Bromopropenes, reaction

Unlike the acid-catalyzed ether cleavage reaction discussed in the previous section, which is general to all ethers, the Claisen rearrangement is specific to allyl aryl ethers, Ar—O—CH2CH = CH2. Treatment of a phenoxide ion with 3-bromopropene (allyl bromide) results in a Williamson ether synthesis and formation of an allyl aryl ether. Heating the allyl aryl ether to 200 to 250 °C then effects Claisen rearrangement, leading to an o-allylphenol. The net result is alkylation of the phenol in an ortho position. 

Similar reactions were also carried out with 3-bromopropene, l-bromo-2-butene, 3-bromocy-clohcxcnc. and 3-bromocyclooctene in refluxing ethanol as solvent24 . Zinc-mediated Barbier reactions can be accomplished with high yields in a mixture of saturated aqueous NH4C1/THF (5 1), at room temperature or below using a 1.2 to 2-fold excess of the halide over the carbonyl compound25. 

Change of base (pyridine, aniline, water) had no obvious effect on the steric course of the reaction (59). Rather surprisingly, it appears that the reactions of the cis and trans isomers of 1-bromopropene with the hydride [Co(CN)5H] give different products this eliminates an initial addition of Co—H to the double bond, since this would lead to the same product for the two isomers 105). Vinyl chloride apparently reacts only with [Co salen] , and not with the hydride, to form the CH2=CHCo complex 43) the same may be the case with the BAE complexes 40). 

Entry 9 uses the oxaborazolidine catalysts discussed on p. 505 with 2-bromopropenal as the dienophile. The aldehyde adopts the exo position in each case, which is consistent with the proposed TS model. Entry 10 illustrates the use of a cationic oxaborazolidine catalyst. The chirality is derived from trans-1,2-diaminocyclohcxanc. Entry 12 shows the use of a TADDOL catalyst in the construction of the steroid skeleton. Entry 13 is an intramolecular D-A reaction catalyzed by a Cu-Ws-oxazoline. Entries 14 and 15 show the use of the oxazaborolidinone catalyst with more complex dienes. 

In solution, products of central and terminal Br addition to propadiene (la) are formed (Scheme 11.3) [13, 37]. The latter are promoted by high reactant ratios [HBr] [C3H4] and low reaction temperatures. Under conditions of kinetic control, the reaction between diene la and HBr furnishes a 67 33 ratio of allyl bromide 4a versus 2-bromopropene 5a. These investigations also revealed that a-addition of Br is reversible, but the /3-addition is not. The reversible addition to Q has been used to explain the preference for allyl bromide formation from substrate la and H Br at low temperatures, since the Br loss profits from elevated temperatures. 

Reaction of the chiral lithium enolate of meso-2,6-dimethylcyclohexanone (6), generated by deprotonation with (R)-l-phenylethylamine and (/ )-camphor/(R)-l-phenylethylaniine derived chiral lithium amides (Table 1, entries 17 and 64) with 3-bromopropene, leads to homoallyl ketones of opposite absolute configuration in acceptable yield with poor to modest enantiomeric excess14, which can be determined directly by H-NMR spectroscopy in the presence of tris [3-(heptafluorohydroxymethylene)-D-camphorato]europium(III) [Eu(hfc)3]. 

The reaction of 1-phenylethyl-, 2-octyl-, and 2-butyl-magnesium chloride (36a, b, c) with vinyl bromide (37a), (E)-p-bromostyrene (37b), 2-bromopropene (37c), and bromobenzene (37d) was carried out in the presence of 0.5 mol- % of a nickel catalyst prepared in situ from nickel chloride and the chiral ligand (35). 

In 1971, Tamura and Kochi described the reaction of aUcyl Grignard reagents with alkenyl bromides in the presence of iron(III) chloride in THF. Only very reactive snbstrates such as vinyl and propenyl bromide were nsed (Scheme 16). Yields of conpling product are moderate to good but, unfortunately, a large excess of alkenyl bromide is required (3 to 9 equivalents). ( )-Bromopropene reacts 15 times faster than the (Z)-isomer. It should be noted that the reaction is stereoselective. 

