Stereoselective Diels-Alder Reactions of gem-Diborylalkenes: Toward the Synthesis of gem-Diboron-Based Polymers

Article abstract of DOI:10.1021/jacs.1c01471

Although gem-diborylalkenes are known to be among the most valuable reagents in modern organic synthesis, providing a rapid access to a wide array of transformations, including the construction of C-C and C-heteroatom bonds, their use as dienophile-reactive groups has been rare. Herein we report the Diels-Alder (DA) reaction of (unsymmetrical) gem-diborylalkenes. These reactions provide a general and efficient method for the stereoselective conversion of gem-diborylalkenes to rapidly access 1,1-bisborylcyclohexenes. Using the same DA reaction manifold with borylated-dienes and gem-diborylalkenes, we also developed a concise, highly regioselective synthesis of 1,1,2-tris- A nd 1,1,3,4-tetrakis(boronates)cyclohexenes, a family of compounds that currently lack efficient synthetic access. Furthermore, DFT calculations provided insight into the underlying factors that control the chemo-, regio-, and stereoselectivity of these DA reactions. This method also provides stereodivergent syntheses of gem-diborylnorbornenes. The utility of the gem-diborylnorbornene building blocks was demonstrated by ring-opening metathesis polymerization (ROMP), providing a highly modular approach to the first synthesis of the gem-diboron-based polymers. Additionally, these polymers have been successfully submitted to postpolymerization modification reactions. Given its simplicity and versatility, we believe that this novel DA and ROMP approach holds great promise for organoboron synthesis as well as organoboron-based polymers and that it will result in more novel transformations in both academic and industrial research.

Full text of DOI:10.1021/jacs.1c01471

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Stereoselective Diels−Alder Reactions of gem-Diborylalkenes:
Toward the Synthesis of gem-Diboron-Based Polymers
Nadim Eghbarieh,# Nicole Hanania,# Alon Zamir, Molhm Nassir, Tamar Stein,* and Ahmad Masarwa*
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ABSTRACT: Although gem-diborylalkenes are known to be among the most
valuable reagents in modern organic synthesis, providing a rapid access to a wide
array of transformations, including the construction of C−C and C-heteroatom
bonds, their use as dienophile-reactive groups has been rare. Herein we report the
Diels−Alder (DA) reaction of (unsymmetrical) gem-diborylalkenes. These reactions
provide a general and efficient method for the stereoselective conversion of gem-
diborylalkenes to rapidly access 1,1-bisborylcyclohexenes. Using the same DA
reaction manifold with borylated-dienes and gem-diborylalkenes, we also developed a
concise, highly regioselective synthesis of 1,1,2-tris- and 1,1,3,4-tetrakis(boronates)-
cyclohexenes, a family of compounds that currently lack efficient synthetic access.
Furthermore, DFT calculations provided insight into the underlying factors that
control the chemo-, regio-, and stereoselectivity of these DA reactions. This method
also provides stereodivergent syntheses of gem-diborylnorbornenes. The utility of the
gem-diborylnorbornene building blocks was demonstrated by ring-opening meta-
thesis polymerization (ROMP), providing a highly modular approach to the first synthesis of the gem-diboron-based polymers.
Additionally, these polymers have been successfully submitted to postpolymerization modification reactions. Given its simplicity and
versatility, we believe that this novel DA and ROMP approach holds great promise for organoboron synthesis as well as
organoboron-based polymers and that it will result in more novel transformations in both academic and industrial research.
