Highly syn selective addition of aqueous HBr to hydrophobically shielded arylalkynes

Article abstract of DOI:10.1016/j.tet.2014.05.002

Hydrophobically shielded alkynes HC≡C-aryl, carrying 2,6-di- and 2,4,6-tri-tert-butylphenyl as the aryl group, can add aqueous HBr on heating in moist chloroform solutions to produce pure H₂C=C(-Br)-aryl (isolated yields 96%, no hydrolysis). Em

Full text of DOI:10.1016/j.tet.2014.05.002

Accepted Manuscript
Highly syn selective addition of aqueous HBr to hydrophobically shielded arylalkynes
Rudolf Knorr , Eva C. Rossmann , Monika Knittl , Petra Böhrer
PII:
S0040-4020(14)00662-0
10.1016/j.tet.2014.05.002
TET 25551
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 5 April 2014
Revised Date: 1 May 2014
Accepted Date: 2 May 2014
Please cite this article as: Knorr R, Rossmann EC, Knittl M, Böhrer P, Highly syn selective addition
of aqueous HBr to hydrophobically shielded arylalkynes, Tetrahedron (2014), doi: 10.1016/
j.tet.2014.05.002.
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Highly syn selective addition of aqueous
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HBr to hydrophobically shielded arylalkynes
Rudolf Knorr*, Eva C. Rossmann, Monika Knittl, Petra Böhrer
Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstrasse 5–13 (Haus F), 81377
München, Germany
R = H, tert-butyl
tBu
tBu
tBu
tBu
tBu
conc.
HBr
D
+
CHD
H
D
H
-
R
C
C
D
R
C
C
R
R
Br
Sn(CH3)3
LiSnMe3
Br
tBu
tBu
tBu
E only

1
ACCEPTED MANUSCRIPT
Tetrahedron
journal homepage: www.elsevier.com
Highly syn selective addition of aqueous HBr to hydrophobically shielded
arylalkynes§
Rudolf Knorr*, Eva C. Rossmann, Monika Knittl, Petra Böhrer
Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstrasse 5–13 (Haus F), 81377 München, Germany
* E-mail: rhk@cup.uni-muenchen.de
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Hydrophobically shielded alkynes HC≡C–aryl, carrying 2,6-di- and 2,4,6-tri-tert-butylphenyl as
the aryl group, can add aqueous HBr on heating in moist chloroform solutions to produce pure
H2C=C(–Br)–aryl (isolated yields 96%, no hydrolysis). Employment of DC≡C–aryl furnished
initially only the E stereoisomer of DHC=C(–Br)–aryl (stereospecific syn-addition), which was
slowly both dedeuteriated and partially transformed to the Z stereoisomer by HBr. The strongly
retarded HBr addition to H3C–C≡C–aryl in moist chloroform produced again more E than Z
product, whereas a thermodynamic E/Z ratio of 10:87 was found in moist acetic acid.
Substitution of Br by LiSnMe3 produced H2C=C(–SnMe3)–aryl with well resolved long range
119Sn NMR coupling constants.
Available online
Keywords:
bromoalkenes
reaction mechanisms
stannylation
steric hindrance
vinyl cations
2009 Elsevier Ltd. All rights reserved
.
1. Introduction
Ar
Br
1
The addition of anhydrous HBr to arylacetylenes in aprotic
solvents (CH2Cl2, CHCl3) is a useful method of preparing 1-aryl-
1-bromoalkenes (1). Scheme 1 depicts a collection of reaction
modes that might be significant for the stereochemical course of
HBr addition to a deuteriated starting alkyne 7. The frequently
postulated ion pair intermediate 5 should be formed from 7
through syn addition and will generate the product 2 with E
configuration, whereas the Z product 3 might formally be
ascribed to an anti addition mode. Unfortunately, the E/Z
selectivity is only moderate if not poor1–4 in carefully dried
solvents; this may be due to configurational interconversion of
the products (2 → 3) or of the intermediate ion pairs (5 → 6). It
may also be caused by ionic dissociation of 5 that generates the
free vinyl cation 9 which should1,2 furnish a practically 1:1
mixture of products 2 and 3 (bottom line of Scheme 1). The
presence of water should lead to a partial or total hydrolysis of 9
with formation of a ketone 10, unless a faster reaction can save 9
from such a fate.
2
H
cis
H
trans
1
Ar
Br
Br
Ar
1
2
1
2
-
DBr
+ HBr
+ HBr
D
cis
H
trans
D
trans
H
cis
Ar
1
Br
C
+
-
Br
2
(E)
3 (Z)
DCH2
?
4
Ar
Ar
1
-
+
-
1
+
C
C
Br
C
Br
+ HBr
?
?
-
DBr
C
Ar
C
Ar
C
syn
D
H
?
D
H
5
(E)
6
(Z)
-
-
-
C
H
C
D
Br
-
Br
+
?
+ H
+
+
7
8
-
D
-
H
Ar
1
+
C
C
?
-
DBr
external
Ar
O
?
+ H2O
-
+ Br
C
D
H
?
6
3
9
free
DCH2
10
cation
5
2
Scheme 1. Mechanistic possibilities related to ionic HBr
addition to the alkyne 7. Reaction modes labeled with “?”

2
Tetrahedron
ACCEPTED MANUSCRtB
were not detected with the present substituents Ar = 2,6-di-
I
u
PT
1
C(CH3)3
and 2,4,6-tri-tert-butylphenyl.
+ HBr
2
1
C
2
C
1´
1´
R´
H
R
C
C
R´
R
+
Simple C1–CH2D single bond rotation in the carbenium ion
pair 4 can explain the HBr-catalyzed1 equilibration of products 2
and 3 along with their partial dedeuteriation that affords some
unlabeled bromoalkene 1. In contrast, high energy barriers in 5
and 6 oppose rotations of both the C1=CHD double bond and
the Ar–C1 single bond (even5 in a linear free vinyl cation such
as 9); instead, the direct interconversion of 5 and 6 should require
-
Br
tBu
C(CH3)3
_
R
R´
11 13
_
_
11HH 11buD
12HH 12buD
HH
HD
H
H
H
D
buH tBu H
buD tBu D
migration1,2 of Br
¯ within these ion pairs. If the aryl group in 5
is sufficiently bulky to impede such bromide migration, the
dedeuteriated alkyne 8 can be generated from 5 only via the free
vinyl cation 9, provided that the products are formed irreversibly
(as they usually are) under the reaction conditions and that 5
cannot lose DBr directly through an anti elimination in this
acidic milieu. The latter proviso can be confirmed by means of
the principle of microscopic reversibility which assures that anti
elimination and anti addition share the same transition state.
Therefore, an anti elimination from 5 to give 8 can be excluded if
we can show that the reverse anti addition (8 → 5) does not occur
even though this would be thermodynamically (but not
kinetically) almost equal to the observed 7 → 5 addition (Scheme
1). Using arylalkynes that are two-sidedly shielded by tert-butyl
(t-Bu) substituents, we will seek mechanistic evidence and
tBu
C(CH3)3
2´
3´
5´
2´
3´
R´
2
1
C
2
C
1´
H
4´
1´
4´
R
Li
&2THF
(
)
R
1
2
Br
6´
6´
5´
tBu
C(CH3)3
14H: R = H
14bu: R = tBu
13HH, (E)-13HD
13buH, (E)-13buD
Scheme 2. HBr syn addition via vinyl ion pairs 12.
The alkyne 11HH in chloroform (or CDCl3) solution had to be
heated together with an excess (≤ 8 equiv.) of conc. HBr (“48%”)
at 60 °C during at least 18 hours for complete conversion into the
analytically pure bromoalkene 13HH (yield 96%). With a third
t-Bu substituent in para position, however, the alkyne 11buH
added concentrated HBr under the same conditions but within
only two hours, and the crude product 13buH (yield 96%) was
NMR-spectroscopically pure. This acceleration by the electron-
demonstrate that high yields and
E or Z stereocontrol are
possible with an experimentally simple two-phase system
consisting of moist chloroform and concentrated (aqueous) HBr.
donating,12,13
remote substituent
R
=
t-Bu points to
2. Results and discussion
intermediates with an increased electron demand, presumably
the vinyl carbenium ion pairs 12HH and 12buH, whose
irreversible collapse created the products 13HH and 13buH,
respectively, but no solvolysis products (acetophenone
derivatives like 10) even on prolonged heating in the aqueous
two-phase system. Such a product stability permitted operating
under unusual reaction conditions (excess concentrated HBr in
chloroform), which served to increase the overall reaction rates to
a convenient magnitude.
2.1. Kinetic E (syn) selectivity affording two-sidedly
shielded vinyl bromides
In the 2,4,6-tri-tert-butylphenyl series, the known6,7 alkyne
11buH (Scheme 2) and its precursors were prepared8 in analogy
with the efficient protocols published9 for 11HH in the 2,6-di-
tert-butylphenyl family. The deuterio alkynes 11HD and 11buD
were obtained through deuteriolysis of the lithium acetylides8
14H and 14bu, which had been prepared through deprotonation
of 11HH and 11buH, respectively. We seized the opportunity of
measuring low-temperature NMR spectra of the hitherto
uncharacterized 14H in THF solution, finding8 that [6Li]14H is
dimeric in accord with other lithium acetylides.10,11
Delocalization of negative electric charge from the carbanionic
center C-2 in 14H into the aromatic ring was indicated by
moderate upfield 13C NMR lithiation shifts ∆δ = δ(14H) –
δ(11HH) (for example, –4.2 ppm at C-para).8
The syn mode of HBr addition became evident with deuterium
labeling: The deuterio alkyne 11HD in moist chloroform
afforded 13HH and (E)-13HD (42:58) along with some 11HH,
but no (Z)-13HD. This observation excludes both an intervention
of a Z ion pair (like 6) and a product formation from a free vinyl
cation (like 9) alone, the latter because that would have furnished
a very close to 50:50 E/Z ratio.1,4 As explained in the
Introduction, however, the observation of some 11HH points to
the occurrence of some free vinyl cation whose dedeuteriation
(like 9 → 8) was faster than the formation of (Z)-13HD (like 9 →
6 → 3). An almost 50:50 E/Z ratio would also result under
thermodynamic product control; but the absence of (Z)-13HD
established that this had not taken place, so that the experiment
had been conducted with kinetic control. Similarly, the faster
consumption of 11buD by concentrated HBr (3.4 equiv.) in
chloroform furnished 13buH, (E)-13buD, and initially no (Z)-
13buD (39:61:0), indicating again kinetic control; in fact, a later
snapshot (65 min at 60 °C) exhibited a changing ratio of 44:48:8.
This later joint accumulation of the unlabeled product 13buH
and emergence of (Z)-13buD point to the HBr-catalyzed1
dedeuteriation and isomerization, respectively, of the initial
product (E)-13buD. Another fraction of 13buH arose through
HBr addition to 11buH that had been detected in situ (formed
like 9 → 8). The initial syn (E) preferences are considerably
higher than those reported for the HBr addition to 1-(p-anisyl)-2-

