The Intricate Assembling of gem‐Diphenylpropargylic Units

Abstract

While optimized procedures for selective propargylic – versus allenic – attack (in particular by alkynylsilanes) have proven to be compatible with many substitution patterns at the propargylic center, the case of diarylpropargyl electrophiles has remained problematic. The intrinsic reactivity of 1,1‐diphenylpropargylic alcohols [R–C≡C–C(Ph2)OH (R = TMS, H, Me)] in the presence of various acids (and in the absence of additional nucleophile) has thus been systematically investigated. Whereas the monophenyl analogues [R–C≡C–CH(Ph)OH] afford the expected bis(phenylpropargyl) ethers, the diphenyl versions undergo complex but quite selective processes to afford various structural types: depending on the acid used, a diallene, an allenyne, an indenylallene, an indanone or a condensed tetra‐ and pentacycles were obtained. When the reactions were conducted in the presence of an alkynylsilane capable of playing the role of a competing nucleophile, the expected propargylic substitution products – dialkynyldiphenylmethanes or their isomers – were never observed. The hitherto unknown simple hydrocarbons diethynyl‐ and dipropynyl‐diphenylmethane could, however, be obtained in low yields through a four‐step sequence involving allenylidene‐ and alkynylruthenium intermediates.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Reactions of arylacetylenic compounds with arenes in the presence of aluminum halides

“…Analogous conclusions were drawn previously [24][25][26] XXb ( XXf (40) XXl (12) dual reactivity of ions XIII ↔ XIV were reported in [23,27]. Some results of quantum-chemical calculations of charge and orbital parameters of resonancestabilized cations were reviewed in [23].…”

“…By the action of various Lewis and Brønsted acids alcohols II give rise to propynyl (alkynylcarbenium) cations XIII [23][24][25][26][27] that may also be represented as allenyl cations XIV (Scheme 3). Resonance-stabilized cationic intermediates XIII ↔ XIV are capable of reacting with arenes along two pathways.…”

Direct Nucleophilic SN1‐Type Reactions of Alcohols

“…[25] The use of less-reactive nucleophiles such as alkynylsilanes with alcohols 7 and 8 gave a complicated mixture of products derived from rearrangements of the formed carbocation. [26] The use of PTSA was applied to the synthesis of bicyclo[3.1.0]hexanes. [27,28] In addition, an efficient one-pot propargylation/cycloisomerization tandem process catalyzed by PTSA was developed for the synthesis of substituted oxazole derivatives, starting from propargylic alcohols and amides.…”

The chemistry of bisallenes

“…For example, derivative 83 can be obtained in good yield by a C 3 -coupling route, as demonstrated in Scheme 18, from a trimethylsilyl-protected propargyl alcohol, 82 [67]. …”