Tropylium-Promoted Hydroboration Reactions: Mechanistic Insights Via Experimental and Computational Studies

Via Experimental and Computational Studies

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Tropylium-Promoted Hydroboration Reactions: Mechanistic Insights

Nhan N. H. Ton, Binh Khanh Mai,* and Thanh Vinh Nguyen *

Cite This: J. Org. Chem. 2021, 86, 9117 −9133

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sı Supporting Information

ABSTRACT: Hydroboration reaction of alkynes is one of the

most synthetically powerful tools to access organoboron com-

pounds, versatile precursors for cross-coupling chemistry. This type

of reaction has traditionally been mediated by transition-metal or

main group catalysts. Herein, we report a novel method using

tropylium salts, typically known as organic oxidants and Lewis

acids, to promote the hydroboration reaction of alkynes. A broad

range of vinylboranes can be easily accessed via this metal-free

protocol. Similar hydroboration reactions of alkenes and epoxides can also be eﬃciently catalyzed by the same tropylium catalysts.

Experimental studies and DFT calculations suggested that the reaction follows an uncommon mechanistic pathway, which is

triggered by the hydride abstraction of pinacolborane with tropylium ion. This is followed by a series of in situ counterion-activated

substituent exchanges to generate boron intermediates that promote the hydroboration reaction.

INTRODUCTION

Hydroboration of C−C multiple bonds is one of the most

frequently used reactions in organic synthesis. It produces alkyl

or alkenyl boranes, which are stable but versatile synthetic

precursors for a wide range of coupling reactions. Since the

could abstract a hydride from borane reagents to trigger the

reaction via the formation of borenium cation (Scheme 1c). If

this works, it will oﬀer a new catalytic approach in hydroboration

chemistry, especially when the electrophilic tropylium ion is

distinctively diﬀerent from existing catalysts, many of which are

nucleophiles or hydride donors.

■

early examples of uncatalyzed addition of borane to oleﬁns by

Brown, the ﬁeld has evolved signiﬁcantly over the last few

decades to develop more eﬃcient and practical catalytic systems

for more challenging hydroborative processes. Among them, the

transition-metal-catalyzed hydroboration reaction remains the

RESULTS AND DISCUSSIONS

■

We started our investigation by looking at the catalytic activity of

tropylium tetraﬂuoroborate (1a) in the reaction between

phenylacetylene (2a) and pinacolborane (HBpin, 3). Gratify-

ingly, the initial reaction met with successful results. We

proceeded further to optimize the reaction conditions on

solvent, temperature, the stoichiometry of HBpin, and tropylium

catalyst loading (for full optimization details, see the Supporting

most thoroughly explored direction (Scheme 1a) with the focus

slowly shifting from precious metals to more abundant ones.

More recently, main group catalytic hydroboration has emerged

as a new research ﬁeld, with organoborane catalysts being the

most attractive options (Scheme 1b). The quest to make

hydroboration reaction completely metal-free ventures further

into some scattered examples of organocatalytic systems such as

Information). The optimal conditions are presented in Table

1, where the best reaction outcomes were achieved with 2 mol %

1a at 70 °C in neat condition. The use of relatively non-polar

solvents such as 1,2-dichloroethane or toluene also led to

excellent yields of hydroboration adduct 4a, while acetonitrile or

THF completely turned oﬀ the reaction (entries 6−7, Table 1).

Our initial thought is that this can be attributed to the fact that

these solvents can solvate or coordinate very well to tropylium or

carboxylic acids or amides. While the mechanism of transition-

metal- and main group element-catalyzed hydroboration

reactions has been extensively studied, there is a limited

understanding of the activation mode and reaction design in

organocatalytic systems. Therefore, a reliable and eﬃcient

organocatalyst, which is also robust and simple for necessary

mechanistic studies, is in demand.

Given our ongoing interest in the chemistry of tropylium

ion and our recent recognition of the similarity in Lewis acid

Received: May 24, 2021

Published: June 17, 2021

catalyst activity of tropylium ion to boron Lewis acids B(Ar ) ,

which are known as eﬃcient catalysts for hydroboration

c,e−g,i,k

reactions,

we envisioned that tropylium salts could

potentially serve as suitable organocatalysts for the same

chemical transformation. In addition to its Lewis acidity,

tropylium ion is also known as an organic oxidant, which

2021 American Chemical Society

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Scheme 1. Tropylium-Catalyzed Hydroboration of Alkynes, Alkenes, and Epoxides

boron intermediates, rendering them much less reactive for the

reaction to proceed.

Without the tropylium catalyst, it only led to 15% yield of 4a

as the background reaction (entry 1). Tropylium hexaﬂuor-

ophosphate seemed to be much less eﬀective for this reaction

ensure that good eﬃciency is achieved across the whole

substrate scope. Most arylacetylene substrates reacted smoothly

to give the (E)-alkenyl boronic ester products in good to

excellent yield (4a−4o, Scheme 2). The reaction worked

eﬃciently with heterocyclic as well as bis and tris(acetylenyl)

substrates (4p−4s). Non-terminal aromatic alkynes also reacted

but led to lower product yields (4t−4v) and a mixture of

regioisomers (4t1 and 4t2). Under these reaction conditions,

alkyl-substituted alkynes resulted in generally lower conversions

to the products than their aryl alkyne counterparts.

(

entry 8). Other tropylium salts such as chloride, bromide (1b),

or triﬂate (1c) also proved to be capable of catalyzing the

reaction with similar eﬃciency; however, tropylium chloride is

too hygroscopic to be considered a practical catalyst. The

important role of tropylium ion was highlighted by the fact that

potassium or tetrabutylammonium salts of the same counterions

either showed no catalytic activity or hindered the reaction.

Furthermore, other hydride abstractors or organic Lewis acids

Initial Mechanistic Studies. Apart from a demonstration of

practicality, it is also important to elucidate the mechanistic

pathway of our newly developed tropylium salt-catalyzed

hydroboration of alkynes. While we carried out this project,

the Thomas group reported important recognitions of hidden

such as tritylium or nitrosonium salts could also catalyze the

reaction but with lower eﬃciencies than tropylium salts (entry

). Tritylium salt was previously used in sub-stoichiometric

pathways in hydroboration chemistry, which actually involve

quantities to initiate NHC−borane-mediated hydroboration of

the catalytic activity of in situ generated BH from nucleophilic

alkene via hydride abstraction. The regioselectivity and

stereochemistry of the reaction product were conﬁrmed by

comparison to literature data and also by a simple isotope

labeling study. This study used our developed optimal reaction

conditions on deuterated phenylacetylene (2a-D, 95%-D,

Scheme 2), which gave product 4a-D in 87% yield with 93%

deuterium at the terminal carbon, conﬁrming that the HBpin

added to the C−C triple bond in cis-fashion with the indicated

regioselectivity.

catalysts or ligands and HBpin. These reports prompted us to

carefully reconsider our original hypotheses and perform an in-

depth investigation into the activation mode of our tropylium

catalysts (Scheme 3). As HBpin and alkynes do not react readily

with each other on their own, the uncatalyzed background

reaction (entry 1, Table 1) does suggest the possible presence of

BH . Thomas and co-workers nominated a qualitative “hidden

BH catalysis” test, involving the addition of tetramethylethy-

lenediamine (TMEDA) to the reaction. This additive will

We subsequently applied the optimized conditions toward the

hydroboration of other alkyne substrates (Scheme 2). While

these conditions work well for most substrates we tested, some

of them showed unsatisfactory conversions. Therefore, we

settled with a more suitable set of conditions from entry 4, Table

supposedly complex with any BH formed in the reaction

mixture, making it easier to identify by characteristic signals in

NMR spectra. Thus, this was the ﬁrst mechanistic control study

that we did and we learned straightaway that it turned oﬀ the

reaction completely, as no hydroboration product was formed in

the reaction (Scheme 3a). In hindsight, this was not surprising,

, with 5 mol % tropylium catalyst and 12 h reaction time to

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Reaction conditions: 0.5 mmol 2a , 0.6 mmol 3 (1.2 equiv), and 0.01

0e,12a,b

Article

Table 1. Optimization of Tropylium-Catalyzed Reaction

amines.

The coordination of tropylium to TMEDA was

Between 2a and 3

observed by NMR spectroscopy (see page S10 in the

experimental Supporting Information ); this presumably ren-

dered our catalytic system inactive. Therefore, Thomas’s

qualitative test is, unfortunately, not applicable to our reaction

conditions.

Our ﬁrst original hypothesis was that the Lewis-acidic

tropylium ion could coordinate with the oxygen centers on

HBpin, enhancing the polarization of the B−H bond and

promote the addition to the triple bond (Scheme 3b). This will

likely produce the cis-selective addition product (trans-isomer),

which agrees with our actual results. Our complexation NMR

studies between tropylium salts and HBpin did show some

supporting evidence for this Lewis coordination (Scheme 3d).

However, there is a more predominant transformation happen-

entry

variations from optimal conditions

4a (%)

no catalyst (also see entry 8)

4 h

1 mol % catalyst 1a

5 mol % catalyst 1a

50 °C

solvent DCE or toluene (10 mol % 1a)

solvent MeCN or THF (10 mol % 1a)

variation of catalyst

94 or 95

traces

see below

ing between these two species. The H NMR spectrum (in

CD CN) of a 1:4 mixture of Trop.BF 1a or Trop.Br 1b with

HBpin (to mimic 5 mol % catalyst vs. 0.2 equiv = 20 mol % in

excess of HBpin in optimal reaction conditions) also showed

clear evidence of the formation of cycloheptatriene, even when

the analysis was done instantly after mixing the two compounds.

Similar outcomes were observed with longer reaction times or at

0 °C (see Scheme 3d for a representative spectrum of the

mixture after 30 min at room temperature). It beckoned another

hypothesis we had at the beginning of this work that tropylium

ion could act as a hydride abstractor

12a,b

to generate a borenium

ion from HBpin, as observed by Crudden, Thomas, and

e,18

others.

a vinyl cation intermediate,

hydride from another molecule of HBpin to generate the target

This could then quickly react with the alkyne to form

18b,19

which subsequently takes a

product and start another reaction cycle (Scheme 3c). There

were also known examples of borenium-mediated haloboration

reactions via analogous reaction pathways.

However, the B NMR of the Trop.BF 1a/HBpin mixture

(Scheme 3e) did not show the presence of a borenium species.

This is most likely due to the fact that the borenium ion would

immediately combine with a nucleophilic species in the reaction

mixture, such as the ﬂuoride from tetraﬂuoroborate or the

bromide, to form more stable boranes. It is also possible that this

borenium species was undetected in our experimental settings.

mmol 1a neat or in the indicated solvent (2 mL) at the indicated

temperature for the indicated time.

catalysts are anhydrous.

