Hydroalumination of Alkenes and Alkynes by Primary Aluminum

Oct 15, 2014 - In keeping with this view, studies of the reactions of the three primary alanes (ArPri8AlH2)2, (ArPri4AlH2)2 (ArPri4 = C6H3-2,6(C6H3-2,...
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Hydroalumination of Alkenes and Alkynes by Primary Aluminum Hydrides under Mild Conditions James C. Fettinger,† Paul A. Gray,‡ Christopher E. Melton,† and Philip P. Power*,† †

Department of Chemistry, University of California at Davis, One Shields Avenue, Davis, California 95616, United States Department of Chemistry, University of Victoria, Victoria, British Columbia V8W 3 V6, Canada

‡

S Supporting Information *

ABSTRACT: The reactions of the sterically crowded primary alane (ArPri8AlH2)2 (ArPri8 = C6H-2,6(C6H2-2,4,6-Pri3)2-3,5-Pri2) with alkynes and alkenes are described. It is shown that hydroalumination of the terminal alkynes HCCSiMe3 and HCCPh readily occurs under mild conditions via the cis-addition of the Al−H moiety across the CC triple bond with no evidence of hydrogen elimination. Hydroalumination was observed also with a range of terminal olefins, but no reactivity was observed with internal alkenes or alkynes. The relatively high reactivity of (ArPri8AlH2)2 was attributed to the steric crowding of the large terphenyl substituent, which favors dissociation of the alane and increases the availability of the more reactive three-coordinate aluminum site in the monomer. In keeping with this view, studies of the reactions of the three primary alanes (ArPri8AlH2)2, (ArPri4AlH2)2 (ArPri4 = C6H3-2,6(C6H3-2,6-Pri2)2), and (ArMe6AlH2)2 (ArMe6 = C6H3-2,6(C6H2-2,4,6-Me3)2) with alkenes showed that the reaction rates are inversely proportional to the size of the terphenyl substituent, consistent with higher reactivity of the aluminum monomer. The structures of the alkenyl insertion products, ArPri8Al(CHCHPh)2 and ArPri8Al(CHCHSiMe3)2, the alkylated derivative, ArPri8Al(CH2CH2SiMe3)2, and the precursor aluminates {Li(OEt2)H3AlArPri8·Li(OEt2)2H3AlArPri8}, (LiH3AlArPri8)2, and alanes (ArPri8AlH2)2, and (ArPri4AlH2)2 were determined by X-ray crystallography.

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INTRODUCTION Since Schlesinger and co-workers reported the synthesis of lithium aluminum hydride (LiAlH4) and alane (AlH3) in 1947,1 aluminum hydrides have become indispensable reducing agents for chemical synthesis.2 The facile reactivity of ionic or neutral aluminum compounds such as diisobutylaluminum hydride3−6 and Na{H2Al(OCH2CH2OCH3)2}5,6 (Red-Al) has made aluminum hydrides reagents of choice for the reduction of many types of unsaturated organic moieties, including those having carbon−carbon double and triple bonds.2,5−10 Primary aluminum hydrides, compounds of general formula RAlH2 (R = aryl, akyl, alkoxy, amido), and the related aluminates M[RAlH3] also have the potential to act as useful reagents for inorganic and organic syntheses. They have not been studied widely because when R is a small alkyl or aryl organic group, they generally conproportionate as shown in eqs 1 and 2.11 2RAlH 2 → AlH3 + R 2AlH

(1)

2[RAlH3]− → [AlH4 ]− + [R 2AlH 2]−

(2)

his group reported the synthesis and structures of the aryloxo and hydrazido aluminates {Li(THF)2H3AlOC6H3-2,6-tBu2}16 and {Li(Et2O)2H3AlN(Ph)N(SiMe3)2}2.17 In addition, the neutral primary aluminum hydrides (Mes*AlH2)2,14 (ArMe6AlH2)215 (ArMe6 = C6H3-2,6(C6H2-2,4,6-Me3)2) and (ArPri6AlH2)215 (ArPri6 = C6H3-2,6(C6H2-2,4,6-iPr3)2) were obtained from the corresponding aluminate species by treatment with organic or silyl halides. Despite these advances, reports on the reactivity of primary aluminum hydrides remain relatively limited,18−24 and there have been no studies of their reactivity toward alkenes or alkynes in contrast to the numerous studies of secondary alanes with these substrates.4 In principle, either hydrogen elimination or hydroalumination reactions are possible when terminal alkynes are reacted with secondary alanes. For example, the reaction of dimethyl or di-tert-butylalane with phenylacetylene (PhCCH) readily affords H2 elimination and formation of dialkylaluminum alkynes.25,26 However, the more crowded secondary alane HAl{CH(SiMe3)2}2 gave hydroalumination and cis-addition to the Al−H bond, rather than hydrogen elimination, when treated with PhCCH.27,28 We show now that the steric shielding provided by a single sterically crowding aryl group is sufficient to favor hydroalumination instead of hydrogen elimination upon reaction of the primary arylalane with terminal alkynes PhCCH or trimethylsilyl acetylene (Me3SiCCH). Furthermore, we demonstrate that there is an inverse correlation between the size of the

However, through the use of bulky aryl substituents, it is possible to stabilize primary organoalanes and alanates sufficiently to allow their characterization at room temperature. Early examples include the related amido hydride {Li(OEt 2 ) 2 H 3 AlN(SiMe 3 ) 2 } 2 , by Stalke and co-workers, 12 [(THF)2 LiH3Al{C(SiMe2Ph)3}]2, by Eaborn, Smith, and their co-workers,13 and the aryl aluminates {Li(THF)2H3AlMes*}214 (Mes* = C6H2-2,4,6-tBu3), {Li(OEt2)1.5H3AlTripph}2 (Tripph = C6H2-2,4,6-(C6H5)3),12 and {(Et2O)LiH3AlArMe6}n.15 Nöth and © 2014 American Chemical Society

