Assessing Ligand and Counterion Effects in the Noble Metal

Research Article pubs.acs.org/acscatalysis

Assessing Ligand and Counterion Effects in the Noble Metal Catalyzed Cycloisomerization Reactions of 1,6-Allenynes: a Combined Experimental and Theoretical Approach Florian Jaroschik,†,∥ Antoine Simonneau,†,∥ Gilles Lemière,† Kevin Cariou,† Nicolas Agenet,† Hani Amouri,† Corinne Aubert,† Jean-Philippe Goddard,† Denis Lesage,† Max Malacria,† Yves Gimbert,*,‡ Vincent Gandon,*,§ and Louis Fensterbank*,† †

Sorbonne Universités, UPMC Univ Paris 06 and CNRS, IPCM (UMR 8232), F-75005 Paris, France Université Grenoble Alpes and CNRS, DCM (UMR 5250), F-38000 Grenoble, France § Institut de Chimie Moléculaire et des Matériaux d’Orsay, CNRS UMR 8182, Université Paris-Sud, Université Paris-Saclay, Bâtiment 420, 91405 Orsay cedex, France ‡

S Supporting Information *

ABSTRACT: 1,6-Allenynes are useful mechanistic probes in noblemetal catalysis, since they can give rise to very distinct products in a highly selective fashion. Various cycloisomerization reactions have been described, and discrete mechanisms have been postulated. Of particular interest, whereas Alder-ene types of products can be obtained in a variety of ways using noble-metal catalysts (Au, Pt, Rh, ...), hydrindienes have been reported solely with gold and platinum under specific conditions. It was shown in a previous study that this intriguing transformation required the presence of chloride ligands at the active catalytic species. Herein, the factors governing the fate of 1,6-allenynes under cycloisomerization conditions have been studied more thoroughly, revealing a much more complex scenario. The nature of ligands, counterions, and metals was examined, showing that hydrindienes can be isolated in the absence of halides, using electron-rich, bulky triorganophosphines or carbene ligands. This crucial finding could also be used to access hydrindienes in high yields, not only with gold or platinum but also with silver. On the basis of mass spectrometry, NMR spectroscopy, and computations, refined mechanistic scenarios have been put forward, also rationalizing counterion effects. Notably, a metal vinylidene intermediate has been proposed for the formation of the hydrindiene derivatives. Finally, in the presence of tris((triphenylphosphine)gold)oxonium tetrafluoroborate as catalyst, a new pathway has been unveiled, involving gold alkyne σ,π complexes and leading to previously unobserved [2 + 2] cycloaddition compounds. KEYWORDS: allenyne, gold catalysis, cycloisomerization, counterion effect, vinylidene, DFT calculations, Alder-ene

1. INTRODUCTION The selectivity of organic transformations relying on homogeneous catalysis can be dramatically influenced by the experimental conditions (metals, organoligands, counterions, solvent, atmosphere, pressure, light, additives, ...). All of these parameters have been thoroughly studied for reactions of industrial importance, notably those based on palladium or rhodium catalysis.1 On the other hand, the flourishing field of gold and platinum catalysis2−4 is still barely understood, especially with respect to ligand and counterion effects.3c,q Recently, impressive progress has been made in the understanding of mechanisms governing cycloisomerization reactions.5 Enynes have been intensely used as mechanistic probes because of the various types of skeletal rearrangements that they can undergo.5,6 Even though these studies allowed the postulation of mechanistic rationales, characterizations of © XXXX American Chemical Society

putative intermediates that could support these hypotheses are still scarce.7 1,n-Allenynes, a particular class of enynes, have also received much attention in transition-metal and maingroup catalysis.8,9 Our previous studies on 1,6-allenynes A showed that these compounds could be converted either into the unusual hydrindiene type of products B or into the more commonly observed Alder-ene products of type C (Scheme 1).10 While the former was only observed when gold or platinum chlorides were used as catalysts, complexes exhibiting weakly coordinating anions only gave rise to the latter. Because our initial study was rather limited in terms of “halide-free” catalysts,10b we decided to take a closer look at the Received: November 27, 2015 Revised: June 20, 2016

5146

DOI: 10.1021/acscatal.5b02696 ACS Catal. 2016, 6, 5146−5160

Research Article

ACS Catalysis Scheme 1. Gold- and Platinum-Catalyzed Cycloisomerization of 1,6-Allenynes into Hydrindienes B or Alder-ene Products C

Table 1. Gold- and Platinum-Catalyzed Cyclizations of Allenyne 1

cat.a

entry

influence of ligands (phosphines and carbenes) and counterions. Deuterium labeling and mass spectrometry experiments, as well as DFT computations, were also conducted. This scrutiny led to unexpected discoveries which shed a new light on this reaction.

2. RESULTS AND DISCUSSION 2.1. Catalyst Scope. Focusing on 1,6-allenyne 1, we examined a variety of catalysts (Tables 1−3) and notably the gold bipyridine, phosphine, and carbene complexes C1−C5 (Chart 1). In several cases, the bulky triorganophosphine L1 was also introduced into the reaction mixtures as an external ligand. Chart 1. Catalysts and Ligands Surveyed in Tables 1−3

1b 2 3b 4b 5

AuCl Cl AuCl3 NaAuCl4 AuBr3

6

AuI

7

AuCN

e

8

PtCl2

9b

PtCl4

10

PtBr2

11 12 13b 14 15 16 17 18b 19b

K2PtBr6 PtI2 Ph3PAuCl + AgSbF6f Ph3PAuCl + AgPF6f Ph3PAuCl + AgBF4f Ph3PAuCl + AgOTff Ph3PAuCl + AgClO4f Ph3PAuNTf2 [Pt(PhCN)2dppp] [BF4]2

T (°C) 0 0 0 0 room temp room temp room temp room temp room temp room temp 80 80 0 0 0 0 0 0 35

time (h)

yield of 2 (%)

0.5 0.25 0.5 1 2

79 73 80 75 80

yield of 3 (%) g g g g g

1

c

24

d

24

80

g

1

80

g

24

75

g

24 3 0.25 0.5 0.5 0.5 0.5 6 0.3

6 78

g g 70 70 72 84 75 68 57

5

(1/1)h (1/1)h (1/1)h (1/1)h (1/1)h (2/1)h (3/1)h

a

Conditions unless specified otherwise: [Au] (1 mol %), CH2Cl2, [Pt] (5 mol %), toluene. bSee ref 10b. cComplex mixture. dNo reaction. e See ref 10a. fLAuCl (1 mol %) and AgX (1 mol %). gTrace amounts observed. hEndo/exo ratio.