Yield of the isolated 4a. All

as tropylium ion is known to coordinate to the nitrogen center of

the amino group and even remove hydride from tertiary

Apart from the remaining HBpin and BF anion, we initially

Scheme 2. Substrate Scope of the Hydroboration of Alkynes

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Scheme 3. Experimental Mechanistic Studies and Original Hypotheses of Reaction Mechanism: (a) Control Study With TMEDA;

(

b) Lewis Acid Activation Hypothesis; (c) Hydride Abstraction Activation Hypothesis; (d) H NMR Complexation Studies; and

(

e) B NMR Complexation Studies Between HBpin and 1a

Scheme 4. Third Mechanistic Hypothesis and Supporting Computational and Experimental Studies

could only identify the presence of BF , plus the formation of

Mechanistic Pathway Involving Tropylium Ion. In our

some unidentiﬁed boranes in the B NMR spectrum (Scheme

substrate scope study discussed above in Scheme 2, tropylium

bromide (1b) and triﬂate (1c) catalysts were also employed for a

selected number of substrates and gave comparable or slightly

lower eﬃciencies than tropylium tetraﬂuoroborate. It should be

noted that we chose not to carry out more examples with these

catalysts due to their hygroscopic and diﬃcult-to-handle nature.

On the other hand, as discussed earlier in Table 1, potassium or

tetrabutylammonium salts of the same counterions either

showed no catalytic activity or hindered the reaction, while

tritylium or nitrosonium salts could also catalyze the reaction

but with lower eﬃciencies than tropylium salts. It prompted us

to investigate the possibility of a more direct role of tropylium

ion in triggering the reaction, which is depicted in Scheme 4. We

hypothesized that the electrophilic tropylium ion could add to

the C−C triple bond to generate a vinyl cation intermediate 5,

e, also see B− F NMR studies and discussion later). On the

other hand, we have carried out an extensive density functional

theory (DFT) calculation to validate our hypotheses in either

Scheme 3b or 3c. Our DFT calculations showed that the

activation barrier for the uncatalyzed hydroboration between

HBpin and alkyne 2a is approximately 42.5 kcal/mol, which is

consistent with the fact that this hydroboration cannot occur

without a catalyst (see Figure S1 in the computational

Supporting Information for details). However, all transition

states and intermediates related to mechanistic hypotheses in

Scheme 3b,c are too energetically unfavored to be feasible (see

Figure S1 in the computational Supporting Information);

therefore, they could be considered unlikely.

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which can take a hydride from HBpin similar to the borenium

pathway in Scheme 3c. This would result in closely associated

pair 6, which would lead to the formation of carbocation 7.

Intermediate 7 could then eliminate a tropylium ion to form the

hydroboration product.

A combination of computational and experimental studies

was then carried out to ﬁnd supporting arguments for this new

hypothesis (Scheme 4). The addition of tropylium ion to the C−

C triple bond of 2a has an activation energy of 22.3 kcal/mol and

that of the subsequent hydride abstraction step between vinyl

cation 5 and HBpin is calculated to be 28.4 kcal/mol (Scheme

). The hydride transfer presumably leads to the formation of

the associated pair 6. Our attempt to optimize 6 always

generated carbocation 7, which suggests that 6 is unstable and

will transform into 7 barrierlessly. To conﬁrm this result, Born-

Oppenheimer molecular dynamics (BOMD) simulations were

carried out. We found that after TS-2, 6 is formed and rapidly

converted to 7 without crossing any barrier at around 248 fs (see

Figure S2 in the computational Supporting Information), which

is consistent with our DFT calculations on this species.

Figure 1. Kinetic studies of the conversion of 2a to reaction product 4a

with diﬀerent catalyst loadings, (inset) zoom-in version of initial

reaction stage.

While TS-2 is associated with high activation energy, making

this a challenging pathway, the rest of this reaction proﬁle looks

rather feasible. Because of the steric repulsion between Bpin and

phenyl moieties, TS-3 is calculated to be 6.4 kcal/mol lower in

energy than TS-4, resulting in the cis-selective 4a as the favored

product, which is consistent with our experiments. However, we

failed to ﬁnd experimental evidence of the formation of vinyl

cation 5 by trapping it with other nucleophiles. As a

representative example, we could not ﬁnd the methoxy-

cycloheptatrienylation adduct when we replaced the borane

reagent with methanol (Scheme 4, bottom-left). Interestingly, a

similar trapping reaction with the electron-rich aromatic alkyne

of 1a) seemed to undergo a catalyst pre-activation period of 5−

0 min. The lower catalyst loading was, the longer the reaction

took to start showing some conversion. The pre-activation time

of 5 mol % tropylium bromide was similar to that of 10 mol %

tropylium tetraﬂuoroborate. This prompted us to look deeper

into the reaction between tropylium salts and HBpin, which was

presumably the only chemical process that caused this pre-

activation period.

As discussed before, it was obviously evident that tropylium

ion abstracted a hydride from HBpin and turned into

cycloheptatriene, but what became of the rest eluded us earlier.

The pinacolatoborenium ion intermediate (see Scheme 3c) was

never observed so we believed it was captured by some

nucleophiles in the reaction mixture. This is presumably where

the counterions from tropylium salts came into play. With

tropylium tetraﬂuoroborate, it was previously reported that the

BF anion can dissociate to form BF and ﬂuoride, which can

o successfully led to the formation of adduct 9 in decent

eﬃciency without optimization of reaction conditions (Scheme

, bottom-right, also see page S16 in the experimental

Supporting Information ). The formation of 9 indicates that

when there is a strong stabilization eﬀect, in this case coming

from the conjugation of a p-NMe group, it is possible to form a

vinyl cation intermediate (5o), which reacts with methanol to

form 9. However, this type of reactivity might not be general for

the other alkyne substrates, as observed with the standard

substrate 2a. In addition to the high activation energy to TS-2,

although this does not rule out completely but casts some doubts

over our third hypothesis of this tropylium-promote hydro-

boration reaction mechanism.

capture the borenium ion into the corresponding ﬂuoropinaco-

10f

lato borane FBpin. A similar reaction can presumably happen

with tropylium bromide to form BrBpin.

Therefore, we decided to carry out further B and F NMR

spectroscopic analyses of the reaction between tropylium salts

and HBpin to get more insights (Figure 2). The pertinent

sections of B NMR spectra of HBpin and tropylium

Kinetic and B− F NMR Studies. Another revisit to our

catalyst scope in Table 1 revealed that tropylium salts with

chloride, bromide, and triﬂate and tetraﬂuoroborate counterions

catalyzed the reaction eﬃciently, while those with hexﬂuor-

ophosphate or BArF counterions gave unsatisfactory results.

Furthermore, while tritylium tetraﬂuoroborate led to almost the

same eﬃciency as the tropylium counterpart, tritylium

hexaﬂuorophosphate completely suppressed this hydroboration

reaction. These results indicated that while tropylium ion is an

essential component, the counterions have an undeniable role in

their catalytic activity. Hence, to further unravel the mechanism

of our chemistry, we carried out kinetic studies of reactions

catalyzed by 5 mol % of tropylium bromide (1b) and diﬀerent

loadings of tropylium tetraﬂuoroborate (1a) under optimal

conditions. The conversion to the hydroboration product was

tetraﬂuoroborate (1a) mixture at 4/1 ratio after 30 min at

room temperature are presented in Figure 2. After matching

signals with known boron species in the literature, we were able

to identify FBpin (∼21 ppm in B and ∼−153 ppm in

NMR) at all ratios. We did not observe any signals

corresponding to [FHBpin] , formed by the coordination of

−

ﬂuoride with HBpin, which was reported by the Thomas group

in their work with iron and cobalt tetraﬂuoroborate salts.

There were signals corresponding to some remaining BF anions

(∼−1 and ∼−151.7 ppm) and the byproduct BF (∼−1 and

∼−155.3 ppm). There were two more new boron compounds

formed in this mixture. Although we could not fully interpret

these signals, we believe that they correspond to BHF₂and

BH F (never been characterized by experimental NMR

spectroscopy before), which were presumably produced via

monitored by H NMR spectroscopy over time (Figure 1). The

substituent exchange reactions between HBpin and BF (vide

7e,27

interesting thing we learned from these studies is that most

reactions (except the one with high catalyst loading−15 mol %

infra).

It should be noted that in F NMR spectra, there are

signals corresponding to B (small peaks or shoulders) and B

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Figure 2. ¹¹B (128 MHz, CD CN, 298 K) and F (376.5 MHz, CD CN, 298 K) NMR spectra of HBpin and 1a (pure and mixture at 4/1 ratio, rt, 30

min).

(major peaks) isotopes for each species (see page S11 in the

whereas potassium and tetrabutylammonium salts show no

experimental Supporting Information ).

activity (Table 1). We also found that BF species can further

Mechanistic DFT Calculations. With our experimental

data pointing toward the formation of a complex mixture of a

range of borane species in the presence of HBpin and tropylium

tetraﬂuoroborate, we carried out computational studies to

react with HBpin via two transborylation transition states TS-6

and TS-7 (B−H/B−F σ-bond metathesis), giving BHF and

BH F species with the activation barriers calculated to be 27.6

kcal/mol relative to the BF intermediate (Figure 3a).

rationalize these results. Interestingly, our DFT calculations

Our DFT calculations demonstrate that for this hydro-

show that HBpin can indeed react with tropylium tetraﬂuor-

oborate via a hydride transfer transition state TS-5 to form

boration of alkyne, the formation of in situ BH F species is very

important and BH F can potentially act as a hidden boron

cycloheptatriene, FBpin, and BF species with the calculated

catalyst. The catalytic cycle for hydroboration is shown in Figure

barrier of 25.3 kcal/mol (Figure 3a). This result is consistent

with the experimental ﬁnding that other hydride acceptors,

including tritylium and nitrosonium tetraﬂuoroborate, can also

promote this hydroboration reaction albeit with lower eﬃciency,

3b. From BH F, the 1,2-syn-addition to 2a via TS-8 is found to

occur giving boron alkene intermediate 10 with the calculated

barrier of 9.9 kcal/mol relative to BH F. To proceed,

transborylation (B−C/B−H σ-bond metathesis) between 10

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hydroboration of alkyne catalyzed by BH F. In Figure 3 b, energy values (kcal/mol) are relative to BH F. All bond distances are in Ångstrom.

Article

Figure 3. (a) Free-energy proﬁle (kcal/mol) for the formation of BH F from HBpin and tropylium tetraﬂuoroborate and (b) catalytic cycle for the

Figure 4. (a) Free-energy proﬁle (kcal/mol) for the formation of BH Br from HBpin and tropylium bromide and (b) catalytic cycle for the

hydroboration of alkyne catalyzed by BH F (energy values are relative to BH Br).

and HBpin can then take place in a stepwise fashion via TS-9 and

TS-10 giving product 4a with the retention of E-conﬁguration.

The activation barriers of TS-9 and TS-10 are calculated to be

This result gives a rationalization for the negative eﬀect of

solvents on this hydroboration reaction (see Table 1).

Here, it should be mentioned that we have also carried out

DFT calculations for various hydroboration pathways that could

be triggered by each boron intermediate of the reaction,

2.4 and 22.7 kcal/mol relative to intermediate 10. The

transborylation involves a transient μ-H bridged species 11

consisting of two three-center two-electron (“3c−2e”) bonds,

which are employed by the sp hybrid orbital of bridging carbon

atom and the 1s orbital of bridging hydrogen atom (see Figure

S3 in the computational Supporting Information for the intrinsic

bond orbital analysis of intermediate 11). Our DFT calculations

are in agreement with a recent study by Thomas and Lloyd-

including the possibilities that (i) BHF intermediate acts as the

hydroboration catalyst and (ii) BH₃forms from further

reduction of BH F by HBpin and subsequently acts as the

catalyst. The catalytic cycle for the hydroboration of alkyne

catalyzed by BHF is shown in Figure S4 in the computational

Supporting Information. The overall activation barrier for this

catalytic cycle is calculated to be 30.4 kcal/mol, which is indeed

Jones.

