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The use of transition metals to catalyse the addition of hydridoboranes to unsaturated organic molecules was initially realised several decades ago. Although this area of chemistry received considerable attention at the time, interest in this reaction and its use in organic synthesis waned for several years. Like a phoenix rising from the ashes, this amazing catalytic reaction has grown to include the use of earth-abundant metal catalysts and a much wider range of organic substrates. Indeed, it is now commonly utilised as a diagnostic tool to assess the reactivity and catalytic ability of newly generated transition metal and main group complexes. As this field is progressing so rapidly, this review highlights some important advances up to the end of 2021 and into early 2022. Excluded from this review are 'hydroboration' reactions using diboron sources.

This study reports on the exploration of the phosphinoboration reaction with several thiosemicarbazones (R5R4NC(S)NR3N=CR1R2). Reactions between either Ph2PBpin (pin = 1,2-O2C2Me4) or Ph2PBcat (cat = 1,2-O2C6H4) with thiosemicarbazones containing a terminal primary or secondary amine afforded boron-containing heterocyclic 1,3,4-thiadiazoline products in excellent yield. The addition of Ph2PBpin to thiosemicarbazones containing an NMe2 group in the terminal position generated novel five-membered heterocycles in moderate yield, which included boron, sulfur, and nitrogen atoms. Heterocyclization of the thiosemicarbazones occurs preferentially in the presence of functional groups such as acetyl and pyridyl groups.

The uncatalyzed addition of catecholborane to α-diimines has been examined. Reactions proceed smoothly at room temperature in the absence of a catalyst or additive, providing bis-hydroboration products with 3-coordinate boron centres or mono-hydroboration products with 4-coordinate boron centres, depending on the nature of the diimine reagent.

We report the first trans phosphinoboration of internal alkynes. With an organophosphine catalyst, alkynoate esters and the phosphinoboronate Ph2 P-Bpin are efficiently converted into the corresponding trans-α-phosphino-β-boryl acrylate products in moderate to good yield with high regio- and Z-selectivity. This reaction operates under mild conditions and demonstrates good atom economy, requiring only a modest excess of the phosphinoboronate. X-ray crystallography experiments allowed structural assignment of the unprecedented and densely functionalized (Z)-α-phosphino-β-boryl acrylate products.

This investigation examines the reactivity of phosphinoboronate esters Ph2PBpin (pin = 1,2-O2C2Me4) and Ph2PBcat (cat = 1,2-O2C6H4), as well as other phosphinoboron species, with various aryl and aliphatic acyl chlorides. These reactions proceed smoothly to give acyl phosphines of the type RC(O)PR'2 along with loss of a boron-chloride compound. In some cases, a second equivalent of the phosphinoboron species can add to the C[double bond, length as m-dash]O double bond at elevated temperatures to give the corresponding diphosphines RC(OBR''2)(PR'2)2. These ambiphilic diphosphines behave like substituted (1,1-bis(diphenylphosphino)methane) derivatives in a reaction of PhC(OBpin)(PPh2)2 (2a) with (η5-C9H7)Rh(η2-coe)2 (coe = cis-cyclooctene) affording the indenyl rhodium complex (η5-C9H7)Rh(PhC(OBpin)(PPh2)2) (3a) where the phosphines are bound to the metal centre in a κ2-P,P bidentate manner.

In this study we have studied the synthesis, characterization and catalytic activity of phosphinocopper(I) complexes [Cu(PPh3)3(NCCH3)][B(O2C6H4-4-R)2] (3a: R = H; 3b: R = Me; 3c: R = NO2) and [Cu(PPh3)3(NCCH3)][B(O2C6H3-3,5-di-tBu)2] (3d) containing weakly coordinating arylspiroborate ligands bearing various electronic and sterically defined characteristics. All new compounds have been characterized fully including single crystal X-ray diffraction studies for 3a and 3c and confirm that the arylspiroborate ligands do not coordinate to the copper atoms. Using these new metal complexes as precatalysts in the cyclisation of short chain alkynoic acids gave the corresponding exo-dig cyclic lactones exclusively.

