A Comparison of Two Zinc Hydride Catalysts for Terminal Alkyne C-H Borylation/Hydroboration and the Formation of 1,1,1-Triborylalkanes by Tandem Catalysis Using Zn-H and B-H Compounds

Marina Uzelac, Kang Yuan, and Michael J. Ingleson*

Article

A Comparison of Two Zinc Hydride Catalysts for Terminal Alkyne C−

H Borylation/Hydroboration and the Formation of 1,1,1-

Triborylalkanes by Tandem Catalysis Using Zn−H and B−H

Compounds

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sı Supporting Information

ABSTRACT: The synthesis of 1,1,1-triborylalkanes from terminal alkynes and

pinacolborane (HBPin) is reported. This transformation proceeds via initial Zn-

catalyzed alkyne C−H borylation, which can be achieved using a NacNacZnH

complex. Combinations of a NacNacZn-alkynyl formed via C−H zincation of a

terminal alkyne and HBPin exist in equilibrium with the alkynyl-BPin and

NacNacZnH. The consumption of NacNacZnH by irreversible reaction with a

terminal alkyne evolving H is essential for driving alkyne C−H borylation to

completion. The alkynyl-BPin compounds undergo hydroboration catalyzed by Zn−H complexes at raised temperatures with a

{

7DIPP}ZnH(NTf ) complex (7DIPP = 1,3-bis(2,6-diisopropylphenyl)-4,5,6,7-tetrahydro-1H-1,3-diazepin-3-ium-2-ide) a more

active catalyst for hydroboration than a NacNacZnH complex. Calculations indicate the {7DIPP}Zn−H congener has a more

pronounced biphilic character than that of NacNacZnH (greater electrophilicity at Zn while maintaining a basic hydride). Of the

two hydroboration steps, the hydroboration of alkynylBPin is catalyzed by Zn−H complexes, while the hydroboration of 1,1-

diborylalkenes is catalyzed more eﬀectively by B−H-containing species, including boranes formed in situ from HBPin. These

observations led to a one-pot protocol being developed for converting terminal alkynes into 1,1,1-triborylalkanes that utilizes

{

7DIPP}ZnPh(NTf ) as a precatalyst for the formation of 1,1-diborylated alkenes with subsequent addition of BH -THF as catalyst

for the ﬁnal step.

INTRODUCTION

inexpensive, low-toxicity earth-abundant-metal-based catalysts

are desirable for the wider uptake of these compounds in

synthesis.

■

Organoboron compounds have become staple reagents in

chemical synthesis as they possess a desirable balance between

stability and facile transformation into other functional

Terminal alkynes are attractive precursors to 1,1,1-

triborylated alkanes as they are readily available inexpensive

feedstocks. In this area, Chirik et al. utilized two diﬀerent

cobalt catalysts (Figure 1, top) to aﬀord triborylated alkanes by

ﬁrst performing the 1,1-diboration of terminal alkynes with

bis(pinacolato)diboron (B Pin ) followed by hydroboration

groups. Multiply borylated organic compounds embody

especially attractive reagents because of the potential for the

stepwise, selective conversion of the boryl groups into C−C

and C-heteroatom bonds. While compounds such as geminal

bisboryl-alkanes have been extensively utilized in synthetic

with pinacolborane (HBPin). While notable work, cobalt has

low permitted daily exposure (PDE) limits thus catalysts based

on earth-abundant 3d metals with much less stringent PDE

endeavors, 1,1,1-triboryl alkanes have received much less

attention. This is partly due to the greater complexity of

making these compounds selectively and eﬃciently, a challenge

that only recently has begun to be addressed. While the ﬁrst

selective route to 1,1,1-triborylalkanes is over 40 years old this

multiple boronation of alkyl halides required excess lithium

values, such as Fe, Cu, or Zn would be preferable. During the

course of this study, Marder et al. reported a Cu-based catalyst

capable of the 1,1,1-triborylation of terminal alkynes in the

presence of stoichiometric KF. This work (Figure 1b)

represents a highly eﬃcient method to access these

metal. Recently, more eﬃcient routes to 1,1,1-triborylalkanes

have been achieved using transition metal catalysis. For

example, Ir catalyzed C(sp )−H triborylation of 2-ethyl-

Received: February 7, 2020

pyridines, and Ni catalyzed triborylation of benzylic C−H

bonds. Styrenes are also eﬀective precursors to 1,1,1-

triborylalkanes via Co-catalyzed borylation/hydroboration of

vinylarenes and Rh-catalyzed borylation/hydroboration of

(

E)-styrylboronates. Nonetheless, general routes to 1,1,1-

triborylalkanes using simple hydrocarbon starting materials and

XXXX American Chemical Society

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

Scheme 1. Stepwise Transformation of Terminal Alkynes

into 1,1-Diborylated Alkenes Using Zn−H III and

Precatalyst II which Forms I In Situ on Metathesis with

HBPin

) with HBPin catalyzed by I or II (as a more readily

accessible precatalyst), required heating to 90 °C for signiﬁcant

conversion of the alkynylBPin 2a to 3a. Compound III was

explored to determine if it was a more active catalyst than I in

this transformation. However, from multiple attempts using III

to catalyze the formation of 3a from alkyne 1a, it became

evident that III is a less eﬀective catalyst than I/II is for this

conversion. This is illustrated by there being no observable

formation of 3a (by multinuclear NMR spectroscopy) after 25

h at 90 °C using 10 mol % of III in C D (in a sealed tube). In

contrast, using I under identical conditions 3a is formed in

good yield (and up to 73% yield after 36 h). Only when the

reaction temperature is increased to 110 °C does III catalyze

the formation of 3a to any observable extent, albeit requiring

Figure 1. Select previous work. Inset this work on tandem Zn−H/B−

H catalysis for forming 1,1,1-triborylated alkanes.

compounds using a low toxicity and earth abundant metal

based catalyst.

There is an increasing interest in using zinc complexes as

20 h at 110 °C to reach 90% conversion of 2a to 3a.

To explore the disparity between the two Zn−H catalysts we

borylation catalysts, and recently we demonstrated that a

explored the initial step, formation of 2a by C−H borylation.

Comparing the activity of III to that of I for alkyne C−H

borylation under identical conditions (1.1 equiv of HBPin, 10

mol % catalyst, 1 M THF solution, 60 °C) after only 120 min,

NHC-Zn complex {7DIPP}ZnH(NTf ) (I, Figure 1c) or its

precursor, {7DIPP}ZnPh(NTf ) (II), is capable of catalyzing

both the C−H borylation of terminal alkynes and the

hydroboration of the alkynylBPin product aﬀording 1,1-

diborylated alkenes. In this work, the hydroboration step

proceeded via alkyne hydrozincation then H−B/Zn−C

8% of 2a was formed using III, making it faster than the

borylation catalyzed by I. However, if the reaction is performed

in C D , then I is slightly more active than III (though both

metathesis. Experimental ﬁndings supported by DFT

calculations revealed that I contains a highly electrophilic

zinc center and a hydridic (thus Brønsted basic) Zn−H

moiety; thus, I is biphilic in character. Arguably the most well

studied low-coordinate zinc hydride complexes use the β-

diketiminate ligand (NacNac), with a N-DIPP substituted

NacNac derivative enabling formation of a three coordinate

zinc hydride, termed herein (DIPPNacNac)ZnH, III (Scheme

are eﬀective catalysts). We attribute this solvent disparity to

the diﬀering propensities of I and III to bind THF due to their

relative electrophilicity (vide infra).

To gain further insight into the reactivity of III,

stoichiometric reactions were performed. When III was reacted

with terminal alkyne 1a in a 1:1 ratio at room temperature in

benzene, alkyne deprotonation takes place as evidenced by the

consumption of the alkynyl CC−H resonance in the H

, DIPPNacNac = {2,6- Pr H C N(CH )C} CH)). While

2 3 6 3 2

NMR spectrum and concomitant formation of H (δ = 4.47

III and other low-coordinate zinc hydrides have been reported

to catalyze the hydrosilylation and hydroboration of ketones

ppm). The reaction aﬀords a single major new (DIPPNacNac)

Zn containing a product consistent with a (DIPPNacNac)Zn-

and aldehydes, as well as the hydrosilylation of CO , III has to

(

alkynyl) species (IV, Scheme 2). The subsequent addition

the best of our knowledge not been used to catalyze alkyne

5−17

borylation.

Herein we report our studies comparing the

reactivity of low-coordinate zinc hydride complexes I and III in

the borylation of terminal alkynes. This led to the development

of a tandem Zn−H/B−H catalytic system for the one-pot

transformation of terminal alkynes into 1,1,1-triborylalkanes.

Scheme 2. Deprotonation of Alkyne 1a by III and Reversible

σ-Bond Metathesis between IV and HBPin

RESULTS AND DISCUSSION

Comparison of the Reactivity of Zn Catalysts. The

reported 1,1-diboration of 4-ethynyltoluene (1a, right Scheme

■

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Article

of 1 equiv of HBPin to IV resulted in formation of 2a, as

indicated by growth of a diagnostic resonance at 24.9 ppm in

the B NMR spectrum, and III concomitantly (indicated by

the growth of the diagnostic Zn−H resonance at 4.37 ppm in

the H NMR spectrum). However, despite Zn−C /H−BPin

metathesis taking place at room temperature, this trans-

formation does not go to completion (after 24 h at room

temperature). This is not due to kinetic factors as after

prolonged heating and then cooling back to room temperature

the same ratio of IV: 2a was observed, indicating an

equilibrium process. As the equilibrium position was reached

at room temperature, Zn−C/H−B metathesis proceeds with a

relatively low barrier for this system. Consistent with this,

combining III and 2a in benzene led to a mixture containing

III, 2a, and the metathesis products HBPin and IV at room

temperature conﬁrming reversibility. This is comparable to

previously reported reactivity observed on combining complex

I with alkynylBPin species (where a σ-bond metathesis

equilibrium was also observed). This suggests that reversible

metathesis between Zn−H and (sp)C−B compounds with ΔG

close to zero may be a common phenomenon for low-

coordinate zinc complexes.

To further assess the relative reactivity of {7DIPP}Zn and

NacNacZn systems calculations at the M06−2x/lanl2dz/6-

11G/PCM(THF) level were performed. For the NacNacZn

system, while alkyne deprotonation is thermodynamically

favored (eq 1 in Figure 2), the metathesis of the resultant

zinc-alkynyl species with HBPin is energetically uphill (eq 2 in

−

Figure 2), albeit by <2 kcal mol . Thus, the subsequent

reaction of III with additional terminal alkyne is key to drive

the reaction to completion and thus ensures high conversions

to the alkynylBPin. Again, this is comparable with observations

previously reported for the {7DIPP}Zn congener (see Figure 2

inset).

Comparison of the frontier molecular orbitals of [I-THF]⁺

(

see ref 19) and III indicated that as expected [I-THF] is

more electrophilic than III. The LUMO of [I-THF] (Figure

Figure 2. Top, the calculated energy changes for C−H zincation and

for Zn−C/H−BPin metathesis for NacNacZn complexes and inset for

) has signiﬁcant zinc character and at −0.74 eV is signiﬁcantly

lower in energy than the LUMO+1 of III (+0.46 eV) (the

LUMO for III has no zinc contribution). However, the energy

of the highest occupied molecular orbital with signiﬁcant Zn−

comparison for NHC-Zn complexes. Bottom, comparison of key

select frontier molecular orbitals of III and [I-THF] at isosurface

value = 0.04.

H character is comparable for both: For [I-THF] , it is the

Table 1. Optimization for Synthesis of 1,1,1-

HOMO (−8.56 eV), and for III, it is the HOMO−5 (−8.50

Triborylalkane

eV). Also notably, comparison of the NBO charges of the Zn−

H moiety in [I-THF] and III found that the hydride in [I-

THF] has a magnitude of negative charge comparable to that

of the hydride in III (−0.544 and −0.460, respectively). The

positive charge on Zn is also comparable in [I-THF] and III

(

+1.155 and +1.104, respectively). The energy of the key

entry

T (°C)

conc [mol/L]

time [h]

frontier orbitals and the charge distribution both support the

hypothesis that low-coordinate {7DIPP}Zn−H cations com-

bine considerable electrophilicity at zinc with a hydridic

moiety. The more pronounced biphilic character (due to the

lower LUMO energy) is possibly the key to the superior

catalytic performance of I relative to III.

Formation of 1,1,1-Triborylated Alkanes. During the

comparison of I and III in alkyne borylation, varying quantities

of 1,1,1-triborylated alkane 4a were observed at raised

temperatures when using I (or II, e.g., Table 1, entries 1−3).

No signiﬁcant amount (<5%) of 4a was observed in any

reaction using III as the catalyst (even after 160 h at 110 °C in

a sealed tube). Using precatalyst II, changing the solvent to

other arenes such as toluene did not aﬀect the outcome of this

100

110

<1%

<5%

Reaction conditions as shown unless otherwise indicated. Using 4

equiv of HBPin, as in previous work (ref 13). Using toluene instead

of benzene. Using 5 mol % of I. Refers to the concentration of

terminal alkyne 1a. Yields determined by in situ H NMR

spectroscopy vs CH Br as an internal standard.

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

transformation signiﬁcantly (entry 3 vs 4). Therefore, C D

observed in reactions using III under identical conditions.

Thus, the disparity between the reactions using I and III is

attributed to the absence of an in situ formed B−H catalyst

in reactions using (DIPPNacNac)Zn species due to the lower

Lewis acidity of III relative to I (and derivatives).

was used purely to facilitate in situ reaction monitoring.

Decreasing the catalyst loading had a detrimental eﬀect while

increasing the concentration helped to increase the yield

(Table 1, entries 6−7). Despite some improvements, reactions

required long times at high temperature to form a signiﬁcant

amount of 4a, and unreacted diborylated alkene 3a was

observed in these reactions indicating incomplete conversion

even after 24 h.

Hydroboration of 1,1-Diborylalkenes by B−H-Con-

taining Species. While THF-BH was previously shown to be

poor at catalyzing hydroboration of alkynylBPin 2a to 3a

(particularly relative to I), we surmised that the increased

steric bulk present in 1,1-diborylalkene 3a aﬀorded by the two

BPin units may signiﬁcantly disfavor hydrozincation of 3a with

sterically encumbered (7-DIPP)Zn−H species. Therefore, the

ability of some common boranes to eﬀect hydroboration of the

1,1-diborylated alkene 3a to 4a was explored (Table 2).

During reactions using I or II, signiﬁcant HBPin

decomposition was observed, with growth in ¹¹B resonances

^o^b^s^e^r^v^e^d^b^e^t^w^e^eⁿ²⁰^aⁿ^d²²^p^p^m^c^oⁿ^sⁱ^s^t^eⁿ^t^wⁱ^t^h⁽^R^O⁾3^B

species. Concomitantly, new H−B containing species were

observed (as indicated by multiplets with J

couplings,

B−H

including at −13 and −40 ppm in B NMR spectra). It is

feasible that under forcing conditions NHC dissociation from

Table 2. Hydroboration of 1,1-Diborylalkene 3a Using

Zn is occurring, and this Lewis base may facilitate

Borane Catalysts

degradation of HBPin. However, an alternative decom-

position pathway is also proceeding as indicated by formation

of 2,2-dimethyl-3-OBPin-butane (Scheme 3), indicated by a

Scheme 3. Electrophile (E )-Initiated Formation of 2,2-

Dimethyl-3-OBPin-butane

entry

BH-cat

BEt₃

T (°C)

concentration

time [h]

110

3 M

BH ·SMe₂

BH ·THF

Reaction conditions as shown unless otherwise indicated. BH SMe

added after alkynylBPin (2a) formation instead of after formation of

a. Yields determined by in situ H NMR spectroscopy based on the

ratio of product vs CH Br added after the time shown as an internal

standard.

BEt was initially explored as it reacts in aromatic solvents

with HBPin to form H BEt (x + y = 3) species, and we

surmised that low steric bulk, base-free R BH/RBH species

may be required for catalyzing hydroboration of bulky 3a via a

mechanism related to that recently identiﬁed. Substoichio-

diagnostic resonance at 3.95 ppm (q, ³J_H_H= 6.3 Hz). This is

presumably formed by electrophilic activation of a BPin species

inducing a pinacol rearrangement (methyl migration, see

Scheme 3) with subsequent hydroboration yielding the

observed product, 2,2-dimethyl-3-OBPin-butane. Indeed,

pinacolone was hydroborated to this product using HBPin

metric amounts of BEt were added to the standard reaction

mixture (which still contains excess HBPin) after the formation

of diborylated alkene 3a using I (or II). This led to formation

of EtBPin, H BEt_x(x + y = 3) species (by B NMR

spectroscopy) and desired product 4a, albeit in modest yield

under the reaction conditions catalyzed by I or BH -THF

(Table 2, entry 1). Next, guided by the work of Thomas,

(Scheme 3).

Cowley, and co-workers, BH compounds were explored and

The extensive HBPin degradation observed in the presence

found to be more eﬀective catalysts (entry 2). However, if BH₃

species were added earlier, speciﬁcally after formation of the

alkynylBPin 2a under otherwise identical conditions, then

lower yields of 1,1,1-triborylalkane 4a were obtained (compare

entries 2 and 3). This is consistent with the importance of Zn−

H in eﬃcient formation of the diborylated alkene (3a) as

of I/II at raised temperatures suggested that B−H species

eﬀective for catalyzing hydroboration may be formed in situ.

Analysis of the decomposition of HBPin during catalysis using

precatalyst II at 60, 90, and 110 °C (see the Supporting

Information) revealed that although formation of (RO)₃B

species and 2,2-dimethyl-3-OBPin-butane is observed at all

three temperatures it takes much longer at the two lower

temperatures. The same analysis also showed that the

formation of 4a (monitored using the characteristic ArCH₂

resonance at 3.19 ppm) has an induction period (despite

signiﬁcant amounts of 3a being present in solution after short

reaction times at 110 °C). Together these observations

suggested in situ formation of a B−H species that acts as a

catalyst enabling formation of triborylated alkane 4a. This

borane is presumably formed by reaction of a {7DIPP}Zn

species with HBPin as minimal HBPin degradation was

previously discussed. BH -THF proved the most eﬀective

catalyst when added post formation of the 1,1-diborylalkene

(formed using I or II) with 10 mol % BH -THF aﬀording

desired product 4a in much shorter reaction times and with

higher yields than using just I or II (entry 4 vs 5).

With eﬀective conditions for the synthesis of 1,1,1-

triborylated alkanes (4x) from terminal alkynes identiﬁed

using combined Zn−H/B−H catalysts, triborylation of a range

of terminal alkynes was investigated. The substrates selected

were determined by our previous report which documents the

functional groups tolerated by species I/II. Aromatic alkynes

Organometallics XXXX, XXX, XXX−XXX

Organometallics

good yields (Figure 3 ). It should be noted that electron-

Article

with both electron-donating and -withdrawing substituents

were amenable providing desired products 4x in moderate to

Scheme 4. Formation of 4a and 5a during Terminal Alkyne

Borylation/Hydroboration Catalyzed by Zn−H and B−H

Species

Figure 3. Scope of tandem Zn−H/B−H-catalyzed triborylation of

terminal alkynes. Yields by in situ H NMR spectroscopy vs CH Br

as internal standard.

HBPin on heating (110 °C, 5 h) led to high-yield conversion

to 4a, conﬁrming that a zinc species is not required for this

withdrawing substituents required slightly longer reaction

times in the Zn-catalyzed transformation of terminal alkynes

step. In contrast, attempting the BH -THF-catalyzed hydro-

to diborylated alkenes consistent with our previous report.

Haloaryl-substituted alkynes were also tolerated, and no C−X

X = Br, Cl, and F) bond cleavage was detected. Furthermore,

boration of 6a aﬀorded no detectable (by H NMR

spectroscopy) formation of 5a (even at 110 °C). In contrast,

compound II catalyzed production of 5a from 6a using HBPin

(61% yield after 15 h at 110 °C). Thus, B−H species, such as

(

heteroaromatic and polyaromatic substrates such as 3-

ethynylthiophene or 2-ethynylnaphthalene were suitable

substrates for this methodology. However, in the case of 3-

ethynylthiophene, despite prolonged heating only 30% of 4j

was obtained with signiﬁcant amount of 1,1-diborylalkene 3j

BH -THF, are eﬀective for the transformation of 3a to 4a but

are ineﬀective for the transformation of 6a to 5a, with the

opposite selectivity observed using I.

It was hypothesized that the failure of BH -THF to eﬀect

remaining unreacted. Thus, the BH -catalyzed hydroboration

catalytic hydroboration of 6a with HBPin is due to the reaction

is not eﬃcient with this substrate. Furthermore, 1-ethynyl

cyclohexene did not react to produce any signiﬁcant amount

of BH -THF with multiple equivalents of 6a to form sterically

hindered compounds such as A/B (and isomers thereof,

(

<10%) of triborylated product. It should be noted that for

Scheme 5). Hindered R B compounds have been demon-

several of substrates in Figure 3 the omission of BH -THF was

tested in the ﬁnal step, but this led to lower yields of 4x in all

cases under otherwise identical conditions.

The mass balance in these reactions was composed largely of

Scheme 5. Possible Products from the Reaction of BH -

THF with 6a (A/B and Isomers Thereof) and 3a (C)

a second borylated alkane product, 5x, indicated by a

diagnostic doublet ( J ≈ 8 Hz) at ca. 2.7−2.9 ppm in the

H NMR spectra, consistent with a 1,1-diborylated alkane. The

formation of 5x (Scheme 4) is presumably via double

hydroboration of the terminal alkyne as the 1,1,1-triborylated

alkanes are stable under these reaction conditions (i.e., they do

not undergo protodeboronation in situ). It was previously

shown that in aromatic solvents while dehydrogenative

borylation of 1x to form 2x dominates using I or II minor

quantities of hydroboration products are observed (e.g., 2-

These compounds may also be THF adducts/hydride bridged

dimers of A−C.

[

(

(E)-2-p-tolylethenyl]-4,4,5,5-tetramethyl-1,3,2-dioxaborolane

6a)). In situ analysis of the reaction starting from 1a under

strated to not undergo metathesis with HBPin; thus, it is

feasible that species like A or B would not react (or only react

very slowly) with HBPin even on heating. The reaction of

standard conditions (110 °C, 3 M benzene solution, 5 equiv of

HBPin, 10 mol % of II, sealed tube) revealed formation and

BH -THF with 2 equiv of 6a in the absence of HBPin at

then consumption of E-styrylboronate 6a. Compound 6a has

ambient temperature led to a complex mixture, but key

observations include the following: the complete consumption

an indicative resonance at 6.10 ppm (d, J = 18.3 Hz) that is

replaced by the appearance of a new doublet as the reaction

of 6a; the formation of alkylBpin species (indicated by δ₁₁_B=

proceeds at 2.87 ppm ( J = 8.2 Hz) consistent with 5a.