The activation barrier for the formation of the BH F catalyst

from HBpin and tropylium tetraﬂuoroborate is calculated to be

much less favorable than the same process with BH F (Figure

3b) and unlikely to happen. The reaction mechanism for the

7.6 kcal/mol (Figure 3a), whereas the rate-determining step for

hydroboration catalyzed by BH is shown in Figure S5 in the

computational Supporting Information. It is obvious that BH₃

the hydroboration of alkyne by the BH F catalyst is 22.7 kcal/

mol (Figure 3b). These results suggest that the reaction has to

suspend until the formation of BH F. Once BH F species is

generated, the hydroboration of alkyne can take place eﬃciently.

This ﬁnding is consistent with our experimental data that an

can also eﬃciently catalyze the conversion of 2a to 4a. However,

because of the high reactivity of BH species, the reaction is

unlikely to stop there. Intrinsic reaction coordinate (IRC)

calculation and BOMD simulations indicate that at around 271

fs after the transborylation giving BH and 4a, BH species can

“activation period” exists at the beginning of the reaction (Figure

). Furthermore, solvents that can strongly bind to the boron

quickly react to the C−C double bond of 4a without crossing

any barrier, giving an alkane intermediate (see Figures S6 and S7

in the computational Supporting Information). This is followed

by a transborylation with HBpin giving gem-diboron alkane as

the favorable product. This is inconsistent with our experimental

intermediates in Figure 3a are likely to hinder the reaction. For

example, the binding energy of THF to BF is calculated to be

2.9 kcal/mol, which thus increases the activation barrier of TS-

in THF to be 40.5 kcal/mol relative to the BF −THF complex.

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Scheme 5. (a) Competitive Reactions Between Alkyne and Alkene; (b) Substrate Scope of the Hydroboration of Alkenes

ﬁndings in that we can only observe the semi-hydroboration

adduct 4a with our tropylium catalysts.

From BrBpin, two transborylation steps between BrBpin and

two HBpin molecules then take place giving BH Br, and B pin

A control study using BH₃(commercially available in

is produced as the by-product, which will act as a catalyst for the

hydroboration of the alkyne (Figure 4a). The catalytic cycle for

complexes with Me S or THF) as the catalyst indeed led to

the formation of the gem-diboron alkane product. Moreover, the

the hydroboration of alkyne 2a by BH Br is shown in Figure 4b,

activation barrier for the transborylation between BH F and

and the reaction mechanism is found to be similar to the

HBpin giving BH is calculated to be 8.1 kcal/mol higher than

hydroboration with BH F. The reaction starts with a 1,2-syn-

that of the 1,2-syn-addition between BH F and alkyne 2a (see

addition via TS-14 between BH Br and alkyne, which is

Figure S1 in the computational Supporting Information for

followed by a stepwise transborylation via TS-15 and TS-16

giving alkenylboron product 4a. The hydroboration catalytic

detail), which indicates that BH is unlikely to be the catalyst for

our hydroboration reaction. Our mechanism, therefore, can be

cycle by BH Br in Figure 4b has lower energy barriers than the

considered a hybridization of the hidden catalysis by borane

and the counterion-activated catalysis by Fe/Co tetraﬂuorborate

one by BH F in Figure 3b, which is consistent with our

experimental data that Trop.Br 1b led to a faster reaction than

salts. Both of these were recently reported by the Thomas

group as game-changing factors in hydroboration catalysis.

While our mechanistic proposal still needs further validation in

future work, the fact that a simple species such as tropylium

tetraﬂuoroborate 1a could potentially trigger hydroboration in

several diﬀerent ways is intriguing.

the same catalyst loading of Trop.BF 1a (Figure 1).

It should be noted here that we have put tremendous amount

of work to attempt to directly catalyze the hydroboration

reaction under investigation with BH F or BH Br, which can be

synthesized in situ by mixing appropriate BHal with BH in a 1:2

ratio. These studies led to positive reaction outcomes. For

Mechanism of Reaction with Tropylium Bromide. As

the hydroboration reaction can also work with tropylium

bromide 1b as catalyst (Scheme 2), it has to activate the system

example, approximately 10 mol % of the prepared [BH Hal] can

catalyze the conversion of 2a to 4a in 40−80% yields. However,

these experiments were rather operationally diﬃcult to set up,

hence leading to varying outcomes which are hard to reproduce.

Therefore, while giving supporting evidence for our mechanistic

hypothesis, the method is a lot more practical using tropylium

salts as reaction promoters.

through a diﬀerent pathway since there is no BF anion. The

experimental ﬁnding suggests that 5 mol % tropylium bromide

can promote the hydroboration with similar kinetics and

eﬃciency as 10 mol % tropylium tetraﬂuoroborate (see Figure

). Our DFT calculations suggest that HBpin can react with

Hydroboration of Alkenes and Epoxides. Thus, a

combination of experiments and DFT calculations suggest

that HBpin can react with tropylium salts, that is, tropylium

tropylium bromide via TS-11 to form BrBpin with the calculated

barrier of 21.7 kcal/mol (Figure 4a).

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Scheme 6. (a) Substrate Scope of Epoxide Hydroboration; (b) Free-Energy Proﬁle (kcal/mol) for the Hydroboration of Epoxide

15 Catalyzed by BH₂F

tetraﬂuoroborate or bromide, to generate in situ boron catalysts,

opening transition state TS-17 giving zwitterionic species 21,

followed by a hydride transfer from boron moiety to the

carbocationic center via TS-18 to form intermediate 22. To

proceed, transborylation (B−O/B−H σ-bond metathesis)

between 20 and HBpin in a concerted manner via TS-19 occurs

to give product 16. We could not locate any transient μ-H

bridged species in the transborylation step. The activation

barriers of the hydride transfer TS-18 and the transborylation

TS-19 are calculated to be 20.9 and 23.3 kcal/mol relative to

those of 18 and 20, respectively.

that is, BH F and BH Br. We were curious to see if their catalytic

activity can be amenable to other types of hydroboration

reactions. We chose to investigate this on alkenes and epoxides,

as they are more challenging substrates than carbonyl or imine

b,3a,c,6l

compounds.

A competitive reaction between phenyl-

acetylene 2a and styrene 13a with limiting the amount of HBpin

showed that the hydroboration of alkenes is less favorable than

that of alkynes (Scheme 5a). However, with the same set of

reaction conditions used in Scheme 2, we were able to

hydroborylate a range of aromatic as well as aliphatic alkenes

to the desired products with moderate to high yields. Tropylium

salts 1a and 1b promoted the reaction with similar eﬃciencies on

the substrates we selected for comparative studies (Scheme 5b).

We have also calculated the reaction mechanism for the

CONCLUSIONS

■

In conclusion, we report a novel hydroboration protocol using

tropylium salts as eﬃcient reaction promoters. This method is

amenable to alkynes, alkenes, and epoxides to produce a broad

range of vinyl- and alkylboranes as well as alcohols. Apart from

the synthetic value of this metal-free protocol, tropylium salts

proved to be attractive and versatile probes to explore

mechanistic details of the hydroboration reaction. This work

oﬀers insights into the controversial topic of hidden or true

catalysis for hydroboration reactions. Experimental studies and

DFT calculations suggested that the reaction likely follows some

interesting pathways, which are triggered by a hydride

abstraction of pinacolborane with tropylium ion. This is

followed by a series of in situ counterion-activated substituent

exchanges to generate boron intermediates that promote the

hydroboration reaction. Further applications of this new

protocol to multi-boration reactions or with other boranes

such as HBdan are currently being developed in our lab and will

be reported in due course.

hydroboration of alkene catalyzed by BH F species. The

mechanism of this reaction is indeed similar to the hydro-

boration of alkyne where the ﬁrst step is the 1,2-syn-addition to

alkene 13, followed by transborylation giving a boron alkane

product 14 (see Figure S8 in the computational Supporting

Information ). Our calculated energy barriers were also

consistent with the fact that hydroboration of alkenes is more

challenging than alkynes.

Similar to alkenes, tropylium-catalyzed hydroboration of

epoxides was slightly more challenging than the reaction with

alkynes. It required a higher catalyst loading of tropylium

catalyst and longer reaction time to achieve complete conversion

(

Scheme 6a). It should be noted that the boronic ester products

6 were hydrolyzed to alcohols 17 during reaction workup for a

better puriﬁcation process. However, due to this extra step,

yields of the isolated alcohol products are not reﬂective of the

reaction eﬃciency. Indeed, H and B NMR analyses of the

crude reaction mixtures before workup showed a 90%

quantitative conversion of the epoxides to their corresponding

boronic esters.

EXPERIMENTAL SECTION

General Methods. Reactions, unless otherwise stated, were

conducted in an oven-dried vial equipped with a stirrer bar under a

dry nitrogen atmosphere. 4,4,5,5-Tetramethyl-1,3,2-dioxaborolane

■

We then focused on elucidating the reaction mechanism for

the hydroboration of epoxides. The computed free-energy

(HBpin) was purchased from Sigma-Aldrich and was freshly distilled

before use. Tropylium tetraﬂuoroborate was purchased from Sigma-

Aldrich and was freshly recrystallized before use. The other tropylium

salts were synthesized in situ by ion-exchanging with tropylium

proﬁle is shown in Scheme 6b. It turns out that BH F can also act

as a catalyst for this reaction. The reaction starts with a ring−

125

J. Org. Chem. 2021, 86, 9117−9133

The Journal of Organic Chemistry

Article

115.2, 83.3, 24.9, and 21.4 ppm; ¹¹B{ H} NMR (128 MHz, CDCl ): δ

tetraﬂuoroborate. Solvents for reaction setup were obtained from a

solvent puriﬁcation system and tested for water content (<30 ppm).

Other commercially available reagents and solvents were used as

purchased unless otherwise noted. Analytical thin-layer chromatog-

raphy was performed using silica gel plates precoated with silica gel 60

F₂₅₄(0.2 mm). Flash chromatography employed 230−400 mesh silica

gel. Solvents used for chromatography are quoted as volume/volume

ratios.

30.47 ppm. The NMR is consistent with literature data.

