Unique Insertion Mechanisms of Bis-dehydro-β-diketiminato Silylene

Published: September 22, 2011

r 2011 American Chemical Society 5344 dx.doi.org/10.1021/om200424k |Organometallics 2011, 30, 5344–5351

ARTICLE

pubs.acs.org/Organometallics

Unique Insertion Mechanisms of Bis-dehydro-β-diketiminato SilyleneTibor Szilv�asi, Kinga Nyíri, and Tam�as Veszpr�emi*

Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics (BUTE), Szent Gell�ert t�er 4,1521 Budapest, Hungary

bS Supporting Information

’ INTRODUCTION

In the past twenty years several stable silylenes have beensynthesized and characterized.1 Their stabilizing effects, boththermodynamic and kinetic, and their reactivity have been deeplyinvestigated.2 Much less information is known about the me-chanisms of their reactions.

The synthesis of the first six-membered-ring silylene in 2006(1; Scheme 1) opened up a new field of silylene chemistry.3 Thespecialty of 1 is the seven electrons in the π-system of the ring,which suggests a zwitterionic structure (1b). In our previousstudy,4 we proved that the aromatic 1b has no relevant role indescribing the structure of silylene; however, it is important insome reactions.5

The reactions of 1 have been widely studied since its pre-paration.6One interesting type of reaction is insertionwhen a singleX�Y bond splits and two new silicon bonds form, saturating in thisway the silicon center (Scheme 2).

Several insertion reactions of 1 have been described so far: e.g.with H2O,

6a H2S,6q NH3,

6h N2H4,6j C2H2,

6e P4,6b haloalkanes,

and halosilanes.6g In addition to the common 1,1-insertionproduct (product a, Pra) two other surprising products were ob-served (Scheme 3). In the case of H2S,

6q instead of the expected1,1-insertion a silicon�sulfur double bond was found, forming apentavalent silicon center, while the second hydrogen saturatedthe methylene group of the silicon ring (product b, Prb). In thereaction with water after the 1,4-addition a peculiar second stepwas also observed in which the OH group of the 1,4-adductreacted with a second silylene molecule (1), forming a dimer(product c, Prc).

In our previous study,7 the direct, one-step insertion reactionsof seven different silylenes, including 1, into eight differentreactant molecules were investigated. We set up the trend ofthe reactivity of silylenes and proved that the reactivities of thesilicon center are similar in 1 and the well-known saturated five-membered N-heterocyclic silylene 3. We found that the sum of

the reaction free energy and the activation free energy dependedonly on the reactants and did not depend on the silylenes.

Considering, however, the experimental conditions the directinsertion mechanism provides activation barriers that are toohigh, suggesting that 1 has different complex reaction pathwayswhich are not located only on the silicon center. The reaction of 1andMe3SiOTf leads to a 1,4-adduct as an intermediate in the firststep, which then rearranges to the thermodynamic 1,1-product(OTfdOSO2CF3).

3 In the reaction of 1 with B(C6F5)3 the1,4-adduct was isolated as the final product.5 Similarly to this trainof thought, Driess et al. proposed a 1,4-mechanism (Scheme 4) forthe addition of haloalkanes and halosilanes.6g According to this

Scheme 1

Scheme 2

Received: May 19, 2011

ABSTRACT: The unique insertion reactions of the first, stablesix-membered-ring silylene ({HC[CMeN(R)]2}Si, R = 2,6-diisopropylphenyl) with eight reactants were investigated bythe B3LYP/cc-pVTZ method. The initial step (IS) of all thereactions is the formation of an intermediate 1,4-adduct, IM,which will be then the starting point toward the different finalstates (FS). In this study three different mechanisms were foundand studied to the 1,4-adduct and six reaction paths from the 1,4-adduct to the final products. On the basis of the results, the differentreaction paths, the experimental insertion products, and the special reactivity of the six-membered-ring silylene have been explained.

5345 dx.doi.org/10.1021/om200424k |Organometallics 2011, 30, 5344–5351

Organometallics ARTICLE

scheme, a concerted step leads to an 1,4-adduct (4) and then arearrangement results in the corresponding 1,1-insertion product.

The aim of this work is to explore and prove the uniquemechanisms of 1 in different insertion reactions and, with thehelp of them, support the experimental data. In order to analyzethese mechanisms, eight different reactants (A�H) were se-lected (Scheme 5), for which experimental data were available.

