Non-metal-templated approaches to bis(borane) derivatives of macrocyclic dibridgehead diphosphines via alkene metathesis

Article abstract of DOI:10.3762/bjoc.14.211

Two routes to the title compounds are evaluated. First, a ca. 0.01 M CH₂Cl₂ solution of H₃B·P((CH₂)₆CH=CH₂)₃ (1·BH₃) is treated with 5 mol % of Grubbs' first generation catalyst (0 °C to reflux), followed by H₂ (5 bar) and Wilkinson's catalyst (55 °C). Column chromatography affords H₃B·P(n-C₈H₁₇)₃ (1%), H₃B·P((CH₂)₁₃CH₂)(n-C₈H₁₇) (8%; see text for tie bars that indicate additional phosphorus–carbon linkages, which are coded in the abstract with italics), H₃B·P((CH₂)₁₃CH₂)((CH₂)₁₄)P((CH₂)₁₃CH₂)·BH₃ (6·2BH₃, 10%), in,out-H₃B·P((CH₂)₁₄)₃P·BH₃ (in,out-2·2BH₃, 4%) and the stereoisomer (in,in/out,out)-2·2BH₃ (2%). Four of these structures are verified by independent syntheses. Second, 1,14-tetradecanedioic acid is converted (reduction, bromination, Arbuzov reaction, LiAlH₄) to H₂P((CH₂)₁₄)PH₂ (10; 76% overall yield). The reaction with H₃B·SMe₂ gives 10·2BH₃, which is treated with n-BuLi (4.4 equiv) and Br(CH₂)₆CH=CH₂ (4.0 equiv) to afford the tetraalkenyl precursor (H₂C=CH(CH₂)₆)₂(H₃B)P((CH₂)₁₄)P(BH₃)((CH₂)₆CH=CH₂)₂ (11·2BH₃; 18%). Alternative approaches to 11·2BH₃ (e.g., via 11) were unsuccessful. An analogous metathesis/hydrogenation/chromatography sequence with 11·2BH₃ (0.0010 M in CH₂Cl₂) gives 6·2BH₃ (5%), in,out-2·2BH₃ (6%), and (in,in/out,out)-2·2BH₃ (7%). Despite the doubled yield of 2·2BH₃, the longer synthesis of 11·2BH₃ vs 1·BH₃ renders the two routes a toss-up; neither compares favorably with precious metal templated syntheses.

Full text of DOI:10.3762/bjoc.14.211

Non-metal-templated approaches to bis(borane) derivatives of
macrocyclic dibridgehead diphosphines via alkene metathesis
Tobias Fiedler1,2, Michał Barbasiewicz2,3, Michael Stollenz1,4 and John A. Gladysz*1,2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2018, 14, 2354–2365.
1Department of Chemistry, Texas A&M University, PO Box 30012,
doi:10.3762/bjoc.14.211
College Station, Texas 77842-3012, USA, 2Institut für Organische
Chemie and Interdisciplinary Center for Molecular Materials,
Friedrich-Alexander-Universität Erlangen-Nürnberg, Henkestraße 42,
Received: 14 June 2018
Accepted: 24 August 2018
Published: 07 September 2018
9
1054 Erlangen, Germany, 3Present address: Faculty of Chemistry,
University of Warsaw, Pasteura 1, 02-093 Warszawa, Poland and
Present address: Department of Chemistry and Biochemistry,
4
This article is part of the thematic issue "Progress in metathesis chemistry
III".
Kennesaw State University, 370 Paulding Building NW, MD#1203,
Kennesaw, Georgia 30144, USA
Guest Editor: K. Grela
Email:
John A. Gladysz* - gladysz@mail.chem.tamu.edu
© 2018 Fiedler et al.; licensee Beilstein-Institut.
License and terms: see end of document.
*
Corresponding author
Keywords:
alkene metathesis; crystal structures; homeomorphic isomerization;
hydrogenation; in/out isomers; metathesis; phosphine boranes
Abstract
Two routes to the title compounds are evaluated. First, a ca. 0.01 M CH2Cl2 solution of H3B·P((CH2)6CH=CH2)3 (1·BH3) is treated
with 5 mol % of Grubbs' first generation catalyst (0 °C to reflux), followed by H2 (5 bar) and Wilkinson's catalyst (55 °C). Column
chromatography affords H3B·P(n-C8H17)3 (1%), H3B·P((CH2)13CH2)(n-C8H17) (8%; see text for tie bars that indicate additional
phosphorus–carbon linkages, which are coded in the abstract with italics), H3B·P((CH2)13CH2)((CH2)14)P((CH2)13CH2)·BH3
(
6·2BH3, 10%), in,out-H3B·P((CH2)14)3P·BH3 (in,out-2·2BH3, 4%) and the stereoisomer (in,in/out,out)-2·2BH3 (2%). Four of these
structures are verified by independent syntheses. Second, 1,14-tetradecanedioic acid is converted (reduction, bromination, Arbuzov
reaction, LiAlH4) to H2P((CH2)14)PH2 (10; 76% overall yield). The reaction with H3B·SMe2 gives 10·2BH3, which
is treated with n-BuLi (4.4 equiv) and Br(CH2)6CH=CH2 (4.0 equiv) to afford the tetraalkenyl precursor
(
H2C=CH(CH2)6)2(H3B)P((CH2)14)P(BH3)((CH2)6CH=CH2)2 (11·2BH3; 18%). Alternative approaches to 11·2BH3 (e.g., via 11)
were unsuccessful. An analogous metathesis/hydrogenation/chromatography sequence with 11·2BH3 (0.0010 M in CH2Cl2) gives
6
1
·2BH3 (5%), in,out-2·2BH3 (6%), and (in,in/out,out)-2·2BH3 (7%). Despite the doubled yield of 2·2BH3, the longer synthesis of
1·2BH3 vs 1·BH3 renders the two routes a toss-up; neither compares favorably with precious metal templated syntheses.
