A series of 2-methyl-1,3-propenylene-bridged (η-diarylphosphinocyclopentadienyl)(phosphine)manganese(I) dicarbonyl complexes 2 have been developed as a new class of phosphine-olefin ligands based on a planar-chiral transition-metal scaffold, which show better robustness as well as higher enantioselectivity over phosphine-olefin ligands 1 with a planar-chiral (η-arene)chromium(0) framework. The practical enantiospecific and scalable synthesis of 2 has been established. Phosphine-olefin ligands 2 enable construction of an effective chiral environment around a transition-metal center upon coordination, and thus their rhodium(I) complexes exhibit excellent catalytic performance in the various asymmetric addition reactions of arylboron nucleophiles. Complex 2b, which has a bis(3,5-dimethylphenyl)phosphino group on the cyclopentadienyl ring, is found to be a superior chiral ligand in the rhodium-catalyzed asymmetric 1,4-addition reactions of arylboronic acids to various cyclic/acyclic enones giving the corresponding arylation products in over 99% ee. On the other hand, 2c and 2d, which have bis[3,5-bis(trifluoromethyl)phenyl]phosphino and bis(3,5-di-tert-buthyl-4-methoxyphenyl)phosphino groups, respectively, are highly efficient chiral ligands in the rhodium-catalyzed asymmetric 1,2-addition reactions of the arylboron nucleophiles to imines or aldehydes showing up to 99.9% ee. The X-ray crystallographic studies of (R)-2b and [RhCl((S*)-2b)] reveal the absolute configuration of 2b and its phosphine-olefin bidentate coordination to a rhodium(I) cation. Structural comparison with [RhCl((R*)-1b)] postulates the origins of the higher enantioselectivity of newly developed phosphine-olefin ligands 2.
Enantioselective desymmetrization of C s -symmetric (η 5 -2,5-dialkenylphospholyl)(allyldiphenylphosphine)manganese(I) dicarbonyl complexes 1 was realized by molybdenum-catalyzed asymmetric ringclosing metathesis (ARCM), and the corresponding bridged planar-chiral phosphacymantrene derivatives 2 were obtained in good yields with excellent enantioselectivity. The enantioselectivity of the ARCM reaction was strongly influenced by the structures of the phospholyl-bound alkenyl groups, and the highest enantioselectivity of up to 99% ee was achieved in the reaction of 1d,e, which possess the 2-methylpropenyl substituents at the 2-and 5-positions of the η 5 -phospholides. Single-enantiomeric planarchiral 2d, which was obtained by the recrystallization of the highly enantiomerically enriched ARCM product, can serve as a chiral ligand for the palladium-catalyzed asymmetric allylic alkylation to show good enantioselectivity in up to 74% ee.
Kinetic resolution
of racemic planar-chiral (η5-1-alkenyl-2-bromocyclopentadienyl)manganese(I)
complexes 1 was realized by the molybdenum-catalyzed
asymmetric ring-closing
metathesis. While vinyl-Cp derivative 1a was resolved
in excellent enantioselectivity with the k
rel values of up to 127, the selectivity in the ARCM reaction of allyl-Cp
derivative 1b was modest (k
rel = 3.0). ARCM product 2a, which was obtained in an enantiomerically
pure form by the two successive ARCM reactions or recrystallization
of the enantiomerically enriched products, was found to be a versatile
synthetic precursor to various planar-chiral cymantrene derivatives.
-(w-alkenyl)cyclopentadienyl](w-alkenylphosphine)manganese(I) dicarbonyl complexes were examined.T he RCM reactionb etween an h 5 -allyl-or an h 5 -vinyl-cyclopentadienyll iganda nd an allyldiphenylphosphine ligand in the manganese coordination spheresg ave the corresponding cyclized products in good to excellent yields.T he X-ray analyses of the two cyclized productsr evealed the structural differences between the C 3 -a nd the C 4 -bridged complexes.T he RCM reactions of the o-bromo-substituted manganese complexesw ere also operative andt he cyclized products were obtainedinexcellent yields of > 95%. Thepreferential formation of the C 4 -bridged species over the C 3 -bridged analogues in the RCM of the manganese complexes was confirmed by an intramolecular competitiveR CM experiment with the [h 5 -1-(methallyl)-2-(2-propenyl)cyclopentadienyl](allyldiphenylphosphine)manganese(I) dicarbonyl complex.
Commercial [PdCl 2 (NH 3 ) 2 ]w as appliedt oc atalyze the Mizoroki-Heck coupling of aryl halides with dialkyl allylphosphonates using iPr 2 NH as the base in water under aerobic conditions. The reactionw as conducted at8 08C( for aryl iodides) or 120 8C( for aryl bromides) for 36 h, giving the correspondinga ryl-substituted allylphosphonates in good to excellent yields, and both water-soluble and insoluble aryl halidesc ould be utilized in this reaction. After extracting the reaction solution with ethyl acetate, the residual aqueous solution was reused for the same reaction several times, and the true catalystwas found to be palladium nanoparticles. In addition, ao ne-pot synthesis of 1,4-diarylbuta-1,3-dienes, which combinedt his catalytic reactionw ith the Horner-Wadsworth-Emmons-Wittig reaction, was also performed.
Results and DiscussionAt the outset, iodobenzene (1a)a nd diethyl allylphosphonate (2a)w eres elected as representativer eactants for optimization of the reactionc onditions (Table 1). When [PdCl 2 (NH 3 ) 2 ]a ssociated with aw ater-soluble cationic 2,2'-bipyridyl ligand [43] [a] W.
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