Ketone structural model

By changing from the simplest to larger aliphatic and cyclic ketones, structural factors may be introduced which favor alternative unimolecular primary photoprocesses or provide pathways to products not available to the simple model compound. In addition, both the increase in molecular size and irradiation in solution facilitate rapid vibrational relaxation of the electronically excited reactant as well as the primary products to thermally equilibrated species. In this way the course of primary and secondary reactions will also become increasingly structure-selective. In a,a -unsym-metrically substituted ketones, the more substituted bond undergoes a-cleavage preferentially. 

Fig. 5.8. Optimized (HF/3-21G) structures of the exo and endo transition states for reduction of f-butyl methyl ketone by model catalyst. The exo structure is favored by 2.1 kcal, in accord with an experimental e.e of 88%. Reproduced from J. Am. Chem. Soc., 116, 8516 (1994), by permission of die American Chemical Society.

First, peak heights are measured at five points in the NMR spectra (Figure 2). All NMR spectra of fulvic acids described in this study were determined as the sodium salt in D20 at pH 8 (21). Peak heights were used rather than peak areas to minimize overlapping spectral contributions from various proton structures. From structural-model considerations, peak 1 appears to be a combination of methylene and methine protons in aliphatic alicyclic rings and branched methyl groups located beta to carbonyl groups of a carboxylic acid, ester, or ketone. The structural model rules out meth-... 

The relationship between structure and photoinitiation activity has been examined for polymeric systems bearing side-chain 1-substituted cyclohexyl-phenyl ketone moieties in the UV curing of the HDDA/BA equimolar mixture [19,20]. Indeed, the activity of poly[(l-acryloxycyclohexyl)phenyl ketone] [poly (APK)] and styrene/4-chloromethyl-styrene/l-(4-styrylmethyloxy)cyclohexyl phenyl ketone copolymers (PABOK) has been compared with that of the corresponding low-molecular-weight structural models such as 1-hydroxy-cyclohexyl phenyl ketone (HPK), 1-acetoxy-cyclohexyl phenyl ketone (ACPK) and l-(4-isopropyl-benzyloxy) cyclohexyl phenyl ketone (PIBOK). 

Kinetic studies of the Midland reduction confirmed that the reduction of aldehydes is a bimolecular process and the changes in ketone structure have a marked influence on the rate of the reaction (e.g., the presence of an EWG in the para position of aryl ketones increases the rate compared to an EDG in the same position). However, when the carbonyl compound is sterically hindered, the rate becomes independent of the ketone concentration and the structure of the substrate. The mechanism with sterically unhindered substrates involves a cyclic boatlike transition structure (similar to what occurs in the Meerwein-Ponndorf-Verley reduction). The favored transition structure has the larger substituent (Rl) in the equatorial position, and this model correctly predicts the absolute stereochemistry of the product. 

The authors used as structural models bis-tcrf-alkyl ketones rearranging in an add medium according to the following scheme, the limiting step being that of the 1,2-shift of an alkyl group (CHj or CjHj) in a-hydroxycarbonium ions of type (39) ... 

Structural models of the simplest aldehyde (formaldehyde) and the simplest ketone (acetone) are shown in h Figure 4.1. 

The strueture of poly(ethylene-co-carbon oxide) (1.4 wt% CO) was studied by 50.3 MHz carbon-13 NMR spectroscopy [296]. With model compounds, assigments of new structures and previously undetected products of photodegradation and photo-oxidation in the solid state were made. Four per cent of the CO groups were aecounted for as ethyl ketones with the remainder being randomly distributed along the polymer backbone ehain. Evidence was presented for the formation of cis, trans-cyclobutanols. A novel a-branched ketonic structure was also found. The reactivity of ketonic groups is affected by polymer matrix morphology. 

Figure 5 (a) Structure of the five- and six-membered lactol rings in GO. (b) New structural model of GO, considering different functionalities such as the five- and six-membered lactol rings (blue), ester of a tertiary alcohol (purple), hydroxyl (black), epoxy (red), and ketone (green). Reprinted with permission from Gao, W. Alemany, L. B. Ci, L. J. Ajayan, P. M. Nat. Chem. 2009, 1,403-408. Copyright 2009 Macmillan Publishers Ltd. 

Give the structures or build molecular models of both alcohol products for each ketone... 

Hydride reduction (with LiAlH4 or NaBH4) of each of the following ketones has been reported in the chemical literature and gives a mixture of two diastereomeric alcohols in each case. Give the structures or build molecular models of both alcohol products for each ketone. 

The following molecular model represents a tetrahedral intermediate resulting from addition of a nucleoph ile to an aldehyde or ketone. Identify the reactants, and write the structure of the final product when the nucleophilic addition reaction is complete. 

There have been ab initio studies of the transition structure using several model catalysts and calculations attheHF/3-21G, HF/6-31G(4), and MP2/6-31G(4) levels.155 The enantioselectivity is attributed to the preference for an exo rather than an endo approach of the ketone, as shown in Figure 5.8. 

It has also been inferred that differences found between crystallinities measured by density and those from heat of fusion by DSC area determination, as given for polyethylenes in the example of Figure 4 [72], may be related to baseline uncertainties, or not accounting for the temperature correction of AHc. Given that similar differences in crystallinity from density and heat of fusion were reported for isotactic poly(propylene) [43] and polyfaryl ether ether ketone ketone), PEEKK [73], other features of phase structure that deviate from the two-phase model may be involved in the crystallinity discrepancy. 

We first examined ketone 48 and hydroxy ketone 56 because (1) no reduction of C5a-carbonyl in 48 occurred when using L-Selectride to reduce the C3-ketone and (2) NaBH4 did not touch the C5a-carbonyl group in hydroxy ketone 56. By using minimized Spartan model of vinylogous ester 48 and the X-ray structure of hydroxy ketone 56, we found unique conformational elements (Fig. 8.10) in addition to fully validating the aforementioned NMR analysis of possible positions for H8a when C8a is ip2-hybridzied and H7 when C8a is sp3-hybridized. In 48,... 

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