We have investigated the addition of a simple phosphinoboronate ester, Ph2PBpin (pin = 1,2-O2C2Me4), to 2-diphenylphosphinobenzaldehyde (2-Ph2PC6H4C(O)H) and related aldimine derivatives (2-Ph2PC6H4C(NR)H) as a simple and effective strategy for generating unique diphosphine ligands bearing a pendant Lewis-acid Bpin group. These reactions proceed selectively to give one new product where the phosphide fragment has added to the aldehyde (or imine) carbon atom and the electron-deficient boron group has added to the electron-rich heteroatom. Preliminary studies show these new compounds can ligate to Pd(II) and Pt(II) metal centres. These novel metal complexes, as well as the organic soluble [MCl2(coe)]2 (M = Pd, Pt, coe = cis-cyclooctene) compounds, have been shown to be effective precatalysts in the cyclisation of alkynoic acids to give the corresponding exo-dig cyclic lactones. Reactions employing these metal complexes also generated unusual endo-dig cyclic lactones not traditionally observed in these cyclisation reactions. For instance, reactions of 4-pentynoic acid also afforded significant amounts of α-angelica lactone, a biologically-important compound traditionally prepared via the catalytic dehydration and cyclisation of levulinic acid.

In this study we have expanded upon a family of substituted pentacyclic oxadiazadiborinane derivatives from the condensation of carbohydrazide with ortho-formylphenyl boronic acid derivatives containing a variety of chemical and physical properties. All new complexes have been characterized fully including two single crystal X-ray diffraction studies which confirm the solid-state structure of these species along with an unusual methanol activation product, which appears to be a minor solid-state side product. The lack of inherent solubility of these compounds in common physiological media precluded us from doing traditional antimicrobial activities. To circumvent this problem, we incorporated these boron compounds into chitosan films. Only weak or moderate activities against Gram-negative and Gram-positive bacteria were observed in this study.

This preliminary study reports on the synthesis of two new boron-capsaicin derivatives containing either a short or long chain aliphatic tail group using an iridium catalyzed hydroboration reaction with pinacolborane. The boronate ester groups reside on the terminal position of the tail group and are necessary for the bioactivity of these compounds. Indeed, both compounds showed considerable activity against two Gram-positive bacteria, including Vancomycin-resistant Enterococcus. Vancomycin is considered the last resort medication for the treatment of septicemia, and new antibacterial agents that can treat sepsis are of paramount importance. The more lipophilic boron compound with the longer aliphatic chain also showed antifungal activity against Saccharomyces cerevisiae.

The addition of 4-ethyl-3-thiosemicarbazide to benzaldehyde and boronic acid containing derivatives afforded the corresponding thiosemicarbazones (1–3) or benzodiazaborines (4–6) depending on the position of the boronic acid within the ring. All compounds have been characterized fully including an X-ray diffraction study of the methoxy-containing benzodiazaborine 6. Attempts to coordinate thiosemicarbazones 2 and 3 to palladium(II) acetate were unsuccessful; however, addition of the non-boron-containing derivative 1 to palladium afforded complex 7 whose molecular structure was determined by an X-ray diffraction study. The initial bioactivities of compounds 1–7 were examined against two fungi, Aspergillus niger and Saccharomyces cerevisiae, and two bacteria, Bacillus cereus and Pseudomonas aeruginosa.

The stereoselective anti-addition of selenoboranes to α,β-acetylenic esters and amides was achieved in a transition metal-free context using catalytic amounts of PCy3. The reaction provides anti-3,4-selenoboration with concomitant delivery of α-vinyl selenides by protodeboronation with MeOH. Interestingly, in the absence of phosphine the selenoboration switches towards the formation of β-vinyl selenides. Theoretical calculations rationalize the regio- and stereoselectivity of the reaction by discovering a new mechanism for the anti-3,4-selenoboration. While the selenoborane is activated via the “push–pull” effect of B, the phosphine interacts with the β position of the alkynoate switching the polarity of the triple bond and favoring 1,3-selenoboration which provides the α-addition of the selenyl group. Then, the autocatalytic action of a second selenoborane reagent, which coordinates to the phosphorus ylide intermediate, determines the stereoselectivity and completes the catalytic process. Finally, the comparison of selenoborane reagents with diboranes and silaboranes, which have exhibited analogous reactivity, shows that the selenium moiety has a larger nucleophilic character favoring the performance of the reaction under mild conditions.

