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N–N bond cleavage in diazoalkanes with a titanium alkylidene
Polyhedron 84 (2014) 177–181
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N–N bond cleavage in diazoalkanes with a titanium alkylidene Jun-ichi Ito, Marco G. Crestani, Brad C. Bailey, Xinfeng Gao, Daniel J. Mindiola ⇑ Contribution from the Department of Chemistry, Indiana University, 800 E. Kirkwood Avenue, Bloomington, IN 47405, USA
a r t i c l e
i n f o
Article history: Received 9 April 2014 Accepted 23 July 2014 Available online 1 August 2014 Keywords: Schrock carbene Alkylidene Titanium Diazoalkane Imido
a b s t r a c t The titanium alkylidene-triflate complex (PNP)Ti@CHtBu(OTf) was found to promote N–N bond cleavage of 9-diazofluorene and ditolyldiazomethane to give the imido complexes, (PNP)Ti@N[C13H9](OTf) (1) and (PNP)Ti@N[CHtolyl2](OTf) (2), respectively. The molecular structure of 2 was determined by single-crystal X-ray diffraction studies. Along with imido formation leading to 1 and 2, the alkylidene Ti@CHtBu ligand in (PNP)Ti@CHtBu(OTf) was found to eliminate with the a-N atom of the diazoalkane to form the nitrile NCtBu, which was confirmed by a combination of 1H NMR spectroscopy and GC–MS. The reaction mechanism of the N–N bond cleavage promoted by the reactive Ti@CHtBu ligand is also discussed. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Diazoalkanes are often sought reagents for carbene sources due to their propensity to extrude N2 [1,2]. Taking advantage of such feature, it is of no surprise that transition metal catalysts can aid in the delivery of the carbene unit via the use of N2CR2 (R = alkyl, aryl, H, C(O)OR0 , etc) reagents [3]. Such process has found widespread use in cyclopropanation [4], C–H insertion [5], and ringexpansion (Büchner-type) reactions [4]. These metals, often of the coinage type, can be applied catalytically, and in some cases even stereoselectively, for efficient delivery of the carbene unit. However, the reactivity of N2CR2 reagents towards early transition metals differs greatly, due to the inherent stability of the M@N bond, and quite possibly the coordination mode of such substrate to the metal-center [6]. As a result, N2CR2 compounds often form diazoalkane adducts enjoying from a strong M@N multiple bond to form M@N–N@CR2 type ligands. In rare cases, can N2 elimination be promoted to generate the transient carbene species [6a,g]. Examples of powerful Lewis acids promoting N2 elimination of diazoalkane reagents in the presence of a suitable trap have been also reported [7]. In contrast to N–C bond cleavage in a diazoalkane, N–N bond rupture is a much rarer phenomenon [6b,8–10]. In 2005 we reported the first example of N–N bond cleavage of N2CPh2 via the use of a well-defined four-coordinate titanium alkylidene (nacnac)Ti@CHtBu(OTf) (nacnac@[ArNC(CH3)]2CH; Ar@2,6-iPr2C6H3) ⇑ Corresponding author. Present address: University of Pennsylvania, Department of Chemistry, 231 South 34th Street, Philadelphia, PA 19104, USA. Tel.: +1 (215) 898 5247; fax: +1 (215) 573 9711. E-mail address: [email protected] (D.J. Mindiola). http://dx.doi.org/10.1016/j.poly.2014.07.036 0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.
to form the imido (nacnac)Ti@N–NCHPh2(NCtBu)(OTf) (Scheme 1) [8]. Such reaction proceeded smoothly and cleanly with N2CPh2 at 25 °C without any observable intermediates (when the reaction was gauged by 1H NMR spectroscopy). In 2008 Chirik and co-workers reported a similar reaction (although differing in stoichiometry) in which an intermediary species [(iPrPDI)Fe@CHR] (iPrPDI@2,6-(2,6-iPr2C6H3N@CMe2)2C5H3N) was proposed to promote the N–N bond cleavage of various N2CHR substrates to form equal molar mixtures of nitrile and aldimine iron adducts (Scheme 2) [9]. In the absence of a metal carbene or alkylidene ligand, examples of N–N bond cleavage of the coordinated N2CR2 ligand on Mo2 [10] or Ti [6c] centers have been reported when such compounds are exposed to CO to afford ketimide-isocyanato complexes of the type Mx(N@CR2)(NCO) (x = 1, 2). In this study we show that an alkylidene ligand is indeed involved in the activation and cleavage of the N–N bond of arylated N2CR2 (R = aryl). We explored the neopentylidene complex (PNP)Ti = CHtBu(OTf) (PNP@N[2-PiPr2-4-methylphenyl]2) [11] with the two diazoalkanes, 9-diazofluorene and ditolyldiazomethane, and hence propose a pathway for the activation and transformation of the N–N bond into a titanium imido and nitrile. The involvement of a kinetically stable Ti@CHtBu ligand in the cleavage of the N–N bond in diazoalkanes as well as the transfer of the a-N of such reagent to ‘‘CtBu’’ to form a nitrile are presented and discussed. 2. Results and discussion 2.1. N-N bond breakage of aryldiazoalkanes Treatment of the alkylidene-triflate complex (PNP)Ti@CHtBu (OTf) with one equiv of 9-diazofluorene over one hour in pentane
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i
iPr tBuHC
N2
OTf
OTf Ti N N
Pr
N 2CPh2
i Pr
t
Ti
BuCN
N N
N Ph2 HC
PiPr2
i Pr
N
P iPr2
Scheme 3. Synthesis of complexes 1 and 2.
imido-alkyl species, (PNP)Ti@N[Ar](CHt2Bu) [11], (PNP)Ti@N[1adamantyl](CHt2Bu) [12] and (PNP)Ti@N[SiMe3](OTf) [13].
