Intramolecular Frustrated Lewis Pair Research Articles

Are intramolecular frustrated Lewis pairs also intramolecular catalysts? A theoretical study on H2 activation.

We investigate computationally a series of intramolecular frustrated Lewis pairs (FLPs), with the general formula Mes2PCHRCH2B(C6F5)2, that are known from the literature to either activate molecular hydrogen (FLPs with R = H (1) or Me (4)), or remain inert (FLPs with R = Ph (2) or SiMe3 (3)). The prototypical system Mes2PCH2CH2B(C6F5)2 (1) has been described in the literature (Grimme et al., Angew. Chem., Int. Ed., 2010; Rokob et al., J. Am. Chem. Soc., 2013) as an intramolecular reactant that triggers the reaction with H2 in a bimolecular concerted fashion. In the current study, we show that the concept of intramolecular H2 activation by linked FLPs is not able to explain the inertness of the derivative compounds 2 and 3 towards H2. To cope with this, we propose an alternative intermolecular mechanism for the investigated reaction, assuming stacking of two open-chain FLP conformers, and formation of a dimeric reactant with two Lewis acid–base domains, that can split up to two hydrogen molecules. Using quantum-chemical methods, we compute the reaction profiles describing these alternative mechanisms, and compare the derived predictions with earlier reported experimental results. We show that only the concept of intermolecular H2 activation could explain both the activity of the FLPs having small substituents in the bridging molecular region, and the inertness of the FLPs with a bulkier substitution, in a consistent way. Importantly, the intermolecular H2 activation driven by intramolecular FLPs indicates the key role of steric factors and noncovalent interactions for the design of metal-free systems that can efficiently split H2, and possibly serve as metal-free hydrogenation catalysts.

1,1-Carboboration to tellurium-boron intramolecular frustrated Lewis pairs.

The tellurium acetylide Ph(CH2)2TeCCPh reacts with boranes via 1,1-carboboration to give a series of borylated vinyl telluroethers. The product Ph(CH2)2TeC(Ph)C(C6F5) B(C6F5)2 reacts with phenylacetylene and iodine to give novel tellurium-boron heterocyclic compounds.

Intramolecular frustrated Lewis pair with the smallest boryl site: reversible H2 addition and kinetic analysis.

Ansa-aminoborane 1 (ortho-TMP-C6H4-BH2; TMP = 2,2,6,6-tetramethylpiperid-1-yl), a frustrated Lewis pair with the smallest possible Lewis acidic boryl site (-BH2), is prepared. Although it is present in quenched forms in solution, and BH2 represents an acidic site with reduced hydride affinity, 1 reacts with H2 under mild conditions producing ansa-ammonium trihydroborate 2. The thermodynamic and kinetic features as well as the mechanism of this reaction are studied by variable-temperature NMR spectroscopy, spin-saturation transfer experiments, and DFT calculations, which provide comprehensive insight into the nature of 1.

Intramolecular Frustrated Lewis Pair with the Smallest Boryl Site: Reversible H2 Addition and Kinetic Analysis

AbstractAnsa‐aminoborane 1 (ortho‐TMPC6H4BH2; TMP=2,2,6,6‐tetramethylpiperid‐1‐yl), a frustrated Lewis pair with the smallest possible Lewis acidic boryl site (BH2), is prepared. Although it is present in quenched forms in solution, and BH2 represents an acidic site with reduced hydride affinity, 1 reacts with H2 under mild conditions producing ansa‐ammonium trihydroborate 2. The thermodynamic and kinetic features as well as the mechanism of this reaction are studied by variable‐temperature NMR spectroscopy, spin‐saturation transfer experiments, and DFT calculations, which provide comprehensive insight into the nature of 1.

ChemInform Abstract: Intramolecular Frustrated Lewis Pairs: Formation and Chemical Features

AbstractReview: 148 refs.

ChemInform Abstract: Carbonylation Reactions of Intramolecular Vicinal Frustrated Phosphane/Borane Lewis Pairs.

The intramolecular frustrated Lewis pair (FLP) Mes2PCH2CH2B(C6F5)2 4 adds cooperatively to carbon monoxide to form the five-membered heterocyclic carbonyl compound 5. The intramolecular FLP 7 contains an exo-3-B(C6F5)2 Lewis acid and an endo-2-PMes2 Lewis base functionality coordinated at the norbornane framework. This noninteracting FLP adds carbon monoxide in solution at -35 °C cooperatively to yield a five-membered heterocyclic FLP-carbonyl compound 8. In contrast, FLP 7 is carbonylated in a CO-doped argon matrix at 25 K to selectively form a borane carbonyl 9 without involvement of the adjacent phosphanyl moiety. The free FLP 7 was generated in the gas phase from its FLPH2 product 10. A DFT study has shown that the phosphonium hydrido borate zwitterion 10 is formed exergonically in solution but tends to lose H2 when brought into the gas phase.