3
deuterioacetylene (up to 4:1)1 or to 1-(naphth-1-yl)-2-
formed with external H2O despite the treatment with aqueous
ACCEPTED MANUSCRIPT
deuterioacetylene (4:1)3 or to Ph–C≡CD (3:1)2.
HBr. We hoped and will establish in Section 2.3 that the latter
impediment is not large enough to baffle the substitution of
bromine in 13 by the external nucleophile LiSnMe3.
The various ionization events described above will occur more
rapidly in a more polar milieu. Thus, a solution of HBr in moist
acetic acid converted 11buD at 23 °C quickly (≤ 10 min) into a
mixture of 13buH, (E)-13buD, and (Z)-13buD; the ratio of
42:36:22 indicated that these products had been generated under
E/Z kinetic control rather than under thermodynamic control and
also not via the free cation derived from 12buD alone. For other
examples of a more polar milieu, a saturated CDCl3 solution of
2.2. Thermodynamic Z selectivity of a two-sidedly shielded
1-bromo-1-arylpropyne
The addition of concentrated HBr to the propyne9 16 in moist
chloroform solution at 60 °C was far from being complete after
6.5 days. The crude material contained residual propyne 16
along with (E)-15 and (Z)-15 in a ratio of 36:46:18 but no 2,6-t-
Bu2-propiophenone9 (17). The E/Z configurational assignments
¯ ¯ along
Et4N+Br or a CH2Cl2 solution containing n-Bu4N+Br
with 11buD and F3C–CO2H furnished 13buH, (E)-13buD, and
(Z)-13buD in less than 15 min at 23 °C in the ratio of 78:11:11.
As far as F3C–CO2H may not be sufficiently acidic to perform
product dedeuteriation and E/Z interconversion as mentioned
above, this product mixture may be ascribed to a free vinyl
cation alone: In contrast to the above experiments with HBr, the
3
followed from the JC,H coupling constants of C-1´ to 2-H (7.2
Hz and 3.6 Hz, respectively). This E preference resembled the
4:1 ratio reported18 for 1-phenylpropyne and would be
compatible with both a dominant syn addition mechanism and an
intermedicacy of a free cation (such as 9); for the latter variant,
the recombinant approach of Br¯ as illustrated in 12 (R = H, R´
free cation was trapped here by the exuberant Br¯ (→ E/Z =
= CH3) would preferably occur from the less hindered side to
give more E than Z product. In either case, however, the above
product ratio does not fall into the realm of thermodynamic
control, as will be shown in the sequel.
11:11) in competition with the characteristic dedeuteriation
(11buD → two-sidedly shielded free cation like 9 → 11buH
→→ 13buH) that was still faster (78%) than trapping by Br¯ .
Similar types of experimental setup have been routinely applied
in the literature for preparations of bromoalkenes from less
congested terminal alkynes with HBr: For examples, the
3´
3´
2´
tBu
tBu
2´
4´
5´
4´
5´
tBu
necessity of working under strictly anhydrous conditions1,2 in
1
C
HBr
1´
HBr
1´
Br
H
1
C
Br
C
C
CH3
6´
14,15
CH2Cl2
or acetic acid16 was either emphasized15,16 or
in CHCl3
6´
in HOAc
tBu
tBu
3
C
3
C
2
implied, especially when a tetraalkylammonium bromide in
H3C
cis
(E)-15
2
H
CH3
16
tBu
15
trans
cis
trans
CH2Cl2 with HBr gas was employed, whereas the frequently
HBr + H2O
(Z)-15
annoying hydrolytic ketone formation was surprisingly not
noted17 on heating 1,2-diethynylbenzene with concentrated HBr
(2 equiv.) in moist 3-pentanone as the solvent. Even though our
moist two-phase systems preclude precise kinetic quantifications
of Scheme 2, the dramatic acceleration of all these reactions in
the more polar solvents supports the suggested ionization events
with carbenium ion intermediates during and after the formation
of products 13. The necessary E/Z assignments of 13HD and
13buD were not easily accomplished in view of the small
olefinic cis/trans 1H NMR shift differences of down to 0.01 ppm
(as for 13buH in CH2Cl2); therefore, these olefinic resonances
were firmly assigned through 1H NMR Overhauser enhancement
not observed
tBu
preferred
under
thermodynamic
preferred
under
kinetic
control
O
17
C
CH2-CH3
control
tBu
Scheme 3. Kinetic and thermodynamic termination of HBr
addition to 16.
The above product ratio changed to 3:10:87 when 16 was
treated with a solution of HBr (2
M) in moist acetic acid at 60 °C
for seven days; ketone 17 was detected in such a mixture only if
substantially higher amounts of water were present. The latter
E/Z ratio of 10:87 was the same after only three days at 60 °C
and is thought to represent thermodynamic product control, to be
compared with the E/Z equilibrium ratio19 of 12:88 for 1-
anisyl-1-bromopropene. Obviously, the 2-CH3 substituent is
repelled by the bromine atom less strongly than by the 2´,6´-di-
tert-butylphenyl group. The latter bulky substituent may thus be
characterized by an empirical substituent parameter20 of λd = ca.
3.9, based upon λd(Br) = ca. 1.6; these two parameters will
reproduce the above experimental E/Z ratio of 0.115 through
the relationship8,20 of ln(E/Z) = 0.95(3.9 – 1.6)/(0 – 1) = –2.18,
which amounts to a calculated E/Z ratio of 0.113. Thus, our
conditions for preparing (Z)-bromoalkenes may be suitable for
alkynes carrying bulky aryl groups and a second substituent that
(NOE) differences,
13C satellites of the olefinic proton
resonances, deuterium-induced upfield 1H NMR shifts, trans
3
2
and cis JC,H coupling constants to C-1´, JC,H coupling to C-1,
or auxiliary results from 13C NMR spectra under {1H} selective
decoupling.
The apparently unprecedented, initially strict syn mode of HBr
addition to our model alkynes 11 simplifies the mechanistic
multiplicity in Scheme 1, so that the reaction modes labeled with
”?” therein may be disregarded for our hydrophobically shielded
system. We explain this by blockades of both of the two obvious
possibilities of stereoinversion of the ion pair 5 (E) to give 6 (Z):
As outlined in the Introduction, rotation about the Ar–C1 single
bond in the cations is energetically too expensive.5 Secondly,
bromide anion migration1,2 within the ion pairs 5, 6, or 12
(excluding dissociation) is impeded by the 2´-/6´-t-Bu
substituents. On the other hand, the products 13 (first E, then Z)
could not have been generated by the E/Z unselective (because
dissociated) free vinyl cations (such as 9) which preferred to
react only (namely, faster) with regeneration of the arylalkynes
(11) under the presently used reaction conditions. This may be
is at least as “large”20
as methyl.
For comparison,
pentamethylphenyl (λd = 0.30) is “smaller” than methyl (λd
=
1.0) when connected to a double bond.20
2.3. Vinylic Br/SnMe3 Substitution
Lithium trimethylstannide, LiSnMe3, is a useful (albeit
somewhat unstable)21,22 reagent for replacing a Br (or Cl)10 atom
with SnMe3. Within less than 15 min at below rt, bromoalkene
due to a low concentration of external Br
system; moreover, the required in-plane approach (as in 12) of
external Br toward C-1 of the free vinyl cation might also be
¯ in our two-phase
23
13HH and LiSnMe3 in THF (Scheme 4) furnished the vinyltin
¯
compound 18HH along with the “parent” olefin 19HH (64:36).
slowed down through the adverse two-sided shielding. This
possibility would also explain why ketones (such as 10) were not
Similarly, the vinyltin derivative 18buH and the known24,25

4
Tetrahedron
olefin 19buH (7:3) were obtained from bromoalkene 13buH.
13C nuclei and all sp2-bound protons in 18HH and even those of
ACCEPTED MANUSCRIPT
The parent olefins 19 point to formation of alkenyllithiums 20
either as products of side-reactions or as inadvertently trapped
intermediates. It appears obvious that LiSnMe3 can approach the
bromine atom of 13 only from close to the σ-bond plane of the
olefinic double bond (C-1/C-2), that is, in a similar manner as
the p-t-Bu carbons in 18buH were well resolved.
3. Conclusion
Connected to C-1 of a C-1/C-2 double bond, 2´-/6´-di-tert-
butylaryl (aryl*) substituents such as “supermesityl” (mes*)28 or
“super-2,6-xylyl” (xyl*)9 are much “larger” than bromine,
depicted for Br
¯ in 12 (Scheme 2). An alternative initiation of
methyl, or pentamethylphenyl20
when viewed from the
an addition-elimination process through contacts of LiSnMe3
with the π-orbital lobes of C-1 is impeded since both π-faces are
shielded by the 2´-/6´-t-Bu substituents in 13 (Scheme 4) as far
as the aryl group maintains a close to orthogonal orientation with
respect to the olefinic double-bond. In fact, evasive rotational
moves about the C-1´/C-1 single bond should be more strongly
opposed than by the free-enthalpy barrier of ∆G‡(162 °C) > 23.8
kcal mol–1 that was observed26 for 1-(2´,6´-diisopropylphenyl)-
1-bromoethene and -1-methylethene which are similar to but less
strongly shielded than 13.
standpoint of a substituent at C-2. Despite their two-sided
shielding of the π-faces of the olefinic double bond, these two
aryl* groups leave enough free space in the olefinic σ-plane for
a rather strainfree connection of C-1 to an SnMe3 group. On the
other hand, aryl* disfavors the following events close to C-1 of
the double bond.
(i) Single-bond rotation of aryl* in H2C=C(–X)–aryl* is
strongly impeded if X is at least as “large” as bromíne or methyl.
Consequently, the close to orthogonal conformations of the aryl*
plane with respect to the olefinic double bond plane belong to an
energetic minimum with rather steep walls.
H3C
H3C
H3C
CH3
CH3
CH3
3´
3´
3´
R
2´
R
2´
R
C
2´
C
C
CH3
4´
4´
4´
H3C
H3C Sn
H3C
CH3
5´
Br
H
5´
5´
H
H
+
1´
1´
-
LiBr
1´
6´tBu
R´
6´tBu
R´
cis
1
6´tBu
R´
cis
CH3
1
1
(ii) For electronic reasons, the above single-bond rotation
2
+ LiSnMe3
2
2
H
remains inhibited also in the more linear cation topology of the
ion pairs DHC=C –aryl* Br
+
trans
trans
13HH, 13buH
(E)-13HD
¯
(12HD or 12buD) that are
18HH, 18buH
(E)-18HD
19HH, 19buH
(Z)-19HD
generated through protonation of the alkynes DC≡C–aryl* by
HBr. In other words, 5 (E) with Ar = aryl* cannot rotate into the
6 (Z) configuration on the preparative time scale.
+ LiSnMe3
_
?
?
18 20
R
R´
-
BrSnMe3
+ BrSnMe3
HH
HD
H
H
H
D
(iii) The 2´-/6´-t-Bu groups in an ion pair 5 (E) with Ar =
aryl* impede Br
H3C
CH3
buH tBu H
¯ migration across the nonrotating, orthogonal
R
C
THF
aryl* group as a second possibility of E/Z stereoinversion.
Together with the disfavored single-bond rotation described in
(ii), such an inhibited migration entails the unprecedentedly strict
stereospecificity of ionic addition of HBr to DC≡C–aryl* that
afforded the (initially) configurationally pure (E)-bromoalkenes.
H3C
H3C
CH3
3´
CH3
H3C
Li
3´
THF
THF
R
R
2´
2´
C
C
CH3
1
4´
5´
4´
tBu
H3C Sn
H3C
1
CH3
CH3
5´
H
D
R´
cis
H
trans
1´
1´
6´tBu
6´tBu
1
2
2
H
H
D
20HH, 20buH, (Z)-20HD
cis
trans
cis
trans
(iv)
Although stereospecific (since faster than
Br¯
(Z)-20HD (E)-20HD
(Z)-18HD
(E)-19HD
migration), the nucleophilic addition of Br
ion pair 5 (E, Ar = aryl*) or 12 appears sufficiently retarded, so
that a portion of that 5 (E) can dissociate and form the free
cation DHC=C –aryl* (like 9). Now however, the recombinant
¯
to C-1 within the
Scheme 4. Me3Sn derivatives 18 and olefins 19 from
bromoalkenes 13.
+
addition of external Br
¯ to this 9 should be even less probable
A bona-fide specimen of alkenyllithium 20buH, generated
from 13buH and n-BuLi in Et2O, reacted readily with ClSnMe3
to furnish clean 18buH. This may be taken to confirm that a
Br/Sn interchange reaction of 13 with LiSnMe3 generated 20 as a
primary intermediate that subsequently coupled to its co-product
BrSnMe3 with formation of 18. Because the observed olefin 19
suggested that trapping of 20 by BrSnMe3 competed with proton
transfer from THF or adventitious sources, one might anticipate
that the high stereolability to be expected27 for 20 should lead to
stereorandom stannane mixtures. Thus, the bromoalkene (E)-
13HD (as obtained in Section 2.1) and LiSnMe3 would generate
the primary intermediate (Z)-20HD (Scheme 4) which (if
microsolvated by three THF ligands27 at lithium) would
equilibrate with (E)-20HD before the concomitant BrSnMe3
produced 18HD. In fact, configurationally pure (E)-13HD with
an admixture of 13HH reacted with LiSnMe3 in THF to furnish
a 32:34:34 product ratio of 18HH, (E)-18HD, and (Z)-18HD
along with the parent olefins 19HH, (E)-19HD, and (Z)-19HD in
a ratio of 35:30:35. These stereo-unselective product mixtures
are compatible with (albeit not definitive evidence for) a pathway
via stereolabile, trisolvated 20HD. Although C-1 in 20 is heavily
shielded by the 2´-/6´-t-Bu substituents, there should be enough
space left for the Li(THF)3 group, judging by comparison with
Sn(CH3)3 in 18 as a model: The NMR spectra of 18HH and
18buH showed no line broadening that would indicate hindered
internal rotations at rt, so that the 119Sn coupling constants of all
than within the already preformed ion pairs 5 or 12; indeed, 9
with Ar = aryl* appears to decay with formation of unlabeled
HC≡C–aryl* rather than by forming bromoalkenes 13.
(v) This decay mode of 9 (Ar = aryl*) and its unlabeled
+
analogues H2C=C –aryl* apparently occurred also faster than
their hydrolysis to give ketones such as Me–C(=O)–aryl* (like
10 or 17). This points to a sufficiently retarded approach of H2O
toward the cationic center C-1; in practice, it permits the
convenient use of aqueous (concentrated) HBr in our two-phase
system or even in an almost homogeneous milieu.
(vi) A sufficiently retarded approach of BrSnMe3 toward the
carbanionic center C-1 in the supposedly stereolabile
alkenyllithium DHC=C(–Li)–aryl* (20HD or 20buD, if indeed
intermediates) might explain why the considered Br/Li
interchange reaction of DHC=C(–Br)–aryl* with LiSnMe3 was
stereo-unselective. This supposition suggests to perform more
detailed investigations of bona-fide 20HH, to be prepared from
H2C=C(–Br)–aryl* or H2C=C(–SnMe3)–aryl*, in particular since
the above effects of two-sided shielding let us anticipate an
unusually low tendency of 20HH to dimerize or to form higher
aggregates.