The addition of phosphinoboronate ester Ph2PBpin (pin = 1,2-O2C2Me4) (1) to a number of different N-heterocycles has been investigated. Reaction of 1 with pyridine resulted in highly selective formation of the corresponding 1,4-addition product, with addition of the electron-deficient Bpin group to the pyridine nitrogen atom and the phosphido group to the para carbon atom. Conversely, reactions of para-substituted pyridine derivatives oc-curred predominately to afford 1,2-addition products while quinoline reacted to afford the 1,2-adduct which ultimately isomerized to afford the corresponding 1,4-addition product. Preliminary computational studies have been undertaken to explore possible pathways for these transformations including transfer of the PPh2- anion from [B(PPh2)2pin]- to the 4-position of a borenium/boronium activated pyridine and concerted pathways for 1,2-addition via intramolecular nucleophilic attack of PPh2 at C2 of a Ph2PBpin-coordinated pyridine via a four-centered transition state and intramolecular transfer of PPh2 to the 2-position of a boron-activated pyridine in a phosphido-bridged dimer involving a six-centered transition state.

A series of iminophosphineplatinum(II) complexes have been prepared from pro-ligands derived from aniline derivatives containing electron-donating methoxy groups or electron-withdrawing fluorides and [PtCl2(η 2 − coe)]2 (coe = cis-cyclooctene). All new pro-ligands and metal complexes have been fully characterized, including an X-ray diffraction study for compound 11 (derived from para-methoxyaniline). Additionally, the molecular structure of a di-iminophosphineplatinum dication 11a has been determined. The platinum compounds showed no appreciable cytotoxic properties against two glioma cell lines using the MTT method.

Six Schiff base compounds have been prepared from the condensation of o-vanillin, 2,3-dihydroxybenzaldehyde and 2,3,4-trihydroxybenzaldehyde with 4-aminosalicylic acid and 5-aminosalicylic acid (5-ASA). Addition of these Schiff bases to [Pd(OAc)2] afforded the corresponding bis(salicylaldiminato)palladium(II) complexes in moderate to excellent yields. All new palladium complexes have been characterized fully using standard spectroscopic methods, elemental analyses and a single-crystal X-ray diffraction study in the case of 2e, the palladium complex containing Schiff base ligands derived from 5-ASA and 2,3-dihydroxybenzaldehyde. All derivatives of 5-ASA were examined for potential antimicrobial activities against two species of fungi, Aspergillus niger and Saccharomyces cerevisiae, as well as two species of bacteria, Bacillus cereus (Gram-positive) and Pseudomonas aeruginosa (Gram-negative).

Three new iminophosphines containing pinacol-derived boronate esters have been prepared and ligated to dichloridoplatinum(II) fragments. All compounds have been characterized fully, including an X-ray diffraction study carried out for the platinum complex 8, which is derived from 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)aniline. These three new platinum complexes, along with the non-boron containing control, have been examined for their initial cytotoxic properties against two glioma cell lines using the MTT method.

The insertion of the diazo derivative Me3SiCHN2 into pinB-SR σ bonds (R = Ph, Tol, Bn) allows a direct synthesis of multisubstituted H-C(SR)(Bpin)(SiMe3) compounds. Consecutive base-assisted transformations of HC(S)(B) (Si) systems lead to deborylative alkylations, Sommelet-Haüser rearrangements, and deprotoalkylations. Intramolecular cyclizations can be selectively performed either via desilylative or deborylative manifolds by fine-tuning the base employed.

We have prepared eleven pyridinecarboxaldimines (N-N’R) from the condensation of 2-pyridinecarboxaldehyde and the corresponding lipophilic primary amines. Addition of these ligands to [PtCl 2 (η 2 -coe)] 2 (coe = cis-cyclooctene) gave complexes of the type cis-PtCl 2 (N-N’R) ( 2a - k ) in moderate to high yields. The molecular structure of the N-butylphenyl derivative ( 2h ) has been confirmed by an X-ray diffraction study. Crystals of 2h were monoclinic with a = 8.5414(9) A, b = 11.6774(12) A, c = 16.1235(16) A, β = 92.7610(10)° in the space group P 2(1)/ c . All platinum complexes were examined for their in vitro cytotoxic properties against two human glioblastoma multiforme cell lines LN18 and LN405. The DNA binding properties of one of the most cytotoxic compounds was compared with that of cisplatin, but no effect was observed under the conditions tested.