’THEORETICAL SECTION

All calculations were carried out using the Gaussian 038 quantumchemical program package. In our last study,7 we fully characterized the

B3LYP method in conjunction with the cc-pVTZ basis set for theinsertion reaction of silylenes, using MP2 and CCSD ab initio methodsand explaining all the geometrical discrepancies between the com-puted and the solid-state structure. Since 2 and the selected reactants

Scheme 3

Scheme 4

Scheme 5

Table 1. Calculated Reaction Energies, Gibbs Free Energies,Activation Energies, and Activation Gibbs Free Energies(in kJ/mol) for the Direct Mechanism of 2 + (A�H)a

H2

(A)

H2O

(B)

CH3OH

(C)

H2S

(D)

NH3

(E)

N2H4

(F)

CH3Cl

(G)

CH3I

(H)

TSdirect ΔEq 201 116 111 103 157 151 196 193

ΔGq 235 148 145 140 191 184 234 230

Pra ΔE �96 �202 �197 �141 �150 �143 �256 �219

ΔG �56 �158 �151 �94 �108 �100 �206 �167aUsing the B3LYP/cc-pVTZ level of theory.

Table 2. Reaction Energies, Gibbs Free Energies, ActivationEnergies, and Activation Gibbs Free Energies (in kJ/mol) forthe Initial Step Mechanisms (ISI�FSIII)

a

H2

(A)

H2O

(B)

CH3OH

(C)

H2S

(D)

NH3

(E)

N2H4

(F)

CH3Cl

(G)

CH3I

(H)

ISIISI-TS1 ΔEq 185 54 53 59 71 87 196 186

ΔGq 222 94 93 105 117 132 237 231

ISIIISII-TS1 ΔEq 168 46 47 75 77 89 211 221

ΔGq 205 86 87 117 92 129 248 264

ISII-IM1 ΔE 47 �40 �34 �12 11 16 �46 �23

ΔG 101 9 17 47 61 68 14 40

ISII-TS2 ΔEq 256 176 183 201 225 228 217 231

ΔGq 293 210 218 242 259 263 252 268

ISIIIISIII-TS1 ΔEq 124 91

ΔGq 171 138

IM ΔE �43 �142 �135 �114 �84 �85 �179 �160

ΔG 6 �93 �86 �59 �37 �37 �121 �101aUsing the B3LYP/cc-pVTZ level of theory.



(exceptD) were part of that study, we assumed that the B3LYP/cc-pVTZmethod is suitable for exploring the unique insertion reactions of 2.To consider relativistic effects, we employed the cc-pVTZ-PP basis set oniodine atoms.9

Stationary points on the potential energy surface (PES) werecharacterized by harmonic vibrational frequency calculations. Transitionstates (TS), with one imaginary frequency, were confirmed by intrinsicreaction coordinate (IRC) calculations. The MOLDEN program10 wasutilized for visualization and drawing.

’RESULTS AND DISCUSSION

First we summarize the results of the directmechanism (Table 1).7

On this path the new Si�X and Si�Y bonds are formed in onestep before the transition state. With all reactants TSdirectprovides similar geometries: the X�Si�Y angle is between 41and 59�, and the silicon slightly lifts out from the ring plane (thedihedral angle C6�N7�Si1�N2 is between 3 and 8�; for

numbering, see Scheme 1), forming a slightly distorted tetra-hedral geometry.

Table 1 indicates that the activation barriers calculated for thismechanism are generally too high, which contradicts the mildconditions applied during the reactions. Therefore, we studieddifferent mechanisms via the 1,4-addition suggested by Driess atal.3,5 In this process the initial step (IS) of all the reactions is theformation of an intermediate 1,4-adduct, IM, which will be thenthe starting point toward the final state (FS). Our discussionfollows this logic. First we describe and characterize the possiblereaction paths from the reactants to IM and then the differentsteps between IM and the final products (Pra,b,c). The geome-tries of all the investigated derivatives is summarized in TablesS5�S72 (see the Supporting Information).1. Steps toward IM. Table 2 shows the calculated reaction

energies, Gibbs free energies, activation energies, and activationGibbs free energies of the initial steps of all the reactions towardIM. Three different mechanisms could be found.