INTRODUCTION
■
Organoboron reagents have had an enormous impact on the
development of new chemical reactions1 and have extended
the scope of accessible complex molecular scaffolds.2−4
Organoboronate compounds are particularly attractive owing
to their wide availability and air stability,5 making them
versatile reagents in organic synthesis.6,7
Although many synthetic methods utilize transformations of
C−B bonds,2 the development of polyborylated reagents
would enable greater structural diversity, an important
objective.8 Therefore, over the past decade, much effort has
been expended to synthesize new functionalized classes of
polyboronates, which have been shown to be excellent building
blocks for the modular construction of new compounds.9−12
Figure 1. General scheme of the work. (A) Classifying of
Among polyboron-containing structural motifs, (unsym-
metrical)13 gem-diboron derivatives (2, 3′) are a well-known
emerging class with good potential for novel synthetic
applications (Figure 1A).14−17 The special properties and
structures of bisnucleophile gem-diboryl compounds 2 and 3′
(termed geminated organodimetallics)18,19 have attracted
increasing attention from synthetic chemists, particularly in
constructing C−C/C−heteroatom bonds. In recent years, gem-
diboryl compounds 2 and 3′ have been widely adopted as
coupling partners in synthetic chemistry.11,20 For example,
gem-diborylalkenes 2 have served as building blocks for
Suzuki−Miyaura cross-coupling, nucleophilic partners, reduc-
organoboron compounds. (B) The well-known boron-based polymer
I. (C) The unprecedented gem-diboron based polymer II. (D)
General scheme of the Diels−Alder reaction of polyboronated
compounds 3 with 2 and the application of their cycloaddition
product 4 in the ROMP reaction to generate the gem-diborylpolymer
II′. B = boron group.
Received: February 6, 2021
Published: April 14, 2021
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Figure 2. Initial study and work plan. (A) The well-known DA reaction using the vinylborantes 1 with diene 3. (B) Description of our DA reaction
of polyboronated compounds. (C) DFT calculations indicate that gem-diborylalkene 2 can undergo the DA reaction with diene 3e. (D) CYLview
structures of the TSs of Bpin-exo-TS, Bpin-endo-TS, and gemBpinBpin-TS; the calculations were performed with Gaussian 16 software using the
M06-2X.41 TS structures were visualized with CYLview.1.0b.42 B = boron group, Bpin = pinacolato-boron, Bdan = B-1,8-diaminonaphthalene.
tion approaches, Michael additions,21 radical chemistry,22 and
other reactions.13,23
strategies to efficiently construct complex molecules (Figure
2B).34,36
Recently, we reported a photoredox-mediated reaction of
gem-diborylalkenes22 and showed that gem-diborylalkenes 2
have similar electron deficiency as vinylboron 1;22 hence, 2
should serve as a suitable dienophile for this type of
cycloaddition reaction. However, key challenges include (1)
the steric repulsion introduced by the two groups of the bulky
Bpin units in the TS of the cycloaddition reaction;37 (2)
whether the regio- and stereoselectivity of the cycloaddition
can be controlled when two unsymmetrical boron groups are
placed on the geminated carbon of dienophile 2′;13 and (3)
whether the reaction can proceed in a regioselective manner
when borylated dienes react with 2 (Figure 2B).34
To answer these questions, we first conducted computa-
tional studies on the Diels−Alder (DA) reaction to predict
whether gem-diborylalkenes 2 could be used as dienophile
reactive partners for the DA reaction.35,38 According to our
computational studies of the energy profiles of the cyclo-
addition reactions of diene-Cp 3e with dienophiles 1 and 2 at
room temperature (rt), the transition state of gemBpinBpin-
TS is likely to be more energetically stabilized compared to
vinyboronate TS Bpin-exo-TS by 1.4 kcal/mol (Figure 2C,D).
Considerations for the relative stabilities of the different
transition states are discussed in the SI (pp S65, S66).36,38,39
Moreover, the CHelpG population analysis40 on the carbons of
the reactive double bond of the dienophiles (1 and 2) shows
that the double bond of 2 has less electron concentration than
the double bond in 1 (full details are given in the SI (pp S65,
Despite the fact that organoborons 1 and 1′ (Figure 1A)
have been applied in many fields including materials,
polymer24−29 I (Figure 1B), drugs,30 and, in industry, gem-
diboryl units 2 and 3′ have seldom been employed in these
fields, e.g., polymer II (Figure 1C).30 To this end, we contend
that a new paradigm of research is needed to complementarily
propel this gem-diboryl class of compounds to reach the same
application level as their monoboron analogues.