5
ppm, C-2), 126.3, 128.3, 132.6 (2∆ = –0.0348 ppm, C-1), 138.5,
148.4 ppm.
4. Experimental section
4.1. General information
ACCEPTED MANUSCRIPT
4.4. 1-Bromo-1-(2´,4´,6´-tri-tert-butylphenyl)ethene (13buH)
1H and 13C NMR spectra were referenced to Si(CH3)4 as an
internal standard. Deuterium-induced isotope shifts transmitted
over n chemical bonds, n∆ = δ(deuteriated) – δ(nondeuteriated),
provide separate 1H or 13C NMR signals for a deuteriated
compound and its unlabeled parent in a nonequivalent mixture.
All n∆ values measured in this work are negative, because in
each case the labeled component absorbed upfield from the
parent.
The alkyne 11buH (380 mg, 1.41 mmol),8 conc. HBr (1.52
mL, 13.5 mmol), and chloroform (6.4 mL) were heated in a
tightly stoppered Schlenk flask (100 mL) at 60 °C with magnetic
stirring for 2 h. The mixture was poured into aqueous NaOH (12
mL, 12 mmol) and rinsed out with CH2Cl2 (15 mL). The
CHCl3/CH2Cl2 layer was washed with dist. water (3 ×), dried
with MgSO4, and evaporated to yield solid, crude 13buH (478
mg, 96%) containing only a tiny trace of residual 11buH. The
analytically pure, cottony needles of 13buH (m.p. 113–114 °C)
crystallized from hexane, EtOH, or methanol and should not be
stored in a desiccator with solid KOH because of re-conversion
to the alkyne 11buH. IR (film) ν 2964, 2909, 2870, 1626 (w,
C=C), 1601 (w), 1363, 1040, 569 cm–1. 1H NMR (400 MHz,
CDCl3) δ 1.31 (s, 9H, 4´-CMe3), 1.55 (s, 18H, 2´-/6´-CMe3),
6.002 (d, 2J 1.58 Hz, 1H, olefinic H cis to aryl), 6.024 (d, 2J 1.58
Hz, 1H, trans-H), 7.44 (s, 2H, 3´-/5´-H) ppm, assigned through
an NOE difference spectrum with irradiation of the 2´-/6´-t-Bu
protons generating a stronger enhancement of the cis-H (upfield)
doublet signal. 13C NMR (100.6 MHz, CDCl3) δ 31.26 (qm, 1J
125.8 Hz, 3J 4.9 Hz, 4´-CMe3), 33.70 (qm, 1J 125.8 Hz, 3J 4.9
Hz, 2´-/6´-CMe3), 34.98 (m, 2J 3.8 Hz, quart. 4´-C), 38.46
(broadened m, quart. 2´-/6´-C), 123.47 (dd, 1J 153.7 Hz, 3J 6.8
Hz, C-3´/-5´), 126.08 (dd, 1J 159.0 Hz to cis-H and 1J 164.4 Hz
4.2. 1-Bromo-1-(2´,6´-di-tert-butylphenyl)ethene (13HH)
A Schlenk flask (100 mL) was charged with a magnetic
stirring bar, (2´,6´-di-tert-butylphenyl)acetylene9 (2.14 g, 9.98
mmol), chloroform (32.0 mL), and conc. HBr (“48%”, 9.80 mL,
87 mmol). The flask was closed with a glass stopper (held by a
spring), warmed up with a short venting of the stopper at 50 °C,
and heated at 60 °C with stirring for 21 h. The mixture was
poured into aqueous NaOH (0.67
M, 150 mL) and diluted with
chloroform (40 mL). The aqueous layer was extracted with
chloroform (50 mL), and the combined CHCl3 layers were
washed with dist. water (100 mL), dried with MgSO4, filtered,
and evaporated. The remaining, slightly brown liquid 13HH
(2.84 g, 96%) was analytically pure; b.p. 90–93 °C (bath
temp.)/0.1 mbar. IR (film) ν 2965, 2870, 1632 (C=C), 1473,
1362, 1185, 1047, 904, 806, 754, 642 cm–1. 1H NMR (400 MHz,
2
2
to trans-H, C-2), 133.16 (dd, J (–)6.8 Hz29 to trans-H, J (+)5.3
Hz to cis-H, C-1), 135.45 (ddt, 3J 8.0 Hz to trans-H, 3J 3.5 Hz
to cis-H, 3J 7.5 Hz to 3´-/5´-H, C-1´), 147.70 (m, 3J 3.5 Hz to
CH3, C-2´/-6´), 149.92 (m, 3J 3.4 Hz, C-4´) ppm, assigned
through comparison with 13HH and selective {3´-/5´-H}
decoupling which showed the quart. 2´,6´-C sharpened to a >
sextet with 2J 3.8 Hz, C-1´ as a well resolved dd with 3J 8 and
3.5 Hz. Anal. Calcd for C20H31Br (351.38): C, 68.36; H, 8.89;
Br, 22.74. Found: C, 68.68; H, 8.66; Br, 22.31.
2
CDCl3) δ 1.54 (s, 18H, 2´-/6´-CMe3), 6.02 (d, J 1.68 Hz, 1H,
2
13C satellites 1J 159.0 Hz, olefinic H cis to aryl), 6.05 (d, J 1.68
Hz, 1H, 13C satellites 1J 164.8 Hz, trans-H), 7.20 (t, 3J 8.0 Hz,
1H, 4´-H), 7.41 (d, 3J 8.0 Hz, 2H, 3´-/5´-H) ppm, assigned
through an NOE difference spectrum with irradiation of the t-Bu
protons, which intensified the doublet at δ 6.02 ppm (cis-H) and
even stronger also the 3´-/5´-H doublet. 13C NMR (100.6 MHz,
CDCl3) δ 33.58 (qm, 1J 125.8 Hz, 3J 4.9 Hz, 2´-/6´-CMe3),
38.19 (pseudo-dm; computer-simulated with 3J 4.1 Hz to one of
2
4
5
3´-/5´-H, J 3.8 Hz to CH3, J 0.4 Hz to 4´-H, and J +0.3 Hz to
the other one of 3´-/5´-H; quart. 2´-/6´-C), 126.05 (dd, 1J 159.0
Hz to cis-H and 1J 164.8 Hz to trans-H, C-2), 126.27 (dm;
4.5. (E)-[2-D]-1-Bromo-1-(2´,4´,6´-tri-tert-butylphenyl)ethene
(E-13buD)
computer-simulated with 1J 157.3 Hz, J 1.9 Hz, and J 7.8 Hz;
C-3´/-5´), 128.30 (dm; computer-simulated with 1J 159.4 Hz and
2J 0.5 Hz; C-4´), 132.60 (ddt; computer-simulated with 2J –7.3
Hz29 to trans-H, 2J +5.8 Hz to cis-H, and 4J 1.3 Hz to 3´,5´-H;
C-1), 138.49 (m; computer-simulated with 3J 8.0 Hz to trans-H,
3J 3.4 Hz to cis-H, 3J 7.3 Hz to 3´-/5´-H, and 4J 1.5 Hz to 4´-H;
C-1´), 148.38 (dm; 3J 7.5 Hz to 4´-H, 3J 3.4 Hz to CH3; C-2´/-
6´) ppm, assigned through selective {1H} decoupling as follows:
{CH3} → C-2´/-6´ as a d 3J 7.5 Hz, quart. 2´-/6´-C as the X-part
1H NMR (400 MHz, CDCl3) δ 6.008 (olefinic H trans to aryl,
2∆ = –0.0154) ppm. 13C NMR (100.6 MHz, CDCl3) δ 33.70 (6∆
= –0.0064 ppm, 2´-/6´-CMe3), 38.46 (5∆ = –0.0062 ppm, quart.
2´-/6´-C), 123.45 (5∆ = –0.0076 ppm, C-3´/-5´), 125.82 (dt, 1J
2
3
1
164.5 Hz to trans-H only, JCD 24.4 Hz, 1∆ = –0.257 ppm, C-2),
133.13 (2∆ = –0.035 ppm, C-1), 135.43 (3∆ = –0.0171 ppm, C-
1´) ppm, assigned through selective {3´-/5´-H} decoupling
2
showing C-1 as a d with J 6.8 Hz to trans-H only.
5
of an AA´X spin system with 3J + J = 4.2 Hz, and C-1´ well
4.6. (Z)-[2-D]-1-Bromo-1-(2´,4´,6´-tri-tert-butylphenyl)ethene
(Z-13buD)
4
resolved; {cis- and trans-H} → C-1 as a t J 1.3 Hz, C-1´ as a td
4
3J 7.3 Hz and J 1.5 Hz; {4´-H} → C-1´ as a ddt; {3´-/5´-H} →
An NMR tube (5 mm) was charged with the deuteriated
alkyne 11buD (31 mg, 0.12 mmol), [D12]cyclohexane (0.06 mL),
and conc. HBr (0.88 mmol) in moist acetic acid (0.44 mL, see 15
for the preparation). The alkyne was completely consumed in
this two-phase system in less than 10 min at 23 °C, and another
in-situ spectrum taken after 1 h showed 13buH, (E)-13buD, and
(Z)-13buD in the ratio of 42:36:22. The mixture was diluted
with iced water and Et2O, and the Et2O layer was extracted with
2
3
quart. 2´-/6´-C as a sharpened > octet J 3.8 Hz, C-1´ as a dd J
8.0 and 3.4 Hz, C-1 as a dd 2J 7.3 and 5.8 Hz. Anal. Calcd for
C16H23Br (295.25): C, 65.09; H, 7.85. Found: C, 65.49; H, 7.75.
4.3. (E)-[2-D]-1-Bromo-1-(2´,6´-di-tert-butylphenyl)ethene
(E-13HD)
The preceding protocol was applied to the deuteriated alkyne
11HD8 (185 mg, 0.86 mmol) in chloroform (3.10 mL) with conc.
HBr (0.84 mL, 7.6 mmol) at 60 °C for 18.5 h. After workup, the
unpurified liquid (215 mg, 84%) contained only 13HH and (E)-
13HD (42:58) with no more than a trace of residual 11HD. 1H
NMR (400 MHz, CDCl3) δ 1.54, 6.03 (olefinic H trans to aryl
with 2∆ = –0.0155 ppm), 7.21, 7.41 ppm. 13C NMR (100.6
2
M NaOH, washed with dist. water until neutral, dried over
Na2SO4, and concentrated to give the solid crude product mixture
(36 mg, 85%) as above. 1H NMR (400 MHz, CDCl3) δ 5.986
(olefinic H cis to aryl, 2∆ = –0.0166) ppm, assigned through an
NOE difference spectrum with irradiation of the 2´-/6´-t-Bu
protons generating a strong enhancement of the cis-H (upfield)
2
MHz, CDCl3) δ 33.6, 38.2, 125.8 (t, JCD 24.4 Hz, 1∆ = –0.2565