Introduction
We have found that a variety of metal complexes with trans- metatheses to give, after hydrogenations, metal complexes of
phosphine ligands of the formula P((CH2)mCH=CH2)3 in,in isomers of macrocyclic dibridgehead diphosphines [1-13].
(
1; m = 4–14) undergo threefold interligand ring closing alkene Representative examples with square planar complexes are
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shown in Scheme 1. Analogous sequences with trigonal bipyra- but the routes are stoichiometric in precious metals. Although
midal substrates proceed in somewhat higher overall yields, as the metals can be recovered as species such as K2Pt(CN)4 or
analyzed elsewhere [1-4]. Setaka has developed a similar chem- RhCl(PMe3)3, we have nonetheless sought to develop more
istry in which the phosphorus atoms are replaced by silicon and economical protocols.
the metal fragment by p-phenylene (p-C6H4) or related aromat-
ic moieties [14-19]. These types of compounds are viewed as The analogous Fe(CO)3 adducts are easily prepared [1-4], but in
promising candidates for molecular gyroscopes [14-21].
efforts to date it has not been possible to efficiently remove the
dibridgehead diphosphine ligands from the low cost iron frag-
We subsequently developed an interest in the free dibridgehead ment. Oxidations that lead to the corresponding dibridgehead
diphosphine ligands P((CH2)n)3P (n = 14, 2; 18, 3), prompted in diphosphine dioxides (O=)P((CH2)n)3P(=O) have exhibited
part by the unexpected discovery of the facile demetalations promise, but purification has been problematic [24]. Indeed,
shown in Scheme 1 [5,6,10,22]. Such compounds were previ- phosphine oxides are everyday precursors to phosphines, so we
ously known only for much smaller ring sizes (n < 4) [23]. have considered various non-metal-templated routes to 2·2(=O),
These reactions require excesses of certain nucleophiles, and 3·2(=O), and related species. However, as described in the
the mechanisms remain under study. The yields are quite good, discussion section, the yields have not been competitive [25].
Scheme 1: Syntheses of gyroscope like platinum and rhodium complexes and dibridgehead diphosphines derived therefrom.
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Another preliminary point concerns the ability of macrocyclic
dibridgehead diphosphorus compounds to exhibit in/out
isomerism [26]. As shown in Scheme 1, there are three limiting
configurations for 2 and 3: in,in, out,out, and in,out (identical to
out,in). The first two, as well as the degenerate in,out pair, can
rapidly interconvert by a process termed homeomorphic isomer-
ization [26,27], which is akin to turning the molecules inside
out. Readers are referred to earlier publications in this series for
additional details [22,25,28-30]. Interconversions between the
in,in/out,out and in,out/out,in manifolds require phosphorus
inversion and temperatures considerably in excess of 100 °C.
In this paper, we describe two non-metal-templated approaches
to 2 that are based upon metatheses of phosphine boranes of
alkene containing phosphines. The first involves the monophos-
phorus precursor H3B·P((CH2)6CH=CH2)3 (1·BH3) [31], and
the second a diphosphorus precursor in which one of the methy-
lene chains linking the two phosphorus atoms has already been
installed. The advantages and limitations of each are analyzed
in detail. Some of the results (Scheme 2) have appeared in the
supporting information of a preliminary communication [28],
and others in a dissertation [32].
Results
1. Monophosphorus precursors
As reported earlier [31], the alkene containing phosphine
P((CH2)6CH=CH2)3 (1) can be prepared in 87% yield from the
reaction of PCl3 and MgBr(CH2)6CH=CH2. Following the addi-
tion of H3B·SMe2, the phosphine borane 1·BH3 can be isolated
in 65–85% yields [31], as shown in Scheme 2. It is critical to
avoid an excess of H3B·SMe2, as this brings the C=C units into
play. In fact, when substoichiometric amounts of H3B·SMe2 are
added to THF solutions of purified 1·BH3, gels immediately
form.
A ca. 0.01 M CH2Cl2 solution of 1·BH3 and a ca. 0.002 M
CH2Cl2 solution of Grubbs' first generation catalyst (3 mol %)
were combined at 0 °C. The mixture was warmed to room tem-
perature, and a second charge of Grubbs' catalyst added
(
2 mol %). The sample was refluxed, and then filtered through
silica gel. The filtrate was concentrated and treated with H2
5 bar) and Wilkinson's catalyst (55 °C). The mixture was taken
(
to dryness and the residue tediously chromatographed on a
silica gel column. Numerous fractions were collected and
analyzed by TLC. The mass recovery from the column was
3
3% of theory (for complete metathesis).
More than ten mobile products could be discerned, but only five
could be isolated in pure form and ultimately identified. These
are described in order of elution. Each was analyzed by NMR
Scheme 2: Synthesis and alkene metathesis of the monophosphorus
precursor 1·BH3.
