DOI: 10.1002/chem.201102729

Catalytic Non-Conventional trans-Hydroboration: A Theoretical andExperimental Perspective

Jessica Cid, Jorge J. Carb�,* and Elena Fern�ndez*[a]

Introduction

The hydroboration of alkynes is a useful method for the syn-thesis of 1-alkenylboronate compounds, which are versatileintermediates in organic synthesis.[1] The conventional cis-hydroboration reaction of alkynes with catecholborane(HBcat),[2] pinacolborane (HBpin),[3] and 4,4,6-trimethyl-1,3,2-dioxaborinane[4] to yield (E)-1-alkenylboronates wasfirst studied through stoichiometric syn-addition approaches.The activation of dialkoxyboranes by transition-metal com-plexes opened up a new perspective on hydroboration reac-tions when Kono et al.[5] demonstrated that catecholboranecould be oxidatively added to the rhodium(I) center in[RhCl ACHTUNGTRENNUNG(PPh3)3] to form the rhodium ACHTUNGTRENNUNG(III) complex [RhClH-ACHTUNGTRENNUNG(Bcat) ACHTUNGTRENNUNG(PPh3)3]. Since then, transition-metal catalyzed hydro-boration has been shown to alter the chemo-, regio-, anddiastereoselectivity of the stoichiometric hydroboration re-

actions, but always with the formation of cis-hydroborationproducts.[6] However, Miyaura and co-workers reported thefirst rhodium- and iridium-catalyzed non-conventional trans-hydroboration reaction to yield (Z)-1-alkenylboronates.[7]

They postulated that at-least two dominant factors could re-verse the conventional cis-hydroboration to prefer the trans-hydroboration: 1) the presence of Et3N as an additive and2) the use of an excess of the alkyne over the borane re-agent, (Scheme 1). The lack of further studies on the non-conventional trans-hydroboration approach prompted us tostudy its mechanism from an experimental and a theoreticalpoint of view in order to establish the basis of the desiredselectivity in the trans-hydroboration reaction. Thus, we ex-plored the influence of the ligand in modifying the metalspecies and the influence of reaction conditions in order tomake general the methodology. Furthermore, the mecha-nism proposed by Miyaura and co-workers has been investi-gated using density functional theory (DFT) calculations,which focused on the origin of the trans-selectivity.

Results and Discussion

The first catalytic cis-hydroboration reaction was observedon alkenes by M�nnig and Nçth.[6a] They proposed a mecha-nism for the rhodium-catalyzed hydroboration reaction that

Abstract: We have studied the non-conventional trans-hydroboration reac-tion of alkynes both experimentallyand theoretically. A catalytic systembased on the in situ mixture of [{Rh-ACHTUNGTRENNUNG(cod)Cl}2]/PCy3 (cod=1,5-cycloocta-diene, Cy=cyclohexyl) has been ableto activate pinacolborane and catechol-borane and transfer boryl and hydridegroups onto the same unhinderedcarbon atom of the terminal alkynes.The presence of a base (Et3N) favoredthe non-conventional trans-hydrobora-tion over the traditional cis-hydrobora-tion. Varying the substrate had a signif-icant influence on the reaction, with upto 99 % conversion and 94 % regiose-lectivity observed for para-methyl-phe-nylacetylene. Both DFT and quantum

mechanical/molecular mechanicalONIOM calculations were carried outon the [RhClACHTUNGTRENNUNG(PR3)2] system. To explainthe selectivity towards the (Z)-alkenyl-boronate we explored several alterna-tive mechanisms to the traditional cis-hydroboration, using propyne as amodel alkyne. The proposed mecha-nism can be divided into four stages:1) isomerization of the alkyne into thevinylidene, 2) oxidative addition of theborane reagent, 3) vinylidene insertioninto the Rh�H bond, and finally 4) re-ductive elimination of the C�B bond to

yield the 1-alkenylboronate. Calcula-tions indicated that the vinylidene in-sertion is the selectivity-determiningstep. This result was consistent with theobserved Z selectivity when the steri-cally demanding phosphine groups,such as PCy3 and PiPr3, were intro-duced. Finally, we theoretically ana-lyzed the effect of the substrate on theselectivity; we identified several factorsthat contribute to the preference foraryl alkynes over aliphatic alkynes forthe Z isomer. The intrinsic electronicproperties of aryl substituents favoredthe Z-pathway over the E-pathway,and the aryl groups containing electrondonating substituents favored the oc-currence of the vinylidene reactionchannel.

Keywords: boranes · density func-tional calculations · homogeneouscatalysis · hydroboration · rhodium

[a] J. Cid, Dr. J. J. Carb�, Dr. E. Fern�ndezDepartament de Qu�mica F�sica i Inorg�nicaUniversitat Rovira i VirgiliC/Marcel.l� Domingo s/n, 43007, Tarragona (Spain)Fax: (+34) 977-559563E-mail : j.carbo@urv.cat

mariaelena.fernandez@urv.cat

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/chem.201102729.

