Intramolecular [4+2] Diels−Alder Cycloaddition of a 2H-Phosphole to Coordinated Unsaturated Phosphines, Phospholes, and an Arsine

Abstract

Stereoselective [4+2] Diels−Alder cycloadditions occur between unsaturated tertiary phosphine, phosphole, and arsine molybedenum carbonyl complexes, e.g., (Ph2E-trans-CH2CHCHCH3)Mo(CO)5 [E = P, As] and 3,4-dimethyl-1-phenylphosphole (DMPP). The coordinated DMPP undergoes a [1,5] phenyl migration at about 145 °C prior to or concomitant with the [4+2] intramolecular cycloaddition reaction with the alkene moiety of the dienophile to produce a new class of conformationally rigid bidentate ligands containing the 1-phosphanorbornene bicyclic ring system. The characteristic 1H, 13C{1H}, and 31P{1H} NMR spectroscopic features of these compounds are described. Crystal structures of most of the new compounds are reported.

Synthesis and Characterization of Unsymmetrical (cis-(Phosphino−arsino)ethene)tetracarbonylmolybdenum Complexes: Hydroarsination Reactions of (H3CC⋮CPPh2)Mo(CO)5 and [H2CCCH(DBP)]Mo(CO)5 and Hydrophosphination Reactions of (Ph2AsCH2C⋮CH)Mo(CO)5

“…So far we have not been able to identify or detect the trans mixed arsine−phospholyl geometrical isomer of H in the product mixture. Analogous dimolybdenum trans -diphosphine and mixed phosphole−phosphine complexes have been reported earlier. , In a similar way, the addition of the As−H bond in the Markovnikov sense across the internal vinyl fuctional group of 2a ‘ will produce the dangling arsine complex J , which may undergo base promoted [1,3] hydrogen migration to produce any of the geometrical isomers of H . Alternatively, the driving force of chelation may also cause the intermediate J to form the five-membered chelate ring following the elimination of a CO molecule to produce the exo methylene complex K .…”

“…31 P{ 1 H}, 13 C{ 1 H}, and 1 H NMR Spectroscopy. The 31 P{ 1 H} NMR spectra for the rigid chelate complexes E , F , and M each display a single resonance at δ 55.03, 51.95, and 55.34 ppm, respectively, in the region expected for Mo(0) complexes. , The large downfield shifts for the phosphorus nuclei in each case is consistent with the formation of the five-membered chelate ring . The 13 C{ 1 H} NMR spectra for these chelate complexes display two distinct sets of resonances for each of the mutually cis equatorial carbonyl carbons, and the most downfield resonance corresponds to the carbonyl group trans to the arsino unit.…”

“…The latter may subsequently undergo the [1,3] hydrogen shift to form the thermodynamically more stable complex F . Although we have not been able to identify the intermediates J and K for this particular reaction, we have earlier reported the formation of analogous dimolybdenum diphosphine, diphosphole, and mixed phosphine−phosphole complexes obtained as products of similar hydrophosphination reactions. , The formation of the cis -diphosphole complex I in a minor amount indicates that some ligand dissociation followed by the substitution of a CO unit may occur to form the cis -bis(phosphole) intermediate L , which then rapidly undergoes the hydroarsination reaction at the terminal vinyl group in the Markovnikov sense to give the resultant cis -bis( cis -DBP−As) complex I . It may also be argued that the hydrophosphination reaction precedes the formation of the cis -bis(phosphole) intermediate, in which case H is formed first, and then two such molecules of H condense to form I following the elimination of a unit of Mo(CO) 6 .…”

“…There is evidence for some (arsine)pentacarbonylchromium complexes to undergo rapid ligand dissociation followed by substitution with stronger nucleophiles, such as a phosphine. We have encountered such ligand dissociation followed by substitution reactions in the case of mixed-phosphine−phosphole complexes . In contrast, the hydroarsination reactions of the phosphinoalkyne and phospholylalleneyl complexes 2b ‘‘ and 2a ‘, respectively, with the free secondary arsine gave clean products, and no ligand exchange products were observed in any of these cases.…”

“…Such addition reactions may provide an efficient route for the syntheses of unsymmetrical, rigid, chelating diphosphine ligands which have potential applications in the area of asymmetric catalysis B , C , and D 5 (Scheme ).

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