Figure 1. Mechanisms ISI and ISII for the reaction of H2O and 2 at the B3LYP/cc-pVTZ level of theory.

Figure 2. Mechanism ISIII, FSV, and FSVI for the reaction of CH3I and 2 at the B3LYP/cc-pVTZ level of theory.



Mechanism ISI (Figure 1) is a one-step concerted process, inwhich the X�Y bond of the reactant molecule splits and a newSi�X and C4�Y bond is formed in one step (where X = OH,OCH3, SH, NH, NNH2, Cl, I and Y = H, CH3). ISI-TS1 is amoderate energy six-membered ring transition state. The X�Ybond length increases about 35% in ISI-TS1, while the formingSi�X and C4�Y bonds are only 13% and 36% longer on average

than those in the resulting product (IM). In the case of G andHthe C4�Y bond is 78% longer than in IM. In the transition stateISI-TS1 the molecule is highly distorted, the C6�N7�Si1�N2dihedral angle (for numbering, see Scheme 1) is between 18 and23�, and the dihedral angle of the butadiene moiety (C6�C5�C3�C4) alters between 127� and 165�. The activation barriersin the reactions of 2 with B�F are significantly lower than

Table 3. Reaction Energies, Gibbs Free Energies, Activation Energies, and Activation Gibbs Free Energies in kJ/mol of Final StepMechanisms (FSI�FSVI) Starting from IMa

H2 (A) H2O (B) CH3OH (C) H2S (D) NH3 (E) N2H4 (F) CH3Cl (G) CH3I (H)

IM ΔE �43 �142 �135 �114 �84 �85 �179 �160

ΔG 6 �93 �86 �59 �37 �37 �121 �101

FSIFSI-TS1 ΔEq 211 101 108 143 151 152 193 211

ΔGq 247 138 146 187 188 191 231 250

Pra ΔE �96 �202 �197 �141 �150 �143 �256 �219

ΔG �56 �158 �151 �94 �108 �100 �206 �167

FSIIFSII-TS1 ΔEq 274 189 197 217 246 234 b b

ΔGq 310 223 231 258 278 270 b b

FSII-IM1 ΔE 47 �40 �34 �12 11 16 �52 �30

ΔG 101 9 17 47 61 68 7 32

FSII-TS2 ΔEq 209 106 114 144 154 163 146 165

ΔGq 241 141 150 185 188 197 193 212

Pra ΔE �96 �202 �197 �141 �150 �143 �256 �219

ΔG �56 �158 �151 �94 �108 �100 �206 �167

FSIIIFSIII-TS1 ΔEq 81 48 184 120

ΔGq 116 95 217 153

Prb ΔE �176 �198 �51 �38

ΔG �129 �137 �8 2

FSIVc

FSIV-TS1 ΔEq �38 �17 65 62

ΔGq 46 88 154 154

Prc ΔE �352 �261 �235 �221

ΔG �256 �150 �142 �127

FSVFSV-TS1 ΔEq 123 140

ΔGq 150 168

FSV-IM1 ΔE 110 127

ΔG 92 110

FSV-TS2 ΔEq 150 155

ΔGq 179 186

Pra ΔE �256 �219

ΔG �206 �167

FSVIFSVI-TS1 ΔEq 2 �205 �183 �127 �92 �85 �152 �112

ΔGq 140 �62 �44 23 43 53 4 45

FSVI-IM1 ΔE �265 �224

ΔG �108 �72

FSVI-TS2 ΔEq �220 �169

ΔGq �82 �24

Pra ΔE �96 �202 �197 �141 �150 �143 �256 �219

ΔG �56 �158 �151 �94 �108 �100 �206 �167aUsing B3LYP/cc-pVTZ level of theory. bTS cannot be found. c Energies and Gibbs free energies refer to the 2 � 2 + (B�F) reactions.



