Neopentyl Substituents Research Articles

Turn-on mode fluorescent diarylethene containing neopentyl substituents that undergoes all-visible-light switching.

This paper presents a strategy for improving the all-visible-light switching response of turn-on mode fluorescent diarylethene derivatives. Introduction of neopentyl or isobutyl substituents at the reactive carbons (2- and 2'-positions) of an oxidized bis(benzothienyl)perfluorocyclopentene derivative, which undergoes both cyclization and cycloreversion reactions upon irradiation with visible light, was effective in increasing the cycloreversion quantum yield by one or two orders of magnitude in comparison with the yield of an ethyl-substituted derivative. Any significant influence on the cyclization and fluorescence quantum yields was not observed by the introduction of neopentyl or isobutyl substituents.

Revisiting the Ullman's Radical Chemistry for Phthalocyanine Derivatives.

Phthalocyanine derivatives do not cease to gain attention due to their numerous properties and applications (e.g., sensor, PDT). This makes them a unique scaffold for the design of new material. In this context, we were interested to develop the synthesis of an imino nitroxide-substituted phthalocyanine by Ullman's procedure; a challenge due to the intrinsic low solubility of most phthalocyanine derivative in much solvents. To overcome this solubility problem, we designed a phthalocyanine with bulky neopentyl substituents in peripheral positions as counterpart to the imino nitroxide moieties. The imino nitroxide-substituted phthalocyanine was obtained by condensation of a monoformyl-substituted phthalocyanine with 2,3-bis(hydroxylamino)-2,3-dimethylbutane in refluxing THF-MeOH (2:1) mixture in the presence of p-toluenesulfonic acid monohydrate, follow by oxidation with PbO2 . Characterization was performed by electrochemistry, UV/Vis and EPR spectroscopy in solution as well as SQUID in solid state.

Effect of bulky substituents on the photochromic properties of indoline spiropyrans containing an annelated aromatic or heteroaromatic fragment

Novel indoline spiropyrans containing bulky neopentyl substituents at the nitrogen atom of the indoline fragment were prepared. The structure of the obtained compounds was established by a set of physicochemical methods of analysis. Comparison was made for the spectral absorption and kinetic parameters of the obtained compounds and their analogs containing methyl and benzyl substituents instead of neopentyl ones.

Synthesis of methyl C132-alkylated pheophorbides a/a′ and their stereochemical characterization

C132-Alkylations of a 9:1 epimeric mixture of methyl pheophorbide a/a′, derived from naturally occurring chlorophyll a, were examined by using primary, secondary, and tertiary alkyl iodides. The sterically undemanding C132-alkylation of the methyl pheophorbide epimers successfully afforded their major (132R)- and minor (132S)-adducts, while the isopropylation primarily gave the (132S)-product and no desired adducts were obtained for tert-butyl and neopentyl substituents. The C132-alkylated epimers were separated by reverse phase-HPLC techniques and their stereochemistry was characterized by 1D/2D NMR spectroscopy.

Isolation and characterization of a new bacteriochlorophyll-c bearing a neopentyl substituent at the 8-position from the bciD-deletion mutant of the brown-colored green sulfur bacterium Chlorobaculum limnaeum

We recently constructed the mutant of the brown-colored green sulfur bacterium Chlorobaculum limnaeum lacking BciD which was responsible for formation of a formyl group at the 7-position in bacteriochlorophyll(BChl)-e biosynthesis. This mutant exclusively gave BChl-c, but not BChl-e, as the chlorosome pigments (Harada et al. in PLoS One 8(4):e60026, 2013). By the mutation, the homolog and epimer composition of the pigment was drastically altered. The methylation at the 8(2)-position in the mutant cells proceeded to create BChl-c carrying large alkyl substituents at this position. Correspondingly, the content of BChls-c having the (S)-configuration at the chiral 3(1)-position remarkably increased and accounted for 80.6% of the total BChl-c. Based on the alteration of the pigment composition in the mutant cells, a new BChl-c bearing the bulkiest, triple 8(2)-methylated neopentyl substituent at the 8-position ([N,E]BChl-c) was identified. The molecular structure of [N,E]BChl-c was fully determined by its NMR, mass, and circular dichroism spectra. The newly identified [N,E]BChl-c was epimerically pure at the chiral 3(1)-position and its stereochemistry was determined to be an (S)-configuration by modified Mosher's method. Further, the effects of the C8(2)-methylation on the optical absorption properties of monomeric BChls-c were investigated. The Soret but not Qy absorption bands shifted to longer wavelengths by the extra methylation (at most 1.4nm). The C8(2)-methylation induced a slight but apparent effect on absorption properties of BChls-c in their monomeric states.