Received: September 2, 2014 Published: October 15, 2014 6232

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Scheme 1. Synthesis of (LiArPri8AlH3)2, {Li(Et2O)ArPri8AlH2·Li(Et2O)2ArPri8AlH3}, and (ArPri8AlH2)2

Scheme 2. Reactions of (ArPri8AlH2)2 with Substituted Acetylenes at 25 °C in Pentane

terphenyl substituent and the rate of hydroalumination of alkenes. Additionally, we report the structures of the recently synthesized29 ArPri4AlH2 and ArPri8AlH2 and their alanate precursors and show that even in the case of the latter, where the substituent is the bulkiest currently available terphenyl group ArPri8, the structure of the alane remains dimeric in the solid state.

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RESULTS AND DISCUSSION Synthesis. The syntheses of (ArPri4AlH2)2 and (ArPri8AlH2)2 were described recently by this group in connection with their reactivity toward ammonia and phosphine29 and are similar to earlier preparations of less crowded m-terphenyl alanes.15 The synthetic route (Scheme 1) to the bulkiest primary alane (ArPri8AlH2)2 proceeds via the dimeric alanate (LiH3AlArPri8)2 or the ether-complexed alanate {Li(Et 2 O) 1.5 H 3 AlAr Pri 8 ·Li(OEt2)2AlArPri8} obtained by reaction of AlH3·NMe330 with the aryl lithium LiArPri8.31 Upon treatment with one equivalent of Me3SiCl, both the monomeric and dimeric aluminate salts were found to eliminate LiCl with evolution of Me3SiH to afford the unsolvated primary alane (ArPri8AlH2)2. The synthesis of (ArPri4AlH2)2 was accomplished under very similar conditions to those given in Scheme 1.29 Crystallization of the crude alane product from toluene produced crystals of {ArPri4AlH2}2 in good yield that were suitable for X-ray crystallography. Reactivity of (ArPri8AlH2)2 with Alkynes. Scheme 2 shows the reactions of (ArPri8AlH2)2 with some simple alkynes. Two equivalents of phenylacetylene or trimethylsilylacetylene react with half an equivalent of the dimer (ArPri8AlH2)2 in pentane solution at room temperature to afford the alkenyl alanes ArPri8Al(CHCHPh)2 or ArPri8Al(CHCHSiMe3)2, respectively, via Al−H addition, in high yield as the only products. Reactions with one equivalent of the acetylenes afforded product mixtures that could not be easily separated. In contrast, the treatment of (ArPri8AlH2)229 with the internal alkynes bis(trimethylsilyl)acetylene (Me3SiCCSiMe3) and diphenylacetylene (PhCCPh) did not produce any hydroalumination products even after extended reaction times. After stirring at room temperature for 4 days in C6D6, no consumption of the alane or either Me3SiCCSiMe3 or PhCCPh could be observed by 1H NMR spectroscopy. In addition, no reaction could be detected upon heating the reactants in hexane to 55 °C for 12 h.

It seems likely that the lack of hydroalumination observed for the reaction of PhCCPh or Me3SiCCSiMe3 with (ArPri8AlH2)2 is due mainly to steric effects. It could be argued that the phenyl substituents in PhCCPh lower the nucleophilic character of the triple bond, which suppresses its reactivity by lowering its tendency to coordinate to aluminum. However, for Me3SiCCSiMe3, the SiMe3 substituents are expected to slightly raise the electron density in the triple bond and increase its tendency to coordinate to the Lewis acidic aluminum. As no reaction was observed with either disubstituted acetylene, we infer that the lack of reactivity toward PhCCPh and Me3SiCCSiMe3 is steric, rather than electronic, in origin. In contrast, the dialkyl aluminum hydride HAl{CH(SiMe3)2}2, featuring the bis(trimethylsilyl)methyl substituents, undergoes hydroalumination with the alkyne Ph2Si(CCPh)2 under mild conditions,28 suggesting that the steric crowding in this alane is insufficient to inhibit the reaction. Reactivity of (ArPri8AlH2)2 with Alkenes. In an initial experiment, (ArPri8AlH2)2 was stirred with the 3,3-dimethyl-16233

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Table 1. Insertion Experiments Using Various m-TerphenylSubstituted Alanes and Olefins

