Martin Stanford

Four-membered rings with a P₂BCh core (Ch=S, Se) have been synthesized by the reaction of phosphinidene chalcogenide (Ar*P=Ch) and phosphaborene (Mes*P=BNR₂). The mechanistic pathways towards these rings are explained by detailed computational work that confirmed the preference for the formation of P−P, not P−B, bonded systems, which seems counterintuitive given that both phosphorus atoms contain bulky ligands. The reactivity of the newly synthesized heterocycles, as well as that of the known (RPCh)_n rings (n=2, 3), was probed by the addition of N-heterocyclic carbenes, which revealed that all investigated compounds can act as sources of low-coordinate phosphorus species.

Some serious (P₂)BS: The combination of a phosphinidene chalcogenide and phosphaborene has created a unique heterocycle containing Group 13, 15, and 16 elements. Detailed theoretical work describes the likely pathway in generating P₂BCh heterocycles. The reactivity of these, and other (RPCh)_n rings, with N-heterocyclic carbenes is reported.

Reactions of a magnesium-based pinacolatoboryl nucleophile with the electrophilic organoboranes, 9-BBN and Ph₃B, provide facile B−B′ single bond formation. Although the Ph₃B derivative is thermally stable, when heated, the unsymmetrical diborane(5) anion derived from 9-BBN is found to isomerize to two regioisomeric species via a proposed mechanism involving dehydroboration of the borabicyclo[3.3.1]nonane and syn-diboration of the resultant alkenyl carbocycle.

B-B King: Reactions of a magnesium-based pinacolatoboryl nucleophile with electrophilic organoboranes, 9-BBN or Ph₃B, provide facile B−B′ single bond formation.

A series of NHC-supported 1,2-dithienyldiborenes was synthesized from the corresponding (dihalo)thienylborane NHC precursors. NMR and UV/Vis spectroscopic data, as well as X-ray crystallographic analyses, were used to assess the electronic and steric influences on the B=B double bond of various NHCs and electron-donating substituents on the thienyl ligands. Crystallographic data showed that the degree of coplanarity of the diborene core and thienyl groups is highly dependent on the sterics of the substituents. Furthermore, any increase in the electron-donating ability of the substituents resulted in the destabilization of the HOMO and greater instability of the resulting diborenes.

Mind the gap: In order to increase the reactivity of the B=B double bond, electron-donating groups were installed on the heteroaryl rings of NHC-supported 1,2-dithienyldiborenes. While the coplanarity of the thiophenes and the diborene core proved highly dependent on sterics, any increase in the electron-donating ability of the substituents resulted in the destabilization of the HOMO and greater instability of the resulting diborenes.

The incorporation of carbene (R₂C:) and silylene (R₂Si:) functionalities in the same molecule is a challenging task owing to their inherent reactivity. The synthesis of a stable compound (Si−NHC) featuring an unmasked carbene as well as a silylene is accomplished for the first time by installing a silylene functionality on the backbone of a 1,3-imidazolium-derived NHC. The Si−NHC compound offers significant potential as a ligand in the design of new catalysts, and as a building block for the preparation of new molecules and materials. More information can be found in the Full Paper by R. S. Ghadwal et al. (DOI: 10.1002/chem.201703530).

The preparation of an unprecedented Ge^I-Ge^I bonded digermylene [K₂{Ge₂(μ-κ²:η²:η⁴-2,6-(2,6-ⁱPr₂C₆H₃-N)₂-4-CH₃C₅H₂N)₂}] in an eclipsed conformation stabilized by two bridging diamidopyridyl ligands is presented. Although it exhibits an eclipsed conformation, the Ge−Ge bond length is 2.5168(6) Å, which is shorter than those in the trans-bent and gauche digermylenes. In combination with two pendant amido groups, the Ge^I₂ motif is employed as a building block to assemble the first example of octagermylene [Ge₄(μ-κ²:κ¹-2,6-(2,6-ⁱPr₂C₆H₃-N)₂-4-CH₃C₅H₂N)₂]₂ showing a cyclic configuration and containing three distinct types of Ge^I−Ge^I bonds.

Eclipse at the heart: In contrast to many examples of trans-bent digermylenes with a staggered configuration, a digermylene with an eclipsed geometry has now been characterized. The eclipsed digermylene was employed as a building block to assemble a cyclic octagermylene containing three types of Ge^I−Ge^I bonds (see picture; Ge brown, N pink).

