Ditungsten Siloxide Hydrides, [(silox)(2)WH(n)()](2) (n = 1, 2; silox = (t)BuSiO), and Related Complexes

The addition of 4.0 equiv of Na(silox) to Na[W(2)Cl(7)(THF)(5)] afforded (silox)(2)ClW&tbd1;WCl(silox)(2) (1, 65%). Treatment of 1 with 2.0 equiv of MeMgBr in Et(2)O provided (silox)(2)MeW&tbd1;WMe(silox)(2) (2, 81%). In the presence of 1 atm of H(2), reduction of 1 with 2.0 equiv of Na/Hg in DME provided (silox)(2)HW&tbd1;WH(silox)(2) (3, 70%), characterized by a hydride resonance at delta 19.69 (J(WH) = 325 Hz, (1)H NMR). Exposure of 2 to 1 atm of H(2) yielded 3 and CH(4) via (silox)(2)HW&tbd1;WMe(silox)(2) (4); use of D(2) led to [(silox)(2)WD](2) (3-d(2)). Exposure of 3 to ethylene ( approximately 1 atm, 25 degrees C) in hexanes generated (silox)(2)EtW&tbd1;WEt(silox)(2) (5), but solutions of 5 reverted to 3 and free C(2)H(4) upon standing. NMR spectral data are consistent with a sterically locked, gauche, C(2) symmetry for 1-5. Thermolysis of 3 at 100 degrees C (4 h) resulted in partial conversion to (silox)(2)HW&tbd1;W(OSi(t)Bu(2)CMe(2)CH(2))(silox) (6a, approximately 60%) and free H(2), while extended thermolysis with degassing (5 d, 70 degrees C) produced a second cyclometalated rotational isomer, 6b (6a:6b approximately 3:1). When left at 25 degrees C (4 h) in sealed NMR tubes, 6 and free H(2) regenerated 3. Reduction of 1 with 2.0 equiv of Na/Hg in DME also afforded 6a (25%). When 3 was exposed to approximately 3 atm of H(2), equilibrium amounts of [(silox)(2)WH(2)](2) (7) were observed by (1)H NMR spectroscopy (3 + H(2) right harpoon over left harpoon 7; 25.9-88.7 degrees C, DeltaH = -9.6(4) kcal/mol, DeltaS = -21(2) eu). Benzene solutions of 3 and 1-3 atm of D(2) revealed incorporation of deuterium into the silox ligands, presumably via intermediate 6. In sealed tubes containing [(silox)(2)WCl](2) (1) and dihydrogen (1-3 atm), (1)H NMR spectral evidence for [(silox)(2)WCl](2)(&mgr;-H)(2) (8) was obtained, suggesting that formation of 3 from 1 proceeded via reduction of 8. Alternatively, 3 may be formed from direct reduction of 1 to give [(silox)(2)W](2) (9), followed by H(2) addition. Hydride chemical shifts for 7 are temperature dependent, varying from delta 1.39 (-70 degrees C, toluene-d(8)), to delta 3.68 (90 degrees C). (29)Si{(1)H} NMR spectra revealed a similar temperature dependence of the silox (delta 12.43, -60 degrees C, to delta 13.64, 45 degrees C) resonances. These effects may arise from thermal population of a low-lying, deltadelta, paramagnetic excited state of D(2)(d)() [(silox)(2)W](2)(&mgr;-H)(4) (DeltaE approximately 2.1 kcal/mol, chi(7a) approximately 0.03), an explanation favored over thermal equilibration with an energetically similar but structurally distinct isomer (e.g., [(silox)(2)WH(2)](2)(&mgr;-H)(2), DeltaG degrees approximately 0.69 kcal/mol, chi(7b) approximately 0.25) on the basis of spectral arguments. Extended Hückel and ab initio molecular orbital calculations on model complexes [(H(3)SiO)(2)W](2)(&mgr;-H)(4) (staggered bridged 7a', EHMO), [(H(3)SiO)(2)WH(2)](2) (all-terminal 7b', EHMO), [(H(3)SiO)(2)W](2) (9', EHMO), (HO)(4)W(2)(H(4)) (staggered-bridged 7", ab initio), and (HO)(4)W(2)(H(4)) (bent-terminal 7, ab initio) generally support the explanation of a thermally accessible excited state and assign 7 a geometry intermediate between the all-terminal and staggered-bridged forms.     

