O-Isotoluenes

To promote Peterson olefination, the use of KH results in the formation of o-isotoluene derivatives [2d, 7], On the other hand, treatment of allenic alcohols with concentrated sulfuric acid produces the diene-allenes. The preference for Z geometry of the methyl-substituted a C=C in silylated allenic alcohol is attributed to the allylic methyl group at the C-1 position, favoring the axial position in the chair-like transition state, thus avoiding the severe steric interaction between the equatorial allylic methyl and the equatorial rigid bicyclic ligand on the boron. 

Similarly, condensation of allylborane formed with bicyclohexylborane with 4-methyl-2,3-pentadienal leads to o-isotoluene and the corresponding diene-al-lene with E geometry (>98%) of the a C=C double bond (Scheme 24.19) [4]. 

Conjugated systems, including 1,3-butadienes, diene-allenes, enyne-allenes, enediynes, dienediynes, and enediallenes, were synthesized via organoboranes. A wide array of cascade cyclization reactions involving high energy intermediates, such as o-isotoluenes, biradicals, o-quinodimethanes, benzocyclobutadienes, and enyne-ketenes, were initiated from these unsaturated compounds. 

Hydroboration of 1-hexyne with dicyclohexylborane produced the alkenylborane 55, which on treatment with the lithium acetylide 56 furnished the organoborate complex 57 (Scheme 14) (24). Trimethyltin chloride then induced a stereoselective migration of the 1-hexenyl group to the adjacent acetylenic carbon to afford the diene-allene 58 and subsequently the o-isotoluenes 59 and 60. A variety of other o-isotoluenes were likewise synthesized. 

Tetrachloro-o-benzoquinone adds to allene to give the a-diketone (14) which after irradiation yields tetrachloro-isotoluene... 

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