(E)-2-(4-Propylstyryl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane

(4c). Prepared according to the general procedure from 1-propyl-4-

ethynylbenzene (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxabor-

olane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by ﬂash

column chromatography eluting with hexanes/DCM (7:3) to provide

the title compound as a yellow oil (250 mg, 0.92 mmol, 92% yield). H

NMR (400 MHz, CDCl ): δ 7.56−7.37 (m, 3H), 7.25−7.13 (m, 2H),

NMR spectroscopy was performed at 298 K using an AVANCE III

.17 (d, J = 18.4 Hz, 1H), 2.61 (dd, J = 8.5, 6.7 Hz, 2H), 1.82−1.60 (m,

HD 400 (400.1 MHz, H; 100.6 MHz, C; 128 MHz, B; 376.5 MHz,

13 1

H), 1.35 (s, 12H), and 0.97 (t, J = 7.3 Hz, 3H) ppm; C{ H} NMR

F), an AVANCE III 300 (300 MHz, H; 75 MHz, C; 96 MHz, B;

82.5 MHz, F), or an AVANCE III 400 (400.1 MHz, H; 100.6 MHz,

C; 128 MHz, B; 376.5 MHz, F). Data are expressed in parts per

(101 MHz, CDCl ): δ 149.6, 143.8, 135.1, 128.7, 127.0, 115.3, 83.3,

83.1, 37.9, 24.8, 24.4, and 13.8 ppm; B{ H} NMR (128 MHz,

CDCl ): δ 30.36 ppm. The NMR is consistent with literature data.

E)-4,4,5,5-Tetramethyl-2-(4-pentylstyryl)-1,3,2-dioxaborolane

4d). Prepared according to the general procedure from 1-ethynyl-4-

pentylbenzene (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxaboro-

lane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by ﬂash column

chromatography eluting with hexanes/DCM (7:3) to provide the title

million (ppm) downﬁeld shift from tetramethylsilane with residual

solvent as an internal reference (δ 7.26 ppm for chloroform) and is

reported as position (δ in ppm), multiplicity (s = singlet, d = doublet, t =

triplet, q = quartet, m = multiplet), coupling constant (J in Hz), and

(

integration (number of protons). C NMR spectra were recorded at

298 K with complete proton decoupling. Data are expressed in parts per

compound as a light yellow oil (276 mg, 0.92 mmol, 92% yield). H

million (ppm) downﬁeld shift relative to the internal reference (δ 77.2

ppm for the central peak of deuterated chloroform).

NMR (400 MHz, CDCl ): δ 7.60−7.32 (m, 3H), 7.26−7.13 (m, 2H),

(

.16 (d, J = 18.4 Hz, 1H), 2.75−2.54 (m, 2H), 1.75−1.58 (m, 2H), 1.35

Infrared spectra were obtained on a ThermoNicolet Avatar 370 FT-

13 1

s, 16H), and 1.08−0.86 (m, 3H) ppm; C{ H} NMR (101 MHz,

−

IR spectrometer and are reported in wavenumbers (cm ). HRMS were

performed at the Bioanalytical Mass Spectrometry Facility within the

Mark Wainwright Analytical Centre at the University of New South

Wales on an Orbitrap LTQ XL (Thermo Fisher Scientiﬁc, San Jose, CA,

USA) ion trap mass spectrometer.

CDCl ): δ 149.6, 144.1, 135.1, 128.7, 127.1, 115.2, 83.3, 35.8, 31.5,

1.02, 24.8, 22.6, and 14.0 ppm; B{ H} NMR (128 MHz, CDCl ): δ

9.95 ppm. The NMR is consistent with literature data.

(

E)-2-(2-([1,1′-Biphenyl]-4-yl)vinyl)-4,4,5,5-tetramethyl-1,3,2-di-

oxaborolane (4e). Prepared according to the general procedure from

General Procedure for Substrate Scope. Alkynes (Scheme 2) or

alkenes (Scheme 5). To a mixture of alkyne or alkene substrate (1.0

mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (HBpin, ∼153.6

mg, 1.2 mmol, 1.2 equiv) in a reaction vial equipped with a stirrer bar

was added tropylium tetraﬂuoroborate catalyst (∼9.0 mg, 0.05 mmol, 5

mol %, or the equivalent molar amount if tropylium bromide or

tropylium triﬂate was used). The reaction mixture was heated to 70 °C

on a heating mantle for 12 h, with the reaction progress monitored by

TLC. The product was directly puriﬁed from the reaction mixture by

ﬂash column chromatography (silica-gel, DCM/hexanes).

-ethynyl-1,1′-biphenyl (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-

dioxaborolane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by

ﬂash column chromatography eluting with hexanes/DCM (7:3) to

provide the title compound as a white solid (165 mg, 0.54 mmol, 54%

yield). H NMR (400 MHz, CDCl ): δ 7.67−7.60 (m, 6H), 7.53−7.44

(

m, 3H), 7.41−7.35 (m, 1H), 6.26 (d, J = 18.4 Hz, 1H), and 1.37 (s,

13 1

2H) ppm; C{ H} NMR (101 MHz, CDCl ): δ 149.1, 141.6, 140.6,

36.5, 128.8, 127.6, 127.5, 127.3, 127.0, 83.4, and 24.9 ppm; B{ H}

NMR (128 MHz, CDCl ): δ 30.79 ppm. The NMR is consistent with

literature data.

Epoxides (Scheme 6). To a mixture of epoxide (1.0 mmol) and

,4,5,5-tetramethyl-1,3,2-dioxaborolane (HBpin, ∼153.6 mg, 1.2

(E)-4,4,5,5-Tetramethyl-2-(2-(phenanthren-9-yl)vinyl)-1,3,2-diox-

aborolane (4f). Prepared according to the general procedure from 9-

ethynylphenanthrene (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-

dioxaborolane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by

ﬂash column chromatography eluting with hexanes/DCM (1:1) to

provide the title compound as a yellow oil (214 mg, 0.65 mmol, 65%

yield). H NMR (400 MHz, CDCl ): δ 8.79−8.70 (m, 1H), 8.71−8.61

mmol, 1.2 equiv) in a reaction vial equipped with a stirrer bar was

added tropylium tetraﬂuoroborate catalyst (∼18.0 mg, 0.1 mmol, 10

mol %). The reaction mixture was heated to 70 °C on a heating mantle

for 24 h, with the reaction progress monitored by TLC. The product

was directly puriﬁed from the reaction mixture by ﬂash column

chromatography (silica-gel, EtOAc/hexanes).

(

m, 1H), 8.40−8.35 (m, 1H), 8.31 (d, J = 18.0 Hz, 1H), 8.03 (s, 1H),

Characterization Data of Products in Scheme 2. (E)-4,4,5,5-

Tetramethyl-2-styryl-1,3,2-dioxaborolane (4a). Prepared according

to the general procedure from ethynylbenzene (1.0 mmol, ∼102 mg)

and 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (HBpin, ∼153.6 mg, 1.2

mmol, 1.2 equiv), puriﬁed by ﬂash column chromatography eluting

with hexanes/DCM (7:3) to provide the title compound as a light

yellow oil (225 mg, 0.98 mmol, 98% yield). The procedure could be

scaled up using ethynylbenzene (10.0 mmol, ∼1.02 g) and 4,4,5,5-

tetramethyl-1,3,2-dioxaborolane (HBpin, ∼1.536 g, 12 mmol, 1.2

.94 (dd, J = 7.6, 1.7 Hz, 1H), 7.73−7.61 (m, 5H), 6.46 (d, J = 18.1 Hz,

H), and 1.45 (s, 12H) ppm; C{ H} NMR (101 MHz, CDCl ): δ

47.3, 134.6, 131.7, 130.7, 130.4, 129.1, 127.0, 126.8, 126.7, 126.6,

25.4, 124.7, 123.1, 122.6, 83.5, 48.8, and 25.0 ppm; B{ H} NMR

(128 MHz, CDCl ): δ 31.16 ppm. The NMR is consistent with

literature data.

(E)-4,4,5,5-Tetramethyl-2-(4-(triﬂuoromethyl)styryl)-1,3,2-dioxa-

borolane (4g). Prepared according to the general procedure from 1-

ethynyl-4-(triﬂuoromethyl)benzene (1.0 mmol) and 4,4,5,5-tetrameth-

yl-1,3,2-dioxaborolane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv),

puriﬁed by ﬂash column chromatography eluting with hexanes/DCM

(7:3) to provide the title compound as a pale yellow solid (259 mg, 0.87

mmol, 87% yield). H NMR (400 MHz, CDCl ): δ 7.64−7.56 (m, 4H),

7.43 (d, J = 18.4 Hz, 1H), 6.28 (d, J = 18.4 Hz, 1H), and 1.34 (s, 12H)

equiv) to yield the product (1.95 g, 8.5 mmol, 85%). H NMR (400

MHz, CDCl ): δ 7.52 (dt, 2H), 7.44 (d, J = 18.4 Hz, 1H), 7.40−7.29

(

m, 3H), 6.21 (d, J = 18.4 Hz, 1H), and 1.35 (s, 12H) ppm; C{ H}

NMR (101 MHz, CDCl ): δ 149.5, 137.5, 128.9, 128.6, 127.1, 116.4,

3.4, and 24.8 ppm; ¹¹B{ H} NMR (128 MHz, CDCl ): δ 30.62 ppm.

The NMR is consistent with literature data.

ppm; C{ H} NMR (101 MHz, CDCl ): δ 147.7, 140.8, 130.5 (q, J =

(

E)-4,4,5,5-Tetramethyl-2-(4-methylstyryl)-1,3,2-dioxaborolane

32.4 Hz), 127.2, 125.6 (d, J = 3.7 Hz), 122.8, 120.1, 83.6, and 24.8 ppm;

(

4b). Prepared according to the general procedure from 1-ethynyl-4-

B{ H} NMR (128 MHz, CDCl ): δ 29.95 ppm; F{ H} NMR (376

methylbenzene (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxabor-

olane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by ﬂash

column chromatography eluting with hexanes/DCM (7:3) to provide

MHz, CDCl ): δ −62.65 ppm. The NMR is consistent with literature

data.

(E)-2-(3,5-Bis(triﬂuoromethyl)styryl)-4,4,5,5-tetramethyl-1,3,2-di-

the title compound as a pale yellow oil (227 mg, 0.93 mmol, 93% yield).

oxaborolane (4h). Prepared according to the general procedure from

1-ethynyl-3,5 bis(triﬂuoromethyl)benzene (1.0 mmol) and 4,4,5,5-

tetramethyl-1,3,2-dioxaborolane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2

equiv), puriﬁed by ﬂash column chromatography eluting with hexanes/

H NMR (400 MHz, CDCl ): δ 7.62−7.29 (m, 3H), 7.18 (d, J = 7.9 Hz,

H), 6.18 (d, J = 18.5 Hz, 1H), 2.37 (s, 3H), and 1.35 (s, 12H) ppm;

C{ H} NMR (101 MHz, CDCl ): δ 149.6, 138.9, 134.9, 129.3, 127.1,

126

J. Org. Chem. 2021, 86, 9117−9133

The Journal of Organic Chemistry

Article

DCM (7:3) to provide the title compound as a yellow oil (296 mg, 0.81

methoxybenzene (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-

dioxaborolane(HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by

ﬂash column chromatography eluting with hexanes/DCM (7:3) to

mmol, 81% yield). H NMR (300 MHz, CDCl ): δ 7.90 (d, J = 1.7 Hz,

H), 7.84−7.75 (m, 1H), 7.43 (d, J = 18.4 Hz, 1H), 6.34 (d, J = 18.4 Hz,

H), and 1.33 (s, 12H) ppm; ¹³C{ H} NMR (76 MHz, CDCl ): δ

provide the title compound as a yellow oil (216 mg, 0.83 mmol, 83%

45.7, 139.5, 132.0 (q, J = 33.3 Hz), 128.0−125.8 (m), 125.0, 122.5−

yield). H NMR (400 MHz, CDCl ): δ 7.47−7.42 (m, 2H), 7.38 (d, J =

21.7 (m), 121.4, 83.8, and 24.7 ppm; B{ H} NMR (96 MHz,

18.4 Hz, 1H), 6.87 (d, J = 8.8 Hz, 2H), 6.04 (d, J = 18.4 Hz, 1H), 3.79

(s, 3H), and 1.32 (s, 12H) ppm; C{ H} NMR (101 MHz, CDCl ): δ

160.3, 149.1, 130.4, 128.5, 114.0, 83.2, 55.2, and 24.8 ppm; B{ H}

CDCl ): δ 30.13 ppm; F{ H} NMR (283 MHz, CDCl ): δ −63.22

ppm. The NMR is consistent with literature data.

