Lewis Acid Catalyzed Transfer Hydromethallylation for the Construction of Quaternary Carbon Centers

Article abstract of DOI:10.1002/anie.201909852

The design and gram-scale synthesis of a cyclohexa-1,4-diene-based surrogate of isobutene gas is reported. Using the highly electron-deficient Lewis acid B(C₆F₅)₃, application of this surrogate in the hydromethallylation of electron-rich styrene derivatives provided sterically congested quaternary carbon centers. The reaction proceeds by C(sp³)?C(sp³) bond formation at a tertiary carbenium ion that is generated by alkene protonation. The possibility of two concurrent mechanisms is proposed on the basis of mechanistic experiments using a deuterated surrogate.

Full text of DOI:10.1002/anie.201909852

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Accepted Article
Title: Lewis Acid-Catalyzed Transfer Hydromethallylation for the
Construction of Quaternary Carbon Centers
Authors: Johannes C. L. Walker and Martin Oestreich
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201909852
Angew. Chem. 10.1002/ange.201909852
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10.1002/anie.201909852
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Lewis Acid-Catalyzed Transfer Hydromethallylation for the
Construction of Quaternary Carbon Centers
Johannes C. L. Walker and Martin Oestreich*
Abstract: The design and gram-scale synthesis of a cyclohexa-1,4-
diene-based surrogate of isobutene gas is reported. Using the highly
electron-deficient Lewis acid B(C6F5)3, its application in the
hydromethallylation of electon-rich styrene derivatives to provide
sterically congested quaternary carbon centers was achieved. The
reaction proceeds by C(sp3)–C(sp3) bond formation at a tertiary
carbenium ion that is generated by alkene protonation. The
possibility of two concurrent mechanisms is proposed on the basis of
mechanistic experiments using a deuterated surrogate.
The catalytic preparation of quaternary carbon centers using
C(sp3)–C(sp3) bond-forming reactions has been identified as a
key challenge in organic synthesis.[1] Recent progress in this
area has mainly centered on transition-metal-catalyzed
transformations.[2–4] Metal-free approaches are less common.[5,6]
One strategy to achieve C(sp3)–C(sp3) bond formation is the
attack of a carbon nucleophile onto a tertiary carbenium ion.[7,8]
Although this has been exploited in the context of carbenium-ion
generation by dehydration of tertiary alcohols with catalytic
amounts of strong acid,[5,6,9] the complementary process by
catalytic protonation of alkenes is far less well explored.[10,11]
Our laboratory has been investigating cyclohexa-1,4-diene-
based surrogates of difficult-to-handle compounds for metal-free
transfer reactions.[12–15] As part of this programme we previously
developed cyclohexa-1,4-dienes 5 and 6 as surrogates of
isobutane gas and reported their use in the transfer hydro-tert-
butylation of alkenes using the strong boron Lewis acid B(C6F5)3
(Scheme 1, top).[14,16] Hydride abstraction from the bisallylic
position of the surrogates led to formation of the tert-butyl-
substituted Wheland intermediates 7+ and transfer of the
electrofugal tert-butyl group to the terminus of 1,1-diaryl-
Scheme 1. Cyclohexa-1,4-diene-based surrogates of isobutane and isobutene
gas for metal-free transfer hydroalkylation and -allylation, respectively.
substituted alkenes. Borohydride addition to the resulting
benzylic tertiary carbenium ion delivered formally anti-
Markovnikov alkylation products such as 2. However, this
process was hampered by side reactions to give 3 and 4, and
the substrate scope was quite limited.
We wondered whether an appropriately substituted
cyclohexa-1,4-diene would facilitate abstraction of a nucleofugal
hydrocarbon group and then lead to the complementary
Markovnikov hydroalkylation.[17] In seeking
a
plausible
hydrocarbon unit, we were interested in a report by Ménard und
Stephan who had achieved the stoichiometric C–H activation of
isobutene with the frustrated Lewis pair tBu3P/B(C6F5)3 (Scheme
1, middle).[18] The methallyl borate 8– formed represents a
potential nucleophilic source of the methallyl group,[19,20] and we
hoped that a cyclohexa-1,4-diene surrogate of isobutene gas
could be developed to allow for the abstraction of the methallyl
nucleofuge by B(C6F5)3 to give 8– and
a Brønsted-acidic
[*]
Dr. J. C. L. Walker, Prof. Dr. M. Oestreich
Institut für Chemie, Technische Universität Berlin
Strasse des 17. Juni 115, 10623 Berlin (Germany)
E-mail: martin.oestreich@tu-berlin.de
Wheland intermediate. Interception of this complex by an alkene
might then allow for transfer hydromethallylation and the
formation of a quaternary carbon center by way of C(sp3)–C(sp3)
bond formation (Scheme 1, bottom).[21–23]
Homepage: http://www.organometallics.tu-berlin.de
Supporting information for this article is given via a link at the end of
the document.