We next turned our attention to the nature of the [LAu] moiety (Table 2). Gold complexes incorporating the ligands triethyl (C2)-, tri-tert-butyl (C3)-, and biphenyl-2-yl-di-tertbutylphosphine (C4)14 as well as the N-heterocyclic carbene IPr (C5)15 were successively tested in combination with various counterions (entries 1−16). In the SbF6− series (entries 1−4), while Ph3P delivered 3 as the sole product (see entry 13 of Table 1), a mixture comprising 19% of 2 and 57% of 3 was obtained when Et3P was used (entry 1). Surprisingly, a reversal of chemoselectivity took place with t-Bu3P with respect to Ph3P, compound 2 being obtained selectively in 80% yield (entry 2). This result represents the first case of a selective hydrindiene formation with a cationic catalyst. Similarly, C4 and C5 allowed the transformation of 1 into 2 (entries 3 and 4). Interestingly, in sharp contrast with the Ph3P series, the nature of the anion proved crucial for the reaction outcome. With t-Bu3P (C3), while PF6− led to 2 like SbF6− (entry 5), a 1.3:1 mixture of 2 and 3 was isolated with BF4− (Table 2, entry 6). On the other hand, it was possible to annihilate the bulky phosphine effect by selecting the TfO− or the ClO4− anions, 3 being formed again as the sole product (entries 7 and 8). A similar trend was observed in the C4 (entries 9−12) and C5 series (entries 13−16), the major or sole product being 2 with PF6− and BF4− and 3 with TfO− or ClO4−. Thus, with [LAu][Y]

The first set of cycloisomerization reactions of allenyne 1 are summarized in Table 1. As expected, this compound transformed into either hydrindiene 2 or the Alder-ene product 3. In line with what we had previously observed, Au(I) and Au(III) chlorides led to 2 exclusively (entries 1−4). Full conversion of 1 was reached in less than 1 h at 0 °C using 1 mol % of these salts. Gold(III) bromide gave the same result, yet in 2 h at room temperature (entry 5). A complex mixture ensued when gold(I) iodide was used (entry 6), and no reaction took place with gold(I) cyanide (entry 7). Platinum(II) and -(IV) chlorides, bromides, and iodides also led to 2 exclusively (entries 8−12). Whereas K2PtBr6 gave 2 in poor yield (entry 11), other halide salts fully converted 1 and gave 2 in good isolated yields at room temperature (entries 8−10) or at 80 °C (entry 12). Following a previously observed trend,11 PtCl4 proved to be the most reactive species (entry 9). Some cationic complexes were investigated next (entries 13−19). They were generated in situ from Ph3PAuCl and AgY (Y = SbF6−, PF6−, BF 4 − , TfO − , ClO 4 − ), prepared independently ([Pt(PhCN)2dppp][BF4]2),12 or purchased ([Ph3PAu][NTf2]).13 Apart from the last species, with which traces of 2 were isolated (entry 18), these species gave rise to an inseparable regioisomeric mixture of Alder-ene products 3 exclusively, in 57−84% yield, whatever the counterion. 5147


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ACS Catalysis Table 2. Screening of Au/Ag Two-Component Mixtures for the Cyclization of Allenyne 1

cat.a

entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

C2 C3 C4 C5 C3 C3 C3 C3 C4 C4 C4 C4 C5 C5 C5 C5

+ + + + + + + + + + + + + + + +

AgSbF6 AgSbF6 AgSbF6 AgSbF6 AgPF6 AgBF4 AgOTf AgClO4 AgPF6 AgBF4 AgOTf AgClO4 AgPF6 AgBF4 AgOTf AgClO4

yield of 2 (%)

yield of 3 (%)b

19c 80 83 80 65 47c

57 (1/1)c d d d d 35 (1/1)c 75 (1/1) 85 (1/1) d d 69 (1/1)c 53 (1/1)c 27c 10c 53 (1/1)c 82 (1/2)

76 85 14c 27c 53c 71c 27c

Table 3. Silver-Catalyzed Cyclizations of Allenyne 1 (X = C(CH2OMe)2)

entry 1 2 3 4 5 6 7 8 9 10 11 12 13

a

Conditions: LAuCl (1 mol %) and AgX (1 mol %), CH2Cl2. bExo isomer; otherwise, endo/exo ratio in parentheses. cDeduced from overall yield and 2:3 1H NMR ratio. dTrace amounts observed.

14

cat.a AgSbF6 AgSbF6c AgSbF6d AgPF6 AgBF4 AgOTf AgClO4 AgNO3 AgCl AgI AgSbF6/L1 (1/1) AgPF6/L1 (1/1) AgBF4/L1 (1/1) AgOTf/L1 (1/1)

time (h) 0.25 1 1 0.25 1 1 4 4 4 4 1

yield of 2 (%)

yield of 3 (%)b

yield of 4 (%) 60

82 (1/1) 82 (1/1) 55 38

37 (1/1) 73 (1/1) e e e e

85

f

1

85

f

1

85

f

1

85

f

a

Conditions: [Ag] (5 mol %), CH2Cl2 unless stated otherwise. Endo/exo ratio in parentheses. cConditions: [Ag] (5 mol %), toluene. dConditions: [Ag] (5 mol %), Et2O. eNo reaction. fTrace amounts observed.

b

species, cooperative effects between L and the weakly coordinating anion Y− are possible. One may argue that these catalysts were generated in situ in the presence of silver and that the latter may also be involved. On the basis of some recent literature data, it seems clear that a potential interplay between gold and silver should not be ruled out too lightly.16 Although we reported in our previous study10b the inactivity of AgSbF6 and AgOTf toward the cycloisomerization reaction of a TMS analogue of 1, we counterchecked blank experiments with a freshly opened container of AgSbF6 under rigorous exclusion of air. Unexpectedly, when it was mixed with 5 mol % of AgSbF6 in dichloromethane, allenyne 1 was completely converted within 15 min at room temperature into the previously unobserved vinyl chloride 4 (Table 3, entry 1). In toluene or diethyl ether, the reaction furnished 3 in 82% yield (entries 2 and 3). We were then again confronted with a fascinating weakly coordinating anion effect. In dichloromethane, while PF6− gave the same result as SbF6− (entry 4), BF4− partially diverted the selectivity toward 2 (entry 5). On the other hand, no reaction took place when AgClO4, AgNO3, AgCl, or AgI were used (entries 7−10). Keeping in mind that the introduction of bulky ligands could favor the formation of 2 in the gold series, (biphenyl-2-yl)(t-Bu)2P (L1) was added to the silver salts17 before introducing 1 in the reaction mixture (entries 11−14). This time, it was the counterion effect that was annihilated, these reactions leading exclusively to 2. Finally, it is worth noting that the transformations employing silver salts as catalysts required higher catalyst loadings, prolonged reaction times, and higher temperatures in comparison to the respective gold-catalyzed experiments, suggesting that [LAu]+ is the active species when LAuCl and AgX are used jointly. Nevertheless, the expedient alternative of silver shows once more the dramatic influence of counterions and organoligands on the reaction outcome.

2.2. Substrate Scope. The generality of these new ligand, metal, and counterion effects was then investigated with differently substituted allenynes (Table 4). In the case of allenyne 5, bearing a gem-diester group instead of the gemdiether group in the tether, it had been shown previously that PtCl2 catalysis led to the selective formation of hydrindiene product 6.10a It is now shown that AuCl3 also leads to 6 in very good yield (entry 1). As expected, the cationic gold(I) catalyst PPh3AuNTf2 led to the exclusive formation of Alder-ene product 7 in 80% yield (entry 3). The gold and silver catalysts based on the Johnphos ligand yielded again selectively hydrindiene 6 (entries 4 and 6). AgSbF6 alone led to the chlorinated compound 8 (entry 5), analogously to 4. Thus, all these findings are fully consistent with the results obtained with precursor 1. Heteroatom-containing allenynes can bring substantial variation in the reaction outcomes due to modified stereoelectronic constraints.3 Allenyne 9 with a NTs group in the linker led to a mixture of products under Pt catalysis, among which the hydrindiene 10 and the Alder-ene product 11 could be identified (entry 8). With gold catalysts, no reaction was observed with simple gold halide complexes or PPh3AuX derivatives (entries 7 and 9). However, the gold catalysts C2/ AgSbF6 and C4/AgSbF6 yielded hydrindiene 10, showing once again the importance of the electronic and steric influence of the phosphine ligand (entries 10 and 11). The same observation was made under silver catalysis. Whereas no reaction was observed with AgSbF6 alone, the combination of this salt with Johnphos ligand L1 led to the selective formation of 10 (entries 12 and 13). Finally, we also attempted the transformation of the trisubstituted allenyne 12. The formation of hydrindiene products from such substrates had not been 5148


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ACS Catalysis Table 4. Cycloisomerization Reactions with Various Allenynes

entry

allenyne

cat.a

1 2c 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

5 5 5 5 5 5 9 9 9 9 9 9 9 12 12 12 12 12 12

AuCl3 PtCl2 Ph3PAuNTf2 C4 + AgSbF6d AgSbF6e AgSbF6/L1f AuCl3g PtCl2 Ph3PAuSbF6d C2 + AgSbF6d C4 + AgSbF6d AgSbF6g AgSbF6/L1f AuCl3 PtCl2 Ph3PAuSbF6d C4 + AgSbF6f,h AgSbF6i AgSbF6/L1f,g

T (°C) 0 room 0 0 10 room room 80 room room room room room room 80 room 0 room room

temp

temp temp temp temp temp temp temp temp temp temp temp

time (h)

yield of A (%)

0.5 24 0.5 0.5 0.5 0.5 24 8 3 1 1 24 4 6 6 3 4 2 4

85 60

yield of B (%)b

yield of C (%)

80 (5/2) 90 45 90 21

21 (0/1)

42 71

43 (0/1) 15 (0/1)

50 60

75 10 74 20

(0/1) (0/1) (0/1) (0/1) 50

a

Unless specified otherwise: [Au] (1 mol %), CH2Cl2; [Pt] (5 mol %), toluene; [Ag] (5 mol %), CH2Cl2. bEndo/exo ratio. cSee ref 10a. Conditions: LAuCl (1 mol %) and AgSbF6 (1 mol %). eConditions: [Ag] (10 mol %). fConditions: [Ag] (5 mol %), L1 (5 mol %), CH2Cl2. gNo reaction. hComplex reaction mixture. iConditions: [Ag] (20 mol %). d

CD3OD in the case of 1 → [CD3OD]-3, while the M−C bond would be cleaved by D+ with retention of configuration.

reported before. Consistent with the results already mentioned involving metal halide complexes, PtCl2 led to the selective formation of hydrindiene 13 at higher temperature (entry 15). AuCl3 and PPh3AuX salts yielded selectively the Alder-ene product 14 (entries 14 and 16). This contrasts with the reaction of allenyne 5 with AuCl3, which gives hydrindiene 6. Very likely, allylic strain between the R1 group and the terminal methyl groups favors the hydrindiene pathway.18 In contrast also to the other substrates, gold and silver catalysts with the Johnphos ligand did not result in productive reactions (entries 17 and 19). However, with AgSbF6 (20 mol %) alone, the expected chloride product 15 was obtained (entry 18). In conclusion, even though some substrate dependence is evidenced in these reactions, a general trend can be drawn: metal halide catalysts are prone to give hydrindiene products, as is the case with cationic gold and silver catalysts carrying bulky electron-rich phosphine ligands. Cationic PPh3AuX complexes either lead to Alder-ene type products or do not react at all. AgSbF6 alone in dichloromethane gives rise to new cycloisomerization products containing a chlorine atom with most substrates. 2.3. Mechanistic Rationalization. 2.3.1. A Common Intermediate? In our previous study,10b we performed a NMR monitoring of the cycloisomerization of 1 with 1 mol % of AuCl3 in CD3OD, providing selectively [CD3OD]-3. This finding implies that these cycloisomerizations transit via a carbocationic intermediate of type I2, formed by nucleophilic attack of the external allene double bond onto the goldcomplexed alkyne (I1) in a 6-exo-dig fashion (Scheme 2). The allylic carbocation I2 would be regioselectively trapped by

