Rhodium-Mediated Dehydrogenative BorylationâHydroborylation of

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Rhodium-Mediated Dehydrogenative Borylation−Hydroborylation of Bis(alkyl)alkynes: Intermediates and Mechanism Sheila G. Curto, Miguel A. Esteruelas,* Montserrat Olivań , and Enrique Oñate Departamento de Química Inorgánica−Instituto de Síntesis Química y Catálisis Homogénea (ISQCH), Centro de Innovación en Química Avanzada (ORFEO-CINQA), Universidad de ZaragozaCSIC, 50009 Zaragoza, Spain

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S Supporting Information *

ABSTRACT: Complex Rh(Bpin){κ3-P,O,P-[xant(PiPr2)2]} (Bpin = pinacolboryl; xant(PiPr2)2 = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) catalyzes the addition of B2pin2 to 3-hexyne and 4octyne to give equimolecular mixtures of conjugated boryldienes and borylolefins, as a result of the addition of the B−B bond of the diborane to different molecules of alkynes and hydride transfer from one to the other. Both the dehydrogenative borylation and hydroborylation reactions form a catalytic cycle that has been deduced on the basis of stoichiometric studies. Complex Rh(Bpin){κ3-P,O,P-[xant(PiPr2)2]} promotes the dehydrogenative borylation of alkynes by means of reactions of insertion of the alkyne into the Rh−B bond, Z−E isomerization of the β-borylalkenyl ligand of the resulting Rh−alkenyl species, and Cγ−H bond activation of the alkyl substituent attached to the alkenyl Cα atom. As a consequence of the formation of boryldienes, the monohydride RhH{κ3P,O,P-[xant(PiPr2)2]} is generated. The latter in a sequential manner reacts with the alkynes and the diborane to give the borylolefin hydroborylation products, via Rh−alkenyl intermediates, and regenerates the initial Rh−boryl compound. The latter also promotes stoichiometric cycles to prepare diboryl-2-olefins via allyl intermediates. In addition, the stoichiometric rhodiummediated formation of 1-boryl-2-olefins is shown.

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INTRODUCTION Unsaturated organic molecules bearing C−B bonds are established members of relevant tools in organic synthesis.1 That is why, substitutions of hydrogen in C−H bonds by BR2 groups have awakened great interest in recent years, not only those of aromatic C(sp2)−H bonds but also other classes.2 In 2013, Ozerov’s group observed that iridium complexes stabilized by Si,N,N-pincer ligands efficiently promote the hydrogen substitution in C(sp)−H bonds to give molecular hydrogen and alkynylboronates (eq 1),3 which are particularly

The transition-metal-catalyzed diborylation of alkynes systematically displays 1,2-diborylalkenes with Z-stereochemistry in a selective manner,12 with a few exceptions that show mixtures of Z and E isomers13 or the formation of 1,1diborylalkenes when the alkyne is terminal.14 For a M−BR2 catalyst (Scheme 1), the key intermediate for the formation of 1,2-diborylalkenes is a (R2B)2M{(Z)-β-C(R′)C(R″)BR2}species,15 which can be formed by oxidative addition of diborane to a M{(Z)-β-C(R′)C(R″)BR2} compound and by insertion of the alkyne into a M−BR2 bond of a M(BR2)3 derivative. The formation of isomer Z is favored because the reductive elimination of Z-1,2-diborylalkene is very fast. However, when the formation of the abovementioned key intermediate, by oxidative addition of diborane, and/or the reductive elimination of Z-1,2-diborylalkene occurs at rates comparable to the rate of the Z−E isomerization of the βborylalkenyl group or are slower steps, the formation of E-1,2diborylalkene is observed.

useful for providing molecules with C−B bonds at positions difficult of borylating by alternative methods.4 Since then, in addition to Ir,5 catalysts of Fe,6 Pd,7 Cu,8 and Zn9 have been reported for this dehydrogenative borylation of terminal alkynes. Additions of σ-B−E bonds (E = H, B, Si, Sn, etc.) to the C− C triple bond of alkynes are alternative straightforward methods for generating C−B bonds in unsaturated molecules. These borylation reactions are catalyzed by complexes of late transition metals, mainly of groups 8, 9, 10, and 11.10 Although the syn products are generally favored because of the concerted character of the elemental steps involved in the catalysis, the four possible alkenylborane isomers are usually observed for the addition to asymmetrical alkynes (eq 2).11 © XXXX American Chemical Society

Received: February 14, 2019

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DOI: 10.1021/acs.organomet.9b00104 Organometallics XXXX, XXX, XXX−XXX

Organometallics

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RESULTS AND DISCUSSION Catalytic Borylation of 3-Hexyne and 4-Octyne. Complex Rh(Bpin){κ3-P,O,P-[xant(PiPr2)2]} (1) catalyzes the borylation of both alkynes. However, the formation of diborylalkenes is not observed, in contrast to that expected according to the previous reaction of 2-butyne. In the presence of 6.0 × 10−3 M 1, the treatment of 0.2 M 3-hexyne with 0.2 M B2pin2, in n-octane, at 70 °C, under argon atmosphere leads after 20 h to the transformation of the 80% of the alkyne into an equimolecular mixture of 4-pinacolboryl-(3E)-1,3-hexadiene and 3-pinacolboryl-3-hexene. The latter is composed by the isomers E and Z in a 2:1 molar ratio (eq 3). Under the

Scheme 1. Mechanisms for the Synthesis of 1,2Diborylalkenes

The ether-diphosphine 4,5-bis(diisopropylphosphino)xanthene (xant(PiPr2)2) is a flexible ligand, which adapts its coordination mode to the electronic and steric requirements of each particular complex,16 in agreement with that observed for other POP ligands.17 This flexibility allows a rapid Z−E isomerization of the β-borylalkenyl group (Z)-C(Me) C(Me)Bpin coordinated with the fragment Rh{xant(PiPr2)2}, via rhodacyclopropene intermediates bearing a κ2-P,O,Pdiphosphine.16n In spite of that, complex Rh(Bpin){κ3-P,O,P[xant(PiPr2)2]} catalyzes the syn-addition of B2pin2 to 2butyne, via the short-lived Rh(Bpin)3{κ3-P,O,P-[xant(PiPr2)2]} intermediate, to exclusively give Z-pinBC(Me)C(Me)Bpin (Scheme 2). The selectivity is a consequence of the slower rate of the Z−E isomerization with regard to the very fast reaction of the alkyne with Rh(Bpin)3{κ3-P,O,P-[xant(PiPr2)2]} to give the Z-1,2-diborylalkene product. The Z−E isomerization of the β-borylalkenyl group in the square-planar complex and the formation of the intermediate Rh(Bpin)3{κ3-P,O,P-[xant(PiPr2)2]} are competitive processes. The former is sterically controlled, whereas the second one is both sterically and electronically controlled. In the search for the size influence of the substituents of the alkyne in the selectivity of the diborylation, we have studied the behavior of 3-hexyne and 4-octyne, which has given rise to the discovery of a new type of borylation reaction. In this paper, we report the catalytic addition of a diborane to different molecules of alkynes, along with the hydride transfer from one to the other, and rationalize the result inferring the catalytic cycle on the basis of stoichiometric observations.

same conditions, the 90% of 4-octyne is transformed into equimolecular amounts of 5-pinacolboryl-2,4-octadiene and 4pinalcolboryl-4-octene. In this case, the diene is formed by isomers 2E,4E and 2Z,4E in a 2.7:1 molar ratio, whereas the olefin is obtained as a mixture of isomers E and Z also in a 2.7:1 molar ratio (eq 4).

The catalysis can be viewed, from a formal point of view, as reactions which coexist and take place in different molecules at about 50% yield. The formation of the diene could be at first glance described as a tandem process involving a hydrogen transfer from an alkyl substituent to the C−C triple bond, followed by the substitution of a hydrogen atom in the C−C double bond, resulting from the C−C triple bond reduction, by a pinacolboryl group. The monoolefins appear to be the result of a classical hydroborylation of internal alkynes.

Scheme 2. Diborylation of 2-Butyne

B


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Organometallics

shows two sets of doublets of doublets with 1JP−Rh and 2JP−P values of about 190 and 24.4 and 19.1 Hz, respectively, at 41.4 and 39.7 ppm (minor) and 28.1 and 25.1 ppm (major), corresponding to inequivalent PiPr2 groups of the two possible diastereoisomers that can be generated as a consequence of the chirality of both the metal center and the carbon atom bearing the boryl substituent. At room temperature, these signals are converted into two broad resonances, as a consequence of the ring-flip of the xanthene linker. In benzene-d6, complex 4 reacts with B2pin2 to quantitatively give 1,4-dipinacolboryl-(E)-2hexene and regenerate 1 (Scheme 3), closing a cycle for the stoichiometric rhodium-mediated 1,4-addition of bis(pinacolato)diboron to 3-hexyne. The cycle shown in Scheme 3 does not replicate the catalytic products. However, the allyl complex 4 points out the formation pathway of 4-pinacolboryl-(3E)-1,3-hexadiene, as this diolefin is an intermediate in its generation. Scheme 4

The addition of diboranes to internal alkynes usually takes place on two atoms of the same molecule. The bidirectional splitting of the B−B bond observed in this case is rare. In order to understand the surprising reactions shown in eqs 3 and 4 and to know what is really going on, we decided to study the stoichiometric reactions of these alkynes with 1. Stoichiometric Reactions of 1 with 3-Hexyne and 4Octyne. Addition of 1.0 equiv of 3-hexyne to toluene solutions of 1 at 253 K leads to Rh{(Z)-C(Et)C(Et)Bpin}{κ3-P,O,P[xant(PiPr2)2]} (2) as a result of the insertion of the alkyne into the Rh−Bpin bond (Scheme 3). At room temperature, the Scheme 3. Rhodium-Mediated Stoichiometric Bis(borylation) of 3-Hexyne