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involved the oxidative addition of a B�H bond to the coor-dinatively unsaturated metal center followed by alkene coor-dination, insertion, and hydride migration onto the coordi-nated alkene, with subsequent reductive elimination to formthe B�C bond. Scheme 2 (top) has adapted the catalyticcycle of the cis-hydroboration reaction for the reaction with

alkynes. Elucidation by quantum mechanical methods com-plemented the mechanistic studies on the rhodium-mediatedcis-hydroboration reaction, although, to the best of ourknowledge, the theoretical studies reported so far have onlyconsidered alkene substrates.[6d, 8] When Miyaura and co-workers developed the first trans-hydroboration reaction ofalkynes, they observed that the b-hydrogen atom in the (Z)-1-alkenylboronate unexpectedly did not derive from theborane reagent, thus representing an example of non-con-ventional trans-hydroboration. By means of a rhodium-mediated hydroboration of [1-D]-1-octyne, they observedthat the deuterium-labeled terminal-carbon atom selectivelyshifted onto the b-carbon atom. On the basis of this interest-ing observation, the authors suggested a plausible mecha-nism for both the acetylenic hydrogen migration and thegem-addition of the B�H bond. A rhodium complex modi-fied with an electron-donating ligand could favor the oxida-tive addition of the terminal C�H bond on the substrate andthe resulting stable vinylidene complex might be the key in-termediate. This process would be followed by the oxidativeaddition of the borane and 1,2-migration of the boryl grouponto the a-carbon atom, thus providing the (Z)-1-alkenyl-boronate via reductive elimination (Scheme 2, on bottom).The presence of Et3N might suppress the cis-hydroborationof the borane because a parallel experiment demonstratedthat the treatment of [RhClH ACHTUNGTRENNUNG(Bcat) ACHTUNGTRENNUNG(PiPr3)2] with Et3N ledto the complete reductive elimination of catecholborane.[7]

Conventional trans-hydrosilylation of alkynes has been ex-plained by the formation of carbene-type metal species asthe key intermediates.[9] To justify the observed unique ste-reoselectivity, Ojima and co-workers[9a] postulated a siliconshift onto the acetylenic bond and a zwitterionic carbene–rhodium complex that undergoes isomerization (Scheme 3),whereas Tanke and Crabtree[9b, c] proposed that the key in-termediate is actually the closely related h2-vinyl complex(Scheme 4). However, to the best of our knowledge, therehave been no reported examples of the conventional trans-hydroboration of alkynes. To gain more insight into thedominant mechanism in the cis- and non-conventional trans-hydroboration reactions, we performed a series of rhodium-and iridium-mediated hydroboration reactions of alkynes in

Scheme 1. Transition-metal-catalyzed cis- and trans-hydroboration reac-tions.

Scheme 2. Suggested mechanistic cycles for cis-hydroboration and thenon-conventional trans-hydroboration reaction.

Scheme 3. Suggested catalytic cycle for the conventional rhodium-cata-lyzed trans-hydrosilylation reaction.

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parallel to DFT analysis. A catalytic system based on the insitu mixture of [{Rh ACHTUNGTRENNUNG(cod)Cl}2]/PCy3 (cod= 1,5-cycloocta-diene, Cy=cyclohexyl) transformed the model substrate 1-octyne into a mixture of isomers 2–4, depending on the sub-strate/borane ratio. Isomer 2 was expected to be formed viathe non-conventional trans-hydroboration pathway, whilstisomers 3 and 4 are the two regioisomers formed from thecis-hydroboration reaction. The alkyne/borane ratio seemsto play a role in determining the regioselectivity for isomer2 (Table 1, entries 1–3). In this case, the optimal result wasobtained when the pinacolborane reagent (1 a) was presentin a slight excess. Apart from tetrahydrofuran, other sol-vents like cyclohexane and dichloromethane were exploredwithout any significant improvement in the regioselectivitytowards product 2, (Table 1, entries 4–6). When the analo-

gous catalytic system [{Ir ACHTUNGTRENNUNG(cod)Cl}2]/PCy3 was used instead, alower percentage of isomer 2 was achieved, to the benefit ofisomer 4 (Table 1, entries 7 and 8). Next, we examined theinfluence of the phosphine ligands at a fixed substrate/borane ratio of 1:1.2 in tetrahydrofuran. Under these reac-tion conditions, several monophosphines, such as PACHTUNGTRENNUNG(nBu)3,PMe3, and PPh3, and diphosphines 1,4-bis(diphenylphosphi-no)butane (dppb) and 1,1’-bis(diphenylphosphino)ferrocene(dppf) favored the formation of cis-hydroboration-products3 and 4 more-strongly than monophosphine PCy3, withisomer 3 being the major product (Table 1, entries 9–13).When diphosphines were used in the catalytic system, theactivity was also lower, particularly when dppf was used asthe bidentate ligand (Table 1, entry 13). Because the modifi-cation of the rhodium complex with monophosphine PCy3

provided the highest regioselectivity towards isomer 2(Table 1, entry 3), we explored the influence of the Rh/PCy3

ratio and temperature on the reaction outcome. Figure 1 il-lustrates the inverse relationship between activity and regio-selectivity towards isomer 2 when the ratio of Rh/PCy3

