Syntheses and chemistry of very robust phosphiranes

Article abstract of DOI:10.1080/10426500212274

BABAR-Phos is a very stable polycyclic phosphirane that readily forms complexes with rhodium and platinum. Depending on the oxidation state and further co-ligands, either phosphirane complexes or metallaphosphetanes (as products of a metal insertion) prevail. However, these insertion reactions are reversible. These rhodium BABAR-Phos complexes are active catalysts for the selective hydroboration of olefins.

Full text of DOI:10.1080/10426500212274

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Phosphorus, Sulfur, and Silicon and the Related
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Syntheses and Chemistry of Very Robust Phosphiranes
Hansjörg Grützmacher a , Jürgen Liedtke a , Gionata Frasca a , Florian Läng a & Nicola Pè a
a ETH-Hönggerberg , HCI H131, Zürich, CH-8093, Switzerland
Published online: 27 Oct 2010.
To cite this article: Hansjörg Grützmacher , Jürgen Liedtke , Gionata Frasca , Florian Läng & Nicola Pè (2002) Syntheses and
Chemistry of Very Robust Phosphiranes, Phosphorus, Sulfur, and Silicon and the Related Elements, 177:6-7, 1771-1774, DOI:
10.1080/10426500212274
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Phosphorus, Sulfur and Silicon, 2002, Vol. 177:1771–1774
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Copyright 2002 Taylor & Francis
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DOI: 10.1080/10426500290093793
SYNTHESES AND CHEMISTRY OF VERY ROBUST
PHOSPHIRANES
Hansjo¨rg Gru¨tzmacher, Ju¨rgen Liedtke, Gionata Frasca,
Florian La¨ng, and Nicola Pe`
ETH-Ho¨nggerberg, HCI H131, CH-8093 Zu¨rich, Switzerland
(Received July 29, 2001; accepted December 25, 2001)
BABAR-Phos is a very stable polycyclic phosphirane that readily forms
complexes with rhodium and platinum. Depending on the oxidation
state and further co-ligands, either phosphirane complexes or met-
allaphosphetanes (as products of a metal insertion) prevail. How-
ever, these insertion reactions are reversible. These rhodium BABAR-
Phos complexes are active catalysts for the selective hydroboration of
olefins.
Keywords: Catalysis; hydroboration; phosphiranes; rhodium com-
plexes
INTRODUCTION
In many homogeneously catalyzed reactions, phosphane transition
metal complexes are used as catalyst precursors. A severe problem is
the sensitivity of the non-complexed phosphanes and their complexes
towards oxygen. The Wilkinson catalyst, [RhCl(PPh3)3] (1) becomes
rapidly oxidized upon exposure to oxygen, and Ph3P O is formed. Apart
from catalyst destruction, this may also significantly alter the selectiv-
ity of the catalyzed reaction. In the hydroboration reaction of styrene
with catechol borane, HBcat, the ratio of 1-phenylethanol (1-PhEtOH)
to 2-phenylethanol (2-PhEtOH) is highly dependent on the quality of 1
(1-PhEtOH prevails with non-oxidised 1).1 A simple molecular orbital
(MO) diagram for an eight valence electron configurated: AB3 species
shows that with increasing pyramidalisation at A, the energy of the
highest occupied orbital (HOMO) decreases, thus rendering AB3 more
difficult to oxidize. On the other hand, AB3 becomes a poorer donor
Address correspondence to H. Gru¨tzmacher, ETH-Ho¨nggerberg, HCI H131, CH-8093
Zu¨rich, Switzerland. E-mail: gruetzmacher@inorg.chem.ethz.ch
1771

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H. Gru¨tzmacher et al.
towards a metal center, and therefore may be only a weakly binding
ligand. In phosphiranes, compounds containing a three-membered PC2
heterocycle, a very small 6 (P) can be realized, and we decided to
explore their coordination chemistry to transition metals relevant to
3
catalysis.2
RESULTS AND DISCUSSION
The synthesis of BABAR-Phos is shown in Scheme 1. Starting from
chlorocycloheptatrienes (dibenzotropylidenylchlorides) 1, the amines
2 and after lithiation and reaction with PCl3, the (tropylideny-
lamino)dichlorophosphanes 3 are obtained in excellent yields. Usu-
ally the dehalogenation reaction, RPCl2 + Mg → (RP)∞ + MgCl2, leads
to mixture of polyphosphanes; however, due to the proximity of
the PCl2 group to the C C unit, the intermediate [tropNR-P Mg]
is intercepted in a [2 + 1] cycloaddition reaction, giving BABAR-
Phos in very high yields (>90%).4 BABAR-Phos is robust and
does not react with O2, S8, alkylating reagents, aqueous base, or
acid.
SCHEME 1 Synthesis of BABAR-Phos.
The co-ordination chemistry of BABAR-Phos platinum and rhodium
complexes was investigated (Scheme 2). While the reaction with
PtMe2(cod) leads quantitatively to the stable platinum(II) complex 4,
reaction with platinum(O) complexes like Pt(nbn)3 (nbn = norbornene)
furnishes the platinaphosphetane 5.5 However, the metal insertion
can be reversed, and when 5 is reacted with malenic anhydride

Syntheses and Chemistry of Very Robust Phosphiranes
1773
(malan), the uncomplexed BABAR-Phos is formed beside Pt(PPh3)2
(malan).
SCHEME 2 Synthesis of BABAR-Phos metal complexes.
These unusual insertion/de-insertion reactions are dependent on the
reaction conditions, as is illustrated by the reversible transformation
6
7.6 When L is a labile ligand (i.e., acetonitrile), addition of Cl to 6
→
leads to the quantitative formation of 7. When a strongly binding ligand
L = PPh3 is added to 7 and Cl is exchanged for the weakly coordinat-
ing PF6 anion, the complex 6 (3 L = 2 PPh3, MeCN) is reconstituted
quantitatively.
The influence of the BABAR-Phos substituents R and R1 on the se-
lectivity in the catalyzed hydroboration of styrene was investigated
(Figure 1). The selectivity and activity depends strongly on steric factors
and the BABAR-Phos/Rh ratios. The highest selectivity for 1-PhEtOH
was obtained with the electron withdrawing aryl substituent R = 3,5-
(CF3)2C6H3 resting in conjugation via the nitrogen center with the
PC2 ring and R1 = Ph at the ring carbon atom. Both the activity and
selectivity augment with increasing BABAR-Phos/Rh ratios. When R
stands for a bulky alkyl or 2,6-substituted aryl group and no substituent
is bonded to the PC2 ring, the selectivity is rather low. In any case,
the catalyst remained active and could be used for several catalytic
runs.

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H. Gru¨tzmacher et al.
FIGURE 1 Hydroboration of styrene with catechol borane, HBcat, using
2 mol% [Rh(cod)2]O3SCF3/BABAR-Phos catalyst precursors in thf at T = 298 K
for 1 h. The substituents R, R1, and/or BABAR-Phos/Rh ratio were varied: En-
try 1: ratio 1/1; Entries 2–5: ratios 1/1, 2/1, 3/1, 4/1; Entries 6–9: ratios 1/1,
2/1, 3/1, 4/1; Entry 10: preparation with [Rh( 2-Cl)(cod)]2, ratio 3/1; Entry 11:
ratio 3/1.
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