10 research outputs found
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 [{(μ<sub>3</sub>-BH)(Cp*Ru)Fe(CO)<sub>3</sub>}<sub>2</sub>(μ-CO)] (<b>1</b>; Cp* = η<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>) 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)<sub>3</sub>-μ-2,3-(CO)-2,3-(Cp*)<sub>2</sub>-4,6-Ph<sub>2</sub>-<i>closo</i>-1,2,3,4,6-FeRu<sub>2</sub>C<sub>2</sub>BH] (<b>2</b>), [1,8-(Cp*)<sub>2</sub>-2,2,7,7-(CO)<sub>4</sub>-μ-2,8-(CO)-μ-7,8-(CO)-4-Me-5-Ph-<i>pileo</i>-1,2,7,4,5-RuFe<sub>2</sub>C<sub>2</sub>(BH)<sub>2</sub>] (<b>3</b>), [1,8-(Cp*)<sub>2</sub>-2,2,7,7-(CO)<sub>4</sub>-μ-2,8-(CO)-μ-7,8-(CO)-4,5-Me<sub>2</sub>-<i>pileo</i>-1,2,7,4,5-RuFe<sub>2</sub>C<sub>2</sub>(BH)<sub>2</sub>] (<b>4</b>), and [1,2-(Cp*)<sub>2</sub>-6,6,7,7-(CO)<sub>4</sub>-μ-2,7-(CO)-<i>exo</i>-μ-5,6-(PPh<sub>2</sub>)-μ<sub>3</sub>-1,2,6-(BH)-4-Ph-<i>pileo</i>-1,2,6,7,4,5-Ru<sub>2</sub>Fe<sub>2</sub>C<sub>2</sub>BH] (<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 PPh<sub>2</sub> group. All the cluster compounds <b>2</b>–<b>5</b> have been characterized by IR and <sup>1</sup>H, <sup>11</sup>B,
and <sup>13</sup>C 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)<sub>2</sub>B<sub>5</sub>H<sub>11</sub>] (X = Cl, Br, and I)
Mild thermolysis of tantalaborane [(Cp*Ta)<sub>2</sub>B<sub>5</sub>H<sub>11</sub>], <b>1</b> (Cp* = η<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>) in presence of halogen sources affords
the
open cage clusters [(Cp*TaX)<sub>2</sub>B<sub>5</sub>H<sub>11</sub>], <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)<sub>2</sub>B<sub>4</sub>H<sub>9</sub>(μ-BH<sub>4</sub>)], <b>5</b>, under the same
reaction conditions forms the B–H substituted cluster [(Cp*Ta)<sub>2</sub>B<sub>4</sub>H<sub>8</sub>I(μ-BH<sub>4</sub>)], <b>6</b>. All the new metallaboranes have been characterized by mass
spectrometry, <sup>1</sup>H, <sup>11</sup>B, <sup>13</sup>C 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 <sup>11</sup>B 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> M<sub>2</sub>B<sub>5</sub> clusters which can be generated from a 7-sep 9-vertex <i>oblatocloso</i> M<sub>2</sub>B<sub>7</sub> 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> M<sub>2</sub>B<sub>4</sub> cluster derived from an 8-vertex <i>oblatocloso</i> hexagonal bipyramidal cluster, in which BH<sub>4</sub><sup>–</sup> anion is weakly bonded in a bidentate
mode
Synthesis and Characterization of Hypoelectronic Tantalaboranes: Comparison of the Geometric and Electronic Structures of [(Cp*TaX)<sub>2</sub>B<sub>5</sub>H<sub>11</sub>] (X = Cl, Br, and I)
Mild thermolysis of tantalaborane [(Cp*Ta)<sub>2</sub>B<sub>5</sub>H<sub>11</sub>], <b>1</b> (Cp* = η<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>) in presence of halogen sources affords
the
open cage clusters [(Cp*TaX)<sub>2</sub>B<sub>5</sub>H<sub>11</sub>], <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)<sub>2</sub>B<sub>4</sub>H<sub>9</sub>(μ-BH<sub>4</sub>)], <b>5</b>, under the same