As a part of a general program to investigate the reactivity
and selectivity of gem-diboryl compounds in new synthetic
applications, we sought to prepare variants bearing the gem-
diboryl-norbornene group (4) because these strained com-
pounds (∼27 kcal/mol of inherent strain) might offer new
opportunities toward the ring-opening metathesis polymer-
ization (ROMP) reaction31 and lead to unprecedented gem-
diboryl-based polymer II′ (Figure 1D).
RESULTS AND DISCUSSION
■
We posited that an efficient way to prepare gem-diboryl-
norbornene structural motifs 4 would be through a [4 + 2]
cycloaddition reaction of gem-diborylalkenes 2 with cyclo-
pentadiene (CP) 3. Although the [4 + 2] cycloaddition
reactions of vinylboranates,32 e.g., 1, are well documented in
the literature (Figure 2A),33−36 to the best of our knowledge,
the use of gem-diborylkenes 2 in these types of reactions is
rare,23 despite the potential to provide new and efficient
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Figure 3. Diels−Alder reaction with 2. (A) General reaction conditions of the DA with 2. (B) examples of the DA products as a result of reaction
of 2 with symmetrical dienes. (C) Examples of the DA products as a result of a regioselective reaction of 2 with unsymmetrical dienes. (D)
Preparation of the 1,1,3,4-tetraBpincyclohexene 4l by the DA reaction of 2 with diene 3l. (E) Regioselective preparation of the 1,1,2-
triBpincyclohexene adduct 4k by the stereospecific DA reaction of 2 with diene E-3k. (F) DFT calculations for the regioselective rationale by the
TSs of 1,1,2-triBpin-TS and 1,1,3-triBpin-TS; the calculations were performed with Gaussian 16 software using M06-2X.41 The relative structures
of 4f and 4g-Si have been confirmed by 2D NMR NOESY. X-ray and TS structures were visualized with CYLview 1.0b.42 rr = regioisomeric ratio,
dr = diastereomeric ratio, yields are isolated. Bpin = pinacolato-boron.
S66). These observations support the fact that 2 is slightly
more “dienophilic” toward the DA reaction than 1, albeit more
bulky (Figure 2C,D).
(4a−g) were isolated in good yields under the established
optimal conditions. The reaction also proceeded in very good
yield on a gram scale, e.g., 4e. The reaction works well with
anthracene derivative 3c, affording the gem-diborylcyclic
adduct 4c, which was confirmed by X-ray crystallographic
analysis (see the CYLview structure of 4c, Figure 3B). In
addition, the reaction works smoothly with hexachlorocyclo-
pentadiene 3e-6Cl, forging the hexachloro cycloaddition
product 4e-Cl.
To investigate our proposed reaction, gem-diborylalkene 2,
along with 3e, was subjected to DA reaction conditions
(Figures 2C-2, 3A,B). We obtained the desired gem-
diborylated cycloaddition product 4e in good yield at rt with
toluene as solvent. Next, we investigated the scope of the DA
reaction using readily available gem-diborylalkene 2 and various
diene substrates (3) bearing aliphatic, aromatic, and
heteroatom substituents (Figure 3B). Generally, the products
Using bulky dienes (i.e., pentamethylcyclopentadiene 3f),
along with adjusting the reaction temperature, played a critical
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Figure 4. Oxidation reaction of 4. Examples of the utility of gem-diborylcyclohexenes 4 in the oxidation reaction that yields the ketone products
4O. The relative structure 4O-d has been confirmed by 2D NMR NOESY. rr = regioisomeric ratio, dr = diastereomeric ratio, yields are isolated.
Bpin = pinacolato-boron.