Anilido imine–magnesium bromide complexes, (L^HMgBr)₂ and (L^MeMgBr)₂, have been prepared [L^H = DIPP-N(C₆H₄)C(H)N-DIPP and L^Me = DIPP-N(C₆H₄)C(Me)N-DIPP (DIPP = 2,6-iPr₂C₆H₃)]. Their dimeric crystal structures are compared to the dimer (DIPP-nacnacMgBr)₂ [DIPP-nacnac = DIPP-NC(Me)C(H)C(Me)N-DIPP]. The anilido imine ligands show some degree of charge delocalization but are clearly more asymmetric than the DIPP-nacnac complex: one of the N atoms has considerable anilido character, whereas the other one is close to an imine. Calculated natural population analysis (NPA) charges (B3PW91/6-311++G**) and an atoms-in-molecules (AIM) study on model systems quantify the asymmetry in charge distribution. Quantification of the Lewis acidity of L^HMgBr and L^MeMgBr by the Gutmann–Beckett test gave acceptor numbers of 58.9 and 58.3, respectively. The complexes themselves are inert to a number of electrophiles; however, in combination with PPh₃ the substrate 1-butene oxide was ring-opened. The resulting betaine-type alkoxide complex is unstable and decomposes at room temperature through a Wittig-type reaction. At higher temperatures also a reverse Wittig-type decomposition is observed.

The frustrated Lewis pair (FLP) type reactivity of (Mg anilido imine)/PPh₃ combinations has been investigated. Whereas the majority of substrates show no reaction, 1-butene oxide led to ring opening. The resulting phosphonium alkoxide complexes are unstable and show temperature-dependent decomposition pathways.

Phosphasilenes are heavier imine analogues, containing very reactive Si=P double bonds. This work summarizes structures, syntheses, electronic, and chemical properties of these and related species with an emphasis on recent achievements. For more information see the Review by S. Inoue and co-workers. For more details, see the Review by S. Inoue et al. on page 12014 ff.

The reactivity of a geometrically constrained phosphorus(III) complex bearing the N,N-bis(3,5-di-tert-butyl-2-phenolate)amide pincer ligand (P(ONO); 1) towards oxidants and reductants is explored. This compound can be readily oxidized to the phosphorus(V) dihalo-derivatives P(ONO)X₂ where X=Cl (2), Br (3) and I (4). Attempts at isolating the analogous difluoride through oxidation of 1 were unsuccessful yielding only the hydrofluoride P(ONO)(H)F (5), however P(ONO)F₂ (6) can be accessed via a halide exchange reaction of 2 with KF. Compound 2 can be employed as a precursor to novel cationic species through chloride ion displacement using strong Lewis bases. Thus, reaction of 2 with two or three molar equivalents of dimethylaminopyridine (DMAP) affords [P(ONO)(Cl)(DMAP)₂]⁺ (7) and [P(ONO)(DMAP)₃]²⁺ (8). Reaction of 2 with the weaker bidentate base 2,2′-bipyridine (bipy) affords [P(ONO)(Cl)(bipy)]⁺ (9), although this species was only accessible upon addition of a halide abstracting agent. The dicationic tris(pyridine) adduct [P(ONO)(py)₃]²⁺ (10) is also accessible by reaction of 4 with pyridine. Oxidation of 1 using oxygen gas proceeds slowly and allows for the observation of two compounds, a mixed valence dimeric phosphorus(III)/phosphorus(V) compound [P(ONO)(μ²-O)(μ²:κ¹,κ²-ONO)P] (11) and the fully oxidized species [P(ONO)(μ²-O)(μ²:κ¹,κ²-ONO)P(O)] (12). Finally, reaction of 1 using KC₈ results in the dimerization of the putative radical anion [P(ONO)]^.− through formation of a P−P bond to afford [P(ONO)]₂²⁻ (13). Reactions with TEMPO result in the formation of the trigonal bipyramidal species P(ONO)(TEMPO)₂ (14).

The reactivity of a geometrically constrained phosphorus(III) complex towards oxidants and reductants is explored. These studies show that this complex can be readily oxidized to afford a series of novel phosphorus(V) compounds, including rare species such as a phosphorus(V) diiodide.