2.2. Proposed mechanism to N–N bond cleavage of aryldiazoalkanes by (PNP)Ti = CHtBu(OTf) The formation of 1 and 2 is interesting due to the way the diazoalkane has been transformed. First, N–N bond breaking has occurred, with the terminal (a-N) nitrogen being expelled as nitrile (NCtBu), whereas the internal nitrogen (b-N) has been transformed into an imido ligand. Second, the former alkylidene hydrogen has migrated to the carbenoid motif of the reagent N2CR2. Such transformation would be unexpected given the formal charges for N2CR2 or its possible canonical forms (Scheme 4), where the a-N or carbene C would be anticipated to react with the electrophilic site; the b-N likely reacting with the nucleophilic site [14]. Nonetheless, the carbene carbon of N2CR2 could also exhibit a more ambiphilic behavior depending on the operative resonance canonical form. Irrespective of this, one can assume that coordination of N2CPh2 to the titanium center in (PNP)Ti@CHtBu(OTf) proceeds via the terminal nitrogen or carbene carbon rather than by the internal nitrogen in the diazoalkane motif. Scheme 5 depicts how the neopentylidene ligand in (PNP)Ti@CHtBu(OTf) most likely participates in the N–N bond rupture of N2CR2. We first speculate that a [2 + 2]-cycloaddition of the diazo unit in N2CPh2 across the alkylidene ligand would give rise to
N 2 N2 CHR
Ar N
N
Fe
NCR
+
N
Fe
-4 N 2
R
Ar
Ar
Ar
H N
N
N
N
+ NC tBu OTf
2 83% yield
Ar
N2
H N
Ti PiPr2
N N2
+ NC tBu OTf
1 89% yield, X -ray
N
Ar
Fe
H N
Ti PiPr2
toluene 25 oC, 2h
results in clean formation of the imido (PNP)Ti@N[C13H9](OTf) (1) along with NCtBu (Scheme 3). When (PNP)Ti@CHtBu(OTf) and N2C(tolyl)2 are mixed (tolyl = 4-MeC6H4), the imido relative (PNP)Ti@N[CHtolyl2](OTf) (2) is formed in addition to NCtBu (Scheme 3). Reactions are quantitative based on 31P NMR spectroscopy with 1 and 2 being isolated in 89% and 83% yield, respectively. Unfortunately no intermediates could be detected by NMR spectroscopy (1H and 31P), even when the reaction was monitored at low-temperatures in thawing toluene (95 °C). Quantitative formation of NCtBu was gauged by 1H NMR spectroscopy, and also corroborated by GC–MS. Compounds 1 and 2 display similar spectroscopic features such as two doublets in the 31P NMR spectrum with a 2JPP of 61 Hz. In addition, the 1H NMR spectrum evidences a sharp resonance at 6–5 ppm (1H) for the imido methine. In the case of complex 1, a 4JHP coupling of 4 Hz could be resolved. Each of the methine resonances (b-carbon on imido ligand) was clearly correlated to a resonance at 84.6 ppm (1) or 85.4 (2) ppm in the 13 C NMR spectrum with the aid of an HMQC experiment. Lastly, the 19F NMR spectrum revealed the retention of the triflate ligand in 1 and 2, resonating at 77.5 ppm. Table 1 lists salient multinuclear NMR spectroscopic features for compounds 1 and 2. To corroborate the structures of 1 or 2, we collected X-ray diffraction data on a single crystal of the former. Fig. 1 displays the solid state structure of 1 with thermal ellipsoids at the 50% probability level. Inspection of the structure reveals a five-coordinate titanium complex possessing a terminal imide ligand (Ti1@N31, 1.6903(16) Å), which is clearly shorter to the amide-Ti distance observed in the PNP ligand (Ti1–N10, 2.0463(16) Å). Because of the long Ti–P distances, the P–Ti–P angle is highly distorted from a trans configuration (151.24(2)°). The triflate ligand coordinates to the titanium center, while carbon C32 is sp3 hybridized due to hydrogen transfer from the alkylidene-triflate complex to this carbon in the former diazoalkane substrate. Consequently, the N–C distance has elongated to that expected for a single bond (1.441(3) Å). Overall, complex 1 resembles the reported
2
pentane 25 oC, 1h
OTf
N2
Scheme 1. N–N bond rupture of N2CPh2 by a titanium neopentylidene. Ar represents the sterically encumbering ligand 2,6-iPr2C6H3.
N
N
Ti PiPr2
Ar
Ar
PiPr2
CH tBu
Scheme 2. N-N bond rupture of N2CHR by a transient iron carbene. Ar represents the sterically encumbering ligand 2,6-iPr2C6H3, and R = cyclohexyl, tBu, Ph, tolyl, mesityl, 2MeOC6H4, and 2-iPrOC6H4.
Table 1 Salient NMR spectroscopic features of compounds 1 and 2 (ppm). Compound
1
13
31
1 2
5.02 (t, 4JPH = 4.4 Hz, NCH) 6.08 (NCH)
84.6 (s, NCH) 85.4 (s, NCH)
26.6 (d, 2JPP = 61.4 Hz)19.3 (d, 2JPP = 61.4 Hz) 27.5 (d, 2JPP = 60.6 Hz) 18.1 (d, 2JPP = 60.6 Hz)
H NMR
C NMR
P NMR (2JPP in Hz)
19
F NMR
77.5 77.5
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the intermediate zwitterion A (other canonical forms are not illustrated for clarity), which can isomerize to a g2 bound azine B. Although structures A and B are not necessarily critical to the formation of product, we show different bonding forms of an azinelike ligand generated by [2 + 2]-cycloaddition of the diazoalkane to illustrate how diverse this ligand type can be. However, the most likely pathway to formation of the imide and nitrile involves a 1,3dipolar cycloaddition (also referred to as a [4 + 2]-cycloaddition reaction by recent IUPAC notation) of N2CR2 to the alkylidene ligand in (PNP)Ti@CHtBu(OTf), akin to that reported by Chirik and co-workers, to form intermediate C [9]. In contrast to Chirik’s system [9], which undergoes a slow 1,3-migration of a proton to the b-N, complex C would be poised to tautomerize via the a-carbons (or undergo an intramolecular a-hydrogen abstraction) to form a disubstituted alkylidene species labeled D. Isomerization of the alkylidene ligand in D to a (C,N)-chelating ligand would render E a tautomer of A, in which the hydrogen atom is now positioned on the arylated carbon. The last step, and perhaps the fastest one due to the driving force to form a strong Ti@N bond, would involve [2 + 2]-retrocycloaddition to extrude the nitrile NCtBu to form the titanium-imide. Very likely, the tautomerization or abstraction step converting C to D is the slowest step of the overall process, although we cannot rule out the cycloaddition or isomerization processes to be also sluggish since we don’t observe intermediates in route to 1 or 2. Based on deuterium labeling studies using N2CDR, Chirik and co-workers have suggested the 1,3-hydrogen migration to be the rate determining step in the formation of the imine and nitrile [9].