those of the direct mechanism. This suggests the importance of1,4-mechanisms. In the reactions with H2, CH3Cl, and CH3I,however,ΔE is high enough to justify the existence of some otherprocesses.Mechanism ISII (Figure 1) is a two-step process. The first step

is the formation of an 1,2-adduct in which the TS is a four-membered Si�X�Y�N ring. The new Si�X and N�Y bondsare formed before the first transition state. It seems straight-forward to reach the final product (Pra) from ISII-TS1 with a2,1-hydrogen migration; however, in real systems, the bulky2,6-(i-Pr)2C6H3 substituents on the nitrogens prevent thismotion. This step might be possible only after the rearrangementof the ligand sphere of the nitrogen, which can occur with theconsecutive 2,4- and 4,2-migration of Y. Therefore, the secondstep of this process is a 2,4-migration of Y.While the molecule in the first transition state (ISII-TS1) is

nearly planar, the 2,4-migration step (ISII-TS2) proceeds via ahighly distorted geometry (with reactants A�F the dihedralangle of the butadiene moiety (C6�C5�C3�C4) varies be-tween 145 and 153�). Probably this is the main reason that thefirst step requires a low barrier but the second step needs a fairlyhigh activation energy. Because of the second energetic step, thismechanism is not likely.For the reactions with G and H a different one-step mechan-

ism (ISIII) with relatively low energy barriers is suggested(Figure 2). In this process the first step is an electrophilic attackto the exocyclic double bond (C4), in which the partially positivemethyl group turns toward the electron-rich C4 atom of the

butadiene moiety. In the transition state ISIII-TS1 the C�Cdistance between the approaching methyl group of the reactantand the methylene group of silylene is 2.02 Å (G) and 2.09 Å(H), respectively. The methyl group is almost planar and theC�X distance elongates from 1.80 to 2.42 Å (X = Cl) and from2.16 to 2.73 Å (X = I). This geometry suggests a typical SN2 typemechanism (Figure 2). Then the halogen anion migrates to thesilicon center, forming the stable intermediate IM.The Si�Y bond angle is nearly perpendicular to the molecular

plane in IM. The C6�N7�Si1�N2 dihedral angle is between 9and 13�. IM forms a nearly planar ring; however, the calculatedNICS values are found to be between 0.2 and 1.8 ppm, whichsuggests the lack of aromaticity in the 1,4-adducts. Probably thisfact explains that IM is observable only in very few examples.2. Steps from IM toward the Final Product. Table 3 shows

the calculated reaction energies, Gibbs free energies, activationenergies and activation Gibbs free energies of all the proposedmechanisms starting from IM toward the final products. Thepossible reaction paths between IM and the products are plottedon Figure 3 for the example of H2O.In FSI the 1,4-intermediate (IM) rearranges in a one-step

concerted process to provide the experimentally isolated 1,1-product (Pra). FSI-TS1 is a highly bent transition state; thedihedral angles C6�C5�C3�C4 and C6�C5�C3�N2 of thebutadiene moiety are about 92 and 54�, respectively, on average(A�F). Y is about halfway between C4 and Si, since themoleculeis strongly bent, and the forming Si�Y bond in FSI-TS1 is only25�30% longer than in Pra, while the C4�Y bond is only 28%(in average) longer than in the starting point (IM).FSII is a two-step, highly energetic process. The first step is a

4,2-migration of Y, resulting in the intermediate FSII-IM1, whichis followed by a 2,1-migration of Y (Figure 3).We note that in thereactions of 2 and A�F intermediates ISII-IM1 are equal to FSII-IM1, since the substituents on the nitrogens are all hydrogens. Inthe case of G and H, when the migrating substituent is a methylgroup, ISII-IM1 and FSII-IM1 are obviously different.If the rearrangement reaction of IM proceeds via mechanism

FSIII, the resulting product is a SidE double-bonded pentavalent

Figure 3. Possible mechanisms FSI�FSIV for the reaction of H2O and 2 at the B3LYP/cc-pVTZ level of theory (FSIV refers to the 2 � 2 + H2Oreaction).

Figure 4. Rate-determining step of FSVI at the 2 + E reaction.



silicon. Although this reaction may take place with B�F, it seemsto be reasonable only with B and D for energetic reasons. In thispath both the transition state FSIII-TS1 and the product Prbrepresent a nearly planar ring.Mechanism FSIV suggests a further insertion reaction between

