Preparation of 2,4-dimethyldecane, 5- n-propylnonane, 2-methyl-5-ethylnonane, and 3-and 4-ethyldecanes

Paraffins 8, 15, 24, 35, and 41 were each prepared in three steps from the respective carbonyl precursors. Structural elucidation of these was performed using detailed 13 C NMR analysis.


Introduction
Simple branched alkanes are common components of petroleum fuels and derivatives.During the 1930's and 1940's, many syntheses of alkanes [1][2][3] were carried out to provide data on the heats of formation, heats of combustion, and general physical properties of paraffins.Recent interest in branched alkanes concerns their use as fuel additives 4 , their importance as moieties in microemulsion-forming surfactants 5 , and their synthesis as components of natural products, particularly insect hormones [6][7][8][9][10] .
Methods of synthesizing alkanes include: i) the classical alcohol-olefin-paraffin conversion [1][2][3] , ii) reduction of olefins prepared from acyl cyanides 11 or ketones 6 , iii) halopolycarbon homologation employing α,ω-dihalides and Grignard reagents 7 , and iv) protonolysis of trialkylboranes 12 .In general, the most accessible route to branched alkanes is the classical route.The appropriately substituted alcohols required can be readily obtained from reaction of Grignard reagents with ketones.Simple dehydration of these alcohols yields olefin mixtures, which can be hydrogenated by a variety of catalysts.Reduction of olefins can be carried out in a hydrogen atmosphere over Raney-nickel 11,13 , Pd-C 6,14 , PBI-PdCl 2 15 , polymer-supported Rh(CO 2 )(pd) 16 , HNaY zeolite 17 , or platinum oxide 1,18 .Use of Raney-nickel or HNaY zeolite often requires high pressures, while platinum oxide and palladium-based catalysts function at or near 1 atm.

Results and Discussion
Synthesis of these alkanes was designed through a disconnection approach.The initial disconnected fragments for 2,4-dimethyldecane were 2-octanone and isobutylmagnesium bromide.However, addition of isobutylmagnesium bromide to 2-octanone gave a considerable amount of the reduction product, 2-octanol, as well the desired addition product 3.So this disconnection was abandoned and 4-methyl-2-pentanone and hexylmagnesium chloride were selected as the new fragments, which gave the alcohol 3 in good yield.The alcohol intermediate 3 was subjected to acid catalyzed dehydration to give a mixture of alkenes, 4-7; which after hydrogenation provided only the desired alkane 8 (Scheme 1).In the case of 24, alcohol 18 was prepared by the reaction of Grignard reagent 17 with the carbonyl component 16.However, since 18 is a secondary alcohol, the elimination reaction was unsuccessful.Thus, 18 was converted into the corresponding chloride 19, by a standard method 19 , and was subjected to dehydrohalogenation by treatment with KOH in ethanol.Finally, the mixture of alkenes 20-23 was hydrogenated to the paraffin 24, under the same conditions as for the previous compounds (Scheme 3).

Characterization by NMR spectra
In the 1 H NMR spectra, the number of CH, CH 2 and CH 3 protons corresponded to those expected from the structures of the alkanes, but the multiplicity patterns provided no useful information.The structural determination of paraffins by 13 C NMR is particularly useful as the carbon chemical shifts are spread over 50 ppm.The structures of all the paraffins were confirmed through detailed 13 C investigations. 13C-DEPT experiments differentiated the CH, CH 2 and CH 3 carbons.Two-dimensional carbon-hydrogen correlation spectra also provided useful information about each structure.

Experimental Section
General procedure for the preparation of alkanes.A solution of the appropriate ketone or aldehyde (0.5 mol) in anhydrous ether (50 mL) was added dropwise to the appropriate Grignard reagent (2M sol. in ether, 250 mL, 0.5 mol) at rt.After the addition, the reaction mixture was stirred for 5 h; after which, the reaction mixture was poured into saturated ammonium chloride solution and extracted with diethyl ether (2 x 50 mL).The combined ethereal layers were dried over anhydrous sodium sulfate, and the ether was removed under vacuum.The corresponding alcohols were obtained as colorless liquids in 63−76% yields.The crude alcohol (for 8 and 15, 70 g, 0.38 mol) was dissolved in toluene (300 mL) and a catalytic amount (0.5 g) of p-toluenesulfonic acid (PTSA) was added.The mixture was refluxed overnight under a Dean-Stark trap.Ethyl acetate was added and the organic layer washed with water, dried over sodium sulfate and concentrated.The alkenes were obtained in 88−95% yields.In the case of 24, the crude alcohol (0.5 mol scale) was treated with SOCl 2 and the intermediate chloride was treated with KOH in refluxing ethanol to afford a mixture of alkenes 20-23.

Table I (
Scheme 6)reports the detailed assignment of carbon signals to the paraffins.

Table 1 .
13C Assignments to the structures of paraffins