Anchimeric assistance by γ-aryl groups in reactions of organosilicon iodides

Abstract

The rates of reaction of the iodides (Me,Si),C(SiMe,C,H,Y) (SiMe,l), 1 (Y = p-OMe, p -M e , H, p-CI, or m-CF,), with (CF,),CHOH and CF,CH,OH (both containing CDCI,), MeOH (containing CCI,), and H, O (mixed with dioxane and CCI,) have been measured. In the reaction with the two fluorinated alcohols the spread of rates is very large, 1, Y = p -O M e being 1.9 x 1 O5 and 6.5 x 1 O4 times, respectively as reactive as 1, Y = rn-CF,, showing that the aryl group provides anchimeric assistance to the leaving of the iodide ion in the rate-determining step, thought t o be formation of the 1,3-aryl-bridged cation II. Unexpectedly, since this process is analogous to an electrophilic aromatic substitution, the substituent effects show excellent correlations w i t h a-constants. In the methanolysis and hydrolysis of 1 such an SN1 process appears to operate alongside an S, 2 process involving direct nucleophilic displacement of iodide b y the solvent, the S, 2 process being facilitated b y electron-withdrawal and the SN1 by electron-release b y Y; estimated values of the rate constants for the SN1 processes show reasonable correlation with o+-constants. Rates of the SN2 reactions of 1 with KSCN in MeCN have also been measured, and the substituent effects, involving an increase in rate b y a factor of only 2.3 o n going from I , Y = p -O M e t o 1, Y = m-CF,, are found t o be consistent with normal through-bond transmission of electronic effects.It was shown previously that in reactions of the iodides (Me,Si),C(SiR,I) with electrophiles, such as Ag' salts, iodine monochloride, or trifluoroacetic acid, rearranged products of the type (Me,Si),C(SiR,Me)(SiMe,X) can be formed, either exclusively or along with unrearranged products (Me,Si),-CSiR,X, apparently via a cation of the type I, Z = Me.' Analogous 1,3-migrations of a vinyl group were observed in the reaction of the iodide (Me,Si),C(SiMe,CH=CH,)(SiEt,I) with Ag' salts, and the much greater ease of reaction of the iodide (Me3Si),C(SiMe,CH=CH,)(SiMe,I) than of (Me,Si),CSiMe,I towards CF,CH,OH or CF,CO,H was attributed to nucleophilic assistance by the vinyl group to the ratedetermining departure of the iodide ion to form the cation I, R = Me,Z = CH=CH,.,The corresponding migration of a phenyl group was subsequently observed in the reaction of (Me,Si),C(SiMe,Ph)-(SiEt,I) with Ag' salts, and attributed to formation of the cation 11, R = Et, Y = H., That observation did not, however, show

Unusual mechanistic pathways. The novel chemistry of compounds with tris(trimethylsilyl)methyl or related ligands on siliconElectronic supplementary information (ESI) available: a complete list of the author's publications, with titles, on compounds with trisyl or related ligands on silicon (and for comparison, on germanium or tin) or species containing trisyl-type groups not on a metal or metalloid. See http://www.rsc.org/suppdata/dt/b1/b106509m/

“…(An explanation is offered in ref. 27 of why the correlation does not require use of σ ϩ -constants, as was expected since the bridged cation closely resembles a Wheland intermediate in classical electrophilic aromatic substitutions.) The data for the reactions in MeOH and H 2 O-dioxane, also shown in Table 1, are considered later.…”

“…20 The chloride (Me 26 That Ph and other aryl groups also provide substantial anchimeric assistance to the ionization was very effectively confirmed by the very large effects of the substituents Y in 1). 27 Values of log k, where k is the observed rate constant, against σ-constants are impressively linear in both cases, with unusually high ρ-values of Ϫ7.6 and Ϫ6.6, respectively. (An explanation is offered in ref.…”

“…30 ) In addition, as we have seen, the anchimeric assistance by aryl groups YC 6 H 4 varies over a wide range, in solvolysis of the iodides (Me 3 Si) 2 (YC 6 H 4 Me 2 Si)CSiMe 2 I in (CF 3 ) 2 CHOH. 27 The strongest assistance is provided by an NMe 2 group, but the reactivity of even the chloride (Me 3 Si) 2 -(Me 2 NMe 2 Si)CSiMe 2 Cl is so great that quantitative assessment has not yet been possible. 35 Hydrogen has been shown to bridge (see the section on crystallographic evidence) and can be expected to provide anchimeric assistance, but there is no conclusive evidence that it does so.…”

Structures of Tetrasilylmethane Derivatives C(SiXMe2)4 (X=H, F, Cl, Br) in the Gas Phase and their Dynamic Structures in Solution

“…[1][2][3][4] The most widely studied tetrasilylmethane derivatives have the general structures (XMe 2 Si) 4 C, (Me 3 Si) 3 CSiRR X, (PhMe 2 Si) 3 CSiRR X, and (Me 3 Si) 2 C(SiMe 2 X)(SiR 2 Y) (where R and R = Me, Et, Ph etc., and X and Y = H, halide, OAc etc.). [1][2][3][4][5] A range of simple bis-functionalized tetrasilylmethanes (XMe 2 Si) 2 C(SiMe 3 ) 2 (for example, X = H, 6-9 F, [9][10][11] Cl, 7,[9][10][11][12][13][14] Br, 6,7,9,15,16 I, 7,9,11,12,17 OH, 7,9,10,18,19 OMe, 10,[20][21][22] OAc, 7,10,20 O 2 CCF 3 , [10][11][12]…”

The gas-phase structure and some reactions of the bulky primary silane (Me3Si)3CSiH3 and the solid-state structure of the bulky dialkyl disilane [(Me3Si)3CSiH2]2

“…has parameters similar to those of (Me 3 Si) 3 CBPh 2 . However, this structure possesses shorter average Si-C central bonds of 1.914 (8) Å as well as slightly tilted trimethylsilyl groups and C methyl -Si-C methyl angles that appear to be less compressed at 107.0(11) • . Table 3 summarises some geometric parameters of compounds bearing the (Me 3 Si) 3 C-substituent discussed above.…”