Chemistry of a geminal frustrated Lewis pair featuring electron withdrawing C6F5 substituents at both phosphorus and boron

Article abstract of DOI:10.1039/c1cc10241a

Hydroboration of the electron poor phosphine (1-propenyl)P(C ₆F₅)₂ with Piers' borane [HB(C ₆F₅)₂] gave the geminal frustrated Lewis pair (C₆F₅)₂P-CH(Et)-B(C₆F ₅)₂. It undergoes 1,2-addition reactions to an alkene and an alkyne and to the CN bond of an isocyanate. With mesityl azide it undergoes a 1,3-addition reaction.

Full text of DOI:10.1039/c1cc10241a

View Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: DOI: 10.1039/c1cc10241a
www.rsc.org/chemcomm
COMMUNICATION
Chemistry of a geminal frustrated Lewis pair featuring electron
withdrawing C6F5 substituents at both phosphorus and boronw
¨
Annika Stute, Gerald Kehr, Roland Frohlichz and Gerhard Erker*
Received 13th January 2011, Accepted 9th February 2011
DOI: 10.1039/c1cc10241a
Hydroboration of the electron poor phosphine (1-propenyl)-
P(C6F5)2 with Piers’ borane [HB(C6F5)2] gave the geminal
frustrated Lewis pair (C6F5)2P–CH(Et)–B(C6F5)2. It undergoes
1,2-addition reactions to an alkene and an alkyne and to the
CQN bond of an isocyanate. With mesityl azide it undergoes a
1,3-addition reaction.
electronically. The resulting decreased Lewis basicity would
undoubtedly diminish the self-quenching ability of the Lewis
pair, but it remained to be shown whether a sufficient residual
Lewis base reactivity would remain to still observe FLP
chemical behavior. We have prepared such an electronically
modified intramolecular frustrated P/B Lewis pair system and
observed a remarkable new FLP addition chemistry.
We prepared vinylP(C6F5)2 (4a) from (C6F5)2PCl6 and vinyl-
Frustrated Lewis pair (FLP) chemistry is developing rapidly.1
Various combinations of bulky Lewis acids and bases have led
to non-quenched ‘‘antagonistic’’ pairs,2 that often feature
remarkable reactivities and reaction modes with various small
molecules. The ethylene-linked Mes2P–CH2–CH2–B(C6F5)2
system (1) is one of the most reactive and versatile intra-
molecular frustrated Lewis pairs.3 Due to the steric bulk of its
substituents it features only a weak interaction between the
phosphorus Lewis base and the boron Lewis acid.3a The
system 1 rapidly splits dihydrogen at ambient conditions
and reacts with various unsaturated substrates, sometimes
in remarkable ways (e.g. with phenyl azide or with phenyl iso-
cyanate to yield 2 or 3, respectively, see Chart 1).4
magnesium chloride. Subsequent hydroboration with Piers’
7
borane HB(C6F5)2 went smoothly. However, the H–[B] addi-
tion was not regioselective and we obtained the ‘‘Markovnikov’’
(5b) and ‘‘anti-Markovnikov’’ products (5a) in a ca. 1 : 2 ratio
(see Scheme 1, for details see the ESIw). We figured that
introduction of an electron-donating b-alkyl substituent should
then favour the formation of the ‘‘Markovnikov product’’.
Therefore, we reacted (C6F5)2PCl with 1-propenylmagnesium
chloride to give a mixture of the propenylP(C6F5)2 products
E- and Z-4b. Their hydroboration with HB(C6F5)2 proceeded
rapidly by regioselective ‘‘Markovnikov addition’’ to generate
the geminal P/B Lewis pair 6 (see Scheme 1).
