Novel benzo-and pyrido-anellated 1, 3-azaphospholes

Article abstract of DOI:10.1080/10426500701807905

We present the synthesis of OH-functional and bulky N-substituted benzazaphospholes, novel pyrido-azaphospholes, addition versus CH-metalation by tBuLi and reactions with electrophiles yielding a novel asymmetric P,N-heterocyclic ethylene-1,2-bis(phosphin

Full text of DOI:10.1080/10426500701807905

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Phosphorus, Sulfur, and Silicon
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Novel Benzo-and Pyrido-
Anellated 1, 3-Azaphospholes
J. Heinicke a , B. R. Aluri a , M. S.S. Adam a & P. G.
Jones b
a Institut für Biochemie-Anorganische Chemie ,
Universität Greifswald , Greifswald, Germany
b Institut füur Anorganische und Analytische
Chemie , Technische Universität Braunschweig ,
Braunschweig, Germany
Published online: 20 Jun 2008.
To cite this article: J. Heinicke , B. R. Aluri , M. S.S. Adam & P. G. Jones (2008) Novel
Benzo-and Pyrido-Anellated 1, 3-Azaphospholes, Phosphorus, Sulfur, and Silicon and
the Related Elements, 183:2-3, 779-782, DOI: 10.1080/10426500701807905
To link to this article: http://dx.doi.org/10.1080/10426500701807905
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Phosphorus, Sulfur, and Silicon, 183:779–782, 2008
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500701807905
Novel Benzo-and Pyrido-Anellated 1, 3-Azaphospholes
J. Heinicke,1 B. R. Aluri,1 M. S. S. Adam, 1 and P. G. Jones2
1Institut fu¨r Biochemie–Anorganische Chemie, Universita¨t Greifswald,
Greifswald, Germany
2Institut fu¨r Anorganische und Analytische Chemie, Technische
Universita¨t Braunschweig, Braunschweig, Germany
We present the synthesis of OH-functional and bulky N-substituted benzazaphosp-
holes, novel pyrido-azaphospholes, addition versus CH-metalation by tBuLi and re-
actions with electrophiles yielding a novel asymmetric P,N-heterocyclic ethylene-1,2-
bis(phosphine) and phosphino-functional benzazaphospholes for hemilabile σ3,σ2–
P,Pꢀ coordination.
Keywords Ethylenebis(phosphine) ligands; heterophospholes; organolithium reagents;
palladium catalysis; phosphaalkenes
Heterophospholes with planar 6π-electron systems are diagonal rel-
atives of pyrrols, furans, or thiophenes. Like these, they are strongly
stabilized by aromatic delocalization but provide at the double-bonded
phosphorus atom (σ2) a neutral coordination site1 that differs strongly
from the coordination behavior of carbanionic or carbene donor cen-
ters. Anellation by carbo- or heterocycles2 provides steric protection
and a tool to tune electronic properties at the σ2-phosphorus donor site.
For 1H-1,3-benzazaphospholes (BAPs),3,4 P-C diagonalogues of indoles,
we recently reported a novel convenient synthesis by nickel-catalyzed
phosphonylation of 2-bromoanilides and subsequent reductive cycliza-
tion with LiAlH4.5,6 This method has now been applied to the synthe-
sis of σ2-P biaryl ligands (e.g., 2-(naphth-1-yl)-1,3-benzazaphosphol 1)
as well as, alkoxy- and hydroxy-functional BAPs 2 and 3 (Scheme 1)
all characterized by X-ray crystal structure analysis. 2-Pyridyl 1H-1,3-
benzazaphosphols could not be obtained in this way.
Attempts to extend the two-step procedure to pyrido-1,3-
azaphospholes failed in both steps, the nickel-catalyzed P-C cou-
pling and the reductive cyclization. These heterocycles were then
Address correspondence to J. Heinicke, Institut fu¨r Biochemie–Anorganische
Chemie, Universita¨t Greifswald, D-17487 Greifswald, Germany. E-mail: heinicke@
uni-greifswald.de
779

780
J. Heinicke et al.
SCHEME 1
synthesized by PdCl2-catalyzed C-P coupling of amino- or amido-
bromopyridines with triethylphosphite (e.g. to 4, reduction with
LiAlH4) to the respective aminophosphinopyridines 5 and con-
densation with Me2NCH(OMe)2, proceeding via phosphaalkenes to
the azaphospholo[5,4-b]pyridines 6 (Scheme 2). Azaphospholo[4,5-
b]pyridines, potential P,N alternative or hybrid ligands with pyridine-
N and phosphorus on the same side, are analogously available from
3-amino-2-bromopyridines.
SCHEME 2
The latter route was also applied to the synthesis of novel
bulky N-alkyl and N-aryl benzazaphospholes from N-secondary
2-phosphinoanilines and Me2NCH(OMe)2. The precursor anilines
(2-BrC6H4NHR) were obtained by Pd-catalyzed amination of o-
dibromobenzene (1-adamantyl, mesityl, 2,6-diisopropylphenyl) or re-
duction of 2-bromoanilides (neopentyl). The cyclization of the phos-
phaalkenes 7 is strongly hindered by bulky N-aryl groups but can be
achieved by catalysis with a small amount of concentrated aqueous
hydrochloric acid, which, surprisingly, did not add to the P = C bond
but gave the stable BAPs 8. Metallation of 8 by tBuLi is influenced by
the steric bulk at nitrogen and strongly retarded for adamantyl and dip
substituents. For N-neopentyl and mesityl groups, formation of 2-lithio-
benzazaphosphols is preferred, as already described for N-methyl and

Novel Benzo-and Pyrido-Anellated 1, 3-Azaphospholes
781
N-ethylbenzazaphosphole.6,7 The 2-lithio-reagents were coupled with
tBu2PCl providing 2-di-tert-butylphosphino-BAPs 9 (Scheme 3),
SCHEME 3
a novel class of bulky and basic σ3-P,σ2-Pꢀ ligands that are intended
to stabilize late transition metal catalysts (after the reductive elimina-
tion step) by hemilabile coordination of the σ2-P coordination site to the
zero-valent metal. Related catalytic studies are in progress. The coordi-
nation strength at σ2-P is weak for cationic transition metals; even (0.5)
[Rh(COD)Cl]2 was not added. However, as shown recently,8,9 M0(CO)5
fragments (M = Cr, Mo, W) are bound firmly via strong back-bonding,
indicated by the low downfield (Cr) or even upfield (Mo, W) coordination
chemical shift of the 31P NMR signals.
Despite the usual preference for CH-metalation of BAPs, tBuLi
can also add at the P C bond. Thus, conversion of 1-neopentyl-
benzazaphosphole to a novel heterocyclic 1,2-ethylenebis(phosphine)
ligand 10 was observed. This can be explained by a normal/inverse
FIGURE 1 Molecular structure of
bis(phosphine) (SSSS-configuration).
a novel P,N-heterocyclic ethylene

782
J. Heinicke et al.
two-step addition, first of a semiequivalent of tBuLi followed by the
1
primary adduct. The observation of only two doublets in the 31P{ H}
3
NMR spectra (D8-THF, δ: –81.48 (d), 5.02 (d), JPP = 65.7 Hz) gives
evidence that the reaction proceeds with high diastereoselectivity. X-
Ray crystal structure analysis revealed the isomers with SSSS- and
RRRR-configuration (Figure 1).
For the electron-withdrawing pyrido-anellated azaphospholes the
addition of tBuLi is the preferred reaction. This paves the way to syn-
thesize 2-functionally substituted dihydro-pyrido-azaphospholes as P-
asymmetric P, X hybrid ligands.
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