N2 systems. Notably, the monoboron systems are much less reactive in alkyne hydroboration than the B2N2-based bis-boranes, despite the former being three coordinate at boron while the latter are four coordinate at boron. Finally, these B2N2 electrophiles are much more prone to dissociate into mono-borane species than pyrazabole [H2B(µ-N2C3H3)]2 analogues, making them less useful for the directed diborylation of a single substrate.

Borylation Directed Borylation of Indoles Using Pyrazabole Electrophiles: A One-Pot Route to C7-Borylated-Indolines.

Pyrazabole (1) is a readily accessible diboron compound that can be transformed into ditopic electrophiles. In 1 (and derivatives), the B⋅⋅⋅B separation is ca. 3 Å, appropriate for one boron centre bonding to N and one to the C7 of indoles and indolines. This suitable B⋅⋅⋅B separation enables double E-H (E=N/C) functionalisation of indoles and indolines. Specifically, the activation of 1 with HNTf2 generates an electrophile that transforms N-H indoles and indolines into N/C7-diborylated indolines, with N-H borylation directing subsequent C7-H borylation. Indole reduction to indoline occurs before C-H borylation and our studies indicate this proceeds via hydroboration-C3-protodeboronation to produce an intermediate that then undergoes C7 borylation. The borylated products can be converted in situ into C7-BPin-N-H-indolines. Overall, this represents a transient directed C-H borylation to form useful C7-BPin-indolines.

Two Directing Groups Used for Metal Catalysed Meta-C-H Functionalisation Only Effect Ortho Electrophilic C-H Borylation.

Two templates used in meta-directed C-H functionalisation under metal catalysis do not direct meta-C-H borylation under electrophilic borylation conditions. Using BCl3 only Lewis adduct formation with Lewis basic sites in the template is observed. While combining BBr3 and the template containing an amide linker only led to amide directed ortho C-H borylation, with no pyridyl directed meta borylation. The amide directed borylation is selective for the ortho borylation of the aniline derived unit in the template, with no ortho borylation of the phenylacetyl ring - which would also form a six membered boracycle - observed. In the absence of other aromatics amide directed ortho borylation on to phenylacetyl rings can be achieved. The absence of meta-borylation using two templates indicates a higher barrier to pyridyl directed meta borylation relative to amide directed ortho borylation and suggests that bespoke templates for enabling meta-directed electrophilic borylation may be required.

Retro-Diels-Alder decomposition of norbornadiene mediated by a cationic magnesium complex.

First evidence for the coordination of norbornadiene (nbd) and dicyclopentadiene (dcpd) with the main group metal Mg is provided by the crystal structures of adducts with cationic ß-diketiminate (BDI) Mg complexes. While the dcpd complex is thermally stable, [(BDI)Mg+·nbd][B(C6F5)4-] shows slow room temperature retro-Diels-Alder decomposition to give a complex with the cation (BDI)Mg(C5H5)Mg(BDI)+.

Cationic Aluminium Complexes as Catalysts for Imine Hydrogenation.

Strongly Lewis acidic cationic aluminium complexes, stabilized by ß-diketiminate (BDI) ligands and free of Lewis bases, have been prepared as their B(C6 F5 )4 - salts and were investigated for catalytic activity in imine hydrogenation. The backbone (R1) and N (R2) substituents on the R1,R2 BDI ligand (R1,R2 BDI=HC[C(R1)N(R2)]2 ) influence sterics and Lewis acidity. Ligand bulk increases along the row Me,DIPP BDI

Unsupported Mg-Alkene Bonding.

The first intermolecular early main group metal-alkene complexes were isolated. This was enabled by using highly Lewis acidic Mg centers in the Lewis base-free cations (Me BDI)Mg+ and (tBu BDI)Mg+ with B(C6 F5 )4 - counterions (Me BDI=CH[C(CH3 )N(DIPP)]2 , tBu BDI=CH[C(tBu)N(DIPP)]2 , DIPP=2,6-diisopropylphenyl). Coordination complexes with various mono- and bis-alkene ligands, typically used in transition metal chemistry, were structurally characterized for 1,3-divinyltetramethyldisiloxane, 1,5-cyclooctadiene, cyclooctene, 1,3,5-cycloheptatriene, 2,3-dimethylbuta-1,3-diene, and 2-ethyl-1-butene. In all cases, asymmetric Mg-alkene bonding with a short and a long Mg-C bond is observed. This asymmetry is most extreme for Mg-(H2 C=CEt2 ) bonding. In bromobenzene solution, the Mg-alkene complexes are either dissociated or in a dissociation equilibrium. A DFT study and AIM analysis showed that the C=C bonds hardly change on coordination and there is very little alkene→Mg electron transfer. The Mg-alkene bonds are mainly electrostatic and should be described as Mg2+ ion-induced dipole interactions.

