3- Bromo-l-alkynes

Preparation of 1 -Bromo-l-alkynes from AlkynyUfrhium and Cyanogen Bromide... 

Alkynes are simply brominated in high yields by concentrated aqueous potassium hypobromite at 0-40 °C to yield the corresponding 1-bromo-l-alkynes. 

The ketones can also be prepared from 1-halo-l-alkynes. Dialkylboranes, generated in situ via hydridation of dialkylhaloboranes, hydroborate 1-bromo-l-alkynes to... 

The stereoselective synthesis of the title compounds has been achieved. Thexylborane on reaction with 2 molar equivalents of 1-iodo-l-alkyne at 0 °C proceeds to near completion (88% for 1-iodo-l-hexyne) to form fully substituted organoborane (33), which upon treatment with 2 molar equivalents of sodium methoxide at 0 °C readily produces trans-1,2,3-butatrienes (Eq. 102) 157). The same reaction, however, with either 1-chloro or 1-bromo-l-alkynes is sluggish to form the thexyl-l-halo-l-alkenyl-borane31). 

Alkenylboron compounds couple with the representative organic halides or triflates (Scheme 28). Hexaalkylbenzene was synthesized by sixfold alkenylation (55) of hexabromobenzene followed by catalytic hydrogenation of the double bonds 11521. The reaction of 1-alkenylborane with 1-bromo-l-alkyne stereose-lectively provided ( )-enyne (56) which was then converted into ( ,Z)-hexa-deca-10,12-dienal, a sex pheromone of the melonworm 11531. Due to the difficulty of purification of a geometrical mixture, the stereoselective synthesis is critical for such dienes or trienes. The PGEi derivatives (57) were synthesized... 

Conjugated enynes are of importance for themselves, as well as for the synthesis of conjugated dienes. The cross-coupling reaction of 1-alkenyl(disiamyl)boranes (3c) with 1-bromo-l-alkynes (Scheme 2-34) provides conjugated enynes in high yields [45]. 

Z)-1-Alkenylboronates are prepared by hydroboration of 1-bromo-l-alkynes with dibromoborane/dimethyl sulfide complex, followed by treatment of the intermediate (Z)-(l-bromo-l-alkenyl)boronates with potassium triisopropoxyborohydride. H. C. Brown and T. Imai, Organometallics, 1984, 3, 1392. 

Synthesis of trans-disuhstituted olefins. Brown et al,3 have reported a new stereoselective synthesis of nvws-disubstituted olefins (4) utilizing thexylmonoalkylboranes (1) as intermediates. These are prepared by treating an olefin with thexylborane under N2 at -25° for 1 hr. A 1-bromo-l-alkyne is then added, and after 1 hr. the resulting... 

Alkyl-l-alkynes. Reaction of 1-bromo-l-alkynes (1) with at least 2 eq. of n-butyllithium in hexane gives 3-butyl-1-alkynes (2) in excellent yields. The favored mechanism involves metal-halogen exchange (a) and further reaction with n-butyllithium to give the dUithioalkyne (b), which is then alkylated at the propargylic carbon atom ... 

An alternative route to vinylborinates adds the dimethyl sulfide complex of HBBr2 to 1-bromo-l-alkynes. Reaction of 1-bromo-l-hexyne (52) gave 53 in 87% yield, after treatment with an alcohol (2-propanol in this case).Subsequent reduction of the bromide with potassium triisopropoxyborohydride [KBH(0/-Pr)3] generated cis-vinylborane (54) in 89% yield. 

Hydroboration of terminal alkynes, e.g. 1 -hexyne, 1 -octyne or cyclohexylacetylene, with a dialkylborane, such as bis(l, 2-dimethylpropyl)borane, followed by copper(I)cyanide and copper(II) acetate in HMPA containing a trace of water, gives isomerically pure ( )-l-cyanoalk-l-enes (equation 29)133. Successive treatment of 1-bromo-l-alkynes with dialkylboranes and sodium methoxide results in the borinate esters 208, which are converted into ( )-alkenes of greater than 99% isomeric purity by protonolysis. The action of alkaline hydrogen peroxide on the borinates produces ketones (equation 30)134. 

When n = 4, the yield is close to 80% when n = 3, this is lowered to 40%. Quenching with D2O instead of H2O leads to incorporation of D instead of H attached to the double bond. The complexation process also activates the acetylenic bond. Formation of a palladium complex has been used to react a y-acetylenic carboxylate with a 1-bromo-l-alkyne to give an ynenol lactone (70). Palladium has also been used to cyclize a zinc substituted alkyne. ... 

B.iv.c. trans-a,pSubstituted Alkenylmetals. The accessibility of trani-a,/3-substituted alkenyhnetals is still rather limited, and only a few examples of this type of crosscoupling reactions have been reported (Table 6). Notably, trtmi-a,/3-substituted alkenylboronates can be prepared by hydroboration of 1-bromo-l-alkynes followed by subsequent treatments with organolithiums and bases, a procedure similar to that for the preparation of (Z)-/3-substituted alkenylboron compounds.t " Et The Pd-catalyzed crosscoupling reaction of the resultant organoboranes with aryl halides proceeds with complete retention of alkenyl stereochemistry to give trisubstituted alkenesP (Scheme 33). 

Reaction of 1-bromo-l-alkynes with alkoxide ion allows the generation of vinyli-dene carbencs (Scheme 5). ... 

Hydroboration of 1-bromo-l-alkynes with chloro(thexyl)-borane leads to the synthesis of alkynyl ketones in 61-63% yields by the sequence of reactions shown in eq 5. Sequential treatment of alkenylchloro(thexyl)boranes, which are formed by the reaction of chloro(thexyl)borane with alkynes, with lithium chloro-propargylide and aldehydes affords 1,3-enynols or 1,2,4-trienols depending on the reaction conditions. ... 

The reaction of potassium salts of 4-pentynoic acids—prepared from the add and KOBu-t or KH—with 1-bromo-l-alkynes affords 6-( )-alkylidene-y-butyrolactonest (Scheme 21). Lithium and sodium carboxylates have proved unsuccessful. Addition of potassium bromide, use of DMSO, and excess 4-pentynoic acid have been found to increase the yield, whereas 1-iodo-l-alkynes lead to the formation of the expected lactone along with major amounts of S-iodo- y methylene-y-butyrolactone. Tri(o-tolyl)phosphine and tri(2-furyl)phosphine can promote an effident transformation. The latter, however, has been found to give the best results. 

Dialkylvinyl bromides have been prepared stereoselectively from 1-bromo-l-alkynes by hydroboration with dialkylboranes as shown in Scheme43. ... 

Negishi and coworkers have developed a method for preparation of conjugated -enynes via alkenylboranes. In one synthesis (227), acetylene (50i) was treated with disiamylborane and then with the lithium salt of acetylene (503) to afford complex (504), which after treatment with iodine and sodium hydroxide stereoselectively afforded (505) (Scheme 88). Syntheses of other conjugated E,Z dienes have been achieved by this method (228, 229). By use of thexylborane and 1-bromo-l-alkynes, conjugated E,E dienes can be readily prepared (230) (Scheme 89). 

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