Thermal stability of the silica-aminopropylsilane-polyimide interface

Thermal degradation of the silica–aminopropylsilane–amic acid/imide interface was studied by modifying a high-surface-area, neutral silica gel with a number of substituted aminopropylsilanes (APS). These substrates were reacted further with phthalic anhydride or aromatic amic acid monomers and the thermal decomposition of the adsorbed/reacted materials was monitored by thermogravimetric analysis (TGA) and infrared (IR) spectroscopy. The 3-aminopropyltriethoxysilane/poly[N,N′-(p,p′-oxydiphenylene)pyromellitimide] interface was also evaluated by this method. Comparison clearly distinguishes the thermal decomposition of surface-bound APS from surface-bound alkylphthalimides, the adhesion product of alkylamines and aromatic amic acids. Alkylamine imidization with the elimination of aromatic amine (analogous to polymer scission) and the decomposition of the surface-bound imide are shown in the amic acid TGA profiles. This imidization and the accompanying aniline elimination begin at about 130°C, under nitrogen, to form the surface alkyl imide which slowly decomposes at 400°C. TGA analysis indicates that the surface-bound imide undergoes minimal degradation under nitrogen at 370 ± 10°C; temperatures above this threshold range produce changes in the APS–imide interface.     

Additions of enantiopure alpha-sulfinyl carbanions to (S)-N-sulfinimines: asymmetric synthesis of beta-amino sulfoxides and beta-alphamino alcohols

The addition of the lithium anions derived from (R)- and (S)-methyl and -ethyl p-tolyl sulfoxides to (S)-N-benzylidene-p-toluenesulfinamide provides an easy access route to enantiomerically pure beta-(N-sulfinyl)amino sulfoxides. Stereoselectivity can be achieved when the configurations at the sulfur atoms of the two reagents are opposite (matched pair), thus resulting in only one diastereoisomer, even for the case in which two new chiral centers are created. The N-sulfinyl group primarily controls the configuration of the carbon bonded to the nitrogen, whereas the configuration of the alpha-sulfinyl carbanion seems to be responsible for the level of asymmetric induction, as well as for the configuration of the new stereogenic C-SO carbon in the reactions with ethyl p-tolyl sulfoxides. An efficient method for transforming the obtained beta-(N-sulfinyl)amino sulfoxides into optically pure beta-amino alcohols, based on the stereoselective non-oxidative Pummerer reaction, is also reported.     

A Total Synthesis of FK-506(1)

A total synthesis of FK-506 (1) is presented. The synthesis features a highly convergent approach utilizing a block coupling strategy. Top and bottom half sections of the molecule are coupled by addition of a vinyl cuprate with a spiroenone. The alpha-allyl aldol functionality is revealed by a reductive opening of the spiroenone system. The labile alpha,beta-diketoamide hemiketal portion of the molecule is prepared by a late stage generation and oxidation of a masked enediol. Top and bottom half segments are themselves derived by coupling of smaller subunits, resulting in a very convergent route.     

Nuclear analogs of β-lactam antibiotics. 7. The synthesis of 3-thia- and 3-aza-1-dethiaceph-1-em esters

The total synthesis of a 3-thia- and a 3-aza-1-dethiacepham ($\text{9}$ and $\text{16}$) are described. Neither compound possessed antibacterial activity $\text{vs}$$\text{B}$. $\text{subtilis}$. An unsaturated analog, a 3-aza-1-dethiaceph-1-em exhibited weak antibacterial activity.     

Comparison of the electrophilic and free-radical addition of halogens with hexafluoro-1,3-butadiene and 1,3-butadiene

Ionic and photochemical reaction of chlorine (Cl₂), bromine (Br₂) and iodine monochloride (ICl) to hexafluoro-1,3-butadiene (1) and 1,3-butadiene (2) were carried out under conditions that would provide product distributions under controlled ionic or free-radical conditions. Product distributions for ionic reaction of Cl₂ and Br₂ with 1 are similar and suggest a weakly-bridged halonium ion species. Theoretical calculations support weakly-bridged chloronium and bromonium ions for both dienes 1 and 2. There are more of the 1,4-dihalo-2-butene products from ionic halogenation of 1 than 2 which correlates with the greater charge density on carbon-4 of halonium ions from 1. Ionic and free-radical reactions of ICl with 1 give 8 and 2% of 3-chloro-4-iodohexafluoro-1-butene and 4-chloro-3-iodohexafluoro-1-butene, respectively. The minor cis-1,4-dihalo-2-butene products from 1 and 2 are reported when formed.     

(Z)-3-p-tolylsulfinylacrylonitrile as a chiral dienophile: diels-alder reactions with furan and acyclic dienes

The behavior of (Z)-3-p-tolylsulfinylacrylonitrile (1) as a chiral dienophile has been evaluated from its reactions with furan and acyclic dienes. Electrostatic interactions of the cyano group with the sulfinyl one restrict the conformational mobility around the C-S bond, thus controlling the pi-facial selectivity, which is almost complete in all cases, the approach of the diene from the less-hindered face of the dienophile (that bearing the lone electron pair) in the predominant rotamer being the favored one. The regioselectivity is also completely controlled by the cyano group. Additionally, the reactivity of compound 1 as well as its endo-selectivity are both higher than those observed for the corresponding (Z)-3-sulfinylacrylates, thus proving the potential of sulfinylnitriles as chiral dienophiles.     

Density functional theory calculation of (29)Si NMR chemical shifts of organosiloxanes

Density functional theory at the B3LYP/6-311++G(d,p) level is applied to calculate the (29)Si NMR chemical shifts of a variety of organosiloxane moieties including monomers or precursors for polymerization and representative segments of organosiloxane polymers or thin films. The calculated shifts of two linear dimethylsiloxane compounds, hexamethylcyclotrisiloxane (D3) and octamethylcyclotetrasiloxane (D4), compare well with their known values, having an average error of 3.4 ppm. The same method is applied to structures believed to occur in organosilicate glass thin films deposited using hot-filament chemical vapor deposition (HFCVD) from D3 and D4. The chemical shift at -15 ppm is identified as a cross-linking Si-Si bond between two strained D groups and has not previously been reported. Retention of the strained ringed structure in HFCVD films deposited from D3 is confirmed. The rings are bonded to the matrix through either Si-O or Si-Si bonds, with the latter only becoming prevalent when higher filament temperatures are employed. The strained ring structure is also observed in films deposited from a precursor with a larger unstrained ring structure, D4. These observations suggest that the known gas-phase conversion pathways of D4 to D3 and dimethylsilanone as well as the methyl abstraction reaction from D3 operate in the HFCVD reaction chemistry.