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Showing posts with label Surprises. Show all posts
Showing posts with label Surprises. Show all posts

Sunday 29 November 2015

Surprises in the Study of Ruthenium-catalyzed Stereo- and Chemoselective Aldolizations

http://www.orientjchem.org/wp-content/uploads/2015/10/Vol31_No4_Sur_Elah_Sch1.jpg.

 Typical procedure for ruthenium-catalyzed aldol reaction
A mixture of aldehyde (1 mmol), ketone (3 mmol), RuCl3.nH2O(0.02 mmol) and KOH (28 mg, 0.5 mmol) was stirred at 15 °C and monitored by TLC. After the indicated reaction time, the reaction mixture was purified by thin layer chromatography (silica gel, EtOAc-petroleum ether, 4:12) providing the aldol adduct.

Selected Spectral Data of the Products
Product (3aa)
Yellow oil; FT-IR (neat) (υmax/cm-1): 1666, 3415. Only syn isomer was isolated. 1H NMR (500 MHz, CDCl3): δH(ppm) 1.69-2.65 (m, 7H), 2.52 (s, 3H), 3.03 (d, J = 3.2 Hz, OH), 5.39 (m, 1H), 7.28 (m, 4H). 13C NMR (125 MHz, CDCl3): δC(ppm) 12.2, 14.0, 14.9, 41.1, 58.1, 72.2, 126.5, 128.5, 133.5, 135.2, 218.4. Anal Calcd for C13H16O2S (236): C, 66.10; H, 6.77; S, 13.55. Found: C, 66.10; H, 6.79; S, 13.56.


Table 1: Ruthenium-catalyzed cross aldol reactions of aldehydes with cycloalkanones 3aa-ec.
Entrya R Aldol Yield (%)b Time (h)
1 1a 4-MeSC6H4 3aa 81 5
2 1b 3-Methylthiophen-2-yl 3ba 86 1.5
3 1c 5-Methylthiophen-2-yl 3ca 83 1
4 1d Thiophen-2-yl 3da 91 3
5 1a 4-MeSC6H4 3ab 79 4.5
6 1b 3-Methylthiophen-2-yl 3bb 76 2.5
7 1c 5-Methylthiophen-2-yl 3cb 77 3
8 1a 4-MeSC6H4 3ac 72 4.5
9 1b 3-Methylthiophen-2-yl 3bc 80 2.5
10 1c 5-Methylthiophen-2-yl 3cc 78 2.5
11 12c 1d Thiophen-2-yl 1e  4-NO2C6H4 3dc 3ec 73 55 2 5
aAll products were characterized by 1H NMR, 13C NMR and IR. b Yields after purification by chromatography. c Identified by comparison with authentic sample.38





 http://www.orientjchem.org/wp-content/uploads/2015/10/Vol31_No4_Sur_Elah_Fig1.jpghttp://www.orientjchem.org/wp-content/uploads/2015/10/Vol31_No4_Sur_Elah_Fig2.jpg

I t is a well understood phenomenon that the lower values of the 1H NMR coupling constants of the carbinol protons than 1 Hz, for example 3cc, along with very weak correlation between the 1-H and 2-H (Figure 1) in the NOESY spectrum clearly indicate the relative stereochemistry of the aldol adduct in favor of the syn geometry. Also, in the IR spectrum of 3cc, the broad band of OH was observed with a maximum at 3500 cm-1, which indicates the existence of hydrogen bonding of aldol adduct (Figure 2). In fact, the structure has been fixed in a special stereochemistry which dominated by hydrogen   bonding observed in IR spectra and the tendency to minimize the steric crowding between the 5-Methylthiophen-2-yl and cycloheptanone rings. These results illuminate that 1-C, 2-C, 1-H and 2-H has the same chemical environment, that is to say 3cc is exclusively of syn stereochemistry.


Product (3cc)
Yellow oil; FT-IR (neat) (υmax/cm-1): 1674, 3465. Only syn isomer was isolated. 1H NMR (500 MHz, CDCl3): δH(ppm) 2.92-1.54 (m, 11H), 2.44 (s, 3H), 3.37 (d, J = 3.9 Hz, OH), 5.25 (s, 1H), 6.60 (d, J = 2.4 Hz, 1H), 6.72 (d, J = 3.2 Hz, 1H). 13C NMR (125 MHz, CDCl3): δC(ppm) 15.2, 23.6, 25.3, 28.4, 28.6, 44.0, 57.8, 71.5, 124.5, 124.6, 138.6, 143.3, 217.2. Anal Calcd for C13H18O2S (238): C, 65.54; H, 7.56; S, 13.44. Found: C, 65.53; H, 7.56; S, 13.44.
Oriental Journal of Chemistry
Keshavarz E, Tabatabaeian K, Mamaghani M, Mahmoodi N. O. Surprises in the Study of Ruthenium-catalyzed Stereo- and Chemoselective Aldolizations. Orient J Chem 2015;31(4).


Elahe Keshavarz,1,* Khalil Tabatabaeian2, Manouchehr Mamaghani2 and Nosrat O. Mahmoodi2
1Department of Sciences, Farhangian University, P.O. Box 1998963341, Rasht, Iran.
2Department of Chemistry, Faculty of Sciences, Guilan University, P.O. Box 41335-1914, Rasht, Iran.
Corresponding Author E-Mail: keshavarz@guilan.ac.ir
ABSTRACT: A convenient and diastereoselective method was developed for the synthesis of aldol derivatives in the presence of a catalytic amount of RuCl3.nH2O under solvent-free conditions. Aldol adducts were obtained in good yields and with high chemoselectivity  in short reaction times. In this protocol, aromatic and heteroaromatic aldehydes readily participate as electrophilic cross-aldol partners with a range of cycloalkanones as ketone donors.
KEYWORDS: catalytic aldol reaction; diastereoselective aldolization; green synthesis

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