The required surrogate would likely necessitate a quaternary
center adjacent to the methallyl group to prevent undesired side
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10.1002/anie.201909852
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COMMUNICATION
reactions such as transfer hydrogenation,[24] as well as to aid
stabilization of the positively charged Wheland intermediate.
Surrogates 9 and 10 fulfill this requirement and were easily
accessible by Birch alkylation from benzoic acid and biphenyl,
respectively (see the Supporting Information for experimental
details). We then subjected the surrogates to a model reaction
with 1,1-diphenylethene (1a) and 10 mol% B(C6F5)3 in CH2Cl2
(Table 1, entry 1). Benzyl ether-based surrogate 9 did not give
any of the desired hydromethallylation product 11a, instead
providing indane 12a in 6% yield; 12a presumably results from
the intramolecular Friedel–Crafts alkylation of 11a.[25] Biphenyl-
based surrogate 10 was more reactive, forming the intended
product 11a in 3% and indane 12a in 40% yield (entry 2).
Switching the solvent to toluene improved the selectivity for 11a
(entry 3). We eventually established that electron-rich styrene
derivatives allowed the reaction to proceed to full conversion,
and we could suppress cyclization to indanes 12 by modifying
the steric environment of the alkene. Using para-anisyl-
substituted alkene 1b, we were initially able to form 11b in 82%
yield with no indane 12b observed (entry 4). The loading of both
surrogate 10 and B(C6F5)3 could be reduced with no detrimental
impact on the yield, and 11b was then isolated in 82% yield
(entry 5). The reaction could also be scaled up to 1.00 mmol
(entry 6). No reaction occurred in the absence of the catalyst
(entry 7), but interestingly, B(C6F5)3 was able to partially
decompose surrogate 10 to biphenyl in the absence of the
alkene starting material (entry 8).
8[f]
10
5.0
PhMe
<1[f]
<1
1b
[a] Unless otherwise noted, reactions were performed on a 0.10 mmol scale
with 1.3 equiv 9 or 10 in 0.25 mL (0.4 M) of the indicated solvent. [b]
Determined by 1H NMR spectroscopy by the addition of 1,2-dibromomethane
as an internal standard. [c] 1.5 equiv surrogate 10 used. [d] Isolated yield. [e]
On a 1.0 mmol scale. [f] Alkene 1b was not added. 22% conversion of
surrogate 10 to biphenyl.
With optimized conditions in hand, we proceeded to
investigate the substrate scope (Scheme 3). A range of electron-
rich α-substituted styrenes could be used with benzyl ethers as
in 11c and cyclic ethers as in 11d both tolerated. The indane-
based alkene 11e could be prepared in 92% yield and, using this
framework, an exocyclic trisubstituted alkene could also be
reacted to give 11f in 92% yield. As previously observed in other
transfer reactions, tetrasubstituted alkenes were unreactive.[12,13]
Products bearing two contiguous quaternary carbon centers
could be accessed, with 11g formed in 94% yield. Here, slightly
higher loadings of B(C6F5)3 and surrogate 10 were required to
push the reaction to completion. These conditions found further
application with other less reactive substrates. Increased
substitution at the α-position of the styrene was also tolerated,
as in 11h–j, although in these cases cyclization to indanes 12h–j
could not be completely suppressed; the proportion of indanes
12h–j increased with the size of the α-substituent. Performing
the reaction with two equivalents of surrogate 10 did help to
reduce the extent of cyclization; 11h and 12h were however
formed in a 3:1 mixture with 1.3 equiv surrogate 10. A substrate
bearing the bulky tert-butyl group in the α-position was
unreactive. Cyclic alkanes containing quaternary carbon centers
could also be synthesized using this methodology with
cyclohexane 11k and cycloheptane 11l being formed in 89%
and 95% yield, respectively. Finally, a series of compounds
bearing pendant aryl groups was prepared. Phenyl-substituted
11m was formed in 85% yield, and products with halogen
substituents as in 11n and 11o as well as an electron-donating
methoxy substituent as in 11p could also be accessed. To
illustrate the utility of these products, a selection of these
compounds was further derivatized (see the Supporting
Information for details).