Scheme 4. Formation of 4

β-boryl alkenyl group isomerizes to afford Rh{(E)-C(Et) C(Et)Bpin}{κ3-P,O,P-[xant(PiPr2)2]} (3). Both insertion and isomerization reactions are quantitative processes. As a consequence, complex 3 was isolated in a high yield of 90%. The presence of a β-borylalkenyl group in these compounds is strongly supported by the presence of a doublet (1JC−Rh ≈ 39 Hz) of triplets (2JC−P ≈ 11 Hz), at 190.5 ppm for 2 and 197.9 ppm for 3, in the 13C{1H} NMR spectra and a broad resonance, at 32.4 ppm for 2 and 30.6 ppm for 3, in the 11B NMR spectra, corresponding to the Rh−C carbon atom and the Bpin substituent of the β-borylalkenyl group. The stereochemistry at the C−C double bond was inferred from the behavior of the CH2 resonances of the ethyl groups in the 1 1 H, H-NOESY NMR spectra (see Supporting Information). The 31P{1H} NMR spectra show a doublet (1JP−Rh ≈ 195 Hz) between 30 and 33 ppm for the equivalent PiPr2 groups of the diphosphine. Complex 3 undergoes a second isomerization, in pentane, at 50 °C. In these conditions, it is quantitatively transformed, after 24 h, into the allyl isomer Rh{η3-CH2CHCHCH(Bpin)Et}{κ2-P,P-[xant(PiPr2)2]} (4). In agreement with the formation of the allyl moiety, the 1H NMR spectrum of the new compound, in benzene-d6, at 298 K contains four characteristic allyl signals at 4.90 (Hc), 4.00 (Hi), 3.28 (Ht), and 1.83 (Ht) ppm, which fit with 13C-resonances at 93.1 (Cc), 66.6 (Ci), and 42.9 (Ct) ppm in the 1H,13C-HSQC spectrum. The η3-coordination of the allyl group induces a change in the coordination mode of the diphosphine, which happens to act as a κ2-P,P. This is strongly supported by the 31P{1H} NMR spectrum, at 193 K, in toluene-d8. Under these conditions, it

summarizes the process. The C−H bond activation of the methyl group of the ethyl substituent at the Cα atom of the βborylalkenyl ligand of 3 should afford intermediate A, which could evolve into B by migration of the hydride ligand from the metal center to Cα. Then, the subsequent β-hydrogen elimination should lead to hydride C, bearing 4-pinalcolboryl(3E)-1,3-hexadiene η2-coordinated through the terminal C−C double bond. Thus, the coordination of the internal C−C double bond of the generated diene followed by the migration of the hydride ligand from the rhodium atom to the boronsubstituted carbon atom in the resulting intermediate D would give the allyl complex 4. Under catalytic conditions, the displacement of the coordinated C−C double bond of C by an alkyne molecule yields 4-pinalcolboryl-(3E)-1,3-hexadiene, whereas the generated hydride-π-alkyne species leads to the hydroborylation products (vide infra). The behavior of 4-octyne is similar to that of 3-hexyne, with some peculiarities due to the presence of an additional CH2 group in the alkyl substituents (Scheme 5). The C−C triple bond inserts into the Rh−B bond of 1 to initially afford the Zβ-borylalkenyl derivative Rh{(Z)-C(Pr)C(Pr)Bpin}{κ3P,O,P-[xant(PiPr2)2]} (5), which evolves into the E-isomer Rh{(E)−C(Et)C(Et)Bpin}{κ3-P,O,P-[xant(PiPr2)2]} (6), in pentane, at room temperature. In agreement with the C


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Organometallics Scheme 5. Rhodium-Mediated Stoichiometric Bis(borylation) of 4-Octyne

Scheme 6. Formation of 7

ethyl counterpart, at 50 °C, complex 6 is quantitatively transformed into the allyl derivative Rh{η3CH2CHCHCH2CH(Bpin)Pr}{κ 2-P,P-[xant(PiPr2) 2]} (7), which reacts with B2pin2 to give 1,5-dipinacolboryl-(E)-2octene and 1, closing a cycle for the stoichiometric rhodiummediated 1,5-addition of the diborane to the alkyne. Diene isomers 2E,4E- and 2Z,4E-5-pinacolboryl-2,4-octadiene obtained in the catalysis are intermediate stages in the way of the allyl complex 7. Therefore, its formation in the absence of alkyne is consistent with the catalytic results. Complex 7 is generated in a similar manner to 4 through intermediates E−K (Scheme 6). The key intermediate for the formation of the dienes is F. This species can exist as a mixture of conformers Fa and Fb, resulting from the free rotation of the alkenyl moiety around the RhCH−CH2 bond. They would generate Ga and Gb, respectively, containing the diolefin η2coordinated to the rhodium atom. As in C, under catalytic conditions, the alkyne reaches the dienes and the resulting hydride-π-alkyne species works to form 4-pinacolboryl-4octene. Stoichiometric Reactions of RhH{κ3-P,O,P-[xanti (P Pr2)2]} with 3-Hexyne and 4-Octyne. According to Schemes 4 and 6, in toluene, the allyl complexes 4 and 7 should be in equilibrium with spectroscopically undetectable amounts of the corresponding hydride-Rh(η2-boryldiene) derivatives C and G. In order to confirm this, we added 1.0 equiv of 3-hexyne to a toluene-d8 solution of 4 contained in an NMR tube. As expected, at 50 °C, 4-pinacolboryl-(3E)-1,3hexadiene was reached and a sequence of three new organometallic compounds was observed. Because they should be a consequence of the interaction of the alkyne with the hydride ligand resulting from the hydrogen β-elimination reaction in intermediate B, we decided to study the reactions of the previously reported monohydride RhH{κ3-P,O,P[xant(PiPr2)2]} (8)16b with 3-hexyne and 4-octyne (Scheme 7). Treatment of pentane solutions of 8 with 1.0 equiv of the alkynes, at room temperature, instantaneously leads to Rh{(Z)C(R)C(R)H}{κ3-P,O,P-[xant(PiPr2)2]} (R = Et (9), Pr (10)). These compounds, which were isolated as red solids in about 80% yield, are the result of the syn-addition of the Rh− H bond of 8 to the C−C triple bond of the respective alkyne.

Similarly to the β-borylalkenyl groups of 2 and 5, the alkenyl ligands of 9 and 10 undergo Z−E isomerization, in pentane, at room temperature to give Rh{(E)-C(R)C(R)H}{κ3-P,O,P[xant(PiPr2)2]} (R = Et (11), Pr (12)). The isomerization is slower than in the β-borylalkenyl counterparts and is favored by the size of the R substituent at the C−C double bond. Thus, after 1 day, Z/E molar ratios of 50:50 (R = Et) and 30:70 (R = Pr) were reached. Isomers 9 and 11 (R = Et) co-crystallized in a saturated pentane solution to afford crystals suitable for X-ray diffraction analysis. Figure 1a shows the structure of the Z isomer, complex 9, whereas Figure 1b shows the structure of the E isomer, complex 11. The geometry around the rhodium atom of both complexes can be rationalized as square-planar with the alkenyl ligand disposed trans to the oxygen atom of the diphosphine and O−Rh−C angles of 172.34(12)° (Z) and 161.0(3)° (E). The Rh−C bond lengths of 2.026(4) (Z) and 2.027(8) (E) Å compare well with those reported for other rhodium−alkenyl derivatives.16n,18 In agreement with the X-ray diffraction analysis structures, the 13C{1H} NMR spectra of 9− 12, in toluene-d8, show doublets (1JC−Rh ≈ 40 Hz) of triplets (2JC−P ≈ 12 Hz) at about 153 ppm for the Z isomers and about 155 ppm for the E isomers, corresponding to the RhC−carbon atoms. These resonances appear shifted about 40 ppm toward higher field with regard to those of the β-borylalkenyl analogues, suggesting that the replacement of the boryl substituent by a hydrogen atom weakens the Rh−C bond while reinforces the C−C double bond, which is consistent with the observed decrease of the isomerization rate. The D


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Organometallics Scheme 7. Stoichiometric Reactions of 8 with 3-Hexyne and 4-Octyne

Figure 2. Molecular diagram of complex 14 (ellipsoids shown at 50% probability). All hydrogen atoms (except those of the allyl moiety) are omitted for clarity. Selected bond distances (Å) and angles (deg): Rh−P(1) = 2.3255(8), Rh−P(2) = 2.3004(8), Rh−C(1) = 2.176(3), Rh−C(2) = 2.109(3), Rh−C(3) = 2.190(4), C(1)−C(2) = 1.405(5), and C(2)−C(3) = 1.411(5); P(1)−Rh−P(2) = 112.27(3), C(1)− Rh−C(3) = 67.56(13), and C(1)−C(2)−C(3) = 119.1(3).

C(1)−Rh−C(3) angle of 67.53(13)° and the pentyl substituent in antiposition with regard to the meso carbon atom C(2). In agreement with other rhodium−allyl compounds,19 the coordination of the C3-skeleton is asymmetric. The separation between the central carbon atom C(2) and the metal center (2.109(3) Å) is shorter than the distances between the metal center and the terminal (2.176(3) Å) and internal (2.190(4) Å) carbon atoms C(1) and C(3). The carbon−carbon bond lengths within the allyl skeleton are 1.405(5) Å for C(1)−C(2) and 1.411(5) Å for C(2)−C(3). The 1H, 13C{1H}, and 31P{1H} NMR spectra for both 13 and 14 are consistent with the structure shown in Figure 2 and agree well with those of 4 and 7, bearing a pinacolboryl substituted allyl ligand. The resonances corresponding to the hydrogen atoms of the C3-skeleton appear at about 4.9 (Hc), 4.0 (Hi), 3.3, and 1.8 (Ht) ppm in the 1H NMR spectra. These signals fit with 13C-resonances at about 93 (Cc), 67 (Ci), and 43 (Ct) ppm in the 1H,13C-HSQC spectra. At 193 K, in toluene-d8, the 31P{1H} NMR spectra show two doublets (1JP−Rh = 199−190 Hz) of doublets (2JP−P ≈ 19 Hz) at 33.3 and 30.3 ppm for 13 and at 28.2 and 25.1 ppm for 14. Allyl complexes 13 and 14 show a similar behavior to 4 and 7 in the presence of B2pin2. The addition of 1.5 equiv of the latter to toluene solutions of 13 and 14 at room temperature affords 1-pinacolboryl-2-hexene and 1-pinacolboryl-2-octene, respectively, along with 1. Stoichiometric reactions summarized in Scheme 7 do not lead to the catalytic hydroborylation products. The main difference is centered on the position of the boryl substituent in the resulting monoolefin, terminal for the stoichiometric processes and internal for the catalytic one. Because the internal substituted carbon atom in the olefin coincides with one of the carbon atoms of the triple bond in the starting alkyne and both alkynes give Z and E isomers of the olefin, one can conclude that the catalytic hydroborylation products result

Figure 1. Molecular diagram of complexes 9 (a) and 11 (b) (ellipsoids shown at 50% probability). All hydrogen atoms (except that of the alkenyl moieties) are omitted for clarity. Selected bond distances (Å) and angles (deg): Rh−P(1) = 2.2425(6), Rh−P(2) = 2.2402(7), Rh−O(1) = 2.2369(16), and P(1)−Rh−P(2) = 163.21(2). For 9: Rh−C(1A) = 2.026(4), C(1A)−C(2A) = 1.348(6); O(1)−Rh−C(1A) = 172.34(12). For 11: Rh−C(1B) = 2.027(8), C(1B)−C(2B) = 1.339(8); O(1)−Rh−C(1B) = 161.0(3).