Discovery of simplified N2-substituted pyrazolo[3,4-d]pyrimidine derivatives as novel adenosine receptor antagonists: Efficient synthetic approaches, biological evaluations and molecular docking studies

In the present study, a molecular simplification approach was employed to design novel bicyclic pyrazolo[3,4-d]pyrimidine (PP) derivatives from tricyclic pyrazolo[4,3-e]-1,2,4-triazolo-[1,5-c]pyrimidines (PTP) as promising human A3 adenosine receptor (hA3AR) antagonists. All the target compounds were synthesized using novel and efficient synthetic schemes and the structure–activity relationship studies of these PPs were explored through the synthesis of a series of PTP analogues with various substituents. Substituents with different lipophilicity and steric hindrance (e.g., alkyl and aryl–alkyl) functions were introduced at N2 position of the pyrazole ring, while acyl groups with different electronic properties were introduced at C6 position of the bicyclic nucleus to probe both electronic and positional effects. Most of the synthesized derivatives of the PP series presented good affinity at the hA3AR, as indicated by the low micromolar range of Ki values and among them, compound 63 with N2 neopentyl substituents showed most potent hA3AR affinity with Ki value of 0.9μM and high selectivity (hA1AR/hA3AR=>111 & hA2AAR/hA3AR=>111) towards other adenosine receptor subtypes. Interestingly, small isopropyl groups at N2 position displayed high affinity at another receptor subtype (hA2AAR, e.g., compound 55, with Ki hA2AAR=0.8μM), while they were less favorable at the hA3AR. Molecular docking analysis was also performed to predict the possible binding mode of target compounds inside the hA3AR and hA2AAR. Overall, PP derivatives represent promising starting points for new AR antagonists.

Group 10 Metal Complexes of a Ferrocene‐Based N‐Heterocyclic Carbene: Syntheses, Structures and Catalytic Applications

AbstractThis paper focuses on complexes of the N‐heterocyclic carbene (NHC) 1‐Np, which contains a 1,1′‐ferrocenediyl backbone and neopentyl substituents at the N atoms flanking the divalent C atom. 1‐Np is a ring‐expanded NHC with a ring size of six atoms. The square‐planar group 10 metal complexes [{PdCl(μ‐Cl)(1‐Np)}2], cis‐[PdCl2(1‐Np)(PPh3)], trans‐[PdCl2(1‐Np)2] and trans‐[NiCl2(1‐Np)(PPh3)] have been prepared by the reaction of 1‐Np with trans‐[PdCl2(PhCN)2], [PdCl2(PPh3)2], cis‐[PdCl2(COD)] (COD = cycloocta‐1,5‐diene) and [NiCl2(PPh3)2], respectively. In addition, the carbene–borane adduct 1‐Np–BF3 has been obtained by serendipity. All new compounds have been structurally characterised by single‐crystal X‐ray diffraction, which demonstrates the pronounced trans influence of 1‐Np. 1‐Np has been compared with the ring‐expanded NHC 1,3‐diisopropyltetrahydropyrimid‐2‐ylidene (THP‐iPr) in the context of homogeneous catalysis, which utilises analogous palladium NHC complexes known to be particularly effective in the case of NHC = THP‐iPr. The catalytic performance of cis‐[PdCl2(1‐Np)(PPh3)] in Suzuki–Miyaura cross‐coupling reactions of PhB(OH)2 with aryl bromides Br–p‐C6H4–R is similar to that reported for cis‐[PdCl2(THP‐iPr)(PPh3)]. With aryl chlorides, however, the latter complex is superior to its 1‐Np analogue. The catalytic performance reported for trans‐[PdCl2(THP‐iPr)2] in Mizoroki–Heck reactions of n‐butyl acrylate with aryl bromides Br–p‐C6H4–R is equal to (R = H) or moderately better than that of trans‐[PdCl2(1‐Np)2] [R = C(O)Me, OMe].

