Azulenylcarbene and Naphthylcarbene Isomerizations. Falling Solid Flash Vacuum Pyrolysis

Article abstract of DOI:10.1021/acs.joc.5b00412

1-Azulenylcarbene 18 has been generated from 5-(1-azulenyl)tetrazole and the sodium salt of azulene-1-carbaldehyde tosylhydrazone using the falling solid flash vacuum pyrolysis (FS-FVP) method. The principal products, which are also formed from both 1- and 2-naphthylcarbenes, cyclobuta[de]naphthalene 6, cyclopenta[cd]indene 16, and benzofulvenallene 17, are explained in terms of two reaction paths, (a) a rearrangement to benzofulvenyl-7-carbene 13 and (b) a rearrangement to 1-naphthylcarbene 1. Moreover, 16 is also formed from 2-azulenylcarbene 30, thereby indicating the occurrence of a 2-azulenylcarbene-1-azulenylcarbene rearrangement. The reaction mechanisms are supported by density functional theory calculations at the B3LYP/6-31G?? level, which indicate that all the rearrangements have activation barriers of <35 kcal/mol, thus making them readily achievable under FVP conditions. Chemical Presented.

Full text of DOI:10.1021/acs.joc.5b00412

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Article
Azulenylcarbene and Naphthylcarbene
Isomerizations. Falling Solid Flash Vacuum Pyrolysis
David Kvaskoff, Jürgen Becker, and Curt Wentrup
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.5b00412 • Publication Date (Web): 30 Apr 2015
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Azulenylcarbene and Naphthylcarbene Isomerizations. Falling Solid Flash
Vacuum Pyrolysis
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David Kvaskoff,† Jürgen Becker‡ and Curt Wentrup†,‡,*
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†School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, Qld
4072, Australia. wentrup@uq.edu.au
‡Fachbereich Chemie der Philipps-Universität Marburg, D-35037 Marburg, Germany
Abstract: 1-Azulenylcarbene 18 has been generated from 5-(1-azulenyl)tetrazole and the sodium
salt of azulene-1-carbaldehyde tosylhydrazone using the falling solid flash vacuum pyrolysis
(FS-FVP) method. The principal products, which are also formed from both 1- and 2-
naphthylcarbenes, cyclobuta[de]naphthalene 6, cyclopenta[cd]indene 16, and benzofulvenallene
17, are explained in terms of two reaction paths, (a) a rearrangement to benzofulvenyl-7-carbene
13, and (b) a rearrangement to 1-naphthylcarbene 1. Moreover, 16 is also formed from 2-
azulenylcarbene 30, thereby indicating the occurrence of a 2-azulenylcarbene – 1-
azulenylcarbene rearrangement. The reaction mechanisms are supported by DFT calculations at
the B3LYP/6-31G** level, which indicate that all the rearrangements have activation barriers
below 35 kcal/mol, thus making them readily achievable under FVP conditions.
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Introduction
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The rich rearrangement chemistry of the naphthylcarbenes has been the subject of detailed
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investigation under both thermal and photochemical reaction conditions. The thermal
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interconversion of 1- and 2-naphthylcarbenes 1 and 5, and the rearrangement of both of them to
cyclobuta[de]naphthalene 6 as the global energy minimum, was established in 1980 and is
summarized in Scheme 1.2,3,4
Scheme 1. 2-Naphthylcarbene – 1-naphthylcarbene – cyclobuta[de]naphthalene rearrangement.
The energy profile has been evaluated at the B3LYP/6-311+G*//B3LYP/6-31G* level,
which indicated that the tricyclic cyclopropenes 2 and 4 are transition states.5 However, the
carbenes can also form two other tricyclic cyclopropenes 7 and 10, which represent energy
minima (Scheme 2).3,5 The cyclopropenes 7 and 10 were identified in Ar matrices under
photochemical conditions.6 The carbenes, tricyclic cyclopropenes, benzocycloheptatetraenes and
the corresponding benzocycloheptatrienylidenes have been the subject of further, extensive
investigation.7,8
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Scheme 2. Naphthylcarbene - benzobicyclo[4.1.0]heptatriene - benzocycloheptatetraene
rearrangements.