Scheme 2. Trapping of the Putative Intermediate I2 (M = Au, Ag, Pt) with CD3OD

In an attempt to detect a less advanced cationic intermediate (before protonolysis of the M−C bond), we performed electrospray ionization (ESI) mass spectrometry experiments with allenyne 1 and C1, a rare type of charged gold(III) chloride,19 but this time with 3% methanol (CH3OD or CD3OD) in dichloromethane as the solvent for infusion during the ESI process (Figure 1). Adducts [1 + bipyAuCl2 + CH3OD]+ and [1 + bipyAuCl2 + CD3OD]+ that would correspond to m/z 692 (16-H) and 695 (16-D) were not detected, probably because the association energy between methanol and the cationic intermediate was too weak. Instead, we observed ions at m/z 655 and 658, 5149


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Scheme 4. From I2 to Hydrindienes (M = Au, AuCl2, PtCl, PtCl3)

However, hydrindienes could also be selectively obtained with bulky phosphine or carbene complexes lacking a halide ligand. These new findings encouraged us to further probe the reaction mechanism using labeled substrates. A series of experiments with the deuterated precursors 1-D and 1-D6 cast new light on this process (Scheme 5). In the presence of

Figure 1. ESI experiments by infusion in dichloromethane/CH3OD (CD3OD) of 1 and C1.

corresponding respectively to 17-H and 17-D (Scheme 3). They are likely obtained from 16-H and 16-D after the loss of

Scheme 5. Reaction with the Deuterated Precursors 1-D and 1-D6

Scheme 3. Formation of Ions 17-H and 17-D at m/z 655 and 658

DCl and before protonolysis of the Au−C bond. Intriguingly, the liberation during the collision-induced dissociation (CID) process of a strong acid in the gas phase (DCl) near the metal center does not trigger, even in trace amounts, the formation of a deaurated species. This observation would suggest that the protodeauration is an energy-demanding process, which is consistent with other literature reports.20 Presumably, it corresponds to the rate-determining step of this type of reaction. It is therefore reminiscent of the type II reactions defined by Hammond and Xu, for which the regeneration of a cationic gold catalyst is also the rate-determining step.20b 2.3.2. Hydrindienes. Our initial mechanistic rationale10b was based on the results we obtained with the few tested catalysts at that time (Scheme 1). We proposed the formation of I2 described above from which, as suggested by DFT computations, intramolecular nucleophilic attack by the vinyl metal fragment would occur to give four-membered-ring complex I3 (Scheme 4). Ring opening of I3 would then lead to I4, and subsequent HCl elimination would provide the vinylgold complex I5. The calculated mechanistic sequence eventually yielded alkylmetal complex I6 after 5-endo-trig carbometalation.21,22 Pertaining to this proposal, all metal halides (except AuI) yielded only the hydrindiene product 2 when starting from allenyne 1 (Table 1). In addition, the mass spectrometry experiments summarized in Figure 1 and Scheme 3 confirmed that HCl can be eliminated from organogold chlorides.

NaAuCl4, 1-D transformed into the anticipated hydrindiene product 2-D, but the deuterium incorporation took place at the unexpected methine position. Similar results were obtained with AuCl3, C1, and C4 + AgSbF6. Moreover, the precursor 1D6 gave rise to hydrindiene 2-D6, showing an unforeseen incorporation of deuterium at the vinylic position when NaAuCl4, KAuCl4, or complexes of gold and silver bearing the bulky (1-biphenyl)(tBu)2P (L1) ligand were used. It is worth noting that a cross experiment with a mixture of 1-D6 and unlabeled allenyne 5 provided both of the anticipated hydrindiene products 2-D6 and 6 without any detectable scrambling of the deuterium atoms. To account for the observation of these unanticipated deuteration patterns, we had to partially revise our initial scenario. Instead of setting the stage for an addition of the 5150


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through TSe. On the other hand, the Alder-ene pathway comprises two elementary steps: the formation of Int2 via TSa as above and the 1,5-proton shift to give Int3 through TSb (red). There are no significant geometrical differences in the carbenium framework of Int2 complexes fitted with phosphine or NHC ligands (see Figure 2 and Figure S1 in the Supporting

vinylmetal, allylic cation I2 could rather evolve toward metal vinylidene I7 via a 1,4-hydride shift (Scheme 6). The latter Scheme 6. Metal Vinylidene Pathway

would then undergo an intramolecular C−H insertion providing hydrindiene products B, which is consistent with the deuterium labeling. Gold, platinum, or silver vinylidenes have been invoked sporadically in the literature.23 Interestingly, they have been generally proposed with halide complexes24 or complexes with bulky ligands, notably bulky NHCs.25 Recently, Aue, Zhang, and Hashmi have reported on the intervention of gold vinylidene complexes and their reactivity. They have notably disclosed an intramolecular C(sp3)−H insertion very similar to that proposed in the formation of products 2 from I7.26 Computations were carried out to validate this mechanism proposal. Following recent literature reports27,28 combined with our own explorations (see the Supporting Information for full computational details), all of the relative energies discussed in this work have been obtained at the B2PLYP/def-2 TZVP (all atoms)//B3LYP/LAN2DZ(ECP, Au), 6-31G(d,p) (other atoms) level. The vinylidene pathway, leading to the hydrindiene, was calculated with gold complexes fitted with various ligands (Scheme 7; Int1 → Int6 (blue)). It requires Scheme 7. Computed Pathways Leading to the Alder-ene Framework (Int3) or to the Hydrindiene Framework (Int6) Figure 2. Geometries of the computed Int2 and complexes with M = IPrAu+ (top) and L1Au+ (bottom) (distances in Å).

