Synthesis and derivatization of highly-functionalized λ5- phospholes

Article abstract of DOI:10.1039/c4cc02089h

A variety of λ⁵-phosphole derivatives bearing up to three distinct peripheral functionalities have been prepared by regiospecific [3+2] cycloaddition reactions of the diphosphinoketenimine (PPh₂) ₂CCNtBu (1) with electron-poor alkenes. Selective derivatization of the exocyclic functional groups, including formation of dimetallic complexes with a phosphole core, was subsequently accomplished. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc02089h

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Synthesis and derivatization of highly-
functionalized k5-phospholes†
Cite this: Chem. Commun., 2014,
50, 5597
Javier Ruiz,*a Marta P. Gonzalo,a Marilın Vivancoa and Santiago Garcıa-Grandab
´
´
Received 20th March 2014,
Accepted 2nd April 2014
DOI: 10.1039/c4cc02089h
www.rsc.org/chemcomm
A variety of k5-phosphole derivatives bearing up to three distinct peri-
pheral functionalities have been prepared by regiospecific [3+2] cyclo-
Q Q
addition reactions of the diphosphinoketenimine (PPh2)2C NtBu (1)
C
with electron-poor alkenes. Selective derivatization of the exocyclic
functional groups, including formation of dimetallic complexes with a
phosphole core, was subsequently accomplished.
Although the chemistry of phosphole derivatives is currently well
developed, relatively few systematic studies on the synthesis of
functionalized phospholes have been reported.1,2 A variety of
phospholes are known to contain different functional groups
attached at phosphorus or carbon atoms, such as carboxylic acid
and derivatives,3 aldehydes and ketones4 or phospholes bearing
halo substituents,5 which are key building blocks for the synthesis
Scheme 1 Formation of 2H-l5-phospholes 2 and 3 by reaction of diphos-
phinoketenimine 1 with acrolein and acrylonitrile.
of other phosphole derivatives. Notable examples are also functio- (Scheme 1). Very likely the reaction proceeds through nucleophilic
nalized phospholes containing p-conjugated systems, such as attack of a phosphorus atom of 1 to the unsubstituted carbon
dithienophospholes,6 arene-fused phospholes,7 dithienylethene- atom of the alkene followed by a 1,5-electrocyclization, which
containing phospholes,8 dithiaphospholes9 and phospholes complete the formal [3+2] cycloaddition reaction observed. Addi-
bearing acetylene functions,10 owing to their unique electronic tionally, a [1,3]-H shift from the endocyclic CH group to the iminic
properties. Considering all the above, the development of experi- nitrogen atom must occur to finally generate compounds 2 and 3.
mental strategies for the introduction of new functional groups at The proposed mechanism is in agreement with the regiospecifi-
the periphery of the phosphole cycle emerges as an important goal. city of the reaction, as only one regioisomer was formed. Further-
Herein we describe the synthesis of a variety of l5-phospholes more, this result is in accord with a HOMO–LUMO interaction
featuring exocyclic functional groups such as phosphane, amine, between 1 and the alkene, as the LUMO in the latter molecule is
aldehyde, nitrile or ester, as well as their selective derivatization, mainly centered in the unsubstituted carbon atom.12 Full char-
including formation of transition-metal complexes.
acterization of compounds 2 and 3 was accomplished by spectro-
Reaction of the diphosphinoketenimine (PPh2)2CQCQNtBu scopic methods. In the 31P{1H} NMR spectrum two doublets
(1)11 with acrolein or acrylonitrile at room temperature led to the for the two nonequivalent phosphorus atoms were observed, the
formation of the 2H-l5-phospholes
2
and 3, respectively high-field signal corresponding to the exocyclic diphenyl-
phosphino group (see ESI† for full characterization data).
The divergent reactivity of 1 with olefins in comparison with that
a
´
´
´
´
Departamento de Quımica Organica e Inorganica, Facultad de Quımica,
Universidad de Oviedo, 33006 Oviedo, Spain. E-mail: jruiz@uniovi.es;
Fax: +34 985103446; Tel: +34 985102977
of the alkynes, previously observed by our group, where two succes-
sive cyclization processes occurred, is worth noting.13 Now the [3+2]
cycloaddition reaction is limited to one alkene molecule, leaving the
exocyclic diphenylphosphino group unreacted. This circumstance
makes compounds 2 and 3 highly functionalized l5-phospholes,
notably containing phosphino and amino substituents available for
further transformations on these phosphaheterocycles.
b
´
´
´
´
Departamento de Quımica Fısica y Analıtica, Facultad de Quımica, Universidad de
Oviedo, 33006 Oviedo, Spain. E-mail: sgg@uniovi.es
† Electronic supplementary information (ESI) available: Experimental details and
analytical and spectroscopic data in pdf format. CCDC 929248 (4), 929250 (5) and
929251 (7). For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c4cc02089h
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Scheme 2 Formation of the 3H-l5-phosphole 4 by reaction of 1 with
dimethyl maleate.
Compound 1 also reacted with an internal electron-poor alkene
such as dimethyl maleate. The electron affinity value of this alkene
is appreciably lower than that of acrolein and acrylonitrile.12
Consequently, stronger reaction conditions were required in this
