Reactivity patterns of benzhydryl(mesityl)phosphane oxide–a potential intermediate in carbonyl-carbonyl coupling reactions?

Article abstract of DOI:10.1080/10426507.2018.1543305

Benzhydryl(mesityl)phosphane oxide 2-H is prepared and its deprotonation and nucleophilicity behavior investigated. Treatment of lithium salts of 2-H with MeI results in methylation at the P-center while the benzyl carbon is not affected. Only upon double lithiation, it is possible to methylate also the benzyl carbon. Di-anion 2-2Li is reactive towards benzaldehyde to afford moderate amounts of triphenylethene after basic work.

Full text of DOI:10.1080/10426507.2018.1543305

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: https://www.tandfonline.com/loi/gpss20
Reactivity patterns of
benzhydryl(mesityl)phosphane oxide – a potential
intermediate in carbonyl-carbonyl coupling
reactions?
Nicolas D’Imperio & Anna I. Arkhypchuk
To cite this article: Nicolas D’Imperio & Anna I. Arkhypchuk (2019) Reactivity patterns of
benzhydryl(mesityl)phosphane oxide – a potential intermediate in carbonyl-carbonyl coupling
reactions?, Phosphorus, Sulfur, and Silicon and the Related Elements, 194:4-6, 575-579, DOI:
10.1080/10426507.2018.1543305
To link to this article: https://doi.org/10.1080/10426507.2018.1543305
© 2018 The Author(s). Published with
license by Taylor & Francis Group, LLC
Published online: 20 Feb 2019.
Submit your article to this journal
Article views: 126
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at
https://www.tandfonline.com/action/journalInformation?journalCode=gpss20

PHOSPHORUS, SULFUR, AND SILICON AND THE RELATED ELEMENTS
2019, VOL. 194, NOS. 4–6, 575–579
https://doi.org/10.1080/10426507.2018.1543305
Reactivity patterns of benzhydryl(mesityl)phosphane oxide – a potential
intermediate in carbonyl-carbonyl coupling reactions?
Nicolas D’Imperio and Anna I. Arkhypchuk
€
Department of Chemistry Ångstrom, Uppsala University, Uppsala, Sweden
ABSTRACT
ARTICLE HISTORY
Received 8 October 2018
Accepted 27 October 2018
Benzhydryl(mesityl)phosphane oxide 2-H is prepared and its deprotonation and nucleophilicity
behavior investigated. Treatment of lithium salts of 2-H with MeI results in methylation at the P-
center while the benzyl carbon is not affected. Only upon double lithiation, it is possible to methy-
late also the benzyl carbon. Di-anion 2-2Li is reactive towards benzaldehyde to afford moderate
amounts of triphenylethene after basic work.
KEYWORDS
Aldehyde-aldehyde
coupling; alkenes;
nucleophilicity; reactiv-
ity study
GRAPHICAL ABSTRACT
Introduction
exhibits an opposite polarization compared to that in the
starting carbonyl compound (Figure 1b). As a consequence of
this Umpolung, treatment of the phosphaalkene with a
Bu4NOH solution results in the formal addition of water to
the P ¼ C double bond with a hydroxyl group placed at the P-
center. The thereby obtained hydrophosphinite can be present
in solution in two forms[15,16] – either as a trivalent (hydro-
phosphinic acid) or a pentavalent (secondary phosphine
oxide) phosphorus compound. In the presence of a second
aldehyde, the secondary phosphine oxide reacts smoothly and
an alkene is easily formed within a few minutes. In case a
second aldehyde is not present, the crucial secondary phos-
phine oxide intermediate can be isolated from the reaction
mixture. As a proof of the proposed mechanism, it can be sub-
jected in its purified form to the reaction conditions in the
presence of the aldehyde to form the olefinic product.