Information for representative cases). In particular, the C1−C2 bond length is always close to 1.35 Å. The C2−C7 distance of ∼1.50 Å suggests that there is no delocalization of the C1−C2 π electrons toward the allyl cation moiety. As expected for an allyl cation, the C7−C6 and C7−C8 bond lengths are virtually identical and are close to 1.40 Å. In Int3, the organic ligand becomes an electron-rich olefin to which the metal is bound in a η1 fashion to C1. The C8−C9 bonds are almost perpendicular to C2−C7 in all cases to avoid steric hindrance with the methyl group at C6 and the metal fragment (see the Supporting Information). The M−C1 bonds are similar for Int4 and Int5 but significantly shorter than those of Int2 and Int3 (see Figure 3 and Table S5 and Table S6 in the Supporting Information for representative examples). Also pertaining to the vinylidene depiction of these complexes, the C1−C2 bond lengths computed for Int4 and Int5 are markedly shorter than for the preceding complexes. The XMC1 (where X is the atom bound to Au in the M fragment, i.e. P or C) and MC1C2 angles are close to 180° and therefore are in agreement with the vinylidene rendition of Int4 and Int5. A significant exception is found with the L1Au+ fragment in the case of Int4. This time,

four elementary steps: (1) a 6-exo-dig nucleophilic attack of the external allene double bond to the gold complexed alkyne moiety to give Int2 through the transition state TSa; (2) a 1,4hydride shift giving rise to the gold vinylidene complex Int4 through TSc; (3) conformational change through TSd to give Int5, which is preorganized for insertion; (4) C−H insertion giving rise to Int6, which contains the hydrindiene framework, 5151


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Figure 4. Geometries of the computed TSc with M = IPrAu+ (top) and L1Au+ (bottom) (distances in Å).

Figure 3. Geometries of the computed Int4 with M = IPrAu+ (top) and L1Au+ (bottom) (distances in Å).

the PAuC1 angle is as low as 167.6°. This could be the consequence of an interaction with one phenyl group.17 In Int4, the distance between gold and the phenyl centroid is 3.38 Å, which is the lowest of all computed C4 complexes. The geometrical description of TSa and TSb has already been discussed elsewhere.10b We will focus here on TSc and TSe (see Table S7 in the Supporting Information). Reaching TSc from Int2 means compression of the C2−C7 bonds, which are shorter than in both Int2 and Int4 (see Figure 4 for representative examples and Table S6 in the Supporting Information). The H transfer is largely asynchronous and seems related to the size of the ligand of the M fragments. With larger ligands, the C1−H bond is shorter and, conversely, the C8−H bond is longer. We can note that similar values are found for PEt3 and PPh3; thus, this effect is unlikely to be of electronic origin. In TSe, the bond order of Au−C1 and C1−C2 clearly diminishes in comparison to Int5. In contrast with TSc, the nature of the ligand of the M fragment exerts only a small influence on the C1−H and C9−H bond lengths. The C1−C9 bond distance is also only modestly affected. As for Int2, the Au−X bonds are relatively short in Int6 and there is delocalization of the π electrons along the C1C2C7C6 framework (Table S8 and Figure S4 in the Supporting Information). Table 5 (see Table S2 in the Supporting Information for solvated values) shows the Gibbs free energies corresponding to Scheme 7. The first line deals with M = AuCl3 and then from H3PAu+ to L1Au+ by order of increasing bulk (see section

2.3.3). The formation of the six-membered-ring complex Int2, which is common to the two reaction pathways, is slower with LAu+ fragments, in which L is a bulky ligand (TSa > 12 kcal/ mol), in comparison to the parent PH3 or AuCl3 (TSa < 4.4 kcal/mol). This process is always exergonic by 6−23 kcal/mol but to a lesser extent with larger ligands such as Me3P (−10.7 kcal/mol), Et3P (−7.5 kcal/mol), Ph3P (−7.6 kcal/mol), tBu3P (−6.4 kcal/mol), IPr (−6.8 kcal/mol), or JohnPhos (C4, −8.2 kcal/mol) in comparison to H3P (−22.1 kcal/mol). The formation of Int3 is also strongly exergonic by 28−37 kcal/mol in each case, with slight differences also attributable to the steric bulk brought about by the ligand: H3P (−27.8 kcal/mol) vs IPr (−36.6 kcal/mol). For TSb, a trend similar to that for TSa is observed: LAu+ fragments result in a much higher free energy (>10 kcal/mol relative to Int1) than for AuCl3 and H3PAu+. Importantly, the free energies of all TSc are much lower than those of TSb (from 13.3 kcal/mol with AuCl3 to 19.9 kcal/mol with PPh3Au+), involving also in all cases lower barriers from Int2. Depending on the ligand, the Int2 → Int4 step can be endergonic (AuCl3, H3PAu+, Me3PAu+, Et3PAu+, Ph3PAu+), thermoneutral (tBu3PAu+, C4Au+), or exergonic (IPrAu+). TSd is also always lower in energy than TSb. Int5 is always more stable than Int4. Finally, TSe is easily accessible and leads to Int6 in a strongly exergonic fashion. Overall, TSc, TSd, and TSe are all markedly lower in energy than TSb. Thus, in all cases, the vinylidene pathway proved to be the preferred one in comparison to the highly demanding intramolecular proton transfer pathway leading to the Alder-ene product. 5152


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Table 5. Computed Gibbs Free Energies (at 298 K; B2PLYP/def2-TZVP//B3LYP/LANL2DZ(Au), 6-31G(d,p), kcal/mol), Relative to Int1 (or to Another Intermediate in Parentheses) M

TSa

Int2

TSb (rel to Int2)

Int3

AuCl3 H3PAu+ Me3PAu+ Et3PAu+ Ph3PAu+ tBu3PAu+ IPrAu+ C4Au+

2.6 4.4 12.2 13.7 15.9 14.7 13.9 12.4

−22.6 −22.1 −10.7 −7.5 −7.6 −6.4 −6.8 −8.2

−2.5 (20.1) 1.4 (23.5) 12.1(22.8) 14.9 (22.4) 16.5 (24.1) 16.2 (22.6) 14.8 (21.6) 13.3 (21.5)

−50.4 −49.7 −42.6 −41.3 −41.2 −40.9 −43.4 −41.6

TSc (rel.to Int2) −15.8 −11.5 −3.6 −1.2 −3.4 −1.2 −1.5 −1.3

(6.8) (10.6) (7.1) (6.3) (4.2) (5.2) (5.3) (6.9)

Int4 −16.1 −15.3 −7.3 −6.1 −6.2 −6.2 −8.7 −8.2

TSd (rel to Int4) −6.2 −9.3 −2.9 −1.1 −0.1 −1.3 −3.8 −2.1

(9.9) (6.0) (4.4) (5.0) (6.1) (4.9) (4.9) (6.1)

Int5

TSe (rel to Int5)

Int6

−19.4 −17.9 −11.3 −12.4 −10.1 −10.0 −13.3 −10.8

−15.3 (4.1) −14.9 (3.0) −6.5(4.8) −4.3 (8.1) −2.5 (7.6) −1.7 (8.3) −5.0 (8.3) −3.9 (6.9)

