11 research outputs found
El rechazo entre iguales: una visión general
III Jornadas Internacionales del Grupo GREIHablar de aceptación y rechazo entre iguales es un tema de gran relevancia en nuestro contexto social actual. Este hecho es consecuencia de la importancia que tiene la atracción interpersonal como índice de adaptación socioemocional, ya que la realidad evidencia que una buena aceptación en el grupo de compañeros y compañeras favorece una buena adaptación y bienestar socioemocional, mientras que niveles bajos de aceptación social son factores de riesgo y de problemática en el tema de interacción socialAcceptance and rejection between peers have become a relevant issue in our
current social context. This is a consequence of the importance that interpersonal
attraction is bestowed as a socio-emotional adaptation system. Indeed, as reality
evidences, strong acceptance levels among peers foster not only better adaptation
within the group but also better levels of socio-emotional well-being. However,
low social acceptance levels stand as problematic risk factors regarding social
interaction
Stereoselective Synthesis of Unsaturated and Functionalized l‑NHBoc Amino Acids, Using Wittig Reaction under Mild Phase-Transfer Conditions
The stereoselective synthesis of a new amino acid phosphonium
salt
was described by quaternization of melting triphenylphosphine with
the γ-iodo NHBoc-amino ester, derived from l-aspartic
acid. The deprotection of the carboxylic acid function to afford the
phosphonium salt with a free carboxylic acid group was achieved by
a palladium-catalyzed desallylation reaction. This phosphonium salt
was used in the Wittig reaction with aromatic or aliphatic aldehydes
and trifluoroacetophenone, under solid–liquid phase-transfer
conditions in chlorobenzene and in the presence of K<sub>3</sub>PO<sub>4</sub> as weak base, to afford the corresponding unsaturated amino
acids without racemization. Thus, the reaction with substituted aldehydes
allows to graft various functionalized groups on the lateral chain
of the amino acid, such as trifluoromethyl, cyano, nitro, ferrocenyl,
boronato, or azido. In addition, the reaction of the amino acid Wittig
reagent with α,β-unsaturated aldehydes leads to amino
acids bearing a diene on the lateral chain. Finally, this amino acid
phosphonium salt appears to be a new powerful tool for the preparation
of unsaturated and non-proteinogenic α-amino acids, directly
usable for the synthesis of customized peptides
<i>o</i>‑(Hydroxyalkyl)phenyl P‑Chirogenic Phosphines as Functional Chiral Lewis Bases
The stereoselective synthesis of P-chirogenic phosphines bearing an <i>o</i>-hydroxyalkyl chelating arm is described. The synthesis is based either on the hydroxyalkylation of P-chirogenic <i>o</i>-bromophenylphosphines (borane) or on their carbonatation and then reduction. The hydroxyalkylation with benzaldehyde or pivalaldehyde affords a mixture of epimers which are isolated by chromatography and characterized by their X-ray structures. Preliminary assays of free P-chirogenic <i>o</i>-(hydroxyalkyl)phenyl phosphines, as new functional Lewis bases in catalyzed asymmetric aza-MBH reaction, lead to β-aminoester derivatives with ee values up to 74%
Stereoselective Synthesis of P-Chirogenic Dibenzophosphole–Boranes via Aryne Intermediates
A new aryne-mediated tandem cross-coupling/P-cyclization
sequence
starting from tertiary phosphine–boranes and 1,2-dibromobenzenes
is reported. P-chirogenic dibenzophospholes become accessible in a
regio-, chemo-, and diastereoselective way
<i>o</i>‑(Hydroxyalkyl)phenyl P‑Chirogenic Phosphines as Functional Chiral Lewis Bases
The stereoselective synthesis of P-chirogenic phosphines bearing an <i>o</i>-hydroxyalkyl chelating arm is described. The synthesis is based either on the hydroxyalkylation of P-chirogenic <i>o</i>-bromophenylphosphines (borane) or on their carbonatation and then reduction. The hydroxyalkylation with benzaldehyde or pivalaldehyde affords a mixture of epimers which are isolated by chromatography and characterized by their X-ray structures. Preliminary assays of free P-chirogenic <i>o</i>-(hydroxyalkyl)phenyl phosphines, as new functional Lewis bases in catalyzed asymmetric aza-MBH reaction, lead to β-aminoester derivatives with ee values up to 74%