Despite all of the advantages of the developed sequence,
several restrictions in substrate scope were observed.[11] Only
secondary phosphine oxides obtained from aldehydes bearing
electron withdrawing (EWG) substituents appeared to be suf-
ficiently active in the olefination step. The second aldehyde
also needs to be selected with care since strongly electron
donating aldehydes are not suitable for this reaction. We rea-
soned that one of the ways to overcome these limitations
could be to increase the reactivity of the deprotonated phos-
pine oxides by decreasing the steric bulk around the phos-
phorus center. Considering all potential synthetic
The importance of C ¼ C double bonds in modern organic
chemistry is difficult to overestimate. Alkenes play key roles
in polymer and material sciences,[1] are present in biologically
active molecules[2] and are essential for pigments.[3] Their
synthesis is one of the most fundamental tasks of organic
chemistry since the double bond was first postulated by
Butlerov in his theory of chemical structure in 1861.[4] Today,
there is a large number of methodologies for the preparation
of C ¼ C double bonds, for example, by dehydrohalogena-
tion[5] or elimination of water,[6] by rearrangements,[7] or ole-
fination reactions (e.g. Peterson olefination,[8] Wittig
reaction,[9] Horner-Wadsworth-Emmons reaction[10]). The
approach that is taken in each specific case depends largely on
the availability of the starting materials, on the position in the
molecule at which the double bond has to be installed, and the
reactivity of the remaining part of the molecules. Recently our
group published a procedure which allows the formation of
C ¼ C doubles bond by the direct coupling of two aldehydes
(Figure 1a, b).[11–13] In contrast to the McMurry protocol[14]
which uses low-valent transitional metals, requires harsh reac-
tion conditions, and does not allow the selective preparation
of unsymmetric alkenes from the coupling of two non-identi-
cal carbonyl compounds, our approach proceeds by a stepwise
anionic mechanism which resolves the drawbacks mentioned
above. In our synthetic methodology,[11] a first aldehyde
reacts with a lithiated phosphanylphosphonate to obtain phos-
phaalkenes in which the C-center of the P ¼ C double bond complications that may arise when preparing phosphaalkenes
CONTACT Anna I. Arkhypchuk
ß 2018 Taylor & Francis Group, LLC
anna.arkhypchuk@kemi.uu.se
This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License (http://creativecommons.org/licenses/by-nc-nd/4.0/),
which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited, and is not altered, transformed, or built upon in any way.

576
N. D’IMPERIO AND A. I. ARKHYPCHUK
Figure 1. (a) Goal of the project – the reductive coupling of two aldehydes to C ¼ C double bonds, (b) Alkene formation via phosphaalkenes and secondary phos-
phine oxides, (c) Phosphine oxides used in previous and present work.
with decreased kinetic stabilization due to the smaller P-sub-
stituents,[17] we were interested in scrutinizing the reactivity
of benzhydrylphosphane oxides that carry a 2,4,6-trimethyl-
phenyl (mesityl, Mes) P-substituent instead of the previously
Mes
Ph
NEt2
Ph
HBr/H2O
Mes
Ph
O
H
P
1
P
Ph
45%
2-H
,
ꢀ
published 2,4,6-tri(tertbutyl)phenyl (Mes ) group (Figure 1c).
Scheme 1. Preparation of target secondary phosphine oxide 2-H.
As described above, the benzylphosphine oxides are crucial
intermediates in the overall coupling sequence, and their
downstream chemistry is thus imperative to investigate and
understand. We thus set out to prepare and study the depro-
tonation behavior of benzylmesitylphosphane oxides, to inves-
tigate their methylation behavior to identify the most
nucleophilic site, and finally check its reactivity towards
benzaldehyde.
þ104 ppm which is consistent with the lithium salt of the
hydrophosphinic acid 2-Li (resonance structure B in Figure
2). Treatment of 2-Li with excess methyl iodide results in the
formation of a single product according to the 31P NMR spec-
troscopy with a chemical shift of þ46 ppm (THF solution). In
the 1H NMR spectrum of the isolated product, the large doub-
let of doublets at 8.10 ppm is not observed any longer, indicat-
ing the absence of a proton that is directly attached to the
phosphorus center, suggesting the formation of the tertiary
phosphine oxide 3. The proton at the a carbon appears as a
Results and discussion
2
Benzhydryl(mesityl)phosphane oxide 2-H was prepared from
literature-known compound 118 by treatment with a concen-
trated aqueous HBr solution (Scheme 1). The exothermic
reaction proceeds smoothly and is completed within several
minutes as clearly visible from 31P NMR spectroscopy were
the peak of the starting material 1 at þ67 ppm completely
disappears and a new peak of the product can be found at
þ24 ppm. Product 2-H is obtained in 45% isolated yield as a
white powder.
