Cyclopentanone selective 2-alkylation

Titanium(IV) is a powerful but selective Lewis acid which can promote the coupling of allylsilanes with carbonyl compounds and derivatives In the presence of titanium tetrachlonde, benzalacetone reacts with allyltnmethylsilane by 1,4-addition to give 4-PHENYL-6-HEPTEN-2-ONE. Similarly, the enol silyl ether of cyclopentanone is coupled with f-pentyl chloride using titanium tetrachlonde to give 2-(tert-PENTYL)CYCLOPENTANONE, an example of a-tert-alkylation of ketones. 

The effect of HMPA on the reactivity of cyclopentanone enolate has been examined.44 This enolate is primarily a dimer, even in the presence of excess HMPA, but the reactivity increases by a factor of 7500 for a tenfold excess of HMPA at -50° C. The kinetics of the reaction with CH3I are consistent with the dimer being the active nucleophile. It should be kept in mind that the reactivity of regio- and stereoisomeric enolates may be different and the alkylation product ratio may not reflect the enolate composition. This issue was studied with 2-heptanone.45 Although kinetic deprotonation in THF favors the 1-enolate, a nearly equal mixture of C(l) and C(3) alkylation was observed. The inclusion of HMPA improved the C(l) selectivity to 11 1 and also markedly accelerated the rate of the reaction. These results are presumably due to increased reactivity and less competition from enolate isomerization in the presence of HMPA. 

The above conditions allow the synthesis of a large variety of a,a-dialkyl substituted ketones including spiro derivatives and cyclopentanones, cyclohexanones, cycloheptanones and cyclododecanones bearing two alkyl groups at die a-posidon (Scheme 162, e Scheme 165, c Scheme 166, e Scheme 186, a and b Scheme 187 Scheme 188, a Scheme 189, a and The reactions are less selective when the dichlorocarbene is generated from bromodichloromethane and lead to gem-dichlorocyclopropanes if the dichlorocarbene is produced from chloroform and potassium r-butoxide (Scheme 162, f Scheme 188, b Scheme 189, c). ... 

Delivery of an electrophile to the less hindered face of an enolate also occurs in intramolecular alkylation reactions. When 500 was treated with potassium fert-butoxide, a mixture of (E) and (Z) enolates (501 and 502. respectively) was obtained. Intramolecular displacement of bromide generated a single isomer (503). In this case, the electrophile can approach the enolate from only one face (the bottom or a face). Because of this conformational constraint, both (E) and (Z) enolates lead to the same product. In cyclopentanone and cyclohexanone enolates. an increase in the size of a facial blocking group increases selectivity. When that group was small, the selectivity decreased. 

Acyclic diazo compounds containing aliphatic chains yield in most cases five-membered rings, e. g., in (8-54) the raAZ5 -cyclopentanone 8.119 is formed diastereo-selectively in 77 9/o yield (Taber and Ruckle, 1986). The diazoketo ester 8.120 containing an alkyl chain with a terminal C = C bond leads to the cyclopentanone 8.121 in 629/o yield (8-55), which demonstrates that the insertion dominates an intra- or inter-molecular cyclopropanation, as found by Wenkert s group (Checcherelli et al., 1990). 

The best alkylating agents for silyl enol ethers are tertiary alkyl halides they form stable carbocations in the presence of Lewis acids such as TiCl4 or SnCl4. Most fortunately, this is just the type of compound that is unsuitable for reaction with lithium enolates or enamines, as elimination results rather than alkylation a nice piece of complementary selectivity. Below is an example the alkylation of cyclopentanone with 2-chloro-2-methylbutane. The ketone was converted to the trimethylsilyl enol ether with triethylamine and trimethylsilylchloride we discussed this step on p. 466 (Chapter 20). Titanium tetrachloride in dry dichloromethane promotes the alkylation step. 

As the aUylated enamine has higher reactivity (i.e., lower ionization potential) the selective mono-aUylation of cyclohexanone required excess (2 equiv.) ketone reagent. Nevertheless 20 equivalents of cyclobutane were necessary for the selective reaction with cyclobutanone and only 2,5-bis-allylated cyclopentanone was obtained, as the second oxidation occurred immediately on the iminium intermediate prior to hydrolysis with this substrate. The allylation reaction was compatible with alkyl and heteroatom substituents at the P and y positions. When non-symmetrical heteroatom containing substrates were used, C(4) allylation occurred selectively in high yields (70-86%) and in high ee (80-99%) (Figure 39.2). 

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