Chemoselective phosphination of titanacyclobutene: A convenient method for synthesis of allylphosphine derivatives

Article abstract of DOI:10.1021/om500084j

Titanacyclobutenes reacted with chlorophosphine to afford titanoallylphosphines with high chemoselectivity, and the resulting titanoallylphosphine could be converted into functionalized allylphosphine sulfides via reactions with various electrophiles.

Full text of DOI:10.1021/om500084j

Communication
pubs.acs.org/Organometallics
Chemoselective Phosphination of Titanacyclobutene: A Convenient
Method for Synthesis of Allylphosphine Derivatives
Yiqing Zhou,† Chao Chen,*,† Xiaoyu Yan,† and Chanjuan Xi*,†,‡
†Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology (Ministry of Education), Department of Chemistry,
Tsinghua University, Beijing 100084, People’s Republic of China
‡State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, People’s Republic of China
S
* Supporting Information
ABSTRACT: Titanacyclobutenes reacted with chlorophosphine to
afford titanoallylphosphines with high chemoselectivity, and the resulting
titanoallylphosphine could be converted into functionalized allylphos-
phine sulfides via reactions with various electrophiles.
Scheme 1
llylphosphine and its derivatives are an important class of
Aorganic compounds that are widely employed both as
ligands in transition-metal complexes and as substrates in
various organic synthesis processes.1,2 Conventionally, proto-
cols for the preparation of allylphosphines included the
reactions of chlorophosphine and allyl Grignard reagents or
allyl chlorides and lithium diphenylphosphanide3 and other
related methods.4 Other ways to prepare allylphosphines are by
Michaelis−Arbuzov and Pudovik rearrangements.5 Nonethe-
less, it is often difficult to prepare substituted allylphosphines
by these methods. Consequently, the development of a versatile
and general method for the preparation of functionalized
allylphosphines is a necessity. Titanacyclobutenes which can be
°C. Then the reaction mixture was warmed to room
temperature and stirred at the same temperature for 3 h. The
reaction mixture was quenched with 3 M HCl, and
allylphosphine 3a was obtained in 86% NMR yield. Purification
of 3a directly by column chromatography led to the formation
of a complex inseparable mixture. Since the trivalent phosphine
3a was sensitive to oxygen, the product of 3a was isolated as
phosphine sulfide 4a in 82% isolated yield after treatment of the
reaction mixture with elemental sulfur. Deuteriolysis of the
reaction mixture instead of hydrolysis afforded the deuterated
compound 3-deuterio-2,3-diphenylallylphosphine sulfide (5a)
in 76% isolated yield with 91% deuterium incorporation
(Scheme 2). This result showed that the product of the
reaction of titanacyclobutene with chlorodiphenylphosphine
before hydrolysis contains one titanocene−sp2 carbon bond.
6
easily prepared by the reaction of alkyne and Cp2TiMe2 have
two markedly different sites of reaction in the sp2- and sp3-
hybridized titanocene−carbon bonds with an allyl skeleton.
Reactions occurring selectively at either site and at both sites
have been reported.7 Among them, the groups of Knobler’ and
Harlow have reported the reaction of titanacyclobutenes with
PhPCl2 or PhAsCl2 to afford phosphacyclobutenes or
arsacyclobutenes.7b,c In these cases, both sp2 carbon−
titanocene and sp3 carbon−titanocene bonds reacted with
two P−Cl or As−Cl moieties. It is interesting to compare the
reactivities of sp2 carbon−titanocene and sp3 carbon−
titanocene bonds in titanacyclobutene toward the P−Cl moiety,
which potentially affords acyclic monophosphine reagents.
Herein, we report the reaction of titanacyclobutenes with a
chlorophosphine (R2PCl) that has one P−Cl moiety. The
reaction occurred at the sp3 carbon−titanocene bond, which
provides a novel preparative method for the preparation of
allylphosphine derivatives (Scheme 1). Furthermore, the
resulting intermediate 2 can be converted into other function-
alized allylphosphines via reaction with electrophiles.
Scheme 2
A typical procedure is as follows. To a toluene solution of
titanacyclobutene 1a (3 mL), prepared from Cp2TiCl2 (150
mg, 0.6 mmol), MeLi (3 M in diethoxymethane solution, 0.4
mL, 1.2 mmol), and diphenylacetylene (107 mg, 0.6 mmol) in
the dark (wrapped in aluminum foil) according to the reported
procedure,6d was added 1 equiv of diphenylchlorophophine at 0
Received: January 24, 2014
Published: February 4, 2014
© 2014 American Chemical Society
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dx.doi.org/10.1021/om500084j | Organometallics 2014, 33, 844−846