doublet at 4.42 ppm with JHP = 9 Hz indicating that C-cen-
tered alkylation has not occured. A new doublet at 1.74 ppm
2
with JHP = 12 Hz and integrating for 3 protons can be
assigned to the methyl group introduced at the phosphorus
center, suggesting reactivity through intermediate A (Figure
2). All together, the NMR spectroscopic data suggests exclu-
sive alkylation at the P-center by a Michaelis - Becker reaction
and, indeed, compound 3 appears to be literature known with
analytical data fully consistent with ours.[19] Treatment of 2-
Li with aldehydes resulted in no reaction and starting materi-
als were isolated even under prolonged reaction times and ele-
vated temperatures. This result indicates the complete absence
of anions that could be described by resonance structures C
and D in solutions of 2-Li (Figure 2).
Treatment of 2-H with two equivalents of BuLi resulted
in a strong color change from colorless to bright orange and
finally red solutions. The reaction mixture showed multiple
very broaden signals at ca. þ105 ppm in the 31P NMR spec-
tra, indicating complex and fast equilibria at room tempera-
ture (Figure 2, resonance structures A’ and B’). Treatment
of 2-2Li with methyl iodide resulted in the formation of
product 4 (Scheme 2, d = þ58 ppm) as the major product
1
The H NMR spectrum of 2-H features a doublet of dou-
1
blets at 8.10 ppm with coupling constants JHP = 477 Hz and
3JHH = 3 Hz. The size of the H-P coupling constant confirms
the domination of the pentavalent structure of 2-H in solu-
tion. A second doublet of doublets can be found at
2
3
4.53 ppm with coupling constants JHP = 14 Hz and JHH
3 Hz that is assigned to the proton at the a benzyl carbon.
=
Having 2-H in hand, a series of experiments were con-
ducted to reveal the compound’s primary deprotonation site.
Secondary phosphine oxide 2-H was thus treated with one
or two equivalents of BuLi in THF (Scheme 2).
Addition of 1 eq. of base resulted in almost no color change
of the reaction mixture and the 31P NMR spectrum indicated
the formation of a new species with a chemical shift of with small trace of 3 also visible in the 31P NMR spectrum.

PHOSPHORUS, SULFUR, AND SILICON AND THE RELATED ELEMENTS
577
O
Mes
Ph
OLi
Mes
P
P
P
1
2
BuLi
BuLi
I
I
CH3
CH3
eq.
eq.
Ph
Ph
Ph
3 50%
,
Mes
Ph
O
H
2-Li
P
Ph
O
Mes
Ph
OLi
Ph
Mes
Ph
P
2-H
Ph
Li
4 13%
,
2-2Li
Scheme 2. Reaction of 2-H with BuLi and CH3I.
H
D
Mes
O
Mes
Ph
OH
Ph
Mes
O
Mes
O
P
P
C
P
P
Ph
Ph
Ph
B
Ph
Ph
Ph
A
BuLi
O
Mes
Ph
O
Mes
Ph
P
P
Ph
Ph
A'
B'
Figure 2. Possible structures of anions present in reaction mixtures after mono and di-lithiation of 2-H.
H
O
Ph
+
5
6
,11%
Ph
O
P
O
Mes
Ph
OLi
Ph
P
Bu
H
4NOH
2O
Mes
Ph
Ph
Ph
Ph
Li
Ph
Ph
2-2Li
E
Scheme 3. Reaction of 2-2Li with benzaldehyde affording 5 in 11% yield.
The 1H NMR spectrum of 4 contains no signals in the presumably aids in its decomposition with the diphenyl-
region from 4 to 6 ppm which could be assigned to the pro-
ton at the a carbon, but features instead two doublets at
1.97 ppm (2JHP = 16 Hz) and 1.62 ppm (3JHP = 7 Hz) from
the methyl groups at the P and C-centers, respectively.