Figure 5. Diels−Alder reaction with 2′. (A) General reaction conditions of the DA with unsymmetrical gem-diborylalkene 2′. (B) Examples of the
DA products as a result of reaction of 2 with different dienes. (C) DFT calculations of the TSs of exo-BpinBdan-TS and endo-BpinBdan-TS; the
calculations were performed with Gaussian 16 software using M06-2X.41 (D) DA reaction of 2′ with diene 3l leads to 1-Bdan-1,3,4-triBpin-
cyclohexene 5l. (E) Stereospecific DA reaction of 2′ with diene E-3K leads regioselectively to diastereomers 5k and 5k′; the two 1,1,2-triboryl
products were easily separated by column chromatography. The 1,2-regioselectivity manner has been determined by the X-ray structure of 5k′. The
relative configuration of 5k has been confirmed by 2D NMR NOESY. X-ray and TS structures were visualized with CYLview 1.0b.42 rr =
regioisomeric ratio, dr = diastereomeric ratio, yields are isolated. Bpin = pinacolato-boron, Bdan = B-1,8-diaminonaphthalene.
role in controlling diastereoselectivity (i.e., the syn vs anti
outcomes on 4f; Figure 3B). Whereas at a high temperature
(185 °C) the reaction proceeds with moderate diastereose-
lectivity (dr = 83:17), at room temperature the reaction
afforded the bis-borylated compound 4f as the exclusive
diastereoisomer (dr > 99:01). Interestingly, the product gem-
diboryl-bicyclo[2.2.2]octane 4g-Si was obtained in a high
regioselective mode, i.e., rr > 99:01 (Figure 3B).
Figure 3C). Notably, the constitutional isomers in 4 (h, h′)
and 4 (i, i′) can be easily separated by column chromatog-
raphy (Figure 3C; see the SI).
Remarkably, the reaction of 2 with 3,4-diboryl diene 3l can
create the exceptional 1,1,3,4-tetraborylcyclohexene skeleton 4l
(Figure 3D). Moreover, we examined the DA reaction of 1-
boron-diene34,43 E-3k with gem-diborylalkene 2, and surpris-
ingly, the reaction generated the rare 1,1,2-triboryl cyclic
adduct 4k as the exclusive isomer with complete stereo-
specificity (Figure 3E).12,43,44 We have obtained unambiguous
The reaction also exhibits moderate regioselectivity when
unsymmetrical dienes are used, giving the para adducts (4h−j,
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support for the structures of 4k using X-ray crystallographic
analysis (see CYLviews in Figure 3E). DFT calculations
indicate that the TS of the 1,1,2-triboryl constitutional isomer
is favored by 3 kcal/mol over the TS of the 1,1,3-triboryl
isomer (1,1,2-triBpin-TS vs 1,1,3-triBpin-TS; Figure 3F).
Next, the polyborylated cycloadducts 4 were subjected to
oxidation reactions using H2O2 (Figure 4).45 Oxygenated
compounds 4O were obtained in a chemoselective manner
(Figure 4). Thus, we have demonstrated that gem-dibor-
ylalkenes serve as ketene equivalents in [4 + 2] cyclo-
additions.46 Of note, 4d has been subjected to two oxidation
reactions that lead chemoselectively to two different products,
4O-d and 4O-d′. In 4O-d, first the double bond was
stereoselectively epoxidized and then the gem-di-Bpin unit
underwent oxidation. However, in 4O-d′ the geminal-Bpin
position first underwent oxidation and then the double bond
isomerizes due to the deprotonation of the acidic benzylic and
allylic proton that is located α to the generated carbonyl
(Figure 4; see proposed pathways in the SI). When triboryl
compound 4k was subjected to the oxidation conditions,
ketone 4O-k was selectively obtained, most likely resulting
from oxidation of the gem-diboryl moiety to give a boron-
enolate,47 which hydrolyzed in situ to finally give 4O-k (Figure
4; see proposed pathways in the SI).