6
Tetrahedron
CCEPTED MANUSCRIPT
singlet signal. 13C NMR (100.6 MHz, CDCl3) δ 125.78 (t,
°C with stirring. After addition of a freshly prepared, green
A
1JCD 25.2 Hz, ∆ = –0.288, C-2) ppm.
solution of LiSnMe3 (10.0 mmol) in THF (10.2 mL), the red-
brown mixture was stirred at –70 °C for 5 min and without
cooling for 10 min, then poured into Et2O (20 mL) and dist.
water (50 mL, slightly exothermic). The aqueous layer was
extracted with Et2O (2 × 20 mL), and the combined Et2O layers
were washed with dist. water, dried over MgSO4, and
concentrated. The crude material containing 18HH and the
parent olefin 19HH (65:35) was separated from tin coproducts
(first fractions) by column chromatography (silicagel, eluent
petroleum ether) and then distilled under 0.001 mbar, yielding
19HH (152 mg, 18%, b.p. 78–83 °C bath temp.), thereafter a
mixture (298 mg) of 19HH and 18HH (7:3), followed by pure
18HH (609 mg, 40%, b.p. 110–112 °C bath temp.). 1H NMR of
18HH (400 MHz, CDCl3) δ +0.183 (s, 9H, 119Sn satellites 2J
52.1 Hz, SnMe3), 1.388 (s, 18H, 2´-/6´-CMe3), 5.732 (d, 2J 2.62
Hz, 1H, 119Sn satellites 3J 73.9 Hz, olefinic H trans to aryl),
5.814 (d, 2J 2.62 Hz, 1H, 119Sn satellites 3J 156.6 Hz, cis-H),
7.042 (t, 3J 8.0 Hz, 1H, 119Sn satellites 6J 5.9 Hz, 4´-H), 7.370
(d, 3J 8.0 Hz, 2H, 119Sn satellites 5J 3.3 Hz, 3´-/5´-H) ppm,
assigned through a {t-Bu2} NOE difference spectrum with strong
enhancements of SnMe3, cis-H, and 3´-/5´-H. 1H NMR of 18HH
(400 MHz, [D6]benzene) δ +0.157 (s, 9H, SnMe3), 1.411 (s,
18H, 2´-/6´-CMe3), 5.597 (d, 2J 2.6 Hz, 1H, trans-H), 5.811 (d,
1
23
4.7. 1-(2´,6´-Di-tert-butylphenyl)-2-lithioacetylene (14H)8
4.8. 1-(2´,4´,6´-Tri-tert-butylphenyl)-2-lithioacetylene (14bu)8
4.9. 1-Bromo-1-(2´,6´-di-tert-butylphenyl)-1-propene (15)
A solution of HBr (2.0
M) in moist acetic acid was prepared
by dropwise addition (2 h) of conc. HBr (“48%”, 8.0 mL, 71
mmol) to acetic anhydride (27.4 mL, 290 mmol, strongly
exothermic) with good stirring that was continued overnight. The
propyne 16 (119 mg, 0.521 mmol)9 and this HBr solution (4.60
mL, 9.2 mmol HBr, ca. 7 mmol of H2O) were stirred in a tightly
stoppered Schlenk flask (10 mL) at 60 °C (homogeneous
solution) for 7 d. The mixture was poured into aqueous NaOH (1
M
, 20 mL) and extracted with CHCl3 (3 × 10 mL). The extracts
were washed with dist. water until neutral, dried with MgSO4,
filtered, and concentrated to yield a liquid mixture (145 mg) of
residual propyne 16, (E)-15, and (Z)-15 (3:10:87) that was
distilled at 105–112 °C (bath temp.)/0.001 mbar. 1H NMR of
(E)-15 (400 MHz, CDCl3) δ 1.505 (s, 18H, 2´-/6´-CMe3), 1.526
3
3
(d, J 7.2 Hz, 3H, cis CH3-3), 6.147 (q, J 7.2 Hz, 1H, olefinic H
3
3
trans to aryl), 7.204 (t, J 8.1 Hz, 1H, 4´-H), 7.419 (d, J 8.1 Hz,
2H, 3´-/5´-H) ppm. 1H NMR of (Z)-15 (400 MHz, CDCl3) δ
1.456 (s, 18H, 2´-/6´-CMe3), 1.866 (d, 3J 6.5 Hz, 3H, trans CH3-
3), 5.944 (q, 3J 6.5 Hz, 1H, cis-H), 7.198 (t, 3J 8.1 Hz, 1H, 4´-
H), 7.407 (d, 3J 8.1 Hz, 2H, 3´-/5´-H) ppm. 13C NMR of (E)-15
(100.6 MHz, CDCl3) δ 18.87 (qd, 1J 127.8 Hz, 2J 2.9 Hz, CH3-
3), 33.59 (qm, 1J 126.0 Hz, 3J 4.9 Hz, 2´-/6´-CMe3), 38.42 (m,
3
3
2J 2.6 Hz, 1H, cis-H), 7.046 (t, J 8.0 Hz, 1H, 4´-H), 7.397 (d, J
8.0 Hz, 2H, 3´-/5´-H) ppm. 13C NMR of 18HH (100.6 MHz,
CDCl3) δ –5.23 (qm, 1J 128.6 Hz, 3J ca. 1 Hz, 119Sn satellites
1J 342.6 Hz, SnMe3), 33.77 (qm, 1J 125.7 Hz, 3J 4.9 Hz, 119Sn
2
satellites 5J 4.8 Hz, 2´-/6´-CMe3), 37.42 (dm, J (–)3.7 Hz, 3J +
5J = ca. 4.4 Hz, 119Sn satellites 4J 3.2 Hz, quart. 2´-/6´-C),
124.92 (sharp d, 1J 158.3 Hz, 119Sn satellites 5J 9.2 Hz, C-4´),
2
2
3J + 5J = 4.5 Hz, J 3.8 Hz, quart. 2´-/6´-C), 124.25 (dq, J (–
)8.2 Hz29 to trans-H, 3J 7.6 Hz to CH3-3, C-1), 127.10 (dm,30 1J
156 Hz, C-3´/-5´), 128.14 (dm,30 1J 158.5 Hz, C-4´), 133.43 (dq,
1J 160.0 Hz, 2J (–)6.9 Hz,29 C-2), 134.85 (dtd, 3J 7.2 Hz to
4
125.11 (ddd, 1J 155.5 Hz, 2J 2.3 Hz, 3J 7.6 Hz, but J to t-Bu
not resolved, 119Sn satellites 4J 6.5 Hz, C-3´/-5´), 130.84 (sharp
1
1
2
dd, J 156.4 Hz and J 155.3 Hz, 119Sn satellites J 25.0 Hz, C-
trans-H, 3J 7.2 Hz to 3´-/5´-H, and J 1.4 Hz to 4´-H, C-1´),
4
2), 145.84 (ddtd, 3J 13.2 Hz to trans-H, 3J 7.7 Hz to cis-H, 3J
148.78 (unresolved dm containing 3J 7.5 Hz to 4´-H, C-2´/-6´)
ppm, assigned through the C,H coupling constants and selective
{1H} decoupling as follows: {t-Bu2 and CH3-3} → quart. 2´-/6´-
C as the broadened X-part of an AA´BX spin system with 3J +
5J = 4.5 Hz, C-2 as a sharp d 1J 160.0 Hz, and C-2´/-6´ as a
broadened d 3J 7.5 Hz; {trans-H} → CH3-3 as a sharp q 1J
6.8 Hz to 3´-/5´-H, and 4J 1.5 Hz to 4´-H, 119Sn satellites J 36.0
2
Hz, C-1´), 146.50 (dm, 3J 7.5 Hz to 4´-H, 3J 3.5 Hz to CH3,
119Sn satellites 3J 17.5 Hz, C-2´/-6´), 160.55 (ddm, 2J (–)5.6
Hz29 to cis-H, 2J ca. (–)1 Hz29 to trans-H, unresolved 3J to the
SnMe3 protons, 119Sn satellites 1J 466.5 Hz, C-1) ppm,
assigned through selective {1H} decoupling as follows: {SnMe3}
→ C-1 as an ill-resolved dd; {2´-/6´-CMe3} → quart. 2´-/6´-C as
127.8 Hz, C-1´ as a td J 7.2 Hz and 4J 1.4 Hz; {3´-/5´-H} → C-
3
1´ as a d 3J 7.2 Hz to trans-H; {3´-/4´-/5´-H} → C-1 as a dq.
13C NMR of (Z)-15 (100.6 MHz, CDCl3) δ 17.51 (qd, 1J 127.8
Hz, 2J 2.9 Hz, CH3-3), 33.67 (qm, 1J 126.1 Hz, 3J 4.9 Hz, 2´-
the X-part of an AA´X spin system with 3J + J = 4.4 Hz, C-
5
2´/-6´ as a d 3J 7.5 Hz, and C-3´/5´ as a sharp ddd; {trans- and
cis-H} → C-1 as a broadened s, C-1´ as a td; {3´-/5´-H} →
quart. 2´-/6´-C as a clean m 2J 3.7 Hz, C-1´ as a clean dd to
/6´-CMe3), 38.22 (m, 3J + J = 4.2 Hz, 2J 3.8 Hz, quart. 2´-
5
/6´-C), 126.32 (dm,30 1J 156 Hz, C-3´/-5´), 127.71 (dqt, 2J 10.1
3
=CH2, and C-2´/-6´ as a sharpened m J 3.4 Hz, {4´-H} → C-1´
Hz to cis-H, J 8.4 Hz to CH3-3, and 4J 1.2 Hz to 3´-/5´-H, C-1),
3
as a ddt. 13C NMR of 18HH (100.6 MHz, [D6]benzene) δ –5.29
128.09 (dm,30 1J 159.1 Hz, C-4´), 130.72 (dq, 1J 155.1 Hz, 2J
7.1 Hz, C-2), 139.09 (dtdq, 3J 3.6 Hz to cis-H, 3J 7.2 Hz to 3´-
/5´-H, 4J 1.5 Hz to 4´-H, and 4J ca. 0.7 Hz to CH3-3, C-1´),
(
119Sn satellites 1J 342 Hz, SnMe3), 34.03 (2´-/6´-CMe3), 37.55
(quart. 2´-/6´-C), 125.55 (C-3´/-5´), 125.61 (C-4´), 130.76 (C-2),
145.75 (C-1´), 146.51 (119Sn satellites 3J 17.5 Hz, C-2´/-6´),
161.21 (C-1) ppm. Anal. Calcd for C19H32Sn (379.17): C, 60.18;
H, 8.51. Found: C, 60.45; H 8.76.
3
148.88 (dm, J 7.5 Hz to 4´-H, C-2´/-6´) ppm, assigned through
the C,H coupling constants and selective {1H} decoupling as
follows: {t-Bu2} → quart. 2´-/6´-C as the X-part of an AA´X
5
spin system with 3J + J = 4.2 Hz, C-2´/-6´ as a d 3J 7.5 Hz;
4.11. [2-D]-[1-(2´,6´-Di-tert-
butylphenyl)vinyl](trimethyl)stannane (18HD)
{CH3-3} → C-1 as a d 2J 10.1 Hz, C-2 as a d 1J 155.1 Hz, C-1´
1
as a dtd; {cis-H} → CH3-3 as a sharp q J 127.8 Hz, C-1 as a qt
4
This was prepared as above from (E)-13HD as a 34:34
mixture of (E)- and (Z)-18HD. 1H NMR of (E)-18HD (400
3J 8.4 Hz and J ca. 1.2 Hz, and C-1´ as a tdq; {3´-/5´-H} → 2´-
/6´-C as a clean m 2J 3.8 Hz, C-1 as a sharpened dq, C-1´ as a
2
3
MHz, [D6]benzene) δ +0.157 (s, 9H, 119Sn satellites J 51.8 Hz,
dm J 3.6 Hz.
SnMe3), 1.411 (s, 18H, 2´-/6´-CMe3), 5.580 (s, 2∆ = –0.0175
3
ppm, 1H, olefinic H trans to aryl), 7.046 (t, J 8.0 Hz, 1H, 4´-H),
4.10. [1-(2´,6´-Di-tert-butylphenyl)vinyl](trimethyl)stannane
(18HH)
7.397 (d, 3J 8.0 Hz, 2H, 3´-/5´-H) ppm. 1H NMR of (Z)-18HD
(400 MHz, [D6]benzene) δ +0.157 (s, 9H, 119Sn satellites 2J
51.8 Hz, SnMe3), 1.411 (s, 18H, 2´-/6´-CMe3), 5.792 (s, 2∆ = –
0.0187 ppm, 1H, cis-H), 7.046 and 7.397 ppm as above. 13C
NMR of (E)- and (Z)- 18HD (100.6 MHz, [D6]benzene) δ –5.29
The bromoalkene 13HH (1.18 g, 4.00 mmol), anhydrous THF
(16.0 mL), and a magnetic stirring bar were placed in a dry
Schlenk flask (50 mL) under argon gas cover and cooled to –70