Encouraged by the results described above, we decided to
treat solution of 2-2Li with benzaldehyde (Scheme 3).
methyl anion being a good leaving group that is then proto-
nated upon aqueous work-up to form 6.
Conclusions
After several hours at room temperature the color of the
reaction mixture changed from bright orange-red to pale
yellow, indicating complete consumption of 2-2Li.
Unfortunately, no triphenylethene could be identified in the
reaction mixture. We hypothesize that intermediate anion E
(Scheme 3) should have been formed, but due to the nega-
tive charge at the P-center, it cannot undergo cyclization to
the oxaphosphetane intermediate that is crucial for alkene
formation. This problem could be overcome by addition of
an aqueous solution of Bu4NOH to the reaction mixture to
provide a pH at which the P–Oꢁ is presumably protonated
to allow the alcohol at the b carbon to act as a nucleophile
at the P-center. Thereby, the oxaphosphetane is formed
which finally collapses to form the desired product 5, as evi-
denced by analytical data that are fully consistent with the
literature values.[20] Unfortunately, compound 5 was only
isolated in 11% yield, with diphenylmethane 6 as the main
A method for the easy preparation of 2-H from literature
known compound was developed. Secondary phosphine
oxide 2-H appears to undergo stepwise deprotonation upon
addition of one or two equivalents of base. First deprotona-
tion is shown to occur at the phosphorus center and the
obtained anion reacts selectively to give the Michaelis –
Becker product 3. The second deprotonation takes place at
the a carbon and the obtained dianion 2-2Li can be success-
fully double methylated. Treatment of 2-2Li with benzalde-
hyde gives rise to the desired triphenylethene, albeit only in
11% isolated yield after basic work up. Overall, 2-H shows
ꢀ
very different reactivity in comparison to its Mes analog
and even though its double lithium salt 2-2Li can be used
for coupling reactions to obtain the alkenes, the current pro-
cedure is not optimal and alternative approaches to increase
the scope of the aldehyde-aldehyde coupling needs to be
byproduct (Scheme 3). The dianionic character of 2-2Li found. These may include future work to decrease the steric

578
N. D’IMPERIO AND A. I. ARKHYPCHUK
demand of the P-substituent further, possibly together with column chromatography (DCM:acetone =18:1, Rf = 0.37) as
an increase of oxygenation level of the P-center.[12,13]
a white solid. Yield 50%, 26 mg. 1H NMR (400 MHz,
CDCl3): d ¼ 7.77–7.72 (m, 2H, Ph), 7.41–7.35 (m, 2H, Ph),
7.34–7.26 (m, 2H, Ph), 7.18–7.14 (m, 2H, Ph), 7.13.10 (m,
Experimental
4
2
2H, Ph), 6.79 (d, JHP = 3.4 Hz, 2H, Mes), 4.42 (d, JHP
=
General experimental procedures. 1H, 13C and 31P NMR
spectra were recorded on a 400 MHz spectrometer (JEOL)
unless noted otherwise. Chemical shifts were referenced to
residual solvent peaks and are given as follows: chemical
shift (d, ppm), multiplicity [s, singlet; br, broad; d, doublet,
t, triplet, q, quartet, m, multiplet, coupling constant (Hz),
integration]. All compounds displayed the expected isotope
distribution patterns. Anhydrous CH2Cl2 was obtained by
distillation from CaH2 under an N2 atmosphere. Anhydrous
THF and Et2O were distilled from sodium under an N2
atmosphere. Compound 1 was prepared according to litera-
ture procedures.[18]
8.4 Hz, 1H, PCH), 2.34 (s, 6H, o-Me-Mes), 2.24 (s, 3H, p-
2
Me-Mes), 1.74 (d, JHP = 12.2 Hz, 3H, PCH3). 31P NMR
(162 MHz, CDCl3): d ¼ 42.7 ppm. 13C NMR (101 MHz,
CDCl3): d ¼ 141.3 (s), 141.3 (s), 137.5 (d, J ¼ 4 Hz), 136.5 (d,
J ¼ 6 Hz), 131.2 (d, J ¼ 11 Hz), 130.1 (s), 130.1 (s), 129.4 (d,
J ¼ 5 Hz), 128.9 (s, J ¼ 2 Hz), 128.2 (d, J ¼ 2 Hz), 127.5 (s),
127.5 (s), 126.9 (d, J ¼ 3 Hz), 125.8 (d, J ¼ 92 Hz), 55.9 (d,
J ¼ 61 Hz), 23.6 (d, J ¼ 3 Hz), 21.0 (d, J ¼ 1 Hz), 20.1 (d,
J ¼ 68 Hz) ppm. HR-MS/ESI (þ): m/z ¼ 349.1887 [M þ H]þ,
calculated [C23H26OP]þ = 349.1721.