−66.7 −62.6 −56.1 −54.2 −56.1 −53.3 −59.0 −52.0

electron density of H bonding is stronger with TfO− than with BF4− or PF6−.36 This should also be the case with ClO4−. To gain some insight into the role of counterions in the proton shift as depicted in case 2 (Scheme 8), DFT calculations were performed on the cycloisomerization of 1 with Me3PAu+ in the presence of a triflate anion (Scheme 9, for calculations with ClO4−, see Scheme S2 in the Supporting Information). In the corresponding Int2, the triflate anion is coordinated to gold with an O−Au bond distance of 3.44 Å (see Figure S5 in the Supporting Information). The latter evolves in a first step by proton abstraction from an allenic methyl group to generate triflic acid in close vicinity to the vinyl gold moiety (Int2-H + TfOH). The energetic cost for this step is 3.2 kcal/mol, giving a strongly exothermic transformation by more than 18 kcal/mol. Finally, in a second step, the protonation by TfOH of the vinyl gold moiety is allowed (barrier of 6.1 kcal/mol from Int2-H + TfOH), giving Int3 + TfO− in an exothermic step by more than 27 kcal/mol. If we compare this energetic profile (shown in red) to the energetic profile (shown in black) corresponding to the hydride shift through TSc + TfO− giving Int4+TfO−, we can observe a similar barrier (3.3 vs 3.2 kcal/mol). However, in the case of the hydride shift, this transformation is virtually thermoneutral, and whatever the energetic cost for the step after Int4, all is definitively drawn at this stage: the Alder-ene formation is favored in the presence of a triflate counterion (Scheme 9A). By comparison with the energetic profile without TfO− (Scheme 9B), some comments can be made. The presence of TfO− results in a decrease of the barrier values for TSb and TSc, but this effect is stronger for TSb, which corresponds to the Hae proton migration (toward Cae), than for TSc, involving the Hh hydride shift (toward Ch). We can understand why, by considering geometrical constraints in Int2 (Figure 5). The fact that TfO− is ligated to Au can be supported by the change in the P−Au−Cae angle, almost 180° in case B, respecting the expected linear coordination and becoming 163° in case A, due to an interaction with an O atom of the triflate. The triflate also interacts through the remaining O atoms with hydrogen atoms Hae (with distances of ∼2 Å between O and Hae) that exhibit an acidic character because of the proximity of the cationic center Ch. Thus, the triflate can ideally act as an efficient proton shuttle from a methyl group and generate a molecule of triflic acid still coordinated to the metallic center. These interactions also have an important consequence, that is the diminishing of the Ch−Hh distance (2.42 Å in form A vs. 2.714 Å in form B), thus rendering the hydride transfer easier. These combined effects could be a satisfactory explanation for the lowering of both TSb and TSc. Focusing on the proton migration, the fact that, in the first time, the triflate can easily generate a double bond by a proximity driven proton abstraction and that, in a second time,

2.3.3. Alder-ene Products. Experimental results contradict the computationally predicted systematic preference for the hydrindiene product (Table 5). We may speculate that intermolecular protoderauration would intervene and compete with the previously proposed intramolecular proton transfer. This intermolecular process would be also strongly dependent on the steric bulk of the ligand. Thus, we suggest two additional mechanistic pathways (Scheme 8). In case 1, I2 eliminates H+ Scheme 8. Mechanistic Pathways Leading to C

to give I9, a precursor of C, through metal−carbon bond protolysis. Case 2 is an intramolecular proton shift which gives C (corresponding to 3 with (X = C(CH2OMe)2) directly. It is similar to the 1,5-proton shift depicted in Scheme 6; however, with the assistance of the counterion, the energy barrier would be lowered. Case 2 accounts for the formation of Alder-ene products when using C3−C5 with AgY and Y = TfO−, ClO4− (Table 2, entries 7, 8, 11, 12, 15, and 16). It is now well established that some counterions can actually assist deprotonations.29 It has been shown by computations that TfO− can serve as a proton shuttle in gold-catalyzed reactions through the establishment of S−O···H hydrogen bonds.22a,30 A recent report by Zhdanko and Maier on the beneficial role of ClO4− and especially TfO− in the Au(I)-catalyzed hydroalkoxylation of alkynes also supports this reasoning.31 In contrast, this assistance would become much less significant with fluorinated anions such as BF4− and SbF6−.32 From the pKa values of the corresponding acids, the following classification of basicity can be made: TfO− > NTf2− > ClO4−.33 The last species would be far more basic than BF4−, PF6−, and SbF6−, which cannot actually be considered as Brønsted bases since the corresponding superacids are elusive compounds.34 In terms of atomic charges in TfO−, ClO4−, BF4−, and PF6−, it was shown that oxygen atoms are more electron rich than fluorine atoms.35 Therefore, the 5153


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Scheme 9. Comparison of Mechanistic Pathways Including Triflate Counterion in the Cases of (A) Me3P-Au+ Catalyst, (B) Me3P-Au+ Catalyst without Triflate, (C) Me3P-Ag+ Catalysta

a

Gibbs free energies are given in kcal/mol.

Figure 5. Geometrical modification (distances in Å) due to the coordination of TfO− on Int2-AuPMe3: (A) Int2 + TfO−; (B) Int2. Atoms tagged “ae” are involved in the Alder-ene pathway (shown in red), Atoms tagged “h” are involved in the hydrindiene pathway (shown in black).

the filled 5d orbitals to the ligands occurs.”3o In our case, the cationic vinyl C ligand might be a determining factor of this coordination mode. Finally, three- and four-coordinate gold(I) complexes are indeed known.38 Since this concerted process is intramolecular and takes place quite far from the ligand, the steric bulk around the metal is probably not very relevant. It is, as mentioned above, unlikely that the nonoxygenated anions may serve as proton shuttles (SbF6−, PF6−, BF4−). With these, H+ can be eliminated and rapidly cleave the metal−carbon

its protonated form yet present in the coordination sphere of gold can play the role of an acid is completely beneficial for the protodemetalation of the vinylgold moiety. The suggested Au···Y coordination stemming from our calculations requires diffuse orbitals at the metal center. It has already been proposed in modeling studies with a triflate30b and a tosylate Y anion.37 For his part, Corma also stated that “Associative addition of an extra ligand is somewhat allowed (trigonal structures) when a significant π-back donation from 5154