2 and IM. The silicon of 2 inserts into the (Si)E�H bond of IM,forming a “dimer” via a Si�E�Si structure. The relative positionsof the two rings in the calculated geometry differ somewhat fromthe experimentally observed data,6a as we substituted the bulky2,6-(i-Pr)2C6H3 groups with hydrogens. Nevertheless, the en-ergy difference between the optimum and the experimental ringpositions is negligible (7 kJ/mol). This reaction is expectablewith E = O (H2O) or E = S (H2S). Although the reaction is alsopossible in the case of E = N (NH3 and N2H4), it requires highactivation energy. We have to mention that the energy scale ofthis mechanism in Figure 3 is not comparable with that of othermechanisms in Figure 3 because of the varying numbers of atoms.This visualization was chosen to simplify the representation ofthese mechanisms.FSV is a two-step radical mechanism which proceeds only in

the reaction of 2 and CH3Cl or CH3I. In this path methyl radicalleaves the exocyclic methylene group of the ring, forming theradical intermediate FSV-IM1 (Figure 2). In FSV-TS1 the C4�Ybond elongates from the starting 1.5 Å (IM) to about 2.5 Å. Thesilicon is slightly under the plane (the dihedral angle ofC6�N7�Si1�N2 is about 10�), while the butadiene moiety isplanar. The methyl group is also nearly planar, and the sum ofH�C�H bond angles around the methyl group is 354�. In theradical intermediate FSV-IM1, the methyl radical leaves and themolecule is almost planar (the silicon is only 7� under the ringplane). In FSV-TS2 the Si�Y distance is about 40% longer (2.5 Å)than in the final product (Pra). Since the mechanism requiresenergy that is too high in comparison to the experimentalconditions, this process is unlikely.Finally, we mention two multistep chainlike mechanisms

when the 1,1-insertion product is formed. First, there is thepossible reaction between two IM intermediates (FSVI) in whichthe ”tail” (C4) of an IM reacts with the silicon center of anotherIM molecule. This kind of mechanism is possible with all theinvestigated reactants. In the reactions of A�F or G and H aproton (Figure 4) or a methyl cation (Figure 2) is transferred,respectively, forming an ion pair. These ions can react with each

other by simply changing the roles. However, a chain reactionlike mechanism with another IM is also possible; the “tail” of thecation interacts with the silicon center of the anion (Figure 2),forming two 1,1-insertion products (Pra). These particles areextremely reactive: the proton migration proceeds without anactivation barrier, and the methyl transfer is also favorable(Table 3). Though the activation barriers of this process areconsiderably influenced by the bulky groups, they should besignificantly lower than those of the other concurrent processes.The surprisingly low energy barriers of this chain mechanismsuggest the general importance of this process in the reactivity ofthe six-membered-ring silylene.3. Chain Mechanism (CM) with Alkyl Halogenides. During

the initial step of ISIII the leaving halogen anion does notnecessarily attack the silicon center of the same silylene; it isable to migrate toward the new silylene 2 and react with it,providingCM-IM1 without an activation barrier (Figure 5). Thismechanism (CM) is probably operating mainly at the early stageof the reaction, since its precondition is the high concentration ofnonreacted silylene 2. The stable anion CM-IM1 formed in thisway reacts further with a methyl halogenide, which dissociatesto a halogen anion and a methyl cation in the transition stateCM-TS1. The reaction of the methyl cation provides the 1,1-insertion product (Pra), while the halogen anion reacts with anew silylene 2. The activation barrier of this mechanism is rathersmall (Table 4) to be operative at room temperature, which mayexplain at least the initial period of the reaction ofG andHwith 2.This result suggests that, with extra halogen ions, the reactionmay proceed without ISIII-TS1: i.e., a catalytic cycle is formed.We can further assume that the reaction of any silylene withhaloalkanes may be explained by this mechanism.4. Selection of theMost Favorable Reaction Paths.Accord-

ing to the general schemes described above, we can easily selectthe most likely mechanisms (Table 5).As recently documented byBertrand and co-workers,11N-hetero-

cyclic carbenes are not sufficiently electrophilic to react withH2 (A). This is true to a greater extent for stable silylenes. Sincethe lowest energy barrier is 185 kJ/mol, no reaction could beassumed between 1 and H2. Indeed, this reaction has never beenobserved.1, however, reacts with water6a (B) and hydrogen sulfide6q

(D) at low temperature and forms the peculiar Prc and Prb. The

Figure 5. Chain mechanism for the reaction of CH3I + 2 at the B3LYP/cc-pVTZ level of theory.



