Efficient Synthesis of Aryl Boronates via Cobalt-Catalyzed Borylation of Aryl Chlorides and Bromides

An efficient catalytic system based on a Co­(II)-NHC precursor has been developed for the cross coupling of <i>bis</i>(pinacolato)­diboron with aryl halides including aryl chlorides, affording the aryl boronates in good to excellent yields. A wide range of functional groups are tolerated under mild reaction conditions. The reaction shows excellent chemoselectivity for bromide over chloride. Preliminary mechanistic investigations show that the catalytic cycle may rely on a cobalt­(I)–(III) redox couple

Synthesis and Characterization of Novel Ruthenaferracarboranes from Photoinsertion of Alkynes into a Ruthenaferraborane

Photolysis of [{(μ₃-BH)­(Cp*Ru)­Fe­(CO)₃}₂(μ-CO)] (<b>1</b>; Cp* = η⁵-C₅Me₅) in the presence of various alkynes such as 1,2-diphenylethyne, 1-phenyl-1-propyne, 2-butyne, and 1-(diphenylphosphino)-2-phenylacetylene led to the formation of four types of novel heterometallic metallacarboranes, [1,1,1-(CO)₃-μ-2,3-(CO)-2,3-(Cp*)₂-4,6-Ph₂-<i>closo</i>-1,2,3,4,6-FeRu₂C₂BH] (<b>2</b>), [1,8-(Cp*)₂-2,2,7,7-(CO)₄-μ-2,8-(CO)-μ-7,8-(CO)-4-Me-5-Ph-<i>pileo</i>-1,2,7,4,5-RuFe₂C₂(BH)₂] (<b>3</b>), [1,8-(Cp*)₂-2,2,7,7-(CO)₄-μ-2,8-(CO)-μ-7,8-(CO)-4,5-Me₂-<i>pileo</i>-1,2,7,4,5-RuFe₂C₂(BH)₂] (<b>4</b>), and [1,2-(Cp*)₂-6,6,7,7-(CO)₄-μ-2,7-(CO)-<i>exo</i>-μ-5,6-(PPh₂)-μ₃-1,2,6-(BH)-4-Ph-<i>pileo</i>-1,2,6,7,4,5-Ru₂Fe₂C₂BH] (<b>5</b>). Cluster compound <b>2</b> exhibits an octahedral structure with adjacent carbon atoms consistent with its skeletal electron pair (sep) count of 7. The cage geometry of <b>3</b> and <b>4</b> is based on a pentagonal bipyramid with one additional {Cp*Ru} vertex capping one of its faces. The solid-state X-ray diffraction results of <b>5</b> suggest that the core geometry is a capped pentagonal bipyramid, with an Fe–C bridging PPh₂ group. All the cluster compounds <b>2</b>–<b>5</b> have been characterized by IR and ¹H, ¹¹B, and ¹³C NMR spectroscopy, and the geometries of the structures were unequivocally established by crystallographic analysis

Approach to the Synthesis of <i>gem</i>-Thiolated Alkylboronates via Cobalt-Catalyzed Diboration of Aldehydes

A new method has been developed for the sequential gem-thioborylation of readily available aldehydes via the cobalt-catalyzed diboration reaction. The N-heterocyclic carbene (NHC)–cobalt complex has been used as a catalyst for the diboration of aldehydes to generate α-oxyl boronic esters, which react with lithium thiolates to form a tetracoordinate boronate species, which undergoes 1,2-metalate rearrangement in the presence of trifluoroacetic anhydride. The stepwise functionalization of the boryl and thiol moiety of the products enriches the chemical toolbox of diverse organic synthesis

Synthesis and Characterization of Hypoelectronic Tantalaboranes: Comparison of the Geometric and Electronic Structures of [(Cp*TaX)₂B₅H₁₁] (X = Cl, Br, and I)

Mild thermolysis of tantalaborane [(Cp*Ta)₂B₅H₁₁], <b>1</b> (Cp* = η⁵-C₅Me₅) in presence of halogen sources affords the open cage clusters [(Cp*TaX)₂B₅H₁₁], <b>2</b>–<b>4</b> (<b>2</b>: X = Cl; <b>3</b>: X = Br; and <b>4</b>: X = I) in good yields. In contrast, the tetraborohydride cluster, [(Cp*Ta)₂B₄H₉(μ-BH₄)], <b>5</b>, under the same reaction conditions forms the B–H substituted cluster [(Cp*Ta)₂B₄H₈I­(μ-BH₄)], <b>6</b>. All the new metallaboranes have been characterized by mass spectrometry, ¹H, ¹¹B, ¹³C NMR spectroscopy, and elemental analysis, and the structural types were established by crystallographic analysis of clusters <b>3</b>, <b>4</b>, and <b>6</b>. Density functional theory (DFT) calculations at the BP86/TZ2P ZORA level reveal geometries in agreement with the structure determinations, large gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in accord with their stabilities. B3LYP-computed ¹¹B chemical shifts accurately reflect the experimentally measured shifts. Clusters <b>2</b>–<b>4</b> can be viewed as 7-sep 7-vertex <i>oblatoarachno</i> M₂B₅ clusters which can be generated from a 7-sep 9-vertex <i>oblatocloso</i> M₂B₇ cluster by removal of two equatorial boron atoms. Cluster <b>6</b> can be considered as an electron-deficient 6-sep 6-vertex <i>oblatoarachno</i> M₂B₄ cluster derived from an 8-vertex <i>oblatocloso</i> hexagonal bipyramidal cluster, in which BH₄^– anion is weakly bonded in a bidentate mode