reaction conditions forms the B–H substituted cluster [(Cp*Ta)<sub>2</sub>B<sub>4</sub>H<sub>8</sub>I(μ-BH<sub>4</sub>)], <b>6</b>. All the new metallaboranes have been characterized by mass
spectrometry, <sup>1</sup>H, <sup>11</sup>B, <sup>13</sup>C 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 <sup>11</sup>B 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> M<sub>2</sub>B<sub>5</sub> clusters which can be generated from a 7-sep 9-vertex <i>oblatocloso</i> M<sub>2</sub>B<sub>7</sub> 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> M<sub>2</sub>B<sub>4</sub> cluster derived from an 8-vertex <i>oblatocloso</i> hexagonal bipyramidal cluster, in which BH<sub>4</sub><sup>–</sup> anion is weakly bonded in a bidentate
mode
Chemistry of Homo- and Heterometallic Bridged-Borylene Complexes
Thermolysis
of [(Cp*RuCO)<sub>2</sub>B<sub>2</sub>H<sub>6</sub>] (<b>1</b>; Cp* = η<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>) with [Ru<sub>3</sub>(CO)<sub>12</sub>] yielded the trimetallaborane
[(Cp*RuCO)<sub>3</sub>(μ<sub>3</sub>-H)BH] (<b>2</b>)
and a number of homometallic boride clusters: [Cp*RuCO{Ru(CO)<sub>3</sub>}<sub>4</sub>B] (<b>3</b>), [(Cp*Ru)<sub>2</sub>{Ru<sub>2</sub>(CO)<sub>8</sub>}BH] (<b>4</b>), and [(Cp*Ru)<sub>2</sub>{Ru<sub>4</sub>(CO)<sub>12</sub>}BH] (<b>5</b>). Compound <b>2</b> is isoelectronic and isostructural with the triply bridged
borylene compounds [(μ<sub>3</sub>-BH)(Cp*RuCO)<sub>2</sub>(μ-CO){Fe(CO)<sub>3</sub>}] (<b>6</b>) and [(μ<sub>3</sub>-BH)(Cp*RuCO)<sub>2</sub>(μ-H)(μ-CO){Mn(CO)<sub>3</sub>}] (<b>7</b>), where the [μ<sub>3</sub>-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)<sub>5</sub>-2,3-(Cp*)<sub>2</sub>-μ-2,3-(CO)-μ<sub>3</sub>-1,2,3-(CO)-5-(SiMe<sub>3</sub>)-<i>pileo</i>-1,7,2,3,4,5-Fe<sub>2</sub>Ru<sub>2</sub>C<sub>2</sub>BH] (<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 Ru<sub>4</sub> 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 <sup>11</sup>B and <sup>1</sup>H chemical shifts
for compound <b>2</b> precisely follow the experimentally measured
values. All the compounds have been characterized by IR and <sup>1</sup>H, <sup>11</sup>B, and <sup>13</sup>C 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*RhCl<sub>2</sub>)<sub>2</sub>], (Cp* = η<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>) with [LiBH<sub>4</sub>·thf] followed by thermolysis with excess of [BH<sub>3</sub>·thf] afforded 16-vertex <i>closo</i>-[(Cp*Rh)<sub>3</sub>B<sub>12</sub>H<sub>12</sub>Rh{Cp*RhB<sub>4</sub>H<sub>9</sub>}], <b>1</b>, 15-vertex [(Cp*Rh)<sub>2</sub>B<sub>13</sub>H<sub>13</sub>], <b>2</b>, 12-vertex [(Cp*Rh)<sub>2</sub>B<sub>10</sub>H<sub><i>n</i></sub>(OH)<sub><i>m</i></sub>],
(<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)<sub>3</sub>B<sub>7</sub>H<sub>7</sub>], <b>4</b>, and