product of the 1 + B reaction is a siloxy silylene, a formal dimer ofthe LSiOH and the SiL (L=HC[CMeN(2,6-isopropenylphenyl)]2)molecules (Scheme 3, Prc). In contrast, the product of the 1 +Dreaction is a LSi(dS)H silathioformamide (Scheme 3, Prb).Indeed, in these cases formal dimerization (FSIV) and the 1,2H-rearrangement (FSIII) were found as the two competing reactionpaths. In the reaction 2 + B, mechanism FSIII (Si�OH fSi(dO)H reorganization) requires a 116 kJ/mol activationbarrier, while FSIV needs only 46 kJ/mol (ΔGq, Table 3). Thecombination of ISI and FSIV can explain the experimentalfindings written above. The computed data suggest that in the2 +B reaction Prc is a product of a two-step mechanism. The first

step is a 1,4-addition of water to 2 (ISI, ΔGq = 94 kJ/mol), and

the second is the insertion of 2 into the OH bond of IM.ISI explains the initial step of the reaction 2 + D to reach the

1,4-intermediate IM (ΔGq = 105 kJ/mol). The activation Gibbsfree energy, ΔGq, in the reaction of 2 and H2S was found to be95 kJ/mol in FSIII and 88 kJ/mol in FSIV. In this case the twoalternative reaction paths (FSIII and FSIV) have comparablebarriers. The isolable product is, however, a silathioformamide(Prb). This might be explained by the effect of the substituentsof 1 and the fact that an internal rearrangement (FSIII) isgenerally faster than a bimolecular reaction (FSIV) with bulkysubstituents.The reaction of silylene 1 with ammonia6h (E) or hydrazine6j

(F) resulted in the isolable 1,1-substituted derivatives of 1 undermild conditions. The first 1,4-addition step provides a moderateactivation barrier (ΔGq = 117 and 132 kJ/mol, respectively), butthe path to the final productPra requires a high barrier on FSI andFSII. The formation of the experimentally found 1,1-product canbe explained by mechanism FSVI. The activation energy of thebimolecular hydrogen transfer was found to be only 43 kJ/mol in2 + E and 53 kJ/mol in 2 + F reactions.FSVI can also explain the reactions of 2with alcohols. Reaction

2 +C proceeds in two steps as a combination of ISI and FSVI. Thecalculated barrier was 93 kJ/mol, almost the same as we found inthe reaction of 2 +B. As a consequence, it is likely that 1 can reactwith methanol under mild conditions.

Table 4. Reaction Energies, Gibbs Free Energies, ActivationEnergies, and Activation Gibbs Free Energies in kJ/mol of theChain Mechanism (CM) Referring to 2 + G/H + Cl�/I� a

CH3Cl (G) CH3I (H)

CM-IM1 ΔE �109 �56

ΔG �81 �30

CM-TS1 ΔEq �89 �65

ΔGq 10 �1

Pra ΔE �256 �219

ΔG �206 �167aUsing the B3LYP/cc-pVTZ level of theory.

Table 5. Suggested Mechanisms of the Reactions between 2 and A�H (Reaction Gibbs Free Energies and Activation Gibbs FreeEnergies in kJ/mol)a

ISI-TS1 IM FSVI-TS1 FSVI-IM1 FSVI-TS2 Pra

H2 (A) 222 6 140 �56

ISI-TS1 IM FSIV-TS1 Prc

H2O (B)b 94 �93 46 �256


CH3OH (C) 93 �86 �44 �151

ISI-TS1 IM FSIII-TS1 Prb

H2S (D) 105 �59 95 �137


NH3 (E) 117 �84 43 �108


N2H4 (F) 132 �85 53 �100

ISIII-TS1 IM FSVI-TS1 FSVI-IM1 FSVI-TS2 Pra

CH3Cl (G) 171 �121 4 �108 �82 �206

ISIII-TS1 M1 M2 Pra

Cl� + CH3Cl (G)c 171 �81 10 �206

ISIII-TS1 IM FSVI-TS1 FSVI-IM1 FSVI-TS2 Pra

CH3I (H) 138 �101 45 �72 �24 �167

ISIII-TS1 M1 M2 Pra

I� + CH3I (H)c 138 �30 �1 �167

aUsing the B3LYP/cc-pVTZ level of theory. bReferring to the to 2 � 2 + B reaction. cReferring to the to 2 + G/H + Cl�/I� reactions.