(1,1-diphenylethyl)(mesityl)(methyl)phosphane oxide 4
To a cooled (ca ꢁ40 ꢂC) solution of 30 mg (0.089 mmol) of
2-H in 5 mL THF, a hexane solution of n-BuLi (1.6 M,
0.19 mmol, 2.2 eq.) was added dropwise. The deep orange
reaction mixture was stirred for 30 min at -40 ꢂC and then it
was stirred at ambient temperature for additional 2.5 hours.
Excess of MeI was added to the reaction mixture at ambient
temperature and an aliquote of the mixture was analyzed by
31P NMR with an internal benzene-d6 capillary. A new peak
of the product 4 (d ¼ þ 58 ppm) is detected, with the pres-
ence also of some 3 (Ratio 4: 3 obtained by 31P NMR is 1:
0.2). The solvent removed under vacuum to afford crude 4
as a white solid. NMR and HRMS measurements were per-
formed directly on the crude mixture, without any further
purification. Crude yield of 4 estimated 13%. 1H NMR
(400 MHz, CDCl3): d ¼ 7.45–7.13 (m, 10 H, Ph), 6.75 (d,
4JHP = 3.4 Hz, 2H, Mes), 2.24 (s, 6H, o-Me-Mes), 2.19 (s,
Benzhydryl(mesityl)phosphane oxide 2-H
To a cooled (ca 4 ꢂC) solution of 4 g (0.01 mol) of 1 in
50 mL DCM, concentrated solution of HBr in water (45%,
0.03 mol, 1.25 eq., 1.5 mL) was added dropwise. The reaction
mixture was stirred for 30 min at r.t., diluted with DCM
(total volume ca 150 mL) and washed with water until the
pH of the organic phase was found neutral. The organic
phase was dried over MgSO4, filtered and the solvent
removed under vacuum to afford crude 2-H as a white solid.
Washing of this solid with 2 ꢃ 25 mL Et2O and drying under
vacuum gave the final product as a white fine powder. Yield
1
1
45%, 1.5 g. H NMR (400 MHz, CDCl3): d ¼ 8.10 (dd, JHP
=
3
477, JHH = 3 Hz, 1H, PH), 7.38–7.33 (m, 4H, Ph), 7.33–7.22
4
2
(m, 6H, Ph), 6.78 (d, JHP = 4 Hz, 2H, Mes), 4.53 (dd, JHP
3
= 14, JHH = 3 Hz, 1H, PCH), 2.27 (s, 3H, p-Me-Mes), 2.17
2
(s, 6H, o-Me-Mes) ppm. 31P NMR (162 MHz, CDCl3):
d ¼ 25.2 ppm. 13C NMR (101 MHz, CDCl3): d ¼ 142.4 (s),
142.4 (s), 142.2 (d, J ¼ 10 Hz), 137.1 (d, J ¼ 3 Hz), 135.7 (d,
J ¼ 6 Hz), 130.3 (d, J ¼ 11 Hz), 130.0 (s), 129. 9 (s), 129.5 (s),
129.4 (m), 128.9 (d, J ¼ 2 Hz), 128.8 (d, J ¼ 1 Hz), 122.8 (d,
J ¼ 97 Hz), 54.2 (d, J ¼ 61 Hz), 21.3 (d, J ¼ 1 Hz), 21.3 (s),
21.2 (s) ppm. HR-MS/ESI (þ): m/z ¼ 335.1657 [M þ H]þ,
calculated [C22H24OP]þ = 335.1565.
