Intramolecular 1,1-carboboration versus intermolecular FLP addition in reactions of boranes and bis(phenylethynyl)telluroether

Article abstract of DOI:10.1039/c4cc09258a

Reactions of boranes with Te(CCPh)₂ proceed via initial intermolecular 1,1-carboboration followed by either an intramolecular carboboration or an FLP addition to a second molecule of the intermediate, yielding 1-bora-4-tellurocyclohexa-2,5-diene heterocycles or tricylic derivatives of 1,4-ditellurocyclohexa-2,5-diene, respectively. The latter species is also shown to convert to the former upon heating.

Full text of DOI:10.1039/c4cc09258a

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Intramolecular 1,1-carboboration versus
intermolecular FLP addition in reactions of
boranes and bis(phenylethynyl)telluroether†
Fu An Tsao,a Alan. J. Lougha and Douglas W. Stephan*ab
Cite this: Chem. Commun., 2015,
51, 4287
Received 19th November 2014,
Accepted 4th February 2015
DOI: 10.1039/c4cc09258a
www.rsc.org/chemcomm
Reactions of boranes with Te(CCPh)2 proceed via initial intermolecular
1,1-carboboration followed by either an intramolecular carboboration
or an FLP addition to a second molecule of the intermediate, yielding
1-bora-4-tellurocyclohexa-2,5-diene heterocycles or tricylic derivatives
of 1,4-ditellurocyclohexa-2,5-diene, respectively. The latter species is
also shown to convert to the former upon heating.
The incorporation of tellurium into heterocycles and polymeric
materials has garnered recent attention for applications in new
optoelectronic materials.1 However, the laborious and low-yielding
(o40%) nature of the syntheses of Te-containing heterocycles have
been noted by many authors in the literature.2,3 Indeed, the
development of new tellurium-containing heterocycles has been
hampered by the lack of synthetic methodologies.
Fig. 1 POV-ray depiction of (a) 2, (b) 4 and (c) 6. C: black, Te: gold, B:
Recognizing that unsaturated heterocycles that incorporated
donor and acceptors sites would be of interest, we considered new
synthetic avenues to Te–B heterocycles. The research groups of
Wrackmeyer,4 Berke,5 and Erker6 have exploited the reactions of
yellow-green, F: pink. Hydrogen atoms have been omitted for clarity.
The bis(phenylethynyl)telluroether Te(CCPh)2 1, was prepared by
alkynes with electrophilic boranes to effect 1,1-carboborations.6,7 a modified literature procedure.13 The addition of a slurry of 1 in
The reactivity affords vinyl-boranes but can be extended to prepare pentane to BPh3 at room temperature was stirred for 16 hours
a variety of boryl-substituted cyclopentandienes, including phos- affording the product 2 which was isolated in 93% yield. The 11B
pholes,8 boroles,9 and plumboles.10 More recently this reactivity NMR spectrum of 2 showed a single peak at 53.3 ppm. Single crystals
has also been employed to generate sulfur-substituted vinyl species,11 of 2 suitable for X-ray analysis grown from a saturated pentane
while we have shown that 1,1-carboration of tellurium-mono- solution confirmed the structure of 2 to be a planar six-membered
acetylides afford boron-substituted vinyl telluroethers.12 In this com- 1-bora-4-tellurocyclohexa-2,5-diene heterocycle (Fig. 1(a)). Phenyl-
munication, we explore related 1,1-carboration reactions of boranes substituents are found on each of the linking olefinic carbons and on
with Te(CCPh)2. These reactions are shown to proceed via double the B atom. The Te–C and B–C distances are 2.075(2) Å, 2.087(2) Å and
carboboration and Te/B frustrated Lewis pair (FLP) additions reac- 1.559(3) Å, 1.552(3) Å and 1.576(3) Å, respectively while the intra-ring
tions, affording 1-bora-4-tellurocyclohexa-2,5-diene and tricylic Te/B C–Te–C and C–B–C angles were found to be 97.08(9)1 and 124.3(2)1,
heterocycles, respectively.