equivalent PiPr2 groups display a doublet (1JP−Rh = 188−184 Hz) between 35 and 32 ppm, in the 31P{1H} NMR spectra. Complexes 11 and 12 undergo a subsequent isomerization into the terminal allyl derivatives Rh(η3-CH2CHCHR){κ2-P,P[xant(PiPr2)2]} (R = Pr (13), Pent (14)), in a similar manner to their β-borylalkenyl counterparts. In pentane, at 70 °C, the transformation is quantitative after 12 h. Complex 14 was characterized by X-ray diffraction analysis. The structure (Figure 2) proves the formation of the allyl group and confirms the change in the coordination mode of the diphosphine, which happens to act as bidentated with a P− Rh−P bite angle of 112.27(3)°. The allyl group coordinates to the rhodium atom to form a square-planar environment with a E


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Organometallics

hydride−rhodium(I)-(η2-diene) intermediates evolve into the allyl derivatives 4 and 7, which afford the bisborylated products 1,4-dipinacolboryl-(E)-2-hexene and 1,5-dipinacolboryl-E-2octene, respectively, by reaction with the diborane. The absence of these olefins in the catalytic processes also indicates that the displacement of the dienes by the alkynes is faster than the reactions of the allyl complexes 4 and 7 with the diborane. The hydride−rhodium(I)-(η2-alkyne) intermediates evolve by means of the migratory insertion of the C−C triple bond of the alkyne into the Rh−H bond, to give Z-alkenyl species (complexes 9 and 10). In contrast to the β-borylalkenyl counterparts, these compounds can be isolated as solids in almost quantitative yield because their transformation into E isomers is slow at room temperature (complexes 11 and 12). Both Z- and E-alkenyl compounds react with diborane to give the Z and E isomers of the hydroborylation products 3pinacolboryl-3-hexene and 4-pinacolboryl-4-octene and to regenerate the catalyst, complex 1. The formation of both isomers of each olefin indicates that in both cases, the rate of the Z−E isomerization is similar to the rate of the reactions of the alkenyl isomers with the diborane. In the absence of the latter, the metal center of the E-alkenyl complexes is also able to promote the C−H bond activation of a Cγ−H bond of the alkyl substituent attached at the alkenyl Cα atom to generate the allyl derivatives 13 and 14, which leads to the internal olefins, boryl substituted at a terminal position, 1-pinacolboryl2-hexene and 1-pinacolboryl-2-octene. Because this C−H bond activation is slower than the reactions of the alkenyl complexes with the diborane, these olefins are not observed under catalytic conditions.

from the reactions of alkenyl complexes 10−12 with the diborane and therefore that the C−H bond activation reactions of the γ-CH2 group of the alkenyl substituents at the Cα atom, which are responsible of the formation of the olefins substituted at a terminal position via the allyl derivatives, are slower than the reactions of the alkenyl species with the diborane. Catalytic Cycle. Scheme 8 shows cycles that rationalize the formation of the catalysis products and introduces the results Scheme 8. Catalytic Cycle

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CONCLUDING REMARKS This study points out that the presence of a boryl substituent at the Cβ atom of an alkenyl ligand coordinated to rhodium has a surprising marked influence on the reactivity of the metal center, proves that the square-planar rhodium(I) complexes Rh(Bpin){κ3-P,O,P-[xant(PiPr2)2]} and RhH{κ3-P,O,P-[xant(PiPr2)2]} collaborate to perform two borylation reactions, involving the addition of the same B−B bond to different molecules of the same alkyne through only one catalytic cycle, and reveals that the second is a consequence of the first. The triple bond of bis(alkyl)alkynes inserts into the Rh−B and Rh−H bonds of Rh(Bpin){κ3-P,O,P-[xant(PiPr2)2]} and RhH{κ3-P,O,P-[xant(PiPr2)2]} to give the corresponding alkenyl derivatives. The alkenyl group of these species undergoes Z−E isomerization, whereas the metal center reacts with diboranes to afford bis(boryl)- or borylolefins. The presence of a boryl substituent at the Cβ atom of the alkenyl group increases the rate of the isomerization and prevents the reaction of the metal center with the diborane. However, it favors the Cγ−H bond activation of the alkyl substituent attached to the Cα atom of the alkenyl group. These abilities of the boryl group make possible a sequence of events which defines a particular catalytic cycle involving reactions of dehydrogenative borylation and hydroborylation together. Complex Rh(Bpin){κ3-P,O,P-[xant(PiPr2)2]} promotes the dehydrogenative borylation of bis(alkyl)alkynes to give conjugated boryldienes by means of a sequence of reactions including insertion, Z−E isomerization, and C−H bond activation. As consequence of the formation of boryldienes, the monohydride RhH{κ3-P,O,P-[xant(PiPr2)2]} is generated. The latter carries out the hydroborylation of a second molecule of bis(alkyl)alkyne with a diborane to yield Z- and E-

of the stoichiometric studies within the catalytic context. Bis(alkyl)alkynes insert into the Rh−Bpin bond of the catalyst, complex 1, to give Z-β-borylalkenyl intermediates (complexes 2 and 5). The β-borylalkenyl group of these species undergoes a rapid Z−E isomerization, which leads to the corresponding E-β-boryl-alkenyl derivatives (complexes 3 and 6). The metal center of these compounds, which can be isolated as solids in almost quantitative yields, has the ability of activating a Cγ−H bond of the alkyl substituent attached to the Cα atom of the βborylalkenyl ligand. The hydride ligand of the resulting intermediates, A and E, migrates from the metal center to the coordinated alkenyl Cα atom. Thus, the subsequent βhydrogen elimination reaction on the β-CH2 group of the alkyl substituent in the generated species (intermediates B and F) gives rise to dienes coordinated to the metal center through a Cγ−Cβ double bond (intermediates C and G). Because free rotation around RhCγ−Cβ single bond is possible, isomers with Z and E stereochemistries at the coordinated Cγ−Cβ double bond are formed. The displacement of the coordinated dienes by a second alkyne molecule affords boryl-diolefin catalytic products and hydride−rhodium(I)-(η2-alkyne) species. The formation of the boryldiolefins 4-pinacol-(3E)-1,3hexadiene, 5-pinacolboryl-(2E,4E)-2,4-octadiene, and 5-pinacolboryl-(2Z,4E)-2,4-octadiene takes place because the Z−E isomerization of the β-borylalkenyl groups, which increases the rate as the steric hindrance of the alkyl substituents increases, and the C−H bond activation of a Cγ−H bond of the latter are faster than the reactions of 1 and the alkenyl complexes 2, 3, 5, and 6 with the diborane. In the absence of alkyne, the F


Article

Organometallics

2H, CH2), 1.78 (dd, 3JH−H = 6.5, 4JH−H = 1.7, 3H, CH3), 1.44−1.33 (m, 2H, CH2), 1.28 (12H, CH3 Bpin), 0.91−0.84 (m, 3H, CH3). The stereochemistry of the double bonds was determined by a NOESY experiment. 13C{1H}-apt plus HSQC and HMBC NMR (128.77 MHz, CDCl3, 298 K): δ 144.9 (CH), 131.7 (CH), 131.4 ( CH), 82.9 (C Bpin), 39.7 (CH2), 25.0 (CH3 Bpin), 23.6 (CH2), 18.2 (CH3), 14.0 (CH3). 11B NMR (96.29 MHz, CDCl3, 298 K): δ 30.5 (s). Spectroscopic Data of 5-Pinacolboryl-(2Z,4E)-octadiene. 1H NMR (500.13 MHz, CDCl3, 298 K): δ 6.87 (d, 3JH−H = 11.7,  CH), 6.73 (ddq, 1H, 3JH−H = 11.7, 3JH−H = 11.0, 4JH−H = 1.8, CH), 5.55 (dq, 1H, 3JH−H = 11.0, 3JH−H = 7.1, CH), 2.17 (t, 3JH−H = 7.3, 2H, CH2), 1.77 (dd, 3JH−H = 7.2, 4JH−H = 1.8, 3H, CH3), 1.44−1.33 (m, 2H, CH2), 1.27 (12H, CH3 Bpin), 0.91−0.84 (m, 3H, CH3). The stereochemistry of the double bonds was determined by a NOESY experiment. 13C{1H}-apt plus HSQC and HMBC NMR (128.77 MHz, CDCl3, 298 K): δ 138.7 (CH), 129.0 (CH), 127.5 ( CH), 83.1 (C Bpin), 39.5 (CH2), 24.5 (CH3 Bpin), 23.6 (CH2), 14.1 (CH3), 13.1 (CHCH3). 11B NMR (96.29 MHz, CDCl3, 298 K): δ 30.5 (s). Spectroscopic Data of 4-Pinacolboryl-(E)-4-octene.23 1H NMR (500.13 MHz, CDCl3, 298 K): δ 5.97 (t, 3JH−H = 7.6, 1H, CH), 2.27 (app q, 3JH−H = 7.6, 2H, CH2), 2.07 (t, 3JH−H = 7.5, 2H, CH2), 1.44−1.33 (m, 4H, CH2), 1.26 (s, 12H, CH3 Bpin), 0.91−0.84 (m, 6H, CH3). 13C{1H}-apt plus HSQC and HMBC NMR (125.77 MHz, CDCl3, 298 K): δ 145.1 (CH), 83.1 (C Bpin), 39.2 (CH2), 33.3 (CH2), 24.9 (CH3 Bpin), 23.6 (CH2), 14.0 (CH3), 13.8 (CH3). 11B NMR (96.29 MHz, CDCl3, 298 K): δ 30.5 (s). Spectroscopic Data of 4-Pinacolboryl-(Z)-4-octene.23 1H NMR (500.13 MHz, CDCl3, 298 K): δ 6.29 (t, 3JH−H = 7.1, 1H, CH), 2.12−2.08 (m, 4H, 2CH2), 1.44−1.33 (m, 4H, 2CH2), 1.25 (s, 12H, CH3 Bpin), 0.91−0.84 (m, 6H, 2CH3). 13C{1H}-apt plus HSQC and HMBC NMR (125.77 MHz, CDCl3, 298 K): δ 146.3 (CH), 83.4 (C Bpin), 39.3 (CH2), 33.3 (CH2), 24.9 (CH3 Bpin), 23.4 (CH2), 14.2 (CH3). 11B NMR (96.29 MHz, CDCl3, 298 K): δ 30.5 (br). Reaction of Rh(Bpin){κ3-P,O,P-[xant(PiPr2)2]} (1) with 3Hexyne at Low Temperature: Spectroscopic Detection of Rh{(Z)−C(Et)C(Et)Bpin}{κ3-P,O,P-[xant(PiPr2)2]} (2). A screw top NMR tube containing a solution of 1 (20 mg, 0.03 mmol) in toluene-d8 (0.5 mL) and cooled at 195 K was treated with 3-hexyne (3.4 μL, 0.15 mmol). The NMR tube was immediately introduced into an NMR probe cooled at 253 K. The immediate and quantitative conversion of 1 to 2, together with a small amount of Rh{(E)− C(Et)C(Et)Bpin}{κ3-P,O,P-[xant(PiPr2)2]} (3, 13%), was observed by 1H and 31P{1H} NMR spectroscopies. 1H NMR (400.13 MHz, C7D8, 253 K): δ 7.17 (m, 2H, CH-arom), 6.90 (d, 3JH−H = 7.5, 2H, CH-arom), 6.84 (t, 3JH−H = 7.5, 2H, CH-arom), 3.50 (q, 3JH−H = 7.4, 2H, Rh−C(CH2CH3)), 2.83 (q, 3JH−H = 7.1, 2H C(Bpin)(CH2CH3)), 2.63 (m, 2H, PCH(CH3)2), 2.52 (m, 2H, PCH(CH3)2), 1.45 (m, 24H, 6H, CH2CH3 + 18H PCH(CH3)2), 1.23 (s, 12H, CH3 Bpin), 1.13 (m, 12H, 6H PCH(CH3)2 + 6H CH3 POP). 13C{1H}-apt plus HSQC and HMBC NMR (100.62 MHz, C7D8, 253 K): δ 190.5 (dt, 1JC−Rh = 38.6, 2JC−P = 11.8, Rh−C), 155.5 (vt, N = 14.9, Carom POP), 131.4 (s, CH-arom POP), 130.6 (vt, N = 4.9, C-arom POP), 127.8 (s, CH-arom POP), 127.4 (resonance inferred from the HMBC spectrum, C-Bpin), 125.5 (vt, N = 15, C-arom POP), 123.8 (s, CH-arom POP), 81.7 (s, C Bpin), 36.5 (s, CH2), 35.1 (s, C(CH3)2), 34.1 (s, C(CH3)2), 31.4 (s, C(CH3)2), 26.3 (dvt, 2JC−Rh = 2.9, N = 14.9, PCH(CH3)2), 26.2 (s, C(Bpin)(CH2CH3)), 25.7 (s, CH3 Bpin), 25.5 (dvt, 2JC−Rh = 2.5, N = 16.2, PCH(CH3)2), 20.3 (vt, N = 6.9, PCH(CH3)2), 20.0 (vt, N = 8.8, PCH(CH3)2), 19.4 (s, PCH(CH3)2), 18.1 (s, PCH(CH3)2), 17.9 and 16.7 (both s,  C(CH2CH3)). 31P{1H} NMR (161.98 MHz, C7D8, 253 K): δ 29.7 (d, 1 JP−Rh = 195.9). 11B{1H} NMR (128.38 MHz, C7D8, 253 K): δ 32.4 (br). Reaction of Rh(Bpin){κ3-P,O,P-[xant(PiPr2)2]} (1) with 3Hexyne at Room Temperature: Preparation of Rh{(E)− C(Et)C(Et)Bpin}{κ3-P,O,P-[xant(PiPr2)2]} (3). 3-Hexyne (17 μL, 0.15 mmol) was added to a solution of 1 (100 mg, 0.15 mmol) in pentane (5 mL) and the resulting mixture was stirred for 30 min at