(

E)-2-(4-Isocyanostyryl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane

NMR (128 MHz, CDCl ): δ 30.12 ppm. The NMR is consistent with

literature data.

(

4i). Prepared according to the general procedure from 1-ethynyl-4-

isocyanobenzene (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxabor-

olane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by ﬂash

column chromatography eluting with hexanes/DCM (7:3) to provide

(E)-N,N-Dimethyl-4-(2-(4,4,5,5-tetramethyl-1,3,2-Dioxaborolan-

2-yl)vinyl)aniline (4o). Prepared according to the general procedure

from 4-ethynyl-N,N-dimethylaniline (1.0 mmol) and 4,4,5,5-tetra-

methyl-1,3,2-dioxaborolane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv),

puriﬁed by ﬂash column chromatography eluting with hexanes/DCM

the title compound as a white solid (153 mg, 0.60 mmol, 60% yield). H

NMR (400 MHz, CDCl ): δ 7.64 (d, J = 8.4 Hz, 2H), 7.56 (d, J = 8.4

Hz, 2H), 7.38 (d, J = 18.4 Hz, 1H), 6.29 (d, J = 18.4 Hz, 1H), and 1.33

(7:3) to provide to yield the title compound as a pale yellow solid (221

s, 12H) ppm; ¹C{ H} NMR (101 MHz, CDCl ): δ 147.1, 141.7,

(

mg, 0.81 mmol, 81% yield). H NMR (400 MHz, CDCl ): δ 7.45−7.41

32.5, 127.4, 118.8, 112.0, 83.7, and 24.8 ppm; ¹¹B{ H} NMR (128

(m, 2H), 7.37 (d, J = 18.4 Hz, 1H), 6.71−6.67 (m, 2H), 5.96 (d, J = 18.3

Hz, 1H), 3.00 (s, 6H), and 1.34 (s, 12H) ppm; ¹³C{ H} NMR (101

MHz, CDCl ): δ 30.33 ppm. The NMR is consistent with literature

data.

E)-2-(4-Fluorostyryl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane

4j). Prepared according to the general procedure from 1-ethynyl-4-

ﬂuorobenzene (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxaborolane

MHz, CDCl ): δ 151.0, 149.8, 128.4, 125.9, 112.0, 83.0, 40.3, and 24.8

ppm; ¹¹B{ H} NMR (128 MHz, CDCl ): δ 30.31 ppm. The NMR is

(

consistent with literature data.

(E)-4,4,5,5-Tetramethyl-2-(2-(thiophen-2-yl)vinyl)-1,3,2-dioxa-

borolane (4p). Prepared according to the general procedure from 2-

ethynylthiophene (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxabor-

olane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by ﬂash

column chromatography eluting with hexanes/DCM (7:3) to provide

(

HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by ﬂash column

chromatography eluting with hexanes/DCM (1:1) to provide the title

compound as a pale yellow oil (208 mg, 0.84 mmol, 84% yield). H

NMR (400 MHz, CDCl ): δ 7.51−7.42 (m, 2H), 7.37 (d, J = 18.4 Hz,

H), 7.02 (t, J = 8.7 Hz, 2H), 6.09 (d, J = 18.4 Hz, 1H), and 1.31 (s,

the title compound as a yellow oil (160 mg, 0.68 mmol, 68% yield). H

2H) ppm; ¹³C{ H} NMR (101 MHz, CDCl ): δ 164.4, 161.9, 148.2,

NMR (400 MHz, CDCl

): δ 7.50 (d, J = 18.1 Hz, 1H), 7.30−7.24 (m,

33.7, 128.7, 115.7, 115.4, 83.4, and 24.8 ppm; B{ H} NMR (128

1H), 7.13−7.09 (m, 1H), 7.01 (dd, 1H), 5.94 (d, J = 18.1 Hz, 1H), and

MHz, CDCl ): δ 30.23 ppm; F{ H} NMR (376 MHz, CDCl ): δ

−

1.32 (s, 12H) ppm; C{ H} NMR (101 MHz, CDCl

): δ 144.0, 127.7,

112.29 ppm. The NMR is consistent with literature data.

127.6, 126.3, 83.4, and 24.8 ppm; B{ H} NMR (128 MHz, CDCl ): δ

30.15 ppm. The NMR is consistent with literature data.

(

E)-2-(4-Chlorostyryl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane

(4k). Prepared according to the general procedure from 1-chloro-4-

1,3,5-Tris((E)-2-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)-

vinyl)benzene (4q). Prepared according to the general procedure from

1,3,5-triethynylbenzene (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-

dioxaborolane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by

ﬂash column chromatography eluting with hexanes/DCM (7:3) to

ethynylbenzene (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxabor-

olane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by ﬂash

column chromatography eluting with hexanes/DCM (7:3) to provide

the title compound as a light yellow solid (190 mg, 0.72 mmol, 72%

yield). H NMR (400 MHz, CDCl ): δ 7.43−7.40 (m, 2H), 7.35 (d, J =

provide the title compound as a white solid (443 mg, 0.83 mmol, 83%

8.4 Hz, 1H), 7.32−7.29 (m, 2H), 6.15 (d, J = 18.4 Hz, 1H), and 1.32

yield). H NMR (400 MHz, CDCl

): δ 7.53 (s, 3H), 7.37 (d, J = 18.4

(

s, 12H) ppm; ¹C{ H} NMR (101 MHz, CDCl ): δ 148.0, 136.0,

Hz, 3H), 6.17 (d, J = 18.4 Hz, 3H), and 1.30 (s, 36H) ppm; C{ H}

34.6, 128.8, 128.2, 117.2, 83.5, and 24.8 ppm; ¹¹B{ H} NMR (128

NMR (101 MHz, CDCl ): δ 148.8, 138.1, 126.1, 117.4, 83.4, and 24.8

ppm; ¹¹B{ H} NMR (128 MHz, CDCl

): δ 31.6 ppm. The NMR is

MHz, CDCl ): δ 29.78 ppm. The NMR is consistent with literature

data.

E)-2-(4-Bromostyryl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane

4l). Prepared according to the general procedure from 1-bromo-4-

consistent with literature data.

(

1,3-Bis((E)-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)vinyl)-

benzene (4r). Prepared according to the general procedure from 1,3-

diethynylbenzene (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxabor-

olane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by ﬂash

column chromatography eluting with hexanes/DCM (7:3) to provide

(

ethynylbenzene (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxabor-

olane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by ﬂash

column chromatography eluting with hexanes/DCM (7:3) to provide

the title compound as a pale yellow solid (262 mg, 0.85 mmol, 85%

the title compound as a white solid (260 mg, 0.68 mmol, 68% yield). H

yield). H NMR (300 MHz, CDCl ): δ 7.47 (d, J = 8.5 Hz, 2H), 7.37 (s,

NMR (400 MHz, CDCl

): δ 7.58 (d, J = 1.8 Hz, 1H), 7.45 (dd, 2H),

H), 7.32 (d, J = 9.0 Hz, 1H), 6.17 (d, J = 18.4 Hz, 1H), and 1.32 (s,

7.40 (d, J = 18.4 Hz, 2H), 7.33 (dd, 1H), 6.19 (d, J = 18.4 Hz, 2H), and

2H) ppm; ¹³C{ H} NMR (76 MHz, CDCl ): δ 148.07, 136.41,

1.33 (s, 24H) ppm; C{ H} NMR (101 MHz, CDCl ): δ 149.2, 137.8,

31.76, 128.52, 122.91, 117.40, 83.46, and 24.83 ppm; B{ H} NMR

128.8, 127.4, 126.1, 116.9, 83.4, and 24.8 ppm; B{ H} NMR (128

(96 MHz, CDCl ): δ 30.26 ppm. The NMR is consistent with literature

MHz, CDCl ): δ 30.49 ppm. The NMR is consistent with literature

data.

(

E)-2-(4-Ethoxystyryl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane

1,4-Bis((E)-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)vinyl)-

benzene (4s). Prepared according to the general procedure from 1,4-

diethynylbenzene (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxabor-

olane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by ﬂash

column chromatography eluting with hexanes/DCM (1:1) to provide

(

4m). Prepared according to the general procedure from 1-ethoxy-4-

ethynylbenzene (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxabor-

olane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by ﬂash

column chromatography eluting with hexanes/DCM (7:3) to provide

the title compound as a yellow solid (249 mg, 0.91 mmol, 91% yield).

the title compound as a white solid (286.5 mg, 0.75 mmol, 75% yield).

H NMR (400 MHz, CDCl ): δ 7.43−7.39 (m, 2H), 7.36 (d, J = 18.4

H NMR (400 MHz, CDCl ): δ 7.47 (s, 4H), 7.39 (d, J = 18.4 Hz, 2H),

Hz, 1H), 6.83 (d, J = 8.8 Hz, 2H), 6.01 (d, J = 18.4 Hz, 1H), 3.99 (q, J =

6.19 (d, J = 18.4 Hz, 2H), and 1.33 (s, 24H) ppm; C{ H} NMR (101

MHz, CDCl ): δ 148.86, 137.99, 127.33, 116.91, 83.39, and 24.82 ppm;

B{ H} NMR (128 MHz, CDCl ): δ 31.47 ppm. The NMR is

.0 Hz, 2H), 1.37 (t, J = 7.0 Hz, 3H), and 1.29 (s, 12H) ppm; ¹³C{ H}

NMR (101 MHz, CDCl ): δ 159.7, 149.2, 130.2, 128.5, 114.5, 144.1,

3.2, 63.4, 24.8, and 14.8 ppm; B{ H} NMR (128 MHz, CDCl ): δ

consistent with literature data.

9.80 ppm. The NMR is consistent with literature data.

E)-2-(4-Methoxystyryl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane

4n). Prepared according to the general procedure from 1-ethynyl-4-

(Z)-4,4,5,5-Tetramethyl-2-(1-phenyl(prop-1-en)-2-yl)-1,3,2-

dioxa-borolane (4t Isomer 1). Prepared according to the general

procedure from 1-phenyl-1-propyne (1.0 mmol) and 4,4,5,5-

(

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tetramethyl-1,3,2-dioxaborolane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2

equiv), puriﬁed by ﬂash column chromatography eluting with hexanes/

DCM (7:3) to provide boronic ester (151.3 mg, 0.62 mmol, 62% (a/b =

16.98, and 7.85 ppm; ¹¹B{ H} NMR (128 MHz, CDCl ): δ 29.72 ppm.

