Five‐Membered Metallacyclic Allenoids: Synthesis and Structure of Remarkably Stable Strongly Distorted Cyclic Allene Derivatives

Abstract

Metallocene lends a hand: Treatment of [bis(alkynyl)HfCp2] complexes with HB(C6F5)2 results in 1,1‐hydroboration and subsequent carbon–carbon coupling to yield the corresponding metallacyclopenta‐1,2‐diene derivatives (see structure of the trimethylsilyl derivative; Hf brown, B red, Si blue, F green, C black). X‐ray diffraction and a quantum chemical analysis reveal an electronic structure between a bent cyclic allene and a coordinated butenyne.

Reversible 1,1‐Hydroboration: Boryl Insertion into a CN Bond and Competitive Elimination of HBR2 or RH

“…B(C 6 F 5 ) 3 ). Although afew examples of formal alkyne 1,1-hydroboration involving the use of activat-ing reagents are known, [13] the straightforward 1,1-hydroboration of alkenes or alkynes by common boranes,s uch as HBR 2 ,r emains unknown. Although afew examples of formal alkyne 1,1-hydroboration involving the use of activat-ing reagents are known, [13] the straightforward 1,1-hydroboration of alkenes or alkynes by common boranes,s uch as HBR 2 ,r emains unknown.…”

Straightforward synthesis of five-membered metallacycloallenes: 1-Zirconacyclopenta-2,3-diene compounds derived from 1,3-enynes

“…Erker and co-workers indicated that complex 2a (M = Hf) showed fast epimerization at 90 °C [9], and so did 2c (M = Zr) at ambient temperature [10]. NMR observation of 4 demonstrated, on the other hand, that two Cp signals did not coalesce at 100 °C, indicating very slow epimerization of 4 ( Fig.…”

Frustrated Lewis Pair Mediated 1,2‐Hydrocarbation of Alkynes

“…[9][10][11][12] Whereas most FLPs contain boron-based Lewis acids,c ationic carbon Lewis acids can be effective in FLPs. [13] Of specific relevance to this study is the reaction of alkynes with [Ph 3 C][BF 4 ]i naFLP with (o-tolyl) 3 Pt hat resulted in either intramolecular Friedel-Crafts reactivity or 1,2-addition to the alkyne (Scheme 3). [14] In these cases,t he absence of aC À H functionality at the electrophilic carbon center precludes hydrocarbation reactivity.A sp art of our studies on carbon Lewis acid based FLPs, [15] we were interested in determining the reaction outcome(s) from combining the carbon Lewis acid N-methylacridinium ([1] + )w ith alkynes,w ith and without additional Lewis bases.T hese experiments led to the discovery of an ew FLP-mediated route for the 1,2-hydrocarbation of alkynes.T he mechanism of this reaction is distinct from that reported by Mayr and co-workers and those established for alkyne hydroboration reactions.…”

“…In contrast, an equimolar mixture of [1][BArCl],2,6-lutidine (which forms an FLP), [15a] and 4-ethynylanisole resulted in aslow reaction. After 48 hat 20 8 8Ci nD CM, only partial consumption of [1][BArCl] had occurred;nevertheless,crystallization (by layering the sample with pentane) and X-ray diffraction studies revealed formation of the Z 1,2-hydrocarbation product [3][BArCl] (Scheme 5, left). Ther eaction was accelerated at higher temperature (60 8 8C, 72 h) but led to the observation of adifferent product in the 1 HNMR spectrum, consistent with the trans isomer, [2] + (which has two diagnostic vinylic doublets with 3 J H,H = 16 Hz).…”