butene (H2CCHCMe3) in pentane at room temperature overnight. Concentration under reduced pressure provided a colorless product, whose solutions become an intense orange upon exposure to air. Several crystallization conditions were explored, but none produced X-ray quality crystals. Nonetheless, complete reaction to afford the bis-insertion product ArPri8Al(CH2CH2CMe3)2 was confirmed by 1H NMR spectroscopy, which indicated that both the olefinic and Al−H signals had disappeared. The reaction of (ArPri8AlH2)2 with ethylene at ca. 1 atm pressure at room temperature affords the bis-insertion product ArPri8Al(CH2CH3)2 in high yield. Complete reaction was confirmed by 1H NMR spectroscopy, which indicated the disappearance of the Al−H signal and the appearance of a welldefined quartet at δ = −0.5 ppm and a triplet at δ = 1.5 ppm, consistent with the formation of Al−Et moieties. The integration ratios of these two signals with respect to those from the ArPri8 ligand hydrogens indicated the presence of two ethyl groups per ArPri8 substituent. The upfield chemical shift of the methylene signal indicates that it is bound to the electropositive aluminum atom. Attempts at crystallization using a variety of solvents under different conditions did not produce material suitable for X-ray crystallography, but the signals observed for ArPri8Al(CH2CH3)2 resemble those of ArPri4Al(CH2CH3)2 (synthesized via salt metathesis by Wehmschulte and co-workers), the structural characterization of which was reported recently.32 The reaction of styrene with (ArPri8AlH2)2 proceeded more slowly than those of ethylene or H2CCH(CMe3); 1H NMR spectroscopy confirmed that complete consumption of hydride occurred only after 24 h at 40 °C. In this case, single crystals of the product ArPri8Al(CH2CH2Ph)2 suitable for X-ray crystallography could be grown readily from toluene solution after storage for ca. 1 week at room temperature (see structural description below). Effects of m-Terphenyl Ligand Size on Primary Aluminum Hydride Insertion Reactivity. We investigated the steric effects of the terphenyl substituent on the rates of hydroalumination using three dimeric primary alanes, (ArMe6AlH2)2,15 (ArPri4AlH2)2,29 and (ArPri8AlH2)2,29 which differ significantly in the degree of steric crowding. The differing steric influence of the structures of m-terphenyl-substituted group 13 element derivatives was studied earlier and shown to lower the degree of aggregation in low-valent metal derivatives with increasing size of the terphenyl ligand.33,34 To investigate the reactivity, each m-terphenyl alane was mixed with an olefin listed in Table 1, and the reactions were monitored by 1H NMR spectroscopy. The results clearly indicate that the size of the terphenyl ligand inversely correlates with reactivity. The bulkiest alane, (ArPri8AlH2)2, reacts more quickly with the substrate olefins than the less bulky (ArPri4AlH2)2, which, in turn, reacts more readily than (ArMe6AlH2)2, the least bulky arylalane of the series. The observation of increased reactivity with increased ligand bulk seems counterintuitive since with larger steric bulk the substrate should have less access to the reactive metal center. However, the same correlation is observed in the reaction of secondary alanes.4 It is generally agreed2,6,11,35−39 that hydroalumination of alkenes proceeds through initial coordination of the olefin to the p-orbital of a monomeric three-coordinate aluminum hydride (Scheme 3), followed by a concerted cisaddition of the Al−H moiety across the unsaturated bond. It follows that the alane that produces the largest concentration of monomer in solution should undergo olefin insertion into the Al−H bond at the fastest rate. As the steric bulk of the mterphenyl ligand is increased, the monomeric form becomes

increasingly stabilized relative to its dimer, which has bridging hydrides and four-coordinate aluminums. Indeed, this can be seen in the 1H NMR of (ArPri8AlH2)2 at slightly elevated temperatures, with the reversible appearance of new resonances likely arising from the dissociation of the dimer in solution. This phenomenon is further exemplified by the behavior of (Et2AlH)2, which is known to hydroaluminate C−C double bonds only at elevated temperatures, where there is dissociation of (Et2AlH)2 into monomers.40−43 In the case of the internal olefin 2,3-dimethyl-2-butene, the reaction is prevented, presumably for steric reasons, as the increased substitution of the olefin likely prevents its approach and coordination to the aluminum center. Despite heating at 50 °C for 48 h, no reaction was observed between any of the mterphenyl alanes and this alkene. Reaction of (ArPri8AlH2)2 with cyclohexene was also attempted, but in agreement with the reaction discussed above no addition was observed for this olefin.

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STRUCTURAL CHARACTERIZATIONS Figure 1 depicts the solid-state structure of the trihydridoaluminate dimer (LiH3AlArPri8)2,29 which was obtained from the reaction of Li(OEt2)ArPri8 with AlH3(NMe3) in pentane. It is one of a rare group of structurally characterized primary alanates unsolvated by Lewis bases,24 and it is the only unsolvated arylsubstituted primary alanate that has been structurally characterized. The related amido-substituted primary alanate, [KH3AlN{C6H2-2,6(CHPh)2-4-Me}(SiMe3)]2, also has a dimeric structure.44 However, it differs from (LiH3AlArPri8)2 in that the three hydrides are in close contact with the alkali metal cation (i.e., K+), which also has an η6-interaction with one of the flanking rings and an η2-interaction with an adjacent ring of the neighboring 6234

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Scheme 3. Proposed Mechanism of C−C Double-Bond Insertion into Al−H Bonds

Table 2. Selected Distances (Å) and Angles (deg) for (LiH3AlArPri8)2 C1−Al1 Al1−H1 Al1−H2 Al1−H3 Al1--Li1 Li1−H1 Li1−H2a Li1−H1a Li1−cent

2.007(1) 1.63(2) 1.56(2) 1.51(2) 2.749 (2) 1.83(2) 1.94(2) 2.19(2) 2.125(4)

C1−Al1−H1 C1−Al1−H2 C1−Al1−H3 H1−Al1−H2 H1−Al1−H3 H2−Al1−H3 H1−Li1−cent H2−Li1−cent H2a−Li1−H1 H1a−Li1−cent H1a−Li1−H2a

