Unexpected Migratory Insertion Reactions of M(alkyl)2 (M=Zn,Cd) and Diamidocarbenes

The electrophilic character of free diamidocarbenes (DACs) allows them to activate inert bonds in small molecules, such as NH₃ and P₄. Herein, we report that metal coordinated DACs also exhibit electrophilic reactivity, undergoing attack by Zn and Cd dialkyl precursors to afford the migratory insertion products [(6‐MesDAC‐R)MR] (M=Zn, Cd; R=Et, Me; Mes=mesityl). These species were formed via the spectroscopically characterised intermediates [(6‐MesDAC)MR₂], exhibiting barriers to migratory insertion which increase in the order MR₂ = ZnEt₂ < ZnMe₂ < CdMe₂. Compound [(6‐MesDAC‐Me)CdMe] showed limited stability, undergoing deposition of Cd metal, by an apparent β‐H elimination pathway. These results raise doubts about the suitability of diamidocarbenes as ligands in catalytic reactions involving metal species bearing nucleophilic ligands (M‐R, M‐H).     

Formation of the chromophore of the pyoverdine siderophores by an oxidative cascade

The pyoverdine chromophore is formed in a reaction involving a two-electron oxidation, a conjugate addition, and a second two-electron oxidation. This oxidative cascade can be carried out with polyphenol oxidase (PPO), MnO(2), and cell-free extracts from Pseudomonas aeruginosa. [reaction: see text]     

Isolation and characterisation of transition and main group metal complexes supported by hydrogen-bonded zwitterionic polyphenolic ligands

Reaction of Zr(O(i)Pr)4 or Sn[N(SiMe3)2]2 with the tris-phenol amine ligand H3L(Me/Me) results in the formation of zirconium or tin complexes containing the new C3-symmetric zwitterionic ammonium-trisphenolate ligand HL2-, while increasing the steric bulk of the ligand results in the isolation of a zirconium complex containing the known trianionic ligand L3-.     

Silver-phosphine complexes of the highly methylated carborane monoanion [closo-1-H-CB11Me11]-

The synthesis of the silver(I) salt of the highly methylated carborane anion [closo-1-H-CB(11)Me(11)](-) is described, Ag[closo-1-H-CB(11)Me(11)] 1, which in the solid state shows close intermolecular Ag...H(3)C contacts. Addition of various monodentate phosphines to 1 results in the formation of the complexes (R(3)P)Ag[closo-1-H-CB(11)Me(11)] [R = Ph, 2; cyclohexyl (C(6)H(11)), 3; (3,5-Me(2)-C(6)H(3)), 4]. All these complexes show close intermolecular Ag.H(3)C contacts in the solid state that are considerably shorter than the sum of the van der Waals radius of methyl (2.00 A) and the ionic radius of silver(I) (1.29 A). For 2 and 3 there are other close intermolecular Ag...H(3)C contacts in the solid state, arising from proximate carborane anions in the crystal lattice. Addition of methyl groups to the periphery of the phosphine ligand (complex 4) switches off the majority of these interactions, leaving essentially a single cage interacting with the cationic silver-phosphine fragment through three CH(3) groups. In solution (CD(2)Cl(2)) Ag...H(3)C contacts remain, as evidenced by both the downfield chemical shift change and the significant line-broadening observed for the cage methyl signals. These studies also show that the metal fragment is fluxional over the surface of the cage. The Ag...H(3)C interactions in solution may be switched off by addition of a stronger Lewis base than [closo-1-H-CB(11)Me(11)](-). Thus, addition of [NBu(4)][closo-1-H-CB(11)H(5)Br(6)] to 2 affords (Ph(3)P)Ag[closo-1-H-CB(11)H(5)Br(6)], while adding Et(2)O or PPh(3) affords the well-separated ion-pairs [(Ph(3)P)(L)Ag][closo-1-H-CB(11)Me(11)] (L = OEt(2) 5, PPh(3) 6,) both of which have been crystallographically characterized. DFT calculations on 2 (at the B3LYP/DZVP level) show small energy differences between the possible coordination isomers of this compound, with the favored geometry being one in which the [(Ph(3)P)Ag](+) fragment interacts with three of the [BCH(3)] vertices on the lower surface of the cage, similar to the experimentally observed structure of 4.     

The biosynthesis of the thiazole phosphate moiety of thiamin (vitamin B1): the early steps catalyzed by thiazole synthase

Thiazole synthase (ThiG) catalyzes an Amadori-type rearrangement of 1-deoxy-d-xylulose-5-phosphate (DXP) via an imine intermediate. In support of this, we have demonstrated enzyme-catalyzed exchange of the C2 carbonyl of DXP. Borohydride reduction of the enzyme DXP imine followed by top-down mass spectrometric analysis localized the imine to lysine 96. On the basis of these observations, a new mechanism for the biosynthesis of the thiazole phosphate moiety of thiamin pyrophosphate in Bacillus subtilis is proposed. This mechanism involves the generation of a ketone at C3 of DXP by an Amadori-type rearrangement of the imine followed by nucleophillic addition of the sulfur carrier protein (ThiS-thiocarboxylate) to this carbonyl group.     

Reconstitution and biochemical characterization of a new pyridoxal-5'-phosphate biosynthetic pathway

The substrates for Bacillus subtilis PLP synthase (YaaD and YaaE) are identified, and the first reconstitution of PLP biosynthesis using this pathway is described. Three partial reactions catalyzed by YaaD are also identified.     

A Proposed Improved Replacement Policy for Army Vehicles

The British Army currently uses a replacement policy for non-armoured vehicles which employs age-based repair limits derived by a sensible but rather arbitrary method. The paper describes how a proposed improved replacement policy was devised: one which uses extensive data to compute optimum age-based repair limits and allows these limits to be adjusted for vehicles having exceptionally high and low mileages. The feasibility of a resale type of policy for some vehicles is examined.After a general introduction, the model is described in detail. There follows an account of how the assumptions of the model were tested. Then the results are presented and discussed, in particular the savings to be expected from implementing the improved policy. Finally, the proposals are described which have now been approved by Army staff and are to be submitted to the Treasury.     

The bis(η-cyclopentadienyl)titanium(IV) – salicylate system revisited and the characterisation of two 3,5-di-t-butylsalicylate analogues. The molecular structure of [Cp2Ti(sal)], (sal = O2CC6H4O2−)

The Cp₂Ti^IV – salicylate system has been re-examined and procedures for the syntheses of [Cp₂Ti(salH)₂] and [Cp₂Ti(sal)] explored, [salH = (O₂CC₆H₄OH-2)^–; sal = (O₂CC₆H₄O-2)^2–]. The new 3,5-di-t-butylsalicylate analogues have been prepared and characterised. An X-ray structure determination of [Cp₂Ti(sal)] confirms the chelating nature of the sal ligand which is bonded to the metal through the phenolic and one of the carboxylate oxygen atoms.     

Selective Deprotection Of tert-Butyldimethylsilyl Ether With Lithium Bromide And 18-Crown-6

Lithium bromide, in the presence of a crown ether, has been found to selectively remove primary tert-butyldimethylsilyl ethers under controlled conditions. This selectivity has been utilized to synthesize 3′-TBDMS-2′-deoxyuridine in one step from the diprotected compound.