(
1H, 31P{1H}, 13C{1H}; always CDCl3) and IR spectroscopy,
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Beilstein J. Org. Chem. 2018, 14, 2354–2365.
mass spectrometry, and microanalysis, as summarized in the ex- as the monocyclic intramolecular metathesis product
perimental section. The 13C{1H} NMR spectra proved to be
(5·BH3; see Scheme 2). The
most diagnostic of structure, and were analyzed in detail. The yield was 8%.
3
1P{1H} NMR spectra were all very similar (broad apparent
doublets due to phosphorus boron coupling).
The third product was also a colorless oil. The 13C{1H} NMR
spectrum exhibited seven signals, three of which were phos-
First, traces of a colorless oil were obtained. The 1H NMR spec- phorus coupled doublets (second spectrum from top, Figure 1).
trum showed a characteristic triplet at 0.83 ppm consistent with Analogous coupling patterns are found with the free dibridge-
a terminal methyl group. The 13C{1H} NMR spectrum exhib- head diphosphines 2 and 3 in Scheme 1. No NMR signals diag-
ited eight signals, two of which were phosphorus coupled nostic of methyl groups were present, and further analysis is
doublets. One of the singlets (14.0 ppm) was typical of a termi- presented along with that for an isomer below.
nal methyl group. Based upon these data, and the integration of
the 1H NMR spectrum, the oil was assigned as the hydro- A white powder was obtained next. The 13C{1H} NMR spec-
genated phosphine borane H3B·P(n-C8H17)3 (4·BH3), a known trum exhibited fourteen signals, half of which were approxi-
compound [33]. The yield was only 1%.
mately twice as intense as the others. Two signals of each set
exhibited phosphorus coupling. The overall pattern was quite
Next, another colorless oil eluted. The 1H NMR and similar to those shown by metal complexes with cis or
3C{1H} NMR spectra again exhibited signals characteristic of trans coordinating diphosphine ligands of the formula
a methyl group (0.86 ppm, t; 14.0 ppm, s). Integration of the (6) [6,7,12,13,35].
H NMR spectrum established a 14:1 area ratio for the methy- This suggested the diphosphine diborane structure 6·2BH3 (see
1
1
lene (1.62–1.19 ppm) and methyl signals. The 13C{1H} NMR Scheme 2), which is derived from one metathesis involving
spectrum featured one set of seven signals and another set of alkenyl moieties on different phosphorus atoms, and two me-
eight with an intensity ratio of approximately 2:1. The less tatheses of alkenyl moieties on identical phosphorus atoms. The
intense set resembled the signals arising from the n-octyl groups yield was 10%. The structure has been confirmed by an
in 4·BH3. The more intense set was very similar to the independent synthesis (detachment of the diphosphine from a
signals arising from the cyclic
substruc- platinum complex followed by borane addition) and a crystal
tures of 6·2BH3 (described below) and a phosphine borane structure [6].
reported earlier [34]. The mass
spectrum exhibited an intense ion at m/z 340 (5+, 93%), Finally, another white powder was obtained. As with the
and no ions of higher mass. Hence, the oil was assigned previous oil isolated above, the 13C{1H} NMR spectrum exhib-
Figure 1: The 13C{1H} NMR spectra (CDCl3, 100 MHz) of in,out-2·2BH3, (in,in/out,out)-2·2BH3, 6·2BH3, and the crude reaction mixture after
hydrogenation from Scheme 5 (top); doublets are marked with an asterisk.
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Beilstein J. Org. Chem. 2018, 14, 2354–2365.
ited seven signals, three of which were phosphorus coupled
doublets (third spectrum from top, Figure 1). Both spectra were
consistent with dibridgehead diphosphine diboranes
H3B·P((CH2)14)3P·BH3 (2·2BH3) derived from threefold inter-
molecular metatheses of 1·BH3. Based upon independent syn-
theses from the dibridgehead diphosphines 2 obtained in
Scheme 1 [6], they were assigned as in,out-2·2BH3 (4%) and
the stereoisomer (in,in/out,out)-2·2BH3 (2%), as shown in
Scheme 2. The depiction of the latter as an out,out (vs in,in)
isomer in Scheme 2 is arbitrary, but represents the form found
in a confirming crystal structure [6].
Parallel reactions were conducted with Grubbs' second genera-
tion catalyst and the nitro-Grela catalyst [36]. However, the
combined yields of 2 diminished.
2. Diphosphorus precursors
Since the yields of the cage like diphosphine diboranes 2·2BH3
in Scheme 2 were – as expected – very low, alternative strate-
gies were considered. The poor mass balance was attributed, at
least in part, to the formation of oligomeric products that were
retained on the column. Improvements might be expected from
precursors in which one of the methylene chains tethering the
two phosphorus atoms was pre-formed. Thus, we set out to
prepare a tetraalkenyl metathesis precursor as shown in
Scheme 3.
The first step, a previously reported reduction of commercial
1
,14-tetradecanedioic acid to 1,14-tetradecanediol (7) [37], was
followed by an Appel reaction to give 1,14-dibromotetradecane
8) [38-43]. An Arbuzov reaction then afforded the diphospho-
(
nate (EtO)2(O=)P((CH2)14)P(=O)(OEt)2 (9) [44]. Subsequent
reduction with LiAlH4 gave the diprimary diphosphine
H2P((CH2)14)PH2 (10) in 76% yield from 7 as a foul smelling
white powder.