113.1(6) 115.3(7) 118.0(7) 93.7(9) 104.1(9) 109.5(1) 120.8(6) 115.4(6) 122.1(8) 137.9(5) 68.3(7)

four-coordinate aluminum and three-coordinate lithium include the lithium aluminates Li(OEt2)H5Al3(OC(CH3)3)5,48 {Li(THF)}H 2 Al{C(SiMe 3 ) 2 (SiMe 2 OMe)}, 49 Li(OEt 2 )HAl(OC 6 H 3 -2,6- i Pr 2 ) 3 , 16 and Li(OEt 2 )AlMe(2,6- t Bu 2 -4-MeC6H2)2(OCCH3Ph2).50 In these cases, the aluminum centers are also coordinated by bulky ligands, which likely impede a higher coordination number at the lithium center. The diethyl ether complexed lithium primary alanate {Li(OEt2)H3AlArPri8·Li(OEt2)2H3AlArPri8},29 illustrated in Figure 2, was obtained from the reaction of ArPri8Li(OEt2) with AlH3· NMe3 in 50% Et2O/hexane. The structure features two separate lithium alanate salts that differ mainly in the solvation of the Li+ ions. In Figure 2a, the Li+ ion is complexed by one ether oxygen and two bridging hydrides and also displays a relatively close approach to the centroid of a flanking aryl ring (Li−centroid = 2.451 Å). In Figure 2b, the Li+ ion is complexed by two ether oxygens and two bridging hydrides, but there are no close approaches to a flanking aryl ring of the terphenyl ligand or their substituents. Structural parameters (Table 3) surrounding the aluminum atom in each complex are almost identical. It is noteworthy that the three Al−H distances in each structure are essentially indistinguishable, with no significant difference between the terminal and bridging Al−H bond lengths. This, combined with the fact that the Li−H distances are up to 0.4 Å longer than the Al−H bond lengths, suggests that the structure is best viewed as a contact ion pair between the (H3AlArPri8)− anion and their ethersolvated lithium cations. The complexes represent a significant addition to the unusual class of monomeric or unassociated lithium trihydroaluminates. To our knowledge, there exist only two other examples that have been structurally characterized: {[Li(tmen) 2 ][H 3 AlC(SiMe3)3]}21 and BmtAlH3Li(DME)251 (Bmt = 4-tBu-2,6bis[(2,2,6,6-tetramethyl-m-terphenyl-2-yl)methyl]phenyl, DME

Figure 1. Thermal ellipsoid plot (30% probability) of (LiH3AlArPri8)2. Carbon-bound H atoms are not shown for clarity. Selected distances and angles are given in Table 2.

monomer unit. The structure of (LiH3AlArPri8)2 bears a resemblance to the structure of m-terphenyl tribromoaluminate, (LiBr3AlArMe6)2, reported by Robinson and co-workers.45 However, the metal hydride core is centrosymmetric and features an eight-membered (AlHLiH)2 ring unlike the Li2Br6 pseudo-octahedral core observed for the bromo derivative. An (AlHLiH)2 core ring structure has also been observed in several solvated lithium trihydroalanates.12,13,16,17,44 In (LiH3AlArPri8)2, Al1 has distorted tetrahedral coordination and is σ-bonded to the m-terphenyl ligand and three hydrides, H1, H2, and H3. The hydride H3 is terminally bound, while H1 and H2 form bridging interactions to Li1 and Li1A, respectively. The distances between the aluminum atom and these hydrides are approximately 1.63 Å, the Cipso−Al distance is 2.007(1) Å, and the Cipso−Al−H angles range from 113.09(58)° to 117.97(71)°. An interesting feature of the molecule is the coordination environment of the lithium ions, which are bound to two hydrogens and to a flanking aryl ring of the terphenyl ligand in an η6 fashion. The Li1−H distances to the bridging hydrides H1 and H2 are 1.83(2) and 1.94(2) Å, respectively. These values are within the known range for lithium alanates such as Li4H4[HAl{N(SiMe3)2}2]3,46 DippN[Al{N(H)Dipp}2H]2{Li(THF)},47 and {Li(THF)2H3AlMes*}2,14 which lie between 1.6 and 1.9 Å.12,13,16,24,46 The CT1−Li1−H1, CT1−Li1−H2A, and H1− Li1−H2A (CT1 = aryl ring centroid) angles are 120.8(6)°, 115.4(6)°, and 122.1(8)°, which total 358.3(8)°. Thus, the coordination geometry at lithium may be regarded as essentially trigonal planar. Examples of alanate structures that contain both 6235

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Figure 2. Thermal ellipsoid plot (30%) of the two differently solvated lithium etherate complexes of LiH3AlArPri8 that cocrystallized in the solid state.

Table 3. Selected Distances (Å) and Angles (deg) for the Diethyl Ether Complexes of LiH3AlArPri8 Illustrated in Figure 2 a C1−Al1 Al1--Li1 Al1−H1 Al1−H2 Al1−H3 Li1−O44 Li1−H2 Li1−H3

2.014(3) 2.616(5) 1.58(3) 1.57(3) 1.55(3) 1.932(6) 1.94(3) 1.96(3)

b C1−Al1−H2 C1−Al1−H3 C1−Al1−H1 H2−Al1−H3 H2−Li1−H3 H1−Li1−H3 O44−Li1−H2 O44−Li1−H3

111.9(9) 110.2(9) 120.7(9) 93.4(13) 71.2(11) 71.2(11) 131.5(8) 115.0(8)

C51−Al2 Al2--Li2 Al2−H51 Al2−H52 Al2−H53 Li2−O94 Li2−O98 Li2−H51 Li2−H52

2.016(3) 2.611(5) 1.57(3) 1.56(3) 1.56(3) 1.972(6) 1.923(6) 1.87(3) 1.99(3)

C51−Al2−H51 C51−Al2−H52 C51−Al2−H53 H51−Al2−H52 H51−Al2−H53 H52−Al2−H53 O94−Li2−O98 H51−Li2−H52 H51−Li2−O94 H52−Li2−O98

115.2(9) 114.9(9) 114.2(11) 92.3(13) 106.7(14) 111.3(14) 104.1(2) 71.7(11) 113.1(9) 110.9(8)

Figure 3. (a) Thermal ellipsoid plot at 30% probability for (ArPri8AlH2)2. C−H hydrogens are not shown for clarity. Selected distances and angles are given in Table 4. (b) 30% probability thermal ellipsoid plot of (ArPri4AlH2)2. Co-crystallized solvent molecules and C−H hydrogens are not shown. Selected distances and angles are given in Table 4.