7
2
3
(SnMe3), 34.03 (2´-/6´-CMe3), 37.56 (quart. 2´-/6´-C), 125.56 (C-
3´/-5´), 125.62 (C-4´), ca. 130.44 (C-2), ca. 145.74 (C-1´),
146.51 (C-2´/-6´), 161.05 (2∆ = –0.131 and –0.167 ppm, C-1)
ppm.
decet J 3.8 Hz, C-1´ as a dd J 10.8 and 6.6 Hz, and C-2´/-6´
ACCEPTED MANUSCRIPT
as a decet 3J 3.5 Hz. 13C NMR (100.6 MHz, [D6]benzene) δ
32.55 (2´-/6´-CMe3), 36.84 (quart. 2´-/6´-C), 119.28 (C-2),
124.53 (C-3´/-5´), 126.75 (C-4´), 139.74 (C-1´), 141.01 (C-1),
148.97 (C-2´/-6´) ppm.
4.12. [1-(2´,4´-,6´-Tri-tert-
butylphenyl)vinyl](trimethylstannane (18buH)
4.14. [2-D]-2´,6´-Di-tert-butylstyrene (19HD)
The bromoalkene 13buH (354 mg, 1.01 mmol) and LiSnMe3
(2.52 mmol)23 in anhydrous THF (5.0 plus 6.0 mL) were treated
and worked up as described above for 18HH. The crude material
(434 mg) containing 18buH and the parent olefin 19buH (7:3)
was distilled twice (b.p. 105–110 °C bath temp./0.1 mbar) to give
pure 18buH (50 mg, 11%), m.p. 101–103 °C. IR(KBr) ν 2957,
2915, 2868, 1602 (w), 1478, 1421, 1390, 1361, 772 cm–1. 1H
This was obtained together with 18HD as a 30:35 mixture of
(E)- and (Z)-19HD. 1H NMR of (E)-19HD (400 MHz,
2
[D6]benzene) δ 1.389 (s, 2´-/6´-CMe3), 4.881 (d, 3J 18.0 Hz,
∆
3
= –0.0158 ppm, H cis to aryl), 7.110 (t, J 8.0 Hz, 4´-H), 7.331
(d, 3J 8.0 Hz, 3´-/5´-H) ppm. 1H NMR of (Z)-19HD (400 MHz,
2
[D6]benzene) δ 1.389 (s, 2´-/6´-CMe3), 5.161 (d, 3J 11.3 Hz,
∆
= –0.0168 ppm, trans-H), 7.110 and 7.331 ppm as above. 13C
NMR of (E)- and (Z)-19HD (30:35, 100.6 MHz, [D6]benzene) δ
32.55 (2´-/6´-CMe3), 36.84 (quart. 2´-/6´-C), 118.96 and 119.00
2
NMR (400 MHz, CDCl3) δ +0.16 (s, 9H, 119Sn satellites J 52.0
Hz, SnMe3), 1.31 (s, 9H, 4´-CMe3), 1.39 (s, 18H, 2´-/6´-CMe3),
5.713 (d, 2J 2.72 Hz, 1H, 119Sn satellites 3J 74.0 Hz, olefinic H
trans to aryl), 5.827 (d, 2J 2.72 Hz, 1H, 119Sn satellites 3J 158.5
(1∆ = –0.313 and –0.281 ppm, both with JCD 24.1 Hz, C-2),
1
124.53 (C-3´/-5´), 126.75 (C-4´), 139.72 and 139.73 (3∆ = –
0.0199 and –0.0117 ppm, both very broad, C-1´), 140.90 and
140.92 (2∆ = –0.115 and –0.092 ppm, C-1), 148.974 (4∆ = –0.005
ppm, C-2´/-6´) ppm.
5
Hz, cis-H), 7.38 (s, 2H, 119Sn satellites J 3.0 Hz, 3´-/5´-H) ppm.
13C NMR (100.6 MHz, CDCl3) δ –5.33 (q, 1J 128.6 Hz, 119Sn
satellites 1J 339.8 Hz, SnMe3), 31.43 (qm, 1J 125.5 Hz, 3J 4.9
Hz, 119Sn satellites 7J 2.6 Hz, 4´-CMe3), 33.86 (qm, 1J 125.5
4.15. 2´,4´,6´-Tri-tert-butylstyrene (19buH).24,25
3
5
Hz, J 4.8 Hz, 119Sn satellites J 4.7 Hz, 2´-/6´-CMe3), 34.73 (m,
119Sn satellites 6J 3.9 Hz, quart. 4´-C), 37.69 (m, 119Sn satellites
4J 3.2 Hz, quart. 2´-/6´-C), 122.12 (dd, 1J 152.5 Hz, 3J 6.8 Hz,
119Sn satellites 4J 6.7 Hz, C-3´/-5´), 130.80 (sharp t, 1J 156 Hz,
1H NMR (400 MHz, CDCl3) δ 1.33 (s, 9H, 4´-CMe3), 1.41 (s,
2
18H, 2´-/6´-CMe3), 5.053 (dd, 3J 18.0 Hz, J 2.3 Hz, 1H, olefinic
H cis to aryl), 5.441 (dd, 3J 11.4 Hz, 2J 2.3 Hz, 1H, trans-H),
119Sn satellites J 26.3 Hz, C-2), 142.46 (ddt, J 13.2 Hz to trans-
2
3
3
7.195 (dd, J 18.0 and 11.4 Hz, 1H, 1-H), 7.386 (s, 2H, 3´-/5´-H)
H, J 7.6 Hz to cis-H, and 3J 6.6 Hz to 3´-/5´-H, 119Sn satellites
3
1
ppm. 13C NMR (100.6 MHz, CDCl3) δ 31.48 (qm, J 125.5 Hz,
2J 36.2 Hz, C-1´), 145.60 (m, 3J ca. 3.5 Hz, 119Sn satellites 3J
17.6 Hz, C-2´/-6´), 146.10 (m, 3J 3.5 Hz, 119Sn satellites 5J 9.9
Hz, C-4´), 160.91 (dm, 2J (–)5.5 Hz,29 119Sn satellites 1J 473.7
Hz, C-1) ppm, assigned through the C,H coupling constants and
selective {1H} decoupling as follows: {SnMe3} → C-1 as a
3J 4.9 Hz, 4´-CMe3), 32.36 (qm, 1J 125.5 Hz, 3J 4.9 Hz, 2´-/6´-
CMe3), 34.90 (m, 2J 3.9 Hz, quart. 4´-C), 37.09 (m, 2J 3.9 Hz,
quart. 2´-/6´-C), 119.27 (td, 1J 156.6 Hz, 2J (–)1.9 Hz29 to 1-H,
C-2), 121.31 (dd, 1J 152.9 Hz, 3J 6.5 Hz, C-3´/-5´), 136.59
3
3
3
(ddtd, J 11.2 Hz to trans-H, J 6.4 Hz to cis-H, J 6.6 Hz to 3´-
/5´-H, and 2J 2.4 Hz to 1-H, C-1´), 140.75 (dd, 1J 156.0 Hz, 2J
(–)2.8 Hz29 to cis-H, and 2J 0 Hz29 to trans-H, C-1), 147.88 (>
sextet, 3J 2.9 Hz, C-4´), 148.13 (> septet, 3J 3.3 Hz, C-2´/-6´)
ppm.
broadened d 2J 5.5 Hz; {all nine CCH3} → quart. 4´-C as a t J
3
0.9 Hz, C-4´ as a sharp s, and C-2´/-6´ as a broadened s; {3´-/5´-
H} → C-1 remained a d, C-1´ as a dd 3J 13.2 and 7.6 Hz;
{trans- and cis-H} → C-1´ as a t 3J 6.6 Hz. Anal. Calcd for
C23H40Sn (435.28): C, 63.47; H, 9.26. Found: C, 63.72; H, 9.43.
4.13. 2´,6´-Di-tert-butylstyrene (19HH)
Colorless liquid, b.p. 78–83 °C/0.001 mbar. 1H NMR (400
3
MHz, CDCl3) δ 1.40 (s, 18H, 2´-/6´-CMe3), 5.014 (dd, J 18.0
Hz, 2J 2.20 Hz, 1H, olefinic H cis to aryl), 5.451 (dd, 3J 11.4
3
Hz, 2J 2.20 Hz, 1H, trans-H), 7.15 (t, J 8.0 Hz, 1H, 4´-H), 7.23
(dd, 3J 18.0 and 11.4 Hz, 1H, 1-H), 7.34 (d, 3J 8.0 Hz, 2H, 3´-
/5´-H) ppm. 1H NMR (400 MHz, [D6]benzene) δ 1.389 (2´-/6´-
3
2
CMe3), 4.897 (dd, J 18.0 Hz, J 2.30 Hz, 1H, cis-H), 5.177 (dd,
3J 11.3 Hz, 2J 2.30 Hz, 1H, trans-H), 7.110 (t, 3J 8.0 Hz, 1H, 4´-
3
H), 7.12 (superposed dd, 1-H), 7.331 (d, J 8.0 Hz, 2H, 3´-/5´-H)
1
ppm. 13C NMR (100.6 MHz, CDCl3) δ 32.31 (qm, J 125.9 Hz,
3J 4.9 Hz, 2´-/6´-CMe3), 36.80 (> nonet, 2J 3.8 Hz to CH3, quart.
2´-/6´-C), 119.35 (td, 1J 157 Hz, 2J (–)2.2 Hz,29 C-2), 124.21
(dm, 1J ca. 156 Hz, C-3´/-5´), 126.23 (dm, 1J 158 Hz, C-4´),
139.73 (ddtm, 3J 10.8 Hz to trans-H, 3J 6.6 Hz to cis-H, 3J 6.7
Hz to 3´-/5´-H, but 4J unresolved, C-1´), 140.37 (dd, 1J 156.4
Hz, 2J (–)3.2 Hz29 to cis-H, and 2J 0 Hz29 to trans-H, C-1),
3
3
148.96 (dm, J 7.6 Hz to 4´-H, J 3.5 Hz to CH3, C-2´/-6´) ppm,
assigned through comparison with the C,H coupling constants of
styrene and selective {1H} decoupling as follows: {CH3} →
quart. 2´-/6´-C as the X-part of an AA´BX spin system with 3J +
5J = 4.0 Hz, C-2´/-6´ as a dm 3J 7.6 Hz; {cis-H} → C-1 as a
sharp d 1J 156.4 Hz, {trans-H} → C-1 as the unchanged dd;
3
{cis- and trans-H} → C-1´ as a tm J 6.7 Hz; {1-H} → C-2 as a
sharp t 1J 157 Hz; {3´-/5´-H} → quart. 2´-/6´-C as a sharpened
m
2J 3.8 Hz; {1-H and 3´-/4´-/5´-H} → quart. 2´-/6´-C as a

8
Tetrahedron
20. Knorr, R. Chem. Ber. 1980, 113, 2441–2461, eq. 1 and Table 1
ACCEPTED MANUSCRIPT
therein.
21. Wells, W. L.; Brown, T. L. J. Organomet. Chem. 1968, 11, 271–
Acknowledgments
280.
22. Kobayashi, K.; Kawanisi, M.; Hitomi, T.; Kozima, S. J.
Organomet. Chem. 1982, 233, 299–311.
23. Knorr, R.; Pires, C.; Freudenreich, J. J. Org. Chem. 2007, 72,
6084–6090, references 20–22 therein, with our protocol detailed
on p. S5 of that Supporting Information.
This article is dedicated to Professor Christoph Rüchardt on
the occasion of his 85th birthday. Financial support of parts of
this work by the Deutsche Forschungsgemeinschaft is gratefully
acknowledged. We thank the Chemistry Department of the LMU
München for supporting the final stages of this project.
24. Rieker, A.; Butsugan, Y.; Shimizu, M. Tetrahedron Lett. 1971,
12, 1905–1908.
25. Watanabe, S.; Kawashima, T.; Tokitoh, N.; Okazaki, R. Bull.
Chem. Soc. Jpn. 1995, 68, 1437–1448, on p. 1444.
26. Knorr, R.; Ruhdorfer, J.; Böhrer, P.; Bronberger, H.; Räpple, E.
Liebigs Ann. Chem. 1994, 433–438, on p. 434.
References and notes
§
Sterically Congested Molecules, Part 29. Part 28: Knorr, R.;
Menke, T.; Hennig, K.-O.; Freudenreich, J.; Böhrer, P.; Schubert,
B. Tetrahedron 2014, 70, 2703–2710.
27. Knorr, R.; Menke, T.; Behringer, C.; Ferchland, K.; Mehlstäubl, J.;
Lattke, E. Organometallics 2013, 32, 4070–4081.
28. Bauer, W.; Winchester, W. R.; Schleyer, P. v. R. Organometallics
1987, 6, 2371–2379.
1. Rappoport, Z.; Apeloig, Y. J. Am. Chem. Soc. 1974, 96, 6428–
6436, Table III and references quoted therein.
2. Marcuzzi, F.; Melloni, G. J. Am. Chem. Soc. 1976, 98, 3295–
3300, Table III and literature cited therein.
3. Nelb II, R. G.; Stille, J. K. J. Am. Chem. Soc. 1976, 98, 2834–
2839, on p. 2836.
29. For negative and zero values of olefinic 2J(13C,H), see: Vögeli,
U.; Herz, D.; von Philipsborn, W. Org. Magn. Reson. 1980, 13,
200–209, Table 1 therein.
30. Multiplet of the same unusual shape as computed for 13HH.
4. Hanack, M.; Weber, E. Chem. Ber. 1983, 116, 777–797, and
references cited therein.
Supplementary Material
5. Apeloig, Y.; Franke, W.; Rappoport, Z.; Schwarz, H.; Stahl, D. J.
Am. Chem. Soc. 1981, 103, 2770–2780, Table IV therein.
6. Zimmerman, H. E.; Dodd, J. R. J. Am. Chem. Soc. 1970, 92,
6507–6515, on p. 6513.
7. Hartke, K.; Gerber, H.-D.; Roegrath, U. Liebigs Ann. Chem. 1991,
903–916, on p. 908.
Empirical calibration of the E/Z equilibrium constant of 15;
preparation and properties of the alkynes 11 and lithium
acetylides 14; precursors in the 2,4,6-tri-tert-butylphenyl
(“supermesityl”) series.
8. See the Supporting Information.
9. Knorr, R.; Rossmann, E. C.; Knittl, M. Synthesis 2010, 2124–
2128.
10. Knorr, R.; Menke, T.; Ferchland, K.; Mehlstäubl, J.; Stephenson,
D. S. J. Am. Chem. Soc. 2008, 130, 14179–14188.
11. Entries 66–70 of Table S1 in the Supporting Information of Ref.
10.
12. McDaniel, D. H.; Brown, H. C. J. Org. Chem. 1958, 23, 420–
427, Table VII therein.
13. Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165–195,
no. 349 of Table I therein.
Legends
Scheme 1: Mechanistic possibilities related to ionic HBr
addition to the alkyne 7. Reaction modes labeled with “?” were
not detected with the present substituents Ar = 2,6-di- and 2,4,6-
tri-tert-butylphenyl.
14. Hiyama, T.; Wakasa, N.; Ueda, T.; Kusumoto, T. Bull. Chem.
Soc. Jpn. 1990, 63, 640–642, compound 7b therein.
15. Marshall, J. A.; Sehon, C. A.; Frank, S. A.; Roush, W. R. Org.
Synth. 1999, 76, 263; Coll. Vol. 2004, 10, 599.
16. Rosiak, A.; Frey, W.; Christoffers, J. Eur. J. Org. Chem. 2006,
4044–4054.
17. Lo, C.-Y.; Kumar, M. P.; Chang, H.-K.; Lush, S.-F.; Liu, R.-S. J.
Org. Chem. 2005, 70, 10482–19487.
18. Weiss, H. M.; Touchette, K. M.; Andersen, F.; Iskhakov, D. Org.
Biomol. Chem. 2003, 1, 2148–2151.
Scheme 2. HBr syn addition via vinyl ion pairs 12.
Scheme 3. Kinetic and thermodynamic termination of HBr
addition to 16.
Scheme 4. Me3Sn derivatives 18 and olefins 19 from
bromoalkenes 13.
Click here to remove instruction text...
19. Grob, C. A.; Nussbaumer, R. Helv. Chim. Acta 1971, 54, 2528–
2535, Table 3 therein.

ACCEPTED MANUSCRIPT
Supporting Information
for
Highly syn selective addition of aqueous HBr to
hydrophobically shielded arylalkynes
Rudolf Knorr,* Eva C. Rossmann, Monika Knittl, Petra Böhrer
Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstrasse 5–13 (Haus F), 81377 München, Germany
Contents
:
page
S2
S1. Empirical calibration of the E/Z equilibrium constant of 15
S2. Preparation and properties of the alkynes 11 and lithium acetylides 14
[2-D]-1-(2´,6´-Di-tert-butylphenyl)acetylene (11HD)
S2
S2
S2
S3
1-(2´,4´,6´-Tri-tert-butylphenyl)acetylene (mes*–C≡CH, 11buH)
[2-D]-1-(2´,4´,6´-Tri-tert-butylphenyl)acetylene (mes*–C≡CD, 11buD)
1-(2´,6´-Di-tert-butylphenyl)-2-lithioacetylene ([6Li]-14H) with Figure S1 and Table S1 S3
S3. Precursors in the 2,4,6-tri-tert-butylphenyl (“supermesityl”, “mes*”) series
Scheme S1. Synthetic plan of the mes* compounds
S4
S4
2,4,6-Tri-tert-butylphenyllithium (“supermesityllithium”, mes*Li, S2)
2,4,6-Tri-tert-butylbenzoic acid (mes*CO2H, S3)
S5
S5
2,4,6-Tri-tert-butylbenzoyl chloride (mes*COCl, S4)
S6
2,4,6,2´,4´,6´-Hexa-tert-butylbenzil (mes*CO–COmes*, S5)
3-(2´,4´,6´-Tri-tert-butylphenyl)-3-hydroxypropenoic acid (S6´ and keto form S6)
2,4,6-Tri-tert-butylacetophenone (mes*CO–CH3, S8)
S6
S6
S7
NMR parameters of acetophenone (Ph–CO–CH3) to be compared with S8
3-Acetyl-2,4,6-tri-tert-butylcyclohexa-1,4-diene (S9)
S8
S9
Diethyl 1-(2´,4´,6´-tri-tert-butylphenyl)vinyl phosphate (S10)
3-(2´,4´,6´-Tri-tert-butylphenyl) propiolic acid (mes*C≡CCO2H, S11)
S4. References
S9
S10
S11
S1

ACCEPTED MANUSCRIPT
S1. Empirical calibration of the E/Z equilibrium constant of 15
Equilibrium constants K = [E]/[Z] for X=Y double bond systems with substituents R1 – R4
may be calculatedS1 by eq S1 from empirical substituent parameters λd(Ri) and sensitivity
factors ρXY. For olefins (where X = Y = carbon), ρCC = 0.95 could predict %E values in a
rather satisfying accord with the experimental values (43 examples in Table 2 of Ref. S1).
ln K = ρXY [λd(R1) – λd(R2)]×[λd(R3) – λd(R4)]
(S1)
It may be noticed that 2,6-disubstituted phenyl (λd = 0.30 up to 0.55)S1 is often “slimmer” than
methyl (λd = 1 in Table 1 of Ref. S1) as a consequence of an orthogonal orientation with respect
to the C=C double bond. However, the very high parameter λd = 3.9 for 2´,6´-di-tert-
butylphenyl in 1-bromo-1-(2´,6´-di-tert-butylphenyl)-1-propene (15) is ascribed to repulsive
interactions between CH3-3 and the t-Bu methyls in spite of the orthogonal aryl orientation.
S2. Preparation and properties of the alkynes 11 and lithium acetylides 14
[2-D]-1-(2´,6´-Di-tert-butylphenyl)acetylene (11HD). The alkyne 11HH (231 mg, 1.08
mmol)S2 in anhydrous THF (5.0 mL) was stirred and cooled at –70 °C when treated with n-
BuLi (1.40 mmol) in hexane (0.58 mL). After further stirring of the pale violet mixture for three
min, the cooling bath was removed and stirring continued for another three min and during the
subsequent addition of D2O (0.058 mL). The decolorized solution was worked up with dist. water
and Et2O, and the washed and dried (MgSO4) ethereal extracts were concentrated, affording the
1
pure and fully deuteriated alkyne 11HD (185 mg, 80%). H NMR (400 MHz, CDCl3)
δ 1.56 (s,
18H, 2´-/6´-CMe3), 7.20 (t, 1H, 4´-H), 7.32 (d, 2H, 3´-/5´-H) ppm, no signal at 3.80 ppm for
1
3
≡CH. H NMR (400 MHz, THF)
δ 1.55 (s, 18H, 2´-/6´-CMe3), 7.12 (t, J 8.0 Hz, 1H, 4´-H),
3
7.25 (d, J 8.0 Hz, 2H, 3´-/5´-H) ppm. 13C NMR (100.6 MHz, THF)
δ
30.7 (2´-/6´-CMe3), 36.8
2
1
(quart. 2´-/6´-C), 86.8 (t, JCD 7.4 Hz, 2∆ = –0.439 ppm, C-1), 93.9 (t, 1JCD 38.5 Hz, ∆ = –0.261
ppm, C-2), 120.3 (3∆ = 0.0 ppm, C-1´), 124.2 (C-3´/-5´), 128.7 (6∆ = –0.008 ppm, C-4´), 154.9
n
(C-2´/-6´) ppm; these deuterium-induced isotope shifts ∆ = δ(11HD) – δ(11HH) acting across n
chemical bonds resemble those reportedS3 for Ph–C≡C–D.
1-(2´,4´,6´-Tri-tert-butylphenyl)acetylene (mes*–C≡CH, 11buH). A dry Schlenk flask (50
mL) was charged with the enol phosphate S10 (1.61g, 3.79 mmol, see Scheme S1 and the
synthetic procedure in Section S3), anhydrous tert-butyl methyl ether (t-BuOMe, 25 mL), and a
S2

ACCEPTED MANUSCRIPT
magnetic stirring bar, then cooled to –70 °C. With stirring under a stream of dry argon gas, t-
BuLi (9.8 mmol) in pentane (5.78 mL) was introduced within five min by pipetting. The clear,
yellow solution was stirred for 10 min at –70 °C and 10 min without cooling, then poured onto
crushed ice. The warmed-up mixture was diluted with Et2O (20 mL), and the separated aqueous
layer was shaken with Et2O (10 mL). The combined Et2O extracts were washed with aqueous
HCl (2
M, 10 mL) and then with dist. water (3 × 30 mL) until neutral, dried over Na2SO4, and
concentrated to furnish practically pure 11buH (1.03 g, 100%) with m.p. 110–115 °C. The
recrystallized sample had m.p. 118–119.5 °C (from hexane or methanol; Ref. S4: 124–125 °C;
1
Ref. S5: 108 °C). H NMR (400 MHz, CDCl3)
δ
1.32 (s, 9H, 4´-CMe3), 1.57 (s, 18H, 2´-/6´-
30.42
CMe3), 3.76 (s, 1H, ≡CH), 7.33 (s, 2H, 3´-/5´-H) ppm. 13C NMR (100.6 MHz, CDCl3)
δ
(2´-/6´-CMe3), 31.28 (4´-CMe3), 35.24 (quart. 4´-C), 36.59 (quart. 2´-/6´-C), 86.19 (quart. C-1),
91.64 (C-2), 116.36 (C-1´), 120.75 (C-3´/-5´), 150.30 (quart. C-4´), 153.87 (quart. C-2´/-6´) ppm,
assigned as in Ref. S5 through comparison with 11HHS2 and 11HD.
[2-D]-1-(2´,4´,6´-Tri-tert-butylphenyl)acetylene (mes*–C≡CD, 11buD). Obtained quantita-
tively from alkyne 11buH by the preceding procedure for 11HD, and purified (63%) through
1
sublimation at 100–160 °C (bath temp.)/0.001 mbar. H NMR (200 MHz, CDCl3)
δ 1.32 (s, 9H,
4´-CMe3), 1.57 (s, 18H, 2´-/6´-CMe3), 7.33 (s, 2H, 3´-/5´-H) ppm.
1-(2´,6´-Di-tert-butylphenyl)-2-lithioacetylene ([6Li]14H). This was repared in an NMR
tube (5 mm) from alkyne 11HH in THF with n-Bu6Li (0.67 equiv). The aggregation-reporting
NMR chemical shifts
δ of 4´-H, C-2, C-4´, and C-1´ (ipso) in Table S1 are almost independent
of the temperature, which indicates that the same species is dominant between +25 °C and –96
°C. This species is not the monomer: The quintet pattern of 13C,6Li spin-spin coupling
6
observed at –96 °C established that the anionic center C-2 is in contact with n = 2 Li nuclei, as
expected for a monocyclic dimer, trimer, or oligomer. For a dimer with a = 2 contacts of a 6Li
1
nucleus with two 13C nuclei, the observed magnitude of J(13C,6Li) = 8.5 Hz with L = 66S6
–1
indicates a microsolvation number of d = 66 × (n × 1JC,Li
)
– a = 1.9 THF ligands per Li,
namely, a tetrasolvated dimer (lithiation shifts ∆δ in Figure S1).
S3

ACCEPTED MANUSCRIPT
+0.14
-0.15
C(CH3)3
H
1
C
2
-4.9
-0.31
H
C
+67.5
Li &2THF
]
2
-5.3
+40.3
-0.9
-2.9
H
C(CH3)3
+0.1
+0.3
Figure S1. Lithiation NMR shifts ∆
δ
=
δ
(14H) – δ(11HH) of the tetrasolvated dimer
of 14H in THF at 25 °C with reference to 11HH in situ (1H italic, 13C regular).
1
n
Table S1. H and 13C NMR shifts
δ
(ppm)a and coupling constants J of the
tetrasolvated dimer [6Li]14H in THF solution at different temperatures
°C
3´-/5´-H
7.105
7.106
7.108
7.111 br
7.113 br
d
4´-H
CH3
C-2
C-2´/-6´
152.01
151.66
151.44
151.45
151.40
C-1
C-4´
C-3´/-5´ C-1´
2´-/6´-C CH3
+25
–10
–45
–80
–96
J
6.812
1.688
1.681
1.691
1.698
1.693
161.38
xbr
127.13
127.00
127.04
126.64
126.59
123.80
123.61
123.63
124.03
124.03
123.30
123.32
123.36
123.48
123.46
115.01
37.06
37.04
37.05
37.07
37.08
30.80
30.60
30.43
30.30
30.27
6.814
114.49
114.39
114.63
114.59
brd
6.827
xbr
6.859 br
6.866 br
158.87
158.82
t
qi (6Li)
unre-
solved
unre-
solved
dm 156 d 155
d 3J = 7
> sept
q 125
m 4.8
[Hz] 3J = 7.9
3J = 7.9
= 8.5 b
2J = 3.8
a
b
br = broad, brd = broadened, xbr = extremely broad. At –96 °C.
S3. Precursors in the 2,4,6-tri-tert-butylphenyl (“supermesityl”, “mes*”) series
tBu
tBu
tBu
R
tBu
tBu
tBu
tBu
tBu
O
C
tBu
Cl
MeLi
CO2H
C
O
C
O
tBu
tBu
tBu
tBu
tBu
tBu
S1: R = Br
S2: R = Li
S4
S3
S5
LiCuMe2
tBu
in Et2O
tBu
tBu
tBu
tBu
tBu
tBu
tBu
Li
in EtNH2
H
CH3
CH2CO2H
CH2
CH3
C
C
O
C
C
O
CO2
nBuLi
H
O
tBu
tBu
tBu
tBu
OLi
170 °C
_
S8
S9
S6
S7
CO2
nBuLi
Cl-PO(OEt)2
tBu
tBu
S8
CLi
tBu
C
C
R´
tBu
tBu
tBu
tBuLi
CHCO2H
14bu
CH2
tBu
C
O
C
tBu
tBu
OH
14bu: R´= Li
11buH: R´= H
P(OEt)2
O
S11: R´= CO2H 11buD: R´= D
S6´
S10
Scheme S1. Synthetic plan of the mes* compounds.
S4