ISI-ISII and FSI-FSV cannot properly explain the observedinsertion reactions of 1 to aliphatic halogenides at room tem-perature. For example, the reaction of 1 with MeI (H) wascomplete after 2 h at room temperature. The direct radicalmechanism was found to be unlikely by Driess at al.6g On theother hand, we found a relatively low barrier ionic mechanismfor the 1,4-addition of 2 to the C�I bond (ISIII). Startingfrom ISIII-TS1, however, mechanisms FSVI and CM also providelow activation barriers in the case of reactants G and H. Bothmechanisms may be operative in 2 +G and 2 +H reactions, sincethe rate-determining step is certainly ISIII.

’CONCLUSION

In this work the possible mechanisms of the insertion reac-tions of silylene 2 into H2, H2O, CH3OH, H2S, NH3, N2H4,CH3Cl, and CH3I (A�H) were characterized by the DFTmethod. We found that the direct mechanism cannot properlyexplain the experimental results. The isolated products andthe observed intermediates suggest that the reaction via an1,4-adduct is more likely. We investigated three possible reactionpaths to reach the 1,4-intermediate (IM) and six differentmechanism from IM to the different final products (Pra,b,c).The experimental findings can be well explained on the basis ofthe calculated reaction energies.

The activation barriers of the one-step 1,4-addition (ISI) inthe reactions of 2 and B�F are significantly lower than those ofthe direct mechanism. Starting from IM, a consecutive protontransfer (FSVI) was found to be the most likely to reach the final1,1-insertion product (Pra) in the reactions with C, E, F, and H.The reaction of 2with B can be explained by the formation of the1,4-adduct (IM) and the subsequent insertion of 2 into the OHbond of IM (FSIV). In the case ofD the alternative reaction pathshave comparable barriers.

For the reactions with alkyl halogenides (G andH) a one-stepionic mechanism (ISIII) with relatively low energy barriers issuggested as an initial step. Depending on the reaction partner ofthe ion formed in ISIII, two alternative reaction paths (FSVI andCM) were found.

’ASSOCIATED CONTENT

bS Supporting Information. Tables giving optimized geo-metries and the computed energies. This material is available freeof charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

’ACKNOWLEDGMENT

We thank the Hungarian Scientific Research Foundation(OTKA) for financial support under the grant K76806.

’REFERENCES

(1) (a) Denk, M.; Lennon, J. R.; Hayashi, R.; West, R.; Belyakov,A. V.; Verne, H. P.; Haaland, A.; Wagner, M.; Metzler, N. J. Am. Chem.Soc. 1994, 116, 2691. (b) Gehrhus, B.; Lappert, M. F.; Heinicke, J.;Boese, R.; Bl€aser, D. J. Chem. Soc., Chem. Commun. 1995, 193, 1932.(c) West, R.; Denk, M. Pure Appl. Chem. 1996, 68, 785. (d) Kira, M.;Ishida, S.; Iwamoto, T.; Kabuto, C. J. Am. Chem. Soc. 1999, 121, 9722.

(2) For reviews, see: (a) Karni, L. M.; Kapp, J.; Schleyer, P. v. R.;Apeloig, Y. In The Chemistry of Organic Silicon Compounds (TheChemistry of Functional Groups); Rappoport, Z., Apeloig, Y., Eds.; Wiley:Chichester, U.K., 2001; Vol. 3, p 1. (b) Gaspar, P. P.; West, R. In TheChemistry of Organic Silicon Compounds (The Chemistry of FunctionalGroups); Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K., 2001;Vol. 3, p 2463. (c) Haaf, M.; Schmedake, T. A.; West, R. Acc. Chem.Res. 2000, 33, 704. (d) Tokitoh, N.; Okazaki, R. Coord. Chem. Rev.2000, 210, 251.(e) Veszpr�emi, T. In Molecular Structure Research;Hargittai, M., Hargittai, I., Eds.; JAI Press, Stanford, CT, 2000; Vol. 6,p 267. (f) Gehrhus, B.; Lappert, M. F. J. Organomet. Chem. 2001, 617�618, 209. (g) Hill, N. J.; West, R. J. Organomet. Chem. 2004, 689, 4165.

(3) Driess, M.; Yao, S.; Brym, M.; van W€ullen, C.; Lentz, D. J. Am.Chem. Soc. 2006, 128, 9628.