Notably, in spite of their non-aromatic character and consequential high energies,
rearrangements through the transition structures 2TS and 4TS is efficient under flash vacuum
pyrolysis (FVP) conditions.2,3 Our calculations of Gibbs free energies at the B3LYP/6-31G**
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level affords an overall barrier for the interconversion of 1 and 5 at 600 C as ca. 30 kcal/mol,
and it is only ~16 kcal/mol for the cyclization of 1 to 6 on the singlet energy surface (see Scheme
3).
Moreover, rearrangement to the tricyclic cyclobuta[cd]indene 16 and benzofulvenallene
17 also takes place to a minor extent in the FVP reactions of both 1- and 2-naphthylcarbenes.3
These reactions can be formulated as indicated in Scheme 3 based on analogy with the ring
contraction of phenylcarbene to fulvenallene.9 The lowest-energy paths to these products have
activation barriers of no more than 35 kcal/mol (Scheme 3). Benzofulvenylcarbene 13 has been
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generated independently by FVP of the diazo compound 14, which yielded 15 at the lowest
temperatures, and 16 and 17 at higher temperatures.9,10
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Scheme 3. Thermal rearrangements of the naphthylcarbenes. Numbers in normal font are Gibbs
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∆
G in kcal/mol at 873 K calculated at the B3LYP/6-31G** level, relative to 18E
= 0 (see Scheme 6). Relative energies of transition states are indicated above the arrows. See the
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Supporting Information for an analogous scheme with electronic energies E and computational
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details .
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In contrast to these extensive studies, little is known about the isomeric azulenylcarbenes.
However, Sander and co-workers recently published an intriguing reaction, the tunneling 1,4- (or
1,10)-H shift converting 1-azulenylcarbene 18 to the fulvene derivative 19 under matrix isolation
condition (Scheme 4).11 The calculated classical barrier for the 1,9-H shift at the RCCSD(T)/cc-
pVTZ//(U)B3LYP/cc-pVTZ level was only ~23 kcal/mol.11 Although this is far too high for the
classical process in Ar or Ne matrix at 3-10 K, it will be very facile under FVP conditions This
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observation prompts us to report our investigation of the pyrolytic behavior of 1- and 2-
azulenylcarbenes.
Scheme 4. The tunneling 1-azulenylcarbene rearrangement reported by Sander et al.11
Results and Discussion
1-Azulenylcarbene 18 was generated from two different precursors, the 1-azulenecarbaldehyde
tosylhydrazone sodium salt and 5-(1-azulenyl)tetrazole (Scheme 5). Both precursors are
expected to generate 1-diazomethylazulene, and this was
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Scheme 5. Products (6, 16, and 17) of the falling solid flash vacuum pyrolysis (FS-FVP) of
precursors of 1-azulenylcarbene 18.
supported by the observation of an IR absorption at 2040 cm-1 in the 77 K IR spectrum of the
products resulting from mild pyrolysis. However both these precursors are too involatile for
conventional FVP, with the consequence that only very limited amounts of products can be
obtained in a reasonable time. Instead, we applied the falling solid flash vacuum pyrolysis (FS-
FVP).12 Using this technique, the solid tosylhydrazone sodium salt was pyrolyzed at 600 oC, and
5-(1-azulenyl)tetrazole at 800 oC. The products were isolated in liquid N2 traps and examined by
gas chromatography and NMR spectroscopy. Comparison with the samples characterized
previously3,4,10 permitted the identification of 6, 16, and 17, i.e. the same products as obtained
from the naphthylcarbenes (formation of 15 would not be observable at these reaction
temperatures, as it reverts to 13 and hence 16 and 17). In addition, the pyrolysis of 5-substituted
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tetrazoles always results in some cycloreversion to HN3 and a nitrile,9 in this case azulene-1-
carbonitrile.
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The formation of 16 is readily explained in terms of the Sander rearrangement 18ꢀ19
and then 19ꢀ20ꢀ21 (path a, Scheme 6). The latter reaction, 19ꢀ20ꢀ21, is simply a
cycloheptatetraene-phenylcarbene rearrangement, 21 ꢀ 22 is an electrocyclization, and 22 ꢀ 16
is a 1,2-H shift. All these steps have very modest calculated activation barriers of no more than
23 kcal/mol, which will be very readily accessible under FVP conditions.