Magnesium-halobenzene bonding: mapping the halogen sigma-hole with a Lewis-acidic complex.

Complexes of the Lewis base-free cations (MeBDI)Mg+ and ( tBuBDI)Mg+ with Ph-X ligands (X = F, Cl, Br, I) have been studied (MeBDI = HC[C(Me)N-DIPP]2 and tBuBDI = HC[C(tBu)N-DIPP]2; DIPP = 2,6-diisopropylphenyl). For the smaller ß-diketiminate ligand (MeBDI) only complexes with PhF could be isolated. Heavier Ph-X ligands could not compete with bonding of Mg to the weakly coordinating anion B(C6F5)4 -. For the cations with the bulkier tBuBDI ligand, the full series of halobenzene complexes was structurally characterized. Crystal structures show that the Mg⋯X-Ph angle strongly decreases with the size of X: F 139.1°, Cl 101.4°, Br 97.7°, I 95.1°. This trend, which is supported by DFT calculations, can be explained with the σ-hole which increases from F to I. Charge calculation and Atoms-In-Molecules analyses show that Mg⋯F-Ph bonding originates from electrostatic attraction between Mg2+ and the very polar C δ+-F δ- bond. For the heavier halobenzenes, polarization of the halogen atom becomes increasingly important (Cl < Br < I). Complexation with Mg leads in all cases to significant Ph-X bond activation and elongation. This unusual coordination of halogenated species to early main group metals is therefore relevant to C-X bond breaking.

Complexation and Versatile Reactivity of a Highly Lewis Acidic Cationic Mg Complex with Alkynes and Phosphines.

[(BDI)Mg+ ][B(C6 F5 )4 - ] (1; BDI=CH[C(CH3 )NDipp]2 ; Dipp=2,6-diisopropylphenyl) was prepared by reaction of (BDI)MgnPr with [Ph3 C+ ][B(C6 F5 )4 - ]. Addition of 3-hexyne gave [(BDI)Mg+ ⋅(EtC≡CEt)][B(C6 F5 )4 - ]. Single-crystal X-ray analysis, NMR investigations, Raman spectra, and DFT calculations indicate a significant Mg-alkyne interaction. Addition of the terminal alkynes PhC≡CH or Me3 SiC≡CH led to alkyne deprotonation by the BDI ligand to give [(BDI-H)Mg+ (C≡CPh)]2 ⋅2 [B(C6 F5 )4 - ] (2, 70 %) and [(BDI-H)Mg+ (C≡CSiMe3 )]2 ⋅2 [B(C6 F5 )4 - ] (3, 63 %). Addition of internal alkynes PhC≡CPh or PhC≡CMe led to [4+2] cycloadditions with the BDI ligand to give {Mg+ C(Ph)=C(Ph)C[C(Me)=NDipp]2 }2 ⋅ 2 [B(C6 F5 )4 - ] (4, 53 %) and {Mg+ C(Ph)=C(Me)C[C(Me)=NDipp]2 }2 ⋅2 [B(C6 F5 )4 - ] (5, 73 %), in which the Mg center is N,N,C-chelated. The (BDI)Mg+ cation can be viewed as an intramolecular frustrated Lewis pair (FLP) with a Lewis acidic site (Mg) and a Lewis (or Brønsted) basic site (BDI). Reaction of [(BDI)Mg+ ][B(C6 F5 )4 - ] (1) with a range of phosphines varying in bulk and donor strength generated [(BDI)Mg+ ⋅PPh3 ][B(C6 F5 )4 - ] (6), [(BDI)Mg+ ⋅PCy3 ][B(C6 F5 )4 - ] (7), and [(BDI)Mg+ ⋅ PtBu3 ][B(C6 F5 )4 - ] (8). The bulkier phosphine PMes3 (Mes=mesityl) did not show any interaction. Combinations of [(BDI)Mg+ ][B(C6 F5 )4 - ] and phosphines did not result in addition to the triple bond in 3-hexyne, but during the screening process it was discovered that the cationic magnesium complex catalyzes the hydrophosphination of PhC≡CH with HPPh2 , for which an FLP-type mechanism is tentatively proposed.