(4) Nyíri, K.; Veszpr�emi, T. Organometallics 2009, 28, 5909.(5) Driess, M.; Yao, S.; Brym, M.; van W€ullen, C. Angew. Chem., Int.

Ed. 2006, 45, 6730.(6) (a) Yao, S.; Brym, M.; van W€ullen, C.; Driess, M. Angew. Chem.,

Int. Ed. 2007, 46, 4159. (b) Xiong, Y.; Yao, S.; Brym, M.; Driess, M.Angew. Chem., Int. Ed. 2007, 46, 4511. (c) Yao, S.; Xiong, Y.; Brym, M.;Driess, M. J. Am. Chem. Soc. 2007, 129, 7268. (d) Yao, S.; Xiong, Y.;Brym, M.; Driess, M. Chem. Asian J. 2008, 3, 113. (e) Yao, S.; vanW€ullen, C.; Sun, X.-Y.; Driess, M. Angew. Chem., Int. Ed. 2008, 47, 3250.(f) Yao, S.; Xiong, Y.; van W€ullen, C.; Driess, M. Organometallics 2009,28, 1610. (g) Xiong, Y.; Yao, S.; Driess, M. Organometallics 2009,28, 1927. (h) Jana, A.; Schulzke, C.; Roesky, H. W. J. Am. Chem. Soc.2009, 131, 4600. (i) Xiong, Y.; Yao, S.; Driess, M. Chem. Eur. J. 2009,15, 5545. (j) Jana, A.; Roesky, H. W.; Schulzke, C.; Samuel, P. P.Organometallics 2009, 28, 6574. (k) Xiong, Y.; Yao, S.; Bill, E.; Driess, M.Inorg. Chem. 2009, 48, 7522. (l) Xiong, Y.; Yao, S.; Driess, M. J. Am.Chem. Soc. 2009, 131, 7562. (m)Xiong, Y.; Yao, S.; Driess,M.Chem. Eur.J. 2009, 15, 8542. (n) Xiong, Y.; Yao, S.; Driess, M. Chem. Asian J. 2010,5, 322. (o) Xiong, Y.; Yao, S.; Driess, M. Organometallics 2010, 29, 987.(p) Yao, S.; Xiong, Y.; Driess, M. Chem. Eur. J. 2010, 16, 1281.(q) Meltzer, A.; Inoue, S.; Pr€asang, C.; Driess, M. J. Am. Chem. Soc.2010, 132, 3038. (r) Jungton, A.-K.; Meltzer, A.; Pr€asang, C.; Braun, T.;Driess, M.; Pennera, A. Dalton Trans. 2010, 39, 5436. (s) Epping, J. D.;Yao, S.; Karni, M.; Apeloig, Y.; Driess, M. J. Am. Chem. Soc. 2010,132, 5443. (t) Xiong, Y.; Yao, S.; M€uller, R.; Kaupp, M.; Driess, M. J. Am.Chem. Soc. 2010, 132, 6912. (u) Xiong, Y.; Yao, S.; Driess, M. DaltonTrans. 2010, 39, 9282. (v) Jana, A.; Samuel, P. P.; Tav�car, G.; Roesky,H. W.; Schulzke, C. J. Am. Chem. Soc. 2010, 132, 10164.

(7) Nyíri, K.; Szilv�asi, T.; Veszpr�emi, T. Dalton Trans. 2010, 39, 9347.(8) (a) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;

Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.;Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.;Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken,V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski,V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick,D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui,Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.;Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith,T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.;Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; andPople, J. A. Gaussian 03, Revision C.02; Gaussian, Inc., Wallingford, CT,2004. (b) Feller, D. J. Comput. Chem. 1996, 17, 1571. (c) Schuchardt,K. L.; Didier, B. T.; Elsethagen, T.; Sun, L.; Gurumoorthi, V.; Chase, J.;Li, J.; Windus, T. L. J. Chem. Inf. Model. 2007, 47, 1045.

(9) Schaftenaar, G.; Noordik, J. H. J. Comput.-Aided Mol. Des. 2000,14, 123.

(10) Frey, G. D.; Lavallo, V.; Donnadieu, B.; Schoeller, W. W.;Bertrand, G. Science 2007, 316, 439.

Unique Insertion Mechanisms of Bis-dehydro-β-diketiminato Silylene

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