Furthermore, we investigated the DA reaction using
unsymmetrical 1,1-bisdiborylalkenes (2′ and 2-F). It was
anticipated that the cycloaddition might proceed with good
stereoselectivity (Figure 5).13 We were happy to discover that
the reaction of 1,1-BpinBdan-ethene (2′) with dienes 3a−d
afforded the unsymmetrical gem-diborylalkane 5a−d (con-
firmed by X-ray crystallographic analyses of 5a and 5c) in good
yield (Figure 5A,B). High diastereoselectivity was observed in
the reaction of 2′ with CP, affording cycloaddition product 5c
(endo:exo = 92:8). Our calculations indicate that the reaction
favors the endo product 5c with the TS energy of endo-
BpinBdan-TS, which is 2 kcal/mol less than that of exo-
BpinBdan-TS (Figure 5C).24,35,38 This is in good agreement
with the fact that the aromatic-planar Bdan group is less bulky
than the Bpin group.13,33,37,48 Therefore, it would appear that
the diastereoselectivity in this case is driven primarily by sterics
(additional considerations for the relative stabilities of the
different transition states are discussed in the SI (pp S65,
S66)).
Moreover, the reaction of 2′ with 3,4-diboryl diene 3l
yielded 1,1,3,4-tetraborylcyclohexene adduct 5l (Figure 5D).
Additionally, the DA reaction of 1-boron-diene34,43 (E-3k)
with gem-diborylalkene 2′ generated regioselectively the
separable diastereomers of the 1,1,2-triboryl cyclic adduct
with complete stereospecificity (5k, 5k′; Figure 5E).43,44 We
have obtained unambiguous support for the structures of 5k′
using X-ray crystallographic analysis (see CYLviews in Figure
5E). These two different boron groups can provide the basis
for selective C−B sequential functionalization, which in turn
reacts differently.13,49
Unlike boronic esters, for example, the Bpin and Bdan
groups, monoalkyl-trifluoroborate salts are known to be easily
activated and to undergo rapid transmetalation with transition-
metal complexes. In general, owing to their air, moisture, shelf,
and thermal stability, as well as their occurrence as free-flowing
crystalline solids, monotrifluoroborate salts have now become
extremely popular reagents in synthesis. Thus, we attempted to
use the gem-BpinBF3Cs alkene 2-F as a dienophile for the DA
reaction (Figure 6A). Unfortunately, the reaction did not
Figure 6. (A) Unsuccessful Diels−Alder reaction with 2-F. (B)
General reaction conditions of the chemoselective trifluorination with
gem-diborylalkanes 4 as an alternative method for the unsuccessful
direct DA of 2-F with diene 3. (C) Examples of the trifluorination
products as a result of reaction of 2 with different dienes. The relative
configurations of 6b and 6d have been confirmed by 2D NMR
NOESY. dr = diastereomeric ratio, yields are isolated. Bpin =
pinacolato-boron.
afford the desired product (gem-diborylcyclohexene BF3Cs
containing 6) and instead led to decomposition (Figure 6A).
Alternatively, we sought to obtain product 6 using our recently
reported conditions for late-stage trifluoroboration of gem-
diborylalkenes by employing CsF (Figure 6B,C).50 We were
pleased to observe the desired mono-BF3Cs products 6a−d in
good yield (Figure 6C). Moreover, the reaction shows
moderate diastereoselectivity in 6b and 6d when 4e and 4g,
respectively, are used. The rationale of the diastereoselectivity
is in good agreement with our reported mechanism that
follows the likelihood that fluorination occurs from the less
sterically encumbered face of norbornenes 4e−g, affording the
exo-disposed BF3Cs group (6b and 6d, respectively).50
Next, we envisioned a stereodivergent synthesis of
norbornene 5f involving controlling exo and endo norbornene
structural motifs as illustrated in Figure 7. Toward this goal,
cyclopentadienyl 3f was reacted in two different scenarios. In
path a (Figure 7), 3f reacted with the unsymmetrical gem-
diborylalkene 2′ to form only the cycloaddition product 5f-
endo (confirmed by X-ray) in 85% yield. In path b (Figure 7),
3f was first reacted with the symmetrical gem-BpinBpin-alkene
2 to yield 4f as a single diastereomer, which was then subjected
to a diastereoselective trifluoroboration favoring the less
sterically hindered face of the norbornene (denoted by arrows
in 4f) to afford 6f in high diastereoselectivity (94:06; Figure
7).50 Finally, the BF3Cs group was converted to the Bdan
group to afford the 5f-exo product (Figure 7).50 Overall, our
method serves as a powerful tool for the diastereocontrolled
synthesis of norbornene motifs.