Preparation and reactivity of an isolable N-heterocyclic carbene–borane

Deprotonation of the N-heterocyclic carbene 1,3-di-tert-butylimidazolin-2-ylidene (1) with n-butyl lithium in the 4-position, followed by addition of bis(2,4,6-trimethylphenyl)boronfluoride, Mes2BF, afforded 4-[bis(2,4,6-trimethylphenyl)boryl]-1,3-di-tert-butylimidazolin-2-ylidene (6), which contains non-interacting carbene and borane moieties and can thus be regarded as a potential intramolecular frustrated Lewis pair (FLP). Alternatively, 6 can be conveniently prepared from 1 by reaction with Mes2BF via the abnormal carbene–borane adduct 7, which could be converted into 6 by treatment with potassium hexamethyldisilazide, K(HMDS). The reactions of 6 with the rhodium(I) complexes [Rh(COD)Cl]2 and [Rh(CO)2Cl]2 furnished [(6)Rh(COD)Cl] (8) and [(6)Rh(CO)2Cl] (9), with the latter allowing an assessment of the donor properties of 6 by IR spectroscopy. 6 reacted with carbon dioxide and tert-butyl isocyanate to form the adducts 6·CO2 (11) and 6·tBuNCO (12), which proved unstable in solution, with partial dissociation into the starting materials. The X-ray crystal structures of 6, 7, 8·2CH2Cl2 and 12 are reported.

Carbene‐9‐BBN Ring Expansions as a Route to Intramolecular Frustrated Lewis Pairs for CO2 Reduction

Reactions of phosphine-derived carbenes with 9-borabicyclo[3.3.1]nonane (9-BBN) result in ring-expansion reactions to generate novel intramolecular frustrated Lewis pairs (FLPs). These FLPs effect the catalytic reduction of CO2 in the presence of boranes to give BOB and methoxy-borate species.

Ethene/alkyne exchange reaction at an intramolecular frustrated Lewis pair

A vicinal ethylene bridged S/B frustrated Lewis pair (FLP) in situ generated by a hydroboration reaction of phenyl vinyl sulfide, reacts with p-tert-butylphenylacetylene by ethene/alkyne exchange and subsequent 1,2-addition of a second alkyne equivalent to give a zwitterionic sulfonium/borate product.

Carbonylation Reactions of Intramolecular Vicinal Frustrated Phosphane/Borane Lewis Pairs

The intramolecular frustrated Lewis pair (FLP) Mes2PCH2CH2B(C6F5)2 4 adds cooperatively to carbon monoxide to form the five-membered heterocyclic carbonyl compound 5. The intramolecular FLP 7 contains an exo-3-B(C6F5)2 Lewis acid and an endo-2-PMes2 Lewis base functionality coordinated at the norbornane framework. This noninteracting FLP adds carbon monoxide in solution at -35 °C cooperatively to yield a five-membered heterocyclic FLP-carbonyl compound 8. In contrast, FLP 7 is carbonylated in a CO-doped argon matrix at 25 K to selectively form a borane carbonyl 9 without involvement of the adjacent phosphanyl moiety. The free FLP 7 was generated in the gas phase from its FLPH2 product 10. A DFT study has shown that the phosphonium hydrido borate zwitterion 10 is formed exergonically in solution but tends to lose H2 when brought into the gas phase.

Haloboration of internal alkynes with boronium and borenium cations as a route to tetrasubstituted alkenes.

Hail boration! 2-Dimethylaminopyridine-ligated dihaloborocations [X2B(2-DMAP)](+) with a strained four-membered boracycle were used for the haloboration of terminal and dialkyl internal alkynes (see scheme). Esterification then provided vinyl boronate esters as useful precursors to tetrasubstituted alkenes. Following mechanistic studies, the scope of the haloboration was expanded simply by variation of the amine. Pin = 2,3-dimethyl-2,3-butanedioxy.

Functional group chemistry at intramolecular frustrated Lewis pairs: substituent exchange at the Lewis acid site with 9-BBN

The vicinal frustrated P/B Lewis pair (FLP) Mes(2)PCH(2)CH(2)B(C(6)F(5))(2) reacts with 9-borabicyclo[3.3.1]nonane (9-BBN) by C(6)F(5)vs. H exchange at boron to give the new [B]-H functionalized FLP Mes(2)PCH(2)CH(2)B(H)(C(6)F(5)) (4) and 9-C(6)F(5)-BBN. The latter was characterized as an isonitrile adduct by X-ray diffraction. The new FLP 4 forms an adduct with pyridine and it undergoes clean hydroboration reactions with 1-pentyne or added styrene or dimesitylvinylphosphane. The products formed stable adducts with pyridine; two such examples were also characterized by X-ray crystal structure analysis. A similar alkyl vs. hydrogen exchange was observed upon treatment of an Al/N based Lewis pair, iBu(2)Al-(Me(3)Si)C=C(H)-N(CH(2)CH(2))(2)NMe (14), with 9-BBN.

