Communication pubs.acs.org/Organometallics
Reactivity of Boryl- and Silyl-Substituted Carbenoids toward Alkynes: Insertion and Cycloaddition Chemistry Andrey V. Protchenko,† Matthew P. Blake,† Andrew D. Schwarz,† Cameron Jones,‡ Philip Mountford,† and Simon Aldridge*,† †
Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, U.K. School of Chemistry, Monash University, P.O. Box 23, Melbourne, Victoria 3800, Australia
‡
S Supporting Information *
ABSTRACT: Three modes of reactivity of phenyl-substituted alkynes toward acyclic tetrelenes are reported, with reaction pathways found to be dependent not only on the nature of the group 14 element but also on the supporting ligand set. Systems featuring Sn−B or Ge−B bonds undergo insertion chemistry, forming borane-appended (vinyl)SnII and GeII species. With a bis(amido)stannylene, phenylacetylene acts as a protic acid, generating a SnII acetylide with a unique bridged structure. Reactivity toward a more strongly reducing SiII system is dominated by the possibility of accessing SiIV via [2 + 1] cycloaddition chemistry.
T
reversibility) as the magnitude of the EII/EIV redox couple becomes smaller. Thus, in the current paper we examine the comparative reactivity of a series of closely related acyclic stannylenes, germylenes, and silylenes toward phenyl- and diphenylacetylene. Bis(boryl)stannylene 1a (Chart 1) has been shown to undergo insertion into a range of E−H bonds (E = H, B, N, O, Si) to give
he genesis of a significant proportion of the subvalent molecular chemistry of the group 14 elements1 owes much to the pioneering work of Lappert and co-workers.2 The development of modern theories of chemical bonding for maingroup elements beyond the straightjacket of the so-called “double bond rule”,3,4 for example, draws significantly on groundbreaking early compounds such as Lappert’s bis(bis(trimethylsilyl)methyl)stannylene (Scheme 1) and West’s
Chart 1. Carbenoid Systems Exploited in the Current Study Scheme 1. Lappert’s Bis(bis(trimethylsilyl)methyl)stannylene
(hydrido)SnIV systems,10f with the role of the ancillary ligand framework being critical not only in tuning the SnII/SnIV potential to facilitate oxidative addition but also in lowering the activation barrier through destabilization of the metal-based HOMO.10f With this in mind, we targeted the reaction of 1a with phenylacetylene as a potential route to analogous C−H activation chemistry.11 However, this reaction, carried out in benzene at 23 °C, proceeds instead via insertion of the alkyne into both Sn−B bonds to give divinylstannylene 4, featuring a pair of pendant borane functions (Scheme 2). That the reaction proceeds via formal stannaboration of PhCCH is evident by the appearance of an alkenic 1H NMR signal at 7.31 ppm, with
5,6
tetramesityldisilene. More recently, interest has focused on the development of main-group compounds displaying some of the key chemical characteristics of transition-metal systems: e.g., relatively small HOMO−LUMO gaps, well-defined radical character, and the ability to accomplish small-molecule capture/activation.7 Among such novel systems are examples of compounds capable of the capture (in some cases reversibly) of unsaturated substrates such as alkenes and alkynes.8,9 The reactivity of tetrelenes toward alkynes, however, stems from much earlier work, with the first examples being reported as long ago as 1976.8k,l In recent work we have been interested in developing five- and six-valence-electron main-group systems and their use in oxidative E−H and E−C bond formation.10 Our interest in the heavier tetrelenes is driven by the possibility for reductive regeneration of the active species (and ultimately for © XXXX American Chemical Society