respectively. The formation of 2 is thought to result from an initial
intermolecular 1,1-carboboration of 1, followed by an intramolecular
1,1-carboboration of the remaining acetylenic group on Te affording the
six-membered ring (Scheme 1). Even though 1,1-carboboration has
been documented extensively, the species (CH2)5Sn(HCQC(Ph))2BPh.4
is the only other reported analogous 6-membered heterocycle that has
been crystallographically characterized.4,14
a Department of Chemistry, 80 St. George Street, University of Toronto, Toronto,
Ontario, M5S 3H6, Canada. E-mail: dstephan@chem.utoronto.ca
b Chemistry Department, Faculty of Science, King Abdulaziz University (KAU),
Jeddah 21589, Saudi Arabia
† Electronic supplementary information (ESI) available: Synthetic and spectro-
scopic details are deposited crystallographic data have been deposited in the
Cambridge database. CCDC 1033757–1033760. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c4cc09258a
The corresponding addition of a slurry of 1 in pentane to
B(C6F5)3 at À35 1C resulted in the immediate color change from
This journal is ©The Royal Society of Chemistry 2015
Chem. Commun., 2015, 51, 4287--4289 | 4287

View Article Online
Communication
ChemComm
Scheme 1 Proposed reaction pathways to 2–7 via (a) 1,1 carboboration,
(b) FLP addition.
yellow to dark red. The 19F NMR spectrum showed complete
consumption of the starting material and the formation of a
minor product 3 which exhibited a sharp 11B peak at À4.1 ppm.
A major product 4 was also seen to exhibit two sets 19F signals
resulting from C6F5 rings in a 2 : 1 intensity ratio (À139.7
À161.9 À163.2, and À132.9, À153.4 À155.9 ppm, respectively)
and a broad 11B peak at 49.5 ppm. Heating this mixture to 80 1C Fig. 2 POV-ray depiction of (a) 5, (b) core of 5; C: black, Te: gold, B:
yellow-green, F: pink, Hydrogen atoms have been omitted for clarity.
for 8 h resulted in the complete disappearance of 3 suggesting
that this species was converted to the thermodynamic product
4. On work-up, the product 4 was isolated in 57% yield. An X-ray the 11B{1H} resonance (À4.1). 19F{1H} NMR data for 3 showed
diffraction study revealed that 4 is directly analogous to 2 with 15 discrete signals suggestive of both inequivalence and hindered
C6F5 rings replacing the Ph substituents of 2 (Fig. 2(b)).
rotation of the C6F5 rings. In a similar fashion, species 5 exhibits
The corresponding reaction of PhB(C6F5)2 and 1 gave about eight distinct 19F resonances in addition to a 11B resonance at
a 1 : 1 mixture of two species, 5 and 6 as evidenced by 11B and À3.5 ppm consistent with a 4-coordinate boron centre. While 3
19F NMR spectra. Again heating the solution to 80 1C for 8 h could not be unambiguously formulated, an X-ray crystallographic
resulted in the conversion of 5 to 6. The species 6 was confirmed to study of 5 revealed it to be a species consisting of three fused
be the 6-membered ring analogous to 2 and 4 (Fig. 1(c)). The solid- 5–6–5 rings featuring a central 1,4-ditellurocyclohexa-2,5-diene
state structures of 2, 4 and 6 are directly analogous, featuring a heterocycle. The tellurium centers of compound 5 both adopt
central 6-membered 1-bora-4-tellurocyclohexa-2,5-diene ring with distorted trigonal pyramidal geometry and are separated by
various aryl substituents. The B–C and CQC bond lengths in the 3.2896(7) Å which is within the range previously reported Te–Te
central ring of 6 are 1.559(3) Å and 1.552(3) Å and 1.360(3) Å and interactions in Te(IV)-based heterocycles (3.15–3.28 Å).16 Fused to
1.363(3) Å, respectively, suggesting some delocalization over the the Te–C bonds of the central rings are adjacent 5-membered
C–C–B–C–C fragment. The Te–C bond distances in 2, 4 and 6 fall TeCBC2 rings which incorporated the 4 coordinate B centers
in the range of 2.063(3)–2.087(2) Å which is significantly shorter than (Fig. 2). It is also noteworthy that the corresponding reaction of
those seen in Ph(CH2)2TeC(Ph)C(C6F5)B(C6F5)2(OCHPh) (2.126(2), MeB(C6F5)2 with 1 affords exclusively the product 7 which exhibits
Te–C(15): 2.151(3) Å)12 indicative of some degree of delocalization similar spectral parameters to 3 and 5, consistent with the analo-
within the 6-membered heterocycle. However, the 13C shifts of the gous formulation of 7 as [(C6F5)2BC(Me)QC(Ph)TeC(CPh)]2.
