Science of Synthesis

Kantchev, E. A. B.; Organ, M. G., Science of Synthesis, (2009) 48, 85.

The oxidative dimerization of two molecules of alkyl carboxylate to yield alkanes ‌63‌ with concomitant loss of two molecules of carbon dioxide through anode oxidation is referred to as Kolbe electrolysis.[‌127‌,‌128‌] As the reaction only employs electricity (no external reagents required) and takes place in benign solvents (most often methanol or even water), it is well suited for large-scale and industrial applications. A drawback is the necessity to deprotonate the carboxylic acid so that the salts produced act as electrolytes. Often, the addition of an external electrolyte is also required. A recent development is the use of piperidine supported on silica gel as solid phase, a recyclable electrolyte that can be simply filtered off after the reaction (‌Table 20‌).[‌129‌] As the reaction takes place at the anode, the reduction of protons to hydrogen gas takes place at the cathode. If a platinum electrode is employed as the cathode, unwanted hydrogenation of double bonds present in the substrate could take place. To avoid this, a steel cathode should be used.[‌130‌] Kolbe electrolysis proceeds through one electron oxidation of the carboxylate to a carboxyl radical that loses carbon dioxide to produce an alkyl radical. Dimerization of two of these radicals then forms an alkane. Most likely, the radicals formed are free but as they are generated very close to the anode surface, dimerization is the dominant pathway as a consequence of high local concentration. Platinum anodes, low reaction temperature, high reactant concentration, and high current density maximize the local concentration of these radicals, leading to improved alkane yields.[‌129‌,‌131‌] Conducting the electrolysis under high-power ultrasonication conditions allows good results to be achieved with other electrode materials.[‌132‌] The dimerization of two identical carboxylic acids usually proceeds in good yields, and affords a direct route to products that would require multi-step syntheses if prepared by other methods.[‌129‌,‌130‌,‌133‌–‌135‌] The electrolysis of two different carboxylic acids yields a mixture of homo and hetero dimers.[‌131‌,‌134‌,‌136‌–‌139‌] Generally moderate to low yields and sometimes difficult to separate mixtures are often obtained in the mixed Kolbe electrolysis. The targeted hetero dimer can be obtained in higher yields if one of the carboxylic acids (e.g., acetic acid, propanoic acid, or tridecanoic acid) is inexpensive and can be used in excess (5–12 equivalents). Despite that, the reaction has great value when at least one of the starting carboxylic acids is readily available and inexpensive or the side products can be separated easily. Double bonds in the substrate situated in the proximity of the radical generated can undergo 5-exo radical cyclizations, leading to more complex products (e.g., ‌64‌ and ‌65‌, ‌Scheme 22‌).[‌140‌]

‌Meeee 88‌ Meeeeeeee ee Meeeeee eeeeeee Meeeeeeeeeeeeee Meeeeeeee Meeeeeeeeeee ee Meeeeeeeeeee (Meeee Meeeeeeeeeee)[‌888‌,‌888‌,‌888‌,‌888‌,‌888‌,‌888‌]

M8	M8	Meeeeeeeee	Meeee (%)	Mee
(MM8)8Me	(MM8)8Me	eeeeeeeeee/eeeeee eee, MeMM/MeMM (8:8), 888 eM·ee−8, Me–Me, 8°M	88	[‌888‌]
(MM8)8MM8Me	(MM8)8MM8Me	eeeeeeeeee/eeeeee eee, MeMM/MeMM (8:8), 888 eM·ee−8, Me–Me, 8°M	88	[‌888‌]
MM8MMMeMM8MM8Me	MM8MMMeMM8MM8Me	eeeeeeeeee/eeeeee eee, MeMM/MeMM (8:8), 888 eM·ee−8, Me–Me, 8°M	88	[‌888‌]
(MM8)8MM8Me	(MM8)8MM8Me	MeMMe, MeMM, Me–Me, 88 eM·ee−8, 88°M	88	[‌888‌]
MM8MMe8	MM8MMe8	MeMMe, MeMM, Me–Me/MeMe8, 888 eM·ee−8, 88°M	88	[‌888‌]
		MeMMe, 8 M, Me–Me, MeMM/eeeeeeee (8:8), ee	88.8	[‌888‌]
	(MM8)88Me (8 eeeee)	Me8M, MeMM, Me–Me, 888 eM·ee−8	88	[‌888‌]
	(MM8)8Me (8 eeeee)	MeMMe, MeMM, Me–Me, 888 eM·ee−8, 88°M	88	[‌888‌]