The unsolvated neutral alane, (ArPri8AlH2)2, shown in Figure 3, can be obtained in good yield and purity by reaction of Li(OEt2)H3AlArPri8 or (LiH3AlArPri8)2 with Me3SiCl, but the compound does not readily crystallize due to its high solubility in hydrocarbon solvents.29 Multiple recrystallization attempts afforded only amorphous powders, but after several months of

= 1,2-dimethoxyethane, tmen = tetramethylethane-1,2-diamine). Both examples feature bidentate donor ligands that coordinate Li+, block hydride bridging between lithium and aluminum, and have Al−H and Al−C distances similar to those of the aryl aluminate complex. 6236

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Table 4. Selected Distances (Å) and Angles (deg) for the Neutral Primary Alanes (ArPri8AlH2)2 and (ArPri4AlH2)2 (ArPri8AlH2)2 Al1−C1 Al1---Al2 Al2−C43 Al1−H1 Al1−H2 Al1−H3 Al2−H2 Al2−H3 Al2−H4

1.989(2) 2.6363(9) 1.996(2) 1.63(2) 1.63(2) 1.66(2) 1.75(2) 1.67(2) 1.64(13)

C1−Al1−H1 C1−Al1−H2 C1−Al1−H3 H2−Al1−H3 H2−Al2−H3

(ArPri4AlH2)2 113.1(6) 125.4(6) 121.1(8) 78.1(10) 74.7(10)

Al1−C1 Al1--Al1A Al1−H1 Al1−H2

1.969(1) 2.6321(7) 1.46(2) 1.76(1)

C1−Al1−H1 C(1)−Al1−H2 H1−Al1−H2

124.1(8) 112.7(4) 108.2(8)

Figure 4. Thermal ellipsoid plots of alkenyl alane products (a) ArPri8Al(CHCHPh)2 and (b) ArPri8Al(CHCHSiMe3)2, which arise from the reaction of (ArPri8AlH2)2 with PhCCH and Me3SiCCH, respectively. Each is drawn at 30% probability, and C−H hydrogens on the m-terphenyl ligands and those on the phenyl rings of ArPri8Al(CHCHPh)2 are not shown for clarity. Selected distances (Å) and angles (deg) for both compounds are given Table 5.

slow crystal growth in a ca. −20 °C freezer, X-ray quality crystals could be obtained from a pentane solution only by addition of free ligand HArPri8. The data indicate that the hydride cocrystallizes with the parent m-terphenyl hydrocarbon as well as two solvent molecules. The compound exists in the solid state as a hydrogen-bridged dimer that contains two aluminum atoms in tetrahedral coordination environments. Selected distances and angles are given in Table 4. The heterocyclic (AlH2Al) core containing the bridging H2 and H3 hydrides has symmetric bonding with Al−H distances that average 1.66(5) Å. Despite the lack of an Al--Al bond, the Al--Al separation, 2.6363(9) Å, is similar to those found for the Al−Al σ-bond distances in the tetraorganodialanes R 2 Al−AlR 2 (R = C 6 H 2 -2,4,6- i Pr 3 , CH(SiMe 3 ) 2 , Si(SiMe3)3).52−54 This small intermetallic separation, which is a consequence of hydride bridging and typical of dimeric alanes,11b,13−16 has a marked effect on the geometry of the dimer. The short Al--Al distance results in a correspondingly short ArPri8−ArPri8 separation, and, presumably, the steric congestion that would result forces a staggered m-terphenyl arrangement, as reflected in the high torsion angle of 91.86(50)° for the array C6−C1−C43−C48. This staggered arrangement is not observed in the less crowded m-terphenyl alane (ArMe6AlH2)2 nor in the bulkier (ArPri4AlH2)2 (see below) or (ArPri6AlH2)2, where attractive dispersion forces between the ring substituents probably favor the eclipsed geometry.15 The solid-state structure of (ArPri4AlH2)2, shown in Figure 3b, is similar to those of the m-terphenyl alanes (ArMe6AlH2)2 and

(ArPri6AlH2)2 and other reported aluminum hydrides.13−16,49,55 However, its structure differs from that of (ArPri8AlH2)2 in Figure 3 in that the flanking rings of the terphenyl ligands are eclipsed. This is contrary to steric expectations, but is consistent with the involvement of attractive dispersion forces between the Pri group of the flanking aryl rings that have been implicated in other terphenyl derivatives.56,57 The C1−Al1 distance is 1.97(4) Å, which is within the 1.9 to 2.1 Å range observed for other known m-terphenyl aluminum species.13−16,46,51,55 The molecule is located on a crystallographically required inversion center, which produces an Al1--Al1A distance of 2.63(3) Å. This value agrees with the distances observed in previously reported m-terphenylsubstituted aluminum hydrides, which range from 2.63 to 2.65 Å for (ArMe6AlH2)2,15 (ArPri6AlH2)2,15 and (ArPri8AlH2)2. The aluminum hydrogens were located in the Fourier difference map and freely refined. Their distances from Al1 are 1.76(2) and 1.80(3) Å. The aluminum shows distorted tetrahedral coordination; the C1−Al1−H1 angle is 113.40(79)° and the C1−Al1−H2 angle is 129.18(58)°. The crystalline products of the hydroalumination reactions were also analyzed by X-ray crystallography. The solid-state structure of ArPri8Al(CHCHPh)2 is illustrated in Figure 4a, and that of ArPri8Al(CHCHSiMe3)2 is illustrated in Figure 4b. The structures reveal that cis Al−H addition4,8,58,59 has occurred to afford the bis(alkenyl)arylaluminum products. This is to be expected on the basis of steric shielding, which prevents formation of the more energetically favored trans-product.60 In both compounds, the ipso-C, para-C, and Al atoms are located on 6237