Bulky cationic ß-diketiminate magnesium complexes.

Cationic ß-diketiminate Mg complexes with the bulky tBuBDI ligand and the weakly coordinating anion B(C6F5)4- have been prepared by the reaction of (tBuBDI)MgnBu with [Ph3C]+[B(C6F5)4]-; tBuBDI = CH[C(tBu)N-Dipp]2 and Dipp = 2,6-diisopropylphenyl. Their structures are compared to the previously reported cationic (MeBDI)Mg+ complexes; MeBDI = CH[C(Me)N-Dipp]2. Crystallization of [(tBuBDI)Mg]+[B(C6F5)4]- from chlorobenzene gave a unique (tBuBDI)Mg+·ClC6H5 cation with a rather short MgCl and consequently long C-Cl bond. Crystallization from chlorobenzene/arene solvent mixtures gave (tBuBDI)Mg+·arene complexes (arene = benzene, toluene, m-xylene) but in the presence of mesitylene the chlorobenzene complex was formed. Due to the greater shielding of the metal, none of these complexes display Mg(F5C6)4B- interactions. Crystal structures of the arene complexes show in all cases η2-coordination of the arene ligands. Ring slippage from a more favorable η2-coordination can be explained by the steric bulk of the tBuBDI ligand. The smaller arenes, benzene and toluene, also bind to (tBuBDI)Mg+ in bromobenzene solution. The Lewis acidity of these cationic Mg complexes was determined by the Gutmann-Beckett test. The acceptor number for (tBuBDI)Mg+ (AN = 76.0) is substantially higher than that estimated for (MeBDI)Mg+ (AN = 70.3). Calculation of the atomic NPA charges by DFT shows that the Mg2+ ion in (tBuBDI)Mg+ is slightly more positively charged than the metal in (MeBDI)Mg+, confirming its higher Lewis acidity. The lower benzene complexation energy calculated for (tBuBDI)Mg+versus (MeBDI)Mg+ is due to steric congestion of the metal in the (tBuBDI)Mg+ cation which allows only for Mg(η1)C6H6 instead of Mg(η6)C6H6 bonding. This ring slippage, however, results in a significant polarization of the electron density in the benzene ring, making it susceptible for nucleophilic attack.

Highly Lewis acidic cationic alkaline earth metal complexes.

Cationic Lewis base-free ß-diketiminate (BDI) complexes of Mg and Ca have been isolated as their B(C6F5)4- salts. The cation (BDI)Mg+ shows an extraordinarily strong Lewis acidity that can compete with strong Lewis acids like B(C6F5)3 and (BDI)AlMe+. Its highly electrophilic nature is exemplified by isolation of an 3-hexyne adduct.

Unsupported metal silyl ether coordination.

Simple silyl ethers like O(SiMe3)2 in contrast to normal ethers are inert to metal bonding; however, a "naked", highly Lewis-acidic, cationic Mg species enforces complexation. DFT calculations indicate that agostic interactions and van der Waals attraction significantly contribute to the stability of this first example of unsupported metal silyl ether coordination.

A Simple Route to Calcium and Strontium Hydride Clusters.

The first strontium hydride complex has been obtained by simply treating Sr[N(SiMe3 )2 ]2 with PhSiH3 in the presence of PMDTA. The Sr complex Sr6 H9 [N(SiMe3 )2 ]3 ⋅(PMDTA)3 crystallizes as an "inverse cryptand": an interstitial H- is surrounded by a Sr6 H84+ cage decorated with amide and PMDTA ligands. The analogous Ca complex could also be obtained and both retain their solid-state structures in solution: 1 H NMR spectra in C6 D6 show two doublets and one nonet (4:4:1). Up to 90 °C, no coalescence is observed. The Ca cluster was investigated by DFT calculations and shows atypically low charges on Ca (+1.14) and H (-0.59) which signifies an unexpectedly low ionicity. AIM analysis shows hydride⋅⋅⋅hydride bond paths with considerable electron densities in the bond critical point. The clusters thermally decompose into larger, undefined, metal hydride aggregates.