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ACS Catalysis bond (case 1). This scenario should take place with the less bulky ligands. A classification according to Tolman’s angles39 gives PEt3 (132°) < PPh3 (145°) < PtBu3 (182°) < (biphen-2yl)tBu2P (246°). Using the percent buried volume (%Vbur)40 instead of the Tolman angle to incorporate the IPr ligand into the list, we obtain the following: PEt3 (31.7) < PPh3 (34.8) < PtBu3 (43.9) < IPr (44.5) < (biphen-2-yl)tBu2P (55.5). With the first two phosphines, the carbon−metal bond is probably less protected, and the cleavage is fast. With the last three ligands, however, the protodemetalation would be slowed down. However, Hammond and Xu also report that protodeauration is sensitive to the sterics of the ligand on gold.20a In sharp contrast with our findings, the Johnphos ligand gives the highest rate of protodeauration in the reaction they studied. This was ascribed to the fact that the orthosubstituted phenyl ring stabilizes the generated cationic gold complex through a η2 interaction. Such a stabilization effect has also been advanced by Hashmi, Yates, and Ariafard:20f electronrich phosphines stabilize the gold cationic fragment and help in its departure. Notwithstanding this, our case differs because the C ligand of gold is cationic, which implies a quite high energy barrier for the protodeauration and levels the influence of the L ligand. As for the intramolecular protodeauration, it would only be operative when a good proton shuttle such as TfO− or ClO4− anion is present. If the addition of a triflate or a perchlorate counterion through a silver salt can influence the outcome of the goldcatalyzed reaction resulting in the privileged formation of the Alder-ene product, the introduction of a bulky ligand (L1) in the case of a silver catalysis with AgOTf changes the end point of the reaction toward the hydrindiene compound by incapacitating the effect of TfO− (see entries 1 and 4, Table 3). Thus, only gold seems to be concerned by case 2 of Scheme 8. The principal difference in the energetic profiles between Au and Ag is that, in the case of gold, the coordination of triflate on L−Au−substrate results in a decrease of the activation energy to TSb (3.2 kcal/mol; see Scheme 9A,B), equaling the value leading to TSc (3.3 kcal/mol). Such an effect is not observed with Ag. Indeed (see Scheme 9C), TSb + TfO− remains higher by 6.7 kcal/mol than TSc + TfO−. The reason for such a difference between Au and Ag for apparently similar intermediates remains a question, even if in Ag+ complexes ion pairs and higher-order ionic aggregates are more likely to be formed with TfO− in comparison to BF4− 41 or with ClO4− in comparison to SbF6−.42 With such a persistent discrimination between these critical determining values, the hydrindiene formation remains favored with Ag. 2.3.4. Vinyl Chloride Product 4. The surprising formation of vinyl chloride 4 (entries 1 and 4 of Table 3) can be attributed to a chemoselective activation of the allene framework as in I9 rather than the alkyne (Scheme 10). Although preferential activation of the allene is favored on the basis of thermodynamicsbecause of the possible generation of a stable allylic cationit rarely defeats the well-described kinetic alkynophilicity.43 The resulting transient vinylic cation would be then trapped by a chloride originating from dichloromethane.44 2.3.5. The Missing Pathway: Dual Activation Mechanism? Substrates exhibiting a terminal alkyne moiety can sometimes follow a dual activation pathway under homogeneous gold catalysis.45 The tris((triphenylphosphine)gold)oxonium tetrafluoroborate complex C6 has been shown by the team of Toste8l and our team46 to favor such pathways, furnishing gold

Scheme 10. Mechanistic Proposal for the Formation of Vinyl Chloride 4

acetylides and their corresponding bimetallic σ,π complexes after rapid deprotonation of the substrate. Complex C6 could completely convert 1,6-allenyne 1, albeit over a considerably prolonged reaction time in comparison to other Ph3PAu+ precursors. Surprisingly, the selectivity was turned toward bicyclo[4.2.0]octadiene product 18, a completely different product than either hydrindiene or Alder-ene compounds, that was isolated in 70% yield (Scheme 11). It arises from a formal Scheme 11. Reaction of 1 in the Presence of Complex C6

alkyne−allene [2 + 2] cycloaddition process.47 We wished to check whether this compound could be a missed intermediate in the cyclization processes described above, but compound 18 slowly decomposed in the presence of catalytic amounts of Ph3PAuNTf2, and no diagnostic 1H NMR signals of either hydrindiene 2 or Alder-ene compounds 3 could be detected while monitoring this experiment. To the best of our knowledge, the gold-catalyzed alkyne− allene [2 + 2] cycloaddition is not known, in contrast to the gold-catalyzed alkyne−alkene [2 + 2] cycloadditions.48 We suspected the formation of gold acetylides and σ,π complexes through dual gold activation44 to be responsible for this mechanistic dichotomy and ran a few experiments to confirm this hypothesis. Indeed, the group of Garcia and Corma has already brought experimental support to the implication of these species in the gold-catalyzed intermolecular alkyne− alkene [2 + 2] cycloaddition.48b We prepared and spectroscopically characterized gold acetylide 19 and the binuclear σ,π complex 20 of allenyne 1 and studied their reactivity in solution (Scheme 12). Gold acetylide 19 proved thermally stable in solution, and its reaction with 1 equiv of cationic gold(I) immediately furnished the binuclear σ,π complex 20. The latter was stable for several hours at room temperature in CD2Cl2 but slowly decomposed upon heating to Au(0) 20b and [(Ph3P)2Au][SbF6] (evidenced by MS as well as 31P NMR), while the organic counterpart was isolated as an intractable mixture of unidentified products. No evidence of C−C bond formation could be collected from this last experiment. However, treating a wet (not distilled) CDCl3 solution of acetylide 19 with a catalytic amount of either Ph3PAuNTf2 or C6 gave rise to cyclobutene 18 in 55% isolated yield after a few hours at reflux, strongly suggesting that acetylide 19 is involved 5155


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ACS Catalysis Scheme 12. Reactivity of the Gold Acetylide 19 and the σ,πComplex 20