3H, p-Me-Mes), 1.97 (d, JHP = 15.8 Hz, 3H, PCH3), 1.62 (d,
3JHP = 7.2 Hz, 3 H, PCCH3) ppm. 31P NMR (162 MHz,
CDCl3): d ¼ 58.4 ppm. HR-MS/ESI (þ): m/z ¼ 363.2031
[M þ H]þ, calculated [C24H28OP]þ = 363.1878.
Triphenylethene 5
To a cooled (ca ꢁ40 ꢂC) solution of 100 mg (0.3 mmol) of
2-H in 15 mL THF, a hexane solution of n-BuLi (2.5 M,
0.6 mmol, 2 eq.) was added dropwise. The deep orange reac-
tion mixture was stirred for 30 min at ꢁ40 ꢂC and then it
was stirred at ambient temperature for additional 2.5 hours.
34 mg (0.32 mmol, 1.05 eq) of benzaldehyde were added to
the reaction mixture that was stirred at ambient temperature
for additional 12 hours. Excess of an aqueous solution of
TBAOH was added. The aqueous phase was extracted with
EtOAc (ꢃ3). The organic phase was dried over MgSO4, fil-
tered and the solvent removed under vacuum to afford a
crude mixture of 5, 6 (relative ratio 5:6 = 1:2) and other
undefined byproducts. The crude was subjected to a silica
Benzhydryl(mesityl)(methyl)phosphane oxide 3
To a cooled (ca ꢁ40 ꢂC) solution of 50 mg (0.15 mmol) of
2-H in 5 mL THF, a hexane solution of n-BuLi (1.6 M,
0.16 mol, 1.1 eq.) was added dropwise. The pale yellow reac-
tion mixture was stirred for 10 min at ꢁ40 ꢂC and then it
was stirred at ambient temperature for an additional hour.
Excess of MeI was added to the reaction mixture at ambient
temperature and an aliquote of the mixture was analyzed by
31P NMR with an internal benzene-d6 capillary. A new peak
of the product 3 (d = þ 46 ppm) is observed. The reaction
is quenched with water and the aqueous phase was extracted gel column chromatography (10% of Et2O in heptane, Rf =
with DCM (ꢃ3). The organic phase was dried over MgSO4, 0.7) to afford a mixture of 5 and 6 in 2: 1 ratio. Yield of 5
1
filtered and the solvent removed under vacuum to afford 11%, 8 mg. H NMR of 5 and 6 are in accordance with lit-
crude 3 as a white solid. Product 3 was isolated via silica gel erature[20] and commercial available product, respectively.

PHOSPHORUS, SULFUR, AND SILICON AND THE RELATED ELEMENTS
579
[12] Mai, J.; Arkhypchuk, A. I.; Gupta, A. K.; Ott, S. Reductive
Coupling of Two Aldehydes to Unsymmetrical E-Alkenes via
Phosphaalkene and Phosphinate Intermediates. Chem.
Commun. (Camb.) 2018, 54, 7163–7166.
Acknowledgments
Financial support for this work from the Swedish Research Council is
gratefully acknowledged.
[13] Arkhypchuk, A. I.; D’Imperio, N.; Ott, S. One-Pot
Intermolecular Reductive Cross-Coupling of Deactivated
Aldehydes to Unsymmetrically 1,2-Disubstituted Alkenes. Org.
Lett. 2018, 20, 5086–5089.
[14] McMurry, J. E. Carbonyl-Coupling Reactions Using Low-Valent
Titanium. Chem. Rev. 1989, 89, 1513–1524.
[15] Christiansen, A.; Li, C.; Garland, M.; Selent, D.; Ludwig, R.;
References
[1] Marder, S. R.; Beratan, D. N.; Cheng, L. T. Approaches for
Optimizing the First Electronic Hyperpolarizability of
Conjugated Organic Molecules. Science 1991, 252, 103–106.
