2-Propyl carbanion

Focusing our attention on anionic molecules, we turn first to the problem of conformational isomerism in the 2-propyl carbanion, CH3CHCH3. The two geometries which we will compare are again the staggered and eclipsed conformations shown below ... 

Recent work [29], involving the use of reticulated vitreous carbon cathodes in DMF containing TMABF4, has shown that the reduction of iodoethane is a two-electron process and that the products are ethane (55%), butane (32%), and ethylene (6%). In the same investigation [29], the electroreduction of 2-iodopropane was found to occur in two steps to give the 2-propyl radical and the 2-propyl carbanion, respectively depending on the potential, propane, propylene, and 2,3-dimethylbutane are obtained in different yields. 

I > Br > Cl > F) is in agreement with the order of the leaving ability of halide ions and also supports an SET reaction mechanism [7]. The intermediacy of the radicals (2) in the chain process is demonstrated by radical trapping with norbornene in the course of the perfluo-roalkylation of 2-nitro-2-propyl carbanion with perfluorooctyl iodide (Scheme 2.33) [8]. 

One way to generate carbanions is to combine an acidic molecule with one equivalent of a very strong base, such as n-butyl lithium (n-BuLi). For example, reaction of the alkyne shown below with n-BuLi leads to a carbanion of formula CsH, 02 , which then undergoes an Sn2 reaction with n-propyl bromide (n-PrBr),... 

C-( -propyl)-N-phenylnitrone to N-phenylmaleimide, 46, 96 semicarbazide hydrochloride to ami-noacetone hydrochloride, 46, 1 side-chain carbanions to arynes, 48, 55... 

The stability of the carbanions has been found to be in the order benzyl > vinyl > phenyl > -C2TI5 > n-propyl > isobutyl > neopentyl. 

Continuing to assume that Gibbs energies and enthalpies are essentially equal, the above reaction is endothermic by 14 kJ moP for R = vinyl. However, it is exothermic for the other R groups cyclopropyl, —6 ethyl, —20 propyl, —22 isobutyl, —26 neopentyl, —31 cyclobutyl, —35 and cyclopentyl, —39 kJmoP. The values are in the order expected for carbanion stability. 

A situation of this kind has been characterised in the competing rearrangement of and elimination from 1 -(2 -acetoxy-2-propyl)indene (32 in Scheme 9.19) induced by methanolic p-nitrophenoxide [ 49 ]. The deuterium isotope effect on disappearance of reactant was kulko = 5.2 but, for the products, ku/kD = 12.2 for the rearrangement to 33 and ku/kD = 3.6 for the elimination to alkene 34. At least one hydrogen-bonded carbanion, 35, is required to account for these observations. Its reprotonation to yield the rearranged product was expected to be isotopically sensitive while the expulsion of acetate was not. 

Schleyer and coworkers calculated the electron affinity of ethyl, 2-propyl, cyclobutyl and cyclopropyl radicals. Apart from the cyclopropyl radical, these radicals have negative electron affinities suggesting that the corresponding anions cannot be observed as long-lived species in the gas phase. For the cyclopropyl radical, an electron affinity of 5.1 kcal moT was predicted in reasonable agreement with the experimental value of 8 kcal moT Accordingly, it is probable that the cyclopropyl anion is the only saturated carbanion that can be observed experimentally in the gas phase. 

The diastereoselectivity in the alkylation of N-substituted 2-oxo-2-propyl-1,3,2-oxazaphosphorinanes (435) is influenced by the bulkiness of the nitrogen substituent. The ot-carbanion derivatives of (435) when R = CHPh2 and R = CPhs are unstable in the presence of DMPU and afford unexpected products (Figure 79). 

However, experiments in the gas phase gave different results. In reactions of OH with alkyltrimethylsilanes, it is possible for either R or Me to cleave. Since the R or Me comes off as a carbanion or incipient carbanion, the product ratio RH/ MeH can be used to establish the relative stabilities of various R groups. From these experiments a stability order of neopentyl > cyclopropyl > ferf-butyl > n-propyl > methyl > isopropyl > ethyl was found. On the other hand, in a different kind of gas-phase experiment, Graul and Squires were able to observe CHs ions, but not the ethyl, isopropyl, or ferf-butyl ions. ° ... 

A sequence starting with methylenebis[phosphonic acid] tetraester carbanions, and consisting of alkylation with an appropriate 3-substituted-propyl halide, side-chain modification, and cyclization, ultimately affords the tetraalkyl esters of cyclopropane-1,l-bis[phosphonic acid], and from which the free acid has been obtained.The reaction between 1,2-dibromoethane and ethyl (diethoxyphosphinyl) acetate or ethyl... 

It also has been found that butyllithiums react faster with 2,2-bis(phenylseleno)propane than with its methylseleno analog. Thus, addition of n-butyllithium to a 1 1 mixture of 2,2-bis(phenylseleno)propane and 2,2-bis(methylseleno)propane leads selectively to the formation of 2-lithio-2-phenylselenopropane under kinetically controlled conditions, providing the a-selenoalkyllithium with the more stable caibanionic center (Scheme 4). Furthermore, 2-methylselenopr(q)yUithium reacts with 2,2-bis(phenyl-seleno)propane and leads, under thermodynamically controlled conditions, to 2-phenylseleno-2-propyl> lithium, thus demonstrating the better propensity of a phenylseleno moiety over a methylseleno moiety to stabilize a carbanionic center (Scheme 5). 

Unlike the carbanions from 1,3-dithians (see Chapter 3, p. 31 and Chapter 6, p. 90), the sulfinyl carbanions have the ability to undergo Michael additions (conjugate or 1,4-additions) with a,p-unsaturated carbonyl compounds. For instance, the secondary carbanion (44) from the ethyl ethylthiomethyl sulfoxide (45) may be sucessively reacted with ethyl iodide and 3-butene-2-one to give heptan-2,5-dione (46) via the tertiary carbanion (47), as shown in Scheme 23. The carbanion (47) may also be condensed with propyl bromide, and hydrolysis of the product yields ethyl propyl ketone (48) (Scheme 23). 

An intramolecular Michael-t)rpe cycUzation of 98 and subsequent trapping of the carbanion intermediate 99 with propyl iodide produced 2,3-disubstituted-l-methyUndole 100 <05OL4641>. The indole ring was formed after elimination of sulfmic acid. 

If the carbanion reacts with acetophenone, 2-hydroxy-2-phenyl-propyl phenyl ketone is formed. This compound is not stable in alkaline solution because the hydrogens alpha to the carbonyl and the hydroxy are acidic. This 3-hydroxy carbonyl compound can undergo an elimination reaction to form a more stable 3 unsaturated compound. The reaction steps are shown below ... 

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