Special Issue: Mike Lappert Memorial Issue Received: December 8, 2014
A
DOI: 10.1021/om501252m Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
palladium-catalyzed stannaboration of phenylacetylene using a Me3SnIV(boryl) reagent.12 5 and 6 represent, to our knowledge, the first structurally characterized examples of vinylstannylenes, or indeed of vinylsubstituted acyclic carbenoids in general.13,14 The molecular structure of 6, for example, features the expected mononuclear two-coordinate tin center, with an angle defined by the two substituents at the metal (106.7(1)°) which is essentially indistinguishable from that of precursor 1b (mean 106.9(2)°). That measured for 5 is similar, if slightly narrower (101.2(1)°). In 6 the stannylene unit lies essentially coplanar with the (planar) amido function (C−Sn−N−Si torsion angle 16.4(1)°), while the corresponding torsion angle for the vinyl substituent (45.9(1)°) implies little interaction of the CC double bond with the formally vacant tin pz orbital. On the other hand, the boryl and vinyl functions are aligned close to coplanar (N−B−C−C torsion angle 17.5(1)°) and the corresponding B−C bond length (1.553(1) Å) reflects a degree of conjugation found in other reported vinylboranes.15 In the case of 5, the analogous boryl/ vinyl components are not so obviously coplanar (mean N−B− C−C torsion angle 36.7(3)°) and the associated B−C bonds are marginally longer (mean 1.576(3) Å). Moreover, in the absence of a strongly π donating amido group, the vinyl substituents in 5 appear to be better oriented for CC−Sn π conjugation (with C−Sn−C−C torsion angles of 15.1(2) and 23.6(2)°) and the associated Sn−C distances (2.223(2), 2.230(3) Å) are slightly shorter than that found in 6 (2.254(1) Å). In the absence of an ancillary Sn−B bond, an alternative mode of reactivity is displayed by stannylenes toward phenylacetylene. Thus, diamidostannylene 1c16 undergoes a formal protonolysis reaction with PhCCH, leading to the formation of the dimeric alkynylstannylene 8 (Scheme 3) and aniline HN(SiMe3)Dipp.
Scheme 2. Insertion of Alkynes into the Sn−B Bonds of (Boryl)stannylenes: Reactions of Bis(boryl)stannylene 1a and Amido(boryl)stannylene 1b toward Terminal and Internal Alkynesa
a
Key reagents and conditions: (i) for 4, PhCCH (2.0 equiv), C6D6, 23 °C, 10 min, ca. 90% by NMR, for 5, PhCCPh (2.2 equiv), C6D6, 23 °C, 1 h, 89%; (ii) for 6, PhCCH (1.0 equiv), C6D6, 23 °C, 2 min, quantitative by NMR; for 7, PhCCPh (1.1 equiv), C6D6, 23 °C, 12 h, quantitative by NMR.
the regiochemistry of the insertion being implied by CH and quaternary carbon resonances in the regions of the 13C{1H} spectrum characteristic of boron- and tin-bound vinyl carbon atoms, respectively (δC 137.2 and 212.0 ppm). Although single crystals of 4 suitable for X-ray diffraction could not be obtained, alkyne insertion into the Sn−B bonds to give a SnII product can also be achieved with diphenylacetylene. In this case, the product so obtained (5; Scheme 2) does prove to be amenable to crystallographic study (Figure 1).
Scheme 3. Reaction of Diamidostannylene 1c with Phenylacetylene (Left) and Molecular Structure of 8 As Determined by X-ray Crystallography (Right)a
Figure 1. Molecular structures of 5 (left) and 6 (right) as determined by X-ray crystallography. Most hydrogen atoms are omitted, and iPr groups are shown in wireframe format for clarity; thermal ellipsoids are set at the 50% probability level. Key bond lengths (Å) and angles (deg): for 5, Sn(1)−C 2.223(2), 2.230(3), C(53)−C(54) 1.363(3), C(67)−C(68) 1.362(3), B(1)−C(54) 1.574(3), B(2)−C(68) 1.577(3), C(53)− Sn(1)−C(67) 101.2(1); for 6, Sn(1)−N(3) 2.096(1), Sn(1)−C(28) 2.254(1), C(27)−C(28) 1.357(1), B(1)−C(27) 1.553(1), N(3)− Sn(1)−C(28) 106.7(1).