Fig. 1. Solid state structure of complex 1 displaying thermal ellipsoids at the 50% probability level. A solvent molecule in the asymmetric unit, as well as hydrogen atoms have been excluded for clarity. Selected distances are reported in Å and angles in degrees. Ti1–N31, 1.6903(16); Ti1–O45, 2.0218(14); Ti1–N10, 2.0463(16); Ti1–P2, 2.5805(7); Ti1–P18, 2.5878(7); N31–Ti1–O45, 111.08(7); N31–Ti1–N10, 111.38(7); O45–Ti1–N10, 137.53(6); N31–Ti1–P2, 100.41(6); O45–Ti1–P2, 95.37(4); N10–Ti1–P2, 76.19(5); N31–Ti1–P18, 97.56(6); O45–Ti1–P18, 98.89(4); N10–Ti1–P18, 76.43(5); P2–Ti1–P18, 151.24(2).
3. Conclusions
R
N
N
N
N
N
N
N
N
R
R
R
R
R
R
Complex (PNP)Ti@CHtBu(OTf) can readily activate and cleave the N–N bond of 9-diazofluorene and ditolyldiazomethane to form a titanium imido and the nitrile NCtBu. As such, the diazo unit of N2CR2 has been transformed significantly, but more notably, without loss of N2. This system behaves similar to our previously reported alkylidene derivative, (nacnac)Ti@CHtBu(OTf), but given the more coordinately saturated nature of the PNP ligand we do not observe coordination of the nitrile that is produced in the reaction. Opposite to Chirik’s proposed carbene species, [(iPrPDI)Fe@CHR], which converts N2CHR to a nitrile and aldimine
R
Scheme 4. Possible resonances for N2CR2.
Pi Pr2 N
CH tBu
Ti Pi Pr2
OTf
N 2 CR 2
t Pi Pr2 H Bu C
N
Ti
N N
N
Pi Pr2 OTf
CR2
Ti
HtBu Pi Pr2 C N Ti
C R2 Pi Pr2 OTf C
N
D
N=CR2
B tBu Pi Pr2 C
Pi Pr2 tBu C N N Ti N Pi Pr2 OTf
N
Pi Pr2 OTf
A
N 2 CR 2
N
t Pi Pr2 H Bu C
NC tBu N
N CHR2
Ti Pi Pr2
N OTf
CH R2
E
Scheme 5. Proposed mechanism to formation of complexes 1 and 2. R2 represents two tolyl groups or a fluorene.
1 or 2
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[9], HN@CHR, tautomerization in our system occurs in the opposite direction: a result of the inherent strength of the Ti@N bond in addition to the more polarized Ti–C bond in a proposed cyclic intermediate such as C. In either case however, the proton selectively tautomerizes to the most available basic site. 4. Experimental 4.1. General considerations Unless otherwise stated, all manipulations involving air- or moisture sensitive compounds were performed in double or single M. Braun Lab Master glove-boxes under purified nitrogen or argon atmospheres or using high vacuum standard Schlenk techniques under an argon atmosphere. Anhydrous hydrocarbon solvents (toluene and n-pentane) were purchased from Aldrich in 20 L stainless-steel sure-sealed reservoirs and directly dispensed into the gloveboxes after passage through activated alumina and Q-5 drying agent columns installed in an M. Braun solvent purifier system (MB-SPS). Benzene-d6 (Cambridge Isotope Laboratories, CIL) was degassed by three consecutive freeze–pump–thaw (FTP) cycles in a Schlenk line and placed over sodium and molecular sieves for at least 12 h prior to use. (PNP)Ti@CHtBu(OTf) [11], 9-diazofluorene [15] and ditolyldiazomethane [16] were prepared according to the literature references. 1H, 13C, 19F, and 31P NMR and 2D (dqfCOSY, gHSQC) NMR spectra were recorded using Varian 500 or 400 MHz NMR spectrometers operating at 25 °C. 1H and 13C NMR chemical shifts are reported referenced to the internal residual proton or carbon resonances of C6D6 (dH = 7.160 ppm, dC = 128 ppm). 19F NMR chemical shifts are reported with respect to external HOCOCF3 (78.5 ppm). 31P NMR chemical shifts are reported relative to external H3PO4 (d = 0.0 ppm). Multiple attempts to obtain satisfactory combustion analysis of 1 and 2 failed as a result of their inherent thermal sensitivity. In lieu of this, we provide high resolution multinuclear NMR spectral data in the Supporting information, as proof of bulk purity of these complexes. 4.2. Reaction of (PNP)Ti@CHtBu(OTf) with 9-diazofluorene to form complex 1 9-Diazofluorene (19.2 mg, 0.104 mmol) was added to a toluene solution (3 ml) of (PNP)Ti@CHtBu(OTf) (69.7 mg, 0.100 mmol), at room temperature. The solution was stirred for 1 h, during which time the color changed from red to wine-red. The solvent was removed under vacuum and the residue extracted using n-pentane (ca. 80 mL). The extract was filtered through a glass filter, concentrated in vacuo and cooled at 35 °C over several days to yield purple crystals of (PNP)Ti@N[C13H9](OTf) (1) (71.9 mg, 0.0893 mmol, 89%). For 1: 1H NMR (25 °C, 400 MHz, C6D6): 8.22 (d, J = 6.4 Hz, 1H, Ar–H), 7.40 (t, J = 7.4 Hz, 2H, Ar–H), 7.22 (t, J = 8.0 Hz, 1H, Ar–H), 7.13 (d, J = 8.4 Hz, 1H, Ar–H), 7.10 (d, J = 6.0 Hz, 1H, Ar–H), 7.00– 6.92 (m, 3H, Ar–H), 6.86 (dd, JPH = 4.2 Hz, J = 8.6 Hz, Ar–H), 6.70– 6.76 (m, 2H, A–H), 6.62 (dd, JPH = 1.6, 6.4 Hz, Ar–H), 5.02 (t, JPH = 4.4 Hz, NCH), 2.32 (sep, J = 7.2 Hz, 1H, CHMe2), 2.26 (s, 3H, Me), 2.13–1.95 (m, 2H, CHMe2), 2.00 (s, 3H, Me), 1.48 (dd, JHH = 7.2 Hz, JPH = 16.0 Hz, 3H, CHMe2), 1.33–1.17 (m, 9H, CHMe2), 0.98 (dd, JHH = 7.2 Hz, JPH = 16.0 Hz, 3H, CHMe2), 0.89–0.76 (m, 6H, CHMe2), 0.65–0.56 (m, 6H, CHMe2). 