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DOI: 10.1055/sos-SD-027-00847

Schobert, R.; Gordon, G. J.Science of Synthesis, (200427986.

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 8095%). 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 CoreyFuchs 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).

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