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Table 5. Selected Distances (Å) and Angles (deg) Observed in ArPri8Al(CHCHPh)2 and ArPri8Al(CHCHSiMe3)2 i

i

ArPr 8Al(CHCHPh)2 Al1−C1 Al1−C23 C23−C24 C24−C25

1.988(2) 1.952(1) 1.326(2) 1.480(2)

C23−Al1−C23A C1−Al1−C23 Al1−C23−C24 C23−C24−C25

ArPr 8Al(CHCHSiMe3)2 112.83(8) 123.59(4) 124.0(1) 127.2(1)

Al1−C1 Al1−C23 C23−C24 C24−Si1

a 2-fold rotational axis, and there is trigonal planar coordination at the aluminum. The alkenyl Al1−C23 distances (Table 5) in the two compounds are essentially identical at 1.952(2) Å and agree with the distances observed in other structurally characterized vinyl alanes such as Me 3 SiCHCHAl{CH(SiMe3)2}2 and CH[CMeNDipp]2Al(CCPh)(CHCPh2) (Dipp = 2,6-iPr2C6H3).27,28,61−64 The sums of angles around the sp2hybridized α-carbon atoms are approximately 360°, as expected for trigonal carbon. The phenyl- and trimethylsilyl-substituted alkenyls display C23−Al1−C23A angles of 112.83(8)° and 114.15(8)°, respectively, which are narrower than the idealized value of 120° due to the size of the ArPri8 substituents. The styrenyl substituents have a torsion angle of 36.40(13)° with respect to the central aromatic ring, and the trimethylsilyl alkenyl derivative exhibits a torsion angle of 40.23(7)°. The differing steric congestion between the compounds can be attributed as the primary reason for the variation in these angles. Single crystals of the aryl dialkyl aluminum species ArPri8Al(CH2CH2Ph)2 suitable for X-ray diffraction were recovered from a supersaturated toluene solution after ca. 1 week at room temperature. The solid-state structure of the product is shown in Figure 5. The ArPri8Al(CH2CH2Ph)2 molecule is monomeric, and

1.982(2) 1.952(1) 1.332(2) 1.862(3)

C1−Al1−C23 C23−Al1−C23A Al1−C23−C24 C23−C24−Si1

122.92(4) 114.15(8) 121.4(1) 127.4(1)

Table 6. Selected Distances (Å) and Angles (deg) for ArPri8Al(CH2CH2Ph)2 C1−Al1 Al1−C43 Al1−C51 C43−C44

1.987(2) 1.972(2) 1.978(2) 1.540(3)

C1−Al1−C43 C1−Al1−C51 C43−Al1−C51 C2−C1−Al1−C43

125.80(8) 125.39(8) 108.80(8) 37.44(6)

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CONCLUSION Reactivity studies of primary organoaluminum hydrides show that they display facile insertion reactivity with both terminal alkynes and terminal alkenes, although no insertion was observed with internal olefins. The products from these reactions were structurally characterized and confirm that insertion, rather than deprotonation, occurs in all cases. The effect of m-terphenyl size on the reactivity of the primary alanes toward insertion was investigated, and the results show that increasing ligand bulk increases reactivity, consistent with the existence of the monomer in solution as the reactive species. The structures of the primary lithium organoaluminum hydride salts (LiH3AlArPri8)2 and the ether solvated {(Et2O)1 or 2 H3AlArPri8}2 were also determined, along with those of the neutral alanes (ArPri4AlH2)2 and (ArPri8AlH2)2.

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EXPERIMENTAL SECTION

All experiments were carried out using modified Schlenk techniques under nitrogen with the rigorous exclusion of oxygen and water. Solvents were dried by the Grubbs method64 and stored over potassium mirrors until needed. Chlorotrimethylsilane was distilled from CaH2 and stored under nitrogen before use. AlH3·NMe330 and ArPri8Li(OEt2)31 were prepared by literature methods. NMR spectra were recorded either on a Varian 300 MHz spectrometer or a Bruker Avance 500 MHz instrument. Melting point data were collected on a Mel-Temp(II) apparatus in glass capillaries sealed under Ar and are uncorrected. The compounds (LiArPri8AlH3),26 (ArPri8AlH2)2,26 and (ArPri4AlH2)226 were prepared as previously described. ArPri8Al(CHCHPh)2. (ArPri8AlH2)2 (0.252 g, 0.493 mmol) was dissolved in pentane (25 mL), and phenylacetylene (0.42 mL, 1.0 mmol) was added as a neat liquid. The reaction was stirred overnight, and the filtrate was reduced in volume to ca. 10 mL and stored for several weeks at −20 °C, during which time colorless crystals were obtained. Yield: 0.292 g (74%). Anal. Calcd for C58H75Al: C, 87.17; H, 9.46. Found: C, 87.21; H, 10.4. 1H NMR (300 MHz, C6D6): δ 0.92 (d, 3JHH = 5.3 Hz, 12H, (CH3)2CH), 0.97 (d, 3JHH = 5.2 Hz, 12H, (CH3)2CH), 0.99 (d, 3JHH = 5.2 Hz, 12H, (CH3)2CH), 1.03 (d, 3JHH = 5.1 Hz, 12H, (CH3)2CH), 2.60 (m, 4H, (CH3)2CH), 2.82 (sept, 3JHH = 5.1 Hz, 4H, (CH3)2CH), 5.31 (d, 3JHH = 15.5 Hz, 2H, CHCHPh), 6.75 (t, 3JHH = 5.8 Hz, 2H), 6.87 (m, 6H, CHCHC6H5 and o-CHCHC6H5), 7.00 (d, J = 5.4 Hz, 4H, m-C6H5), 7.05 (s, 4H, C6H2-Pri3), 7.34 (s, 1H, C6H-Pri2). 13C NMR (75 MHz, C6D6): δ 14.17, 22.87, 24.14, 24.79, 25.46, 25.67, 29.88, 30.30, 31.78, 32.94, 32.96, 34.50, 34.79, 122.24, 126.79, 127.74, 128.37, 128.98, 138.78, 140.05, 145.74, 147.79, 148.76, 151.42. 2962, 1608, 1208. IR, cm−1 ν(Al−H) = 2988 (m), 1610 (m), 1261 (m). Mp: 186 °C (with dec). ArPri8Al(CHCHSiMe3)2. (ArPri8AlH2)2 (0.215 g, 0.362 mmol) was dissolved in pentane (25 mL), and Me3SiCCH (0.071g, 0.724 mmol) was added via syringe at room temperature. The solution was stirred at room temperature for 3 days, after which time the reaction was filtered.