Ten lipophilic amines were prepared from the reductive amination of vanillin and the corresponding primary amines using sodium borohydride in methanol. All compounds have been obtained elementally pure and an X-ray diffraction study on the 4-n-butylaniline derivative has confirmed the molecular structure. Whilst the overall antibiotic activity of the derivatives was low, some of these compounds, particularly the boronate ester 2-methoxy-4-((2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenylamino)methyl)phenol (7), showed a promising degree of antimycobacterial activity against Mycobacterium tuberculosis H37Ra, where activity seemed to vary by the position of the boron substitution on the aniline ring.

A small family of organometallic platinum complexes containing a chloride, cis-cyclooctene, and a Schiff base ligand have been prepared and characterized fully. Three aliphatic amines and four aromatic amines were chosen as representative examples. All complexes were stable in air except for 7, derived from the pinacol-protected 4-aminophenylboronate ester 4-H2NC6H4Bpin (pin = 1,2-O2C2Me4), which decomposed via B–C bond cleavage. Both complexes 4 (derived from aniline) and 7 were further characterized by single-crystal X-ray diffraction studies and confirmed the square planar nature of the platinum centre whereby the chloride ligand lies trans to the deprotonated hydroxyl group of the Schiff base ligand. The imine functionality is trans to the organic cyclooctene group. Complex 3, which contained the longest aliphatic chain studied (an octyl group), was the most promising for inducing apoptosis in the malignant MB231 breast cancer cell line. Conversely, complexes 4–6, which contained aromatic groups, were the most active against renal cell carcinoma (RCC) cell lines.

The reaction of [PdCl2(η2-coe)]2 (coe = cis-cyclooctene) with nine lipophilic iminopyridine ligands (NN′, 1a–i) afforded the corresponding metal complexes cis-[PdCl2(NN′)] (2a–i) with concomitant loss of the labile coe ligands. All iminopyridine complexes were characterized by physicochemical and spectroscopic methods. The molecular structures of two complexes, 2a and 2b, were determined by single crystal X-ray diffraction studies. All complexes were tested for their potential antifungal and antibacterial activities.

This is the peer reviewed version of the following article: Cormier, K.; Curry, R. D.; Betsch, M. P.; Goguen, J. A.; Vogels, C. M.; Decken, A.; Turcotte, S.; Westcott, S. A. Synthesis, Characterization, and Anticancer Activities of Pyrogallol-Based Arylspiroborates J. Heterocyclic Chem. 2016, 53, 1807-1812., which has been published in final form at http://dx.doi.org/10.1002/jhet.2490. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.

The reaction of Cu(II) sulfate or Pd(II) acetate with four salicylaldimines derived from 5-bromosalicylaldehyde and long-chain aliphatic primary amines afforded the corresponding metal complexes 2a–d and 3a–d, respectively. All complexes were characterized by physicochemical and spectroscopic methods. The molecular structure of 2b was determined by a single-crystal X-ray diffraction study. All complexes were tested for their potential antifungal and antibacterial activities.

Two novel boron compounds containing caffeic acid phenethyl ester (CAPE) derivatives have been prepared and characterized fully. These new compounds and CAPE have been investigated for potential antioxidant and antimicrobial properties and their ability to inhibit 5-lipoxygenase and whether chelation to boron improves their biological activity. Sodium salt 4 was generally more active than ammonium salt 5 in the biological assays and surpassed the radical scavenging ability of CAPE. Compounds 4 and 5 were more active than CAPE and Zileuton in human polymorphonuclear leukocytes. These results clearly show the effectiveness of the synthesized salts as transporter of CAPE.

Eight lipophilic 3-hydroxy-4-pyridinones have been prepared from a microwave-mediated reaction along with the corresponding copper(II) complexes. All complexes have been obtained elementally pure and X-ray diffraction studies on two of the copper complexes have confirmed the structure of these compounds. Some of these complexes showed a promising degree of anti-mycobacterial activity against Mycobacterium tuberculosis, where activity seemed to vary by substitution at the pyridinone nitrogen atom.