34.1 ppm); and the persistence of some BH -THF.

Identiﬁcation of Catalytically Active Species. To

determine the catalytically active species for the formation of

Furthermore, these products did not react on further addition

of HBPin and heating. This conﬁrms a nonstoichiometric

a and 5a, compounds 3a and 6a were prepared

reaction occurs between 6a and BH -THF to aﬀord inactive

(toward HBpin metathesis) organoboranes.

independently. Adding 10 mol % BH -THF to 3a and

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Kingdom; orcid.org/0000-0001-9975-8302;

Article

The number of equivalents of an alkene that a BH species

react with is highly dependent on alkene steric bulk. For

AUTHOR INFORMATION

Corresponding Author

■

example, Brown and co-workers showed that BH -THF and

Michael J. Ingleson − EastCHEM School of Chemistry,

University of Edinburgh, Edinburgh EH9 3FJ, United

Email: michael.ingleson@edinburgh.ac.uk

excess 2-methyl-2-butene proceeded rapidly to the dialkylbor-

ane product, while even with an excess of the more bulky

alkene 2,3-dimethyl-2-butene the reaction only proceeded to

the monoalkylborane product. Thus, diﬀerent alkene/BH₃-

THF reaction ratios (and thus diﬀerent degrees of sterics

around boron and thus diﬀerent barriers to metathesis with

HBPin) may be the origin of the reactivity disparity between

Authors

Marina Uzelac − EastCHEM School of Chemistry, University of

Edinburgh, Edinburgh EH9 3FJ, United Kingdom;

orcid.org/0000-0002-5060-7017

a and 6a with BH -THF. Consistent with this, combining 2

equiv of 3a and BH -THF led to a rapid reaction but

Kang Yuan − EastCHEM School of Chemistry, University of

incomplete consumption of 3a (indicating a 1:1 reaction

stoichiometry) and the formation of a product consistent with

C. On combination with HBPin, this reaction mixture yielded

Edinburgh, Edinburgh EH9 3FJ, United Kingdom

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.organomet.0c00086

a on heating. This indicates that the barrier to metathesis with

HBPin for species dervied from 3a/BH (such as C) is

signiﬁcantly lower than that for species derived from 6a/BH₃

Funding

European Research Council (769599).

(

such as A/B). Thus, the ability of BH species to catalyze

Notes

hydroboration using HBPin is highly dependent on the steric

bulk of the alkyne/alkene, which in turn controls the reaction

stoichiometry between alkene/BH₃.

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

This work was made possible by ﬁnancial support from the

Horizon 2020 Research and Innovation Program (grant no.

769599). Dr. Mark Crimmin is thanked for useful discussions

and NacNacZnH.

CONCLUSIONS

■

Direct comparison of {7DIPP}ZnH(NTf ) (I) with a

NacNacZnH (III) complex revealed that both can catalyze

alkyne C−H borylation and alkyne hydroboration and that

Zn−C(sp)/H−BPin metathesis was reversible for both.

However, NacNacZnH (III) was a poorer catalyst for

alkynylBPin hydroboration. This is attributed to the greater

biphilic character of {7DIPP}ZnH species, which are more

electrophilic than the NacNacZn congeners (based on the

energy of the key zinc based unoccupied orbital) while still

being hydridic. At raised temperatures {7DIPP}Zn species

enabled the formation of 1,1,1-triborylated alkanes. However,

this occurs after signiﬁcant decomposition of HBPin,

suggesting catalysis by in situ formed B−H species is key for

the hydroboration of 1,1-diborylated alkenes in this case.

REFERENCES

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