Organometallics
Communication
The result also indicates that the reaction shows good
selectivity for phosphination of titanacyclobutene.
Further study involving the use of various substituted
titanacyclobutenes bearing alkyl, aryl, and TMS groups resulted
in all cases in the formation of allylphosphines in good yields.
Representative results are summarized in Table 1. The
Table 1. CpTiMe2-Mediated Reaction of Alkynes with
Chlorophosphines Leading to Formation of Allylphosphine
Derivatives
Figure 1. ORTEP drawing of 4b with 30% probability thermal
ellipsoids. Hydrogen atoms are omitted for clarity.
in 55% yield (entry 4). Alkyl-substituted titanacyclobutenes
afforded the desired products in moderate yields (entries 5−7).
The titanacyclobutenes prepared from unsymmetrical alkynes
gave allylphosphine sulfides as a mixture (entries 8−12). When
PhCCMe, PhCCEt, and PhCCBu were used (entries
8−10), respectively, two isomers were obtained in a 3:1 ratio
on the basis of NMR analysis. When TMSCCMe and
ThCCBu were used, the ratio of two isomers was 4:1,
respectively (entries 11 and 12). It should be noted that when
chlorodiisopropylphosphine was used instead of chlorodiphe-
nylphosphine, the reaction also proceeded smoothly and the
product was obtained in 48% yield at 80 °C for 8 h (entry 13).
In all the above reactions, we did not observe any
alkenylphosphorus compounds. Therefore, it is interesting to
compare the reactivities of Ti−C(sp3) and Ti−C(sp2) bonds in
titanacyclobutenes toward the P−Cl moiety.
The remaining Ti−C(sp2) bond of titanoallylphosphine 2
was further treated with electrophiles to give functionalized
allylphosphine derivatives (Scheme 3). Treatment of 2a with
Scheme 3
a
b
Isolated yield. A 3:1 mixture of two regioisomers; major isomer is
c
shown. A 4:1 mixture of two regioisomers; major isomer is shown.
d
ClP(i-Pr)2 was used, and the reaction was carried out at 80 °C for 8
iodine and bromine gave the iodo- and bromoallylphosphine
oxides 6a and 7a in 61% and 62% yields after hydrolysis,
respectively. The reaction of 2a with allylic bromide in the
presence of CuCl gave the corresponding cross-coupling
product 8a in 49% yield. When acetyl chloride was added
directly to a solution of 2a in situ, product 9a was obtained in
65% isolated yield.
In summary, we have developed a novel reaction for the
direct preparation of substituted metalloallylphosphines via the
highly chemoselective phosphination of titanacyclobutene. The
resulting titanoallylphosphines could be converted into various
functionalized allylphosphine derivatives.
h.
titanacyclobutenes prepared from symmetrical alkynes afforded
single allylphosphine sulfide products (entries 1−7). Aryl-
substituted titanacyclobutenes afforded the desired products in
excellent yields (entries 1−3). A single-crystal X-ray analysis of
4b verified the formation of allylphosphine, as shown in Figure
1. When a thienyl-substituted titanacyclobutene was used,
allylphosphine sulfide 4c was obtained in good yield (entry 3).
Reaction of a trimethylsilyl-substituted titanacyclobutene with
chlorodiphenylphosphine also afforded the desired product 4d
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Organometallics
Communication
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ASSOCIATED CONTENT
* Supporting Information
Text, figures, and a CIF file giving experimental details and
characterization data for all compounds and crystallographic
data for 4b. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: cjxi@mail.tsinghua.edu.cn (C.X.); chenchao01@mails.
tsinghua.edu.cn (C.C.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the national key basic research
program of China (973 program) (2012CB933402) and the
National Natural Science Foundation of China (21272132,
21032004, and 20972085).
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