borylolefins and regenerates the initial boryl complex, closing the cycle. In summary, a metal-mediated dehydrogenative borylation− hydroborylation of bis(alkyl)alkynes, CH3(CH2)xCC(CH2)xCH3, has been developed and rationalized for x > 0.

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EXPERIMENTAL SECTION

General Information. All reactions were carried out with exclusion of air using Schlenk tube techniques or in a drybox. Instrumental methods and X-ray details are given in the Supporting Information. In the NMR spectra, the chemical shifts (in ppm) are referenced to residual solvent peaks (1H, 13C{1H}), external 85% H3PO4 (31P{1H}), or BF3·OEt2 (11B{1H}). Coupling constants J and N are given in hertz. Rh(Bpin){κ3-P,O,P-[xant(PiPr2)2]} (1)16g and RhH{κ3-P,O,P-[xant(PiPr2)2]} (8)16b were prepared by the published methods. Borylation of 3-Hexyne with B2pin2. In an argon-filled glovebox, an Ace pressure tube was charged with Rh(Bpin){κ3P,O,P-[xant(PiPr2)2]} (1, 20 mg, 0.03 mmol), B2pin2 (153 mg, 0.6 mmol), 3-hexyne (68 μL, 0.6 mmol), and 5 mL of n-octane. The resulting mixture was stirred at 70 °C for 20 h. After this time, the solvent was evaporated under reduced pressure to afford an oil. After adding 1,2-dichloroethane as an internal standard, it was dissolved in CDCl3, and the yield of the reaction determined by 1H NMR. The 1H NMR spectrum shows a mixture of 4-pinacolboryl-(3E)-1,3-hexadiene (39%), 3-pinacolboryl-(E)-3-hexene (27%), and 3-pinacolboryl-(Z)3-hexene (13%). Afterward, the mixture was purified by passing it through a short pad of silica gel, using n-hexane as the eluent. Spectroscopic Data for 4-Pinacolboryl-(3E)-1,3-hexadiene. 1H NMR (500.13 MHz, CDCl3, 298 K): δ 7.12 (ddd, 3JH−H = 16.9, 3JH−H = 11.0, 3JH−H = 10.1, 1H, CH), 6.56 (d, 3JH−H = 11.0, 1H, CH), 5.21 (dd, 3JH−H = 16.9, 2JH−Hgem = 2.2, 1H, CH2), 5.14 (dd, 3JH−H = 10.1, 2JH−Hgem = 2.2, 1H, CH2), 2.19 (q, 3JH−H = 7.5, 2H, CH2), 1.29 (s, 12H, CH3 Bpin), 1.01 (t, 3JH−H = 7.5, 1H, CH3). 13C{1H}-apt plus HSQC and HMBC NMR (75.478 MHz, CDCl3, 298 K): δ 144.0 (CH), 137.0 (CH), 118.5 (CH2), 83.3 (C Bpin), 30.0 (CH2), 25.0 (CH3 Bpin), 14.7 (CH3). 11B NMR (96.29 MHz, CDCl3, 298 K): δ 32.9 (s). Spectroscopic Data of 3-Pinacolboryl-(E)-3-hexene.20 1H NMR (500.13 MHz, CDCl3, 298 K): δ 5.99 (t, 3JH−H = 7.5, 1H, CH), 2.63 (app q, 3JH−H = 7.5, 2H, CH2), 2.30 (m, 2H, CH2), 1.27 (s, 12H, CH3 Bpin), 0.99−0.93 (m, 6H, CH3). 13C{1H}-apt plus HSQC and HMBC NMR (125.77 MHz, CDCl3, 298 K): δ 146.9 (CH), 83.2 (C Bpin), 25.0 (CH3 Bpin), 24.6 (CH2), 23.3 (CH2), 15.1 (CH3), 14.8 (CH3). 11B NMR (96.29 MHz, CDCl3, 298 K): δ 32.0 (s). Spectroscopic Data of 3-Pinacolboryl-(Z)-3-hexene.20,21 1H NMR (500.13 MHz, CDCl3, 298 K): δ 6.20 (t, 3JH−H = 7.2, 1H, CH), 2.17−2.06 (m, 4H, 2CH2), 1.26 (12H, CH3 Bpin), 0.99− 0.93 (m, 6H, CH3). 13C{1H}-apt plus HSQC and HMBC NMR (125.77 MHz, CDCl3, 298 K): δ 147.1 (CH), 83.1 (C Bpin), 24.9 (CH3 Bpin), 19.1 (CH2), 18.7 (CH2), 15.9 (CH3), 15.0 (CH3). 11B NMR (96.29 MHz, CDCl3, 298 K): δ 30.5 (s). Borylation of 4-Octyne with B2pin2. In an argon-filled glovebox, an Ace pressure tube was charged with Rh(Bpin){κ3P,O,P-[xant(PiPr2)2]} (1, 20 mg, 0.03 mmol), B2pin2 (153 mg, 0.6 mmol), 4-octyne (88 μL, 0.6 mmol), and 5 mL of n-octane. The resulting mixture was stirred at 70 °C for 12 h. After this, the solvent was evaporated under reduced pressure to afford an oil. After adding 1,2-dichloroethane as an internal standard, it was dissolved in CDCl3, and the yield of the reaction was determined by 1H NMR. The 1H NMR spectrum shows a mixture of 5-pinacolboryl-(2E,4E)-octadiene (33%), 5-pinacolboryl-(2Z,4E)-octadiene (12%), 4-pinacolboryl-(E)4-octene (33.5%), and 4-pinacolboryl-(Z)-4-octene (12.5%). Afterward, the mixture was purified by passing it through a short pad of silica gel, using n-hexane as the eluent. Spectroscopic Data of 5-Pinacolboryl-(2E,4E)-octadiene.22 1H NMR (500.13 MHz, CDCl3, 298 K): δ 6.78 (ddq, 3JH−H = 14.4, 3JH−H = 11.0, 4JH−H = 1.7, 1H, CH), 6.51 (d, 3JH−H = 11.0, 1H, CH), 5.70 (dq, 3JH−H = 14.4, 3JH−H = 6.5, 1H, CH), 2.11 (t, 3JH−H = 7.5, G