It has been shown that borane adducts of primary phosphines
can be doubly deprotonated, and that the resulting phosphorus
dianions can be bis(alkylated) [45-47]. Thus, the diphosphine
Scheme 3: Synthesis of the diphosphorus precursor 11·2BH3.
1
0 and H3B·SMe2 were reacted to give the diphosphine dibo-
rane H2(H3B)P((CH2)14)P(BH3)H2 (10·2BH3) as a white solid (4.1 equiv) and then Br(CH2)6CH=CH2 (4.0 equiv). Work-
in 87% yield. A subsequent reaction with n-BuLi (4.4 equiv) up gave the target compound (H2C=CH(CH2)6)2P((CH2)14)P-
and Br(CH2)6CH=CH2 (4.0 equiv) gave the tetraalkenyl target ((CH2)6CH=CH2)2 (11) in 72% yield. However, all attempts to
(
(
H2C=CH(CH2)6)2(H3B)P((CH2)14)P(BH3)((CH2)6CH=CH2)2 convert 11 to 11·2BH3 gave only traces of the latter. Mainly
11·2BH3), but in only 18% yield.
insoluble material formed, which was presumed to be
oligomeric and possibly derived from B–H additions to the
Accordingly, two alternative routes to 11·2BH3 were consid- alkenyl groups.
ered. The initial step for the first is depicted in Scheme 4. Pri-
mary phosphines can be doubly deprotonated, analogously to In the second approach, 10 was first converted to the
borane adducts, and the phosphorus dianions subsequently tetrachloride Cl2P((CH2)14)PCl2 (12) in 94% yield using
bis(alkylated) [34,48]. Thus, 10 was treated with n-BuLi triphosgene, a standard reagent for the chlorination of phos-
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Beilstein J. Org. Chem. 2018, 14, 2354–2365.
Scheme 4: Truncated approaches to the diphosphorus precursor
1·2BH3 from 10.
1
phorus–hydrogen bonds [49]. Since a direct reaction with an
excess of the Grignard reagent BrMg(CH2)6CH=CH2 would
give 11, a dead end, initial conversion to the bis(borane) adduct
1
2·2BH3 was envisioned. However, reactions of 12 and
H3B·SMe2 (2.1 equiv) afforded only insoluble material.
Thus, despite the low yield of the final step in Scheme 3, rea-
sonable quantities of the diphosphine diborane 11·2BH3 could
be stockpiled. As shown in Scheme 5, 11·2BH3 was subjected
Scheme 5: Alkene metathesis of the diphosphorus precursor 11·2BH3.
to a metathesis/hydrogenation/column chromatography se- yields were affected by the challenging separation. In particular,
quence similar to that for 1·BH3 in Scheme 2. However, a in,out-2·2BH3 and 6·2BH3 eluted very closely, rendering some
tenfold higher dilution was used in the metathesis step mixed fractions unavoidable and lowering the amounts of pure
(
0.0010 M as compared to 0.010 M).
products.
Figure 1 shows a 13C{1H} NMR spectrum of the crude product Compared to the metathesis/hydrogenation sequence for 1·BH3
after hydrogenation stacked above spectra of the three products (Scheme 2) the yields of in,out-2·2BH3 and (in,in/out,out)-
that could be isolated after the rather tedious column chromato- 2·2BH3 (Scheme 5) are higher but still poor. Taking into
graphy: the dibridgehead diphosphine diborane in,out-2·2BH3, account the overall yields (three steps from PCl3 and
its constitutional isomer 6·2BH3, and its stereoisomer (in,in/ BrMg(CH2)6CH=CH2 in the first synthesis vs seven steps from
out,out)-2·2BH3. It can be inferred from the top spectrum that 1,14-tetradecanedioic acid in the second), the latter route does
the three products were the major components and moreover not offer any advantage, even if one were to improve the
present in approximately equal amounts. However, the isolated conversion of 10·2BH3 to 11·2BH3.
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Beilstein J. Org. Chem. 2018, 14, 2354–2365.
Discussion
high dilution provides less of an advantage in Scheme 2, as one
As contrasted in Scheme 6, Scheme 2 and Scheme 5 present wants to favor intermolecular over intramolecular metatheses in
two conceptually related routes to the isomeric title compound the first step. In Scheme 5, one wants to avoid intermolecular
2
·2BH3. In the first, two trialkenylphosphine boranes metatheses at all stages.
(
1·BH3 = I) must undergo metathesis. The first productive step
is intermolecular, giving a diphosphorus compound with a At present, we have no rationale for the in,out vs (in,in/out,out)
P(CH2)6CH=CH(CH2)6P tether II that is positioned for subse- isomer ratios for 2·2BH3. However, it is easy to map the se-
quent intramolecular ring closing steps. Those involving alkenyl quence leading to each, as shown in Scheme 7. When there is
groups from different phosphorus atoms are productive (leading only one tether between the two phosphorus atoms, the phos-
to 2·2BH3 via hydrogenation of IIIa), and those involving phorus–boron bonds can be arrayed in an anti fashion, as
groups from the same phosphorus atoms are non-productive depicted in VII. When subsequent metatheses join alkenyl
(
leading to 6·2BH3 via hydrogenation of IVa). In the second, groups in the syn positions on each phosphorus atom (front to
the starting material has a preformed P(CH2)14P tether front and rear to rear), (in,in/out,out)-2·2BH3 must result (as
11·2BH3 = V), and the four alkenyl groups have reactivity drawn in Scheme 7, the out,out isomer would be the kinetic
(
options (→ IIIb or IVb) analogous to those of intermediate II
with the P(CH2)6CH=CH(CH2)6P tether. Importantly, all of
these steps are presumed to be largely under kinetic control,
consistent with experience with the types of metatheses in
Scheme 1 [1-13,34].