With these valuable gem-diborylcyclohexenes in hand, e.g.,
gem-diborylnorbornene, we sought to demonstrate their
synthetic utility in selective transformations of the double
bond through the ROMP reaction, as depicted in Figure 8A, to
generate the novel gem-diborylalkene-based polymers.
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Figure 7. Stereodivergent synthesis of norbornene 5f. Path a: Norbornenediastereomer 5f-endo was stereoselectivity synthesized in one step from
DA reaction of cyclopentadiene 3f with the unsymmetrical gem-diborylalkene 2′. The relative configuration of 5f-endo was determined using X-ray
crystallography (see the CYLview structure of 5f-endo). Path b: 5f-exo was synthesized in three steps. Step 1: diastereoselective DA reaction of 3f
with gem-diborolalkene 2, which leads to norbornene 4f. Step 2: stereoselective trifluorination of norbornene 4f from the less hindered si-face,
leading to the exo product 6f. Step 3: the gem-diborylnorbornene-BF3Cs 6f was directly converted to unsymmetrical gem-Bpin Bdan-norbornene 5f-
exo in a stereospecific manner. X-ray structures were visualized with CYLview 1.0b.42 dr = diastereomeric ratio, yields are isolated. Bpin =
pinacolato-boron, Bdan = B-1,8-diaminonaphthalene.
gem-diboryl analogues have not been investigated; thus, they
can provide a new array of polymer properties to forge a wider
diversity of gem-diboron-based polymers. Thus, gem-dibor-
ylnorbornenes hold great promise to serve as a monomer for
ROMP31 reactions that form polymers containing gem-diboryl
units (Figure 8A). It is noteworthy that the ROMP reactions
for the mono-boron-containing monomers are rarely reported,
and there are only a few examples of very special side-chain
boron groups.51
In this regard, we chose the [Ru] Grubbs second-generation
catalyst as a catalyst in the ROMP reactions, because its known
to tolerate broad functional groups, air, and moisture, and this
includes the fact that the Bpin group is expected to be intact
under the Ru catalysis conditions for other types of reactions,
i.e., olefin metathesis catalysts.52
We subjected norbornenes 4e and 5c to ROMP31
Figure 8. Synthesis of gem-diboron-based polymers. (A) General
polymerization reaction conditions using 1 mol % of the
scheme of the catalyzed Ru ROMP of norbornenes 4e and 5c to yield
[Ru] Grubbs second-generation catalyst and tetrahydrofuran
polymers poly-7-BpinBpin and poly-7-BpinBdan with >98%
(THF) as a solvent.53 We were gratified to observe that gem-
conversion. (B) The obtained polymers. (C) Polydispersity indexes
diborylnorbornene 4e and 5c selectively polymerized to afford,
for the first time, gem-diboron-based polymers poly-7-
BpinBpin and poly-7-BpinBdan, respectively, in high
conversions (Figure 8B). This transformation was followed
(PDI) and gel permeation chromatography (GPC) of both polymers.
Thermogravimetric analysis (TGA) for both polymers poly-7-
BpinBpin and poly-7-BpinBdan is provided in the SI (p S51).
[Ru] = Grubbs second-generation catalyst. Bpin = pinacolato-boron,
Bdan = B-1,8-diaminonaphthalene.
