Carboboration of isocyanates with tris(pentafluorophenyl)borane and evidence for dissociative FLP chemistry of an acid–base pair

Abstract

We describe carboelementation reactions involving tris(pentafluorophenyl)borane [B(C6F5)3] with isocyanates, and contrast this reactivity with that of the heavier group 13 Lewis acids Al(C6F5)3 and Ga(C6F5)3.

Linkage Isomerism Leading to Contrasting Carboboration Chemistry: Access to Three Constitutional Isomers of a Borylated Phosphaalkene

“…The transition state TS 2 connects reactants to the E-i somero f2,but the overallbarrier (red pathway) is too high to be consistent with arapid reaction at roomt emperature. [8]: B1ÀO1 1.398(2);O 1 ÀP1 1.593(2), P1ÀC1 1.684(2), C1ÀB2 1.512(2); B1-O1-P1 125.35 (11);O 1-P1-C1 104.06(7), P1-C1-B2 112.07 (12).…”

“…The carboboration of isocyanates, valencei soelectronic analogues of phosphaketenes, was recently found to give rise to 1,2-carboborated productsa cross the C=Ob ond, which insert as econd equivalent of isocyanate to yield six-membered heterocycles. [10,11] In contrast, related isoelectronic compounds, such as hydrazoic acid and organica zides reactw ith BCF via a 1,1-carboboration accompanied by loss of dinitrogen to afford aminoboranes. [23] The PÀCb ond length is again consistent with significant pcharacter (1.692(2) ).…”

“…[7,8] In the case of tris(pentafluorophenyl)borane, related reactions are only possible using more reactive substrates, such as allenyl ketones and isocyanates. [9][10][11][12] It is these latters tudies, as well as relatedw ork exploring the hydroboration of phosphaalkynes, [13][14][15] that prompted us to explore the reactivityo fB CF towardst wo isomericf orms of ab oryl-phosphaethynolate, [B]OCPa nd [B]PCO( where [B] = N,N'-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2-diazaboryl). [16,17] Interestingly,t hese reactions give rise to three different borylatedp hosphaalkenes with the same molecular formula.…”

1,3-Carboboration of iodonium ylides

“…11 1,2-Carboboration of sp 2 -centres has been exemplified through the use of allenyl esters and ketones with B(C 6 F 5 ) 3 12 and recently isocyanates leading to 4-membered cyclic compounds exhibiting frustrated Lewis pair characteristics (Scheme 1b). 13 1,2-Carboboration of cyclic iminium ions with B(C 6 F 5 ) 3 , has also been reported under photochemical conditions leading to highly desirable tertiary amines. 14 These transformations demonstrate the proclivity of B(C 6 F 5 ) 3 to effect selectively 1,2-aryl transfers.…”

New Ga/N‐based Active Lewis Pairs, Trifunctional Ga–Ga Compounds, Reactions with Cyanamide and Dual Insertion of Isocyanate

“…The exceptional reactivity of 2 was demonstrated by C–H bond activation of alkynes, H–C≡C–R,, [8b] the insertion of R–N=C=N–R into the Al‐C(vinyl) bond ( 3 ; Scheme ) or C–C bond formation with t Bu‐N=C=O. [8b] Particularly noteworthy is the capability of these ALPs to oligomerize isocyanates[8b], or cyanamides,[8a] but to prevent the formation of thermodynamically favored cyclic trimers , . Acyclic oligomers of isocyanates are important synthons, while oligomers of nitriles have a backbone of alternating C and N atoms with the one‐dimensional delocalization of π‐electron density.…”