Although the second route intuitively seems more favorable,
after the initial intermolecular metathesis of 1·BH3 (I), both
require an equivalent series of steps to reach (after hydrogena-
tion) 2·2BH3. One reason 1·BH3 is an inferior substrate is that
following the initial generation of a P(CH2)6CH=Ru species,
two P(CH2)6CH=CH2 moieties remain available for non-
productive intramolecular ring closing metathesis (giving VI).
In contrast, with the analogous intermediate derived from
1
1·2BH3 (V), there is only one P(CH2)6CH=CH2 moiety that
Scheme 7: Steps that set the in,in/out,out vs in,out stereochemistry of
·2BH3 in Scheme 2 and Scheme 5.
2
can give non-productive chemistry. It is also worth noting that
Scheme 6: Schematic comparison of the key alkene metathesis steps in Scheme 2 and Scheme 5.
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Beilstein J. Org. Chem. 2018, 14, 2354–2365.
product). When the first metathesis does not join the syn posi- However, as shown in Scheme 8, it has proved possible to
tions, as in VIII (front to rear), one phosphorus–boron bond synthesize the diphosphine dioxides 14, in which the two phos-
must subsequently be rotated by 180° to create a syn orienta- phorus atoms are tethered by a methylene chain, in two
tion for the second metathesis.
steps in 66–68% overall yields from diethyl phosphonate
(O=)PH(OEt)2), Grignard reagents BrMg(CH2)mCH=CH2,
(
Of course, if the first metathesis step does not require a syn rela- base (NaH), and appropriate α,ω-dibromides Br(CH2)nBr [25].
tionship (per VIII), the same possibility can be entertained for Following metathesis and hydrogenation, these afford dibridge-
the second (see IX). This would lead to an isomeric bicyclic head diphosphine oxides 15 and 16 in 14–19% yields. This is
compound with "crossed chains". We have sought to access slightly better than the combined yield of in,out- and (in,in/
such species by conducting metatheses of substrates of the types out,out)-2·2BH3 in Scheme 5, although the data are not strictly
in Scheme 1 that give thirty-three membered macrocycles comparable as the ring sizes differ. It has not yet proved
(
n = 30) [7]. However, none have so far been detected. Other possible to efficiently separate the in/out isomers of 15 and 16.
types of crossed chain in/out isomer systems have in fact been However, byproducts derived from metatheses of alkenyl
realized [25,30].
groups on the same phosphorus atom – such as 17 (comparable
to 6·2BH3) – appear to form in much smaller amounts.
As communicated earlier [28] and will be described more fully
in a later paper, both isomers of 2·2BH3 are easily deprotected To our knowledge, only one macrocyclic dibridgehead diphos-
to give the respective isomers of the dibridgehead diphosphine phine diborane has been previously reported, (in,in/out,out)-
2
in high yields. Since phosphine oxides are also easily 18·2BH3 in Scheme 9 [50,51]. This features triarylphosphorus
converted to phosphines, one could consider parallel bridgeheads and p-phenylene containing tethers that are long
approaches to 2 via metatheses of the phosphine oxide enough to allow rapid homeomorphic isomerization. The pre-
(
(
(
O=)P((CH2)6CH=CH2)3 (1(=O)) or diphosphine dioxide cursor 18·2(=O) was prepared by a threefold Williamson ether
H2C=CH(CH2)6)2(O=)P((CH2)14)P(=O)((CH2)6CH=CH2)2 synthesis in surprisingly high yields (61% in,in/out,out and
11·2(=O)). Given the poor results with 1·BH3 in Scheme 2, no in,out combined) [50,51], likely aided by the geminal dialkyl
attempt has been made to explore similar reactions with 1(=O). effect associated with the quaternary centers [52].
Scheme 8: Another non-metal-templated approach to dibridgehead diphosphorus compounds.
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Scheme 9: Previously synthesized dibridgehead diphosphine diboranes.
Finally, it should be noted that a number of alkene containing compounds that employ alkene metathesis. The new ap-
phosphine boranes have been employed in metathesis reactions proaches (Scheme 2; Scheme 3 and Scheme 5) lack metal tem-
[
53,54]. In particular, the tetraalkenyl diphosphine diborane plates, which differentiates them from the routes presented in
1
1
9·2BH3 in Scheme 10 represents a downsized version of Scheme 1. However, neither is competitive with Scheme 1,
1·2BH3. A species analogous to 6·2BH3, 20·2BH3, is obtained despite eliminating the requirement for stoichiometric amounts
in much higher yield than any of the products in Scheme 5 [53]. of precious metals. Furthermore, preassembling a diphosphine
Hence, selectivities can strongly depend upon the lengths of the diborane substrate per Scheme 3 and Scheme 5 is not competi-
methylene segments in the precursor.
tive with the "shotgun" approach in Scheme 2, and both routes
require comparably demanding preparative column chromato-
graphy. Hence, the most promising direction for future research
would seem to be templated syntheses via non-precious metals
[
55]. This remains an area of ongoing investigation in our labo-
ratory and further results will be reported in due course.