Scheme 13. Reaction Pathway and Gibbs Free Energies (kcal/mol) for the [2 + 2] Gold Acetylide−Allene Cycloaddition of a 1,6-Allenyne

in the mechanism leading to 18. In contrast, neither 1,6allenyne 1 nor its corresponding acetylide 19 could deliver any cyclization product under thermal conditions (refluxing chloroform). One more intriguing finding completed the intricate picture. When it was treated with a catalytic amount of digold complex 20, substrate 1 provided a mixture of regioisomers of 3 in a 79:21 ratio. We can assume here that a minute release of PPh3AuNTf249 from complex 20 triggers the major formation of Alder-ene products 3, as observed in entry 18 of Table 1. These findings are in sharp contrast with Houk’s and Toste’s results, as 1,5-allenynes selectively gave formal Alder-ene compounds through an acetylide/bimetallic σ,π-complex mechanism.8l To shed light on this apparent discrepancy, we performed calculations at the density functional level of theory,50 starting from the H3PAu+-complexed gold acetylide of the parent 1,6-allenyne (Int7, Scheme 13), which corresponds to a homologue of the Houk−Toste 1,5-allenyne system8l (vide infra). To stay as close as possible to their model, we did not introduce the internal allene methyl group in the computations. A first kinetically demanding (∼17.6 kcal/mol) cyclization step leads to gold-stabilized carbocation Int8. From the latter, mechanistic divergence toward the [2 + 2] cycloadduct (Int8 → Int10, blue) was calculated to be ∼18 kcal/mol more favorable than the pathway leading to the Alderene product (Int8 → Int9, red; see Table S3 in the Supporting Information). We then revisited the Houk−Toste system and modeled this mechanistic dichotomy in the case of a 1,5-allenyne (Scheme 14; see Table S4 in the Supporting Information). From the allylic cation Int12 already proposed and computed by these authors, an exergonic step leads to the gold-stabilized cation Int14 similar to Int8, for which ring closure to the gold-stabilized cyclobuten-1-yl anion complex Int15 was calculated to require ∼15 kcal/mol to reach TSl (blue). However, from the common intermediate Int12, a lower barrier of ∼14 kcal/mol is necessary to get the goldstabilized vinyl anion Int13 in a highly exergonic fashion (red). Thus, the peculiar reactivity of 1,6-allenynes compared to 1,5allenynes comes from the fact that the [2 + 2] transition state TSh lies lower in energy than the Alder-ene transition state TSg because of the greater flexibility conferred by the sixmembered ring. On the other hand, in the case of the 1,5allenyne, the barriers to reach TSj and TSl are not really

discriminating. However, the higher strain induced by the fivemembered ring would render the formation of Int15 from Int12 much less exergonic than for Int13 (20.5 vs 41.3 kcal/ mol). Because all these reactions take place at prolonged reaction times and/or with heating (60 °C in CDCl3), thermodynamic control would take place, giving the observed Alder-ene products. Analysis of the bond distances in TSh and TSl shows that the forming C−C bond is shorter in the former (2.26 vs 2.30 Å, see Figure 6). Finally, we envisioned that the putative gold-stabilized cyclobuten-1-yl anion complex Int10 (see Scheme 13)51 that terminates the mechanistic sequence leading to cycloaddition product 18 would be basic enough to deprotonate D2O.52 We repeated the experiment of Scheme 11 in the presence of 10 equiv of D2O, which considerably lengthened the reaction time (Scheme 15). After 12 days at room temperature, we were able to isolate a mixture of cyclobutene 18-D and the starting material (66% overall yield, 18-D:1-D in a 74:26 ratio). 18-D exhibited a 72% D incorporation. Presumably, all of the cyclobutene gold complex 21 which is formed would be transformed into 18-D after final deuterolysis while the recovered 1-D exhibiting 10% D suggests incomplete formation of the gold acetylide 19 and/or incomplete [2 + 2] cycloaddition with concomitant H/D exchange. 5156


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rationalization of these divergent reactivities is proposed. A series of catalysts of either metal halide salts (gold and platinum) or cationic metal complexes (gold and silver) with an assortment of bulky phosphines and NHC ligands provided an otherwise rarely encountered hydrindiene product. In this case, a pathway involving a metal vinylidene intermediate that would evolve following a C(sp3)−H insertion has been proposed. Other complexes bearing less bulky phosphines (PEt3 and PPh3) or cationic complexes with a triflate anion afford the classical Alder-ene cycloisomerization products. An intriguing highlight of this study is the critical role of counterions which can notably neutralize ligand effects. Such a neutralizing effect was not observed in silver catalysis, which could be supported by calculations. The reactivity of the system was also completely diverted when tris((triphenylphosphine)gold)oxonium tetrafluoroborate was used as the catalyst. In that case, an unprecedented gold-catalyzed alkyne−allene [2 + 2] cycloaddition, involving gold dual activation, took place. A theoretical treatment of this reaction allowed to reconcile these findings with Houk’s and Toste’s results with 1,5-allenynes, for which the Alder-ene pathway is more favorable experimentally and theoretically. Overall the mapping of the parameters which control these different evolutions should find applications in gold catalysis broader than this prototypical case. It clearly opens valuable perspectives to a more predictable gold catalysis.

Scheme 14. Reaction Pathway and Gibbs Free Energies (kcal/mol) for the [2 + 2] Gold Acetylide−Allene Cycloaddition of a 1,5-Allenyne

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b02696. Spectral data, synthetic methods, experimental procedures, and computational details (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail for Y.G.: [email protected]. *E-mail for V.G.: [email protected]. *E-mail for L.F.: [email protected]. Author Contributions

Figure 6. Geometry of the computed TSh (left) and TSl (right) (distances in Å).

∥

These authors contributed equally.

Notes

The authors declare no competing financial interest.

Scheme 15. [2 + 2] Cycloaddition Pathway in the Presence of D2O (X = C(CH2OMe)2)

■

ACKNOWLEDGMENTS This work was supported by the UPMC, CNRS, and ANR “allenes”. L.F., V.G., and M.M. thank the IUF. Calculations were performed at the CRIANN (Centre Régional d’Informatique et d’Applications Numériques de Normandie), plan interrégional du bassin parisien (project 2006-013), the CRI of UPS (Orsay, France), and the CECIC of ICMG (Grenoble, France).

■ ■

ABBREVIATIONS IPr, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; dppp, 1,3-bis(diphenylphosphino)propane

3. CONCLUSIONS In this report, the cycloisomerization reaction of model 1,6allenyne substrates has been studied with noble metals catalysis (gold, platinum, and silver) and has been shown to be extremely dependent on the reaction conditions. Nevertheless, by means of deuterium labeling experiments and calculations a

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