The first examples for the rhenium-catalysed hydroboration of aldehydes, ketones and aldimines, including heteroaromatic quinoline, are reported herein. Reactions are remarkably chemoselective and tolerant of several functional groups. A wide array of rhenium complexes were efficient pre-catalysts for these hydroborations, including new low-valent complexes of the formula [Re(N-N)(CO)(L)]X (N-N = bipy derivative, L = labile ligand/solvent, and X = [BAr ] and [B(3,5-di-tBu-cat)]), which have been characterized fully including an X-ray diffraction study for [Re(bipy)(CO)(quin)][BAr ] (2). A new silver spiroboronate ester Ag[B(3,5-di-tBu-cat)](NCCH) (3) was prepared and characterized fully, including an X-ray diffraction study, and used to make one of the new rhenium complexes., The Oviedo authors thank MINECO and FEDER (grant CTQ2015-70231-P) and Principado de Asturias (grant FC-15-GRUPIN14-103) for funding, and Ministerio de Educación for an FPU predoctoral fellowship (to RA). SAW is grateful to the Natural Sciences and Engineering Research Council, the CRC programme and Mount Allison University for funding.

Twelve new (salicylaldiminato)platinum(II) complexes have been prepared, characterized and examined for their in vitro cytotoxic properties against three human glioma cell lines LN18, LN405 and Hs683. Initial biotesting was done on platinum complexes 1–6, which display a wide range of physicochemical properties. Whereas introduction of aromatic rings in the complexes did not seem to have a significant impact on bioactivities, the length of the aliphatic group positively correlated with cytotoxicity in all cell lines. Complexes 2 and 3 were found to be particularly potent against LN18 cells as demonstrated by stronger cell growth inhibition when compared to cisplatin. These initial findings led us to develop an additional series of mono (salicylaldiminato)platinum(II) complexes 7–12, which were subjected to additional cytotoxicity studies but were less cytotoxic compared to the bis Schiff base complexes. These organometallic compounds could serve as interesting starting points for the development of novel therapeutic strategies to treat brain tumors.

We have examined six boron compounds for their potential antituberculosis activities. Addition of 2-formylphenyl pinacol boronate ester to tetraamine derivatives gave three new aminoboron compounds, which were elementally pure when obtained, and one example was characterized by a single crystal X-ray diffraction study. Heterocyclic aminoboron compounds 1–4 showed moderate degrees of antimycobacterial activity against Mycobacterium tuberculosis.

Two sterically diverse iminophosphines have been prepared from the addition of 2-Ph2PC(O)H and 4-H2NC6H4OMe (L1) and 2,6-H2NC6H4(i-Pr)2 (L2), and the addition of boranes to these substrates has been examined in an effort to reduce the imine C N bond. Reactions using borane-dimethylsulfide with the smaller imine L1 afforded adducts arising from initial coordination to the phosphorus atom and then the imine nitrogen. Addition of excess borane-dimethylsulfide eventually led to reduction of the imine and generated an active H2BNRR′ unit, which went on to make several aminophosphine products. The sterically hindered imine L2 was used to retard the reactivity in the resulting aminoborane. Hydroboration studies with a new borane (7) showed good selectivity in reactions with 1-octene and vinyl arenes. Reactions of iminophosphines with alkyl and dialkylborane also gave adducts, making reduction of the imine increasingly difficult. Addition of the dialkoxyboranes, catecholborane and pinacolborane, were promising for the less hindered imine, but harsh reaction conditions (metal catalysts, microwave reactor and elevated temperatures and pressures) were required in reductions of the sterically-hindered imine, giving rise to complicated product distributions.