Experimental
General. Reactions (except hydrogenations) were conducted
under inert atmospheres using standard Schlenk techniques. All
chromatography was carried out under aerobic conditions. Ad-
ditional data are supplied in Supporting Information File 1.
Scheme 10: Alkene metathesis of the tetraalkenyldiphosphine dibo-
rane 19·2BH3.
Conclusion
Metathesis/hydrogenation of H3B·P((CH2)6CH=CH2)3
In conclusion, this work constitutes a further installment in the (1·BH3; Scheme 2 [32]). A Schlenk flask was charged with
evolution of synthetic strategies for dibridgehead diphosphorus 1·BH3 (1.177 g, 3.110 mmol) [31] and CH2Cl2 (320 mL; the re-
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Beilstein J. Org. Chem. 2018, 14, 2354–2365.
sulting solution was 0.0097 M in 1·BH3) and cooled to 0 °C. A (EI) [56]: 340 ([M − BH3]+, 93%), 228 ([M − BH3 − C8H17 +
solution of Grubbs' first generation catalyst (0.077 g, 1]+, 100%).
0
.094 mmol, 3 mol %) in CH2Cl2 (50 mL) was added dropwise
via syringe with stirring over 1 h. The cooling bath was re- in,out-H3B·P((CH2)14)3P·BH3 (in,out-2·2BH3; 039 g,
moved. After 2 h, additional Grubbs' first generation catalyst 0.057 mmol, 4%), colorless oil. Anal. calcd for C42H90B2P2
was added as a solid (0.051 g, 0.062 mmol, 2 mol %). The flask (678.73): C, 74.32; H, 13.37; found: C, 73.86; H, 13.49;
was fitted with a condenser and the mixture was refluxed 1H NMR (500 MHz, CDCl3) δ 1.56–1.51 (m, 12H, PCH2),
overnight, cooled to room temperature, and passed through a 1.49–1.42 (m, 12H, CH2), 1.39–1.33 (m, 12H, CH2), 1.31–1.21
SiO2 pad (3 cm), which was rinsed with CH2Cl2. The eluate (m, 48H, CH2), 0.45 and 0.27 (br apparent d, 6H, BH3);
was concentrated to ca. 20 mL by rotary evaporation, and trans- 13C{1H} NMR (126 MHz, CDCl3) δ 30.5 (d, JCP = 11.3 Hz,
ferred to a Fischer–Porter bottle. Wilkinson's catalyst (0.086 g, CH2), 28.35 (s, CH2), 28.28 (s, CH2), 28.2 (s, CH2), 28.1 (s,
0
.093 mmol, 3 mol %) was added, and the bottle was partially CH2), 23.0 (d, JCP = 34.3 Hz, CH2), 22.2 (d, JCP = 1.9 Hz,
evacuated and charged with hydrogen (5 bar). The sample was CH2); 31P{1H} NMR (202 MHz, CDCl3) δ 15.6 and 15.4 (br
kept at 55 ºC for 60 h. The solvent was removed and the residue apparent d); IR (oil film): 2926 (s), 2853 (m), 2366 (w), 1459
was placed at the top of a chromatography column (SiO2, (w), 1413 (w), 1135 (w), 1061 (m), 803 (w), 722 (w) cm−1; MS
3
1
.5 × 36 cm), which was eluted with hexanes/CH2Cl2 (3:1 to (MALDI+, THAP) [56]: 651.6 ([M – 2BH3 + 1]+, 100%).
:3 v/v) and then CH2Cl2. Fractions were assayed by TLC,
combined where appropriate, and slowly evaporated to dryness
(6·2BH3;
in a fume hood. Some fractions (0.091 g total out of the recov- 0.101 g, 0.149 mmol, 10%), white solid, mp 96 °C (capillary).
ered mass of 0.344 g) consisted of unidentified and/or impure Anal. calcd for C42H90B2P2 (678.73): C, 74.32; H, 13.37;
products, or oligomers and polymers. Products that could be found: C, 73.92; H, 13.47. The identity of this compound,
characterized are as follows (in order of elution).