changed from 1:1 to 1:2 to 1:4. The highest percentage ofproduct achieved for the non-conventional trans-hydrobora-tion reaction was obtained at a Rh/PCy3 ratio of 1:4 and wasindependently formed at 0 8C, 25 8C, and 70 8C. With allthese data in mind, we performed a study of the scope ofthe substrates, taking into consideration different steric andelectronic properties. We first extrapolated the best reactionconditions to promote the non-conventional trans-hydrobo-ration reaction with similar aliphatic alkynes such as 1-hep-tyne and 1-pentyne. In both cases, although the catalytic ac-tivity was improved, the regioselectivity of isomer 2 de-creased slightly (Table 2, entries 1 and 2). Steric demands on

the alkyne substituent favored the formation of thedesired product in the case of tert-butyl acetylenebut not in the case of cyclohexyl acetylene (Table 2,entries 3 and 4). The influence of electronic effectson the rhodium-catalyzed hydroboration reactionwas principally observed when electron-withdraw-ing and electron-donating phenyl acetylenes weretransformed into the alkenyl boronate products.Regioselectivities of up to 94 % for isomer 2 wereachieved when electron-rich alkynes were involved(Table 2, entries 5–7). On the contrary, despitebeing electron poor, para-trifluoromethyl-phenyla-cetylene was converted into the product in almostquantitative yield, but the cis-hydroboration wasvery competitive. Other electron-rich alkynes withhigh steric demands were also mainly convertedinto isomer 2 (Table 2, entries 8 and 9). It is impor-tant to note that despite the successful hydrobora-tion of the para-trifluoromethyl-phenylacetylene, allof the other substrates provided negligible or noformation of isomer 4, owing to the constrained ad-dition of the boryl moiety onto the most-hinderedcarbon in the cis-hydroboration reaction. Interest-ingly enough, when the rhodium-mediated hydrobo-ration of para-methyl-phenyl acetylene was carried

Scheme 4. Suggested catalytic cycle for the conventional iridium-cata-lyzed trans-hydrosilylation reaction.

Table 1. Base-assisted metal-catalyzed hydroboration of 1-octyne.[a]

Entry Catalytic system Solvent Substrate/borane

Conv.[%][b]

2[c] 3[c] 4[c]

1 [{Rh ACHTUNGTRENNUNG(cod)Cl}2]/PCy3 THF 1.2:1 47 38 47 152 [{Rh ACHTUNGTRENNUNG(cod)Cl}2]/PCy3 THF 1:1 87 60 34 63 [{Rh ACHTUNGTRENNUNG(cod)Cl}2]/PCy3 THF 1:1.2 73 66 30 44 [{Rh ACHTUNGTRENNUNG(cod)Cl}2]/PCy3 C6H12 1:1 99 48 46 65 [{Rh ACHTUNGTRENNUNG(cod)Cl}2]/PCy3 C6H12 1:1.2 88 65 28 76 [{Rh ACHTUNGTRENNUNG(cod)Cl}2]/PCy3 CH2Cl2 1:1 71 58 34 87 [{Ir ACHTUNGTRENNUNG(cod)Cl}2]/PCy3 THF 1:1 98 53 37 98 [{Ir ACHTUNGTRENNUNG(cod)Cl}2]/PCy3 THF 1:1.2 63 58 33 99 [{Rh ACHTUNGTRENNUNG(cod)Cl}2]/PPh3 THF 1:1.2 87 30 47 2310 [{Rh ACHTUNGTRENNUNG(cod)Cl}2]/P ACHTUNGTRENNUNG(nBu)3 THF 1:1.2 98 18 56 2611 [{Rh ACHTUNGTRENNUNG(cod)Cl}2]/PMe3 THF 1:1.2 89 9 62 2912 [{Rh ACHTUNGTRENNUNG(cod)Cl}2]/dppb THF 1:1.2 64 21 57 2213 [{Rh ACHTUNGTRENNUNG(cod)Cl}2]/dppf THF 1:1.2 39 33 47 20

[a] Standard conditions: [{M ACHTUNGTRENNUNG(cod)Cl}2]/L (M =Rh or Ir, 0.015 mmol), monophosphine(0.06 mmol), diphosphine (0.03 mmol), Et3N (5 mmol), pinacolborane (1.2 mmol), 1-octyne (1 mmol) for substrate/borane ratio= 1:1.2. Solvent (3 mL), 25 8C, 4 h. [b] Con-version determined by GC analysis of the consumption of 1-octyne. [c] Percentage ofisomeric ratio, determined by GC analysis.

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out with catecholborane (1 b) as the borane reagent, the re-gioselectivity towards isomer 2 remained very high whilstthe conversion increased significantly (Scheme 5). It isknown that borane reagents derived from catechol units aremore reactive owing to their enhanced Lewis acid proper-ties.[10] In order to gain a greater insight into the mechanisticdetails of the trans-hydroboration reaction, a detailed theo-retical study of the rhodium-mediated reaction of alkyneswas performed. In our first approach to the mechanism, weused simplified model systems: PH3 for phosphine ligands,HB ACHTUNGTRENNUNG(O2C2H4) for the pinacolborane reagent, and the sim-plest alkyne substrate, HC�CMe. We also considered the re-

sults from real systems in orderto asses several key variables,such as ligand and substratestructure. We also assumed thatthe two phosphine ligands coor-dinated trans to each other, asobserved in the X-ray structureof the key vinylidene complex[trans- ACHTUNGTRENNUNG(PiPr)2RhCl ACHTUNGTRENNUNG(=C=