The NMR is consistent with literature data.

(E)-2-(2-Cyclohexylvinyl)-4,4,5,5-tetramethyl-1,3,2-dioxaboro-

lane (4z). Prepared according to the general procedure from

ethynylcyclohexane (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxa-

borolane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by ﬂash

column chromatography eluting with hexanes/DCM (7:3) to provide

the title compound as a colorless oil (149 mg, 0.63 mmol, 63% yield).

H NMR (400 MHz, CDCl ): δ 6.60 (dd, J = 18.2, 6.2 Hz, 1H), 5.39

: 1) as a colorless oil. The product was isolated as a mixture of

regioisomers (a + b). H NMR (400 MHz, CDCl ): δ (isomer a) =

.46−7.42 (m, 2H), 7.40 (d, J = 7.4 Hz, 2H), 7.36 (d, J = 7.3 Hz, 1H),

.33−7.29 (m, 1H), 2.06 (d, J = 1.8 Hz, 3H), and 1.37 (s, 12H) ppm;

C{ H} NMR (101 MHz, CDCl ): δ (isomer a) = 142.5, 139.8, 138.0,

129.4, 128.1, 127.1, 125.9, 83.5, 24.9, and 16.0 ppm; B{ H} NMR

(

dd, J = 18.2, 1.5 Hz, 1H), 2.06−2.02 (m, 1H), 1.78−1.72 (m, 4H),

(

128 MHz, CDCl ): δ 30.32 ppm. The NMR is consistent with

.70−1.62 (m, 1H), 1.30−1.29 (m, 12H), 1.19−1.13 (m, 4H), and

literature data.

.12−1.06 (m,1H) ppm; C{ H} NMR (101 MHz, CDCl ): δ 159.9,

(

Z)-Trimethyl(2-phenyl-1-(4,4,5,5-tetramethyl-1,3,2-dioxaboro-

11 1

lan-2-yl)vinyl)silane (4u). Prepared according to the general procedure

from trimethyl(phenylethynyl)silane (1.0 mmol) and 4,4,5,5-tetra-

methyl-1,3,2-dioxaborolane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv),

puriﬁed by ﬂash column chromatography eluting with hexanes/DCM

153.2, 83.0, 43.3, 31.9, 26.2, 26.0, 24.8, and 23.8 ppm; B{ H} NMR

(128 MHz, CDCl ): δ 30.06 ppm. The NMR is consistent with

literature data.

E)-Trimethyl(3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-

allyl)silane (4aa). Prepared according to the general procedure from

trimethyl(prop-2-yn-1-yl)silane (1.0 mmol) and 4,4,5,5-tetramethyl-

1,3,2-dioxaborolane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed

by ﬂash column chromatography eluting with hexanes/DCM (7:3) to

(

7:3) to provide the title compound as a colorless oil (136 mg, 0.45

mmol, 45% yield). H NMR (400 MHz, CDCl ): δ 8.04 (s, 1H), 7.34−

.22 (m, 5H), 1.33 (s, 12H), and 0.03 (s, 9H) ppm; C{ H} NMR

(

101 MHz, CDCl ): δ 157.0, 141.4, 128.2, 127.7, 127.5, 83.3, 24.8, and

.9 ppm; B{ H} NMR (128 MHz, CDCl ): δ 31.99 ppm. The NMR

provide the title compound as a colorless oil (96 mg, 0.40 mmol, 40%

yield). H NMR (400 MHz, CDCl ): δ 6.87−6.56 (m, 1H), 5.28 (dd, J

is consistent with literature data.

17.1 Hz, 1H), 1.74 (dt, J = 17.1, 2H), 1.44−1.14 (m, 12H), and

(

Z)-2-(1,2-Diphenylvinyl)-4,4,5,5-tetramethyl-1,3,2-dioxaboro-

13 1

.27−−0.14 (m, 9H) ppm; C{ H} NMR (101 MHz, CDCl ): δ

lane (4v). Prepared according to the general procedure from 1,2-

adiphenylethyne (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxabor-

olane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by ﬂash

column chromatography eluting with hexanes/DCM (7:3) to provide

11 1

52.1, 82.8, 28.3, 24.8, and −1.8 ppm; B{ H} NMR (128 MHz,

CDCl ): δ 29.83 ppm. The NMR is consistent with literature data.

(

E)-2-(Dec-1-en-1-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane

4bb). Prepared according to the general procedure from 1-decyne (1.0

mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (HBpin, ∼153.6

mg, 1.2 mmol, 1.2 equiv), puriﬁed by ﬂash column chromatography

eluting with hexanes/DCM (7:3) to provide the title compound as a

(

the title compound as a pale yellow solid (229 mg, 0.75 mmol, 75%

yield). H NMR (400 MHz, CDCl ): δ 7.44 (s, 1H), 7.36−7.29 (m,

H), 7.29−7.22 (m, 3H), 7.19−7.15 (m, 3H), 7.14−7.12 (m, 2H), and

.37 (s, 12H) ppm; C{ H} NMR (101 MHz, CDCl ): δ 143.2, 140.5,

37.0, 130.0, 128.9, 128.3, 127.9, 127.64, 126.3, 83.8, and 24.9 ppm;

B{ H} NMR (128 MHz, CDCl ): δ 30.58 ppm. The NMR is

colorless oil (173 mg, 0.65 mmol, 65% yield). H NMR (400 MHz,

CDCl ): δ 6.63 (dt, J = 17.9 Hz, 1H), 5.42 (dt, J = 17.9 Hz, 1H), 2.26−

2.05 (m, 2H), 1.48−1.35 (m, 2H), 1.26 (s, 22H), and 0.99−0.73 (m,

consistent with literature data.

H) ppm; C{ H} NMR (101 MHz, CDCl ): δ 154.8, 128.6, 118.5,

(

E)-4,4,5,5-Tetramethyl-2-(4-phenylbut-1-en-1-yl)-1,3,2-dioxa-

borolane (4w). Prepared according to the general procedure from but-

-yn-1-ylbenzene (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxabor-

2.9, 35.8, 29.4, 29.2, 28.2, 24.75, 22.7, and 14.1 ppm; B{ H} NMR

128 MHz, CDCl ): δ 29.99 ppm. The NMR is consistent with

(

literature data.

(

olane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by ﬂash

column chromatography eluting with hexanes/DCM (7:3) to provide

E)-2-(3-Bromoprop-1-en-1-yl)-4,4,5,5-tetramethyl-1,3,2-dioxa-

borolane (4cc). Prepared according to the general procedure from 3-

bromoprop-1-yne (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxabor-

olane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by ﬂash

column chromatography eluting with hexanes/DCM (7:3) to provide

the title compound as a colorless oil (93.86 mg, 0.38 mmol, 38% yield).

the title compound as a colorless oil (180 mg, 0.70 mmol, 70% yield).

H NMR (400 MHz, CDCl ): δ 7.37−7.28 (m, 2H), 7.24 (d, J = 7.1 Hz,

H), 6.77 (dt, J = 18.0, 6.2 Hz, 1H), 5.57 (dt, J = 18.0, 1.6 Hz, 1H), 2.80

(

dd, J = 9.7, 6.4 Hz, 2H), 2.69−2.44 (m, 2H), and 1.32 (s, 12H) ppm;

C{ H} NMR (101 MHz, CDCl ): δ 153.4, 141.8, 128.4, 125.9, 119.2,

H NMR (400 MHz, CDCl ): δ 6.69 (dt, J = 17.6, 7.0 Hz, 1H), 5.69 (dt,

J = 17.6, 1.2 Hz, 1H), 3.99 (dd, J = 7.0, 1.2 Hz, 2H), and 1.28 (s, 12H)

ppm; C{ H} NMR (101 MHz, CDCl ): δ 146.7, 83.6, 33.6, and 24.8

ppm; B{ H} NMR (128 MHz, CDCl ): δ 29.47 ppm. The NMR is

consistent with literature data.

Z)-4,4,5,5-Tetramethyl-2-(oct-4-en-4-yl)-1,3,2-dioxaborolane

4dd). Prepared according to the general procedure from 4-octyne (1.0

mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (HBpin, ∼153.6

mg, 1.2 mmol, 1.2 equiv), puriﬁed by ﬂash column chromatography

eluting with hexanes/DCM (7:3) to provide the title compound as a

3.1, 37.5, 34.6, and 24.8 ppm; ¹¹B{ H} NMR (128 MHz, CDCl ): δ

0.04 ppm. The NMR is consistent with literature data.

E)-2-(3-Cyclohexylprop-1-en-1-yl)-4,4,5,5-tetramethyl-1,3,2-di-

(

oxaborolane (4x). Prepared according to the general procedure from

prop-2-yn-1-ylcyclohexane (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-

dioxaborolane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by

ﬂash column chromatography eluting with hexanes/DCM (7:3) to

(

provide the title compound as a colorless oil (190 mg, 0.76 mmol, 76%

yield). H NMR (400 MHz, CDCl ): δ 6.61 (dt, J = 17.9, 6.9 Hz, 1H),

.41 (dt, J = 17.9, 1.5 Hz, 1H), 2.05 (td, J = 6.9, 1.5 Hz, 2H), 1.75−1.58

colorless oil (99 mg, 0.42 mmol, 42% yield). H NMR (400 MHz,

(

m, 4H), 1.46−1.33 (m, 1H), 1.27 (s, 12H), 1.25−1.09 (m, 4H), and

CDCl ): δ 6.32 (t, J = 7.1 Hz, 1H), 2.15−2.09 (m, 4H), 1.45−1.40 (m,

.90 (m, 2H) ppm; ¹³C{ H} NMR (101 MHz, CDCl ): δ 153.5, 119.7,

4H), 1.27 (s, 12H), 0.93 (d, J = 7.4 Hz, 3H), and 0.92−0.87 (d, J = 7.4

2.9, 44.1, 37.2, 33.2, 26.5, 26.3, and 24.8 ppm; B{ H} NMR (128

Hz, 3H) ppm; C{ H} NMR (101 MHz, CDCl ): δ 145.99, 82.93,

MHz, CDCl ): δ 29.81 ppm. The NMR is consistent with literature

30.7, 30.6, 24.7, 23.3, 22.4, 14.1, and 14.0 ppm; B{ H} NMR (128

MHz, CDCl ): δ 30.81 ppm. The NMR is consistent with literature

data.

E)-2-(3-Cyclopropylprop-1-en-1-yl)-4,4,5,5-tetramethyl-1,3,2-di-

(

data.

(

oxaborolane (4y). Prepared according to the general procedure from

prop-2-yn-1-ylcyclopropane (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-

dioxaborolane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by

ﬂash column chromatography eluting with hexanes/DCM (7:3) to

Z)-4-(2-(Cyclohepta-2,4,6-trien-1-yl)-1-methoxyvinyl)-N,N-di-

methylaniline (9). To a mixture of p-(N,N-dimethylamino)-

phenylacetylene (1.0 mmol) and NaHCO (0.5 mmol) in methanol

(2 mL) was added tropylium tetraﬂuoroborate (1.0 mmol). The

reaction mixture was stirred at room temperature for 10 min. Methanol

was removed under reduced pressure, and the residues were taken up in

DCM. The solution was washed with water and then brine. The organic

extract was dried over Na SO and concentrated under reduced

provide the title compound as a colorless oil (108 mg, 0.52 mmol, 52%

yield). H NMR (400 MHz, CDCl ): δ 6.06 (dd, J = 17.8, 9.3 Hz, 1H),

5.47 (d, J = 17.8 Hz, 1H), 1.50 (dddd, J = 12.7, 9.2, 8.0, 4.7 Hz, 1H),

.24 (s, 12H), 0.85−0.71 (m, 2H), and 0.58−0.47 (m, 2H) ppm;

C{ H} NMR (101 MHz, CDCl ): δ 158.50, 115.07, 82.86, 24.73,

pressure. The product was puriﬁed using ﬂash column chromatography

128

J. Org. Chem. 2021, 86, 9117−9133

The Journal of Organic Chemistry

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(

silica-gel, EtOAc/hexanes 2:8) to obtain compound 9 (∼25% yield,

dioxaborolane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by

ﬂash column chromatography eluting with hexanes/DCM (1:1) to

containing a minor amount of impurities). H NMR (400 MHz;

CDCl ): δ 7.31−7.15 (m, 2H), 6.88−6.74 (m, 2H), 6.72 (m, 2H), 6.37

provide the title compound as a colorless oil (193 mg, 0.70 mmol, 67%

(

m, 1H), 6.21 (m, 1H), 5.19 (dt, J = 3.5, 1.7 Hz, 1H), 3.81 (tdd, J = 6.6,

yield). H NMR (300 MHz, CDCl ): δ 7.38−6.97 (m, 2H), 7.10−6.56

.8, 1.5 Hz, 2H), 3.73−3.64 (m, 1H), 3.50 (s, 3H), and 2.98 (s, 6H)

(m, 2H), 3.85 (s, 3H), 3.22−2.40 (m, 2H), 1.27 (d, 12H), and 1.24−

ppm; C{ H} NMR (101 MHz; CDCl ): δ 149.8, 147.6, 140.0, 139.2,

1.10 (m, 2H); C{ H} NMR (76 MHz, CDCl ): δ 157.4, 132.8, 129.1,

29.0, 128.5, 126.5, 119.8, 112.3, 74.6, 56.6, 41.4, 40.6, and 39.6 ppm;

126.7, 120.3, 110.1, 83.0, 55.2, 24.9, and 24.4 ppm; B{ H} NMR (96

−

IR (Neat) 2886, 2809, 1603, 1520, and 1443 cm ; HRMS (ESI+) m/

z: [M + H] calcd for C H NO, 268.1696; found, 268.1697.

MHz, CDCl ): δ 34.09 ppm. The NMR is consistent with literature

data.

Characterization Data of Products in Scheme 5. 4,4,5,5-

Tetramethyl-2-phenethyl-1,3,2-dioxaborolane (14a). Prepared ac-

cording to the general procedure from styrene (1.0 mmol) and 4,4,5,5-

tetramethyl-1,3,2-dioxaborolane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2

equiv), puriﬁed by ﬂash column chromatography eluting with hexanes/

4-(2-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)ethyl)phenyl

Acetate (14g). Prepared according to the general procedure from 4-

vinylphenyl acetate (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxa-

borolane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by ﬂash

column chromatography eluting with hexanes/DCM (1:1) to provide

DCM (1:1) to provide the title compound as a colorless oil (197 mg,

the title compound as a colorless oil (104 mg, 0.36 mmol, 36% yield).

.85 mmol, 85% yield). H NMR (400 MHz, CDCl ): δ 7.37−7.27 (m,

H NMR (400 MHz, CDCl ): δ 7.23 (d, J = 8.4 Hz, 2H), 6.99 (d, J = 8.4

H), 7.25−7.18 (m, 1H), 2.88−2.80 (m, 2H), 1.29 (s, 12H), and 1.26−

Hz, 2H), 2.76 (t, J = 8.2 Hz, 2H), 2.30 (s, 3H), 1.24 (s, 12H), and 1.19−

.20 (m, 2H) ppm; ¹³C{ H} NMR (101 MHz, CDCl ): δ 144.5, 128.2,

1.12 (m, 2H) ppm; C{ H} NMR (101 MHz, CDCl ): δ 169.7, 148.5,

28.1, 125.6, 83.1, 30.0, 24.9, and 13.1 ppm; B{ H} NMR (128 MHz,

142.0, 128.9, 121.1, 83.2, 29.4, 24.8, and 21.2 ppm; B{ H} NMR (128

CDCl ): δ 33.92 ppm. The NMR is consistent with literature data.

MHz, CDCl ): δ 34.62 ppm. The NMR is consistent with the literature

,4,5,5-Tetramethyl-2-(4-methylphenethyl)-1,3,2-dioxaborolane

14b). Prepared according to the general procedure from 1-methyl-4-

vinylbenzene (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxaborolane

data.

(

2-(4-Bromophenethyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane

(14h). Prepared according to the general procedure from 1-bromo-4-

vinylbenzene (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxaborolane

(HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by ﬂash column

chromatography eluting with hexanes/DCM (1:1) to provide the title

(

HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by ﬂash column

chromatography eluting with hexanes/DCM (1:1) to provide the title

compound as a colorless oil (192 mg, 0.78 mmol, 78% yield). H NMR

(

400 MHz, CDCl ): δ 7.31−6.93 (m, 4H), 2.90−2.67 (m, 2H), 2.37 (s,

compound as a white solid (233 mg, 0.75 mmol, 75% yield). H NMR

H), 1.29 (s, 12H), and 1.19 (dd, J = 9.0, 7.5 Hz, 2H) ppm; C{ H}

(400 MHz, CDCl ): δ 7.48−7.31 (m, 2H), 7.23−6.98 (m, 2H), 2.72 (t,

NMR (101 MHz, CDCl ): δ 141.4, 134.9, 128.9, 127.9, 83.1, 29.6, 24.9,

J = 8.1 Hz, 2H), 1.24 (d, J = 1.8 Hz, 12H), and 1.13 (dd, J = 9.2, 7.0 Hz,

1.0, and 13.3 ppm; ¹¹B{ H} NMR (128 MHz, CDCl ): δ 34.03 ppm.

2H) ppm; C{ H} NMR (101 MHz, CDCl ): δ 143.4, 131.2, 129.8,

The NMR is consistent with literature data.

119.2, 83.2, 29.4, 24.8, and 12.8 ppm; B{ H} NMR (128 MHz,

,4,5,5-Tetramethyl-2-(2-phenylpropyl)-1,3,2-dioxaborolane

14c). Prepared according to the general procedure from prop-1-en-2-

ylbenzene (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxaborolane

CDCl ): δ 33.81 ppm. The NMR is consistent with literature data.

(

2-(2-(4-Chlorophenyl)propyl)-4,4,5,5-tetramethyl-1,3,2-dioxa-

borolane (14i). Prepared according to the general procedure from 1-

chloro-4-(prop-1-en-2-yl)benzene (1.0 mmol) and 4,4,5,5-tetramethyl-

1,3,2-dioxaborolane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed

by ﬂash column chromatography eluting with hexanes/DCM (1:1) to

(

HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by ﬂash column

chromatography eluting with hexanes/DCM (1:1) to provide the title

compound as a colorless oil (160 mg, 0.65 mmol, 65% yield). H NMR

(

400 MHz, CDCl ): δ 7.32−7.25 (m, 4H), 7.17 (ddd, J = 8.6, 5.9, 2.2

provide the title compound as a colorless oil (115 mg, 0.41 mmol, 41%

Hz, 1H), 3.19−2.96 (m, 1H), 1.31 (d, J = 6.9 Hz, 3H), and 1.21−1.17

yield). H NMR (400 MHz, CDCl ): δ 7.28−7.21 (m, 2H), 7.21−7.15

(m, 14H) ppm; C{ H} NMR (101 MHz, CDCl ): δ 149.2, 128.2,

(m, 2H), 3.03 (h, J = 7.2 Hz, 1H), 1.27 (d, J = 6.9 Hz, 3H), 1.18 (d, J =

26.6, 125.7, 83.0, 35.8, 24.9, and 21.2 ppm; ¹¹B{ H} NMR (128 MHz,

1.8 Hz, 12H), and 1.14 (d, J = 7.8 Hz, 2H) ppm; C{ H} NMR (101

CDCl ): δ 33.59 ppm. The NMR is consistent with literature data.

MHz, CDCl ): δ 147.7, 131.2, 128.2, 128.0, 83.1, 35.3, 24.9, 24.8, 24.7,

(

R)-2-(2-(4-Methoxyphenyl)-1-phenylethyl)-4,4,5,5-tetrameth-

and 21.2 ppm; B{ H} NMR (128 MHz, CDCl ): δ 33.59 ppm. The

yl1,3,2-dioxaborolane (14d). Prepared according to the general

procedure from (E)-1,2-diphenylethene (1.0 mmol) and 4,4,5,5-

tetramethyl-1,3,2-dioxaborolane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2

equiv), puriﬁed by ﬂash column chromatography eluting with hexanes/

NMR is consistent with literature data.

Trimethyl(3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-

propyl)silane (14j). Prepared according to the general procedure from

allyltrimethylsilane (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxabor-

olane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by ﬂash

column chromatography eluting with hexanes/DCM (1:1) to provide

DCM (1:1) to provide the title compound as a colorless oil (123 mg,

.40 mmol, 40% yield). H NMR (400 MHz, CDCl ): δ 7.44−6.98 (m,

0H), 3.18 (dd, J = 13.5, 9.8 Hz, 1H), 2.99 (dd, J = 13.5, 6.9 Hz, 1H),

.72 (dd, J = 9.8, 6.9 Hz, 1H), 1.37−1.20 (m, 6H), and 1.14 (d, J = 3.4

the title compound as a colorless oil (145 mg, 0.60 mmol, 60% yield).

H NMR (400 MHz, CDCl ): δ 1.53−1.36 (m, 2H), 1.26 (s, 12H),

Hz, 12H) ppm; C{ H} NMR (101 MHz, CDCl ): δ 142.6, 141.8,

0.84 (t, J = 7.7 Hz, 2H), 0.58−0.46 (m, 2H), and −0.02 (s, 9H) ppm;

28.9, 128.4, 128.3, 128.0, 125.8, 83.4, 38.9, 24.8, 24.6, and 24.5 ppm;

C{ H} NMR (101 MHz, CDCl ): δ 82.8, 24.8, 20.1, 18.6, and −1.6

B{ H} NMR (128 MHz, CDCl ): δ 33.66 ppm. The NMR is

ppm; B{ H} NMR (128 MHz, CDCl ): δ 34.03 ppm. The NMR is

consistent with literature data.