which has been independently synthesized, has been confirmed
crystallographically [6]. 1H NMR (500 MHz, CDCl3)
H3B·P(n-C8H17)3 (4·BH3 [33]; 0.007 g, 0.018 mmol, 1%), δ 1.65–1.14 (br m, 84H, CH2), 0.49 and 0.26 (br apparent d,
colorless oil. Anal. calcd for C24H54BP (384.47): C, 74.98; H, 6H, BH3); 13C{1H} NMR (126 MHz, CDCl3) δ 31.3 (d,
1
4.16; found: C, 74.93; H, 14.02; 1H NMR (400 MHz, CDCl3) JCP = 12.6 Hz, CH2), 29.54 (s, CH2), 29.53 (s, CH2), 29.4 (s,
δ 1.53–1.37 (m, 12H, CH2), 1.33–1.30 (m, 6H, CH2), 1.26–1.23 CH2), 29.1 (s, CH2), 29.0 (d, JCP = 11.1 Hz, 2CH2), 26.8 (s,
(
(
m, 24H, CH2), 0.83 (t, 3JHH = 6.9 Hz, 9H, CH3), 0.47 and 0.19 2CH2), 26.6 (s, 2CH2), 26.5 (s, 2CH2), 26.1 (s, 2CH2), 23.8 (d,
br apparent d, 3H, BH3); 13C{1H} NMR (101 MHz, CDCl3) JCP = 35.3 Hz, CH2), 22.6 (d, JCP = 1.0 Hz, CH2), 22.3 (d,
δ 31.7 (s, CH2), 31.1 (d, JCP = 12.0 Hz, CH2), 29.0 (s, CH2), JCP = 33.5 Hz, 2CH2), 21.2 (d, JCP = 3.3 Hz, 2CH2);
2
2
8.9 (s, CH2), 22.9 (d, JCP = 34.3 Hz, CH2), 22.50 (s, CH2), 31P {1H} NMR (202 MHz, CDCl3) δ 15.6 and 15.2 (br apparent
2.48 (s, CH2), 14.0 (s, CH3); 31P{1H} NMR (162 MHz, d); IR (powder film): 2922 (s), 2853 (m), 2366 (m), 1459 (m),
CDCl3) δ 15.9 and 15.5 (br apparent d); IR (oil film): 2926 (s), 1417 (w), 1135 (w), 1061 (m), 791 (w), 722 (m) cm−1; MS (EI)
2
1
856 (m), 2366 (m), 1463 (m), 1413 (w), 1378 (w), 1135 (w), [56]: 678 (M+, 9%), 665 ([M − BH3]+, 100%), 652 ([M − 2BH3
061 (m), 1034 (w), 807 (w), 764 (w), 722 (m) cm−1; MS (EI) + 1]+, 72%).
[
56]: 384 (M+, <1%), 370 ([M − BH3]+, 79%).
(
in,in/out,out)-H3B·P((CH2)14)3P·BH3 ((in,in/out,out)-2·2BH3;
(
5·BH3; 0.090 g, 0.25 mmol, 0.016 g, 0.024 mmol, 2%), colorless oil that solidified to give a
8
7
%), colorless oil. Anal. calcd for C22H48BP (354.40): C, white powder, mp 112 °C. Anal. calcd for C42H90B2P2
4.56; H, 13.65; found: C, 74.27; H, 13.52; 1H NMR (678.73): C, 74.32; H, 13.37; found: C, 74.71; H, 13.34;
(
500 MHz, CDCl3) δ 1.62–1.19 (m, 42H, CH2), 0.86 (t, 3H, 1H NMR (500 MHz, CDCl3) δ 1.55–1.50 (m, 12H, CH2),
3
1
JHH = 7.0 Hz, CH3), 0.48 and 0.26 (br apparent d, 3H, BH3); 1.47–1.39 (m, 12H, CH2), 1.37–1.32 (m, 12H, CH2), 1.29–1.21
3C{1H} NMR (126 MHz, CDCl3) δ 31.7 (s, CH2), 31.2 (d, (m, 48H, CH2), 0.38 and 0.26 (br apparent d, 6H, BH3);
JCP = 12.6 Hz, CH2), 29.03 (s, CH2), 29.01 (s, CH2), 28.9 (d, 13C{1H} NMR (126 MHz, CDCl3) δ 30.6 (d, JCP = 12.1 Hz,
JCP = 11.1 Hz, 2CH2), 26.7 (s, 2CH2), 26.53 (s, 2CH2), 26.48 CH2), 29.23 (s, CH2), 29.17 (s, CH2), 28.9 (s, CH2), 28.4 (s,
(
(
s, 2CH2), 26.1 (s, 2CH2), 23.8 (d, JCP = 35.4 Hz, CH2), 22.57 CH2), 22.5 (d, JCP = 34.1 Hz, CH2), 22.1 (d, JCP = 2.7 Hz,
d, JCP = 1.2 Hz, 2CH2), 22.55 (s, CH2), 22.3 (d, JCP = 33.6 Hz, CH2); 31P{1H} NMR (202 MHz, CDCl3) δ 14.9 and 14.7 (br
CH2), 21.2 (d, JCP = 3.3 Hz, 2CH2), 14.0 (s, CH3); apparent d); IR (powder film): 2922 (s), 2853 (s), 2366 (m),
1P {1H} NMR (202 MHz, CDCl3) δ 15.6 and 15.2 (br apparent 1467 (m), 1413 (w), 1131 (w), 1061 (m), 807 (w), 760 (w), 718
d); IR (oil film): 2926 (s), 2856 (m), 2366 (m), 1459 (m), 1417 (m) cm−1; MS (MALDI+, THAP) [56]: 702.0 ([M + Na]+,
w), 1135 (w), 1061 (m), 811 (m), 760 (m), 722 (m) cm−1; MS 98%), 666.0 ([M − BH3 + 1]+, 100%).
3
(
2363

Beilstein J. Org. Chem. 2018, 14, 2354–2365.
6
.
.
Nawara-Hultzsch, A. J.; Stollenz, M.; Barbasiewicz, M.; Szafert, S.;
Metathesis/hydrogenation of (H2C=CH(CH2)6)2(H3B)P-
Lis, T.; Hampel, F.; Bhuvanesh, N.; Gladysz, J. A. Chem. – Eur. J.