Synthesis and Characterization of Hypoelectronic Tantalaboranes: Comparison of the Geometric and Electronic Structures of [(Cp*TaX)₂B₅H₁₁] (X = Cl, Br, and I)

Mild thermolysis of tantalaborane [(Cp*Ta)₂B₅H₁₁], <b>1</b> (Cp* = η⁵-C₅Me₅) in presence of halogen sources affords the open cage clusters [(Cp*TaX)₂B₅H₁₁], <b>2</b>–<b>4</b> (<b>2</b>: X = Cl; <b>3</b>: X = Br; and <b>4</b>: X = I) in good yields. In contrast, the tetraborohydride cluster, [(Cp*Ta)₂B₄H₉(μ-BH₄)], <b>5</b>, under the same reaction conditions forms the B–H substituted cluster [(Cp*Ta)₂B₄H₈I­(μ-BH₄)], <b>6</b>. All the new metallaboranes have been characterized by mass spectrometry, ¹H, ¹¹B, ¹³C NMR spectroscopy, and elemental analysis, and the structural types were established by crystallographic analysis of clusters <b>3</b>, <b>4</b>, and <b>6</b>. Density functional theory (DFT) calculations at the BP86/TZ2P ZORA level reveal geometries in agreement with the structure determinations, large gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in accord with their stabilities. B3LYP-computed ¹¹B chemical shifts accurately reflect the experimentally measured shifts. Clusters <b>2</b>–<b>4</b> can be viewed as 7-sep 7-vertex <i>oblatoarachno</i> M₂B₅ clusters which can be generated from a 7-sep 9-vertex <i>oblatocloso</i> M₂B₇ cluster by removal of two equatorial boron atoms. Cluster <b>6</b> can be considered as an electron-deficient 6-sep 6-vertex <i>oblatoarachno</i> M₂B₄ cluster derived from an 8-vertex <i>oblatocloso</i> hexagonal bipyramidal cluster, in which BH₄^– anion is weakly bonded in a bidentate mode

Chemistry of Homo- and Heterometallic Bridged-Borylene Complexes

Thermolysis of [(Cp*RuCO)₂B₂H₆] (<b>1</b>; Cp* = η⁵-C₅Me₅) with [Ru₃(CO)₁₂] yielded the trimetallaborane [(Cp*RuCO)₃(μ₃-H)­BH] (<b>2</b>) and a number of homometallic boride clusters: [Cp*RuCO­{Ru­(CO)₃}₄B] (<b>3</b>), [(Cp*Ru)₂{Ru₂(CO)₈}­BH] (<b>4</b>), and [(Cp*Ru)₂{Ru₄(CO)₁₂}­BH] (<b>5</b>). Compound <b>2</b> is isoelectronic and isostructural with the triply bridged borylene compounds [(μ₃-BH)­(Cp*RuCO)₂(μ-CO)­{Fe­(CO)₃}] (<b>6</b>) and [(μ₃-BH)­(Cp*RuCO)₂(μ-H)­(μ-CO)­{Mn­(CO)₃}] (<b>7</b>), where the [μ₃-BH] moiety occupies the apical position. To test if compound <b>2</b> undergoes hydroboration reactions with alkynes, as observed with <b>6</b>, we performed the reaction of <b>2</b> with the same set of alkynes under photolytic conditions. However, neither <b>2</b> nor <b>7</b> undergoes hydroboration to yield a vinyl–borylene complex. On the other hand, thermolysis of <b>6</b> with trimethylsilylethylene yielded the novel diruthenacarborane [1,1,7,7,7-(CO)₅-2,3-(Cp*)₂-μ-2,3-(CO)-μ₃-1,2,3-(CO)-5-(SiMe₃)-<i>pileo</i>-1,7,2,3,4,5-Fe₂Ru₂C₂BH] (<b>8</b>). The solid-state X-ray diffraction results suggest that <b>8</b> exhibits a pentagonal -bipyramidal geometry with one additional CO capping one of its faces. Cluster <b>3</b> is a boride cluster where boron is in the interstitial position of a square-pyramidal geometry, whereas compound <b>4</b> can be described as a tetraruthenium boride in which the Ru₄ butterfly skeleton has an interstitial boron atom. Electronic structure calculations of compound <b>2</b> employing density functional theory (DFT) generate geometries in agreement with the structure determinations. The existence of a large HOMO–LUMO gap in <b>2</b> is in agreement with its high stability. Bonding patterns in the structure have been analyzed on the grounds of DFT calculations. Furthermore, the B3LYP-computed ¹¹B and ¹H chemical shifts for compound <b>2</b> precisely follow the experimentally measured values. All the compounds have been characterized by IR and ¹H, ¹¹B, and ¹³C NMR spectroscopy, and the geometries of the structures were unambiguously established by crystallographic analyses of <b>2</b>–<b>4</b> and <b>8</b>