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A cyclobuta[cd]azulene 23 was not isolated (see Scheme 6). This is understandable in
terms of the calculated free energy at the B3LYP/6-31G** level, which is ca. 36 kcal/mol higher
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than that of cyclobuta[de]naphthalene 6 at 600 C. So, it could be that a potentially formed
cyclobuta[cd]azulene 23 rearranges to 6 via an azulene-naphthalene rearrangement;13 but this is a
very high energy process with an activation barrier of the order of 70 kcal/mol, which requires a
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temperature above 1000 C in our apparatus. It was desirable, therefore, to explore lower energy
pathways to 6. Such a path has been found in the transformations 18 ꢀ 24 ꢀ 25 ꢀ 26 ꢀ 27
leading to 1-naphthylcarbene 1 (path b, Scheme 6). The triplet state of carbene 25a was
identified in the matrix photolysis study.10 On the singlet energy surface, the corresponding six-
membered cyclic allene 25b is also a possibility, analogous to heterocyclic allenes investigated
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by Sheridan et al.14 The singlet carbene 25a is very nearly planar, but a twisted allene-type
structure 25b with nearly the same energy also exists (see Scheme 6). A CASSCF calculation
also yielded identical energies for 125a and 25b, 3.3 kcal/mol above 325a.
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Scheme 6. Thermal rearrangements of 1-azulenylcarbene 18. Numbers in normal font are Gibbs
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∆
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= 0. Relative energies of transition states are indicated above the arrows. See the Supporting
Information for an analogous scheme with electronic energies E and computational details .
From 1-naphthylcarbene 1 the familiar products 6 and 16 can now be formed. The
cyclization to cyclobutanaphthalene 6 is very facile (Scheme 6), and the transition state was
confirmed by an IRC calculation. There are in principle two routes from 1 to 16 and 17; one by
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the paths shown in Scheme 3 with a maximum activation barrier of 35 kcal/mol, which is
analogous to the phenylcarbene – fulvenallene rearrangement,9 and the other via direct ring
contraction of 1 to 1-ethynylindene 28 (barrier ~50 kcal/mol, Scheme 6). Thus, the transannular
cyclization path via 11 and/or 12 (Scheme 3) is clearly preferred. From 12 and 28 there are low-
energy paths to benzofulvenylcarbenes 13a and 13b, which then cyclizes easily to 16. Thus,
there are potentially two paths to 16, via routes (a) and (b) (Scheme 6). As in the case of
fulvenallene and 5-ethynylcyclopentadiene,9 any interconversion between 28 and 17 is likely to
be collision-induced rather than a unimolecular, pericyclic 1,3-H shift.
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It is noteworthy that the ratio of cyclopenta[cd]indene 16 to cyclobuta[de]naphthalene 6
is much higher in the 600 oC pyrolysis of the tosylhydrazone salt than in the 800 oC pyrolysis of
the tetrazole. Furthermore, benzofulvenallene 17 was only clearly identified in the high-
temperature pyrolysis of the tetrazole. There may be several reasons for this outcome, but it
might reflect the fact that the first step of route (a) has a considerably lower activation barrier
than path (b). Thus, formation of 16 will be preferred at lower temperatures. Formation of 1-
naphthylcarbene and its rearrangements products will become more competitive at high
temperatures.
2-Azulenylcarbene 30 was examined in a similar manner by subjecting 2-(5-
azulenyl)tetrazole to FS-FVP. The 2-azulenyldiazomethane was detectable by an IR absorption
at 2060 cm-1 when using a pyrolysis temperature of 350 oC. At 600 oC cycloreversion to azulene-
2-carbonitrile and HN3 was the major reaction channel, and consequently the yield of C11H8
compounds was low. Cyclopenta[cd]indene 16 was the only C11H8 compound identified. This
indicates the occurrence of a 2-azulenylcarbene – 1-azulenylcarbene rearrangement (Scheme 7).
Singlet 2-azulenylcarbene is calculated to be higher in energy than the Z and E isomers of singlet
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1-azulenylcarbene by 7 and 11 kcal/mol, respectively, so there is a thermodynamic driving force
for the rearrangement. The calculated free energies make this rearrangement perfectly reasonable
with an overall activation barrier of only ca. 15 kcal/mol (Scheme 7). The species 32 may be
formulated either as a planar carbene (32a) or a twisted six-membered cyclic allene (32b)
similarly to 125a and 25b in Scheme 6.
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Further experimental and computational investigation of these very complex systems is
desirable.