Article

Organometallics

formation of 1,4-dipinacolboryl-(E)-2-hexene were observed by NMR. Spectroscopic Data for 1,4-Dipinacolboryl-(E)-2-hexene. 1H NMR (300.13 MHz, C6D6, 298 K): δ 5.73 (m, 1H, CH), 5.49 (m, 1H, CH), 1.97 (dt, 1H, 3JH−H = 6.8, 3JH−H = 6.3, CHBpin), 1.88 (d, 2H, 3JH−H = 7.3, CH2Bpin), 1.04 (s, 12H, CH3 Bpin), 1.02 (s, 12H, CH3 Bpin), 0.85 (t, 3JH−H = 7.3, 3H, CH3) (one of the CH2 resonances is masked by those of 1). 13C{1H}-apt plus HSQC (75.48 MHz, C6D6, 298 K): δ 130.7 (CH), 126.0 (CH), 83.0 (C Bpin), 35.3 (CH2Bpin), 25.2 (CHBpin), 24.9 (CH3 Bpin), 24.7 (CH3 Bpin), 23.2 (CH2), 13.8 (CH3). 11B NMR (96.29 MHz, C6D6, 298 K): δ 32.9 (s). Reaction of Rh(Bpin){κ3-P,O,P-[xant(PiPr2)2]} (1) with 4Octyne at Low Temperature: Spectroscopic Detection of Rh{(Z)-C(Pr)C(Pr)Bpin}{κ3-P,O,P-[xant(PiPr2)2]} (5). A screw top NMR tube containing a solution of 1 (20 mg, 0.03 mmol) in toluened8 (0.5 mL) and cooled at 195 K was treated with 4-octyne (4.4 μL, 0.15 mmol). The NMR tube was immediately introduced into an NMR probe cooled at 253 K. The immediate and quantitative conversion of 1 to 5, together with a small amount of Rh{(E)C(Pr)C(Pr)Bpin}{κ3-P,O,P-[xant(PiPr2)2]} (6, 6%), was observed by 1H and 31P{1H} NMR spectroscopies. 1H NMR (400.13 MHz, C7D8, 253 K): δ 7.20 (m, 2H, CH-arom POP), 6.95 (d, 3JH−H = 7.6, 2H, CH-arom POP), 6.86 (t, 3JH−H = 7.5, 2H, CH-arom POP), 3.42 (m, 2H, CH2CH2CH3), 2.67 (m, 4H, 2H C(Bpin)(CH2CH2CH3) + 2H PCH(CH3)2), 2.53 (m, 2H, PCH(CH3)2), 1.96 (m, 2H, CH2CH2CH3), 1.85 (m, 2H, C(Bpin)(CH2CH2CH3)), 1.41 (m, 18H, PCH(CH3 )2 ), 1.27 (t, 3 JH−H = 7.3, 3H, C(Bpin)(CH2CH2CH3)), 1.26 (t, 3JH−H = 7.4, 3H, C(Rh)(CH2CH2CH3)), 1.21 (s, 12H, CH3 Bpin), 1.18 (s, 3H, CH3 POP), 1.16 (s, 3H, CH3 POP), 1.14 (dvt, 3JH−H = 6.2, N = 12.3, 6H, PCH(CH3)2). 13C{1H}apt plus HSQC and HMBC NMR (100.62 MHz, C7D8, 253 K): δ 187.9 (dt, 1JC−Rh = 39.3, 2JC−P = 11.7, Rh−C), 155.5 (vt, N = 14.8, C-arom POP), 131.4 (s, CH-arom POP), 131.4 (s, CH-arom POP), 130.8 (vt, N = 4.7, C-arom POP), 127.6 (s, CH-arom POP), 126.0 (resonance inferred from the HMBC spectrum, C-Bpin), 125.7 (vt, N = 15.2, C-arom POP), 123.8 (s, CH-arom POP), 81.7 (s, C Bpin), 47.2 (s, RhC(CH2CH2CH3)), 36.5 (s, C(Bpin)(CH2CH2CH3)), 34.8 (s, C(CH3)2), 34.2, 31.5 (both s, C(CH3)2), 26.6 (s,  C(Bpin)(CH2CH2CH3)), 26.4 (dvt, 2JC−Rh = 2.7, N = 15.0, PCH(CH3)2), 25.8 (s, CH3 Bpin), 25.6 (dvt, 2JC−Rh = 3.2, N = 17.1, PCH(CH3)2), 25.2 (s, C(Rh)(CH2CH2CH3)), 20.3 (vt, N = 7.5, PCH(CH3)2), 20.0 (vt, N = 9.3, PCH(CH3)2), 19.3, 18.0 (both s, PCH(CH3)2), 16.5, 16.1 (both s, C(CH2CH2CH3)). 31P{1H} NMR (161.98 MHz, C7D8, 253 K): δ 29.6 (d, 1JP−Rh = 197.4). 11B NMR (96.29 MHz, C7D8, 267 K): δ 33.3 (br). Reaction of Rh(Bpin){κ3-P,O,P-[xant(PiPr2)2]} (1) with 4Octyne at Room Temperature: Preparation of Rh{(E)-C(Pr) C(Pr)Bpin}{κ3-P,O,P-[xant(PiPr2)2]} (6). 4-Octyne (22 μL, 0.15 mmol) was added to a solution of 1 (100 mg, 0.15 mmol) in pentane (5 mL), and the resulting mixture was stirred for 30 min at room temperature. After this, it was concentrated to dryness to afford a red residue. This residue was washed with the minimum amount of pentane (3 × 0.5 mL) to afford a red solid that was dried in vacuo. Yield: 81 mg (69%). Anal. Calcd for C41H66BO3P2Rh: C, 62.92; H, 8.50. Found: C, 63.39; H, 8.06. HRMS (electrospray, m/z): calcd for C41H66BO3P2Rh [M − H]+, 781.3559; found: 781.3681. IR (cm−1): ν(CC) 1538 (w), ν(C−O−C) 1031 (m). 1H NMR (300.13 MHz, C6D6, 298 K): δ 7.23 (m, 2H, CH-arom POP), 7.03 (dd, JH−H = 7.7, JH−P = 1.6, 2H, CH-arom POP), 6.83 (t, JH−H = 7.6, 2H, CH-arom POP), 3.55 (m, 2H, C(CH2CH2CH3)), 3.46 (m, 2H,  C(CH2CH2CH3)), 2.56 (m, 4H, PCH(CH3)2), 2.17 (m, 4H,  C(CH 2CH2CH 3)), 1.45 (dvt, 3JH−H = 7.7, N = 15.4, 6H, PCH(CH3)2), 1.42 (dvt, 3JH−H = 7.7, N = 14.6, 6H, PCH(CH3)2), 1.41 (m, 3H, C(CH 2 CH 2 CH 3 )), 1.30 (m, 3H, C(CH2CH2CH3)), 1.29 (dvt, 3JH−H = 7.5, N = 14.6, 6H, PCH(CH3)2), 1.24 (s, 12H, CH3 Bpin), 1.19 (s, 6H, CH3 POP), 1.13 (dvt, 3JH−H = 6.4, N = 12.5, 6H, PCH(CH3)2). 13C{1H}-apt plus HSQC and HMBC NMR (75.48 MHz, C6D6, 298 K): δ 195.8 (dt, 1JC−Rh = 39.2, 2 JC−P = 10.9, Rh−C), 155.2 (vt, N = 14.6, C-arom POP), 131.2 (s,

room temperature. After this, it was concentrated to dryness to afford a red residue. This residue was washed with the minimum amount of pentane (3 × 0.5 mL) to afford a red solid that was dried in vacuo. Yield: 100 mg (89%). Anal. Calcd for C39H62BO3P2Rh: C, 62.08; H, 8.28. Found: C, 61.89; H, 7.93. HRMS (electrospray, m/z): calcd for C39H62BO3P2Rh [M]+, 753.3245; found, 753.3340. IR (cm−1): ν(C C) 1509 (m), ν(C−O−C) 1021 (m). 1H NMR (300.13 MHz, C6D6, 298 K): δ 7.22 (m, 2H, CH-arom), 7.02 (d, 3JH−H = 7.7, 2H, CHarom), 6.83 (t, 3JH−H = 7.5, 2H, CH-arom), 3.65 (q, 3JH−H = 7.5, 2H, CH2CH3), 3.61 (q, 3JH−H = 7.2, 2H, CH2CH3), 2.57 (m, 4H, PCH(CH3)2), 1.76 (t, 3JH−H = 7.5, 3H, CH2CH3), 1.71 (t, 3JH−H = 7.5, 3H, CH2CH3), 1.44 (dvt, 3JH−H = 6.9, N = 13.9, 6H, PCH(CH3)2), 1.42 (dvt, 3JH−H = 7.2, N = 12.8, 6H, PCH(CH3)2), 1.27 (dvt, 3JH−H = 7.4, N = 14.5, 6H, PCH(CH3)2), 1.26 (s, 12H, CH3 Bpin), 1.20 (s, 6H, CH3), 1.12 (dvt, 3JH−H = 6.0, N = 11.8, 6H, PCH(CH3)2). 13C{1H}-apt plus HSQC and HMBC NMR (75.48 MHz, C6D6, 298 K): δ 197.9 (dt, 1JC−Rh = 39.0, 2JC−P = 10.9, Rh− C), 155.2 (vt, N = 14.5, C-arom POP), 131.2 (s, CH-arom POP), 130.6 (vt, N = 4.5, C-arom POP), 127.6 (resonance inferred from the HMBC spectrum, C−B), 127.4 (s, CH-arom POP), 125.1 (vt, N = 15.0, C-arom POP), 123.6 (s, CH-arom POP), 80.9 (s, C Bpin), 38.7, 36.8 (both s, C(CH2CH3)), 34.3 (s, C(CH3)2), 33.9 (s, C(CH3)2), 31.6 (s, C(CH3)2), 26.2 (dvt, 2JC−Rh = 2.5, N = 15.4, PCH(CH3)2), 25.2 (s, CH3 Bpin), 25.1 (dvt, 2JC−Rh = 3.5, N = 17.1, PCH(CH3)2), 20.1 (vt, N = 7.9, PCH(CH3)2), 19.1 (vt, N = 9.2, PCH(CH3)2), 19.0 (s, C(CH2CH3)), 18.4 (s, PCH(CH3)2), 17.5 (s, PCH(CH3)2), 16.5 (s, C(CH2CH3)). 31P{1H} NMR (121.49 MHz, C6D6, 298 K): δ 33.4 (d, 1JP−Rh = 193.3). 11B NMR (96.29 MHz, C7D8, 267 K): δ 30.6 (br). Isomerization of 3 to Rh{η3-CH2CHCHCH(Bpin)Et}{κ2-P,P[xant(PiPr2)2]} (4). A Schlenk flask provided with a Teflon closure was filled with a solution of 3 (100 mg, 0.13 mmol) in pentane (5 mL) and heated in an oil bath (50 °C) for 24 h. After this, the resulting solution was checked by 31P{1H} NMR spectroscopy, showing quantitative conversion to complex 4. The solution was then concentrated to dryness to afford a brown oil. After several cycles of addition of pentane/evaporation of pentane, a pale brown foamy solid was obtained. Yield: 43 mg (43%). IR (cm−1): ν(CC) 1461 (m), ν(C−O−C) 1076 (m). 1H NMR (300.13 MHz, C6D6, 298 K): δ 7.26 (m, 2H, CH-arom), 7.09 (m, 2H, CH-arom), 6.96 (m, 2H, CHarom), 4.90 (m, 1H, Hc allyl), 4.00 (br, 1H, Hi allyl), 3.28 (br, 1H, Ht allyl), 2.74 2.42 (both br, 2H each, PCH(CH3)2), 1.83 (m, 1H, Ht allyl), 1.62−1.00 (complex m, 45H, 24H PCH(CH3)2 + 12H Bpin + 6H CH3 POP + 3H allylic chain), 0.91 (t, 3JH−H = 6.8, 3H, CH3 allylic chain). 13C{1H}-apt plus HSQC and HMBC NMR (75.48 MHz, C6D6, 298 K): δ 160.3 (resonance inferred from the HMBC spectrum, C-arom), 137.6 (br, C-arom), 136.2 (br, C-arom), 129.0 (s, CH-arom), 128.9 (br, CH-arom), 124.5, 124.3, 123.4, 122.4 (all s, CH-arom), 93.1 (s, Cc allyl), 81.4 (s, C Bpin), 66.6 (d, JC−Rh = 31.2, Ci allyl), 42.9 (d, JC−Rh = 25.1, Ct allyl), 37.5 (s, C(CH3)2), 33.7 (s, CH2 allylic chain), 30.8 (d, JC−P = 20.5, PCH(CH3)2), 29.4 (d, JC−P = 16.2, PCH(CH3)2), 27.4 (d, JC−P = 17.0, PCH(CH3)2), 27.2 (resonance inferred from the HSQC spectrum, C(CH3)2), 26.8 (s, CH2 allylic chain), 24.2 (resonance inferred from the HSQC spectrum, C(CH3)2), 24.8 (s, CH3 Bpin), 21.4 (d, JC−P = 8.1, PCH(CH3)2), 21.3 (d, JC−P = 7.0, PCH(CH3)2), 21.1 (d, 2JC−P = 7.5, PCH(CH3)2), 20.7 (d, 2JC−P = 10.3, PCH(CH3)2), 20.6 (d, 2JC−P = 8.1, PCH(CH3)2), 19.5 (d, 2JC−P = 6.3, PCH(CH3)2), 14.3 (s, CH3 allylic chain). 31P{1H} NMR (121.49 MHz, C6D6, 298 K): δ 39.5 (br), 29.6 (br). 31P{1H} NMR (161.98 MHz, toluene-d8, 193 K): at this temperature, two sets of peaks are observed. They could correspond to two diastereoisomers. Minor: δ 41.4 (dd, 1JP−Rh = 189.2, 2JP−P = 24.3), 39.7 (dd, 1JP−Rh = 194.3, 2JP−P = 24.3); major: δ 28.1 (dd, 1JP−Rh = 190.3, 2JP−P = 19.1), 25.1 (dd, 1JP−Rh = 198.3, 2JP−P = 19.1). 11B NMR (128.38 MHz, C7D8, 193 K): δ 31.7 (br). Reaction of Complex 4 with B2pin2. B2pin2 (7.5 mg, 0.030 mmol) was added to a solution of 4 (15 mg, 0.020 mmol) in C6D6 (0.5 mL) and the resulting mixture was stirred for 15 minutes at room temperature. After this, the quantitative conversion to 1 and the H


