Figure 4. Reaction cascade of the formation of product 2 from FLP 11, CO, and HB(C 6 F 5 ) 2 calculated by DFT. The values given refer to free reaction enthalpies in toluene solution at 298 K (i.e., they include entropy as well as solvent effects) whereas the numbers in parentheses are pure electronic reaction energies in the gas phase.

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.

|Organometallics 2011, 30, 4211-4219 Organometallics ARTICLE chemistry will be illustrated in this article with this new example of an electronically controlled P/B Lewis pair.' RESULTS AND DISCUSSION Generation of the New Frustrated Lewis Pair. For generating the new FLP (13) we first reacted bis(pentafluorophenyl) chlorophosphine ( 11) 17 with propynyllithium to yield the reagent 12 (Scheme 1). Its treatment with Piers' borane [HB(C 6 F 5 ) 2 ] 15 at room temperature gave a ca. 7:1 mixture of the two regioisomeric products 13 and 14 isolated in a combined yield of close to 90%. The major product is the "Markovnikov" addition product 13. It features a very typical set of 1 H NMR spectral data of the dCHCH 3 moiety [methyl: doublet at δ 1.58 ( 3 J H,H = 6.9 Hz) and doublet of quartets with 3 J P,H ≈ 24 Hz at δ 6.73 (see Figure 1)]. The minor product ( 14) in contrast features a broad methyl signal at δ 2.00 and a broad dCH resonance at δ 7.50. The major product (13) features a 31 P NMR signal at δ À62.8 and a broad 11

The dimesitylpropargylphosphanes mes2 P-CH2 -C≡C-R 6 a (R=H), 6 b (R=CH3 ), 6 c (R=SiMe3 ) and the allene mes2 P-C(CH3 )=C=CH2 (8) were reacted with Piers' borane, HB(C6 F5 )2 . Compound 6 a gave mes2 PCH2 CH=CH(B(C6 F5 )2 ] (9 a). In contrast, addition of HB(C6 F5 )2 to 6 b and 6 c gave mixtures of 9 b (R=CH3 ) and 9 c (R=SiMe3 ) with the regioisomers mes2 P-CH2 -C[B(C6 F5 )2 ]=CRH 2 b (R=CH3 ) and 2 c (R=SiMe3 ), respectively. Compounds 2 b,c underwent rapid phosphane/borane (P/B) frustrated Lewis pair (FLP) reactions under mild conditions. Compound 2 c reacted with nitric oxide (NO) to give the persistent FLP NO radical 11. The systems 2 b,c cleaved dihydrogen at room temperature to give the respective phosphonium/hydridoborate products 13 b,c. Compound 13 c transferred the H(+) /H(-) pair to a small series of enamines. Compound 13 c was also a metal-free catalyst (5 mol %) for the hydrogenation of the enamines. The allene 8 reacted with B(C6 F5 )3 to give the zwitterionic phosphonium/borate 17. The -PPh2 -substituted mes2 P-propargyl system 6 d underwent a typical 1,2-P/B-addition reaction to the C≡C triple bond to form the phosphetium/borate zwitterion 20. Several products were characterized by X-ray diffraction.

A new reaction pathway: Carbon monoxide is readily reduced by Piers' borane at a frustrated Lewis pair (FLP) to yield a formylborane stabilized by the FLP (see picture). This reaction may be considered a typical example of efficient activation of a small molecule by a FLP.