ipso-Ph(Te)CQC phenyl rings in 2, 4, and 6 are À143.9, À141.4 and
The formation of the above compounds from 1 are consis-
À141.4/143.5 ppm, respectively. These values deviate significantly tent with two reaction pathways involving an initial 1,1-carbo-
from those expected for aryl substituents on aromatic fragments boration of one of the alkynyl fragments on Te. A second intra-
(À136.6 to 138.1 ppm).15 Thus, the cyclic compounds 2, 4 and 6 molecular carboboration accounts for the formation of 1-bora-4-
cannot be considered fully aromatic.
tellurocyclohexa-2,5-diene heterocycles 2, 4 and 6 (Scheme 1).
The nature of the minor products 3 and 5 were probed. Alternatively, the intermediate Te–B species can undergo an FLP-
Interestingly, adjusting the polarity of the solvent of the initial addition17 in which the Te of one molecule and B of another add
reaction of 1 with B(C6F5)3 to a 1 : 4 mixture of CH2Cl2 : pentane across alkynyl fragments affording the tricyclic species with the
resulted in the precipitation of a yellow powder in 47% yield. central 1,4-ditellurocyclohexa-2,5-diene ring as in 5 (Scheme 1). It is
This species was confirmed to be exclusively 3 as evidenced by important to note that the thermal conversion (80 1C) of 3 to 4 and
4288 | Chem. Commun., 2015, 51, 4287--4289
This journal is ©The Royal Society of Chemistry 2015

View Article Online
ChemComm
Communication
L. Kang, W. T. Delgado, O. Shynkaruk, M. J. Ferguson, R. McDonald
and E. Rivard, J. Am. Chem. Soc., 2013, 135, 5360–5363.
3 (a) E. I. Carrera, T. M. McCormick, M. J. Kapp, A. J. Lough and
D. S. Seferos, Inorg. Chem., 2013, 52, 13779–13790; (b) E. I. Carrera
and D. S. Seferos, Dalton Trans., 2014; (c) T. M. McCormick, E. I. Carrera,
T. B. Schon and D. S. Seferos, Chem. Commun., 2013, 49, 11182–11184.
4 B. Wrackmeyer, P. Thoma, S. Marx, G. Glatz and R. Kempe, Z. Anorg.
Allg. Chem., 2013, 639, 1205–1213.
5 C. F. Jiang, O. Blacque and H. Berke, Organometallics, 2010, 29,
125–133.
6 G. Kehr and G. Erker, Chem. Commun., 2012, 48, 1839–1850.
7 R. L. Melen, Chem. Commun., 2014, 50, 1161–1174.
5 to 6 illustrate that the FLP addition to alkyne is reversible
presumably a result of destabilizing steric congestion prompting
formation of the thermodynamically stable 4 and 6. It is also
noteworthy that the FLP addition products were only observed
for the systems where the transient B/Te species contains electro-
philic B(C6F5)2 fragments, suggesting that only the more Lewis
acidic systems can effect FLP additions. In the case of 7, apparently
the reduced steric congestion in the FLP addition product and the
increased basicity of the Te center garners stability. Effects to
convert 7 to the analogous 1-bora-4-tellurocyclohexa-2,5-diene
heterocycle were unsuccessful, leading only to thermal degradation
and a mixture of unidentified products. Nonetheless, to the best of
our knowledge, the reactions described herein are the first examples
in which that intramolecular 1,1-carboboration are observed to be in
competition with intermolecular FLP addition.
¨
8 J. Mobus, Q. Bonnin, K. Ueda, R. Frohlich, K. Itami, G. Kehr and
G. Erker, Angew. Chem., Int. Ed., 2012, 51, 1954–1957.
¨
9 (a) A. Feldmann, A. Iida, R. Frohlich, S. Yamaguchi, G. Kehr and
G. Erker, Organometallics, 2012, 31, 2445–2451; (b) F. Ge, G. Kehr,
C. G. Daniliuc and G. Erker, J. Am. Chem. Soc., 2014, 136, 68–71.
10 B. Wrackmeyer and K. Horchler, J. Organomet. Chem., 1990, 399,
1–10.