-(4-Methoxyphenethyl)-4,4,5,5-tetramethyl-1,3,2-dioxaboro-

2-(5-Bromopentyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane

(14k). Prepared according to the general procedure from 5-bromopent-

1-ene (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (HBpin,

∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by ﬂash column

chromatography eluting with hexanes/DCM (1:1) to provide the

lane (14e). Prepared according to the general procedure from 1-

methoxy-4-vinylbenzene (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-

dioxaborolane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by

ﬂash column chromatography eluting with hexanes/DCM (1:1) to

provide the title compound as a colorless oil (152 mg, 0.58 mmol, 58%

yield). H NMR (400 MHz, CDCl ): δ 7.16 (d, J = 8.5 Hz, 2H), 6.83 (d,

title compound as a colorless oil (193 mg, 0.70 mmol, 70% yield). H

NMR (400 MHz, CDCl ): δ 3.39 (t, J = 6.9 Hz, 2H), 2.04−1.70 (m,

J = 8.6 Hz, 2H), 3.80 (s, 3H), 3.00−2.58 (m, 2H), 1.24 (s, 12H), and

2H), 1.56−1.33 (m, 4H), 1.23 (s, 12H), and 0.77 (dt, J = 7.3, 3.8 Hz,

.14 (dd, J = 8.8, 7.5 Hz, 2H) ppm; ¹³C{ H} NMR (101 MHz, CDCl ):

2H) ppm; C{ H} NMR (101 MHz, CDCl ): δ 82.9, 33.9, 32.6, 30.8,

δ 157.6, 136.6, 128.9, 113.6, 83.1, 55.3, 29.1, and 24.8 ppm; B{ H}

NMR (128 MHz, CDCl ): δ 34.09 ppm. The NMR is consistent with

24.8, and 23.2 ppm; B{ H} NMR (128 MHz, CDCl ): δ 34.05 ppm.

The NMR is consistent with literature data.

literature data.

Characterization Data of Products in Scheme 6. 2-Phenyl-

ethan-1-ol (17a). Prepared according to the general procedure from

styrene oxide (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxaborolane

(HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed by ﬂash column

-(2-Methoxyphenethyl)-4,4,5,5-tetramethyl-1,3,2-dioxaboro-

lane (14f). Prepared according to the general procedure from 1-

methoxy-2-vinylbenzene (1.0 mmol) and 4,4,5,5-tetramethyl-1,3,2-

129

J. Org. Chem. 2021, 86, 9117−9133

The Journal of Organic Chemistry

https://pubs.acs.org/doi/10.1021/acs.joc.1c01208.

Article

chromatography eluting with hexanes/EtOAc (9:1) to provide the title

compound as a colorless oil (73 mg, 0.60 mmol, 60% yield). H NMR

1,3,2-dioxaborolane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed

by ﬂash column chromatography eluting with hexanes/EtOAc (9:1) to

(

400 MHz, CDCl ): δ 7.39−7.32 (m, 2H), 7.30−7.24 (m, 3H), 3.88 (t,

provide the title compound as a pale yellow oil (90 mg, 0.66 mmol, 66%

J = 6.6 Hz, 2H), 2.90 (t, J = 6.6 Hz, 2H), and 1.89 (s, 1H); C{ H}

NMR (101 MHz, CDCl ): δ 138.6, 129.1, 128.6, 126.5, 63.7, and 39.2

ppm. The NMR is consistent with literature data.

-(p-Tolyl)ethan-1-ol (17b). Prepared according to the general

procedure from 2-(p-tolyl)oxirane (1.0 mmol) and 4,4,5,5-tetramethyl-

yield). H NMR (400 MHz, CDCl ): δ 7.38−7.27 (m, 2H), 7.27−7.19

(m, 3H), 3.70 (t, J = 6.4 Hz, 2H), 2.74 (dd, J = 8.7, 6.8 Hz, 2H), 2.08−

1.84 (m, 2H), and 1.72 (s, 1H) ppm; C{ H} NMR (101 MHz,

CDCl ): δ 141.9, 128.5, 128.4, 125.9, 62.3, 34.2, and 32.1 ppm. The

NMR is consistent with literature data.

,3,2-dioxaborolane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed

Cyclohexanol (17i). Prepared according to the general procedure

from 7-oxabicyclo[4.1.0]heptane (1.0 mmol) and 4,4,5,5-tetramethyl-

1,3,2-dioxaborolane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2 equiv), puriﬁed

by ﬂash column chromatography eluting with hexanes/EtOAc (9:1) to

provide the title compound as a colorless oil (69 mg, 0.51 mmol, 51%

yield). H NMR (400 MHz, CDCl ): δ 7.16 (s, 4H), 3.86 (t, J = 6.6 Hz,

H), 2.86 (t, J = 6.6 Hz, 2H), 2.37 (s, 3H), and 1.80 (s, 1H); C{ H}

provide the title compound as a pale yellow oil (100 mg, 0.62 mmol,

NMR (101 MHz, CDCl ): δ 136.0, 135.4, 129.3, 129.0, 63.8, 38.8, and

62% yield). H NMR (400 MHz, CDCl

): δ 3.62 (dp, J = 8.7, 4.1 Hz,

1.1 ppm. The NMR is consistent with literature data.

-(4-Bromophenyl)ethan-1-ol (17c). Prepared according to the

general procedure from 2-(4-bromophenyl)oxirane (1.0 mmol) and

1H), 2.07−1.86 (m, 2H), 1.75 (dq, J = 9.0, 5.1, 4.1 Hz, 2H), 1.66 (s,

1H), 1.56 (dddd, J = 12.8, 8.1, 3.4, 2.1 Hz, 1H), 1.37−1.24 (m, 4H),

and 1.24−1.09 (m, 1H) ppm; C{ H} NMR (101 MHz, CDCl ): δ

,4,5,5-tetramethyl-1,3,2-dioxaborolane (HBpin, ∼153.6 mg, 1.2

70.3, 35.5, 25.5, and 24.1 ppm. The NMR is consistent with literature

mmol, 1.2 equiv), puriﬁed by ﬂash column chromatography eluting

data.

with hexanes/EtOAc (9:1) to provide the title compound as a colorless

oil a(92 mg, 0.46 mmol, 46% yield). H NMR (400 MHz, CDCl ): δ

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

■

(

.73−7.31 (m, 2H), 7.21−6.91 (m, 2H), 3.85 (t, J = 6.5 Hz, 2H), 2.84

t, J = 6.5 Hz, 2H), and 1.58 (s, 1H); C{ H} NMR (101 MHz,

CDCl ): δ 137.6, 131.6, 130.8, 120.3, 63.4, and 38.6 ppm. The NMR is

consistent with literature data.

,2-Diphenylethan-1-ol (17d). Prepared according to the general

procedure from 2,3-diphenyloxirane (1.0 mmol) and 4,4,5,5-

tetramethyl-1,3,2-dioxaborolane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2

equiv), puriﬁed by ﬂash column chromatography eluting with hexanes/

13 1

Experimental details and spectroscopic data for all

products, full Gaussian reference, Cartesian coordinates,

and electronic and free energies (PDF)

EtOAc (9:1) to provide the title compound as a colorless oil (103 mg,

.52 mmol, 52% yield). H NMR (300 MHz, CDCl ): δ 7.41−7.34 (m,

AUTHOR INFORMATION

Corresponding Authors

■

H), 7.33−7.24 (m, 6H), 4.26 (dd, J = 8.4, 5.8 Hz, 1H), 4.22−4.18 (m,

H), and 1.68 (s, 1H); ¹³C{ H} NMR (76 MHz, CDCl ): δ 141.5,

Thanh Vinh Nguyen − School of Chemistry, University of New

South Wales, Sydney, New South Wales 2052, Australia;

orcid.org/0000-0002-0757-9970; Email: t.v.nguyen@

unsw.edu.au

Binh Khanh Mai − Department of Chemistry, University of

Pittsburgh, Pittsburgh, Pennsylvania 15260, United States;

orcid.org/0000-0001-8487-1417; Email: binh.mai@

pitt.edu

28.8, 128.4, 126.9, 66.2, and 53.7 ppm. The NMR is consistent with

literature data.

-Phenylpropan-1-ol (17e). Prepared according to the general

procedure from 2-methyl-2-phenyloxirane (1.0 mmol) and 4,4,5,5-

tetramethyl-1,3,2-dioxaborolane (HBpin, ∼153.6 mg, 1.2 mmol, 1.2

equiv), puriﬁed by ﬂash column chromatography eluting with hexanes/

EtOAc (9:1) to provide the title compound as a colorless oil (72 mg,

.53 mmol, 53% yield). H NMR (300 MHz, CDCl ): δ 7.55−7.32 (m,

H), 7.32−7.19 (m, 3H), 3.73 (d, J = 6.8 Hz, 2H), a2.98 (q, J = 6.9 Hz,

H), 1.60−1.38 (m, 1H), and 1.31 (d, J = 7.0 Hz, 3H); C{ H} NMR

Author

(

76 MHz, CDCl ): δ 143.7, 128.7, 127.5, 126.7, 68.7, 42.5, and 17.6

Nhan N. H. Ton − School of Chemistry, University of New

South Wales, Sydney, New South Wales 2052, Australia

ppm. The NMR is consistent with literature data.

-(Naphthalen-1-yl)ethan-1-ol (17f). Prepared according to the

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.joc.1c01208

general procedure from 2-(naphthalen-1-yl)oxirane (1.0 mmol) and

,4,5,5-tetramethyl-1,3,2-dioxaborolane (HBpin, ∼153.6 mg, 1.2

mmol, 1.2 equiv), puriﬁed by ﬂash column chromatography eluting

with hexanes/EtOAc (9:1) to provide the title compound as a pale

yellow oil (105 mg, 0.61 mmol, 61% yield). H NMR (400 MHz,

Author Contributions

The manuscript was written through the contributions of all

authors. All authors have given approval to the ﬁnal version of

the manuscript.

CDCl ): δ 8.09 (dq, J = 8.3, 0.9 Hz, 1H), 7.90 (dd, J = 8.1, 1.5 Hz, 1H),

.79 (dt, J = 7.9, 1.2 Hz, 1H), 7.59−7.49 (m, 2H), 7.49−7.39 (m, 2H),

.03 (t, J = 6.7 Hz, 2H), 3.39 (t, J = 6.7 Hz, 2H), and 1.53 (s, 1H) ppm;

Notes

C{ H} NMR (101 MHz, CDCl ): δ 134.4, 134.0, 132.1, 128.9, 127.4,

The authors declare no competing ﬁnancial interest.

27.2, 126.1, 125.7, 125.5, 123.7, 63.1, and 36.2 ppm. The NMR is

consistent with literature data.

-(2-Methoxyphenyl)ethan-1-ol (17g). Prepared according to the

ACKNOWLEDGMENTS

This work was funded by the Australian Research Council (grant

FT180100260 and DP200100063 to T.V.N.).

■

general procedure from 2-(2-methoxyphenyl)oxirane (1.0 mmol) and

,4,5,5-tetramethyl-1,3,2-dioxaborolane (HBpin, ∼153.6 mg, 1.2

mmol, 1.2 equiv), puriﬁed by ﬂash column chromatography eluting

with hexanes/EtOAc (9:1) to provide the title compound as a pale

yellow oil (100 mg, 0.66 mmol, 66% yield). H NMR (400 MHz,

REFERENCES

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