1
by H NMR spectroscopy of both monomers 4e and 5c and
their corresponding polymers poly-7-BpinBpin and poly-7-
BpinBdan, respectively, as depicted in the SI (pp S45−S49,
S64).25 The resulting polymer, poly-7-BpinBpin, was observed
to have a light yellowish color; it has a high molecular weight
(MW = 4.85 × 104) and polydispersity indexes (PDI) of Mw/
Mn = 1.29, based on gel permeation chromatography (GPC)
(Figure 8B,C). Furthermore, the polymer poly-7-BpinBdan
was dark green and has a lower molecular weight (MW = 9.19
× 103) and a higher PDI of Mw/Mn = 1.61 (Figure
8B,C).24,25,31,39 Although these pristine polymers are expected
to be colorless, the dark color of the poly-7-BpinBdan might
be a result of the conjugated nature of the aromatic Bdan
group. While both polymers have different MW values, the
Organoboron polymers29 have attracted widespread atten-
tion, since they provide unique properties for catalysis, sensory
materials, luminescent materials, and biomedical applica-
tions.26,27,29,30 Among them, polymers with pendant boronic
acids/esters account for the majority, since boronic acids/
esters could serve as responsive sites of sensitive materials or
dynamic cross-linking points of self-assembled polymers and
self-healing materials.26,27,30
Moreover, the presence of the boron moiety on the polymer
offers a great opportunity for postpolymerization modifica-
tions28,29 to achieve new functional groups that are difficult to
achieve in regular polymerizations.24,25,29 Although polymers
that contain monoboron units have been widely used,27 their
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Figure 9. Postpolymerization modifications of gem-diboron-based polymers. (A) General scheme of the postpolymerization modifications of poly-
7. (B) Hydrogenation of polymers poly-7-BpinBpin and poly-7-BpinBdan. (C) Trifluorination, protodeborylation, and deuterodeborylation
reactions of poly-7-BpinBpin. (D) Examples of postpolymerization modifications of poly-8-BpinBpin and poly-11-Bpin-H which include
trifluorination, protodeborylation, deuterodeborylation, and arylation. Bpin = pinacolato-boron, Bdan = B-1,8-diaminonaphthalene.
poly-7-BpinBpin is closer to the one expected at a 1 mol %
catalyst compared with literature.51
Subsequently, we examined the potential of postpolymeriza-
tion modifications of these gem-diboron-based polymers by the
functionalization of the double bond and the replacements of
gem-diboryl units (Figure 9A). These modifications include the
hydrogenation of the ethylene moieties using p-toluenesulfonyl
hydrazide 13 as a source for hydrogen (Figure 9B).54,55 The
hydrogenation reactions proceed very efficiently for both gem-
diboron-based polymers poly-7-BpinBpin and poly-7-BpinB-
dan to obtain the new hydrogenated polymers poly-8-
BpinBpin and poly-8-BpinBdan, respectively, in a complete
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Accession Codes
conversion (Figure 9B). The hydrogenated polymers poly-8-
BpinBpin and poly-8-BpinBdan were distinctly confirmed by
1H and 13C NMR spectroscopy (for full details see the SI).55
Our efforts were also directed to the postpolymerization
transformation of the boron groups through the selective
replacement of one of the boron groups by trifluorination,50
protodeborylation,56 and deuterodeborylation56 as described
in Figure 9C. Remarkably, the trifluorination50 reaction of
polymer poly-7-BpinBpin represents an alternative method to
the unsuccessful direct ROMP reaction of monomer Bpin-
BF3Cs-norbornene 6b. Of note, these new polymers were
CCDC 2058307−2058312 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
Tamar Stein − Institute of Chemistry and Fritz Haber Center
for Molecular Dynamics Research, Institute of Chemistry, The
Hebrew University of Jerusalem, Jerusalem 9190401, Israel;
Email: tamar.stein@mail.huji.ac.il
Ahmad Masarwa − Institute of Chemistry, The Hebrew
University of Jerusalem, Jerusalem 9190401, Israel;
orcid.org/0000-0001-6992-5595;
1
confirmed by H, D, 19F, and 13C NMR spectroscopy (for full
details see the SI).