Calcium Hydride Reactivity: Formation of an Anionic N-Heterocyclic Olefin Ligand.

An anionic N-heterocyclic olefin ligand was serendipitously obtained by reaction of an amidinate calcium hydride complex with 1,3-dimethyl-2-methyleneimidazole (NHO). Instead of anticipated addition to the polarized C=CH2 bond to form an unstabilized alkylcalcium complex, deprotonation of the NHO ligand in the backbone was observed. Preference for deprotonation versus addition is explained by loss of aromaticity in the latter conversion. Theoretical calculations demonstrate the substantially increased ylidic character of this anionic NHO ligand which, like N-heterocyclic dicarbenes, shows strong bifunctional coordination.

Self-Assembly of Magnesium Hydride Clusters Driven by Chameleon-Type Ligands.

While magnesium hydride complexes are generally stabilized by hard, bulky N-donor ligands, softer ligands with a broad variety of coordination modes are shown to efficiently adapt themselves to the large variety of Mg2+ centers in a growing magnesium hydride cluster. A P,N-chelating ligand is introduced that displays coordination modes between that of enamide, aza-allyl, and phosphinomethanide. Slight changes in the ligand bite angle have dramatic consequences for the structure type. The hitherto largest neutral magnesium hydride clusters are isolated either in a nonanuclear sheet-structure (brucite-type) or a dodecanuclear ring structure.

"Masked" Lewis-acidity of an aluminum α-phosphinoamide complex.

The reaction of Ph2P(DIPP)NH with AlMe3 cleanly gives an aluminum amide complex that crystallizes as a centrosymmetric dimer with a six-membered Al-N-P-Al-N-P ring. In aromatic solvents the dimer remains intact but the Al-P bond is readily broken upon addition of THF to form Ph2P(DIPP)NAlMe2·THF. Efforts to use [Ph2P(DIPP)NAlMe2]2 as a "masked" Lewis acidic activator for olefin polymerization catalysts were unsuccessful but the complex showed a Frustrated Lewis pair reactivity instead. The P/Al complex reacts with isocyanates to give the C[double bond, length as m-dash]O inserted product that crystallizes as a five-membered ring system Al-O-C(NR)-P-N. The reaction of [Ph2P(DIPP)NAlMe2]2 with CO2, however, gave an insertion in the N-Al bond and the dimeric product [Ph2P(DIPP)NCO2AlMe2]2 was isolated. The dimer [Ph2P(DIPP)NAlMe2]2 is one of the few Al/P FLPs that can activate C[double bond, length as m-dash]C double bonds irreversibly. A reaction with allyl methyl sulfide and 1-hexene led to the clean formation of the structurally similar activated alkene products [(DIPP)N-Ph2P-CH(CH2SMe)CH2]AlMe2 and [(DIPP)N-Ph2P-CH(C4H9)CH2]AlMe2.

ß-Diketiminate calcium hydride complexes: the importance of solvent effects.

A series of (DIPPnacnac)CaN(SiMe3)2·S complexes (DIPPnacnac = HC[C(Me)N(2,6-iPr-C6H3)]2; S = solvent) could be obtained by the addition of S = THF, DME or N-Me-morpholine (Morph) to (DIPPnacnac)CaN(SiMe3)2·OEt2 or (DIPPnacnac)CaN(SiMe3)2. Crystal structures for complexes with S = DME and Morph are compared to literature-known structures with S = none, THF or Et2O. Bulkier and weaker Lewis bases like the tertiary amines Et3N, TMEDA and DABCO did not interact with (DIPPnacnac)CaN(SiMe3)2. The reaction of (DIPPnacnac)CaN(SiMe3)2 with PhSiH3 gave conversion to a calcium hydride complex that dismutated in (DIPPnacnac)2Ca and CaH2. The reaction of (DIPPnacnac)CaN(SiMe3)2·S with PhSiH3 gave [(DIPPnacnac)CaH·S]2 for S = THF, Et2O or N-Me-morpholine (Morph). For S = DME, high reaction temperatures were needed and dismutation into (DIPPnacnac)2Ca and CaH2 was observed. Extensive NMR investigations (VT-NMR and PGSE) confirm the dimeric nature of [(DIPPnacnac)CaH·THF]2 in aromatic solvents or in THF. Thermal decomposition of [(DIPPnacnac)CaH·THF]2 (release of H2 at 200 °C) is compared to that of Mg and Zn analogues. Weakly coordinating Et2O in [(DIPPnacnac)CaH·OEt2]2 could be replaced by THF, Morph or DABCO but not with Et3N. The addition of TMEDA led to the formation of CaH2 and unidentified products. The addition of DME led to the decomposition of Et2O and complex [(DIPPnacnac)CaOEt]2 was obtained. Crystal structures of the following compounds are presented: (DIPPnacnac)CaN(SiMe3)2·S (S = Morph, DME), [(DIPPnacnac)CaH·S]2 (S = Et2O, Morph and DABCO) and [(DIPPnacnac)CaOEt]2. Although bulky ligands have long been thought to be the key to the stabilization of calcium hydride complexes, the presence of a polar, strongly coordinating, co-solvent is also crucial.