CHMe)].[11] Initially, we exam-ined the mechanism proposedby Miyaura and co-workers(Scheme 2, bottom); the firststep involved the isomerizationof the terminal alkyne groupinto the vinylidene in the coor-dination sphere of rhodiumcomplex. This process has beenobserved previously[11, 12] andextensively studied by computa-

tional methods.[13] The precise mechanism of iso-merization had been the subject of some controver-sy, but in the end all evidence pointed toward a un-imolecular process involving the formation of an al-kynyl hydride intermediate (Scheme 6). The secondstep, 1,3-hydrogen shift to yield the vinylidene, waspostulated to be the rate-limiting step.[14] Angeliset al.[13c] and Grotjahn et al.[13d] have demonstratedthat electron-donating ligands favor the formationof both hydrido-alkynyl and vinylidene species. Wehave also observed this trend in other transition-metal complexes,[15] adding that the steric hindranceof bulky ligands has an influence through the desta-bilization of the alkyne.[15a] We noted that the non-conventional trans-hydroboration was observed forbulky and strongly electron-donating ligands, suchas PCy3 and PiPr3. Thus, in this first part of the dis-cussion, we will assume that isomerization takesplace and we will focus on the addition of boraneto the vinylidene complex. Scheme 7 shows thecomputed energy profile for the Z- and E-pathwaysof the borane addition to the vinylidene group,starting with boryl migration (mechanism A). Thevinylidene complex (7) was calculated to be 1.7 kcalmol�1 lower in energy than the alkyne complex

[RhCl ACHTUNGTRENNUNG(PH3)2ACHTUNGTRENNUNG(HCCMe)] (5). However, at a similar computa-tional level, the preference for the vinylidene group was re-

Figure 1. Influence of the Rh/PCy3 ratio and temperature on the catalytic hydroboration of 1-octyne with pina-colborane.

Table 2. Rhodium-catalyzed hydroboration of alkynes with pinacolborane.[a]

Entry Substrate Conv.

[%][b] 2[c] 3[c] 4[c]

1 80 65 33 22 65 67 31 2

3 84 74 26 0

4 88 51 49 0

5 95 48 32 18

6 75 90 10 0

7 76 94 6 0

8 72 77 23 0

9 44 78 22 0

[a] Standard conditions: [{Rh ACHTUNGTRENNUNG(cod)Cl}2]/L (0.015 mmol), PCy3 (0.12 mmol), Et3N(5 mmol), pinacolborane (1.2 mmol), alkyne (1 mmol), substrate/borane ratio=1:1.2,THF (3 mL), 25 8C, 4 h. [b] Conversion determined by GC analysis of the consumptionof the alkyne. [c] Percentage of isomeric ratio, determined by GC analysis.

Scheme 5. Optimized trans-hydroboration reaction of alkynes with cate-cholborane.

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ported to increase to up to approximately 13 kcal mol�1

using real PiPr3 instead of model PH3 phosphines.[13b] Thisresult further supports the postulation that the process isthermodynamically favorable and essentially irreversible, asdemonstrated in previous experimental studies.[13a] This pro-cess should be followed by oxidative addition of the boraneagent to compound 7 to yield a hydrido-boryl intermediate(8A) with the boryl group cis to the vinylidene ligand. De-spite all of our efforts, we could not locate either species 8Anor the transition state for oxidative addition. Following theprevious discussion about the effect of strongly donating li-gands,[13b,c,15] it is reasonable to think that the species 8Awould be stabilized by basic PCy3 phosphine groups andthat the same effect cannot be achieved by simplified PH3 li-gands. A similar situation was faced in previous theoreticalstudies on the hydroboration of alkenes using a [RhClACHTUNGTRENNUNG(PH3)]

complex, in which not all of the conceivable hydrido-borylspecies and none of the transition states for oxidative addi-tion were located.[8] We located the transition state connect-ing intermediates 8A to vinyl complexes 9A (TSA8–9), whichcorresponded to the 1,2-boryl migration onto the a-carbonatom of the vinylidene group. This migration occurs withinthe vinylidene molecular plane, and the distance of the B�Cforming bond is 1.89 � at the pro-Z transition state. The dis-tance between the boron atom and the hydride moiety isrelatively long (2.58 �), thereby showing that the B�H bondis cleaved when boryl migration takes place rather than theconcerted addition of borane to the vinylidene fragment.The key geometric parameters at the corresponding pro-Etransition state only differ by less than 0.01 �. Accordingly,we found that the two transition states for the Z- and E-pathways lay 30.2 and 28.3 kcal mol�1 above compound 7, re-spectively. These overall energy barriers seem somewhathigh for a process occurring at room temperature. The prod-uct of boron migration (9A) is 21 kcal mol�1 more stablethan compound 3. Subsequent reductive elimination withconcomitant C�H bond-formation leads to the borylated-alkene product via transition state TSA9–10 with low energybarriers (ca. 4 kcal mol�1). For this last part of the mecha-nism, the energy differences between the Z and E paths arewithin 1 kcal mol�1. Alternatively, the sequence of the reac-tion could be inversed, that is, hydride migration followedby reductive elimination of the C�B bond (mechanism B).Scheme 8 shows the computed energy profile for both the Zand E pathways, and Figure 2 shows the structures and maingeometric parameters of selected intermediates and transi-tion states. In this case, it was possible to locate the two hy-drido-boryl intermediates (8B) and the transition states forthe oxidative addition of the B�H bond to the rhodiumcenter (TSB7–8). The formation of complexes 8B is endother-mic by 20 kcal mol�1 and proceeds via transition-state struc-tures that have similar energies, leading to very-low reverseenergy barriers (<3 kcal mol�1). From intermediates 8B, theforward energy barriers for hydride migration via TSB8–9 arealso very low (ca. 4 kcal mol�1). These low energy barriersmean that the existence of intermediate 8 may depend onthe specific conditions. Among them, having a chlorideatom trans to the hydride (8B) instead of trans to the borylligand (8A) stabilizes the intermediate. Nevertheless, the im-portant fact is that the reaction goes uphill from compound7 to reach the transition state for hydride migration (TSB8–