The principles of radical addition reactions of alkenes appear to apply equally to alkynes, although there are fewer documented examples of radical additions to triple bonds. Two molecules of hydrogen bromide can add to propyne first to give cis-1 -bromopropene (by antarafacial addition) and then 1,2-dibromopropane ... 

Compound X, of formula C3H5Br3, with methyllithium formed bromocyclopro-pane and 3-bromopropene. The nmr spectrum of X showed a one-proton triplet at 5.9 ppm, a two-proton triplet at 3.55 ppm, and a complex resonance centered at 2.5 ppm downfield from TMS. What is the structure of X Account for the products observed in its reaction with methyllithium. 

Scheme 3).[21] The reaction of 3-bromopropene with catecholborane at 100 °C gives the 3-bromopropylboronate 11, the first intermediate in one of the pathways for the preparation of boroArg peptidesJ14 3-Bromopropylboronate 11 is readily transesterified with pinacol or... 

The reactions of tertiary allylic amines with vinylic halides are related closely to the allylic alcohol reactions since enamines are often major products. We have just begun work in this area and have few results to report yet. We have seen some significant differences in the products formed from tertiary allylic amines and from the related allylic alcohols. A typical example is the reaction of 2-bromopropene with N-allyl piperidine and piperdine where a 42% yield of a single enamine is obtained (6). The related reaction with allyl alcohol gives a mixture of regioisomeric enamines. 

In addition to alkylation of enamines with a-halocarbonyl compounds66,75,76 (Scheme 10), 1,4-dicarbonyl compounds and derivatives may also be prepared by propenylation of enamines followed by ozonolysis77, propynylation followed by hydration54d,7S and reaction with ketone thioacetal monoxide79, diethyl 3-bromopropene phosphonate80 and dichloroacetaldehyde oxime81 (Scheme 21). 

Dialkylated products included. Reaction with 3-bromopropene. => At 43 C. 

The Mechanism of the cross coupling reaction can be accommodated by an oxidative addition of 1-bromopropene to iron(l) followed by exchange with ethylmagnesium bromide and reductive elimination. Scheme 3 is intended to form a basis for discussion and further study of the catalytic mechanism. In order to maintain the stereospecificity, the oxidative addition of bromo-propene in step a should occur with retention. Similar stereochemistry has been observed in oxidative additions of platinum(O) and nickel(O) complexes.(32)(33) The metathesis of the iron(lll) intermediate in step b is ixp icted to be rapid in analogy with other alkylations.(34) The formation of a new carbon-carbon bond by the redilcTive elimination of a pair of carbon-centered ligands in step c has been demonstrated to occur... 

The reaction appears to be stereoselective because the reaction of methyimagnesium bromide with cis and /ran. -1-bromopropene affords only ds- and fran. -butene-2, respectively. 

The main products of the reaction are hydrogen bromide, propene and benzene, with smaller amounts of 1-bromopropene and 2-bromopropane. The mechanism requires the overall activation energy to be equal to that for the first step. While these two energies are very close for allyl bromide (estimates are 45.5 and 47.5 kcal.mole for reaction 1, and 45.4 kcal.mole" for the overall activation energy ), the C-Br bond dissociation energy of 65 kcal.mole for ethyl bromide is considerably greater than the overall activation energy of 53 kcal. 

A halogen-lithium exchange in the reaction of BuLi and 2-bromopropene (41) affords 2-lithiopropene (42) which then adds to the carbonyl double bond. After aqueous workup, alcohol 22 is obtained as a mixture of isomers. 

In the second step, aldehyde 50 is subjected to a Grignard reaction with isoprenyl Grignard reagent 53 which can be readily obtained from 2-bromopropene (51) via halogen-metal exchange using tert-butyllithium and subsequent transmetallation with magnesium bromide. 

---