The hydrolysis-sensitive compound 6 was not isolated as a
pure solid but freshly generated in situ for the respective
trapping experiments. However, the geminal frustrated Lewis
pair was unequivocally characterized spectroscopically. It
shows the typical NMR signals of a tricoordinated boron
atom [11B NMR: d 71; 19F NMR: d ꢀ128.7 (4F, o), ꢀ144.6
(2F, p) and ꢀ159.7 (4F, m-C6F5) (Ddm,p = 15.1)]. Due to the
chiral center [1H NMR: d 4.40 (1-H)] the C6F5 substituents at
the adjacent prochiral phosphorus atom [31P NMR: d ꢀ40.8]
are diastereotopic [e.g. 19F NMR: d ꢀ147.8/ꢀ148.5 (p-C6F5)]
(for further details see the ESIw).
Introducing steric bulk has been a common way to protect
Lewis pairs from mutual annihilation by stable adduct forma-
tion between its components.5 Attaching electron-withdrawing
C6F5 substituents also at the Lewis base component of the pair
might be an attractive alternative to induce FLP behavior
Frustrated Lewis pairs add to isocyanates. Usually P/B
addition to the reactive carbonyl function is observed
Chart 1
Organisch-Chemisches Institut, Universitat Munster,
¨
¨
Corrensstraße 40, D-48149 Munster, Germany.
¨
E-mail: erker@uni-muenster.de; Fax: +49 251-83 36503
w Electronic supplementary information (ESI) available: Additional
experimental and spectroscopic data. CCDC 809019–809021. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1cc10241a
z X-ray structure analyses.
Scheme 1
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun.

View Online
(see e.g. Chart 1).4,8 The geminal P/B Lewis pair 6 also adds
readily to p-tolyl isocyanate, but here 1,2-P/B attachment to
the imine functionality is observed instead.9 The product 7 was
isolated in close to 80% yield. Single crystals for the X-ray
crystal structure analysis were obtained from dichloromethane
at ꢀ30 1C (see Fig. 1). It shows the formation of the five-
membered heterocyclic structure of compound 7 with bond
lengths P1–C01 1.889(4) A, B1–N1 1.580(5) A and C01–O1
1.212(5) A. Boron and phosphorus are bridged by the
propylidene unit (B1–C1 1.683(6) A, P1–C1 1.790(4) A, angle
B1–C1–P1 102.5(2)1) originating from the FLP reagent 6. In
solution the 13C NMR resonance of the carbonyl carbon of the
addition product 7 was found as a broad doublet at d 158.9
(1JPC B 105 Hz). The 19F NMR spectrum (188 K) shows
16 signals for the diastereotopic pairs of C6F5 substituents at
phosphorus and boron (for details see the ESIw). The IR
Scheme 2
A (bond angles: B1–C1–P1 107.6(2)1,
N1–P1 1.666(2)
C1–B1–N3 108.3(2)1, C1–P1–N1 108.17(11)1; dihedral angles:
B1–N3–N2–N1 3.6(4)1, N3–N2–N1–P1 ꢀ9.6(3)1). The observed
N–N–N bond delocalization indicates a participation of the
phosphinimine resonance structure for the structural descrip-
tion of compound 10. In solution compound 10 shows a NMR
signal of a tetracoordinated boron center [11B NMR: d ꢀ9.3]
and a broad 31P NMR resonance at d 4.0.13 The presence of
the bridging chirality center (C1) renders the pairs of C6F5
substituents at both boron and phosphorus diastereotopic.
We exposed the P/B system 6 to dihydrogen at various
conditions but could not observe the respective splitting
reaction.5a Nevertheless, the addition reactions described in this
communication have shown that the electronically modified
(CQO) band of compound 7 was found at 1700 cmꢀ1
.
