Synthesis of electron-withdrawing butane- and arene-sulfonylamino phosphines and use in rhodium-catalyzed hydroformylationDedicated to the memory of Mohammad Salman Hamdani. Sal worked in our labs at Queens College as an undergraduate student from 1998–2001, and died as part of the rescue effort at the World Trade Center in New York City, September 11, 2001.Electronic supplementary information (ESI) available: details of the syntheses, purification, and NMR and analytical data. See http://www.rsc.org/suppd
“…On the basis of the relative integrations of 2 and 3 (1.4:1 after several days at room temperature) and assuming [CO] = 0.024 M, estimated from the known solubility of CO in DCM (mole fraction, 298 K, 1 atm = 1.54 × 10 –3 ; [DCM] = 15.62 M) the equilibrium constant for formation of 3 from 2 is K = 3.0 × 10 at 1 atm CO and 298 K. Therefore, Δ G ° = −0.3 kcal mol –1 .…”
The interactions of Pt and Pd acyl complexes with Lewis acids are investigated through experiment and computation. Variabletemperature NMR studies indicate BF 3 association with trans-[(PPh 3 ) 2 (CO)Pt(COCH 2 Ph)] [BAr F 4 ] (3; BAr F 4 = tetrakis(3,5-bis-(trifluoromethyl)phenyl)borate) at 298 K is endergonic (ΔG°= 2.0 kcal mol −1 ) yet exothermic (ΔH°= −3.4 kcal mol −1 ), suggesting a Lewis basicity comparable to or greater than aldehydes and ketones. Despite the accelerating effect of Lewis acids in the formation of the Pd congener trans-[(PPh 3 ) 2 (CO)Pd(COCH 2 Ph)][BAr F 4 ] (4), no evidence for adduct formation was obtained. DFT (M06-L/def2-TZVP/QZVP) suggests that BF 3 /trans-[(PPh 3 ) 2 (CO)M(COCH 2 Ph)] + adduct formation is more favored for M = Pt than M = Pd. Natural bond orbital analysis shows that upon Lewis acid coordination, the acyl C−O bond is weakened, the natural charge of the acyl C is more positive, and the π* acyl C−O orbital is lowered in energy relative to other unoccupied orbitals.
■ INTRODUCTIONLewis acids promote metal-centered reactivity through ligandbased interactions with transition metal complexes, 1 such as facilitating migratory insertion of CO. 2 Shriver and co-workers established that Lewis acids can accelerate CO insertion into metal−carbon bonds and stabilize the resulting acyl complexes through association to the acyl oxygen. 3 Since Shriver's seminal work, only a handful of reports 4 address the thermodynamic stability of Lewis acid−acyl adducts. Moreover, Lewis acids are seldom added to transition-metal-catalyzed carbonylations, 5 despite the likelihood of acyl intermediates in these reactions.A Lewis acid should increase acyl electrophilicity for the same reasons that Lewis acids influence organic carbonyl reactivity. Chart 1 shows illustrations comparing the LUMO of organic carbonyls and acyls. 6 Early metal η 2 -acyls are highly electrophilic at the acyl carbon, 7 and Lewis acid O-coordination to η 1 -acyl complexes should similarly polarize the acyl carbon and decrease the LUMO (π* CO ) energy. Rare, late transition metal η 2 -acyl complexes have reduced electrophilicity due to dπ/π* CO interactions. 7 However, such overlap is minimized in η 1 coordination shown in Chart 1, bottom.A lower LUMO energy could (a) make π* CO more available for reactions with exogenous nucleophiles and (b) promote internal attack by nucleophilic coligands, such as amines or alkyl ligands. The latter phenomenon resembles the accelerating effect of ligand−Lewis acid interactions on reductive elimination, for which there are both stoichiometric 8 and catalytic examples, such as hydrocyanation. 9 We stress that this qualitative picture is based on an understanding of Lewis acid interactions with aldehydes and ketones. Shriver and co-workers 3 rationalized the C−O acyl bond lengthening of CpFeL 2 (COR) upon BF 3 binding as a resonance effect, shortening the M−C bond and increasing Fischer carbene character. Later theoretical work by Green and co-workers 10 provided a more nuanced picture: the...
“…On the basis of the relative integrations of 2 and 3 (1.4:1 after several days at room temperature) and assuming [CO] = 0.024 M, estimated from the known solubility of CO in DCM (mole fraction, 298 K, 1 atm = 1.54 × 10 –3 ; [DCM] = 15.62 M) the equilibrium constant for formation of 3 from 2 is K = 3.0 × 10 at 1 atm CO and 298 K. Therefore, Δ G ° = −0.3 kcal mol –1 .…”
The interactions of Pt and Pd acyl complexes with Lewis acids are investigated through experiment and computation. Variabletemperature NMR studies indicate BF 3 association with trans-[(PPh 3 ) 2 (CO)Pt(COCH 2 Ph)] [BAr F 4 ] (3; BAr F 4 = tetrakis(3,5-bis-(trifluoromethyl)phenyl)borate) at 298 K is endergonic (ΔG°= 2.0 kcal mol −1 ) yet exothermic (ΔH°= −3.4 kcal mol −1 ), suggesting a Lewis basicity comparable to or greater than aldehydes and ketones. Despite the accelerating effect of Lewis acids in the formation of the Pd congener trans-[(PPh 3 ) 2 (CO)Pd(COCH 2 Ph)][BAr F 4 ] (4), no evidence for adduct formation was obtained. DFT (M06-L/def2-TZVP/QZVP) suggests that BF 3 /trans-[(PPh 3 ) 2 (CO)M(COCH 2 Ph)] + adduct formation is more favored for M = Pt than M = Pd. Natural bond orbital analysis shows that upon Lewis acid coordination, the acyl C−O bond is weakened, the natural charge of the acyl C is more positive, and the π* acyl C−O orbital is lowered in energy relative to other unoccupied orbitals.