9), and the calculated overall barriers are 25.9 and 25.4 kcalmol�1 for the Z and E pathways, respectively. Interestingly,these values are significantly lower than those correspondingto mechanism A. In the TSB8–9 structures (Figure 2), the vi-nylidene ligand bends away from its position in the reactants(8B) to approach the hydride group (Cl-Rh-Ca bond anglechanges from 1138 in 8B to 1338 in TSB8–9 for both the Zand E pathways), thereby ending up trans to the chlorideatom in the products (9B). In this migratory insertion, theformation of the C�H bond is co-planar with the vinylidenemoiety. Once the transition state for migratory insertion isreached, the reaction drops significantly in energy (ca.

Scheme 6. Mechanism for alkyne–vinylidene isomerization in electron-rich metal (e.g., rhodium) centers. Relative energies shown are in kcalmol�1.

Scheme 7. A calculated potential-energy profile (kcal mol�1) for themechanism suggested by Miyaura and co-workers involving an initial iso-merization of the alkyne to the vinylidene (mechanism A). Hydrobora-tion of propyne with HB ACHTUNGTRENNUNG(O2C2H4) catalyzed by a [RhCl ACHTUNGTRENNUNG(PH3)2] complex.Solid lines correspond to the Z path (R1 =Me), dashed lines correspondto the E path (R2 =Me).

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50 kcal mol�1) to give the rhodium/vinyl intermediates (9B),which are 24–25 kcal mol�1 lower in energy than compound7. The pro-E intermediate 9B, which contains the alkyne

substituent and the rhodium fragment trans to one other, is1.4 kcal mol�1 thermodynamically more stable than the pro-Z intermediate. Notably, the difference in energy betweenthe E and Z paths has increased on going from TSB8–9 to 9Bin favor of the E path. However, the high reverse barriersfor intermediates 9B prevent sterospecific product forma-tion from being achieved thermodynamically, provided thatthe two intermediates 9B do not interconvert (see below).Finally, the reaction proceeds by reductive elimination ofthe C�B bond to give the product via TSB9–10. Again, thebarrier for the termination step was calculated to be verylow, only 2.4 kcal mol�1 for the Z pathway.[16] By analogywith the proposed mechanisms for trans-hydrosilylation, weexplored a new mechanism that was consistent with the ob-served trans-hydroboration products. The reaction couldproceed via migratory insertion of the alkyne into the Rh�Bbond leading to a complex in which the boryl and alkyl sub-stituents of the vinyl ligand were trans to one another. Then,prior to the reductive C�H coupling step, the pro-Z inter-mediate could be formed via vinylic C�C rotation(Scheme 9). In addition, these calculations will evaluate thefeasibility of interconversion between the Z and E isomersin intermediate 9B. The attempts to obtain the zwitterioniccarbene/rhodium complex separately, as proposed by Ojimaet al.[9a] (Scheme 3), and the related h2-vinyl complex pro-posed by Crabtree and co-workers[9b–e] (Scheme 4) ended upwith the same structure, which resembles the proposal byCrabtree and co-workers (Scheme 9). This species was char-acterized as a transition state by a single imaginary frequen-cy, whose normal mode corresponds to the rotation of thevinylic C�C carbon bond. The computed energy barrier wasnot very high (20.5 kcal mol�1), but it was significantlyhigher (ca. 5 kcal mol�1) than of the reductive elimination ofthe C�H bond. Obviously, these results do not necessarilypreclude the proposed mechanism for the hydrosilylation ofalkynes, but they discard the C�C rotation in the trans-hy-droboration reaction catalyzed by phosphine-modified rho-dium complexes. Moreover, they indicate that the Z and Eisomers of intermediates 9B would not readily interconvertinto one other, leading to the thermodynamically more-stable E species. With all of these results in hand, we canconsider which could be the most-plausible selectivity-deter-mining step. Although the energy profiles might be tuned bythe inclusion of real ligands, some features are already clear.Comparing mechanism A and B, we observed the same en-ergetic pattern: the reaction goes uphill in energy until theboryl- and hydride-migration transition states, respectively.The resulting rhodium/vinyl complex is low in energy, andhas a very high reverse barrier and a low barrier to give theproduct. This pattern indicates that both boryl and hydridemigration are irreversible steps to give the product as de-fined Z and E stereoisomers. Because we have shown thatthe Z and E isomers do not interconvert easily, we can statethe either the boryl or hydride migration is the selectivity-determining step. As a consequence, the selectivity of theprocess is determined by kinetic control and can be de-scribed by applying transition-state theory (TST). The hy-

Scheme 8. A calculated potential-energy profile (kcal mol�1) for a varianton the mechanism suggested by Miyaura, in which hydrogen migrationonto the vinylidene carbon atom occurs first (mechanism B). Hydrobora-tion of propyne with HB ACHTUNGTRENNUNG(O2C2H4) catalyzed by a [RhCl ACHTUNGTRENNUNG(PH3)2] complex.Solid lines correspond to the Z path (R1 =Me), dashed lines correspondto the E path (R2 =Me).

Figure 2. Molecular structures and main geometric parameters of transi-tion state TSB8–9 and intermediate 9B in the pro-E and pro-Z propyne-hydroboration pathways using PH3 ligands. Distances in � and relativeenergies in kcal mol�1.

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dride migration in mechanism B has the lowest energy barri-er, and consequently, it is the most likely. Moreover, as wewill discuss below, mechanism B is fully consistent with theexpected selectivity. Thus, we focused on analyzing thismechanism by determining the transition states for hydridemigration to the vinylidene group. To assess the effect of thereal ligands and experimental selectivity, we performedhybrid quantum mechanics/molecular mechanics (ONIOM)calculations on the key TSB8–9 transition states. The MMregion included the phosphine substituents and methylgroups of pinacolborane.[17] We used ONIOM calculationsbecause they allowed us to screen several ligands and sub-strates. Moreover, we felt that non-bonding ligand–substrateinteractions are key factors governing selectivity. In thesecases, DFT/MM methods can give superior results to pureDFT calculations because dispersion forces are not properlydescribed for standard gradient-corrected density functionalapproaches.[18] Table 3 compares the relative energies of thekey pro-Z and pro-E transition states for different ligandsand substrates with the experimentally observed selectivity.For propyne and monophosphine ligands PH3 and PMe3

(Table 3, entries 1 and 2), the calculated energy differencebetween the Z and E transition states is low, with the Eisomer slightly favored. Upon introduction of the steric ef-fects of the bulky PCy3 and PiPr3 phosphine groups, thetrend inverts and the Z path becomes more favorable by 1.6and 0.5 kcal mol�1, respectively (Table 3, entries 3 and 4).These results are fully consistent with the observed majorproducts in our work and the work of Miyaura and co-work-ers,[7] thus supporting mechanism B for non-conventionaltrans-hydroboration reactions. If we take a closer look at thegeometry of the pro-E transition state with PCy3 (Figure 3),we can observe that the alkyne substituent is pointing to-wards the metal center, thereby establishing a repulsivesteric interaction with the auxiliary ligands. On the otherhand, the alkyne substituent in the corresponding pro-Z

transition state, points awayfrom the metal center. Thus, itis reasonable to think that in-creasing the bulk of the phos-phine or alkyne substituent willincrease the preference for theZ isomer by destabilizing the Epath. Next, we analyzed theorigin of substrate effects onthe reaction outcome. To under-stand the higher selectivity ob-served for aryl alkynes com-pared with aliphatic alkynes, weconsidered three different as-pects: 1) the intrinsic prefer-ence of the alkyne substituentsin the Z or E pathways, 2) thehigher sensitivity to the sterichindrance of the phosphine

Scheme 9. Schematic representation of an alternative trans-hydroboration reaction through vinylic C�C bond-rotation; this mechanism is analogous to those proposed for the trans-hydrosilylation reaction. The transitionstate involves an h2 interaction of the vinylic fragment with the rhodium center. The hydride reductive-elimi-nation cis mechanism is shown for comparison. Energies shown are in kcal mol�1.

Table 3. Calculated relative energies of pro-E and pro-Z transition statesfor different type of phosphines. Comparison with the experimental re-sults.

Entry Substrate Phosphine TSB8�9 Z/E[c]

pro-Z[kcal mol�1]

pro-E[kcal mol�1]

1 PH3 +0.5 0.0 –

2 PMe3 +0.6 0.0 9:62(a)

3 PCy3 0.0 + 1.6 66:30(a)

4 PiPr3 0.0 + 0.5 91:7(a,b)

5 PH3 0.0 + 0.2 –

6 PCy3 0.0 + 2.7 90:10

7 PiPr3 0.0 + 1.1 97:2(b)

8 PH3 0.0 + 0.1 –

9 PH3 0.0 + 0.4 –

[a] Results for 1-octyne; [b] Values taken from the paper by Miyaura andco-workers, see Ref [7]. [c] Ratio determined experimentally.

Figure 3. Molecular structures of the selectivity-determining pro-E andpro-Z transition states for vinylidene insertion into the Rh�H bondduring propyne hydroboration using PCy3 ligands at the B3LYP:UFFlevel. Hydrogen atoms from the phosphine groups and the borane agentare omitted for clarity. The atoms in the MM part are represented bysticks. Relatives energies shown are in kcal mol�1.