The frustrated Lewis pair 6 reacts with 1-pentyne to regio-
selectively give the five-membered addition product 8 [31P
NMR: d 29.8, 11B NMR: d ꢀ8.6]. It shows the 1H NMR
signals of the newly introduced n-propyl substituent. The 1-H
3
NMR signal (d 8.33) shows a typical large JPH coupling
constant of ca. 68 Hz.10 Similarly, the FLP 6 adds cleanly to
ethylene11 to yield the five-membered P/B heterocycle 9
[31P NMR: d 37.5, 11B NMR: d ꢀ8.7). The diastereotopic
methylene 1H NMR signals of the ethylene bridge occur at
d 3.38 (m)/2.91 (m) (PCH2) and 2.05 (3JPH = 42 Hz)/1.40 (m)
(BCH2), respectively. The diastereotopic 1H NMR resonances
of the CH2 group of the ethyl substituent were found at d 1.75
(3JPH = 33 Hz) and 1.23 (m). Both the compounds 8 and 9
show typical 19F NMR sets of signals of their pairs of
diastereotopic C6F5 groups at both boron and phosphorus
(for details see the ESIw) (Scheme 2).
Eventually, we reacted the FLP 6 with mesityl azide. Again
a rapid addition reaction occurred and we isolated the product
10 in 50% yield. The X-ray crystal structure analysis revealed
1,3-addition of the frustrated P/B Lewis pair to the 1,3-dipolar
reagent (Fig. 2; Scheme 3).12 In the product 10 the strongly
Lewis acidic borane was found attached at the nitrogen atom
bearing the bulky aryl substituent. Inside the six-membered
heterocycle compound 10 exhibits bond lengths B1–N3
1.615(3) A, N3–N2 1.306(3) A, N2–N1 1.304(3) A and
Fig. 2 Molecular structure of compound 10.
Fig. 1 Molecular structure of compound 7.
Scheme 3
c
Chem. Commun.
This journal is The Royal Society of Chemistry 2011

View Online
P/B system 6 shows pronounced frustrated Lewis pair reactivity
despite the small spatial separation of its Lewis acid and base
components in their geminal arrangement at its hydrocarbon
backbone.14 The electronically modified FLP showed some
marked differences in the P/B addition reactions in detail such
as a preferred addition to the CQN bond of the isocyanate
as opposed to the usual CQO addition or the preferred
1,3-addition to an azide in contrast to the usual FLP 1,1- or
1,2-addition reactions to the N3–R reagents.4,15 This behavior
indicates that electronic modification in addition to steric bulk
is likely to become a powerful tool in the further development
of frustrated Lewis pair construction and their characteristic
chemistry.
(e) R. C. Neu, E. Otten, A. Lough and D. W. Stephan, Chem.
Sci., 2011, 2, 170–176.
6 D. D. Magnelli, G. Tesi, J. U. Lowe and W. E. McQuiston, Inorg.
Chem., 1966, 5, 457–461; G. Mancino, A. J. Ferguson, A. Beeby,
N. J. Long and T. S. Jones, J. Am. Chem. Soc., 2005, 127, 524–525.
7 (a) D. J. Parks, R. E. v. H. Spence and W. E. Piers, Angew. Chem.,
1995, 107, 895–897 (Angew. Chem., Int. Ed. Engl., 1995,
34, 809–811); (b) D. J. Parks, W. E. Piers and G. P. A. Yap,
Organometallics, 1998, 17, 5492–5503.
8 S. Moebs-Sanchez, G. Bouhadir, N. Saffon, L. Maron and
D. Bourissou, Chem. Commun., 2008, 3435–3437.
9 For related rare examples see e.g.: B. A. Arbuzov, G. N. Nikonov,
A. S. Balueva, R. M. Kamalov, M. A. Pudikov, R. R. Shagidullin,
A. Kh. Plyamovatyi and R. Sh. Khadiullin, Izv. Akad. Nauk.
SSSR, Ser. Khim., 1991, 2393–2396 (Bull. Acad. Sci. USSR, Div.
Chem. Sci. (Engl. Transl.), 1991, 40, 2099).
