Alkynes bromoboration

Haloboration In view of the brief discussion of the current scope and limitations of alkyne carbometallation presented above, recent development of the alkyne bromoboration-Pd-catalyzed alkenylation protocol, especially those involving the use of propyne [56a] and arylethynes [56b] highlighted earlier and summarized in... 

Some of the basic results of the alkyne bromoboration-Pd-catalyzed alkeny-lation protocol were presented in Scheme 3.8. Its applications to some natural products syntheses are shown in Scheme 3.29. 

Trisubstituted alkenes. The (Z)-2-bromo-l-alkenylboranes (1), obtained by bromoboration of 1-alkynes with BBr, (13, 43), undergo coupling with organozinc chlorides in the presence of Cl2Pd[P(C6H5)3]2 to provide, after protonolysis, disub-stituted alkenes (3). However, the intermediate alkenylborane (2) can undergo a... 

Bromoboration (Z)-/>2-dihalo-l -alkenes1 Reaction of a terminal alkyne (1 -octyne) with BBr3 in CH2C12 results in a p-bromoalkenylborane, which undergoes brominolysis or iodinolysis with retention to provide (Z)-l,2-dihalo-l-alkenes (equation I). 

The use of alkenyl boronic acid derivatives 50, which are readily prepared via hydroboration or bromoboration of alkynes, affords the corresponding p,y-unsaturated amino acids (e.g. 52-57) in a geometrically pure form [34], A variety of amines 48, including primary and secondary amines, anilines, amino alcohols and hydroxylamines can effectively participate in this process, while the alkenyl boronic acid can contain alkyl, aryl or bromo-substituents. Although the alkenyl amino acid side chain is introduced through the boronic acid component, the use of more substituted a-keto acids 49 allows the simultaneous incorporation of an additional a-substituent (e.g. 57). 

In an attempt to synthesize (Z)-l,2-dihalo-l-alkenes from bromoboration adducts of 1-alkynes with B-X-9-BBN, all efforts have been unsuccessful. Fortunately, it has found that the haloboration of 1-alkynes with tribromoborane, followed by the reaction with iodine or bromine chloride in the presence of sodium acetate gives the expected (Z)-l,2-dihalo- 1-alkenes stereospecifically (>98%) in good yields (Eq. 122)187>. 

Bromoboration of terminal alkynes 4 into the /J-bromo-l-alkenyl-boronic esters 5 [26], followed by a palladium-catalyzed displacement of the /J-halogen with organozinc reagents [27] is a known strategy for the preparation of metalated diene 6 (Scheme 3). An additional approach is the diboration of symmetrically disubstituted alkynes 7 with bis(pinacolato)diboron 8, followed... 

Temiinal 2,2-diorgano-1-alkenylboronates (9) are prepared by bromoboration of a terminal alkyne to the /5-bromo-l-alkenylboronic ester (8) [22], followed by palladium-catalyzed displacement of the /5-halogen with organozinc reagents, which proceeds strictly with retention of configuration [23] (Scheme 2-4). 

Haloboranes add to terminal alkynes in a cij-Markovnikov manner however, the bromoboration of acetylene itself exceptionally provides a rrans-adduct which gives the corresponding ( )-l-alkenylborates (10) by reaction with organozinc halides [24] (Scheme... 

Terminal 2-bromo-l-alkenylboronates are readily obtained by bromoboration of terminal alkynes with tribromoborane. The subsequent displacement of the bromine atom with organozinc reagents proceeds w ith strict retention of configuration in the presence of a palladium catalyst [27] (eq (16)). 

Formal carboboration of a terminal alkyne could be achieved by bromoboration and subsequent palladium-catalysed substitution of alkyl for bromine with alkylzinc chloride. 

Bromoboration of alkynes facile and broadly applicable A wide range of Type VII and VIII alkenyl derivatives are accessible... 

As mostly discussed earlier. Type III alkenyl derivatives, that is, ( )-R CH= CHM(or X), are widely and satisfactorily generated by (i) alkyne hydrometallation (M = B, Zr or, in some cases, Al, etc.) (Table 3.2, Scheme 3.6), (ii) polar halogenation reactions ofalkynes (Eqs. (1), (2), and (7), Scheme 3.15), and additionally, (iii) anti bromoboration of ethyne [53] followed by Negishi coupling (Eq. (1), Scheme 3.12). On the other hand. Type IV alkenyl derivatives may be prepared by (i) Normant alkylcupration of ethyne [67, 68] (Eqs. (5) and (6), Scheme 3.11), (ii) Zr-catalyzed alkylalumination of ethyne, (iii) syn hydroboration of 1-halo-l-alkynes followed by hydride-induced inversion of configuration [59] (Scheme 3.9), (iv) hydroboration of 1-alkynes followed by brominolysis (but not iodinolysis) with inversion [95], and (v) syn hydrozirconation or syn hydroalumination of 1-boryl- or 1-silyl-l-alkynes followed by protonolysis of the C-Al or C-Zr bond [96-98]. 

Hyuga, S., Takinami, S., Hara, S., Suzuki, A. 1986. Organic synthesis using haloboration reaction. Part 9. A direct and selective synthesis of ( Z)-l-bromo-l,3-dienes and ( ,Z)-1,3-dienes by the hydrobora-tion-bromoboration sequence of two alkynes. Tetrahedron Lett. 27 977-980. 

SCHEME 7.112 Synthesis of vinyl iodides and bromides based upon initial bromoboration of an alkyne [177],... 

Lewis acid used for deprotection of OH and NH groups cleaves ethers or esters to alkyl bromides bromoborates allene and alkynes)... 

Bromoboration Reactions. BBrj does not add to isolated double bonds, but reacts with allene spontaneously even at low temperature to give (2-bromoallyl)dibromoborane, which provides stable (2-bromoallyl)diphenoxyborane by the addition of anisole. The diphenoxyborane derivative reacts with carbonyl compounds to give 2-bromohomoallylic alcohols in high yields (eq 12). Bromoboration of 1-alkynes provides (Z)-(2-bromo-l-alkenyl)dibromoboranes stereo- and re-gioselectively (eq 13), which are applied for the synthesis of trisubstituted alkenes, a,p-unsaturated esters, and 7,8-unsaturated ketones, bromodienes, 1,2-dihalo-l-alkenes, 2-bromoalkanals, and (3-bromo-oi,3-unsaturated amides. ... 

---