Figure 5. Thermal ellipsoid plot of ArPri8Al(CH2CH2Ph)2 at 30% probability. The m-terphenyl C−H hydrogens, phenyl C−H hydrogens, and cocrystallized hexane molecule are not shown for clarity. Selected distances (Å) and angles (deg) observed in ArPri8Al(CH2CH2Ph)2 are given in Table 6.

the sum of the interligand angles at aluminum (Table 6) is almost exactly 360°. The Al−C−C angles Al−C43−C44 = 114.84(19)° and Al1−C51−C52 = 115.71(19)° are wider than those for ideal tetrahedral coordination, consistent with the electropositive character of aluminum. The C51−C52 and C43−C44 distances are both near 1.54 Å. 6238

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was selected, attached to a glass fiber on a copper pin, and placed in the N2 stream of the diffractometer. Data were collected based upon a single component, processed with SAINT, and corrected for Lorentz and polarization effects and absorption using Blessing’s method65 as incorporated into the program SADABS.66 The structures were determined by direct methods using the program XS.67 Refinement of the structure was achieved using the program XL.67 All of the nonhydrogen atoms were located initially or from one difference-Fourier map least-squares cycle, and convergence proceeded quickly with all of the hydrogen atoms located from a subsequent difference-Fourier map. Single-crystal X-ray diffraction data were recorded on a Bruker APEX diffractometer or Bruker Apex II Duo under a stream of cold nitrogen at 90(2) K using Mo Kα or Cu Kα radiation, respectively, using a Nonius CCD detector. Non-hydrogen atoms and hydrogens bonded to Al were refined anisotropically, while the remaining hydrogen positions were calculated using a riding model. Selected data for the refinements are presented in the SI (Table S1).