1
10 For JPH coupling constants [1J(HP(C6F5)3+) = 594 Hz] see:
G. G. Furin, S. A. Krupoder, A. I. Rezvukhin, T. M. Kilina and
G. G. Yakobson, J. Fluorine Chem., 1983, 22, 345–375.
Notes and references
11 See for a comparison: J. S. J. McCahill, G. C. Welch and
D. W. Stephan, Angew. Chem., 2007, 119, 5056–5059 (Angew.
Chem., Int. Ed., 2007, 46, 4968–4971); see for DFT calculations:
1 D. W. Stephan and G. Erker, Angew. Chem., 2010, 122, 50–81
(Angew. Chem., Int. Ed., 2010, 49, 46–76).
2 (a) W. Tochtermann, Angew. Chem., 1966, 78, 355–375
(Angew. Chem., Int. Ed. Engl., 1966, 5, 351–371); (b) G. Wittig
and E. Benz, Chem. Ber., 1959, 92, 1999–2013; see also:
H. C. Brown, H. I. Schlesinger and S. Z. Cardon, J. Am. Chem.
Soc., 1942, 64, 325–329.
A. Stirling, A. Hamza, T. A. Robok and I. Papaı
2008, 3148–3150; Y. Guo and S. Li, Eur. J. Inorg. Chem., 2008,
2501–2505; see for a comparison: C. M. Momming, S. Fromel,
´
, Chem. Commun.,
¨
¨
G. Kehr, R. Frohlich, S. Grimme and G. Erker, J. Am. Chem. Soc.,
¨
2009, 131, 12280–12289.
3 (a) P. Spies, G. Erker, G. Kehr, K. Bergander, R. Frohlich,
¨
12 Review about ‘‘click chemistry’’: C. R. Becer, R. Hoogenboom and
U. S. Schubert, Angew. Chem., 2009, 121, 4998–5006 (Angew. Chem.,
Int. Ed., 2009, 48, 4900–4908); Review about R–N3 metal coordina-
tion: S. Cenini, E. Gallo, A. Caselli, F. Ragaini, S. Fantauzzi and
C. Piangiolino, Coord. Chem. Rev., 2006, 250, 1234–1253.
13 H.-G. Horn, M. Gersemann and U. Niemann, Chem.-Ztg., 1976,
100, 197–198.
14 F. Lavigne, E. Maerten, G. Alcaraz, N. Saffon-Merceron,
C. Acosta-Silva, V. Branchadell and A. Baceiredo, J. Am.
Chem. Soc., 2010, 132, 8864–8865; A. Schnurr, H. Vitze,
M. Bolte, H.-W. Lerner and M. Wagner, Organometallics, 2010,
29, 6012–6019.
S. Grimme and D. W. Stephan, Chem. Commun., 2007,
5072–5074; (b) P. Spies, S. Schwendemann, S. Lange, G. Kehr,
R. Frohlich and G. Erker, Angew. Chem., 2008, 120, 7654–7657
¨
(Angew. Chem., Int. Ed., 2008, 47, 7543–7546); see also:
(c) K. V. Axenov, C. M. Momming, G. Kehr, R. Frohlich and
¨
G. Erker, Chem.–Eur. J., 2010, 16, 14069–14073.
¨
¨
4 (a) P. Spies, G. Kehr, K. Bergander, B. Wibbeling, R. Frohlich and
G. Erker, Dalton Trans., 2009, 1534–1541; (b) C. M. Momming,
¨
G. Kehr, B. Wibbeling, R. Frohlich and G. Erker, Dalton Trans.,
2010, 39, 7556–7564.
¨
5 (a) G. C. Welch and D. W. Stephan, J. Am. Chem. Soc., 2007, 129,
1880–1881; (b) G. C. Welch, R. R. S. Juan, J. D. Masuda and
D. W. Stephan, Science, 2006, 314, 1124–1126; for reviews see:
(c) D. W. Stephan, Dalton Trans., 2009, 3129–3136;
(d) D. W. Stephan, Org. Biomol. Chem., 2008, 6, 1535–1539;
15 M. W. P. Bebbington, S. Bontemps, G. Bouhadir and D. Bourissou,
Angew. Chem., 2007, 119, 3397–3400 (Angew. Chem., Int. Ed., 2007,
46, 3333–3336).
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun.