Allenyl bromide

Because of this large rate factor in favor of the solvolysis of an allenyl system, it was of interest to examine the solvolytic behavior of the unsubstituted parent allenyl halide. Stang and co-workers (205) have observed good pseudo-first-order rates for the solvolysis of allenyl bromide, 244, R) = = Rj = H,... 

A summary of the solvolysis of allenyl and related substrates is given in Table XIX. As the data in this table indicate, the rates of solvolysis of allenyl bromide and 1-bromomethylenecyclopropane, 247a, are very comparable. Their rate of solvolysis, however, is about 10 slower than the rate of solvolysis of triphenylchloroallene under similar conditions. This rate difference between the parent and triphenyl allenyl system is not unlike the rate difference in the solvolysis of (C6Hs)3CX and CH3X. 

Allenyl bromide accepts the nucleophilic attack of amines or potassium thio-acetate leading to propargylic amines or thioesters via the alkenylidenecarbene 132 or zwitterion intermediate 133 [66],... 

In the early 1980s, one of the first preparations of substituted allenes was reported, which employed a palladium-catalyzed cross-coupling reaction of allenyl halides [9]. In this study, allenyl bromides 13 and various Grignard reagents 14 were coupled in the presence of catalytic amounts of a Pd(0) species, generated in situ by reduction of a Pd(II) salt. Trisubstituted allenes 15 were obtained with high regioselectivity (allene 15 alkyne 16 = 90 10 to 99 1) (Scheme 14.5). 

A convenient route for the preparation of yne-allenes was recently described by Saalfrank et al. [19]. Products 29a/b were formed by Stille cross-coupling of allenyl bromides 27a/b with alkynylstannanes such as 28 (Scheme 14.9). Allenyl phospho-nates such as 30 were also suitable substrates in palladium-catalyzed couplings with propargylstannane 31 (Eq. 14.1). Bisstannylated acetylene 33 as alkyne component furnished the expected yne-bisallene 34 in reasonable yield, but without any diaster-eoselectivity (meso-34 (R,R)-/(S,S)-34 =50 50) (Eq. 14.2). 

The dehydrogenation of bis(a-bromobenzyl) sulfide (41) with Fe2(CO)9 in the presence of furan affords (43). The iron-stabilized sulfur ylide (42) has been proposed as intermediate. Reaction of C -di-phenylnitrone (44) with l,2-bis(methylene)-3,3.4,4,5,5-hexamethylcyclopentane (45) in benzene at 80 C gives a [4 -I- 3] cycloadduct (46 18%), along with some other products. Landor et al. have shown that the dehydrobromination of allenyl bromide (47) in the presence of furan affords bicyclic ketone (49) in 9% yield. Apparently, a strain alkyne intermediate reacts with r-butyl alcohol to afford an enol ether (48), which is then converted to (49) (Scheme 11). 

Useful structures arise from regioselective coupling of alkenylstannanes with benzylic bromides, that of alkynylstannanes with allenyl bromides, and hydrosilanes with alkenyl iodides. ... 

Despite being a less obvious starting material than a l,3-butadiene-2-yl coupling partner, l,2-butadien-4-yl precursors (such as 166 in Suzuki s pioneering example in Scheme 1.26) have seen the most use in dendralene synthesis [118, 131-136]. A couple of more recent examples include the palladium-catalyzed cross-coupling reaction of alkenyl bromides 179 with, for example, the organoindium derived from allenyl bromide 181, or 1,1-dimethyl allene (183) (via a Mizoroki-Heckreaction) (Scheme 1.28) [132,135]. Palladium(0)-catalyzed dimerizations or homocouplings can also furnish the C2-C3 bond [138-142], as can nickel(O)- [143,144] and rhodium(I)-catalyzed ones [137]. 

In the synthesis of the allenyl bromide moiety of panacene 65 (isolated from Aplysia brasiliana ) and isolaurallene 38, a Sn2 strategy was applied (Scheme 42.20). LiCuBra was used to provide the nucleophilic Br source. For instance, alkynyl O-mesylate 64 was converted into the... 

Silverstein followed an older procedure that results in rearrangement of a secondary allylic alcohol to a terminal bromide (eq 6). In a similar fashion, the conversion of propargylic alcohols to allenyl bromides has been noted (eq 7). ... 

---