Scheme 7. 2-Azulenylcarbene – 1-azulenylcarbene rearrangement. Numbers in normal font are
Gibbs free energies ∆G in kcal/mol at 873 K calculated at the B3LYP/6-31G** level, relative to
118E = 0. Relative energies of transition states are indicated above the arrows. See the
Supporting Information for an analogous scheme with electronic energies E and computational
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Conclusion
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The two reaction pathways of 1-azulenylcarbene 18, routes a and b in Scheme 6, satisfactorily
explain the formation of the thermal rearrangement products cyclopenta[cd]indene 16,
cyclobuta[de]naphthalene 6, and benzofulvenallene 17 with modest calculated activation barriers
of no more than 35 kcal/mol, which are easily achieved under FVP conditions. In addition, the
formation of cyclopenta[cd]indene 16 from the FVP of the precursor of 2-azulenylcarbene 30
indicates the occurrence of a 2-azulenylcarbene – 1-azulenylcarbene rearrangement, with an
overall activation barrier as low as 15 kcal/mol.
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Experimental Section
Azulene-1-carbaldehyde Tosylhydrazone
Azulene-1-carbaldehyde (4.0 g; 25.6 mmol) was dissolved in 500 mL of a mixture of dry ether
and dry ethanol 2:1 and treated with 4.76 g (25.6 mmol) of p-toluenesulfonyl hydrazine. The
resulting mixture was stirred at RT under N2 for 48 h. The solvent was then removed in vacuo
and the resulting solid was dried in vacuo at 10-2- hPa to yield 6.5 g (72%) of the tosylhydrazone
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as green crystals, mp 171-172 C; H NMR (DMSO-d6, 400 MHz) 11.2 (s, 1H), 8.88 (d, J=9.7
Hz, 1H), 8.46 (s, 1H), 8.40 (d, J=9.7 Hz, 1H), 8.05 (d. J=4.0 Hz, 1H), 7.87 (d, J=8.2 Hz, 2H),
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7.75 (t, J=9.7 Hz, 1H), 7.40-7.29 (m, 5H), 2.29 (s, 3H); C NMR (DMSO-d6, 100 MHz) 144.4,
143.3, 143.2, 139.3, 137.0, 137.0, 136.3, 136.1, 135.6, 129.5, 127.5, 125.6, 125.5, 121.8, 118.7,
20.9; MS (EI) m/z (%) 324 (10), 169 (82), 156 (22), 142 (20), 140 (15), 139 (42), 115 (25), 91
(46), 65 (22), 39 (11); IR (KBr) 3180 s, 1590 s, 1575 s, 1495 m, 1460 m, 1390 s, 1350 s, 1330 s,
1285 s, 1165 vs, 905 s, 805 s, 765 s, 670 s cm-1. The spectroscopic data are in agreement with
those reported by Sander et al.11 Anal. Calcd for C18H16N2SO2: C, 66.65; H, 4.97; N, 8.64 %.
Found: C, 66.64; H, 4.85; N, 8.53.
Sodium Salt of Azulene-1-carbaldehyde Tosylhydrazone11
The foregoing hydrazone (2.0 g; 6.2 mmol) was dissolved in 100 mL of a mixture of
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dry ether and dry methanol 1:1, and 150 mg (6.2 mmol) of dry NaH under ether was added under
N2. The resulting mixture was stirred under N2 at RT for 24 h. Except for a couple of mL, the
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solvent was then removed in vacuo, and the remainder was allowed to crystallize at 0 C; the
solid was filtered under N2 and dried at 10-2 hPa at 20 oC for 8 h to yield 1.2 g (56%) of the salt
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as green crystals, which were used as such in the pyrolyses.
5-(1-Azulenyl)tetrazole.
A mixture of 1-cyanoazulene (3.06 g; 20 mmol), 1.43 g (22 mmol) sodium azide and 1.16 g (22
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mmol) ammonium chloride was stirred in 20 mL dry DMF at 120 C for 12 d under N2. After
cooling, the solvent was removed in vacuo, and the remaining oil was treated with 150 mL of
water and slowly acidified with HCl. The resulting precipitate was filtered, dissolved in a small
amount of ethanol, chromatographed on silica gel and eluted with ethanol. The first fraction
consisted of unchanged 1-cyanoazulene (Rf = 0.75). The second fraction (Rf = 0.58) yielded the
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product (2.5 g; 64 %) as violet crystals, mp 166-168 C; H NMR (DMSO-d6) 9.63 (d, J = 9.8
Hz, 1H), 8.59 (d, J = 9.8 Hz, 1H), 8.50 (d, J = 4.0 Hz, 1H), 7.02 (d, J = 9.8 Hz, 1H), 7.58 (d, J =
9.8 Hz, 1H), 7.53 (d, J = 4.0 Hz, 1H), 7.48 (d, J = 9.8 Hz, 1H), 7.50 (very broad, 1H); 13C NMR
(DMSO-d6) 152.3, 143.0, 138.8, 137.4, 136.4, 126.5, 126.3, 118.6, 110.1; IR (KBr) 3600-2000
(very broad), 1595 s, 1540 s, 1400 s, 1300 m, 1290 m, 1260 m, 1235 m, 1165 m, 1085 s, 910 m,
860 s, 775 s, 750 cm-1; MS (EI) m/z 153 (M+.-HN3); MS (Field desorption) m/z 196 (M+.). Anal.