Article

Organometallics

(electrospray, m/z): calcd for C33H51OP2Rh [M − H + O2]+, 661.2441; found: 661.2526. IR (cm−1): ν(CC) 1591 (w), ν(C− O−C) 1019 (m). 1H NMR (500.12 MHz, C6D6, 298 K): δ 7.32 (m, 2H, CH-arom), 7.06 (d, 3JH−H = 7.7, 2H, CH-arom), 6.88 (t, 3JH−H = 7.5, 2H, CH-arom), 5.50 (t, 3JH−H = 6.5, 1H, CCH), 2.84 (q, 3JH−H = 7.8, 2H, CH2CH3), 2.55 (m, 6H, 2H CH2CH3 + 4H PCH(CH3)2), 1.74 (t, 3JH−H = 7.5, 3H, CH2CH3), 1.44 (dvt, 3JH−H = 7.7, N = 15.5, 12H, PCH(CH3)2), 1.29 (t, 3JH−H = 7.5, 3H, CH2CH3), 1.23 (m, 18H, 12H PCH(CH3)2 + 6H CH3 POP). 13C{1H}-apt plus HSQC and HMBC NMR (75.48 MHz, C7D8, 253 K): δ 156.9 (vt, N = 15.5, C-arom POP), 153.9 (dt, 1JC−Rh = 40.8, 2JC−P = 12.7, Rh−C), 131.3 (vt, N = 5.0, C-arom POP), 130.7 (s, CH-arom POP), 126.3 (s, CHarom POP), 125.7 (dt, 2JC−Rh = 1.3, 3JC−P = 4.8, CCH), 124.9 (vt, N = 15.6, C-arom POP), 123.6 (s, CH-arom POP), 34.2 (s, C(CH3)2), 33.8 (s, C(CH2CH3)), 22.7 (s, C(CH2CH3)), 19.3 (br, PCH(CH3)2), 18.2 (s, C(CH2CH3)), 18.0 (br, PCH(CH3)2), 17.9 (br, PCH(CH3)2), 16.6 (s, C(CH2CH3)). 31P{1H} NMR (202.46 MHz, C6D6, 298 K): δ 32.5 (d, 1JP−Rh = 187.3). Reaction of RhH{κ3-P,O,P-[xant(PiPr2)2]} (8) with 4-Octyne: Preparation of Rh{(Z)-C(Pr)C(Pr)H}{κ3-P,O,P-[xant(PiPr2)2]} (10). 4-Octyne (27 μL, 0.18 mmol) was added to a solution of 8 (100 mg, 0.18 mmol) in pentane (5 mL). An instantaneous change of color from black to red was observed. Immediately, it was concentrated to dryness to afford a red residue. This residue was washed with a minimum amount of pentane (3 × 0.5 mL) to afford a red solid that was dried in vacuo. Yield: 93 mg (78%). Anal. Calcd for C35H55OP2Rh: C, 64.02; H, 8.44. Found: C, 63.98; H, 8.25. HRMS (electrospray, m/z): calcd for C35H55O3P2Rh [M + O2]+, 689.2754; found, 689.2856. IR (cm−1): ν(CC) 1600 (w), ν(C−O−C) 1099 (m). 1H NMR (500.13 MHz, C6D6, 298 K): δ 7.33 (m, 2H, CHarom), 7.06 (d, JH−H = 7.6, 2H, CH-arom), 6.88 (t, JH−H = 7.5, 2H, CH-arom), 5.54 (t, JH−H = 3.5, 1H, CH), 2.78 (m, 2H,  CCH2CH2CH3), 2.56 (m, 4H, PCH(CH3)2), 2.51 (m, 2H,  CCH2CH2CH3), 2.32 (m, 2H, CCH2CH2CH3), 1.70 (m, 2H,  CCH2CH2CH3), 1.45 (dvt, JH−H = 7.7, N = 15.5, 12H, PCH(CH3)2), 1.33 (t, 3JH−H = 7.3, 3H, CCH2CH2CH3), 1.24 (br, 18H, PCH(CH 3 )2 + CH3 POP), 1.18 (t, 3 JH−H = 7.4, 3H,  CCH2CH2CH3). 13C{1H}-apt plus HSQC and HMBC NMR (75.48 MHz, toluene-d8, 253 K): δ 157.5 (vt, N = 15.3, C-arom POP), 153.1 (dt, 1JC−Rh = 40.9, 2JC−P = 12.4, Rh−C), 131.9 (vt, N = 5.3, C-arom POP), 131.2 (s, CH-arom POP), 126.7 (s, CH-arom), 125.5 (vt, N = 15.3, C-arom POP), 124.7 (dt, 2JC−Rh = 1.4, 3JC−P = 4.9, CCH), 124.1 (s, CH-arom POP), 44.8 (s, CCH2CH2CH3), 34.7 (s, C(CH3)2), 32.9 (s, CCH2CH2CH3), 27.9 (s,  CCH2CH2CH3), 26.7 (br, PCH(CH3)2), 25.7 (s, CCH2CH2CH3), 23.4 (br, PCH(CH3)2), 18.8 (br, PCH(CH3)2), 16.8, 15.3 (both s,  CCH2CH2CH3). 31P{1H} NMR (121.49 MHz, C6D6, 298 K): δ 32.4 (d, 1JP−Rh = 188.0). Isomerization of 9 to Rh{(E)-C(Et)C(Et)H}{κ3-P,O,P-[xant(PiPr2)2]} (11). A solution of 9 (100 mg, 0.16 mmol) in pentane (5 mL) was stirred at room temperature for 3 days. After this, a red suspension was obtained. The red solid was decanted, washed with pentane (3 × 0.5 mL), and dried in vacuo. The 1H and 31P{1H} NMR spectra of this solid in benzene-d6 show a mixture of the Z/E isomers 9:11 in a ratio 10:90 ratio. Spectroscopic data of Rh{(E)-C(Et) C(Et)H}{κ3-P,O,P-[xant(PiPr2)2]} (11): HRMS (electrospray, m/z): calcd for C33H51OP2Rh [M]+, 629.2543; found: 629.2534. IR (cm−1): ν(CC) 1585 (w), ν(C−O−C) 1032 (m). 1H NMR (300.13 MHz, C6D6, 298 K): δ 7.26 (m, 2H, CH-arom), 7.04 (dd, JH−H = 7.7, JH−P = 1.6, 2H, CH-arom), 6.85 (t, 3JH−H = 7.6, 2H, CH-arom), 5.61 (m, 1H, CCH), 3.09 (q, 3JH−H = 7.3, 2H, C(H)(CH2CH3)), 3.00 (q, 3 JH−H = 7.3, 2H, CCH2CH3), 2.51 (m, 4H, PCH(CH3)2), 1.59 (t, 3 JH−H = 7.3, 3H, CH2CH3), 1.38−1.47 (m, 15H, 12H PCH(CH3)2 + 3H C(H)(CH2CH3)), 1.27 (dvt, 3JH−H = 7.1, N = 14.2, 6H, PCH(CH3)2), 1.25 (s, 3H, CH3), 1.23 (s, 3H, CH3), 1.16 (dvt, 3JH−H = 6.4, N = 12.7, 6H, PCH(CH3)2). 13C{1H}-apt plus HSQC and HMBC NMR (75.48 MHz, C6D6, 298 K): δ 155.5 (vt, N = 15.2, Carom POP), 155.4 (dt, 1JC−Rh = 40.3, 2JC−P = 12.2, Rh−C), 131.1 (s, CH-arom POP), 130.6 (vt, N = 5.0, C-arom POP), 127.5 (s, CHarom POP), 124.9 (dvt, JC−Rh = 1.6, N = 14.2, C-arom POP), 123.6