Reversible Heterolytic Si–H Bond Activation by an Intramolecular Frustrated Lewis Pair

The intramolecular frustrated P/B Lewis pair Mes2PCH2CH2B(C6F5)2 (7) reacts readily with phenylsilane by heterolytic cleavage of the Si-H bond to give the zwitterion [Mes2(PhH2Si)P+CH2CH2B-H(C6F5)2] (8a), which has been fully characterized and its structure confirmed by X-ray crystal structure analysis. Variable-temperature NMR studies revealed that the reaction is reversible. Adduct 8a is the predominant species (ca. 98%) in CD2Cl2 solution at low temperature (193 K), whereas at ambient temperature (299 K) it exists in a ca. 7 : 3 equilibrium with unreacted 7 and PhSiH3. Diphenylsilane reacted similarly with the frustrated Lewis pair 7, however, the equilibrium was found to favor the starting materials in the investigated temperature range.

Halogen Addition to a Frustrated Lewis Pair

AbstractThe reactive intramolecular frustrated Lewis pair (FLP) Mes2PCH2CH2B(C6F5)2 reacts rapidly with thionyl chloride to yield the zwitterionic chlorination product [Mes2PCl+]CH2–CH2[BCl(C6F5)2–]. Analogous zwitterionic products were obtained by the reaction of the FLP with chlorine or bromine. Compounds [Mes2PX+]CH2CH2[BX(C6F5)2–] (X = Cl, Br) were characterized by X‐ray crystal structure analyses.

Capture of NO by a Frustrated Lewis Pair: A New Type of Persistent N‐Oxyl Radical

NO your pairs: The intramolecular frustrated Lewis pair (FLP) Mes2PCH2CH2B(C6F5)2 captures NO to give the novel N-oxyl radical P/B-FLP-NO. (see scheme). Coordination of NO to the FLP incites H-atom abstraction reactivity with cyclohexene and ethylbenzene to give P/B-FLP-NOH and the O-functionalized P/B-FLP-NOR species.

Structure and Dynamic Features of an Intramolecular Frustrated Lewis Pair

Di(mesityl)cyclohexenylphosphine undergoes hydroboration with Piers' borane [HB(C(6)F(5))(2)] to yield the cyclohexylene-anellated frustrated Lewis pair 5. This P/B pair splits H(2) with the formation of the product 4 and adds to the C=O double bond of phenyl isocyanate to yield 6. In the crystal, compound 5 features a puckered four-membered heterocyclic core structure with a long P-B bond (av. 2.197(5) Å). The activation energy of the P-B cleavage of the frustrated Lewis pair 5 was determined by dynamic (19)F NMR spectroscopy at ΔG(≠)(298 K)=12.1±0.3 kcal mol(-1).

Mechanistic Insight on the Hydrogenation of Conjugated Alkenes with H2Catalyzed by Early Main-Group Metal Catalysts

Density functional theory calculations have been performed to investigate the molecular mechanism of the hydrogenation reactions of 1,1-diphenylethylene and myrcene catalyzed by the actual calcium hydride catalyst, CaH(dipp-nacnac)(thf) (dipp-nacnac = CH{(CMe)(2,6-iPr(2)-C(6)H(3)N)}(2)). The hydrogenation reactions of these two alkenes proceed via a similar pathway, which includes three steps. First, the hydride migrates from the calcium center to one olefinic carbon in the substrate. Then, the hydride transfer product can easily transform into a key ion-pair intermediate. This intermediate provides an intramolecular frustrated Lewis pair, in which the calcium center acts as a Lewis acid, and one olefinic carbon acts as a Lewis base. Next, the H-H bond is heterolytically cleaved by this frustrated Lewis pair through a concerted Lewis acid-Lewis base mechanism, producing the hydrogenation product and regenerating the catalyst. For these two reactions, the rate-limiting step is the hydride transfer step, with free energy barriers of 29.2 kcal for both substrates. In addition, our calculations indicate that the hydrogenation reaction of 1,1-diphenylethylene catalyzed by the analogous strontium hydride complex may readily occur, but the similar magnesium-mediated hydrogenation reaction is less likely to take place under similar conditions as adopted by the calcium hydride catalyst. The results can give satisfactory descriptions of experimental facts observed for these two hydrogenation reactions. The hydrogenation mechanism proposed here is different from that of the late transition metal-catalyzed alkene hydrogenation or the organolanthanide-catalyzed alkene hydrogenation.

Reactions of an Intramolecular Frustrated Lewis Pair with Unsaturated Substrates: Evidence for a Concerted Olefin Addition Reaction

The intramolecular frustrated Lewis pair (mesityl)(2)P-CH(2)-CH(2)-B(C(6)F(5))(2) was generated in situ by HB(C(6)F(5))(2) hydroboration of dimesitylvinylphosphine. The compound reacts with 1-pentyne by C-H bond cleavage. It undergoes a 1,2-addition to the carbonyl group of trans-cinnamic aldehyde to yield a zwitterionic six-membered heterocycle by B-O and P-C bond formation. The Lewis pair regioselectively adds to the electron-rich C=C double bond of ethyl vinyl ether, and it selectively undergoes an exo-cis-2,3-addition to norbornene. A combined experimental/theoretical study suggests that this reaction takes place in an asynchronous concerted fashion with the B-C bond being formed in slight preference to the P-C bond. The addition products were characterized by X-ray crystal structure analyses.