The single-site activation of strong σ bonds (such as that of H−H, P−H, and N−H) remains a significant challenge in main-group chemistry, with only a few cases reported to date. In this regard, recent exciting experiments performed with aluminum^I complexes hold significance because they have been seen to activate a variety of strong σ bonds. Such chemistry is generally seen to occur in aromatic solvents. Current computational studies with DFT reveal the interesting reason for this: an explicit aromatic solvent molecule acts as a catalyst by converting the aluminum^I complex into aluminum^III during the process. Different cases of σ-bond activation by aluminum^I complexes have been investigated and the efficiency of H−X (X=H, NHtBu, PPh₂) bond activation in the presence of an explicit benzene solvent molecule is orders of magnitude higher than that in its absence. The current work therefore reveals the chemistry of aluminum^I complexes to be richer and more complex than that previously realized, and shows it to be dependent on metal–solvent cooperativity; the first known example of its kind in main-group chemistry.

More than a spectator: Computational investigations with DFT show that the efficiency of H−H, H−N, and H−P σ-bond activation by the NacNac Al^I complex in the presence of explicit benzene solvent molecules is several orders of magnitude higher than that in its absence (see figure). This indicates metal–solvent cooperativity in main-group chemistry.

This work showcases a new catalytic cyclization reaction using a highly Lewis acidic borane with concomitant C−H or C−C bond formation. The activation of alkyne-containing substrates with B(C₆F₅)₃ enabled the first catalytic intramolecular cyclizations of carboxylic acid substrates using this Lewis acid. In addition, intramolecular cyclizations of esters enable C−C bond formation as catalytic B(C₆F₅)₃ can be used to effect formal 1,5-alkyl migrations from the ester functional groups to unsaturated carbon–carbon frameworks. This metal-free method was used for the catalytic formation of complex dihydropyrones and isocoumarins in very good yields under relatively mild conditions with excellent atom efficiency.

The Lewis acidic borane B(C₆F₅)₃ activates alkynes towards an intramolecular cyclization with carboxylic acids. Analogous intramolecular cyclizations of esters via C−C bond formation proceed in good to excellent yields in the presence of catalytic amounts of B(C₆F₅)₃. This process involves a 1,5-alkyl migration from the ester functional group to the unsaturated carbon framework.

The metal-free catalyst tris(2,4,6-trifluorophenyl)borane has demonstrated its extensive applications in the 1,2-hydroboration of numerous unsaturated reagents, namely alkynes, aldehydes and imines, consisting of a wide array of electron-withdrawing and donating functionalities. A range of over 50 borylated products are reported, with many reactions proceeding with low catalyst loading under ambient conditions. These pinacol boronate esters, in the case of aldehydes and imines, can be readily hydrolyzed to leave the respective alcohol and amine, whereas alkynyl substrates result in vinyl boranes. This is of great synthetic use to the organic chemist.

What a boron discovery: The use of tris(2,4,6-trifluorophenyl)borane in the metal-free 1,2-syn-hydroboration of various unsaturated moieties such as alkynes, aldehydes and imines generates a plethora of over 50 borylated products with many reactions proceeding with low catalyst loading under ambient conditions.

A 1,3-digerma-2-silacyclopenta-1,2-diene, that is, a 1,3-digerma-2-silaallene incorporated into a five-membered ring system, was synthesized and obtained as a stable orange solid. Owing to incorporation into a cyclic framework, the 1,3-digerma-2-silaallene moiety is highly bent, exhibiting a Si⁰ character for the central silicon moiety.

The bends: A 1,3-digerma-2-silacyclopenta-1,2-diene, that is, a 1,3-digerma-2-silaallene incorporated into a five-membered ring system, was synthesized and obtained as a stable orange solid. Owing to incorporation into a cyclic framework, the 1,3-digerma-2-silaallene moiety is highly bent, exhibiting a Si⁰ character for the central silicon moiety.

The substituted monomeric phosphanylboranes Ph₂P−BH₂⋅NMe₃ (1) and tBuHP−BH₂⋅NMe₃ (2) have been used for the synthesis of cationic chain compounds built up by R₂P−BH₂ units. With a simple synthesis route, the highly stable cations [Me₃N⋅H₂B−PR₁R₂−BH₂⋅NMe₃]⁺ (1 a, 2 a) and [Me₃N⋅H₂B−PR₁R₂−BH₂−PR₁R₂−BH₂⋅NMe₃]⁺ (1 b, 2 b) (R₁=R₂=Ph; R₁=H, R₂=tBu) are obtained as iodide (I⁻) salts. The reaction of H₂As−BH₂⋅NMe₃ (3) with IBH₂⋅SMe₂ leads to [Me₃N⋅H₂B−AsH₂−BH₂−AsH₂−BH₂⋅NMe₃][I] (3 a), the longest so far known arsanylborane chain. Compound 3 a reacts with acetonitrile through a formal hydroarsination reaction to form [cyclo-{As(BH₂⋅NMe₃)(CMe=NH)₂(BH₂)}][I] (4). The reported synthetic strategy has proved to be a powerful tool for the formation of small, cationic oligomeric units. All products were comprehensively characterized by X-ray structure analysis, NMR, IR spectroscopy, and mass spectrometry in cooperation with DFT calculations.