Efficient Synthesis of (P-Chirogenic) <i>o</i>‑Boronated Phosphines from <i>sec</i>-Phosphine Boranes
An efficient synthesis
of boronated phosphines with an <i>o</i>-phenylene-bridge
prepared from <i>sec</i>-phosphine
boranes and using benzyne chemistry is reported. Successive reactions
of <i>sec</i>-phosphine boranes with <i>n</i>-BuLi
and 1,2-dibromobenzene, and then with boron reagents, afford the <i>o</i>-boronatophenylphosphine derivatives in 71%
yields. The use of P-chirogenic <i>sec</i>-phosphine boranes
leads to the free boronated phosphines with retention of configuration
at the P-center after decomplexation. The reaction of P-chirogenic <i>o</i>-boronatophenylphosphine with KHF<sub>2</sub> affords the corresponding trifluoroborated phosphine with ee >98%
Modular <i>P</i>‑Chirogenic Phosphine-Sulfide Ligands: Clear Evidence for Both Electronic Effect and <i>P</i>‑Chirality Driving Enantioselectivity in Palladium-Catalyzed Allylations
Using the ephedrine methodology,
modular stereoselective syntheses
of a new class of <i>P</i>-chirogenic phosphines bearing
a sulfur-chelating arm (P*,S-hybrid ligand) are described. A first
series of syntheses based on a Fries-like rearrangement of <i>P</i>-chirogenic phosphinite-boranes, which are prepared from
2-bromobenzyl or 2-bromophenethyl alcohol and are mediated by metal–halide
exchange, have been performed. This rearrangement affords phosphine-boranes
stereospecifically with an <i>o</i>-hydroxyalkylphenyl
substituent. The latter residue is subsequently converted into a sulfur-containing
group. In a second series, the stereoselective syntheses were achieved
according to a new strategy involving a reaction of a thiophenyllithium
reagent with a <i>P</i>-chirogenic phosphinite. The X-ray
structures of the P*,S ligands and their palladium complexes allow
us to address the absolute configuration at both the phosphorus and
sulfur centers. The P*,S ligands were used in palladium-catalyzed
allylic alkylations, as tests, affording asymmetric inductions up
to 96% ee. Computer modeling corroborates the regio- and enantioselectivity
of the Pd-catalyzed allylations and the low influence of the substituent
carried by the sulfur moiety, particularly when the chelate forms
a six-membered ring with the metal
Modular Phosphole-Methano-Bridged-Phosphine(Borane) Ligands. Application to Rhodium-Catalyzed Reactions
The synthesis of the phospholyl(phosphinoborane)methane
air- and
moisture-stable hybrid ligands <b>4a</b>–<b>f</b>, starting from 1-phenylphospholes <b>1a</b>–<b>d</b>, was performed via P–C bond coupling on the methano bridge.
Two strategies were investigated, according to the phospholyl moiety
used as a nucleophilic or an electrophilic reagent. In the first pathway,
the phospholyl anions react with the easily available (chloromethyl)diphenylphosphine–borane <b>3</b> to afford ligands <b>4a</b>–<b>d</b> in
29–67% isolated yields. In the second pathway, the phospholyl(dicyclohexylphosphinoborane)methane
ligands <b>4e</b>,<b>f</b> were synthesized in 18–23%
yields, through a nucleophilic substitution on the cyanophosphole.
Removal of the BH<sub>3</sub> moiety on <b>4a</b>–<b>c</b> assisted by DABCO leads to the hybrid phospholyl(diphenylphosphino)methanes <b>5a</b>–<b>c</b>. Compounds <b>4</b> and <b>5</b> were fully characterized by multinuclear NMR spectroscopy
(<sup>1</sup>H, <sup>31</sup>P, <sup>13</sup>C, <sup>11</sup>B), mass
spectrometry, and elemental analysis, and the X-ray crystal structures
of <b>4a</b>,<b>c</b> and <b>5a</b>,<b>b</b> have been established. Ligands <b>5a</b>,<b>b</b> were
used to prepare the cationic rhodium complexes [Rh(η<sup>4</sup>-COD)(<b>5a</b>)]<sup>+</sup> (<b>8a′</b>), [Rh(η<sup>4</sup>-COD)(<b>5b</b>)]<sup>+</sup> (<b>8b</b>), [Rh(<b>5a</b>)<sub>2</sub>]<sup>+</sup> (<b>9a′</b>), and
[Rh(<b>5b</b>)<sub>2</sub>]<sup>+</sup> (<b>9b</b>), containing
four-membered chelate rings with BF<sub>4</sub><sup>–</sup> or CF<sub>3</sub>SO<sub>3</sub><sup>–</sup> as counterions.