13C NMR (25 °C, 125 MHz, C6D6): 160.0 (d, JPC = 21.5, C6H3), 157.6 (d, JPC = 21.5, C6H3), 147.3 (s, C12H8), 147.1 (C12H8), 139.7 (C12H8), 139.4 (C12H8), 133.2, 132.8, 132.7, 131.6, 131.49 (C6H3), 131.45 (C6H3), 125.7, 125.1, 120.1 (d, J = 6.3 Hz, C6H3), 119.3 (d, J = 33.1 Hz, C6H3), 119.3 (s, C12H8), 119.1 (s, C12H8), 117.4 (d, J = 29.5 Hz, C6H3), 116.7 (d, J = 8.1 Hz, C6H3), 24.1 (d, J = 13.5 Hz, CHMe2), 23.6 (d, J = 9.0 Hz, CHMe2),
21.4 (d, J = 17.0 Hz, CHMe2), 20.9 (Me), 20.6 (d, J = 16.1 Hz, CHMe2), 20.4 (Me), 20.11 (CHMe2), 20.10 (d, J = 16.1 Hz, CHMe2), 18.2 (d, J = 8.0 Hz, CHMe2), 18.0 (d, J = 4.5 Hz, CHMe2), 17.6 (d, J = 7.1 Hz, CHMe2), 17.5 (CHMe2), 16.7 (d, J = 5.3 Hz, CHMe2), 16.0 (d, J = 4.5 Hz, CHMe2). 19F NMR (25 °C, 376 MHz, C6D6): 77.5. 31P NMR (25 °C, 162 MHz, C6D6): 26.6 (d, JPP = 61.4 Hz), 19.3 (d, JPP = 61.4 Hz). 4.3. Reaction of (PNP)Ti@CHtBu(OTf) with ditolyldiazomethane to form complex 2 Ditolyldiazomethane (16.4 mg, 0.0738 mmol) was added to a toluene solution (2 ml) of (PNP)Ti@CHtBu(OTf) (50.0 mg, 0.0719 mmol) at room temperature. The solution was stirred for 2 h upon which, the color changed a dark red to wine-red. The solvent was removed under vacuum and the residue extracted with npentane. The extract was filtered through Celite, cooled at 35 °C for 2–3 days to yield purple crystals of (PNP)Ti@N[CHtolyl2](OTf) (2) (49.7 mg, 0.0595 mmol, 83%). For 2: 1H NMR (25 °C, 400 MHz, C6D6): 7.41 (d, JHH = 8.0 Hz, 2H, C6H4), 7.36 (dd, JHH = 8.6 Hz, JPH = 4.2 Hz, 1H, C6H3), 7.04 (d, JHH = 8.4 Hz, 1H, C6H3), 6.98 (d, JHH = 7.6 Hz, 2H, C6H4), 6.95 (d, JHH = 8.0 Hz, 2H, C6H4), 6.92 (dd, JHH = 8.8 Hz, JPH = 4.4 Hz, 1H, C6H3), 6.83–6.75 (m, 4H), 6.70 (d, JPH = 6.4 Hz, 1H, C6H3), 6.08 (s, 1H, NCH), 2.22 (s, 3H, Me), 2.16–2.00 (m, 4H, CHMe2), 2.10 (s, 6H, Me), 2.06 (s, 3H, Me), 1.40 (dd, JHH = 7.2 Hz, JPH = 16.0 Hz, 3H, CHMe2), 1.28 (dd, JHH = 6.6 Hz, JPH = 17.0 Hz, 3H, CHMe2), 1.19 (d, JPH = 13.6 Hz, 3H, CHMe2), 1.06 (dd, JHH = 7.2 Hz, JPH = 16.4 Hz, 3H, CHMe2), 0.94 (dd, JHH = 7.2 Hz, JPH = 17.6 Hz, 3H, CHMe2), 0.85 (dd, JHH = 7.4 Hz, JPH = 16.6 Hz, 3H, CHMe2), 0.78–0.72 (m, 6H, CHMe2). 13 C NMR (25 °C, 125 MHz, C6D6): 160.4 (d, JPC = 20.6 Hz, C6H3), 157.6 (d, JPC = 16.1 Hz, C6H3), 143.6 (C6H4Me), 141.5 (C6H4Me), 135.7 (C6H4Me), 135.2 (C6H4Me), 133.3 (C6H3), 132.9 (C6H3), 132.8 (C6H3), 132.2 (C6H3), 131.5 (d, JPC = 4.5 Hz, C6H3), 128.9 (C6H4Me), 128.8 (C6H4Me), 128.5 (C6H4Me), 127.6 (C6H4Me), 120.2 (d, JPC = 30.5 Hz, C6H3), 120.0 (d, JPC = 7.1 Hz, C6H3), 116.6 (d, JPC = 28.6 Hz, C6H3), 116.6 (d, JPC = 8.0 Hz, C6H3), 85.4 (s, NCH), 23.6 (d, JPC = 8.0 Hz, CHMe2), 22.8 (d, JPC = 11.6 Hz, CHMe2), 21.1 (d, JPC = 18.0 Hz, CHMe2), 21.0 (Me), 20.9 (Me), 20.5 (Me), 20.4 (d, JPC = 16.1 Hz, CHMe2), 20.2 (d, JPC = 7.1 Hz, CHMe2), 19.7 (d, JPC = 4.5 Hz, CHMe2), 19.5 (d, JPC = 8.0 Hz, CHMe2), 18.3 (d, JPC = 7.0 Hz, CHMe2), 17.7 (d, JPC = 10.8 Hz, CHMe2), 17.6 (d, JPC = 5.4 Hz, CHMe2), 16.8 (d, JPC = 5.4 Hz, CHMe2), 15.3 (d, JPC = 7.3 Hz, CHMe2). 19 F NMR (25 °C, 376 MHz, C6D6): 77.5. 31P NMR (25 °C, 162 MHz, C6D6): 27.5 (d, JPP = 60.6 Hz), 18.1 (d, JPP = 18.1 Hz). Acknowledgments Financial support of this research was provided by the National Science Foundation (CHE-0848248 and CHE-1152123). M.G.C. acknowledges CONACYT for a postdoctoral fellowship and J.-I.I. acknowledges financial support from the JSPS (Japan Society for the Promotion of Science). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.poly.2014.07.036. References [1] (a) D. Seyferth, Chem. Rev. 55 (1955) 1155; (b) W.A. Herrmann, Angew. Chem., Int. Ed. Engl. 17 (1978) 800. [2] (a) Examples of carbene complexes formed with diazoalkanes: P. Schwab, R.H. Grubbs, J.W. Ziller, J. Am. Chem. Soc. 118 (1996) 100; (b) W.A. Herrmann, Angew. Chem., Int. Ed. Engl. 13 (1974) 599; (c) W.A. Herrmann, Chem. Ber. 108 (1975) 486;
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N–N bond cleavage in diazoalkanes with a titanium alkylidene Jun-ichi Ito, Marco G. Crestani, Brad C. Bailey, Xinfeng Gao, Daniel J. Mindiola ⇑ Contribution from the Department of Chemistry, Indiana University, 800 E. Kirkwood Avenue, Bloomington, IN 47405, USA
a r t i c l e
i n f o
Article history: Received 9 April 2014 Accepted 23 July 2014 Available online 1 August 2014 Keywords: Schrock carbene Alkylidene Titanium Diazoalkane Imido
