Intermolecular allylboration

Intermolecular Allylboration. A tandem aza[4+2] cycloaddition/allyl-boration three-component reaction has been designed based on the prece-dented carbocyclic [4- -2] cycloaddition/allylboration and a snbsequent one-pot variant. Thns, the thermal reaction between hydrazonobutadienes 138, A-substitnted maleimides, and aldehydes provides polysnbstituted a-hydroxy-alkylpiperidines 141 via the cyclic allylboronate intermediate 139 and the proposed chairlike transition stmctnre 140 (Eq. 103). Monoactivated dienophiles like acrylates fail to react with heterodienes 138 bnt the scope of aldehydes is very broad both ahphatic and aromatic aldehydes are snitable, inclnding electron-rich ones. An inverse electron-demand variant to access the corresponding dihy-dropyran derivatives via the intermediacy of enantiomerically enriched pyranyl allylic boronate 76 has been snbsequently developed (see Eq. 64). ° ... 

Intermolecular allylboration of aldehydes proceeds with high simple diastereoselectivity to give either anti- or syn-B-methyl homoallylic alcohols 5 and 6. (J)... 

Wrackmeyer et al. obtained ( )-3,4-diallyl-2-(but-3-enylidene)-l,l,5-trimethyl-2,3-dihydro-l/7-l,3-silaborole 28 selectively by the reaction of dimethyldi(prop-l-ynyl)silane 63 with triallylborane in CDCI3 at room temperature for several days (Equation 7) <2002JOM146>. The reaction was monitored by H and Si NMR spectroscopy and is believed to take place via an intermolecular 1,1-allylboration followed by an intramolecular 1,2-allylboration. 

Among the possible intramolecular allylmetallation reactions the intermolecular allylstannation (2) has been most widely studied by the group of Y. Yamamoto. (/) These reactions can be carried out under Lewis acid catalysis to proceed via transition states of the type 3, thermally via transition states of the type 4, and under Broensted acid catalysis, a reaction for which the nature of the transition state is not yet clear. In these reactions high to very high stereoselectivity has been attained. But there is not yet a uniform solution to reach predictable stereoselectivity in terms of attaining each of a number of possible stereoisomers selectively. It is for this reason that we wanted to evaluate the scope of the intramolecular allylboration reaction as a route to substituted tetrahydropyrans and piperidines. 

The intramolecular allylboration step was very slow in DMSO, which is the usual solvent for these borylations of allylic acetates (Section 6.2.1.6). Alternatively, a non-co-ordinating solvent like toluene was more suitable for the overall process, provided an arsine or phosphine ligand is added to stabilize the active Pd(0) species during the borylation. With cyclic ketones such as 109 the intramolecular allylation provided cis-fused bicyclic products (110), in agreement with the involvement of the usual chairlike transition state (Equation 55). An intermolecular tandem borylation/allylation was also reported [59]. 

Intermolecular C kinetic isotope effects (KEs) determined for Rouse allylboration of p-anisaldehyde (Scheme 22) are indicative of a rate-limiting cyclic ttansition state... 

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