Dibromides geminal

Identify the reagents you would use to convert 1-pentene into a geminal dibromide ("geminal" indicates that both bromine atoms are connected to the same carbon atom). 

Geminal (Section 19.5) Referring to two groups attached to the same carbon atom. For example, 1,1-dibromo-propane is a geminal dibromide. 

Benzyl- und Allyl-halogenide werden durch Lithiumalanat/Chrom(III)-, Vanadium(III)-, Titan(III)- und Wolfram(VI)-chlorid-Systeme zu substituicrten Athanen, geminate Dihalogenide zu substituierten Athylenen gekoppelt, vicinale Dibromide zu Alkenen debromiert8. 

During the synthesis of [2.2]paracyclophanediene, Dewhirst and Cram 83> prepared the bis-geminal dibromides 4a and 4b, which were subsequently converted into the diketones 5 a and 5 b. The UV spectra of the bromides 4a and 4 b show maxima at 236 nm. A comparison with the Amax values of other [2.2]paracyclophanes brominated at the bridges suggests that the absorption at 236 nm is the 235 nm band of [2.2]para-cyclophane, shifted bathochromically by the inductive effect of the... 

Methylenedithiol was used to construct cyclic meat flavor compounds, such as l,3-dithian-5-one 217 <1998FFJ177>. The reaction of the geminal dithiol with a 1,3-dibromide proceeds with pyridine as base in 44% yield (Equation 81). 

There are numerous examples of the addition of HBr to alkynes.10 Addition to acetylene is difficult, tends to produce mixtures of bromide products, and requires a catalyst.94,10 1 22 Simple terminal alkynes react with HBr in the absence of peroxides to produce mixtures of 2-bromo-l -alkenes and geminal dibromides (equation 110).85,159 The latter can be produced in high yield when excess HBr is used. 60... 

Attempts to brominate the free amine (148) were disappointing (Scheme 3.28). Either incomplete reaction was observed or else a mixture of mono-, di- and tribrominated compounds was obtained. Nonetheless, treatment of this crude mixture with p - am i n o b en z oy lglutam ic acid generated the amide (149), which presumably resulted from bromide displacement in the tribromomethyl precursor, followed by hydrolysis of the remaining geminal dibromide during work-up. 

Another common method for the conversion of an aldehyde into an alkyne is the transformation of the aldehyde via a Corey-Fuchs reaction into a geminal vinyl dibromide followed by reaction with nbutyllithium and aqueous work-up.22... 

Subsequent exposure of geminal dibromide 62 to BU3S11H provided the desired ester 63 via a stereocontrolled cyclization-debromination tandem sequence. Finally, Barbier-Wieland degradation and hydrogenolytic debenzylation of ester 63 afforded 5a-carba-D-fructofuranose 64 in excellent yields. In a preliminary enzymological study, its 6-phosphate derivative showed to be an interesting substrate for the enzymes of the glycolysis pathway. 

Finally, a key method for the preparation of allenes is the dehalogenation of cyclopropane geminal-dibromides using methyllithium. This procedure has been reviewed previously341 and a recent series of papers concerning the synthesis of branched triangu-lanes have used this methodology (equation 39)343,343. 

A cyclopropane ring fused to a further ring as in 245 has proved to be a particularly apposite substrate for the detection of the so-called halolithiocarbenoids such as 246 (this compound is formed from the geminal dibromide with n-butyl lithium). At — 100°C in... 

Pd - and PT-catalyzed carbonylation of geminal vinylic dibromides gives diynes or dicarboxylic acids using benzene or t-amyl alcohol as a solvent, respectively (eq. (10)) [135]. 

Double substitution of geminal dibromides is best accomplished by treatment with an organocuprate, followed by the addition of an alkyl iodide to the crude reaction mixture. " The reaction has found especial application for the synthesis of dimethylcyclopropanes, ubiquitous in natural product chemistry. Excellent yields are often obtained with Me2CuLi, but with less reactive substrates higher order cyano-and thiocyano-cuprates sometimes provide better results. Spiro dialkylation of geminal dibromides has also been accomplished. Vicinal dibromides, on the other hand, afford alkenes upon treatment with dialkyl cuprates. ... 

A geminal dibromide can be prepared from an alkyne. 3-Hexyne is the desired alkyne because it will form one geminal bromide, whereas 2-hexyne would form two different geminal dibromides. 3-Hexyne can be prepared from 1-butyne and ethyl bromide, and 1-butyne can be prepared from ethyne and ethyl bromide. 

Geminal dimethyl substituents affect the yields of macrocyclic products obtained by alkylation of diynes by dibromides. For example, 5,5-dimethylnona-l,8-diyne... 

Coupling of trimethylchlorosilane and geminal dibromides gave a mixture of products [104] ... 

The reaction of geminal dibromides and carbonyl compounds in the presence of butyl-lithium or lithium metal can provide a general route to a wide range of oxirans. Stevens and Pillai have used a Darzens cyclization reaction to form the first example of an isolable epoxy-amine (121). 2-(l-Aziridinyl)-2-phenyl-l-oxaspiro[2,5]octane (121) was relatively stable and rearranged to the corresponding cycloheptanone (122) only after... 

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