■ INTRODUCTIONLewis acids promote metal-centered reactivity through ligandbased interactions with transition metal complexes, 1 such as facilitating migratory insertion of CO. 2 Shriver and co-workers established that Lewis acids can accelerate CO insertion into metal−carbon bonds and stabilize the resulting acyl complexes through association to the acyl oxygen. 3 Since Shriver's seminal work, only a handful of reports 4 address the thermodynamic stability of Lewis acid−acyl adducts. Moreover, Lewis acids are seldom added to transition-metal-catalyzed carbonylations, 5 despite the likelihood of acyl intermediates in these reactions.A Lewis acid should increase acyl electrophilicity for the same reasons that Lewis acids influence organic carbonyl reactivity. Chart 1 shows illustrations comparing the LUMO of organic carbonyls and acyls. 6 Early metal η 2 -acyls are highly electrophilic at the acyl carbon, 7 and Lewis acid O-coordination to η 1 -acyl complexes should similarly polarize the acyl carbon and decrease the LUMO (π* CO ) energy. Rare, late transition metal η 2 -acyl complexes have reduced electrophilicity due to dπ/π* CO interactions. 7 However, such overlap is minimized in η 1 coordination shown in Chart 1, bottom.A lower LUMO energy could (a) make π* CO more available for reactions with exogenous nucleophiles and (b) promote internal attack by nucleophilic coligands, such as amines or alkyl ligands. The latter phenomenon resembles the accelerating effect of ligand−Lewis acid interactions on reductive elimination, for which there are both stoichiometric 8 and catalytic examples, such as hydrocyanation. 9 We stress that this qualitative picture is based on an understanding of Lewis acid interactions with aldehydes and ketones. Shriver and co-workers 3 rationalized the C−O acyl bond lengthening of CpFeL 2 (COR) upon BF 3 binding as a resonance effect, shortening the M−C bond and increasing Fischer carbene character. Later theoretical work by Green and co-workers 10 provided a more nuanced picture: the...
“…As shown in eq 1, for instance, the compound that we have named TosL is readily prepared from the corresponding sulfonamide and phosphorus dichloride, despite the presumed low nucleophilicity of the nitrogen lone-pairs. Compounds such as TosL can serve as ligands that are exceptionally electron-withdrawing, and consistent with this electronic property, these phosphorus compounds function as effective promoters of rhodium-catalyzed hydroformylation. , …”
Reaction of the N-toluenesulfonyl derivatives of (S)-alanine, phenylalanine, and valine (4-6) with PhPCl(2) gave in high yield the 4-methyl, benzyl, and isopropyl derivatives (7-9) of 2-phenyl-1-p-toluenesulfonyl-1,3,2-oxazaphospholidin-5-one. The ratios of the (2S,4S)/(2R,4S) diastereomers (cis/trans isomers) were 1:1, 2:1, and 10:1 for the methyl, benzyl, and isopropyl derivatives 7a,b, 8a,b, and 9a,b, respectively. For 7a,b, both isomers could be crystallized, but for the others only the major isomers were isolable. The X-ray crystal structure of 9a shows that the isopropyl and phenyl groups are mutually cis and that the tolyl moiety is oriented s-trans to both the isopropyl and phenyl groups. Reaction of 6 with Cl(2)PCH(2)CH(2)PCl(2) (10) gave a 56:38:7 mixture of the cis/cis, cis/trans, and trans/trans diphosphorus heterocycles 11a-c. The major isomer could be crystallized and isolated free of the other diastereomers. Reaction of 6 with EtPCl(2) gave a 6:1 mixture of cis/trans isomers of the ethyl-substituted heterocycles 12a,b as an inseparable oil but allowed confirmation of the structure of 11a. Slow epimerization at phosphorus may occur by inversion but more likely by ring opening/closure, since 7b, 9a, and 11a give rise upon standing in solution to mixtures containing starting material and 7a, 9b, and 11b, respectively, along with the free amino acid derivatives 4 and 6. The NMR spectra, and in particular the coupling constants between the alpha-hydrogen atom of the amino acid moiety and phosphorus, were used to establish the identities of the cis and trans isomers. Reaction of 9a with (THF)W(CO)(5) gave the phosphorus-ligated adduct (9a)W(CO)(5) (13), and the IR spectrum of this complex shows that 9a is a strongly electron-withdrawing ligand. The geometry of the sulfonamide moiety is discussed in detail, as are the (1)H NMR coupling constants. The data are consistent with the presence of little steric interaction between the cis isopropyl and phosphorus substituent in 9a, 11a, and 12a and orientation of the tolyl moiety s-cis to the isopropyl group in 9b, 12b, and 13.
Reaktionen zur selektiven Veredelung von Alkenen zählen zu den wichtigsten Methoden in der Organischen Synthese. Durch die Entwicklung von Übergangsmetallkatalysatoren in den letzten Jahrzehnten können heute alle Arten von Selektivität bei einer Vielzahl von Transformationen effizient gesteuert werden. Auch 100 Jahre nach dem Tod von Vladimir Markownikow – dem Pionier auf diesem Gebiet – ist die regioselektive Funktionalisierung von Alkenen und Alkinen immer noch ein wichtiges Forschungsziel: Besonders die Anti‐Markownikow‐Addition von Sauerstoff‐ und Stickstoff‐Nucleophilen an aliphatische Alkene und Alkine verfügt über großes industrielles Potenzial. In diesem Aufsatz werden neuere Entwicklungen auf diesem Gebiet beschrieben. Eine Auswahl aktueller Beispiele zeigt, wie die Regioselektivität durch die Wahl des Katalysators gesteuert werden kann.
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