Neopentyl-substituted PNP-pincer Ligand: Complexation with Iridium to Form an Iridacycle via Alkyl C–H Activation

Abstract A new PNP-pincer ligand 1 bearing neopentyl substituents reacted with [Ir(cod)2]BF4 to form hydridoiridacycle 3 via oxidative addition of a C–H bond of a neopentyl group to IrI. Addition of π-acceptor ligands induced C–H reductive elimination to form cationic iridium(I) complexes 5 ligated by the π-acceptor ligand. Although 3 did not give a benzene adduct, reaction of 3 with nitrobenzene or acetophenone afforded the C–H activated 7a or 8a.

Bulky Alkylphosphines with Neopentyl Substituents as Ligands in the Amination of Aryl Bromides and Chlorides.

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Bulky Alkylphosphines with Neopentyl Substituents as Ligands in the Amination of Aryl Bromides and Chlorides

Di(tert-butyl)neopentylphosphine (DTBNpP) in combination with palladium sources provided catalysts with comparable or better activity for the Hartwig-Buchwald amination of aryl bromides than tri(tert-butyl)phosphine (TTBP) under mild conditions. DTBNpP also provided effective catalysts for amination reactions of aryl chlorides at elevated temperatures. Further replacement of tert-butyl groups with neopentyl substituents resulted in less effective ligands for amination reactions. Computationally derived cone angles showed that replacement of a tert-butyl group with a neopentyl group significantly increased the cone angle of the phosphine. The larger cone angle of DTBNpP than TTBP appears to correlate with the higher activity of catalysts derived from DTBNpP in the amination of aryl bromides. TTBP is a stronger electron donor than DTBNpP, which may explain the higher activity for TTBP-derived catalysts toward aryl chlorides.

Atmospheric pressure chemical ionisation liquid chromatography/multistage mass spectrometry for assignment of sedimentary bacteriochlorophyll derivatives

Atmospheric pressure chemical ionisation liquid chromatography/multistage mass spectrometry (APCI-LC/MSn) provides a rapid, on-line method for the assignment of individual bacteriophaeophorbide c and d methyl esters (BPMEs) in complex mixtures. The MS2 spectrum for each component is diagnostic of the type of BPME (c or d), and characteristic losses in MS5 and MS6 permit assignment of the alkyl substituents at positions C-8 and C-12 of the macrocycle. MS5 mass chromatograms permit the deconvolution of coeluting isobaric BPMEs, revealing the true profiles of the individual components. The distributions are different in lake sediments from la Salada de Chiprana (Spain) and Kirisjes Pond (Antarctica), and a novel BPME c with a neo-pentyl substituent has been observed in the Kirisjes Pond sediment.

Chromatographic enantiomer separation and circular dichroism spectra of chiral rhodanines

Rhodanines [2-thioxo-2,3-dihydro-1,3-thiazol-4(5H)-ones] with isopropyl, sec-butyl and neopentyl substituents in position 3 and methyl and phenyl substituents in position 5 have been resolved into enantiomers by chromatography on microcrystalline, swollen triacetylcellulose. While the 5-methyl compounds showed good optical stability, the 5-phenyl analogues underwent racemization in ethanol solution with half-lives of 1–2 h at ambient temperature and were therefore resolved at –3 °C.The UV spectra resemble those of N-acetyldithiocarbamates but with hypsochromic shifts because of the inclusion of the chromophore in a five-membered ring. Transition energies and absorption intensities were reasonably well reproduced by CNDO/S calculations.The CD spectra showed bands corresponding to the UV bands but also another band at ca. 260 nm, which may be ascribed to an n→π* transition located in the carbonyl group. The absolute configurations of the 5-methyl compounds were assigned on the basis of the signs of the n→π* bands, using Ripperger's quadrant rule for dithiocarbamates. The mechanisms for generation of rotational strengths in the 5-phenyl compounds are more complex and no safe assignments of absolute configurations could be made.The CD spectrum of 3-neopentyl-5-methylrhodanine in ethanol was found to be insensitive to the temperature from ambient to –97 °C, indicating a 1:1 equilibrium between the anti and syn forms, whereas the 5-phenyl analogue showed a considerable temperature dependence, indicating a more biased equilibrium.

