Science of Synthesis

Schobert, R.; Gordon, G. J., Science of Synthesis, (2004) 27, 986.

Corey and Fuchs have devised an efficient two-step process for the conversion of aldehydes into monosubstituted alkynes by chain extension via the corresponding geminal dibromoalkenes ‌29‌ (‌Scheme 10‌).[‌130‌] The latter are readily obtained by addition of the aldehyde to a mixture of carbon tetrabromide and either 2 equivalents of triphenylphosphine or 1 equivalent each of triphenylphosphine and zinc dust. In either case, (dibromomethylene)triphenylphosphorane (‌24‌, X = Br) is formed in situ and this is used to alkenate the aldehyde. The second variant is preferred, since less phosphine is required, the isolation procedure is simpler; moreover, the yields of dibromoalkenes are somewhat higher (range 80–95%). A major shortcoming is the long reaction time necessary for the complete formation of the dibrominated ylides; however, a modification has been described[‌131‌] employing potassium tert-butoxide/tribromomethane in toluene at −20°C, which generates dibromocarbene in the presence of triphenylphosphine. Upon the addition of an aldehyde this mixture produces the corresponding geminal dibromoalkenes typically within minutes. The second step of the Corey–Fuchs sequence (the treatment of the dibromoalkenes ‌29‌ with 2 equivalents of butyllithium) may give initially a vinylidene lithium carbenoid that rapidly undergoes an E2cb elimination of hydrogen bromide to yield the lithium acetylide.[‌132‌] The last compound is normally quenched with water giving rise to the respective terminal alkyne ‌30‌, but it can also be alkylated, acylated, halogenated,[‌133‌] or transmetalated;[‌134‌] typical products then include halides ‌31‌[‌133‌] and acids ‌32‌[‌130‌] (‌Scheme 10‌).

‌Meeeee 88‌ Meeee–Meeee Meeeeeeee ee Meeeeee eeee Meeeeeeee[‌888‌,‌888‌,‌888‌]

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

M eeeeeee ee Me8M (8.88 e, 8.8 eeee), Me eeee (8.88 e, 8.8 eeee), eee MMe8 (8.88 e, 8.8 eeee) ee eeeee MM8Me8 (88 eM) eee eeeeeee ee ee eeeee eeeeeeeee ee eee eee eeeeeeee eee 88–88 e eee eeee eeeeeee eeee eeeeeeee (8.88 e, 8.8 eeee). Mee eeeeeee eee eeeeeee eee e eeeeeee 8–8 e ee eeee eeeeeeeeeee, eee eeee ee eee eeeeeee eeee eeeeeee (888 eM) eee eeeeeeee ee eeeeee eeeeeeeee eeeeeeee. Mee eeeeee eeee eee eeeeeeeeee eeeeeeee ee eeeeeeeeee eeeeee ee MM8Me8 eeeeeeeeee, eee eeeeeee eeeeeeeeeeeee ee eeeeee eee ee eee eeeeeee eeeeeee. Mee eeeeeeee eeeeeeeee eeee eeeeeeeeee ee eeeee eeeeeeeeeee eeee 8,8-eeeeeeeeee-8-eee; eeeee: 8.8 e (88%). M eeee ee eeee eeeeeee (888 ee, 8.8 eeee) ee eeeee MMM (88 eM) ee −88°M eeeee M8 eee eeeeeee eeee 8.88 M MeMe ee eeeeeee (8.8 eM, 8.8 eeee). Meeee eeeeeeee eee 8 e ee −88°M, eee eeeeeee eee eeeeee ee 88°M eee eeeeeeeeee eee 8 e ee eeee eeeeeeeeeee. Meeeeeee ee M8M, eeeeeeeeee eeee eeeeeee, eee eeeeeeeeeeee eeeeeeee eee-8-eee; eeeee: 888 ee (88%); ee 88°M/88 Meee.

References

Meeeeee Meeeeeeeeee

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