ACCEPTED MANUSCRIPT
2,4,6-Tri-tert-butylphenyllithium (“supermesityllithium”, mes*Li, S2).
1
General. H NMR studies revealed that “supermesityl bromide” (mes*Br, S1)S7 and n-BuLi
coexist in t-BuOMe over more than 18 h at room temperature, while this solvent is very slowly
deprotonated by n-BuLi with formation of isobutene. Similarly, t-BuLi deprotonates t-BuOMe,
leaving S1 intact.
When prepared from S1 with n-BuLi, mes*Li (S2) is not stable in a THF solution at ambient
temperature. However, its stability is greatly enhanced on dilution of the THF with pentane.
Moreover, a portion of S2 will crystallize from pentane/THF (4:1)S8 and may be washed with dry
petroleum ether if pure mes*Li is required.
In Et2O at room temperature, the formation of mes*Li (S2) from S1 and n-BuLi was
complete within 10 min; therefore, the traditional long reaction periods of this Br/Li interchange
reaction in the solvent Et2O can be considerably shortened, and the presence of a promoter (such
as TMEDA) is unnecessary. This Et2O solution containing S2 and 1-bromobutane was stable for
several hours at ambient temperature and for at least 18 hours under argon gas in a refrigerator.
Although S2 in Et2O was protonated very slowly by N,N-diisopropylamine at room temperature,
proton transfer from cyclohexylamine (pKa = 41.6) to S2 was fast and quantitative.
Procedure. A dry NMR tube (5 mm) was charged with mes*BrS7 (S1, 80 mg, 0.24 mmol),
pentane (0.40 mL), and anhydrous THF (0.10 mL), then cooled to –70 °C during the addition of
n-BuLi (0.37 mmol) in hexanes (0.14 mL) and for further 60 min. The colorless precipitate of S2
was washed with pentane (2 ×) and then dissolved in anhydrous Et2O (0.50 mL), showing δH
=
7.01 ppm for 3-/5-H of S2. The addition of cyclohexylamine (0.027 mL, 0.24 mmol) at –70 °C
converted S2 completely into 1,3,5-tri-tert-butylbenzene (δH = 7.31 ppm), which was isolated
after quenching with solid CO2 and aqueous workup (no acid S3) as a pure, colorless powder (35
mg, 58%). This indicates that pKa > 41.6 for S2.
A similar run with N,N-diisopropylamine (ca. 2 equiv, pKa = 34.4) in place of cyclohexyl-
amine disclosed a very slow proton transfer at room temperature (ca. 30% in 10 min).
2,4,6-Tri-tert-butylbenzoic acid (mes*CO2H, S3). The Br/Li interchange reaction between
mes*Br and n-BuLi in Et2O (see above) was carried out according to the protocol of Staab and
LauerS9 but with a reaction period shortened to less than 1 hour at room temperature. After
pouring onto solid CO2 and aqueous workup, the acid fraction furnished mes*CO2H (S3, crude
yield up to 75%); m.p. 284–286 °C (toluene), Ref. S9: 298–300 °C. 1H NMR (80 MHz,
S5

ACCEPTED MANUSCRIPT
[D6]acetone)
δ
1.31 (s, 9H, 4-CMe3), 1.44 (s, 18H, 2-/6-CMe3), 7.45 (s, 2H, 3-/5-H), 11.56
1
(broad s, 1H, CO2H) ppm. H NMR (80 MHz, CDCl3):
δ 1.33, 1.48, 7.47 ppm.
2,4,6-Tri-tert-butylbenzoyl chloride (mes*COCl, S4). With slight modifications of the
protocol of Staab and Lauer,S9 mes*CO2H (S3, 7.26 g, 25.0 mmol) was dissolved in anhydrous
Et2O (135 mL), then treated with SOCl2 (only 3.63 mL, 50.0 mmol) and anhydrous pyridine (3
drops only), which deposited a colorless precipitate. After less than 15 h at room temperature,
the clear solution was separated from the precipitate by pipette and concentrated in vacuo, which
afforded a colorless solid that was dried in a desiccator over KOH for not more than 2 h: crude
1
yield of S4 7.39 g (96%), m.p. 148–152 °C (Ref. S9: 151–155 °C). H NMR (80 MHz, CCl4):
δ
1.32 (s, 9H, 4-CMe3), 1.48 (s, 18H, 2-/6-CMe3), 7.34 (s, 2H, 3-/5-H) ppm.
1H NMR studies revealed that this reaction can be complete within 10 min at room
temperature, whereas it requires three equivalents of SOCl2 and 25 hours in the absence of
pyridine. With N,N-dimethylformamide as a catalyst in place of pyridine, the formation of S4
took less than one hour.
2,4,6,2´,4´,6´-Hexa-tert-butylbenzil (mes*CO–COmes*, S5). The addition of methyllithium
(1.2 equiv, in place of CH3MgBr)S10 in Et2O to mes*COCl (S4) at –65 °C furnished a non-
acidic crude product fraction that contained the ketone S8 and the α-diketone S5 in a ca. 4:6
ratio. Extraction of S8 by boiling methanol left the yellow diketone S5 with m.p. 199–201 °C
(Ref. S10: 205–207 °C) with the typicalS10 1H NMR resonances broadened by hindered
rotation.S11 1H NMR (400 MHz, CDCl3, 25 °C)
CMe3), 1.46 (broad s, 6-CMe3), 7.18 (broad s, 3-H), 7.45 (broad s, 5-H) ppm. 13C NMR (100.6
MHz, CDCl3, 25 °C) 31.15 (sharp, 4-CMe3), 32.16 (broad, 2-CMe3), 33.20 (broad, 6-CMe3),
δ 0.85 (broad s, 2-CMe3), 1.28 (sharp s, 4-
δ
34.77 (quart. 4-C), 37.28 and 39.42 (2 × broad, quart. 2-/6-C), 121.67 and 124.64 (C-3/-5),
128.31 (quart. C-4), 147.15 and 150.13 (2 × broad, quart. C-2/-6), 150.36 (sharp, quart. C-1),
203.31 (sharp, C=O) ppm.
3-(2´,4´,6´-Tri-tert-butylphenyl)-3-hydroxypropenoic acid (S6´ and keto form S6). The
ketone mes*CO–CH3 (S8, 71 mg, 0.25 mmol) was placed in an NMR tube (5 mm) and
dissolved in pentane (0.50 mL) with a trace of TMS. The tube was cooled at –70 °C under a
stream of dry argon gas during the introduction of n-BuLi (0.29 mmol) in hexane (0.12 mL).
(Lithium N,N-diisopropylamide in THF deprotonated too slowly.) After 10 min at room
1
temperature, a H NMR spectrum indicated the complete consumption of S8 with formation of
S6

ACCEPTED MANUSCRIPT
the enolate S7 (three s, 1:1:2, at δH = 3.94, 4.03, and 7.39 ppm). The solution was poured onto
solid CO2, warmed up, and dissolved in Et2O (10 mL) with aqueous NaOH (1
M, 10 mL). The
Et2O layer was shaken with NaOH (2 , 3 mL), washed until neutral, dried over Na2SO4, and
M
concentrated to give the pure ketone S8 (28 mg, 39%). The combined NaOH layers were
acidified and shaken with Et2O (2 × 10 mL). These Et2O extracts were combined, washed with
dist. water (3 × 8 mL), dried over Na2SO4, and evaporated to furnish the crude enol form S6´ (31
mg, 38%) with m.p. 169–174 °C (effervescence); the completely degassed (CO2) sample had the
new m.p. 108–113 °C (ketone S8). Recrystallized S6´ had m.p. 174–175.5 °C (from hot toluene);
IR (KBr) of S6´
ν
3300–2500 (O–H), 2964, 1629 (broad), 1608, 1589, 1462 (broad), 1257
1
(broad), 1216, 1197, 825 cm–1. H NMR of S6´ (400 MHz, CDCl3)
δ 1.323 (s, 9H, 4´-CMe3),
1.44 (s, 18H, 2´-/6´-CMe3), 5.45 (s, 1H, =CH), 7.47 (s, 2H, 3´-/5´-H), 10.95 (broad s, OH and
CO2H) ppm. 13C NMR of S6´ (100.6 MHz, CDCl3)
31.26 (4´-CMe3), 33.08 (2´-/6´-CMe3),
δ
35.01 (quart. 4´-C), 37.44 (quart. 2´-/6´-C), 97.23 (C-2), 123.19 (C-3´/-5´), 129.53 (C-1´), 145.09
(C-2´/-6´), 150.62 (C-4´), 168.52 (C-3), 174.75 (CO2H) ppm. Some of the assignments of the
mes* part may be interchanged with those of S6. Anal. Calcd for C21H32O3 (332.48): C, 75.86;
H, 9.70. Found: C, 76.11; H, 9.60.
The keto form S6 increased slowly at ambient temperature in CDCl3 solution up to 47% of the
1
equilibrium mixture: H NMR of S6 (400 MHz, CDCl3)
δ 1.320 (s, 9H, 4´-CMe3), 1.39 (s, 18H,
2´-/6´-CMe3), 3.98 (s, 2H, CH2), 7.42 (s, 2H, 3´-/5´-H), 12.31 (CO2H) ppm. 13C NMR of S6
(100.6 MHz, CDCl3) 31.21 (4´-CMe3), 32.91 (2´-/6´-CMe3), 35.05 (quart. 4´-C), 37.81 (quart.
δ
2´-/6´-C), 52.91 (C-2), 122.62 (C-3´/-5´), 133.97 (C-1´), 147.84 (C-2´/-6´), 150.95 (C-4´), 179.95
(CO2H), 209.65 (C-3) ppm. Some of the assignments of the mes* part may be interchanged
with those of S6´.
2,4,6-Tri-tert-butylacetophenone (mes*CO–CH3, S8). In a modification of the ideas of
Neckers et al.,S12 a suspension of CuI (15.95 g, 83.7 mmol) in anhydrous Et2O (160 mL) was
stirred in a Schlenk flask (500 mL, fitted with a pressure-equalizing dropping funnel) at –10 °C
under argon cover gas during the dropwise addition of methyllithium (135 mmol) in Et2O (90.0
mL). The intensely stirred yellow suspension was cooled to –50 °C and became greenish-yellow
on the dropwise addition of a solution of mes*COCl (S4, 7.39 g, 23.9 mmol) in anhydrous Et2O
(50 mL) within 30 min. After stirring for 2.5 hours at room temperature, the mixture was cooled
in an ice-bath for extermination of unconsumed CuCH3 through the slow introduction of 1,2-
diaminoethane (16.8 mL, 251.2 mmol) under argon. (Caution! Vivid evolution of CH4 gas!)
Upon further stirring at ambient temperature overnight, the mixture was diluted with dist. water
S7

ACCEPTED MANUSCRIPT
(40 mL), and the layers were separated notwithstanding the bronze-colored precipitate. The deep-
blue aqueous layer was shaken with Et2O (50 mL), and the combined Et2O extracts were washed
with dist. water (2 ×), aqueous HCl (2
M, 30 mL), then water until neutral, dried over Na2SO4,
filtered, and concentrated to yield a yellowish solid (5.70 g) containing S8 and the diketone S5
(8:2). Hot methanol extracted S8 from the mixture, leaving the insoluble yellow diketone S5
(674 mg, 1.23 mmol, 10 %). The methanol filtrate was evaporated, and the remaining, almost
colorless needles were dried in a desiccator, affording practically pure ketone S8 (5.00 g, 17.3
1
mmol, 72%) with m.p. 108–115 °C (Ref. S10: 121–123 °C; Ref. S12: 120.5–122 °C). H NMR
(400 MHz, CDCl3)
δ 1.32 (s, 9H, 4-CMe3), 1.38 (s, 18H, 2-/6-CMe3), 2.64 (s, 3H, acetyl), 7.39
1
3
(s, 2H, 3-/5-H) ppm. 13C NMR (100.6 MHz, CDCl3)
δ
31.30 (qm, J 125.5 Hz, J 4.8 Hz, 4-
1
3
2
CMe3), 32.92 (qm, J 125.8 Hz, J 4.9 Hz, 2-/6-CMe3), 34.88 (m, J 3.7 Hz, quart. 4-C), 37.19
2
1
1
(m, J 3.7 Hz, quart. 2-/6-C), 38.19 (sharp q, J 127.5 Hz, CO–CH3), 122.70 (dd, J 153.5 Hz,
3
3
3J 6.6 Hz, C-3/-5), 137.55 (t, J 6.8 Hz but q 3J not resolved, C-1), 144.60 (> octet, J 3.6 Hz but
4
3
2J and J not resolved, C-2/-6), 149.34 (> octet, J 3.6 Hz, C-4), 211.57 (q, 2J 5.7 Hz, C=O)
ppm, assigned through comparison with Ph–CO–CH3 (see below) and selective {3-/5-H}
decoupling which sharpened 4-C and 2-/6-C.
Ketone S8 could not be reduced to the alcohol (wanted for a planned dehydration) with LiAlH4
(no conversion) or with elemental Li0 in either EtNH2 (only S9 obtained) or THF plus 4,4´-di-
tert-butylbiphenyl.
The acetyl group can be cleaved from ketone S8 by the slow action of conc. HBr in the
biphasic system with CDCl3 at 60 °C, affording 1,3-di-tert-butylbenzene and acetic acid within
two days. Therefore, the direct conversionS13 of S8 into the vinyl bromide 13buH with
formation of HBr was unsuccessful and led to deacylation of S8. Instead, we had to take the
longer route via the enol phosphate S10 of S8 and thence alkyne 11buH.
NMR Parameters of acetophenone (Ph–CO–CH3), to be compared with S8. For
confirmation of the signs and magnitudes of the “aromatic” JHH and JCH coupling constants,
widely expanded 1H and 13C spectra of Ph–CO–CH3 in CDCl3 were measured at room
temperature and simulated by the computer program LAOCOON with the following results.
1H NMR (simulated but not optimized, with JHH values averaged over available literature
3
3
data)
7.40 Hz, J(o,o´) 1.90 Hz, J(m,m´) = J(o,p) = 1.30 Hz, J(o,m´) = J(m,o´) = 0.60 Hz. 13C
NMR (JHH fixed as above, only δC and JCH simulated and optimized): 26.55 (sharp q, CH3),
δ 7.44 (tm, m-/m´-H), 7.54 (tt, p-H), 7.94 (dm, o-/o´-H) ppm; J(o,m) 7.90 Hz, J(m,p)
4
4
4
5
5
δ
128.28 (dtm, C-o/-o´), 128.55 (ddm, C-m/-m´), 133.06 (dtt, C-p), 137.14 (tqd, C-ipso), 198.01
S8