Steric factors in the cyclization of 4-azaheptane-2,6-diones to dehydropiperid-3-ones and the stereochemistry of reduction of 2,2-dimethyl- and 5-neopentylpiperid-3-ones

1. The neopentyl substituent in the azadiketones of the type (Ib) completely blocks the cyclization to the dehydropiperidine systems with the participation of the acetonyl carbonyl as the electrophile. 2. In such azadiketones, the geminal substitution at the carbon atom adjoining the nitrogen is less effective in its influence on the regioselectivity of the cyclization by comparison with the influence of substitution at the nitrogen. 3. In the saturated 3-ketopiperidines, the axial substituent at C2 largely blocks the e-coordinate of the approach of the hydride nucleophiles to the carbonyl or the e-adsorption In catalytic hydrogenation. The steric influence of the neopentyl substituent at C2 on the carbonyl depends on the specificity of the particular reagent and can surpass the influence of the tert-butyl group.

Bacteriochlorophylls-d from Chlorobium vibrioforme: chromatographic separations and structural assignments of the methyl bacteriopheophorbides

Bacteriochlorophylls-d (2) have been isolated from Chlorobium vibrioforme forma thiosulfatophilum, a green sulphur bacterium. Treatment of the bacteriochlorophylls with sulphuric acid in methanol affords the methyl bacteriopheophorbides (1) which are separated by high performance liquid chromatography (h.p.l.c.) into six major homologous fractions differing in the nature of 4-(ethyl, n-propyl, isobutyl) and 5-(methyl, ethyl) substituents. Using a combination of h.p.l.c., n.m.r., and X-ray crystallography, the 4-ethyl and 4-n-propyl homologues (1a), (1b), (lc), and (1f) are shown to have the (R) absolute stereochemistry at the 2-(1-hydroxyethyl) substituent while the 4-isobutyl compounds (lc) and (1g) are shown to have the opposite (S) chirality at this position. Two new methyl bacteriopheophorbides-d bearing a 4-neopentyl substituent (1d) and (1h) are isolated and characterized; these appear to be the first natural products with a neopentyl substituent to be isolated and are also assigned the (S) absolute stereochemistry at the 2-position on the basis of n.m.r., h.p.l.c., and X-ray structural evidence. The methyl bacteriopheophorbides-d from a new strain of green sulphur bacteria [Chlorobium vibrioforme(B1-20)] have also been studied; initially, these bacteria, which possess an abnormally short wavelength absorption in living cells, appeared to be producing major amounts of only two methyl bacteriopheophorbides-d (la) and (1e), but after they had grown continually for more than a year, the pigment composition began to contain larger quantities of more highly methylated homologues. The bacteriopheophorbides with R4= Et and R4= Bui again had the (R) and (S) absolute stereochemistries, respectively, at the 2-position, but the chirality at the 2-(1-hydroxyethyl) group in the homologues with R4= Prn is shown to be a mixture of both (R) and (S), this situation resembling that in the bacteriopheophorbides-c.

Isolation and characterization of two new bacteriochlorophylls d bearing neopentyl substituents

The isolation and characterization of two new pigments of Chlorobium vibrioforme forma thiosulfatophilum (NCIB No. 8327) are described. The new bacteriochlorophylls possess a neopentyl substituent at position 4, and the S stereochemistry at the 2-(1-hydroxyethyl) group.

ChemInform Abstract: ELECTROPHILIC AROMATIC SUBSTITUTION. PART 30. THE EFFECTS OF THE P‐BICYCLO(2.2.2)OCTAN‐1‐YL, ADAMANTAN‐1‐YL, EXO‐ AND ENDO‐NORBORNAN‐2‐YL, AND NEOPENTYL SUBSTITUENTS IN DETRITIATION. STERIC ACCELERATION OF HYPERCONJUGATION