A Soft Grip: Magnesium Complexes with a Phosphine-Modified Phosphonium Diylidic Lewis Base.

Simple strategies to obtain magnesium complexes with the soft chelating diylidic ligand [Ph2 PCHPPh2 (fluorenylidene)]- (dppmflu- ) were developed to evaluate the influence of the hard acid (cation) and soft base (anion) mismatch on the stability and reactivity of the formed derivatives. Deprotonation of the precursor Ph2 PCH2 PPh2 (flu) (dppmfluH) by an alkylmagnesium derivative or magnesium amide provided access to [{Mg(dppmflu)(µ-nBu)}2 ], [Mg(dppmflu){N(SiMe3 )2 }], and [{Mg(dppmflu)(µ-Me)}2 ], which were used as starting materials for further investigations. The reaction of [{Mg(dppmflu)(µ-nBu)}2 ] with PhSiH3 in the presence of THF allowed isolation of the magnesium hydride complex [{Mg(dppmflu)(µ-H)(thf)}2 ] without a stabilizing nitrogen donor ligand. Prolonged heating enforced ligand redistribution and [{Mg(dppmflu)(µ-H)(thf)}2 ] was converted to [Mg(dppmflu)2 ] and MgH2 . The homoleptic derivative [Mg(dppmflu)2 ], in which the magnesium center is in a very soft ligand environment, can open a THF molecule by frustrated Lewis pair reactivity to give [{Mg(dppmflu)(µ-OC4 H8 dppmflu)}2 ].

Calcium Hydride Catalyzed Highly 1,2-Selective Pyridine Hydrosilylation.

Reaction of the calcium hydride complex (DIPPnacnac-CaH⋅THF)2 with pyridine is much faster and selective than that of the corresponding magnesium hydride complex (DIPPnacnac = [(2,6-iPr2 C6 H3 )NC(Me)]2 CH). With a range of pyridine, picoline and quinoline substrates, exclusive transfer of the hydride ligand to the 2-position is observed and also at higher temperatures no 1,2→1,4 isomerization is found. The heteroleptic product DIPPnacnac-Ca(1,2-dihydropyridide)⋅(pyridine) shows fast ligand exchange into homoleptic calcium complexes and therefore could not be isolated. Calcium hydride reduction of isoquinoline gave well-defined homoleptic products which could be characterized by X-ray diffraction: Ca(1,2-dihydroisoquinolide)2 ⋅(isoquinoline)4 and Ca3 (1,2-dihydroisoquinolide)6 ⋅(isoquinoline)6 . The striking selectivity difference in the dearomatization of pyridines by Mg or Ca complexes could be explained by DFT theory and was utilized in catalysis. Whereas hydroboration of pyridine with pinacol borane with a calcium hydride catalyst gave only minor conversion, the hydrosilylation of pyridine and quinolines with PhSiH3 yields exclusively 1,2-dihydropyridine and 1,2-dihydroquinoline silanes with 80-90 % conversion. Similar results can be achieved with the catalyst Ca[N(SiMe3 )2 ]2 ⋅(THF)2 . These calcium complexes represent the first catalysts for the 1,2-selective hydrosilylation of pyridines.