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groups, and 3) the preference of the vinylidene path (non-conventional trans) over the classical path (cis) that involvesdirect addition of the borane group to the alkyne. In the ab-sence of steric effects with the PH3 ligand, we observed aclear difference on going from methyl- to phenylacetylene.The relative energies of the pro-Z and pro-E transitionstates invert, with the Z isomer becoming preferred(Table 3, entries 1 and 5). Nevertheless, the energy differ-ence between the Z and E isomers for phenylacetylene isalso small (ca. 0.2 kcal mol�1), and consequently subtle ef-fects are expected to be responsible for them. Figure 4 col-lects the structures of the pro-Z and pro-E transition statesfor the phenylacetylene substrate. When comparing the twoisomers for each substrate type, we observed very similargeometric and electronic parameters (Figures 2 and 4). The

inverse trend should be related to the balance between thecis and trans disposition of the alkyne substituents and be-tween the cis and trans stabilization of the developing nega-tive charge at the vinylidene a-carbon atom. Other aryl-sub-stituted alkyne substrates show similar relative energies ofthe transition states for PH3 ligand (Table 3, entries 8 and9). Thus, the intrinsic electronic properties of phenyl acety-lene have a positive influence on the selectivity towards theZ product as compared with alkyl acetylenes. For phenylace-tylene, addition of the steric hindrance of PCy3 ligandcauses the energetic preference of the Z isomer to increaseby 2.5 kcal mol�1 (Table 3, entries 5 and 6). The latter valueis similar to that observed for methyl-acetylene (+2.1 kcalmol�1), thus showing that both types of substrates have simi-lar sensitivities to the steric effect exerted by the phosphineligands. The last aspect that we considered was whether thevinylidene formation becomes more favored than the cis-hy-droboration channel for phenylacetylene. Thus, assumingthat the 1,3-shift of the hydrogen onto the alkynyl ligand isthe rate-determining step in vinylidene formation,[14] we firstcalculated the energy barriers for different substrates withthe PH3 ligand. The overall energy barriers from the vinyli-dene complex for HC�CR (R=C6H4CF3, Me, Ph, and

C6H4CH3) were 30.1, 29.1, 28.5, and 28.0 kcal mol�1, respec-tively, which nicely correlated with the percentages of ob-tained Z isomer: 48 %, 66 %, 90 %, and 94 %, respectively.The lower the energy barrier, the more favored the non-con-ventional trans reaction channel is, and consequently, thehigher the selectivity towards the Z-alkene isomer. We iden-tified a linear relationship between the observed selectivityfor the Z isomer and the overall energy barriers for vinyli-dene formation (Figure 5). Lynam and co-workers[13a] havedetermined the rate constants for the isomerization of thetwo-step process for n-butyl- and phenyl-substituted alkynes.They obtained values for the 1,3-hydrogen shift that weresomewhat lower (22.8 and 22.1 kcal mol�1 for R=nBu andPh, respectively) than ours (29.1 and 28.5 kcal mol�1 for R=

CH3 and Ph, respectively) owing to our use of the simplified

less-basic PH3 ligands.[13a] However, the trend in substituenteffects observed by the same authors was reflected in thecalculations on our model systems, that is, that isomerizationis easier for aryl than for alkyl substituents. These resultsalso support the idea that the higher selectivity for the Zisomer in aryl-substituted alkynes is also related to the pre-dominance of vinylidene over the cis-hydroboration reactionchannel. We note that the rigorous analysis of this aspectwould require full characterization of both reaction chan-nels, thereby accounting for the electronic effects of PR3

ligand and the role of the base. Detailed studies on these as-pects are in progress in our laboratory, but we believed thatit is worthwhile reporting the first results on the simplifiedmodel systems.

Conclusions

We can conclude that the catalytic system based on the insitu mixture of [{Rh ACHTUNGTRENNUNG(cod)Cl}2] and basic and bulky phos-phine groups, such as PCy3, favored the non-conventionaltrans-hydroboration over the cis-hydroboration in the pres-ence of Et3N. The optimized reaction conditions for the hy-

Figure 4. Molecular structures and main geometric parameters of the se-lectivity-determining pro-E and pro-Z transition states for vinylidene in-sertion into the Rh�H bond during phenylacetylene hydroboration withPH3 ligands. Distances in � and relative energies shown are in kcalmol�1.

Figure 5. Correlation between the observed Z isomer (%) and the overallactivation barriers for vinylidene formation.

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FULL PAPERCatalytic Non-Conventional trans-Hydroboration