CH-arom POP), 130.6 (vt, N = 5.2, C-arom POP), 127.4 (s, CHarom POP), 125.0 (vt, N = 15.3, C-arom POP), 123.5 (s, CH-arom POP), 122.8 (this resonance is inferred from the HMBC, C-Bpin), 80.9 (s, C Bpin), 49.2 (s, C(CH2CH2CH3)), 47.4 (s,  C(CH2CH2CH3)), 34.4 (s, C(CH3)2), 33.9 (s, C(CH3)2), 31.5 (s, C(CH3)2), 28.0 (s, C(CH2CH2CH3)), 26.1 (dvt, 2JC−Rh = 2.6, N = 15.6, PCH(CH3)2), 25.2 (s, CH3 Bpin), 25.2 (m, PCH(CH3)2), 24.7 (s, C(CH2CH2CH3)), 20.0 (vt, N = 8.2, PCH(CH3)2), 19.3 (vt, N = 9.2, PCH(CH3)2), 18.3, 17.6 (both s, PCH(CH3)2), 15.9, 15.6 (both s, C(CH2CH2CH3)). 31P{1H} NMR (121.49 MHz, C6D6, 298 K): δ 33.4 (d, 1JP−Rh = 193.8). 11B NMR (96.29 MHz, C7D8, 267 K): δ 30.5 (br). Isomerization of 6 to Rh{η3-CH2CHCHCH(Bpin)Pr}{κ2-P,P[xant(PiPr2)2]} (7). A Schlenk flask provided with a Teflon closure was filled with a solution of 6 (100 mg, 0.13 mmol) in pentane (5 mL) and heated in an oil bath (50 °C) for 24 h. After this, the resulting solution was checked by 31P{1H} NMR spectroscopy, showing quantitative conversion to complex 7. The solution was concentrated then to dryness to afford a brown oil. All attempts to obtain a solid failed because of the high solubility of the complex. 1H NMR (300.13 MHz, C6D6, 298 K): δ 7.26 (m, 2H, CH-arom), 7.11 (t, JH−H = 6.5, 1H, CH-arom), 6.96 (dd, JH−H = 7.6, JH−P = 2.2, 1H, CH-arom), 6.93 (d, JH−H = 7.7, JH−P = 2.2, 1H, CH-arom), 4.90 (m, 1H, Hc allyl), 4.02 (m, 1H, Hi allyl), 3.28 (m, 1H, Ht allyl), 2.75 (m, 2H, PCH(CH3)2), 2.52 (m, 4H, PCH(CH3)2 and CH2), 1.69−1.00 (complex m, 47H, 24H PCH(CH3)2 + 12H Bpin + 6H CH3 POP + 5H allylic chain), 0.91 (m, 3H, CH3 allylic chain). 13C{1H}-apt plus HSQC and HMBC NMR (75.48 MHz, C6D6, 298 K): δ 160.4 (resonance inferred from the HMBC spectrum, C-arom), 137.8, 137.6 (both s, C-arom), 128.9, 124.0, 122.3 (all s, CH-arom), 93.1 (s, Cc allyl), 82.0 (s, C Bpin), 67.0 (d, JC−Rh = 19.1, Ci allyl), 43.0 (d, JC−Rh = 28.4, Ct allyl), 37.5 (s, C(CH3)2), 33.7 (d, JC−Rh = 4.2, CH2 allylic chain), 31.4 (s, CH2 allylic chain), 30.9 (d, JC−P = 21.9, PCH(CH3)2), 29.5 (d, JC−P = 16.6, PCH(CH3)2), 29.2, 23.0 (both resonances inferred from the HSQC spectrum, C(CH3)2), 27.6 (br m, PCH(CH3)2), 24.9 (s, CH3 Bpin), 23.1 (s, CH2 allylic chain), 21.4 (d, 2JC−P = 7.5, PCH(CH3)2), 21.3 (d, 2JC−P = 7.1, PCH(CH3)2), 21.1 (d, 2JC−P = 7.0, PCH(CH3)2), 20.7 (d, 2JC−P = 9.6, PCH(CH3)2), 20.6 (d, JC−P = 7.1, PCH(CH3)2), 14.4 (s, CH3 allylic chain). 31P{1H} NMR (121.49 MHz, C6D6, 298 K): δ 29.0 (very br). 31P{1H} NMR (161.98 MHz, toluene-d8, 193 K): δ 28.1 (dd, 1JP−Rh = 190.4, 2JP−P = 19.1), 25.1 (dd, 1JP−Rh = 198.4, 2JP−P = 19.1). 11B NMR (96.29 MHz, C7D8, 267 K): δ 30.7 (br). Reaction of Complex 7 with B2pin2. B2pin2 (7.5 mg, 0.030 mmol) was added to a solution of 7 (16 mg, 0.020 mmol) in C6D6 (0.5 mL) and the resulting mixture was stirred for 15 min at room temperature. After this, the quantitative conversion to 1 and the formation of 1,5-dipinacolboryl-(E)-2-octene were observed by NMR. Spectroscopic Data for 1,5-Dipinacolboryl-(E)-2-octene. 1H NMR (300.13 MHz, C6D6, 298 K): δ 5.74 (m, 1H, CH), 5.53 (m, 1H, CH), 2.02 (dt, 1H, 3JH−H = 7.1, 3JH−H = 7.0, CH2), 1.90 (d, 2H, 3JH−H = 7.8, CH2Bpin), 1.05 (s, 12H, CH3 Bpin), 1.01 (s, 12H, CH3 Bpin), 0.87 (t, 3H, 3JH−H = 7.3, CH3) (a CH and two CH2 resonances are masked by those of 1). 11B NMR (96.29 MHz, C6D6, 298 K): 32.7. Reaction of Rh{η 3 -CH 2 CHCHCH(Bpin)Et}{κ 2 -P,P-[xant(PiPr2)2]} (4) with 3-Hexyne. 3-Hexyne (2.3 μL, 0.02 mmol) was added to a solution of 4 (15 mg, 0.02 mmol) in benzene-d6. After heating for 2 h at 50 °C, 1H and 31P{1H} NMR spectroscopies show a mixture of complexes 9, 11, and 13 in a ratio 54:31:15 and the formation of 4-pinacolboryl-(3E)-1,3-hexadiene. Reaction of RhH{κ3-P,O,P-[xant(PiPr2)2]} (8) with 3-Hexyne: Preparation of Rh{(Z)-C(Et)C(Et)H}{κ3-P,O,P-[xant(PiPr2)2]} (9). 3-Hexyne (21 μL, 0.18 mmol) was added to a solution of 8 (100 mg, 0.18 mmol) in pentane (5 mL). An instantaneous change of color from black to red was observed. Immediately, it was concentrated to dryness to afford a red residue. This residue was washed with a minimum amount of pentane (3 × 0.5 mL) to afford a red solid that was dried in vacuo. Yield: 93 mg (82%). Anal. Calcd. for C33H51OP2Rh: C, 63.05; H, 8.18. Found: C, 63.29; H, 8.41. HRMS I


Article

Organometallics (s, CH-arom POP), 120.3 (t, 3JC−P = 3.8, CCH), 37.2 (dt, 2JC−Rh = 2.1, 3JC−P = 2.5, CCH2CH3), 34.0 (s, C(CH3)2), 33.7, 32.6 (both s, C(CH3)2), 31.5 (d, 3JC−Rh = 3.6, CCH2CH3), 25.9 (dvt, 2JC−Rh = 2.9, N = 15.2, PCH(CH3)2), 20.0 (dvt, 2JC−Rh = 3.2, N = 16.6, PCH(CH3)2), 19.7 (vt, N = 8.7, PCH(CH3)2), 18.9 (vt, N = 8.7, PCH(CH3)2), 18.6, 18.0 (both s, PCH(CH3)2), 16.0 (s, C CH2CH3), 15.4 (s, CCH2CH3). 31P{1H} NMR (121.49 MHz, C6D6, 298 K): δ 34.6 (d, 1JP−Rh = 184.9). Reaction of RhH{κ3-P,O,P-[xant(PiPr2)2]} (8) with 4-Octyne: Spectroscopic Detection of Rh{(E)-C(Pr)C(Pr)H}{κ3-P,O,P[xant(PiPr2)2]} (12). A solution of complex 8 (100 mg, 0.18 mmol) in pentane (5 mL) was treated with 4-octyne (27 μL, 0.18 mmol) and the resulting solution was stirred at room temperature for 5 days. After this, it was concentrated to dryness to afford a red residue. The 1 H and 31P{1H} NMR spectra of this residue in benzene-d6 show a mixture of Rh{(Z)-C(Pr)C(Pr)H}{κ3-P,O,P-[xant(PiPr2)2]} (10), Rh{(E)-C(Pr)C(Pr)H}{κ3-P,O,P-[xant(PiPr2)2]} (12) and Rh(η3CH2CHCHPent){κ2-P,P-[xant(PiPr2)2]} (14) in a ratio 20:50:30. Spectroscopic data for Rh{(E)-C(Pr)C(Pr)H}{κ3-P,O,P-[xant(PiPr2)2]} (12). 1H NMR (300.13 MHz, C6D6, 298 K): δ 7.26 (m, 2H, CH-arom), 7.04 (dd, JH−H = 7.1, JH−H = 1.6, 2H, CH-arom), 6.85 (t, JH−H = 7.6, 2H, CH-arom), 5.68 (m, 1H, CH), 3.03 (m, 2H,  CCH2CH2CH3), 2.97 (m, 2H, CCH2CH2CH3), 2.53 (m, 4H, PCH(CH3)2), 2.13 (m, 2H, CCH2CH2CH3), 1.85 (m, 2H,  CCH2CH2CH3), 1.46 (dvt, JH−H = 7.0, N = 13.6, 6H, PCH(CH3)2), 1.44 (s, 3H, CH3 POP), 1.43 (dvt, JH−H = 6.4, N = 15.2, 6H, PCH(CH3)2), 1.29−1.16 (m, 21H, 12H PCH(CH3)2 + 3H CH3 POP + 6H CCH2CH2CH3). 13C{1H}-apt plus HSQC and HMBC NMR (100.62 MHz, toluene-d8, 273 K): δ 155.9 (vt, N = 15.1, C-arom), 155.0 (dt, 1JC−Rh = 40.4, 2JC−P = 12.0, Rh−C), 131.4 (s, CH-arom), 130.8 (vt, N = 5.0, C-arom), 127.8 (s, CH-arom), 125.7 (m, C-arom, inferred from the HMBC spectrum), 123.9 (s, CH-arom), 119.4 (t, 3 JC−P = 4.0, CCH), 48.6 (dt, 3JC−P = 2.3, 2JC−Rh = 2.5,  CCH2CH2CH3), 41.7 (d, 3JC−Rh = 3.6, CCH2CH2CH3), 34.1 (s, C(CH3)2), 32.5 (s, C(CH3)2), 26.2 (dvt, 2JC−Rh = 3.2, N = 15.8, PCH(CH3)2), 25.7 (dvt, 2JC−Rh = 3.7, N = 17.4, PCH(CH3)2), 25.4 (s, CCH2CH2CH3), 25.0 (d, 3JC−Rh = 1.1, CCH2CH2CH3), 20.0 (vt, N = 9.0, PCH(CH3)2), 19.3 (vt, N = 9.1, PCH(CH3)2), 18.8 (vt, N = 6.0, PCH(CH3)2), 18.2 (s, PCH(CH3)2), 16.2, 15.6 (both s,  CCH2CH2CH3). 31P{1H} NMR (121.49 MHz, C6D6, 298 K): δ 34.2 (d, 1JP−Rh = 185.3). Isomerization of 9 to Rh(η3-CH2CHCHPr){κ2-P,P-[xant(PiPr2)2]} (13). A Schlenk flask provided with a Teflon closure was filled with a solution of 9 (100 mg, 0.16 mmol) in pentane (5 mL) and heated in an oil bath (70 °C) overnight. After this, the resulting solution was concentrated to dryness to afford a golden oil. After several cycles of addition of pentane/evaporation of pentane, a golden foamy solid was obtained. Yield: 94 mg (94%). Anal. Calcd for C33H51OP2Rh: C, 63.05; H, 8.18. Found: C, 63.26; H, 8.49. HRMS (electrospray, m/z): calcd for C35H51OP2Rh [M]+, 629.2543; found, 629.2538. IR (cm−1): ν(CC) 1584 (w), ν(C−O−C) 1025 (m). 1H NMR (500.12 MHz, C6D6, 298 K): δ 7.26 (m, 2H, CH-arom), 7.12 (d, JH−H = 7.7, 1H, CH-arom), 7.10 (d, JH−H = 7.7, 1H, CH-arom), 6.94 (t, JH−H = 7.7, 2H, CH-arom), 4.91 (m, 1H, Hc allyl), 4.01 (br, 1H, Ci allyl), 3.29 (br, 1H, Ht allyl), 2.78 (m, 1H, PCH(CH3)2), 2.72 (m, 2H, PCH(CH3)2), 2.47 (br, 2H, PCH(CH3)2), 1.83 (m, 1H, Ht allyl), 1.48, 1.44 (both s, 3H each, CH3 POP), 1.39 (dd, 3JH−H = 7.1, 3 JH−P = 16.1, 3H, PCH(CH3)2), 1.31 (m, 5H, 3H PCH(CH3)2 + 2H CH2), 1.24 (dd, 3JH−H = 6.9, 3JH−P = 13.9, 3H, PCH(CH3)2), 1.13− 1.03 (m, 14H, 12H PCH(CH3)2 + 2H CH2), 0.97 (m, 3H, CH2CH2CH3), 0.84 (br m, 3H, PCH(CH3)2). 13C{1H}-apt plus HSQC and HMBC NMR (75.48 MHz, C6D6, 298 K): δ 160.7 (br, Carom), 137.8 (s, C-arom), 137.6 (s, C-arom), 129.7, 128.9, 124.4, 124.3 (all s, CH-arom), 123.4 (br, C-arom), 122.4 (s, CH-arom), 93.0 (s, Cc allyl), 66.6 (d, JC−Rh = 20.5, Ci allyl), 42.9 (d, JC−Rh = 31.3, Ct allyl), 37.5 (s, C(CH3)2), 33.7 (s, CH2 allylic chain), 30.8 (d, JC−P = 21.0, PCH(CH3)2), 29.4 (d, JC−P = 17.8, PCH(CH3)2), 27.5 (m, PCH(CH3)2), 27.1 (very broad, inferred from the HSQC, CH3 POP), 26.8 (s, CH2 allylic chain), 24.0 (very broad, inferred from the HSQC, CH3 POP), 21.4 (d, JC−P = 7.0, PCH(CH3)2), 21.3 (d, JC−P = 7.4,