a b s t r a c t The titanium alkylidene-triflate complex (PNP)Ti@CHtBu(OTf) was found to promote N–N bond cleavage of 9-diazofluorene and ditolyldiazomethane to give the imido complexes, (PNP)Ti@N[C13H9](OTf) (1) and (PNP)Ti@N[CHtolyl2](OTf) (2), respectively. The molecular structure of 2 was determined by single-crystal X-ray diffraction studies. Along with imido formation leading to 1 and 2, the alkylidene Ti@CHtBu ligand in (PNP)Ti@CHtBu(OTf) was found to eliminate with the a-N atom of the diazoalkane to form the nitrile NCtBu, which was confirmed by a combination of 1H NMR spectroscopy and GC–MS. The reaction mechanism of the N–N bond cleavage promoted by the reactive Ti@CHtBu ligand is also discussed. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Diazoalkanes are often sought reagents for carbene sources due to their propensity to extrude N2 [1,2]. Taking advantage of such feature, it is of no surprise that transition metal catalysts can aid in the delivery of the carbene unit via the use of N2CR2 (R = alkyl, aryl, H, C(O)OR0 , etc) reagents [3]. Such process has found widespread use in cyclopropanation [4], C–H insertion [5], and ringexpansion (Büchner-type) reactions [4]. These metals, often of the coinage type, can be applied catalytically, and in some cases even stereoselectively, for efficient delivery of the carbene unit. However, the reactivity of N2CR2 reagents towards early transition metals differs greatly, due to the inherent stability of the M@N bond, and quite possibly the coordination mode of such substrate to the metal-center [6]. As a result, N2CR2 compounds often form diazoalkane adducts enjoying from a strong M@N multiple bond to form M@N–N@CR2 type ligands. In rare cases, can N2 elimination be promoted to generate the transient carbene species [6a,g]. Examples of powerful Lewis acids promoting N2 elimination of diazoalkane reagents in the presence of a suitable trap have been also reported [7]. In contrast to N–C bond cleavage in a diazoalkane, N–N bond rupture is a much rarer phenomenon [6b,8–10]. In 2005 we reported the first example of N–N bond cleavage of N2CPh2 via the use of a well-defined four-coordinate titanium alkylidene (nacnac)Ti@CHtBu(OTf) (nacnac@[ArNC(CH3)]2CH; Ar@2,6-iPr2C6H3) ⇑ Corresponding author. Present address: University of Pennsylvania, Department of Chemistry, 231 South 34th Street, Philadelphia, PA 19104, USA. Tel.: +1 (215) 898 5247; fax: +1 (215) 573 9711. E-mail address: [email protected] (D.J. Mindiola). http://dx.doi.org/10.1016/j.poly.2014.07.036 0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.
to form the imido (nacnac)Ti@N–NCHPh2(NCtBu)(OTf) (Scheme 1) [8]. Such reaction proceeded smoothly and cleanly with N2CPh2 at 25 °C without any observable intermediates (when the reaction was gauged by 1H NMR spectroscopy). In 2008 Chirik and co-workers reported a similar reaction (although differing in stoichiometry) in which an intermediary species [(iPrPDI)Fe@CHR] (iPrPDI@2,6-(2,6-iPr2C6H3N@CMe2)2C5H3N) was proposed to promote the N–N bond cleavage of various N2CHR substrates to form equal molar mixtures of nitrile and aldimine iron adducts (Scheme 2) [9]. In the absence of a metal carbene or alkylidene ligand, examples of N–N bond cleavage of the coordinated N2CR2 ligand on Mo2 [10] or Ti [6c] centers have been reported when such compounds are exposed to CO to afford ketimide-isocyanato complexes of the type Mx(N@CR2)(NCO) (x = 1, 2). In this study we show that an alkylidene ligand is indeed involved in the activation and cleavage of the N–N bond of arylated N2CR2 (R = aryl). We explored the neopentylidene complex (PNP)Ti = CHtBu(OTf) (PNP@N[2-PiPr2-4-methylphenyl]2) [11] with the two diazoalkanes, 9-diazofluorene and ditolyldiazomethane, and hence propose a pathway for the activation and transformation of the N–N bond into a titanium imido and nitrile. The involvement of a kinetically stable Ti@CHtBu ligand in the cleavage of the N–N bond in diazoalkanes as well as the transfer of the a-N of such reagent to ‘‘CtBu’’ to form a nitrile are presented and discussed. 2. Results and discussion 2.1. N-N bond breakage of aryldiazoalkanes Treatment of the alkylidene-triflate complex (PNP)Ti@CHtBu (OTf) with one equiv of 9-diazofluorene over one hour in pentane
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i
iPr tBuHC
N2
OTf
OTf Ti N N
Pr
N 2CPh2
i Pr
t
Ti
BuCN
N N
N Ph2 HC
PiPr2
i Pr
N
P iPr2
Scheme 3. Synthesis of complexes 1 and 2.
imido-alkyl species, (PNP)Ti@N[Ar](CHt2Bu) [11], (PNP)Ti@N[1adamantyl](CHt2Bu) [12] and (PNP)Ti@N[SiMe3](OTf) [13].