Cationic inorganic chains: The synthesis of oligomeric, cationic group 13/15 compounds was successfully extended to organosubstituted phosphanylboranes. Furthermore, the longest so far known chain with a backbone consisting of alternating B and As atoms was obtained, which was converted into a heterocycle by a hydroarsination reaction with acetonitrile.

Herein a convenient one-pot route to a sterically demanding superbasic pyridine is presented. Functionalization of the 2- and 6-positions with the strongly σ-donating boryl-groups shifts the calculated gas phase basicity of the pyridine nitrogen atom to 1012 kJ mol⁻¹, which outperforms the “proton sponge” 1,8-bis(dimethylamino)naphthalene (996 kJ mol⁻¹). The diazaboryl groups are oriented orthogonally to the pyridine ring and do not block the N-position, which resembles the geometry of commonly used N-heterocyclic carbenes. This allows the substituted pyridine to be used as a neutral N-donor ligand in coordination chemistry that is demonstrated herein with the Lewis adducts of haloboranes. Contrary to NHCs, which can form extraordinarily stable adducts, the pyridine ligand is intended to act as a weaker-coordinating alternative and could allow for alternative ligand chemistry.

A superbasic pyridine ligand: A pyridine substituted with diazaboryl groups at the 2- and 6-positions is presented. The strongly σ-donating diazaboryl groups increase the energy of the pyridine nitrogen lone pair and make it become more basic than the “proton sponge” 1,8-bis(dimethylamino)naphthalene. The N-position is not blocked, but is sterically protected, which renders this pyridine a promising ligand for coordination chemistry.

The synthesis of mono-NHC alane adducts of the type (NHC)⋅AlH₃ (NHC=Me₂Im (1), Me₂Im^Me (2), iPr₂Im (3 and [D₃]-3), iPr₂Im^Me (4), Dipp₂Im (10); Im=imidazolin-2-ylidene, Dipp=2,6-diisopropylphenyl) and (NHC)⋅AliBu₂H (NHC=iPr₂Im (11), Dipp₂Im (12)) as well as their reactivity towards different types of carbenes is presented. Although the mono-NHC adducts remained stable at elevated temperatures, ring expansion occurred when (iPr₂Im)⋅AlH₃ (3) was treated with a second equivalent of the carbene iPr₂Im to give (iPr₂Im)⋅AlH(RER-iPr₂ImH₂) (6). In 6, {(iPr₂Im}AlH} is inserted into the NHC ring. In contrast, ring opening was observed with the sterically more demanding Dipp₂Im with the formation of (iPr₂Im)⋅AlH₂(ROR-Dipp₂ImH₂)H₂Al⋅(iPr₂Im) (9). In 9, two {(iPr₂Im)⋅AlH₂} moieties stabilize the ring-opened Dipp₂Im. If two hydridic sites are blocked, the adducts are stable with respect to further ring expansion or ring opening, as exemplified by the adducts (iPr₂Im)⋅AliBu₂H (11) and (Dipp₂Im)⋅AliBu₂H (12). The adducts (NHC)⋅AlH₃ and (iPr₂Im)⋅AliBu₂H reacted with cAAC^Me by insertion of the carbene carbon atom into the Al−H bond to give (NHC)⋅AlH₂/iBu₂(cAAC^MeH) (13–18) instead of ligand substitution, ring-expansion, or ring-opened products.

Versatile alanes: (NHC)⋅AlH₃ and (NHC)⋅AliBu₂H have been synthesized and treated with N-heterocyclic carbenes and cyclic (alkyl)(amino)carbenes (cAACs). The reactions were found to proceed by NHC ring expansion, NHC ring opening, or cAAC-mediated Al−H bond cleavage (see Scheme). Mechanistic considerations are presented.