droboration of 1-octyne showed that the most-successfulsubstrate/borane ratio was 1:1.2, and tetrahydrofuranseemed to be the solvent of choice. The highest percentageof non-conventional trans-hydroborated product was ach-ieved with a ratio of [Rh]/PCy3 =1:4 and was successfullyformed at 0 8C, 25 8C, and 70 8C. Subtle changes in thenature of the substrate indicated that electron-rich alkyneswith high steric demands were mainly converted into the de-sired organoboron isomers. We have computationally char-acterized a plausible reaction mechanism for the non-con-ventional trans-hydroboration reaction through an initialalkyne to vinylidene isomerization, following the suggestionof Miyaura and co-workers based on their deuterium-la-beled experiment. Unlike their mechanism, we propose a se-quence of vinylidene insertion into the Rh�H bond followedby reductive elimination of the C�B bond. Thus, the mecha-nism can be divided into four stages: 1) isomerization of thealkyne into the vinylidene to yield a rhodium/vinylidenecomplex, 2) oxidative addition of the borane reagent, 3) vi-nylidene insertion into the Rh�H bond, and finally 4) reduc-tive elimination of the C�B bond to yield the 1-alkenylboro-nate. Calculations indicated that the insertion of vinylideneinto the Rh�H bond is the selectivity-determining step. In-troducing the steric effects of real ligands, we were able toreproduce the experimental outcome, thus supporting theconsistency of the proposed mechanism. Our calculationsalso indicated that bulky ligands are required to selectivelyobtain (Z)-1-alkenylboronates and that increasing the sterichindrance of the ligands causes an increase in selectivity ofthe Z isomer (PCy3>PiPr3>PMe3>PH3). The higher selec-tivity observed for aryl alkynes compared with aliphatic al-kynes can be explained by the analysis of different factors.The intrinsic electronic properties of aryl substituents aremore favorable for the Z pathway than alkyl substituents.For electron-donating substituents, the formation of the vi-nylidene complex is favored, which seems to facilitate theoccurrence of the vinylidene reaction channel over the clas-sical cis-hydroboration pathway.

Experimental Section

General : All reactions and manipulations were conducted using standardvacuum-line techniques under an atmosphere of dry nitrogen. All organicsolvents were distilled over dehydrating agents and degassed with nitro-gen before use. [{Rh ACHTUNGTRENNUNG(cod)Cl}2], [{Ir ACHTUNGTRENNUNG(cod)Cl}2], and all the substrates wereused as purchased from Sigma Aldrich. Deuterated solvents for routineNMR measurements were used as purchased from SDS. NMR spectrawere obtained on either a Varian Goku 400 or a Varian Mercury 400spectrometer. 1H NMR and 13C {1H} NMR chemical shifts are reported inppm (d) relative to tetramethylsilane. 11B {1H} NMR chemical shifts arereported in ppm (d) relative to BF3·Et2O. GC analysis was performed onan Agilent Technologies 6850 apparatus with a flame-ionization detectorequipped with an achiral column HP-5 (30 m, 0.25 mm ID, 0.25 mm thick-ness) using H2 as the carrier gas.

General procedure for the rhodium-catalyzed trans-hydroboration of al-kynes : Catalyst precursor ([{Rh ACHTUNGTRENNUNG(m-Cl) ACHTUNGTRENNUNG(cod)}2] or [{Ir ACHTUNGTRENNUNG(m-Cl) ACHTUNGTRENNUNG(cod)}2] =

0.015 mmol) and the ligand (0.06 mmol) were introduced into a previous-ly purged Schlenk tube under a nitrogen atmosphere and dissolved inTHF (3 mL) and NEt3 (3 mL, 5 mmol). The mixture was stirred for 5 min

to reach complete dissolution and complete formation of the catalyticcomplex in situ. Next, freshly distilled catecholborane (1.2 mmol) or pi-nacolborane (1.2 mmol) was added to the solution of catalyst followed bythe substrate (1 mmol). The mixture was stirred at RT for 4 h. The prod-ucts were characterized by 1H NMR spectroscopy and GC to determinethe degree of conversion and the selectivity obtained.

Computational details : All calculations were performed using the Gaussi-an09 series of programs.[19] Full quantum mechanical calculations onmodel systems were performed within the framework of density function-al theory (DFT)[20] using the B3LYP functional.[21] A quasi-relativistic ef-fective-core potential operator was used to represent the 28 innermostelectrons of the Rh atom, as well as the 10 innermost electrons of the Patoms.[22] The basis set for Rh and P atoms was that associated with thepseudopotential,[22] with a standard double-x LAN L2DZ contraction,[19]

and, in the case of P atoms, supplemented by a d shell.[23] The C, H, O,Cl, and B atoms were represented by means of the 6–31G ACHTUNGTRENNUNG(d,p) basisset.[24] All geometry optimizations were full, with no restrictions. Station-ary points located in the potential-energy hypersurface were character-ized as true minima through vibrational analysis. Transition states locatedin the potential-energy hypersurface were characterized through vibra-tional analysis, having one and only one imaginary frequency, whosenormal mode corresponded to the expected motion. For the hybrid quan-tum mechanics/molecular mechanics (QM/MM) calculations, we appliedthe ONIOM method as implemented in the Gaussian 09 package.[25] TheQM region included the [RhCl ACHTUNGTRENNUNG(PH3)2] complex, the methyl- and arylacetylene substrates, and the HB ACHTUNGTRENNUNG(O2C2H4) reagent. The MM region con-stituted of the methyl substituents of borane and the substituents (Me,iPr, and Cy3) of the phosphines. The QM level was the same as men-tioned above. UFF force field[26] was used as implemented in Gaussian 09to describe the atoms included in the MM part.

Acknowledgements

The authors are grateful for financial support from the MEC of Spain(CTQ2010–16226, CTQ2011–29054-C02–01, CTQ2005–08351), from theConsolider Ingenio 2010 (CSD2006–0003), and from the Direcci� Gener-al de Recerca (DGR) of the Autonomous Government of Catalonia(2009SGR462 and XRQTC). J.C. thanks the URV for a grant.

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Received: August 31, 2011Published online: December 30, 2011

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FULL PAPERCatalytic Non-Conventional trans-Hydroboration