Isomerization of Secondary Phosphirane into Terminal Phosphinidene Complexes: An Analogy between Monovalent Phosphorus and Transition Metals

Article abstract of DOI:10.1002/anie.201504869

Secondary phosphirane complexes isomerize above 100 °C to give the corresponding terminal phosphinidene complexes, which can be trapped by alkenes and alkynes. This reaction is a rare instance of the isomerization of a P^III derivative into a P^I derivative. It appears to mimic the reductive elimination of alkanes from transition-alkylmetal hydrides.

Full text of DOI:10.1002/anie.201504869

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201504869
German Edition: DOI: 10.1002/ange.201504869
Phosphorous Compounds
Isomerization of Secondary Phosphirane into Terminal Phosphinidene
Complexes: An Analogy between Monovalent Phosphorus and
Transition Metals
Jonathan Wong, Yongxin Li, Yanwei Hao, Rongqiang Tian,* and FranÅois Mathey*
Abstract: Secondary phosphirane complexes isomerize above
1008C to give the corresponding terminal phosphinidene
complexes, which can be trapped by alkenes and alkynes.
This reaction is a rare instance of the isomerization of a PIII
derivative into a PI derivative. It appears to mimic the reductive
elimination of alkanes from transition-alkylmetal hydrides.
E
lectrophilic terminal phosphinidene complexes [RP-M-
(CO)5] (M = Cr, Mo, W) are now recognized as a powerful
synthetic tool in organophosphorus chemistry.[1] They have
been generated with a variety of substituents, but the parent
species are still not accessible. We have recently shown that
the phosphirane complexes derived from cyclohexene are
good precursors for [RP-W(CO)5] (R = Cl, Fc).[2,3] This
observation led us to investigate the reduction of the chloro
derivatives 1[2] to obtain a potential precursor of [HP-
W(CO)5]. After several attempts (NaH, LiAlH4), we found
that Bu3SnH was the reagent of choice, thus giving 2a,b in
59% yield [Eq. (1)].
Figure 1. X-ray crystal structure of a secondary phosphirane (2b).
Thermal ellipsoids shown at 50% probability. Main bond lengths [Š]
and angles [deg.]: P1–W1 2.490(2), P1–C4 1.85(4), P1–C9 1.82(4),
C4–C9 1.535(17); C4-P1-C9 49.5(5).
The structure of the major isomer (2b) was confirmed by
X-ray crystal analysis (Figure 1).
Having in hand a potential precursor of [HP-W(CO)5], we
tested its reaction with alkenes and alkynes. The results were
completely unexpected, as shown in Equation (2). It appeared
that 2 isomerized to the terminal cyclohexylphosphinidene
pentacarbonyltungsten complex 5 and gives the characteristic
Figure 2. X-ray crystal structure of the phosphirene 3. Thermal ellip-
soids shown at 50% probability. Main bond lengths [Š] and angles
[deg.]: P1–W1 2.5061(12), P1–C6 1.851(4), P1–C18 1.792(4), P1–C19
1.793(4); C18-C19 1.325(6); C6-P1-C18 108.6(2); C6-P1-C19 109.7(2);
C18-P1-C19 43.40(19).
[*] J. Wong, Y. Li, F. Mathey
Division of Chemistry & Biological Chemistry, School of Physical &
Mathematical Sciences, Nanyang Technological University
21 Nanyang Link, Singapore 637371 (Singapore)
E-mail: fmathey@ntu.edu.sg
phosphirane and phosphirene [1+2] cycloaddition products.
The formulation of 3 and 4b was checked by X-ray crystal
structure analysis (Figures 2 and 3). To check the feasibility of
such an isomerization, we performed DFT calculations for 2b
and 5 at the B3LYP/6-31G(d)-lanl2dz(W) level of theory.[4]
The results indicate that the isomerization is feasible since
5 lies only 11.1 kcalmolÀ1 above 2b in energy. The next step
was to check the generality of this transformation. We
prepared the parent phosphirane complex 6,[5] from b-
chloroethylphosphine as shown,[6] and reacted it with tolan
[Eq. (3)].
Y. Hao, R. Tian
College of Chemistry and Molecular Engineering, International
Phosphorus Laboratory, Joint Research Laboratory for Functional
Organophosphorus Materials of Henan Province, Zhengzhou Uni-