Chemischer InformationsdienstVolume 13, Issue 22 Preparative Organic Chemistry ChemInform Abstract: ELECTROPHILIC AROMATIC SUBSTITUTION. PART 30. THE EFFECTS OF THE P-BICYCLO(2.2.2)OCTAN-1-YL, ADAMANTAN-1-YL, EXO- AND ENDO-NORBORNAN-2-YL, AND NEOPENTYL SUBSTITUENTS IN DETRITIATION. STERIC ACCELERATION OF HYPERCONJUGATION W. J. ARCHER, W. J. ARCHERSearch for more papers by this authorM. A. HOSSAINI, M. A. HOSSAINISearch for more papers by this authorR. TAYLOR, R. TAYLORSearch for more papers by this author W. J. ARCHER, W. J. ARCHERSearch for more papers by this authorM. A. HOSSAINI, M. A. HOSSAINISearch for more papers by this authorR. TAYLOR, R. TAYLORSearch for more papers by this author First published: June 1, 1982 https://doi.org/10.1002/chin.198222154Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume13, Issue22June 1, 1982 RelatedInformation

Electrophilic aromatic substitution. Part 30. The effects of the p-bicyclo[2.2.2]octan-1-yl, adamantan-1-yl, exo- and endo-norbornan-2-yl, and neopentyl substituents in detritiation. Steric acceleration of hyperconjugation

Rate coefficients have been measured for detritiation of the compounds [p-3H]C6H4R by anhydrous trifluroacetic acid at 70 °C, and lead to the following partial rate factors (R =): bicyclo[2.2.2]octan-1-yl, 2 650; adamantan-1-yl, 2 000; exo-norbornan-2-yl, 1 750; endo-norbornan-2-yl, 1 340; and neopentyl, 472. The corresponding σ+ values are –0.391, –0.377, –0.371, –0,358, and –0.306. These bulky substituents are much more electron releasing than they are in reactions carried out in highly solvating media where steric hindrance to solvation masks their true electronic effect; in trifluoroacetic acid, solvation is very poor, and steric hindrance to solvation therefore largely absent. Among alkyl groups, bicyclo[2.2.2]octan-1-yl is second only to cyclopropyl in its electron releasing ability which probably derives in part from steric acceleration of hyperconjugation. The neopentyl substituent is more electron releasing than methyl due to the greater importance of C–C over C–H hyperconjugation. The concept of ‘secondary hyperconjugation’ proposed to account for the low activation by neopentyl in molecular bromination is disproved, the phenomenon arising simply from steric hindrance to solvation. Detritiation of endo-norbornan-2-ylbenzene is accompanied by protiodealkylation, which is sterically accelerated by the interaction between the aryl-ring ortho-hydrogen, and the endo-hydrogen on C-6.

Electron donor and acceptor properties of alkyl substituents

The polar effects of alkyl substituents in electrophilic and nucleophilic chemical and electronic transitions is discussed. The question of the importance of hyperconjugation in the electron donor properties of alkyl substituents is raised. In view of the cogent arguments of Dewar, it is doubtful whether quantum mechanical calculations embodying hyperconjugation constitute proof of this effect. That the art of quantum mechanics may not yet be sufficiently developed to be used as proof for or against secondary resonance effects also is evidenced by the calculations of Simpson, who found that an internal dispersion force model (in which conjugation was neglected) reproduced the properties of butadiene just as satisfactorily as the models embodying conjugation. The experimental facts do not unequivocally support the hyperconjugation hypothesis and indeed are, at least in part, contradictory to it. In particular, the demonstration that the Baker-Nathan Effect 2 may be due to the influence of alkyl substituents on the differential solvation of ground and transition states casts doubt on the interpretation that this experimental effect is due to a dominant role of C-H hyperconjugation. In nucleophilic chemical reactions, rate or equilibrium constants for para (or meta) alkyl derivatives are somewhat smaller than those of the corresponding hydrogen compounds. A number of authors have interpreted this in terms of a permanent electron donor role of alkyl substituents (e.g. by hyperconjugation) relative to the hydrogen substituent. However, this static viewpoint of substituent effects fails to account for the finding that p-alkyl substituents function as apparent electron acceptors (relative to the p-hydrogen substituent) in appreciably lowering the energy of the nucleophilic principal electronic transition of phenol, anisole, aniline and N,N-dimethylaniline. These results are qualitatively rationalized in terms of ‘substituent-polarizability” and electronegativity. The p-neopentyl substituent lowers the energy of both electrophilic and nucleophilic electronic transitions to an appreciably greater extent than either the p-methyl or p-t-butyl substituent. This extra stabilizing effect of the neopentyl substituent on both electron deficient and electron rich centers may be due to an internal dispersion force interaction, since the geometry of the neopentyl compounds is particularly favorable for such an interaction.