A convenient one-pot synthesis of 1,8-naphthyridones

In this paper, we disclose an efficient one-pot procedure for the preparation of substituted 1,8-naphthyridin-4-one analogues. Previous efforts to effect this type of transformation were complicated by the formation of benzene tricarboxylate. Via the use of excess base, the impurity formation was completely inhibited. This allowed for the clean preparation of the desired intermediate and subsequent formation of naphthyridone analogues in a single flask, which could then be crystallized directly from the reaction mixture in good yield and high purity.     

Temperature-sensitive luminescence of tris (β-diketone) europium chelates for monitoring high-speed cationic photopolymerizations

The temperature-dependent luminescence of tris (β-diketone) chelates of europium was used for in situ temperature measurements during cationic photopolymerizations of vinyl ethers. These molecular-level luminescent probes provided a real-time, noninvasive method for monitoring temperature during these high-speed polymerizations. Two specific probes, tris (benzoyl-1, 1, 1-trifluoroacetone) europium and tris (1,1,1,5,5,5,-hexafluoroacetylacetone) europium, met several stringent spectral and performance requirements for application in our system. The luminescence from these probes exhibits a reproducible temperature dependence over a wide temperature range and is not sensitive to changes in viscosity. In situ temperature profiles obtained using this novel technique verified the importance of thermal effects during these highly exothermic photopolymerizations. These studies have demonstrated the utility of the tris(β-diketone) europium chelates for characterizing the temperature during high-speed photopolymerizations that cannot be monitored using conventional techniques. © 1995 John Wiley & Sons, Inc.     

Development of a custom high-throughput preparative liquid chromatography/mass spectrometer platform for the preparative purification and analytical analysis of compound libraries

Solution-phase parallel synthesis has had a profound impact on the speed of compound synthesis delivering relatively pure compounds (>80%) in short order. However, to develop structure activity relationships (SAR) for a compound series, each library member should preferably be >95% pure. Historically, achieving and quantifying such high-purity criteria for each library member proved to be the slow step for most lead discovery groups. To address this issue, significant modifications have been made to a commercial Agilent preparative LC/MS system to allow for the general mass-guided purification of diverse compound libraries. The custom modifications include (1) the "DMSO slug" approach for the purification of samples with poor solubility; (2) an active splitter to reduce system back-pressure, reduce the delay volume, and allow for a variable split ratio; (3) a sample loading pump for the quick purification of large, dilute samples; (4) a preparative column-selection valve to quickly change column selectivity or sample loading; and (5) an analytical injector with a separate flow path for crude reaction or fraction analyses.     

A copper-catalyzed C-N bond formation involving sp-hybridized carbons. A direct entry to chiral ynamides via N-alkynylation of amides

A copper-catalyzed new C-N bond formation involving a sp-hybridized carbon is described here leading to a facile entry for syntheses of chiral ynamides. This direct N-alkynylation of amides should have a significant impact on the future development of synthetic methodologies employing ynamides.     

Synthesis of sodium tantalate nanorods by alkalide reduction

NaTaO3 nanorods were synthesized with high (>90%) yield by reduction of TaCl5 with THF solutions of the alkalide K+(15C5)2Na-, followed by product annealing under dynamic vacuum at 250 and 600 degrees C. In addition to the nanorods, the product is comprised of 5-10% approximately 10-nm diameter spheroidal NaTaO3 nanocrystals. The nanorods are generally longer than 500 nm, with some exceeding 1 mum, and 10-100 nm wide, with aspect ratios that vary between 10 and 20:1. Select area electron diffraction patterns of individual nanorods indicate that each nanorod is a single crystal with its axis oriented in the [010] direction.