Table 1. Optimization of the B(C6F5)3-catalyzed transfer hydromethallylation.[a]
B(C6F5)3
(x mol%)
9 or 10
(1.3 equiv)
H
H
R2
R2
Me
R2
solvent
RT for 18 h
R1
R1
R1
Me
Me
12a,b
1a: R1 = H, R2 = Ph
11a,b
1b: R1 = OMe, R2 = Me
H
H
H
H
H
or
Me
MeO
Ph
Me
Me
10
9
B(C6F5)3
[mol%]
Yield of
11 [%][b]
Yield of
12 [%][b]
Entry
Alkene
Surrogate
Solvent
1
2
3
4
1a
1a
1a
1b
9
10
10
10
10
CH2Cl2
CH2Cl2
PhMe
PhMe
<1
3
6
10
40
11
<1
10
32
82
10[c]
85
(82)[d]
1b
5
10
5.0
PhMe
<1
6[e]
7
10
10
5.0
—
PhMe
PhMe
74[d]
<1
<1
<1
1b
1b
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10.1002/anie.201909852
Angewandte Chemie International Edition
COMMUNICATION
alkene α-11b-d2 with formation of
a
C(sp3)–C(sp3) bond.
H
R3
R3
H
Me
R2
R3
Alternatively, direct transfer of the methallyl fragment from α-10-
d2 to carbenium ion 13+ would result in the formation of the
regioisomer γ-11b-d2 with deuteration in the α-position (Pathway
2). In this scenario, B(C6F5)3 operates as an initiator and
methallyl borate [γ-8-d2]– as a spectator counteranion. The dual
role of B(C6F5)3 as catalyst and initiator in transfer chemistry has
previously been discussed on the basis of computational
calculations.[26] Another pathway to arrive at γ-11b-d2 is the
transfer of the methallyl group between two boron centers (see
Pathway 3 in the Supporting Information). This would lead to γ-
deuterated α-8-d2, and attack of the methallyl group onto
carbenium ion 13+ then provides γ-11b-d2. It has not yet been
possible to distinguish between the two possible pathways
leading to γ-11b-d2.
Method A–C
R2
R2
PhMe (0.4 M)
RT for 18 h
R1
R1
R1
Me
Me
12b–p
1b–p
11b–p
H
Me
H
Me
H
Me
Me
Me
Me
MeO
BnO
O
11b: 82%
Method A
11c: 62%
Method A
11d: 27%
Method A
Me
Me
H
H
H
Me
Me
Me
Me
H
H
MeO
MeO
MeO
11e: 92%
Method A
11f: 92%
Method A
11g: 94%
Method C
H
2H
B(C6F5)3
(5 mol%)
2H
H
H
2H
2H
H
Me
Me
Me
Me
H
H
H
H
H
Ar
Me
Ph
Me
PhMe
RT, 18 h
72%
Me
Me
Ar
Me
Ar
Me
2H
Me
2H
-11b-d2
-11b-d2
65 : 28 : 7
-11b-d2
Me
MeO
MeO
MeO
1b
Ar = 4-anisyl
-10-d2
/ -11b-d2 / 11b
(93% 2H)
11h:12h (93:7): 79%
11i:12i (3:1): 71%
11j:12j (2.2:1): 87%
Method B
Method B
Method B
Me
2H
H
Me
2H
2H
H
H
H
H
Me
2H
Me
Me
-11b-d2
Pathway 1
Me
Me
MeO
MeO
B(C6F5)3
MeO
MeO
MeO
-11b-d2
-10-d2
11k: 89%
Method A
11l: 95%
Method C
11m: 85%
Method A
-10-d2
Br
F
MeO
H
H
H
Pathway 2
Me
Me
Me
H
H
H
Me
MeO
MeO
MeO
11n: 92%
11o: 93%
11p: 85%
Method C
Method C
Method C
MeO
Me
Ph
Scheme 2. Scope of the B(C6F5)3 catalyzed transfer hydromethallylation of
alkenes. Method A: B(C6F5)3 (5.0 mol%), surrogate 10 (1.3 equiv); Method B:
B(C6F5)3 (5.0 mol%), surrogate 10 (2.0 equiv); Method C: B(C6F5)3 (7.5 mol%),
surrogate 10 (2.0 equiv).