2.2. Proposed mechanism to N–N bond cleavage of aryldiazoalkanes by (PNP)Ti = CHtBu(OTf) The formation of 1 and 2 is interesting due to the way the diazoalkane has been transformed. First, N–N bond breaking has occurred, with the terminal (a-N) nitrogen being expelled as nitrile (NCtBu), whereas the internal nitrogen (b-N) has been transformed into an imido ligand. Second, the former alkylidene hydrogen has migrated to the carbenoid motif of the reagent N2CR2. Such transformation would be unexpected given the formal charges for N2CR2 or its possible canonical forms (Scheme 4), where the a-N or carbene C would be anticipated to react with the electrophilic site; the b-N likely reacting with the nucleophilic site [14]. Nonetheless, the carbene carbon of N2CR2 could also exhibit a more ambiphilic behavior depending on the operative resonance canonical form. Irrespective of this, one can assume that coordination of N2CPh2 to the titanium center in (PNP)Ti@CHtBu(OTf) proceeds via the terminal nitrogen or carbene carbon rather than by the internal nitrogen in the diazoalkane motif. Scheme 5 depicts how the neopentylidene ligand in (PNP)Ti@CHtBu(OTf) most likely participates in the N–N bond rupture of N2CR2. We first speculate that a [2 + 2]-cycloaddition of the diazo unit in N2CPh2 across the alkylidene ligand would give rise to
N 2 N2 CHR
Ar N
N
Fe
NCR
+
N
Fe
-4 N 2
R
Ar
Ar
Ar
H N
N
N
N
+ NC tBu OTf
2 83% yield
Ar
N2
H N
Ti PiPr2
N N2
+ NC tBu OTf
1 89% yield, X -ray
N
Ar
Fe
H N
Ti PiPr2
toluene 25 oC, 2h
results in clean formation of the imido (PNP)Ti@N[C13H9](OTf) (1) along with NCtBu (Scheme 3). When (PNP)Ti@CHtBu(OTf) and N2C(tolyl)2 are mixed (tolyl = 4-MeC6H4), the imido relative (PNP)Ti@N[CHtolyl2](OTf) (2) is formed in addition to NCtBu (Scheme 3). Reactions are quantitative based on 31P NMR spectroscopy with 1 and 2 being isolated in 89% and 83% yield, respectively. Unfortunately no intermediates could be detected by NMR spectroscopy (1H and 31P), even when the reaction was monitored at low-temperatures in thawing toluene (95 °C). Quantitative formation of NCtBu was gauged by 1H NMR spectroscopy, and also corroborated by GC–MS. Compounds 1 and 2 display similar spectroscopic features such as two doublets in the 31P NMR spectrum with a 2JPP of 61 Hz. In addition, the 1H NMR spectrum evidences a sharp resonance at 6–5 ppm (1H) for the imido methine. In the case of complex 1, a 4JHP coupling of 4 Hz could be resolved. Each of the methine resonances (b-carbon on imido ligand) was clearly correlated to a resonance at 84.6 ppm (1) or 85.4 (2) ppm in the 13 C NMR spectrum with the aid of an HMQC experiment. Lastly, the 19F NMR spectrum revealed the retention of the triflate ligand in 1 and 2, resonating at 77.5 ppm. Table 1 lists salient multinuclear NMR spectroscopic features for compounds 1 and 2. To corroborate the structures of 1 or 2, we collected X-ray diffraction data on a single crystal of the former. Fig. 1 displays the solid state structure of 1 with thermal ellipsoids at the 50% probability level. Inspection of the structure reveals a five-coordinate titanium complex possessing a terminal imide ligand (Ti1@N31, 1.6903(16) Å), which is clearly shorter to the amide-Ti distance observed in the PNP ligand (Ti1–N10, 2.0463(16) Å). Because of the long Ti–P distances, the P–Ti–P angle is highly distorted from a trans configuration (151.24(2)°). The triflate ligand coordinates to the titanium center, while carbon C32 is sp3 hybridized due to hydrogen transfer from the alkylidene-triflate complex to this carbon in the former diazoalkane substrate. Consequently, the N–C distance has elongated to that expected for a single bond (1.441(3) Å). Overall, complex 1 resembles the reported
2
pentane 25 oC, 1h
OTf
N2
Scheme 1. N–N bond rupture of N2CPh2 by a titanium neopentylidene. Ar represents the sterically encumbering ligand 2,6-iPr2C6H3.
N
N
Ti PiPr2
Ar
Ar
PiPr2
CH tBu
Scheme 2. N-N bond rupture of N2CHR by a transient iron carbene. Ar represents the sterically encumbering ligand 2,6-iPr2C6H3, and R = cyclohexyl, tBu, Ph, tolyl, mesityl, 2MeOC6H4, and 2-iPrOC6H4.
Table 1 Salient NMR spectroscopic features of compounds 1 and 2 (ppm). Compound
1
13
31
1 2
5.02 (t, 4JPH = 4.4 Hz, NCH) 6.08 (NCH)
84.6 (s, NCH) 85.4 (s, NCH)
26.6 (d, 2JPP = 61.4 Hz)19.3 (d, 2JPP = 61.4 Hz) 27.5 (d, 2JPP = 60.6 Hz) 18.1 (d, 2JPP = 60.6 Hz)
H NMR
C NMR
P NMR (2JPP in Hz)
19
F NMR
77.5 77.5
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the intermediate zwitterion A (other canonical forms are not illustrated for clarity), which can isomerize to a g2 bound azine B. Although structures A and B are not necessarily critical to the formation of product, we show different bonding forms of an azinelike ligand generated by [2 + 2]-cycloaddition of the diazoalkane to illustrate how diverse this ligand type can be. However, the most likely pathway to formation of the imide and nitrile involves a 1,3dipolar cycloaddition (also referred to as a [4 + 2]-cycloaddition reaction by recent IUPAC notation) of N2CR2 to the alkylidene ligand in (PNP)Ti@CHtBu(OTf), akin to that reported by Chirik and co-workers, to form intermediate C [9]. In contrast to Chirik’s system [9], which undergoes a slow 1,3-migration of a proton to the b-N, complex C would be poised to tautomerize via the a-carbons (or undergo an intramolecular a-hydrogen abstraction) to form a disubstituted alkylidene species labeled D. Isomerization of the alkylidene ligand in D to a (C,N)-chelating ligand would render E a tautomer of A, in which the hydrogen atom is now positioned on the arylated carbon. The last step, and perhaps the fastest one due to the driving force to form a strong Ti@N bond, would involve [2 + 2]-retrocycloaddition to extrude the nitrile NCtBu to form the titanium-imide. Very likely, the tautomerization or abstraction step converting C to D is the slowest step of the overall process, although we cannot rule out the cycloaddition or isomerization processes to be also sluggish since we don’t observe intermediates in route to 1 or 2. Based on deuterium labeling studies using N2CDR, Chirik and co-workers have suggested the 1,3-hydrogen migration to be the rate determining step in the formation of the imine and nitrile [9].