Tl(acac) reacts quantitatively with B2butcat3 (butcat = 3,5-di-tert-butylcatecholato) to give butcatB(acac) and Tl(Bbutcat 2) (1), the latter of which is soluble in common organic solvents such as THF and dichloromethane. Addition of 1 to a mixture of [RhCl(coe) 2]2 (coe = cis-cyclooctene) and diphosphine in THF gave the thallium bridged dinuclear complexes [(P2)Rh(μ-Cl) 2(μ-Tl)Rh(P2)][Bbutcat2] (2-5, P2 is dppm = 1,1′-bis(diphenylphosphino)methane, dppe = 1,2- bis(diphenylphosphino)ethane, dppp = 1,3-bis(diphenylphosphino)propane and dppb = 1,4-bis(diphenylphosphino)butane) as the only new rhodium containing species. Unsymmetrical binding of the thallium atom is observed in some cases owing to a secondary interaction with the phenyl rings of the phosphine ligands. Mononuclear cationic species of the type [(P2)Rh(NCCH 3)2][Bbutcat2] (6-9) were formed when reactions were carried out in acetonitrile, where preferential binding of the solvent was observed over the bulky arylspiroborate ligand. The zwitterionic species Rh(η6-butcatBbutcat)(P2), 11 and 12, could be generated via a different route, by addition of B2butcat3 to Rh(acac)(P2), but only for the sterically-constrained diphosphines dppm and dppe, respectively. While all new complexes effectively catalyzed the addition of catecholborane to 4-vinylanisole and α-methylstyrene, product distributions suffered from a competing dehydrogenative borylation pathway.

The insertion of the diazo derivative Me3SiCHN2 into pinB–SR σ bonds (R = Ph, Tol, Bn) allows a direct synthesis of multisubstituted H–C(SR)(Bpin)(SiMe3) compounds. Consecutive base-assisted transformations of HC(S)(B) (Si) systems lead to deborylative alkylations, Sommelet–Hauser rearrangements, and deprotoalkylations. Intramolecular cyclizations can be selectively performed either via desilylative or deborylative manifolds by fine-tuning the base employed.

Schiff bases derived from condensation of 2,3-dihydroxybenzaldehyde with various primary amines, such as 1-adamantanamine hydrochloride, 2,6-dimethylaniline, 2,6-diethylaniline and 2,6-diisopropylaniline, react with palladium(II) acetate to give the corresponding bis(N-arylsalicylaldiminato)palladium(II) complexes. These complexes have been found to be active catalyst precursors for the Suzuki–Miyaura cross-coupling of aryl bromides and iodides with aryl boronic acids using water as a solvent.

We have prepared ruthenium complexes containing the Bbutcat2- anion (butcat = 3,5-di-tert-butylcatecholato) and the first single crystal X-ray diffraction study of a ruthenium arylspiroborate complex is presented. These new ruthenium complexes have been examined for their ability to catalyse the addition of diorganyloxyboranes to vinylarenes, and results suggest that these compounds generate the corresponding alkenylboronate esters via a competing dehydrogenative borylation pathway

The compound barium bis{bis-(4,6-di-tert-butyl[1,2-benzenediolato(2-)-O,O′]borate)} has been prepared by the addition of 3,5-di-tert-butylcatechol to a solution of boric acid and Ba(OH)2 and characterized by a single crystal X-ray diffraction study. The title compound crystallized in the triclinic space group P-1, with cell parameters a = 13.280(2) Å, b = 15.755(3) Å, and c = 16.980(3) Å, α = 71.691(2)o, β = 79.528(3)o, γ = 80.741(3)o, Z = 1, and V = 3296.1(10) Å3. The structure was solved by direct methods and refined to a final R = 0.0459 for 14370 reflections with I > 2σ(I). One of the arylspiroboronate ester counterions is bound to the barium atom in a rare example of the η1 bonding mode via a single oxygen of one of the catecholato groups. The coordination sphere around the barium is complemented by four molecules of water, one molecule of acetone and two bridging water molecules, connecting to an adjacent barium atom.

Iridium phosphane arylspiroboronate esters have been prepared by addition of B2cat3 (cat = 1,2-O2C6H4) to the iridium precursors Ir(acac)(coe)2/PR3 (acac = acetylacetonato, coe = cis-cyclooctene) or Ir(acac)(coe)2/P-P (chelating bidentate ligands) at elevated temperatures. These novel complexes have been characterized by a number of physical methods including X-ray diffraction studies for the PCy3 (2) and dppp (6, 1,3-diphenylphosphanopropane) derivatives. The arylspiroboronate esters, as seen with the more common rhodium complexes, are bound to the metal centre via one of thebenzene catecholato rings. These organometallic complexes can be used as precatalysts for the selective hydroboration of vinylarene derivatives with HBpin (pin = 1,2-O2C2Me4) to give the corresponding linear hydroboration products. Reactions with methyl oleate proceed at elevated temperatures to give the isomerised/hydroboration product selectively along with equal amounts of hydrogenation product.

