4.4- dimethyl-2-pentanone, enolate

Dimethyl- 3-pentanone Enolate 5-Hydroxy-2,2,4-trimethyl-3-heptanone (81%)... 

Fig. 7.3. Crystal structures of some lithium etiolates of ketones. (A) Unsolvated hexameric enolate of methyl t-butyl ketone (B) tetrahydrofuran solvate of tetramer of enolate of methyl r-butyl ketone (C) tetrahydrofuran solvate of tetramer of enolate of cyclopentanone (D) dimeric enolate of 3,3-dimethyl-4-(r-butyldimethylsiloxy)-2-pentanone. (Structural diagrams are reproduced from Refs. 66-69.) by permission of the American Chemical Society and Verlag Helvetica Chimica Acta AG.

Compare electrostatic potential maps of enolates derived from 2-butanone, 4,4-dimethyl-2-pentanone, 4,4,4-trifluoro-2-butanone and l-phenyl-2-propanone with those of acetone. Which substituents cause significant changes in the electronic structure of these enolates and what are the nature of these changes Justify your answers by making drawings of any important resonance contributors. 

The first element of stereocontrol in aldol addition reactions of ketone enolates is the enolate structure. Most enolates can exist as two stereoisomers. In Section 1.1.2, we discussed the factors that influence enolate composition. The enolate formed from 2,2-dimethyl-3-pentanone under kinetically controlled conditions is the Z-isomer.5 When it reacts with benzaldehyde only the syn aldol is formed.4 The product stereochemistry is correctly predicted if the TS has a conformation with the phenyl substituent in an equatorial position. 

Z-Boron enolates can also be obtained from silyl enol ethers by reaction with the bromoborane derived from 9-BBN (9-borabicyclo[3.3.1]nonane). This method is necessary for ketones such as 2,2-dimethyl-3-pentanone, which give E-boron enolates by other methods. The Z-stereoisomer is formed from either the Z- or E-silyl enol ether.20... 

Several enolates of 4,4-dimethyl-3-(trimethylsiloxy)-2-pentanone have been investigated.106 The lithium enolate reacts through a chelated TS with high 2,2 -anti stereoselectivity, based on the steric differentiation by the f-butyl group. 

SAMPLE SOLUTION (a) Remember that enolization involves the a-carbon atom. The ketone 2,4-dimethyl-3-pentanone gives a single enol because the two a carbons are equivalent. 

Even when the reductive processes, so evident in metal-stimulated processes, are avoided, several side reactions can still cause reductions in the yield of the desired a-arylated ketones. The first, abstraction of 3-hydrogen atoms from the enolate ion by the aryl radical, has already been mentioned (Section 2.2.2.1) and is sometimes a serious, chain-terminating process.43 5 This abstraction reaction, however, appears to be quite unpredictable. 3-Hydrogen abstraction from the enolate of 2,4-dimethyl-3-pentanone (PriCOPr Table 1) which severely disrupts the reaction with iodobenzene, does not prevent high-yielding reactions of the same enolate (and those from other ketones with a-branching) with many other substrates. In in-... 

The reductive dehalogenation predominates in the photoinduced reactions of 32 with 3-pentanone, 2-methyl-3-pentanone, and 2,4-dimethyl-3-pentanone. Although, substitution is favored over reduction in all of the reactions of 32 with cyclic ketone enolates, a competing reduction is responsible for the lower yields observed with cyclopentanone and cyclohexanone compared to cyclooctanone (Scheme 10.59) [73]. 

Bromopyridine (3.16 g, 20 mmol) is added to an enolate solution prepared from 8.56 g (75 mmol) of 2,4-dimethyl-3-pentanone and 4.12 g (75 mmol) of KNH2 in 300 mL of liquid ammonia. After the mixture has been irradiated for 1 h, diethyl ether is added and the ethereal suspension remaining after evaporation of the ammonia is decanted through a filter and the residual salts are washed with diethyl ether. This reaction affords high yield of the substitution product (Sch. 37). 

For example, condensation of silyl enol ether (3.21) of 3-pentanone with 2-methylbutanal in the presence of TiC gives the Ti-complex 3.22, which on hydrolysis yields an aldol product, manicone (4,6-dimethyl-4-octen-3-one) (3.23), an alarm pheromone. Treatment of 3-pentanone with LDA results in the formation of an enolate, which is trapped with chlorotrimethylsilane to give 3.21. Other Lewis acids such as tin tetrachloride (SnCU) and boron trifluoride etherate (BF3-OEt2) can also be used. 

Owing to the dependence of the product stereochemistry on enolate configuration, control of the stereochemistry of enolate formation is important. For ketones with one relatively bulky group, the Z-enolate is favored, resulting in formation of the xyn-aldol product. This is the case, for example, in the reaction of 2,2-dimethyl-3-pentanone and... 

A complicating factor in the analysis is the observation that aldolates can undergo syn/anti equilibration by enolization or by reverse aldolization. Aldols such as 407 can be deprotonated to the dianion (408) and this undergoes alkylation with iodomethane to give the anti product (409), as shown.230 This equilibration is clearly the basis of the aldol-transfer reaction discussed in 143 to 145 in Section 9.4.A.i. If 409 forms a new enolate, equilibration can lead to a mixture of syn and anti products. The primary mechanism for syn/anti equilibration appears to be reverse aldolization.23 A retro-aldol will convert the syn diastereomer (410) into the aldehyde and enolate components, which can regenerate 410 or form the anti diastereomer 411. Syn/anti equilibration can be much slower than reverse aldolization, as with the (Z) enolate of 2,2-dimethyl-3-pentanone).227 Aldolates derived from the more basic ketone enolates are more likely to suffer reverse aldolization than aldolates derived from the less basic enolates of esters, amides, or carboxylate salts. Steric crowding in an aldolate promotes reverse aldolization. The metal is very important, and some metals form... 

Sample Solution (a) Only one of the a carbons of 2,2-dimethyl-3-pentanone has an attached hydrogen, so only one constitutional isomer is possible for the enol. E and Z stereoisomers are possible. 

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