PCH(CH3)2), 21.1 (t, JC−P = 7.0, PCH(CH3)2), 20.7 (d, JC−P = 10.2, PCH(CH3)2), 20.6 (t, JC−P = 7.3, PCH(CH3)2), 19.6 (d, JC−P = 5.3, PCH(CH3)2), 14.2 (s, CH3 allylic chain). 31P{1H} NMR (121.49 MHz, C6D6, 298 K): δ 29.0 (br). 31P{1H} NMR (161.98 MHz, toluene-d8, 193 K): δ 33.3 (dd, 1JP−Rh = 190.2, 2JP−P = 19.1), 30.3 (dd, 1 JP−Rh = 198.1, 2JP−P = 19.1). Isomerization of 10 to Rh(η3-CH2CHCHPent){κ2-P,P-[xant(PiPr2)2]} (14). A Schlenk flask provided with a Teflon closure was filled with a solution of 10 (100 mg, 0.15 mmol) in pentane (5 mL) and heated in an oil bath (70 °C) overnight. After this, the resulting solution was concentrated to dryness to afford a pale brown oil. After several cycles of addition of pentane/evaporation of pentane, a pale brown foamy solid was obtained. Yield: 92 mg (92%). Anal. Calcd for C35H55OP2Rh: C, 64.02; H, 8.44. Found: C, 63.96; H, 8.29. HRMS (electrospray, m/z): calcd for C35H55OP2Rh [M − H]+, 657.2856; found: 657.2804. IR (cm−1): ν(CC) 1583 (w), ν(C−O−C) 1024 (m). 1H NMR (500.12 MHz, C6D6, 298 K): δ 7.26 (m, 2H, CHarom), 7.12 (dd, 3JH−H = 7.6, 5JH−H = 1.2, 1H, CH-arom), 7.10 (d, 3 JH−H = 7.6, 5JH−H = 1.2, 1H, CH-arom), 6.96 (t, 3JH−H = 7.6, 1H, CH-arom), 6.95 (t, 3JH−H = 7.7, 1H, CH-arom), 4.92 (m, 1H, Hc allyl), 4.03 (br, 1H, Hi allyl), 3.30 (br, 1H, Ht allyl), 2.78 (m, 1H, PCH(CH 3 ) 2 ), 2.72 (m, 1H, PCH(CH 3 ) 2 ), 2.49 (br, 2H, PCH(CH3)2), 1.85 (m, 1H, Ht allyl), 1.52 (very br, 2H, CH2 Pent), 1.49, 1.45 (both s, 3H each, CH3 POP), 1.40 (dd, 3JH−H = 7.2, 3JH−P = 16.0, 3H, PCH(CH3)2), 1.31 (dd, 3JH−H = 7.3, 3JH−P = 14.6, 3H, PCH(CH3)2), 1.31 (m, 6H, CH2 Pent), 1.24 (dd, 3JH−H = 6.9, 3JH−P = 13.8, 3H, PCH(CH3)2), 1.12−1.03 (m, 12H, PCH(CH3)2), 0.89 (t, 3H, CH3 Pent), 0.85 (very br, 3H, PCH(CH3)2). 13 C{1H}-apt plus HSQC and HMBC NMR (100.62 MHz, C6D6, 298 K): δ 160.3 (very br, inferred from the HMBC spectrum, C-arom POP), 137.8 (s, C-arom POP), 137.6 (s, C-arom POP), 128.9 (br, CH-arom), 124.5, 124.3, 122.4 (all s, CH-arom), 93.1 (s, Ct allyl), 67.1 (br d, JC−Rh = 26.3, Ci allyl), 42.9 (br d, JC−Rh = 32.7, Ct allyl), 37.5 (s, C(CH3)2), 33.6 (d, JC−Rh = 4.3, CH2 Pent), 32.1 (s, CH2 Pent), 31.5 (s, CH2 Pent), 30.9 (d, 1JC−P = 21.6, PCH(CH3)2), 29.5 (d, 1JC−P = 17.6, PCH(CH3)2), 27.6 (br, PCH(CH3)2), 27.0, 23.9 (both very broad, inferred from the HSQC spectrum, C(CH3)2), 23.1 (s, CH2 Pent), 21.4 (d, JC−P = 8.0, PCH(CH3)2), 21.3 (d, 2JC−P = 8.0, PCH(CH3)2), 21.1 (d, 2JC−P = 7.0, PCH(CH3)2), 20.7 (d, 2JC−P = 11.0, PCH(CH3)2), 20.6 (d, 2JC−P = 9.0, PCH(CH3)2), 19.5 (t, JC−P = 4.9, PCH(CH3)2), 14.4 (s, CH3 Pent). 31P{1H} NMR (121.49 MHz, C6D6, 298 K): δ 29.5 (very br). 31P{1H} NMR (161.98 MHz, toluene-d8, 193 K): δ 28.2 (dd, 1JP−Rh = 190.1, 2JP−P = 19.1), 25.1 (dd, 1 JP−Rh = 198.5, 2JP−P = 19.1). Reaction of Complex 13 with B2pin2. B2pin2 (7.5 mg, 0.030 mmol) was added to a solution of 13 (13 mg, 0.020 mmol) in C6D6 (0.5 mL) and the resulting mixture was stirred for 15 min at room temperature. After this, the quantitative conversion to 1 and the formation of 1-pinacolboryl-2-hexene were observed by NMR spectroscopy. Spectroscopic Data for 1-Pinacolboryl-2-hexene.24 1H NMR (400.13 MHz, C6D6, 298 K): δ 5.73 (m, 1H, CH), 5.50 (m, 1H,  CH), 1.97 (dt, 3JH−H = 7.1, 3JH−H = 6.9, 1H, CH2), 1.88 (d, 3JH−H = 7.3, 2H, CH2Bpin), 1.34 (m, 2H, CH2), 1.04 (s, 12H, CH3 Bpin), 0.85 (t, 3JH−H = 7.4, 3H, CH3). 13C{1H}-apt plus HSQC NMR (100.62 MHz, C6D6, 298 K): δ 130.7 (CH), 126.0 (CH), 82.9 (C Bpin), 35.9 (CH2), 24.5 (CH3 Bpin), 22.8 (CH2), 13.4 (CH3). 11B NMR (96.29 MHz, C6D6, 298 K): δ 32.9 (s). Reaction of Complex 14 with B2pin2. B2pin2 (7.5 mg, 0.030 mmol) was added to a solution of 14 (14 mg, 0.020 mmol) in C6D6 (0.5 mL) and the resulting mixture was stirred for 15 min at room temperature. After this, the quantitative conversion to 1 and the formation of 1-pinacolboryl-2-octene were observed by NMR spectroscopy. Spectroscopic Data for 1-Pinacolboryl-2-octene.24 1H NMR (300.13 MHz, C6D6, 298 K): δ 5.75 (m, 1H, CH), 5.51 (m, 1H,  CH), 2.01 (dt, 2H, 3JH−H = 7.7, 3JH−H = 6.6, CH2), 1.89 (d, 2H, 3JH−H = 7.3, CH2Bpin), 1.34 (m, 2H, CH2), 1.25 (m, 2H, CH2), 1.02 (s, 12H, CH3 Bpin), 0.85 (t, 3H, 3JH−H = 7.0, CH3). One of the CH2 resonances is masked by those of 1. 13C{1H}-apt plus HSQC NMR J


Article

Organometallics (75.48 MHz, C6D6, 298 K): δ 131.0 (CH), 125.8 (CH), 83.1 (C Bpin), 33.2 (CH2), 31.7 (CH2), 29.8 (CH2), 24.5 (CH3 Bpin), 23.0 (CH2), 14.3 (CH3). 11B NMR (96.29 MHz, C6D6, 298 K): δ 32.9.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.9b00104. General information, crystallographic data, and NMR spectra (PDF) Accession Codes

CCDC 1886050−1886051 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Miguel A. Esteruelas: 0000-0002-4829-7590 Montserrat Oliván: 0000-0003-0381-0917 Enrique Oñate: 0000-0003-2094-719X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the MINECO of Spain (Projects CTQ2017-82935-P and Red de Excelencia Consolider CTQ2016-81797-REDC), the Diputación General de Aragón (E06_17R), FEDER, and the European Social Fund is acknowledged.

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REFERENCES

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