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43.1.1.1.17 Method 17: Buta-1,3-diyne Formation from Carbenes and Carbenoids (Fritsch–Buttenberg–Wiechell Rearrangement)

DOI: 10.1055/sos-SD-043-00002

Hirsch, A.; Vostrowsky, O.Science of Synthesis, (20084377.

The rearrangement of a carbene or a carbenoid intermediate to form an acetylene, known as the FritschButtenbergWiechell rearrangement (see also Sections 43.8.1.2.2 and 43.8.2.1.5),[‌268‌‌270‌] is highly suitable for the formation of polyynes and polyyne frameworks within highly conjugated organic materials.[‌271‌] Thus, treating a 2,2-disubstituted 1,1-dibromoethene with butyllithium under mild conditions generates a carbene or carbenoid, which subsequently rearranges by a 1,2-migration of one of its substituents into a 1,2-disubstituted alkyne. Mixed rearrangements, in which one of the migrating groups is an alkyne and the other a vinyl, aryl, or hetaryl group, offer a convenient synthesis route to conjugated alka-1,3-diynes 123 (Table 8). Here, the dibromoethenecarbene precursors 122 are best prepared through the initial formation of an alkynyl ketone, followed by dibromoalkenation under CoreyFuchs conditions.[‌272‌] The corresponding ketones can be obtained by three different routes, namely Grignard type reactions, FriedelCrafts acylations, and CoreyFuchsWittig alkenations.[‌272‌] The success of the FritschButtenbergWiechell rearrangement depends on the choice of solvent, and apolar hydrocarbon solvents such as hexanes, toluene, or benzene work quite well. The FritschButtenbergWiechell rearrangement can be applied to the formation of silyl-, alkyl-, alkenyl-, and arylated polyynes, including di-, tri-, and tetrayne products, as well as the construction of two-dimensional carbon-rich molecules.[‌271‌] A series of dibromoalkenes are converted into alka-1,3-diynes by treatment with butyllithium in dry hexanes at low temperature, and typically such reactions are over by the time the mixture has been allowed to warm to 40°C (Table 8, entries 111).[‌273‌,‌274‌] Among the rearrangement products (entry 11) is the dienediyne atractylodin that may be isolated from medicinal plants of the Atractylodes genus.[‌274‌] When 4,4-dibromo-3-phenylbut-3-en-1-yne is treated with butyllithium (2.2 equiv) in toluene/hexane the lithium salt of phenylbuta-1,3-diyne is obtained and this compound is reacted in situ with electrophiles thereby forming unsymmetrically functionalized buta-1,3-diynes. Three examples using benzaldehydes as the electrophiles are included in Table 8 (entries 1214).[‌275‌] Additionally, the lithium acetylide intermediates may be transmetalated to provide zinc or tin derivatives and these are subsequently transformed into polyynes or arylated ynones via palladium-catalyzed cross-coupling reactions.[‌275‌]

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