transferred to the phosphine to give the phosphorane Ph 3 P=CHCOOEt. The olefin is
then formed from the reaction of aldehyde and the ylide, accompanied by phosphine oxide. The metal–carbene intermediate may also be attacked by substrates
like alkene, amine, or alcohol directly to generate respective products of cyclopropanation, N–C, or O–C bond formation reactions.
Catalyst 67-BAr
F affords cyclopropanated products with lower catalyst loading
and in shorter reaction time compared to 1. It is important to note that catalyst 67BAr
F has two potential catalytic sites whereas 68-BAr
F offers only one axial site.
However, it is unlikely that catalyst 67-BAr
F utilizes both axial sites during
reaction. The formation of the [Ru–Ru]=CHCO 2 Et adduct reduces the electrophilicity of the second Ru. This in turn diminishes the possibility of the formation
of the bis-carbene adduct. DFT calculations on the model species [67 · CHCO 2 Me]
and [68 · CHCO 2 Me] show that although NPA charges on Ru bonded to CHCO 2 Me
are similar, carbenoid carbons show very different charges (À0.12 and À0.25).
Poor electrophilic nature of the carbenoid carbene in 68 · CHCO 2 Me causes the
reduced activity of catalyst 68-BAr
F . Further, this assertion is supported by the
higher negative charge on the second Ru (À0.16) in 67 · CHCO 2 Me. It is thus
concluded that the additional reaction site on 67-BAr
F does not influence its greater
reactivity, rather higher electrophilicity of the carbenoid carbon has a more pronounced effect.
8 Carbenoid C–H Insertion
Metal-catalyzed decomposition of α-diazocarbonyls followed by intramolecular
carbenoid C–H insertion is an effective means to access important heterocyclic
compounds [36, 221–223]. A variety of β- and γ-lactams have been synthesized
from α-diazoacetamides. Several dirhodium catalysts are used for this transformation [224–228]. Diruthenium catalysts with acetate (1), pyridonate (60), saccharinate (63), and triazenide (69) bridges were employed as potential catalysts for this
reaction. A new class of compounds containing calix [4]arenedicarboxylate moiety
(70–72) were also used (Scheme 42) [67]. The catalytic activity of all these
diruthenium(I,I) complexes are compared with dirhodium(II,II) complexes 37 and
73 (Scheme 43).
Carbenoid C–H insertion is dependent on the nature of C–H bond, and the
reactivity follows the sequence: methine > methylene ) methyl [229]. γ-lactam
being a five-membered ring is the favored product. β-lactams which are generated
by activation of the C–H bond adjacent to the nitrogen atom are also observed
[230]. For substrates having aromatic substituents, carbenoid attack at the aromatic
ring is preferred over γ-lactam formation [231, 232]. The results of diruthenium(I,I)
catalyzed carbenoid C–H insertion of N,N-diethyldiazoacetamide (74),
N,N-dibutyldiazoacetamide (75), and N,N-diisopropyldiazoacetamide (76) are
shown in Scheme 44. Using diruthenium(I,I) catalysts, 74 and 75 gave only γlactam whereas β-lactams are also formed when 76 is used. In fact, β-lactam was the
major product for substrate 76. The dirhodium catalysts 37 and 73 gave higher
Reactivity and Catalysis at Sites Trans to the [Ru–Ru] Bond
85
then formed from the reaction of aldehyde and the ylide, accompanied by phosphine oxide. The metal–carbene intermediate may also be attacked by substrates
like alkene, amine, or alcohol directly to generate respective products of cyclopropanation, N–C, or O–C bond formation reactions.
Catalyst 67-BAr
F affords cyclopropanated products with lower catalyst loading
and in shorter reaction time compared to 1. It is important to note that catalyst 67BAr
F has two potential catalytic sites whereas 68-BAr
F offers only one axial site.
However, it is unlikely that catalyst 67-BAr
F utilizes both axial sites during
reaction. The formation of the [Ru–Ru]=CHCO 2 Et adduct reduces the electrophilicity of the second Ru. This in turn diminishes the possibility of the formation
of the bis-carbene adduct. DFT calculations on the model species [67 · CHCO 2 Me]
and [68 · CHCO 2 Me] show that although NPA charges on Ru bonded to CHCO 2 Me
are similar, carbenoid carbons show very different charges (À0.12 and À0.25).
Poor electrophilic nature of the carbenoid carbene in 68 · CHCO 2 Me causes the
reduced activity of catalyst 68-BAr
F . Further, this assertion is supported by the
higher negative charge on the second Ru (À0.16) in 67 · CHCO 2 Me. It is thus
concluded that the additional reaction site on 67-BAr
F does not influence its greater
reactivity, rather higher electrophilicity of the carbenoid carbon has a more pronounced effect.
8 Carbenoid C–H Insertion
Metal-catalyzed decomposition of α-diazocarbonyls followed by intramolecular
carbenoid C–H insertion is an effective means to access important heterocyclic
compounds [36, 221–223]. A variety of β- and γ-lactams have been synthesized
from α-diazoacetamides. Several dirhodium catalysts are used for this transformation [224–228]. Diruthenium catalysts with acetate (1), pyridonate (60), saccharinate (63), and triazenide (69) bridges were employed as potential catalysts for this
reaction. A new class of compounds containing calix [4]arenedicarboxylate moiety
(70–72) were also used (Scheme 42) [67]. The catalytic activity of all these
diruthenium(I,I) complexes are compared with dirhodium(II,II) complexes 37 and
73 (Scheme 43).
Carbenoid C–H insertion is dependent on the nature of C–H bond, and the
reactivity follows the sequence: methine > methylene ) methyl [229]. γ-lactam
being a five-membered ring is the favored product. β-lactams which are generated
by activation of the C–H bond adjacent to the nitrogen atom are also observed
[230]. For substrates having aromatic substituents, carbenoid attack at the aromatic
ring is preferred over γ-lactam formation [231, 232]. The results of diruthenium(I,I)
catalyzed carbenoid C–H insertion of N,N-diethyldiazoacetamide (74),
N,N-dibutyldiazoacetamide (75), and N,N-diisopropyldiazoacetamide (76) are
shown in Scheme 44. Using diruthenium(I,I) catalysts, 74 and 75 gave only γlactam whereas β-lactams are also formed when 76 is used. In fact, β-lactam was the
major product for substrate 76. The dirhodium catalysts 37 and 73 gave higher
Reactivity and Catalysis at Sites Trans to the [Ru–Ru] Bond
85