The volume was reduced to incipient crystallization (ca. 20 mL), and the mixture was warmed gently to redissolve the precipitate. Standing at room temperature for 3 weeks afforded the product as large, colorless, Xray quality crystals. Yield: 0.25 g (88%). Anal. Calcd for C52H83AlSi2: C, 78.92; H, 10.99. Found: C, 78.30; H, 10.61. 1H NMR (300 MHz, C6D6): δ 0.15 (s, 18H, Si(CH3)3), 1.20 (m, 48H, (CH3)2CH), 2.95 (m, 8H, (CH3)2CH), 6.26 (d, 3JHH = 24.7 Hz, 2H, CHCHSi Si(CH3)3), 6.79 (d, 3 JHH = 24.7 Hz, 2H, CHCHSi(CH3)3), 7.23 (s, 4H, C6H2-Pri3), 7.55 (s, 1H, C6H-Pri2). 13C NMR (75 MHz, C6D6): δ 1.76, 24.48, 24.65, 25.10, 25.82, 26.37, 26.51, 29.23, 30.20, 30.47, 30.78, 31.05, 35.04, 35.14, 121.37, 122.85, 143.41, 147.28, 147.86, 148.04, 148.11, 148.14, 149.06. Hydroalumination of Olefins. The insertion experiments were carried out one of two ways: General Preparation. A sample (ca. 1 mmol) of m-terphenyl alane was placed in a 100 mL flask along with the olefin and dissolved in 30 mL of toluene. In the case of ethylene addition, the vessel was evacuated and backfilled with ethylene to a pressure of ∼1 atm, closed, and stirred overnight. 1 H NMR Spectroscopy of the Reaction of (ArPri8AlH2)2 with Olefins. (ArPri8AlH2)2 (0.02 g) was added to a J. Young NMR tube and dissolved in 1 mL of C6D6. The olefin was added via syringe, and the sample heated. Reaction progress was recorded via a Bruker 500 MHz instrument. ArPri8Al(CH2CH2Ph)2. (ArPri8AlH2)2 (0.345 g, 0.290 mmol) was dissolved in 30 mL of pentane, and styrene (0.133 mL, 1.16 mmol) was added at room temperature. The mixture was stirred for 3 d at 40 °C, after which time the solution was concentrated to ca. 5 mL under reduced pressure and stored at −20 °C for 1 week to produce X-ray quality crystals. Yield: 0.3 g (60%). Anal. Calcd for C58H79Al: C, 86.73; H, 9.91. Found: C, 86.81; H, 9.82. 1H NMR (C6D6): δ 0.06 (m, 4H, CH2CH2C6H5), 1.16 (d, 12H, 3JHH = 6.5 Hz, (CH3)2CH), 1.19 (d, 24H, 3 JHH = 6.5 Hz (CH3)2CH), 1.23 (d, 12H, 3JHH = 7.0 Hz, (CH3)2CH), 2.30 (m, 4H, CH2CH2C6H5), 2.78 (m, 4H, (CH3)2CH), 3.03 (sept, 4H, 3 J HH = 6.5 Hz (CH 3 ) 2 CH), 7.11 (m, 14H, C 6 H 2 - i Pr 3 and CH2CH2C6H5), 7.53 (s, 1H, C6H-Pri2). 13C NMR (75 MHz, C6D6): δ 24.17, 25.47, 25.96, 30.24, 34.30, 34.82, 38.61, 121.03, 122.30, 124.01, 135.92, 136.08, 139.22, 146.93, 147.52, 147.73, 147.77, 147.95, 148.10, 148.35, 148.54, 148.83, 148.95, 151.20. Mp: 145 °C (with dec). ArPri8Al(CH2CH2C(CH3)3)2. This preparation was carried out similarly to that described above. Anal. Calcd for C54H87Al: C, 84.98; H, 11.49. Found: C, 85.01; H, 10.62. 1H NMR (300 MHz, C6D6): δ −0.52 (m, 4H, CH2CH2C(CH3)3), 0.79 (s, 4H), 0.99 (s, 18H), 1.03 (m, 2H, mC6H-(CH3)2CH), 1.22 (m, 36H, C6H2-(CH3)2CH), 1.34 (d, 6H, pC6H2-(CH3)2CH), JHH = 7.4 Hz, 1.40 (d, 6H, C6H2-(CH3)2CH), JHH = 6.9 Hz, 2.94 (m, 8H), 7.22 (s, 4H, C6H2-iPr3), 7.56 (s, 1H, C6H-Pri2), 13 C NMR (125 MHz, C6D6): δ 1.44, 6.04, 24.18, 24.42, 25.75, 26.14, 29.29, 29.32, 30.01, 30.35, 30.64, 32.41, 34.59, 39.40, 121.04, 122.20, 122.36, 122.90, 123.48, 124.43, 128.35, 138.87, 143.69, 145.65, 147.35, 147.61, 147.81, 148.45, 149.49, 149.74. ArPri8AlEt2. This preparation was carried out similarly to that described above. Anal. Calcd for C46H71Al: C, 84.86; H, 10.99. Found: C, 85.37; H, 10.61. 1H NMR: δ −0.43 (q, 4H, 3JHH = 8.13 Hz, CH2CH3), 0.85 (t, 6H, 3 JHH = 8.13 Hz, CH2CH3), 1.19 (d, 12H, 3JHH = 6.9 Hz, (CH3)2CH), 1.20 (d, 12H, 3JHH = 6.8 Hz, (CH3)2CH), 1.25 (d, 24H, 3JHH = 6.9 Hz, (CH3)2CH), 2.88−2.80 (m, 4H), 2.87 (sept, 4H, 3JHH = 6.8 Hz, (CH3)2CH), 3.01 (sept, 4H, 3JHH = 6.8 Hz, (CH3)2CH), 7.12 (s, 4H, C6H2-iPr3), 7.66 (s, 1H, C6H-iPr2) 13C NMR (75 MHz, C6D6): δ 1.44, 12.85, 24.17, 24.30, 24.66, 25.79, 26.19, 26.53, 29.78, 30.72, 121.04, 121.95, 127.74, 136.09, 138.05, 147.17, 147.52, 148.12. Mp: 145 °C (with dec). ArMe6Al(CH2CH2C(CH3)3)2. 1H NMR (300 MHz, C6D6): −0.27 (m, 4H), 0.30 (s, 9H), 0.73−0.79 (m, 2H), 0.90 (s, 12H), 1.01−1.07 (m, 2H), 2.19 (s, 6H), 3.103−3.28 (m, 2H), 6.87 (s, 4H), 7.01 (d, 2H, 3JHH = 7.56 Hz), 7.32 (t, 1H, 3JHH = 7.50 Hz). Mp: 167 °C (with dec). X-ray Crystallographic Data Collection. Crystals of (LiArPri8AlH3)2, Li(OEt2)(ArPri8AlH3), (ArPri8AlH2)2, ArPri8Al(CHCHSiMe3)2, ArPri8Al(CHCHPh)2, and ArPri8CH2CH2Ph suitable for single-crystal X-ray diffractometry were removed from a Schlenk flask under a stream of N2 and immediately covered with a layer of hydrocarbon oil. A single crystal

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ASSOCIATED CONTENT

S Supporting Information *

CIF files for (LiAlH 3 Ar Pri 8 ) 2 , {Li(OEt 2 )H 3 AlAr Pri 8 ·Li(OEt) 2 H 3 AlAr Pri 8 }·2C 6 H 14 , (Ar Pri 8 AlH 2 ) 2 ·Ar Pri 8 H·C 6 H 14 , (Ar Pri 4 AlH 2 ) 2 , Ar Pri 8 Al(CHCHC 6 H 5 ) 2 ·0.7·C 6 H 6 , Ar Pri 8 Al(CHCHSiMe3)2, and ArPri8(CH2CH2C6H5)2·0.8C5H12. Table summary of crystallographic data and refinement. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the U.S. Department of Energy (DE-FG0207ER46475) for financial support. P.A.G. also wishes to thank the Natural Sciences and Engineering Research Council of Canada for a Michael Smith Foreign Study Supplement.

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REFERENCES

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