Ligands <b>4a</b>–<b>f</b> were also used to synthesize
the [Rh(η<sup>4</sup>-COD)(<b>4</b>)]<sup>+</sup> chelate
complexes <b>10a</b>–<b>f</b>, resulting from coordination
of the phospholyl part and the BH<sub>3</sub> group via a η<sup>2</sup> mode with two bridging B–H–Rh 3c–2e
bonds, as shown by the X-ray crystal structures of the complexes <b>10b</b>,<b>c</b>. Rhodium complexes <b>8</b> and <b>10</b> isolated or formed in situ with ligands <b>4</b> and <b>5</b> were studied for catalytic hydrogenation of methyl 2-(acetamidomethyl)acrylate
(<b>11</b>) and hydroboration of styrene (<b>13</b>) with
catecholborane. In both reactions, the catalytic systems prepared
either from the phospholyl(phosphinoborane)methane ligands <b>4</b> or the corresponding free ligands <b>5</b>, gave good to excellent
conversions. In addition, the regioselectivity of the catalyzed hydroboration
is slightly influenced using these ligands. Finally, the use of hybrid
phospholyl(phosphinoborane)methanes as ligands offers a new, efficient
way to improve catalytic processes, for designing both the structure
and the electronic properties of the catalyst, or still to implement
it without removing the borane protecting group
P‑Chirogenic Phosphines Supported by Calix[4]arene: New Insight into Palladium-Catalyzed Asymmetric Allylic Substitution
The
first P-chirogenic mono- and diphosphine ligands supported
on the upper rim of a calix[4]arene moiety were synthesized using
the ephedrine methodology. The lithiated calix[4]arene mono- and dianions
both react with the oxazaphospholidine–borane, prepared from
ephedrine, to afford regio- and stereoselectively the corresponding
calix[4]arenyl aminophosphine–boranes, by cleavage of the heterocyclic
ring at the P–O bond position. Subsequent reactions with HCl
and then organolithium reagent and finally decomplexation with DABCO
lead to the corresponding calix[4]arenyl mono- or diphosphines. Both
enantiomers of the calix[4]arenyl phosphines were obtained either
by using (+)- or (−)-ephedrine or by changing the addition
order of the organolithium reagents during the synthesis. The enantiomeric
excesses of the phosphines were determined either by HPLC on a chiral
column of their borane complexes or by <sup>31</sup>P NMR in the presence
of a chiral palladium complex. The absolute configurations of the
mono- and diphosphinocalix[4]arenes were assigned by X-ray analysis
of their crystalline borane complexes. The P-chirogenic calix[4]arenyl
phosphines were tested for asymmetric palladium-catalyzed allylic
substitution of (<i>E</i>)-1,3-diphenylprop-2-en-1-yl acetate,
by dimethyl malonate or benzylamine. When the bis-methylphenylphosphino
calix[4]arene was used, the allylic products were obtained with 82%
and 79% ee, respectively. In both cases, the use of a diphosphine
affords better results than using 2 equivalents of monophosphine.
Despite the <i>C</i><sub>2</sub> symmetry of the P-chirogenic
diphosphine calix[4]arene ligand, computer modeling of the corresponding
Pd(allyl) complex shows a clear dissymmetry of the LUMO, which is
in good agreement with a complexed η<sup>1</sup>-allyl moiety
and with the regio- and enantioselectivity of the Pd-catalyzed allylations
P‑Chirogenic Phosphines Supported by Calix[4]arene: New Insight into Palladium-Catalyzed Asymmetric Allylic Substitution
The
first P-chirogenic mono- and diphosphine ligands supported
on the upper rim of a calix[4]arene moiety were synthesized using
the ephedrine methodology. The lithiated calix[4]arene mono- and dianions
both react with the oxazaphospholidine–borane, prepared from
ephedrine, to afford regio- and stereoselectively the corresponding
calix[4]arenyl aminophosphine–boranes, by cleavage of the heterocyclic
ring at the P–O bond position. Subsequent reactions with HCl
and then organolithium reagent and finally decomplexation with DABCO
lead to the corresponding calix[4]arenyl mono- or diphosphines. Both
enantiomers of the calix[4]arenyl phosphines were obtained either
by using (+)- or (−)-ephedrine or by changing the addition
order of the organolithium reagents during the synthesis. The enantiomeric
excesses of the phosphines were determined either by HPLC on a chiral
column of their borane complexes or by <sup>31</sup>P NMR in the presence
of a chiral palladium complex. The absolute configurations of the
mono- and diphosphinocalix[4]arenes were assigned by X-ray analysis
of their crystalline borane complexes. The P-chirogenic calix[4]arenyl
phosphines were tested for asymmetric palladium-catalyzed allylic
substitution of (<i>E</i>)-1,3-diphenylprop-2-en-1-yl acetate,
by dimethyl malonate or benzylamine. When the bis-methylphenylphosphino
calix[4]arene was used, the allylic products were obtained with 82%
and 79% ee, respectively. In both cases, the use of a diphosphine
affords better results than using 2 equivalents of monophosphine.
Despite the <i>C</i><sub>2</sub> symmetry of the P-chirogenic
diphosphine calix[4]arene ligand, computer modeling of the corresponding
Pd(allyl) complex shows a clear dissymmetry of the LUMO, which is
in good agreement with a complexed η<sup>1</sup>-allyl moiety
and with the regio- and enantioselectivity of the Pd-catalyzed allylations