η3-α-Silabenzyl complexes Cp*W(CO)2{η3(Si,C,C)-Si(p-Tol)2R} (1a, R = Me; 1b, R = p-Tol; Cp* = η5-C5Me5, p-Tol = p-tolyl), the first silicon analogues of η3-benzyl complexes, were synthesized by abstraction of DMAP (4-(dimethylamino)pyridine) with BPh3 either from (aryl)(DMAP·silylene)tungsten complexes 2a,b at room temperature or from (arylsilyl)(DMAP)tungsten complexes 3a,b under irradiation. Complex 1 was demonstrated to be a key intermediate for the interconversion between 2 and 3 and to serve as a synthetic equivalent for both base-free silylene complexes and coordinatively unsaturated silyl complexes.

Reactions of Cp*(CO) 2 W(DMAP)Me [1, Cp* = η 5 -C 5 Me 5 , DMAP = 4-(dimethylamino)pyridine] with trialkylsilanes HSiR 2 Et (R = Me, Et) afforded (DMAP)(trialkylsilyl)tungsten complexes Cp*-(CO) 2 W(DMAP)(SiR 2 Et) (2a, R = Me; 2b, R = Et).On the other hand, a reaction of 1 with dimethoxy(methyl)silane, HSi(OMe) 2 Me, gave a DMAP-stabilized (dimethoxysilylene)tungsten complex, cis-Cp*(CO) 2 W(Me){dSi(OMe) 2 3 DMAP} (cis-3), through 1,2-migration of a methyl group from silicon to tungsten. Silylene complex cis-3 isomerized to its trans-isomer trans-3.

η 3 -α-Silabenzyl complexes Cp*M(CO) 2 -{η 3 (Si,C,C)-Si(p-Tol) 3 } (M = Mo (1-Mo), W (1-W)) reacted with 2-substituted pyridines NC 5 H 4 (2-EH n ) (E = O, S (n = 1); N (n = 2)) under mild conditions to give M−Si−E−C−N (E = O, N), W−Si−N−C−S, and M−E−C−N (E = S, N)metallacycles depending on the metal M or heteroatom E. These three kinds of metallacycles were characterized by spectroscopy, elemental analysis, and X-ray crystallography. The first two silametallacycles take on some silylene complex character and are considered to form via (aryl)silylene complex intermediates generated by cleavage of the Si−C(aryl) bond in the η 3 -α-silabenzyl ligand of 1-Mo and 1-W. ■ INTRODUCTIONActivation of robust Si−C(aryl) bonds induced by transitionmetal complexes is of considerable interest because this can be applied to the direct functionalization of arylsilanes. 1 Many catalytic and stoichiometric reactions via Si−C(aryl) bond activation have been developed using late-transition-metal complexes. 1,2 In these reactions, oxidative addition of Si−C(aryl) bonds to low-valent metal centers plays a crucial role. 2 In contrast, the counterparts using early-transition-metal complexes have received less attention.In addition to the oxidative addition, 1,2-migration of aryl substituents on silicon to metal in coordinatively unsaturated arylsilyl complexes producing (aryl)silylene complexes has been proposed as a crucial process for a few Si−C(aryl) bond activation reactions. 2b,3 We have recently found a reversible 1,2-aryl migration in tungsten−silicon complexes 4 and finally succeeded in isolation of key intermediates of this migration reaction, i.e. η 3 -α-silabenzyl complexes Cp*W(CO) 2 {η 3 (Si,C,C)-Si(p-Tol) 2 R} (R = p-Tol (1-W), Me), the first silicon analogues of η 3 -benzyl complexes. 5a We also found that the reaction of these complexes with 4-(dimethylamino)pyridine (DMAP) gave DMAP-stabilized (aryl)silylene complexes Cp*W(CO) 2 (pTol){Si(p-Tol)R·DMAP} via Si−C(aryl) bond cleavage (1,2-aryl migration) (Scheme 1). This result prompted us to examine the reactions of the η 3 -α-silabenzyl tungsten complex 1-W and its molybdenum analogue 1-Mo 5b with 2-substituted pyridines NC 5 H 4 (2-EH n ) (E = O, S (n = 1); E = N (n = 2)). These substrates bear multiple nucleophilic heteroatoms (N and E) that can attack the electrophilic silicon atom as well as protic hydrogen(s) on E that can protonate the metal center. As a result, we developed unique conversion reactions via Si−C(aryl) bond cleavage and arene elimination to give silametallacycles. We report here the direct conversion of a Si−C(aryl) bond to Si−heteroatom bonds starting from 1-W and 1-Mo. ■ RESULTS AND DISCUSSIONReactions of η 3 -α-Silabenzyl Complexes 1-Mo and 1-W with 2-Substituted Pyridines. 1-Mo and 1-W reacted with 2-substituted pyridines (2-hydroxypyridine, 2-mercaptopyridine, and 2-aminopyridine) to give three kinds of metallacycles 2−6 depending on the metal and the 2-EH n substituent (routes A−C in Scheme 2). Complexes 1-Mo and 1-W reacted with 2-hydroxypy...

The (η3-α-silabenzyl)molybdenum complex Cp*Mo(CO)2{η3(Si,C,C)-Si(p-Tol)3} (1; Cp* = η5-C5Me5, p-Tol = p-C6H4Me) was synthesized in high yield by removal of 4-(dimethylamino)pyridine (DMAP) from the (arylsilyl)(DMAP)molybdenum complex Cp*Mo(CO)2(DMAP){Si(p-Tol)3} (2) with BPh3. The precursor, complex 2, was readily prepared by reaction of the (DMAP)(methyl)molybdenum complex Cp*Mo(CO)2(DMAP)Me (3) with tri-p-tolylsilane (HSi(p-Tol)3) through methane elimination. Study on the reactivity of 1 toward DMAP and nitrile revealed that complex 1 serves as a synthetic equivalent of the 16-electron silyl complex Cp*Mo(CO)2{Si(p-Tol)3}. Thus, complex 1 reacted with DMAP quantitatively at room temperature to reproduce arylsilyl complex 2 through dissociation of the arene carbon atoms coordinated to molybdenum. Complex 1 also reacted with acetonitrile at room temperature to give the N-silyliminoacyl complex Cp*Mo(CO)2{η2(C,N)-C(Me)NSi(p-Tol)3} (4) exclusively via cleavage of the Mo–C(arene) bonds followed by insertion of nitrile into the Mo–Si bond.

DMAP-stabilized (p-tolyl)(silylene) complexes Cp*(CO) 2 W(p-Tol){dSi(p-Tol)R 0 3 DMAP} [1a, R 0 = Me; 1b, R 0 = p-Tol; DMAP = 4-(dimethylamino)pyridine] reacted with excess nitrile RCN (R = Me, Et, i-Pr, t-Bu) in toluene to give κ 2 C,N-N-silyliminoacyl tungsten complexes Cp*(CO) 2 W{κ 2 C,N-C(R)dNSi-(p-Tol) 2 R 0