No abstract

A useful synthetic procedure for the incorporation of the potentially multidentate monoanionic 1-[C 6 H 2 (CH 2 NMe 2 ) 2 -3,5] -(dNCN) and 1-[C 6 H 3 (CH 2 NMe 2 )-4] -(dCN) ligands via the para-position on the periphery of carbosilane (CS) dendrimers has been developed. Lithiation of suitable brominated precursors [C 6 H 3 (CH 2 NMe 2 ) 2 -3,5-Br-1], 1, and [C 6 H 4 -(CH 2 NMe 2 )-4-Br-1], 2, in Et 2 O at -78°C with 2 equiv of t-BuLi gives the monolithiated aminoaryl compounds Li[C 6 H 3 (CH 2 NMe 2 ) 2 -3,5], 1-Li, and Li[C 6 H 4 (CH 2 NMe 2 )-4], 2-Li, which can be subsequently treated in situ with the silicon chlorides ClSiMe 2 (CH 2 ) n SiMe 2 Cl (n ) 2 or 6) to yield the functionalized CS-ligand systems [CH 2 SiMe 2 C 6 H 3 (CH 2 NMe 2 ) 2 -3,5] 2 , 3, and [(CH 2 ) n SiMe 2 C 6 H 4 (CH 2 NMe 2 )-4] 2 (for 4, n ) 1; for 5, n ) 3). The ligand systems 3-5 react with 2 equiv of t-BuLi to give the dilithiated derivatized carbosilane molecules [CH 2 SiMe 2 C 6 H 2 -(CH 2 NMe 2 ) 2 -3,5-Li-4] 2 and [(CH 2 ) n SiMe 2 C 6 H 4 (CH 2 NMe 2 )-4-Li-3] 2 (7, n ) 1) in good chemical yields. Quench reactions of the in situ prepared lithiated derivatives with either D 2 O, Me 3 SiCl/Me 3 SiOTf, or MeSSMe clearly showed that the CS-ligand systems have been quantitatively converted into their lithiated derivatives. In addition, the functionalized carbosilane dendrimers G0-SiMe 2 -(N)CN and G1-SiMe 2 -(N)CN (13-16), which can be prepared in high yield by using the same synthetic approach used for the model compounds, can also be quantitatively converted into highly aggregated, polylithium derivatives. An X-ray molecular structure determination of 7 revealed this compound to be a unique dimeric aggregate {[CH 2 SiMe 2 C 6 H 3 (CH 2 NMe 2 )-4-Li-3] 2 } 2 with their respective 2-electron-4-center (4c-2e) anionic C ipso centers each bonded to a Li 3 face. As a result the (Me) 2 SiCH 2 CH 2 Si(Me) 2 CS backbone exhibits a nonideal trans conformation with a vicinal dihedral angle of -161°.

The limited availability of catalytic reaction components may represent a major hurdle for the practical application of many catalytic procedures in organic synthesis. In this work, we demonstrate that the mixture of isomeric iron complexes [Fe(OTf)2(mix-BPBP)] (mix-1), composed of Λ-α-[Fe(OTf)2(S,S-BPBP)] (S,S-1), Δ-α-[Fe(OTf)2(R,R-BPBP)] (R,R-1) and Δ/Λ-β-[Fe(OTf)2(R,S-BPBP)] (R,S-1), is a practical catalyst for the preparative oxidation of various aliphatic compounds including model hydrocarbons and optically pure natural products using hydrogen peroxide as an oxidant. Among the species present in mix-1, S,S-1 and R,R-1 are catalytically active, act independently and represent ca. 75% of mix-1. The remaining 25% of mix-1 is represented by mesomeric R,S-1 which nominally plays a spectator role in both C-H and C[double bond, length as m-dash]C bond oxidation reactions. Overall, this mixture of iron complexes displays the same catalytic profile as its enantiopure components that have been previously used separately in sp(3) C-H oxidations. In contrast to them, mix-1 is readily available on a multi-gram scale via two high yielding steps from crude dl/meso-2,2'-bipyrrolidine. Next to its use in C-H oxidation, mix-1 is active in chemospecific epoxidation reactions, which has allowed us to develop a practical catalytic protocol for the synthesis of epoxides.

The reaction of tin dichloride with catalytically-relevant group 10 metal precursors [M(Cl)(X)(2-PyPPh(2))(2)] (M = Ni, Pd, Pt; 2-PyPPh(2) = 2-pyridyldiphenylphosphine; X = Cl, Me) provides easy access to unprecedented cationic dichlorostannylene complexes [M(X)(2-PyPPh(2))(2)(SnCl(2))](+) where the M-Sn bond is bridged by two head-to-head coordinated 2-PyPPh(2) ligands. The formation of such species instead of the classical neutral trichlorostannyl derivatives [M(X)(SnCl(3))(2-PyPPh(2))(2)] offers a new insight on the specific effect of the SnCl(2) cocatalyst in group 10 metal catalyzed transformations.

A series of Grubbs-type catalysts that contain lipase-inhibiting phosphoester functionalities have been synthesized and reacted with the lipase cutinase, which leads to artificial metalloenzymes for olefin metathesis. The resulting hybrids comprise the organometallic fragment that is covalently bound to the active amino acid residue of the enzyme host in an orthogonal orientation. Differences in reactivity as well as accessibility of the active site by the functionalized inhibitor became evident through variation of the anchoring motif and substituents on the N-heterocyclic carbene ligand. Such observations led to the design of a hybrid that is active in the ring-closing metathesis and the cross-metathesis of N,N-diallyl-p-toluenesulfonamide and allylbenzene, respectively, the latter being the first example of its kind in the field of artificial metalloenzymes.