case, namely refluxing toluene for 8 h. The 3H-l5-phosphole 4 was
formed under these conditions, a type of phosphaheterocycle very
scarcely encountered in the literature,13,14 which in addition
contains functional groups at all carbon atoms of the heterocycle.
As shown in Scheme 2, generation of 4 might imply a [3+2]
cycloaddition reaction as for compounds 2 and 3, but in this case
a further [1,5]-H shift instead of a [1,3]-H shift occurred to yield the
observed phosphaheterocycle. The structure of 4 at the solid state
was confirmed by an X-ray diffraction study (Fig. 1). The inter-
atomic distances within the N1–C2–C1–P2–C4–C10 skeleton are, to
a variable extent, intermediate between double and single bonds,
thus showing p-electron delocalization, being largely extended to
the functional groups placed on the C2 and C4 carbon atoms of
the heterocycle.
Scheme 3 Derivatization of 2 at the exocyclic heteroatoms to afford
compounds 5–7.
may promote nucleophilic attack from either the nitrogen or
the phosphorus atoms to electrophilic substrates, enlarging
further the functionalization of these rather sophisticated
molecules. Thus, compound 2 was reacted with a terminal
alkyne such as methyl propiolate to give 5 resulting from regio-
and stereo-specific Michael addition of the amino substituent
to the alkyne, whereas reaction of 2 with methyl iodide afforded
the phosphonium salt 6 after selective quaternization of the
exocyclic phosphorus atom (Scheme 3). The structure of 5 was
definitively confirmed by X-ray crystallography (Fig. 2), which
shows the newly formed alkenyl substituent at nitrogen as the
E-isomer, with the exocyclic diphenylphosphino substituent of
the l5-phosphole remaining unreacted. The coordination geo-
metry around the nitrogen atom N1 is distorted trigonal planar,
and the alkenyl and carboxylate skeletons are arranged at the
same plane, suggesting a strong delocalization of the nitrogen
lone pair through these groups. Accordingly, the N1–C6
Compounds 2–4 still retain some additional reactivity
mainly based on the amino and phosphino substituents, which
Fig. 2 Molecular structure of 5, shown with 20% thermal ellipsoids.
Hydrogen atoms of phenyl and methyl groups are omitted for clarity.
Selected interatomic distances (Å) and angles (deg): C1–C2 1.424(8),
C2–C3 1.408(9), C3–C4 1.492(8), P2–C4 1.803(6), P2–C1 1.760(7),
P1–C1 1.805(7), N1–C2 1.427(8), N1–C6 1.368(9), C6–C7 1.336(9),
C7–C9 1.444(11), O2–C9 1.203(9), C3–C5 1.394(10), O1–C5 1.249(9);
C1–C2–C3 118.3(8), C2–C3–C4 112.2(7), C3–C4–P2 103.3(5), C1–P2–C4
96.3(3), C2–C1–P2 104.8(6), P2–C1–P1 133.9(4), C2–C1–P1 121.0(6),
C1–C2–N1 121.2(8), C3–C2–N1 120.5(7), C6–N1–C2 115.9(7).
Fig. 1 Molecular structure of 4, shown with 30% thermal ellipsoids. Hydrogen
atoms of phenyl and methyl groups are omitted for clarity. Selected interatomic
distances (Å) and angles (deg): C1–C2 1.385(4), C2–C3 1.545(4),
C3–C4 1.504(4), P2–C4 1.719(3), P2–C1 1.769(3), P1–C1 1.825(3),
C3–C9 1.501(4), C4–C10 1.429(5), N1–C2 1.343(4), N1–C5 1.493(4);
C1–C2–C3 115.6(3), C2–C3–C4 107.4(2), C3–C4–P2 110.0(2), C4–P2–C1
96.61(14), P2–C1–C2 109.0(2), P2–C1–P1 119.03(17), C2–C1–P1 131.6(2),
N1–C2–C1 122.5(3), N1–C2–C3 122.0(3), C2–N1–C5 135.6(3).
5598 | Chem. Commun., 2014, 50, 5597--5599
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In summary, we have developed a facile approach for the
synthesis of new highly functionalized l5-phospholes, based on
[3+2] cycloaddition reactions of diphosphinoketenimine
(PPh2)2CQCQNtBu with electron-poor alkenes such as acrolein,
acrylonitrile and dimethyl maleate. 2H-l5-phospholes 2 and 3,
and 3H-l5-phosphole 4 have been prepared in this way.
Compound 2 can be post-functionalized selectively at the
exocyclic nitrogen or phosphorus atoms by treatment with methyl
propiolate or methyl iodide, affording the l5-phospholes 5 and
6, respectively. Finally, 2 can serve as a polydentate ligand
in transition-metal complexes, as illustrated in the synthesis
of compound 7, which contains two {Ru(p-cym)Cl2} units
bonded to peripheral diphenylphosphino and aldehyde
functionalities.
Fig. 3 Molecular structure of 7, shown with 30% thermal ellipsoids.
Hydrogen atoms of phenyl, p-cym and methyl groups are omitted for
clarity. Selected interatomic distances (Å) and angles (deg): C1–C2 1.426(9),
C2–C3 1.45(1), C3–C4 1.516(9), P2–C4 1.810(7), P2–C1 1.759(7), P1–C1 1.815(7),
N1–C2 1.357(8), C3–C5 1.34(1), O1–C5 1.302(9), Ru1–P1 2.398(2);
C1–C2–C3 115.3(6), C2–C3–C4 111.1(6), C3–C4–P2 102.8(4), C1–P2–C4
96.0(3), C2–C1–P2 107.7(5), C3–C5–O1 123.5(7), C1–P1–Ru1 112.9(2).
This work was supported by the Spanish Ministerio de
´
Economıa y Competitividad (Project CTQ2012-32239).
Notes and references
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´
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The UV/Vis spectra of compounds 2–6 reveal that the values
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ligands in transition-metal complexes, with donor properties
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equivalent of [Ru(p-cym)Cl2]2 readily affording the dimetallic
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This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 5597--5599 | 5599