Stoichiometric and Catalytic B−C Bond Formation from Unactivated Hydrocarbons and Boranes

Stoichiometric and Catalytic B−C Bond Formation from Unactivated Hydrocarbons and Borane

Mechanistic Investigation of Stoichiometric Alkyne Insertion into Pt−B Bonds and Related Chemistry Bearing on the Catalytic Diborylation of Alkynes Mediated by Platinum(II) Diboryl Complexes^†

The insertion reactivity of alkynes with the diboryl complex (Ph3P)2Pt(BCat)2 (1, Cat ≡ {C6H4O2}2-) has been investigated. Under stoichiometric conditions 1 mediates cis-diborylation of alkynes and the (PPh3)2Pt fragment is trapped by alkyne to give the corresponding Pt−alkyne complexes. Kinetic studies under pseudo first-order conditions of alkyne indicate that the reaction is first order in 1. In the absence of added phosphine, no alkyne dependence is observed. The stoichiometric reaction is inhibited by phosphine addition, and under these conditions, a first-order dependence on alkyne concentration is observed for the disappearance of 1. The stoichiometric results exclude simple, bimolecular insertion of an alkyne into Pt−B bonds of 1, and the observed dependence on phosphine and alkyne strongly favors a mechanism where phosphine dissociation generates a three-coordinate intermediate that mediates alkyne insertion. Activation parameters for the stoichiometric alkyne insertion were derived from the temperature dependence of kobs (70−110 °C). An Eyring plot yielded the following:  ΔH⧧ = 25.9(7) kcal/mol and ΔS⧧ = 4(2) eu. The rates of alkyne diborylation are also sensitive to the nature of the alkyne as 4-octyne reacts much more readily than diphenylacetylene. For para-substituted diarylacetylenes, the rate for the bis(p-trifluoromethyl) derivative is accelerated and the rate for the bis(p-methoxy) derivative is retarded relative to diphenylacetylene. The reactivity of the related diboryl complex, (PPh3)2Pt(BPin)2 (9, Pin ≡ {(CH3)2CO−CO(CH3)2}2-), is much more complex as reductive elimination of PinB−BPin is observed before the onset of the diborylation reaction. This appears to be a general feature for this compound as elimination is promoted by various reagents (e.g., CO, PPh3, Me3Sn−SnMe3, and CatB−BCat). The catalytic diborylation of alkynes mediated by 1 (in the presence of added triphenylphosphine) was investigated. Kinetics experiments revealed many similarities to the stoichiometric reaction as an inverse dependence on [PPh3] and first-order dependence on [alkyne] and [1] were observed. Expressions that directly relate the catalytic and stoichiometric observed rate constants were derived, and the measured values for these two systems were identical within experimental error. Thus, the data are consistent with a catalytic manifold that is identical to that observed in the stoichiometric reaction. Under catalytic conditions, the rate of alkyne diborylation exhibited no dependence on [CatB−BCat]

Efficient Olefin Diboration by a Base-Free Platinum Catalyst

Pt(NBE)3 and Pt(COD)2 (NBE = norbornene, COD = 1,5−cyclooctadiene) catalyze the addition of 2,2‘-bis(1,3,2-benzodioxaborole) (CatB−BCat) to α-olefins. The reactions proceed smoothly under ambient conditions to give 1,2-diborylalkanes in high yield, and the catalyst is compatible with common functional groups

Cyclohexyl-Substituted Polyglycolides with High Glass Transition Temperatures

The substituted glycolides rac-dicyclohexylglycolide (rac-3,6-dicyclohexyl-1,4-dioxane-2,5-dione), meso-dicyclohexylglycolide, R,R-dicyclohexylglycolide, and rac-methylcyclohexylglycolide (rac-3-cyclohexyl-6-methyl-1,4-dioxane-2,5-dione) have been synthesized, and both solution and bulk polymerizations of these monomers are reported. The polymerization kinetics of these new monomers were studied and compared to data for rac-diisopropylglycolide (rac-3,6-diisopropyl-1,4-dioxane-2,5-dione) and rac-lactide. The solution polymerization rates followed the order:  rac-dicyclohexylglycolide rac-diisopropylglycolide rac-methylcyclohexylglycolide rac-lactide. The glass transition temperature of poly(rac-dicyclohexylglycolide) is 98 °C, consistent with a stiff polyglycolide backbone. meso-Dicyclohexylglycolide and R,R-dicyclohexylglycolide were synthesized and polymerized to study the effect of stereochemistry on the polymer properties. The glass transition temperature of meso-dicyclohexylglycolide was 96 °C, while that of poly(R,R-dicyclohexylglycolide) increased to 104 °C. While NMR spectroscopy indicated that poly(R,R-dicyclohexylglycolide) undergoes minimal racemization during polymerization, differential scanning calorimetry (DSC), X-ray diffraction (XRD), and polarized optical microscopy confirmed that the polymer was amorphous

Group 5 Metallocene Complexes as Models for Metal-Mediated Hydroboration: Synthesis of a Reactive Borane Adduct, <i>endo</i>-Cp*₂Nb(H₂BO₂C₆H₄), via Hydroboration of Coordinated Olefins

The olefin complexes Cp*2M(CH2CH(R))(H) (R = H (1), CH3 (2); M = Nb (a), Ta (b)) react cleanly with catecholborane (HBCat) and HBO2C6H3-4-tBu (HBCat‘) to give Cp*2M(H2BCat) (M = Nb (5a), Ta (5b)) and Cp*2M(H2BCat‘) (M = Nb (6a), Ta (6b)) and the anti-Markovnikov hydroboration products CatBCH2CH2R and Cat‘BCH2CH2R. Compounds 2a and 2b react with DBCat‘ (DBO2C6H3-4-tBu) to afford the deuterated analogs Cp*2M(D2BCat‘) (M = Nb (6a), Ta (6b)) where the deuterium label is incorporated exclusively in the metal complex. The hydride resonances in 6a exhibit large perturbations in chemical shift when deuterium is incorporated. On the basis of this isotopic labeling experiment, a mechanism is proposed where HBCat reacts with the 16-electron alkyl intermediates, Cp*2MCH2CH2R (R = H (3), Me (4)), via σ-bond metathesis or oxidative-addition/reductive-elimination sequences, to generate the alkylboranes and an intermediate hydride, Cp*2MH, that is trapped by additional borane to give Cp*2M(H2BCat) (5a and 5b). The solid-state structures for Cp*2Nb(η2-H2BO2C6H3-3-tBu) (16) and Cp2*Nb(η2-BH4) (17) were determined by X-ray diffraction. The metal−boron distances in these two compounds are identical within experimental error. While related group 5 catecholateboryl compounds have pronounced boryl character, the structural parameters for the hydride and boryl ligands in 16 are consistent with formulation as either a borohydride complex or a borane adduct of “Cp*2NbH”. In contrast to other group 5 boryl complexes, 6a reacts readily with various two-electron ligands with elimination of HBCat‘. For example, H2 reacts reversibly to form Cp*2NbH3 and HBCat‘, while “BH3” and CO react irreversibly to yield Cp*Nb(BH4) and Cp*2Nb(H)(CO) with elimination of HBCat‘, respectively. Ethylene and propylene react at 40 °C to regenerate 1a and 2a, with elimination of HBCat‘. When excess olefin is present, the liberated borane is converted to CatBCH2CH2R. Solutions of 1a and 2a catalyze olefin hydroboration under mild conditions. Relationships between the reactivity of 1a and 2a and other early metal and lanthanide catalysts are discussed

Aromatic Borylation/Amidation/Oxidation: A Rapid Route to 5-Substituted 3-Amidophenols

5-Substituted 3-amidophenols are prepared by subjecting 3-substituted halobenzenes to an Ir-catalyzed aromatic borylation, followed by a Pd-catalyzed amidation, and finally an oxidation of the boronic ester intermediate. The entire C−H activation borylation/amidation/oxidation sequence can be accomplished without isolation of any intermediate arenes. Usefully, amide partners can include lactams, carbamates, and ureas

Group 5 Metallocene Complexes as Models for Metal-Mediated Hydroboration: Synthesis of a Reactive Borane Adduct, <i>endo</i>-Cp*₂Nb(H₂BO₂C₆H₄), via Hydroboration of Coordinated Olefins

The olefin complexes Cp*2M(CH2CH(R))(H) (R = H (1), CH3 (2); M = Nb (a), Ta (b)) react cleanly with catecholborane (HBCat) and HBO2C6H3-4-tBu (HBCat‘) to give Cp*2M(H2BCat) (M = Nb (5a), Ta (5b)) and Cp*2M(H2BCat‘) (M = Nb (6a), Ta (6b)) and the anti-Markovnikov hydroboration products CatBCH2CH2R and Cat‘BCH2CH2R. Compounds 2a and 2b react with DBCat‘ (DBO2C6H3-4-tBu) to afford the deuterated analogs Cp*2M(D2BCat‘) (M = Nb (6a), Ta (6b)) where the deuterium label is incorporated exclusively in the metal complex. The hydride resonances in 6a exhibit large perturbations in chemical shift when deuterium is incorporated. On the basis of this isotopic labeling experiment, a mechanism is proposed where HBCat reacts with the 16-electron alkyl intermediates, Cp*2MCH2CH2R (R = H (3), Me (4)), via σ-bond metathesis or oxidative-addition/reductive-elimination sequences, to generate the alkylboranes and an intermediate hydride, Cp*2MH, that is trapped by additional borane to give Cp*2M(H2BCat) (5a and 5b). The solid-state structures for Cp*2Nb(η2-H2BO2C6H3-3-tBu) (16) and Cp2*Nb(η2-BH4) (17) were determined by X-ray diffraction. The metal−boron distances in these two compounds are identical within experimental error. While related group 5 catecholateboryl compounds have pronounced boryl character, the structural parameters for the hydride and boryl ligands in 16 are consistent with formulation as either a borohydride complex or a borane adduct of “Cp*2NbH”. In contrast to other group 5 boryl complexes, 6a reacts readily with various two-electron ligands with elimination of HBCat‘. For example, H2 reacts reversibly to form Cp*2NbH3 and HBCat‘, while “BH3” and CO react irreversibly to yield Cp*Nb(BH4) and Cp*2Nb(H)(CO) with elimination of HBCat‘, respectively. Ethylene and propylene react at 40 °C to regenerate 1a and 2a, with elimination of HBCat‘. When excess olefin is present, the liberated borane is converted to CatBCH2CH2R. Solutions of 1a and 2a catalyze olefin hydroboration under mild conditions. Relationships between the reactivity of 1a and 2a and other early metal and lanthanide catalysts are discussed

Aromatic Borylation/Amidation/Oxidation: A Rapid Route to 5-Substituted 3-Amidophenols

5-Substituted 3-amidophenols are prepared by subjecting 3-substituted halobenzenes to an Ir-catalyzed aromatic borylation, followed by a Pd-catalyzed amidation, and finally an oxidation of the boronic ester intermediate. The entire C−H activation borylation/amidation/oxidation sequence can be accomplished without isolation of any intermediate arenes. Usefully, amide partners can include lactams, carbamates, and ureas

Stereoselective Polymerization of a Racemic Monomer with a Racemic Catalyst: Direct Preparation of the Polylactic Acid Stereocomplex from Racemic Lactide

Stereoselective Polymerization of a Racemic Monomer with a Racemic Catalyst:  Direct Preparation of the Polylactic Acid Stereocomplex from Racemic Lactid

Synthesis, Structure, and Reactivity of β-Diketiminato Aluminum Complexes

The preparation and reaction chemistry of β-diketiminato aluminum complexes are described. (TTP)AlCl2 (1) (TTPH = 2-(p-tolylamino)-4-(p-tolylimino)-2-pentene) is formed by the treatment of AlCl3 with LiTTP. Sequential alkylation of 1 with CH3Li results in the formation of the mono- and dimethyl aluminum complexes (TTP)AlMeCl (2) and (TTP)AlMe2 (3), respectively. Only monoalkyl complexes are produced when more hindered alkyllithium reagents are used. Compounds 2 and 3 are more conveniently prepared by treating Al(CH3)3 with TTPH·HCl and TTPH, respectively. The more sterically hindered β-diketimine ligand 2-((2,6-diisopropylphenyl)amino)-4-((2,6-diisopropylphenyl)imino)-2-pentene (DDPH) also reacts smoothly with Al(CH3)3 to yield (DDP)Al(CH3)2 (4). Compound 3 undergoes methyl abstraction reactions upon addition of B(C6F5)3 or AgOTf. Cationic species formed from 3 and B(C6F5)3 are unstable and decompose to (TTP)Al(CH3)(C6F5) and MeB(C6F5)2. In contrast, (TTP)Al(CH3)(OTf) (6) is thermally stable, but the triflate group is surprisingly inert toward displacement by Lewis bases. Compounds 1, 3, 4, and 6 were crystallographically characterized. The structures all indicate that the β-diketiminato backbone is essentially planar. The pseudotetrahedral aluminum center is displaced from the plane formed by the ligand backbone in 4 by 0.72 Å