Moreover, the hydrogenated polymer poly-8-BpinBpin
underwent selective protodeborylation,56 and deuterodebor-
ylation,56 which were then followed by a trifluorination
reaction22,50 of poly-11-Bpin-H (Figure 9D). Overall, poly-
7-BpinBpin underwent three chemoselective sequential
postpolymerization modifications, i.e., hydrogenation, proto-
deborylation, and trifluorination, which eventually gave poly-7-
BF3K-H (Figure 9B and D). Finally, poly-11-Bpin-H was
subjected to the Pd-catalyzed Suzuki−Miyaura cross-coupling
reaction, to replace the C−B bond with the new C−C bond,
which forms the arylated polymer poly-14-Ar-H (Figure 9D).
These preliminary postpolymerization transformations have
demonstrated the utility of a replaceable-main-chain-based
strategy using boron and the double bond as the key elements
for opening new opportunities for the synthesis of a variety of
new polymers.
Email: Ahmad.Masarwa1@mail.huji.ac.il
Authors
Nadim Eghbarieh − Institute of Chemistry, The Hebrew
University of Jerusalem, Jerusalem 9190401, Israel
Nicole Hanania − Institute of Chemistry, The Hebrew
University of Jerusalem, Jerusalem 9190401, Israel
Alon Zamir − Institute of Chemistry and Fritz Haber Center
for Molecular Dynamics Research, Institute of Chemistry, The
Hebrew University of Jerusalem, Jerusalem 9190401, Israel
Molhm Nassir − Institute of Chemistry, The Hebrew
University of Jerusalem, Jerusalem 9190401, Israel
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c01471
SUMMARY
■
In conclusion, we have developed a method that addresses the
long-standing challenge of regio- and stereoselective Diels−
Alder cycloadditions with poly-alkenylboranes. This was
achieved by introducing a new method that enables the use
of (unsymmetrical) gem-diborylalkenes as a reasonable reactive
dienophile for the DA reaction. The products of these
reactions enable the formal synthesis of polyborylated
cycloadducts, particularly the 1,1,2-tri- and 1,1,3,4-tetrabor-
ylcyclic adduct, which would be difficult to accomplish with
the existing strategies. In addition, the reaction offers the
stereodivergent synthesis of norbornenes by using a diaster-
eoselective trifluorination reaction. We demonstrate the use of
the gem-diborylalkenes as ketene equivalents in [4 + 2]
cycloadditions. Moreover, we utilized gem-diborylnorbornene
in the synthesis, for the first time, of gem-diboryl-based
polymers through ROMP. These polymers underwent
successful postpolymerization modifications to access new
polymers, which also demonstrates the potential diversity of
the main chain replacement. Studies to achieve an
enantioselective DA transformation using chiral gem-dibor-
ylalkenes33 as well as new postpolymerization transformations
of gem-diborylcycloalkene-based polymers are currently under
investigation and will be reported in due course.
Author Contributions
#N.E. and N.H. contributed equally to the work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by grants from the Azrieli
Foundation, The Casali Foundation, and The Hebrew
University of Jerusalem. We would like to thank Prof. Raed
Abu-Reziq and Prof. Roy Shenhar for helpful discussions and
input on the polymer part. We are grateful to Prof. Roy
Shenhar and Dr. Ishay Columbus for the GPC measurements.
We thank Dr. Benny Bogoslavsky (The Hebrew University of
Jerusalem, Israel) for the X-ray structure determination. We
also thank Abeer Ali for the measurements of the
thermogravimetric analysis. N.E., N.H., and M.N. are grateful
for a fellowship from The Hebrew University of Jerusalem,
Israel. T.S. and A.Z. are grateful for support from the Israel
Science Foundation (Grant Number 1941/20).
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