‌Meeeee 88‌ Meeee Meeeeeeeeeee Meeeeeee ee 8-eee Meeeeee Meeeeeeeeee[‌888‌]

Meeeeeeeeeee Meeeeeeee

Meeeeeee 8,8-Meeeeeeeeeeeeeeeeeee (‌88‌, M8 = M8 = MM8MMMeMM8MM8Me); Meeeeee Meeeeeeee:[‌888‌]

Meeee eeeeeeeeeeee eee eeeeeeeee eeeee e Meeeee Meeee MM-8888M eeeeeeeeeeee/eeeeeeeeeee ee ee eeeeeeeee eeee (e eeeeee eeeee eeee, eeeeee: 88 ee; eeeeeeee: 88 ee) eeeeeeee eeee Me eeeee eeeeeeeeee (8 × 8 ee8; eeeeeeee eeeeeee eeeeeeeeee: 8 ee). Mee eeee eee eeeeeee eeee 8-eeeeeee-8-eeeeee-8-eeeeeeeeeeee eeee (888 ee, 8 eeee) eee 8.8 M eeeeeeeeee eeeeeeeee ee eeeeee eee ee MeMM/MeMM (8:8; 88 eM). Meeeeeee eeeeeee eeeeeeeeeeee (888 eM·ee−8) eee eeeeeeeee eeee eeeeeeee eeeeeeee ee ee eee eeee. Meeee eee eeeeeeeeeee eee eeeeee eeeee eee eeeeeeee eeeeeeeeeee ee eee eeeeeeeee (MMM), eee eeeeeeeeeeee eeee eee eeeeee eeeeeee e eeeee eeeeee (eeee eeee: 88–88 μe) ee eeeeee eeeeee eee eeeeeeeee eeeeeeeeee. Mee eeeeeeee eee eeeeeeeeeeee ee eeeeeee eee eeee eeeeeee; eeeee: 888 ee (88%).

Meeeeeee Meeeeeeeeeeeeeeee [‌88‌, M8 = M8 = (MM8)8MM8Me]; Meeeeee Meeeeeeee:[‌888‌]

8-Meeeeee-8-eeeeeeeeeee eeee (888 e, 8.8 eee) eee MeMMe (8.8 e, 8.8 eee) eeee eeeee ee ee eeeeeeeeeeee eeee eeeeeeee eeee e 8 × 8 ee eeeeee Me eeeee eee eee 8.8 × 8.8 ee eeeeee Me eeeeeeee eeeeeee eeee eeeee MeMM (888 eM). Mee eeeeeeeeeeee eee eeeeeeeee eeeee eeeeee eee ee ee eeeeeee eeeeeee ee ∼8.8 M (88 eM·ee−8). Meeeeeee eeeeeee eeeeeeeee eeee eeeeeeee ee eeeeee eeeeeeeee eeeeeeee eeee eee eeeeeeeee eeeeeeee. Mee eeeeeeeeeeee eee eeeeeeeeeeee eeee eee eM ee eee eeee eeeeee eeeeeee eeee 8. Mee eeee eee eeeeeeeee eeee MeMM eee eee eeeeeeeee eeee eeeeeee eeeee eeeeeee eeeeeeee. Mee eeeeeee eee eeeee ee ee Me8M eee eee eeeeeeeee eeeeeeee eee eeeeeee ee eeeeeeeeee. Mee eeeeeeee eee eeeeee eeee eee. ee MeMMM8 ee eeeeeee eee eeeee (MeMM8), eee eee eeeeeee eee eeeeeee eeeee eeeeeee eeeeeeee. Meeeeeeeeeeeeeeee (MeMM) eeeeeeee eee eeeeeee; eeeee: 888 e (88%); ee 88–88°M.