4,8-Di- tert -butyl-2,10-dimethyl-12 H -dibenzo[ d , g ][1,3,2]dioxaborocine ( 1 ) has been prepared in high yield by the addition of H 3 B · SMe 2 to 6,6′-methylene(2- tert -butyl-4-methylphenol). Dioxaborocine 1 is a relatively stable solid that reacts with a variety of aliphatic alkenes in the presence of catalytic amounts of [Cp*IrCl 2 ] 2 to give the terminal hydroboration products. Analogous reactions with vinylarenes, however, afford the corresponding alkenylboronate esters along with equal amounts of the hydrogenation products. Boron products have been characterized by a number of physical and analytical methods, including single-crystal X-ray diffraction studies.

The iridium complex [Ir(acac)(η 2 -coe) 2 ] (1) has been easily prepared by the addition of Tl(acac) to a suspension of [Ir(μ-Cl)(η 2 -coe) 2 ] 2 in hexane. Complex 1 has been characterized fully including an X-ray diffraction study. It is moderately stable and is a useful precursor to a number of different acetylacetonato(phosphane)iridium complexes via addition of monodentate and bidentate phosphanes. Reactions proceeded at room temperature to give high yields of the desired square planar phosphane complexes [Ir(acac)(P)] (2: P = 2 PPh 3 , 3: P = 2 PMePh 2 , 4: P = dppm, 5: P = dppe, 6: P = dppp, 7: P = dppb, 8: P = dppf, 9: P = dcpe) except for those involving dppe, which gave a complicated mixture of products. Complex [Ir(acac)(dppe)] (5) was prepared in a microwave reactor at 125 °C. All complexes prepared in this study were efficient in the intramolecular catalyzed cyclization of 4-pentynoic acid to give exclusive formation of the exocyclic γ-methylene-γ-butyrolactone. Complex 1 was found to be the most active and selective catalyst precursor for the intramolecular cyclization of both 4-pentynoic acid and 5-hexynoic acid.

Addition of pinacolborane (HBO2C2Me4) to 2-iodoaniline can be catalyzed using a number of palladium complexes, including [(t-Bu)2PCl]2PdCl2 (PXPd), to give the corresponding boronate ester 2-H2NC6H4(BO2C2Me4) in excellent yields. The PXPd system could also be used in the catalyzed borylation of substituted anilines to give the corresponding aminoboronate esters in moderate to high yields.

We have examined the reaction of hydantoin (=imidazolidine-2,4-dione) with (formylphenyl)boronic acids, where the addition of a boronic acid group is hoped to increase bioactivities. Addition of (2-formylphenyl)boronic acid to hydantoin gave an unexpected azaborine compound, which presumably arises by initial formation of the (phenylmethylidene)hydantoin, with subsequent loss of H2O to give the cyclized product. Reactions of (3-formylphenyl)- and (4-formylphenyl)boronic acids with hydantoin gave the corresponding [(Z)-phenylmethylidene]hydantoins in good-to-excellent yields. Attempts to use (3-formylthiophen-2-yl)boronic acid gave a product where the boronic acid group has been cleaved.

The addition of catecholborane (HBcat, cat = 1,2-O2C6H4) to 9-vinylcarbazole can give either the branched or linear hydroboration product depending upon the judicious choice of metal catalyst used in these reactions. Analogous reactions with pinacolborane (HBpin, pin = 1,2-O2C2Me4) and HBBzpin (Bzpin = 1,2-O2C2Ph4) using catalytic amounts (5 mol%) of either Rh(acac)(dppb) or [Cp∗IrCl2]2 gave the linear hydroboration product selectively. Hydroborations of 1-pyrrolidino-1-cyclopentene and 1-pyrrolidino-1-cyclohexene were complicated by a competing dehydrogenative borylation pathway. The branched isomer was not observed to any significant extent in these reactions, suggesting that the directing effect of the nitrogen atom is negligible. Although catalyzed additions of HBcat to 1-vinyl-2-pyrrolidinone gave complicated product distributions arising from competing reactions, addition of HBpin effectively generated the corresponding linear hydroboration product in good yields.