Calcd for C11H8N4: C, 67.34; H, 4.11; N, 28.55. Found: C, 67.25; H, 4.08; N, 28.35.
5-(2-Azulenyl)tetrazole
A mixture of 2-cyanoazulene (2.0 g; 13.07 mmol), 936 mg (14.4 mmol) sodium azide and 762
mg (14.4 mmol) ammonium chloride was stirred in 50 mL dry DMF at 120 oC for 5 d under N2.
After cooling, the solvent was removed in vacuo, and the remaining oil was treated with 50 mL
2N HCl. The resulting precipitate was filtered, dissolved in a small amount of ethanol,
chromatographed on basic aluminum oxide and eluted with ethanol. The first fraction consisted
of the unchanged, violet-red 2-cyanoazulene (Rf = 0.75). The second fraction (Rf = 0.58) yielded
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the product (802 g; 31 %) as dark blue crystals, mp 130-135 oC; 1H NMR (DMSO-d6) 8.46 (d, J
3
4
5
6
13
= 9.6 Hz, 2H), 7.90 (t, J = 9.6 Hz, 1H), 7.26 (7, J = 9.6 Hz, 2H), 5.57 (very broad s, 1H); C
7
8
9
NMR (DMSO-d6) 155.5 (s), 140.4 (m, J < 10 Hz), 138.1 (t, J = 10 and 154 Hz) 137.5 (d, J = 10
and 154 Hz), 135.7 (s), 124. 1 (d, J = 10 and 157 Hz), 115.8 (d, J = 6 and 170 Hz); IR (film on
KBr) 3700-2000 (very broad), 1660 s, 1585 m, 1400 s, 1470 m, 1390 s, 1090 s, 1055 s, 1025 m,
880 m, 765 s cm-1; MS (Field desorption) m/z (%) 197 (13, M+.+1), 196 (100, M+.). Anal. Calcd
for C11H8N4: C, 67.34; H, 4.11; N, 28.55. Found: C, 67.07; H, 4.15; N, 28.69.
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Pyrolysis of Azulene-1-carbaldehyde Tosylhydrazone Sodium Salt
(a) A portion of 0.8 g of the salt described above was subjected to falling solid pyrolysis (FS-
o
FVP)12 at 600 C at a pressure varying between 10-3 and 10-1 hPa in the course of 5 h. The
o
resulting pyrolyzate was distilled bulb-to-bulb at 90 C/10-3-10-2 hPa in order to remove
involatile components. The distillate was dissolved in CDCl3 and examined by GC (SE 52, 160
1
oC, isothermally) and H NMR spectroscopy. The following products were obtained: azulene
(6%), 1H-cyclopenta[cd]indene 16 (13%), cyclobuta[de]naphthalene 6 (3%), 1-
methylazulene (15%).
(b) A small sample of the tosylhydrazone sodium salt was heated to 130 oC, and the vapors were
led through the pyrolysis apparatus at 10-3 hPa and condensed on a KBr target at 77 K for IR
spectroscopy, which revealed an absorption at 2040 cm-1 ascribed to 1-(diazomethyl)azulene.