versity,Zhengzhou 450001 (P.R. China)
E-mail: tianrq@zzu.edu.cn
Supporting information and ORCID(s) from the author(s) for this
article are available on the WWW under http://dx.doi.org/10.1002/
anie.201504869.
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radical H-P dissociation/recombination mechanism. The only
possibility was a concerted H-shift, which explained the
exclusive formation of 10 from 8. The computed structure of
the corresponding transition-state TS 1 (one negative fre-
quency) is shown in Figure 4. TS 1 lies 29 kcalmolÀ1 higher in
energy [zero-point energy (ZPE) included] than the starting
phosphirane 8. The magnitude of this barrier is compatible
with the experimental conditions (boiling toluene). TS 2 lies
34.9 kcalmolÀ1 higher in energy (ZPE included) than 8, thus
the formation of 9 is clearly disfavored.
Figure 3. X-ray crystal structure of the phosphirane 4b. Thermal
ellipsoids shown at 50% probability. Main bond lengths [Š] and angles
[deg.]: P1–W1 2.5079(17), P1–C6 1.824(7), P1–C7 1.858(7), P1–C14
1.856(6), C6–C7 1.529(10); C6-P1-C14 107.6(3); C7-P1-C14 108.3(3);
C6-P1-C7 49.1(3).
Figure 4. Computed transition state (TS 1) for the formation of 10
from 8. Main distances [Š] and angles [deg.]: P–W 2.550, P–C1 1.929,
P–C16 2.290, P–H17 1.523, C16–H17 1.540; P-H17-C16 96.75.
This isomerization of a trivalent P into a monovalent P
derivative is a rare example. Most of the preceding examples
concern the [2+1] cycloreversion of polycyclic phosphiranes
into alkenes and phosphinidenes.[8] A single example con-
cerns the rearrangement of an azaphosphiridine into a phos-
phinidene complex in which some stabilization is gained by
[9]
=
interaction with a furan lone pair and a distant C C bond.
The reported reaction provides a valuable route to terminal
phosphinidene complexes without generating by-products,
contrary to the known methods. From another standpoint,
this isomerization is equivalent to the reductive elimination of
an alkane from an alkylmetal hydride in transition-metal
À
chemistry and is the reverse of the C H activation by
phosphinidene complexes, which have been described by
The formation of the 1-ethylphosphirene complex 7[7]
suggested that this isomerization was general. To confirm
this generality and to get useful data to identify a possible
mechanism, we also studied the 2,2-dimethylphosphirane
complex 8, whose isomerization might give either the tert-
butyl phosphinidene 9 or the isobutyl phosphinidene 10
[Eq. (4)]. The complex 8 was prepared from the correspond-
ing chloro derivative 8Cl by reduction with Bu3SnH [see
Eq. (1)]. The trapping reaction with tolan exclusively pro-
duced the 1-isobutylphosphirene 11.
several authors.[10]
Supporting Information: Complete experimental section.
CCDC 1403838 (2b), 1403839 (3) and 1403840 (4b) contain
the supplementary crystallographic data for this paper. These
data are provided free of charge by The Cambridge
Crystallographic Data Centre.
Acknowledgements
We then performed a series of computations on a potential
concerted H-shift mechanism, as well as on either an ionic or
The authors thank the Nanyang Technological University in
Singapore, Zhengzhou University in China, and the National
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J. Cioslowski, and D. J. Fox, Gaussian, Inc., Wallingford CT,
2013.
Natural Science Foundation of China (21302174) for the
financial support of this work.
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Keywords: coordination modes ·
density-functional calculations · isomerization · phosphorous ·
structure elucidation
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Angew. Chem. 2015, 127, 13083–13085
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Received: May 28, 2015
Published online: September 4, 2015
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