Supraicosahedral Polyhedra in Metallaboranes: Synthesis and Structural Characterization of 12‑, 15‑, and 16-Vertex Rhodaboranes

Syntheses and structural characterization of supraicosahedral rhodaborane clusters are reported. Reaction of [(Cp*RhCl₂)₂], (Cp* = η⁵-C₅Me₅) with [LiBH₄·thf] followed by thermolysis with excess of [BH₃·thf] afforded 16-vertex <i>closo</i>-[(Cp*Rh)₃B₁₂H₁₂Rh­{Cp*RhB₄H₉}], <b>1</b>, 15-vertex [(Cp*Rh)₂B₁₃H₁₃], <b>2</b>, 12-vertex [(Cp*Rh)₂B₁₀H_<i>n</i>(OH)_<i>m</i>], (<b>3a</b>: <i>n</i> = 12, <i>m</i> = 0; <b>3b</b>: <i>n</i> = 9, <i>m</i> = 1; <b>3c</b>: <i>n</i> = 8, <i>m</i> = 2) and 10-vertex [(Cp*Rh)₃B₇H₇], <b>4</b>, and [(Cp*Rh)₄B₆H₆], <b>5</b>. Cluster <b>1</b> is the unprecedented 16-vertex cluster, consists of a sixteen-vertex {Rh₄B₁₂} with an <i>exo</i>-polyhedral {RhB₄} moiety. Cluster <b>2</b> is the first example of a carbon free 15-vertex supraicosahedral metallaborane, exhibits icosihexahedron geometry (26 triangular faces) with three degree-six vertices. Clusters <b>3a</b>-<b>c</b> have 12-vertex <i>isocloso</i> geometry, different from that of icosahedral one. Clusters <b>4</b> and <b>5</b> are attributed to the 10-vertex <i>isocloso</i> geometry based on 10-vertex bicapped square antiprism structure. In addition, quantum-chemical calculations with DFT methods at the BP86 level of theory have been used to provide further insight into the electronic structure and stability of the optimized structures which are in satisfactory agreement with the structure determinations. All the compounds have been characterized by IR, ¹H, ¹¹B, ¹³C NMR spectroscopy in solution, and the solid state structures were established by crystallographic analysis of compounds <b>1</b>–<b>5</b>

Supraicosahedral Polyhedra in Metallaboranes: Synthesis and Structural Characterization of 12‑, 15‑, and 16-Vertex Rhodaboranes

Syntheses and structural characterization of supraicosahedral rhodaborane clusters are reported. Reaction of [(Cp*RhCl₂)₂], (Cp* = η⁵-C₅Me₅) with [LiBH₄·thf] followed by thermolysis with excess of [BH₃·thf] afforded 16-vertex <i>closo</i>-[(Cp*Rh)₃B₁₂H₁₂Rh­{Cp*RhB₄H₉}], <b>1</b>, 15-vertex [(Cp*Rh)₂B₁₃H₁₃], <b>2</b>, 12-vertex [(Cp*Rh)₂B₁₀H_<i>n</i>(OH)_<i>m</i>], (<b>3a</b>: <i>n</i> = 12, <i>m</i> = 0; <b>3b</b>: <i>n</i> = 9, <i>m</i> = 1; <b>3c</b>: <i>n</i> = 8, <i>m</i> = 2) and 10-vertex [(Cp*Rh)₃B₇H₇], <b>4</b>, and [(Cp*Rh)₄B₆H₆], <b>5</b>. Cluster <b>1</b> is the unprecedented 16-vertex cluster, consists of a sixteen-vertex {Rh₄B₁₂} with an <i>exo</i>-polyhedral {RhB₄} moiety. Cluster <b>2</b> is the first example of a carbon free 15-vertex supraicosahedral metallaborane, exhibits icosihexahedron geometry (26 triangular faces) with three degree-six vertices. Clusters <b>3a</b>-<b>c</b> have 12-vertex <i>isocloso</i> geometry, different from that of icosahedral one. Clusters <b>4</b> and <b>5</b> are attributed to the 10-vertex <i>isocloso</i> geometry based on 10-vertex bicapped square antiprism structure. In addition, quantum-chemical calculations with DFT methods at the BP86 level of theory have been used to provide further insight into the electronic structure and stability of the optimized structures which are in satisfactory agreement with the structure determinations. All the compounds have been characterized by IR, ¹H, ¹¹B, ¹³C NMR spectroscopy in solution, and the solid state structures were established by crystallographic analysis of compounds <b>1</b>–<b>5</b>

Chemistry of Homo- and Heterometallic Bridged-Borylene Complexes

Thermolysis of [(Cp*RuCO)₂B₂H₆] (<b>1</b>; Cp* = η⁵-C₅Me₅) with [Ru₃(CO)₁₂] yielded the trimetallaborane [(Cp*RuCO)₃(μ₃-H)­BH] (<b>2</b>) and a number of homometallic boride clusters: [Cp*RuCO­{Ru­(CO)₃}₄B] (<b>3</b>), [(Cp*Ru)₂{Ru₂(CO)₈}­BH] (<b>4</b>), and [(Cp*Ru)₂{Ru₄(CO)₁₂}­BH] (<b>5</b>). Compound <b>2</b> is isoelectronic and isostructural with the triply bridged borylene compounds [(μ₃-BH)­(Cp*RuCO)₂(μ-CO)­{Fe­(CO)₃}] (<b>6</b>) and [(μ₃-BH)­(Cp*RuCO)₂(μ-H)­(μ-CO)­{Mn­(CO)₃}] (<b>7</b>), where the [μ₃-BH] moiety occupies the apical position. To test if compound <b>2</b> undergoes hydroboration reactions with alkynes, as observed with <b>6</b>, we performed the reaction of <b>2</b> with the same set of alkynes under photolytic conditions. However, neither <b>2</b> nor <b>7</b> undergoes hydroboration to yield a vinyl–borylene complex. On the other hand, thermolysis of <b>6</b> with trimethylsilylethylene yielded the novel diruthenacarborane [1,1,7,7,7-(CO)₅-2,3-(Cp*)₂-μ-2,3-(CO)-μ₃-1,2,3-(CO)-5-(SiMe₃)-<i>pileo</i>-1,7,2,3,4,5-Fe₂Ru₂C₂BH] (<b>8</b>). The solid-state X-ray diffraction results suggest that <b>8</b> exhibits a pentagonal -bipyramidal geometry with one additional CO capping one of its faces. Cluster <b>3</b> is a boride cluster where boron is in the interstitial position of a square-pyramidal geometry, whereas compound <b>4</b> can be described as a tetraruthenium boride in which the Ru₄ butterfly skeleton has an interstitial boron atom. Electronic structure calculations of compound <b>2</b> employing density functional theory (DFT) generate geometries in agreement with the structure determinations. The existence of a large HOMO–LUMO gap in <b>2</b> is in agreement with its high stability. Bonding patterns in the structure have been analyzed on the grounds of DFT calculations. Furthermore, the B3LYP-computed ¹¹B and ¹H chemical shifts for compound <b>2</b> precisely follow the experimentally measured values. All the compounds have been characterized by IR and ¹H, ¹¹B, and ¹³C NMR spectroscopy, and the geometries of the structures were unambiguously established by crystallographic analyses of <b>2</b>–<b>4</b> and <b>8</b>

Synthesis of Functionalized 1,4-Azaborinines by the Cyclization of Di-<i>tert</i>-butyliminoborane and Alkynes

Di-<i>tert</i>-butyliminoborane is found to be a very useful synthon for the synthesis of a variety of functionalized 1,4-azaborinines by the Rh-mediated cyclization of iminoboranes with alkynes. The reactions proceed via [2 + 2] cycloaddition of iminoboranes and alkynes in the presence of [RhCl­(P<i>i</i>Pr₃)₂]₂, which gives a rhodium η⁴-1,2-azaborete complex that yields 1,4-azaborinines upon reaction with acetylene. This reaction is compatible with substrates containing more than one alkynyl unit, cleanly affording compounds containing multiple 1,4-azaborinines. The substitution of terminal alkynes for acetylene also led to 1,4-azaborinines, enabling ring substitution at a predetermined location. We report the first general synthesis of this new methodology, which provides highly regioselective access to valuable 1,4-azaborinines in moderate yields. A mechanistic rationale for this reaction is supported by DFT calculations, which show the observed regioselectivity to arise from steric effects in the B–C bond coupling en route to the rhodium η⁴-1,2-azaborete complex and the selective oxidative cleavage of the B–N bond of the 1,2-azaborete ligand in its subsequent reaction with acetylene