[(Cp*Rh)<sub>4</sub>B<sub>6</sub>H<sub>6</sub>], <b>5</b>. Cluster <b>1</b> is the unprecedented 16-vertex cluster, consists of a sixteen-vertex
{Rh<sub>4</sub>B<sub>12</sub>} with an <i>exo</i>-polyhedral
{RhB<sub>4</sub>} 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, <sup>1</sup>H, <sup>11</sup>B, <sup>13</sup>C 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*RhCl<sub>2</sub>)<sub>2</sub>], (Cp* = η<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>) with [LiBH<sub>4</sub>·thf] followed by thermolysis with excess of [BH<sub>3</sub>·thf] afforded 16-vertex <i>closo</i>-[(Cp*Rh)<sub>3</sub>B<sub>12</sub>H<sub>12</sub>Rh{Cp*RhB<sub>4</sub>H<sub>9</sub>}], <b>1</b>, 15-vertex [(Cp*Rh)<sub>2</sub>B<sub>13</sub>H<sub>13</sub>], <b>2</b>, 12-vertex [(Cp*Rh)<sub>2</sub>B<sub>10</sub>H<sub><i>n</i></sub>(OH)<sub><i>m</i></sub>],
(<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)<sub>3</sub>B<sub>7</sub>H<sub>7</sub>], <b>4</b>, and
[(Cp*Rh)<sub>4</sub>B<sub>6</sub>H<sub>6</sub>], <b>5</b>. Cluster <b>1</b> is the unprecedented 16-vertex cluster, consists of a sixteen-vertex
{Rh<sub>4</sub>B<sub>12</sub>} with an <i>exo</i>-polyhedral
{RhB<sub>4</sub>} 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, <sup>1</sup>H, <sup>11</sup>B, <sup>13</sup>C 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)<sub>2</sub>B<sub>2</sub>H<sub>6</sub>] (<b>1</b>; Cp* = η<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>) with [Ru<sub>3</sub>(CO)<sub>12</sub>] yielded the trimetallaborane
[(Cp*RuCO)<sub>3</sub>(μ<sub>3</sub>-H)BH] (<b>2</b>)
and a number of homometallic boride clusters: [Cp*RuCO{Ru(CO)<sub>3</sub>}<sub>4</sub>B] (<b>3</b>), [(Cp*Ru)<sub>2</sub>{Ru<sub>2</sub>(CO)<sub>8</sub>}BH] (<b>4</b>), and [(Cp*Ru)<sub>2</sub>{Ru<sub>4</sub>(CO)<sub>12</sub>}BH] (<b>5</b>). Compound <b>2</b> is isoelectronic and isostructural with the triply bridged
borylene compounds [(μ<sub>3</sub>-BH)(Cp*RuCO)<sub>2</sub>(μ-CO){Fe(CO)<sub>3</sub>}] (<b>6</b>) and [(μ<sub>3</sub>-BH)(Cp*RuCO)<sub>2</sub>(μ-H)(μ-CO){Mn(CO)<sub>3</sub>}] (<b>7</b>), where the [μ<sub>3</sub>-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)<sub>5</sub>-2,3-(Cp*)<sub>2</sub>-μ-2,3-(CO)-μ<sub>3</sub>-1,2,3-(CO)-5-(SiMe<sub>3</sub>)-<i>pileo</i>-1,7,2,3,4,5-Fe<sub>2</sub>Ru<sub>2</sub>C<sub>2</sub>BH] (<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 Ru<sub>4</sub> 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 <sup>11</sup>B and <sup>1</sup>H chemical shifts
for compound <b>2</b> precisely follow the experimentally measured
values. All the compounds have been characterized by IR and <sup>1</sup>H, <sup>11</sup>B, and <sup>13</sup>C 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>Pr<sub>3</sub>)<sub>2</sub>]<sub>2</sub>, which gives a rhodium η<sup>4</sup>-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 η<sup>4</sup>-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