Key reagents and conditions: (i) PhCCH (1.1 equiv), hexane, 23 °C, 12 h, 55% isolated yield. In the X-ray structure, hydrogen atoms are omitted, and iPr groups are shown in wireframe format for clarity; thermal ellipsoids are set at the 50% probability level. Key bond lengths (Å) and angles (deg): Sn(1)−N(1) 2.101(1), Sn(1)−C(1) 2.334(1), Sn(1)−C(1′) 2.307(1), C(1)−C(2) 1.220(2), C(2)−C(1)− Sn(1) 126.0(1), C(2)−C(1)−Sn(1′) 139.0(1). a
In contrast to the case for 1a, amido(boryl)stannylene 1b has been shown to be unreactive toward the activation of dihydrogen, a finding that is related, at least in part, to its lower HOMO energy and consequently higher singlet−triplet gap.10f 1b does, however, prove to be reactive toward phenylacetylene, albeit slightly slower than 1a, and yields the related vinylstannylene 6, which retains the ancillary amido ligand, while incorporating 1 equiv of PhCCH via stannaboration. The very high apparent selectivity of this insertion step is signaled by the appearance of only a single set of signals in in situ NMR experiments, and the preference (as with 1a) for syn addition/boryl attachment to the least hindered carbon is in this case confirmed crystallographically (Figure 1). Such behavior mirrors that reported by Tanaka and co-workers for the
Such chemistry mirrors that reported by Fulton and co-workers in the reaction of phenylacetylene with β-diketiminatosupported SnII amide.17 Other routes to SnII alkynyls have previously been reported, notably metathesis reactions of lithium alkynyls with SnII halides,18 but 8 is unique in featuring bridging hydrocarbyl ligands. The alkynyl ligand bridges in a nearsymmetrical fashion across the two tin centers, with Sn(1)−C(1) and Sn(1′)-C(1) distances (2.334(1) and 2.307(1) Å, respectively) which are >0.1 Å longer than those found in terminally bound SnII alkynyls.18 The central Sn2C2 core is planar (angles of 94.7(1) and 85.3(1)° at C(1) and Sn(1), respectively), B
DOI: 10.1021/om501252m Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
characterize the highly labile yellow intermediate species and so targeted instead isolable EIV species through cycloaddition of alkynes at a more strongly reducing (SiII) center. With this in mind, we explored the reactivity of silylsilylene 3 toward phenyland diphenylacteylene,8t with the pendant silyl group being preferred to its boryl counterpart on the basis of reduced lability of the Si−Si (vs Si−B) bond.10f These reactions proceed readily to generate the corresponding silacyclopropene species 10 and 11, both of which are stable with respect to further rearrangement. Each has been characterized by standard spectroscopic/ analytical techniques and by X-ray crystallography (Scheme 5).
with the exocyclic Sn−N bond being projected approximately perpendicular to this plane (∠N(1)−Sn(1)···Sn(1′) 100.2(1)°). This mode of dimerization contrasts markedly with the structures reported by Power and co-workers for systems of the type [ArSn(CCR)]2 (Ar = C6H3Dipp2-2,6), which adopt symmetrical distannene (R = SiMe3) or unsymmetrical stannylstannylene structures (R = tBu) featuring direct tin−tin bonds.18 As with stannaboration, there are (to our knowledge) no previous reports of uncatalyzed alkyne germaboration.19 Thus, with a view to establishing whether chemistry similar to that observed for 1a,b could be achieved with GeII, the reaction of borylgermylene 2 with diphenylacetylene was examined. Accordingly, the related boryl-functionalized vinylgermylene complex 9 could be synthesized and isolated in 28% yield after recrystallization from hexane (Scheme 4). The structure of 9
Scheme 5. [2 + 1] Cycloaddition of Phenyl- and Diphenylacetylene at Silylene 3 (Top) and Molecular Structures of 10 (Bottom Left) and 11 (Bottom Right) As Determined by X-ray Crystallographya
Scheme 4. Reaction of Amido(boryl)germylene 2 with Phenylacetylene (Left) and Molecular Structure of 9 As Determined by X-ray Crystallography (Right)a
Key reagents and conditions: (i) PhCCPh (1.2 equiv), C6D6, 23 °C, 10 min, 28% isolated yield. In the X-ray structure, hydrogen atoms are omitted, and iPr groups are shown in wireframe format for clarity; thermal ellipsoids set at the 50% probability level. Key bond lengths (Å) and angles (deg): Ge(1)−N(3) 1.881(1), Ge(1)−C(28) 2.012(1), C(27)−C(28) 1.360(2), B(1)−C(27) 1.579(2), N(3)− Sn(1)−C(28) 104.5(1).
a
a Key reagents and conditions: (i) for 10, PhCCH (equiv), C6D6, 23 °C,