ACCEPTED MANUSCRIPT
(qtt, C=O) ppm, assigned through selective {1H} decoupling experiments; 1J(CH3) 127.5 Hz,
1J(C-o) 160.66(5) Hz, 1J(C-m) 161.30(5) Hz, 1J(C-p) 160.32(4) Hz, 2J(C-o,m-H) 1.15(5) Hz,
2J(C-m,o-H) 1.00(7) Hz, 2J(C-m,p-H) –0.03(5) Hz, 2J(C-p,m-H) 1.18(3) Hz, 2J(C-ipso,o-H)
0.00(0) Hz, 2J(C=O,CH3) –5.883(3) Hz, 3J(C-o,p-H) 7.48(5) Hz, 3J(C-o,o´-H) 6.49(5) Hz,
3
3
3
3J(C-m,m´-H) 7.77(5) Hz, J(C-p,o-H) 7.69(3) Hz, J(C-ipso,m-H) 7.276(4) Hz, J(C-ipso,CH3)
3
4
4
1.097(3) Hz, J(C=O,o-H) 3.852(4) Hz, J(C-o,m´-H) –1.49(5) Hz, J(C-m,o´-H) –0.38(7) Hz,
4
5
4J(C-ipso,p-H) –1.23(1) Hz, J(C=O,m-H) 0.434(3) Hz, J(C=O,p-H) –0.007(5) Hz.
3-Acetyl-2,4,6-tri-tert-butylcyclohexa-1,4-diene (S9). EtNH2 (3.0 mL) was condensed into
a dry Schlenk flask (25 mL) and cooled to –60 °C under argon cover gas. Mes*CO–CH3 (S8,
290 mg, 1.00 mmol) was added and dissolved readily. The clear solution was treated with a
ribbon of elemental lithium (35 mg, 5.04 mmol) which turned black and dissolved quickly on
stirring at –45 °C. The dark blue solution was warmed up to –6 °C, whereupon all EtNH2 was
removed in a current of argon. The pale violet residue was treated successively with pentane (15
mL), methanol (2 mL), and water (5 mL). After extraction of the aqueous layer with Et2O (5
mL), the combined organic layers were washed with dist. water (3 × 5 mL), dried over Na2SO4,
and evaporated to dryness, leaving almost pure S9 as a slightly yellowish liquid (211 mg, 73%).
1H NMR (400 MHz, CDCl3)
δ 0.99 (s, 9H, 6-CMe3), 1.09 (s, 18H, 2-/4-CMe3), 1.99 (s, 3H,
5
3
5
3
acetyl), 2.61 (dt, 1H, J 3.7 Hz, J 1.8 Hz, 6-H), 4.24 (d, 1H, J 3.7 Hz, 3-H), 5.89 (d, 2H, J
1.8 Hz, 1-/5-H) ppm, assigned through the JHH values. 13C NMR (100.6 MHz, CDCl3)
δ 24.5
1
3
1
3
(qd, J 127.5 Hz, J 2 Hz, CO–CH3), 27.7 (q, J 124.3 Hz, > sextet J ca. 4.6 Hz, 6-CMe3), 29.9
(q, 1J 124.8 Hz, > septet 3J ca. 4.6 Hz, 2-/4-CMe3), 34.0 (m, 2J ca. 3 Hz, quart. 6-C), 37.0
1
3
1
3
(unresolved, quart. 2-/4-C), 47.8 (d, J 126 Hz, > octet J 3.6 Hz, C-6), 56.0 (dtd, J 132 Hz, J
4
1
3
6.8 Hz, J ca. 2.8 Hz, C-3), 121.8 (ddt, J 154 Hz, J 7.2 Hz, apparent J ca. 5.4 Hz to 6-H and to
3
5- or 1-H, C-1/-5), 145.4 (dm, J 6 Hz, apparent J ca. 3.5 Hz, C-2/-4) ppm.
Diethyl 1-(2´,4´,6´-tri-tert-butylphenyl)vinyl phosphate (S10). In view of the impossibility
of converting ketone S8 directly into the bromoalkene 13buH, we turned to the enol phosphate
method of Negishi et al.S14 However, we had to use n-BuLi (in pentane or better in THF) as the
base in place of lithium N,N-diisopropylamideS14 which reacts too slowly with S8, as mentioned
in the preparation of the acid S6´.
A Schlenk flask (100 mL) was charged with mes*CO–CH3 (S8, 2.50 g, 8.67 mmol),
anhydrous THF (20 mL), and a magnetic stirring bar, then cooled to –70 °C under argon cover
gas and stirred during the addition of n-BuLi (16.4 mmol) in hexane (6.24 mL) from a pressure-
S9

ACCEPTED MANUSCRIPT
equalizing dropping funnel within 10 min. The red solution was stirred for another 15 min,
whereafter Cl-PO(OEt)2 (2.49 mL, 17.2 mmol) was slowly added from a pipette. The supply of
argon gas was stopped and the flask left overnight in the slowly warming bath. The mixture was
poured into aqueous NaOH (2
M, 30 mL) and the flask was rinsed out with dist. water. The
separated aqueous layer was shaken with Et2O (20 mL), and the combined organic layers were
washed with dist. water (3 × 50 mL) until neutral, dried over Na2SO4, and concentrated. The
solidifying crude material (3.20 g, 87%) may be used as such; the analytically pure sample had
m.p. 72.5–74 °C (from pentane at low temperature). IR (KBr)
ν
2962, 1641, 1364, 1269, 1254,
1
1056, 1026, 990, and 833 cm–1. H NMR (400 MHz, CDCl3)
δ
1.32 (s, 9H, 4´-CMe3), 1.35 (td,
4
3JHH 7.1 Hz, JPH 1.2 Hz, 2 × ethyl-CH3), 1.50 (s, 18H, 2´-/6´-CMe3), ca. 4.19 and 4.22 (m, 4H, 2
3
2
4
× diastereotopic OCH including JPH), 4.81 (dd, 1H, JHH 2.6 Hz, JPH 1.7 Hz, olefinic H cis to
2
aryl), 5.40 (d, 1H, JHH 2.6 Hz, olefinic trans-H), 7.46 (s, 2H, 3´-/5´-H) ppm. 13C NMR (100.6
1
2
3
1
MHz, CDCl3)
δ 16.10 (qtd, J 127.0 Hz, J 2.6 Hz, JPC 7.2 Hz, 2 × POCCH3), 31.33 (q, J
3
1
3
125.5 Hz, > septet J 4.9 Hz, 4´-CMe3), 33.18 (qm, J 126.0 Hz, J 4.9 Hz, 2´-/6´-CMe3), 35.03
1
(not resolved, quart. 4´-C), 37.77 (> septet, apparent J ca. 3.8 Hz, quart. 2´-/6´-C), 64.19 (tqd, J
3
2
1
3
148 Hz, J 4.5 Hz, JPC (–)6 Hz, 2 × OCH2), 104.04 (ddd, apparent J 160 and 162 Hz, JPC 2.1
1
3
3
Hz, olefinic =CH2), 122.45 (dd, J 153.8 Hz, J 6.7 Hz, C-3´/-5´), 130.64 (ddtd, J 7.4 Hz to
3
3
3
3
trans-H, J 2.8 Hz to cis-H, J 7.0 Hz to 3´-/5´-H, JPC 10.7 Hz, C-1´), 149.60 (> octet, J 3.4
3
2
2
Hz, C-2´/-6´), 150.03 (> octet, J 3.7 Hz, C-4´), 152.32 (ddtd, J 6.1 Hz to trans-H, J 5.7 Hz to
4
3
cis-H, J ca. 1 Hz to 3´-/5´-H, JPC 8.8 Hz, C-1) ppm, assigned through comparison with H3C–
PO(OEt)2 and selective {1H} decoupling as follows: {2´-/6´-C(CH3)3} → C-2´/-6´ as a sharp s,
quart. 2´-/6´-C as the X-part of an AA´X spin system with 3J + 5J = 4.7 Hz; {cis-H} → C-1´
2
2
2
as a dtd, C-1 as a dd J 6.1 Hz and JPC 8.8 Hz; {trans-H} → C-1´ as a dtd, C-1 as a dd J 5.7
2
Hz and JPC 8.8 Hz; {3´-/5´-H} → C-1´ as a ddd, C-1 as a ddd. Anal. Calcd for C24H41O4P
(424.56): C, 67.90; H 9.73. Found: C, 67.88; H, 9.57.
3-(2´,4´,6´-Tri-tert-butylphenyl)propiolic acid (mes*C≡CCO2H, S11). Alkyne (11buH)
formation by elimination of diethyl phosphate from our enol phosphate S10 with the originallyS14
employed base lithium N,N-diisopropylamide in THF proceeded slowly at room temperature and
not at all at –70 °C. The stronger base n-BuLi deprotonated S10 also slowly in pentane solution
yet fast in THF at or below room temperature; however, fissions of the C–O (elimination) and P–
O bonds (enolate formation) occurred with similar speed in THF. The desired elimination was
fast and strongly preferred with t-BuLi (> 2 equiv) as the base in both pentane and t-BuOMe at
ambient temperature or at –70 °C. Residual t-BuLi was destroyed by t-BuOMe with formation
S10

ACCEPTED MANUSCRIPT
of isobutene even at –70 °C, as shown by the absence of pivalic acid (t-BuCO2H) after
carboxylation.
Procedure: The enol phosphate S10 (380 mg, 0.895 mmol), anhydrous t-BuOMe (2.70 mL),
and a magnetic stirring bar were placed in a Schlenk flask (10 mL) and cooled to –70 °C. With
stirring under a stream of dry argon gas, t-BuLi (ca. 2.6 mmol) in pentane (2.68 mL) was added
by pipette, forming a red and turbid mixture which turned pale and clear within 15 min. After 60
min at –70 °C, the solution was poured onto crushed solid CO2, warmed up, and dissolved in
Et2O (25 mL) and aqueous NaOH (2 M, 8 mL). The organic layer was shaken with NaOH (2 M, 4
× 10 mL), washed with dist. water until neutral, dried over Na2SO4, and evaporated to leave the
non-acidic fraction (22 mg) containing the alkyne 11buH and a trace of ketone S8. The combined
NaOH layers were acidified with conc. HCl, shaken with Et2O (3 × 15 mL), and discarded. The
latter Et2O extracts were combined, washed with dist. water (2 × 30 mL), dried over Na2SO4, and
concentrated to afford crude mes*–C≡C–CO2H (S11, 309 mg) which was recrystallized from
toluene (4.3 mL) in a flask with reflux condensor. The colorless needles of pure S11 (180 mg,
64%) began to sublime at 235 °C in a sealed capillary and had m.p. >290 °C (Ref. S4: m.p. 200
1
°C with decomposition). IR (KBr)
ν
2201 cm–1. H NMR (400 MHz, CDCl3)
δ
1.33 (s, 9H,
4´-CMe3), 1.57 (s, 18H, 2´-/6´-CMe3), 7.37 (s, 2H, 3´-/5´-H), 8.68 (broad s, 1H, CO2H) ppm. 13C
NMR (100.6 MHz, CDCl3) 30.70 (2´-/6´-CMe3), 31.14 (4´-CMe3), 35.55 (quart. 4´-C), 36.74
δ
(quart. 2´-/6´-C), 91.98 and 93.41 (C≡C), 113.0 (C-1´), 121.09 (C-3´/-5´), 153.40 (C-4´), 156.60
(C-2´/-6´), 158.90 (CO2H) ppm.
S4. References
S1. Knorr, R. Chem. Ber. 1980, 113, 2441–2461, eq. 1 and Table 1 therein.
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