(
(CH2)14)P(BH3)((CH2)6CH=CH2)2 (11·2BH3; Scheme 5
32]). Diphosphine diborane 11·2BH3 (1.222 g, 1.672 mmol),
2
014, 20, 4617–4637. doi:10.1002/chem.201304419
Kharel, S.; Joshi, H.; Bhuvanesh, N.; Gladysz, J. A. Organometallics
018, 173, in press. doi:10.1021/acs.organomet.8b00345
1·2BH3), Grubbs' first generation catalyst (0.069 g, 8. Wang, L.; Hampel, F.; Gladysz, J. A. Angew. Chem., Int. Ed. 2006, 45,
[
7
CH2Cl2 (1700 mL; the resulting solution was 0.0010 M in
2
1
0
0
.083 mmol, 5 mol %), Wilkinson's catalyst (0.046 g,
.050 mmol, ca. 3 mol %), and H2 were combined in a proce-
4372–4375. doi:10.1002/anie.200601191
Angew. Chem. 2006, 118, 4479–4482. doi:10.1002/ange.200601191
. Wang, L.; Shima, T.; Hampel, F.; Gladysz, J. A. Chem. Commun.
9
dure analogous to that used for 1·BH3. An identical work-up
gave in,out-2·2BH3 (0.072 g, 0.106 mmol, 6%, minor impuri-
ties evident by 13C{1H} NMR), 6·2BH3 (0.056 g, 0.083 mmol,
2006, 4075–4077. doi:10.1039/B606728J
10.Estrada, A. L.; Jia, T.; Bhuvanesh, N.; Blümel, J.; Gladysz, J. A.
Eur. J. Inorg. Chem. 2015, 5318–5321. doi:10.1002/ejic.201500953
5
%, minor impurities evident by 13C{1H} NMR), and (in,in/
11.Hess, G. D.; Hampel, F.; Gladysz, J. A. Organometallics 2007, 26,
129–5131. doi:10.1021/om700625u
5
out,out)-2·2BH3 (0.075 g, 0.111 mmol, 7%), along with several
fractions consisting of unidentified and/or impure products, or
oligomers and polymers. Spectroscopic data for in,out-2·2BH3,
1
2.Fiedler, T.; Bhuvanesh, N.; Hampel, F.; Reibenspies, J. H.;
Gladysz, J. A. Dalton Trans. 2016, 45, 7131–7147.
doi:10.1039/C6DT00692B
(
in,in/out,out)-2·2BH3, and 6·2BH3 matched those reported
1
3.Hess, G. D.; Fiedler, T.; Hampel, F.; Gladysz, J. A. Inorg. Chem. 2017,
56, 7454–7469. doi:10.1021/acs.inorgchem.7b00909
4.Setaka, W.; Yamaguchi, K. J. Am. Chem. Soc. 2013, 135,
above.
1
14560–14563. doi:10.1021/ja408405f
Supporting Information
and earlier work cited therein.
1
5.Setaka, W.; Inoue, K.; Higa, S.; Yoshigai, S.; Kono, H.; Yamaguchi, K.
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J. Org. Chem. 2014, 79, 8288–8295. doi:10.1021/jo501539h
Additional experimental data.
16.Setaka, W.; Higa, S.; Yamaguchi, K. Org. Biomol. Chem. 2014, 12,
354–3357. doi:10.1039/C4OB00470A
3
[https://www.beilstein-journals.org/bjoc/content/
1
7.Shionari, H.; Inagaki, Y.; Yamaguchi, K.; Setaka, W.
Org. Biomol. Chem. 2015, 13, 10511–10516.
doi:10.1039/C5OB01644D
supplementary/1860-5397-14-211-S1.pdf]
18.Nishiyama, Y.; Inagaki, Y.; Yamguchi, K.; Setaka, W. J. Org. Chem.
Acknowledgements
2
015, 80, 9959–9966. doi:10.1021/acs.joc.5b01489
The authors thank the National Science Foundation (CHE-
19.Masuda, T.; Arase, J.; Inagaki, Y.; Kawahata, M.; Yamaguchi, K.;
Ohhara, T.; Nakao, A.; Momma, H.; Kwon, E.; Setaka, W.
Cryst. Growth Des. 2016, 16, 4392–4401.
1
153085, CHE-1566601) and Deutsche Forschungsgemein-
schaft (DFG, GL 300/9-1) for support. As noted above, many of
the experiments can be found in the doctoral dissertation of Dr.
Tobias Fiedler.
doi:10.1021/acs.cgd.6b00508
2
0.Kottas, G. S.; Clarke, L. I.; Horinek, D.; Michl, J. Chem. Rev. 2005,
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ORCID® iDs
1892–1910. doi:10.1039/C1CS15197E
Tobias Fiedler - https://orcid.org/0000-0001-7169-305X
Michał Barbasiewicz - https://orcid.org/0000-0002-0907-7034
Michael Stollenz - https://orcid.org/0000-0002-8635-164X
John A. Gladysz - https://orcid.org/0000-0002-7012-4872
22.Kharel, S.; Joshi, H.; Bierschenk, S.; Stollenz, M.; Taher, D.;
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Angew. Chem. 2002, 114, 4210–4212.
doi:10.1002/1521-3757(20021104)114:21<4210::AID-ANGE4210>3.0.
CO;2-J
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