Me
B(C6F5)3
B(C6F5)3
2H 2
13+[ -8-d2]
H
2H 2
[HꞏC12H10]+[ -8-d2]
H
To gain insight into the reaction mechanism, we prepared
deuterated surrogate α-10-d2 (Scheme 3; see the Supporting
Information for experimental details). Despite this surrogate
being deuterated solely at the allyl terminus (γ-position), its
reaction with alkene 1b produced 11b-d2 with deuterium
incorporation in both the α- and γ-positions. This suggests the
presence of at least two concurrent mechanisms (see the
Supporting Information for more elaborate catalytic cycles). First,
based on the work of Ménard and Stephan,[18] we propose that
alkene attack from surrogate α-10-d2 to B(C6F5)3 results in
abstraction of the methallyl group and formation of α-deuterated
methallyl borate complex [γ-8-d2]– along with protonated
biphenyl [HꞏC12H10]+ (Scheme 3, Pathway 1). Protonation of
alkene 1b by the Brønsted-acidic Wheland complex yields the
tertiary carbenium ion 13+ with concomitant aromatization of the
surrogate core to biphenyl. Transfer of the methallyl group from
methallyl borate [γ-8-d2]– to 13+ then provides γ-deuterated
biphenyl
1b
Scheme 3. Mechanistic experiment with deuterated surrogate and proposed
mechanism.
To summarize, a cyclohexa-1,4-diene-based surrogate of
isobutene gas has been developed and utilized in the transfer
hydromethallylation of electron-rich styrene derivatives. The
method enables the catalytic formation of sterically congested
quaternary carbon atoms and represents a rare example of the
formation of C(sp3)–C(sp3) bonds from carbenium ions that have
been formed by the protonation of an alkene. A range of
different scaffolds could be incorporated, and the utility of the
products was demonstrated by their derivatization.
Acknowledgements
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10.1002/anie.201909852
Angewandte Chemie International Edition
COMMUNICATION
5203–5208; b) G. Li, H. Liu, Y. Wang, S. Zhang, S. Lai, L. Tang, J.
Zhao, Z. Tang, Chem. Comm. 2016, 52, 2304–2306; c) C. D.-T.
Nielsen, A. J. P. White, D. Sale, J. Bures, A. C. Spivey, ChemRxiv
preprint, 2019, DOI 10.26434/chemrxiv.8052623.v1.
J.C.L.W. gratefully acknowledges the Alexander von Humboldt
Foundation for a Theodor Heuss fellowship (2018–2019). M.O.
is indebted to the Einstein Foundation Berlin for an endowed
professorship.
[11] For an example of catalytic protonation of alkenes for C(sp3)–C(sp3)
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K. Laxminarayana, K. Damodar, D. N. Kumar, Chem. Lett. 2008, 38,
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Conflict of interest
[12] For overviews of early work, see: a) S. Keess, M. Oestreich, Chem. Sci.
2017, 8, 4688–4695; b) M. Oestreich, Angew. Chem. Int. Ed. 2016, 55,
494–499; Angew. Chem. 2016, 128, 504‒509.
The authors declare no conflict of interest.
[13] For key publications, see: a) A. Simonneau, M. Oestreich, Angew.
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Soc. 2019, 141, 1135–1140.
Keywords: boron
• carbocations • carbon–carbon bond
formation • transfer processes • Lewis acids
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[25] The cyclization of systems similar to 15a to indanes under both
Brønsted- and Lewis-acid catalysis has previously been reported. For
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2004, 6, 2341–2343; b) C. Peppe, E. S. Lang, F. M. de Andrade, L. B.
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[10] For examples of catalytic protonation of alkenes for C(sp2)–C(sp3) bond
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Osiak, R. M. Vaden, B. E. Welm, M. S. Sigman, Tetrahedron 2012, 68,
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Tremblay, Tetrahedron Lett. 2012, 53, 4600–4603; d) B. V. Ramulu, L.
Mahendar, G. Satyanarayana, Asian J. Org. Chem. 2016, 5, 207–212.
[26] S. Banerjee, K. Vanka, ACS Catal. 2018, 8, 6163–6176.
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10.1002/anie.201909852
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Suggestion for the Entry for the Table of Contents
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J. C. L. Walker, M. Oestreich*
Page No. – Page No.
Lewis Acid-Catalyzed Transfer
Hydromethallylation for the
Construction of Quaternary Carbon
Centers
Quat a wonderful world. A surrogate of isobutene gas based on cyclohexa-1,4-
diene has been designed that enables the transfer hydromethallylation of alkenes
under Lewis-acidic conditions. The method allows for the construction of a
congested quaternary carbon center and a C(sp3)–C(sp3) bond by way of a tertiary
carbenium ion generated by alkene protonation.
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