¨
11 C. Eller, G. Kehr, C. G. Daniliuc, R. Frohlich and G. Erker, Organo-
metallics, 2013, 32, 384–386.
In summary, we have demonstrated that the reaction between
bis(phenylethynyl)telluroether and a series of boranes leads to two
products. Following initial intermolecular 1,1-carboboration, the
intermediate can either undergo intramolecular 1,1-carboboration
or intermolecular FLP addition. Compounds 2, 4 and 6 are of
particular interest as they present new tellurium-containing hetero-
cycles with an electrophilic element embedded within the hetero-
cycle. These facile and high-yielding syntheses are especially
attractive for future applications in optoelectronic materials. Such
investigations are currently underway in our laboratory.
12 F. A. Tsao and D. W. Stephan, Dalton Trans., 2015, 44, 71–74.
13 M. J. Dabdoub and J. V. Comasseto, Organometallics, 1988, 7, 84–87.
14 (a) L. Killian and B. Wrackmeyer, J. Organomet. Chem., 1978, 153,
153–164; (b) B. Wrackmeyer, K. Horchler and R. Boese, Angew.
Chem., Int. Ed., 1989, 28, 1500–1502; (c) B. Wrackmeyer and
G. Kehr, Polyhedron, 1991, 10, 1497–1506; (d) R. Koster, G. Seidel,
I. Klopp, C. Kruger, G. Kehr, J. Suss and B. Wrackmeyer, Chem. Ber./Recl.,
1993, 126, 1385–1396; (e) B. Wrackmeyer, S. Kundler and R. Boese,
Chem. Ber./Recl., 1993, 126, 1361–1370; ( f ) B. Wrackmeyer, G. Kehr and
D. Wettinger, Inorg. Chim. Acta, 1994, 220, 161–173; (g) B. Wrackmeyer,
S. Kundler, W. Milius and R. Boese, Chem. Ber., 1994, 127, 333–342;
(h) B. Wrackmeyer, S. Kundler and A. Arizacastolo, Phosphorus, Sulfur
Silicon Relat. Elem., 1994, 91, 229–239; (i) B. Wrackmeyer, A. Pedall,
W. Milius, O. L. Tok and Y. N. Bubnov, J. Organomet. Chem., 2002, 649,
232–245; ( j) B. Wrackmeyer, B. H. Kenner-Hofmann, W. Milius,
P. Thoma, O. L. Tok and M. Herberhold, Eur. J. Inorg. Chem., 2006,
101–108.
NSERC of Canada is thanked for financial support. DWS is
grateful for the award of a Canada Research Chair. FAT is
grateful for an NSERC CGS-D.
15 Z. Garkani-Nejad and M. Poshteh-Shirani, Can. J. Chem., 2011, 89,
598–607.
16 (a) J. Beck, Chem. Ber., 1995, 128, 23; (b) J. Beck and K. Mueller-
Buschbaum, Z. Anorg. Allg. Chem., 1997, 623, 409.
17 (a) M. A. Dureen, C. C. Brown and D. W. Stephan, Organometallics, 2010,
29, 6594–6607; (b) M. A. Dureen, C. C. Brown and D. W. Stephan,
Organometallics, 2010, 29, 6422–6432; (c) M. A. Dureen and
D. W. Stephan, J. Am. Chem. Soc., 2009, 131, 8396–8397.
Notes and references
1 (a) A. A. Jahnke and D. S. Seferos, Macromol. Rapid Commun., 2011,
32, 943–951; (b) T. Baumgartner, Acc. Chem. Res., 2014, 47, 1613.
2 (a) G. He, W. T. Delgado, D. J. Schatz, C. Merten, A. Mohammadpour,
L. Mayr, M. J. Ferguson, R. McDonald, A. Brown, K. Shankar and
E. Rivard, Angew. Chem., Int. Ed., 2014, 53, 4587–4591; (b) G. He,
This journal is ©The Royal Society of Chemistry 2015
Chem. Commun., 2015, 51, 4287--4289 | 4289