[2] De Filippis, B.; Ammazzalorso, A.; Fantacuzzi, M.; Giampietro,
L.; Maccallini, C.; Amoroso, R. Anticancer Activity of Stilbene-
based Derivatives. Chem. Med. Chem. 2017, 12, 558–570.
[3] Manfredi, N.; Cecconi, B.; Abbotto, A. Multi-Branched Multi-
Anchoring Metal-Free Dyes for Dye-Sensitized Solar Cells. Eur.
J. Org. Chem. 2014, 2014, 7069–7086.
[4] Larder, D. F.; Kluge, F. F. Alexander Mikhailovich Butlerov’s
Theory of Chemical Structure. J. Chem. Educ. 1971, 48, 287–289.
[5] Bissember, A. C.; Levina, A.; Fu, G. C. A Mild, Palladium-
Catalyzed Method for the Dehydrohalogenation of Alkyl
Bromides: Synthetic and Mechanistic Studies. J. Am. Chem. Soc.
2012, 134, 14232–14237.
[6] Friesen, J. B.; Schretzman, R. Dehydration of 2-Methyl-1-
Cyclohexanol: New Findings from a Popular Undergraduate
Laboratory Experiment. J. Chem. Educ. 2011, 88, 1141–1147.
[7] Enders, D.; Knopp, M.; Schiffers, R. Asymmetric [3.3]-
Sigmatropic Rearrangements in Organic Synthesis. Tetrahedron:
Asymmetry 1996, 7, 1847–1882.
[8] Staden, L. F. v.; Gravestock, D.; Ager, D. New Developments in
the Peterson Olefination Reaction. Chem. Soc. Rev. 2002, 31,
195–200.
€
Spannenberg, A.; Baumann, W.; Franke, R.; Borner, A. On the
Tautomerism of Secondary Phosphane Oxides. Eur. J. Org.
Chem. 2010, 2010, 2733–2741.
[16] Kurscheid, B.; Wiebe, W.; Neumann, B.; Stammler, H. G.;
Hoge, B. Investigations of the Tautomeric Equilibria between
Phosphane Oxides and Their Corresponding Phosphinous
Acids Bearing Electron-Withdrawing Perfluoroaryl Groups. Eur.
J. Inorg. Chem. 2011, 2011, 5523–5529.
[17] Wang, S.; Samedov, K.; Serin, S. C.; Gates, D. P. PhP ¼ CPh 2
and Related Phosphaalkenes: A Solution Equilibrium Between a
Phosphaalkene and a 1,2-Diphosphetane. Eur. J. Inorg. Chem.
2016, 2016, 4144–4151.
[18] Van Der Knaap, T. A.; Klebach, T. C.; Visser, F.; Bickelhaupt,
F.; Ros, P.; Baerends, E. J.; Stam, C. H.; Konijn, M. Synthesis
and Structure of Aryl-Substituted Phospha-Alkenes. M.
Tetrahedron 1984, 40, 765–776.
[19] Van der Knap, T. A.; Bickelhaupt, F. A Nucleophilic Reaction
of
a
Phosphaalkene:
The
Methylation
of
Mesityldiphenylmethylenephosphine. Tetrahedron Lett. 1982,
23, 2037–2040.
[9] Wittig, G. From Diyls to Ylides to my Idyll. Science 1980, 210,
600–604.
[20] Bi Jie, L.; Yi Zhou, L.; Xing Yu, L.; Jia, L.; Bing Tao, G.; Zhang
Jie, S. Cross-Coupling of Aryl/Alkenyl Pivalates with
Organozinc Reagents through Nickel-Catalyzed C-O Bond
Activation under Mild Reaction Conditions. Angew. Chem. Int.
Ed. Engl. 2008, 47, 10124–10127.
[10] Boutagy, J.; Thomas, R. Olefin Synthesis with Organic
Phosphonate Carbanions. Chem. Rev. 1974, 74, 87–99.
[11] Esfandiarfard, K.; Mai, J.; Ott, S. Unsymmetrical E-Alkenes
from the Stereoselective Reductive Coupling of Two Aldehydes.
J. Am. Chem. Soc. 2017, 139, 2940–2943.