Pyrolysis of 5-(1-Azulenyl)tetrazole
o
A sample of 1 g of the solid, powdered tetrazole was subjected to FS-FVP12 at 600 C at a
pressure varying between 10-3 and 10-1 hPa in the course of 60 min. The resulting products were
o
1
examined by GC (SE52, 160 C isothermally) and H NMR spectroscopy and identified by
comparison with the compounds isolated previously.10 The following products were obtained:
naphthalene
(7%),
azulene
(20%),
1H-cyclopenta[cd]indene
16
(11%),
cyclobuta[de]naphthalene 6 (21%), 1-cyanoazulene (40%). 1-Vinylideneindene 17 did
1
not survive GC but was identified by its H NMR signal at 5.43 ppm and its IR
absorption at 1935 cm-1.10
Pyrolysis of 5-(2-Azulenyl)tetrazole
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(a) A sample of 500 mg of the solid, powdered tetrazole was subjected to FS-FVP at 600 oC at a
pressure varying between 10-3 and 10-1 hPa in the course of 50 min. The resulting products were
3
4
5
6
7
8
9
o
1
dissolved in CDCl3 and examined by GC-MS (SE30, 160 C isothermally) and H NMR
spectroscopy and identified by comparison with the products isolated previously.10 The
following products were obtained: naphthalene (9%), azulene (8%), 1H-cyclopenta[cd]indene
10
11
12
13
14
15
16
17
18
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16 (5%), and 2-cyanoazulene (62%). There was no evidence for cyclobuta[de]naphthalene
6.
(b) A small sample of 5-(2-azulenyltetrazole ws sublimed at 100 oC and pyrolyzed at 350 oC/10-4
hPa. The products was condensed on a KBr target at 77 K for IR spectroscopy, which revealed
an absorption at 2060 cm-1 ascribed to 2-(diazomethyl)azulene.
ASSOCIATED CONTENT
Supporting Information
NMR spectra of pyrolysis products 6 and 16. Schemes 3, 6 and 7 with electronic energies at
273.15 K. Computational details, Cartesian coordinates, energies, and imaginary frequencies
(where applicable) of calculated structures. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author: wentrup@uq.edu.au
Notes
The authors declare no competing financial interests.
Acknowledgements
This work was supported by the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen
Industrie, the National Computing Infrastructure facility financed by the Australian Government
(MAS grant g01), and the Queensland Cyber Infrastructure Foundation at The University of
Queensland.
References
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1 (a) Jones, W. M. Acc. Chem. Res. 1977, 10, 353. (b) Wentrup, C. Top. Curr. Chem. 1976, 62,
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2 Becker, J.; Wentrup, C. J. Chem. Soc., Chem. Commun. 1980, 190.
3 Wentrup, C.; Mayor, C.; Becker, J.; Lindner, H. J. Tetrahedron 1985, 41, 1601.
4 See also the related work using pyrolysis of methoxy(trimethylsilyl)methylnaphthalenes as
carbene sources: T. A. Engler, H. Shechter, J. Org. Chem. 1999, 64, 4247.
5 Xie, Y.; Schreiner, P. R.; Schleyer, P. von R.; Schaefer, H. F. J. Am. Chem. Soc. 1997, 119,
1370.
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8 Kuhn, A.; Vosswinkel, M.; Wentrup, C. J. Org. Chem. 2002, 67, 9023.
9 Kvaskoff, D.; Lüerssen, H.; Bednarek, P.; Wentrup, C. J. Am. Chem. Soc. 2014, 136, 15203.
10 Wentrup, C.; Benedikt, J. J. Org. Chem. 1980, 45, 1407.
11 Henkel, S.; Huynh, Y-am.; Neuhaus, P.; Winkler, M.; Sander, W. J. Am. Chem. Soc. 2012, 134,
13204.
12 (a) Wentrup, C.; Becker, J.; Winter, H.-W. Angew. Chem. Int. Ed. 2015, 54,
DOI 10.1002/anie.201412431. (b) Wentrup, C. Aust. J. Chem. 2014, 67, 1150.
13 (a) Becker, J.; Wentrup, C.; Katz, E.; Zeller, K.-P. J. Am. Chem. Soc. 1980, 102, 5110. (b)
Scott, L. T. Acc. Chem. Res. 1982, 15, 52. (c) Alder, R. W.; East, S. P.; Harvey, J. N.; Oakley,
M. T. J. Am. Chem. Soc. 2003, 125, 5375. (d) Dyakov, Yu. A.; Mebel, A. M.; Lin, S. H.; Lee, Y.
T.; Ni, C.-K. J. Chin. Chem. Soc. 2006, 53, 161.
14 (a) Nikitina, A. F.; Sheridan, R. J. Org. Lett. 2006, 7, 4467. (b) Sheridan, R. J. Chem. Rev.
2013, 113, 7179.
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