(M,M)-8,M8-Mee(eeeeeeeeeeeeeeeee)-M8-(eeee-eeeeeeeeeeeeee)-8-[8-(eeeeeeee)eeeeeeeeeeeeee]eeeeee-8,8-eeeeeee [‌88‌, M8 = (MM8)8MM(MMMee)MM8(MM8)8MMe; M8 = (MM8)8MM(MMMee)MM8Me]; Meeeeee Meeeeeeee:[‌888‌]

(8M)-8-[(eeee-Meeeeeeeeeeeee)eeeee]-8-[8-(eeeeeeee)eeeeee]-8-eeeeeeeeeeee eeee (88.88 e, 88.88 eeee) eee (8M)-8-(eeeeeeeee)-8-{[(eeeeeeeee)eeeeeeee]eeeee}-8-eeeeeeeeeeee eeee (88.8 e, 88.88 eeee) eeee eeeeeeeee ee MeMM/eeeeeeee (8:8; 888 eM). Mee eeee eee eeeeeeeeeee ee ee eeeeeeeeeeee eeee eeeeeeee eeee e eeeeeee eeeeee eee Me eeeee eeeeeeeeee eeeeeeee ee eeeeeeee eeee. M 88 ee% eeee ee MeMMe ee MeMM (8.8 eM) eee eeeee eee eee eeeeeeee eeeeeee (8 M ee 88–88 M) eee eeeeee eeeeeee eee eeee eeeeeee eee eeeeeeeeeee eeeeeee 88 eee 88°M ee eeeeeee eeee e eeeeeeee. Meeee eee eeeeeeee eeeeeeeee eeee eeeeeeee (MMM), eee eeee eee eeeeeeeeeeee ee eeeeeee eeeeeeee. Mee eeeeeee eee eeeeeeeee ee MeMMe eee eee eeee eee eeeeee eeee eee MMe, eee. ee MeMMM8, eee eeeee eeeee eeeeeee. Mee eeeeeee eeeee eee eeeee (Me8MM8) eee eee eeeeeeee eeee eeeeeee eeeee eeeeeee eeeeeeee. Mee eeeeeee eee eeeeeeee eeee eeeeee eee (eeeeeee/MeMMe 8:8). Meeeee eeeeeeeeeeeeee [eeeeeee-eeeee eeeeee eee (M88), MeMM/M8M 8:8] eeeeeeee eee eeeeeee; eeeee: 8.88 e (88.8%).

8-Meeeeeee-8-eeeeeeeeeeeeeeeeeeee-8-eee [‌88‌, M8 = eee-(8-Mee-8-eeeeeeeeeeeeeeeeeeee-8-ee)eeeeee; M8 = (MM8)88Me]; Meeeeee Meeeeeeee:[‌888‌]

M eeee ee eee-(8-eee-8-eeeeeeeeeeeeeeeeeeee-8-ee)eeeeee eeee (88 ee, 888 μeee), eeeeeeeeeee eeee (888 ee, 8.88 eeee), eee Me8M (8.8 ee, 88 μeee) ee MeMM (8.8 eM) eee eeeeeeeeeeee ee ee eeeeeeeee eeee eeee Me eeee eeeeeeeeee (eeeeeee eeeeeee 888 eM·ee−8). Meeee eee eeeeeeee eee eeeeeeee (MMM), eee eeeeeee eee eeeeeee. Mee eeeeeee eee eeeeeeeee ee MeMMe eee eeeeee eeee 8 M ee MeMM. Mee eeeeeee eeeee eee eeeee (Me8MM8) eee eee eeeeeeee eeee eeeeeee eeeee eeeeeee eeeeeeee. Meeee eeeeeeeeeeeeee (eeeeee eee, MeMMe/eeeeee 8:8) eeeeeeee eee eeeeeee; eeeee: 88 ee (88%).

References

Meeeeee Meeeeeeeeee

• 8.Meeeee-Meee, (8888) 8/8, 888.
• 8.Meeeee-Meee, (8888) M 8e, 888.
• 8.Meeeee-Meee, (8888) M 8e, 88.