Fig. 1. Solid state structure of complex 1 displaying thermal ellipsoids at the 50% probability level. A solvent molecule in the asymmetric unit, as well as hydrogen atoms have been excluded for clarity. Selected distances are reported in Å and angles in degrees. Ti1–N31, 1.6903(16); Ti1–O45, 2.0218(14); Ti1–N10, 2.0463(16); Ti1–P2, 2.5805(7); Ti1–P18, 2.5878(7); N31–Ti1–O45, 111.08(7); N31–Ti1–N10, 111.38(7); O45–Ti1–N10, 137.53(6); N31–Ti1–P2, 100.41(6); O45–Ti1–P2, 95.37(4); N10–Ti1–P2, 76.19(5); N31–Ti1–P18, 97.56(6); O45–Ti1–P18, 98.89(4); N10–Ti1–P18, 76.43(5); P2–Ti1–P18, 151.24(2).
3. Conclusions
R
N
N
N
N
N
N
N
N
R
R
R
R
R
R
Complex (PNP)Ti@CHtBu(OTf) can readily activate and cleave the N–N bond of 9-diazofluorene and ditolyldiazomethane to form a titanium imido and the nitrile NCtBu. As such, the diazo unit of N2CR2 has been transformed significantly, but more notably, without loss of N2. This system behaves similar to our previously reported alkylidene derivative, (nacnac)Ti@CHtBu(OTf), but given the more coordinately saturated nature of the PNP ligand we do not observe coordination of the nitrile that is produced in the reaction. Opposite to Chirik’s proposed carbene species, [(iPrPDI)Fe@CHR], which converts N2CHR to a nitrile and aldimine
R
Scheme 4. Possible resonances for N2CR2.
Pi Pr2 N
CH tBu
Ti Pi Pr2
OTf
N 2 CR 2
t Pi Pr2 H Bu C
N
Ti
N N
N
Pi Pr2 OTf
CR2
Ti
HtBu Pi Pr2 C N Ti
C R2 Pi Pr2 OTf C
N
D
N=CR2
B tBu Pi Pr2 C
Pi Pr2 tBu C N N Ti N Pi Pr2 OTf
N
Pi Pr2 OTf
A
N 2 CR 2
N
t Pi Pr2 H Bu C
NC tBu N
N CHR2
Ti Pi Pr2
N OTf
CH R2
E
Scheme 5. Proposed mechanism to formation of complexes 1 and 2. R2 represents two tolyl groups or a fluorene.
1 or 2
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[9], HN@CHR, tautomerization in our system occurs in the opposite direction: a result of the inherent strength of the Ti@N bond in addition to the more polarized Ti–C bond in a proposed cyclic intermediate such as C. In either case however, the proton selectively tautomerizes to the most available basic site. 4. Experimental 4.1. General considerations Unless otherwise stated, all manipulations involving air- or moisture sensitive compounds were performed in double or single M. Braun Lab Master glove-boxes under purified nitrogen or argon atmospheres or using high vacuum standard Schlenk techniques under an argon atmosphere. Anhydrous hydrocarbon solvents (toluene and n-pentane) were purchased from Aldrich in 20 L stainless-steel sure-sealed reservoirs and directly dispensed into the gloveboxes after passage through activated alumina and Q-5 drying agent columns installed in an M. Braun solvent purifier system (MB-SPS). Benzene-d6 (Cambridge Isotope Laboratories, CIL) was degassed by three consecutive freeze–pump–thaw (FTP) cycles in a Schlenk line and placed over sodium and molecular sieves for at least 12 h prior to use. (PNP)Ti@CHtBu(OTf) [11], 9-diazofluorene [15] and ditolyldiazomethane [16] were prepared according to the literature references. 1H, 13C, 19F, and 31P NMR and 2D (dqfCOSY, gHSQC) NMR spectra were recorded using Varian 500 or 400 MHz NMR spectrometers operating at 25 °C. 1H and 13C NMR chemical shifts are reported referenced to the internal residual proton or carbon resonances of C6D6 (dH = 7.160 ppm, dC = 128 ppm). 19F NMR chemical shifts are reported with respect to external HOCOCF3 (78.5 ppm). 31P NMR chemical shifts are reported relative to external H3PO4 (d = 0.0 ppm). Multiple attempts to obtain satisfactory combustion analysis of 1 and 2 failed as a result of their inherent thermal sensitivity. In lieu of this, we provide high resolution multinuclear NMR spectral data in the Supporting information, as proof of bulk purity of these complexes. 4.2. Reaction of (PNP)Ti@CHtBu(OTf) with 9-diazofluorene to form complex 1 9-Diazofluorene (19.2 mg, 0.104 mmol) was added to a toluene solution (3 ml) of (PNP)Ti@CHtBu(OTf) (69.7 mg, 0.100 mmol), at room temperature. The solution was stirred for 1 h, during which time the color changed from red to wine-red. The solvent was removed under vacuum and the residue extracted using n-pentane (ca. 80 mL). The extract was filtered through a glass filter, concentrated in vacuo and cooled at 35 °C over several days to yield purple crystals of (PNP)Ti@N[C13H9](OTf) (1) (71.9 mg, 0.0893 mmol, 89%). For 1: 1H NMR (25 °C, 400 MHz, C6D6): 8.22 (d, J = 6.4 Hz, 1H, Ar–H), 7.40 (t, J = 7.4 Hz, 2H, Ar–H), 7.22 (t, J = 8.0 Hz, 1H, Ar–H), 7.13 (d, J = 8.4 Hz, 1H, Ar–H), 7.10 (d, J = 6.0 Hz, 1H, Ar–H), 7.00– 6.92 (m, 3H, Ar–H), 6.86 (dd, JPH = 4.2 Hz, J = 8.6 Hz, Ar–H), 6.70– 6.76 (m, 2H, A–H), 6.62 (dd, JPH = 1.6, 6.4 Hz, Ar–H), 5.02 (t, JPH = 4.4 Hz, NCH), 2.32 (sep, J = 7.2 Hz, 1H, CHMe2), 2.26 (s, 3H, Me), 2.13–1.95 (m, 2H, CHMe2), 2.00 (s, 3H, Me), 1.48 (dd, JHH = 7.2 Hz, JPH = 16.0 Hz, 3H, CHMe2), 1.33–1.17 (m, 9H, CHMe2), 0.98 (dd, JHH = 7.2 Hz, JPH = 16.0 Hz, 3H, CHMe2), 0.89–0.76 (m, 6H, CHMe2), 0.65–0.56 (m, 6H, CHMe2). 13C NMR (25 °C, 125 MHz, C6D6): 160.0 (d, JPC = 21.5, C6H3), 157.6 (d, JPC = 21.5, C6H3), 147.3 (s, C12H8), 147.1 (C12H8), 139.7 (C12H8), 139.4 (C12H8), 133.2, 132.8, 132.7, 131.6, 131.49 (C6H3), 131.45 (C6H3), 125.7, 125.1, 120.1 (d, J = 6.3 Hz, C6H3), 119.3 (d, J = 33.1 Hz, C6H3), 119.3 (s, C12H8), 119.1 (s, C12H8), 117.4 (d, J = 29.5 Hz, C6H3), 116.7 (d, J = 8.1 Hz, C6H3), 24.1 (d, J = 13.5 Hz, CHMe2), 23.6 (d, J = 9.0 Hz, CHMe2),
21.4 (d, J = 17.0 Hz, CHMe2), 20.9 (Me), 20.6 (d, J = 16.1 Hz, CHMe2), 20.4 (Me), 20.11 (CHMe2), 20.10 (d, J = 16.1 Hz, CHMe2), 18.2 (d, J = 8.0 Hz, CHMe2), 18.0 (d, J = 4.5 Hz, CHMe2), 17.6 (d, J = 7.1 Hz, CHMe2), 17.5 (CHMe2), 16.7 (d, J = 5.3 Hz, CHMe2), 16.0 (d, J = 4.5 Hz, CHMe2). 19F NMR (25 °C, 376 MHz, C6D6): 77.5. 31P NMR (25 °C, 162 MHz, C6D6): 26.6 (d, JPP = 61.4 Hz), 19.3 (d, JPP = 61.4 Hz). 4.3. Reaction of (PNP)Ti@CHtBu(OTf) with ditolyldiazomethane to form complex 2 Ditolyldiazomethane (16.4 mg, 0.0738 mmol) was added to a toluene solution (2 ml) of (PNP)Ti@CHtBu(OTf) (50.0 mg, 0.0719 mmol) at room temperature. The solution was stirred for 2 h upon which, the color changed a dark red to wine-red. The solvent was removed under vacuum and the residue extracted with npentane. The extract was filtered through Celite, cooled at 35 °C for 2–3 days to yield purple crystals of (PNP)Ti@N[CHtolyl2](OTf) (2) (49.7 mg, 0.0595 mmol, 83%). For 2: 1H NMR (25 °C, 400 MHz, C6D6): 7.41 (d, JHH = 8.0 Hz, 2H, C6H4), 7.36 (dd, JHH = 8.6 Hz, JPH = 4.2 Hz, 1H, C6H3), 7.04 (d, JHH = 8.4 Hz, 1H, C6H3), 6.98 (d, JHH = 7.6 Hz, 2H, C6H4), 6.95 (d, JHH = 8.0 Hz, 2H, C6H4), 6.92 (dd, JHH = 8.8 Hz, JPH = 4.4 Hz, 1H, C6H3), 6.83–6.75 (m, 4H), 6.70 (d, JPH = 6.4 Hz, 1H, C6H3), 6.08 (s, 1H, NCH), 2.22 (s, 3H, Me), 2.16–2.00 (m, 4H, CHMe2), 2.10 (s, 6H, Me), 2.06 (s, 3H, Me), 1.40 (dd, JHH = 7.2 Hz, JPH = 16.0 Hz, 3H, CHMe2), 1.28 (dd, JHH = 6.6 Hz, JPH = 17.0 Hz, 3H, CHMe2), 1.19 (d, JPH = 13.6 Hz, 3H, CHMe2), 1.06 (dd, JHH = 7.2 Hz, JPH = 16.4 Hz, 3H, CHMe2), 0.94 (dd, JHH = 7.2 Hz, JPH = 17.6 Hz, 3H, CHMe2), 0.85 (dd, JHH = 7.4 Hz, JPH = 16.6 Hz, 3H, CHMe2), 0.78–0.72 (m, 6H, CHMe2). 13 C NMR (25 °C, 125 MHz, C6D6): 160.4 (d, JPC = 20.6 Hz, C6H3), 157.6 (d, JPC = 16.1 Hz, C6H3), 143.6 (C6H4Me), 141.5 (C6H4Me), 135.7 (C6H4Me), 135.2 (C6H4Me), 133.3 (C6H3), 132.9 (C6H3), 132.8 (C6H3), 132.2 (C6H3), 131.5 (d, JPC = 4.5 Hz, C6H3), 128.9 (C6H4Me), 128.8 (C6H4Me), 128.5 (C6H4Me), 127.6 (C6H4Me), 120.2 (d, JPC = 30.5 Hz, C6H3), 120.0 (d, JPC = 7.1 Hz, C6H3), 116.6 (d, JPC = 28.6 Hz, C6H3), 116.6 (d, JPC = 8.0 Hz, C6H3), 85.4 (s, NCH), 23.6 (d, JPC = 8.0 Hz, CHMe2), 22.8 (d, JPC = 11.6 Hz, CHMe2), 21.1 (d, JPC = 18.0 Hz, CHMe2), 21.0 (Me), 20.9 (Me), 20.5 (Me), 20.4 (d, JPC = 16.1 Hz, CHMe2), 20.2 (d, JPC = 7.1 Hz, CHMe2), 19.7 (d, JPC = 4.5 Hz, CHMe2), 19.5 (d, JPC = 8.0 Hz, CHMe2), 18.3 (d, JPC = 7.0 Hz, CHMe2), 17.7 (d, JPC = 10.8 Hz, CHMe2), 17.6 (d, JPC = 5.4 Hz, CHMe2), 16.8 (d, JPC = 5.4 Hz, CHMe2), 15.3 (d, JPC = 7.3 Hz, CHMe2). 19 F NMR (25 °C, 376 MHz, C6D6): 77.5. 31P NMR (25 °C, 162 MHz, C6D6): 27.5 (d, JPP = 60.6 Hz), 18.1 (d, JPP = 18.1 Hz). Acknowledgments Financial support of this research was provided by the National Science Foundation (CHE-0848248 and CHE-1152123). M.G.C. acknowledges CONACYT for a postdoctoral fellowship and J.-I.I. acknowledges financial support from the JSPS (Japan Society for the Promotion of Science). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.poly.2014.07.036. References [1] (a) D. Seyferth, Chem. Rev. 55 (1955) 1155; (b) W.A. Herrmann, Angew. Chem., Int. Ed. Engl. 17 (1978) 800. [2] (a) Examples of carbene complexes formed with diazoalkanes: P. Schwab, R.H. Grubbs, J.W. Ziller, J. Am. Chem. Soc. 118 (1996) 100; (b) W.A. Herrmann, Angew. Chem., Int. Ed. Engl. 13 (1974) 599; (c) W.A. Herrmann, Chem. Ber. 108 (1975) 486;
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